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Versions: (draft-johansson-avt-rtcp-avpf-non-compound) 00 01 02 03 04 05 06 07 08 09 RFC 5506

Network Working Group                                       I. Johansson
Internet-Draft                                             M. Westerlund
Updates: 3550,3711,4585                                      Ericsson AB
(if approved)                                               Feb 20, 2009
Intended status: Standards Track
Expires: August 24, 2009


     Support for Reduced-Size RTCP, Opportunities and Consequences
                  draft-ietf-avt-rtcp-non-compound-09

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on August 24, 2009.

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Abstract

   This memo discusses benefits and issues that arise when allowing RTCP
   packets to be transmitted with reduced size.  The size can be reduced
   if the rules on how to create compound packets outlined in RFC3550
   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 profile (RFC 4585).  This document updates [RFC3550], [RFC3711]
   and [RFC4585].


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 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
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17










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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 profile AVPF [RFC4585] specifies new RTCP packet types for
   feedback messages.  Some of these feedback messages would benefit
   from being transmitted with minimal delay.  AVPF does provide 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) is
   motivated by the fact that reduced-size RTCP is mainly beneficial for
   event driven feedback purposes and that the AVPF early and immediate
   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 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 TCP or DCCP payload or
      something else.  Synonymous to "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 a 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 packets 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 or 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 an 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
   (SSRC).  Below is a short description 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 Synchronisation
       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 quickly can 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 analysis 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 to not 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 longer canonical identifier of a particular end-
       point instance that is bound to an SSRC.  If that binding isn't
       received and kept current the receiver may not detect a SSRC
       collision, i.e. two different CNAMEs using the same SSRC.  It
       also can't detect a RTP level routing loop with the result that
       the same SSRC and CNAME arrives from multiple lower-layer source
       addresses.

   Reviewing the above it is obvious that both SR/RR and the CNAME are
   very important for new session participants to be able to utilize any



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   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 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:  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 GSM
      GPRS.

   Codec Control Signaling:  An example that can be used with reduced-
      size RTCP is e.g TMMBR messages as specified in [RFC5104] which
      signal a request for a change in codec bitrate.  The benefit of
      its use for these messages is 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 be able 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 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



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      smaller risk that the report packets will affect the channel that
      they report upon.  Another benefit is that it is possible, with
      reduced-size RTCP, to allow e.g anonymous status reporting to be
      transmitted unencrypted.  This is something that may be beneficial
      for e.g network monitoring purposes.

3.3.  Benefits of Reduced-Size RTCP

   As mentioned in the introduction, most advantages of using reduced-
   size RTCP packets exists in cases when the available RTCP bitrate is
   limited.  This 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 be 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 a 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 on the link layer, i.e. commonly
      into 2 or 3 fragments.

   o  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 an link
      interface capable of 30 kbps takes 32 milliseconds (ms) assuming
      uniform transmission rate.

   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



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   packets compared to 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 with 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 or immediate 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
   [I-D.ietf-avt-tfrc-profile], the size of compound RTCP 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 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 RFC3550.  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.  This in particular 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 done 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 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 use a reduced
      size NACK message (see Section 6.2.1 of [RFC4585]) to 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 between old ones
      only seeing the compound RTCP feedback messages and the newer ones
      seeing both, where the old ones 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 the following way: 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-sized 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

   SRTP presents a problem for reduced-size RTCP.  Section 3.4 in
   [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 process 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 which multiplex RTP and RTCP on the same port, as
   defined in [I-D.ietf-avt-rtp-and-rtcp-mux], care must be taken to
   ensure that the 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
   [I-D.ietf-avt-rtp-and-rtcp-mux] 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 RoHC header compression
      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 RTCP are transmitted at
      regular intervals to ensure that the mechanisms maintained by the
      compound packets, like feedback reporting works.  The tracking of
      session size and number of participants warrants mentioning again
      as this ensures that the RTCP bandwidth remain bounded independent
      of the number of session participants.

   o  Second, as the compound RTCP 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 or Immediate
   mode.  Reduced-size RTCP SHALL NOT be sent until at least one
   compound RTCP has been sent.  In Immediate mode all feedback messages
   MAY be sent as reduced-size RTCP.  In early mode a feedback message
   scheduled for transmission as an 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



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   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 makes it deviate from the compound RTCP packet
   definition given in section 6.1 in [RFC3550]:

   o  Contains one or more RTCP packet(s)

   o  Any RTCP packet type allowed, however see section 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 end-point.

   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 end-point 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 if the reduced-size RTCP
      are delivered or not.  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.

      *  Feedback RTCP packets: In many cases the feedback messages sent
         using reduced-size RTCP will result in either explicit or



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         implicit indications that they have been received.  An example
         of is the RTP retransmission [RFC4588] that results from a NACK
         message [RFC4585].  Another example is the Temporary Maximum
         Media Bitrate Notification message resulting from a Temporary
         Maximum Media Bitrate Request [RFC5104].  A third example is
         the presence of a Decoder Refresh Point [RFC5104] in the video
         media stream resulting from the Full Intra Request 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 it.  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 RFC3550 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 draft.

   In general, as the benefit with large sized 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
   defined in Section 3.5 in [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 mode.





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4.2.4.  Immediate Mode

   Section 3.3 in RFC4585 gives the option to use AVPF Immediate mode as
   long as the groupsize is below a certain limit.  As transmission
   using reduced-size RTCP may reduce the bandwidth demand it also opens
   up the possibility of a more liberal use of immediate mode.


5.  Signaling

   This document defines the "a=rtcp-rsize" SDP [RFC4566] attribute to
   indicate if the session participant is capable of supporting reduced-
   size RTCP for applications that uses 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 RTSP [RFC2326] and 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 provide authentication; one such mechanism is SRTP
   [RFC3711].


7.  IANA Considerations

   Following the guidelines in [RFC4566], the IANA is requested to
   register one new SDP attribute:

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





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   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 RFC3550.  It is a property attribute, which
   does not take a value.

   Note to RFC Editor: please replace "RFC XXXX" above with the RFC
   number of this memo, and remove this note.


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 contain some text copied from [RFC3550],
   [RFC4585]and [RFC3711].  We take the 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



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              Real-time Transport Control Protocol (RTCP)-Based Feedback
              (RTP/SAVPF)", RFC 5124, February 2008.

9.2.  Informative References

   [I-D.ietf-avt-rtp-and-rtcp-mux]
              Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port",
              draft-ietf-avt-rtp-and-rtcp-mux-07 (work in progress),
              August 2007.

   [I-D.ietf-avt-tfrc-profile]
              Gharai, L., "RTP with TCP Friendly Rate Control",
              draft-ietf-avt-tfrc-profile-10 (work in progress),
              July 2007.

   [MTSI-3GPP]
              3GPP, "Specification : 3GPP TS 26.114 (v7.4.0), http://
              www.3gpp.org/ftp/Specs/archive/26_series/26.114/
              26114-740.zip", March 2007.

   [OMA-PoC]  Open Mobile Alliance, "Specification : Push to talk Over
              Cellular User Plane, http://www.openmobilealliance.org/
              release_program/docs/PoC/V1_0_1-20061128-A/
              OMA-TS-PoC-UserPlane-V1_0_1-20061128-A.pdf",
              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.



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              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.


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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