Network Working Group                                       I. Johansson
Internet-Draft                                             M. Westerlund
Intended status: Standards Track                             Ericsson AB
Expires: August 28, 2008                                    Feb 25, 2008

     Support for non-compound RTCP, opportunities and consequences

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   This memo discusses benefits and issues that arise when allowing RTCP
   packets to be transmitted as non-compound packets, i.e not follow the
   rules of RFC 3550.  Based on that analysis this memo proposes changes
   to the rules to allow feedback messages to be sent as non-compound
   RTCP packets when using the RTP AVPF profile (RFC 4585) under certain

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Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  RTCP Compound Packets  . . . . . . . . . . . . . . . . . . . .  3
   3.  Benefits with non-compound packets . . . . . . . . . . . . . .  4
   4.  Issues with non-compound RTCP packets  . . . . . . . . . . . .  6
     4.1.  Middle boxes . . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  Packet Validation  . . . . . . . . . . . . . . . . . . . .  6
       4.2.1.  Old RTCP Receivers . . . . . . . . . . . . . . . . . .  6
       4.2.2.  Weakened Packet Validation . . . . . . . . . . . . . .  7
       4.2.3.  Bandwidth consideration  . . . . . . . . . . . . . . .  7
       4.2.4.  Computation of avg_rtcp_size . . . . . . . . . . . . .  7
     4.3.  Header compression . . . . . . . . . . . . . . . . . . . .  7
     4.4.  RTP and RTCP multiplex on the same port  . . . . . . . . .  7
     4.5.  Encryption/authentication  . . . . . . . . . . . . . . . .  8
   5.  Use cases for non-compound RTCP  . . . . . . . . . . . . . . .  8
     5.1.  Control plane signaling  . . . . . . . . . . . . . . . . .  8
     5.2.  Codec control signaling  . . . . . . . . . . . . . . . . .  8
     5.3.  Feedback . . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.4.  Status reports . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Rules and guidelines for non-compound packets in AVPF  . . . . 10
     6.1.  Verification of the delivery of non-compound packets . . . 10
     6.2.  Algorithm modifications  . . . . . . . . . . . . . . . . . 11
       6.2.1.  Distinction between compound and non-compound RTCP . . 11
       6.2.2.  Modified bandwidth algorithms  . . . . . . . . . . . . 12
       6.2.3.  Immediate mode . . . . . . . . . . . . . . . . . . . . 12
       6.2.4.  Enforcing compound RTCP  . . . . . . . . . . . . . . . 12
     6.3.  Open issues  . . . . . . . . . . . . . . . . . . . . . . . 13
     6.4.  SDP Signalling Attribute . . . . . . . . . . . . . . . . . 13
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     10.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
   Intellectual Property and Copyright Statements . . . . . . . . . . 16

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1.  Introduction

   In RTP [RFC3550] it is currently mandatory to always use RTCP
   compound packets containing at least Sender Reports or Receiver
   reports, and a SDES packet containing at least the CNAME item.  There
   are good reasons for this as discussed below (see Section 2).
   However this do result in that the minimal RTCP packets are 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 and AVPF do provide
   some mechanism to enable this.  However for environments with low-
   bitrate links this still consumes quite large amount of resources and
   introduce extra delay in the time it takes to completely send the
   compound packet in the network.  There are also other benefits as
   discussed in Section 3.

   The use of non-compound packets is not without issues.  This is
   discussed in Section 4.  These issues needs to be considered and are
   part of the motivation for this document.

   In addition this document proposes how AVPF could be updated to allow
   the transmission of non-compound packets in a way that would not
   substantially affect the mechanisms that compound packets provide.
   The connection to AVPF is motivated by the fact that non-compound
   RTCP is mainly intended for event driven feedback purposes and that
   the AVPF early and immediate modes make this possible.

2.  RTCP Compound Packets

   Section 6.1 in RFC3550 [RFC3550] specifies that an RTCP packet must
   be sent in a compound packet consisting of at least two individual
   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).  Lets examine what these RTCP packet types are used for.

   1.  The sender and receiver reports (see Section 6.4 of RFC 3550
       [RFC3550]) provides the RTP session participant with the Sender
       Source Identifier (SSRC) of all RTCP senders.  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 they quickly
       can establish a good estimate of the group size.  Failure to do
       this would result in RTCP senders consuming to much bandwidth.

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   2.  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 become more useful the more frequently
       it is received, at least until one report per round-trip time
       (RTT) is achieved.  Therefore there are most cases no reason to
       not include the sender or receiver report in all RTCP packets.

   3.  The CNAME SDES item (See Section 6.5.1 of RFC 3550 [RFC3550])
       exists to allow receivers to determine which media flows that
       should be synchronized with each other 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.

   4.  Sender Reports (SR) is 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

   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
   received media and to avoid flooding the network with RTCP reports.
   In addition, if not sent regularly the dynamic nature of the
   information provided would make it less and less useful.

3.  Benefits with non-compound packets

   As mentioned in the introduction, most advantages of using non-
   compound packets exists in cases when the available RTCP bit-rate is
   limited.  This because non-compound packets will be 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.  And if the recommended form of user@host is used, then
   most strings will be longer than 20 characters.  Thus a non-compound
   packets can become at least 70-80 bytes smaller than the compound

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   The following benefits exist for the smaller non-compound packets:

   1.  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.

   2.  For links where the packet loss rate grows with the packet size,
       smaller packets will be less likely to be dropped.  An example of
       such links are radio links.  In the cellular world there exist
       links that are optimized to handle RTP packets sized for carrying
       compressed speech, which increases the capacity and coverage for
       voice services in a given wireless network.  Minimum sized
       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.

   3.  Independently of the link type there are additional benefits with
       sending feedback in small non-compound RTCP.  One such example is
       applications that use RTCP AVPF in early or immediate mode to
       send frequent event driven feedback.  Under these circumstances
       non-compound RTCP reduces the risk that the RTCP bandwidth
       becomes too high during periods of heavy adaptation feedback

   4.  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 non-compound RTCP gives a
       very substantial improvement.

   In cases when non-compound packets carry 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 compared to media packets hurts when trying to perform media
   adaptation, to for example handle the changed performance present at
   the cell border in cellular system.

   For high bit-rate applications there is usually no problem of
   supplying RTCP with sufficient bit-rates.  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

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   cases there are no reasons to provide regular reports with higher
   density than this.  Any additional bandwidth can then be used for
   feedback messages.  The benefit of non-compound packets in this case
   is limited, but exists.  One typical example is video using generic
   NACK in cases where the RTT is low.  Using non-compound packets would
   reduce the total amount of bits used for RTCP.  This is primarily
   applicable if the number of non-compound packets 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.

4.  Issues with non-compound RTCP packets

   This section describes some of the known issues with non-compound
   RTCP packets

4.1.  Middle boxes

   Middle boxes in the network may discard RTCP packets that do not
   follow the rules outlined in section 6.1 of RFC3550.  The effect of
   this might for instance be that compound RTCP packets would get
   through while the non-compound feedback packets would be lost.

4.2.  Packet Validation

   A non-compound packet will be discarded by the packet validation code
   in Appendix A of RFC 3550 [RFC3550].  This has several impacts as
   described in the following sub sections.

4.2.1.  Old RTCP Receivers

   Any RTCP receiver without updated packet validation code will discard
   the non-compound packets.  Thus these receivers will not see the
   feedback contained in the these non-compound packets.  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 non-compound NACK message (see Section
   6.2.1 of RFC 4585 [RFC4585]) to non updated media sender in a session
   using the retransmission scheme defined by RFC 4588 [RFC4588].

   This type of discarding would also effect 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 non-
   compound packets.

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4.2.2.  Weakened Packet Validation

   The packet validation code needs to be rewritten to accept non-
   compound packets.  One potential effect of this change is much weaker
   validation that received packets actually are RTCP packets, 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 non-compound packets to
   contain only some specific RTCP packet types, that is preferably
   signalled on a session basis.  A solution to this is presented in
   Section 6.2

4.2.3.  Bandwidth consideration

   The discarding of non-compound RTCP packets would effect the RTCP
   transmission calculation in the following way; the avg_rtcp_size
   value would become larger than for RTP receivers that exclude the
   non-compound in this calculation (assuming that non-compound packets
   are smaller than compound ones).  Therefore these senders would
   under-utilize the available bit-rate 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 non-compound
   packets may result in that the updated receivers time out non-updated
   senders prematurely.  A solution to this is presented in Section 6.2.

4.2.4.  Computation of avg_rtcp_size

   Long intervals between compound RTCP packets and many non-compound
   RTCP packets in between may lead to a computation of a value for
   avg_rtcp_size that varies greatly over time.  This is discussed more
   in Section 6.2.

4.3.  Header compression

   The classifiers for header compression algorithms such as RoHC
   [RFC3095] and its profiles must be aware of the fact that, with the
   proposed non-compound RTCP packets, the first RTCP packet type might
   differ from 200 or 201.  Otherwise they would likely wrongly classify
   the packets as something else than RTCP.  However, as no header
   compression technology defined in IETF compresses RTCP, this should
   have no real impact.

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 RTCP
   packets are non-compound.

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4.5.  Encryption/authentication

   SRTP presents a problem for non-compound 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".

   However the same text also states that the encryption prefix that is
   present in the receiver and sender reports should not be used by
   SRTP.  The conclusion is therefore that it is possible to use non-
   compound RTCP with SRTP.

   Non-compound RTCP also 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 packet may differ from 200 or 201
   (SR or RR).

5.  Use cases for non-compound RTCP

   Below are listed a few use cases for non-compound RTCP.  It is worth
   noting here that the current uses of non-compound RTCP are thoroughly
   specified in other standardization bodies and are limited to specific
   services such as PoC or 3GPP-MTSI.  A general definition of the use
   of non-compound RTCP for e.g control plane or codec control signaling
   would probably need to be specified within the IETF.

5.1.  Control plane signaling

   Open Mobile Alliance (OMA) Push-to-talk over Cellular (PoC) [OMA-PoC]
   makes use of non-compound packets when transmitting certain events.
   The OMA POC service is primarily used over cellular links capable of
   IP transport, such as the GSM GPRS.

5.2.  Codec control signaling

   Examples of codec control usage for non-compound RTCP are found in

   Another example that can be used with non-compound RTCP is e.g TMMBR
   messages as specified in [I-D.ietf-avt-avpf-ccm] which signal a
   request for a change in codec bitrate.  The benefit of non-compound
   RTCP for these messages is that in bad channel conditions, a non-
   compound RTCP can be considerably more likely to be received than
   larger compound RTCP messages.  This is critical as these messages
   predominantly occur when channel conditions are poor.

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5.3.  Feedback

   The feedback scenario is best presented as a Video stream 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 play out quality considerably.  If
   the generic NACK packets are transmitted as non-compound packets, the
   bandwidth requirement for RTCP will be minimal, enabling more
   frequent feedback.  Like in the Codec control case it is important
   that these packets can be transmitted with as little delay as
   possible.  The RTCP bandwidth reduction and transmission speed are
   equally useful when retransmission is not used for loss recovery.

   Another interesting use for non-compound RTCP is 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 for the feedback when the round trip time is short.  For
   this particular application non-compound RTCP may give a very
   substantial improvement.

5.4.  Status reports

   One idea proposed is to transmit small measurement or status reports
   in non-compound RTCP, and to be able to split the sub-packets of a
   minimum compound RTCP and transmit them 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 smaller risk
   that the report packets will affect the channel that they report

   Even though this may be an interesting use case a few issues needs to
   be considered.

   o  A risk exists that it opens up for a whole set of incompatible
      metrics and reports devised in various standardization fora
      leading to a potential interoperability problems.

   o  Middle boxes or third party network monitoring equipment may fail
      to understand the new reports or even discard these new report

   o  There may arise a need to verify that these "special" reports
      reach the intended recipient in case middle boxes in the network

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      discards unknown reports.  In many cases it is difficult to verify
      that for instance sender reports are received correctly, missing
      SR may as well be an indication that the other endpoint has

6.  Rules and guidelines for non-compound packets in AVPF

   Based on the above analysis it seems feasible to allow transmission
   of non-compound RTCP under some restrictions.  First of all it is
   important that compound packets are regularly sent to ensure the
   feedback reporting works.  The tracking of session size and number of
   participants is also important as this ensures that the RTCP
   bandwidth remain bounded independent of the number of session
   participants.  As the compound packets also are used to establish the
   synchronization, any newly joining participant in a session would
   need to receive a compound packet from the media sender.  In summary
   the regular usage of compound packets must be maintained throughout
   the complete session.  Thus non-compound packets should be restricted
   to be used as extra feedback packets sent in cases when a regular
   compound packet would not have been sent.

   The usage of non-compound RTCP packet SHALL only be done in RTP
   sessions operating in AVPF [RFC4585] Early RTCP or Immediate feedback
   mode.  Non-compound packets SHALL NOT be sent until at least one
   compound packet has been sent.  In Immediate feedback mode all
   feedback messages MAY be sent as non-compound packets.  In early RTCP
   mode a feedback message scheduled for transmission as an Early RTCP
   packet, i.e not a Regular RTCP packet, MAY be sent as a non-compound
   packet.  All packets that scheduled for transmission as Regular RTCP
   packets SHALL be sent as (full) compound RTCP packets as indicated by
   AVPF [RFC4585].

6.1.  Verification of the delivery of non-compound packets

   If an application is to use non-compound packets it is important to
   verify that they actually reaches the session participants.  As
   outlined above in Section 4.1 and Section 4.2 packets may be
   discarded along the path or in the end-point.  The end-points can be
   resolved by introducing signaling that informs if all session
   participants are capable of non-compound packets or not.  The middle
   box issue is more difficult and here one will be required to use
   heuristics to determine if the non-compound packets are delivered or
   not.  However in many cases the feedback messages sent using non-
   compound packets will result in either explicit or implicit
   indications that they have been received.  Example of such are the
   RTP retransmission [RFC4588] that result from a NACK message
   [RFC4585], the Temporary Maximum Media Bit-rate Notification message

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   resulting from a Temporary Maximum Media Bit-rate Request
   [I-D.ietf-avt-avpf-ccm], or the presence of a Decoder Refresh Point
   [I-D.ietf-avt-avpf-ccm] in the video media stream resulting from the
   Full Intra Request sent.

   A proposed algorithm to detect consistent failure of delivery of non-
   compound packets needs to be written.  The details of this algorithm
   is application dependent and therefore outside the scope of this

   An optional method to detect if non-compound RTCP are discarded is to
   send a non-compound SR packet, then check that the timestamp is
   echoed back in the corresponding RR packet.

   If the verification fails it is strongly RECOMMENDED that only
   compound RTCP according to the rules outlined in RFC3550 is

6.2.  Algorithm modifications

6.2.1.  Distinction between compound and non-compound RTCP

   One question that arise is how to distinguish between small (non-
   compound) and large (compound) RTCP.  A few alternatives:

   o  Payload type: A non-compound RTCP may have a (first) PT number
      that differs from the PT numbers for SR or RR.  This may be a weak
      alternative as some interest to be able to split minimum compound
      RTCP is expressed, see Status reports (Section 5.4).  A possible
      problem here is also that this distinction does not actually tell
      the size of the RTCP.

   o  Fixed size, set in specification.  For instance one may base the
      distinction on the likely minimum size of a minimal compound RTCP.
      Assuming that such a packet will contain at least an SR (32 bytes)
      and a SDES CNAME (likely 16 bytes or more) one can conclude that
      48 bytes (+IP/UDP overhead) is probably the smallest realistic
      size of a compound RTCP.

   o  Fixed size, set in session setup : Some sessions may e.g use
      RTCP-XR or some other RTCP reporting on occasions that may give
      very large packet sizes, it may be desirable to adjust the

   o  Variable size: As non-compound RTCP is by definition RTCP that
      does not follow the rules for compound RTCP as they are specified
      in RFC3550, the size can be determined "on the fly".

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   o  Comparison against total UDP packet size: If the size of the first
      RTCP packet is + possible SRTCP overhead is smaller than the UDP
      packet size it is a compound RTCP.  If the sizes match then it is
      a non-compound RTCP.  This method is an indication of the number
      of RTCP packets in a given UDP payload, if 2 RTCP packets or more
      then it is a compound RTCP.

   Of the above alternatives the last one is the most straightforward
   and simple.  The exception is if one need to know if it is a non-
   compound SRTCP without decrypting it (for instance in a middle box).

6.2.2.  Modified bandwidth algorithms

   The use of non-compound RTCP does not imply any specific need for
   algorithm modifications.  A few possible algorithm modifications have
   been tested and even though the modifications may improve the
   performance when feedback is transmitted the benefits are not judged
   large enough to justify the relatively large changes in the

   A more extensive report covering various test-cases and different
   algorithm modifications can be downloaded from:

   The report also covers tests with other averaging factors than the
   specified 1/16, and it shows that it is beneficial to use slower
   averaging (1/32 or 1/64) as it makes the estimate avg_rtcp_size more
   stable and does not degrade the feedback transmission performance.
   This modification is however not critical.

6.2.3.  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 feedback using
   non-compound RTCP becomes smaller it opens up for a more liberal use
   of immediate mode.

6.2.4.  Enforcing compound RTCP

   As discussed earlier it is important that the sending of compound
   RTCP packets do happen at regular interval.  However, this will occur
   as long as the RTCP senders follow the AVPF scheduling algorithm
   defined in Section 3.5 in [RFC4585].  This as all regular RTCP
   packets must be full compound RTCP packets.  Note that also in
   immediate mode is there a requirement on sending regular RTCP

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6.3.  Open issues

   This section contains a list of the yet unresolved issues.  Currently
   no open issues.

6.4.  SDP Signalling Attribute

   We request to define the a "a=rtcp-nc" [RFC4566] attribute to
   indicate if the session participant is capable of supporting non-
   compound packets.  It is a required that a participant that proposes
   the use of non-compound RTCP itself supports the reception of non-
   compound RTCP.

   An offering client that wish to use non-compound RTCP MUST include
   the attribute "a=rtcp-nc" in the SDP offer.  If the other client does
   not support non-compound RTCP the attribute MUST be removed from the
   answer SDP.

7.  IANA Considerations

   IANA will be required to register the SDP signalling attribute
   defined in Section 6.4.

8.  Security Considerations

   The security considerations of RTP [RFC3550] and AVPF [RFC4585] will
   apply also to non-compound packets.  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 packets.  This
   vulnerability can mostly be addressed by usage of an security
   mechanism that provide authentication, e.g.  SRTP[RFC3711].

9.  Acknowledgements

   The authors would like to thank all the people who gave feedback on
   this document.

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

10.  References

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10.1.  Normative References

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

10.2.  Informative References

              3GPP, "Specification : 3GPP TS 26.114 (v7.1.0
              Specs_update_after_SA36/", March 2007.

              Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback  (AVPF)", draft-ietf-avt-avpf-ccm-10 (work
              in progress), October 2007.

              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.

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

   [OMA-PoC]  Open Mobile Alliance, "Specification : Push to talk Over
              Cellular User Plane,
              November 2006.

   [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.

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   [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.

Authors' Addresses

   Ingemar Johansson
   Ericsson AB
   Laboratoriegrand 11
   SE-971 28 Lulea

   Phone: +46 73 0783289

   Magnus Westerlund
   Ericsson AB
   Torshamnsgatan 21-23
   SE-164 83 Stockholm

   Phone: +46 8 7190000
   Email: magnus.westerlund (AT)

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