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+Network Working Group D. Schremp
+Request for Comments: 1975 J. Black
+Category: Informational J. Weiss
+ Magnalink
+ August 1996
+
+ PPP Magnalink Variable Resource Compression
+
+Status of This Memo
+
+ This memo provides information for the Internet community. This memo
+ does not specify an Internet standard of any kind. Distribution of
+ this memo is unlimited.
+
+Abstract
+
+ The Point-to-Point Protocol (PPP) [1] provides a standard method of
+ encapsulating multiple protocol datagrams over point-to-point links.
+ The PPP Compression Control Protocol [2] provides a method for
+ negotiating data compression over PPP links.
+
+ The Magnalink Variable Resource Compression Algorithm (MVRCA) allows
+ a wide range of interoperable compression implementations whose
+ performance characteristics are a function of available CPU and
+ memory resources.
+
+Introduction
+
+ The Magnalink variable resource compression algorithm defines a
+ family of interoperable compression solutions with compression
+ performance as a function of available CPU and memory resources. It
+ addresses the need for an algorithm which can be tailored to the
+ system on which it is implemented without compromising
+ interoperability.
+
+Licensing
+
+ Source licenses are available on a non-discriminatory basis.
+
+ The contact person for evaluation under NDA and Licensing is:
+
+ Director of OEM Sales
+ Magnalink Communications Division
+ Telco Systems Inc.
+ 63 Nahatan Street
+ Norwood, Mass. 02062
+ Phone: (617) 255-9400, Fax: (617) 255-5885
+ oem@magna.telco.com
+
+
+
+Schremp, Black & Weiss Informational [Page 1]
+
+RFC 1975 PPP Magnalink Variable Resource Compression August 1996
+
+
+MVRCA Packets
+
+ Before any MVRCA packets may be communicated, PPP must reach the
+ Network-Layer Protocol phase[1], and the Compression Control Protocol
+ must reach the Opened state.
+
+ The text of a Packet to be compressed begins with PPP Protocol
+ number. The Packet header including the PPP Protocol number may have
+ already been compressed when Protocol-Field-Compression has been
+ negotiated.
+
+Reliability and Sequencing
+
+ MVRCA packets may be sent across an unreliable link or may use a
+ reliable link as described in "PPP Reliable Transmission"[3] if the
+ reliable link has been negotiated. If frames are delivered out of
+ order or a frame is dropped, the decompressor will detect this and
+ requests a resynchronization using the Reset-Req and Reset-Ack types
+ of the CCP[2], with the compressor for the affected context.
+
+Data Expansion
+
+ Although the compression algorithm may occasionally expand a data
+ packet, there is no expansion in MVRCA since any expanded data is
+ instead sent uncompressed. Dictionary synchronization is maintained
+ across uncompressed packets.
+
+Encapsulation
+
+ The encapsulation consists of the PPP Protocol Identifier, a bit to
+ indicate if the data is compressed, the Context Identifier(CID), a
+ proprietary flag bit (E), a Packet Integrity Byte(PIB), and the
+ Compressed data.
+
+ 0 1
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | PPP Protocol Identifier |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ |C|E| CID | PIB | C compressed flag
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 1 data is compressed
+ | Compressed data ... 0 data is not compressed
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+
+
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+Schremp, Black & Weiss Informational [Page 2]
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+RFC 1975 PPP Magnalink Variable Resource Compression August 1996
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+Compressed/Uncompressed Flag (C)
+
+ When attempting to compress certain types of Packets or Fragments the
+ compressor may not be effective. When this occurs the uncompressed
+ data is added to the compression History Buffer and sent across the
+ link in frame with the Compressed/Uncompressed Flag(C) set to 0.
+
+Context Identifier (CID)
+
+ Since PPP will transport multiple protocol datagrams it may be
+ advantageous to compress each protocol or each virtual circuit in a
+ different History Buffer or Context. The CID allows the compressor to
+ indicate to the decompressor which History Buffer the compressor
+ decided to use for a given Packet. The basis of this decision is up
+ to the implementor. The number of buffers and size of each buffer is
+ negotiated.
+
+ A CID of 0 indicates that the Packet by Packet context will be used
+ if it has been negotiated. The Packet by Packet context is cleared
+ between Packets so that this History Buffer is not maintained across
+ Packet boundaries.
+
+Packet Integrity Byte (PIB)
+
+ To ensure that Packets are being compressed and decompressed
+ correctly and to ensure History Buffer synchronization is maintained,
+ a Packet Integrity Byte is added to the packet header.
+
+ The packet integrity byte is defined in the full protocol
+ specification.
+
+Configuration Option Format
+
+ Description
+
+ The CCP MVRCA Configuration Option negotiates the use of MVRCA on the
+ link. By default or ultimate disagreement, no compression is used.
+
+ 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
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | Type | Length |FE |P| History | # Contexts |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+
+
+
+
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+Schremp, Black & Weiss Informational [Page 3]
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+RFC 1975 PPP Magnalink Variable Resource Compression August 1996
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+ Type
+
+ 24
+
+ Length
+
+ 4
+
+
+ FE - Features
+
+ Negotiates features specific to this compression algorithm.
+
+
+ History
+
+ Defines the size of the compression history buffer. Valid values are
+ defined in the full protocol specification.
+
+
+ # Contexts
+
+ This is the number of contexts. Each context implies the creation of
+ a History Buffer for that context of the size indicated in the
+ Context History field. Values are 1-63. This value includes both
+ the Packet by Packet context and the number of contexts for which
+ history is maintained. Therefore, when this value is 1 and the P
+ (Packet by Packet) flag is also 1, then only in packet compression is
+ supported and history context is not retained across packet
+ boundaries. The Context Identifier (CID) starts with 1 for contexts
+ where the history is maintained.
+
+
+ P - Packet by Packet flag
+
+ When 1, packet by packet compression is enabled for the context whose
+ context ID is 0. When P is 0, packet by packet compression is not
+ supported.
+
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+Schremp, Black & Weiss Informational [Page 4]
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+RFC 1975 PPP Magnalink Variable Resource Compression August 1996
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+Security Considerations
+
+ Security issues are not discussed in this memo.
+
+References
+
+ [1] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51,
+ RFC 1661, July 1994.
+
+ [2] Rand, D., "The PPP Compression Control Protocol (CCP)", RFC 1962,
+ June 1996.
+
+ [3] Rand, D., "PPP Reliable Transmission", RFC 1663, July 1994.
+
+Acknowledgments
+
+Chair's Address
+
+ The working group can be contacted via the current chair:
+
+ Karl Fox
+ Ascend Communications
+ 3518 Riverside Drive, Suite 101
+ Columbus, Ohio 43221
+
+ EMail: karl@ascend.com
+
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+Schremp, Black & Weiss Informational [Page 5]
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+RFC 1975 PPP Magnalink Variable Resource Compression August 1996
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+Authors' Addresses
+
+ Comments about this document may also be directed to the authors.
+
+ Doug Schremp
+ Telco Systems, Inc.
+ Magnalink Communications Division
+ 63 Nahatan Street
+ Norwood Ma. 02062
+
+ Phone: (617) 255-9400
+ EMail: dhs@magna.telco.com
+
+
+ Jeffrey Black
+ Telco Systems, Inc.
+ Magnalink Communications Division
+ 63 Nahatan Street
+ Norwood Ma. 02062
+
+ Phone: (617) 255-9400
+ EMail: jtb@magna.telco.com
+
+
+ Jeffrey Weiss
+ Telco Systems, Inc.
+ Magnalink Communications Division
+ 63 Nahatan Street
+ Norwood Ma. 02062
+
+ Phone: (617) 255-9400
+ EMail: jaw@magna.telco.com
+
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+Schremp, Black & Weiss Informational [Page 6]
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