Guidelines for End-to-End Support of the RTP Control Protocol (RTCP) in Back-to-Back User Agents (B2BUAs)

Session Initiation Protocol (SIP) Back-to-Back User Agents (B2BUAs) are SIP entities that can act as a logical combination of both a User Agent Server (UAS) and a User Agent Client (UAC). As such, their behaviour is not always completely adherent to standards and can lead to unexpected situations. presents a taxonomy of the most commonly deployed B2BUA implementations and describes how they differ in terms of the functionality and features they provide. Such components often do not only act on the signalling plane (intercepting and possibly modifying SIP messages), but also on the media plane. This means that, in order to receive and manage all RTP and RTCP packets in a session, these components also manipulate the session descriptions in the related offer/answer exchanges . The reasons for such behaviour can be different. The B2BUA may want, for instance, to provide transcoding functionality for participants with incompatible codecs, or it may need the traffic to be directly handled for different reasons. This can lead to several different topologies for RTP-based communication, as documented in . Whatever the reason, such behaviour does not come without a cost. In fact, whenever a media-aware component is placed on the path between two or more participants that want to communicate by means of RTP/RTCP, the end-to-end nature of such protocols is broken. While this may not be a problem for RTP packets, which can be quite easily relayed, it definitely can cause serious issue for RTCP messages, which carry important information and feedback on the communication quality the participants are experiencing. Consider, for instance, the simple scenario only involving two participants and a single RTP session depicted in :

| |=== SSRC3 ===>| | | Alice | | B2BUA | | Bob | | |<=== SSRC2 ===| |<=== SSRC4 ===| | +--------+ +---------+ +---------+ ]]> In this common scenario, a participant (Alice) is communicating with another participant (Bob) as a result of a signalling session managed by a B2BUA: this B2BUA is also on the media path between the two and is acting as a Media Relay. This means that two separate RTP sessions are involved (one per side), each carrying two RTP streams (one per media direction). As part of this process, the B2BUA is also rewriting some of the RTP header information on the way. In this example, just the Synchronization Source (SSRC) of the incoming RTP streams is changed, but more information may be modified as well (e.g., sequence numbers, timestamps, etc.). In particular, whenever Alice sends an RTP packet, she sets her SSRC (SSRC1) in the RTP header of her RTP source stream. The B2BUA rewrites the SSRC (SSRC3) before relaying the packet to Bob. At the same time, RTP packets sent by Bob (SSRC4) get their SSRC rewritten as well (SSRC2) before being relayed to Alice. Assuming now that Alice needs to inform Bob that she has lost several packets in the last few seconds, she will place the related received RTP stream SSRC she is aware of (SSRC2) together with her own (SSRC1) in RTCP Reports and/or NACKs. Since the B2BUA is making use of different SSRCs for the RTP streams in the RTP session it established with each participant, blindly relaying Alice's incoming RTCP messages to Bob would cause issues. These RTCP messages would reference SSRCs Bob doesn't know about, which would result in precious feedback being dropped. In fact, Bob is only aware of SSRC4 (the one his source RTP stream uses) and SSRC3 (the one he's receiving from the B2BUA in the received RTP stream) and knows nothing about SSRC1 and SSRC2 in the messages he received instead. Considering the feedback being dropped because of this may contain precious information (e.g., related to packet loss, congestion, and other network issues or considerations), the inability to take them into account may lead to severe issues. For instance, Bob may flood Alice with more media packets she can handle and/or not retransmit the packets she missed and asked for. This may easily lead to a very bad communication experience, if not eventually to an unwanted termination of the communication itself. This is just a trivial example that, together with additional scenarios, will be addressed in the following sections. Nevertheless, it is a valid example of how such a simple mishandling of precious information may lead to serious consequences. This is especially true if we picture more complex scenarios involving several participants at the same time, multiple RTP sessions (e.g., a video stream along audio) rather than a single one, redundancy RTP streams, SSRC multiplexing, and so on. Considering how common B2BUA deployments are, it is very important for them to properly address RTCP messages in order to be sure that their activities on the media plane do not break or interfere with anything relevant to the session.

As described in the Introduction (), it's very common for B2BUA deployments to act on the media plane rather than just on the signalling plane alone. In particular, describes three different categories of such B2BUAs: (1) a simple Media Relay that is effectively unaware of anything that is transported; (2) a Media-aware Relay that inspects and/or modifies RTP and RTCP messages as they flow by; and (3) a full-fledged media termination entity that terminates and generates RTP and RTCP messages as needed. and already mandate some specific behaviours in the presence of certain topologies. However, due to their mixed nature, B2BUAs sometimes can't or won't implement all relevant specifications. This means that it's not rare to encounter issues that may be avoided with more disciplined behaviour in that regard, that is, if the B2BUAs followed at least a set of guidelines to ensure no known problems occur. For this reason, the following subsections describe the proper behaviour that B2BUAs, whatever above category they fall in, should follow in order not to impact any end-to-end RTCP effectiveness.

A Media Relay, as identified in , simply forwards all RTP and RTCP messages it receives without either inspecting or modifying them. Using the terminology in "RTP Topologies" , this can be seen as an RTP Transport Translator. As such, B2BUAs acting as Media Relays are not aware of what traffic they're handling. This means that both packet payloads and packet headers are opaque to them. Many Session Border Controllers (SBCs) implement this kind of behaviour, e.g., when acting as a bridge between an inner and outer network. Considering that all headers and identifiers in both RTP and RTCP are left untouched, issues like the SSRC mismatch described in the previous section would not occur. However, similar problems could still happen for different reasons, for instance, if the session description prepared by the B2BUA, whether it has been modified or not, ends up providing incorrect information. This may happen, for example, if the Session Description Protocol (SDP) on either side contains 'ssrc' attributes that don't match the actual SSRC being advertised on the media plane or if the B2BUA advertised support for NACK because it implements it while the original INVITE didn't. Such issues might occur, for instance, when the B2BUA acting as a Media Relay is generating a new session description when bridging an incoming call rather than using the original session description. This may cause participants to find a mismatch between the SSRCs advertised in the SDP and the ones actually observed in RTP and RTCP messages or to have them either ignore or generate RTCP feedback packets that were not explicitly advertised as supported. In order to prevent such an issue, a Media Relay B2BUA SHOULD forward all the SSRC- and RTCP-related SDP attributes when handling a multimedia session setup between participants: this includes attributes like 'ssrc' , 'rtcp-fb' , 'rtcp-xr-attrib' , and others. However, certain SDP attributes may lead to call failures when forwarded by a Media Relay, as they have an implied assumption that the attribute describes the immediate peer. A clear example of this is the 'rtcp' attribute, which describes the expected RTCP peer port. Other attributes might include the immediate peer's IP address, preferred transport, etc. In general, the guideline is to require rewriting of attributes that are implicitly describing the immediate peer. B2BUAs SHOULD forward all other SDP attributes in order to avoid breaking additional functionality that endpoints may be relying on. If implementors have doubts about whether this guidance applies to a specific attribute, they should test to determine if call failures occur. The cited 'rtcp' example is also relevant whenever RTP/RTCP multiplexing support is being negotiated. If the B2BUA acting as a Media Relay is unaware of the specifics of the traffic it is handling, and as such may not have RTP/RTCP parsing capabilities, it SHOULD reject RTP/RTCP multiplexing by removing the 'rtcp-mux' SDP attribute. If instead the Media Relay is able to parse RTP/RTCP, and can verify that demultiplexing can be performed without any RTP Payload Type rewrites (i.e., no overlap between any RTP Payload Types and the RTCP Payload Type space has been detected), then the B2BUA SHOULD negotiate RTP/RTCP multiplexing support if advertised. It is worth mentioning that, by leaving RTCP messages untouched, a Media Relay may also leak information that, according to policies, may need to be hidden or masqueraded, e.g., domain names in CNAME items. Besides, these CNAME items may actually contain IP addresses: this means that, should a NAT be involved in the communication, this may actually result in CNAME collisions, which could indeed break the end-to-end RTCP behaviour. While can prevent this from happening, there may be implementations that don't make use of it. As such, a B2BUA MAY rewrite CNAME items if any potential collision is detected, even in the Media Relay case. If a B2BUA does indeed decide to rewrite CNAME items, then it MUST generate new CNAMEs following . The same SHOULD be done if RTP extensions involving CNAMEs are involved (e.g., "urn:ietf:params:rtp-hdrext:sdes:cname" ). If that is not possible, e.g., because the Media Relay does not have RTP header editing capabilities or does not support these extensions, then the B2BUA MUST reject the negotiation of such extensions when negotiating the session.

A Media-aware Relay, unlike the Media Relay addressed in the previous section, is aware of the media traffic it is handling. This means it inspects RTP and RTCP messages flowing by and may even modify their headers. Using the terminology in , this can be seen as an RTP Translator. A B2BUA implementing this role typically does not inspect the RTP payloads, which would be opaque to them: this means that the actual media would not be manipulated (e.g., transcoded). This makes them quite different from the Media Relays previously discussed, especially in terms of the potential issues that may occur at the RTCP level. In fact, being able to modify the RTP and RTCP headers, such B2BUAs may end up modifying RTP-related information like SSRC / Contributing Source (CSRC), sequence numbers, timestamps, and others in an RTP stream before forwarding the modified packets to the other interested participants. This means that, if not properly disciplined, such behaviour may easily lead to issues like the one described in the introductory section. For this reason, it is very important for a B2BUA modifying RTP-related information across two related RTP streams to also modify, in a coherent way, the same information in RTCP messages. It is worthwhile to point out that such a B2BUA may not necessarily forward all the packets it receives. Selective Forwarding Units (SFUs) , for instance, may be implemented to aggregate or drop incoming RTCP messages while at the same time originating new ones on their own. It is important to clarify that a B2BUA SHOULD NOT randomly drop or forward RTCP feedback of the same type (e.g., a specific XR block type or specific Feedback messages) within the context of the same session as that may lead to confusing, if not broken, feedback to the recipients of the message due to gaps in the communication. As to the messages that are forwarded and/or aggregated, it's important to make sure the information is coherent. Besides the behaviour already mandated for RTCP translators in Section 7.2 of , a media-aware B2BUA MUST handle incoming RTCP messages to forward following these guidelines: If the B2BUA has changed the SSRC of the sender RTP stream a Sender Report refers to, it MUST update the SSRC in the SR packet header as well. If the B2BUA has changed the SSRCs of other RTP streams too, and any of these streams are addressed in any of the SR Report Blocks, it MUST update the related values in the SR Report Blocks as well. If the B2BUA has also changed the base RTP sequence number when forwarding RTP packets, then this change MUST be reflected in the 'extended highest sequence number received' field in the Report Blocks. In case the B2BUA is acting as a Selective Forwarding Unit (SFU) , it needs to track in the outgoing SR, the relevant number of packets sent, and the total amount of bytes sent to the receiver. The guidelines for SR apply to RR as well. If the B2BUA has changed the SSRC of any RTP stream addressed in any of the chunks of an incoming SDES message, it MUST update the related SSRCs in all the chunks. The same considerations made with respect to CNAME collisions at the end of apply here as well. If the B2BUA has changed the SSRC of any RTP stream addressed in the SSRC/CSRC identifiers included in a BYE packet, it MUST update them in the message. If the B2BUA has changed the SSRC of any RTP stream addressed in the header of an APP packet, it MUST update the identifier in the message. Should the B2BUA be aware of any specific APP message format that contains additional information related to SSRCs, it SHOULD update them accordingly as well. If the B2BUA has changed the SSRC of the RTP stream associated with the originator of an XR packet, it MUST update the SSRC in the XR message header. The same guidelines given for SR/RR, with respect to SSRC identifiers in Report Blocks, apply to all the Report Block types in the XR message as well. If the B2BUA has also changed the base RTP sequence number when forwarding RTP packets, then this change MUST be reflected in the 'begin_seq' and 'end_seq' fields that are available in most of the Report Block types that are part of the XR specification. If the B2BUA has changed any SSRC of RTP streams addressed in an RSI packet, it MUST update the SSRC identifiers in the message. This includes the distribution source SSRC, which MUST be rewritten with the one the B2BUA uses to send RTP packets to each sender participant, the summarized SSRC, and when a Collision Sub-Report Block is available, the SSRCs in the related list. If the B2BUA has changed any SSRC of RTP streams addressed in a TOKEN packet, it MUST update the SSRC identifiers in the message. This includes the Packet Sender SSRC, which MUST be rewritten with the one the B2BUA uses to send RTP packets to each sender participant, and the Requesting Client SSRC when the message is a response, which MUST be rewritten using the related sender participant(s) SSRC. All Feedback messages have a common packet format, which includes the SSRC identifier of the Packet Sender and the SSRC identifier of the media source the feedback is related to. Just as described for the previous messages, these SSRC identifiers MUST be updated in the message if the B2BUA has changed the SSRC of the RTP streams addressed there. It MUST NOT, however, change a media source SSRC that was originally set to zero, unless zero is actually the SSRC that was chosen by one of the involved endpoints, in which case the above-mentioned rules as to SSRC rewriting apply. Considering that many Feedback messages also include additional data as part of their specific Feedback Control Information (FCI), a media-aware B2BUA MUST take care of them accordingly, if it can parse and regenerate them, according to the following guidelines: A media-aware B2BUA MUST properly rewrite the Packet ID (PID) of all addressed lost packets in the NACK FCI if it changed the RTP sequence numbers. A media-aware B2BUA MUST properly rewrite the additional SSRC identifier in the specific FCI if it changed the related RTP SSRC of the media sender. describes an RTCP payload-specific Feedback message that reports the receiver's available bandwidth to the sender. As of the time of this writing, REMB has been widely deployed but has not been standardized. The REMB mechanism will not function correctly across a media-aware B2BUA that changes the SSRC of the media sender unless it also changes the SSRC values in the REMB packet. The same guidelines given for SR/RR management apply, considering the presence of sequence numbers in the ECN Feedback Report format. For the management of RTCP XR ECN Summary Report messages, the same guidelines given for generic XR messages apply. Apart from the generic guidelines related to Feedback messages, no additional modifications are needed for Picture Loss Indication (PLI), Slice Lost Indication (SLI), and Reference Picture Selection Indication (RPSI) Feedback messages. Of course, the same considerations about the need for SDP and RTP/RTCP information to be coherent applies to media-aware B2BUAs. This means that, if a B2BUA changes any SSRC, it MUST update the related 'ssrc' attributes, if present, before sending it to the recipient. Besides, it MUST rewrite the 'rtcp' attribute if provided. At the same time, while a media-aware B2BUA is typically able to inspect/modify RTCP messages, it may not support all RTCP messages. This means that a B2BUA may choose to drop RTCP messages it can't parse. In that case, a media-aware B2BUA MUST advertise its RTCP level of support in the SDP in a coherent way in order to prevent, for instance, a UAC from sending NACK messages that would never reach the intended recipients. It's important to point out that, in case a compound RTCP packet was received and any RTCP message in it needs to be dropped, then the B2BUA SHOULD NOT drop the whole compound RTCP packet, but only the selected messages. The same considerations on CNAMEs made in regard to Media Relays apply to Media-aware Relays as well. Specifically, if RTP extensions involving CNAMEs are involved (e.g., "urn:ietf:params:rtp-hdrext:sdes:cname" ) and negotiated because the B2BUA supports them, then the B2BUA MUST update the CNAME value in there as well, if it was changed. It is worth pointing out that, if the new CNAME is larger than the old one, this would result in a larger RTP packet than originally received. If the length of the updated packet exceeds the MTU of any of the networks the packet will traverse, this can result in the packet being dropped and lost by the recipient. A different set of considerations is worthwhile for RTP/RTCP multiplexing and Reduced-Size RTCP . While the former allows for a better management of network resources by multiplexing RTP packets and RTCP messages over the same transport, the latter allows for a compression of RTCP messages, thus leading to less network traffic. For RTP/RTCP multiplexing, a B2BUA acting as a Media Relay may use it on either RTP session independently. This means that, for instance, a Media Relay B2BUA may use RTP/RTCP multiplexing on one side of the communication and not use it on the other side, if the endpoint does not support it. This allows for a better management of network resources on the side that does support it. In case any of the parties in the communications supports it and the B2BUA does too, the related 'rtcp-mux' SDP attribute MUST be forwarded on the other side(s). If the B2BUA detects that any of the parties in the communication do not support the feature, it may decide to either disable it entirely or still advertise it for the RTP sessions with parties that do support it. In case the B2BUA decides to involve RTP/RTCP multiplexing, it MUST ensure that there are no conflicting RTP Payload Type numbers on either side. When there are, it MUST rewrite RTP Payload Type numbers to prevent conflicts in the session where the RTP/RTCP multiplexing is applied. Should RTP Payload Types be rewritten, the related information in the SDP MUST be updated accordingly. For Reduced-Size RTCP, the considerations are a bit different. In fact, while a Media Relay B2BUA may choose to use it on the side that supports it and not on the side that doesn't, there are several reasons for discouraging such behaviour. While Reduced-Size allows for less network traffic related to RTCP messaging in general, this gain may lead a Reduced-Size RTCP implementation to also issue a higher rate of RTCP Feedback messages. This would result in increased RTCP traffic on the side that does not support Reduced-Size and could, as a consequence, actually be counterproductive if the available bandwidth is different on the two sides. Negotiating a session with both sides would allow the B2BUA to discover which one supports Reduced-Size and which doesn't and decide whether or not to allow the sides to independently use Reduced-Size. Should the B2BUA decide to disable the feature on all sides, which is suggested in case Reduced-Size is not supported by all parties involved, it MUST NOT advertise support for the Reduced-Size RTCP functionality on either side, by removing the 'rtcp-rsize' attribute from the SDP.

A Media Terminator B2BUA, unlike simple Media Relays and media-aware ones, is able to terminate media itself. As such, it can inspect and/or modify RTP payloads as well. This means that such components, for instance, can act as media transcoders and/or originate specific RTP media. Using the terminology in "RTP Topologies" , this can be seen as an RTP Media Translator. Such a topology can also be seen as a back-to-back RTP session through a middlebox, as described in Section 3.2.2 of . Such a capability makes them quite different from the previously introduced B2BUA typologies. Since such a B2BUA would terminate RTP itself, it can take care of the related statistics and feedback functionality directly, with no need to simply relay any message between the participants in the multimedia session. For this reason, no specific guideline is needed to ensure a proper end-to-end RTCP behaviour in such scenarios, because most of the time, there would be no end-to-end RTCP interaction among the involved participants in the first place. Nevertheless, should any RTCP message actually need to be forwarded to another participant in the multimedia session, the same guidelines provided for the media-aware B2BUA case apply. For RTP/RTCP multiplexing support, the same considerations already given for the Media Relay management also apply to Media Terminators. Some different considerations might be given as to the Reduced-Size RTCP functionality instead. In fact, in the Media Terminator case, it is safe to use the feature independently on each side, as the B2BUA would terminate RTCP. In that case, the B2BUA SHOULD advertise and negotiate support for Reduced-Size if available and MUST NOT otherwise.