Guidelines for Extending the RTP Control Protocol (RTCP)
RFC 5968
Document | Type | RFC - Informational (September 2010) | |
---|---|---|---|
Authors | Colin Perkins , Joerg Ott | ||
Last updated | 2015-10-14 | ||
RFC stream | Internet Engineering Task Force (IETF) | ||
Formats | |||
Additional resources | Mailing list discussion | ||
IESG | Responsible AD | Robert Sparks | |
Send notices to | (None) |
RFC 5968
4. Issues with RTCP Extensions Issues that have come up in the past with extensions to RTP and RTCP include (but are probably not limited to) the following: o Defining RTP or RTCP extensions only or primarily for unicast two- party sessions. RTP is inherently a group communication protocol, even when operating on a unicast connection. Extensions may become useful in the future well outside their originally intended area of application, and should consider this. Stating that something works for unicast only is not acceptable, particularly since various flavours of multicast have become relevant again, and as middleboxes such as repair servers, mixers, and RTCP- supporting Multipoint Control Units (MCUs) [RFC5117] become more widely used. o Assuming reliable (instant) state synchronisation. RTCP reports are sent irregularly and may be lost. Hence, there may be a significant time lag (several seconds) between intending to send a state update to the RTP peer(s) and the packet being received; in some cases, the packet may not be received at all. o Requiring reliable delivery of RTCP reports. While reliability can be implemented on top of RTCP using acknowledgements, this will come at the cost of significant additional delay, which may defeat the purpose of providing the feedback in the first place. Moreover, for scalability reasons due to the group-based nature of RTCP, these ACKs need to be adaptively rate limited or targeted to a subgroup or individual entity to avoid implosion as group sizes increase. RTCP is not intended or suitable for use as a reliable control channel. o Issuing commands, rather than giving hints. RTCP is about reporting observations -- in a best-effort manner -- between RTP entities. Causing actions on the remote side requires some form of reliability (see above), and adherence cannot be verified. o Expanding RTCP reporting, to use it as a network management tool. RTCP is sensitive to the size of RTCP reports as the latter determines the mean reporting interval given a certain bitrate share for RTCP (yet, RTCP may also be used to report information that has fine-grained temporal characteristics, if summarisation or data reduction by the endpoint would lose essential resolution). The information going into RTCP reports should primarily target the peer(s) (and thus include information that can be meaningfully reacted upon); nevertheless, such reports may Ott & Perkins Informational [Page 9] RFC 5968 Guidelines for RTCP Extensions September 2010 provide useful information to augment other network management tools. Gathering and reporting statistics beyond this is not an RTCP task and should be addressed by out-of-band protocols. o Creating serious complexity. Related to the previous item, RTCP reports that convey all kinds of data need to gather and calculate/infer this information to begin with (which requires very precise specifications). Given that it already seems to be difficult to even implement baseline RTCP, any added complexity can only discourage implementers, may lead to buggy implementations (in which case the reports do not serve their intended purpose), and hinder interoperability. o Introducing architectural issues. Extensions are written without considering the architectural concepts of RTP. For example, point-to-point communication is assumed, yet third-party monitors are expected to listen in. Besides being a bad idea to rely on eavesdropping entities on the path, this is obviously not possible if Secure RTP (SRTP) is being used with encrypted SRTCP packets. This list is surely not exhaustive. Also, the authors do not claim that the suggested extensions (even if using acknowledgements) would not serve a legitimate purpose. We rather want to draw attention to the fact that the same results may be achievable in a way that is architecturally cleaner and conceptually more RTP/RTCP-compliant. The following section contains a first attempt to provide some guidelines on what to consider when thinking about extensions to RTP and RTCP. 5. Guidelines Designing RTCP extensions requires consideration of a number of issues, as well as in-depth understanding of the operation of RTP mechanisms. While it is expected that there are many aspects not yet covered by RTCP reporting and operation, quite a bit of functionality is readily available for use. Other mechanisms should probably never become part of the RTP family of specifications, despite the existence of their equivalents in other environments. In the following, we provide some guidance to consider when (and before!) developing an extension to RTCP. We begin with a short checklist concerning the applicability of RTCP in the first place: o Check what can be done with the existing mechanisms, exploiting the information that is already available in RTCP. Is the need for an extension only perceived (e.g., due to lazy implementers, or artificial constraints in endpoints), or is the function or Ott & Perkins Informational [Page 10] RFC 5968 Guidelines for RTCP Extensions September 2010 data really not available (or derivable from existing reports)? It is worthwhile remembering that redundant information supplied by a protocol runs the risk of being inconsistent at some point, and various implementations may handle such situations differently (e.g., give precedence to different values). Similarly, there should be exactly one (well-specified) way of performing every function and operation of the protocol. o Is the extension applicable to RTP entities running anywhere in the Internet, or is it a link- or environment-specific extension? In the latter cases, local extensions (e.g., header compression, or non-RTP protocols) may be preferable. RTCP should not be used to carry information specific to a particular (access) link. o Is the extension applicable in a group communication environment, or is it specific to point-to-point communications? RTP and RTCP are inherently group communication protocols, and extensions must scale gracefully with increasing group sizes. From a conceptual viewpoint, the designer of every RTCP extension should ask -- and answer(!) -- at least the following questions: o How will this new building block complement and work with the other components of RTCP? Are all interactions fully specified? o Will this extension work with all different profiles (e.g., the Secure RTP profile [RFC3711], and the extended RTP profile for RTCP-based feedback [RFC4585])? Are any feature interactions expected? o Should this extension be kept in-line with baseline RTP and its existing profiles, or does it deviate so much from the base RTP operation that an incompatible new profile must be defined? Use and definition of incompatible profiles are strongly discouraged, but if they prove necessary, how do nodes using the different profiles interact? What are the failure modes, and how is it ensured that the system fails in a safe manner? o How does this extension interoperate with other nodes when the extension is not understood by the peer(s)? o How will the extension deal with different networking conditions (e.g., how does performance degrade with increases in losses and latency, possibly across orders of magnitude)? Ott & Perkins Informational [Page 11] RFC 5968 Guidelines for RTCP Extensions September 2010 o How will this extension work with group communication scenarios, such as multicast? Will the extensions degrade gracefully with increasing group sizes? What will be the impact on the RTCP report frequency and bitrate allocation? For the specific design, the following considerations should be taken into account (they're a mixture of common protocol design guidelines, and specifics for RTCP): o First of all, if there is (and for RTCP this applies quite often) a mechanism from a different networking environment, don't try to directly recreate this mechanism in RTP/RTCP. The Internet environment is extremely heterogeneous, and will often have drastically different properties and behaviour to other network environments. Instead, ask what the actual semantics and the result required to be perceived by the application or the user are. Then, design a mechanism that achieves this result in a way that is compatible with RTP/RTCP. (And do not forget that every mechanism will break when no packets get through -- the Internet does not guarantee connectivity or performance.) o Target re-usability of the specification. That is, think broader than a specific use case, and try to solve the general problem in cases where it makes sense to do so. Point solutions need a very good motivation to be dealt with in the IETF in the first place. This essentially suggests developing building blocks whenever possible, allowing them to be combined in different environments than initially considered. Where possible, avoid mechanisms that are specific to particular payload formats, media types, link or network types, etc. o For everything (packet format, value, procedure, timer, etc.) being defined, make sure that it is defined properly, so that independent interoperable implementation can be built. It is not sufficient that you can implement the feature: it has to be implemented in several years by someone unfamiliar with the working group discussion and industry context. Remember that fields need to be both generated and reacted upon, that mechanisms need to be implemented, etc., and that all of this increases the complexity of an implementation. Features that are too complex won't get implemented (correctly) in the first place. o Extensions defining new metrics and parameters should reference existing standards whenever possible, rather than try to invent something new and/or proprietary. Ott & Perkins Informational [Page 12] RFC 5968 Guidelines for RTCP Extensions September 2010 o Remember that not every bit or every action must be represented or signalled explicitly. It may be possible to infer the necessary pieces of information from other values or their evolution (a very prominent example is TCP congestion control). As a result, it may be possible to de-couple bits on the wire from local actions and reduce the overhead. o Particularly with media streams, reliability can often be "soft". Rather than implementing explicit acknowledgements, receipt of a hint may also be observed from the altered behaviour (e.g., the reception of a requested intra-frame, or changing the reference frame for video, changing the codec, etc.). The semantics of messages should be idempotent so that the respective message may be sent repeatedly. Requiring hard reliability does not scale with increasing group sizes, and does not degrade gracefully as network performance reduces. o Choose the appropriate extension point. Depending on the type of RTCP extension being developed, new data items can be transported in several different ways: * A new RTCP Source Description (SDES) item is appropriate for transporting data that describes the source, or the user represented by the source, rather than the ongoing media transmission. New SDES items may be registered to transport source description information of general interest (see [RFC3550], Section 15), or the PRIV item ([RFC3550], Section 6.5.8) may be used for proprietary extensions. * A new RTCP XR block type is appropriate for transporting new metrics regarding media transmission or reception quality (see [RFC3611], Section 6.2). * New RTP profiles may define a profile-specific extension to RTCP SR and/or RR packets, to give additional feedback (see [RFC3550], Section 6.4.3). It is important to note that while extensions using this mechanism have low overhead, they are not backwards compatible with other profiles. Where compatibility is needed, it's generally more appropriate to define a new RTCP XR block or a new RTCP packet type instead. * New RTCP AVPF (Audio-Visual Profile with Feedback) transport- layer feedback messages should be used to transmit general- purpose feedback information that will be generated and processed by the RTP transport. Examples include (negative) Ott & Perkins Informational [Page 13] RFC 5968 Guidelines for RTCP Extensions September 2010 acknowledgements for particular packets, or requests to limit the transmission rate. This information is intended to be independent of the codec or application in use (see [RFC4585], Sections 6.2 and 9). * New RTCP AVPF payload-specific feedback messages should be used to convey feedback information that is specific to a particular media codec, RTP payload format, or category of RTP payload formats. Examples include video picture loss indication or reference picture selection, which are useful for many video codecs (see [RFC4585], Sections 6.3 and 9). * New RTCP AVPF application layer feedback messages should be used to convey higher-level feedback, from one application to another, above the level of codecs or transport (see [RFC4585], Sections 6.4 and 9). * A new RTCP application-defined, or APP, packet is appropriate for private use by applications that don't need to interoperate with others, or for experimentation before registering a new RTCP packet type ([RFC3550], Section 6.7). It is not appropriate to define a new RTCP APP packet in a standards document: use one of the other extension points, or define a new RTCP packet type instead. * Finally, new RTCP packet types may be registered with IANA if none of the other RTCP extension points are appropriate (see [RFC3550], Section 15). The RTP framework was designed following the principle of application level framing with integrated layer processing, proposed by Clark and Tennenhouse [ALF]. Effective use of RTP requires that extensions and implementations be designed and built following the same philosophy. That philosophy differs markedly from many previous systems in this space, and making effective use of RTP requires an understanding of those differences. 6. Security Considerations This memo does not specify any new protocol mechanisms or procedures, and so raises no explicit security considerations. When designing RTCP extensions, it is important to consider the following points: Ott & Perkins Informational [Page 14] RFC 5968 Guidelines for RTCP Extensions September 2010 o Privacy: RTCP extensions, in particular new Source Description (SDES) items, can potentially reveal information considered to be sensitive by end users. Extensions should carefully consider the uses to which information they release could be put, and should be designed to reveal the minimum amount of additional information needed for their correct operation. o Congestion control: RTCP transmission timers have been carefully designed such that the total amount of traffic generated by RTCP is a small fraction of the media data rate. One consequence of this is that the individual RTCP reporting interval scales with both the media data rate and the group size. The RTCP timing algorithms have been shown to scale from two-party unicast sessions to groups with tens of thousands of participants, and to gracefully handle flash crowds and sudden departures [TimerRecon]. Proposals that modify the RTCP timer algorithms must be careful to avoid congestion, potentially leading to denial of service, across the full range of environments where RTCP is used. o Denial of service: RTCP extensions that change the location where feedback is sent must be carefully designed to prevent denial of service attacks against third-party nodes. When such extensions are signalled, for example in the Session Description Protocol (SDP), this typically requires some form of authentication of the signalling messages (e.g., see the security considerations of [RFC5760]). The security considerations of the RTP specification [RFC3550] apply, along with any applicable profile (e.g., [RFC3551]). 7. Acknowledgements This document has been motivated by many discussions in the AVT WG. The authors would like to acknowledge the active members in the group for providing the inspiration. 8. References 8.1. Normative References [RFC2198] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse- Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, September 1997. [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, April 1998. Ott & Perkins Informational [Page 15] RFC 5968 Guidelines for RTCP Extensions September 2010 [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. [RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556, July 2003. [RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control Protocol Extended Reports (RTCP XR)", RFC 3611, November 2003. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004. [RFC4571] Lazzaro, J., "Framing Real-time Transport Protocol (RTP) and RTP Control Protocol (RTCP) Packets over Connection-Oriented Transport", RFC 4571, July 2006. [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. [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. Hakenberg, "RTP Retransmission Payload Format", RFC 4588, July 2006. [RFC5109] Li, A., "RTP Payload Format for Generic Forward Error Correction", RFC 5109, December 2007. [RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced- Size Real-Time Transport Control Protocol (RTCP): Opportunities and Consequences", RFC 5506, April 2009. 8.2. Informative References [RFC1925] Callon, R., "The Twelve Networking Truths", RFC 1925, April 1996. [RFC5117] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117, January 2008. Ott & Perkins Informational [Page 16] RFC 5968 Guidelines for RTCP Extensions September 2010 [RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control Protocol (RTCP) Extensions for Single-Source Multicast Sessions with Unicast Feedback", RFC 5760, February 2010. [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and Control Packets on a Single Port", RFC 5761, April 2010. [RFC5762] Perkins, C., "RTP and the Datagram Congestion Control Protocol (DCCP)", RFC 5762, April 2010. [ALF] Clark, D. and D. Tennenhouse, "Architectural Considerations for a New Generation of Protocols", Proceedings of ACM SIGCOMM 1990, September 1990. [TimerRecon] Schulzrinne, H. and J. Rosenberg, "Timer Reconsideration for Enhanced RTP Scalability", Proceedings of IEEE Infocom 1998, March 1998. Authors' Addresses Joerg Ott Aalto University School of Science and Technology Otakaari 5 A Espoo, FIN 02150 Finland EMail: jo@netlab.tkk.fi Colin Perkins University of Glasgow Department of Computing Science Glasgow G12 8QQ United Kingdom EMail: csp@csperkins.org Ott & Perkins Informational [Page 17]