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diff --git a/doc/rfc/rfc3095.txt b/doc/rfc/rfc3095.txt new file mode 100644 index 0000000..b89088e --- /dev/null +++ b/doc/rfc/rfc3095.txt @@ -0,0 +1,9411 @@ + + + + + + +Network Working Group C. Bormann, Editor, TZI/Uni Bremen +Request for Comments: 3095 C. Burmeister, Matsushita +Category: Standards Track M. Degermark, Univ. of Arizona + H. Fukushima, Matsushita + H. Hannu, Ericsson + L-E. Jonsson, Ericsson + R. Hakenberg, Matsushita + T. Koren, Cisco + K. Le, Nokia + Z. Liu, Nokia + A. Martensson, Ericsson + A. Miyazaki, Matsushita + K. Svanbro, Ericsson + T. Wiebke, Matsushita + T. Yoshimura, NTT DoCoMo + H. Zheng, Nokia + July 2001 + + + RObust Header Compression (ROHC): + Framework and four profiles: RTP, UDP, ESP, and uncompressed + +Status of this Memo + + This document specifies an Internet standards track protocol for the + Internet community, and requests discussion and suggestions for + improvements. Please refer to the current edition of the "Internet + Official Protocol Standards" (STD 1) for the standardization state + and status of this protocol. Distribution of this memo is unlimited. + +Copyright Notice + + Copyright (C) The Internet Society (2001). All Rights Reserved. + +Abstract + + This document specifies a highly robust and efficient header + compression scheme for RTP/UDP/IP (Real-Time Transport Protocol, User + Datagram Protocol, Internet Protocol), UDP/IP, and ESP/IP + (Encapsulating Security Payload) headers. + + Existing header compression schemes do not work well when used over + links with significant error rates and long round-trip times. For + many bandwidth limited links where header compression is essential, + such characteristics are common. + + + + + + +Bormann, et al. Standards Track [Page 1] + +RFC 3095 Robust Header Compression July 2001 + + + This is done in a framework designed to be extensible. For example, + a scheme for compressing TCP/IP headers will be simple to add, and is + in development. Headers specific to Mobile IPv4 are not subject to + special treatment, but are expected to be compressed sufficiently + well by the provided methods for compression of sequences of + extension headers and tunneling headers. For the most part, the same + will apply to work in progress on Mobile IPv6, but future work might + be required to handle some extension headers, when a standards track + Mobile IPv6 has been completed. + +Table of Contents + + 1. Introduction....................................................6 + 2. Terminology.....................................................8 + 2.1. Acronyms.....................................................13 + 3. Background.....................................................14 + 3.1. Header compression fundamentals..............................14 + 3.2. Existing header compression schemes..........................14 + 3.3. Requirements on a new header compression scheme..............16 + 3.4. Classification of header fields..............................17 + 4. Header compression framework...................................18 + 4.1. Operating assumptions........................................18 + 4.2. Dynamicity...................................................19 + 4.3. Compression and decompression states.........................21 + 4.3.1. Compressor states..........................................21 + 4.3.1.1. Initialization and Refresh (IR) State....................22 + 4.3.1.2. First Order (FO) State...................................22 + 4.3.1.3. Second Order (SO) State..................................22 + 4.3.2. Decompressor states........................................23 + 4.4. Modes of operation...........................................23 + 4.4.1. Unidirectional mode -- U-mode..............................24 + 4.4.2. Bidirectional Optimistic mode -- O-mode....................25 + 4.4.3. Bidirectional Reliable mode -- R-mode......................25 + 4.5. Encoding methods.............................................25 + 4.5.1. Least Significant Bits (LSB) encoding .....................25 + 4.5.2. Window-based LSB encoding (W-LSB encoding).................28 + 4.5.3. Scaled RTP Timestamp encoding .............................28 + 4.5.4. Timer-based compression of RTP Timestamp...................31 + 4.5.5. Offset IP-ID encoding......................................34 + 4.5.6. Self-describing variable-length values ....................35 + 4.5.7. Encoded values across several fields in compressed headers 36 + 4.6. Errors caused by residual errors.............................36 + 4.7. Impairment considerations....................................37 + 5. The protocol...................................................39 + 5.1. Data structures..............................................39 + 5.1.1. Per-channel parameters.....................................39 + 5.1.2. Per-context parameters, profiles...........................40 + 5.1.3. Contexts and context identifiers ..........................41 + + + +Bormann, et al. Standards Track [Page 2] + +RFC 3095 Robust Header Compression July 2001 + + + 5.2. ROHC packets and packet types................................41 + 5.2.1. ROHC feedback .............................................43 + 5.2.2. ROHC feedback format ......................................45 + 5.2.3. ROHC IR packet type .......................................47 + 5.2.4. ROHC IR-DYN packet type ...................................48 + 5.2.5. ROHC segmentation..........................................49 + 5.2.5.1. Segmentation usage considerations........................49 + 5.2.5.2. Segmentation protocol....................................50 + 5.2.6. ROHC initial decompressor processing.......................51 + 5.2.7. ROHC RTP packet formats from compressor to decompressor....53 + 5.2.8. Parameters needed for mode transition in ROHC RTP..........54 + 5.3. Operation in Unidirectional mode.............................55 + 5.3.1. Compressor states and logic (U-mode).......................55 + 5.3.1.1. State transition logic (U-mode)..........................55 + 5.3.1.1.1. Optimistic approach, upwards transition................55 + 5.3.1.1.2. Timeouts, downward transition..........................56 + 5.3.1.1.3. Need for updates, downward transition..................56 + 5.3.1.2. Compression logic and packets used (U-mode)..............56 + 5.3.1.3. Feedback in Unidirectional mode..........................56 + 5.3.2. Decompressor states and logic (U-mode).....................56 + 5.3.2.1. State transition logic (U-mode)..........................57 + 5.3.2.2. Decompression logic (U-mode).............................57 + 5.3.2.2.1. Decide whether decompression is allowed................57 + 5.3.2.2.2. Reconstruct and verify the header......................57 + 5.3.2.2.3. Actions upon CRC failure...............................58 + 5.3.2.2.4. Correction of SN LSB wraparound........................60 + 5.3.2.2.5. Repair of incorrect SN updates.........................61 + 5.3.2.3. Feedback in Unidirectional mode..........................62 + 5.4. Operation in Bidirectional Optimistic mode...................62 + 5.4.1. Compressor states and logic (O-mode).......................62 + 5.4.1.1. State transition logic...................................63 + 5.4.1.1.1. Negative acknowledgments (NACKs), downward transition..63 + 5.4.1.1.2. Optional acknowledgments, upwards transition...........63 + 5.4.1.2. Compression logic and packets used.......................63 + 5.4.2. Decompressor states and logic (O-mode).....................64 + 5.4.2.1. Decompression logic, timer-based timestamp decompression.64 + 5.4.2.2. Feedback logic (O-mode)..................................64 + 5.5. Operation in Bidirectional Reliable mode.....................65 + 5.5.1. Compressor states and logic (R-mode).......................65 + 5.5.1.1. State transition logic (R-mode)..........................65 + 5.5.1.1.1. Upwards transition.....................................65 + 5.5.1.1.2. Downward transition....................................66 + 5.5.1.2. Compression logic and packets used (R-mode)..............66 + 5.5.2. Decompressor states and logic (R-mode).....................68 + 5.5.2.1. Decompression logic (R-mode).............................68 + 5.5.2.2. Feedback logic (R-mode)..................................68 + 5.6. Mode transitions.............................................69 + 5.6.1. Compression and decompression during mode transitions......70 + + + +Bormann, et al. Standards Track [Page 3] + +RFC 3095 Robust Header Compression July 2001 + + + 5.6.2. Transition from Unidirectional to Optimistic mode..........71 + 5.6.3. From Optimistic to Reliable mode...........................72 + 5.6.4. From Unidirectional to Reliable mode.......................72 + 5.6.5. From Reliable to Optimistic mode...........................72 + 5.6.6. Transition to Unidirectional mode..........................73 + 5.7. Packet formats...............................................74 + 5.7.1. Packet type 0: UO-0, R-0, R-0-CRC .........................78 + 5.7.2. Packet type 1 (R-mode): R-1, R-1-TS, R-1-ID ...............79 + 5.7.3. Packet type 1 (U/O-mode): UO-1, UO-1-ID, UO-1-TS ..........80 + 5.7.4. Packet type 2: UOR-2 ......................................82 + 5.7.5. Extension formats..........................................83 + 5.7.5.1. RND flags and packet types...............................88 + 5.7.5.2. Flags/Fields in context..................................89 + 5.7.6. Feedback packets and formats...............................90 + 5.7.6.1. Feedback formats for ROHC RTP............................90 + 5.7.6.2. ROHC RTP Feedback options................................91 + 5.7.6.3. The CRC option...........................................92 + 5.7.6.4. The REJECT option........................................92 + 5.7.6.5. The SN-NOT-VALID option..................................92 + 5.7.6.6. The SN option............................................93 + 5.7.6.7. The CLOCK option.........................................93 + 5.7.6.8. The JITTER option........................................93 + 5.7.6.9. The LOSS option..........................................94 + 5.7.6.10. Unknown option types....................................94 + 5.7.6.11. RTP feedback example....................................94 + 5.7.7. RTP IR and IR-DYN packets..................................96 + 5.7.7.1. Basic structure of the IR packet.........................96 + 5.7.7.2. Basic structure of the IR-DYN packet.....................98 + 5.7.7.3. Initialization of IPv6 Header [IPv6].....................99 + 5.7.7.4. Initialization of IPv4 Header [IPv4, section 3.1].......100 + 5.7.7.5. Initialization of UDP Header [RFC-768]..................101 + 5.7.7.6. Initialization of RTP Header [RTP]......................102 + 5.7.7.7. Initialization of ESP Header [ESP, section 2]...........103 + 5.7.7.8. Initialization of Other Headers.........................104 + 5.8. List compression............................................104 + 5.8.1. Table-based item compression..............................105 + 5.8.1.1. Translation table in R-mode.............................105 + 5.8.1.2. Translation table in U/O-modes..........................106 + 5.8.2. Reference list determination..............................106 + 5.8.2.1. Reference list in R-mode and U/O-mode...................107 + 5.8.3. Encoding schemes for the compressed list..................109 + 5.8.4. Special handling of IP extension headers..................112 + 5.8.4.1. Next Header field.......................................112 + 5.8.4.2. Authentication Header (AH)..............................114 + 5.8.4.3. Encapsulating Security Payload Header (ESP).............115 + 5.8.4.4. GRE Header [RFC 2784, RFC 2890].........................117 + 5.8.5. Format of compressed lists in Extension 3.................119 + 5.8.5.1. Format of IP Extension Header(s) field..................119 + + + +Bormann, et al. Standards Track [Page 4] + +RFC 3095 Robust Header Compression July 2001 + + + 5.8.5.2. Format of Compressed CSRC List..........................120 + 5.8.6. Compressed list formats...................................120 + 5.8.6.1. Encoding Type 0 (generic scheme)........................120 + 5.8.6.2. Encoding Type 1 (insertion only scheme).................122 + 5.8.6.3. Encoding Type 2 (removal only scheme)...................123 + 5.8.6.4. Encoding Type 3 (remove then insert scheme).............124 + 5.8.7. CRC coverage for extension headers........................124 + 5.9. Header compression CRCs, coverage and polynomials...........125 + 5.9.1. IR and IR-DYN packet CRCs.................................125 + 5.9.2. CRCs in compressed headers................................125 + 5.10. ROHC UNCOMPRESSED -- no compression (Profile 0x0000).......126 + 5.10.1. IR packet................................................126 + 5.10.2. Normal packet............................................127 + 5.10.3. States and modes.........................................128 + 5.10.4. Feedback.................................................129 + 5.11. ROHC UDP -- non-RTP UDP/IP compression (Profile 0x0002)....129 + 5.11.1. Initialization...........................................130 + 5.11.2. States and modes.........................................130 + 5.11.3. Packet types.............................................131 + 5.11.4. Extensions...............................................132 + 5.11.5. IP-ID....................................................133 + 5.11.6. Feedback.................................................133 + 5.12. ROHC ESP -- ESP/IP compression (Profile 0x0003)............133 + 5.12.1. Initialization...........................................133 + 5.12.2. Packet types.............................................134 + 6. Implementation issues.........................................134 + 6.1. Reverse decompression.......................................134 + 6.2. RTCP........................................................135 + 6.3. Implementation parameters and signals.......................136 + 6.3.1. ROHC implementation parameters at compressor..............137 + 6.3.2. ROHC implementation parameters at decompressor............138 + 6.4. Handling of resource limitations at the decompressor........139 + 6.5. Implementation structures...................................139 + 6.5.1. Compressor context........................................139 + 6.5.2. Decompressor context......................................141 + 6.5.3. List compression: Sliding windows in R-mode and U/O-mode..142 + 7. Security Considerations.......................................143 + 8. IANA Considerations...........................................144 + 9. Acknowledgments...............................................145 + 10. Intellectual Property Right Claim Considerations.............145 + 11. References...................................................146 + 11.1. Normative References.......................................146 + 11.2. Informative References.....................................147 + 12. Authors' Addresses...........................................148 + Appendix A. Detailed classification of header fields.............152 + A.1. General classification......................................153 + A.1.1. IPv6 header fields........................................153 + A.1.2. IPv4 header fields........................................155 + + + +Bormann, et al. Standards Track [Page 5] + +RFC 3095 Robust Header Compression July 2001 + + + A.1.3. UDP header fields.........................................157 + A.1.4. RTP header fields.........................................157 + A.1.5. Summary for IP/UDP/RTP....................................159 + A.2. Analysis of change patterns of header fields................159 + A.2.1. IPv4 Identification.......................................162 + A.2.2. IP Traffic-Class / Type-Of-Service........................163 + A.2.3. IP Hop-Limit / Time-To-Live...............................163 + A.2.4. UDP Checksum..............................................163 + A.2.5. RTP CSRC Counter..........................................164 + A.2.6. RTP Marker................................................164 + A.2.7. RTP Payload Type..........................................164 + A.2.8. RTP Sequence Number.......................................164 + A.2.9. RTP Timestamp.............................................164 + A.2.10. RTP Contributing Sources (CSRC)..........................165 + A.3. Header compression strategies...............................165 + A.3.1. Do not send at all........................................165 + A.3.2. Transmit only initially...................................165 + A.3.3. Transmit initially, but be prepared to update.............166 + A.3.4. Be prepared to update or send as-is frequently............166 + A.3.5. Guarantee continuous robustness...........................166 + A.3.6. Transmit as-is in all packets.............................167 + A.3.7. Establish and be prepared to update delta.................167 + Full Copyright Statement..........................................168 + +1. Introduction + + During the last five years, two communication technologies in + particular have become commonly used by the general public: cellular + telephony and the Internet. Cellular telephony has provided its + users with the revolutionary possibility of always being reachable + with reasonable service quality no matter where they are. The main + service provided by the dedicated terminals has been speech. The + Internet, on the other hand, has from the beginning been designed for + multiple services and its flexibility for all kinds of usage has been + one of its strengths. Internet terminals have usually been general- + purpose and have been attached over fixed connections. The + experienced quality of some services (such as Internet telephony) has + sometimes been low. + + Today, IP telephony is gaining momentum thanks to improved technical + solutions. It seems reasonable to believe that in the years to come, + IP will become a commonly used way to carry telephony. Some future + cellular telephony links might also be based on IP and IP telephony. + Cellular phones may have become more general-purpose, and may have IP + stacks supporting not only audio and video, but also web browsing, + email, gaming, etc. + + + + + +Bormann, et al. Standards Track [Page 6] + +RFC 3095 Robust Header Compression July 2001 + + + One of the scenarios we are envisioning might then be the one in + Figure 1.1, where two mobile terminals are communicating with each + other. Both are connected to base stations over cellular links, and + the base stations are connected to each other through a wired (or + possibly wireless) network. Instead of two mobile terminals, there + could of course be one mobile and one wired terminal, but the case + with two cellular links is technically more demanding. + + Mobile Base Base Mobile + Terminal Station Station Terminal + + + | ~ ~ ~ \ / \ / ~ ~ ~ ~ | + | | | | + +--+ | | +--+ + | | | | | | + | | | | | | + +--+ | | +--+ + | | + |=========================| + + Cellular Wired Cellular + Link Network Link + + Figure 1.1 : Scenario for IP telephony over cellular links + + It is obvious that the wired network can be IP-based. With the + cellular links, the situation is less clear. IP could be terminated + in the fixed network, and special solutions implemented for each + supported service over the cellular link. However, this would limit + the flexibility of the services supported. If technically and + economically feasible, a solution with pure IP all the way from + terminal to terminal would have certain advantages. However, to make + this a viable alternative, a number of problems have to be addressed, + in particular problems regarding bandwidth efficiency. + + For cellular phone systems, it is of vital importance to use the + scarce radio resources in an efficient way. A sufficient number of + users per cell is crucial, otherwise deployment costs will be + prohibitive. The quality of the voice service should also be as good + as in today's cellular systems. It is likely that even with support + for new services, lower quality of the voice service is acceptable + only if costs are significantly reduced. + + + + + + + + +Bormann, et al. Standards Track [Page 7] + +RFC 3095 Robust Header Compression July 2001 + + + A problem with IP over cellular links when used for interactive voice + conversations is the large header overhead. Speech data for IP + telephony will most likely be carried by RTP [RTP]. A packet will + then, in addition to link layer framing, have an IP [IPv4] header (20 + octets), a UDP [UDP] header (8 octets), and an RTP header (12 octets) + for a total of 40 octets. With IPv6 [IPv6], the IP header is 40 + octets for a total of 60 octets. The size of the payload depends on + the speech coding and frame sizes being used and may be as low as + 15-20 octets. + + From these numbers, the need for reducing header sizes for efficiency + reasons is obvious. However, cellular links have characteristics + that make header compression as defined in [IPHC,CRTP] perform less + than well. The most important characteristic is the lossy behavior + of cellular links, where a bit error rate (BER) as high as 1e-3 must + be accepted to keep the radio resources efficiently utilized. In + severe operating situations, the BER can be as high as 1e-2. The + other problematic characteristic is the long round-trip time (RTT) of + the cellular link, which can be as high as 100-200 milliseconds. An + additional problem is that the residual BER is nontrivial, i.e., + lower layers can sometimes deliver frames containing undetected + errors. A viable header compression scheme for cellular links must + be able to handle loss on the link between the compression and + decompression point as well as loss before the compression point. + + Bandwidth is the most costly resource in cellular links. Processing + power is very cheap in comparison. Implementation or computational + simplicity of a header compression scheme is therefore of less + importance than its compression ratio and robustness. + +2. Terminology + + The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", + "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this + document are to be interpreted as described in RFC 2119. + + BER + + Bit Error Rate. Cellular radio links can have a fairly high BER. + In this document BER is usually given as a probability, but one + also needs to consider the error distribution as bit errors are + not independent. + + + + + + + + + +Bormann, et al. Standards Track [Page 8] + +RFC 3095 Robust Header Compression July 2001 + + + Cellular links + + Wireless links between mobile terminals and base stations. + + Compression efficiency + + The performance of a header compression scheme can be described + with three parameters: compression efficiency, robustness and + compression transparency. The compression efficiency is + determined by how much the header sizes are reduced by the + compression scheme. + + Compression transparency + + The performance of a header compression scheme can be described + with three parameters: compression efficiency, robustness, and + compression transparency. The compression transparency is a + measure of the extent to which the scheme ensures that the + decompressed headers are semantically identical to the original + headers. If all decompressed headers are semantically identical + to the corresponding original headers, the transparency is 100 + percent. Compression transparency is high when damage propagation + is low. + + Context + + The context of the compressor is the state it uses to compress a + header. The context of the decompressor is the state it uses to + decompress a header. Either of these or the two in combination + are usually referred to as "context", when it is clear which is + intended. The context contains relevant information from previous + headers in the packet stream, such as static fields and possible + reference values for compression and decompression. Moreover, + additional information describing the packet stream is also part + of the context, for example information about how the IP + Identifier field changes and the typical inter-packet increase in + sequence numbers or timestamps. + + Context damage + + When the context of the decompressor is not consistent with the + context of the compressor, decompression may fail to reproduce the + original header. This situation can occur when the context of the + decompressor has not been initialized properly or when packets + have been lost or damaged between compressor and decompressor. + + + + + + +Bormann, et al. Standards Track [Page 9] + +RFC 3095 Robust Header Compression July 2001 + + + Packets which cannot be decompressed due to inconsistent contexts + are said to be lost due to context damage. Packets that are + decompressed but contain errors due to inconsistent contexts are + said to be damaged due to context damage. + + Context repair mechanism + + Context repair mechanisms are mechanisms that bring the contexts + in sync when they were not. This is needed to avoid excessive + loss due to context damage. Examples are the context request + mechanism of CRTP, the NACK mechanisms of O- and R-mode, and the + periodic refreshes of U-mode. + + Note that there are also mechanisms that prevent (some) context + inconsistencies from occurring, for example the ACK-based updates + of the context in R-mode, the repetitions after change in U- and + O-mode, and the CRCs which protect context updating information. + + CRC-DYNAMIC + + Opposite of CRC-STATIC. + + CRC-STATIC + + A CRC over the original header is the primary mechanism used by + ROHC to detect incorrect decompression. In order to decrease + computational complexity, the fields of the header are + conceptually rearranged when the CRC is computed, so that it is + first computed over octets which are static (called CRC-STATIC in + this document) and then over octets whose values are expected to + change between packets (CRC-DYNAMIC). In this manner, the + intermediate result of the CRC computation, after it has covered + the CRC-STATIC fields, can be reused for several packets. The + restarted CRC computation only covers the CRC-DYNAMIC octets. See + section 5.9. + + Damage propagation + + Delivery of incorrect decompressed headers, due to errors in + (i.e., loss of or damage to) previous header(s) or feedback. + + Loss propagation + + Loss of headers, due to errors in (i.e., loss of or damage to) + previous header(s)or feedback. + + + + + + +Bormann, et al. Standards Track [Page 10] + +RFC 3095 Robust Header Compression July 2001 + + + Error detection + + Detection of errors. If error detection is not perfect, there + will be residual errors. + + Error propagation + + Damage propagation or loss propagation. + + Header compression profile + + A header compression profile is a specification of how to compress + the headers of a certain kind of packet stream over a certain kind + of link. Compression profiles provide the details of the header + compression framework introduced in this document. The profile + concept makes use of profile identifiers to separate different + profiles which are used when setting up the compression scheme. + All variations and parameters of the header compression scheme + that are not part of the context state are handled by different + profile identifiers. + + Packet + + Generally, a unit of transmission and reception (protocol data + unit). Specifically, when contrasted with "frame", the packet + compressed and then decompressed by ROHC. Also called + "uncompressed packet". + + Packet Stream + + A sequence of packets where the field values and change patterns + of field values are such that the headers can be compressed using + the same context. + + Pre-HC links + + The Pre-HC links are all links that a packet has traversed before + the header compression point. If we consider a path with cellular + links as first and last hops, the Pre-HC links for the compressor + at the last link are the first cellular link plus the wired links + in between. + + Residual error + + Error introduced during transmission and not detected by lower- + layer error detection schemes. + + + + + +Bormann, et al. Standards Track [Page 11] + +RFC 3095 Robust Header Compression July 2001 + + + Robustness + + The performance of a header compression scheme can be described + with three parameters: compression efficiency, robustness, and + compression transparency. A robust scheme tolerates loss and + residual errors on the link over which header compression takes + place without losing additional packets or introducing additional + errors in decompressed headers. + + RTT + + The RTT (round-trip time) is the time elapsing from the moment the + compressor sends a packet until it receives feedback related to + that packet (when such feedback is sent). + + Spectrum efficiency + + Radio resources are limited and expensive. Therefore they must be + used efficiently to make the system economically feasible. In + cellular systems this is achieved by maximizing the number of + users served within each cell, while the quality of the provided + services is kept at an acceptable level. A consequence of + efficient spectrum use is a high rate of errors (frame loss and + residual bit errors), even after channel coding with error + correction. + + String + + A sequence of headers in which the values of all fields being + compressed change according to a pattern which is fixed with + respect to a sequence number. Each header in a string can be + compressed by representing it with a ROHC header which essentially + only carries an encoded sequence number. Fields not being + compressed (e.g., random IP-ID, UDP Checksum) are irrelevant to + this definition. + + Timestamp stride + + The timestamp stride (TS_STRIDE) is the expected increase in the + timestamp value between two RTP packets with consecutive sequence + numbers. + + + + + + + + + + +Bormann, et al. Standards Track [Page 12] + +RFC 3095 Robust Header Compression July 2001 + + +2.1. Acronyms + + This section lists most acronyms used for reference. + + AH Authentication Header. + CID Context Identifier. + CRC Cyclic Redundancy Check. Error detection mechanism. + CRTP Compressed RTP. RFC 2508. + CTCP Compressed TCP. Also called VJ header compression. RFC 1144. + ESP Encapsulating Security Payload. + FC Full Context state (decompressor). + FO First Order state (compressor). + GRE Generic Routing Encapsulation. RFC 2784, RFC 2890. + HC Header Compression. + IPHC IP Header Compression. RFC 2507. + IPX Flag in Extension 2. + IR Initiation and Refresh state (compressor). Also IR packet. + IR-DYN IR-DYN packet. + LSB Least Significant Bits. + MRRU Maximum Reconstructed Reception Unit. + MTU Maximum Transmission Unit. + MSB Most Significant Bits. + NBO Flag indicating whether the IP-ID is in Network Byte Order. + NC No Context state (decompressor). + O-mode Bidirectional Optimistic mode. + PPP Point-to-Point Protocol. + R-mode Bidirectional Reliable mode. + RND Flag indicating whether the IP-ID behaves randomly. + ROHC RObust Header Compression. + RTCP Real-Time Control Protocol. See RTP. + RTP Real-Time Protocol. RFC 1889. + RTT Round Trip Time (see section 2). + SC Static Context state (decompressor). + SN (compressed) Sequence Number. Usually RTP Sequence Number. + SO Second Order state (compressor). + SPI Security Parameters Index. + SSRC Sending source. Field in RTP header. + CSRC Contributing source. Optional list of CSRCs in RTP header. + TC Traffic Class. Octet in IPv6 header. See also TOS. + TOS Type Of Service. Octet in IPv4 header. See also TC. + TS (compressed) RTP Timestamp. + U-mode Unidirectional mode. + W-LSB Window based LSB encoding. See section 4.5.2. + + + + + + + + +Bormann, et al. Standards Track [Page 13] + +RFC 3095 Robust Header Compression July 2001 + + +3. Background + + This chapter provides a background to the subject of header + compression. The fundamental ideas are described together with + existing header compression schemes. Their drawbacks and + requirements are then discussed, providing motivation for new header + compression solutions. + +3.1. Header compression fundamentals + + The main reason why header compression can be done at all is the fact + that there is significant redundancy between header fields, both + within the same packet header but in particular between consecutive + packets belonging to the same packet stream. By sending static field + information only initially and utilizing dependencies and + predictability for other fields, the header size can be significantly + reduced for most packets. + + Relevant information from past packets is maintained in a context. + The context information is used to compress (decompress) subsequent + packets. The compressor and decompressor update their contexts upon + certain events. Impairment events may lead to inconsistencies + between the contexts of the compressor and decompressor, which in + turn may cause incorrect decompression. A robust header compression + scheme needs mechanisms for avoiding context inconsistencies and also + needs mechanisms for making the contexts consistent when they were + not. + +3.2. Existing header compression schemes + + The original header compression scheme, CTCP [VJHC], was invented by + Van Jacobson. CTCP compresses the 40 octet IP+TCP header to 4 + octets. The CTCP compressor detects transport-level retransmissions + and sends a header that updates the context completely when they + occur. This repair mechanism does not require any explicit signaling + between compressor and decompressor. + + A general IP header compression scheme, IP header compression [IPHC], + improves somewhat on CTCP and can compress arbitrary IP, TCP, and UDP + headers. When compressing non-TCP headers, IPHC does not use delta + encoding and is robust. When compressing TCP, the repair mechanism + of CTCP is augmented with a link-level nacking scheme which speeds up + the repair. IPHC does not compress RTP headers. + + CRTP [CRTP, IPHC] by Casner and Jacobson is a header compression + scheme that compresses 40 octets of IPv4/UDP/RTP headers to a minimum + of 2 octets when the UDP Checksum is not enabled. If the UDP + Checksum is enabled, the minimum CRTP header is 4 octets. CRTP + + + +Bormann, et al. Standards Track [Page 14] + +RFC 3095 Robust Header Compression July 2001 + + + cannot use the same repair mechanism as CTCP since UDP/RTP does not + retransmit. Instead, CRTP uses explicit signaling messages from + decompressor to compressor, called CONTEXT_STATE messages, to + indicate that the context is out of sync. The link round-trip time + will thus limit the speed of this context repair mechanism. + + On lossy links with long round-trip times, such as most cellular + links, CRTP does not perform well. Each lost packet over the link + causes several subsequent packets to be lost since the context is out + of sync during at least one link round-trip time. This behavior is + documented in [CRTPC]. For voice conversations such long loss events + will degrade the voice quality. Moreover, bandwidth is wasted by the + large headers sent by CRTP when updating the context. [CRTPC] found + that CRTP did not perform well enough for a lossy cellular link. It + is clear that CRTP alone is not a viable header compression scheme + for IP telephony over cellular links. + + To avoid losing packets due to the context being out of sync, CRTP + decompressors can attempt to repair the context locally by using a + mechanism known as TWICE. Each CRTP packet contains a counter which + is incremented by one for each packet sent out by the CRTP + compressor. If the counter increases by more than one, at least one + packet was lost over the link. The decompressor then attempts to + repair the context by guessing how the lost packet(s) would have + updated it. The guess is then verified by decompressing the packet + and checking the UDP Checksum -- if it succeeds, the repair is deemed + successful and the packet can be forwarded or delivered. TWICE + derives its name from the observation that when the compressed packet + stream is regular, the correct guess is to apply the update in the + current packet twice. [CRTPC] found that even with TWICE, CRTP + doubled the number of lost packets. TWICE improves CRTP performance + significantly. However, there are several problems with using TWICE: + + 1) It becomes mandatory to use the UDP Checksum: + + - the minimal compressed header size increases by 100% to 4 + octets. + + - most speech codecs developed for cellular links tolerate errors + in the encoded data. Such codecs will not want to enable the + UDP Checksum, since they do want damaged packets to be + delivered. + + - errors in the payload will make the UDP Checksum fail when the + guess is correct (and might make it succeed when the guess is + wrong). + + + + + +Bormann, et al. Standards Track [Page 15] + +RFC 3095 Robust Header Compression July 2001 + + + 2) Loss in an RTP stream that occurs before the compression point + will make updates in CRTP headers less regular. Simple-minded + versions of TWICE will then perform badly. More sophisticated + versions would need more repair attempts to succeed. + +3.3. Requirements on a new header compression scheme + + The major problem with CRTP is that it is not sufficiently robust + against packets being damaged between compressor and decompressor. A + viable header compression scheme must be less fragile. This + increased robustness must be obtained without increasing the + compressed header size; a larger header would make IP telephony over + cellular links economically unattractive. + + A major cause of the bad performance of CRTP over cellular links is + the long link round-trip time, during which many packets are lost + when the context is out of sync. This problem can be attacked + directly by finding ways to reduce the link round-trip time. Future + generations of cellular technologies may indeed achieve lower link + round-trip times. However, these will probably always be fairly + high. The benefits in terms of lower loss and smaller bandwidth + demands if the context can be repaired locally will be present even + if the link round-trip time is decreased. A reliable way to detect a + successful context repair is then needed. + + One might argue that a better way to solve the problem is to improve + the cellular link so that packet loss is less likely to occur. Such + modifications do not appear to come for free, however. If links were + made (almost) error free, the system might not be able to support a + sufficiently large number of users per cell and might thus be + economically infeasible. + + One might also argue that the speech codecs should be able to deal + with the kind of packet loss induced by CRTP, in particular since the + speech codecs probably must be able to deal with packet loss anyway + if the RTP stream crosses the Internet. While the latter is true, + the kind of loss induced by CRTP is difficult to deal with. It is + usually not possible to completely hide a loss event where well over + 100 ms worth of sound is completely lost. If such loss occurs + frequently at both ends of the end-to-end path, the speech quality + will suffer. + + A detailed description of the requirements specified for ROHC may be + found in [REQ]. + + + + + + + +Bormann, et al. Standards Track [Page 16] + +RFC 3095 Robust Header Compression July 2001 + + +3.4. Classification of header fields + + As mentioned earlier, header compression is possible due to the fact + that there is much redundancy between header field values within + packets, but especially between consecutive packets. To utilize + these properties for header compression, it is important to + understand the change patterns of the various header fields. + + All header fields have been classified in detail in appendix A. The + fields are first classified at a high level and then some of them are + studied more in detail. Finally, the appendix concludes with + recommendations on how the various fields should be handled by header + compression algorithms. The main conclusion that can be drawn is + that most of the header fields can easily be compressed away since + they never or seldom change. Only 5 fields, with a combined size of + about 10 octets, need more sophisticated mechanisms. These fields + are: + + - IPv4 Identification (16 bits) - IP-ID + - UDP Checksum (16 bits) + - RTP Marker (1 bit) - M-bit + - RTP Sequence Number (16 bits) - SN + - RTP Timestamp (32 bits) - TS + + The analysis in Appendix A reveals that the values of the TS and IP- + ID fields can usually be predicted from the RTP Sequence Number, + which increments by one for each packet emitted by an RTP source. + The M-bit is also usually the same, but needs to be communicated + explicitly occasionally. The UDP Checksum should not be predicted + and is sent as-is when enabled. + + The way ROHC RTP compression operates, then, is to first establish + functions from SN to the other fields, and then reliably communicate + the SN. Whenever a function from SN to another field changes, i.e., + the existing function gives a result which is different from the + field in the header to be compressed, additional information is sent + to update the parameters of that function. + + Headers specific to Mobile IP (for IPv4 or IPv6) do not receive any + special treatment in this document. They are compressible, however, + and it is expected that the compression efficiency for Mobile IP + headers will be good enough due to the handling of extension header + lists and tunneling headers. It would be relatively painless to + introduce a new ROHC profile with special treatment for Mobile IPv6 + specific headers should the completed work on the Mobile IPv6 + protocols (work in progress in the IETF) make that necessary. + + + + + +Bormann, et al. Standards Track [Page 17] + +RFC 3095 Robust Header Compression July 2001 + + +4. Header compression framework + +4.1. Operating assumptions + + Cellular links, which are a primary target for ROHC, have a number of + characteristics that are described briefly here. ROHC requires + functionality from lower layers that is outlined here and more + thoroughly described in the lower layer guidelines document [LLG]. + + Channels + + ROHC header-compressed packets flow on channels. Unlike many + fixed links, some cellular radio links can have several channels + connecting the same pair of nodes. Each channel can have + different characteristics in terms of error rate, bandwidth, etc. + + Context identifiers + + On some channels, the ability to transport multiple packet streams + is required. It can also be feasible to have channels dedicated + to individual packet streams. Therefore, ROHC uses a distinct + context identifier space per channel and can eliminate context + identifiers completely for one of the streams when few streams + share a channel. + + Packet type indication + + Packet type indication is done in the header compression scheme + itself. Unless the link already has a way of indicating packet + types which can be used, such as PPP, this provides smaller + compressed headers overall. It may also be less difficult to + allocate a single packet type, rather than many, in order to run + ROHC over links such as PPP. + + Reordering + + The channel between compressor and decompressor is required to + maintain packet ordering, i.e., the decompressor must receive + packets in the same order as the compressor sent them. + (Reordering before the compression point, however, is dealt with, + i.e., there is no assumption that the compressor will only receive + packets in sequence.) + + + + + + + + + +Bormann, et al. Standards Track [Page 18] + +RFC 3095 Robust Header Compression July 2001 + + + Duplication + + The channel between compressor and decompressor is required to not + duplicate packets. (Duplication before the compression point, + however, is dealt with, i.e., there is no assumption that the + compressor will receive only one copy of each packet.) + + Packet length + + ROHC is designed under the assumption that lower layers indicate + the length of a compressed packet. ROHC packets do not contain + length information for the payload. + + Framing + + The link layer must provide framing that makes it possible to + distinguish frame boundaries and individual frames. + + Error detection/protection + + The ROHC scheme has been designed to cope with residual errors in + the headers delivered to the decompressor. CRCs and sanity checks + are used to prevent or reduce damage propagation. However, it is + RECOMMENDED that lower layers deploy error detection for ROHC + headers and do not deliver ROHC headers with high residual error + rates. + + Without giving a hard limit on the residual error rate acceptable + to ROHC, it is noted that for a residual bit error rate of at most + 1E-5, the ROHC scheme has been designed not to increase the number + of damaged headers, i.e., the number of damaged headers due to + damage propagation is designed to be less than the number of + damaged headers caught by the ROHC error detection scheme. + + Negotiation + + In addition to the packet handling mechanisms above, the link + layer MUST provide a way to negotiate header compression + parameters, see also section 5.1.1. (For unidirectional links, + this negotiation may be performed out-of-band or even a priori.) + +4.2. Dynamicity + + The ROHC protocol achieves its compression gain by establishing state + information at both ends of the link, i.e., at the compressor and at + the decompressor. Different parts of the state are established at + different times and with different frequency; hence, it can be said + that some of the state information is more dynamic than the rest. + + + +Bormann, et al. Standards Track [Page 19] + +RFC 3095 Robust Header Compression July 2001 + + + Some state information is established at the time a channel is + established; ROHC assumes the existence of an out-of-band negotiation + protocol (such as PPP), or predefined channel state (most useful for + unidirectional links). In both cases, we speak of "negotiated + channel state". ROHC does not assume that this state can change + dynamically during the channel lifetime (and does not explicitly + support such changes, although some changes may be innocuous from a + protocol point of view). An example of negotiated channel state is + the highest context ID number to be used by the compressor (MAX_CID). + + Other state information is associated with the individual packet + streams in the channel; this state is said to be part of the context. + Using context identifiers (CIDs), multiple packet streams with + different contexts can share a channel. The negotiated channel state + indicates the highest context identifier to be used, as well as the + selection of one of two ways to indicate the CID in the compressed + header. + + It is up to the compressor to decide which packets to associate with + a context (or, equivalently, which packets constitute a single + stream); however, ROHC is efficient only when all packets of a stream + share certain properties, such as having the same values for fields + that are described as "static" in this document (e.g., the IP + addresses, port numbers, and RTP parameters such as the payload + type). The efficiency of ROHC RTP also depends on the compressor + seeing most RTP Sequence Numbers. + + Streams need not share all characteristics important for compression. + ROHC has a notion of compression profiles: a compression profile + denotes a predefined set of such characteristics. To provide + extensibility, the negotiated channel state includes the set of + profiles acceptable to the decompressor. The context state includes + the profile currently in use for the context. + + Other elements of the context state may include the current values of + all header fields (from these one can deduce whether an IPv4 header + is present in the header chain, and whether UDP Checksums are + enabled), as well as additional compression context that is not part + of an uncompressed header, e.g., TS_STRIDE, IP-ID characteristics + (incrementing as a 16-bit value in network byte order? random?), a + number of old reference headers, and the compressor/decompressor + state machines (see next section). + + This document actually defines four ROHC profiles: One uncompressed + profile, the main ROHC RTP compression profile, and two variants of + this profile for compression of packets with header chains that end + + + + + +Bormann, et al. Standards Track [Page 20] + +RFC 3095 Robust Header Compression July 2001 + + + in UDP and ESP, respectively, but where RTP compression is not + applicable. The descriptive text in the rest of this section is + referring to the main ROHC RTP compression profile. + +4.3. Compression and decompression states + + Header compression with ROHC can be characterized as an interaction + between two state machines, one compressor machine and one + decompressor machine, each instantiated once per context. The + compressor and the decompressor have three states each, which in many + ways are related to each other even if the meaning of the states are + slightly different for the two parties. Both machines start in the + lowest compression state and transit gradually to higher states. + + Transitions need not be synchronized between the two machines. In + normal operation it is only the compressor that temporarily transits + back to lower states. The decompressor will transit back only when + context damage is detected. + + Subsequent sections present an overview of the state machines and + their corresponding states, respectively, starting with the + compressor. + +4.3.1. Compressor states + + For ROHC compression, the three compressor states are the + Initialization and Refresh (IR), First Order (FO), and Second Order + (SO) states. The compressor starts in the lowest compression state + (IR) and transits gradually to higher compression states. The + compressor will always operate in the highest possible compression + state, under the constraint that the compressor is sufficiently + confident that the decompressor has the information necessary to + decompress a header compressed according to that state. + + +----------+ +----------+ +----------+ + | IR State | <--------> | FO State | <--------> | SO State | + +----------+ +----------+ +----------+ + + Decisions about transitions between the various compression states + are taken by the compressor on the basis of: + + - variations in packet headers + - positive feedback from decompressor (Acknowledgments -- ACKs) + - negative feedback from decompressor (Negative ACKs -- NACKs) + - periodic timeouts (when operating in unidirectional mode, i.e., + over simplex channels or when feedback is not enabled) + + + + + +Bormann, et al. Standards Track [Page 21] + +RFC 3095 Robust Header Compression July 2001 + + + How transitions are performed is explained in detail in chapter 5 for + each mode of operation. + +4.3.1.1. Initialization and Refresh (IR) State + + The purpose of the IR state is to initialize the static parts of the + context at the decompressor or to recover after failure. In this + state, the compressor sends complete header information. This + includes all static and nonstatic fields in uncompressed form plus + some additional information. + + The compressor stays in the IR state until it is fairly confident + that the decompressor has received the static information correctly. + +4.3.1.2. First Order (FO) State + + The purpose of the FO state is to efficiently communicate + irregularities in the packet stream. When operating in this state, + the compressor rarely sends information about all dynamic fields, and + the information sent is usually compressed at least partially. Only + a few static fields can be updated. The difference between IR and FO + should therefore be clear. + + The compressor enters this state from the IR state, and from the SO + state whenever the headers of the packet stream do not conform to + their previous pattern. It stays in the FO state until it is + confident that the decompressor has acquired all the parameters of + the new pattern. Changes in fields that are always irregular are + communicated in all packets and are therefore part of what is a + uniform pattern. + + Some or all packets sent in the FO state carry context updating + information. It is very important to detect corruption of such + packets to avoid erroneous updates and context inconsistencies. + +4.3.1.3. Second Order (SO) State + + This is the state where compression is optimal. The compressor + enters the SO state when the header to be compressed is completely + predictable given the SN (RTP Sequence Number) and the compressor is + sufficiently confident that the decompressor has acquired all + parameters of the functions from SN to other fields. Correct + decompression of packets sent in the SO state only hinges on correct + decompression of the SN. However, successful decompression also + requires that the information sent in the preceding FO state packets + has been successfully received by the decompressor. + + + + + +Bormann, et al. Standards Track [Page 22] + +RFC 3095 Robust Header Compression July 2001 + + + The compressor leaves this state and goes back to the FO state when + the header no longer conforms to the uniform pattern and cannot be + independently compressed on the basis of previous context + information. + +4.3.2. Decompressor states + + The decompressor starts in its lowest compression state, "No Context" + and gradually transits to higher states. The decompressor state + machine normally never leaves the "Full Context" state once it has + entered this state. + + +--------------+ +----------------+ +--------------+ + | No Context | <---> | Static Context | <---> | Full Context | + +--------------+ +----------------+ +--------------+ + + Initially, while working in the "No Context" state, the decompressor + has not yet successfully decompressed a packet. Once a packet has + been decompressed correctly (for example, upon reception of an + initialization packet with static and dynamic information), the + decompressor can transit all the way to the "Full Context" state, and + only upon repeated failures will it transit back to lower states. + However, when that happens it first transits back to the "Static + Context" state. There, reception of any packet sent in the FO state + is normally sufficient to enable transition to the "Full Context" + state again. Only when decompression of several packets sent in the + FO state fails in the "Static Context" state will the decompressor go + all the way back to the "No Context" state. + + When state transitions are performed is explained in detail in + chapter 5. + +4.4. Modes of operation + + The ROHC scheme has three modes of operation, called Unidirectional, + Bidirectional Optimistic, and Bidirectional Reliable mode. + + It is important to understand the difference between states, as + described in the previous chapter, and modes. These abstractions are + orthogonal to each other. The state abstraction is the same for all + modes of operation, while the mode controls the logic of state + transitions and what actions to perform in each state. + + + + + + + + + +Bormann, et al. Standards Track [Page 23] + +RFC 3095 Robust Header Compression July 2001 + + + +----------------------+ + | Unidirectional Mode | + | +--+ +--+ +--+ | + | |IR| |FO| |SO| | + | +--+ +--+ +--+ | + +----------------------+ + ^ ^ + / \ + / \ + v v + +----------------------+ +----------------------+ + | Optimistic Mode | | Reliable Mode | + | +--+ +--+ +--+ | | +--+ +--+ +--+ | + | |IR| |FO| |SO| | <--------------> | |IR| |FO| |SO| | + | +--+ +--+ +--+ | | +--+ +--+ +--+ | + +----------------------+ +----------------------+ + + The optimal mode to operate in depends on the characteristics of the + environment of the compression protocol, such as feedback abilities, + error probabilities and distributions, effects of header size + variation, etc. All ROHC implementations MUST implement and support + all three modes of operation. The three modes are briefly described + in the following subsections. + + Detailed descriptions of the three modes of operation regarding + compression and decompression logic are given in chapter 5. The mode + transition mechanisms, too, are described in chapter 5. + +4.4.1. Unidirectional mode -- U-mode + + When in the Unidirectional mode of operation, packets are sent in one + direction only: from compressor to decompressor. This mode therefore + makes ROHC usable over links where a return path from decompressor to + compressor is unavailable or undesirable. + + In U-mode, transitions between compressor states are performed only + on account of periodic timeouts and irregularities in the header + field change patterns in the compressed packet stream. Due to the + periodic refreshes and the lack of feedback for initiation of error + recovery, compression in the Unidirectional mode will be less + efficient and have a slightly higher probability of loss propagation + compared to any of the Bidirectional modes. + + Compression with ROHC MUST start in the Unidirectional mode. + Transition to any of the Bidirectional modes can be performed as soon + as a packet has reached the decompressor and it has replied with a + feedback packet indicating that a mode transition is desired (see + chapter 5). + + + +Bormann, et al. Standards Track [Page 24] + +RFC 3095 Robust Header Compression July 2001 + + +4.4.2. Bidirectional Optimistic mode -- O-mode + + The Bidirectional Optimistic mode is similar to the Unidirectional + mode. The difference is that a feedback channel is used to send + error recovery requests and (optionally) acknowledgments of + significant context updates from decompressor to compressor (not, + however, for pure sequence number updates). Periodic refreshes are + not used in the Bidirectional Optimistic mode. + + O-mode aims to maximize compression efficiency and sparse usage of + the feedback channel. It reduces the number of damaged headers + delivered to the upper layers due to residual errors or context + invalidation. The frequency of context invalidation may be higher + than for R-mode, in particular when long loss/error bursts occur. + Refer to section 4.7 for more details. + +4.4.3. Bidirectional Reliable mode -- R-mode + + The Bidirectional Reliable mode differs in many ways from the + previous two. The most important differences are a more intensive + usage of the feedback channel and a stricter logic at both the + compressor and the decompressor that prevents loss of context + synchronization between compressor and decompressor except for very + high residual bit error rates. Feedback is sent to acknowledge all + context updates, including updates of the sequence number field. + However, not every packet updates the context in Reliable mode. + + R-mode aims to maximize robustness against loss propagation and + damage propagation, i.e., minimize the probability of context + invalidation, even under header loss/error burst conditions. It may + have a lower probability of context invalidation than O-mode, but a + larger number of damaged headers may be delivered when the context + actually is invalidated. Refer to section 4.7 for more details. + +4.5. Encoding methods + + This chapter describes the encoding methods used for header fields. + How the methods are applied to each field (e.g., values of associated + parameters) is specified in section 5.7. + +4.5.1. Least Significant Bits (LSB) encoding + + Least Significant Bits (LSB) encoding is used for header fields whose + values are usually subject to small changes. With LSB encoding, the + k least significant bits of the field value are transmitted instead + of the original field value, where k is a positive integer. After + receiving k bits, the decompressor derives the original value using a + previously received value as reference (v_ref). + + + +Bormann, et al. Standards Track [Page 25] + +RFC 3095 Robust Header Compression July 2001 + + + The scheme is guaranteed to be correct if the compressor and the + decompressor each use interpretation intervals + + 1) in which the original value resides, and + + 2) in which the original value is the only value that has the + exact same k least significant bits as those transmitted. + + The interpretation interval can be described as a function f(v_ref, + k). Let + + f(v_ref, k) = [v_ref - p, v_ref + (2^k - 1) - p] + + where p is an integer. + + <------- interpretation interval (size is 2^k) -------> + |-------------+---------------------------------------| + v_ref - p v_ref v_ref + (2^k-1) - p + + + The function f has the following property: for any value k, the k + least significant bits will uniquely identify a value in f(v_ref, k). + + The parameter p is introduced so that the interpretation interval can + be shifted with respect to v_ref. Choosing a good value for p will + yield a more efficient encoding for fields with certain + characteristics. Below are some examples: + + a) For field values that are expected always to increase, p can be + set to -1. The interpretation interval becomes + [v_ref + 1, v_ref + 2^k]. + + b) For field values that stay the same or increase, p can be set to + 0. The interpretation interval becomes [v_ref, v_ref + 2^k - 1]. + + c) For field values that are expected to deviate only slightly from a + constant value, p can be set to 2^(k-1) - 1. The interpretation + interval becomes [v_ref - 2^(k-1) + 1, v_ref + 2^(k-1)]. + + d) For field values that are expected to undergo small negative + changes and larger positive changes, such as the RTP TS for video, + or RTP SN when there is misordering, p can be set to 2^(k-2) - 1. + The interval becomes [v_ref - 2^(k-2) + 1, v_ref + 3 * 2^(k-2)], + i.e., 3/4 of the interval is used for positive changes. + + The following is a simplified procedure for LSB compression and + decompression; it is modified for robustness and damage propagation + protection in the next subsection: + + + +Bormann, et al. Standards Track [Page 26] + +RFC 3095 Robust Header Compression July 2001 + + + 1) The compressor (decompressor) always uses v_ref_c (v_ref_d), the + last value that has been compressed (decompressed), as v_ref; + + 2) When compressing a value v, the compressor finds the minimum value + of k such that v falls into the interval f(v_ref_c, k). Call this + function k = g(v_ref_c, v). When only a few distinct values of k + are possible, for example due to limitations imposed by packet + formats (see section 5.7), the compressor will instead pick the + smallest k that puts v in the interval f(v_ref_c, k). + + 3) When receiving m LSBs, the decompressor uses the interpretation + interval f(v_ref_d, m), called interval_d. It picks as the + decompressed value the one in interval_d whose LSBs match the + received m bits. + + Note that the values to be encoded have a finite range; for example, + the RTP SN ranges from 0 to 0xFFFF. When the SN value is close to 0 + or 0xFFFF, the interpretation interval can straddle the wraparound + boundary between 0 and 0xFFFF. + + The scheme is complicated by two factors: packet loss between the + compressor and decompressor, and transmission errors undetected by + the lower layer. In the former case, the compressor and decompressor + will lose the synchronization of v_ref, and thus also of the + interpretation interval. If v is still covered by the + intersection(interval_c, interval_d), the decompression will be + correct. Otherwise, incorrect decompression will result. The next + section will address this issue further. + + In the case of undetected transmission errors, the corrupted LSBs + will give an incorrectly decompressed value that will later be used + as v_ref_d, which in turn is likely to lead to damage propagation. + This problem is addressed by using a secure reference, i.e., a + reference value whose correctness is verified by a protecting CRC. + Consequently, the procedure 1) above is modified as follows: + + 1) a) the compressor always uses as v_ref_c the last value that has + been compressed and sent with a protecting CRC. + b) the decompressor always uses as v_ref_d the last correct + value, as verified by a successful CRC. + + Note that in U/O-mode, 1) b) is modified so that if decompression of + the SN fails using the last verified SN reference, another + decompression attempt is made using the last but one verified SN + reference. This procedure mitigates damage propagation when a small + CRC fails to detect a damaged value. See section 5.3.2.2.3 for + further details. + + + + +Bormann, et al. Standards Track [Page 27] + +RFC 3095 Robust Header Compression July 2001 + + +4.5.2. Window-based LSB encoding (W-LSB encoding) + + This section describes how to modify the simplified algorithm in + 4.5.1 to achieve robustness. + + The compressor may not be able to determine the exact value of + v_ref_d that will be used by the decompressor for a particular value + v, since some candidates for v_ref_d may have been lost or damaged. + However, by using feedback or by making reasonable assumptions, the + compressor can limit the candidate set. The compressor then + calculates k such that no matter which v_ref_d in the candidate set + the decompressor uses, v is covered by the resulting interval_d. + + Since the decompressor always uses as the reference the last received + value where the CRC succeeded, the compressor maintains a sliding + window containing the candidates for v_ref_d. The sliding window is + initially empty. The following operations are performed on the + sliding window by the compressor: + + 1) After sending a value v (compressed or uncompressed) protected by + a CRC, the compressor adds v to the sliding window. + + 2) For each value v being compressed, the compressor chooses k = + max(g(v_min, v), g(v_max, v)), where v_min and v_max are the + minimum and maximum values in the sliding window, and g is the + function defined in the previous section. + + 3) When the compressor is sufficiently confident that a certain value + v and all values older than v will not be used as reference by the + decompressor, the window is advanced by removing those values + (including v). The confidence may be obtained by various means. + In R-mode, an ACK from the decompressor implies that values older + than the ACKed one can be removed from the sliding window. In + U/O-mode there is always a CRC to verify correct decompression, + and a sliding window with a limited maximum width is used. The + window width is an implementation dependent optimization + parameter. + + Note that the decompressor follows the procedure described in the + previous section, except that in R-mode it MUST ACK each header + received with a succeeding CRC (see also section 5.5). + +4.5.3. Scaled RTP Timestamp encoding + + The RTP Timestamp (TS) will usually not increase by an arbitrary + number from packet to packet. Instead, the increase is normally an + integral multiple of some unit (TS_STRIDE). For example, in the case + of audio, the sample rate is normally 8 kHz and one voice frame may + + + +Bormann, et al. Standards Track [Page 28] + +RFC 3095 Robust Header Compression July 2001 + + + cover 20 ms. Furthermore, each voice frame is often carried in one + RTP packet. In this case, the RTP increment is always n * 160 (= + 8000 * 0.02), for some integer n. Note that silence periods have no + impact on this, as the sample clock at the source normally keeps + running without changing either frame rate or frame boundaries. + + In the case of video, there is usually a TS_STRIDE as well when the + video frame level is considered. The sample rate for most video + codecs is 90 kHz. If the video frame rate is fixed, say, to 30 + frames/second, the TS will increase by n * 3000 (= n * 90000 / 30) + between video frames. Note that a video frame is often divided into + several RTP packets to increase robustness against packet loss. In + this case several RTP packets will carry the same TS. + + When using scaled RTP Timestamp encoding, the TS is downscaled by a + factor of TS_STRIDE before compression. This saves + + floor(log2(TS_STRIDE)) + + bits for each compressed TS. TS and TS_SCALED satisfy the following + equality: + + TS = TS_SCALED * TS_STRIDE + TS_OFFSET + + TS_STRIDE is explicitly, and TS_OFFSET implicitly, communicated to + the decompressor. The following algorithm is used: + + 1. Initialization: The compressor sends to the decompressor the value + of TS_STRIDE and the absolute value of one or several TS fields. + The latter are used by the decompressor to initialize TS_OFFSET to + (absolute value) modulo TS_STRIDE. Note that TS_OFFSET is the + same regardless of which absolute value is used, as long as the + unscaled TS value does not wrap around; see 4) below. + + 2. Compression: After initialization, the compressor no longer + compresses the original TS values. Instead, it compresses the + downscaled values: TS_SCALED = TS / TS_STRIDE. The compression + method could be either W-LSB encoding or the timer-based encoding + described in the next section. + + 3. Decompression: When receiving the compressed value of TS_SCALED, + the decompressor first derives the value of the original + TS_SCALED. The original RTP TS is then calculated as TS = + TS_SCALED * TS_STRIDE + TS_OFFSET. + + 4. Offset at wraparound: Wraparound of the unscaled 32-bit TS will + invalidate the current value of TS_OFFSET used in the equation + above. For example, let us assume TS_STRIDE = 160 = 0xA0 and the + + + +Bormann, et al. Standards Track [Page 29] + +RFC 3095 Robust Header Compression July 2001 + + + current TS = 0xFFFFFFF0. TS_OFFSET is then 0x50 = 80. Then if + the next RTP TS = 0x00000130 (i.e., the increment is 160 * 2 = + 320), the new TS_OFFSET should be 0x00000130 modulo 0xA0 = 0x90 = + 144. The compressor is not required to re-initialize TS_OFFSET at + wraparound. Instead, the decompressor MUST detect wraparound of + the unscaled TS (which is trivial) and update TS_OFFSET to + + TS_OFFSET = (Wrapped around unscaled TS) modulo TS_STRIDE + + 5. Interpretation interval at wraparound: Special rules are needed + for the interpretation interval of the scaled TS at wraparound, + since the maximum scaled TS, TSS_MAX, (0xFFFFFFFF / TS_STRIDE) may + not have the form 2^m - 1. For example, when TS_STRIDE is 160, + the scaled TS is at most 26843545 which has LSBs 10011001. The + wraparound boundary between the TSS_MAX may thus not correspond to + a natural boundary between LSBs. + + interpretation interval + |<------------------------------>| + + unused scaled TS + ------------|--------------|----------------------> + TSS_MAX zero + + When TSS_MAX is part of the interpretation interval, a number of + unused values are inserted into it after TSS_MAX such that their + LSBs follow naturally upon each other. For example, for TS_STRIDE + = 160 and k = 4, values corresponding to the LSBs 1010 through + 1111 are inserted. The number of inserted values depends on k and + the LSBs of the maximum scaled TS. The number of valid values in + the interpretation interval should be high enough to maintain + robustness. This can be ensured by the following rule: + + Let a be the number of LSBs needed if there was no + wraparound, and let b be the number of LSBs needed to + disambiguate between TSS_MAX and zero where the a LSBs of + TSS_MAX are set to zero. The number of LSB bits to send + while TSS_MAX or zero is part of the interpretation interval + is b. + + This scaling method can be applied to many frame-based codecs. + However, the value of TS_STRIDE might change during a session, for + example as a result of adaptation strategies. If that happens, the + unscaled TS is compressed until re-initialization of the new + TS_STRIDE and TS_OFFSET is completed. + + + + + + +Bormann, et al. Standards Track [Page 30] + +RFC 3095 Robust Header Compression July 2001 + + +4.5.4. Timer-based compression of RTP Timestamp + + The RTP Timestamp [RFC 1889] is defined to identify the number of the + first sample used to generate the payload. When 1) RTP packets carry + payloads corresponding to a fixed sampling interval, 2) the sampling + is done at a constant rate, and 3) packets are generated in lock-step + with sampling, then the timestamp value will closely approximate a + linear function of the time of day. This is the case for + conversational media, such as interactive speech. The linear ratio + is determined by the source sample rate. The linear pattern can be + complicated by packetization (e.g., in the case of video where a + video frame usually corresponds to several RTP packets) or frame + rearrangement (e.g., B-frames are sent out-of-order by some video + codecs). + + With a fixed sample rate of 8 kHz, 20 ms in the time domain is + equivalent to an increment of 160 in the unscaled TS domain, and to + an increment of 1 in the scaled TS domain with TS_STRIDE = 160. + + As a consequence, the (scaled) TS of headers arriving at the + decompressor will be a linear function of time of day, with some + deviation due to the delay jitter (and the clock inaccuracies) + between the source and the decompressor. In normal operation, i.e., + no crashes or failures, the delay jitter will be bounded to meet the + requirements of conversational real-time traffic. Hence, by using a + local clock the decompressor can obtain an approximation of the + (scaled) TS in the header to be decompressed by considering its + arrival time. The approximation can then be refined with the k LSBs + of the (scaled) TS carried in the header. The value of k required to + ensure correct decompression is a function of the jitter between the + source and the decompressor. + + If the compressor knows the potential jitter introduced between + compressor and decompressor, it can determine k by using a local + clock to estimate jitter in packet arrival times, or alternatively it + can use a fixed k and discard packets arriving too much out of time. + + The advantages of this scheme include: + + a) The size of the compressed TS is constant and small. In + particular, it does NOT depend on the length of silence intervals. + This is in contrast to other TS compression techniques, which at + the beginning of a talkspurt require sending a number of bits + dependent on the duration of the preceding silence interval. + + b) No synchronization is required between the clock local to the + compressor and the clock local to the decompressor. + + + + +Bormann, et al. Standards Track [Page 31] + +RFC 3095 Robust Header Compression July 2001 + + + Note that although this scheme can be made to work using both scaled + and unscaled TS, in practice it is always combined with scaled TS + encoding because of the less demanding requirement on the clock + resolution, e.g., 20 ms instead of 1/8 ms. Therefore, the algorithm + described below assumes that the clock-based encoding scheme operates + on the scaled TS. The case of unscaled TS would be similar, with + changes to scale factors. + + The major task of the compressor is to determine the value of k. Its + sliding window now contains not only potential reference values for + the TS but also their times of arrival at the compressor. + + 1) The compressor maintains a sliding window + + {(T_j, a_j), for each header j that can be used as a reference}, + + where T_j is the scaled TS for header j, and a_j is the arrival + time of header j. The sliding window serves the same purpose as + the W-LSB sliding window of section 4.5.2. + + 2) When a new header n arrives with T_n as the scaled TS, the + compressor notes the arrival time a_n. It then calculates + + Max_Jitter_BC = + + max {|(T_n - T_j) - ((a_n - a_j) / TIME_STRIDE)|, + for all headers j in the sliding window}, + + where TIME_STRIDE is the time interval equivalent to one + TS_STRIDE, e.g., 20 ms. Max_Jitter_BC is the maximum observed + jitter before the compressor, in units of TS_STRIDE, for the + headers in the sliding window. + + 3) k is calculated as + + k = ceiling(log2(2 * J + 1), + + where J = Max_Jitter_BC + Max_Jitter_CD + 2. + + Max_Jitter_CD is the upper bound of jitter expected on the + communication channel between compressor and decompressor (CD-CC). + It depends only on the characteristics of CD-CC. + + + + + + + + + +Bormann, et al. Standards Track [Page 32] + +RFC 3095 Robust Header Compression July 2001 + + + The constant 2 accounts for the quantization error introduced by + the clocks at the compressor and decompressor, which can be +/-1. + + Note that the calculation of k follows the compression algorithm + described in section 4.5.1, with p = 2^(k-1) - 1. + + 4) The sliding window is subject to the same window operations as in + section 4.5.2, 1) and 3), except that the values added and removed + are paired with their arrival times. + + Decompressor: + + 1) The decompressor uses as its reference header the last correctly + (as verified by CRC) decompressed header. It maintains the pair + (T_ref, a_ref), where T_ref is the scaled TS of the reference + header, and a_ref is the arrival time of the reference header. + + 2) When receiving a compressed header n at time a_n, the + approximation of the original scaled TS is calculated as: + + T_approx = T_ref + (a_n - a_ref) / TIME_STRIDE. + + 3) The approximation is then refined by the k least significant bits + carried in header n, following the decompression algorithm of + section 4.5.1, with p = 2^(k-1) - 1. + + Note: The algorithm does not assume any particular pattern in the + packets arriving at the compressor, i.e., it tolerates reordering + before the compressor and nonincreasing RTP Timestamp behavior. + + Note: Integer arithmetic is used in all equations above. If + TIME_STRIDE is not equal to an integral number of clock ticks, + time must be normalized such that TIME_STRIDE is an integral + number of clock ticks. For example, if a clock tick is 20 ms and + TIME_STRIDE is 30 ms, (a_n - a_ref) in 2) can be multiplied by 3 + and TIME_STRIDE can have the value 2. + + Note: The clock resolution of the compressor or decompressor can + be worse than TIME_STRIDE, in which case the difference, i.e., + actual resolution - TIME_STRIDE, is treated as additional jitter + in the calculation of k. + + Note: The clock resolution of the decompressor may be communicated + to the compressor using the CLOCK feedback option. + + Note: The decompressor may observe the jitter and report this to + the compressor using the JITTER feedback option. The compressor + may use this information to refine its estimate of Max_Jitter_CD. + + + +Bormann, et al. Standards Track [Page 33] + +RFC 3095 Robust Header Compression July 2001 + + +4.5.5. Offset IP-ID encoding + + As all IPv4 packets have an IP Identifier to allow for fragmentation, + ROHC provides for transparent compression of this ID. There is no + explicit support in ROHC for the IPv6 fragmentation header, so there + is never a need to discuss IP IDs outside the context of IPv4. + + This section assumes (initially) that the IPv4 stack at the source + host assigns IP-ID according to the value of a 2-byte counter which + is increased by one after each assignment to an outgoing packet. + Therefore, the IP-ID field of a particular IPv4 packet flow will + increment by 1 from packet to packet except when the source has + emitted intermediate packets not belonging to that flow. + + For such IPv4 stacks, the RTP SN will increase by 1 for each packet + emitted and the IP-ID will increase by at least the same amount. + Thus, it is more efficient to compress the offset, i.e., (IP-ID - RTP + SN), instead of IP-ID itself. + + The remainder of section 4.5.5 describes how to compress/decompress + the sequence of offsets using W-LSB encoding/decoding, with p = 0 + (see section 4.5.1). All IP-ID arithmetic is done using unsigned + 16-bit quantities, i.e., modulo 2^16. + + Compressor: + + The compressor uses W-LSB encoding (section 4.5.2) to compress a + sequence of offsets + + Offset_i = ID_i - SN_i, + + where ID_i and SN_i are the values of the IP-ID and RTP SN of + header i. The sliding window contains such offsets and not the + values of header fields, but the rules for adding and deleting + offsets from the window otherwise follow section 4.5.2. + + Decompressor: + + The reference header is the last correctly (as verified by CRC) + decompressed header. + + When receiving a compressed packet m, the decompressor calculates + Offset_ref = ID_ref - SN_ref, where ID_ref and SN_ref are the + values of IP-ID and RTP SN in the reference header, respectively. + + + + + + + +Bormann, et al. Standards Track [Page 34] + +RFC 3095 Robust Header Compression July 2001 + + + Then W-LSB decoding is used to decompress Offset_m, using the + received LSBs in packet m and Offset_ref. Note that m may contain + zero LSBs for Offset_m, in which case Offset_m = Offset_ref. + + Finally, the IP-ID for packet m is regenerated as + + IP-ID for m = decompressed SN of packet m + Offset_m + + Network byte order: + + Some IPv4 stacks do use a counter to generate IP ID values as + described, but do not transmit the contents of this counter in + network byte order, but instead send the two octets reversed. In + this case, the compressor can compress the IP-ID field after + swapping the bytes. Consequently, the decompressor also swaps the + bytes of the IP-ID after decompression to regenerate the original + IP-ID. This requires that the compressor and the decompressor + synchronize on the byte order of the IP-ID field using the NBO or + NBO2 flag (see section 5.7). + + Random IP Identifier: + + Some IPv4 stacks generate the IP Identifier values using a + pseudo-random number generator. While this may provide some + security benefits, it makes it pointless to attempt compressing + the field. Therefore, the compressor should detect such random + behavior of the field. After detection and synchronization with + the decompressor using the RND or RND2 flag, the field is sent + as-is in its entirety as additional octets after the compressed + header. + +4.5.6. Self-describing variable-length values + + The values of TS_STRIDE and a few other compression parameters can + vary widely. TS_STRIDE can be 160 for voice and 90 000 for 1 f/s + video. To optimize the transfer of such values, a variable number of + octets is used to encode them. The number of octets used is + determined by the first few bits of the first octet: + + First bit is 0: 1 octet. + 7 bits transferred. + Up to 127 decimal. + Encoded octets in hexadecimal: 00 to 7F + + First bits are 10: 2 octets. + 14 bits transferred. + Up to 16 383 decimal. + Encoded octets in hexadecimal: 80 00 to BF FF + + + +Bormann, et al. Standards Track [Page 35] + +RFC 3095 Robust Header Compression July 2001 + + + First bits are 110: 3 octets. + 21 bits transferred. + Up to 2 097 151 decimal. + Encoded octets in hexadecimal: C0 00 00 to DF FF FF + + First bits are 111: 4 octets. + 29 bits transferred. + Up to 536 870 911 decimal. + Encoded octets in hexadecimal: E0 00 00 00 to FF FF FF FF + +4.5.7. Encoded values across several fields in compressed headers + + When a compressed header has an extension, pieces of an encoded value + can be present in more than one field. When an encoded value is + split over several fields in this manner, the more significant bits + of the value are closer to the beginning of the header. If the + number of bits available in compressed header fields exceeds the + number of bits in the value, the most significant field is padded + with zeroes in its most significant bits. + + For example, an unscaled TS value can be transferred using an UOR-2 + header (see section 5.7) with an extension of type 3. The Tsc bit of + the extension is then unset (zero) and the variable length TS field + of the extension is 4 octets, with 29 bits available for the TS (see + section 4.5.6). The UOR-2 TS field will contain the three most + significant bits of the unscaled TS, and the 4-octet TS field in the + extension will contain the remaining 29 bits. + +4.6. Errors caused by residual errors + + ROHC is designed under the assumption that packets can be damaged + between the compressor and decompressor, and that such damaged + packets can be delivered to the decompressor ("residual errors"). + + Residual errors may damage the SN in compressed headers. Such damage + will cause generation of a header which upper layers may not be able + to distinguish from a correct header. When the compressed header + contains a CRC, the CRC will catch the bad header with a probability + dependent on the size of the CRC. When ROHC does not detect the bad + header, it will be delivered to upper layers. + + Damage is not confined to the SN: + + a) Damage to packet type indication bits can cause a header to be + interpreted as having a different packet type. + + + + + + +Bormann, et al. Standards Track [Page 36] + +RFC 3095 Robust Header Compression July 2001 + + + b) Damage to CID information may cause a packet to be interpreted + according to another context and possibly also according to + another profile. Damage to CIDs will be more harmful when a large + part of the CID space is being used, so that it is likely that the + damaged CID corresponds to an active context. + + c) Feedback information can also be subject to residual errors, both + when feedback is piggybacked and when it is sent in separate ROHC + packets. ROHC uses sanity checks and adds CRCs to vital feedback + information to allow detection of some damaged feedback. + + Note that context damage can also result in generation of + incorrect headers; section 4.7 elaborates further on this. + +4.7. Impairment considerations + + Impairments to headers can be classified into the following types: + + (1) the lower layer was not able to decode the packet and did not + deliver it to ROHC, + + (2) the lower layer was able to decode the packet, but discarded + it because of a detected error, + + (3) ROHC detected an error in the generated header and discarded + the packet, or + + (4) ROHC did not detect that the regenerated header was damaged + and delivered it to upper layers. + + Impairments cause loss or damage of individual headers. Some + impairment scenarios also cause context invalidation, which in turn + results in loss propagation and damage propagation. Damage + propagation and undetected residual errors both contribute to the + number of damaged headers delivered to upper layers. Loss + propagation and impairments resulting in loss or discarding of single + packets both contribute to the packet loss seen by upper layers. + + Examples of context invalidating scenarios are: + + (a) Impairment of type (4) on the forward channel, causing the + decompressor to update its context with incorrect information; + + + + + + + + + +Bormann, et al. Standards Track [Page 37] + +RFC 3095 Robust Header Compression July 2001 + + + (b) Loss/error burst of pattern update headers: Impairments of + types (1),(2) and (3) on consecutive pattern update headers; a + pattern update header is a header carrying a new pattern + information, e.g., at the beginning of a new talk spurt; this + causes the decompressor to lose the pattern update + information; + + (c) Loss/error burst of headers: Impairments of types (1),(2) and + (3) on a number of consecutive headers that is large enough to + cause the decompressor to lose the SN synchronization; + + (d) Impairment of type (4) on the feedback channel which mimics a + valid ACK and makes the compressor update its context; + + (e) a burst of damaged headers (3) erroneously triggers the "k- + out-of-n" rule for detecting context invalidation, which + results in a NACK/update sequence during which headers are + discarded. + + Scenario (a) is mitigated by the CRC carried in all context updating + headers. The larger the CRC, the lower the chance of context + invalidation caused by (a). In R-mode, the CRC of context updating + headers is always 7 bits or more. In U/O-mode, it is usually 3 bits + and sometimes 7 or 8 bits. + + Scenario (b) is almost completely eliminated when the compressor + ensures through ACKs that no context updating headers are lost, as in + R-mode. + + Scenario (c) is almost completely eliminated when the compressor + ensures through ACKs that the decompressor will always detect the SN + wraparound, as in R-mode. It is also mitigated by the SN repair + mechanisms in U/O-mode. + + Scenario (d) happens only when the compressor receives a damaged + header that mimics an ACK of some header present in the W-LSB window, + say ACK of header 2, while in reality header 2 was never received or + accepted by the decompressor, i.e., header 2 was subject to + impairment (1), (2) or (3). The damaged header must mimic the + feedback packet type, the ACK feedback type, and the SN LSBs of some + header in the W-LSB window. + + Scenario (e) happens when a burst of residual errors causes the CRC + check to fail in k out of the last n headers carrying CRCs. Large k + and n reduces the probability of scenario (e), but also increases the + number of headers lost or damaged as a consequence of any context + invalidation. + + + + +Bormann, et al. Standards Track [Page 38] + +RFC 3095 Robust Header Compression July 2001 + + + ROHC detects damaged headers using CRCs over the original headers. + The smallest headers in this document either include a 3-bit CRC + (U/O-mode) or do not include a CRC (R-mode). For the smallest + headers, damage is thus detected with a probability of roughly 7/8 + for U/O-mode. For R-mode, damage to the smallest headers is not + detected. + + All other things (coding scheme at lower layers, etc.) being equal, + the rate of headers damaged by residual errors will be lower when + headers are compressed compared when they are not, since fewer bits + are transmitted. Consequently, for a given ROHC CRC setup the rate + of incorrect headers delivered to applications will also be reduced. + + The above analysis suggests that U/O-mode may be more prone than R- + mode to context invalidation. On the other hand, the CRC present in + all U/O-mode headers continuously screens out residual errors coming + from lower layers, reduces the number of damaged headers delivered to + upper layers when context is invalidated, and permits quick detection + of context invalidation. + + R-mode always uses a stronger CRC on context updating headers, but no + CRC in other headers. A residual error on a header which carries no + CRC will result in a damaged header being delivered to upper layers + (4). The number of damaged headers delivered to the upper layers + depends on the ratio of headers with CRC vs. headers without CRC, + which is a compressor parameter. + +5. The protocol + +5.1. Data structures + + The ROHC protocol is based on a number of parameters that form part + of the negotiated channel state and the per-context state. This + section describes some of this state information in an abstract way. + Implementations can use a different structure for and representation + of this state. In particular, negotiation protocols that set up the + per-channel state need to establish the information that constitutes + the negotiated channel state, but it is not necessary to exchange it + in the form described here. + +5.1.1. Per-channel parameters + + MAX_CID: Nonnegative integer; highest context ID number to be used by + the compressor (note that this parameter is not coupled to, but in + effect further constrained by, LARGE_CIDS). + + + + + + +Bormann, et al. Standards Track [Page 39] + +RFC 3095 Robust Header Compression July 2001 + + + LARGE_CIDS: Boolean; if false, the short CID representation (0 bytes + or 1 prefix byte, covering CID 0 to 15) is used; if true, the + embedded CID representation (1 or 2 embedded CID bytes covering CID 0 + to 16383) is used. + + PROFILES: Set of nonnegative integers, each integer indicating a + profile supported by the decompressor. The compressor MUST NOT + compress using a profile not in PROFILES. + + FEEDBACK_FOR: Optional reference to a channel in the reverse + direction. If provided, this parameter indicates which channel any + feedback sent on this channel refers to (see 5.7.6.1). + + MRRU: Maximum reconstructed reception unit. This is the size of the + largest reconstructed unit in octets that the decompressor is + expected to reassemble from segments (see 5.2.5). Note that this + size includes the CRC. If MRRU is negotiated to be 0, no segment + headers are allowed on the channel. + +5.1.2. Per-context parameters, profiles + + Per-context parameters are established with IR headers (see section + 5.2.3). An IR header contains a profile identifier, which determines + how the rest of the header is to be interpreted. Note that the + profile parameter determines the syntax and semantics of the packet + type identifiers and packet types used in conjunction with a specific + context. This document describes profiles 0x0000, 0x0001, 0x0002, + and 0x0003; further profiles may be defined when ROHC is extended in + the future. + + Profile 0x0000 is for sending uncompressed IP packets. See section + 5.10. + + Profile 0x0001 is for RTP/UDP/IP compression, see sections 5.3 + through 5.9. + + Profile 0x0002 is for UDP/IP compression, i.e., compression of the + first 12 octets of the UDP payload is not attempted. See section + 5.11. + + Profile 0x0003 is for ESP/IP compression, i.e., compression of the + header chain up to and including the first ESP header, but not + subsequent subheaders. See section 5.12. + + Initially, all contexts are in no context state, i.e., all packets + referencing this context except IR packets are discarded. If defined + by a "ROHC over X" document, per-channel negotiation can be used to + pre-establish state information for a context (e.g., negotiating + + + +Bormann, et al. Standards Track [Page 40] + +RFC 3095 Robust Header Compression July 2001 + + + profile 0x0000 for CID 15). Such state information can also be + marked read-only in the negotiation, which would cause the + decompressor to discard any IR packet attempting to modify it. + +5.1.3. Contexts and context identifiers + + Associated with each compressed flow is a context, which is the state + compressor and decompressor maintain in order to correctly compress + or decompress the headers of the packet stream. Contexts are + identified by a context identifier, CID, which is sent along with + compressed headers and feedback information. + + The CID space is distinct for each channel, i.e., CID 3 over channel + A and CID 3 over channel B do not refer to the same context, even if + the endpoints of A and B are the same nodes. In particular, CIDs for + any pairs of forward and reverse channels are not related (forward + and reverse channels need not even have CID spaces of the same size). + + Context information is conceptually kept in a table. The context + table is indexed using the CID which is sent along with compressed + headers and feedback information. The CID space can be negotiated to + be either small, which means that CIDs can take the values 0 through + 15, or large, which means that CIDs take values between 0 and 2^14 - + 1 = 16383. Whether the CID space is large or small is negotiated no + later than when a channel is established. + + A small CID with the value 0 is represented using zero bits. A small + CID with a value from 1 to 15 is represented by a four-bit field in + place of a packet type field (Add-CID) plus four more bits. A large + CID is represented using the encoding scheme of section 4.5.6, + limited to two octets. + +5.2. ROHC packets and packet types + + The packet type indication scheme for ROHC has been designed under + the following constraints: + + a) it must be possible to use only a limited number of packet sizes; + b) it must be possible to send feedback information in separate ROHC + packets as well as piggybacked on forward packets; + c) it is desirable to allow elimination of the CID for one packet + stream when few packet streams share a channel; + d) it is anticipated that some packets with large headers may be + larger than the MTU of very constrained lower layers. + + + + + + + +Bormann, et al. Standards Track [Page 41] + +RFC 3095 Robust Header Compression July 2001 + + + These constraints have led to a design which includes + + - optional padding, + - a feedback packet type, + - an optional Add-CID octet which provides 4 bits of CID, and + - a simple segmentation and reassembly mechanism. + + A ROHC packet has the following general format (in the diagram, + colons ":" indicate that the part is optional): + + --- --- --- --- --- --- --- --- + : Padding : variable length + --- --- --- --- --- --- --- --- + : Feedback : 0 or more feedback elements + --- --- --- --- --- --- --- --- + : Header : variable, with CID information + --- --- --- --- --- --- --- --- + : Payload : + --- --- --- --- --- --- --- --- + + Padding is any number (zero or more) of padding octets. Either of + Feedback or Header must be present. + + Feedback elements always start with a packet type indication. + Feedback elements carry internal CID information. Feedback is + described in section 5.2.2. + + Header is either a profile-specific header or an IR or IR-DYN header + (see sections 5.2.3 and 5.2.4). Header either + + 1) does not carry any CID information (indicating CID zero), or + 2) includes one Add-CID Octet (see below), or + 3) contains embedded CID information of length one or two octets. + + Alternatives 1) and 2) apply only to compressed headers in channels + where the CID space is small. Alternative 3) applies only to + compressed headers in channels where the CID space is large. + + Padding Octet + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 1 1 0 0 0 0 0 | + +---+---+---+---+---+---+---+---+ + + + + + + + +Bormann, et al. Standards Track [Page 42] + +RFC 3095 Robust Header Compression July 2001 + + + Add-CID Octet + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 1 1 0 | CID | + +---+---+---+---+---+---+---+---+ + + CID: 0x1 through 0xF indicates CIDs 1 through 15. + + Note: The Padding Octet looks like an Add-CID octet for CID 0. + + Header either starts with a packet type indication or has a packet + type indication immediately following an Add-CID Octet. All Header + packet types have the following general format (in the diagram, + slashes "/" indicate variable length): + + 0 x-1 x 7 + --- --- --- --- --- --- --- --- + : Add-CID octet : if (CID 1-15) and (small CIDs) + +---+--- --- --- ---+--- --- ---+ + | type indication | body | 1 octet (8-x bits of body) + +---+--- ---+---+---+--- --- ---+ + : : + / 0, 1, or 2 octets of CID / 1 or 2 octets if (large CIDs) + : : + +---+---+---+---+---+---+---+---+ + / body / variable length + +---+---+---+---+---+---+---+---+ + + The large CID, if present, is encoded according to section 4.5.6. + +5.2.1. ROHC feedback + + Feedback carries information from decompressor to compressor. The + following principal kinds of feedback are supported. In addition to + the kind of feedback, other information may be included in profile- + specific feedback information. + + ACK : Acknowledges successful decompression of a packet, + which means that the context is up-to-date with a high + probability. + + NACK : Indicates that the dynamic context of the + decompressor is out of sync. Generated when several + successive packets have failed to be decompressed + correctly. + + + + + +Bormann, et al. Standards Track [Page 43] + +RFC 3095 Robust Header Compression July 2001 + + + STATIC-NACK : Indicates that the static context of the decompressor + is not valid or has not been established. + + It is anticipated that feedback to the compressor can be realized in + many ways, depending on the properties of the particular lower layer. + The exact details of how feedback is realized is to be specified in a + "ROHC over X" document, for each lower layer X in question. For + example, feedback might be realized using + + 1) lower-layer specific mechanisms + + 2) a dedicated feedback-only channel, realized for example by the + lower layer providing a way to indicate that a packet is a + feedback packet + + 3) a dedicated feedback-only channel, where the timing of the + feedback provides information about which compressed packet caused + the feedback + + 4) interspersing of feedback packets among normal compressed packets + going in the same direction as the feedback (lower layers do not + indicate feedback) + + 5) piggybacking of feedback information in compressed packets going + in the same direction as the feedback (this technique may reduce + the per-feedback overhead) + + 6) interspersing and piggybacking on the same channel, i.e., both 4) + and 5). + + Alternatives 1-3 do not place any particular requirements on the ROHC + packet type scheme. Alternatives 4-6 do, however. The ROHC packet + type scheme has been designed to allow alternatives 4-6 (these may be + used for example over PPP): + + a) The ROHC scheme provides a feedback packet type. The packet type + is able to carry variable-length feedback information. + + b) The feedback information sent on a particular channel is passed + to, and interpreted by, the compressor associated with feedback on + that channel. Thus, the feedback information must contain CID + information if the associated compressor can use more than one + context. The ROHC feedback scheme requires that a channel carries + feedback to at most one compressor. How a compressor is + associated with feedback on a particular channel needs to be + defined in a "ROHC over X" document. + + + + + +Bormann, et al. Standards Track [Page 44] + +RFC 3095 Robust Header Compression July 2001 + + + c) The ROHC feedback information format is octet-aligned, i.e., + starts at an octet boundary, to allow using the format over a + dedicated feedback channel, 2). + + d) To allow piggybacking, 5), it is possible to deduce the length of + feedback information by examining the first few octets of the + feedback. This allows the decompressor to pass piggybacked + feedback information to the associated same-side compressor + without understanding its format. The length information + decouples the decompressor from the compressor in the sense that + the decompressor can process the compressed header immediately + without waiting for the compressor to hand it back after parsing + the feedback information. + +5.2.2. ROHC feedback format + + Feedback sent on a ROHC channel consists of one or more concatenated + feedback elements, where each feedback element has the following + format: + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 1 1 1 0 | Code | feedback type octet + +---+---+---+---+---+---+---+---+ + : Size : if Code = 0 + +---+---+---+---+---+---+---+---+ + / feedback data / variable length + +---+---+---+---+---+---+---+---+ + + Code: 0 indicates that a Size octet is present. + 1-7 indicates the size of the feedback data field in + octets. + + Size: Optional octet indicating the size of the feedback data + field in octets. + + feedback data: Profile-specific feedback information. Includes + CID information. + + The total size of the feedback data field is determinable upon + reception by the decompressor, by inspection of the Code field and + possibly the Size field. This explicit length information allows + piggybacking and also sending more than one feedback element in a + packet. + + When the decompressor has determined the size of the feedback data + field, it removes the feedback type octet and the Size field (if + present) and hands the rest to the same-side associated compressor + + + +Bormann, et al. Standards Track [Page 45] + +RFC 3095 Robust Header Compression July 2001 + + + together with an indication of the size. The feedback data received + by the compressor has the following structure (feedback sent on a + dedicated feedback channel MAY also use this format): + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + : Add-CID octet : if for small CIDs and (CID != 0) + +---+---+---+---+---+---+---+---+ + : : + / large CID (4.5.6 encoding) / 1-2 octets if for large CIDs + : : + +---+---+---+---+---+---+---+---+ + / feedback / + +---+---+---+---+---+---+---+---+ + + The large CID, if present, is encoded according to section 4.5.6. + CID information in feedback data indicates the CID of the packet + stream for which feedback is sent. Note that the LARGE_CIDS + parameter that controls whether a large CID is present is taken from + the channel state of the receiving compressor's channel, NOT from + that of the channel carrying the feedback. + + It is REQUIRED that the feedback field have either of the following + two formats: + + FEEDBACK-1 + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | profile specific information | 1 octet + +---+---+---+---+---+---+---+---+ + + FEEDBACK-2 + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + |Acktype| | + +---+---+ profile specific / at least 2 octets + / information | + +---+---+---+---+---+---+---+---+ + + Acktype: 0 = ACK + 1 = NACK + 2 = STATIC-NACK + 3 is reserved (MUST NOT be used. Otherwise unparseable.) + + The compressor can use the following logic to parse the feedback + field. + + + +Bormann, et al. Standards Track [Page 46] + +RFC 3095 Robust Header Compression July 2001 + + + 1) If for large CIDs, the feedback will always start with a CID + encoded according to section 4.5.6. If the first bit is 0, the + CID uses one octet. If the first bit is 1, the CID uses two + octets. + + 2) If for small CIDs, and the size is one octet, the feedback is a + FEEDBACK-1. + + 3) If for small CIDs, and the size is larger than one octet, and the + feedback starts with the two bits 11, the feedback starts with an + Add-CID octet. If the size is 2, it is followed by FEEDBACK-1. + If the size is larger than 2, the Add-CID is followed by + FEEDBACK-2. + + 4) Otherwise, there is no Add-CID octet, and the feedback starts with + a FEEDBACK-2. + +5.2.3. ROHC IR packet type + + The IR header associates a CID with a profile, and typically also + initializes the context. It can typically also refresh (parts of) + the context. It has the following general format. + + 0 1 2 3 4 5 6 7 + --- --- --- --- --- --- --- --- + : Add-CID octet : if for small CIDs and (CID != 0) + +---+---+---+---+---+---+---+---+ + | 1 1 1 1 1 1 0 | x | IR type octet + +---+---+---+---+---+---+---+---+ + : : + / 0-2 octets of CID / 1-2 octets if for large CIDs + : : + +---+---+---+---+---+---+---+---+ + | Profile | 1 octet + +---+---+---+---+---+---+---+---+ + | CRC | 1 octet + +---+---+---+---+---+---+---+---+ + | | + / profile specific information / variable length + | | + +---+---+---+---+---+---+---+---+ + + x: Profile specific information. Interpreted according to the + profile indicated in the Profile field. + + + + + + + +Bormann, et al. Standards Track [Page 47] + +RFC 3095 Robust Header Compression July 2001 + + + Profile: The profile to be associated with the CID. In the IR + packet, the profile identifier is abbreviated to the 8 least + significant bits. It selects the highest-number profile in the + channel state parameter PROFILES that matches the 8 LSBs given. + + CRC: 8-bit CRC computed using the polynomial of section 5.9.1. Its + coverage is profile-dependent, but it MUST cover at least the + initial part of the packet ending with the Profile field. Any + information which initializes the context of the decompressor + should be protected by the CRC. + + Profile specific information: The contents of this part of the IR + packet are defined by the individual profiles. Interpreted + according to the profile indicated in the Profile field. + +5.2.4. ROHC IR-DYN packet type + + In contrast to the IR header, the IR-DYN header can never initialize + an uninitialized context. However, it can redefine what profile is + associated with a context, see for example 5.11 (ROHC UDP) and 5.12 + (ROHC ESP). Thus the type needs to be reserved at the framework + level. The IR-DYN header typically also initializes or refreshes + parts of a context, typically the dynamic part. It has the following + general format: + + 0 1 2 3 4 5 6 7 + --- --- --- --- --- --- --- --- + : Add-CID octet : if for small CIDs and (CID != 0) + +---+---+---+---+---+---+---+---+ + | 1 1 1 1 1 0 0 0 | IR-DYN type octet + +---+---+---+---+---+---+---+---+ + : : + / 0-2 octets of CID / 1-2 octets if for large CIDs + : : + +---+---+---+---+---+---+---+---+ + | Profile | 1 octet + +---+---+---+---+---+---+---+---+ + | CRC | 1 octet + +---+---+---+---+---+---+---+---+ + | | + / profile specific information / variable length + | | + +---+---+---+---+---+---+---+---+ + + Profile: The profile to be associated with the CID. This is + abbreviated in the same way as with IR packets. + + + + + +Bormann, et al. Standards Track [Page 48] + +RFC 3095 Robust Header Compression July 2001 + + + CRC: 8-bit CRC computed using the polynomial of section 5.9.1. + Its coverage is profile-dependent, but it MUST cover at least + the initial part of the packet ending with the Profile field. + Any information which initializes the context of the + decompressor should be protected by the CRC. + + Profile specific information: This part of the IR packet is + defined by individual profiles. It is interpreted according + to the profile indicated in the Profile field. + +5.2.5. ROHC segmentation + + Some link layers may provide a much more efficient service if the set + of different packet sizes to be transported is kept small. For such + link layers, these sizes will normally be chosen to transport + frequently occurring packets efficiently, with less frequently + occurring packets possibly adapted to the next larger size by the + addition of padding. The link layer may, however, be limited in the + size of packets it can offer in this efficient mode, or it may be + desirable to request only a limited largest size. To accommodate the + occasional packet that is larger than that largest size negotiated, + ROHC defines a simple segmentation protocol. + +5.2.5.1. Segmentation usage considerations + + The segmentation protocol defined in ROHC is not particularly + efficient. It is not intended to replace link layer segmentation + functions; these SHOULD be used whenever available and efficient for + the task at hand. + + ROHC segmentation should only be used for occasional packets with + sizes larger than what is efficient to accommodate, e.g., due to + exceptionally large ROHC headers. The segmentation scheme was + designed to reduce packet size variations that may occur due to + outliers in the header size distribution. In other cases, + segmentation should be done at lower layers. The segmentation scheme + should only be used for packet sizes that are larger than the maximum + size in the allowed set of sizes from the lower layers. + + In summary, ROHC segmentation should be used with a relatively low + frequency in the packet flow. If this cannot be ensured, + segmentation should be performed at lower layers. + + + + + + + + + +Bormann, et al. Standards Track [Page 49] + +RFC 3095 Robust Header Compression July 2001 + + +5.2.5.2. Segmentation protocol + + Segment Packet + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 1 1 1 1 1 1 | F | + +---+---+---+---+---+---+---+---+ + / Segment / variable length + +---+---+---+---+---+---+---+---+ + + F: Final bit. If set, it indicates that this is the last segment of + a reconstructed unit. + + The segment header may be preceded by padding octets and/or feedback. + It never carries a CID. + + All segment header packets for one reconstructed unit have to be sent + consecutively on a channel, i.e., any non-segment-header packet + following a nonfinal segment header aborts the reassembly of the + current reconstructed unit and causes the decompressor to discard the + nonfinal segments received on this channel so far. When a final + segment header is received, the decompressor reassembles the segment + carried in this packet and any nonfinal segments that immediately + preceded it into a single reconstructed unit, in the order they were + received. The reconstructed unit has the format: + + Reconstructed Unit + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | | + / Reconstructed ROHC packet / variable length + | | + +---+---+---+---+---+---+---+---+ + / CRC / 4 octets + +---+---+---+---+---+---+---+---+ + + The CRC is used by the decompressor to validate the reconstructed + unit. It uses the FCS-32 algorithm with the following generator + polynomial: x^0 + x^1 + x^2 + x^4 + x^5 + x^7 + x^8 + x^10 + x^11 + + x^12 + x^16 + x^22 + x^23 + x^26 + x^32 [HDLC]. If the reconstructed + unit is 4 octets or less, or if the CRC fails, or if it is larger + than the channel parameter MRRU (see 5.1.1), the reconstructed unit + MUST be discarded by the decompressor. + + + + + + +Bormann, et al. Standards Track [Page 50] + +RFC 3095 Robust Header Compression July 2001 + + + If the CRC succeeds, the reconstructed ROHC packet is interpreted as + a ROHC Header, optionally followed by a payload. Note that this + means that there can be no padding and no feedback in the + reconstructed unit, and that the CID is derived from the initial + octets of the reconstructed unit. + + (It should be noted that the ROHC segmentation protocol was inspired + by SEAL by Steve Deering et al., which later became ATM AAL5. The + same arguments for not having sequence numbers in the segments but + instead providing a strong CRC in the reconstructed unit apply here + as well. Note that, as a result of this protocol, there is no way in + ROHC to make any use of a segment that has residual bit errors.) + +5.2.6. ROHC initial decompressor processing + + The following packet types are reserved at the framework level in the + ROHC scheme: + + 1110: Padding or Add-CID octet + 11110: Feedback + 11111000: IR-DYN packet + 1111110: IR packet + 1111111: Segment + + Other packet types can be used at will by individual profiles. + + The following steps is an outline of initial decompressor processing + which upon reception of a ROHC packet can determine its contents. + + 1) If the first octet is a Padding Octet (11100000), + strip away all initial Padding Octets and goto next step. + + 2) If the first remaining octet starts with 1110, it is an Add-CID + octet: + + remember the Add-CID octet; remove the octet. + + 3) If the first remaining octet starts with 11110, and an Add-CID + octet was found in step 2), + + an error has occurred; the header MUST be discarded without + further action. + + 4) If the first remaining octet starts with 11110, and an Add-CID + octet was not found in step 2), this is feedback: + + find the size of the feedback data, call it s; + remove the feedback type octet; + + + +Bormann, et al. Standards Track [Page 51] + +RFC 3095 Robust Header Compression July 2001 + + + remove the Size octet if Code is 0; + send feedback data of length s to the same-side associated + compressor; + if packet exhausted, stop; otherwise goto 2). + + 5) If the first remaining octet starts with 1111111, this is a + segment: + + attempt reconstruction using the segmentation protocol + (5.2.5). If a reconstructed packet is not produced, this + finishes the processing of the original packet. If a + reconstructed packet is produced, it is fed into step 1) + above. Padding, segments, and feedback are not allowed in + reconstructed packets, so when processing them, steps 1), + 4), and 5) are modified so that the packet is discarded + without further action when their conditions match. + + 6) Here, it is known that the rest is forward information (unless the + header is damaged). + + 7) If the forward traffic uses small CIDs, there is no large CID in + the packet. If an Add-CID immediately preceded the packet type + (step 2), it has the CID of the Add-CID; otherwise it has CID 0. + + 8) If the forward traffic uses large CIDs, the CID starts with the + second remaining octet. If the first bit(s) of that octet are not + 0 or 10, the packet MUST be discarded without further action. If + an Add-CID octet immediately preceded the packet type (step 2), + the packet MUST be discarded without further action. + + 9) Use the CID to find the context. + + 10) If the packet type is IR, the profile indicated in the IR packet + determines how it is to be processed. If the CRC fails to verify + the packet, it MUST be discarded. If a profile is indicated in + the context, the logic of that profile determines what, if any, + feedback is to be sent. If no profile is noted in the context, + no further action is taken. + + 11) If the packet type is IR-DYN, the profile indicated in the IR-DYN + packet determines how it is to be processed. + + a) If the CRC fails to verify the packet, it MUST be discarded. + If a profile is indicated in the context, the logic of that + profile determines what, if any, feedback is to be sent. If no + profile is noted in the context, no further action is taken. + + + + + +Bormann, et al. Standards Track [Page 52] + +RFC 3095 Robust Header Compression July 2001 + + + b) If the context has not been initialized by an IR packet, the + packet MUST be discarded. The logic of the profile indicated + in the IR-DYN header (if verified by the CRC), determines what, + if any, feedback is to be sent. + + 12) Otherwise, the profile noted in the context determines how the + rest of the packet is to be processed. If the context has not + been initialized by an IR packet, the packet MUST be discarded + without further action. + + The procedure for finding the size of the feedback data is as + follows: + + Examine the three bits which immediately follow the feedback packet + type. When these bits are + 1-7, the size of the feedback data is given by the bits; + 0, a Size octet, which explicitly gives the size of the + feedback data, is present after the feedback type octet. + +5.2.7. ROHC RTP packet formats from compressor to decompressor + + ROHC RTP uses three packet types to identify compressed headers, and + two for initialization/refresh. The format of a compressed packet + can depend on the mode. Therefore a naming scheme of the form + + <modes format is used in>-<packet type number>-<some property> + + is used to uniquely identify the format when necessary, e.g., UOR-2, + R-1. For exact formats of the packet types, see section 5.7. + + Packet type zero: R-0, R-0-CRC, UO-0. + + This, the minimal, packet type is used when parameters of all SN- + functions are known by the decompressor, and the header to be + compressed adheres to these functions. Thus, only the W-LSB + encoded RTP SN needs to be communicated. + + R-mode: Only if a CRC is present (packet type R-0-CRC) may the + header be used as a reference for subsequent decompression. + + U-mode and O-mode: A small CRC is present in the UO-0 packet. + + Packet type 1: R-1, R-1-ID, R-1-TS, UO-1, UO-1-ID, UO-1-TS. + + This packet type is used when the number of bits needed for the SN + exceeds those available in packet type zero, or when the + parameters of the SN-functions for RTP TS or IP-ID change. + + + + +Bormann, et al. Standards Track [Page 53] + +RFC 3095 Robust Header Compression July 2001 + + + R-mode: R-1-* packets are not used as references for subsequent + decompression. Values for other fields than the RTP TS or IP-ID + can be communicated using an extension, but they do not update the + context. + + U-mode and O-mode: Only the values of RTP SN, RTP TS and IP-ID can + be used as references for future compression. Nonupdating values + can be provided for other fields using an extension (UO-1-ID). + + Packet type 2: UOR-2, UOR-2-ID, UOR-2-TS + + This packet type can be used to change the parameters of any SN- + function, except those for most static fields. Headers of packets + transferred using packet type 2 can be used as references for + subsequent decompression. + + Packet type: IR + + This packet type communicates the static part of the context, + i.e., the value of the constant SN-functions. It can optionally + also communicate the dynamic part of the context, i.e., the + parameters of the nonconstant SN-functions. + + Packet type: IR-DYN + + This packet type communicates the dynamic part of the context, + i.e., the parameters of nonconstant SN-functions. + +5.2.8. Parameters needed for mode transition in ROHC RTP + + The packet types IR (with dynamic information), IR-DYN, and UOR-2 are + common for all modes. They can carry a mode parameter which can take + the values U = Unidirectional, O = Bidirectional Optimistic, and R = + Bidirectional Reliable. + + Feedback of types ACK, NACK, and STATIC-NACK carry sequence numbers, + and feedback packets can also carry a mode parameter indicating the + desired compression mode: U, O, or R. + + As a shorthand, the notation PACKET(mode) is used to indicate which + mode value a packet carries. For example, an ACK with mode parameter + R is written ACK(R), and an UOR-2 with mode parameter O is written + UOR-2(O). + + + + + + + + +Bormann, et al. Standards Track [Page 54] + +RFC 3095 Robust Header Compression July 2001 + + +5.3. Operation in Unidirectional mode + +5.3.1. Compressor states and logic (U-mode) + + Below is the state machine for the compressor in Unidirectional mode. + Details of the transitions between states and compression logic are + given subsequent to the figure. + + Optimistic approach + +------>------>------>------>------>------>------>------>------+ + | | + | Optimistic approach Optimistic approach | + | +------>------>------+ +------>------>------+ | + | | | | | | + | | v | v v + +----------+ +----------+ +----------+ + | IR State | | FO State | | SO State | + +----------+ +----------+ +----------+ + ^ ^ | ^ | | + | | Timeout | | Timeout / Update | | + | +------<------<------+ +------<------<------+ | + | | + | Timeout | + +------<------<------<------<------<------<------<------<------+ + +5.3.1.1. State transition logic (U-mode) + + The transition logic for compression states in Unidirectional mode is + based on three principles: the optimistic approach principle, + timeouts, and the need for updates. + +5.3.1.1.1. Optimistic approach, upwards transition + + Transition to a higher compression state in Unidirectional mode is + carried out according to the optimistic approach principle. This + means that the compressor transits to a higher compression state when + it is fairly confident that the decompressor has received enough + information to correctly decompress packets sent according to the + higher compression state. + + When the compressor is in the IR state, it will stay there until it + assumes that the decompressor has correctly received the static + context information. For transition from the FO to the SO state, the + compressor should be confident that the decompressor has all + parameters needed to decompress according to a fixed pattern. + + + + + + +Bormann, et al. Standards Track [Page 55] + +RFC 3095 Robust Header Compression July 2001 + + + The compressor normally obtains its confidence about decompressor + status by sending several packets with the same information according + to the lower compression state. If the decompressor receives any of + these packets, it will be in sync with the compressor. The number of + consecutive packets to send for confidence is not defined in this + document. + +5.3.1.1.2. Timeouts, downward transition + + When the optimistic approach is taken as described above, there will + always be a possibility of failure since the decompressor may not + have received sufficient information for correct decompression. + Therefore, the compressor MUST periodically transit to lower + compression states. Periodic transition to the IR state SHOULD be + carried out less often than transition to the FO state. Two + different timeouts SHOULD therefore be used for these transitions. + For an example of how to implement periodic refreshes, see [IPHC] + chapters 3.3.1-3.3.2. + +5.3.1.1.3. Need for updates, downward transition + + In addition to the downward state transitions carried out due to + periodic timeouts, the compressor must also immediately transit back + to the FO state when the header to be compressed does not conform to + the established pattern. + +5.3.1.2. Compression logic and packets used (U-mode) + + The compressor chooses the smallest possible packet format that can + communicate the desired changes, and has the required number of bits + for W-LSB encoded values. + +5.3.1.3. Feedback in Unidirectional mode + + The Unidirectional mode of operation is designed to operate over + links where a feedback channel is not available. If a feedback + channel is available, however, the decompressor MAY send an + acknowledgment of successful decompression with the mode parameter + set to U (send an ACK(U)). When the compressor receives such a + message, it MAY disable (or increase the interval between) periodic + IR refreshes. + +5.3.2. Decompressor states and logic (U-mode) + + Below is the state machine for the decompressor in Unidirectional + mode. Details of the transitions between states and decompression + logic are given subsequent to the figure. + + + + +Bormann, et al. Standards Track [Page 56] + +RFC 3095 Robust Header Compression July 2001 + + + Success + +-->------>------>------>------>------>--+ + | | + No Static | No Dynamic Success | Success + +-->--+ | +-->--+ +--->----->---+ +-->--+ + | | | | | | | | | + | v | | v | v | v + +--------------+ +----------------+ +--------------+ + | No Context | | Static Context | | Full Context | + +--------------+ +----------------+ +--------------+ + ^ | ^ | + | k_2 out of n_2 failures | | k_1 out of n_1 failures | + +-----<------<------<-----+ +-----<------<------<-----+ + +5.3.2.1. State transition logic (U-mode) + + Successful decompression will always move the decompressor to the + Full Context state. Repeated failed decompression will force the + decompressor to transit downwards to a lower state. The decompressor + does not attempt to decompress headers at all in the No Context and + Static Context states unless sufficient information is included in + the packet itself. + +5.3.2.2. Decompression logic (U-mode) + + Decompression in Unidirectional mode is carried out following three + steps which are described in subsequent sections. + +5.3.2.2.1. Decide whether decompression is allowed + + In Full Context state, decompression may be attempted regardless of + what kind of packet is received. However, for the other states + decompression is not always allowed. In the No Context state only IR + packets, which carry the static information fields, may be + decompressed. Further, when in the Static Context state, only + packets carrying a 7- or 8-bit CRC can be decompressed (i.e., IR, + IR-DYN, or UOR-2 packets). If decompression may not be performed the + packet is discarded, unless the optional delayed decompression + mechanism is used, see section 6.1. + +5.3.2.2.2. Reconstruct and verify the header + + When reconstructing the header, the decompressor takes the header + information already stored in the context and updates it with the + information received in the current header. (If the reconstructed + header fails the CRC check, these updates MUST be undone.) + + + + + +Bormann, et al. Standards Track [Page 57] + +RFC 3095 Robust Header Compression July 2001 + + + The sequence number is reconstructed by replacing the sequence number + LSBs in the context with those received in the header. The resulting + value is then verified to be within the interpretation interval by + comparison with a previously reconstructed reference value v_ref (see + section 4.5.1). If it is not within this interval, an adjustment is + applied by adding N x interval_size to the reconstructed value so + that the result is brought within the interpretation interval. Note + that N can be negative. + + If RTP Timestamp and IP Identification fields are not included in the + received header, they are supposed to be calculated from the sequence + number. The IP Identifier usually increases by the same delta as the + sequence number and the timestamp by the same delta times a fixed + value. See chapters 4.5.3 and 4.5.5 for details about how these + fields are encoded in compressed headers. + + When working in Unidirectional mode, all compressed headers carry a + CRC which MUST be used to verify decompression. + +5.3.2.2.3. Actions upon CRC failure + + This section is written so that it is applicable to all modes. + + A mismatch in the CRC can be caused by one or more of: + + 1. residual bit errors in the current header + + 2. a damaged context due to residual bit errors in previous headers + + 3. many consecutive packets being lost between compressor and + decompressor (this may cause the LSBs of the SN in compressed + packets to be interpreted wrongly, because the decompressor has + not moved the interpretation interval for lack of input -- in + essence, a kind of context damage). + + (Cases 2 and 3 do not apply to IR packets; case 3 does not apply to + IR-DYN packets.) The 3-bit CRC present in some header formats will + eventually detect context damage reliably, since the probability of + undetected context damage decreases exponentially with each new + header processed. However, residual bit errors in the current header + are only detected with good probability, not reliably. + + When a CRC mismatch is caused by residual bit errors in the current + header (case 1 above), the decompressor should stay in its current + state to avoid unnecessary loss of subsequent packets. On the other + hand, when the mismatch is caused by a damaged context (case 2), the + decompressor should attempt to repair the context locally. If the + local repair attempt fails, it must move to a lower state to avoid + + + +Bormann, et al. Standards Track [Page 58] + +RFC 3095 Robust Header Compression July 2001 + + + delivering incorrect headers. When the mismatch is caused by + prolonged loss (case 3), the decompressor might attempt additional + decompression attempts. Note that case 3 does not occur in R-mode. + + The following actions MUST be taken when a CRC check fails: + + First, attempt to determine whether SN LSB wraparound (case 3) is + likely, and if so, attempt a correction. For this, the algorithm of + section 5.3.2.2.4 MAY be used. If another algorithm is used, it MUST + have at least as high a rate of correct repairs as the one in + 5.3.2.2.4. (This step is not applicable to R-mode.) + + Second, if the previous step did not attempt a correction, a repair + should be attempted under the assumption that the reference SN has + been incorrectly updated. For this, the algorithm of section + 5.3.2.2.5 MAY be used. If another algorithm is used, it MUST have at + least as high a rate of correct repairs as the one in 5.3.2.2.5. + (This step is not applicable to R-mode.) + + If both the above steps fail, additional decompression attempts + SHOULD NOT be made. There are two possible reasons for the CRC + failure: case 1 or unrecoverable context damage. It is impossible to + know for certain which of these is the actual cause. The following + rules are to be used: + + a. When CRC checks fail only occasionally, assume residual errors in + the current header and simply discard the packet. NACKs SHOULD + NOT be sent at this time. + + b. In the Full Context state: When the CRC check of k_1 out of the + last n_1 decompressed packets have failed, context damage SHOULD + be assumed and a NACK SHOULD be sent in O- and R-mode. The + decompressor moves to the Static Context state and discards all + packets until an update (IR, IR-DYN, UOR-2) which passes the CRC + check is received. + + c. In the Static Context state: When the CRC check of k_2 out of the + last n_2 updates (IR, IR-DYN, UOR-2) have failed, static context + damage SHOULD be assumed and a STATIC-NACK is sent in O- and R- + mode. The decompressor moves to the No Context state. + + d. In the No Context state: The decompressor discards all packets + until a static update (IR) which passes the CRC check is received. + (In O-mode and R-mode, feedback is sent according to sections + 5.4.2.2 and 5.5.2.2, respectively.) + + + + + + +Bormann, et al. Standards Track [Page 59] + +RFC 3095 Robust Header Compression July 2001 + + + Note that appropriate values for k_1, n_1, k_2, and n_2, are related + to the residual error rate of the link. When the residual error rate + is close to zero, k_1 = n_1 = k_2 = n_2 = 1 may be appropriate. + +5.3.2.2.4. Correction of SN LSB wraparound + + When many consecutive packets are lost there will be a risk of + sequence number LSB wraparound, i.e., the SN LSBs being interpreted + wrongly because the interpretation interval has not moved for lack of + input. The decompressor might be able to detect this situation and + avoid context damage by using a local clock. The following algorithm + MAY be used: + + a. The decompressor notes the arrival time, a(i), of each incoming + packet i. Arrival times of packets where decompression fails are + discarded. + + b. When decompression fails, the decompressor computes INTERVAL = + a(i) - a(i - 1), i.e., the time elapsed between the arrival of the + previous, correctly decompressed packet and the current packet. + + c. If wraparound has occurred, INTERVAL will correspond to at least + 2^k inter-packet times, where k is the number of SN bits in the + current header. On the basis of an estimate of the packet inter- + arrival time, obtained for example using a moving average of + arrival times, TS_STRIDE, or TS_TIME, the decompressor judges if + INTERVAL can correspond to 2^k inter-packet times. + + d. If INTERVAL is judged to be at least 2^k packet inter-arrival + times, the decompressor adds 2^k to the reference SN and attempts + to decompress the packet using the new reference SN. + + e. If this decompression succeeds, the decompressor updates the + context but SHOULD NOT deliver the packet to upper layers. The + following packet is also decompressed and updates the context if + its CRC succeeds, but SHOULD be discarded. If decompression of + the third packet using the new context also succeeds, the context + repair is deemed successful and this and subsequent decompressed + packets are delivered to the upper layers. + + f. If any of the three decompression attempts in d. and e. fails, the + decompressor discards the packets and acts according to rules a) + through c) of section 5.3.2.2.3. + + Using this mechanism, the decompressor may be able to repair the + context after excessive loss, at the expense of discarding two + packets. + + + + +Bormann, et al. Standards Track [Page 60] + +RFC 3095 Robust Header Compression July 2001 + + +5.3.2.2.5. Repair of incorrect SN updates + + The CRC can fail to detect residual errors in the compressed header + because of its limited length, i.e., the incorrectly decompressed + packet can happen to have the same CRC as the original uncompressed + packet. The incorrect decompressed header will then update the + context. This can lead to an erroneous reference SN being used in + W-LSB decoding, as the reference SN is updated for each successfully + decompressed header of certain types. + + In this situation, the decompressor will detect the incorrect + decompression of the following packet with high probability, but it + does not know the reason for the failure. The following mechanism + allows the decompressor to judge if the context was updated + incorrectly by an earlier packet and, if so, to attempt a repair. + + a. The decompressor maintains two decompressed sequence numbers: the + last one (ref 0) and the one before that (ref -1). + + b. When receiving a compressed header the SN (SN curr1) is + decompressed using ref 0 as the reference. The other header + fields are decompressed using this decompressed SN curr1. (This + is part of the normal decompression procedure prior to any CRC + test failures.) + + c. If the decompressed header generated in b. passes the CRC test, + the references are shifted as follows: + + ref -1 = ref 0 + ref 0 = SN curr1. + + d. If the header generated in b. does not pass the CRC test, and the + SN (SN curr2) generated when using ref -1 as the reference is + different from SN curr1, an additional decompression attempt is + performed based on SN curr2 as the decompressed SN. + + e. If the decompressed header generated in b. does not pass the CRC + test and SN curr2 is the same as SN curr1, an additional + decompression attempt is not useful and is not attempted. + + f. If the decompressed header generated in d. passes the CRC test, + ref -1 is not changed while ref 0 is set to SN curr2. + + g. If the decompressed header generated in d. does not pass the CRC + test, the decompressor acts according to rules a) through c) of + section 5.3.2.2.3. + + + + + +Bormann, et al. Standards Track [Page 61] + +RFC 3095 Robust Header Compression July 2001 + + + The purpose of this algorithm is to repair the context. If the + header generated in d. passes the CRC test, the references are + updated according to f., but two more headers MUST also be + successfully decompressed before the repair is deemed successful. Of + the three successful headers, the first two SHOULD be discarded and + only the third delivered to upper layers. If decompression of any of + the three headers fails, the decompressor MUST discard that header + and the previously generated headers, and act according to rules a) + through c) of section 5.3.2.2.3. + +5.3.2.3. Feedback in Unidirectional mode + + To improve performance for the Unidirectional mode over a link that + does have a feedback channel, the decompressor MAY send an + acknowledgment when decompression succeeds. Setting the mode + parameter in the ACK packet to U indicates that the compressor is to + stay in Unidirectional mode. When receiving an ACK(U), the + compressor should reduce the frequency of IR packets since the static + information has been correctly received, but it is not required to + stop sending IR packets. If IR packets continue to arrive, the + decompressor MAY repeat the ACK(U), but it SHOULD NOT repeat the + ACK(U) continuously. + +5.4. Operation in Bidirectional Optimistic mode + +5.4.1. Compressor states and logic (O-mode) + + Below is the state machine for the compressor in Bidirectional + Optimistic mode. The details of each state, state transitions, and + compression logic are given subsequent to the figure. + + Optimistic approach / ACK + +------>------>------>------>------>------>------>------>------+ + | | + | Optimistic appr. / ACK Optimistic appr. /ACK ACK | + | +------>------>------+ +------>--- -->-----+ +->--+ + | | | | | | | + | | v | v | v + +----------+ +----------+ +----------+ + | IR State | | FO State | | SO State | + +----------+ +----------+ +----------+ + ^ ^ | ^ | | + | | STATIC-NACK | | NACK / Update | | + | +------<------<------+ +------<------<------+ | + | | + | STATIC-NACK | + +------<------<------<------<------<------<------<------<------+ + + + + +Bormann, et al. Standards Track [Page 62] + +RFC 3095 Robust Header Compression July 2001 + + +5.4.1.1. State transition logic + + The transition logic for compression states in Bidirectional + Optimistic mode has much in common with the logic of the + Unidirectional mode. The optimistic approach principle and + transitions occasioned by the need for updates work in the same way + as described in chapter 5.3.1. However, in Optimistic mode there are + no timeouts. Instead, the Optimistic mode makes use of feedback from + decompressor to compressor for transitions in the backward direction + and for OPTIONAL improved forward transition. + +5.4.1.1.1. Negative acknowledgments (NACKs), downward transition + + Negative acknowledgments (NACKs), also called context requests, + obviate the periodic updates needed in Unidirectional mode. Upon + reception of a NACK the compressor transits back to the FO state and + sends updates (IR-DYN, UOR-2, or possibly IR) to the decompressor. + NACKs carry the SN of the latest packet successfully decompressed, + and this information MAY be used by the compressor to determine what + fields need to be updated. + + Similarly, reception of a STATIC-NACK packet makes the compressor + transit back to the IR state. + +5.4.1.1.2. Optional acknowledgments, upwards transition + + In addition to NACKs, positive feedback (ACKs) MAY also be used for + UOR-2 packets in the Bidirectional Optimistic mode. Upon reception + of an ACK for an updating packet, the compressor knows that the + decompressor has received the acknowledged packet and the transition + to a higher compression state can be carried out immediately. This + functionality is optional, so a compressor MUST NOT expect to get + such ACKs initially. + + The compressor MAY use the following algorithm to determine when to + expect ACKs for UOR-2 packets. Let an update event be when a + sequence of UOR-2 headers are sent to communicate an irregularity in + the packet stream. When ACKs have been received for k_3 out of the + last n_3 update events, the compressor will expect ACKs. A + compressor which expects ACKs will repeat updates (possibly not in + every packet) until an ACK is received. + +5.4.1.2. Compression logic and packets used + + The compression logic is the same for the Bidirectional Optimistic + mode as for the Unidirectional mode (see section 5.3.1.2). + + + + + +Bormann, et al. Standards Track [Page 63] + +RFC 3095 Robust Header Compression July 2001 + + +5.4.2. Decompressor states and logic (O-mode) + + The decompression states and the state transition logic are the same + as for the Unidirectional case (see section 5.3.2). What differs is + the decompression and feedback logic. + +5.4.2.1. Decompression logic, timer-based timestamp decompression + + In Bidirectional mode (or if there is some other way for the + compressor to obtain the decompressor's clock resolution and the + link's jitter), timer-based timestamp decompression may be used to + improve compression efficiency when RTP Timestamp values are + proportional to wall-clock time. The mechanisms used are those + described in 4.5.4. + +5.4.2.2. Feedback logic (O-mode) + + The feedback logic defines what feedback to send due to different + events when operating in the various states. As mentioned above, + there are three principal kinds of feedback; ACK, NACK and STATIC- + NACK. Further, the logic described below will refer to different + kinds of packets that can be received by the decompressor; + Initialization and Refresh (IR) packets, IR packets without static + information (IR-DYN) and type 2 packets (UOR-2), or type 1 (UO-1) and + type 0 packets (UO-0). A type 0 packet carries a packet header + compressed according to a fixed pattern, while type 1, 2 and IR-DYN + packets are used when this pattern is broken. + + Below, rules are defined stating which feedback to use when. If the + optional feedback is used once, the decompressor is REQUIRED to + continue to send optional feedback for the lifetime of the packet + stream. + + State Actions + + NC: - When an IR packet passes the CRC check, send an ACK(O). + - When receiving a type 0, 1, 2 or IR-DYN packet, or an IR + packet has failed the CRC check, send a STATIC-NACK(O), + subject to the considerations at the beginning of section + 5.7.6. + + SC: - When an IR packet is correctly decompressed, send an ACK(O). + - When a type 2 or an IR-DYN packet is correctly decompressed, + optionally send an ACK(O). + - When a type 0 or 1 packet is received, treat it as a + mismatching CRC and use the logic of section 5.3.2.2.3 to + decide if a NACK(O) should be sent. + + + + +Bormann, et al. Standards Track [Page 64] + +RFC 3095 Robust Header Compression July 2001 + + + - When decompression of a type 2 packet, an IR-DYN packet or an + IR packet has failed, use the logic of section 5.3.2.2.3 to + decide if a STATIC-NACK(O) should be sent. + + FC: - When an IR packet is correctly decompressed, send an ACK(O). + - When a type 2 or an IR-DYN packet is correctly decompressed, + optionally send an ACK(O). + - When a type 0 or 1 packet is correctly decompressed, no + feedback is sent. + - When any packet fails the CRC check, use the logic of + 5.3.2.2.3 to decide if a NACK(O) should be sent. + +5.5. Operation in Bidirectional Reliable mode + +5.5.1. Compressor states and logic (R-mode) + + Below is the state machine for the compressor in Bidirectional + Reliable mode. The details of each state, state transitions, and + compression logic are given subsequent to the figure. + + ACK + +------>------>------>------>------>------>------>------+ + | | + | ACK ACK | ACK + | +------>------>------+ +------>------>------+ +->-+ + | | | | | | | + | | v | v | v + +----------+ +----------+ +----------+ + | IR State | | FO State | | SO State | + +----------+ +----------+ +----------+ + ^ ^ | ^ | | + | | STATIC-NACK | | NACK / Update | | + | +------<------<------+ +------<------<------+ | + | | + | STATIC-NACK | + +------<------<------<------<------<------<------<------<------+ + +5.5.1.1. State transition logic (R-mode) + + The transition logic for compression states in Reliable mode is based + on three principles: the secure reference principle, the need for + updates, and negative acknowledgments. + +5.5.1.1.1. Upwards transition + + The upwards transition is determined by the secure reference + principle. The transition procedure is similar to the one described + in section 5.3.1.1.1, with one important difference: the compressor + + + +Bormann, et al. Standards Track [Page 65] + +RFC 3095 Robust Header Compression July 2001 + + + bases its confidence only on acknowledgments received from the + decompressor. This ensures that the synchronization between the + compression context and decompression context will never be lost due + to packet losses. + +5.5.1.1.2. Downward transition + + Downward transitions are triggered by the need for updates or by + negative acknowledgment (NACKs and STATIC_NACKs), as described in + section 5.3.1.1.3 and 5.4.1.1.1, respectively. Note that NACKs + should rarely occur in R-mode because of the secure reference used + (see fourth paragraph of next section). + +5.5.1.2. Compression logic and packets used (R-mode) + + The compressor starts in the IR state by sending IR packets. It + transits to the FO state once it receives a valid ACK for an IR + packet sent (an ACK can only be valid if it refers to an SN sent + earlier). In the FO state, it sends the smallest packets that can + communicate the changes, according to W-LSB or other encoding rules. + Those packets could be of type R-1*, UOR-2, or even IR-DYN. + + The compressor will transit to the SO state after it has determined + the presence of a string (see section 2), while also being confident + that the decompressor has the string parameters. The confidence can + be based on ACKs. For example, in a typical case where the string + pattern has the form of non-SN-field = SN * slope + offset, one ACK + is enough if the slope has been previously established by the + decompressor (i.e., only the new offset needs to be synchronized). + Otherwise, two ACKs are required since the decompressor needs two + headers to learn both the new slope and the new offset. In the SO + state, R-0* packets will be sent. + + Note that a direct transition from the IR state to the SO state is + possible. + + The secure reference principle is enforced in both compression and + decompression logic. The principle means that only a packet carrying + a 7- or 8-bit CRC can update the decompression context and be used as + a reference for subsequent decompression. Consequently, only field + values of update packets need to be added to the encoding sliding + windows (see 4.5) maintained by the compressor. + + Reasons for the compressor to send update packets include: + + 1) The update may lead to a transition to higher compression + efficiency (meaning either a higher compression state or smaller + packets in the same state). + + + +Bormann, et al. Standards Track [Page 66] + +RFC 3095 Robust Header Compression July 2001 + + + 2) It is desirable to shrink sliding windows. Windows are only + shrunk when an ACK is received. + + The generation of a CRC is infrequent since it is only needed for + an update packet. + + One algorithm for sending update packets could be: + + * Let pRTT be the number of packets that are sent during one + round-trip time. In the SO state, when (64 - pRTT) headers have + been sent since the last acked reference, the compressor will + send m1 consecutive R-0-CRC headers, then send (pRTT - m1) R-0 + headers. After these headers have been sent, if the compressor + has not received an ACK to at least one of the previously sent + R0-CRC, it sends R-0-CRC headers continuously until it receives a + corresponding ACK. m1 is an implementation parameter, which can + be as large as pRTT. + + * In the FO state, m2 UOR-2 headers are sent when there is a + pattern change, after which the compressor sends (pRTT - m2) + R-1-* headers. m2 is an implementation parameter, which can be + as large as pRTT. At that time, if the compressor has not + received enough ACKs to the previously sent UOR-2 packets in + order to transit to SO state, it can repeat the cycle with the + same m2, or repeat the cycle with a larger m2, or send UOR-2 + headers continuously (m2 = pRTT). The operation stops when the + compressor has received enough ACKs to make the transition. + + An algorithm for processing ACKs could be: + + * Upon reception of an ACK, the compressor first derives the + complete SN (see section 5.7.6.1). Then it searches the sliding + window for an update packet that has the same SN. If found, that + packet is the one being ACKed. Otherwise, the ACK is invalid and + MUST be discarded. + + * It is possible, although unlikely, that residual errors on the + reverse channel could cause a packet to mimic a valid ACK + feedback. The compressor may use a local clock to reduce the + probability of processing such a mistaken ACK. After finding the + update packet as described above, the compressor can check the + time elapsed since the packet was sent. If the time is longer + than RTT_U, or shorter than RTT_L, the compressor may choose to + discard the ACK. RTT_U and RTT_L correspond to an upper bound + and lower bound of the RTT, respectively. (These bounds should + be chosen appropriately to allow some variation of RTT.) Note + that the only side effect of discarding a good ACK is slightly + reduced compression efficiency. + + + +Bormann, et al. Standards Track [Page 67] + +RFC 3095 Robust Header Compression July 2001 + + +5.5.2. Decompressor states and logic (R-mode) + + The decompression states and the state transition logic are the same + as for the Unidirectional case (see section 5.3.2). What differs is + the decompression and feedback logic. + +5.5.2.1. Decompression logic (R-mode) + + The rules for when decompression is allowed are the same as for U- + mode. Although the acking scheme in R-mode guarantees that non- + decompressible packets are never sent by the compressor, residual + errors can cause delivery of unexpected packets for which + decompression should not be attempted. + + Decompression MUST follow the secure reference principle as described + in 5.5.1.2. + + CRC verification is infrequent since only update packets carry CRCs. + A CRC mismatch can only occur due to 1) residual bit errors in the + current header, and/or 2) a damaged context due to residual bit + errors in previous headers or feedback. Although it is impossible to + determine which is the actual cause, case 1 is more likely, as a + previous header reconstructed according to a damaged packet is + unlikely to pass the 7- or 8-bit CRC, and damaged packets are + unlikely to result in feedback that damages the context. The + decompressor SHOULD act according to section 5.3.2.2.3 when CRCs + fail, except that no local repair is performed. Note that all the + parameter numbers, k_1, n_1, k_2, and n_2, are applied to the update + packets only (i.e., exclude R-0, R-1*). + +5.5.2.2. Feedback logic (R-mode) + + The feedback logic for the Bidirectional Reliable mode is as follows: + + - When an updating packet (i.e., a packet carrying a 7- or 8-bit CRC) + is correctly decompressed, send an ACK(R), subject to the sparse + ACK mechanism described below. + + - When context damage is detected, send a NACK(R) if in Full Context + state, or a STATIC-NACK(R) if in Static Context state. + + - In No Context state, send a STATIC-NACK(R) when receiving a non-IR + packet, subject to the considerations at the beginning of section + 5.7.6. The decompressor SHOULD NOT send STATIC-NACK(R) when + receiving an IR packet that fails the CRC check, as the compressor + will stay in IR state and thus continue sending IR packets until a + valid ACK is received (see section 5.5.1.2). + + + + +Bormann, et al. Standards Track [Page 68] + +RFC 3095 Robust Header Compression July 2001 + + + - Feedback is never sent for packets not updating the context (i.e., + packets that do not carry a CRC) + + A mechanism called "Sparse ACK" can be applied to reduce the feedback + overhead caused by a large RTT. For a sequence of ACK-triggering + events, a minimal set of ACKs MUST be sent: + + 1) For a sequence of R-0-CRC packets, the first one MUST be ACKed. + + 2) For a sequence of UOR-2, IR, or IR-DYN packets, the first N of + them MUST be ACKEd, where N is the number of ACKs needed to give + the compressor confidence that the decompressor has acquired the + new string parameters (see second paragraph of 5.5.1.2). In case + the decompressor cannot determine the value of N, the default + value 2 SHOULD be used. If the subsequently received packets + continue the same change pattern of header fields, sparse ACK can + be applied. Otherwise, each new pattern MUST be treated as a new + sequence, i.e., the first N packets that exhibit a new pattern + MUST be ACKed. + + After sending these minimal ACKs, the decompressor MAY choose to ACK + only k subsequent packets per RTT ("Sparse ACKs"), where k is an + implementation parameter. To achieve robustness against loss of + ACKs, k SHOULD be at least 1. + + To avoid ambiguity at the compressor, the decompressor MUST use the + feedback format whose SN field length is equal to or larger than the + one in the compressed packet that triggered the feedback. + + Context damage is detected according to the principles in 5.3.2.2.3. + + When the decompressor is capable of timer-based compression of the + RTP Timestamp (e.g., it has access to a clock with sufficient + resolution, and the jitter introduced internally in the receiving + node is sufficiently small) it SHOULD signal that it is ready to do + timer-based compression of the RTP Timestamp. The compressor will + then make a decision based on its knowledge of the channel and the + observed properties of the packet stream. + +5.6. Mode transitions + + The decision to move from one compression mode to another is taken by + the decompressor and the possible mode transitions are shown in the + figure below. Subsequent chapters describe how the transitions are + performed together with exceptions for the compression and + decompression functionality during transitions. + + + + + +Bormann, et al. Standards Track [Page 69] + +RFC 3095 Robust Header Compression July 2001 + + + +-------------------------+ + | Unidirectional (U) mode | + +-------------------------+ + / ^ \ ^ + / / Feedback(U) \ \ Feedback(U) + / / \ \ + / / \ \ + Feedback(O) / / Feedback(R) \ \ + v / v \ + +---------------------+ Feedback(R) +-------------------+ + | Optimistic (O) mode | ----------------> | Reliable (R) mode | + | | <---------------- | | + +---------------------+ Feedback(O) +-------------------+ + +5.6.1. Compression and decompression during mode transitions + + The following sections assume that, for each context, the compressor + and decompressor maintain a variable whose value is the current + compression mode for that context. The value of the variable + controls, for the context in question, which packet types to use, + which actions to be taken, etc. + + As a safeguard against residual errors, all feedback sent during a + mode transition MUST be protected by a CRC, i.e., the CRC option MUST + be used. A mode transition MUST NOT be initiated by feedback which + is not protected by a CRC. + + The subsequent subsections define exactly when to change the value of + the MODE variable. When ROHC transits between compression modes, + there are several cases where the behavior of compressor or + decompressor must be restricted during the transition phase. These + restrictions are defined by exception parameters that specify which + restrictions to apply. The transition descriptions in subsequent + chapters refer to these exception parameters and defines when they + are set and to what values. All mode related parameters are listed + below together with their possible values, with explanations and + restrictions: + + Parameters for the compressor side: + + - C_MODE: + Possible values for the C_MODE parameter are (U)NIDIRECTIONAL, + (O)PTIMISTIC and (R)ELIABLE. C_MODE MUST be initialized to U. + + - C_TRANS: + Possible values for the C_TRANS parameter are (P)ENDING and + (D)ONE. C_TRANS MUST be initialized to D. When C_TRANS is P, + it is REQUIRED + + + +Bormann, et al. Standards Track [Page 70] + +RFC 3095 Robust Header Compression July 2001 + + + 1) that the compressor only use packet formats common to all + modes, + + 2) that mode information is included in packets sent, at least + periodically, + + 3) that the compressor not transit to the SO state, + + 4) that new mode transition requests be ignored. + + Parameters for the decompressor side: + + - D_MODE: + Possible values for the D_MODE parameter are (U)NIDIRECTIONAL, + (O)PTIMISTIC and (R)ELIABLE. D_MODE MUST be initialized to U. + + - D_TRANS: + Possible values for the D_TRANS parameter are (I)NITIATED, + (P)ENDING and (D)ONE. D_TRANS MUST be initialized to D. A + mode transition can be initiated only when D_TRANS is D. While + D_TRANS is I, the decompressor sends a NACK or ACK carrying a + CRC option for each received packet. + +5.6.2. Transition from Unidirectional to Optimistic mode + + When there is a feedback channel available, the decompressor may at + any moment decide to initiate transition from Unidirectional to + Bidirectional Optimistic mode. Any feedback packet carrying a CRC + can be used with the mode parameter set to O. The decompressor can + then directly start working in Optimistic mode. The compressor + transits from Unidirectional to Optimistic mode as soon as it + receives any feedback packet that has the mode parameter set to O and + that passes the CRC check. The transition procedure is described + below: + + Compressor Decompressor + ---------------------------------------------- + | | + | ACK(O)/NACK(O) +-<-<-<-| D_MODE = O + | +-<-<-<-<-<-<-<-+ | + C_MODE = O |-<-<-<-+ | + | | + + If the feedback packet is lost, the compressor will continue to work + in Unidirectional mode, but as soon as any feedback packet reaches + the compressor it will transit to Optimistic mode. + + + + + +Bormann, et al. Standards Track [Page 71] + +RFC 3095 Robust Header Compression July 2001 + + +5.6.3. From Optimistic to Reliable mode + + Transition from Optimistic to Reliable mode is permitted only after + at least one packet has been correctly decompressed, which means that + at least the static part of the context is established. An ACK(R) or + a NACK(R) feedback packet carrying a CRC is sent to initiate the mode + transition. The compressor MUST NOT use packet types 0 or 1 during + transition. The transition procedure is described below: + + Compressor Decompressor + ---------------------------------------------- + | | + | ACK(R)/NACK(R) +-<-<-<-| D_TRANS = I + | +-<-<-<-<-<-<-<-+ | + C_TRANS = P |-<-<-<-+ | + C_MODE = R | | + |->->->-+ IR/IR-DYN/UOR-2(SN,R) | + | +->->->->->->->-+ | + |->-.. +->->->-| D_TRANS = P + |->-.. | D_MODE = R + | ACK(SN,R) +-<-<-<-| + | +-<-<-<-<-<-<-<-+ | + C_TRANS = D |-<-<-<-+ | + | | + |->->->-+ R-0*, R-1* | + | +->->->->->->->-+ | + | +->->->-| D_TRANS = D + | | + + As long as the decompressor has not received an UOR-2, IR-DYN, or IR + packet with the mode transition parameter set to R, it must stay in + Optimistic mode. The compressor must not send packet types 1 or 0 + while C_TRANS is P, i.e., not until it has received an ACK for a + UOR-2, IR-DYN, or IR packet sent with the mode transition parameter + set to R. When the decompressor receives packet types 0 or 1, after + having ACKed an UOR-2, IR-DYN, or IR packet, it sets D_TRANS to D. + +5.6.4. From Unidirectional to Reliable mode + + The transition from Unidirectional to Reliable mode follows the same + transition procedure as defined in section 5.6.3 above. + +5.6.5. From Reliable to Optimistic mode + + Either the ACK(O) or the NACK(O) feedback packet is used to initiate + the transition from Reliable to Optimistic mode and the compressor + MUST always run in the FO state during transition. The transition + procedure is described below: + + + +Bormann, et al. Standards Track [Page 72] + +RFC 3095 Robust Header Compression July 2001 + + + Compressor Decompressor + ---------------------------------------------- + | | + | ACK(O)/NACK(O) +-<-<-<-| D_TRANS = I + | +-<-<-<-<-<-<-<-+ | + C_TRANS = P |-<-<-<-+ | + C_MODE = O | | + |->->->-+ IR/IR-DYN/UOR-2(SN,O) | + | +->->->->->->->-+ | + |->-.. +->->->-| D_MODE = O + |->-.. | + | ACK(SN,O) +-<-<-<-| + | +-<-<-<-<-<-<-<-+ | + C_TRANS = D |-<-<-<-+ | + | | + |->->->-+ UO-0, UO-1* | + | +->->->->->->->-+ | + | +->->->-| D_TRANS = D + | | + + As long as the decompressor has not received an UOR-2, IR-DYN, or IR + packet with the mode transition parameter set to O, it must stay in + Reliable mode. The compressor must not send packet types 0 or 1 + while C_TRANS is P, i.e., not until it has received an ACK for an + UOR-2, IR-DYN, or IR packet sent with the mode transition parameter + set to O. When the decompressor receives packet types 0 or 1, after + having ACKed the UOR-2, IR-DYN, or IR packet, it sets D_TRANS to D. + +5.6.6. Transition to Unidirectional mode + + The decompressor can force a transition back to Unidirectional mode + if it desires to do so. Regardless of which mode this transition + starts from, a three-way handshake MUST be carried out to ensure + correct transition on the compressor side. The transition procedure + is described below: + + + + + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 73] + +RFC 3095 Robust Header Compression July 2001 + + + Compressor Decompressor + ---------------------------------------------- + | | + | ACK(U)/NACK(U) +-<-<-<-| D_TRANS = I + | +-<-<-<-<-<-<-<-+ | + C_TRANS = P |-<-<-<-+ | + C_MODE = U | | + |->->->-+ IR/IR-DYN/UOR-2(SN,U) | + | +->->->->->->->-+ | + |->-.. +->->->-| + |->-.. | + | ACK(SN,U) +-<-<-<-| + | +-<-<-<-<-<-<-<-+ | + C_TRANS = D |-<-<-<-+ | + | | + |->->->-+ UO-0, UO-1* | + | +->->->->->->->-+ | + | +->->->-| D_TRANS = D, D_MODE= U + + After ACKing the first UOR-2(U), IR-DYN(U), or IR(U), the + decompressor MUST continue to send feedback with the Mode parameter + set to U until it receives packet types 0 or 1. + +5.7. Packet formats + + The following notation is used in this section: + + bits(X) = the number of bits for field X present in the compressed + header (including extension). + + field(X) = the value of field X in the compressed header. + + context(X) = the value of field X as established in the context. + + value(X) = field(X) if X is present in the compressed header; + = context(X) otherwise. + + hdr(X) = the value of field X in the uncompressed or + decompressed header. + + Updating properties: Lists the fields in the context that are + directly updated by processing the compressed header. Note + that there may be dependent fields that are implicitly also + updated (e.g., an update to context(SN) often updates + context(TS) as well). See also section 5.2.7. + + + + + + +Bormann, et al. Standards Track [Page 74] + +RFC 3095 Robust Header Compression July 2001 + + + The following fields occur in several headers and extensions: + + SN: The compressed RTP Sequence Number. + + Compressed with W-LSB. The interpretation intervals, see section + 4.5.1, are defined as follows: + + p = 1 if bits(SN) <= 4 + p = 2^(bits(SN)-5) - 1 if bits(SN) > 4 + + IP-ID: A compressed IP-ID field. + + IP-ID fields in compressed base headers carry the compressed IP-ID + of the innermost IPv4 header whose corresponding RND flag is not + 1. The rules below assume that the IP-ID is for the innermost IP + header. If it is for an outer IP header, the RND2 and NBO2 flags + are used instead of RND and NBO. + + If value(RND) = 0, hdr(IP-ID) is compressed using Offset IP-ID + encoding (see section 4.5.5) using p = 0 and default-slope(IP-ID + offset) = 0. + + If value(RND) = 1, IP-ID is the uncompressed hdr(IP-ID). IP-ID is + then passed as additional octets at the end of the compressed + header, after any extensions. + + If value(NBO) = 0, the octets of hdr(IP-ID) are swapped before + compression and after decompression. The value of NBO is ignored + when value(RND) = 1. + + TS: The compressed RTP Timestamp value. + + If value(TIME_STRIDE) > 0, timer-based compression of the RTP + Timestamp is used (see section 4.5.4). + + If value(Tsc) = 1, Scaled RTP Timestamp encoding is used before + compression (see section 4.5.3), and default-slope(TS) = 1. + + If value(Tsc) = 0, the Timestamp value is compressed as-is, and + default-slope(TS) = value(TS_STRIDE). + + The interpretation intervals, see section 4.5.1, are defined as + follows: + + p = 2^(bits(TS)-2) - 1 + + + + + + +Bormann, et al. Standards Track [Page 75] + +RFC 3095 Robust Header Compression July 2001 + + + CRC: The CRC over the original, uncompressed, header. + + For 3-bit CRCs, the polynomial of section 5.9.2 is used. + For 7-bit CRCs, the polynomial of section 5.9.2 is used. + For 8-bit CRCs, the polynomial of section 5.9.1 is used. + + M: RTP Marker bit. + + Context(M) is initially zero and is never updated. value(M) = 1 + only when field(M) = 1. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 76] + +RFC 3095 Robust Header Compression July 2001 + + + The general format for a compressed RTP header is as follows: + + 0 1 2 3 4 5 6 7 + --- --- --- --- --- --- --- --- + : Add-CID octet : if for small CIDs and CID 1-15 + +---+---+---+---+---+---+---+---+ + | first octet of base header | (with type indication) + +---+---+---+---+---+---+---+---+ + : : + / 0, 1, or 2 octets of CID / 1-2 octets if large CIDs + : : + +---+---+---+---+---+---+---+---+ + / remainder of base header / variable number of bits + +---+---+---+---+---+---+---+---+ + : : + / Extension (see 5.7.5) / extension, if X = 1 in base header + : : + --- --- --- --- --- --- --- --- + : : + + IP-ID of outer IPv4 header + 2 octets, if value(RND2) = 1 + : : + --- --- --- --- --- --- --- --- + / AH data for outer list / variable (see 5.8.4.2) + --- --- --- --- --- --- --- --- + : : + + GRE checksum (see 5.8.4.4) + 2 octets, if GRE flag C = 1 + : : + --- --- --- --- --- --- --- --- + : : + + IP-ID of inner IPv4 header + 2 octets, if value(RND) = 1 + : : + --- --- --- --- --- --- --- --- + / AH data for inner list / variable (see 5.8.4.2) + --- --- --- --- --- --- --- --- + : : + + GRE checksum (see 5.8.4.4) + 2 octets, if GRE flag C = 1 + : : + --- --- --- --- --- --- --- --- + : : + + UDP Checksum + 2 octets, + : : if context(UDP Checksum) != 0 + --- --- --- --- --- --- --- --- + + Note that the order of the fields following the optional extension is + the same as the order between the fields in an uncompressed header. + + In subsequent sections, the position of the large CID in the diagrams + is indicated using this notation: + + + +Bormann, et al. Standards Track [Page 77] + +RFC 3095 Robust Header Compression July 2001 + + + +===+===+===+===+===+===+===+===+ + + Whether the UDP Checksum field is present or not is controlled by the + value of the UDP Checksum in the context. If nonzero, the UDP + Checksum is enabled and sent along with each packet. If zero, the + UDP Checksum is disabled and not sent. Should hdr(UDP Checksum) be + nonzero when context(UDP Checksum) is zero, the header cannot be + compressed. It must be sent uncompressed or the context + reinitialized using an IR packet. Context(UDP Checksum) is updated + only by IR or IR-DYN headers, never by UDP checksums sent in headers + of type 2, 1, or 0. + + When an IPv4 header is present in the static context, for which the + corresponding RND flag has not been established to be 1, the packet + types R-1 and UO-1 MUST NOT be used. + + When no IPv4 header is present in the static context, or the RND + flags for all IPv4 headers in the context have been established to be + 1, the packet types R-1-ID, R-1-TS, UO-1-ID, and UO-1-TS MUST NOT be + used. + + While in the transient state in which an RND flag is being + established, the packet types R-1-ID, R-1-TS, UO-1-ID, and UO-1-TS + MUST NOT be used. This implies that the RND flag(s) of the Extension + 3 may have to be inspected before the format of a base header + carrying an Extension 3 can be determined. + +5.7.1. Packet type 0: UO-0, R-0, R-0-CRC + + Packet type 0 is indicated by the first bit being 0: + + R-0 + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 0 0 | SN | + +===+===+===+===+===+===+===+===+ + + Updating properties: R-0 packets do not update any part of the + context. + + + + + + + + + + + +Bormann, et al. Standards Track [Page 78] + +RFC 3095 Robust Header Compression July 2001 + + + R-0-CRC + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 0 1 | SN | + +===+===+===+===+===+===+===+===+ + |SN | CRC | + +---+---+---+---+---+---+---+---+ + + Note: The SN field straddles the CID field. + + Updating properties: R-0-CRC packets update context(RTP Sequence + Number). + + UO-0 + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 0 | SN | CRC | + +===+===+===+===+===+===+===+===+ + + Updating properties: UO-0 packets update the current value of + context(RTP Sequence Number). + +5.7.2. Packet type 1 (R-mode): R-1, R-1-TS, R-1-ID + + Packet type 1 is indicated by the first bits being 10: + + R-1 + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 0 | SN | + +===+===+===+===+===+===+===+===+ + | M | X | TS | + +---+---+---+---+---+---+---+---+ + + Note: R-1 cannot be used if the context contains at least one IPv4 + header with value(RND) = 0. This disambiguates it from R-1-ID and + R-1-TS. + + + + + + + + + + + +Bormann, et al. Standards Track [Page 79] + +RFC 3095 Robust Header Compression July 2001 + + + R-1-ID + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 0 | SN | + +===+===+===+===+===+===+===+===+ + | M | X |T=0| IP-ID | + +---+---+---+---+---+---+---+---+ + + Note: R-1-ID cannot be used if there is no IPv4 header in the + context or if value(RND) and value(RND2) are both 1. + + R-1-TS + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 0 | SN | + +===+===+===+===+===+===+===+===+ + | M | X |T=1| TS | + +---+---+---+---+---+---+---+---+ + + Note: R-1-TS cannot be used if there is no IPv4 header in the + context or if value(RND) and value(RND2) are both 1. + + X: X = 0 indicates that no extension is present; + X = 1 indicates that an extension is present. + + T: T = 0 indicates format R-1-ID; + T = 1 indicates format R-1-TS. + + Updating properties: R-1* headers do not update any part of the + context. + +5.7.3. Packet type 1 (U/O-mode): UO-1, UO-1-ID, UO-1-TS + + UO-1 + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 0 | TS | + +===+===+===+===+===+===+===+===+ + | M | SN | CRC | + +---+---+---+---+---+---+---+---+ + + Note: UO-1 cannot be used if the context contains at least one + IPv4 header with value(RND) = 0. This disambiguates it from UO- + 1-ID and UO-1-TS. + + + + +Bormann, et al. Standards Track [Page 80] + +RFC 3095 Robust Header Compression July 2001 + + + UO-1-ID + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 0 |T=0| IP-ID | + +===+===+===+===+===+===+===+===+ + | X | SN | CRC | + +---+---+---+---+---+---+---+---+ + + Note: UO-1-ID cannot be used if there is no IPv4 header in the + context or if value(RND) and value(RND2) are both 1. + + UO-1-TS + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 0 |T=1| TS | + +===+===+===+===+===+===+===+===+ + | M | SN | CRC | + +---+---+---+---+---+---+---+---+ + + Note: UO-1-TS cannot be used if there is no IPv4 header in the + context or if value(RND) and value(RND2) are both 1. + + X: X = 0 indicates that no extension is present; + X = 1 indicates that an extension is present. + + T: T = 0 indicates format UO-1-ID; + T = 1 indicates format UO-1-TS. + + Updating properties: UO-1* packets update context(RTP Sequence + Number). UO-1 and UO-1-TS packets update context(RTP Timestamp). + UO-1-ID packets update context(IP-ID). Values provided in + extensions, except those in other SN, TS, or IP-ID fields, do not + update the context. + + + + + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 81] + +RFC 3095 Robust Header Compression July 2001 + + +5.7.4. Packet type 2: UOR-2 + + Packet type 2 is indicated by the first bits being 110: + + UOR-2 + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 1 0 | TS | + +===+===+===+===+===+===+===+===+ + |TS | M | SN | + +---+---+---+---+---+---+---+---+ + | X | CRC | + +---+---+---+---+---+---+---+---+ + + Note: UOR-2 cannot be used if the context contains at least one + IPv4 header with value(RND) = 0. This disambiguates it from UOR- + 2-ID and UOR-2-TS. + + Note: The TS field straddles the CID field. + + UOR-2-ID + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 1 0 | IP-ID | + +===+===+===+===+===+===+===+===+ + |T=0| M | SN | + +---+---+---+---+---+---+---+---+ + | X | CRC | + +---+---+---+---+---+---+---+---+ + + Note: UOR-2-ID cannot be used if there is no IPv4 header in the + context or if value(RND) and value(RND2) are both 1. + + UOR-2-TS + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 1 0 | TS | + +===+===+===+===+===+===+===+===+ + |T=1| M | SN | + +---+---+---+---+---+---+---+---+ + | X | CRC | + +---+---+---+---+---+---+---+---+ + + Note: UOR-2-TS cannot be used if there is no IPv4 header in the + context or if value(RND) and value(RND2) are both 1. + + + +Bormann, et al. Standards Track [Page 82] + +RFC 3095 Robust Header Compression July 2001 + + + X: X = 0 indicates that no extension is present; + X = 1 indicates that an extension is present. + + T: T = 0 indicates format UOR-2-ID; + T = 1 indicates format UOR-2-TS. + + Updating properties: All values provided in UOR-2* packets update + the context, unless explicitly stated otherwise. + +5.7.5. Extension formats + + (Note: the term extension as used for additional information + contained in the ROHC headers does not bear any relationship to the + term extension header used in IP.) + + Fields in extensions are concatenated with the corresponding field in + the base compressed header, if there is one. Bits in an extension + are less significant than bits in the base compressed header (see + section 4.5.7). + + The TS field is scaled in all extensions, as it is in the base + header, except optionally when using Extension 3 where the Tsc flag + can indicate that the TS field is not scaled. Value(TS_STRIDE) is + used as the scale factor when scaling the TS field. + + In the following three extensions, the interpretation of the fields + depends on whether there is a T-bit in the base compressed header, + and if so, on the value of that field. When there is no T-bit, +T + and -T both mean TS. This is the case when there are no IPv4 headers + in the static context, and when all IPv4 headers in the static + context have their corresponding RND flag set (i.e., RND = 1). + + If there is a T-bit, + + T = 1 indicates that +T is TS, and + -T is IP-ID; + + T = 0 indicates that +T is IP-ID, and + -T is TS. + + Extension 0: + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 0 0 | SN | +T | + +---+---+---+---+---+---+---+---+ + + + + + +Bormann, et al. Standards Track [Page 83] + +RFC 3095 Robust Header Compression July 2001 + + + Extension 1: + + +---+---+---+---+---+---+---+---+ + | 0 1 | SN | +T | + +---+---+---+---+---+---+---+---+ + | -T | + +---+---+---+---+---+---+---+---+ + + Extension 2: + + +---+---+---+---+---+---+---+---+ + | 1 0 | SN | +T | + +---+---+---+---+---+---+---+---+ + | +T | + +---+---+---+---+---+---+---+---+ + | -T | + +---+---+---+---+---+---+---+---+ + + Extension 3 is a more elaborate extension which can give values for + fields other than SN, TS, and IP-ID. Three optional flag octets + indicate changes to IP header(s) and RTP header, respectively. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 84] + +RFC 3095 Robust Header Compression July 2001 + + + Extension 3: + + 0 1 2 3 4 5 6 7 + +-----+-----+-----+-----+-----+-----+-----+-----+ + | 1 1 | S |R-TS | Tsc | I | ip | rtp | (FLAGS) + +-----+-----+-----+-----+-----+-----+-----+-----+ + | Inner IP header flags | ip2 | if ip = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + | Outer IP header flags | if ip2 = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + | SN | if S = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + / TS (encoded as in section 4.5.6) / 1-4 octets, + ..... ..... ..... ..... ..... ..... ..... ..... if R-TS = 1 + | | + / Inner IP header fields / variable, + | | if ip = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + | IP-ID | 2 octets, if I = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + | | + / Outer IP header fields / variable, + | | if ip2 = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + | | + / RTP header flags and fields / variable, + | | if rtp = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + + S, R-TS, I, ip, rtp, ip2: Indicate presence of fields as shown to + the right of each field above. + + Tsc: Tsc = 0 indicates that TS is not scaled; + Tsc = 1 indicates that TS is scaled according to section + 4.5.3, using value(TS_STRIDE). + Context(Tsc) is always 1. If scaling is not desired, the + compressor will establish TS_STRIDE = 1. + + SN: See the beginning of section 5.7. + + TS: Variable number of bits of TS, encoded according to + section 4.5.6. See the beginning of section 5.7. + + IP-ID: See the beginning of section 5.7. + + + + + + + +Bormann, et al. Standards Track [Page 85] + +RFC 3095 Robust Header Compression July 2001 + + + Inner IP header flags + + These correspond to the inner IP header if there are two, and the + single IP header otherwise. + + 0 1 2 3 4 5 6 7 + ..... ..... ..... ..... ..... ..... ..... ..... + | TOS | TTL | DF | PR | IPX | NBO | RND | ip2 | if ip = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + + TOS, TTL, PR, IPX: Indicates presence of fields as shown to the + right of the field in question below. + + DF: Don't Fragment bit of IP header. + + NBO: Indicates whether the octets of hdr(IP identifier) of this IP + header are swapped before compression and after decompression. + + NBO = 1 indicates that the octets need not be swapped. NBO = 0 + indicates that the octets are to be swapped. See section 4.5.5. + + RND: Indicates whether hdr(IP identifier) is not to be compressed + but instead sent as-is in compressed headers. + + IP2: Indicates presence of Outer IP header fields. Unless the + static context contains two IP headers, IP2 is always zero. + + Inner IP header fields + + ..... ..... ..... ..... ..... ..... ..... ..... + | Type of Service/Traffic Class | if TOS = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + | Time to Live/Hop Limit | if TTL = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + | Protocol/Next Header | if PR = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + / IP extension headers / variable, + ..... ..... ..... ..... ..... ..... ..... ..... if IPX = 1 + + Type of Service/Traffic Class: That field in the uncompressed IP + header (absolute value). + + Time to Live/Hop Limit: That field in the uncompressed IP header. + + Protocol/Next Header: That field in the uncompressed IP header. + + IP extension header(s): According to section 5.8.5. + + + + +Bormann, et al. Standards Track [Page 86] + +RFC 3095 Robust Header Compression July 2001 + + + Outer IP header flags + + The fields in this part of the Extension 3 header refer to the + outermost IP header: + + 0 1 2 3 4 5 6 7 + ..... ..... ..... ..... ..... ..... ..... ..... | TOS2| TTL2| + DF2 | PR2 |IPX2 |NBO2 |RND2 | I2 | if ip2 = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + + These flags are the same as the Inner IP header flags, but refer + to the outer IP header instead of the inner IP header. The + following flag, however, has no counterpart in the Inner IP header + flags: + + I2: Indicates presence of the IP-ID field. + + Outer IP header fields + + ..... ..... ..... ..... ..... ..... ..... ..... + | Type of Service/Traffic Class | if TOS2 = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + | Time to Live/Hop Limit | if TTL2 = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + | Protocol/Next Header | if PR2 = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + / IP extension header(s) / variable, + ..... ..... ..... ..... ..... ..... ..... ..... if IPX2 = 1 + | IP-ID | 2 octets, + ..... ..... ..... ..... ..... ..... ..... ..... if I2 = 1 + + The fields in this part of Extension 3 are as for the Inner IP + header fields, but they refer to the outer IP header instead of + the inner IP header. The following field, however, has no + counterpart among the Inner IP header fields: + + IP-ID: The IP Identifier field of the outer IP header, unless + the inner header is an IPv6 header, in which case I2 is always + zero. + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 87] + +RFC 3095 Robust Header Compression July 2001 + + + RTP header flags and fields + + 0 1 2 3 4 5 6 7 + ..... ..... ..... ..... ..... ..... ..... ..... + | Mode |R-PT | M | R-X |CSRC | TSS | TIS | if rtp = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + | R-P | RTP PT | if R-PT = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + / Compressed CSRC list / if CSRC = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + / TS_STRIDE / 1-4 oct if TSS = 1 + ..... ..... ..... ..... ..... ..... ..... .... + / TIME_STRIDE (milliseconds) / 1-4 oct if TIS = 1 + ..... ..... ..... ..... ..... ..... ..... ..... + + Mode: Compression mode. 0 = Reserved, + 1 = Unidirectional, + 2 = Bidirectional Optimistic, + 3 = Bidirectional Reliable. + + R-PT, CSRC, TSS, TIS: Indicate presence of fields as shown to the + right of each field above. + + R-P: RTP Padding bit, absolute value (presumed zero if absent). + + R-X: RTP eXtension bit, absolute value. + + M: See the beginning of section 5.7. + + RTP PT: Absolute value of RTP Payload type field. + + Compressed CSRC list: See section 5.8.1. + + TS_STRIDE: Predicted increment/decrement of the RTP Timestamp + field when it changes. Encoded as in section 4.5.6. + + TIME_STRIDE: Predicted time interval in milliseconds between + changes in the RTP Timestamp. Also an indication that the + compressor desires to perform timer-based compression of the RTP + Timestamp field: see section 4.5.4. Encoded as in section 4.5.6. + +5.7.5.1. RND flags and packet types + + The values of the RND and RND2 flags are changed by sending UOR-2 + headers with Extension 3, or IR-DYN headers, where the flag(s) have + their new values. The establishment procedure of the flags is the + normal one for the current mode, i.e., in U-mode and O-mode the + values are repeated several times to ensure that the decompressor + + + +Bormann, et al. Standards Track [Page 88] + +RFC 3095 Robust Header Compression July 2001 + + + receives at least one. In R-mode, the flags are sent until an + acknowledgment for a packet with the new RND flag values is received. + + The decompressor updates the values of its RND and RND2 flags + whenever it receives an UOR-2 with Extension 3 carrying values for + RND or RND2, and the UOR-2 CRC verifies successful decompression. + + When an IPv4 header for which the corresponding RND flag has not been + established to be 1 is present in the static context, the packet + types R-1 and UO-1 MUST NOT be used. + + When no IPv4 header is present in the static context, or the RND + flags for all IPv4 headers in the context have been established to be + 1, the packet types R-1-ID, R-1-TS, UO-1-ID, and UO-1-TS MUST NOT be + used. + + While in the transient state in which an RND flag is being + established, the packet types R-1-ID, R-1-TS, UO-1-ID, and UO-1-TS + MUST NOT be used. This implies that the RND flag(s) of Extension 3 + may have to be inspected before the exact format of a base header + carrying an Extension 3 can be determined, i.e., whether a T-bit is + present or not. + +5.7.5.2. Flags/Fields in context + + Some flags and fields in Extension 3 need to be maintained in the + context of the decompressor. Their values are established using the + mechanism appropriate to the compression mode, unless otherwise + indicated in the table below and in referred sections. + + Flag/Field Initial value Comment + --------------------------------------------------------------------- + Mode Unidirectional See section 5.6 + + NBO 1 See section 4.5.5 + RND 0 See sections 4.5.5, 5.7.5.1 + + NBO2 1 As NBO, but for outer header + RND2 0 As RND, but for outer header + + TS_STRIDE 1 See section 4.5.3 + TIME_STRIDE 0 See section 4.5.4 + Tsc 1 Tsc is always 1 in context; + can be 0 only when an Extension 3 + is present. See the discussion of the + TS field in the beginning of section + 5.7. + + + + +Bormann, et al. Standards Track [Page 89] + +RFC 3095 Robust Header Compression July 2001 + + +5.7.6. Feedback packets and formats + + When the round-trip time between compressor and decompressor is + large, several packets can be in flight concurrently. Therefore, + several packets may be received by the decompressor after feedback + has been sent and before the compressor has reacted to feedback. + Moreover, decompression may fail due to residual errors in the + compressed header. + + Therefore, + + a) in O-mode, the decompressor SHOULD limit the rate at which + feedback on successful decompression is sent (if it is sent at + all); + b) when decompression fails, feedback SHOULD be sent only when + decompression of several consecutive packets has failed, and when + this occurs, the feedback rate SHOULD be limited; + c) when packets are received which belong to a rejected packet + stream, the feedback rate SHOULD be limited. + + A decompressor MAY limit the feedback rate by sending feedback only + for one out of every k packets provoking the same (kind of) feedback. + The appropriate value of k is implementation dependent; k might be + chosen such that feedback is sent 1-3 times per link round-trip time. + + See section 5.2.2 for a discussion concerning ways to provide + feedback information to the compressor. + +5.7.6.1. Feedback formats for ROHC RTP + + This section describes the format for feedback information in ROHC + RTP. See also 5.2.2. + + Several feedback formats carry a field labeled SN. The SN field + contains LSBs of an RTP Sequence Number. The sequence number to use + is the sequence number of the header which caused the feedback + information to be sent. If that sequence number cannot be + determined, for example when decompression fails, the sequence number + to use is that of the last successfully decompressed header. If no + sequence number is available, the feedback MUST carry a SN-NOT-VALID + option. Upon reception, the compressor matches valid SN LSBs with + the most recent header sent with a SN with matching LSBs. The + decompressor must ensure that it sends enough SN LSBs in its feedback + that this correlation does not become ambiguous; e.g., if an 8-bit SN + LSB field could wrap around within a round-trip time, the FEEDBACK-1 + format cannot be used. + + + + + +Bormann, et al. Standards Track [Page 90] + +RFC 3095 Robust Header Compression July 2001 + + + FEEDBACK-1 + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | SN | + +---+---+---+---+---+---+---+---+ + + A FEEDBACK-1 is an ACK. In order to send a NACK or a STATIC-NACK, + FEEDBACK-2 must be used. FEEDBACK-1 does not contain any mode + information; FEEDBACK-2 must be used when mode information is + required. + + FEEDBACK-2 + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + |Acktype| Mode | SN | + +---+---+---+---+---+---+---+---+ + | SN | + +---+---+---+---+---+---+---+---+ + / Feedback options / + +---+---+---+---+---+---+---+---+ + + Acktype: 0 = ACK + 1 = NACK + 2 = STATIC-NACK + 3 is reserved (MUST NOT be used for parseability) + + Mode: 0 is reserved + 1 = Unidirectional mode + 2 = Bidirectional Optimistic mode + 3 = Bidirectional Reliable mode + + + Feedback options: A variable number of feedback options, see + section 5.7.6.2. Options may appear in any order. + +5.7.6.2. ROHC RTP Feedback options + + A ROHC RTP Feedback option has variable length and the following + general format: + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | Opt Type | Opt Len | + +---+---+---+---+---+---+---+---+ + / option data / Opt Len octets + +---+---+---+---+---+---+---+---+ + + + +Bormann, et al. Standards Track [Page 91] + +RFC 3095 Robust Header Compression July 2001 + + + Sections 5.7.6.3-9 describe the currently defined ROHC RTP feedback + options. + +5.7.6.3. The CRC option + + The CRC option contains an 8-bit CRC computed over the entire + feedback payload, without the packet type and code octet, but + including any CID fields, using the polynomial of section 5.9.1. If + the CID is given with an Add-CID octet, the Add-CID octet immediately + precedes the FEEDBACK-1 or FEEDBACK-2 format. For purposes of + computing the CRC, the CRC fields of all CRC options are zero. + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | Opt Type = 1 | Opt Len = 1 | + +---+---+---+---+---+---+---+---+ + | CRC | + +---+---+---+---+---+---+---+---+ + + When receiving feedback information with a CRC option, the compressor + MUST verify the information by computing the CRC and comparing the + result with the CRC carried in the CRC option. If the two are not + identical, the feedback information MUST be ignored. + +5.7.6.4. The REJECT option + + The REJECT option informs the compressor that the decompressor does + not have sufficient resources to handle the flow. + + +---+---+---+---+---+---+---+---+ + | Opt Type = 2 | Opt Len = 0 | + +---+---+---+---+---+---+---+---+ + + When receiving a REJECT option, the compressor stops compressing the + packet stream, and should refrain from attempting to increase the + number of compressed packet streams for some time. Any FEEDBACK + packet carrying a REJECT option MUST also carry a CRC option. + +5.7.6.5. The SN-NOT-VALID option + + The SN-NOT-VALID option indicates that the SN of the feedback is not + valid. A compressor MUST NOT use the SN of the feedback to find the + corresponding sent header when this option is present. + + +---+---+---+---+---+---+---+---+ + | Opt Type = 3 | Opt Len = 0 | + +---+---+---+---+---+---+---+---+ + + + + +Bormann, et al. Standards Track [Page 92] + +RFC 3095 Robust Header Compression July 2001 + + +5.7.6.6. The SN option + + The SN option provides 8 additional bits of SN. + + +---+---+---+---+---+---+---+---+ + | Opt Type = 4 | Opt Len = 1 | + +---+---+---+---+---+---+---+---+ + | SN | + +---+---+---+---+---+---+---+---+ + +5.7.6.7. The CLOCK option + + The CLOCK option informs the compressor of the clock resolution of + the decompressor. This is needed to allow the compressor to estimate + the jitter introduced by the clock of the decompressor when doing + timer-based compression of the RTP Timestamp. + + +---+---+---+---+---+---+---+---+ + | Opt Type = 5 | Opt Len = 1 | + +---+---+---+---+---+---+---+---+ + | clock resolution (ms) | + +---+---+---+---+---+---+---+---+ + + The smallest clock resolution which can be indicated is 1 + millisecond. The value zero has a special meaning: it indicates that + the decompressor cannot do timer-based compression of the RTP + Timestamp. Any FEEDBACK packet carrying a CLOCK option SHOULD also + carry a CRC option. + +5.7.6.8. The JITTER option + + The JITTER option allows the decompressor to report the maximum + jitter it has observed lately, using the following formula which is + very similar to the formula for Max_Jitter_BC in section 4.5.4. + + Let observation window i contain the decompressor's best + approximation of the sliding window of the compressor (see section + 4.5.4) when header i is received. + + Max_Jitter_i = + + max {|(T_i - T_j) - ((a_i - a_j) / TIME_STRIDE)|, + for all headers j in observation window i} + + Max_Jitter = + + max { Max_Jitter_i, for a large number of recent headers i } + + + + +Bormann, et al. Standards Track [Page 93] + +RFC 3095 Robust Header Compression July 2001 + + + This information may be used by the compressor to refine the formula + for determining k when doing timer-based compression of the RTP + Timestamp. + + +---+---+---+---+---+---+---+---+ + | Opt Type = 6 | Opt Len = 1 | + +---+---+---+---+---+---+---+---+ + | Max_Jitter | + +---+---+---+---+---+---+---+---+ + + The decompressor MAY ignore the oldest observed values of + Max_Jitter_i. Thus, the reported Max_Jitter may decrease. + Robustness will be reduced if the compressor uses a jitter estimate + which is too small. Therefore, a FEEDBACK packet carrying a JITTER + option SHOULD also carry a CRC option. Moreover, the compressor MAY + ignore decreasing Max_Jitter values. + +5.7.6.9. The LOSS option + + The LOSS option allows the decompressor to report the largest + observed number of packets lost in sequence. This information MAY be + used by the compressor to adjust the size of the reference window + used in U- and O-mode. + + +---+---+---+---+---+---+---+---+ + | Opt Type = 7 | Opt Len = 1 | + +---+---+---+---+---+---+---+---+ + | longest loss event (packets) | + +---+---+---+---+---+---+---+---+ + + The decompressor MAY choose to ignore the oldest loss events. Thus, + the value reported may decrease. Since setting the reference window + too small can reduce robustness, a FEEDBACK packet carrying a LOSS + option SHOULD also carry a CRC option. The compressor MAY choose to + ignore decreasing loss values. + +5.7.6.10. Unknown option types + + If an option type unknown to the compressor is encountered, it must + continue parsing the rest of the FEEDBACK packet, which is possible + since the length of the option is explicit, but MUST otherwise ignore + the unknown option. + +5.7.6.11. RTP feedback example + + Feedback for CID 8 indicating an ACK for SN 17 and Bidirectional + Reliable mode can have the following formats. + + + + +Bormann, et al. Standards Track [Page 94] + +RFC 3095 Robust Header Compression July 2001 + + + Assuming small CIDs: + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 1 1 1 0 | 0 1 1 | feedback packet type, Code = 3 + +---+---+---+---+---+---+---+---+ + | 1 1 1 0 | 1 0 0 0 | Add-CID octet with CID = 8 + +---+---+---+---+---+---+---+---+ + | 0 0 | 1 1 | SN MSB = 0 | AckType = ACK, Mode = Reliable + +---+---+---+---+---+---+---+---+ + | SN LSB = 17 | + +---+---+---+---+---+---+---+---+ + + The second, third, and fourth octet are handed to the compressor. + + The FEEDBACK-1 format may also be used. Assuming large CIDs: + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 1 1 1 0 | 0 1 0 | feedback packet type, Code = 2 + +---+---+---+---+---+---+---+---+ + | 0 0 0 0 1 0 0 0 | large CID with value 8 + +---+---+---+---+---+---+---+---+ + | SN LSB = 17 | + +---+---+---+---+---+---+---+---+ + + The second and third octet are handed to the compressor. + + Assuming small CIDs: + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 1 1 1 0 | 0 1 0 | feedback packet type, Code = 2 + +---+---+---+---+---+---+---+---+ + | 1 1 1 0 | 1 0 0 0 | Add-CID octet with CID = 8 + +---+---+---+---+---+---+---+---+ + | SN LSB = 17 | + +---+---+---+---+---+---+---+---+ + + The second and third octet are handed to the compressor. + + + + + + + + + + + +Bormann, et al. Standards Track [Page 95] + +RFC 3095 Robust Header Compression July 2001 + + + Assuming small CIDs and CID 0 instead of CID 8: + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 1 1 1 0 | 0 0 1 | feedback packet type, Code = 1 + +---+---+---+---+---+---+---+---+ + | SN LSB = 17 | + +---+---+---+---+---+---+---+---+ + + The second octet is handed to the compressor. + +5.7.7. RTP IR and IR-DYN packets + + The subheaders which are compressible are split into a STATIC part + and a DYNAMIC part. These parts are defined in sections 5.7.7.3 + through 5.7.7.7. + + The structure of a chain of subheaders is determined by each header + having a Next Header, or Protocol, field. This field identifies the + type of the following header. Each Static part below that is + followed by another Static part contains the Next Header/Protocol + field and allows parsing of the Static chain; the Dynamic chain, if + present, is structured analogously. + + IR and IR-DYN packets will cause a packet to be delivered to upper + layers if and only if the payload is non-empty. This means that an + IP/UDP/RTP packet where the UDP length indicates a UDP payload of + size 12 octets cannot be represented by an IR or IR-DYN packet. Such + packets can instead be represented using the UNCOMPRESSED profile + (section 5.10). + +5.7.7.1. Basic structure of the IR packet + + This packet type communicates the static part of the context, i.e., + the values of the constant SN functions. It can optionally also + communicate the dynamic part of the context, i.e., the parameters of + nonconstant SN functions. It can also optionally communicate the + payload of an original packet, if any. + + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 96] + +RFC 3095 Robust Header Compression July 2001 + + + 0 1 2 3 4 5 6 7 + --- --- --- --- --- --- --- --- + | Add-CID octet | if for small CIDs and CID != 0 + +---+---+---+---+---+---+---+---+ + | 1 1 1 1 1 1 0 | D | + +---+---+---+---+---+---+---+---+ + | | + / 0-2 octets of CID info / 1-2 octets if for large CIDs + | | + +---+---+---+---+---+---+---+---+ + | Profile | 1 octet + +---+---+---+---+---+---+---+---+ + | CRC | 1 octet + +---+---+---+---+---+---+---+---+ + | | + | Static chain | variable length + | | + +---+---+---+---+---+---+---+---+ + | | + | Dynamic chain | present if D = 1, variable length + | | + - - - - - - - - - - - - - - - - + | | + | Payload | variable length + | | + - - - - - - - - - - - - - - - - + + D: D = 1 indicates that the dynamic chain is present. + + Profile: Profile identifier, abbreviated as defined in section + 5.2.3. + + CRC: 8-bit CRC, computed according to section 5.9.1. + + Static chain: A chain of static subheader information. + + Dynamic chain: A chain of dynamic subheader information. What + dynamic information is present is inferred from the Static + chain. + + Payload: The payload of the corresponding original packet, if any. + The presence of a payload is inferred from the packet length. + + + + + + + + + +Bormann, et al. Standards Track [Page 97] + +RFC 3095 Robust Header Compression July 2001 + + +5.7.7.2. Basic structure of the IR-DYN packet + + This packet type communicates the dynamic part of the context, i.e., + the parameters of nonconstant SN functions. + + 0 1 2 3 4 5 6 7 + --- --- --- --- --- --- --- --- + : Add-CID octet : if for small CIDs and CID != 0 + +---+---+---+---+---+---+---+---+ + | 1 1 1 1 1 0 0 0 | IR-DYN packet type + +---+---+---+---+---+---+---+---+ + : : + / 0-2 octets of CID info / 1-2 octets if for large CIDs + : : + +---+---+---+---+---+---+---+---+ + | Profile | 1 octet + +---+---+---+---+---+---+---+---+ + | CRC | 1 octet + +---+---+---+---+---+---+---+---+ + | | + / Dynamic chain / variable length + | | + +---+---+---+---+---+---+---+---+ + : : + / Payload / variable length + : : + - - - - - - - - - - - - - - - - + + Profile: Profile identifier, abbreviated as defined in section 5.2.3. + + CRC: 8-bit CRC, computed according to section 5.9.1. + + NOTE: As the CRC checks only the integrity of the header + itself, an acknowledgment of this header does not signify that + previous changes to the static chain in the context are also + acknowledged. In particular, care should be taken when IR + packets that update an existing context are followed by IR-DYN + packets. + + Dynamic chain: A chain of dynamic subheader information. What + dynamic information is present is inferred from the Static chain of + the context. + + Payload: The payload of the corresponding original packet, if any. + The presence of a payload is inferred from the packet length. + + + + + + +Bormann, et al. Standards Track [Page 98] + +RFC 3095 Robust Header Compression July 2001 + + + Note: The static and dynamic chains of IR or IR-DYN packets for + profile 0x0001 (ROHC RTP) MUST end with the static and dynamic parts + of an RTP header. If not, the packet MUST be discarded and the + context MUST NOT be updated. + + Note: The static or dynamic chains of IR or IR-DYN packets for + profile 0x0002 (ROHC UDP) MUST end with the static and dynamic parts + of a UDP header. If not, the packet MUST be discarded and the + context MUST NOT be updated. + + Note: The static or dynamic chains of IR or IR-DYN packets for + profile 0x0003 (ROHC ESP) MUST end with the static and dynamic parts + of an ESP header. If not, the packet MUST be discarded and the + context MUST NOT be updated. + +5.7.7.3. Initialization of IPv6 Header [IPv6] + + Static part: + + +---+---+---+---+---+---+---+---+ + | Version = 6 |Flow Label(msb)| 1 octet + +---+---+---+---+---+---+---+---+ + / Flow Label (lsb) / 2 octets + +---+---+---+---+---+---+---+---+ + | Next Header | 1 octet + +---+---+---+---+---+---+---+---+ + / Source Address / 16 octets + +---+---+---+---+---+---+---+---+ + / Destination Address / 16 octets + +---+---+---+---+---+---+---+---+ + + Dynamic part: + + +---+---+---+---+---+---+---+---+ + | Traffic Class | 1 octet + +---+---+---+---+---+---+---+---+ + | Hop Limit | 1 octet + +---+---+---+---+---+---+---+---+ + / Generic extension header list / variable length + +---+---+---+---+---+---+---+---+ + + Eliminated: + + Payload Length + + + + + + + +Bormann, et al. Standards Track [Page 99] + +RFC 3095 Robust Header Compression July 2001 + + + Extras: + + Generic extension header list: Encoded according to section + 5.8.6.1, with all header items present in uncompressed form. + + CRC-DYNAMIC: Payload Length field (octets 5-6). + + CRC-STATIC: All other fields (octets 1-4, 7-40). + + CRC coverage for extension headers is defined in section 5.8.7. + + Note: The Next Header field indicates the type of the following + header in the static chain, rather than being a copy of the Next + Header field of the original IPv6 header. See also section 5.7.7.8. + +5.7.7.4. Initialization of IPv4 Header [IPv4, section 3.1]. + + Static part: + + Version, Protocol, Source Address, Destination Address. + + +---+---+---+---+---+---+---+---+ + | Version = 4 | 0 | + +---+---+---+---+---+---+---+---+ + | Protocol | + +---+---+---+---+---+---+---+---+ + / Source Address / 4 octets + +---+---+---+---+---+---+---+---+ + / Destination Address / 4 octets + +---+---+---+---+---+---+---+---+ + + Dynamic part: + + Type of Service, Time to Live, Identification, DF, RND, NBO, + extension header list. + + +---+---+---+---+---+---+---+---+ + | Type of Service | + +---+---+---+---+---+---+---+---+ + | Time to Live | + +---+---+---+---+---+---+---+---+ + / Identification / 2 octets + +---+---+---+---+---+---+---+---+ + | DF|RND|NBO| 0 | + +---+---+---+---+---+---+---+---+ + / Generic extension header list / variable length + +---+---+---+---+---+---+---+---+ + + + + +Bormann, et al. Standards Track [Page 100] + +RFC 3095 Robust Header Compression July 2001 + + + Eliminated: + + IHL (IP Header Length, must be 5) + Total Length (inferred in decompressed packets) + MF flag (More Fragments flag, must be 0) + Fragment Offset (must be 0) + Header Checksum (inferred in decompressed packets) + Options, Padding (must not be present) + + Extras: + + RND, NBO See section 5.7. + + Generic extension header list: Encoded according to section + 5.8.6.1, with all header items present in uncompressed form. + + CRC-DYNAMIC: Total Length, Identification, Header Checksum + (octets 3-4, 5-6, 11-12). + + CRC-STATIC: All other fields (octets 1-2, 7-10, 13-20) + + CRC coverage for extension headers is defined in section 5.8.7. + + Note: The Protocol field indicates the type of the following header + in the static chain, rather than being a copy of the Protocol field + of the original IPv4 header. See also section 5.7.7.8. + +5.7.7.5. Initialization of UDP Header [RFC-768]. + + Static part: + + +---+---+---+---+---+---+---+---+ + / Source Port / 2 octets + +---+---+---+---+---+---+---+---+ + / Destination Port / 2 octets + +---+---+---+---+---+---+---+---+ + + Dynamic part: + + +---+---+---+---+---+---+---+---+ + / Checksum / 2 octets + +---+---+---+---+---+---+---+---+ + + + + + + + + + +Bormann, et al. Standards Track [Page 101] + +RFC 3095 Robust Header Compression July 2001 + + + Eliminated: + + Length + + The Length field of the UDP header MUST match the Length field(s) + of the preceding subheaders, i.e., there must not be any padding + after the UDP payload that is covered by the IP Length. + + CRC-DYNAMIC: Length field, Checksum (octets 5-8). + + CRC-STATIC: All other fields (octets 1-4). + +5.7.7.6. Initialization of RTP Header [RTP]. + + Static part: + + SSRC. + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + / SSRC / 4 octets + +---+---+---+---+---+---+---+---+ + + Dynamic part: + + P, X, CC, PT, M, sequence number, timestamp, timestamp stride, + CSRC identifiers. + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | V=2 | P | RX| CC | (RX is NOT the RTP X bit) + +---+---+---+---+---+---+---+---+ + | M | PT | + +---+---+---+---+---+---+---+---+ + / RTP Sequence Number / 2 octets + +---+---+---+---+---+---+---+---+ + / RTP Timestamp (absolute) / 4 octets + +---+---+---+---+---+---+---+---+ + / Generic CSRC list / variable length + +---+---+---+---+---+---+---+---+ + : Reserved | X | Mode |TIS|TSS: if RX = 1 + +---+---+---+---+---+---+---+---+ + : TS_Stride : 1-4 octets, if TSS = 1 + +---+---+---+---+---+---+---+---+ + : Time_Stride : 1-4 octets, if TIS = 1 + +---+---+---+---+---+---+---+---+ + + + + + +Bormann, et al. Standards Track [Page 102] + +RFC 3095 Robust Header Compression July 2001 + + + Eliminated: + + Nothing. + + Extras: + + RX: Controls presence of extension. + + Mode: Compression mode. 0 = Reserved, + 1 = Unidirectional, + 2 = Bidirectional Optimistic, + 3 = Bidirectional Reliable. + + X: Copy of X bit from RTP header (presumed 0 if RX = 0) + + Reserved: Set to zero when sending, ignored when received. + + Generic CSRC list: CSRC list encoded according to section + 5.8.6.1, with all CSRC items present. + + CRC-DYNAMIC: Octets containing M-bit, sequence number field, + and timestamp (octets 2-8). + + CRC-STATIC: All other fields (octets 1, 9-12, original CSRC list). + +5.7.7.7. Initialization of ESP Header [ESP, section 2] + + This is for the case when the NULL encryption algorithm [NULL] is NOT + being used with ESP, so that subheaders after the ESP header are + encrypted (see 5.12). See 5.8.4.3 for compression of the ESP header + when NULL encryption is being used. + + Static part: + + +---+---+---+---+---+---+---+---+ + / SPI / 4 octets + +---+---+---+---+---+---+---+---+ + + Dynamic part: + + +---+---+---+---+---+---+---+---+ + / Sequence Number / 4 octets + +---+---+---+---+---+---+---+---+ + + Eliminated: + + Other fields are encrypted, and can neither be located nor + compressed. + + + +Bormann, et al. Standards Track [Page 103] + +RFC 3095 Robust Header Compression July 2001 + + + CRC-DYNAMIC: Sequence number (octets 5-8) + + CRC-STATIC: All other octets. + + Note: No encrypted data is considered to be part of the header for + purposes of computing the CRC, i.e., octets after the eight octet are + not considered part of the header. + +5.7.7.8. Initialization of Other Headers + + Headers not explicitly listed in previous subsections can be + compressed only by making them part of an extension header chain + following an IPv4 or IPv6 header, see section 5.8. + +5.8. List compression + + Header information from the packet stream to be compressed can be + structured as an ordered list, which is largely constant between + packets. The generic structure of such a list is as follows. + + +--------+--------+--...--+--------+ + list: | item 1 | item 2 | | item n | + +--------+--------+--...--+--------+ + + This section describes the compression scheme for such information. + The basic principles of list-based compression are the following: + + 1) While the list is constant, no information about the list is sent + in compressed headers. + + 2) Small changes in the list are represented as additions (Insertion + scheme), or deletions (Removal scheme), or both (Remove Then + Insert scheme). + + 3) The list can also be sent in its entirety (Generic scheme). + + There are two kinds of lists: CSRC lists in RTP packets, and + extension header chains in IP packets (both IPv4 and IPv6). + + IPv6 base headers and IPv4 headers cannot be part of an extension + header chain. Headers which can be part of extension header chains + include + + a) the AH header + b) the null ESP header + c) the minimal encapsulation header [RFC2004, section 3.1] + d) the GRE header [GRE1, GRE2] + e) IPv6 extension headers. + + + +Bormann, et al. Standards Track [Page 104] + +RFC 3095 Robust Header Compression July 2001 + + + The table-based item compression scheme (5.8.1), which reduces the + size of each item, is described first. Then it is defined which + reference list to use in the insertion and removal schemes (5.8.2). + List encoding schemes are described in section 5.8.3, and a few + special cases in section 5.8.4. Finally, exact formats are described + in sections 5.8.5-5.8.6. + +5.8.1. Table-based item compression + + The Table-based item compression scheme is a way to compress + individual items sent in compressed lists. The compressor assigns + each item in a list a unique identifier Index. The compressor + conceptually maintains a table with all items, indexed by Index. The + (Index, item) pair is sent together in compressed lists until the + compressor gains enough confidence that the decompressor has observed + the mapping between the item and its Index. Such confidence is + obtained by receiving an acknowledgment from the decompressor in R- + mode, and in U/O-mode by sending L (Index, item) pairs (not + necessarily consecutively). After that, the Index alone is sent in + compressed lists to indicate the corresponding item. The compressor + may reassign an existing Index to a new item, and then needs to re- + establish the mapping in the same manner as above. + + The decompressor conceptually maintains a table that contains all + (Index, item) pairs it knows about. The table is updated whenever an + (Index, item) pair is received (and decompression is verified by a + CRC). The decompressor retrieves the item from the table whenever an + Index without an accompanying item is received. + +5.8.1.1. Translation table in R-mode + + At the compressor side, an entry in the Translation Table has the + following structure. + + +-------+------+---------------+ + Index i | Known | item | SN1, SN2, ... | + +-------+------+---------------+ + + The Known flag indicates whether the mapping between Index i and item + has been established, i.e., if Index i alone can be sent in + compressed lists. Known is initially zero. It is also set to zero + whenever Index i is assigned to a new item. Known is set to one when + the corresponding (Index, item) pair is acknowledged. + Acknowledgments are based on the RTP Sequence Number, so a list of + RTP Sequence Numbers of all packets which contain the (Index, item) + pair is included in the translation table. When a packet with a + sequence number in the sequence number list is acknowledged, the + Known flag is set, and the sequence number list can be discarded. + + + +Bormann, et al. Standards Track [Page 105] + +RFC 3095 Robust Header Compression July 2001 + + + Each entry in the Translation Table at the decompressor side has the + following structure: + + +-------+------+ + Index i | Known | item | + +-------+------+ + + All Known fields are initialized to zero. Whenever the decompressor + receives an (Index, item) pair, it inserts item into the table at + position Index and sets the Known flag in that entry to one. If an + index without an accompanying item is received for which the Known + flag is zero, the header MUST be discarded and a NACK SHOULD be sent. + +5.8.1.2. Translation table in U/O-modes + + At the compressor side, each entry in the Translation Table has the + following structure: + + +-------+------+---------+ + Index | Known | item | Counter | + +-------+------+---------+ + + The Index, Known, and item fields have the same meaning as in section + 5.8.1.1. + + Known is set when the (Index, item) pair has been sent in L + compressed lists (not necessarily consecutively). The Counter field + keeps track of how many times the pair has been sent. Counter is set + to 0 for each new entry added to the table, and whenever Index is + assigned to a new item. Counter is incremented by 1 whenever an + (Index, item) pair is sent. When the counter reaches L, the Known + field is set and after that only the Index needs to be sent in + compressed lists. + + At the decompressor side, the Translation Table is the same as the + Translation Table defined in R-mode. + +5.8.2. Reference list determination + + In reference based compression schemes (i.e., addition or deletion + based schemes), compression and decompression of a list (curr_list) + are based on a reference list (ref_list) which is assumed to be + present in the context of both compressor and decompressor. The + compressed list is an encoding of the differences between curr_list + and ref_list. Upon reception of a compressed list, the decompressor + applies the differences to its reference list in order to obtain the + original list. + + + + +Bormann, et al. Standards Track [Page 106] + +RFC 3095 Robust Header Compression July 2001 + + + To identify the reference list (to be) used, each compressed list + carries an identifier (ref_id). The reference list is established by + different methods in R-mode and U/O-mode. + +5.8.2.1. Reference list in R-mode and U/O-mode + + In R-mode, the choice of reference list is based on acknowledgments, + i.e., the compressor uses as ref_list the latest list which has been + acknowledged by the decompressor. The ref_list is updated only upon + receiving an acknowledgment. The least significant bits of the RTP + Sequence Number of the acknowledged packet are used as the ref_id. + + In U/O-mode, a sequence of identical lists are considered as + belonging to the same generation and are all assigned the same + generation identifier (gen_id). Gen_id increases by 1 each time the + list changes and is carried in compressed and uncompressed lists that + are candidates for being used as reference lists. Normally, Gen_id + must have been repeated in at least L headers before the list can be + used as a ref_list. However, some acknowledgments may be sent in O- + mode (and also in U-mode), and whenever an acknowledgment for a + header is received, the list of that header is considered known and + need not be repeated further. The least significant bits of the + Gen_id is used as the ref_id in U/O-mode. + + The logic of the compressor and decompressor for reference based list + compression is similar to that for SN and TS. The principal + difference is that the decompressor maintains a sliding window with + candidates for ref_list, and retrieves ref_list from the sliding + window using the ref_id of the compressed list. + + Logic of compressor: + + a) In the IR state, the compressor sends Generic lists (see 5.8.5) + containing all items of the current list in order to establish or + refresh the context of the decompressor. + + In R-mode, such Generic lists are sent until a header is + acknowledged. The list of that header can be used as a reference + list to compress subsequent lists. + + In U/O-mode, the compressor sends generation identifiers with the + Generic lists until + + 1) a generation identifier has been repeated L times, or + + 2) an acknowledgment for a header carrying a generation identifier + has been received. + + + + +Bormann, et al. Standards Track [Page 107] + +RFC 3095 Robust Header Compression July 2001 + + + The repeated (1) or acknowledged (2) list can be used as a + reference list to compress subsequent lists and is kept together + with its generation identifier. + + b) When not in the IR state, the compressor moves to the FO state + when it observes a difference between curr_list and the previous + list. It sends compressed lists based on ref_list to update the + context of the decompressor. (However, see d).) + + In R-mode, the compressor keeps sending compressed lists using the + same reference until it receives an acknowledgment for a packet + containing the newest list. The compressor may then move to the + SO state with regard to the list. + + In U/O-mode, the compressor keeps sending compressed lists with + generation identifiers until + + 1) a generation identifier has been repeated L times, or + + 2) an acknowledgment for a header carrying the latest generation + identifier has been received. + + The repeated or acknowledged list is used as the future reference + list. The compressor may move to the SO state with regard to the + list. + + c) In R-mode, the compressor maintains a sliding window containing + the lists which have been sent to update the context of the + decompressor and have not yet been acknowledged. The sliding + window shrinks when an acknowledgment arrives: all lists sent + before the acknowledged list are removed. The compressor may use + the Index to represent items of lists in the sliding window. + + In U/O-mode, the compressor needs to store + + 1) the reference list and its generation identifier, and + + 2) if the current generation identifier is different from the + reference generation, the current list and the sequence + numbers with which the current list has been sent. + + (2) is needed to determine if an acknowledgment concerns the + latest generation. It is not needed in U-mode. + + d) In U/O-mode, the compressor may choose to not send a generation + identifier with a compressed list. Such lists without generation + identifiers are not assigned a new generation identifier and must + + + + +Bormann, et al. Standards Track [Page 108] + +RFC 3095 Robust Header Compression July 2001 + + + not be used as future reference lists. They do not update the + context. This feature is useful when a new list is repeated few + times and the list then reverts back to its old value. + + Logic of decompressor: + + e) In R-mode, the decompressor acknowledges all received uncompressed + or compressed lists which establish or update the context. (Such + compressed headers contain a CRC.) + + In O-mode, the decompressor MAY acknowledge a list with a new + generation identifier, see section 5.4.2.2. + + In U-mode, the decompressor MAY acknowledge a list sent in an IR + packet, see section 5.3.2.3. + + f) The decompressor maintains a sliding window which contains the + lists that may be used as reference lists. + + In R-mode, the sliding window contains lists which have been + acknowledged but not yet used as reference lists. + + In U/O-mode, the sliding window contains at most one list per + generation. It contains all generations seen by the decompressor + newer than the last generation used as a reference. + + g) When the decompressor receives a compressed list, it retrieves the + proper ref_list from the sliding window based on the ref_id, and + decompresses the compressed list obtaining curr_list. + + In R-mode, curr_list is inserted into the sliding window if an + acknowledgment is sent for it. The sliding window is shrunk by + removing all lists received before ref_list. + + In U/O-mode, curr_list is inserted into the sliding window + together with its generation identifier if the compressed list had + a generation identifier and the sliding window does not contain a + list with that generation identifier. All lists with generations + older than ref_id are removed from the sliding window. + +5.8.3. Encoding schemes for the compressed list + + Four encoding schemes for the compressed list are described here. + The exact formats of the compressed CSRC list and compressed IP + extension header list using these encoding schemes are described in + sections 5.8.5-5.8.6. + + + + + +Bormann, et al. Standards Track [Page 109] + +RFC 3095 Robust Header Compression July 2001 + + + Generic scheme + + In contrast to subsequent schemes, this scheme does not rely on a + reference list having been established. The entire list is sent, + using table based compression for each individual item. The + generic scheme is always used when establishing the context of the + decompressor and may also be used at other times, as the + compressor sees fit. + + Insertion Only scheme + + When the new list can be constructed from ref_list by adding + items, a list of the added items is sent (using table based + compression), along with the positions in ref_list where the new + items will be inserted. An insertion bit mask indicates the + insertion positions in ref_list. + + Upon reception of a list compressed according to the Insertion + Only scheme, curr_list is obtained by scanning the insertion bit + mask from left to right. When a '0' is observed, an item is + copied from the ref_list. When a '1' is observed, an item is + copied from the list of added items. If a '1' is observed when + the list of added items has been exhausted, an error has occurred + and decompression fails: The header MUST NOT be delivered to upper + layers; it should be discarded, and MUST NOT be acknowledged nor + used as a reference. + + To construct the insertion bit mask and the list of added items, + the compressor MAY use the following algorithm: + + 1) An empty bit list and an empty Inserted Item list are generated + as the starting point. + + 2) Start by considering the first item of curr_list and ref_list. + + 3) If curr_list has a different item than ref_list, + + a set bit (1) is appended to the bit list; + the first item in curr_list (represented using table-based + item compression) is appended to the Inserted Item list; + advance to the next item of curr_list; + + otherwise, + + a zero bit (0) is appended to the bit list; + + advance to the next item of curr_list; + advance to the next item of ref_list. + + + +Bormann, et al. Standards Track [Page 110] + +RFC 3095 Robust Header Compression July 2001 + + + 4) Repeat 3) until curr_list has been exhausted. + + 5) If the length of the bit list is less than the required bit + mask length, append additional zeroes. + + Removal Only scheme + + This scheme can be used when curr_list can be obtained by removing + some items in ref_list. The positions of the items which are in + ref_list, but not in curr_list, are sent as a removal bit mask. + + Upon reception of the compressed list, the decompressor obtains + curr_list by scanning the removal bit mask from left to right. + When a '0' is observed, the next item of ref_list is copied into + curr_list. When a '1' is observed, the next item of ref_list is + skipped over without being copied. If a '0' is observed when + ref_list has been exhausted, an error has occurred and + decompression fails: The header MUST NOT be delivered to upper + layers; it should be discarded, and MUST NOT be acknowledged nor + used as a reference. + + To construct the removal bit mask and the list of added items, the + compressor MAY use the following algorithm: + + 1) An empty bit list is generated as the starting point. + + 2) Start by considering the first item of curr_list and ref_list. + + 3) If curr_list has a different item than ref_list, + + a set bit (1) is appended to the bit list; + advance to the next item of ref_list; + + otherwise, + + a zero bit (0) is appended to the bit list; + advance to the next item of curr_list; + advance to the next item of ref_list. + + 4) Repeat 3) until curr_list has been exhausted. + + 5) If the length of the bit list is less than the required bit + mask length, append additional ones. + + + + + + + + +Bormann, et al. Standards Track [Page 111] + +RFC 3095 Robust Header Compression July 2001 + + + Remove Then Insert scheme + + In this scheme, curr_list is obtained by first removing items from + ref_list, and then inserting items into the resulting list. A + removal bit mask, an insertion bit mask, and a list of added items + are sent. + + Upon reception of the compressed list, the decompressor processes + the removal bit mask as in the Removal Only scheme. The resulting + list is then used as the reference list when the insertion bit + mask and the list of added items are processed, as in the + Insertion Only scheme. + +5.8.4. Special handling of IP extension headers + + In CSRC list compression, each CSRC is assigned an index. In + contrast, in IP extension header list compression an index is usually + associated with a type of extension header. When there is more than + one IP header, there is more than one list of extension headers. An + index per type per list is then used. + + The association with a type means that a new index need not always be + used each time a field in an IP extension header changes. However, + when a field in an extension header changes, the mapping between the + index and the new value of the extension header needs to be + established, except in the special handling cases defined in the + following subsections. + +5.8.4.1. Next Header field + + The next header field in an IP header or extension header changes + whenever the type of the immediately following header changes, e.g., + when a new extension header is inserted after it, when the immediate + subsequent extension header is removed from the list, or when the + order of extension headers is changed. Thus it may not be uncommon + that, for a given header, the next header field changes while the + remaining fields do not change. + + Therefore, in the case that only the next header field changes, the + extension header is considered to be unchanged and rules for special + treatment of the change in the next header field are defined below. + + All communicated uncompressed extension header items indicate their + own type in their Next Header field. Note that the rules below + explain how to treat the Next Header fields while showing the + conceptual reference list as an exact recreation of the original + uncompressed extension header list. + + + + +Bormann, et al. Standards Track [Page 112] + +RFC 3095 Robust Header Compression July 2001 + + + a) When a subsequent extension header is removed from the list, the + new value of the next header field is obtained from the reference + extension header list. For example, assume that the reference + header list (ref_list) consists of headers A, B and C (ref_ext_hdr + A, B, C), and the current extension header list (curr_list) only + consists of extension headers A and C (curr_ext_hdr A, C). The + order and value of the next header fields of these extension + headers are as follows. + + ref_list: + +--------+-----+ +--------+-----+ +--------+-----+ + | type B | | | type C | | | type D | | + +--------+ | +--------+ | +--------+ | + | | | | | | + +--------------+ +--------------+ +--------------+ + ref_ext_hdr A ref_ext_hdr B ref_ext_hdr C + + curr_list: + +--------+-----+ +--------+-----+ + | type C | | | type D | | + +--------+ | +--------+ | + | | | | + +--------------+ +--------------+ + curr_ext_hdr A curr_ext_hdr C + + Comparing the curr_ext_hdr A in curr_list and the ref_ext_hdr A in + ref_list, the value of next header field is changed from "type B" + to "type C" because of the removal of extension header B. The new + value of the next header field in curr_ext_hdr A, i.e., "type C", + does not need to be sent to the decompressor. Instead, it is + retrieved from the next header field of the removed ref_ext_hdr B. + + b) When a new extension header is inserted after an existing + extension header, the next header field in the communicated item + will carry the type of itself, rather than the type of the header + that follows. For example, assume that the reference header list + (ref_list) consists of headers A and C (ref_ext_hdr A, C), and the + current header list (curr_list) consists of headers A, B and C + (curr_ext_hdr A, B, C). The order and the value of the next + header fields of these extension headers are as follows. + + + + + + + + + + + +Bormann, et al. Standards Track [Page 113] + +RFC 3095 Robust Header Compression July 2001 + + + ref_list: + +--------+-----+ +--------+-----+ + | type C | | | type D | | + +--------+ | +--------+ | + | | | | + +--------------+ +--------------+ + ref_ext_hdr A ref_ext_hdr C + + curr_list: + +--------+-----+ +--------+-----+ +--------+-----+ + | type B | | | type C | | | type D | | + +--------+ | +--------+ | +--------+ | + | | | | | | + +--------------+ +--------------+ +--------------+ + curr_ext_hdr A curr_ext_hdr B curr_ext_hdr C + + Comparing the curr_list and the ref_list, the value of the next + header field in extension header A is changed from "type C" to + "type B". + + The uncompressed curr_ext_hdr B is carried in the compressed + header list. However, it carries "type B" instead of "type C" in + its next header field. When the decompressor inserts a new header + after curr_ext_hdr A, the next header field of A is taken from the + new header, and the next header field of the new header is taken + from ref_ext_hdr A. + + c) Some headers whose compression is defined in this document do not + contain Next Header fields or do not have their Next Header field + in the standard position (first octet of the header). The GRE and + ESP headers are such headers. When sent as uncompressed items in + lists, these headers are modified so that they do have a Next + Header field as their first octet (see 5.8.4.3 and 5.8.4.4). This + is necessary to enable the decompressor to decode the item. + +5.8.4.2. Authentication Header (AH) + + The sequence number field in the AH [AH] contains a monotonically + increasing counter value for a security association. Therefore, when + comparing curr_list with ref_list, if the sequence number in AH + changes and SPI field does not change, the AH is not considered as + changed. + + If the sequence number in the AH linearly increases as the RTP + Sequence Number increases, and the compressor is confident that the + decompressor has obtained the pattern, the sequence number in AH need + not be sent. The decompressor applies linear extrapolation to + reconstruct the sequence number in the AH. + + + +Bormann, et al. Standards Track [Page 114] + +RFC 3095 Robust Header Compression July 2001 + + + Otherwise, a compressed sequence number is included in the IPX + compression field in an Extension 3 of an UOR-2 header. + + The authentication data field in AH changes from packet to packet and + is sent as-is. If the uncompressed AH is sent, the authentication + data field is sent inside the uncompressed AH; otherwise, it is sent + after the compressed IP/UDP/RTP and IPv6 extension headers and before + the payload. See beginning of section 5.7. + + Note: The payload length field of the AH uses a different notion of + length than other IPv6 extension headers. + +5.8.4.3. Encapsulating Security Payload Header (ESP) + + When the Encapsulating Security Payload Header (ESP) [ESP] is present + and an encryption algorithm other than NULL is being used, the UDP + and RTP headers are both encrypted and cannot be compressed. The ESP + header thus ends the compressible header chain. The ROHC ESP profile + defined in section 5.12 MAY be used for the stream in this case. + + A special case is when the NULL encryption algorithm is used. This + is the case when the ESP header is used for authentication only, and + not for encryption. The payload is not encrypted by the NULL + encryption algorithm, so compression of the rest of the header chain + is possible. The rest of this section describes compression of the + ESP header when the NULL encryption algorithm is used with ESP. + + It is not possible to determine whether NULL encryption is used by + inspecting a header in the stream, this information is present only + at the encryption endpoints. However, a compressor may attempt + compression under the assumption that the NULL encryption algorithm + is being used, and later abort compression when the assumption proves + to be false. + + The compressor may, for example, inspect the Next Header fields and + the header fields supposed to be static in subsequent headers in + order to determine if NULL encryption is being used. If these change + unpredictably, an encryption algorithm other than NULL is probably + being used and compression of subsequent headers SHOULD be aborted. + Compression of the stream is then either discontinued, or a profile + that compresses only up to the ESP header may be used (see 5.12). + While attempting to compress the header, the compressor should use + the SPI of the ESP header together with the destination IP address as + the defining fields for determining which packets belong to the + stream. + + + + + + +Bormann, et al. Standards Track [Page 115] + +RFC 3095 Robust Header Compression July 2001 + + + In the ESP header [ESP, section 2], the fields that can be compressed + are the SPI, the sequence number, the Next Header, and the padding + bytes if they are in the standard format defined in [ESP]. (As + always, the decompressor reinserts these fields based on the + information in the context. Care must be taken to correctly reinsert + all the information as the Authentication Data must be verified over + the exact same information it was computed over.) + + ESP header [ESP, section 2]: + + 0 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Security Parameters Index (SPI) | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Sequence Number | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Payload Data (variable) | + ~ ~ + | | + + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | Padding (0-255 octets) | + +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | Pad Length | Next Header | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Authentication Data | + + (variable length, but assumed to be 12 octets) + + | | + + + + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + SPI: Static. If it changes, it needs to be reestablished. + + Sequence Number: Not sent when the offset from the sequence number + of the compressed header is constant. When the offset is not + constant, the sequence number may be compressed by sending + LSBs. See 5.8.4. + + Payload Data: This is where subsequent headers are to be found. + Parsed according to the Next Header field. + + Padding: The padding octets are assumed to be as defined in [ESP], + i.e., to take the values 1, 2, ..., k, where k = Pad Length. + If the padding in the static context has this pattern, padding + in compressed headers is assumed to have this pattern as well + and is removed. If padding in the static context does not + have this pattern, the padding is not removed. + + + +Bormann, et al. Standards Track [Page 116] + +RFC 3095 Robust Header Compression July 2001 + + + Pad Length: Dynamic. Always sent. 14th octet from end of packet. + + Next Header: Static. 13th octet from end of packet. + + Authentication Data: Can have variable length, but when compression + of NULL-encryption ESP header is attempted, it is assumed to have + length 12 octets. + + The sequence number in ESP has the same behavior as the sequence + number field in AH. When it increases linearly, it can be compressed + to zero bits. When it does not increase linearly, a compressed + sequence number is included in the IPX compression field in an + Extension 3 of an UOR-2 header. + + The information which is part of an uncompressed item of a compressed + list is the Next Header field, followed by the SPI and the Sequence + Number. Padding, Pad Length, Next Header, and Authentication Data + are sent as-is at the end of the packet. This means that the Next + Header occurs in two places. + + Uncompressed ESP list item: + + +---+---+---+---+---+---+---+---+ + | Next Header ! 1 octet (see section 5.8.4.1) + +---+---+---+---+---+---+---+---+ + / SPI / 4 octets + +---+---+---+---+---+---+---+---+ + / Sequence Number / 4 octets + +---+---+---+---+---+---+---+---+ + + When sending Uncompressed ESP list items, all ESP fields near the + the end of the packet are left untouched (Padding, Pad Length, + Next Header, Authentication Data). + + A compressed item consists of a compressed sequence number. When an + item is compressed, Padding (if it follows the 1, 2, ..., k pattern) + and Next Header are removed near the end of the packet. + Authentication Data and Pad Length remain as-is near the end of the + packet. + +5.8.4.4. GRE Header [RFC 2784, RFC 2890] + + The GRE header is a set of flags, followed by a mandatory Protocol + Type and optional parts as indicated by the flags. + + + + + + + +Bormann, et al. Standards Track [Page 117] + +RFC 3095 Robust Header Compression July 2001 + + + The sequence number field in the GRE header contains a counter value + for a GRE tunnel. Therefore, when comparing curr_list with ref_list, + if the sequence number in GRE changes, the GRE is not considered as + changed. + + If the sequence number in the GRE header linearly increases as the + RTP Sequence Number increases and the compressor is confident that + the decompressor has received the pattern, the sequence number in GRE + need not be sent. The decompressor applies linear extrapolation to + reconstruct the sequence number in the GRE header. + + Otherwise, a compressed sequence number is included in the IPX + compression field in an Extension 3 of an UOR-2 header. + + The checksum data field in GRE, if present, changes from packet to + packet and is sent as-is. If the uncompressed GRE header is sent, + the checksum data field is sent inside the uncompressed GRE header; + otherwise, if present, it is sent after the compressed IP/UDP/RTP and + IPv6 extension headers and before the payload. See beginning of + section 5.7. + + In order to allow simple parsing of lists of items, an uncompressed + GRE header sent as an item in a list is modified from the original + GRE header in the following manner: 1) the 16-bit Protocol Type field + that encodes the type of the subsequent header using Ether types (see + Ether types section in [ASSIGNED]) is removed. 2) A one-octet Next + Header field is inserted as the first octet of the header. The value + of the Next Header field corresponds to GRE (this value is 47 + according to the Assigned Internet Protocol Number section of + [ASSIGNED]) when the uncompressed item is to be inserted in a list, + and to the type of the subsequent header when the uncompressed item + is in a Generic list. Note that this implies that only GRE headers + with Ether types that correspond to an IP protocol number can be + compressed. + + Uncompressed GRE list item: + + +---+---+---+---+---+---+---+---+ + | Next Header ! 1 octet (see section 5.8.4.1) + +---+---+---+---+---+---+---+---+ + / C | | K | S | | Ver | 1 octet + +---+---+---+---+---+---+---+---+ + / Checksum / 2 octets, if C=1 + +---+---+---+---+---+---+---+---+ + / Key / 4 octets, if K=1 + +---+---+---+---+---+---+---+---+ + / Sequence Number / 4 octets, if S=1 + +---+---+---+---+---+---+---+---+ + + + +Bormann, et al. Standards Track [Page 118] + +RFC 3095 Robust Header Compression July 2001 + + + The bits left blank in the second octet are set to zero when + sending and ignored when received. + + The fields Reserved0 and Reserved1 of the GRE header [GRE2] must + be all zeroes; otherwise, the packet cannot be compressed by this + profile. + +5.8.5. Format of compressed lists in Extension 3 + +5.8.5.1. Format of IP Extension Header(s) field + + In Extension 3 (section 5.7.5), there is a field called IP extension + header(s). This section describes the format of that field. + + 0 1 2 3 4 5 6 7 + +-----+-----+-----+-----+-----+-----+-----+-----+ + | CL | ASeq| ESeq| Gseq| res | 1 octet + +-----+-----+-----+-----+-----+-----+-----+-----+ + : compressed AH Seq Number, 1 or 4 octets : if ASeq = 1 + ----- ----- ----- ----- ----- ----- ----- ----- + : compressed ESP Seq Number, 1 or 4 octets : if Eseq = 1 + ----- ----- ----- ----- ----- ----- ----- ----- + : compressed GRE Seq Number, 1 or 4 octets : if Gseq = 1 + ----- ----- ----- ----- ----- ----- ----- ----- + : compressed header list, variable length : if CL = 1 + ----- ----- ----- ----- ----- ----- ----- ----- + + ASeq: indicates presence of compressed AH Seq Number + ESeq: indicates presence of compressed ESP Seq Number + GSeq: indicates presence of compressed GRE Seq Number + CL: indicates presence of compressed header list + res: reserved; set to zero when sending, ignored when received + + When Aseq, Eseq, or Gseq is set, the corresponding header item (AH, + ESP, or GRE header) is compressed. When not set, the corresponding + header item is sent uncompressed or is not present. + + The format of compressed AH, ESP and GRE Sequence Numbers can each be + either of the following: + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 119] + +RFC 3095 Robust Header Compression July 2001 + + + 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+ + | 0 | LSB of sequence number | | 1 | | + +---+---+---+---+---+---+---+---+ +---+ + + | | + + LSB of sequence number + + | | + + + + | | + +---+---+---+---+---+---+---+---+ + + The format of the compressed header list field is described in + section 5.8.6. + +5.8.5.2. Format of Compressed CSRC List + + The Compressed CSRC List field in the RTP header part of an Extension + 3 (section 5.7.5) is as in section 5.8.6. + +5.8.6. Compressed list formats + + This section describes the format of compressed lists. The format is + the same for CSRC lists and header lists. In CSRC lists, the items + are CSRC identifiers; in header lists, they are uncompressed or + compressed headers, as described in 5.8.4.2-4. + +5.8.6.1. Encoding Type 0 (generic scheme) + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | ET=0 |GP |PS | CC = m | + +---+---+---+---+---+---+---+---+ + : gen_id : 1 octet, if GP = 1 + +---+---+---+---+---+---+---+---+ + | XI 1, ..., XI m | m octets, or m * 4 bits + / --- --- --- ---/ + | : Padding : if PS = 0 and m is odd + +---+---+---+---+---+---+---+---+ + | | + / item 1, ..., item n / variable + | | + +---+---+---+---+---+---+---+---+ + + ET: Encoding type is zero. + + PS: Indicates size of XI fields: + PS = 0 indicates 4-bit XI fields; + PS = 1 indicates 8-bit XI fields. + + + +Bormann, et al. Standards Track [Page 120] + +RFC 3095 Robust Header Compression July 2001 + + + GP: Indicates presence of gen_id field. + + CC: CSRC counter from original RTP header. + + gen_id: Identifier for a sequence of identical lists. It is + present in U/O-mode when the compressor decides that it may use + this list as a future reference list. + + XI 1, ..., XI m: m XI items. The format of an XI item is as + follows: + + +---+---+---+---+ + PS = 0: | X | Index | + +---+---+---+---+ + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + PS = 1: | X | Index | + +---+---+---+---+---+---+---+---+ + + X = 1 indicates that the item corresponding to the Index + is sent in the item 0, ..., item n list. + X = 0 indicates that the item corresponding to the Index is + not sent. + + When 4-bit XI items are used and m > 1, the XI items are placed in + octets in the following manner: + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | XI k | XI k + 1 | + +---+---+---+---+---+---+---+---+ + + Padding: A 4-bit padding field is present when PS = 0 and m is + odd. The Padding field is set to zero when sending and ignored + when receiving. + + Item 1, ..., item n: + + Each item corresponds to an XI with X = 1 in XI 1, ..., XI m. + + + + + + + + + + + +Bormann, et al. Standards Track [Page 121] + +RFC 3095 Robust Header Compression July 2001 + + +5.8.6.2. Encoding Type 1 (insertion only scheme) + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | ET=1 |GP |PS | XI 1 | + +---+---+---+---+---+---+---+---+ + : gen_id : 1 octet, if GP = 1 + +---+---+---+---+---+---+---+---+ + | ref_id | + +---+---+---+---+---+---+---+---+ + / insertion bit mask / 1-2 octets + +---+---+---+---+---+---+---+---+ + | XI list | k octets, or (k - 1) * 4 bits + / --- --- --- ---/ + | : Padding : if PS = 0 and k is even + +---+---+---+---+---+---+---+---+ + | | + / item 1, ..., item n / variable + | | + +---+---+---+---+---+---+---+---+ + + Unless explicitly stated otherwise, fields have the same meaning and + values as for encoding type 0. + + ET: Encoding type is one (1). + + XI 1: When PS = 0, the first 4-bit XI item is placed here. + When PS = 1, the field is set to zero when sending, and + ignored when receiving. + + ref_id: The identifier of the reference CSRC list used when the + list was compressed. It is the 8 least significant bits of + the RTP Sequence Number in R-mode and gen_id (see section + 5.8.2) in U/O-mode. + + insertion bit mask: Bit mask indicating the positions where new + items are to be inserted. See Insertion Only scheme in + section 5.8.3. The bit mask can have either of the + following two formats: + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 122] + +RFC 3095 Robust Header Compression July 2001 + + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 0 | 7-bit mask | bit 1 is the first bit + +---+---+---+---+---+---+---+---+ + + +---+---+---+---+---+---+---+---+ + | 1 | | bit 1 is the first bit + +---+ 15-bit mask + + | | bit 7 is the last bit + +---+---+---+---+---+---+---+---+ + + XI list: XI fields for items to be inserted. When the insertion + bit mask has k ones, the total number of XI fields is k. When + PS = 1, all XI fields are in the XI list. When PS = 0, the + first XI field is in the XI 1 field, and the remaining k - 1 + XI fields are in the XI list. + + Padding: Present when PS = 0 and k is even. + + item 1, ..., item n: One item for each XI field with the X bit + set. + +5.8.6.3. Encoding Type 2 (removal only scheme) + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | ET=2 |GP |res| Count | + +---+---+---+---+---+---+---+---+ + : gen_id : 1 octet, if GP = 1 + +---+---+---+---+---+---+---+---+ + | ref_id | + +---+---+---+---+---+---+---+---+ + / removal bit mask / 1-2 octets + +---+---+---+---+---+---+---+---+ + + Unless explicitly stated otherwise, fields have the same meaning + and values as in section 5.8.5.2. + + ET: Encoding type is 2. + + res: Reserved. Set to zero when sending, ignored when + received. + + Count: Number of elements in ref_list. + + + + + + + +Bormann, et al. Standards Track [Page 123] + +RFC 3095 Robust Header Compression July 2001 + + + removal bit mask: Indicates the elements in ref_list to be + removed in order to obtain the current list. See section + 5.8.3. The removal bit mask has the same format as the + insertion bit mask of section 5.8.6.3. + +5.8.6.4. Encoding Type 3 (remove then insert scheme) + + See section 5.8.3 for a description of the Remove then insert + scheme. + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | ET=3 |GP |PS | XI 1 | + +---+---+---+---+---+---+---+---+ + : gen_id : 1 octet, if GP = 1 + +---+---+---+---+---+---+---+---+ + | ref_id | + +---+---+---+---+---+---+---+---+ + / removal bit mask / 1-2 octets + +---+---+---+---+---+---+---+---+ + / insertion bit mask / 1-2 octets + +---+---+---+---+---+---+---+---+ + | XI list | k octets, or (k - 1) * 4 bits + / --- --- --- ---/ + | : Padding : if PS = 0 and k is even + +---+---+---+---+---+---+---+---+ + | | + / item 1, ..., item n / variable + | | + +---+---+---+---+---+---+---+---+ + + The fields in this header have the same meaning and formats as in + section 5.8.5.2, except when explicitly stated otherwise below. + + ET: Encoding type is 3. + + removal bit mask: See section 5.8.6.3. + +5.8.7. CRC coverage for extension headers + + All fields of extension headers are CRC-STATIC, with the following + exceptions which are CRC-DYNAMIC. + + 1) Entire AH header. + 2) Entire ESP header. + 3) Sequence number in GRE, Checksum in GRE + + + + + +Bormann, et al. Standards Track [Page 124] + +RFC 3095 Robust Header Compression July 2001 + + +5.9. Header compression CRCs, coverage and polynomials + + This chapter describes how to calculate the CRCs used in packet + headers defined in this document. (Note that another type of CRC is + defined for reconstructed units in section 5.2.5.) + +5.9.1. IR and IR-DYN packet CRCs + + The CRC in the IR and IR-DYN packet is calculated over the entire IR + or IR-DYN packet, excluding Payload and including CID or any Add-CID + octet. For purposes of computing the CRC, the CRC field in the + header is set to zero. + + The initial content of the CRC register is to be preset to all 1's. + + The CRC polynomial to be used for the 8-bit CRC is: + + C(x) = 1 + x + x^2 + x^8 + +5.9.2. CRCs in compressed headers + + The CRC in compressed headers is calculated over all octets of the + entire original header, before compression, in the following manner. + + The octets of the header are classified as either CRC-STATIC or CRC- + DYNAMIC, and the CRC is calculated over: + + 1) the concatenated CRC-STATIC octets of the original header, placed + in the same order as they appear in the original header, followed + by + + 2) the concatenated CRC-DYNAMIC octets of the original header, placed + in the same order as they appear in the original header. + + The intention is that the state of the CRC computation after 1) will + be saved. As long as the CRC-STATIC octets do not change, the CRC + calculation will then only need to process the CRC-DYNAMIC octets. + + In a typical RTP/UDP/IPv4 header, 25 octets are CRC-STATIC and 15 are + CRC-DYNAMIC. In a typical RTP/UDP/IPv6 header, 49 octets are CRC- + STATIC and 11 are CRC-DYNAMIC. This technique will thus reduce the + computational complexity of the CRC calculation by roughly 60% for + RTP/UDP/IPv4 and by roughly 80% for RTP/UDP/IPv6. + + Note: Whenever the CRC-STATIC fields change, the new saved CRC state + after 1) is compared with the old state. If the states are + identical, the CRC cannot catch the error consisting in the + decompressor not having updated the static context. In U/O-mode the + + + +Bormann, et al. Standards Track [Page 125] + +RFC 3095 Robust Header Compression July 2001 + + + compressor SHOULD then for a while use packet types with another CRC + length, for which there is a difference in CRC state, to ensure error + detection. + + The initial content of the CRC register is preset to all 1's. + + The polynomial to be used for the 3 bit CRC is: + + C(x) = 1 + x + x^3 + + The polynomial to be used for the 7 bit CRC is: + + C(x) = 1 + x + x^2 + x^3 + x^6 + x^7 + + The CRC in compressed headers is calculated over the entire original + header, before compression. + +5.10. ROHC UNCOMPRESSED -- no compression (Profile 0x0000) + + In ROHC, compression has not been defined for all kinds of IP + headers. Profile 0x0000 provides a way to send IP packets without + compressing them. This can be used for IP fragments, RTCP packets, + and in general for any packet for which compression of the header has + not been defined, is not possible due to resource constraints, or is + not desirable for some other reason. + + After initialization, the only overhead for sending packets using + Profile 0x0000 is the size of the CID. When uncompressed packets are + frequent, Profile 0x0000 should be associated with a CID with size + zero or one octet. There is no need to associate Profile 0x0000 with + more than one CID. + +5.10.1. IR packet + + The initialization packet (IR packet) for Profile 0x0000 has the + following format: + + + + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 126] + +RFC 3095 Robust Header Compression July 2001 + + + 0 1 2 3 4 5 6 7 + --- --- --- --- --- --- --- --- + : Add-CID octet : if for small CIDs and (CID != 0) + +---+---+---+---+---+---+---+---+ + | 1 1 1 1 1 1 0 |res| + +---+---+---+---+---+---+---+---+ + : : + / 0-2 octets of CID info / 1-2 octets if for large CIDs + : : + +---+---+---+---+---+---+---+---+ + | Profile = 0 | 1 octet + +---+---+---+---+---+---+---+---+ + | CRC | 1 octet + +---+---+---+---+---+---+---+---+ + : : (optional) + / IP packet / variable length + : : + --- --- --- --- --- --- --- --- + + res: Always zero. + + Profile: 0. + + CRC: 8-bit CRC, computed using the polynomial of section 5.9.1. + The CRC covers the first octet of the IR packet through the + Profile octet of the IR packet, i.e., it does not cover the + CRC itself or the IP packet. + + IP packet: An uncompressed IP packet may be included in the IR + packet. The decompressor determines if the IP packet is + present by considering the length of the IR packet. + +5.10.2. Normal packet + + A Normal packet is a normal IP packet plus CID information. When the + channel uses small CIDs, and profile 0x0000 is associated with a CID + > 0, an Add-CID octet is prepended to the IP packet. When the + channel uses large CIDs, the CID is placed so that it starts at the + second octet of the Normal packet. + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 127] + +RFC 3095 Robust Header Compression July 2001 + + + 0 1 2 3 4 5 6 7 + --- --- --- --- --- --- --- --- + : Add-CID octet : if for small CIDs and (CID != 0) + +---+---+---+---+---+---+---+---+ + | first octet of IP packet | + +---+---+---+---+---+---+---+---+ + : : + / 0-2 octets of CID info / 1-2 octets if for large CIDs + : : + +---+---+---+---+---+---+---+---+ + | | + / rest of IP packet / variable length + | | + +---+---+---+---+---+---+---+---+ + + Note that the first octet of the IP packet starts with the bit + pattern 0100 (IPv4) or 0110 (IPv6). This does not conflict with any + reserved packet types. Hence, no bits in addition to the CID are + needed. The profile is reasonably future-proof since problems do not + occur until IP version 14. + +5.10.3. States and modes + + There are two modes in Profile 0x0000: Unidirectional mode and + Bidirectional mode. In Unidirectional mode, the compressor repeats + the IR packet periodically. In Bidirectional mode, the compressor + never repeats the IR packet. The compressor and decompressor always + start in Unidirectional mode. Whenever feedback is received, the + compressor switches to Bidirectional mode. + + The compressor can be in either of two states: the IR state or the + Normal state. It starts in the IR state. + + a) IR state: Only IR packets can be sent. After sending a small + number of IR packets (only one when refreshing), the compressor + switches to the Normal state. + + b) Normal state: Only Normal packets can be sent. When in + Unidirectional mode, the compressor periodically transits back to + the IR state. The length of the period is implementation + dependent, but should be fairly long. Exponential backoff may be + used. + + c) When feedback is received in any state, the compressor switches to + Bidirectional mode. + + + + + + +Bormann, et al. Standards Track [Page 128] + +RFC 3095 Robust Header Compression July 2001 + + + The decompressor can be in either of two states: NO_CONTEXT or + FULL_CONTEXT. It starts in NO_CONTEXT. + + d) When an IR packet is received in the NO_CONTEXT state, the + decompressor first verifies the packet using the CRC. If the + packet is OK, the decompressor 1) moves to the FULL_CONTEXT state, + 2) delivers the IP packet to upper layers if present, 3) MAY send + an ACK. If the packet is not OK, it is discarded without further + action. + + e) When any other packet is received in the NO_CONTEXT state, it is + discarded without further action. + + f) When an IR packet is received in the FULL_CONTEXT state, the + packet is first verified using the CRC. If OK, the decompressor + 1) delivers the IP packet to upper layers if present, 2) MAY send + an ACK. If the packet is not OK, no action is taken. + + g) When a Normal packet is received in the FULL_CONTEXT state, the + CID information is removed and the IP packet is delivered to upper + layers. + +5.10.4. Feedback + + The only kind of feedback in Profile 0x0000 is ACKs. Profile 0x0000 + MUST NOT be rejected. Profile 0x0000 SHOULD be associated with at + most one CID. ACKs use the FEEDBACK-1 format of section 5.2. The + value of the profile-specific octet in the FEEDBACK-1 ACK is 0 + (zero). + +5.11. ROHC UDP -- non-RTP UDP/IP compression (Profile 0x0002) + + UDP/IP headers do not have a sequence number which is as well-behaved + as the RTP Sequence Number. For UDP/IPv4, there is an IP-ID field + which may be echoed in feedback information, but when no IPv4 header + is present such feedback identification becomes problematic. + + Therefore, in the ROHC UDP profile, the compressor generates a 16-bit + sequence number SN which increases by one for each packet received in + the packet stream. This sequence number is thus relatively well- + behaved and can serve as the basis for most mechanisms described for + ROHC RTP. It is called SN or UDP SN below. Unless stated otherwise, + the mechanisms of ROHC RTP are used also for ROHC UDP, with the UDP + SN taking the role of the RTP Sequence Number. + + + + + + + +Bormann, et al. Standards Track [Page 129] + +RFC 3095 Robust Header Compression July 2001 + + + The ROHC UDP profile always uses p = -1 when interpreting the SN, + since there will be no repetitions or reordering of the compressor- + generated SN. The interpretation interval thus always starts with + (ref_SN + 1). + +5.11.1. Initialization + + The static context for ROHC UDP streams can be initialized in either + of two ways: + + 1) By using an IR packet as in section 5.7.7.1, where the profile is + two (2) and the static chain ends with the static part of an UDP + packet. At the compressor, UDP SN is initialized to a random + value when the IR packet is sent. + + 2) By reusing an existing context where the existing static chain + contains the static part of a UDP packet, e.g., the context of a + stream compressed using ROHC RTP (profile 0x0001). This is done + with an IR-DYN packet (section 5.7.7.2) identifying profile + 0x0002, where the dynamic chain corresponds to the prefix of the + existing static chain that ends with the UDP header. UDP SN is + initialized to the RTP Sequence Number if the earlier profile was + profile 0x0001, and to a random number otherwise. + + For ROHC UDP, the dynamic part of a UDP packet is different from + section 5.7.7.5: a two-octet field containing the UDP SN is added + after the Checksum field. This affects the format of dynamic chains + in IR and IR-DYN packets. + + Note: 2) can be used for packet streams which were initially assumed + to be RTP streams, so that compression started with profile 0x0001, + but were later found evidently not to be RTP streams. + +5.11.2. States and modes + + ROHC UDP uses the same states and modes as ROHC RTP. Mode + transitions and state logic are the same except when explicitly + stated otherwise. Mechanisms dealing with fields in the RTP header + (except the RTP SN) are not used. The decompressed UDP SN is never + included in any header delivered to upper layers. The UDP SN is used + in place of the RTP SN in feedback. + + + + + + + + + + +Bormann, et al. Standards Track [Page 130] + +RFC 3095 Robust Header Compression July 2001 + + +5.11.3. Packet types + + The general format of a ROHC UDP packet is the same as for ROHC RTP + (see beginning of section 5.7). Padding and CIDs are the same, as is + the feedback packet type (5.7.6.1) and the feedback. IR and IR-DYN + packets (5.7.7) are changed as described in 5.11.1. + + The general format of compressed packets is also the same, but there + are differences in specific formats and extensions as detailed below. + The differences are caused by removal of all RTP specific information + except the RTP SN, which is replaced by the UDP SN. + + Unless explicitly stated below, the packet formats are as in sections + 5.7.1-6. + + R-1 + + The TS field is replaced by an IP-ID field. The M flag has become + part of IP-ID. The X bit has moved. The formats R-1-ID and R-1- + TS are not used. + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 0 | SN | + +===+===+===+===+===+===+===+===+ + | X | IP-ID | + +---+---+---+---+---+---+---+---+ + + UO-1 + + The TS field is replaced by an IP-ID field. The M flag has become + part of SN. Formats UO-1-ID and UO-1-TS are not used. + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 0 | IP-ID | + +===+===+===+===+===+===+===+===+ + | SN | CRC | + +---+---+---+---+---+---+---+---+ + + UOR-2 + + + + + + + + + + +Bormann, et al. Standards Track [Page 131] + +RFC 3095 Robust Header Compression July 2001 + + + New format: + + 0 1 2 3 4 5 6 7 + +---+---+---+---+---+---+---+---+ + | 1 1 0 | SN | + +===+===+===+===+===+===+===+===+ + | X | CRC | + +---+---+---+---+---+---+---+---+ + +5.11.4. Extensions + + Extensions are as in 5.7.5, with the following exceptions: + + Extension 0: + + +---+---+---+---+---+---+---+---+ + | 0 0 | SN | IP-ID | + +---+---+---+---+---+---+---+---+ + + Extension 1: + + +---+---+---+---+---+---+---+---+ + | 0 1 | SN | IP-ID | + +---+---+---+---+---+---+---+---+ + | IP-ID | + +---+---+---+---+---+---+---+---+ + + Extension 2: + + +---+---+---+---+---+---+---+---+ + | 1 0 | SN | IP-ID2 | + +---+---+---+---+---+---+---+---+ + | IP-ID2 | + +---+---+---+---+---+---+---+---+ + | IP-ID | + +---+---+---+---+---+---+---+---+ + + IP-ID2: For outer IP-ID field. + + Extension 3 is the same as Extension 3 in section 5.7.5, with the + following exceptions. + + 1) The initial flag octet has the following format: + + 0 1 2 3 4 5 6 7 + +-----+-----+-----+-----+-----+-----+-----+-----+ + | 1 1 | S | Mode | I | ip | ip2 | + +-----+-----+-----+-----+-----+-----+-----+-----+ + + + +Bormann, et al. Standards Track [Page 132] + +RFC 3095 Robust Header Compression July 2001 + + + Mode: Replaces R-TS and Tsc of 5.7.5. Provides mode information + as was earlier done in RTP header flags and fields. + + ip2: Replaces rtp bit of 5.7.5. Moved here from the Inner IP + header flags octet. + + 2) The bit which was the ip2 flag in the Inner IP header flags in + 5.7.5 is reserved. It is set to zero when sending and ignored + when receiving. + +5.11.5. IP-ID + + Treated as in ROHC RTP, but the offset is from UDP SN. + +5.11.6. Feedback + + Feedback is as for ROHC RTP with the following exceptions: + + 1) UDP SN replaces RTP SN in feedback. + 2) The CLOCK option (5.7.6.6) is not used. + 3) The JITTER option (5.7.6.7) is not used. + +5.12. ROHC ESP -- ESP/IP compression (Profile 0x0003) + + When the ESP header is being used with an encryption algorithm other + than NULL, subheaders after the ESP header are encrypted and cannot + be compressed. Profile 0x0003 is for compression of the chain of + headers up to and including the ESP header in this case. When the + NULL encryption algorithm is being used, other profiles can be used + and could give higher compression rates. See section 5.8.4.3. + + This profile is very similar to the ROHC UDP profile. It uses the + ESP sequence number as the basis for compression instead of a + generated number, but is otherwise very similar to ROHC UDP. The + interpretation interval (value of p) for the ESP-based SN is as with + ROHC RTP (profile 0x0001). Apart from this, unless stated explicitly + below, mechanisms and formats are as for ROHC UDP. + +5.12.1. Initialization + + The static context for ROHC ESP streams can be initialized in either + of two ways: + + 1) by using an IR packet as in section 5.7.7.1, where the profile is + three (3) and the static chain ends with the static part of an ESP + header. + + + + + +Bormann, et al. Standards Track [Page 133] + +RFC 3095 Robust Header Compression July 2001 + + + 2) by reusing an existing context, where the existing static chain + contains the static part of an ESP header. This is done with an + IR-DYN packet (section 5.7.7.2) identifying profile 0x0003, where + the dynamic chain corresponds to the prefix of the existing static + chain that ends with the ESP header. + + In contrast to ROHC UDP, no extra sequence number is added to the + dynamic part of the ESP header: the ESP sequence number is the only + element. + + Note: 2) can be used for streams where compression has been initiated + under the assumption that NULL encryption was being used with ESP. + When it becomes obvious that an encryption algorithm other than NULL + is being used, the compressor may send an IR-DYN according to 2) to + switch to profile 0x0003 without having to send an IR packet. + +5.12.2. Packet types + + The packet types for ROHC ESP are the same as for ROHC UDP, except + that the ESP sequence number is used instead of the generated + sequence number of ROHC UDP. The ESP header is not part of any + compressed list in ROHC ESP. + +6. Implementation issues + + This document specifies mechanisms for the protocol and leaves many + details on the use of these mechanisms to the implementers. This + chapter is aimed to give guidelines, ideas and suggestions for + implementing the scheme. + +6.1. Reverse decompression + + This section describes an OPTIONAL decompressor operation to reduce + the number of packets discarded due to an invalid context. + + Once a context becomes invalid (e.g., when more consecutive packet + losses than expected have occurred), subsequent compressed packets + cannot immediately be decompressed correctly. Reverse decompression + aims at decompressing such packets later instead of discarding them, + by storing them until the context has been updated and validated and + then attempting decompression. + + Let the sequence of stored packets be i, i + 1, ..., i + k, where i + is the first packet and i + k is the last packet before the context + was updated. The decompressor will attempt to recover the stored + packets in reverse order, i.e., starting with i + k, and working back + toward i. When a stored packet has been reconstructed, its + correctness is verified using its CRC. Packets not carrying a CRC + + + +Bormann, et al. Standards Track [Page 134] + +RFC 3095 Robust Header Compression July 2001 + + + must not be delivered to upper layers. Packets where the CRC + succeeds are delivered to upper layers in their original order, i.e., + i, i + 1, ..., i + k. + + Note that this reverse decompression introduces buffering while + waiting for the context to be validated and thereby introduces + additional delay. Thus, it should be used only when some amount of + delay is acceptable. For example, for video packets belonging to the + same video frame, the delay in packet arrivals does not cause + presentation time delay. Delay-insensitive streaming applications + can also be tolerant of such delay. If the decompressor cannot + determine whether the application can tolerate delay, it should not + perform reverse decompression. + + The following illustrates the decompression procedure in some detail: + + 1. The decompressor stores compressed packets that cannot be + decompressed correctly due to an invalid context. + + 2. When the decompressor has received a context updating packet and + the context has been validated, it proceeds to recover the last + packet stored. After decompression, the decompressor checks the + correctness of the reconstructed header using the CRC. + + 3. If the CRC indicates successful decompression, the decompressor + stores the complete packet and attempts to decompress the + preceding packet. In this way, the stored packets are recovered + in reverse order until no compressed packets are left. For each + packet, the decompressor checks the correctness of the + decompressed headers using the header compression CRC. + + 4. If the CRC indicates an incorrectly decompressed packet, the + reverse decompression attempt MUST be terminated and all remaining + uncompressed packets MUST be discarded. + + 5. Finally, the decompressor forwards all the correctly decompressed + packets to upper layers in their original order. + +6.2. RTCP + + RTCP is the RTP Control Protocol [RTP]. RTCP is based on periodic + transmission of control packets to all participants in a session, + using the same distribution mechanism as for data packets. Its + primary function is to provide feedback from the data receivers on + the quality of the data distribution. The feedback information may + be used for issues related to congestion control functions, and + directly useful for control of adaptive encodings. + + + + +Bormann, et al. Standards Track [Page 135] + +RFC 3095 Robust Header Compression July 2001 + + + In an RTP session there will be two types of packet streams: one with + the RTP header and application data, and one with the RTCP control + information. The difference between the streams at the transport + level is in the UDP port numbers: the RTP port number is always even, + the RTCP port number is that number plus one and therefore always odd + [RTP, section 10]. The ROHC header compressor implementation has + several ways at hand to handle the RTCP stream: + + 1. One compressor/decompressor entity carrying both types of streams + on the same channel, using CIDs to distinguish between them. For + sending a single RTP stream together with its RTCP packets on one + channel, it is most efficient to set LARGE_CIDS to false, send the + RTP packets with the implied CID 0 and use the Add-CID mechanism + to send the RTCP packets. + + 2. Two compressor/decompressor entities, one for RTP and another one + for RTCP, carrying the two types of streams on separate channels. + This means that they will not share the same CID number space. + + RTCP headers may simply be sent uncompressed using profile 0x0000. + More efficiently, ROHC UDP compression (profile 0x0002) can be used. + +6.3. Implementation parameters and signals + + A ROHC implementation may have two kinds of parameters: configuration + parameters that are mandatory and must be negotiated between + compressor and decompressor peers, and implementation parameters that + are optional and, when used, stipulate how a ROHC implementation is + to operate. + + Configuration parameters are mandatory and must be negotiated between + compressor and decompressor, so that they have the same values at + both compressor and decompressor, see section 5.1.1. + + Implementation parameters make it possible for an external entity to + stipulate how an implementation of a ROHC compressor or decompressor + should operate. Implementation parameters have local significance, + are optional to use and are thus not necessary to negotiate between + compressor and decompressor. Note that this does not preclude + signaling or negotiating implementation parameters using lower layer + functionality in order to set the way a ROHC implementation should + operate. Some implementation parameters are valid only at either of + compressor or decompressor. Implementation parameters may further be + divided into parameters that allow an external entity to describe the + way the implementation should operate and parameters that allow an + external entity to trigger a specific event, i.e., signals. + + + + + +Bormann, et al. Standards Track [Page 136] + +RFC 3095 Robust Header Compression July 2001 + + +6.3.1. ROHC implementation parameters at compressor + + CONTEXT_REINITIALIZATION -- signal + This parameter triggers a reinitialization of the entire context at + the decompressor, both the static and the dynamic part. The + compressor MUST, when CONTEXT_REINITIALIZATION is triggered, back off + to the IR state and fully reinitialize the context by sending IR + packets with both the static and dynamic chains covering the entire + uncompressed headers until it is reasonably confident that the + decompressor contexts are reinitialized. The context + reinitialization MUST be done for all contexts at the compressor. + This parameter may for instance be used to do context relocation at, + e.g., a cellular handover that results in a change of compression + point in the radio access network. + + NO_OF_PACKET_SIZES_ALLOWED -- value: positive integer + This parameter may be set by an external entity to specify the number + of packet sizes a ROHC implementation may use. However, the + parameter may be used only if PACKET_SIZES is not used by an external + entity. With this parameter set, the ROHC implementation at the + compressor MUST NOT use more different packet sizes than the value + this parameter stipulates. The ROHC implementation must itself be + able to determine which packet sizes will be used and describe these + to an external entity using PACKET_SIZES_USED. It should be noted + that one packet size might be used for several header formats, and + that the number of packet sizes can be reduced by employing padding + and segmentation. + + NO_OF_PACKET_SIZES_USED _- value: positive integer + This parameter is set by the ROHC implementation to indicate how many + packet sizes it will actually use. It can be set to a large value to + indicate that no particular attempt is made to minimize that number. + + PACKET_SIZES_ALLOWED -- value: list of positive integers (bytes) + This parameter, if set, governs which packet sizes in bytes may be + used by the ROHC implementation. Thus, packet sizes not in the set + of values for this parameter MUST NOT be used. Hence, an external + entity can mandate a ROHC implementation to produce packet sizes that + fit pre-configured lower layers better. If this parameter is used to + stipulate which packet sizes a ROHC implementation can use, the + following rules apply: + + - A packet large enough to hold the entire IR header (both static and + dynamic chain) MUST be part of the set of sizes, unless MRRU is set + to a large enough value to allow segmentation. + - The packet size likely to be used most frequently in the SO state + SHOULD be part of the set. + + + + +Bormann, et al. Standards Track [Page 137] + +RFC 3095 Robust Header Compression July 2001 + + + - The packet size likely to be used most frequently in the FO state + SHOULD be part of the set. + + PACKET_SIZES_USED -- values: set of positive integers (bytes) + This parameter describes which packet sizes a ROHC implementation + uses if NO_OF_PACKET_SIZES_ALLOWED or PACKET_SIZES_ALLOWED is used by + an external entity to stipulate how many packet sizes a ROHC + implementation should use. The information about used packet sizes + (bytes) in this parameter, may then be used to configure lower + layers. + + PAYLOAD_SIZES -_ values: set of positive integer values (bytes) + This parameter is set by an external entity that wants to make use of + the PACKET_SIZES_USED parameter to indicate which payload sizes can + be expected. + + When a ROHC implementation has a limited set of allowed packet sizes, + and the most preferable header format has a size that is not part of + the set, it has the following options: + + - Choose the next larger header format from the allowed set. This is + probably the most efficient choice. + - Use the most preferable header format as if there were no + restrictions on size, and then add padding octets to complete a + packet of the next larger size in the allowed set. + - Use segmentation to fragment the packet into pieces that would make + up packets of sizes that are permissible (possibly after the + addition of padding to the last segment). + + It should be noted that even if the two last parameters introduce the + possibility of restricting the number of packet sizes used, such + restrictions will have a negative impact on compression performance. + +6.3.2. ROHC implementation parameters at decompressor + + MODE -- values: [U-mode, O-mode, R-mode] + This parameter triggers a mode transition using the mechanism + described in chapter 5 when the parameter changes value, i.e., to U- + mode (Unidirectional mode), O-mode (Bidirectional Optimistic mode) or + R-mode (Bidirectional Reliable mode). The mode transition is made + from the current mode to the new mode as signaled by the + implementation parameter. For example, if the current mode is + Bidirectional Optimistic mode, MODE should have the value O-mode. If + the MODE is changed to R-mode, a mode transition MUST be made from + Bidirectional Optimistic mode to Bidirectional Reliable mode. MODE + should not only serve as a trigger for mode transitions, but also + make it visible which mode ROHC operates in. + + + + +Bormann, et al. Standards Track [Page 138] + +RFC 3095 Robust Header Compression July 2001 + + + CLOCK_RESOLUTION -- value: nonnegative integer + This parameter indicates the system clock resolution in units of + milliseconds. A zero (0) value means that there is no clock + available. If nonzero, this parameter allows the decompressor to use + timer-based TS compression (section 4.5.4) and SN wraparound + detection (section 5.3.2.2.4). In this case, its specific value is + also significant for correctness of the algorithms. + + REVERSE_DECOMPRESSION_DEPTH -- value: nonnegative integer + This parameter determines whether reverse decompression as described + in section 6.1 should be used or not, and if used, to what extent. + The value indicates the maximum number of packets that can be + buffered, and thus possibly be reverse decompressed by the + decompressor. A zero (0) value means that reverse decompression MUST + NOT be used. + +6.4. Handling of resource limitations at the decompressor + + In a point-to-point link, the two nodes can agree on the number of + compressed sessions they are prepared to support for this link. It + may, however, not be possible for the decompressor to accurately + predict when it will run out of resources. ROHC allows the + negotiated number of contexts to be larger than could be accommodated + in the worst case. Then, as context resources are consumed, an + attempt to set up a new context may be rejected by the decompressor, + using the REJECT option of the feedback payload. + + Upon reception of a REJECT option, the compressor SHOULD wait for a + while before attempting to compress additional streams destined for + the rejecting node. + +6.5. Implementation structures + + This section provides some explanatory material on data structures + that a ROHC implementation will have to maintain in one form or + another. It is not intended to constrain the implementations. + +6.5.1. Compressor context + + The compressor context consists of a static part and a dynamic part. + The content of the static part is the same as the static chain + defined in section 5.7.7. The dynamic part consists of multiple + elements which can be categorized into four types. + + a) Sliding Window (SW) + b) Translation Table (TT) + c) Flag + d) Field + + + +Bormann, et al. Standards Track [Page 139] + +RFC 3095 Robust Header Compression July 2001 + + + These elements may be common to all modes or mode specific. The + following table summarizes all these elements. + + +--------+---------------------------+-------------+----------------+ + | | Common to | Specific to | Specific to | + | | all modes | R-mode | U/O-mode | + +--------+---------------------------+-------------+----------------+ + | SWs | GSW | R_CSW | UO_CSW | + | | | R_IESW | UO_IESW | + +--------+---------------------------+-------------+----------------+ + | TTs | | R_CTT | UO_CTT | + | | | R_IETT | UO_IETT | + +--------+---------------------------+-------------+----------------+ + | Flags | UDP Chksum | | ACKED | + | | TSS, TIS | | | + | | RND, RND2 | | | + | | NBO, NBO2 | | | + +--------+---------------------------+-------------+----------------+ + | Fields | Profile | | CSRC_REF_ID | + | | C_MODE | | CSRC_GEN_ID | + | | C_STATE | | CSRC_GEN_COUNT | + | | C_TRANS | | IPEH_REF_ID | + | | TS_STRIDE (if TSS = 1) | | IPEH_GEN_ID | + | | TS_OFFSET (if TSS = 1) | | IPEH_GEN_COUNT | + | | TIME_STRIDE (if TIS = 1) | | | + | | CURR_TIME (if TIS = 1) | | | + | | MAX_JITTER_CD (if TIS = 1)| | | + | | LONGEST_LOSS_EVENT(O) | | | + | | CLOCK_RESOLUTION(O) | | | + | | MAX_JITTER(O) | | | + +--------+---------------------------+-------------+----------------+ + + 1) GSW: Generic W_LSB Sliding Window + + Each element in GSW consists of all the dynamic fields in the + dynamic chain (defined in section 5.7.7) plus the fields specified + in a) but excluding the fields specified in b). + + a) Packet Arrival Time (if TIS = 1) + Scaled RTP Time Stamp (if TSS = 1) (optional) + Offset_i (if RND = 0) (optional) + + b) UDP Checksum, TS Stride, CSRC list, IPv6 Extension Headers + + 2) R_CSW: CSRC Sliding Window in R-mode + + R_IESW: IPv6 Extension Header Sliding Window in R-mode + + + + +Bormann, et al. Standards Track [Page 140] + +RFC 3095 Robust Header Compression July 2001 + + + UO_CSW: CSRC Sliding Window in U/O-mode + + UO_IESW: IPv6 Extension Header Sliding Window in U/O-mode + + Each element in R_CSW, R_IESW, UO_CSW and UO_IESW is defined in + section 6.5.3. + + 3) R_CTT: CSRC Translation Table in R-mode + + R_IETT: IPv6 Extension Header Translation Table in U/O-mode + + UO_CTT: CSRC Translation Table in U/O-mode + + UO_IETT: IPv6 Extension Header Translation Table in U/O-mode + + Each element in R_CTT and R_IETT is defined in section 5.8.1.1. + Each element in UO_CTT and UO_IETT is defined in section 5.8.1.2. + + 4) ACKED: Indicates whether or not the decompressor has ever acked + + 5) CURR_TIME: The current time value (used for context relocation + when timer-based timestamp compression is used) + + 6) All the other flags and fields are defined elsewhere in the ROHC + document. + +6.5.2. Decompressor context + + The decompressor context consists of a static part and a dynamic + part. The content of the static part is the same as the static chain + defined in section 5.7.7. The dynamic part consists of multiple + elements, one of which is the nonstatic reference header that + includes all the nonstatic fields. These nonstatic fields are the + fields in the dynamic chain defined in section 5.7.7, excluding UDP + Checksum and TS_Stride. All the remaining elements can be + categorized into four types: + + a) Sliding Window (SW) + b) Translation Table (TT) + d) Flag + e) Field + + These elements may be mode specific or common to all modes. The + following table summarizes all these elements. + + + + + + + +Bormann, et al. Standards Track [Page 141] + +RFC 3095 Robust Header Compression July 2001 + + + +--------+---------------------------+-------------+----------------+ + | | Common to | Specific to | Specific to | + | | all modes | R-mode | U/O-mode | + +--------+---------------------------+-------------+----------------+ + | SWs | | R_CSW | UO_CSW | + | | | R_IESW | UO_IESW | + +--------+---------------------------+-------------+----------------+ + | TTs | | R_CTT | UO_CTT | + | | | R_IETT | UO_IETT | + +--------+---------------------------+-------------+----------------+ + | Flags | UDP Checksum | | ACKED | + | | TSS, TIS | | | + | | RND, RND2 | | | + | | NBO, NBO2 | | | + +--------+---------------------------+-------------+----------------+ + | Fields | Profile | | CSRC_GEN_ID | + | | D_MODE | | IPEH_GEN_ID | + | | D_STATE | | PRE_SN_V_REF | + | | D_TRANS | | | + | | TS_STRIDE (if TSS = 1) | | | + | | TS_OFFSET (if TSS = 1) | | | + | | TIME_STRIDE (if TIS = 1) | | | + | | PKT_ARR_TIME (if TIS = 1) | | | + | | LONGEST_LOSS_EVENT(O) | | | + | | CLOCK_RESOLUTION(O) | | | + | | MAX_JITTER(O) | | | + +--------+---------------------------+-------------+----------------+ + + 1) ACKED: Indicates whether or not ACK has ever been sent. + + 2) PKT_ARR_TIME: The arrival time of the packet that most recently + decompressed and verified using CRC. + + PRE_SN_V_REF: The sequence number of the packet verified before + the most recently verified packet. + + CSRC_GEN_ID: The CSRC gen_id of the most recently received packet. + + IPEH_GEN_ID: The IPv6 Extension Header gen_id of the most recently + received packet. + + 3) The remaining elements are as defined in the compressor context. + +6.5.3. List compression: Sliding windows in R-mode and U/O-mode + + In R-mode list compression (see section 5.8.2.1), each entry in the + sliding window, both at the compressor side and at the decompressor + side, has the following structure: + + + +Bormann, et al. Standards Track [Page 142] + +RFC 3095 Robust Header Compression July 2001 + + + +---------------------+--------+------------+ + | RTP Sequence Number | icount | index list | + +---------------------+--------+------------+ + + The table index list contains a list of index. Each of these index + corresponds to the item in the original list carried in the packet + identified by the RTP Sequence Number. The mapping between the index + and the item is identified in the translation table. The icount + field carries the number of index in the following index list. + + In U/O-mode list compression, each entry in the sliding window at + both the compressor side and decompressor side has the following + structure. + + +--------+--------+------------+ + | Gen_id | icount | index list | + +--------+--------+------------+ + + The icount and index list fields are the same as defined in R-mode. + Instead of using the RTP Sequence Number to identify each entry, the + Gen_id is included in the sliding window in U/O-mode. + +7. Security Considerations + + Because encryption eliminates the redundancy that header compression + schemes try to exploit, there is some inducement to forego encryption + of headers in order to enable operation over low-bandwidth links. + However, for those cases where encryption of data (and not headers) + is sufficient, RTP does specify an alternative encryption method in + which only the RTP payload is encrypted and the headers are left in + the clear. That would still allow header compression to be applied. + + ROHC compression is transparent with regard to the RTP Sequence + Number and RTP Timestamp fields, so the values of those fields can be + used as the basis of payload encryption schemes (e.g., for + computation of an initialization vector). + + A malfunctioning or malicious header compressor could cause the + header decompressor to reconstitute packets that do not match the + original packets but still have valid IP, UDP and RTP headers and + possibly also valid UDP checksums. Such corruption may be detected + with end-to-end authentication and integrity mechanisms which will + not be affected by the compression. Moreover, this header + compression scheme uses an internal checksum for verification of + reconstructed headers. This reduces the probability of producing + decompressed headers not matching the original ones without this + being noticed. + + + + +Bormann, et al. Standards Track [Page 143] + +RFC 3095 Robust Header Compression July 2001 + + + Denial-of-service attacks are possible if an intruder can introduce + (for example) bogus STATIC, DYNAMIC or FEEDBACK packets onto the link + and thereby cause compression efficiency to be reduced. However, an + intruder having the ability to inject arbitrary packets at the link + layer in this manner raises additional security issues that dwarf + those related to the use of header compression. + +8. IANA Considerations + + The ROHC profile identifier is a non-negative integer. In many + negotiation protocols, it will be represented as a 16-bit value. Due + to the way the profile identifier is abbreviated in ROHC packets, the + 8 least significant bits of the profile identifier have a special + significance: Two profile identifiers with identical 8 LSBs should be + assigned only if the higher-numbered one is intended to supersede the + lower-numbered one. To highlight this relationship, profile + identifiers should be given in hexadecimal (as in 0x1234, which would + for example supersede 0x0A34). + + Following the policies outlined in [IANA-CONSIDERATIONS], the IANA + policy for assigning new values for the profile identifier shall be + Specification Required: values and their meanings must be documented + in an RFC or in some other permanent and readily available reference, + in sufficient detail that interoperability between independent + implementations is possible. In the 8 LSBs, the range 0 to 127 is + reserved for IETF standard-track specifications; the range 128 to 254 + is available for other specifications that meet this requirement + (such as Informational RFCs). The LSB value 255 is reserved for + future extensibility of the present specification. + + The following profile identifiers are already allocated: + + Profile Document Usage + identifier + + 0x0000 RFCthis ROHC uncompressed + 0x0001 RFCthis ROHC RTP + 0x0002 RFCthis ROHC UDP + 0x0003 RFCthis ROHC ESP + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 144] + +RFC 3095 Robust Header Compression July 2001 + + +9. Acknowledgments + + Earlier header compression schemes described in [CJHC], [IPHC], and + [CRTP] have been important sources of ideas and knowledge. + + The editor would like to extend his warmest thanks to Mikael + Degermark, who actually did a lot of the editing work, and Peter + Eriksson, who made a copy editing pass through the document, + significantly increasing its editorial consistency. Of course, all + remaining editorial problems have then been inserted by the editor. + + Thanks to Andreas Jonsson (Lulea University), who supported this work + by his study of header field change patterns. + + Finally, this work would not have succeeded without the continual + advice in navigating the IETF standards track, garnished with both + editorial and technical comments, from the IETF transport area + directors, Allison Mankin and Scott Bradner. + +10. Intellectual Property Right Claim Considerations + + The IETF has been notified of intellectual property rights claimed in + regard to some or all of the specification contained in this + document. For more information consult the online list of claimed + rights. + + The IETF takes no position regarding the validity or scope of any + intellectual property or other rights that might be claimed to + pertain to the implementation or use of the technology described in + this document or the extent to which any license under such rights + might or might not be available; neither does it represent that it + has made any effort to identify any such rights. Information on the + IETF's procedures with respect to rights in standards-track and + standards-related documentation can be found in BCP-11. Copies of + claims of rights made available for publication and any assurances of + licenses to be made available, or the result of an attempt made to + obtain a general license or permission for the use of such + proprietary rights by implementors or users of this specification can + be obtained from the IETF Secretariat. + + The IETF invites any interested party to bring to its attention any + copyrights, patents or patent applications, or other proprietary + rights which may cover technology that may be required to practice + this standard. Please address the information to the IETF Executive + Director. + + + + + + +Bormann, et al. Standards Track [Page 145] + +RFC 3095 Robust Header Compression July 2001 + + +11. References + +11.1. Normative References + + [UDP] Postel, J., "User Datagram Protocol", STD 6, + RFC 768, August 1980. + + [IPv4] Postel, J., "Internet Protocol", STD 5, RFC + 791, September 1981. + + [IPv6] Deering, S. and R. Hinden, "Internet Protocol, + Version 6 (IPv6) Specification", RFC 2460, + December 1998. + + [RTP] Schulzrinne, H., Casner, S., Frederick, R. and + V. Jacobson, "RTP: A Transport Protocol for + Real-Time Applications", RFC 1889, January + 1996. + + [HDLC] Simpson, W., "PPP in HDLC-like framing", STD + 51, RFC 1662, July 1994. + + [ESP] Kent, S. and R. Atkinson, "IP Encapsulating + Security Payload", RFC 2406, November 1998. + + [NULL] Glenn, R. and S. Kent, "The NULL Encryption + Algorithm and Its Use With Ipsec", RFC 2410, + November 1998. + + [AH] Kent, S. and R. Atkinson, "IP Authentication + Header", RFC 2402, November 1998. + + [MINE] Perkins, C., "Minimal Encapsulation within IP", + RFC 2004, October 1996. + + [GRE1] Farinacci, D., Li, T., Hanks, S., Meyer, D. and + P. Traina, "Generic Routing Encapsulation + (GRE)", RFC 2784, March 2000. + + [GRE2] Dommety, G., "Key and Sequence Number + Extensions to GRE", RFC 2890, August 2000. + + [ASSIGNED] Reynolds, J. and J. Postel, "Assigned Numbers", + STD 2, RFC 1700, October 1994. + + + + + + + +Bormann, et al. Standards Track [Page 146] + +RFC 3095 Robust Header Compression July 2001 + + +11.2. Informative References + + [VJHC] Jacobson, V., "Compressing TCP/IP Headers for + Low-Speed Serial Links", RFC 1144, February + 1990. + + [IPHC] Degermark, M., Nordgren, B. and S. Pink, "IP + Header Compression", RFC 2507, February 1999. + + [CRTP] Casner, S. and V. Jacobson, "Compressing + IP/UDP/RTP Headers for Low-Speed Serial Links", + RFC 2508, February 1999. + + [CRTPC] Degermark, M., Hannu, H., Jonsson, L.E., + Svanbro, K., "Evaluation of CRTP Performance + over Cellular Radio Networks", IEEE Personal + Communication Magazine, Volume 7, number 4, pp. + 20-25, August 2000. + + [REQ] Degermark, M., "Requirements for robust + IP/UDP/RTP header compression", RFC 3096, June + 2001. + + [LLG] Svanbro, K., "Lower Layer Guidelines for Robust + RTP/UDP/IP Header Compression", Work in + Progress. + + [IANA-CONSIDERATIONS] Alvestrand, H. and T. Narten, "Guidelines for + Writing an IANA Considerations Section in + RFCs", BCP 26, RFC 2434, October 1998. + + + + + + + + + + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 147] + +RFC 3095 Robust Header Compression July 2001 + + +12. Authors' Addresses + + Carsten Bormann, Editor + Universitaet Bremen TZI + Postfach 330440 + D-28334 Bremen, Germany + + Phone: +49 421 218 7024 + Fax: +49 421 218 7000 + EMail: cabo@tzi.org + + + Carsten Burmeister + Panasonic European Laboratories GmbH + Monzastr. 4c + 63225 Langen, Germany + + Phone: +49-6103-766-263 + Fax: +49-6103-766-166 + EMail: burmeister@panasonic.de + + + Mikael Degermark + The University of Arizona + Dept of Computer Science + P.O. Box 210077 + Tucson, AZ 85721-0077, USA + + Phone: +1 520 621-3498 + Fax: +1 520 621-4642 + EMail: micke@cs.arizona.edu + + + Hideaki Fukushima + Matsushita Electric Industrial Co., + Ltd006, Kadoma, Kadoma City, + Osaka, Japan + + Phone: +81-6-6900-9192 + Fax: +81-6-6900-9193 + EMail: fukusima@isl.mei.co.jp + + + + + + + + + + +Bormann, et al. Standards Track [Page 148] + +RFC 3095 Robust Header Compression July 2001 + + + Hans Hannu + Box 920 + Ericsson Erisoft AB + SE-971 28 Lulea, Sweden + + Phone: +46 920 20 21 84 + Fax: +46 920 20 20 99 + EMail: hans.hannu@ericsson.com + + + Lars-Erik Jonsson + Box 920 + Ericsson Erisoft AB + SE-971 28 Lulea, Sweden + + Phone: +46 920 20 21 07 + Fax: +46 920 20 20 99 + EMail: lars-erik.jonsson@ericsson.com + + + Rolf Hakenberg + Panasonic European Laboratories GmbH + Monzastr. 4c + 63225 Langen, Germany + + Phone: +49-6103-766-162 + Fax: +49-6103-766-166 + EMail: hakenberg@panasonic.de + + + Tmima Koren + Cisco Systems, Inc. + 170 West Tasman Drive + San Jose, CA 95134, USA + + Phone: +1 408-527-6169 + EMail: tmima@cisco.com + + + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 149] + +RFC 3095 Robust Header Compression July 2001 + + + Khiem Le + 2-700 + Mobile Networks Laboratory + Nokia Research Center + 6000 Connection Drive + Irving, TX 75039, USA + + Phone: +1-972-894-4882 + Fax: +1 972 894-4589 + EMail: khiem.le@nokia.com + + + Zhigang Liu + 2-700 + Mobile Networks Laboratory + Nokia Research Center + 6000 Connection Drive + Irving, TX 75039, USA + + Phone: +1 972 894-5935 + Fax: +1 972 894-4589 + EMail: zhigang.liu@nokia.com + + + Anton Martensson + Ericsson Radio Systems AB + Torshamnsgatan 23 + SE-164 80 Stockholm, Sweden + + Phone: +46 8 404 3881 + Fax: +46 8 757 5550 + EMail: anton.martensson@era.ericsson.se + + + Akihiro Miyazaki + Matsushita Electric Industrial Co., Ltd + 1006, Kadoma, Kadoma City, Osaka, Japan + + Phone: +81-6-6900-9192 + Fax: +81-6-6900-9193 + EMail: akihiro@isl.mei.co.jp + + + + + + + + + + +Bormann, et al. Standards Track [Page 150] + +RFC 3095 Robust Header Compression July 2001 + + + Krister Svanbro + Box 920 + Ericsson Erisoft AB + SE-971 28 Lulea, Sweden + + Phone: +46 920 20 20 77 + Fax: +46 920 20 20 99 + EMail: krister.svanbro@ericsson.com + + + Thomas Wiebke + Panasonic European Laboratories GmbH + Monzastr. 4c + 63225 Langen, Germany + + Phone: +49-6103-766-161 + Fax: +49-6103-766-166 + EMail: wiebke@panasonic.de + + + Takeshi Yoshimura + NTT DoCoMo, Inc. + 3-5, Hikarinooka + Yokosuka, Kanagawa, 239-8536, Japan + + Phone: +81-468-40-3515 + Fax: +81-468-40-3788 + EMail: yoshi@spg.yrp.nttdocomo.co.jp + + + Haihong Zheng + 2-700 + Mobile Networks Laboratory + Nokia Research Center + 6000 Connection Drive + Irving, TX 75039, USA + + Phone: +1 972 894-4232 + Fax: +1 972 894-4589 + EMail: haihong.zheng@nokia.com + + + + + + + + + + + +Bormann, et al. Standards Track [Page 151] + +RFC 3095 Robust Header Compression July 2001 + + +Appendix A. Detailed classification of header fields + + Header compression is possible thanks to the fact that most header + fields do not vary randomly from packet to packet. Many of the + fields exhibit static behavior or change in a more or less + predictable way. When designing a header compression scheme, it is + of fundamental importance to understand the behavior of the fields in + detail. + + In this appendix, all IP, UDP and RTP header fields are classified + and analyzed in two steps. First, we have a general classification + in A.1 where the fields are classified on the basis of stable + knowledge and assumptions. The general classification does not take + into account the change characteristics of changing fields because + those will vary more or less depending on the implementation and on + the application used. A less stable but more detailed analysis of + the change characteristics is then done in A.2. Finally, A.3 + summarizes this appendix with conclusions about how the various + header fields should be handled by the header compression scheme to + optimize compression and functionality. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 152] + +RFC 3095 Robust Header Compression July 2001 + + +A.1. General classification + + At a general level, the header fields are separated into 5 classes: + + INFERRED These fields contain values that can be inferred from + other values, for example the size of the frame + carrying the packet, and thus do not have to be + handled at all by the compression scheme. + + STATIC These fields are expected to be constant throughout + the lifetime of the packet stream. Static information + must in some way be communicated once. + + STATIC-DEF STATIC fields whose values define a packet stream. + They are in general handled as STATIC. + + STATIC-KNOWN These STATIC fields are expected to have well-known + values and therefore do not need to be communicated + at all. + + CHANGING These fields are expected to vary in some way: + randomly, within a limited value set or range, or in + some other manner. + + + In this section, each of the IP, UDP and RTP header fields is + assigned to one of these classes. For all fields except those + classified as CHANGING, the motives for the classification are also + stated. In section A.2, CHANGING fields are further examined and + classified on the basis of their expected change behavior. + +A.1.1. IPv6 header fields + + +---------------------+-------------+----------------+ + | Field | Size (bits) | Class | + +---------------------+-------------+----------------+ + | Version | 4 | STATIC | + | Traffic Class | 8 | CHANGING | + | Flow Label | 20 | STATIC-DEF | + | Payload Length | 16 | INFERRED | + | Next Header | 8 | STATIC | + | Hop Limit | 8 | CHANGING | + | Source Address | 128 | STATIC-DEF | + | Destination Address | 128 | STATIC-DEF | + +---------------------+-------------+----------------+ + + + + + + +Bormann, et al. Standards Track [Page 153] + +RFC 3095 Robust Header Compression July 2001 + + + Version + + The version field states which IP version is used. Packets with + different values in this field must be handled by different IP + stacks. All packets of a packet stream must therefore be of the + same IP version. Accordingly, the field is classified as STATIC. + + Flow Label + + This field may be used to identify packets belonging to a specific + packet stream. If not used, the value should be set to zero. + Otherwise, all packets belonging to the same stream must have the + same value in this field, it being one of the fields that define + the stream. The field is therefore classified as STATIC-DEF. + + Payload Length + + Information about packet length (and, consequently, payload + length) is expected to be provided by the link layer. The field + is therefore classified as INFERRED. + + Next Header + + This field will usually have the same value in all packets of a + packet stream. It encodes the type of the subsequent header. + Only when extension headers are sometimes present and sometimes + not, will the field change its value during the lifetime of the + stream. The field is therefore classified as STATIC. + + Source and Destination addresses + + These fields are part of the definition of a stream and must thus + be constant for all packets in the stream. The fields are + therefore classified as STATIC-DEF. + + Total size of the fields in each class: + + +--------------+--------------+ + | Class | Size (octets)| + +--------------+--------------+ + | INFERRED | 2 | + | STATIC | 1.5 | + | STATIC-DEF | 34.5 | + | CHANGING | 2 | + +--------------+--------------+ + + + + + + +Bormann, et al. Standards Track [Page 154] + +RFC 3095 Robust Header Compression July 2001 + + +A.1.2. IPv4 header fields + + +---------------------+-------------+----------------+ + | Field | Size (bits) | Class | + +---------------------+-------------+----------------+ + | Version | 4 | STATIC | + | Header Length | 4 | STATIC-KNOWN | + | Type Of Service | 8 | CHANGING | + | Packet Length | 16 | INFERRED | + | Identification | 16 | CHANGING | + | Reserved flag | 1 | STATIC-KNOWN | + | Don't Fragment flag | 1 | STATIC | + | More Fragments flag | 1 | STATIC-KNOWN | + | Fragment Offset | 13 | STATIC-KNOWN | + | Time To Live | 8 | CHANGING | + | Protocol | 8 | STATIC | + | Header Checksum | 16 | INFERRED | + | Source Address | 32 | STATIC-DEF | + | Destination Address | 32 | STATIC-DEF | + +---------------------+-------------+----------------+ + + Version + + The version field states which IP version is used. Packets with + different values in this field must be handled by different IP + stacks. All packets of a packet stream must therefore be of the + same IP version. Accordingly, the field is classified as STATIC. + + Header Length + + As long no options are present in the IP header, the header length + is constant and well known. If there are options, the fields + would be STATIC, but it is assumed here that there are no options. + The field is therefore classified as STATIC-KNOWN. + + Packet Length + + Information about packet length is expected to be provided by the + link layer. The field is therefore classified as INFERRED. + + Flags + + The Reserved flag must be set to zero and is therefore classified + as STATIC-KNOWN. The Don't Fragment (DF) flag will be constant + for all packets in a stream and is therefore classified as STATIC. + + + + + + +Bormann, et al. Standards Track [Page 155] + +RFC 3095 Robust Header Compression July 2001 + + + Finally, the More Fragments (MF) flag is expected to be zero + because fragmentation is NOT expected, due to the small packet + size expected. The More Fragments flag is therefore classified as + STATIC-KNOWN. + + Fragment Offset + + Under the assumption that no fragmentation occurs, the fragment + offset is always zero. The field is therefore classified as + STATIC-KNOWN. + + Protocol + + This field will usually have the same value in all packets of a + packet stream. It encodes the type of the subsequent header. + Only when extension headers are sometimes present and sometimes + not, will the field change its value during the lifetime of a + stream. The field is therefore classified as STATIC. + + Header Checksum + + The header checksum protects individual hops from processing a + corrupted header. When almost all IP header information is + compressed away, there is no point in having this additional + checksum; instead it can be regenerated at the decompressor side. + The field is therefore classified as INFERRED. + + Source and Destination addresses + + These fields are part of the definition of a stream and must thus + be constant for all packets in the stream. The fields are + therefore classified as STATIC-DEF. + + Total size of the fields in each class: + + +--------------+----------------+ + | Class | Size (octets) | + +--------------+----------------+ + | INFERRED | 4 | + | STATIC | 1 oct + 5 bits | + | STATIC-DEF | 8 | + | STATIC-KNOWN | 2 oct + 3 bits | + | CHANGING | 4 | + +--------------+----------------+ + + + + + + + +Bormann, et al. Standards Track [Page 156] + +RFC 3095 Robust Header Compression July 2001 + + +A.1.3. UDP header fields + + +------------------+-------------+-------------+ + | Field | Size (bits) | Class | + +------------------+-------------+-------------+ + | Source Port | 16 | STATIC-DEF | + | Destination Port | 16 | STATIC-DEF | + | Length | 16 | INFERRED | + | Checksum | 16 | CHANGING | + +------------------+-------------+-------------+ + + Source and Destination ports + + These fields are part of the definition of a stream and must thus + be constant for all packets in the stream. The fields are + therefore classified as STATIC-DEF. + + Length + + This field is redundant and is therefore classified as INFERRED. + + Total size of the fields in each class: + + +------------+---------------+ + | Class | Size (octets) | + +------------+---------------+ + | INFERRED | 2 | + | STATIC-DEF | 4 | + | CHANGING | 2 | + +------------+---------------+ + +A.1.4. RTP header fields + + +-----------------+-------------+----------------+ + | Field | Size (bits) | Class | + +-----------------+-------------+----------------+ + | Version | 2 | STATIC-KNOWN | + | Padding | 1 | STATIC | + | Extension | 1 | STATIC | + | CSRC Counter | 4 | CHANGING | + | Marker | 1 | CHANGING | + | Payload Type | 7 | CHANGING | + | Sequence Number | 16 | CHANGING | + | Timestamp | 32 | CHANGING | + | SSRC | 32 | STATIC-DEF | + | CSRC | 0(-480) | CHANGING | + +-----------------+-------------+----------------+ + + + + +Bormann, et al. Standards Track [Page 157] + +RFC 3095 Robust Header Compression July 2001 + + + Version + + Only one working RTP version exists, namely version 2. The field + is therefore classified as STATIC-KNOWN. + + Padding + + The use of this field is application-dependent, but when payload + padding is used it is likely to be present in all packets. The + field is therefore classified as STATIC. + + Extension + + If RTP extensions are used by the application, these extensions + are likely to be present in all packets (but the use of extensions + is very uncommon). However, for safety's sake this field is + classified as STATIC and not STATIC-KNOWN. + + SSRC + + This field is part of the definition of a stream and must thus be + constant for all packets in the stream. The field is therefore + classified as STATIC-DEF. + + Total size of the fields in each class: + + +--------------+---------------+ + | Class | Size (octets) | + +--------------+---------------+ + | STATIC | 2 bits | + | STATIC-DEF | 4 | + | STATIC-KNOWN | 2 bits | + | CHANGING | 7.5(-67.5) | + +--------------+---------------+ + + + + + + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 158] + +RFC 3095 Robust Header Compression July 2001 + + +A.1.5. Summary for IP/UDP/RTP + + Summarizing this for IP/UDP/RTP one obtains + + +----------------+----------------+----------------+ + | Class \ IP ver | IPv6 (octets) | IPv4 (octets) | + +----------------+----------------+----------------+ + | INFERRED | 4 | 6 | + | STATIC | 1 oct + 6 bits | 1 oct + 7 bits | + | STATIC-DEF | 42.5 | 16 | + | STATIC-KNOWN | 2 bits | 2 oct + 5 bits | + | CHANGING | 11.5(-71.5) | 13.5(-73.5) | + +----------------+----------------+----------------+ + | Total | 60(-120) | 40(-100) | + +----------------+----------------+----------------+ + +A.2. Analysis of change patterns of header fields + + To design suitable mechanisms for efficient compression of all header + fields, their change patterns must be analyzed. For this reason, an + extended classification is done based on the general classification + in A.1, considering the fields which were labeled CHANGING in that + classification. Different applications will use the fields in + different ways, which may affect their behavior. For the fields + whose behavior is variable, typical behavior for conversational audio + and video will be discussed. + + The CHANGING fields are separated into five different subclasses: + + STATIC These are fields that were classified as + CHANGING on a general basis, but are classified + as STATIC here due to certain additional + assumptions. + + SEMISTATIC These fields are STATIC most of the time. + However, occasionally the value changes but + reverts to its original value after a known + number of packets. + + RARELY-CHANGING (RC) These are fields that change their values + occasionally and then keep their new values. + + ALTERNATING These fields alternate between a small number + of different values. + + IRREGULAR These, finally, are the fields for which no + useful change pattern can be identified. + + + + +Bormann, et al. Standards Track [Page 159] + +RFC 3095 Robust Header Compression July 2001 + + + To further expand the classification possibilities without increasing + complexity, the classification can be done either according to the + values of the field and/or according to the values of the deltas for + the field. + + When the classification is done, other details are also stated + regarding possible additional knowledge about the field values and/or + field deltas, according to the classification. For fields classified + as STATIC or SEMISTATIC, the case could be that the value of the + field is not only STATIC but also well KNOWN a priori (two states for + SEMISTATIC fields). For fields with non-irregular change behavior, + it could be known that changes usually are within a LIMITED range + compared to the maximal change for the field. For other fields, the + values are completely UNKNOWN. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 160] + +RFC 3095 Robust Header Compression July 2001 + + + Table A.1 classifies all the CHANGING fields on the basis of their + expected change patterns, especially for conversational audio and + video. + + +------------------------+-------------+-------------+-------------+ + | Field | Value/Delta | Class | Knowledge | + +========================+=============+=============+=============+ + | Sequential | Delta | STATIC | KNOWN | + | -----------+-------------+-------------+-------------+ + | IPv4 Id: Seq. jump | Delta | RC | LIMITED | + | -----------+-------------+-------------+-------------+ + | Random | Value | IRREGULAR | UNKNOWN | + +------------------------+-------------+-------------+-------------+ + | IP TOS / Tr. Class | Value | RC | UNKNOWN | + +------------------------+-------------+-------------+-------------+ + | IP TTL / Hop Limit | Value | ALTERNATING | LIMITED | + +------------------------+-------------+-------------+-------------+ + | Disabled | Value | STATIC | KNOWN | + | UDP Checksum: ---------+-------------+-------------+-------------+ + | Enabled | Value | IRREGULAR | UNKNOWN | + +------------------------+-------------+-------------+-------------+ + | No mix | Value | STATIC | KNOWN | + | RTP CSRC Count: -------+-------------+-------------+-------------+ + | Mixed | Value | RC | LIMITED | + +------------------------+-------------+-------------+-------------+ + | RTP Marker | Value | SEMISTATIC | KNOWN/KNOWN | + +------------------------+-------------+-------------+-------------+ + | RTP Payload Type | Value | RC | UNKNOWN | + +------------------------+-------------+-------------+-------------+ + | RTP Sequence Number | Delta | STATIC | KNOWN | + +------------------------+-------------+-------------+-------------+ + | RTP Timestamp | Delta | RC | LIMITED | + +------------------------+-------------+-------------+-------------+ + | No mix | - | - | - | + | RTP CSRC List: -------+-------------+-------------+-------------+ + | Mixed | Value | RC | UNKNOWN | + +------------------------+-------------+-------------+-------------+ + + Table A.1 : Classification of CHANGING header fields + + The following subsections discuss the various header fields in + detail. Note that table A.1 and the discussions below do not + consider changes caused by loss or reordering before the compression + point. + + + + + + + +Bormann, et al. Standards Track [Page 161] + +RFC 3095 Robust Header Compression July 2001 + + +A.2.1. IPv4 Identification + + The Identification field (IP ID) of the IPv4 header is there to + identify which fragments constitute a datagram when reassembling + fragmented datagrams. The IPv4 specification does not specify + exactly how this field is to be assigned values, only that each + packet should get an IP ID that is unique for the source-destination + pair and protocol for the time the datagram (or any of its fragments) + could be alive in the network. This means that assignment of IP ID + values can be done in various ways, which we have separated into + three classes. + + Sequential jump + + This is the most common assignment policy in today's IP stacks. A + single IP ID counter is used for all packet streams. When the + sender is running more than one packet stream simultaneously, the + IP ID can increase by more than one between packets in a stream. + The IP ID values will be much more predictable and require less + bits to transfer than random values, and the packet-to-packet + increment (determined by the number of active outgoing packet + streams and sending frequencies) will usually be limited. + + Random + + Some IP stacks assign IP ID values using a pseudo-random number + generator. There is thus no correlation between the ID values of + subsequent datagrams. Therefore there is no way to predict the IP + ID value for the next datagram. For header compression purposes, + this means that the IP ID field needs to be sent uncompressed + with each datagram, resulting in two extra octets of header. IP + stacks in cellular terminals SHOULD NOT use this IP ID assignment + policy. + + Sequential + + This assignment policy keeps a separate counter for each outgoing + packet stream and thus the IP ID value will increment by one for + each packet in the stream, except at wrap around. Therefore, the + delta value of the field is constant and well known a priori. + When RTP is used on top of UDP and IP, the IP ID value follows + the RTP Sequence Number. This assignment policy is the most + desirable for header compression purposes. However, its usage is + not as common as it perhaps should be. The reason may be that it + can be realized only when UDP and IP are implemented together so + that UDP, which separates packet streams by the Port + identification fields, can make IP use separate ID counters for + each packet stream. + + + +Bormann, et al. Standards Track [Page 162] + +RFC 3095 Robust Header Compression July 2001 + + + In order to avoid violating [IPv4], packets sharing the same IP + address pair and IP protocol number cannot use the same IP ID + values. Therefore, implementations of sequential policies must + make the ID number spaces disjoint for packet streams of the same + IP protocol going between the same pair of nodes. This can be + done in a number of ways, all of which introduce occasional + jumps, and thus makes the policy less than perfectly sequential. + For header compression purposes less frequent jumps are + preferred. + + It should be noted that the ID is an IPv4 mechanism and is therefore + not a problem for IPv6. For IPv4 the ID could be handled in three + different ways. First, we have the inefficient but reliable solution + where the ID field is sent as-is in all packets, increasing the + compressed headers by two octets. This is the best way to handle the + ID field if the sender uses random assignment of the ID field. + Second, there can be solutions with more flexible mechanisms + requiring less bits for the ID handling as long as sequential jump + assignment is used. Such solutions will probably require even more + bits if random assignment is used by the sender. Knowledge about the + sender's assignment policy could therefore be useful when choosing + between the two solutions above. Finally, even for IPv4, header + compression could be designed without any additional information for + the ID field included in compressed headers. To use such schemes, it + must be known which assignment policy for the ID field is being used + by the sender. That might not be possible to know, which implies + that the applicability of such solutions is very uncertain. However, + designers of IPv4 stacks for cellular terminals SHOULD use an + assignment policy close to sequential. + +A.2.2. IP Traffic-Class / Type-Of-Service + + The Traffic-Class (IPv6) or Type-Of-Service (IPv4) field is expected + to be constant during the lifetime of a packet stream or to change + relatively seldom. + +A.2.3. IP Hop-Limit / Time-To-Live + + The Hop-Limit (IPv6) or Time-To-Live (IPv4) field is expected to be + constant during the lifetime of a packet stream or to alternate + between a limited number of values due to route changes. + +A.2.4. UDP Checksum + + The UDP checksum is optional. If disabled, its value is constantly + zero and could be compressed away. If enabled, its value depends on + the payload, which for compression purposes is equivalent to it + changing randomly with every packet. + + + +Bormann, et al. Standards Track [Page 163] + +RFC 3095 Robust Header Compression July 2001 + + +A.2.5. RTP CSRC Counter + + This is a counter indicating the number of CSRC items present in the + CSRC list. This number is expected to be almost constant on a + packet- to-packet basis and change by small amounts. As long as no + RTP mixer is used, the value of this field is zero. + +A.2.6. RTP Marker + + For audio the marker bit should be set only in the first packet of a + talkspurt, while for video it should be set in the last packet of + every picture. This means that in both cases the RTP marker is + classified as SEMISTATIC with well-known values for both states. + +A.2.7. RTP Payload Type + + Changes of the RTP payload type within a packet stream are expected + to be rare. Applications could adapt to congestion by changing + payload type and/or frame sizes, but that is not expected to happen + frequently. + +A.2.8. RTP Sequence Number + + The RTP Sequence Number will be incremented by one for each packet + sent. + +A.2.9. RTP Timestamp + + In the audio case: + + As long as there are no pauses in the audio stream, the RTP + Timestamp will be incremented by a constant delta, corresponding + to the number of samples in the speech frame. It will thus mostly + follow the RTP Sequence Number. When there has been a silent + period and a new talkspurt begins, the timestamp will jump in + proportion to the length of the silent period. However, the + increment will probably be within a relatively limited range. + + In the video case: + + Between two consecutive packets, the timestamp will either be + unchanged or increase by a multiple of a fixed value corresponding + to the picture clock frequency. The timestamp can also decrease + by a multiple of the fixed value if B-pictures are used. The + delta interval, expressed as a multiple of the picture clock + frequency, is in most cases very limited. + + + + + +Bormann, et al. Standards Track [Page 164] + +RFC 3095 Robust Header Compression July 2001 + + +A.2.10. RTP Contributing Sources (CSRC) + + The participants in a session, which are identified by the CSRC + fields, are expected to be almost the same on a packet-to-packet + basis with relatively few additions and removals. As long as RTP + mixers are not used, no CSRC fields are present at all. + +A.3. Header compression strategies + + This section elaborates on what has been done in previous sections. + On the basis of the classifications, recommendations are given on how + to handle the various fields in the header compression process. + Seven different actions are possible; these are listed together with + the fields to which each action applies. + +A.3.1. Do not send at all + + The fields that have well known values a priori do not have to be + sent at all. These are: + + - IPv6 Payload Length + - IPv4 Header Length + - IPv4 Reserved Flag + - IPv4 Last Fragment Flag + - IPv4 Fragment Offset + + - UDP Checksum (if disabled) + - RTP Version + +A.3.2. Transmit only initially + + The fields that are constant throughout the lifetime of the packet + stream have to be transmitted and correctly delivered to the + decompressor only once. These are: + + - IP Version + - IP Source Address + - IP Destination Address + - IPv6 Flow Label + - IPv4 May Fragment Flag + - UDP Source Port + - UDP Destination Port + - RTP Padding Flag + - RTP Extension Flag + - RTP SSRC + + + + + + +Bormann, et al. Standards Track [Page 165] + +RFC 3095 Robust Header Compression July 2001 + + +A.3.3. Transmit initially, but be prepared to update + + The fields that are changing only occasionally must be transmitted + initially but there must also be a way to update these fields with + new values if they change. These fields are: + + - IPv6 Next Header + - IPv6 Traffic Class + - IPv6 Hop Limit + - IPv4 Protocol + - IPv4 Type Of Service (TOS) + - IPv4 Time To Live (TTL) + - RTP CSRC Counter + - RTP Payload Type + - RTP CSRC List + + Since the values of the IPv4 Protocol and the IPv6 Next Header fields + are in effect linked to the type of the subsequent header, they + deserve special treatment when subheaders are inserted or removed. + +A.3.4. Be prepared to update or send as-is frequently + + For fields that normally either are constant or have values deducible + from some other field, but that frequently diverge from that + behavior, there must be an efficient way to update the field value or + send it as-is in some packets. These fields are: + + - IPv4 Identification (if not sequentially assigned) + - RTP Marker + - RTP Timestamp + +A.3.5. Guarantee continuous robustness + + For fields that behave like a counter with a fixed delta for ALL + packets, the only requirement on the transmission encoding is that + packet losses between compressor and decompressor must be tolerable. + If several such fields exist, all these can be communicated together. + Such fields can also be used to interpret the values for fields + listed in the previous section. Fields that have this counter + behavior are: + + - IPv4 Identification (if sequentially assigned) + - RTP Sequence Number + + + + + + + + +Bormann, et al. Standards Track [Page 166] + +RFC 3095 Robust Header Compression July 2001 + + +A.3.6. Transmit as-is in all packets + + Fields that have completely random values for each packet must be + included as-is in all compressed headers. Those fields are: + + - IPv4 Identification (if randomly assigned) + - UDP Checksum (if enabled) + +A.3.7. Establish and be prepared to update delta + + Finally, there is a field that is usually increasing by a fixed delta + and is correlated to another field. For this field it would make + sense to make that delta part of the context state. The delta must + then be initiated and updated in the same way as the fields listed in + A.3.3. The field to which this applies is: + + - RTP Timestamp + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 167] + +RFC 3095 Robust Header Compression July 2001 + + +Full Copyright Statement + + Copyright (C) The Internet Society (2001). All Rights Reserved. + + This document and translations of it may be copied and furnished to + others, and derivative works that comment on or otherwise explain it + or assist in its implementation may be prepared, copied, published + and distributed, in whole or in part, without restriction of any + kind, provided that the above copyright notice and this paragraph are + included on all such copies and derivative works. However, this + document itself may not be modified in any way, such as by removing + the copyright notice or references to the Internet Society or other + Internet organizations, except as needed for the purpose of + developing Internet standards in which case the procedures for + copyrights defined in the Internet Standards process must be + followed, or as required to translate it into languages other than + English. + + The limited permissions granted above are perpetual and will not be + revoked by the Internet Society or its successors or assigns. + + This document and the information contained herein is provided on an + "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING + TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING + BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION + HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF + MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. + +Acknowledgement + + Funding for the RFC Editor function is currently provided by the + Internet Society. + + + + + + + + + + + + + + + + + + + +Bormann, et al. Standards Track [Page 168] + |