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.

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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

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.

   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 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.

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

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:

• 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.

• 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.

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.

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.
   
   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.

3.4.5. Header Compression

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

• 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.

• 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:

• 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.

• 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

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]:

• contains one or more RTCP packet(s)

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

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

• 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:

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

• 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.

• 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.

• 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 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:

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

• Attribute-name: rtcp-rsize

   o  Long-form attribute name: Reduced-Size RTCP
   
   o  Type of attribute: media-level

• 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.

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.

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