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Versions: (draft-miniero-straw-b2bua-rtcp) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 RFC 8079

STRAW Working Group                                           L. Miniero
Internet-Draft                                                  Meetecho
Intended status: Standards Track                       S. Garcia Murillo
Expires: October 21, 2016                                        Medooze
                                                              V. Pascual
                                                                  Quobis
                                                          April 19, 2016


   Guidelines to support RTCP end-to-end in Back-to-Back User Agents
                                (B2BUAs)
                     draft-ietf-straw-b2bua-rtcp-10

Abstract

   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
   messages 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 http://datatracker.ietf.org/drafts/current/.

   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 October 21, 2016.






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Copyright Notice

   Copyright (c) 2016 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
   (http://trustee.ietf.org/license-info) 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 . . . . . . . . . . . . . . . .   4
     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  . . . . . . . . . . . . . . . . . . . . . .  14
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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



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   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 [RFC7667].  These topologies are
   currently being updated to address new commonly encountered scenarios
   as well [RFC7667].

   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 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 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) in the RTP header of her RTP source
   stream.  The B2BUA rewrites the SSRC (SSRC3) before relaying the




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   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, 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, a blind relaying of these RTCP
   messages 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 messages he received
   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.  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
   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   Besides, this document addresses, where relevant, the RTP-related
   terminology as disciplined in [RFC7656].

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: a B2BUA, in fact, may act as a simple



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   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 messages as they flow by; or it may be a full-fledged
   media termination entity, terminating and generating RTP and RTCP
   messages as needed.

   While [RFC3550] and [RFC7667] already mandate some specific
   behaviours in the presence of certain topologies, not all deployments
   strictly adhere to the specifications.  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 not to impact any end-to-end
   RTCP effectiveness.

3.1.  Media Relay

   A media relay, as identified in [RFC7092], basically just forwards
   all RTP and RTCP messages 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.  This means
   that both packet payloads and packet headers are opaque to them.
   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 still happen,
   though, for different reasons, as for instance if the session
   description ends up providing incorrect information.  This may
   happen, for example, if the SDP on either side contains 'ssrc'
   [RFC5576] attributes that don't match the actual SSRC being
   advertized on the media plane, or in case the B2BUA advertized
   support for NACK because it implements it, while the original INVITE
   didn't.  Such issues 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.  This may cause participants to find a
   mismatch between the SSRCs advertized in SDP and the ones actually
   observed in RTP and RTCP messages (which may indeed change during a
   multimedia session anyway, but having them synced during setup would
   help nonetheless), or to have 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



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   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 messages, 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.

   It is worth mentioning that, leaving RTCP messages 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.  Besides, these CNAME items may actually contain IP addresses
   instead: 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 [RFC7022]
   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, though, then it
   MUST generate new CNAMEs following [RFC7022].

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 messages
   flowing by, and may even be able to modify their headers.  Using the
   RFC3550 terminology, this can be seen as a RTP Translator.  A B2BUA
   implementing this role, though, typically does not 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/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 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, in a coherent way, the same information in
   RTCP messages as well.



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   It is worthwile to point out that such a B2BUA may not necessarily
   forward all the packets it is receiving, though.  Selective
   Forwarding Units (SFU) [RFC7667], for instance, may 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 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
      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 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 Section 3.1 apply
      here as well.

   BYE:  [RFC3550]
      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.

   APP:  [RFC3550]
      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 as well accordingly.

   Extended Reports (XR):  [RFC3611]
      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



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      respect to SSRC identifiers in report blocks, apply for 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 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 of RTP streams addressed in a
      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, 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 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 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 identifier of the packet sender and the SSRC identifier
      of the media source the feedack 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, though, 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.  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.

   TMMBR/TMMBN/FIR/TSTR/TSTN/VBCM:  [RFC5104]




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      Besides the common packet format management for feedback messages,
      a media-aware B2BUA MUST also properly rewrite the additional SSRC
      identifier in the 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) in REMB packets, 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 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 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 message needs to be dropped, then
   the B2BUA SHOULD NOT drop the whole compound RTCP message it may
   belong to, but only the RTCP message itself.

   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 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



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   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.  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 available bandwidth is different on the two
   sides.  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 Reduced-Size or not, 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 either 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.  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 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 [RFC7667].  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



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   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, mostly because most of
   the times 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 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 side.  In that case, the same considerations
   about advertizing the support, or lack of, 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 may not be once any security is
   enforced on RTP packets and RTCP messages by means of SRTP [RFC3711].

   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 envisage 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.

   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




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   discouraged.  More considerations on this are provided in
   [I-D.ietf-straw-b2bua-dtls-srtp].

5.  IANA Considerations

   This document makes no request of IANA.

6.  Security Considerations

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

   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 or misleading information to its
   peer.  This may cause the peer to increase the encoder bitrate, maybe
   up to a point where a 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 09 and the 10
   versions of the draft:

   o  Replaced references to obsoleted RFC 5117 with [RFC7667].

   o  Made reference to [RFC7656] normative.

   o  Clarified text across the whole document to address Ben's review.

   The following are the major changes between the 08 and the 09
   versions of the draft:

   o  Updated references to documents which have become RFC in the
      meanwhile, [RFC7667] and [RFC7656].

   The following are the major changes between the 06 and the 07
   versions of the draft:




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   o  Clarified the suggested changed by Colin Perkins on the management
      of CNAME items in SDES, and added reference to [RFC7022].

   o  Addressed comment by Simon Perreault on CNAME collisions
      management.

   The following are the major changes between the 05 and the 06
   versions of the draft:

   o  Addressed comment by Colin Perkins on the management of CNAME
      items in SDES.

   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 message 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 [RFC7667] to Section 3.3.

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

   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 [RFC7656].




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   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
      involved.

   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 message 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, Magnus Westerlund and Simon Perreault 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,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <http://www.rfc-editor.org/info/rfc3261>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <http://www.rfc-editor.org/info/rfc4566>.



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   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <http://www.rfc-editor.org/info/rfc3264>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <http://www.rfc-editor.org/info/rfc3550>.

   [RFC7656]  Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
              for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
              DOI 10.17487/RFC7656, November 2015,
              <http://www.rfc-editor.org/info/rfc7656>.

9.2.  Informative References

   [RFC7092]  Kaplan, H. and V. Pascual, "A Taxonomy of Session
              Initiation Protocol (SIP) Back-to-Back User Agents",
              RFC 7092, DOI 10.17487/RFC7092, December 2013,
              <http://www.rfc-editor.org/info/rfc7092>.

   [RFC7667]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
              DOI 10.17487/RFC7667, November 2015,
              <http://www.rfc-editor.org/info/rfc7667>.

   [I-D.alvestrand-rmcat-remb]
              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,
              DOI 10.17487/RFC4585, July 2006,
              <http://www.rfc-editor.org/info/rfc4585>.

   [RFC5104]  Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
              "Codec Control Messages in the RTP Audio-Visual Profile
              with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
              February 2008, <http://www.rfc-editor.org/info/rfc5104>.

   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
              <http://www.rfc-editor.org/info/rfc5576>.




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   [RFC3605]  Huitema, C., "Real Time Control Protocol (RTCP) attribute
              in Session Description Protocol (SDP)", RFC 3605,
              DOI 10.17487/RFC3605, October 2003,
              <http://www.rfc-editor.org/info/rfc3605>.

   [RFC3611]  Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
              "RTP Control Protocol Extended Reports (RTCP XR)",
              RFC 3611, DOI 10.17487/RFC3611, November 2003,
              <http://www.rfc-editor.org/info/rfc3611>.

   [RFC5760]  Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
              Protocol (RTCP) Extensions for Single-Source Multicast
              Sessions with Unicast Feedback", RFC 5760,
              DOI 10.17487/RFC5760, February 2010,
              <http://www.rfc-editor.org/info/rfc5760>.

   [RFC6284]  Begen, A., Wing, D., and T. Van Caenegem, "Port Mapping
              between Unicast and Multicast RTP Sessions", RFC 6284,
              DOI 10.17487/RFC6284, June 2011,
              <http://www.rfc-editor.org/info/rfc6284>.

   [RFC6679]  Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
              and K. Carlberg, "Explicit Congestion Notification (ECN)
              for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
              2012, <http://www.rfc-editor.org/info/rfc6679>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <http://www.rfc-editor.org/info/rfc3711>.

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, DOI 10.17487/RFC4568, July 2006,
              <http://www.rfc-editor.org/info/rfc4568>.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761,
              DOI 10.17487/RFC5761, April 2010,
              <http://www.rfc-editor.org/info/rfc5761>.

   [RFC5506]  Johansson, I. and M. Westerlund, "Support for Reduced-Size
              Real-Time Transport Control Protocol (RTCP): Opportunities
              and Consequences", RFC 5506, DOI 10.17487/RFC5506, April
              2009, <http://www.rfc-editor.org/info/rfc5506>.






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   [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,
              DOI 10.17487/RFC5764, May 2010,
              <http://www.rfc-editor.org/info/rfc5764>.

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              DOI 10.17487/RFC4588, July 2006,
              <http://www.rfc-editor.org/info/rfc4588>.

   [RFC7022]  Begen, A., Perkins, C., Wing, D., and E. Rescorla,
              "Guidelines for Choosing RTP Control Protocol (RTCP)
              Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022,
              September 2013, <http://www.rfc-editor.org/info/rfc7022>.

   [I-D.ietf-straw-b2bua-dtls-srtp]
              R, R., Reddy, T., Salgueiro, G., Pascual, V., and P.
              Ravindran, "DTLS-SRTP Handling in Session Initiation
              Protocol (SIP) Back-to-Back User Agents (B2BUAs)", draft-
              ietf-straw-b2bua-dtls-srtp-12 (work in progress), April
              2016.

Authors' Addresses

   Lorenzo Miniero
   Meetecho

   Email: lorenzo@meetecho.com


   Sergio Garcia Murillo
   Medooze

   Email: sergio.garcia.murillo@gmail.com


   Victor Pascual
   Quobis

   Email: victor.pascual@quobis.com










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