Network Working Group M. Westerlund
Internet-Draft I. Johansson
Intended status: Standards Track Ericsson
Expires: January 7, 2010 C. Perkins
University of Glasgow
July 6, 2009
Explicit Congestion Notification (ECN) for RTP over UDP
draft-westerlund-avt-ecn-for-rtp-00
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Abstract
This document specifies how explicit congestion notification (ECN)
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can be used with RTP/UDP flows that use RTCP as feedback mechanism.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions, Definitions and Acronyms . . . . . . . . . . . . 3
3. Discussion, Requirements, and Design Rationale . . . . . . . . 4
3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 6
4. Use of ECN with RTP/UDP/IP . . . . . . . . . . . . . . . . . . 9
4.1. Negotiation of ECN Capability . . . . . . . . . . . . . . 11
4.1.1. Signalling ECN Capability using SDP . . . . . . . . . 11
4.1.2. ICE Parameter to Signal ECN Capability . . . . . . . . 12
4.2. Initiation of ECN Use in an RTP Session . . . . . . . . . 12
4.2.1. Detection of ECT using RTP and RTCP . . . . . . . . . 13
4.2.2. Detection of ECT using STUN with ICE . . . . . . . . . 15
4.3. Ongoing Use of ECN Within an RTP Session . . . . . . . . . 17
4.3.1. Transmission of ECT-marked RTP Packets . . . . . . . . 17
4.3.2. Reporting ECN Feedback via RTCP . . . . . . . . . . . 17
4.3.3. Response to Congestion Notifications . . . . . . . . . 18
4.4. Detecting Failures and Receiver Misbehaviour . . . . . . . 20
4.4.1. Fallback mechanisms . . . . . . . . . . . . . . . . . 21
5. RTCP Extension for ECN feedback . . . . . . . . . . . . . . . 22
6. Processing RTCP ECN Feedback in RTP Translators and Mixers . . 24
6.1. Fragmentation and Reassembly in Translators . . . . . . . 25
6.2. Generating RTCP ECN Feedback in Translators . . . . . . . 25
6.3. Generating RTCP ECN Feedback in Mixers . . . . . . . . . . 25
7. Implementation considerations . . . . . . . . . . . . . . . . 26
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
8.1. SDP Attribute Registration . . . . . . . . . . . . . . . . 26
8.2. AVPF Transport Feedback Message . . . . . . . . . . . . . 26
8.3. STUN attribute . . . . . . . . . . . . . . . . . . . . . . 26
8.4. ICE Option . . . . . . . . . . . . . . . . . . . . . . . . 27
9. Security Considerations . . . . . . . . . . . . . . . . . . . 27
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
10.1. Normative References . . . . . . . . . . . . . . . . . . . 29
10.2. Informative References . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32
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1. Introduction
This document outlines how Explicit Congestion Notification (ECN)
[RFC3168] can be used for RTP [RFC3550] flows running over UDP/IP
which use RTCP as feedback mechanism. The solution consists of
feedback of ECN congestion experienced markings to sender using RTCP,
verification of ECN functionality end-to-end, and how to initiate ECN
usage. The initiation process will have some dependencies on the
signalling mechanism used to establish the RTP session, a
specification for mechanisms using SDP is included.
ECN is getting attention as a method to minimise the impact of
congestion on real-time multimedia traffic. This as packet loss can
be avoided if transmission rate adjustments are quick enough.
Including congestion in wireless access networks when radio resources
and coverage is insufficient to maintain the current media rates.
One key benefit with ECN is it is a lightweight mechanism to allow
for each node along the transmission path to set a congestion
notification in the IP header, thereby letting the endpoints know of
the congested situation.
The introduction of ECN into the Internet requires changes to both
the network and transport layers. At the network layer, IP 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 that sender that ECN marked packets are
being received, so it can respond to the congestion. TCP [RFC3168],
SCTP [RFC4960] and DCCP [RFC4340] have been updated to support ECN,
but to date there is no specification how UDP-based transports, such
as RTP [RFC3550], can be used with ECN.
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. The means by which ECN is used with RTP over UDP is
defined in Section 4, along with RTCP extensions for ECN feedback in
Section 5. In Section 6 we discuss how RTCP ECN feedback is handled
in RTP translators. Section 7 discusses some implementation
considerations, Section 8 lists IANA considerations, and Section 9
discusses the security considerations.
2. Conventions, Definitions and Acronyms
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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Abbreviations
ECN: Explicit Congestion Notification
ECT: ECN Capable Transport
ECN-CE: ECN Congestion Experienced
not-ECT: Not ECN Capable Transport
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 Congestion
Experienced (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 some cases).
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.
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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 ASM,
although a recent extension supports SSM with unicast feedback).
These differences will 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. Indeed, in many ways,
ECN support is more important for RTP sessions, since the impact of
packet loss in real-time audio-visual media flows is highly visible
to users. 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 a voice telephony.
3.1. Requirements
Considering ECN and these protocols one can create a set of
requirements that must be satisfied to at least some degree if ECN is
used by an other protocol (such as RTP over UDP)
o REQ 1: A mechanism to negotiate and initiate the usage of ECN for
RTP/UDP/IP sessions is required
o REQ 2: A mechanism to feedback the reception of any packets that
are ECN-CE marked to the packet sender is required
o REQ 3: Provide mechanism to minimise the possibility for cheating
is preferable
o REQ 4: Some detection and fallback mechanism is needed in case an
intermediate node clears ECT or drops packets with ECT set to
avoid loss of communication due to the attempted usage of ECN.
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).
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o REQ 6: Negotiation of ECN should not cause clipping at the start
of a session.
The following sections describes how these requirements can be meet
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 of the network path(s) traversed. RTP is
used in a heterogeneous range of network environments and topologies,
with various different signalling protocols, all of which need to be
verified to support ECN before it can be used.
The usage 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 at the sender by changing the media encoding, at
the receiver by changing the subscription to a layered encoding, or
in 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 is a standard unicast flow. 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
with potentially several active senders and multicast RTCP
feedback, or a source specific multicast (SSM) group with a single
sender 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
[I-D.ietf-avt-rtcpssm]), and can be used by a sender to determine
if all its receivers, and the network paths to those receivers,
support ECN (see Section 4.2). 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. Note that group membership may change during the lifetime of
a multicast RTP session, potentially introducing new receivers
that are not ECN capable. Senders MUST use the mechanisms
described in Section 4.4 to monitor that all receivers continue to
support ECN, and MUST fallback to non-ECN use if they do not.
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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 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 zero out the ECN bits 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 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.
* 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]: 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, and should have a random ECT mark otherwise. When RTCP
ECN feedback packets (Section 5) are received, they must be
rewritten to match the modifications made to the media stream
(see Section 6.1).
* If the translator is a media transcoder, 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
connection into two parts with their own negotiation. Once
negotiation has been completed, the translator must generate
synthetic 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 6.2).
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It is recognised that ECN and RTCP processing in an RTP translator
that modifies the media stream is non-trivial.
Topo-Mixer: This 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.
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
connections 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.
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 depends 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 cannot 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."
These topologies may be combined within a single RTP session.
This ECN mechanism 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 RTP session can be adapted
in a number of ways, such as media sender using TFRC [RFC5348] or
other algorithms, or for multicast sessions either a sender based
scheme with lowest common rate, or receiver driven mechanism based on
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layers to support more heterogeneous paths.
4. Use of ECN with RTP/UDP/IP
The solution for using ECN with RTP consists of a few different
pieces that together makes the solution work:
1. Negotiation of the capability to do ECN with RTP/UDP
2. Initiation and initial verification of ECN capable transport
3. Ongoing use of ECN within an RTP session
4. Failure detection, verification and fallback
Before an RTP session can be created, a signalling protocol is used
to discover the other participants and negotiate session parameters
(see Section 4.1. One of the parameters that can be negotiated is
the capability of a participant to support ECN functionality, or
otherwise. 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 support ECN (for example, a firewall that
blocks traffic with the ECN bits set).
When a sender joins a session for which all participants claim ECN
capability, it must verify if that capability is usable. There are
two ways in which this verification may be done (Section 4.2):
o The sender may generate a subset of its RTP data packets with the
ECN field set to ECT(0) or ECT(1). Each 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 of each
other. If a new receiver join an existing session it also needs
to verify ECN support. If verification fails the sender needs to
stop using ECN. As the sender will not know of the receiver prior
to it sending RTP or RTCP packets, the sender will wait for the
first RTCP packet from the new receiver to determine if that
contains ECN feedback or not.
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 now with the ECN field set to
ECT is performed. If successful the paths capability is verified.
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Through the use of an extra STUN attribute also support for this
solution can be verified through that mechanism.
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.
Once ECN support has been verified to work for all receivers, a
sender marks all its RTP packets as ECT packets, while receivers
feedback any CE marks to the sender using RTCP in RTP/AVPF immediate
or early feedback mode (see Section 4.3). An RTCP feedback report is
sent as soon as possible by the transmission rules for feedback that
are in place. This feedback report contains all the CE marks that
has been received since the last regular report until the sending of
this packet. This is the mechanism to provide the fastest possible
feedback to senders about CE marks. On receipt of an RTCP report
indicating that CE marked packets were received, the sender must
reduce its sending rate as-if packet loss were reported.
RTCP traffic is never ECT marked for the following reason. ECT
marked traffic may be dropped if the path is not ECN compliant. As
RTCP is used to provide feedback about what has been transmitted and
what ECN markings that are received it is important that these are
received in cases when ECT marked traffic is not getting through.
The above feedback is not optimised for reliability, therefore an
additional procedure is used to ensure more reliable but less timely
reporting of the ECN information. The ECN feedback report is also
sent in the regular RTCP receiver reports. In this case they include
the ECN information covering the last three reporting intervals.
That way a loss of ECN-CE report will with high reliability be
eventual reported.
There a 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 are ECN non-compliant, thus
remarking or dropping packets with ECT set. To prevent this from
impacting the application for any longer duration the function of ECN
is constantly monitored using the ECN feedback information. By using
an ECN nonce over all the received packet that where not ECN-CE
marked and reported explicitly the sender can detect if any remarking
happens. If ECT marked packets are being dropped that will evident
from the RTCP receiver report where the "extended highest sequence
number received" field will stop advancing or if the loss is not 100%
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the high reported packet loss rates. A sender detecting a possible
ECN non-compliance issue can then stop sending ECT marked packets to
determine if that allows the packet to be correctly delivered. If
the issues can be connected to ECN, then ECN usage is suspended and
possibly also re-negotiated.
In the below detailed specification of the behaviour for the
different functions the general case will first be discussed. In
cases special considerations are needed for middleboxes, multicast
usage etc, those will be specially discussed in related subsections.
4.1. Negotiation of ECN Capability
The first stage of ECN negotiation for RTP-over-UDP is to signal
support for ECN capability. There are two signalling schemes that
may be used, depending on how ECN usage is to be initiated: an SDP
extension to indicate that ECN support should be negotiated using RTP
and RTCP, and an ICE parameter to indicate that ECN support should be
negotiated using STUN as part of an ICE exchange.
An RTP system that supports ECN MUST implement the SDP extension to
signal ECN capability as described in Section 4.1.1. It MAY also
implement other ECN capability negotiation schemes, such as the ICE
extension described in Section 4.1.2.
4.1.1. Signalling ECN Capability using SDP
One new SDP attribute, "a=ecn-capable-rtp", is defined. This is a
media level attribute, which 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. If all parties have the capability to use ECN then
some on-path mechanism must be used to negotiate its use, and to
check that all middleboxes on the path support ECN (Section 4.2.1
describes such a mechanism).
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 flow for which it
wishes to use ECN. The answering party includes this same attribute
in the media sections of the answer if it has the capability, and
wishes to, use ECN, or removes it for those flows for which it does
not want to use ECN. If the attribute is removed then ECT MUST NOT
be used in any direction for that media flow.
When SDP is used in a declarative manner, for example a multicast
session using SAP, negotiation of session description parameters is
not possible. The "a=ecn-capable-rtp" attribute MAY be added to the
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session description to indicate that the sender will use ECN in the
RTP session. Receivers MUST NOT join such a session unless they have
the capability to understand ECN-marked UDP packets, and can generate
RTCP ECN feedback (note that having the capability to use ECN doesn't
necessarily imply that the underlying network path between sender and
receiver supports ECN).
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 4.3.3, most RTP sessions using ECN require
rapid RTCP ECN feedback, in order that the sender can react to ECN-CE
marked packets. If such rapid feedback is required, the use of the
Extended RTP Profile for RTCP-Based Feedback (RTP/AVPF) [RFC4585]
MUST be signalled.
4.1.2. ICE Parameter to Signal ECN Capability
One new ICE [I-D.ietf-mmusic-ice] 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 has the capability, and
wishes to, use ECN, and removes the attribute if it does not wish to
use ECN, or doesn't have the capability to use ECN.
If both sides in the ICE exchange have the capability to use ECN,
then they will try to initiate ECN usage using the mechanisms we
describe in Section 4.2.2 for any nominated candidate that uses UDP
as transport protocol for an RTP session and which also include the
"a=ecn-capable-rtp" attribute associated with that media line. They
MUST NOT try to initiate ECN usage for RTP sessions using TCP, SCTP,
or DCCP transport, or for non-RTP sessions.
As described in Section 4.3.3, most RTP sessions using ECN require
rapid RTCP ECN feedback, in order that the sender can react to ECN-CE
marked packets. If such rapid feedback is required, the use of the
Extended RTP Profile for RTCP-Based Feedback (RTP/AVPF) [RFC4585]
MUST be signalled, even when ECN capability negotiation is done
through ICE.
4.2. Initiation of ECN Use in an RTP Session
At the start of the RTP session when the first packets with ECT is
sent it is important to verify that IP packets with ECN field values
of ECT or ECN-CE will reach its destination(s). There is some risk
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that the usage 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 receiver exhibits either of these behaviours one
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
during both the initiation and full usage of ECN with RTP. This is
to ensure that packet loss due to ECN marking will not effect the
RTCP traffic and the necessary feedback information.
An RTP system that supports ECN MUST implement the initiation of ECN
using RTP and RTCP described in Section 4.2.1. It MAY also implement
other mechanisms to initiate ECN support, for example the STUN-based
mechanism described in Section 4.2.2. If support for both mechanisms
is signalled, the sender should try ECN negotiation using STUN with
ICE first, and if it fails, fallback to negotiation using RTP and
RTCP ECN feedback.
No matter how ECN usage is initiated, the sender MUST continually
monitor the ability of the network, and all receivers, to support
ECN, following the mechanisms described in Section 4.4. This is
necessary because path changes or changes in the receiver population
may invalidate the ability of the network to support ECN.
4.2.1. 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 fraction of its traffic with ECT marks to act a probe
for ECN support. Then, on receipt of these ECT-marked packets, the
receivers send RTCP ECN feedback packets 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 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. An RTP sender is RECOMMENDED to
send a minimum of two packets with ECT markings per RTCP reporting
interval, one with ECT(0) and one with ECT(1), and will continue
to send some ECT marked traffic as long as the ECN initiation
phase continues. The sender MUST 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 clearly very media dependent, but the usage of RTP NO-OP
payloads [I-D.ietf-avt-rtp-no-op], if supported, would be an
appropriate choice. 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 duplicated in transmission,
since their 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 participants in the session MUST listen for
ECT or ECN-CE marked RTP packets, and generate RTCP ECN feedback
packets (Section 5) to mark their receipt. If the use of the
Extended RTP Profile for RTCP-Based Feedback (RTP/AVPF) has been
negotiated, then an immediate or early (depending on the RTP/AVPF
mode) 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. If RTP/AVPF has not been negotiated, then
the RTCP ECN feedback should be sent in a compound RTCP packet
along with the regular RTCP reports. The RTP/AVPF profile SHOULD
be negotiated where possible, since it greatly speeds up the ECN
initiation phase by ensuring that RTP senders get the earliest
possible indication that ECN works.
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 4.3).
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 becomes stable, provided all known
participants have sent one or more RTCP ECN feedback packets
indicating correct receipt of the ECT-marked RTP packets generated
by the sender.
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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 a single RTCP ECN feedback
report indicating successful receipt of the ECT-marked packets,
with no negative indications, from a single RTP receiver. After
declaring provisional success, the sender MAY generate ECT-marked
packets as described in Section 4.3, 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 is more than one other participant in the session. If other
participants are detected, 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.
Note: One use case that requires further consideration is a
unicast connection with several SSRCs multiplexed onto the same
flow (e.g. SVC video using SSRC multiplexing for the layers).
It is desirable to be able to rapidly negotiate ECN support for
such a session, but the optimisation above fails since the
multiple SSRCs make it appear that this is a group
communication scenario. It's not sufficient to check that all
SSRCs map to a common RTCP CNAME to check if they're actually
located on the same device, because there are implementations
that use the same CNAME for different parts of a distributed
implementation.
ECN initiation is considered to have failed at the instant when
any RTP session participant sends an RTCP packet that doesn't
contain an RTCP ECN feedback report, but 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.
The reception of RTCP ECN feedback packets that indicate greatly
increased packet loss rates for ECT marked packets, compared to
non-ECT marked packets, is a strong indication of problems with
ECN support on the network path. Senders MAY consider such
reports as indications that they should not use ECN on the path,
even though some ECT-marked packets to reach all receivers.
4.2.2. Detection of ECT using STUN with ICE
This section describes an OPTIONAL method that can be used to avoid
media impact and also ensure ECN capable path prior to media
transmission. This method is considered in the context where the
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session participants are using ICE [I-D.ietf-mmusic-ice] 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.
To minimise the impact of set-up delay, and to prioritise the fact
that one has a 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, and provided the chosen
candidate is not a relayed address, one performs an additional
connectivity check including the here defined STUN attribute "ECT
Check" and in an IP/UDP packet that are ECT marked. The STUN server
will upon reception of the packet note the received ECN field value
and in its response send an IP/UDP/STUN Packet with ECN field set to
not-ECT and also include the ECN check STUN attribute.
The STUN ECN check STUN attribute contains one field and a flag. The
flag indicate if 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 STUN attribute is an
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 1: 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 filed 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 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 STUN server receiving a request with the ECN Check attribute
which understand it SHALL read the ECN field value of the IP/UDP
packet the request was received in. Upon forming the response the
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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.
4.3. Ongoing Use of ECN Within an RTP Session
Once ECN usage has been successfully initiated for an RTP sender,
that sender begins actively sending ECT-marked RTP data packets, and
its receivers begin sending ECN feedback via RTCP packets. This
section describes procedures for sending ECT-marked data, providing
ECN feedback via RTCP, responding to ECN feedback, and detecting
failures and misbehaving receivers.
4.3.1. Transmission of ECT-marked RTP Packets
After a sender has successfully initiated ECN usage, it SHOULD mark
all the RTP data packets it sends as ECT. The choice between ECT(0)
and ECT(1) MUST be made randomly for each packet, and the sender MUST
calculate and record the ECN-nonce sum for outgoing packets [RFC3540]
to allow the use of the ECN-nonce to detect receiver misbehaviour
(see Section 4.4). Guidelines on the random choice of ECT values are
provided in Section 8 of [RFC3540].
The sender SHALL NOT include ECT marks on outgoing RTCP packets, and
SHOULD NOT include ECT marks on any outgoing control messages (e.g.
STUN [RFC5389] packets, DTLS [RFC4347] handshake packets, or ZRTP
[I-D.zimmermann-avt-zrtp] control packets, that are multiplexed on
the same UDP port).
4.3.2. 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 to report this back to the
sender. The feedback RTCP packet sent SHALL consist at least one ECN
feedback packet (Section 5) reporting on the packets received since
the last regular RTCP report, and SHOULD contain an RTCP SR or RR
packet. 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, the
RTP receiver MUST include an ECN feedback packet as part of the
compound packet. The ECN feedback packet must report on packets
received during the last three reporting intervals unless that would
cause the compound RTCP packet to exceed the network MTU, in which
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case it MAY be reduced to cover only the last or two last reporting
intervals. It is important to configure the RTCP bandwidth (e.g.
using an SDP "b=" line) such that the bit-rate is sufficient for a
usage that includes ECN-CE events. Each RTCP feedback packet will
report on the ECN-CE marks received since the last report and the
current ECN nonce value.
The multicast feedback implosion problem, that occurs when many
receivers simultaneously send feedback to a single sender, must also
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 reasonably timely, allowing the sender to
react on a single or a few reports.
An open issue is whether we should employ some form of feedback
suppression on ECN-CE feedback for groups? If one can make an
assumption that a sender will react on a few ECN-CE marks then
suppression could be employed successfully and reduce the RTCP
bandwidth usage.
In case a receiver driven congestion control algorithm is to be used
and has through signalling been agreed upon, the algorithm MAY
specify that the immediate scheduling (and later transmission) of
ECN-CE feedback of any received ECN-CE mark is not required and shall
not be done. In that case ECN feedback is only sent using regular
RTCP reports for verification purpose and in response to the
initiation process of any new media senders as specified in
Section 4.2.1.
4.3.3. Response to Congestion Notifications
When RTP packets are received with ECN-CE marks, the sender and/or
receivers MUST react with congestion control as-if those packets had
been lost. 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 may be 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 on it as if it was
packet losses that was reported. The congestion control algorithm
to be used is not specified here, although TFRC [RFC5348] is one
example that might be used.
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Receiver-Driven Congestion Control: If receiver driven congestion
control mechanism is used, the receiver can react to the ECN-CE
marks without contacting the sender. This may allow faster
response than sender-driven congestion control in some
circumstances. Receiver-driven congestion control is usually
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 packet to check if it experiences congestion at the
current number of layers. If congestion is experienced, the
receiver drops one layer, so reducing the 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 ECN feedback 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
feedback is sufficient (without using RTP/AVPF immediate or early
feedback).
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 in 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 having to
settle for the lowest common rate.
Note: There are many additional references that may be cited here.
If this document is accepted as an AVT work item, some discussion
of the appropriate amount of detail to include here would be
worthwhile.
We note that ECN support is not a silver bullet to improving
performance. The use of ECN gives the change 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
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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.
4.4. Detecting Failures and Receiver Misbehaviour
ECN-nonce is defined in RFC3540 as a means to ensure that a TCP
clients does not mask ECN-CE marks, this assumes that the sending
endpoint (server) acts on behalf of the network.
The assumption about the senders acting on the behalf of the network
may be reduced due to the nature of peer-to-peer usage. Still a
large part of RTP senders are infrastructure devices that do have an
interest in protecting both service quality and the network. In
addition as real-time media commonly is more sensitive to increased
delay and packet loss it will be in both media sender and receivers
interest to minimise the number and duration of any congestion events
as it will affect media quality.
In addition ECN with RTP can suffer from path changes resulting in
that a non ECN compliant node becomes part of the path. That node
may perform either of two actions that has effect on the ECN and
application functionality. The gravest is if the node drops packets
with any ECN field values other than 00b. This can be detected by
the receiver when it receives a RTCP SR packet indicating that a
number of packets has not been received. The sender may also detect
it based on the receivers RTCP RR packet where 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 message or RTCP RR will indicate the situation. The
other action is to remark a packet from ECT 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.
ECN nonce is used as part of this solution primarily to detect non-
compliant nodes on the path. Due to its definition it will also
detect receivers attempting to cheat. We can note that it appears
quite counter productive for a receiver to attempt to cheat as it
most likely will have negative impact on its media quality.
The ECN nonce mechanism used is not exactly the same as in RFC 3540
due to the desire to detect also re-markings of ECT to not-ECT. Thus
the nonce is the 2-bit XOR sum of the previous packets Nonce value
and the ECN field. The initial value for the Nonce is 00b.
Thus packet losses and ECN-nonce failures are possible indication of
issues with using ECN over the path. The next section defines both
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sender and receiver reactions to these cases.
4.4.1. 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 to report this to the sender that includes at least
the RTCP RR and the ECN feedback message. Thus speeding up the
detection at the sender of the losses and thus triggering sender side
mitigation.
A Sender that detects high packet loss rates for its RTP packet flow
while sending them marked as ECT, SHOULD immediately remark them as
not-ECT to determine if the losses potentially are due to the ECT
markings. If the losses disappear with the remarking, 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 the two possible actions exist:
1. Periodically retry the ECN initiation to detect if a path change
occurs to a path that are ECN capable.
2. Renegotiating the session to disable ECN support. A choice that
is suitable if the impact of ECT probing on the media quality are
noticeable. If multiple initiations has 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:
o Video switching MCU or similar middleboxes that selects to deliver
media from the sender only intermittently.
o Load balancing devices may in worst case result in that some
packets take a different network path then the others.
o Mobility solutions that switches underlying network path in a
transparent way for the sender or receiver.
o Membership changes in a multicast group.
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5. RTCP Extension for ECN feedback
One AVPF NACK Transport feedback format with the following
functionality is defined:
o ECN Nonce
o Explicit Sequence numbers for ECN-CE marked packets
o Explicit Sequence numbers for lost packets
The usage of this feedback format called "ECN feedback format"
includes in addition to progressive reporting of ECN-CE marking using
Immediate or early feedback also Initiation and verification
procedures.
The RTCP packet starts with the common header defined by AVPF
[RFC4585] which is reproduced here for the readers information:
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 | PT | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Feedback Control Information (FCI) :
: :
Figure 2: AVPF Feedback common header
From Figure 2 it can be determined the identity of the feedback
provider and for which RTP packet sender it applies. Below is the
feedback information format defined that is inserted as FCI for this
particular feedback messages that is identified with an FMT
value=TBA1.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First Sequence Number | Last Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|INV|RNV|Z|C|P| Reserved | Chunk 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: More chunks if needed :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: ECN Feedback Format
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The FCI information for the ECN Feedback format (Figure 3) are the
following:
First Sequence Number: The first RTP sequence number included in the
ECN nonce and base sequence number for the run length encoding.
Last Sequence Number The last RTP sequence number included in the
ECN nonce and the run length encoding.
INV: Initial Nonce Value. Which is the value of Nonce prior to the
XOR addition of the ECN field value for the packet with RTP
sequence number of "First Sequence Number". This to allow running
calculations and only need to save nonce values at reporting
boundaries.
RNV: Resulting Nonce Value. The Nonce sum value resulting after
having XOR the ECN field value for all packets received and not
ECN-CE marked with the INV value.
Z: ECN Non-capable transport value seen. If set to 1, at least one
packet within the feedback interval has had its ECN value set to
00b (Not-ECT). If set to 0, no packets within the reporting
interval has its ECN field value set to Not-ECT.
C: ECN-CE value(s) part of the feedback interval. If set to 1, at
least one packet within the feedback interval was ECN-CE marked,
the sequence numbers of the packets are explicitly encoded using
chunks. If set to 0, no packets within the reporting interval had
their ECN value set to ECN-CE and no chunks are included.
P: Packet loss part of the feedback interval. If set to 1, at least
one packet within the feedback interval was lost in transit, the
sequence numbers of the packets are explicitly encoded using
chunks. If set to 0, no packets within the reporting interval was
lost and no chunks are included.
Each FCI reports on a single source. Multiple sources can be
reported by including multiple RTCP feedback messages in an compound
RTCP packet. The AVPF common header indicates both the sender of the
feedback message and on which stream it relates to.
Both the ECN-CE and packet loss information is structured as bit
vector where the first bit represents the RTP packet with the
sequence number equal to the First Sequence number. The bit-vector
will contain values representing all packets up to and including the
one in the "Last Sequence Number" field. The chunk mechanism used to
represent the bit-vector in an efficient way may appear longer upon
reception if an explicit bit-vector is used as the last chunk. Bit-
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values representing packets with higher sequence number (modulo 16)
than "Last Sequence Number" are not valid and SHALL be ignored.
The RTP sequence number can easily wrap and that needs to be
considered when handling them. The report SHALL NOT report on more
than 32768 consecutive packets. The last sequence number is the
extended sequence number that is equal too or smaller (less than
65535 packets) than the value present in the Receiver Reports
"extended highest sequence number received" field. The "first
sequence number" value is thus is as an extended sequence number
smaller than the "last sequence number". If there is a wrap between
the first sequence number and the last, i.e. First sequence number >
Last sequence number (seen as 16-bit unsigned integers), then the
wrap needs to included in the calculation.
The ECN-CE bit-vector uses values of 1 to represent that the
corresponding packet was marked as ECN-CE, all other ECN values are
represented as a 0. The packet loss bit vector uses value of 1 to
represent that the corresponding packet was received and a value of 0
to represent loss.
The produced bit-vectors are encoded using chunks. The chunks are
any of the three types defined in [RFC3611], Run Length Chunk
(Section 4.1.1 of [RFC3611]), Bit Vector Chunk (Section 4.1.2 of
[RFC3611]), or Terminating Null Chunk (Section 4.1.3 of [RFC3611]).
In the chunk part of the FCI at least one chunk MUST be included to
achieve 32-bit word alignment. The C and P bits are used to indicate
the inclusion of two different information reports in the feedback
message. When both C and P are sent, the chunks reporting if ECN-CE
was set SHALL be sent first, followed by one Terminating Null chunk
followed by the chunks reporting on which packets where lost,
possibly followed by one terminating null chunk to achieve 32-bit
word alignment. If only one of the C and P bits are set the chunks
reports on only that information, the last chunk MAY be a Terminating
Null chunk if necessary to achieve 32-bit word alignment. If none of
the C and P bits are set, only a single Terminating Null Chunk is
included.
(tbd: We also need to register a regular RTCP packet format
containing the same information as the AVPF NACK feedback format, so
that it can be used with in regular compound RTCP packets.)
6. Processing RTCP ECN Feedback in RTP Translators and Mixers
RTP translators and mixers that support ECN feedback are required to
process, and potentially modify or generate, RTCP packets for the
translated and/or mixed streams.
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6.1. Fragmentation and Reassembly in Translators
An RTP translator may fragment or reassemble RTP data packets without
changing the media encoding. 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 induce corresponding gaps in the outgoing RTP sequence
numbers. An RTP translator MUST also rewrite RTCP packets to make
the corresponding changes to their sequence numbers. 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.
(tbd: complete this section)
6.2. Generating RTCP ECN Feedback in Translators
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.
(tbd: complete this section)
6.3. 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. An ECN-aware RTP mixer must send RTCP reports and
provide ECN feedback for the RTP flows it terminates, and must
generate RTCP reports for the RTP flow it originates, and add ECT
marks to the outgoing packets. This section describes how RTCP is
processed in RTP mixers, and how that interacts with ECN feedback.
(tbd: complete this section)
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7. Implementation considerations
To allow the use of ECN with RTP over UDP, the RTP implementation
must be able to set the ECT bits in outgoing UDP datagrams, and must
be able to read the value of the ECT bits on received UDP datagrams.
The standard Berkeley sockets API predates 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.
8. IANA Considerations
Note to RFC Editor: please replace "RFC XXXX" below with the RFC
number of this memo, and remove this note.
8.1. SDP Attribute Registration
Following the guidelines in [RFC4566], the IANA is requested to
register one new 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). This attribute should be put in the SDP offer if
the offering party wishes to receive an ECT flow. The answering
party should include the attribute in the answer if it wish to
receive an ECT flow. If the answerer does not include the attribute
then ECT MUST be disabled in both directions.
8.2. AVPF Transport Feedback Message
A new RTCP Transport feedback message needs a FMT code point
assigned. ...
8.3. STUN attribute
A new STUN attribute in the Comprehension-optional range needs to be
assigned...
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8.4. ICE Option
A new ICE option "rtp+ecn" is registered in the non-existing registry
which needs to be created.
9. Security Considerations
The usage of ECN with RTP over UDP as specified in this document has
the following known security issues that needs to be considered.
External threats to the RTP and RTCP traffic:
Denial of Service affecting RTCP: For 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. 2. Ensure that the sender disable the usage
of ECN by reporting failures to receive ECN by setting the Z bit
or changing the ECN nonce field. Both Issues, can also be
accomplished by injecting false RTCP packets to the media sender.
Reporting a lot of 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 of it.
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 sensitive information. If considered sensitive the
RTCP feedback shall be encrypted.
Changing the ECN bits An on-path attacker that see 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 en end-point to protect itself against. Only network
infrastructure nodes can detect this illicit remarking. 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
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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
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. To mitigate the risk of cheating receivers
the solution include ECN-Nonce that makes it probabilistically
unlikely that a receiver can cheat for more than a few packets
before being found out. See [RFC3168] and [RFC3540] for more
discussion.
Receivers misbehaving: A receiver may prevent the usage of ECN in an
RTP session by reporting itself as non ECN capable or simple
provide invalid ECN-nonce values. Thus 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 doesn't 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. However, this is no magic bullet and failure
to react to congestion will most likely only slightly delay a
buffer under-run, in which its session also will experience packet
loss and increased delay. There are some chance that the media
senders traffic will push other traffic out of the way without
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being effected to 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.
ECN as covert channel: As the ECN fields two bits can be set to two
different values for ECT, it is possible to use ECN as a covert
channel with a possible bit-rate of one or two bits per packet.
For more discussion of this issue please see
[I-D.ietf-tsvwg-ecn-tunnel].
We note that the end-point security functions needs to prevent an
external attacker from affecting the solution easily are source
authentication and integrity protection. To prevent what information
leakage there can be from the feedback 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 despite only providing
authentication if a entity is within the security context or not.
IPsec [RFC4301] and DTLS [RFC4347] can also provide the necessary
security functions.
The signalling protocols used to initiate an RTP session also needs
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 [RFC3851] would be used. However, with the
limited deployment a minimal mitigation strategy is to require use of
SIPS (SIP over TLS) [RFC3261] [I-D.ietf-sip-sips] to at least
accomplish hop-by-hop protection.
We do note that certain mitigation methods will require network
functions.
10. References
10.1. 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.
[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.
10.2. Informative References
[I-D.ietf-avt-rtcpssm]
Schooler, E., Ott, J., and J. Chesterfield, "RTCP
Extensions for Single-Source Multicast Sessions with
Unicast Feedback", draft-ietf-avt-rtcpssm-18 (work in
progress), March 2009.
[I-D.ietf-avt-rtp-no-op]
Andreasen, F., "A No-Op Payload Format for RTP",
draft-ietf-avt-rtp-no-op-04 (work in progress), May 2007.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-19 (work in progress), October 2007.
[I-D.ietf-sip-sips]
Audet, F., "The use of the SIPS URI Scheme in the Session
Initiation Protocol (SIP)", draft-ietf-sip-sips-09 (work
in progress), November 2008.
[I-D.ietf-tsvwg-ecn-tunnel]
Briscoe, B., "Tunnelling of Explicit Congestion
Notification", draft-ietf-tsvwg-ecn-tunnel-02 (work in
progress), March 2009.
[I-D.zimmermann-avt-zrtp]
Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media
Path Key Agreement for Secure RTP",
draft-zimmermann-avt-zrtp-15 (work in progress),
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March 2009.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[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.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 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.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 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.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, April 2009.
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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
Colin Perkins
University of Glasgow
Department of Computing Science
Glasgow G12 8QQ
United Kingdom
Email: csp@csperkins.org
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