Internet DRAFT - draft-ietf-straw-b2bua-rtcp
draft-ietf-straw-b2bua-rtcp
STRAW Working Group L. Miniero
Internet-Draft Meetecho
Intended status: Standards Track S. Garcia Murillo
Expires: June 25, 2017 Medooze
V. Pascual
Oracle
December 22, 2016
Guidelines to support RTCP end-to-end in Back-to-Back User Agents
(B2BUAs)
draft-ietf-straw-b2bua-rtcp-17
Abstract
SIP Back-to-Back User Agents (B2BUAs) are often designed to also be
on the media path, rather than just intercepting signalling. This
means that B2BUAs often implement an RTP/RTCP stack as well, 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 June 25, 2017.
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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
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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 . . . . . . . . . . . . . . . . . . . . 11
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Change Summary . . . . . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
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 completely adherent to the
standards, and can lead to unexpected situations. [RFC7092] presents
a taxonomy of the most commonly 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, in order to receive and manage all RTP
and RTCP [RFC3550] packets in a session, these components also
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manipulate the session descriptions [RFC4566] in the related offer/
answer exchanges [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. This
can lead to several different topologies for RTP-based communication,
as documented in [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. 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, the
B2BUA is also rewriting some of the RTP header information on the
way. In this example, just the 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 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
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(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 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. 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 Alice 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.
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 described 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
media relay (1), effectively unaware of anything that is transported;
it may be a media-aware relay (2), also inspecting and/or modifying
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RTP and RTCP messages as they flow by; or it may be a full-fledged
media termination entity (3), terminating and generating RTP and RTCP
messages as needed.
[RFC3550] and [RFC7667] already mandate some specific behaviours in
the presence of certain topologies. Anyway, 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 a 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
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], simply 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 prepared by the B2BUA, whether it has been modified or
not, 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 when the B2BUA advertized 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 advertized 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 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 participants: this includes
attributes like 'ssrc' [RFC3261], 'rtcp-fb' [RFC4585], 'rtcp-xr-
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attrib' [RFC3611] 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' [RFC3605] 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 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 [RFC5761] 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 advertized.
It is worth mentioning that, 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 [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]. The same SHOULD be done in case RTP
extensions involving CNAMEs are involved (e.g., "urn:ietf:params:rtp-
hdrext:sdes:cname", [RFC7941]). 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.
3.2. Media-aware Relay
A Media-aware relay, unlike the 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 RFC3550 terminology, this can be
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seen as a RTP Translator. A B2BUA implementing this role, though,
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 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. 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 worthwile to point out that such a B2BUA may not necessarily
forward all the packets it receives, though. Selective Forwarding
Units (SFU) [RFC7667], 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, 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 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 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 Units (SFU) [RFC7667], it needs to track in
the outgoing SR the relevant number of packets sent and total
amount of bytes sent to the receiver.
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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
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 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.
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, when 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 when the message is a
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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. 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]
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.
TMMBR/TMMBN/FIR/TSTR/TSTN/VBCM: [RFC5104]
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.
REMB: [I-D.alvestrand-rmcat-remb]
This draft describes an RTCP Payload-Specific feedback message
that reports the receiver's available bandwidth to the 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.
Explicit Congestion Notification (ECN): [RFC6679]
The same guidelines given for SR/RR management apply,
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.
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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 advertize its RTCP level of
support in the SDP in a coherent way, in order to prevent, for
instance, a UAC to 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 when talking of Media Relays
apply for Media-aware Relays as well. Specifically, if RTP
extensions involving CNAMEs are involved (e.g., "urn:ietf:params:rtp-
hdrext:sdes:cname", [RFC7941]) 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 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
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 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 either side. When there are, it MUST
rewrite RTP payload type numbers to prevent conflicts in the session
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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 several reasons for discouraging such a behaviour. 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. Negotiating a session with both sides would allow the B2BUA
to discover which one supports Reduced-Size and which doesn't, and in
case decide whether to allow the sides to independently use Reduced-
Size or not. 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 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
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 also
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apply for a Media Terminator. 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 advertize and negotiate support for
Reduced-Size if available, and MUST NOT otherwise.
4. IANA Considerations
This document makes no request of IANA.
5. Security Considerations
The discussion made in the previous sections on the management of
RTCP messages by a B2BUA 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, when media security is involved, only the Media-
unaware Relay scenario can be properly addressed. Attempting to
cover Media-aware Relay and Media Termination scenarios when
involving secure sessions will inevitably lead to the B2BUA acting as
a man-in-the-middle, and consequently its behaviour is unspecified
and discouraged. More considerations on this are provided in
[RFC7879].
It is also worth pointing out 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
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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
thus result in what looks like a Denial of Service (DOS) attack
towards the user.
6. IANA Considerations
This document has no IANA actions.
7. Change Summary
Note to RFC Editor: Please remove this whole section.
The following are the major changes between the 16 and the 17
versions of the draft:
o Clarified the meaning of a sentence.
The following are the major changes between the 14 and the 15
versions of the draft:
o Several changes addressing the IESG review (list follows).
o Addressed 'rtcp-mux' in 3.1 as well, and not only 3.2.
o Clarified that, if CNAMEs are rewritten, RTP extensions
referencing them (e.g., [RFC7941]) should be updated too.
Clarified that MTU issues can occur if the rewriting results in a
larger RTP packet.
o Clarified that when handling SR packets, the an SFU B2BUA must
track packets/bytes sent.
o Removed references to billing, lawful interception, etc. from the
intro.
o Moved some references (especially those affected by MUSTs in 3.2)
to Normative.
o Rewritten the "Such attributes SHOULD NOT be forwarded" section to
clarify the context of the attributes that may lead to a failure
if not taken care of.
o Clarified that randomly dropping RTCP packets can lead to
confusion on the recipient.
o Updated text related to REMB.
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o Smaller fixes here and there.
The following are the major changes between the 13 and the 14
versions of the draft:
o Removed first paragraph of Security Considerations which was
unclear.
o Added an IANA Considerations section to clarify there are no
actions.
The following are the major changes between the 12 and the 13
versions of the draft:
o Updated authors' affiliations and mail addresses.
The following are the major changes between the 11 and the 12
versions of the draft:
o Addressed remaining points in Ben's second review.
o Updated reference of STRAW's DTLS-SRTP draft to new [RFC7879].
The following are the major changes between the 10 and the 11
versions of the draft:
o Addressed Ben's second review.
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:
o Clarified the suggested changed by Colin Perkins on the management
of CNAME items in SDES, and added reference to [RFC7022].
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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].
o Rephrased the Media Path Security section to take into account the
MITM-related discussion in Toronto.
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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, Simon Perreault and Ben Campbell
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>.
[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>.
[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>.
[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>.
[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>.
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[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>.
[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>.
[RFC7941] Westerlund, M., Burman, B., Even, R., and M. Zanaty, "RTP
Header Extension for the RTP Control Protocol (RTCP)
Source Description Items", RFC 7941, DOI 10.17487/RFC7941,
August 2016, <http://www.rfc-editor.org/info/rfc7941>.
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.
[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>.
[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>.
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[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>.
[RFC7879] Ravindranath, R., Reddy, T., Salgueiro, G., Pascual, V.,
and P. Ravindran, "DTLS-SRTP Handling in SIP Back-to-Back
User Agents", RFC 7879, DOI 10.17487/RFC7879, May 2016,
<http://www.rfc-editor.org/info/rfc7879>.
Authors' Addresses
Lorenzo Miniero
Meetecho
Email: lorenzo@meetecho.com
Sergio Garcia Murillo
Medooze
Email: sergio.garcia.murillo@gmail.com
Victor Pascual
Oracle
Email: victor.pascual.avila@oracle.com
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