Support for non-compound RTCP, opportunities and consequences
draft-ietf-avt-rtcp-non-compound-04

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Abstract

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 conditions.

Requirements Language

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

Table of Contents

1.  Introduction
2.  RTCP Compound Packets
3.  Benefits with non-compound packets
    3.1.  Low birate links
    3.2.  Higher bitrates
    3.3.  Both high and low bitrate links
4.  Use cases for non-compound RTCP
    4.1.  Control plane signaling
    4.2.  Codec control signaling
    4.3.  Feedback
    4.4.  Status reports
5.  Issues with non-compound RTCP packets
    5.1.  Middle boxes
    5.2.  Packet Validation
        5.2.1.  Old RTCP Receivers
        5.2.2.  Weakened Packet Validation
        5.2.3.  Bandwidth considerations
        5.2.4.  Computation of avg_rtcp_size
    5.3.  Encryption/authentication
    5.4.  RTP and RTCP multiplex on the same port
    5.5.  Header compression
6.  Rules and guidelines for non-compound packets in AVPF
    6.1.  Definition of non-compound RTCP
    6.2.  Algorithm considerations
        6.2.1.  Verification of delivery
        6.2.2.  Single vs multiple RTCP in a non-compound RTCP
        6.2.3.  Enforcing compound RTCP
        6.2.4.  Immediate mode
    6.3.  SDP Signalling Attribute
7.  IANA Considerations
8.  Security Considerations
9.  Acknowledgements
10.  References
    10.1.  Normative References
    10.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

In RTP (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) [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 (RTCP Compound Packets)). However this do result in that the minimal RTCP packets are quite large. The RTP profile AVPF (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),” July 2006.) [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 (Benefits with non-compound packets).

The use of non-compound packets is not without issues. This is discussed in Section 5 (Issues with non-compound RTCP packets). 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] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) 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 [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.)) 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.
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 [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.)) 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 synchronization.

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 bitrate 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 packet can become at least 70-80 bytes smaller than the compound packet.

The following benefits exist for the non-compound packets,

3.1.  Low birate links

For low birate links the benefits are as follows.

• For links where the packet loss rate grows with the packet size, smaller packets are 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 speec. This 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.
• 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 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.

3.2.  Higher bitrates

For high bitrate applications there is usually no problem of supplying 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 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.

As message size generally is a smaller issue for higher birates, it is also possible to transmit multiple RTCP in each lower layer datagram in these cases. The motivation behind non-compound RTCP is 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.

3.3.  Both high and low bitrate links

Independently of the link type there are additional benefits with sending feedback in small non-compound RTCP.

• 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 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] (Gharai, L., “RTP with TCP Friendly Rate Control,” July 2007.), 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.

4.  Use cases for non-compound RTCP

Below are listed a few use cases for non-compound RTCP. The current use of non-compound RTCP is very application specific. 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.

4.1.  Control plane signaling

Open Mobile Alliance (OMA) Push-to-talk over Cellular (PoC) [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.) 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.

4.2.  Codec control signaling

Examples of codec control usage for non-compound RTCP are found in [3GPP‑MTSI] (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.).

Another example that can be used with non-compound RTCP is e.g TMMBR messages as specified in [RFC5104] (Wenger, S., Chandra, U., Westerlund, M., and B. Burman, “Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF),” February 2008.) 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 are likely to occur when channel conditions are poor.

4.3.  Feedback

An example of a feedback scenario that would benefit from non-compound 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 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.

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] (Gharai, L., “RTP with TCP Friendly Rate Control,” July 2007.). 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.

4.4.  Status reports

One proposed idea 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 upon.

Another benefit is that it is, with non-compound RTCP, possible to allow e.g anonymous status reportorting to be transmitted unencrypted. Something that may be beneficial for e.g network monitoring purposes.

5.  Issues with non-compound RTCP packets

This section describes the known issues with non-compound RTCP packets and also a brief analysis.

5.1.  Middle boxes

Middle boxes in the network may discard RTCP packets that do not follow the rules outlined in section 6.1 of RFC3550. Newer report types may be interptered 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 packets would get through while the non-compound feedback packets would be lost.

Verification of the delivery of non-compound RTCP is discussed in Section 6.2.1 (Verification of delivery).

5.2.  Packet Validation

A non-compound packet will be discarded by the packet validation code in Appendix A of [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.). This has several impacts as described in the following sub sections.

5.2.1.  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 [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),” July 2006.)) to non updated media sender in a session using the retransmission scheme defined by [RFC4588] (Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. Hakenberg, “RTP Retransmission Payload Format,” July 2006.).

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.

5.2.2.  Weakened Packet Validation

The packet validation code needs to be rewritten to accept non-compound packets. This in particular affects section 9.1 in [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) 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).

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.

5.2.3.  Bandwidth considerations

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 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 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 (Algorithm considerations).

5.2.4.  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 (Algorithm considerations).

5.3.  Encryption/authentication

SRTP presents a problem for non-compound RTCP. Section 3.4 in [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) 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.

5.4.  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] (Perkins, C. and M. Westerlund, “Multiplexing RTP Data and Control Packets on a Single Port,” August 2007.), care must be taken to ensure that the de-multiplexing is done properly even though RTCP packets are non-compound.

5.5.  Header compression

Two issues are related to header compression:

• Payload type number identification: The RoHC header compression algorithm algorithm [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,” July 2001.) 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 is based on payload type numbers. The classification algorithm must here acklowledge the fact that the payload type number for (the first) RTCP may differ from 200 or 201.
• Compression of RTCP packets: No IETF defined header compression method compress RTCP packets, however if such methods are developed in the future, these methods must take non-compound RTCP in account.

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 transmitted at regular intervals to ensure that 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 are also 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 RTCP (e.g feedback) 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 (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),” July 2006.) [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 are scheduled for transmission as Regular RTCP packets SHALL be sent as (full) compound RTCP packets as indicated by AVPF (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),” July 2006.) [RFC4585].

6.1.  Definition of non-compound RTCP

A non-compound RTCP is a packet that deviates from the rules regarding (minimal) compound RTCP given in RFC3550/4585 in the aspect that they don't contain both the mandatory elements SR/RR and SDES-CNAME. The definition does not make any distinction based on size. This means that it is possible to transmit multiple RTCP in one lower layer datagram.

6.2.  Algorithm considerations

6.2.1.  Verification of delivery

If an application is to use non-compound packets it is important to verify that they actually reach the session participants. As outlined above in Section 5.1 (Middle boxes) and Section 5.2 (Packet Validation) 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 (Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. Hakenberg, “RTP Retransmission Payload Format,” July 2006.) [RFC4588] that result from a NACK message (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),” July 2006.) [RFC4585], the Temporary Maximum Media Bitrate Notification message resulting from a Temporary Maximum Media Bitrate Request (Wenger, S., Chandra, U., Westerlund, M., and B. Burman, “Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF),” February 2008.) [RFC5104], or the presence of a Decoder Refresh Point (Wenger, S., Chandra, U., Westerlund, M., and B. Burman, “Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF),” February 2008.) [RFC5104] in the video media stream resulting from the Full Intra Request sent.

An algorithm to detect consistent failure of delivery of non-compound packets must be used by any application using non-compound. The details of this algorithm is application dependent and therefore outside the scope of this document.

A 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. This verification method is not compelety safe however as it SR is still one of the expected packet types.

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

6.2.2.  Single vs multiple RTCP in a non-compound RTCP

The result of the definition in Section 6.1 (Definition of non-compound RTCP) may be that the resulting size of non-compound RTCP can become larger than a normal compound RTCP. For applications that use access types that are sensitive to packet size (see Section 3.1 (Low birate links)) it is strongly RECOMMENDED that the use of non-compound RTCP is limited to the transmission of single RTCP packets in each lower layer datagram.

The methods to detemine the need for this is outside the scope of this draft.

6.2.3.  Enforcing compound RTCP

As discussed earlier it is important that the transmission of compound RTCP packets 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] (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),” July 2006.). 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 packets.

6.2.4.  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 feedback using non-compound RTCP may become smaller it opens up for a more liberal use of immediate mode.

6.3.  SDP Signalling Attribute

We request to define the a "a=rtcp-nc" [RFC4566] (Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” July 2006.) 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 "a=rtcp-nc" is present in the offer SDP, the answerer that supports non-compound RTCP and wishes to use it SHALL include the "a=rtcp-nc" attribute in the answer.

7.  IANA Considerations

Following the guidelines in [RFC4566] (Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” July 2006.), the IANA is requested to register one new SDP attribute:

• Contact name, email address and telephone number: Authors of RFCXXXX
• Attribute-name: rtcp-nc
• Long-form attribute name: Non-compound RTCP
• Type of attribute: media-level
• Subject to charset: no

This attribute defines the support for non-compound RTCP, i.e the possibility to transmit RTCP that does not comform to the rules for compund 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.  Security Considerations

The security considerations of RTP (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) [RFC3550] and AVPF (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),” July 2006.) [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 any security mechanism that provide authentication, one example such mechanism is SRTP [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.).

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] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.), (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),” July 2006.) [RFC4585]and (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) [RFC3711]. We take the opportunity to thank the authors of said documents.

10.  References

10.1. Normative References

10.2. Informative References

Authors' Addresses

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Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.