Internet DRAFT - draft-ietf-avtcore-ecn-for-rtp
draft-ietf-avtcore-ecn-for-rtp
Network Working Group M. Westerlund
Internet-Draft I. Johansson
Intended status: Standards Track Ericsson
Expires: November 15, 2012 C. Perkins
University of Glasgow
P. O'Hanlon
University of Oxford
K. Carlberg
G11
May 14, 2012
Explicit Congestion Notification (ECN) for RTP over UDP
draft-ietf-avtcore-ecn-for-rtp-08
Abstract
This memo specifies how Explicit Congestion Notification (ECN) can be
used with the Real-time Transport Protocol (RTP) running over UDP,
using RTP Control Protocol (RTCP) as a feedback mechanism. It
defines a new RTCP Extended Report (XR) block for periodic ECN
feedback, a new RTCP transport feedback message for timely reporting
of congestion events, and a Session Traversal Utilities for NAT
(STUN) extension used in the optional initialization method using
Interactive Connectivity Establishment (ICE). Signalling and
procedures for negotiation of capabilities and initialization methods
are also defined.
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
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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 November 15, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions, Definitions and Acronyms . . . . . . . . . . . . 5
3. Discussion, Requirements, and Design Rationale . . . . . . . . 6
3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 8
3.3. Interoperability . . . . . . . . . . . . . . . . . . . . . 12
4. Overview of Use of ECN with RTP/UDP/IP . . . . . . . . . . . . 13
5. RTCP Extensions for ECN feedback . . . . . . . . . . . . . . . 16
5.1. RTP/AVPF Transport Layer ECN Feedback packet . . . . . . . 16
5.2. RTCP XR Report block for ECN summary information . . . . . 19
6. SDP Signalling Extensions for ECN . . . . . . . . . . . . . . 21
6.1. Signalling ECN Capability using SDP . . . . . . . . . . . 21
6.2. RTCP ECN Feedback SDP Parameter . . . . . . . . . . . . . 25
6.3. XR Block ECN SDP Parameter . . . . . . . . . . . . . . . . 25
6.4. ICE Parameter to Signal ECN Capability . . . . . . . . . . 26
7. Use of ECN with RTP/UDP/IP . . . . . . . . . . . . . . . . . . 26
7.1. Negotiation of ECN Capability . . . . . . . . . . . . . . 26
7.2. Initiation of ECN Use in an RTP Session . . . . . . . . . 27
7.3. Ongoing Use of ECN Within an RTP Session . . . . . . . . . 34
7.4. Detecting Failures . . . . . . . . . . . . . . . . . . . . 37
8. Processing ECN in RTP Translators and Mixers . . . . . . . . . 41
8.1. Transport Translators . . . . . . . . . . . . . . . . . . 41
8.2. Fragmentation and Reassembly in Translators . . . . . . . 42
8.3. Generating RTCP ECN Feedback in Media Transcoders . . . . 44
8.4. Generating RTCP ECN Feedback in Mixers . . . . . . . . . . 45
9. Implementation considerations . . . . . . . . . . . . . . . . 45
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 46
10.1. SDP Attribute Registration . . . . . . . . . . . . . . . . 46
10.2. RTP/AVPF Transport Layer Feedback Message . . . . . . . . 46
10.3. RTCP Feedback SDP Parameter . . . . . . . . . . . . . . . 47
10.4. RTCP XR Report blocks . . . . . . . . . . . . . . . . . . 47
10.5. RTCP XR SDP Parameter . . . . . . . . . . . . . . . . . . 47
10.6. STUN attribute . . . . . . . . . . . . . . . . . . . . . . 47
10.7. ICE Option . . . . . . . . . . . . . . . . . . . . . . . . 47
11. Security Considerations . . . . . . . . . . . . . . . . . . . 47
12. Examples of SDP Signalling . . . . . . . . . . . . . . . . . . 50
12.1. Basic SDP Offer/Answer . . . . . . . . . . . . . . . . . . 50
12.2. Declarative Multicast SDP . . . . . . . . . . . . . . . . 52
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 53
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 53
14.1. Normative References . . . . . . . . . . . . . . . . . . . 53
14.2. Informative References . . . . . . . . . . . . . . . . . . 54
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 56
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1. Introduction
This memo outlines how Explicit Congestion Notification (ECN)
[RFC3168] can be used for Real-time Transport Protocol (RTP)
[RFC3550] flows running over UDP/IP which use RTP Control Protocol
(RTCP) as a feedback mechanism. The solution consists of feedback of
ECN congestion experienced markings to the sender using RTCP,
verification of ECN functionality end-to-end, and procedures for how
to initiate ECN usage. Since the initiation process has some
dependencies on the signalling mechanism used to establish the RTP
session, a specification for signalling mechanisms using Session
Description Protocol (SDP) [RFC4566] is included.
ECN can be used to minimise the impact of congestion on real-time
multimedia traffic. The use of ECN provides a way for the network to
send congestion control signals to the media transport without having
to impair the media. Unlike packet loss, ECN signals unambiguously
indicate congestion to the transport as quickly as feedback delays
allow, and without confusing congestion with losses that might have
occurred for other reasons such as transmission errors, packet-size
errors, routing errors, badly implemented middleboxes, policy
violations and so forth.
The introduction of ECN into the Internet requires changes to both
the network and transport layers. At the network layer, IP
forwarding has to be updated to allow routers to mark packets, rather
than discarding them in times of congestion [RFC3168]. In addition,
transport protocols have to be modified to inform the sender that ECN
marked packets are being received, so it can respond to the
congestion. The Transmission Control Protocol (TCP) [RFC3168],
Stream Control Transmission Protocol (SCTP) [RFC4960] and Datagram
Congestion Control Protocol (DCCP) [RFC4340] have been updated to
support ECN, but to date there is no specification how UDP-based
transports, such as RTP [RFC3550], can use ECN. This is due to the
lack of feedback mechanisms directly in UDP. Instead the signaling
control protocol on top of UDP needs to provide that feedback. For
RTP that feedback is provided by RTCP.
The remainder of this memo is structured as follows. We start by
describing the conventions, definitions and acronyms used in this
memo in Section 2, and the design rationale and applicability in
Section 3. Section 4 gives an overview of how ECN is used with RTP
over UDP. RTCP extensions for ECN feedback are defined in Section 5,
and SDP signalling extensions in Section 6. The details of how ECN
is used with RTP over UDP are defined in Section 7. In Section 8 we
describe how ECN is handled in RTP translators and mixers. Section 9
discusses some implementation considerations, Section 10 lists IANA
considerations, and Section 11 discusses security considerations.
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2. Conventions, Definitions and Acronyms
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
Abbreviations and Definitions:
Sender: A sender of RTP packets carrying an encoded media stream.
The sender can change how the media transmission is performed by
varying the media coding or packetisation. It is one end-point of
the ECN control loop.
Receiver: A receiver of RTP packets with the intention to consume
the media stream. It sends RTCP feedback on the received stream.
It is the other end-point of the ECN control loop.
ECN Capable Host: A sender or receiver of a media stream that is
capable of setting and/or processing ECN marks.
ECN Capable Transport (ECT): A transport flow where both sender and
receiver are ECN capable hosts. Packets sent by an ECN Capable
Transport will be marked as ECT(0) or ECT(1) on transmission. See
[RFC3168] for the definition of the ECT(0) and ECT(1) marks.
ECN-CE: ECN Congestion Experienced mark (see [RFC3168]).
ECN Capable Packets: Packets with ECN mark set to either ECT(0),
ECT(1) or ECN-CE.
Not-ECT packets: Packets that are not sent by an ECN capable
transport, and are not ECN-CE marked.
ECN Capable Queue: A queue that supports ECN-CE marking of ECN-
Capable Packets to indicate congestion.
ECN Blocking Middlebox: A middlebox that discards ECN-Capable
Packets.
ECN Reverting Middlebox: A middlebox that changes ECN-Capable
Packets to Not-ECT packets by removing the ECN mark.
Note that RTP mixers or translators that operate in such a manner
that they terminate or split the ECN control loop will take on the
role of receivers or senders. This is further discussed in
Section 3.2.
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3. Discussion, Requirements, and Design Rationale
ECN has been specified for use with TCP [RFC3168], SCTP [RFC4960],
and DCCP [RFC4340] transports. These are all unicast protocols which
negotiate the use of ECN during the initial connection establishment
handshake (supporting incremental deployment, and checking if ECN
marked packets pass all middleboxes on the path). ECN-CE marks are
immediately echoed back to the sender by the receiving end-point
using an additional bit in feedback messages, and the sender then
interprets the mark as equivalent to a packet loss for congestion
control purposes.
If RTP is run over TCP, SCTP, or DCCP, it can use the native ECN
support provided by those protocols. This memo does not concern
itself further with these use cases. However, RTP is more commonly
run over UDP. This combination does not currently support ECN, and
we observe that it has significant differences from the other
transport protocols for which ECN has been specified. These include:
Signalling: RTP relies on separate signalling protocols to negotiate
parameters before a session can be created, and doesn't include an
in-band handshake or negotiation at session set-up time (i.e.,
there is no equivalent to the TCP three-way handshake in RTP).
Feedback: RTP does not explicitly acknowledge receipt of datagrams.
Instead, the RTP Control Protocol (RTCP) provides reception
quality feedback, and other back channel communication, for RTP
sessions. The feedback interval is generally on the order of
seconds, rather than once per network RTT (although the RTP/AVPF
profile [RFC4585] allows more rapid feedback in most cases). RTCP
is also very much oriented around counting packets, which makes
byte counting congestion algorithms difficult to utilize.
Congestion Response: While it is possible to adapt the transmission
of many audio/visual streams in response to network congestion,
and such adaptation is required by [RFC3550], the dynamics of the
congestion response may be quite different to those of TCP or
other transport protocols.
Middleboxes: The RTP framework explicitly supports the concept of
mixers and translators, which are middleboxes that are involved in
media transport functions.
Multicast: RTP is explicitly a group communication protocol, and was
designed from the start to support IP multicast (primarily Any
Source Multicast (ASM) [RFC1112], although a recent extension
supports Source Specific Multicast (SSM) [RFC3569] with unicast
feedback [RFC5760]).
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Application Awareness: When ECN support is provided within the
transport protocol, the ability of the application to react to
congestion is limited, since it has little visibility into the
transport layer. By adding support of ECN to RTP using RTCP
feedback, the application is made aware of congestion, allowing a
wider range of reactions in response to that loss.
Counting vs Detecting Congestion: TCP, and the protocols derived
from it, are mainly designed to respond in the same way whether
they experience a burst of congestion indications within one RTT,
or just a single congestion indication. Whereas real-time
applications may be concerned with the amount of congestion
experienced, whether it is distributed smoothly or in bursts.
When feedback of ECN was added to TCP [RFC3168], the receiver was
designed to flip the echo congestion experienced (ECE) flag to 1
for a whole RTT then flop it back to zero. Whereas ECN feedback
in RTCP will need to report a count of how much congestion has
been experienced within an RTCP reporting period, irrespective of
round trip times.
These differences significantly alter the shape of ECN support in
RTP-over-UDP compared to ECN support in TCP, SCTP, and DCCP, but do
not invalidate the need for ECN support.
ECN support is more important for RTP sessions than, for instance, is
the case for many applications over TCP. This is because the impact
of packet loss in real-time audio-visual media flows is highly
visible to users. For TCP-based applications, however, TCP will
retransmit lost packets, and while extra delay is incurred by having
packets dropped rather than ECN-CE marked, the loss is repaired.
Effective ECN support for RTP flows running over UDP will allow real-
time audio-visual applications to respond to the onset of congestion
before routers are forced to drop packets, allowing those
applications to control how they reduce their transmission rate, and
hence media quality, rather than responding to, and trying to conceal
the effects of unpredictable packet loss. Furthermore, widespread
deployment for ECN and active queue management in routers, should it
occur, can potentially reduce unnecessary queueing delays in routers,
lowering the round-trip time and benefiting interactive applications
of RTP, such as voice telephony.
3.1. Requirements
Considering ECN, transport protocols supporting ECN, and RTP based
applications one can create a set of requirements that must be
satisfied to at least some degree if ECN is to be used by RTP over
UDP.
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o REQ 1: A mechanism must exist to negotiate and initiate the use of
ECN for RTP/UDP/IP sessions so that an RTP sender will not send
packets with ECT in the IP header unless it knows that all
potential receivers will understand any ECN-CE indications they
might receive.
o REQ 2: A mechanism must exist to feed back the reception of any
packets that are ECN-CE marked to the packet sender.
o REQ 3: The provided mechanism should minimise the possibility of
cheating (either by the sender or receiver).
o REQ 4: Some detection and fallback mechanism should exist to avoid
loss of communication due to the attempted usage of ECN in case an
intermediate node clears ECT or drops packets that are ECT marked.
o REQ 5: Negotiation of ECN should not significantly increase the
time taken to negotiate and set-up the RTP session (an extra RTT
before the media can flow is unlikely to be acceptable for some
use cases).
o REQ 6: Negotiation of ECN should not cause media clipping at the
start of a session.
The following sections describes how these requirements can be met
for RTP over UDP.
3.2. Applicability
The use of ECN with RTP over UDP is dependent on negotiation of ECN
capability between the sender and receiver(s), and validation of ECN
support in all elements on the network path(s) traversed. RTP is
used in a heterogeneous range of network environments and topologies,
with various different signalling protocols. The mechanisms defined
here make it possible to verify support for ECN in each of these
environments, and irrespective of the topology.
Due to the need for each RTP sender that intends to use ECN with RTP
to track all participants in the RTP session, the sub-sampling of the
group membership as specified by "Sampling of the Group Membership in
RTP" [RFC2762] MUST NOT be used.
The use of ECN is further dependent on a capability of the RTP media
flow to react to congestion signalled by ECN marked packets.
Depending on the application, media codec, and network topology, this
adaptation can occur in various forms and at various nodes. As an
example, the sender can change the media encoding, or the receiver
can change the subscription to a layered encoding, or either reaction
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can be accomplished by a transcoding middlebox. RFC 5117 identifies
seven topologies in which RTP sessions may be configured, and which
may affect the ability to use ECN:
Topo-Point-to-Point: This utilises standard unicast flows. ECN may
be used with RTP in this topology in an analogous manner to its
use with other unicast transport protocols, with RTCP conveying
ECN feedback messages.
Topo-Multicast: This is either an any source multicast (ASM) group
[RFC3569] with potentially several active senders and multicast
RTCP feedback, or a source specific multicast (SSM) group
[RFC4607] with a single distribution source and unicast RTCP
feedback from receivers. RTCP is designed to scale to large group
sizes while avoiding feedback implosion (see Section 6.2 of
[RFC3550], [RFC4585], and [RFC5760]), and can be used by a sender
to determine if all its receivers, and the network paths to those
receivers, support ECN (see Section 7.2). It is somewhat more
difficult to determine if all network paths from all senders to
all receivers support ECN. Accordingly, we allow ECN to be used
by an RTP sender using multicast UDP provided the sender has
verified that the paths to all its known receivers support ECN,
and irrespective of whether the paths from other senders to their
receivers support ECN ("all its known receivers" are all the SSRCs
that the RTP sender has received RTP or RTCP from the last five
reporting intervals, i.e., they have not timed out). Note that
group membership may change during the lifetime of a multicast RTP
session, potentially introducing new receivers that are not ECN
capable or have a path that doesn't support ECN. Senders must use
the mechanisms described in Section 7.4 to check that all
receivers, and the network paths traversed to reach those
receivers, continue to support ECN, and they need to fallback to
non-ECN use if any receivers join that do not.
SSM groups that uses unicast RTCP feedback [RFC5760] do need a few
extra considerations. This topology can have multiple media
senders that provides traffic to the distribution source (DS) and
are separated from the DS. There can also be multiple feedback
targets. The requirement for using ECN for RTP in this topology
is that the media sender must be provided the feedback from the
receivers, it may be in aggregated form from the feedback targets.
We will not mention this SSM use case in the below text
specifically, but when actions are required by the media source,
they do apply also to case of SSM where the RTCP feedback goes to
the Feedback Target.
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The mechanisms defined in this memo support multicast groups, but
are known to be conservative, and don't scale to large groups.
This is primarily because we require all members of the group to
demonstrate that they can make use of ECN before the sender is
allowed to send ECN-marked packets, since allowing some non-ECN
capable receivers causes fairness issues when the bottleneck link
is shared by ECN and non-ECN flows that we have not (yet) been
able to satisfactorily address. The rules regarding Determination
of ECN Support in Section 7.2.1 may be relaxed in a future version
of this specification to improve scaling once these issues have
been resolved.
Topo-Translator: An RTP translator is an RTP-level middlebox that is
invisible to the other participants in the RTP session (although
it is usually visible in the associated signalling session).
There are two types of RTP translator: those that do not modify
the media stream, and are concerned with transport parameters, for
example a multicast to unicast gateway; and those that do modify
the media stream, for example transcoding between different media
codecs. A single RTP session traverses the translator, and the
translator must rewrite RTCP messages passing through it to match
the changes it makes to the RTP data packets. A legacy, ECN-
unaware, RTP translator is expected to ignore the ECN bits on
received packets, and to set the ECN bits to not-ECT when sending
packets, so causing ECN negotiation on the path containing the
translator to fail (any new RTP translator that does not wish to
support ECN may do so similarly). An ECN aware RTP translator may
act in one of three ways:
* If the translator does not modify the media stream, it should
copy the ECN bits unchanged from the incoming to the outgoing
datagrams, unless it is overloaded and experiencing congestion,
in which case it may mark the outgoing datagrams with an ECN-CE
mark. Such a translator passes RTCP feedback unchanged. See
Section 8.1.
* If the translator modifies the media stream to combine or split
RTP packets, but does not otherwise transcode the media, it
must manage the ECN bits in a way analogous to that described
in Section 5.3 of [RFC3168], see Section 8.2 for details.
* If the translator is a media transcoder, or otherwise modifies
the content of the media stream, the output RTP media stream
may have radically different characteristics than the input RTP
media stream. Each side of the translator must then be
considered as a separate transport connection, with its own ECN
processing. This requires the translator interpose itself into
the ECN negotiation process, effectively splitting the
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connection into two parts with their own negotiation. Once
negotiation has been completed, the translator must generate
RTCP ECN feedback back to the source based on its own
reception, and must respond to RTCP ECN feedback received from
the receiver(s) (see Section 8.3).
It is recognised that ECN and RTCP processing in an RTP translator
that modifies the media stream is non-trivial.
Topo-Mixer: A mixer is an RTP-level middlebox that aggregates
multiple RTP streams, mixing them together to generate a new RTP
stream. The mixer is visible to the other participants in the RTP
session, and is also usually visible in the associated signalling
session. The RTP flows on each side of the mixer are treated
independently for ECN purposes, with the mixer generating its own
RTCP ECN feedback, and responding to ECN feedback for data it
sends. Since unicast transport between the mixer and any end-
point are treated independently, it would seem reasonable to allow
the transport on one side of the mixer to use ECN, while the
transport on the other side of the mixer is not ECN capable, if
this is desired. See Section 8.4 for details in how mixers should
process ECN.
Topo-Video-switch-MCU: A video switching MCU receives several RTP
flows, but forwards only one of those flows onwards to the other
participants at a time. The flow that is forwarded changes during
the session, often based on voice activity. Since only a subset
of the RTP packets generated by a sender are forwarded to the
receivers, a video switching MCU can break ECN negotiation (the
success of the ECN negotiation may depend on the voice activity of
the participant at the instant the negotiation takes place - shout
if you want ECN). It also breaks congestion feedback and
response, since RTP packets are dropped by the MCU depending on
voice activity rather than network congestion. This topology is
widely used in legacy products, but is NOT RECOMMENDED for new
implementations and SHALL NOT be used with ECN.
Topo-RTCP-terminating-MCU: In this scenario, each participant runs
an RTP point-to-point session between itself and the MCU. Each of
these sessions is treated independently for the purposes of ECN
and RTCP feedback, potentially with some using ECN and some not.
Topo-Asymmetric: It is theoretically possible to build a middlebox
that is a combination of an RTP mixer in one direction and an RTP
translator in the other. To quote RFC 5117 "This topology is so
problematic and it is so easy to get the RTCP processing wrong,
that it is NOT RECOMMENDED to implement this topology."
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These topologies may be combined within a single RTP session.
The ECN mechanism defined in this memo is applicable to both sender
and receiver controlled congestion algorithms. The mechanism ensures
that both senders and receivers will know about ECN-CE markings and
any packet losses. Thus the actual decision point for the congestion
control is not relevant. This is a great benefit as the rate of an
RTP session can be varied in a number of ways, for example a unicast
media sender might use TFRC [RFC5348] or some other algorithm, while
a multicast session could use a sender based scheme adapting to the
lowest common supported rate, or a receiver driven mechanism using
layered coding to support more heterogeneous paths.
To ensure timely feedback of ECN-CE marked packets when needed, this
mechanism requires support for the RTP/AVPF profile [RFC4585] or any
of its derivatives, such as RTP/SAVPF [RFC5124]. The standard RTP/
AVP profile [RFC3551] does not allow any early or immediate
transmission of RTCP feedback, and has a minimal RTCP interval whose
default value (5 seconds) is many times the normal RTT between sender
and receiver.
3.3. Interoperability
To ensure interoperability for this specification there is need for
at least one common initilization method for all implementations.
Since initialization using RTP and RTCP (Section 7.2.1) is the one
method that works in all cases, although is not optimal for all uses,
it is selected as mandatory to implement this initialisation method.
This method requires both the RTCP XR extension and the ECN feedback
format, which require the RTP/AVPF profile to ensure timely feedback.
When one considers all the uses of ECN for RTP it is clear that there
exist congestion control mechanisms that are receiver driven only
(Section 7.3.3). These congestion control mechanisms do not require
timely feedback of congestion events to the sender. If such a
congestion control mechanism is combined with an initialization
method that also doesn't require timely feedback using RTCP, like the
leap of faith (Section 7.2.3) or the ICE based method (Section 7.2.2)
then neither the ECN feedback format nor the RTP/AVPF profile would
appear to be needed. However, fault detection can be greatly
improved by using receiver side detection (Section 7.4.1) and early
reporting of such cases using the ECN feedback mechanism.
For interoperability we mandate the implementation of the RTP/AVPF
profile, with both RTCP extensions and the necessary signalling to
support a common operations mode. This specification recommends the
use of RTP/AVPF in all cases as negotiation of the common
interoperability point requires RTP/AVPF, mixed negotiation of RTP/
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AVP and RTP/AVPF depending on other SDP attributes in the same media
block is difficult, and the fact that fault detection can be improved
when using RTP/AVPF.
The use of the ECN feedback format is also recommended, but cases
exist where its use is not required due to no need for timely
feedback. These will be explicitly noted using the term "no timely
feedback required", and generally occur in combination with receiver
driven congestion control, and with the leap-of-faith and ICE-based
initialization methods. We also note that any receiver driven
congestion control solution that still requires RTCP for signalling
of any adaptation information to the sender will still require RTP/
AVPF for timeliness.
4. Overview of Use of ECN with RTP/UDP/IP
The solution for using ECN with RTP over UDP/IP consists of four
different pieces that together make the solution work:
1. Negotiation of the capability to use ECN with RTP/UDP/IP
2. Initiation and initial verification of ECN capable transport
3. Ongoing use of ECN within an RTP session
4. Handling of dynamic behavior through failure detection,
verification and fallback
Before an RTP session can be created, a signalling protocol is used
to negotiate or at least configure session parameters (see
Section 7.1). In some topologies the signalling protocol can also be
used to discover the other participants. One of the parameters that
must be agreed is the capability of a participant to support ECN.
Note that all participants having the capability of supporting ECN
does not necessarily imply that ECN is usable in an RTP session,
since there may be middleboxes on the path between the participants
which don't pass ECN-marked packets (for example, a firewall that
blocks traffic with the ECN bits set). This document defines the
information that needs to be negotiated, and provides a mapping to
SDP for use in both declarative and offer/answer contexts.
When a sender joins a session for which all participants claim to
support ECN, it needs to verify that the ECN support is usable.
There are three ways in which this verification can be done:
o The sender may generate a (small) subset of its RTP data packets
with the ECN field of the IP header set to ECT(0) or ECT(1). Each
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receiver will then send an RTCP feedback packet indicating the
reception of the ECT marked RTP packets. Upon reception of this
feedback from each receiver it knows of, the sender can consider
ECN functional for its traffic. Each sender does this
verification independently. When a new receiver joins an existing
RTP session, it will send RTCP reports in the usual manner. If
those RTCP reports include ECN information, verification will have
succeeded and sources can continue to send ECT packets. If not,
verification fails and each sender MUST stop using ECN (see
Section 7.2.1 for details).
o Alternatively, ECN support can be verified during an initial end-
to-end STUN exchange (for example, as part of ICE connection
establishment). After having verified connectivity without ECN
capability an extra STUN exchange, this time with the ECN field
set to ECT(0) or ECT(1), is performed on the candidate path that
is about to be used. If successful the path's capability to
convey ECN marked packets is verified. A new STUN attribute is
defined to convey feedback that the ECT marked STUN request was
received (see Section 7.2.2), along with an ICE signalling option
(Section 6.4) to indicate that the check is to be performed.
o Thirdly, the sender may make a leap of faith that ECN will work.
This is only recommended for applications that know they are
running in controlled environments where ECN functionality has
been verified through other means. In this mode it is assumed
that ECN works, and the system reacts to failure indicators if the
assumption proved wrong. The use of this method relies on a high
confidence that ECN operation will be successful, or an
application where failure is not serious. The impact on the
network and other users must be considered when making a leap of
faith, so there are limitations on when this method is allowed
(see Section 7.2.3).
The first mechanism, using RTP with RTCP feedback, has the advantage
of working for all RTP sessions, but the disadvantages of potential
clipping if ECN marked RTP packets are discarded by middleboxes, and
slow verification of ECN support. The STUN-based mechanism is faster
to verify ECN support, but only works in those scenarios supported by
end-to-end STUN, such as within an ICE exchange. The third one,
leap-of-faith, has the advantage of avoiding additional tests or
complexities and enabling ECN usage from the first media packet. The
downside is that if the end-to-end path contains middleboxes that do
not pass ECN, the impact on the application can be severe: in the
worst case, all media could be lost if a middlebox that discards ECN
marked packets is present. A less severe effect, but still requiring
reaction, is the presence of a middlebox that re-marks ECT marked
packets to non-ECT, possibly marking packets with an ECN-CE mark as
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non-ECT. This could result in increased levels of congestion due to
non-responsiveness, and impact media quality as applications end up
relying on packet loss as an indication of congestion.
Once ECN support has been verified (or assumed) to work for all
receivers, a sender marks all its RTP packets as ECT packets, while
receivers rapidly feed back reports on any ECN-CE marks to the sender
using RTCP in RTP/AVPF immediate or early feedback mode, unless no
timely feedback is required. Each feedback report indicates the
receipt of new ECN-CE marks since the last ECN feedback packet, and
also counts the total number of ECN-CE marked packets as a cumulative
sum. This is the mechanism to provide the fastest possible feedback
to senders about ECN-CE marks. On receipt of an ECN-CE marked
packet, the system must react to congestion as-if packet loss has
been reported. Section 7.3 describes the ongoing use of ECN within
an RTP session.
This rapid feedback is not optimised for reliability, so another
mechanism, RTCP XR ECN summary reports, is used to ensure more
reliable, but less timely, reporting of the ECN information. The ECN
summary report contains the same information as the ECN feedback
format, only packed differently for better efficiency with reports
for many sources. It is sent in a compound RTCP packet, along with
regular RTCP reception reports. By using cumulative counters for
observed ECN-CE, ECT, not-ECT, packet duplication, and packet loss
the sender can determine what events have happened since the last
report, independently of any RTCP packets having been lost.
RTCP reports MUST NOT be ECT marked, since ECT marked traffic may be
dropped if the path is not ECN compliant. RTCP is used to provide
feedback about what has been transmitted and what ECN markings that
are received, so it is important that it is received in cases when
ECT marked traffic is not getting through.
There are numerous reasons why the path the RTP packets take from the
sender to the receiver may change, e.g., mobility, link failure
followed by re-routing around it. Such an event may result in the
packet being sent through a node that is ECN non-compliant, thus re-
marking or dropping packets with ECT set. To prevent this from
impacting the application for longer than necessary, the operation of
ECN is constantly monitored by all senders (Section 7.4). Both the
RTCP XR ECN summary reports and the ECN feedback packets allow the
sender to compare the number of ECT(0), ECT(1), and non-ECT marked
packets received with the number that were sent, while also reporting
ECN-CE marked and lost packets. If these numbers do not agree, it
can be inferred that the path does not reliably pass ECN-marked
packets. A sender detecting a possible ECN non-compliance issue
should then stop sending ECT marked packets to determine if that
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allows the packets to be correctly delivered. If the issues can be
connected to ECN, then ECN usage is suspended.
5. RTCP Extensions for ECN feedback
This memo defines two new RTCP extensions: one RTP/AVPF [RFC4585]
transport layer feedback format for reporting urgent ECN information,
and one RTCP XR [RFC3611] ECN summary report block type for regular
reporting of the ECN marking information.
5.1. RTP/AVPF Transport Layer ECN Feedback packet
This RTP/AVPF transport layer feedback format is intended for use in
RTP/AVPF early or immediate feedback modes when information needs to
urgently reach the sender. Thus its main use is to report reception
of an ECN-CE marked RTP packet so that the sender may perform
congestion control, or to speed up the initiation procedures by
rapidly reporting that the path can support ECN-marked traffic. The
feedback format is also defined with reduced size RTCP [RFC5506] in
mind, where RTCP feedback packets may be sent without accompanying
Sender or Receiver Reports that would contain the Extended Highest
Sequence number and the accumulated number of packet losses. Both
are important for ECN to verify functionality and keep track of when
CE marking does occur.
The RTP/AVPF transport layer feedback packet starts with the common
header defined by the RTP/AVPF profile [RFC4585] which is reproduced
in Figure 1. The FMT field takes the value [TBA1] to indicate that
the Feedback Control Information (FCI) contains ECN Feedback report,
as defined in Figure 2.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| FMT=TBA1| PT=RTPFB=205 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Feedback Control Information (FCI) :
: :
Figure 1: RTP/AVPF Common Packet Format for Feedback Messages
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Highest Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ECT (0) Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ECT (1) Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ECN-CE Counter | not-ECT Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Loss Packet Counter | Duplication Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: ECN Feedback Report Format
The ECN Feedback Report contains the following fields:
Extended Highest Sequence Number: The 32-bit Extended highest
sequence number received, as defined by [RFC3550]. Indicates the
highest RTP sequence number to which this report relates.
ECT(0) Counter: The 32-bit cumulative number of RTP packets with
ECT(0) received from this SSRC.
ECT(1) Counter: The 32-bit cumulative number of RTP packets with
ECT(1) received from this SSRC.
ECN-CE Counter: The cumulative number of RTP packets received from
this SSRC since the receiver joined the RTP session that were
ECN-CE marked, including ECN-CE marks in any duplicate packets.
The receiver should keep track of this value using a local
representation that is at least 32-bits, and only include the 16-
bits with least significance. In other words, the field will wrap
if more than 65535 ECN-CE marked packets have been received.
not-ECT Counter: The cumulative number of RTP packets received from
this SSRC since the receiver joined the RTP session that had an
ECN field value of not-ECT. The receiver should keep track of
this value using a local representation that is at least 32-bits,
and only include the 16-bits with least significance. In other
words, the field will wrap if more than 65535 not-ECT packets have
been received.
Lost Packets Counter: The cumulative number of RTP packets that the
receiver expected to receive minus the number of packets it
actually received that are not a duplicate of an already received
packet, from this SSRC since the receiver joined the RTP session.
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Note that packets that arrive late are not counted as lost. The
receiver should keep track of this value using a local
representation that is at least 32-bits, and only include the 16-
bits with least significance. In other words, the field will wrap
if more than 65535 packets are lost.
Duplication Counter: The cumulative number of RTP packets received
that are a duplicate of an already received packet from this SSRC
since the receiver joined the RTP session. The receiver should
keep track of this value using a local representation that is at
least 32-bits, and only include the 16-bits with least
significance. In other words, the field will wrap if more than
65535 duplicate packets have been received.
All fields in the ECN Feedback Report are unsigned integers in
network byte order. Each ECN Feedback Report corresponds to a single
RTP source (SSRC). Multiple sources can be reported by including
multiple ECN Feedback Reports packets in an compound RTCP packet.
The counters SHALL be initiated to 0 for each new SSRC received.
This to enable detection of ECN-CE marks or Packet loss on the
initial report from a specific participant.
The use of at least 32-bit counters allows even extremely high packet
volume applications to not have wrapping of counters within any
timescale close to the RTCP reporting intervals. However, 32-bits
are not sufficiently large to disregard the fact that wrappings may
happen during the life time of a long-lived RTP session, and
implementations need to be written to handle wrapping of the
counters. It is recommended that implementations uses local
representation of these counters that are longer than 32-bits to
enable easy handling of wraps.
There is a difference in packet duplication reports between the
packet loss counter that is defined in the Receiver Report Block
[RFC3550] and that defined here. To avoid holding state for what RTP
sequence numbers have been received, [RFC3550] specifies that one can
count packet loss by counting the number of received packets and
comparing it to the number of packets expected. As a result a packet
duplication can hide a packet loss. However, when populating the ECN
Feedback report, a receiver needs to track the sequence numbers
actually received and count duplicates and packet loss separately to
provide a more reliable indication. Reordering may however still
result in that packet loss is reported in one report and then removed
in the next.
The ECN-CE counter is robust for packet duplication. Adding each
received ECN-CE marked packet to the counter is not an issue, in fact
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it is required to ensure complete tracking of the ECN state. If one
of the clones was ECN-CE marked that is still an indication of
congestion. Packet duplication has potential impact on the ECN
verification and thus there is a need to count the duplicates.
5.2. RTCP XR Report block for ECN summary information
This unilateral XR report block combined with RTCP SR or RR report
blocks carries the same information as the ECN Feedback Report and is
be based on the same underlying information. However, the ECN
Feedback Report is intended to report on an ECN-CE mark as soon as
possible, while this extended report is for the regular RTCP
reporting and continuous verification of the ECN functionality end-
to-end.
The ECN Summary report block consists of one RTCP XR report block
header, shown in Figure 3 followed by one or more ECN summary report
data blocks, as defined in Figure 4.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=[TBA2] | Reserved | Block Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: RTCP XR Report Header
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Media Sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ECT (0) Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ECT (1) Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ECN-CE Counter | not-ECT Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Loss Packet Counter | Duplication Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: RTCP XR ECN Summary Report
The RTCP XR ECN Summary Report contains the following fields:
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BT: Block Type identifying the ECN summary report block. Value is
[TBA2].
Reserved: All bits SHALL be set to 0 on transmission and ignored on
reception.
Block Length: The length of this XR report block, including the
header, in 32- bit words minus one. Used to indicate the number
of ECN summary report data blocks present in the ECN summary
report. This length will be 5*n, where n is the number of ECN
summary report blocks, since blocks are a fixed size. The block
length MAY be zero if there is nothing to report. Receivers MUST
discard reports where the block length is not a multiple of five,
since these cannot be valid.
SSRC of Media Sender: The SSRC identifying the media sender this
report is for.
ECT(0) Counter: as in Section 5.1.
ECT(1) Counter: as in Section 5.1.
ECN-CE Counter: as in Section 5.1.
not-ECT Counter: as in Section 5.1.
Loss Packet Counter: as in Section 5.1.
Duplication Counter: as in Section 5.1.
The Extended Highest Sequence number counter for each SSRC is not
present in RTCP XR report, in contrast to the feedback version. The
reason is that this summary report will rely on the information sent
in the Sender Report (SR) or Receiver Report (RR) blocks part of the
same RTCP compound packet. The Extended Highest Sequence number is
available from the SR or RR.
All the SSRCs that are present in the SR or RR SHOULD also be
included in the RTCP XR ECN summary report. In cases where the
number of senders are so large that the combination of SR/RR and the
ECN summary for all the senders exceed the MTU, then only a subset of
the senders SHOULD be included so that the reports for the subset
fits within the MTU. The subsets SHOULD be selected round-robin
across multiple intervals so that all sources are periodically
reported. In case there are no SSRCs that currently are counted as
senders in the session, the report block SHALL still be sent with no
report block entry and a zero report block length to continuously
indicate to the other participants the receiver capability to report
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ECN information.
6. SDP Signalling Extensions for ECN
This section defines a number of SDP signalling extensions used in
the negotiation of the ECN for RTP support when using SDP. This
includes one SDP attribute "ecn-capable-rtp" that negotiates the
actual operation of ECN for RTP. Two SDP signalling parameters are
defined to indicate the use of the RTCP XR ECN summary block and the
RTP/AVPF feedback format for ECN. One ICE option SDP representation
is also defined.
6.1. Signalling ECN Capability using SDP
One new SDP attribute, "a=ecn-capable-rtp", is defined. This is a
media level attribute, and MUST NOT be used at the session level. It
is not subject to the character set chosen. The aim of this
signalling is to indicate the capability of the sender and receivers
to support ECN, and to negotiate the method of ECN initiation to be
used in the session. The attribute takes a list of initiation
methods, ordered in decreasing preference. The defined values for
the initiation method are:
rtp: Using RTP and RTCP as defined in Section 7.2.1.
ice: Using STUN within ICE as defined in Section 7.2.2.
leap: Using the leap of faith method as defined in Section 7.2.3.
Further methods may be specified in the future, so unknown methods
MUST be ignored upon reception.
In addition, a number of OPTIONAL parameters may be included in the
"a=ecn-capable-rtp" attribute as follows:
mode: This parameter signals the endpoint's capability to set and
read ECN marks in UDP packets. An examination of various
operating systems has shown that end-system support for ECN
marking of UDP packets may be symmetric or asymmetric. By this we
mean that some systems may allow end points to set the ECN bits in
an outgoing UDP packet but not read them, while others may allow
applications to read the ECN bits but not set them. This
either/or case may produce an asymmetric support for ECN and thus
should be conveyed in the SDP signalling. The "mode=setread"
state is the ideal condition where an endpoint can both set and
read ECN bits in UDP packets. The "mode=setonly" state indicates
that an endpoint can set the ECT bit, but cannot read the ECN bits
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from received UDP packets to determine if upstream congestion
occurred. The "mode=readonly" state indicates that the endpoint
can read the ECN bits to determine if congestion has occurred for
incoming packets, but it cannot set the ECT bits in outgoing UDP
packets. When the "mode=" parameter is omitted it is assumed that
the node has "setread" capabilities. This option can provide for
an early indication that ECN cannot be used in a session. This
would be case when both the offerer and answerer set the "mode="
parameter to "setonly" or both set it to "readonly".
ect: This parameter makes it possible to express the preferred ECT
marking. This is either "random", "0", or "1", with "0" being
implied if not specified. The "ect" parameter describes a
receiver preference, and is useful in the case where the receiver
knows it is behind a link using IP header compression, the
efficiency of which would be seriously disrupted if it were to
receive packets with randomly chosen ECT marks. It is RECOMMENDED
that ECT(0) marking be used.
The ABNF [RFC5234] grammar for the "a=ecn-capable-rtp" attribute is
shown in Figure 5.
ecn-attribute = "a=ecn-capable-rtp:" SP init-list [SP parm-list]
init-list = init-value *("," init-value)
init-value = "rtp" / "ice" / "leap" / init-ext
init-ext = token
parm-list = parm-value *(";" SP parm-value)
parm-value = mode / ect / parm-ext
mode = "mode=" ("setonly" / "setread" / "readonly")
ect = "ect=" ("0" / "1" / "random")
parm-ext = parm-name "=" parm-value-ext
parm-name = token
parm-value-ext = token / quoted-string
quoted-string = ( DQUOTE *qdtext DQUOTE )
qdtext = %x20-21 / %x23-5B / %x5D-7E / quoted-pair / UTF8-NONASCII
; No DQUOTE and no "\"
quoted-pair = "\\" / ( "\" DQUOTE )
UTF8-NONASCII = UTF8-1 / UTF8-2 / UTF8-3 / UTF8-4
; external references:
; token: from RFC 4566
; SP and DQUOTE from RFC 5234
; UTF8-1, UTF8-2, UTF8-3, and UTF8-4 from RFC 3629
Figure 5: ABNF Grammar for the "a=ecn-capable-rtp" attribute
Note the above quoted string construct has an escaping mechanism for
strings containing ". This uses \ (back slash) as escaping
mechanism, i.e. in a string that contains a " it is replaced by \"
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(backslash double quote) and any \ (backslash) is replaced by \\
(backslash backslash) when put into the double quotes as defined by
the above syntax. The string in a quoted string is UTF-8 [RFC3629].
6.1.1. Use of "a=ecn-capable-rtp:" with the Offer/Answer Model
When SDP is used with the offer/answer model [RFC3264], the party
generating the SDP offer MUST insert an "a=ecn-capable-rtp" attribute
into the media section of the SDP offer of each RTP session for which
it wishes to use ECN. The attribute includes one or more ECN
initiation methods in a comma separated list in decreasing order of
preference, with any number of optional parameters following. The
answering party compares the list of initiation methods in the offer
with those it supports in order of preference. If there is a match,
and if the receiver wishes to attempt to use ECN in the session, it
includes an "a=ecn-capable-rtp" attribute containing its single
preferred choice of initiation method, and any optional parameters,
in the media sections of the answer. If there is no matching
initiation method capability, or if the receiver does not wish to
attempt to use ECN in the session, it does not include an "a=ecn-
capable-rtp" attribute in its answer. If the attribute is removed in
the answer then ECN MUST NOT be used in any direction for that media
flow. If there are initialization methods that are unknown, they
MUST be ignored on reception and MUST NOT be included in an answer.
The endpoints' capability to set and read ECN marks, as expressed by
the optional "mode=" parameter, determines whether ECN support can be
negotiated for flows in one or both directions:
o If the "mode=setonly" parameter is present in the "a=ecn-capable-
rtp" attribute of the offer and the answering party is also
"mode=setonly", then there is no common ECN capability, and the
answer MUST NOT include the "a=ecn-capable-rtp" attribute.
Otherwise, if the offer is "mode=setonly" then ECN may only be
initiated in the direction from the offering party to the
answering party.
o If the "mode=readonly" parameter is present in the "a=ecn-capable-
rtp" attribute of the offer and the answering party is
"mode=readonly", then there is no common ECN capability, and the
answer MUST NOT include the "a=ecn-capable-rtp" attribute.
Otherwise, if the offer is "mode=readonly" then ECN may only be
initiated in the direction from the answering party to the
offering party.
o If the "mode=setread" parameter is present in the "a=ecn-capable-
rtp" attribute of the offer and the answering party is "setonly",
then ECN may only be initiated in the direction from the answering
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party to the offering party. If the offering party is
"mode=setread" but the answering party is "mode=readonly", then
ECN may only be initiated in the direction from the offering party
to the answering party. If both offer and answer are
"mode=setread", then ECN may be initiated in both directions.
Note that "mode=setread" is implied by the absence of a "mode="
parameter in the offer or the answer.
o An offer that does not include a "mode=" parameter MUST be treated
as-if a "mode=setread" parameter had been included.
In an RTP session using multicast and ECN, participants that intend
to send RTP packets SHOULD support setting ECT marks in RTP packets
(i.e., should be "mode=setonly" or "mode=setread"). Participants
receiving data need the capability to read ECN marks on incoming
packets. It is important that receivers can read ECN marks (are
"mode=readonly" or "mode=setread"), since otherwise no sender in the
multicast session will be able to enable ECN. Accordingly, receivers
that are "mode=setonly" SHOULD NOT join multicast RTP sessions that
use ECN. If session participants that are not aware of the ECN for
RTP signalling are invited to a multicast session, and simply ignore
the signalling attribute, the other party in the offer/answer
exchange SHOULD terminate the SDP dialogue so that the participant
leaves the session.
The "ect=" parameter in the "a=ecn-capable-rtp" attribute is set
independently in the offer and the answer. Its value in the offer
indicates a preference for the sending behaviour of the answering
party, and its value in the answer indicates a sending preference for
the behaviour of the offering party. It will be the senders choice
to honour the receivers preference for what to receive or not. In
multicast sessions, all senders SHOULD set the ECT marks using the
value declared in the "ect=" parameter.
Unknown optional parameters MUST be ignored on reception, and MUST
NOT be included in the answer. That way new parameters may be
introduced and verified to be supported by the other end-point by
having them include it in any answer.
6.1.2. Use of "a=ecn-capable-rtp:" with Declarative SDP
When SDP is used in a declarative manner, for example in a multicast
session using the Session Announcement Protocol (SAP, [RFC2974]),
negotiation of session description parameters is not possible. The
"a=ecn-capable-rtp" attribute MAY be added to the session description
to indicate that the sender will use ECN in the RTP session. The
attribute MUST include a single method of initiation. Participants
MUST NOT join such a session unless they have the capability to
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receive ECN-marked UDP packets, implement the method of initiation,
and can generate RTCP ECN feedback. The mode parameter MAY also be
included in declarative usage, to indicate the minimal capability is
required by the consumer of the SDP. So for example in a SSM session
the participants configured with a particular SDP will all be in a
media receive only mode, thus mode=readonly will work as the
capability of reporting on the ECN markings in the received is what
is required. However, using "mode=readonly" also in ASM sessions is
reasonable, unless all senders are required to attempt to use ECN for
their outgoing RTP data traffic, in which case the mode needs to be
set to "setread".
6.1.3. General Use of the "a=ecn-capable-rtp:" Attribute
The "a=ecn-capable-rtp" attribute MAY be used with RTP media sessions
using UDP/IP transport. It MUST NOT be used for RTP sessions using
TCP, SCTP, or DCCP transport, or for non-RTP sessions.
As described in Section 7.3.3, RTP sessions using ECN require rapid
RTCP ECN feedback, unless timely feedback is not required due to a
receiver driven congestion control. To ensure that the sender can
react to ECN-CE marked packets timely feedback is usually required.
Thus, the use of the Extended RTP Profile for RTCP-Based Feedback
(RTP/AVPF) [RFC4585] or other profile that inherits RTP/AVPF's
signalling rules, MUST be signalled unless timely feedback is not
required. If timely feedback is not required it is still RECOMMENDED
to use RTP/AVPF. The signalling of an RTP/AVPF based profile is
likely to be required even if the preferred method of initialization
and the congestion control does not require timely feedback, as the
common interoperable method is likely to be signalled or the improved
fault reaction is desired.
6.2. RTCP ECN Feedback SDP Parameter
A new "nack" feedback parameter "ecn" is defined to indicate the
usage of the RTCP ECN feedback packet format (Section 5.1). The ABNF
[RFC5234] definition of the SDP parameter extension is:
rtcp-fb-nack-param = <See section 4.2 of RFC 4585>
rtcp-fb-nack-param /= ecn-fb-par
ecn-fb-par = SP "ecn"
The offer/answer rules for this SDP feedback parameters are specified
in the RTP/AVPF profile [RFC4585].
6.3. XR Block ECN SDP Parameter
A new unilateral RTCP XR block for ECN summary information is
specified, thus the XR block SDP signalling also needs to be extended
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with a parameter. This is done in the same way as for the other XR
blocks. The XR block SDP attribute as defined in Section 5.1 of the
RTCP XR specification [RFC3611] is defined to be extensible. As no
parameter values are needed for this ECN summary block, this
parameter extension consists of a simple parameter name used to
indicate support and intent to use the XR block.
xr-format = <See Section 5.1 of [RFC3611]>
xr-format /= ecn-summary-par
ecn-summary-par = "ecn-sum"
For SDP declarative and offer/answer usage, see the RTCP XR
specification [RFC3611] and its description of how to handle
unilateral parameters.
6.4. ICE Parameter to Signal ECN Capability
One new ICE [RFC5245] option, "rtp+ecn", is defined. This is used
with the SDP session level "a=ice-options" attribute in an SDP offer
to indicate that the initiator of the ICE exchange has the capability
to support ECN for RTP-over-UDP flows (via "a=ice-options: rtp+ecn").
The answering party includes this same attribute at the session level
in the SDP answer if it also has the capability, and removes the
attribute if it does not wish to use ECN, or doesn't have the
capability to use ECN. If the ICE initiation method (Section 7.2.2)
is actually going to be used, it is also needs to be explicitly
negotiated using the "a=ecn-capable-rtp" attribute. This ICE option
SHALL be included when the ICE initiation method is offered or
declared in the SDP.
Note: This signalling mechanism is not strictly needed as long as
the STUN ECN testing capability is used within the context of this
document. It may however be useful if the ECN verification
capability is used in additional contexts.
7. Use of ECN with RTP/UDP/IP
In the detailed specification of the behaviour below, the different
functions in the general case will first be discussed. In case
special considerations are needed for middleboxes, multicast usage
etc, those will be specially discussed in related subsections.
7.1. Negotiation of ECN Capability
The first stage of ECN negotiation for RTP-over-UDP is to signal the
capability to use ECN. An RTP system that supports ECN and uses SDP
for its signalling MUST implement the SDP extension to signal ECN
capability as described in Section 6.1, the RTCP ECN feedback SDP
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parameter defined in Section 6.2, and the XR Block ECN SDP parameter
defined in Section 6.3. It MAY also implement alternative ECN
capability negotiation schemes, such as the ICE extension described
in Section 6.4. Other signalling systems will need to define
signalling parameters corresponding to those defined for SDP.
The "ecn-capable-rtp" SDP attribute MUST be used when employing ECN
for RTP according to this specification in systems using SDP. As the
RTCP XR ECN summary report is required independently of the
initialization method or congestion control scheme, the "rtcp-xr"
attribute with the "ecn-sum" parameter MUST also be used. The
"rtcp-fb" attribute with the "nack" parameter "ecn" MUST be used
whenever the initialization method or a congestion control algorithm
requires timely sender side knowledge of received CE markings. If
the congestion control scheme requires additional signalling, this
should be indicated as appropriate.
7.2. Initiation of ECN Use in an RTP Session
Once the sender and the receiver(s) have agreed that they have the
capability to use ECN within a session, they may attempt to initiate
ECN use. All session participants connected over the same transport
MUST use the same initiation method. RTP mixers or translators can
use different initiation methods to different participants that are
connected over different underlying transports. The mixer or
translator will need to do individual signalling with each
participant to ensure it is consistent with the ECN support in those
cases where it does not function as one end-point for the ECN control
loop.
At the start of the RTP session, when the first few packets with ECT
are sent, it is important to verify that IP packets with ECN field
values of ECT or ECN-CE will reach their destination(s). There is
some risk that the use of ECN will result in either reset of the ECN
field, or loss of all packets with ECT or ECN-CE markings. If the
path between the sender and the receivers exhibits either of these
behaviours, the sender needs to stop using ECN immediately to protect
both the network and the application.
The RTP senders and receivers SHALL NOT ECT mark their RTCP traffic
at any time. This is to ensure that packet loss due to ECN marking
will not effect the RTCP traffic and the necessary feedback
information it carries.
An RTP system that supports ECN MUST implement the initiation of ECN
using in-band RTP and RTCP described in Section 7.2.1. It MAY also
implement other mechanisms to initiate ECN support, for example the
STUN-based mechanism described in Section 7.2.2, or use the leap of
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faith option if the session supports the limitations provided in
Section 7.2.3. If support for both in-band and out-of-band
mechanisms are signalled, the sender when negotiating SHOULD offer
detection of ECT using STUN with ICE with higher priority than
detection of ECT using RTP and RTCP.
No matter how ECN usage is initiated, the sender MUST continually
monitor the ability of the network, and all its receivers, to support
ECN, following the mechanisms described in Section 7.4. This is
necessary because path changes or changes in the receiver population
may invalidate the ability of the system to use ECN.
7.2.1. Detection of ECT using RTP and RTCP
The ECN initiation phase using RTP and RTCP to detect if the network
path supports ECN comprises three stages. Firstly, the RTP sender
generates some small fraction of its traffic with ECT marks to act as
probe for ECN support. Then, on receipt of these ECT-marked packets,
the receivers send RTCP ECN feedback packets and RTCP ECN summary
reports to inform the sender that their path supports ECN. Finally,
the RTP sender makes the decision to use ECN or not, based on whether
the paths to all RTP receivers have been verified to support ECN.
Generating ECN Probe Packets: During the ECN initiation phase, an
RTP sender SHALL mark a small fraction of its RTP traffic as ECT,
while leaving the reminder of the packets unmarked. The main
reason for only marking some packets is to maintain usable media
delivery during the ECN initiation phase in those cases where ECN
is not supported by the network path. A secondary reason to send
some not-ECT packets are to ensure that the receivers will send
RTCP reports on this sender, even if all ECT marked packets are
lost in transit. The not-ECT packets also provide a base-line to
compare performance parameters against. A fourth reason for only
probing with a small number of packets is to reduce the risk that
significant numbers of congestion markings might be lost if ECT is
cleared to Not-ECT by an ECN-Reverting Middlebox. Then any
resulting lack of congestion response is likely to have little
damaging effect on others. An RTP sender is RECOMMENDED to send a
minimum of two packets with ECT markings per RTCP reporting
interval. In case a random ECT pattern is intended to be used, at
least one packet with ECT(0) and one with ECT(1) should be sent
per reporting interval; in case a single ECT marking is to be
used, only that ECT value SHOULD be sent. The RTP sender SHALL
continue to send some ECT marked traffic as long as the ECN
initiation phase continues. The sender SHOULD NOT mark all RTP
packets as ECT during the ECN initiation phase.
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This memo does not mandate which RTP packets are marked with ECT
during the ECN initiation phase. An implementation should insert
ECT marks in RTP packets in a way that minimises the impact on
media quality if those packets are lost. The choice of packets to
mark is very media dependent. For audio formats, if would make
sense for the sender to mark comfort noise packets or similar.
For video formats, packets containing P- or B-frames (rather than
I-frames) would be an appropriate choice. No matter which RTP
packets are marked, those packets MUST NOT be sent in duplicate,
with and without ECT, since the RTP sequence number is used to
identify packets that are received with ECN markings.
Generating RTCP ECN Feedback: If ECN capability has been negotiated
in an RTP session, the receivers in the session MUST listen for
ECT or ECN-CE marked RTP packets, and generate RTCP ECN feedback
packets (Section 5.1) to mark their receipt. An immediate or
early (depending on the RTP/AVPF mode) ECN feedback packet SHOULD
be generated on receipt of the first ECT or ECN-CE marked packet
from a sender that has not previously sent any ECT traffic. Each
regular RTCP report MUST also contain an ECN summary report
(Section 5.2). Reception of subsequent ECN-CE marked packets MUST
result in additional early or immediate ECN feedback packets being
sent unless no timely feedback is required.
Determination of ECN Support: RTP is a group communication protocol,
where members can join and leave the group at any time. This
complicates the ECN initiation phase, since the sender must wait
until it believes the group membership has stabilised before it
can determine if the paths to all receivers support ECN (group
membership changes after the ECN initiation phase has completed
are discussed in Section 7.3).
An RTP sender shall consider the group membership to be stable
after it has been in the session and sending ECT-marked probe
packets for at least three RTCP reporting intervals (i.e., after
sending its third regularly scheduled RTCP packet), and when a
complete RTCP reporting interval has passed without changes to the
group membership. ECN initiation is considered successful when
the group membership is stable, and all known participants have
sent one or more RTCP ECN feedback packets or RTCP XR ECN summary
reports indicating correct receipt of the ECT-marked RTP packets
generated by the sender.
As an optimisation, if an RTP sender is initiating ECN usage
towards a unicast address, then it MAY treat the ECN initiation as
provisionally successful if it receives an RTCP ECN feedback
report or an RTCP XR ECN summary report indicating successful
receipt of the ECT-marked packets, with no negative indications,
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from a single RTP receiver (where a single RTP receiver is
considered as all SSRCs used by a single RTCP CNAME). After
declaring provisional success, the sender MAY generate ECT-marked
packets as described in Section 7.3, provided it continues to
monitor the RTCP reports for a period of three RTCP reporting
intervals from the time the ECN initiation started, to check if
there are any other participants in the session. Thus as long as
any additional SSRC that report on the ECN usage are using the
same RTCP CNAME as the previous reports and they are all
indicating functional ECN the sender may continue. If other
participants are detected, i.e., other RTCP CNAMEs, the sender
MUST fallback to only ECT-marking a small fraction of its RTP
packets, while it determines if ECN can be supported following the
full procedure described above. Different RTCP CNAMEs received
over an unicast transport may occur when using translators in a
multi-party RTP session (e.g., when using a centralised conference
bridge).
Note: The above optimization supports peer to peer unicast
transport with several SSRCs multiplexed onto the same flow
(e.g., a single participant with two video cameras, or SSRC
multiplexed RTP retransmission [RFC4588]). It is desirable to
be able to rapidly negotiate ECN support for such a session,
but the optimisation above can fail if there are
implementations that use the same CNAME for different parts of
a distributed implementation that have different transport
characteristics (e.g., if a single logical endpoint is split
across multiple hosts).
ECN initiation is considered to have failed at the instant the
initiating RTP sender received an RTCP packet that doesn't contain
an RTCP ECN feedback report or ECN summary report from any RTP
session participant that has an RTCP RR with an extended RTP
sequence number field that indicates that it should have received
multiple (>3) ECT marked RTP packets. This can be due to failure
to support the ECN feedback format by the receiver or some
middlebox, or the loss of all ECT marked packets. Both indicate a
lack of ECN support.
If the ECN negotiation succeeds, this indicates that the path can
pass some ECN-marked traffic, and that the receivers support ECN
feedback. This does not necessarily imply that the path can robustly
convey ECN feedback; Section 7.3 describes the ongoing monitoring
that must be performed to ensure the path continues to robustly
support ECN.
When a sender or receiver detects ECN failures on paths they should
log these to enable follow up and statistics gathering regarding
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broken paths. The logging mechanism used is implementation
dependent.
7.2.2. Detection of ECT using STUN with ICE
This section describes an OPTIONAL method that can be used to avoid
media impact and also ensure an ECN capable path prior to media
transmission. This method is considered in the context where the
session participants are using ICE [RFC5245] to find working
connectivity. We need to use ICE rather than STUN only, as the
verification needs to happen from the media sender to the address and
port on which the receiver is listening.
Note that this method is only applicable to sessions when the remote
destinations are unicast addresses. In addition, transport
translators that do not terminate the ECN control loop and may
distribute received packets to more than one other receiver must
either disallow this method (and use the RTP/RTCP method instead), or
implement additional handling as discussed below. This is because
the ICE initialization method verifies the underlying transport to
one particular address and port. If the receiver at that address and
port intends to use the received packets in a multi-point session
then the tested capabilities and the actual session behavior are not
matched.
To minimise the impact of set-up delay, and to prioritise the fact
that one has working connectivity rather than necessarily finding the
best ECN capable network path, this procedure is applied after having
performed a successful connectivity check for a candidate, which is
nominated for usage. At that point an additional connectivity check
is performed, sending the "ECN Check" attribute in a STUN packet that
is ECT marked. On reception of the packet, a STUN server supporting
this extension will note the received ECN field value, and send a
STUN/UDP/IP packet in reply with the ECN field set to not-ECT and
including an ECN-CHECK attribute. A STUN server that doesn't
understand the extension, or is incapable of reading the ECN values
on incoming STUN packets, should follow the rule in the STUN
specification for unknown comprehension-optional attributes, and
ignore the attribute, resulting in the sender receiving a STUN
response without the ECN Check STUN attribute.
The ECN STUN checks can be lost on the path, for example due to the
ECT marking, but also due to various other non ECN related reasons
causing packet loss. The goal is to detect when the ECT markings are
rewritten or if it is the ECT marking that causes packet loss so that
the path can be determined as not ECT. Other reasons for packet loss
should not result in a failure to verify the path as ECT. Therefore
a number of retransmissions should be attempted. But, the sender of
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ECN STUN checks will also have to set a criteria for when it gives up
testing for ECN capability on the path. Since the ICE agent has
successfully verified the path an RTT measurement for this path can
be performed. To have a high probability of successfully verifying
the path it is RECOMMENDED that the client retransmit the ECN STUN
check at least 4 times. The transmission for that flow is stopped
when an ECN Check STUN response has been received, which doesn't
indicate a retransmission of the request due to a temporary error, or
the maximum number of retransmissions has been sent. The ICE agent
is recommended to give up on the ECN verification MAX(1.5*RTT, 20 ms)
after the last ECN STUN check was sent.
The transmission of the ECT marked STUN connectivity checks
containing the ECN Check attribute can be done prior as well in
parallel to actual media transmission. Both cases are supported,
where the main difference is how aggressively the transmission of the
STUN checks are done. The reason for this is to avoid adding
additional startup delay until media can flow. If media is required
immeditely after nomination has occured the STUN checks SHALL be done
in parallel. If the application does not require media transmission
immediately the verification of ECT SHOULD start using the aggresive
mode. At any point in the process until ECT has been verified or
found to not work media transmission MAY be started and the ICE agent
SHALL transition from the aggressive mode to the parallel mode.
The aggressive mode uses an interval between the retransmissions be
based on the Ta timer as defined in Section 16.1 for RTP Media
Streams in ICE [RFC5245]. The number of ECN STUN checks needing to
be sent will depend on the number of ECN capable flows (N) that is to
be established. The interval between each transmission of an ECN
check packet MUST be Ta. In other words for a given flow being
verified for ECT the RTO is set to Ta*N.
The parallel mode uses transmission intervals that targets that the
bit-rate increase due to the ECT verification checks shall not
increase the total bit-rate more than 10% in addition to the media.
As ICE's regular transmission schedule is mimicking a common voice
call in amount, to meet that goal for most media flows, setting the
retransmission interval to Ta*N*k where k=10 fulfills that goal.
Thus the default behavior SHALL be to use k=10 when in parallel mode.
In cases where the bit-rate of the STUN connectivity checks can be
determined they MAY be sent with smaller values of k, but k MUST NOT
be smaller than 1, as long as the total bit-rate for the connectivity
checks are less than 10% of the used media bit-rate. The RTP media
packets being sent in parallel mode SHALL NOT be ECT marked prior to
verification of the path as ECT.
The STUN ECN-CHECK attribute contains one field and a flag, as shown
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in Figure 6. The flag indicates whether the echo field contains a
valid value or not. The field is the ECN echo field, and when valid
contains the two ECN bits from the packet it echoes back. The ECN-
CHECK attribute is a comprehension optional attribute.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |ECF|V|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: ECN Check STUN Attribute
V: Valid (1 bit) ECN Echo value field is valid when set to 1, and
invalid when set 0.
ECF: ECN Echo value field (2 bits) contains the ECN field value of
the STUN packet it echoes back when field is valid. If invalid
the content is arbitrary.
Reserved: Reserved bits (29 bits) SHALL be set to 0 on transmission,
and SHALL be ignored on reception.
This attribute MAY be included in any STUN request to request the ECN
field to be echoed back. In STUN requests the V bit SHALL be set to
0. A compliant STUN server receiving a request with the ECN Check
attribute SHALL read the ECN field value of the IP/UDP packet the
request was received in. Upon forming the response the server SHALL
include the ECN-CHECK attribute setting the V bit to valid and
include the read value of the ECN field into the ECF field. If the
STUN responder was unable to ascertain, due to temporary errors, the
ECN value of the STUN request, it SHALL set the V bit in the response
to 0. The STUN client may retry immediately.
The ICE based initialization method does require some special
consideration when used by a translator. This is especially for
transport translators and translators that fragment or reassemble
packets, since they do not separate the ECN control loops between the
end-points and the translator. When using ICE-based initiation, such
a translator must ensure that any participants joining an RTP session
for which ECN has been negotiated are successfully verified in the
direction from the translator to the joining participant.
Alternatively, it must correctly handle remarking of ECT RTP packets
towards that participant. When a new participant joins the session,
the translator will perform a check towards the new participant. If
that is successfully completed the ECT properties of the session are
maintained for the other senders in the session. If the check fails
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then the existing senders will now see a participant that fails to
receive ECT. Thus the failure detection in those senders will
eventually detect this. However to avoid misusing the network on the
path from the translator to the new participant, the translator SHALL
remark the traffic intended to be forwarded from ECT to non-ECT. Any
packet intended to be forward that are ECN-CE marked SHALL be
discarded and not sent. In cases where the path from a new
participant to the translator fails the ECT check then only that
sender will not contribute any ECT marked traffic towards the
translator.
7.2.3. Leap of Faith ECT initiation method
This method for initiating ECN usage is a leap of faith that assumes
that ECN will work on the used path(s). The method is to go directly
to "ongoing use of ECN" as defined in Section 7.3. Thus all RTP
packets MAY be marked as ECT and the failure detection MUST be used
to detect any case when the assumption that the path was ECT capable
is wrong. This method is only recommended for controlled
environments where the whole path(s) between sender and receiver(s)
has been built and verified to be ECT.
If the sender marks all packets as ECT while transmitting on a path
that contains an ECN-blocking middlebox, then receivers downstream of
that middlebox will not receive any RTP data packets from the sender,
and hence will not consider it to be an active RTP SSRC. The sender
can detect this and revert to sending packets without ECT marks,
since RTCP SR/RR packets from such receivers will either not include
a report for sender's SSRC, or will report that no packets have been
received, but this takes at least one RTCP reporting interval. It
should be noted that a receiver might generate its first RTCP packet
immediately on joining a unicast session, or very shortly after
joining a RTP/AVPF session, before it has had chance to receive any
data packets. A sender that receives RTCP SR/RR packet indicating
lack of reception by a receiver SHOULD therefore wait for a second
RTCP report from that receiver to be sure that the lack of reception
is due to ECT-marking. Since this recovery process can take several
tens of seconds, during which time the RTP session is unusable for
media, it is NOT RECOMMENDED that the leap-of-faith ECT initiation
method be used in environments where ECN-blocking middleboxes are
likely to be present.
7.3. Ongoing Use of ECN Within an RTP Session
Once ECN has been successfully initiated for an RTP sender, that
sender begins sending all RTP data packets as ECT-marked, and its
receivers send ECN feedback information via RTCP packets. This
section describes procedures for sending ECT-marked data, providing
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ECN feedback information via RTCP, and responding to ECN feedback
information.
7.3.1. Transmission of ECT-marked RTP Packets
After a sender has successfully initiated ECN use, it SHOULD mark all
the RTP data packets it sends as ECT. The sender SHOULD mark packets
as ECT(0) unless the receiver expresses a preference for ECT(1) or
random using the "ect" parameter in the "a=ecn-capable-rtp"
attribute.
The sender SHALL NOT include ECT marks on outgoing RTCP packets, and
SHOULD NOT include ECT marks on any other outgoing control messages
(e.g., STUN [RFC5389] packets, DTLS [RFC6347] handshake packets, or
ZRTP [RFC6189] control packets) that are multiplexed on the same UDP
port. For control packets there might be exceptions, like the STUN
based ECN check defined in Section 7.2.2.
7.3.2. Reporting ECN Feedback via RTCP
An RTP receiver that receives a packet with an ECN-CE mark, or that
detects a packet loss, MUST schedule the transmission of an RTCP ECN
feedback packet as soon as possible (subject to the constraints of
[RFC4585] and [RFC3550]) to report this back to the sender unless no
timely feedback is required. The feedback RTCP packet SHALL consist
of at least one ECN feedback packet (Section 5.1) reporting on the
packets received since the last ECN feedback packet, and will contain
(at least) an RTCP SR/RR packet and an SDES packet, unless reduced
size RTCP [RFC5506] is used. The RTP/AVPF profile in early or
immediate feedback mode SHOULD be used where possible, to reduce the
interval before feedback can be sent. To reduce the size of the
feedback message, reduced size RTCP [RFC5506] MAY be used if
supported by the end-points. Both RTP/AVPF and reduced size RTCP
MUST be negotiated in the session set-up signalling before they can
be used.
Every time a regular compound RTCP packet is to be transmitted, an
ECN-capable RTP receiver MUST include an RTCP XR ECN summary report
as described in Section 5.2 as part of the compound packet.
The multicast feedback implosion problem, that occurs when many
receivers simultaneously send feedback to a single sender, must be
considered. The RTP/AVPF transmission rules will limit the amount of
feedback that can be sent, avoiding the implosion problem but also
delaying feedback by varying degrees from nothing up to a full RTCP
reporting interval. As a result, the full extent of a congestion
situation may take some time to reach the sender, although some
feedback should arrive in a reasonably timely manner, allowing the
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sender to react on a single or a few reports.
7.3.3. Response to Congestion Notifications
The reception of RTP packets with ECN-CE marks in the IP header is a
notification that congestion is being experienced. The default
reaction on the reception of these ECN-CE marked packets MUST be to
provide the congestion control algorithm with a congestion
notification that triggers the algorithm to react as if packet loss
had occurred. There should be no difference in congestion response
if ECN-CE marks or packet drops are detected.
Other reactions to ECN-CE may be specified in the future, following
IETF review. Detailed designs of such alternative reactions MUST be
specified in a Standards Track RFC, and be reviewed to ensure they
are safe for deployment under any restrictions specified. A
potential example for an alternative reaction could be emergency
communications (such as that generated by first responders, as
opposed to the general public) in networks where the user has been
authorized. A more detailed description of these other reactions, as
well as the types of congestion control algorithms used by end-nodes,
is outside of the scope of this document.
Depending on the media format, type of session, and RTP topology
used, there are several different types of congestion control that
can be used:
Sender-Driven Congestion Control: The sender is responsible for
adapting the transmitted bit-rate in response to RTCP ECN
feedback. When the sender receives the ECN feedback data it feeds
this information into its congestion control or bit-rate
adaptation mechanism so that it can react as if packet loss was
reported. The congestion control algorithm to be used is not
specified here, although TFRC [RFC5348] is one example that might
be used.
Receiver-Driven Congestion Control: In a receiver driven congestion
control mechanism, the receivers can react to the ECN-CE marks
themselves without providing ECN-CE feedback to the sender. This
may allow faster response than sender-driven congestion control in
some circumstances and also scale to large number of receivers and
multicast usage. One example of receiver-driven congestion
control is implemented by providing the content in a layered way,
with each layer providing improved media quality but also
increased bandwidth usage. The receiver locally monitors the
ECN-CE marks on received packets to check if it experiences
congestion with the current number of layers. If congestion is
experienced, the receiver drops one layer, so reducing the
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resource consumption on the path towards itself. For example, if
a layered media encoding scheme such as H.264 SVC is used, the
receiver may change its layer subscription, and so reduce the bit
rate it receives. The receiver MUST still send RTCP XR ECN
Summary to the sender, even if it can adapt without contact with
the sender, so that the sender can determine if ECN is supported
on the network path. The timeliness of RTCP feedback is less of a
concern with receiver driven congestion control, and regular RTCP
reporting of ECN summary information is sufficient (without using
RTP/AVPF immediate or early feedback).
Hybrid: There might be mechanisms that utilize both some receiver
behaviors and some sender side monitoring, thus requiring both
feedback of congestion events to the sender and taking receiver
decisions and possible signalling to the sender. In this case the
congestion control algorithm needs to use the signalling to
indicate which features of ECN for RTP are required.
Responding to congestion indication in the case of multicast traffic
is a more complex problem than for unicast traffic. The fundamental
problem is diverse paths, i.e., when different receivers don't see
the same path, and thus have different bottlenecks, so the receivers
may get ECN-CE marked packets due to congestion at different points
in the network. This is problematic for sender driven congestion
control, since when receivers are heterogeneous in regards to
capacity, the sender is limited to transmitting at the rate the
slowest receiver can support. This often becomes a significant
limitation as group size grows. Also, as group size increases the
frequency of reports from each receiver decreases, which further
reduces the responsiveness of the mechanism. Receiver-driven
congestion control has the advantage that each receiver can choose
the appropriate rate for its network path, rather than all receivers
having to settle for the lowest common rate.
We note that ECN support is not a silver bullet to improving
performance. The use of ECN gives the chance to respond to
congestion before packets are dropped in the network, improving the
user experience by allowing the RTP application to control how the
quality is reduced. An application which ignores ECN Congestion
Experienced feedback is not immune to congestion: the network will
eventually begin to discard packets if traffic doesn't respond. It
is in the best interest of an application to respond to ECN
congestion feedback promptly, to avoid packet loss.
7.4. Detecting Failures
Senders and receivers can deliberately ignore ECN-CE and thus get a
benefit over behaving flows (cheating). The ECN Nonce [RFC3540] is
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an addition to TCP that attempts to solve this issue as long as the
sender acts on behalf of the network. The assumption that senders
act on behalf of the network may be false due to the nature of peer-
to-peer use of RTP. Still a significant portion of RTP senders are
infrastructure devices (for example, streaming media servers) that do
have an interest in protecting both service quality and the network.
Even though there may be cases where the nonce may be applicable for
RTP, it is not included in this specification. This is because a
receiver interested in cheating would simply claim to not support the
nonce, or even ECN itself. It is, however, worth mentioning that, as
real-time media is commonly sensitive to increased delay and packet
loss, it will be in both the media sender and receivers interest to
minimise the number and duration of any congestion events as they
will adversely affect media quality.
RTP sessions can also suffer from path changes resulting in a non-ECN
compliant node becoming part of the path. That node may perform
either of two actions that has an effect on the ECN and application
functionality. The gravest is if the node drops packets with the ECN
field set to ECT(0), ECT(1), or ECN-CE. This can be detected by the
receiver when it receives an RTCP SR packet indicating that a sender
has sent a number of packets that it has not received. The sender
may also detect such a middlebox based on the receiver's RTCP RR
packet, when the extended sequence number is not advanced due to the
failure to receive packets. If the packet loss is less than 100%,
then packet loss reporting in either the ECN feedback information or
RTCP RR will indicate the situation. The other action is to re-mark
a packet from ECT or ECN-CE to not-ECT. That has less dire results,
however it should be detected so that ECN usage can be suspended to
prevent misusing the network.
The RTCP XR ECN summary packet and the ECN feedback packet allow the
sender to compare the number of ECT marked packets of different types
received with the number it actually sent. The number of ECT packets
received, plus the number of ECN-CE marked and lost packets, should
correspond to the number of sent ECT marked packets plus the number
of received duplicates. If these numbers don't agree there are two
likely reasons, a translator changing the stream or not carrying the
ECN markings forward, or that some node re-marks the packets. In
both cases the usage of ECN is broken on the path. By tracking all
the different possible ECN field values a sender can quickly detect
if some non-compliant behavior is happening on the path.
Thus packet losses and non-matching ECN field value statistics are
possible indications of issues with using ECN over the path. The
next section defines both sender and receiver reactions to these
cases.
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7.4.1. Fallback mechanisms
Upon the detection of a potential failure, both the sender and the
receiver can react to mitigate the situation.
A receiver that detects a packet loss burst MAY schedule an early
feedback packet that includes at least the RTCP RR and the ECN
feedback message to report this to the sender. This will speed up
the detection of the loss at the sender, thus triggering sender side
mitigation.
A sender that detects high packet loss rates for ECT-marked packets
SHOULD immediately switch to sending packets as not-ECT to determine
if the losses are potentially due to the ECT markings. If the losses
disappear when the ECT-marking is discontinued, the RTP sender should
go back to initiation procedures to attempt to verify the apparent
loss of ECN capability of the used path. If a re-initiation fails
then two possible actions exist:
1. Periodically retry the ECN initiation to detect if a path change
occurs to a path that is ECN capable.
2. Renegotiate the session to disable ECN support. This is a choice
that is suitable if the impact of ECT probing on the media
quality is noticeable. If multiple initiations have been
successful, but the following full usage of ECN has resulted in
the fallback procedures, then disabling of the ECN support is
RECOMMENDED.
We foresee the possibility of flapping ECN capability due to several
reasons: video switching MCU or similar middleboxes that selects to
deliver media from the sender only intermittently; load balancing
devices may in worst case result in that some packets take a
different network path than the others; mobility solutions that
switch underlying network path in a transparent way for the sender or
receiver; and membership changes in a multicast group. It is however
appropriate to mention that there are also issues such as re-routing
of traffic due to a flappy route table or excessive reordering and
other issues that are not directly ECN related but nevertheless may
cause problems for ECN.
7.4.2. Interpretation of ECN Summary information
This section contains discussion on how the ECN summary report
information can be used to detect various types of ECN path issues.
We first review the information the RTCP reports provide on a per
source (SSRC) basis:
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ECN-CE Counter: The number of RTP packets received so far in the
session with an ECN field set to CE.
ECT (0/1) Counters: The number of RTP packets received so far in the
session with an ECN field set to ECT (0) and ECT (1) respectively.
not-ECT Counter: The number of RTP packets received so far in the
session with an ECN field set to not-ECT.
Lost Packets counter: The number of RTP packets that where expected
based on sequence numbers but never received.
Duplication Counter: The number of received RTP packets that are
duplicates of already received ones.
Extended Highest Sequence number: The highest sequence number seen
when sending this report, but with additional bits, to handle
disambiguation when wrapping the RTP sequence number field.
The counters will be initialised to zero to provide values for the
RTP stream sender from the first report. After the first report, the
changes between the last received report and the previous report are
determined by simply taking the values of the latest minus the
previous, taking wrapping into account. This definition is also
robust to packet losses, since if one report is missing, the
reporting interval becomes longer, but is otherwise equally valid.
In a perfect world, the number of not-ECT packets received should be
equal to the number sent minus the lost packets counter, and the sum
of the ECT(0), ECT(1), and ECN-CE counters should be equal to the
number of ECT marked packet sent. Two issues may cause a mismatch in
these statistics: severe network congestion or unresponsive
congestion control might cause some ECT-marked packets to be lost,
and packet duplication might result in some packets being received,
and counted in the statistics, multiple times (potentially with a
different ECN-mark on each copy of the duplicate).
The rate of packet duplication is tracked, allowing one to take the
duplication into account. The value of the ECN field for duplicates
will also be counted and when comparing the figures one needs to take
some fraction of packet duplicates that are non-ECT and some fraction
of packet duplicates being ECT into account into the calculation.
Thus when only sending non-ECT then the number of sent packets plus
reported duplicates equals the number of received non-ECT. When
sending only ECT then number of sent ECT packets plus duplicates will
equal ECT(0), ECT(1), ECN-CE and packet loss. When sending a mix of
non-ECT and ECT then there is an uncertainty if any duplicate or
packet loss was an non-ECT or ECT. If the packet duplication is
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completely independent of the usage of ECN, then the fraction of
packet duplicates should be in relation to the number of non-ECT vs
ECT packet sent during the period of comparison. This relation does
not hold for packet loss, where higher rates of packet loss for non-
ECT is expected than for ECT traffic.
Detecting clearing of ECN field: If the ratio between ECT and not-ECT
transmitted in the reports has become all not-ECT, or has
substantially changed towards not-ECT, then this is clearly an
indication that the path results in clearing of the ECT field.
Dropping of ECT packets: To determine if the packet drop ratio is
different between not-ECT and ECT marked transmission requires a mix
of transmitted traffic. The sender should compare if the delivery
percentage (delivered / transmitted) between ECT and not-ECT is
significantly different. Care must be taken if the number of packets
are low in either of the categories. One must also take into account
the level of CE marking. A CE marked packet would have been dropped
unless it was ECT marked. Thus, the packet loss level for not-ECT
should be approximately equal to the loss rate for ECT when counting
the CE marked packets as lost ones. A sender performing this
calculation needs to ensure that the difference is statistically
significant.
If erroneous behavior is detected, it should be logged to enable
follow up and statistics gathering.
8. Processing ECN in RTP Translators and Mixers
RTP translators and mixers that support ECN for RTP are required to
process, and potentially modify or generate ECN marking in RTP
packets. They also need to process, and potentially modify or
generate RTCP ECN feedback packets for the translated and/or mixed
streams. This includes both downstream RTCP reports generated by the
media sender, and also reports generated by the receivers, flowing
upstream back towards the sender.
8.1. Transport Translators
Some translators only perform transport level translations, like
copying packets from one address domain, like unicast to multicast.
It may also perform relaying like copying an incoming packet to a
number of unicast receivers. This section details the ECN related
actions for RTP and RTCP.
For the RTP data packets the translator, which does not modify the
media stream, SHOULD copy the ECN bits unchanged from the incoming to
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the outgoing datagrams, unless the translator itself is overloaded
and experiencing congestion, in which case it may mark the outgoing
datagrams with an ECN-CE mark.
A Transport translator does not modify RTCP packets. It however MUST
perform the corresponding transport translation of the RTCP packets
as it does with RTP packets being sent from the same source/
end-point.
8.2. Fragmentation and Reassembly in Translators
An RTP translator may fragment or reassemble RTP data packets without
changing the media encoding, and without reference to the congestion
state of the networks it bridges. An example of this might be to
combine packets of a voice-over-IP stream coded with one 20ms frame
per RTP packet into new RTP packets with two 20ms frames per packet,
thereby reducing the header overheads and so stream bandwidth, at the
expense of an increase in latency. If multiple data packets are re-
encoded into one, or vice versa, the RTP translator MUST assign new
sequence numbers to the outgoing packets. Losses in the incoming RTP
packet stream may also induce corresponding gaps in the outgoing RTP
sequence numbers. An RTP translator MUST rewrite RTCP packets to
make the corresponding changes to their sequence numbers, and to
reflect the impact of the fragmentation or reassembly. This section
describes how that rewriting is to be done for RTCP ECN feedback
packets. Section 7.2 of [RFC3550] describes general procedures for
other RTCP packet types.
The processing of arriving RTP packets for this case is as follows.
If an ECN marked packet is split into two, then both the outgoing
packets MUST be ECN marked identically to the original; if several
ECN marked packets are combined into one, the outgoing packet MUST be
either ECN-CE marked or dropped if any of the incoming packets are
ECN-CE marked. If the outgoing combined packet is not ECN-CE marked,
then it MUST be ECT marked if any of the incoming packets were ECT
marked.
RTCP ECN feedback packets (Section 5.1) contain seven fields that are
rewritten in an RTP translator that fragments or reassembles packets:
the extended highest sequence number, the duplication counter, the
lost packets counter, the ECN-CE counter, and not-ECT counter, the
ECT(0) counter, and the ECT(1) counter. The RTCP XR report block for
ECN summary information (Section 5.2) includes all of these fields
except the extended highest sequence number which is present in the
report block in an SR or RR packet. The procedures for rewriting
these fields are the same for both RTCP ECN feedback packet and the
RTCP XR ECN summary packet.
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When receiving an RTCP ECN feedback packet for the translated stream,
an RTP translator first determines the range of packets to which the
report corresponds. The extended highest sequence number in the RTCP
ECN feedback packet (or in the RTCP SR/RR packet contained within the
compound packet, in the case of RTCP XR ECN summary reports)
specifies the end sequence number of the range. For the first RTCP
ECN feedback packet received, the initial extended sequence number of
the range may be determined by subtracting the sum of the lost
packets counter, the ECN-CE counter, the not-ECT counter, the ECT(0)
counter and the ECT(1) counter minus the duplication counter, from
the extended highest sequence number. For subsequent RTCP ECN
feedback packets, the starting sequence number may be determined as
being one after the extended highest sequence number of the previous
RTCP ECN feedback packet received from the same SSRC. These values
are in the sequence number space of the translated packets.
Based on its knowledge of the translation process, the translator
determines the sequence number range for the corresponding original,
pre-translation, packets. The extended highest sequence number in
the RTCP ECN feedback packet is rewritten to match the final sequence
number in the pre-translation sequence number range.
The translator then determines the ratio, R, of the number of packets
in the translated sequence number space (numTrans) to the number of
packets in the pre-translation sequence number space (numOrig) such
that R = numTrans / numOrig. The counter values in the RTCP ECN
feedback report are then scaled by dividing each of them by R. For
example, if the translation process combines two RTP packets into
one, then numOrig will be twice numTrans, giving R=0.5, and the
counters in the translated RTCP ECN feedback packet will be twice
those in the original.
The ratio, R, may have a value that leads to non-integer multiples of
the counters when translating the RTCP packet. For example, a VoIP
translator that combines two adjacent RTP packets into one if they
contain active speech data, but passes comfort noise packets
unchanged, would have an R values of between 0.5 and 1.0 depending on
the amount of active speech. Since the counter values in the
translated RTCP report are integer values, rounding will be necessary
in this case.
When rounding counter values in the translated RTCP packet, the
translator should try to ensure that they sum to the number of RTP
packets in the pre-translation sequence number space (numOrig). The
translator should also try to ensure that no non-zero counter is
rounded to a zero value, unless the pre-translated values are zero,
since that will lose information that a particular type of event has
occurred. It is recognised that it may be impossible to satisfy both
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of these constraints; in such cases, it is better to ensure that no
non-zero counter is mapped to a zero value, since this preserves
congestion adaptation and helps the RTCP-based ECN initiation
process.
One should be aware of the impact this type of translators have on
the measurement of packet duplication. A translator performing
aggregation and most likely also an fragmenting translator will
suppress any duplication happening prior to itself. Thus the reports
and what is being scaled will only represent packet duplication
happening from the translator to the receiver reporting on the flow.
It should be noted that scaling the RTCP counter values in this way
is meaningful only on the assumption that the level of congestion in
the network is related to the number of packets being sent. This is
likely to be a reasonable assumption in the type of environment where
RTP translators that fragment or reassemble packets are deployed, as
their entire purpose is to change the number of packets being sent to
adapt to known limitations of the network, but is not necessarily
valid in general.
The rewritten RTCP ECN feedback report is sent from the other side of
the translator to that which it arrived (as part of a compound RTCP
packet containing other translated RTCP packets, where appropriate).
8.3. Generating RTCP ECN Feedback in Media Transcoders
An RTP translator that acts as a media transcoder cannot directly
forward RTCP packets corresponding to the transcoded stream, since
those packets will relate to the non-transcoded stream, and will not
be useful in relation to the transcoded RTP flow. Such a transcoder
will need to interpose itself into the RTCP flow, acting as a proxy
for the receiver to generate RTCP feedback in the direction of the
sender relating to the pre-transcoded stream, and acting in place of
the sender to generate RTCP relating to the transcoded stream, to be
sent towards the receiver. This section describes how this proxying
is to be done for RTCP ECN feedback packets. Section 7.2 of
[RFC3550] describes general procedures for other RTCP packet types.
An RTP translator acting as a media transcoder in this manner does
not have its own SSRC, and hence is not visible to other entities at
the RTP layer. RTCP ECN feedback packets and RTCP XR report blocks
for ECN summary information that are received from downstream relate
to the translated stream, and so must be processed by the translator
as if it were the original media source. These reports drive the
congestion control loop and media adaptation between the translator
and the downstream receiver. If there are multiple downstream
receivers, a logically separate transcoder instance must be used for
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each receiver, and must process RTCP ECN feedback and summary reports
independently to the other transcoder instances. An RTP translator
acting as a media transcoder in this manner MUST NOT forward RTCP ECN
feedback packets or RTCP XR ECN summary reports from downstream
receivers in the upstream direction.
An RTP translator acting as a media transcoder will generate RTCP
reports upstream towards the original media sender, based on the
reception quality of the original media stream at the translator.
The translator will run a separate congestion control loop and media
adaptation between itself and the media sender for each of its
downstream receivers, and must generate RTCP ECN feedback packets and
RTCP XR ECN summary reports for that congestion control loop using
the SSRC of that downstream receiver.
8.4. Generating RTCP ECN Feedback in Mixers
An RTP mixer terminates one-or-more RTP flows, combines them into a
single outgoing media stream, and transmits that new stream as a
separate RTP flow. A mixer has its own SSRC, and is visible to other
participants in the session at the RTP layer.
An ECN-aware RTP mixer must generate RTCP ECN feedback packets and
RTCP XR report blocks for ECN summary information relating to the RTP
flows it terminates, in exactly the same way it would if it were an
RTP receiver. These reports form part of the congestion control loop
between the mixer and the media senders generating the streams it is
mixing. A separate control loop runs between each sender and the
mixer.
An ECN-aware RTP mixer will negotiate and initiate the use of ECN on
the mixed RTP flows it generates, and will accept and process RTCP
ECN feedback reports and RTCP XR report blocks for ECN relating to
those mixed flows as if it were a standard media sender. A
congestion control loop runs between the mixer and its receivers,
driven in part by the ECN reports received.
An RTP mixer MUST NOT forward RTCP ECN feedback packets or RTCP XR
ECN summary reports from downstream receivers in the upstream
direction.
9. Implementation considerations
To allow the use of ECN with RTP over UDP, an RTP implementation
desiring to support receiving ECN controlled media streams must
support reading the value of the ECT bits on received UDP datagrams,
and an RTP implementation desiring to support sending ECN controlled
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media streams must support setting the ECT bits in outgoing UDP
datagrams. The standard Berkeley sockets API pre-dates the
specification of ECN, and does not provide the functionality which is
required for this mechanism to be used with UDP flows, making this
specification difficult to implement portably.
10. IANA Considerations
Note to RFC Editor: please replace "RFC XXXX" below with the RFC
number of this memo, and remove this note.
10.1. SDP Attribute Registration
Following the guidelines in [RFC4566], the IANA is requested to
register one new media-level SDP attribute:
o Contact name, email address and telephone number: Authors of
RFCXXXX
o Attribute-name: ecn-capable-rtp
o Type of attribute: media-level
o Subject to charset: no
This attribute defines the ability to negotiate the use of ECT (ECN
capable transport) for RTP flows running over UDP/IP. This attribute
is put in the SDP offer if the offering party wishes to receive an
ECT flow. The answering party then include the attribute in the
answer if it wishes to receive an ECT flow. If the answerer does not
include the attribute then ECT MUST be disabled in both directions.
10.2. RTP/AVPF Transport Layer Feedback Message
The IANA is requested to register one new RTP/AVPF Transport Layer
Feedback Message in the table of FMT values for RTPFB Payload Types
[RFC4585] as defined in Section 5.1:
Name: RTCP-ECN-FB
Long name: RTCP ECN Feedback
Value: TBA1
Reference: RFC XXXX
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10.3. RTCP Feedback SDP Parameter
The IANA is requested to register one new SDP "rtcp-fb" attribute
"nack" parameter "ecn" in the SDP ("ack" and "nack" Attribute Values)
registry.
Value name: ecn
Long name: Explicit Congestion Notification
Usable with: nack
Reference: RFC XXXX
10.4. RTCP XR Report blocks
The IANA is requested to register one new RTCP XR Block Type as
defined in Section 5.2:
Block Type: TBA2
Name: ECN Summary Report
Reference: RFC XXXX
10.5. RTCP XR SDP Parameter
The IANA is requested to register one new RTCP XR SDP Parameter "ecn-
sum" in the "RTCP XR SDP Parameters" registry.
Parameter name XR block (block type and name)
-------------- ------------------------------------
ecn-sum TBA2 ECN Summary Report Block
10.6. STUN attribute
A new STUN [RFC5389] attribute in the Comprehension-optional range
under IETF Review (0x8000-0xFFFF) is request to be assigned to the
ECN-CHECK STUN attribute defined in Section 7.2.2. The STUN
attribute registry can currently be found at: http://www.iana.org/
assignments/stun-parameters/stun-parameters.xhtml.
10.7. ICE Option
A new ICE option "rtp+ecn" is registered in the registry that "IANA
Registry for Interactive Connectivity Establishment (ICE) Options"
[RFC6336] creates.
11. Security Considerations
The use of ECN with RTP over UDP as specified in this document has
the following known security issues that need to be considered.
External threats to the RTP and RTCP traffic:
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Denial of Service affecting RTCP: An attacker that can modify the
traffic between the media sender and a receiver can achieve either
of two things: 1) Report a lot of packets as being Congestion
Experience marked, thus forcing the sender into a congestion
response; or 2) Ensure that the sender disables the usage of ECN
by reporting failures to receive ECN by changing the counter
fields. This can also be accomplished by injecting false RTCP
packets to the media sender. Reporting a lot of ECN-CE marked
traffic is likely the more efficient denial of service tool as
that may likely force the application to use lowest possible bit-
rates. The prevention against an external threat is to integrity
protect the RTCP feedback information and authenticate the sender.
Information leakage: The ECN feedback mechanism exposes the
receivers perceived packet loss, what packets it considers to be
ECN-CE marked and its calculation of the ECN-none. This is mostly
not considered as sensitive information. If it is considered
sensitive the RTCP feedback should be encrypted.
Changing the ECN bits: An on-path attacker that sees the RTP packet
flow from sender to receiver and who has the capability to change
the packets can rewrite ECT into ECN-CE thus forcing the sender or
receiver to take congestion control response. This denial of
service against the media quality in the RTP session is impossible
for an end-point to protect itself against. Only network
infrastructure nodes can detect this illicit re-marking. It will
be mitigated by turning off ECN, however, if the attacker can
modify its response to drop packets the same vulnerability exist.
Denial of Service affecting the session set-up signalling: If an
attacker can modify the session signalling it can prevent the
usage of ECN by removing the signalling attributes used to
indicate that the initiator is capable and willing to use ECN with
RTP/UDP. This attack can be prevented by authentication and
integrity protection of the signalling. We do note that any
attacker that can modify the signalling has more interesting
attacks they can perform than prevent the usage of ECN, like
inserting itself as a middleman in the media flows enabling wire-
tapping also for an off-path attacker.
The following are threats that exist from misbehaving senders or
receivers:
Receivers cheating: A receiver may attempt to cheat and fail to
report reception of ECN-CE marked packets. The benefit for a
receiver cheating in its reporting would be to get an unfair bit-
rate share across the resource bottleneck. It is far from certain
that a receiver would be able to get a significant larger share of
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the resources. That assumes a high enough level of aggregation
that there are flows to acquire shares from. The risk of cheating
is that failure to react to congestion results in packet loss and
increased path delay.
Receivers misbehaving: A receiver may prevent the usage of ECN in an
RTP session by reporting itself as non ECN capable, forcing the
sender to turn off usage of ECN. In a point-to-point scenario
there is little incentive to do this as it will only affect the
receiver. Thus failing to utilise an optimisation. For multi-
party session there exist some motivation why a receiver would
misbehave as it can prevent also the other receivers from using
ECN. As an insider into the session it is difficult to determine
if a receiver is misbehaving or simply incapable, making it
basically impossible in the incremental deployment phase of ECN
for RTP usage to determine this. If additional information about
the receivers and the network is known it might be possible to
deduce that a receiver is misbehaving. If it can be determined
that a receiver is misbehaving, the only response is to exclude it
from the RTP session and ensure that is does not any longer have
any valid security context to affect the session.
Misbehaving Senders: The enabling of ECN gives the media packets a
higher degree of probability to reach the receiver compared to
not-ECT marked ones on a ECN capable path. However, this is no
magic bullet and failure to react to congestion will most likely
only slightly delay a network buffer over-run, in which its
session also will experience packet loss and increased delay.
There is some possibility that the media senders traffic will push
other traffic out of the way without being affected too
negatively. However, we do note that a media sender still needs
to implement congestion control functions to prevent the media
from being badly affected by congestion events. Thus the
misbehaving sender is getting a unfair share. This can only be
detected and potentially prevented by network monitoring and
administrative entities. See Section 7 of [RFC3168] for more
discussion of this issue.
We note that the end-point security functions needed to prevent an
external attacker from interfering with the signalling are source
authentication and integrity protection. To prevent information
leakage from the feedback packets encryption of the RTCP is also
needed. For RTP there exist multiple solutions possible depending on
the application context. Secure RTP (SRTP) [RFC3711] does satisfy
the requirement to protect this mechanism. Note, however, that when
using SRTP in group communication scenarios, different parties might
share the same security context; in this case, the authentication
mechanism only shows that one of those parties is involved, not
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necessarily which one. IPsec [RFC4301] and DTLS [RFC6347] can also
provide the necessary security functions.
The signalling protocols used to initiate an RTP session also need to
be source authenticated and integrity protected to prevent an
external attacker from modifying any signalling. Here an appropriate
mechanism to protect the used signalling needs to be used. For SIP/
SDP ideally S/MIME [RFC5751] would be used. However, with the
limited deployment a minimal mitigation strategy is to require use of
SIPS (SIP over TLS) [RFC3261] [RFC5630] to at least accomplish hop-
by-hop protection.
We do note that certain mitigation methods will require network
functions.
12. Examples of SDP Signalling
This section contain a few different examples of the signalling
mechanism defined in this specification in an SDP context. If there
are discrepancies between these examples and the specification text,
the specification text is definitive.
12.1. Basic SDP Offer/Answer
This example is a basic offer/answer SDP exchange, assumed done by
SIP (not shown). The intention is to establish a basic audio session
point to point between two users.
The Offer:
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v=0
o=jdoe 3502844782 3502844782 IN IP4 10.0.1.4
s=VoIP call
i=SDP offer for VoIP call with ICE and ECN for RTP
b=AS:128
b=RR:2000
b=RS:2500
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6
a=ice-options:rtp+ecn
t=0 0
m=audio 45664 RTP/AVPF 97 98 99
c=IN IP4 192.0.2.3
a=rtpmap:97 G719/48000/1
a=fmtp:97 maxred=160
a=rtpmap:98 AMR-WB/16000/1
a=fmtp:98 octet-align=1; mode-change-capability=2
a=rtpmap:99 PCMA/8000/1
a=maxptime:160
a=ptime:20
a=ecn-capable-rtp: ice rtp ect=0 mode=setread
a=rtcp-fb:* nack ecn
a=rtcp-fb:* trr-int 1000
a=rtcp-xr:ecn-sum
a=rtcp-rsize
a=candidate:1 1 UDP 2130706431 10.0.1.4 8998 typ host
a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr
10.0.1.4 rport 8998
This SDP offer offers a single media stream with 3 media payload
types. It proposes to use ECN with RTP, with the ICE based
initialization as being preferred over the RTP/RTCP one. Leap of
faith is not suggested to be used. The offerer is capable of both
setting and reading the ECN bits. In addition the use of both the
RTCP ECN feedback packet and the RTCP XR ECN summary report are
supported. ICE is also proposed with two candidates. It also
supports reduced size RTCP and can to use it.
The Answer:
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v=0
o=jdoe 3502844783 3502844783 IN IP4 198.51.100.235
s=VoIP call
i=SDP offer for VoIP call with ICE and ECN for RTP
b=AS:128
b=RR:2000
b=RS:2500
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
a=ice-options:rtp+ecn
t=0 0
m=audio 53879 RTP/AVPF 97 99
c=IN IP4 198.51.100.235
a=rtpmap:97 G719/48000/1
a=fmtp:97 maxred=160
a=rtpmap:99 PCMA/8000/1
a=maxptime:160
a=ptime:20
a=ecn-capable-rtp: ice ect=0 mode=readonly
a=rtcp-fb:* nack ecn
a=rtcp-fb:* trr-int 1000
a=rtcp-xr:ecn-sum
a=candidate:1 1 UDP 2130706431 198.51.100.235 53879 typ host
The answer confirms that only one media stream will be used. One RTP
Payload type was removed. ECN capability was confirmed, and the
initialization method will be ICE. However, the answerer is only
capable of reading the ECN bits, which means that ECN can only be
used for RTP flowing from the offerer to the answerer. ECT always
set to 0 will be used in both directions. Both the RTCP ECN feedback
packet and the RTCP XR ECN summary report will be used. Reduced size
RTCP will not be used as the answerer has not indicated support for
it in the answer.
12.2. Declarative Multicast SDP
The below session describes an any source multicast using session
with a single media stream.
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v=0
o=jdoe 3502844782 3502844782 IN IP4 198.51.100.235
s=Multicast SDP session using ECN for RTP
i=Multicasted audio chat using ECN for RTP
b=AS:128
t=3502892703 3502910700
m=audio 56144 RTP/AVPF 97
c=IN IP4 233.252.0.212/127
a=rtpmap:97 g719/48000/1
a=fmtp:97 maxred=160
a=maxptime:160
a=ptime:20
a=ecn-capable-rtp: rtp mode=readonly; ect=0
a=rtcp-fb:* nack ecn
a=rtcp-fb:* trr-int 1500
a=rtcp-xr:ecn-sum
In the above example, as this is declarative we need to require
certain functionality. As it is ASM the initialization method that
can work here is the RTP/RTCP based one. So that is indicated. The
ECN setting and reading capability to take part of this session is at
least read. If one is capable of setting that is good, but not
required as one can skip using ECN for anything one sends oneself.
The ECT value is recommended to be set to 0 always. The ECN usage in
this session requires both ECN feedback and the XR ECN summary
report, so their use is also indicated.
13. Acknowledgments
The authors wish to thank the following persons for their reviews and
comments: Thomas Belling, Bob Briscoe, Roni Even, Kevin P. Flemming,
Tomas Frankkila, Christian Groves, Christer Holmgren, Cullen Jennings
Tom Van Caenegem, Simo Veikkolainen, Bill Ver Steeg, Dan Wing, Qin
Wu, and Lei Zhu.
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
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[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611,
November 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 5348, September 2008.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC6336] Westerlund, M. and C. Perkins, "IANA Registry for
Interactive Connectivity Establishment (ICE) Options",
RFC 6336, July 2011.
14.2. Informative References
[RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, August 1989.
[RFC2762] Rosenberg, J. and H. Schulzrinne, "Sampling of the Group
Membership in RTP", RFC 2762, February 2000.
[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
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June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
RFC 3540, June 2003.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
[RFC3569] Bhattacharyya, S., "An Overview of Source-Specific
Multicast (SSM)", RFC 3569, July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, August 2006.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, February 2008.
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
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and Consequences", RFC 5506, April 2009.
[RFC5630] Audet, F., "The Use of the SIPS URI Scheme in the Session
Initiation Protocol (SIP)", RFC 5630, October 2009.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.
[RFC5760] Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
Protocol (RTCP) Extensions for Single-Source Multicast
Sessions with Unicast Feedback", RFC 5760, February 2010.
[RFC6189] Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media
Path Key Agreement for Unicast Secure RTP", RFC 6189,
April 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
Authors' Addresses
Magnus Westerlund
Ericsson
Farogatan 6
SE-164 80 Kista
Sweden
Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com
Ingemar Johansson
Ericsson
Laboratoriegrand 11
SE-971 28 Lulea
SWEDEN
Phone: +46 73 0783289
Email: ingemar.s.johansson@ericsson.com
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Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
United Kingdom
Email: csp@csperkins.org
Piers O'Hanlon
University of Oxford
Oxford Internet Institute
1 St Giles
Oxford OX1 3JS
United Kingdom
Email: piers.ohanlon@oii.ox.ac.uk
Ken Carlberg
G11
1600 Clarendon Blvd
Arlington VA
USA
Email: carlberg@g11.org.uk
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