path: root/doc/rfc/rfc5506.txt

Network Working Group                                       I. Johansson
Request for Comments: 5506                                 M. Westerlund
Updates: 3550, 3711, 4585                                    Ericsson AB
Category: Standards Track                                     April 2009


 Support for Reduced-Size Real-Time Transport Control Protocol (RTCP):
                     Opportunities and Consequences

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) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
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   Without obtaining an adequate license from the person(s) controlling
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.












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Abstract

   This memo discusses benefits and issues that arise when allowing
   Real-time Transport Protocol (RTCP) packets to be transmitted with
   reduced size.  The size can be reduced if the rules on how to create
   compound packets outlined in RFC 3550 are removed or changed.  Based
   on that analysis, this memo defines certain changes to the rules to
   allow feedback messages to be sent as Reduced-Size RTCP packets under
   certain conditions when using the RTP/AVPF (Real-time Transport
   Protocol / Audio-Visual Profile with Feedback) profile (RFC 4585).
   This document updates RFC 3550, RFC 3711, and RFC 4585.

Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. Use Cases and Design Rationale ..................................4
      3.1. RTCP Compound Packets (Background) .........................4
      3.2. Use Cases for Reduced-Size RTCP ............................6
      3.3. Benefits of Reduced-Size RTCP ..............................7
      3.4. Issues with Reduced-Size RTCP ..............................8
           3.4.1. Middle Boxes ........................................9
           3.4.2. Packet Validation ...................................9
           3.4.3. Encryption/Authentication ..........................10
           3.4.4. RTP and RTCP Multiplex on the Same Port ............10
           3.4.5. Header Compression .................................11
   4. Use of Reduced-Size RTCP with AVPF .............................11
      4.1. Definition of Reduced-Size RTCP ...........................12
      4.2. Algorithm Considerations ..................................12
           4.2.1. Verification of Delivery ...........................12
           4.2.2. Single vs Multiple RTCP in a Reduced-Size RTCP .....13
           4.2.3. Enforcing Compound RTCP ............................13
           4.2.4. Immediate Feedback Mode ............................14
   5. Signaling ......................................................14
   6. Security Considerations ........................................14
   7. IANA Considerations ............................................14
   8. Acknowledgements ...............................................15
   9. References .....................................................15
      9.1. Normative References ......................................15
      9.2. Informative References ....................................16











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1.  Introduction

   In RTP [RFC3550] it is currently mandatory to send RTP Control
   Protocol (RTCP) packets as compound packets containing at least a
   sender report (SR) or receiver report (RR), followed by a source
   description (SDES) packet containing at least the CNAME item.  There
   are good reasons for this, as discussed below (see Section 3.1);
   however, it does result in the minimal RTCP packets being quite
   large.

   The RTP/AVPF profile [RFC4585] specifies new RTCP packet types for
   feedback messages.  Some of these feedback messages would benefit
   from being transmitted with minimal delay.  AVPF provides some
   mechanisms to support this; however, for environments with low-
   bitrate links, these messages can still consume a large amount of
   resources and can introduce extra delay in the time it takes to
   completely send the compound packet in the network.  It is therefore
   desirable to send just the feedback, without the other parts of a
   compound RTCP packet.  This memo proposes such a mechanism for this
   and other use cases, as discussed in Section 3.2.

   There are a number of benefits with Reduced-Size RTCP; these are
   discussed in Section 3.3.

   The use of Reduced-Size RTCP is not without issues.  This is
   discussed in Section 3.4.  These issues need to be considered and are
   part of the motivation for this document.

   Finally, this document defines how AVPF is updated to allow for the
   transmission of Reduced-Size RTCP in a way that would not
   substantially affect the mechanisms that compound packets provide;
   see Section 4 for more details.  The connection to AVPF (or SAVPF
   [RFC5124]) is motivated by the fact that Reduced-Size RTCP is mainly
   beneficial for event-driven feedback purposes and that the AVPF Early
   RTCP and Immediate Feedback modes make this possible.

   This document updates [RFC3550], [RFC3711], and [RFC4585].

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 [RFC2119].

   The naming convention for RTCP is often confusing.  Below is a list
   of RTCP terms and what they mean.  See also Section 6.1 in [RFC3550]
   and Section 3.1 in [RFC4585] for details.




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   RTCP packet:  Can be of different types, contains a fixed header part
      followed by structured elements depending on RTCP packet type.

   Lower layer datagram:  Can be interpreted as the UDP payload.  It may
      however, depending on the transport, be a TCP or Datagram
      Congestion Control Protocol (DCCP) payload or something else.
      Synonymous to the "underlying protocol" defined in Section 3 in
      [RFC3550].

   Compound RTCP packet:  A collection of two or more RTCP packets.  A
      compound RTCP packet is transmitted in a lower-layer datagram.  It
      must contain at least an RTCP RR or SR packet and an SDES packet
      with the CNAME item.  Often "compound" is left out, the
      interpretation of RTCP packet is therefore dependent on the
      context.

   Minimal compound RTCP packet:  A compound RTCP packet that contains
      the RTCP RR or SR packet and the SDES packet with the CNAME item
      with a specified ordering.

   (Full) compound RTCP packet:  A compound RTCP packet that conforms to
      the requirements on minimal compound RTCP packets and contains
      more RTCP packets.

   Reduced-Size RTCP packet:  May contain one or more RTCP packets but
      does not follow the compound RTCP rules defined in Section 6.1 in
      [RFC3550] and is thus neither a minimal nor a full compound RTCP.
      See Section 4.1 for a full definition.

3.  Use Cases and Design Rationale

3.1.  RTCP Compound Packets (Background)

   Section 6.1 in [RFC3550] specifies that an RTCP packet must be sent
   as a compound RTCP packet consisting of at least two individual RTCP
   packets, first a sender report (SR) or receiver report (RR), followed
   by additional packets including a mandatory SDES packet containing a
   CNAME item for the transmitting source identifier.  Below is a short
   description of what these RTCP packet types are used for.

   1.  The sender and receiver reports (see Section 6.4 of [RFC3550])
       provide the RTP session participant with the synchronization
       source (SSRC) identifier of all RTP session participants.  Having
       all participants send these packets periodically allows everyone
       to determine the current number of participants.  This
       information is used in the transmission scheduling algorithm.
       Thus, this is particularly important for new participants so that




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       they can quickly establish a good estimate of the group size.
       Failure to do this would result in RTCP senders consuming too
       much bandwidth.

   2.  Before a new session participant has sent any RTP or RTCP packet,
       it can also avoid SSRC collisions with all the SSRCs it sees
       prior to that transmission.  So the possibility to see a
       substantial proportion of the participating sources minimizes the
       risk of any collision when selecting SSRC.

   3.  The sender and receiver reports contain some basic statistics
       usable for monitoring of the transport and thus enable
       adaptation.  These reports become more useful if sent regularly,
       as the receiver of a report can perform analyses to find trends
       between the individual reports.  When used for media transmission
       adaptation, the information becomes more useful the more
       frequently it is received, at least until one report per round-
       trip time (RTT) is achieved.  Therefore, there is, in most cases,
       no reason not to include the sender or receiver report in all
       RTCP packets.

   4.  The CNAME SDES item (see Section 6.5.1 of [RFC3550]) exists to
       allow receivers to determine which media flows should be
       synchronized with each other, both within an RTP session and
       between different RTP sessions carrying different media types.
       Thus, it is important to quickly receive this for each media
       sender in the session when joining an RTP session.

   5.  Sender reports (SR) are used in combination with the above SDES
       CNAME mechanism to synchronize multiple RTP streams, such as
       audio and video.  After having determined which media streams
       should be synchronized using the CNAME field, the receiver uses
       the sender report's NTP and RTP timestamp fields to establish
       synchronization.

   6.  The CNAME SDES item also allows a session participant to detect
       SSRC collisions and separate them from routing loops.  The 32-
       bit, randomly selected SSRC has some probability of collision.
       The CNAME is used as the longer canonical identifier of a
       particular endpoint instance that is bound to an SSRC.  If that
       binding isn't received and kept current, the receiver may not
       detect an SSRC collision, i.e., two different CNAMEs using the
       same SSRC.  It also can't detect an RTP-level routing loop, with
       the result that the same SSRC and CNAME arrive from multiple
       lower-layer source addresses.






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   Reviewing the above, it is obvious that both SR/RR and the CNAME are
   very important in order for new session participants to be able to
   utilize any received media and to avoid flooding the network with
   RTCP reports.  In addition, the dynamic nature of the information
   provided would make it less useful if not sent regularly.

   The following sections will describe the cases when Reduced-Size RTCP
   is beneficial and will also show the possible issues that must be
   considered.

3.2.  Use Cases for Reduced-Size RTCP

   Below are listed a few use cases for Reduced-Size RTCP.

   Control Plane Signaling:  The Open Mobile Alliance (OMA) Push-to-talk
      over Cellular (PoC) [OMA-PoC] makes use of Reduced-Size RTCP when
      transmitting certain events.  The OMA PoC service is primarily
      used over cellular links capable of IP transport, such as the
      Global System for Mobile Connections (GSM) General Packet Radio
      Service (GPRS).

   Codec Control Signaling:  An example that can be used with Reduced-
      Size RTCP is, e.g., Temporary Maximum Media Stream Bitrate Request
      (TMMBR) messages as specified in [RFC5104], which signal a request
      for a change in codec bitrate.  These messages benefit from the
      usage of Reduced-Size RTCP in bad channel conditions, as Reduced-
      Size RTCP are much more likely to be successfully transmitted than
      larger compound RTCP.  This is critical, as these messages are
      likely to occur when channel conditions are poor.  Other examples
      of codec control usage for Reduced-Size RTCP are found in
      [MTSI-3GPP].

   Feedback:  An example of a feedback scenario that would benefit from
      Reduced-Size RTCP is Video streams with generic NACK.  In cases
      where the RTT is shorter than the receiver buffer depth, generic
      NACK can be used to request retransmission of missing packets,
      thus improving playout quality considerably.  If the generic NACK
      packets are transmitted as Reduced-Size RTCP, the bandwidth
      requirement for RTCP will be minimal, enabling more frequent
      feedback.  As in the codec control case, it is important that
      these packets can be transmitted with as little delay as possible.
      Another interesting use for Reduced-Size RTCP is in cases when
      regular feedback is needed, as described in Section 3.3.

   Status Reports:  One proposed idea is to transmit small measurement
      or status reports in Reduced-Size RTCP, and to split the minimal
      compound RTCP and transmit the individual RTCP separately.  The
      status reports can be used either by the endpoints or by other



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      network monitoring boxes in the network.  The benefit is that,
      with some radio access technologies, small packets are more robust
      to poor radio conditions than large packets.  Additionally, with
      small (report) packets, there is a smaller risk that the report
      packets will affect the channel upon which they report.  Another
      benefit is that it is possible, with Reduced-Size RTCP, to allow,
      for example, anonymous status reporting to be transmitted
      unencrypted.  This is something that may be beneficial, for
      instance, for network monitoring purposes.

3.3.  Benefits of Reduced-Size RTCP

   As mentioned in the introduction, most advantages of using Reduced-
   Size RTCP packets exist in cases when the available RTCP bitrate is
   limited.  This is because they can become substantially smaller than
   compound packets.  A compound packet is forced to contain both an RR
   or an SR and the CNAME SDES item.  The RR containing a report block
   for a single source is 32 bytes, an SR is 52 bytes.  Both may be
   larger if they contain report blocks for multiple sources.  The SDES
   packet containing a CNAME item will be 10 bytes plus the CNAME string
   length.  Here, it is reasonable that the CNAME string is at least 10
   bytes to get a decent collision resistance.  If the recommended form
   of user@host is used, then most strings will be longer than 20
   characters.  Thus, a Reduced-Size RTCP can become at least 70-80
   bytes smaller than the compound packet.

   For low bitrate links, the benefits of this reduction in size are as
   follows:

   o  For links where the packet-loss rate grows with the packet size,
      smaller packets are less likely to be dropped.  Radio links are an
      example of such links.  In the cellular world, there exist links
      that are optimized to handle RTP packets sized for carrying
      compressed speech.  This increases the capacity and coverage for
      voice services in a given wireless network.  Minimal compound RTCP
      packets are commonly 2-3 times the size of an RTP packet carrying
      compressed speech.  If the speech packet over such a bearer has a
      packet-loss probability of p, then the RTCP packet will experience
      a loss probability of 1-(1-p)^x, where x is the number of
      fragments the compound packet will be split into on the link
      layer, i.e., commonly into 2 or 3 fragments.

   o  There is a shorter serialization time, i.e., the time it takes the
      link to transmit the packet.  For slower links, this time can be
      substantial.  For example, transmitting 120 bytes over a link
      interface capable of 30 kbps takes 32 milliseconds (ms), assuming
      uniform transmission rate.




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   In cases when Reduced-Size RTCP carries important and time-sensitive
   feedback, both shorter serialization time and the lower loss
   probability are important to enable the best possible functionality.
   Having a packet-loss rate that is much higher for the feedback
   packets than the media packets hurts when trying to perform media
   adaptation to, for example, handle the changed performance present at
   the cell border in a cellular system.

   For high-bitrate applications, there is usually no problem to supply
   RTCP with sufficient bitrates.  When using AVPF, one can use the
   "trr-int" parameter to restrict the regular reporting interval to
   approximately once per RTT or less often.  As in most cases, there is
   little reason to provide regular reports of higher density than this;
   any additional bandwidth can then be used for feedback messages.  The
   benefit of Reduced-Size RTCP in this case is limited, but exists.
   One typical example is video using generic NACK in cases where the
   RTT is low.  Using Reduced-Size RTCP would reduce the total amount of
   bits used for RTCP.  This is primarily applicable if the number of
   reports is large.  This would also result in lower processing delay
   and less complexity for the feedback packets, as they do not need to
   query the RTCP database to construct the right messages.

   As message size is generally a smaller issue at higher bitrates, it
   is also possible to transmit multiple RTCP in each lower-layer
   datagram in these cases.  The motivation behind Reduced-Size RTCP in
   this case is not size, rather it is to avoid the extra overhead
   caused by inclusion of the SR/RR and SDES CNAME items in each
   transmitted RTCP.

   Independently of the link type, there are additional benefits with
   sending feedback in small Reduced-Size RTCP.  Applications that use
   RTCP AVPF in Early RTCP or Immediate Feedback mode need to send
   frequent event-driven feedback.  Under these circumstances, the risk
   is reduced that the RTCP bandwidth becomes too high during periods of
   heavy feedback signaling.

   In cases when regular feedback is needed, such as the profile under
   development for TCP friendly rate control (TFRC) for RTP
   [TCP-FRIEND], the size of compound RTCPs can result in very high
   bandwidth requirements if the round-trip time is short.  For this
   particular application, Reduced-Size RTCP gives a very substantial
   improvement.

3.4.  Issues with Reduced-Size RTCP

   This section describes the known issues with Reduced-Size RTCP and
   also provides a brief analysis of their effects and mitigation.




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3.4.1.  Middle Boxes

   Middle boxes in the network may discard RTCP that do not follow the
   rules outlined in Section 6.1 of RFC 3550.  Newer report types may be
   interpreted as unknown by the middle box.  For instance, if the
   payload type number is 207 instead of 200 or 201, it may be treated
   as unknown.  The effect of this might, for instance, be that compound
   RTCP would get through while the Reduced-Size RTCP would be lost.

   Verification of the delivery of Reduced-Size RTCP is discussed in
   Section 4.2.1.

3.4.2.  Packet Validation

   A Reduced-Size RTCP packet will be discarded by the packet validation
   code in Appendix A of [RFC3550].  This has several impacts:

   Weakened Packet Validation:  The packet validation code needs to be
      rewritten to accept Reduced-Size RTCP.  In particular, this
      affects Section 9.1 in [RFC3550] in the sense that the header
      verification must take into account that the payload type numbers
      for the (first) RTCP in the lower-layer datagram may differ from
      200 or 201 (SR or RR).  One potential effect of this change is
      much weaker validation that received packets actually are RTCP and
      not packets of some other type being wrongly delivered.  Thus,
      some consideration should be given to ensure the best possible
      validation is available, for example, restricting Reduced-Size
      RTCP to contain only some specific RTCP packet types that are
      preferably signalled on a per-session basis.  However, the
      application of a security mechanism for source authentication on
      the packets will provide much stronger protection.

   Old RTP Receivers:  Any RTCP receiver without an updated packet
      validation code will discard the Reduced-Size RTCP, which means
      that the receiver will not see e.g., the contained feedback
      messages.  The effect of this depends on the type of feedback
      message and the role of the receiver.  For example, this may cause
      complete function loss in the case of attempting to send a
      Reduced-Size NACK message (see Section 6.2.1 of [RFC4585]) to a
      non-updated media sender in a session using the retransmission
      scheme defined by [RFC4588].  This type of discarding would also
      affect the feedback suppression defined in AVPF.  The result would
      be a partitioning of the receivers within the session, with the
      old receivers only seeing the compound RTCP feedback messages and
      the newer ones seeing both.  In this case, the old receivers may
      send feedback messages for events already reported on in Reduced-
      Size RTCP.




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   Bandwidth Considerations:  The discarding of Reduced-Size RTCP would
      affect the RTCP transmission calculation in that the avg_rtcp_size
      value would become larger than for RTP receivers that exclude the
      Reduced-Size RTCP in this calculation (assuming that Reduced-Size
      RTCP are smaller than compound ones).  Therefore, these senders
      would under-utilize the available bitrate and send with a longer
      interval than updated receivers.  For most sessions, this should
      not be an issue.  However, for sessions with a large portion of
      Reduced-Size RTCP, the updated receivers may time out non-updated
      senders prematurely.  This is, however, not likely to occur, as
      the time between compound RTCP transmissions needs to become 5
      times that used by the Reduced-Size RTCP senders for it to happen.

   Computation of avg_rtcp_size:  Long intervals between compound RTCP
      with many Reduced-Size RTCP in between may lead to a computation
      of a value for avg_rtcp_size that varies greatly over time.
      Investigation shows that although it varies, this is not enough of
      a problem to warrant further changes or complexities to the RTCP
      scheduling algorithm.

3.4.3.  Encryption/Authentication

   The Secure Real-time Transport Protocol (SRTP) presents a problem for
   Reduced-Size RTCP.  Section 3.4 of [RFC3711] states, "SRTCP MUST be
   given packets according to that requirement in the sense that the
   first part MUST be a sender report or a receiver report".

   Upon examination of how SRTP processes packets, it becomes obvious
   that SRTP has no real dependency on whether the first packet is an SR
   or an RR packet.  What is needed is the common RTCP packet header,
   which is present in all the packet types, with a source SSRC.  The
   conclusion is therefore that it is possible to use Reduced-Size RTCP
   with SRTP.  Nevertheless, as this implies a change to the rules in
   [RFC3711], changes in SRTP implementations MAY become necessary.

3.4.4.  RTP and RTCP Multiplex on the Same Port

   In applications in which multiplex RTP and RTCP are on the same port,
   as defined in [MULTI-RTP], care must be taken to ensure that de-
   multiplexing is done properly even though the RTCP packets are
   reduced size.  The downside of Reduced-Size RTCP is that more values
   representing RTCP packets exist, reducing the available RTP payload
   type space.  However, Section 4 in [MULTI-RTP] already requires the
   corresponding RTP payload type range not be used when performing this
   multiplexing.






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3.4.5.  Header Compression

   Two issues are related to header compression.  Possible changes are
   left for future work:

   o  Payload type number identification: The Robust Header Compression
      (RoHC) algorithm [RFC3095] needs to create different compression
      contexts for RTP and RTCP for optimum performance.  If RTP and
      RTCP are multiplexed on the same port, the classification may be
      based on payload type numbers.  The classification algorithm must
      here acknowledge the fact that the payload type number for (the
      first) RTCP may differ from 200 or 201.

   o  Compression of RTCP: No IETF-defined header compression method
      compress RTCP; however, if such methods are developed in the
      future, these methods must take Reduced-Size RTCP in account.

4.  Use of Reduced-Size RTCP with AVPF

   Based on the above analysis, it seems feasible to allow transmission
   of Reduced-Size RTCP under some restrictions:

   o  First of all, it is important that compound RTCPs are transmitted
      at regular intervals to ensure that the mechanisms maintained by
      the compound packets, like feedback reporting, work.  The tracking
      of session size and number of participants warrants mentioning
      again, as this ensures that the RTCP bandwidth remains bounded
      independent of the number of session participants.

   o  Second, as the compound RTCP packets are also used to establish
      and maintain synchronization between media, any newly joining
      participant in a session would need to receive compound RTCP from
      the media sender(s).

   This implies that the regular transmission of compound RTCP MUST be
   maintained throughout an RTP session.  Reduced-Size RTCP SHOULD be
   restricted to be used as extra RTCP (e.g., feedback), sent in cases
   when a regular compound RTCP packet would not otherwise have been
   sent.

   The usage of Reduced-Size RTCP SHALL only be done in RTP sessions
   operating in AVPF [RFC4585] or SAVPF [RFC5124] Early RTCP or
   Immediate Feedback mode.  Reduced-Size RTCP SHALL NOT be sent until
   at least one compound RTCP has been sent.  In Immediate Feedback
   mode, all feedback messages MAY be sent as Reduced-Size RTCP.  In
   Early RTCP mode, a feedback message scheduled for transmission as an





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   Early RTCP, i.e., not a Regular RTCP, MAY be sent as Reduced-Size
   RTCP.  All RTCP that are scheduled for transmission as Regular RTCP
   SHALL be sent as compound RTCP as indicated by AVPF [RFC4585].

4.1.  Definition of Reduced-Size RTCP

   A Reduced-Size RTCP packet is an RTCP packet with the following
   properties that make it deviate from the compound RTCP packet
   definition given in Section 6.1 in [RFC3550]:

   o  contains one or more RTCP packet(s)

   o  allows any RTCP packet type; however, see Section 4.2.1

   o  MUST NOT be used for Regular (scheduled) RTCP report purposes

   o  MUST NOT be used with the RTP/AVP profile [RFC3551] or the
      RTP/SAVP profile [RFC3711]

4.2.  Algorithm Considerations

4.2.1.  Verification of Delivery

   If an application is to use Reduced-Size RTCP, it is important to
   verify that the Reduced-Size RTCP packets actually reach the session
   participants.  As outlined above in Section 3.4.1 and Section 3.4.2,
   packets may be discarded along the path or in the endpoint.

   A few verification rules are RECOMMENDED to ensure robust RTCP
   transmission and reception and to solve the identified issues when
   Reduced-Size RTCP is used:

   o  The endpoint issue can be solved by introducing signaling that
      informs if all session participants are capable of Reduced-Size
      RTCP.  See Section 5.

   o  The middle box issue is more difficult, and here one will be
      required to use heuristics to determine whether or not the
      Reduced-Size RTCP packets are delivered.  The method used to
      detect successful delivery of Reduced-Size RTCP packets depends on
      the packet type.  The RTCP packet types for which successful
      delivery can be detected are:

      *  Sender reports (SR): Successful transmission of a sender report
         can be verified by inspection of the echoed timestamp in the
         received receiver report (RR).  This can also be used as a
         method to verify if Reduced-Size RTCP can be used at all.




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      *  Feedback RTCP packets: In many cases, the feedback messages
         sent using Reduced-Size RTCP will result in either explicit or
         implicit indications that they have been received.  An example
         of this is the RTP retransmission [RFC4588] that results from a
         NACK message [RFC4585].  Another example is the Temporary
         Maximum Media Stream Bitrate Notification (TMMBN) message
         resulting from a Temporary Maximum Media Stream Bitrate Request
         (TMMBR) [RFC5104].  A third example is the presence of a
         decoder refresh point [RFC5104] in the video media stream
         resulting from the Full Intra Request that was sent.

      RTCP packet types for which it is not possible to detect
      successful delivery SHOULD NOT be transmitted as Reduced-Size RTCP
      packets unless they are transmitted in the same lower-layer
      datagram as another RTCP packet type for which successful delivery
      can be detected.

   o  An algorithm to detect consistent failure of delivery of Reduced-
      Size RTCP MUST be used by any application using Reduced-Size RTCP.
      The details of this algorithm are application dependent and
      therefore outside the scope of this document.

   If the verification fails, it is strongly RECOMMENDED that only
   compound RTCP, according to the rules outlined in RFC 3550, is
   transmitted.

4.2.2.  Single vs Multiple RTCP in a Reduced-Size RTCP

   The result of the definition in Section 4.1 may be that the resulting
   size of Reduced-Size RTCP can become larger than a regularly
   scheduled compound RTCP packet.  For applications that use access
   types that are sensitive to packet size (see Paragraph 2 in
   Section 3.3), it is strongly RECOMMENDED that the use of Reduced-Size
   RTCP is limited to the transmission of a single RTCP packet in each
   lower-layer datagram.  The method to determine the need for this is
   outside the scope of this document.

   In general, as the benefit with large Reduced-Size RTCP packets is
   very limited, it is strongly RECOMMENDED to transmit large Reduced-
   Size RTCP packets as compound RTCP packets instead.

4.2.3.  Enforcing Compound RTCP

   As discussed earlier, it is important that the transmission of
   compound RTCP occurs at regular intervals.  However, this will occur
   as long as the RTCP senders follow the AVPF scheduling algorithm





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   defined in Section 3.5 of [RFC4585].  This follows as all Regular
   RTCP MUST be full compound RTCP.  Note that there is also a
   requirement on sending Regular RTCP in Immediate Feedback mode.

4.2.4.  Immediate Feedback Mode

   Section 3.3 of [RFC4585] gives the option to use AVPF Immediate
   Feedback mode as long as the group size is below a certain limit.  As
   transmissions using Reduced-Size RTCP may reduce the bandwidth
   demand, such transmissions also open up the possibility of a more
   liberal use of Immediate Feedback mode.

5.  Signaling

   This document defines the "a=rtcp-rsize" Session Description Protocol
   (SDP) [RFC4566] attribute to indicate if the session participant is
   capable of supporting Reduced-Size RTCP for applications that use SDP
   for configuration of RTP sessions.  It is REQUIRED that a participant
   that proposes the use of Reduced-Size RTCP shall itself support the
   reception of Reduced-Size RTCP.

   An offering client that wishes to use Reduced-Size RTCP MUST include
   the attribute "a=rtcp-rsize" in the SDP offer.  If "a=rtcp-rsize" is
   present in the offer SDP, the answerer that supports Reduced-Size
   RTCP and wishes to use it SHALL include the "a=rtcp-rsize" attribute
   in the answer.

   In declarative usage of SDP, such as the Real Time Streaming Protocol
   (RTSP) [RFC2326] and the Session Announcement Protocol (SAP)
   [RFC2974], the presence of the attribute indicates that the session
   participant MAY use Reduced-Size RTCP packets in its RTCP
   transmissions.

6.  Security Considerations

   The security considerations of RTP [RFC3550] and AVPF [RFC4585] will
   apply also to Reduced-Size RTCP.  The reduction in validation
   strength for received packets on the RTCP port may result in a higher
   degree of acceptance of spurious data as real RTCP.  This
   vulnerability can mostly be addressed by usage of any security
   mechanism that provides authentication; one such mechanism is SRTP
   [RFC3711].

7.  IANA Considerations

   Following the guidelines in [RFC4566], the IANA has registered a new
   SDP attribute:




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   o  Contact name, email address, and telephone number: Authors of RFC
      5506

   o  Attribute-name: rtcp-rsize

   o  Long-form attribute name: Reduced-Size RTCP

   o  Type of attribute: media-level

   o  Subject to charset: no

   This attribute defines the support for Reduced-Size RTCP, i.e., the
   possibility to transmit RTCP that does not conform to the rules for
   compound RTCP defined in RFC 3550.  It is a property attribute, which
   does not take a value.

8.  Acknowledgements

   The authors would like to thank all the people who gave feedback on
   this document.  Special thanks go to Colin Perkins.

   This document also contains some text copied from [RFC3550],
   [RFC4585], and [RFC3711].  We take this opportunity to thank the
   authors of said documents.

9.  References

9.1.  Normative References

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3550]     Schulzrinne, H., Casner, S., Frederick, R., and V.
                 Jacobson, "RTP: A Transport Protocol for Real-Time
                 Applications", STD 64, RFC 3550, July 2003.

   [RFC3551]     Schulzrinne, H. and S. Casner, "RTP Profile for Audio
                 and Video Conferences with Minimal Control", STD 65,
                 RFC 3551, July 2003.

   [RFC4585]     Ott, J., Wenger, S., Sato, N., Burmeister, C., and J.
                 Rey, "Extended RTP Profile for Real-time Transport
                 Control Protocol (RTCP)-Based Feedback (RTP/AVPF)",
                 RFC 4585, July 2006.

   [RFC5124]     Ott, J. and E. Carrara, "Extended Secure RTP Profile
                 for Real-time Transport Control Protocol (RTCP)-Based
                 Feedback (RTP/SAVPF)", RFC 5124, February 2008.



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9.2.  Informative References

   [MTSI-3GPP]   3GPP, "Specification : 3GPP TS 26.114 (v8.2.0 or
                 later), http://www.3gpp.org/ftp/Specs/html-info/
                 26-series.htm", March 2007.

   [MULTI-RTP]   Perkins, C. and M. Westerlund, "Multiplexing RTP Data
                 and Control Packets on a Single Port", Work
                 in Progress, August 2007.

   [OMA-PoC]     Open Mobile Alliance, "Specification : Push to talk
                 Over Cellular User Plane, http://
                 member.openmobilealliance.org/ftp/public_documents/poc/
                 Permanent_documents/
                 OMA-TS-PoC_UserPlane-V2_0-20080507-C.zip",
                 November 2006.

   [RFC2326]     Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
                 Streaming Protocol (RTSP)", RFC 2326, April 1998.

   [RFC2974]     Handley, M., Perkins, C., and E. Whelan, "Session
                 Announcement Protocol", RFC 2974, October 2000.

   [RFC3095]     Bormann, C., Burmeister, C., Degermark, M., Fukushima,
                 H., Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T.,
                 Le, K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro,
                 K., Wiebke, T., Yoshimura, T., and H. Zheng, "RObust
                 Header Compression (ROHC): Framework and four profiles:
                 RTP, UDP, ESP, and uncompressed", RFC 3095, July 2001.

   [RFC3711]     Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
                 K. Norrman, "The Secure Real-time Transport Protocol
                 (SRTP)", RFC 3711, March 2004.

   [RFC4566]     Handley, M., Jacobson, V., and C. Perkins, "SDP:
                 Session Description Protocol", RFC 4566, July 2006.

   [RFC4588]     Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
                 Hakenberg, "RTP Retransmission Payload Format",
                 RFC 4588, July 2006.

   [RFC5104]     Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
                 "Codec Control Messages in the RTP Audio-Visual Profile
                 with Feedback (AVPF)", RFC 5104, February 2008.

   [TCP-FRIEND]  Gharai, L., "RTP with TCP Friendly Rate Control", Work
                 in Progress, July 2007.




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Authors' Addresses

   Ingemar Johansson
   Ericsson AB
   Laboratoriegrand 11
   SE-971 28 Lulea
   SWEDEN

   Phone: +46 73 0783289
   EMail: ingemar.s.johansson@ericsson.com


   Magnus Westerlund
   Ericsson AB
   Faeroegatan 6
   SE-164 80 Stockholm
   SWEDEN

   Phone: +46 10 7148287
   EMail: magnus.westerlund@ericsson.com































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