STRAW Working Group                                           L. Miniero
Internet-Draft                                                  Meetecho
Intended status: Standards Track                       S. Garcia Murillo
Expires: September 10, 25, 2015                                      Medooze
                                                              V. Pascual
                                                          March 9, 24, 2015

   Guidelines to support RTCP end-to-end in Back-to-Back User Agents


   SIP Back-to-Back User Agents (B2BUAs) are often envisaged to also be
   on the media path, rather than just intercepting signalling.  This
   means that B2BUAs often implement an RTP/RTCP stack as well, whether
   to act as media transcoders or to just passthrough the media
   themselves, thus leading to separate multimedia sessions that the
   B2BUA correlates and bridges together.  If not disciplined, though,
   this behaviour can severely impact the communication experience,
   especially when statistics and feedback information contained in RTCP
   packets get lost because of mismatches in the reported data.

   This document defines the proper behaviour B2BUAs should follow when
   also acting on the signalling/media plane in order to preserve the
   end-to-end functionality of RTCP.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 10, 25, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Signalling/Media Plane B2BUAs . . . . . . . . . . . . . . . .   5
     3.1.  Media Relay . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Media-aware Relay . . . . . . . . . . . . . . . . . . . .   6
     3.3.  Media Terminator  . . . . . . . . . . . . . . . . . . . .  10
   4.  Media Path Security . . . . . . . . . . . . . . . . . . . . .  11
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  Change Summary  . . . . . . . . . . . . . . . . . . . . . . .  12
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13  14
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   Session Initiation Protocol [RFC3261] 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 completelely adherent to the
   standards, and can lead to unexpected situations the IETF is trying
   to address.  [RFC7092] presents a taxonomy of the most deployed B2BUA
   implementations, describing how they differ in terms of the
   functionality and features they provide.

   Such components often do not only act on the signalling plane, that
   is intercepting and possibly modifying SIP messages, but also on the
   media plane.  This means that, when on the signalling path between
   two or more participants willing to communicate, such components also
   manipulate the session description [RFC4566] in order to have all RTP
   and RTCP [RFC3550] pass through it as well within the context of an
   SDP offer/answer [RFC3264].  The reasons for such a 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 like
   billing, lawful interception, session recording and so on.  This can
   lead to several different topologies for RTP-based communication, as
   documented in [RFC5117].  These topologies are currently being
   updated to address new commonly encountered scenarios as well

   Whatever the reason, such a 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, and
   their effectiveness may be affected as a consequence.  While this may
   not be a problem for RTP packets, which from a protocol point of view
   just contain opaque media packets and as such can be quite easily
   relayed, it definitely can cause serious issue for RTCP packets,
   which carry important information and feedback on the communication
   quality the participants are experiencing.  In fact, RTCP packets
   make use of specific ways to address the media they are referring to.
   Consider, for instance, the simple scenario only involving two
   participants and a single RTP session depicted in Figure 1:

   +--------+              +---------+              +---------+
   |        |=== SSRC1 ===>|         |=== SSRC3 ===>|         |
   | Alice  |              |  B2BUA  |              |   Bob   |
   |        |<=== SSRC2 ===|         |<=== SSRC4 ===|         |
   +--------+              +---------+              +---------+

                   Figure 1: B2BUA modifying RTP headers

   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, though, it is
   also rewriting some of the RTP header information on the way, for
   instance because that's how its RTP relaying stack works: in this
   example, just the SSRC of the incoming RTP audio streams is changed,
   but more information may be changed as well (e.g., sequence numbers,
   timestamps, etc.).  In particular, whenever Alice sends an audio RTP
   packet, she sets her SSRC (SSRC1) to 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 she has lost several
   audio packets in the last few seconds, maybe because of a network
   congestion, she would of course place the related received RTP stream
   SSRC she is aware of (SSRC2), together with her own (SSRC1), in RTCP
   Reports and/or NACKS to do so, hoping for a retransmission [RFC4588]
   or for Bob to slow down.  Since the B2BUA is making use of different
   SSRCs for the RTP streams in the RTP session it established with each
   participant, a blind relaying of the RTCP packets to Bob would in
   this case result, from Bob's perspective, in unknown SSRCs being
   addressed, thus resulting in the precious information being dropped.
   In fact, Bob is only aware of SSRCs 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 SSRCs SSRC1 and SSRC2
   in the RTCP packets he would receive instead.  As a consequence of
   the feedback being dropped, unaware of the issue Bob may continue to
   flood Alice with even more media packets and/or not retransmit Alice
   the packets she missed, which 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 trivial mishandling
   of precious information may lead to serious consequences, especially
   considering that more complex scenarios may involve 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
   such feedback, in order to be sure that their activities on the media
   plane do not break anything they're not supposed to.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Besides, this document addresses, where relevant, the RTP-related
   terminology as disciplined in

3.  Signalling/Media Plane B2BUAs

   As anticipated in the introductory section, it's very common for
   B2BUA deployments to also act on the media plane, rather than just
   signalling alone.  In particular, [RFC7092] describes three different
   categories of such B2BUAs, according to the level of activities
   performed on the media plane: a B2BUA, in fact, may act as a simple
   media relay (1), effectively unaware of anything that is transported;
   it may be a media-aware relay (2), also inspecting and/or modifying
   RTP and RTCP packets as they flow by; or it may be a full-fledged
   media termination entity, terminating and generating RTP and RTCP
   packets as needed.

   While [RFC3550] and [RFC5117] already mandate some specific
   behaviours when specific topologies are deployed, not all deployments
   strictly adhere to the specifications and as such it's not rare to
   encounter issues that may be avoided with a more disciplined
   behaviour in that regard.  For this reason, the following subsections
   will describe the proper behaviour B2BUAs, whatever above category
   they fall in, should follow in order to avoid, or at least minimize,
   any impact on end-to-end RTCP effectiveness.

3.1.  Media Relay

   A media relay as identified in [RFC7092] basically just forwards,
   from an application level point of view, all RTP and RTP packets it
   receives, without either inspecting or modifying them.  Using the RTP
   Topologies terminology, this can be seen as a RTP Transport
   Translator.  As such, B2BUA acting as media relays are not aware of
   what traffic they're handling, meaning that not only the packet
   payloads are opaque to them, but headers as well.  Many Session
   Border Controllers (SBC) implement this kind of behaviour, e.g., when
   acting as a bridge between an inner and outer network.

   Considering 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.  Similar problems could occur, though,
   should the session description end up providing incorrect information
   about the media flowing (e.g., if the SDP on either side contain
   'ssrc' [RFC5576] attributes that don't match the actual SSRC being
   advertized on the media plane) or about the supported RTCP mechanisms
   (e.g., in case the B2BUA advertized support for NACK because it
   implements it, but the original INVITE didn't).  Such an issue might
   occur, for instance, in case the B2BUA acting as a media relay is
   generating a new session description when bridging an incoming call,
   rather than taking into account the original session description in
   the first place.  This may cause the participants to find a mismatch
   between the SSRCs advertized in SDP and the ones actually observed in
   RTP and RTCP packets (which may indeed change during a multimedia
   session anyway, but having them synced during setup would help
   nonetheless), or having them either ignore or generate RTCP feedback
   packets that were not explicitly advertized 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 interested participants: this
   includes attributes like 'ssrc' [RFC3261], 'rtcp-fb' [RFC4585],
   'rtcp-xr-attrib' [RFC3611] and others.  It SHOULD NOT, though,
   blindly forward all SDP attributes, as some of them (e.g.,
   candidates, fingerprints, crypto, etc.) may lead to call failures for
   different reasons out of scope to this document.  One notable example
   is the 'rtcp' [RFC3605] attribute that UAC may make use of to
   explicitly state the port they're willing to use for RTCP:
   considering the B2BUA would relay RTCP packets, the port as seen by
   the other UAC involved in the communication would differ from the one
   negotiated originally, and as such it MUST be rewritten accordingly.

   Besides, it is worth mentioning that, leaving RTCP packets untouched,
   a media relay may also let through information that, according to
   policies, may be best left hidden or masqueraded, e.g., domain names
   in CNAME items.  Nevertheless, that information cannot break the end-
   to-end RTCP behaviour.

3.2.  Media-aware Relay

   A Media-aware relay, unlike the the Media Relay addressed in the
   previous section, is actually aware of the media traffic it is
   handling.  As such, it is able to inspect RTP and RTCP packets
   flowing by, and may even be able to modify the headers in any of them
   before forwarding them.  Using the RFC3550 terminology, this can be
   seen as a RTP Translator.  A B2BUA implementing this role would
   typically not, though, inspect the RTP payloads as well, 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 Relay 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 (and hence CSRC lists, that must of course be updated
   accordingly), sequence numbers, timestamps and the like in an RTP
   stream, before forwarding the modified packets to the other
   interested participants in the multimedia sessions on the RTP streams
   they're using to receive the media.  This means that, if not properly
   disciplined, such a behaviour may easily lead to issues like the one
   described in the introductory section.  As such, it is very important
   for a B2BUA modifying RTP-related information across two related RTP
   streams to also modify the same information in RTCP packets as well,
   and in a coherent way, so that not to confuse any of the participants
   involved in a communication.

   It is worthwile to point out that such a B2BUA would not necessarily
   forward all the packets it is receiving, though: Selective Forwarding
   Units (SFU) [I-D.ietf-avtcore-rtp-topologies-update], for instance,
   could aggregate or drop incoming RTCP messages, while at the same
   time originating new ones on their own.  For the messages that are
   forwarded and/or aggregated, though, it's important to make sure the
   information is coherent.

   Besides the behaviour already mandated for RTCP translators in
   Section 7.2 of [RFC3550], a media-aware B2BUA MUST also handle
   incoming RTCP messages to forward following this guideline:

   SR:  [RFC3550]
      If the B2BUA has changed any SSRC in any RTP streams relation, it
      MUST update the SSRC-related information in the incoming SR packet
      before forwarding it.  This includes the sender SSRC, which MUST
      be rewritten with the one the B2BUA uses in the RTP stream used to
      receive RTP packets from each participant, and the SSRC
      information in all the blocks, which MUST be rewritten using the
      related sender participant(s) SSRC.  If the B2BUA has also changed
      the base RTP sequence number when forwarding RTP packets, then
      this change needs to be properly addressed in the 'extended
      highest sequence number received' field in the Report Blocks.

   RR:  [RFC3550]
      The same guidelines given for SR apply for RR as well.

   SDES:  [RFC3550]
      If the B2BUA has changed any SSRC in any direction, it MUST update
      the SSRC-related information in all the chunks in the incoming
      SDES packet before forwarding it.

   BYE:  [RFC3550]
      If the B2BUA has changed any SSRC in any direction, it MUST update
      the SSRC in the BYE message.

   APP:  [RFC3550]
      If the B2BUA has changed any SSRC in any direction, it MUST update
      the SSRC in the APP message.  Should the B2BUA be aware of any
      specific APP message format that contains additional information
      related to SSRCs, it SHOULD update them as well.

   Extended Reports (XR):  [RFC3611]
      If the B2BUA has changed any SSRC in any direction, it MUST update
      the SSRC-related information in the incoming XR message header
      before forwarding it.  This includes the source SSRC, which MUST
      be rewritten with the one the B2BUA uses to send RTP packets to
      each sender participant, and the SSRC information in all the block
      types that include it, which MUST be rewritten using the related
      sender participant(s) SSRC.  If the B2BUA has also changed the
      base RTP sequence number when forwarding RTP packets, then this
      change needs to be properly addressed 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.

   Receiver Summary Information (RSI):  [RFC5760]
      If the B2BUA has changed any SSRC in any direction, it MUST update
      the SSRC-related information in the incoming RSI message header
      before forwarding it.  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, in
      case a Collision Sub-Report Block is available, the SSRCs in the
      related list.

   Port Mapping (TOKEN):  [RFC6284]
      If the B2BUA has changed any SSRC in any direction, it MUST update
      the SSRC-related information in the incoming TOKEN message before
      forwarding it.  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 in case
      the message is a response, which MUST be rewritten using the
      related sender participant(s) SSRC.

   Feedback messages:  [RFC4585]
      All Feedback messages have a common packet format, which includes
      the SSRC of the packet sender and the one of the media source the
      feedack is related to.  Just as described for the previous
      messages, these SSRC identifiers MUST be updated if the B2BUA has
      changed any SSRC in any direction.  It MUST NOT, though, change a
      media source SSRC that was originally set to zero. 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.  Besides, 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.

   NACK:  [RFC4585]
      Besides the common packet format management for feedback messages,
      a media-aware B2BUA MUST also properly rewrite the Packet ID (PID)
      of all addressed lost packets in the NACK FCI if it changed the
      RTP sequence numbers before forwarding a packet.

      Besides the common packet format management for feedback messages,
      a media-aware B2BUA MUST also properly rewrite the additional SSRC
      identifier all those messages envisage as part of their specific
      FCI if it changed the related RTP SSRC of the media sender.

   REMB:  [I-D.alvestrand-rmcat-remb]
      Besides the common packet format management for feedback messages,
      a media-aware B2BUA MUST also properly rewrite the additional SSRC
      identifier(s) REMB packets envisage as part of their specific FCI
      if it changed the related RTP SSRC of the media sender.

   Explicit Congestion Notification (ECN):  [RFC6679]
      Besides the common packet format management for feedback messages,
      the same guidelines given for SR/RR management apply as well,
      considering the presence of sequence numbers in the ECN Feedback
      Report format.  For what concerns 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 PLI, SLI and RPSI feedback
   messages instead.

   Of course, the same considerations about the need for SDP and RTP/
   RTCP information to be coherent also applies to media-aware B2BUAs.
   This means that, if a B2BUA is going to change any SSRC, it SHOULD
   update the related 'ssrc' attributes if they were present in the
   original description before sending it to the recipient, just as it
   MUST rewrite the 'rtcp' attribute if provided.  At the same time, the
   ability for a media-aware B2BUA to inspect/modify RTCP packets may
   also mean such a B2BUA may choose to drop RTCP packets it can't
   parse: in that case, a media-aware B2BUA MUST also advertize its RTCP
   level of support in the SDP in a coherent way, in order to prevent,
   for instance, a UAC to make use of NACK messages that would never
   reach the intended recipients.  It's important to point out that, in
   case any RTCP packet needs to be dropped, then only the offending
   RTCP packet needs to be dropped, and not the whole compound RTCP
   packet it may belong to.

   A different set of considerations, instead, is worthwhile for what
   concerns RTP/RTCP multiplexing [RFC5761] and Reduced-Size RTCP
   [RFC5506].  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 what concerns
   RTP/RTCP multiplexing, a B2BUA acting as a Media Relay can 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 it's not
   supported.  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 does not
   support the feature, it may decide to either disable it entirely or
   still advertize 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
   both sides, and in case there are, it MUST rewrite RTP payload type
   numbers to ensure no conflict in the domain where the RTP/RTCP
   multiplexing is applied.  Should RTP payload types be rewritten, the
   related information in the SDP MUST be updated accordingly.

   For what concerns Reduced-Size RTCP, instead, 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 other aspects to take into account before doing so.  While
   Reduced-Size allows indeed 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 an increased RTCP traffic on the side that does
   not support Reduced-Size, and could as a consequence be actually
   counterproductive if the bandwidth is different on each side.  That
   said, the B2BUA can choose whether or not to advertize support for
   Reduced-Size RTCP on either side by means of the 'rtcp-rsize' SDP
   attribute.  Should a B2BUA decide to allow the sides to independently
   use or not Reduced-Size, then the B2BUA MUST advertize support for
   the feature on the sides that support it, and MUST NOT advertize it
   on the sides that don't, by removing the related attribute from the
   SDP before forwarding it.  Should the B2BUA decide to disable the
   feature on all sides, instead, it MUST NOT advertize support for the
   Reduced-Size RTCP functionality on any side, by removing the 'rtcp-
   rsize' attribute from the SDP.

3.3.  Media Terminator

   A Media Terminator B2BUA, unlike simple relays and media-aware ones,
   is also able to terminate media itself, that is taking care of RTP
   payloads as well and not only headers.  This means that such
   components, for instance, can act as media transcoders and/or
   originate specific RTP media.  Using the RTP Topologies terminology,
   this can be seen as a RTP Media Translator.  Such a topology can also
   be seen as a Back-to-back RTP sessions through a Middlebox, as
   described in Section 3.2.2 of
   [I-D.ietf-avtcore-rtp-topologies-update].  Such a capability makes
   them quite different from the previously introduced B2BUA typologies,
   as this means they are going to terminate RTCP as well: in fact,
   since the media is terminated by themselves, the related statistics
   and feedback functionality can be taken care directly by the B2BUA,
   and does not need to be relayed to the other 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, mostly because most of
   the times there would be no end-to-end RTCP interaction among the
   involved participants at all, as the B2BUA would terminate them all
   and take care of them accordingly.  Nevertheless, should any RTCP
   packet actually need to be forwarded to another participant in the
   multimedia session, the same guidelines provided for the media-aware
   B2BUA case apply.

   For what concerns RTP/RTCP multiplexing support, the same
   considerations already given for the Media Relay management basically
   apply for a Media Terminator as well.  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 leg.  In that case, the same considerations
   about advertizing the support, or lack of support, of the feature on
   either side as described for the Media Relay case apply here as well.

4.  Media Path Security

   The discussion made in the previous sections on the management of
   RTCP messages by a B2BUA has so far mostly worked under the
   assumption that the B2BUA has actually access to the RTP/RTCP
   information itself.  This is indeed true if we assume that plain RTP
   and RTCP is being handled, but this may not be true once any security
   is enforced on RTP packets and RTCP messages by means of SRTP
   [RFC3711], whether the keying is done using Secure Descriptions
   [RFC4568] or DTLS-SRTP [RFC5764].

   While typically not an issue in the Media Relay case, where RTP and
   RTCP packets are forwarded without any modification no matter whether
   security is involved or not, this could definitely have an impact on
   Media-aware Relays and Media Terminator B2BUAs.  To make a simple
   example, if we think of a SRTP/SRTCP session across a B2BUA where the
   B2BUA itself has no access to the keys used to secure the session,
   there would be no way to manipulate SRTP headers without violating
   the hashing on the packet; at the same time, there would be no way to
   rewrite the RTCP information accordingly either, as most of the
   packet (especially when RTCP compound packets are involved) would be

   For this reason, it is important to point out that the operations
   described in the previous sections are only possible if the B2BUA has
   a way to effectively manipulate the packets and messages flowing by.
   This means that, in case media security is involved, only the Media-
   unaware Relay scenario can be properly addressed.  Attempting to
   cover Media-aware Relay and Media Terminarion scenarios when
   involving secure sessions will inevitably lead to the B2BUA acting as
   a man-in-the-middle, and as such its behaviour is unspecified and

5.  IANA Considerations

   This document makes no request of IANA.

6.  Security Considerations

   This document, being a summary and vest common practice overview that
   covers different standards, does not introduce any additional
   consideration to those described by the aforementioned standard
   documents themselves.

   It is worth pointing out, though, that there are scenarios where an
   improper management of RTCP messaging across a B2BUA may lead,
   willingly or not, to situations not unlike an attack.  To make a
   simple example, an improper management of a REMB feedback message
   containing, e.g., information on the limited bandwidth availability
   for a user, may lead to missing information to its peer, who may end
   up increasing the encoder bitrate up to a point where the user with
   poor connectivity will inevitably be choked by an amount of data it
   cannot process.  This scenario may as such result in what looks like
   a Denial of Service (DOS) attack towards the user.

7.  Change Summary

   Note to RFC Editor: Please remove this whole section.

   The following are the major changes between the 04 and the 05
   versions of the draft:

   o  Clarified behaviour when SSRC is zero.

   o  Fixed a couple of nits found by the Idnits tool.

   The following are the major changes between the 03 and the 04
   versions of the draft:

   o  Addressed review by Magnus Westerlund.

   o  Added guidelines for ECN RTCP messages.

   o  Clarified that if an RTCP packet is dropped because unsupported,
      only the unsupported packet is dropped and not the compound packet
      that contains it.

   o  Added reference to Section 3.2.2 of
      [I-D.ietf-avtcore-rtp-topologies-update] to Section 3.3.

   o  Added considerations on RTP/RTCP multiplexing and Reduced-Size

   The following are the major changes between the 02 and the 03
   versions of the draft:

   o  Rephrased the Media Path Security section to take into account the
      MITM-related discussion in Honolulu.

   o  Added some Security Considerations.

   The following are the major changes between the 01 and the 02
   versions of the draft:

   o  Updated terminology to better adhere to

   o  Rephrased the Media Path Security section to take into account the
      MITM-related discussion in Toronto.

   o  Clarified that NACK management might be trickier when SRTP is

   The following are the major changes between the 00 and the 01
   versions of the draft:

   o  Updated references and mapping per taxonomy RFC (7092).

   o  Added a reference to RTP topologies, and tried a mapping as per-
      discussion in London.

   o  Added more RTCP packet types to the Media-Aware section.

   o  Clarified that fixing the 'rtcp' SDP attribute is important.

   o  Added a new section on the impact of media security.

8.  Acknowledgements


   The authors would like to thank Flavio Battimo and Pierluigi Palma
   for their invaluable feedback in the early stages of the document.
   The authors would also like to thank Colin Perkins, Bernard Aboba,
   Albrecht Schwarz, Hadriel Kaplan, Keith Drage, Jonathan Lennox,
   Stephen Farrell and Magnus Westerlund for their constructive
   comments, suggestions, and reviews that were critical to the
   formulation and refinement of this document.

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.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264, June

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

9.2.  Informative References

   [RFC7092]  Kaplan, H. and V. Pascual, "A Taxonomy of Session
              Initiation Protocol (SIP) Back-to-Back User Agents", RFC
              7092, December 2013.

9.2.  Informative References

   [RFC5117]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 5117,
              January 2008.

              Westerlund, M. and S. Wenger, "RTP Topologies", draft-
              ietf-avtcore-rtp-topologies-update-06 (work in progress),
              March 2015.

              Lennox, J., Gross, K., Nandakumar, S., and G. Salgueiro,
              "A Taxonomy of Grouping Semantics and Mechanisms for Real-
              Time Transport Protocol (RTP) Sources", draft-ietf-avtext-
              rtp-grouping-taxonomy-06 (work in progress), March 2015.

              Alvestrand, H., "RTCP message for Receiver Estimated
              Maximum Bitrate", draft-alvestrand-rmcat-remb-03 (work in
              progress), October 2013.

   [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

   [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.

   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", RFC 5576, June 2009.

   [RFC3605]  Huitema, C., "Real Time Control Protocol (RTCP) attribute
              in Session Description Protocol (SDP)", RFC 3605, October

   [RFC3611]  Friedman, T., Caceres, R., and A. Clark, "RTP Control
              Protocol Extended Reports (RTCP XR)", RFC 3611, November

   [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.

   [RFC6284]  Begen, A., Wing, D., and T. Van Caenegem, "Port Mapping
              between Unicast and Multicast RTP Sessions", RFC 6284,
              June 2011.

   [RFC6679]  Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
              and K. Carlberg, "Explicit Congestion Notification (ECN)
              for RTP over UDP", RFC 6679, August 2012.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, July 2006.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761, April 2010.

   [RFC5506]  Johansson, I. and M. Westerlund, "Support for Reduced-Size
              Real-Time Transport Control Protocol (RTCP): Opportunities
              and Consequences", RFC 5506, April 2009.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              July 2006.

Authors' Addresses

   Lorenzo Miniero


   Sergio Garcia Murillo


   Victor Pascual