Network Working Group M. Westerlund
Internet-Draft I. Johansson
Intended status: Standards Track Ericsson
Expires: April 5, 2010 C. Perkins
University of Glasgow
P. O'Hanlon
UCL
K. Carlberg
G11
October 2, 2009
Explicit Congestion Notification (ECN) for RTP over UDP
draft-westerlund-avt-ecn-for-rtp-01
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Abstract
This document specifies how explicit congestion notification (ECN)
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 . . . . . . . . . . . . . . 12
4.2. Initiation of ECN Use in an RTP Session . . . . . . . . . 17
4.3. Ongoing Use of ECN Within an RTP Session . . . . . . . . . 22
4.4. Detecting Failures and Receiver Misbehaviour . . . . . . . 26
5. RTCP Extensions for ECN feedback . . . . . . . . . . . . . . . 29
5.1. ECN Feedback packet . . . . . . . . . . . . . . . . . . . 29
5.2. RTCP XR Report block for ECN summary information . . . . . 32
5.3. RTCP XR Report Block for ECN Nonce . . . . . . . . . . . . 34
6. Processing RTCP ECN Feedback in RTP Translators and Mixers . . 37
6.1. Fragmentation and Reassembly in Translators . . . . . . . 37
6.2. Generating RTCP ECN Feedback in Translators . . . . . . . 37
6.3. Generating RTCP ECN Feedback in Mixers . . . . . . . . . . 38
7. Implementation considerations . . . . . . . . . . . . . . . . 38
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
8.1. SDP Attribute Registration . . . . . . . . . . . . . . . . 38
8.2. AVPF Transport Feedback Message . . . . . . . . . . . . . 39
8.3. RTCP XR Report blocks . . . . . . . . . . . . . . . . . . 39
8.4. STUN attribute . . . . . . . . . . . . . . . . . . . . . . 39
8.5. ICE Option . . . . . . . . . . . . . . . . . . . . . . . . 39
9. Security Considerations . . . . . . . . . . . . . . . . . . . 39
10. Examples of SDP Signalling . . . . . . . . . . . . . . . . . . 42
11. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 42
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 42
12.1. Normative References . . . . . . . . . . . . . . . . . . . 42
12.2. Informative References . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 45
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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. When ECN is used, the
network can signal to applications that congestion is occurring,
whether that congestion is due to queuing at a congested link,
limited resources and coverage on a radio link, or other reasons.
This congestion signal allows applications to reduce their
transmission rate in a controlled manner, rather than responding to
uncontrolled packet loss, and so improves the user experience while
benefiting the network.
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 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 use ECN. This
is due to the lack of feedback mechanism directly in UDP. Instead
the protocol on top of UDP needs to provide that feedback, which for
RTP is 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. 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 and mixers. 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",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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).
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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 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 desirable
o REQ 4: Some detection and fallback mechanism is needed 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.
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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 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
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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 needs to fallback to non-ECN use if they do not.
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 be ECT marked if any of the incoming packets
are ECT marked. 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
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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 6.2).
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, if
this is desired.
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 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.
The ECN mechanism defined in this memo is applicable to both sender
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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 layers to support more heterogeneous paths.
To ensure timely feedback of CE marked packets, 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.
The control of which RTP data packets are marked as ECT, and whether
ECT(0) or ECT(1) is used, is due to the sender. RTCP packets must
not be ECT marked, whether generated by sender or receivers.
4. Use of ECN with RTP/UDP/IP
The solution for using ECN with RTP over UDP/IP consists of four
different pieces that together makes the solution work:
1. Negotiation of the capability to use 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). 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 ECN
capability, it must verify if that capability is usable. There are
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three ways in which this verification phase may be done
(Section 4.2):
o The sender may generate a (small) 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 joins 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, this time with the ECN field
set to ECT(0) or ECT(1), is performed. 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
request was received.
o Thirdly, make a leap of faith that it will work. This is only
recommended in applications that know they run in controlled
environments where ECN functionality through other means have been
verified. In this mode one assumes ECN to work and then reacts to
failure indicators if the assumption proved wrong. The usage of
this method relies on a high confidence in successful ECN function
or an application where failure are not serious. However, also
the impact on the network and other users must be considered.
Thus there are limitation to when this method is allowed.
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 remarks ECT marked
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packets to non-ECT, possibly marking packets with a CE mark as non-
ECT. This can force the network into heavy congestion due to non-
responsiveness, and seriously impact media quality.
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 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 indicates new
CE marks since last ECN feedback packet and also the number of new CE
marks through a accumulative sum. This is the mechanism to provide
the fastest possible feedback to senders about CE marks. On receipt
of a CE marked packet, the system must react to congestion as-if
packet loss has been reported.
This rapid feedback is not optimised for reliability, therefore an
additional procedure is used to ensure more reliable, but less
timely, reporting of the ECN information. An ECN summary report
should also be sent in regular RTCP reports. The ECN summary report
contains the same information as the ECN feedback format, only packed
differently for better efficiency with large reports. By using
accumulative counters for seen CE, ECT, not-ECT or packet loss the
sender can determine what events has happened since the last report,
independently of any RTCP packets having been lost.
RTCP traffic must not be 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.
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
remarking 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. Both the RTCP ECN
summary reports and the ECN feedback packets allow the sender to
compare the number of ECT(0), ECT(1), and non-ECT marked packet with
those that were sent, while also reporting CE marked and lost
packets. If these numbers do not agree with what was sent, it can be
inferred that the path does not reliably pass ECN-marked packets.
More detailed discussions are presented in Section 4.4 and
Section 4.4.2 on how to interpret different cases. A sender
detecting a possible ECN non-compliance issue should then stop
sending ECT marked packets to determine if that allows the packet to
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be correctly delivered. If the issues can be connected to ECN, then
ECN usage is suspended and possibly also re-negotiated.
This specification offers an option of computing and reporting an ECN
nonce over all received packets that where not ECN-CE marked or
reported explicitly lost. Thus, the sender will have an additional
tool to detect if any remarking happens. It can also based on
statistics detect receivers that suppress reporting of CE marked
packets, i.e. detect cheating. The incentive for a real-time
application to cheat in its ECN reporting is less than for TCP, as
increased congestion levels are likely to cause packet losses that
decrease the media quality. While for TCP lying allows for keeping a
larger congestion window than compliant receivers and any packet
losses will be corrected by TCP's retransmission. The ECN nonce
mechanism also requires more data to function correctly. To enable
the sender to verify the ECN nonce, the sender must learn the
sequence number of all packets that was either CE marked or lost.
Otherwise it can't correctly exclude these packet from the ECN nonce
sum. This is done using a RTCP XR Nonce report, containing the nonce
sums and indicating the lost or ECN-CE marked packets using a run
length encoded bit-vector. Thus ECN nonce has a higher demand for
RTCP bandwidth. Combined with the reduced incentive to cheat, this
mechanism is optional and is only recommended for applications where
the incentive might be higher, such as streaming with
retransmissions.
In the detailed specification of the behaviour below, 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 the
capability to use ECN. This includes negotiating if ECN is to be
used symmetrically, the method for initial ECT verification, and
whether the ECN nonce is to be used. This memo defines the mappings
of this information onto SDP both for declarative and offer/answer
usage. There are one SDP extension to indicate if ECN support should
be used and the method for initiation. In addition there are an ICE
parameter to indicate that ECN initiation using STUN as part of an
ICE exchange is supported.
An RTP system that supports ECN and uses SDP in the signalling MUST
implement the SDP extension to signal ECN capability as described in
Section 4.1.1. It MAY also implement alternative ECN capability
negotiation schemes, such as the ICE extension described in
Section 4.1.2.
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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, and to negotiate the method for ECN initiation to be
used in the session. Thus the attribute take a list of methods for
initiation, which are ordered in decreasing preference. The defined
values for the initiation method are:
rtp: Using RTP and RTCP as defined in Section 4.2.1.
ice: Using STUN within ICE as defined in Section 4.2.2.
leap: Using the leap of faith method as defined in Section 4.2.3.
In addition, a number of OPTIONAL parameters may be included in the
"a=ecn-capable-rtp" attribute as follows:
o The "mode" 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
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 downstream congestion has
occurred, 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 "readonly", or when an RTP sender entity considers
offering "readonly".
o The "nonce" parameter may be used to signal whether the ECN nonce
is to be used in the session. This parameter takes two values;
"nonce=1" for nonce proposed or shall be used, and "nonce=0" for
no nonce.
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o The "ect" parameter makes it possible to express the preferred ECT
marking. This is either random (default), ECT(0) or ECT(1). If
the ECN nonce is used then this parameter MUST be ignored, and
random ECT is implied. 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.
The ABNF [RFC5234] grammar for the "a=ecn-capable-rtp" attribute is
as follows:
ecn-attribute = "a=ecn-capable-rtp" init-list parameter-list
init-list = init-value *("," init-value)
init-value = "rtp" / "ice" / "leap" / init-ext
init-ext = token
parameter-list = *(SP ";" par-value)
par-value = nonce / mode / ect / (parameter "=" value)
mode = "mode=" ("setonly" / "setread" / "readonly")
nonce = "nonce=" ("0" / "1")
ect = "ect=" ("random" / "0" / "1")
parameter = token
value = token / quoted-string
token = 1*(%x21 / %x23-27 / %x2A-2B / %x2D-2E / %x30-39 /
%x41-5A / %x5E-7A / %x7C / %x7E)
quoted-string = ( DQ *qdtext DQ )
qdtext = %x20-21 / %x23-7E / %x80-FF
DQ = %x22 ; US-ASCII double-quote mark (34)
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 attribute includes one or more ECN initiation
methods in a comma separated list in decreasing order of preference,
with some 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 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 then ECN MUST NOT be used in any direction for that media
flow. The answer may also include optional parameters, as discussed
below.
If the "mode=setonly" parameter is present in the "a=ecn-capable-rtp"
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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.
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.
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 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.
If the "nonce=1" parameter is present in the "a=ecn-capable-rtp"
attribute of the offer, the answer MUST explicitly include the
"nonce=" parameter in the "a=ecn-capable-rtp" attribute of the answer
to indicate if it supports the ECN nonce. If the answer indicates
support ("nonce=1") then ECN nonce SHALL be used in the session; if
the answer does not include the "nonce=" parameter, or includes
"nonce=0", then the ECN nonce SHALL NOT be used. The answer MAY
include a "nonce=0" parameter in an answer even if not included in
the offer. This indicates that the answerer supports and is
interested in using ECN-nonce in this session, but it is not
currently enabled. If the offerer supports use of the nonce then it
SHOULD run a second round of offer/answer to enable use of the ECN
nonce.
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 behaviour of the answering party, and
its value in the answer indicates a preference for the behaviour of
the offering party. It will be the senders choice if to honor the
receivers preference or not.
When SDP is used in a declarative manner, for example in a multicast
session using SAP, negotiation of session description parameters is
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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 understand ECN-marked UDP packets, implement
the method of initiation, 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). If the nonce parameter is included the ECN nonce shall be used
in the session. The mode parameter MAY be included also in
declarative usage, to indicate which 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.
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, RTP sessions using ECN require rapid
RTCP ECN feedback, in order that the sender can react to ECN-CE
marked packets. Thus, the use of the Extended RTP Profile for RTCP-
Based Feedback (RTP/AVPF) [RFC4585] MUST be signalled.
When using ECN nonce, the RTCP XR signalling indicating the ECN Nonce
report MUST also be included in the SDP following [RFC3611].
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 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 this initiation method
(Section 4.2.2) actually is going to be used, it is explicitly
negotiated using the "a=ecn-capable-rtp" attribute.
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.
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4.2. 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.
At the start of the RTP session when the first packets with ECT are
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
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 or use the leap of faith option
if the session supports the limitations provided in Section 4.2.3.
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 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.
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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. 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 SHOULD NOT mark all RTP
packets as ECT during the ECN initiation phase.
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. 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 contain an ECN summary report
(Section 5.2). Reception of any ECN-CE marked packets SHOULD also
result in additional early or immediate feedback packet with the
ECN feedback packet.
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
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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.
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 any other participants 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 or ECN summary 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.
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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 do reach all receivers.
The sender must also watch for cases where ECT packets has been
remarked, for example to not-ECT, either explicitly reported in an
ECN feedback packet, or implicit due to a receiver not including
the ECN feedback format in its regular report.
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
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.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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
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.
4.2.3. 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). It is not generally
recommended as the impact on both the application and the network may
be substantial. Applications may experience high packet loss rates,
this is both from dropped ECT marked packets, and the result of
driving the network into higher degrees of congestion by not being
responsive to ECN marks. The network may experience higher degrees
of congestion due to the unresponsiveness of the sender due to lost
ECN-CE marks from non-compliant remarking.
The method is to go directly to "ongoing use of ECN" as defined in
Section 4.3. 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.
Not sending any RTP packets as not-ECT in the case of non-compliant
node dropping ECT marked traffic the RTP receiver will not get any
baseline packets to ensure that it treat this SSRC as an active
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sender. Thus the failure to include the sender in its RTCP sender or
receiver packets report block becomes the indicator for this case.
This is blunter than a receiver report block that indicates
explicitly how many packets actually has been lost. The sender
should be aware that in unicast or under AVPF transmission rules the
first RTCP packet may come immediately upon joining or already after
500 ms. Thus, triggering on reports without any report blocks,
cannot be done reliably on the first RTCP report received from a new
SSRC. Thus delaying detection of lack of functionality substantially
until a second report comes in.
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.
4.2.4. ECN Nonce during initiation
ECN Nonce if enabled SHALL be used during initiation the same way as
ECN nonce is used under ongoing use of ECN as described in
Section 4.3.2.1.
4.3. Ongoing Use of ECN Within an RTP Session
Once ECN usage has been successfully initiated for an RTP sender,
that sender begins sending all RTP data packets as ECT-marked, and
its receivers continue sending ECN feedback information via RTCP
packets. This section describes procedures for sending ECT-marked
data, providing ECN feedback information via RTCP, responding to ECN
feedback information, 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) is determined by the sender having considered the
preferencies expressed by the "ect" parameter in the "a=ecn-capable-
rtp" attribute. If the sender selects a random choice of ECT
marking, the sender MUST record the statistics for the different ECN
values sent. If ECN nonce is activated the sender must record the
value and calculate 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
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[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 ECN feedback packet, 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. ECN Nonce information
SHOULD NOT be included in early or immediate reports, only when
regular reports are sent.
Every time a regular compound RTCP packet is to be transmitted, the
RTP receiver MUST include an RTCP XR ECN summary report Section 5.2
as part of the compound packet. If ECN-nonce is enabled the receiver
MUST also include an RTCP XR Nonce report packet Section 5.3. 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.
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
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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 ("rtp") of any new media senders as specified in
Section 4.2.1.
4.3.2.1. ECN Nonce Reporting
ECN Nonce reporting requires both the ECN nonce sum and the sequence
numbers for packets where the ECN marking has been lost. This
information is variable size as it depends on both the total number
of packet sent per reporting interval and the CE and Packet loss
pattern how many bits are required for reporting.
The RTCP packets may be lost, and to avoid the possibility for
cheating by "losing" the Nonce information for where one is cheating
the nonce coverage needs to be basically complete. Thus the Nonce
reporting SHOULD cover at least the 3 regular reporting intervals.
The only exception allowed is if the reporting information becomes to
heavy and makes the RTCP report packet become larger than the MTU.
In that case a receiver MAY reduced to coverage for the ECN nonce to
only the last or two last reporting intervals. A sender should
consider the received size report for cases where the coverage is not
at least three reporting intervals and determine if this may be done
to cheat or not. Failure to have reported on all intervals MAY be
punished by reducing the congestion safe rate.
The ECN nonce information in the ECN feedback packet consists of both
a start value for the nonce prior to the first packet in the
reporting interval and the final 2-bit XOR sum over all the received
ECN values, both not-ECT and ECT for the report interval. The report
interval is explicitly signalled in the RTCP XR Nonce report packet.
The initial value for the Nonce is 00b.
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
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example that might be used.
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
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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.
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 information 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.
The ECN feedback packet allows the sender to compare the number of
ECT marked packets of different type with the number it actually
sent. The number of ECT packets received plus the number of CE
marked and lost packets should correspond to the number of sent ECT
marked packets. If this number doesn't agree there are two likely
reasons, a translator changing the stream or not carrying the ECN
markings forward or that some node remarks 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 happing on the path.
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Thus packet losses and non-matching ECN field value statistics are
possible indication of issues with using ECN over the path. The next
section defines both 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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4.4.2. Interpretation of ECN Summary information
This section contains discussion on how you can use the ECN summary
report information in detecting various types of ECN path issues.
Lets start to review the information the reports provide on a per
source (SSRC) basis:
CE Counter: The number of RTP packets received so far in the session
with an ECN field set to CE (11b).
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
(10b / 01b).
not-ECT Counter: The number of RTP packets received so far in the
session with an ECN field set to not-ECT (00b)
Packet loss counter: The number of RTP packets that are expected
minus the number received.
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 initiated to zero they provide value for the RTP
stream sender from the very first report. After the first report the
changes between the latest received and the previous one is
determined by simply taking the values of the latest minus the
previous one, taking field wrapping into account. This definition is
also robust to packet losses, as if one report is missing, the period
for which the information is covering becomes longer, but otherwise
equally valid.
In a perfect world the number of not-ECT received should be equal to
the number sent minus some fraction of the lost packets, and the sum
of the ECT, CE should be equal to the number ECT marked sent minus a
fraction of the lost packets. There are however two sources of
uncertainty in this, number of packet losses, and packet duplication.
Packet loss and packet duplication can change the distribution
between ECT(0), ECT(1) and not-ECT. This by having for example a ECT
(0) packet being lost, and then a ECT(1) packet being duplicated and
counted as two, thus making the ECT(1) counter become one bigger and
the ECT(0) one less than expected. To avoid these issues it is
recommended that suppression of duplicate packets are performed
before gathering this statistics.
The level of packet duplication included in the report can be
estimated from the sum over all of fields counting received packets.
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A high level of packet duplication increases the insecurity in the
statistics and firm conclusions becomes more difficult and requires
clearer statics.
Detecting clearing of ECN field: If the ratio between ECT and not-
ECT transmitted in the reports has become all not-ECT or
substantially changed towards not-ECT then this is clearly
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.
4.4.3. Using ECN-nonce
This document offers ECN Nonce as a method of strengthening both the
detection of failures and enable senders to verify the receiver
behavior. We 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. However, certain usages of RTP may
result in a situation that is more similar to TCP, i.e. where packet
losses are repaired and a higher bit-rate is desirable. Thus RTP
sessions that use repair mechanisms as FEC or retransmission may
consider the usage of the ECN nonce to prevent cheating.
5. RTCP Extensions for ECN feedback
This documents defines three different RTCP extensions. One AVPF
NACK Transport feedback format for urgent ECN information. One RTCP
XR ECN summary report block type for regular reporting of the ECN
marking information. And one additional RTCP XR report block type
for ECN nonce.
5.1. ECN Feedback packet
This AVPF NACK feedback format is intended for usage in AVPF early or
immediate feedback modes when information needs to urgently reach the
sender. Thus its main usage is upon reception of a ECN-CE marked RTP
packet, or during the initiation procedures to speed that up. The
feedback format is also defined with reduced size RTCP [RFC5506] in
mind. In reduced size RTCP feedback packets may be sent without
accompanying Sender or Receiver Reports that would contain the
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Extended Highest Sequence number and the accumulated number of packet
losses. Both are important for the ECN functionality to verify
functionality and keep track of when CE marking does occur.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Highest Sequence Number | Lost packets counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CE Counter | not-ECT Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ECT (0) Counter | ECT (1) Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: ECN Feedback Format
The FCI information for the ECN Feedback format (Figure 3) are the
following:
Extended Highest Sequence Number: The least significant 20-bit from
an Extended highest sequence number received value as defined by
[RFC3550]. Used to indicate for which packet this report is valid
upto.
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Lost Packets Counter: The total number of RTP packets from this SSRC
the receiver that it expected minus the number of received, see
Section 6.4.1 of [RFC3550] for the normative definition. This
representation is done using 12-bit signed representation,
compared to 24-bit in RTCP SR or RR packets. It is important to
ensure that the wrapping is handled correctly.
CE Counter: The total number of RTP packets from this SSRC the
receiver has received since the receiver joined the RTP session
that had an ECN field value of CE. The receiver should keep track
of this value using a local representation that is longer than 16-
bits, and only include the 16-bits with least significance. In
other words, the field will wrap to 0 if more than 65535 packets
has been received.
ECT (0) Counter: The total number of RTP packets from this SSRC the
receiver has received since the receiver joined the RTP session
that had an ECN field value of ECT (0). The receiver should keep
track of this value using a local representation that is longer
than 16-bits, and only include the 16-bits with least
significance. In other words, the field will wrap if more than
65535 packets has been received.
ECT (1) Counter: The total number of RTP packets from this SSRC the
receiver has received since the receiver joined the RTP session
that had an ECN field value of ECT (1). The receiver should keep
track of this value using a local representation that is longer
than 16-bits, and only include the 16-bits with least
significance. In other words, the field will wrap if more than
65535 packets has been received.
not-ECT Counter: The total number of RTP packets from this SSRC the
receiver has received 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 longer
than 16-bits, and only include the 16-bits with least
significance. In other words, the field will wrap if more than
65535 packets has been received.
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.
The Counters SHALL be initiated to 0 for a new receiver. This to
enable detection of CE or Packet loss already on the initial report
from a specific participant.
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The Extended Highest sequence number and packet loss fields are both
truncated in comparison to the RTCP SR or RR versions. This is to
save bits as the representation is redundant unless reduced size RTCP
is used in such a way that only feedback packets are transmitted,
with no SR or RR in the compound RTCP packet. Due to that regular
RTCP reporting will include the longer versions of the fields the
wrapping issue will be less unless the packet rate of the application
is so high that the fields will wrap within a regular RTCP reporting
interval. In those case the feedback packet need to be sent in a
compound packet together with the SR or RR packet.
There is an issue with packet duplication in relation to the packet
loss counter. If one avoids holding state for which sequence number
has been received then the way one can count loss is to count the
number of received packets and compare that to the number of packets
expected. As a result a packet duplication can hide a packet loss.
If a receiver is tracking the sequence numbers actually received and
suppresses duplicates it provides for a more reliable packet loss
indication. Reordering may also result in that packet loss is
reported in one report and then removed in the next.
The CE counter is actually more robust for packet duplication.
Adding each received CE marked packet to the counter is not an issue.
If one of the clones was CE marked that is still a indication of
congestion. Packet duplication has potential impact on the ECN
verification. Thus the sum of packets reported may be higher than
the number sent. However, most detections are still applicable.
5.2. RTCP XR Report block for ECN summary information
This report block combined with RTCP SR or RR report blocks carries
the same information as the ECN Feedback Packet and shall be based on
the same underlying information. However, there is a difference in
semantics between the feedback format and this XR version. Where the
feedback format is intended to report on a CE mark as soon as
possible, this extended report is for the regular RTCP report and
continuous verification of the ECN functionality end-to-end.
The ECN Summary report block consists of one report block 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT | Reserved | Block Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
and then followed of one or more of the following report data blocks:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Media Sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CE Counter | not-ECT Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ECT (0) Counter | ECT (1) Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 Lenght: The length of the report block. Used to indicate the
number of report data blocks present in the ECN summary report.
This length will 3*n, where n is the number of data blocks, i.e. 3
for one data block, 6 for two, etc.
SSRC of Media Sender: The SSRC identifying the media sender this
report is for.
CE Counter: The total number of RTP packets from this SSRC the
receiver has received since the receiver joined the RTP session
that had an ECN field value of CE. The receiver should keep track
of this value using a local representation that is longer than 16-
bits, and only include the 16-bits with least significance. In
other words, the field will wrap if more than 65535 packets has
been received.
not-ECT Counter: The total number of RTP packets from this SSRC the
receiver has received 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 longer
than 16-bits, and only include the 16-bits with least
significance. In other words, the field will wrap if more than
65535 packets has been received.
ECT (0) Counter: The total number of RTP packets from this SSRC the
receiver has received since the receiver joined the RTP session
that had an ECN field value of ECT (0). The receiver should keep
track of this value using a local representation that is longer
than 16-bits, and only include the 16-bits with least
significance. In other words, the field will wrap if more than
65535 packets has been received.
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ECT (1) Counter: The total number of RTP packets from this SSRC the
receiver has received since the receiver joined the RTP session
that had an ECN field value of ECT (1). The receiver should keep
track of this value using a local representation that is longer
than 16-bits, and only include the 16-bits with least
significance. In other words, the field will wrap if more than
65535 packets has been received.
The Extended Highest Sequence number and the packet loss 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 always
be sent in a RTCP compound packet where the Extended Highest Sequence
number and the accumulated number of packet losses are present in the
RTCP Sender Report or Receiver Report packet's report block.
5.3. RTCP XR Report Block for ECN Nonce
This RTCP XR block is for ECN Nonce reporting. It consists of an
initial part that contains the ECN nonce XOR sum followed by an Run
length encoded (RLE) bitvector that indicate which RTP sequence
numbers that wasn't included in the ECN nonce sum due to having been
lost or ECN CE marked. The bit-vector uses 1 to indicate that the
packet wasn't included in the ECN nonce sum and 0 for packets that
where.
The bit-vector is expressed using either Run-Length Encoding or 15-
bit explicit bit-vectors. The whole vector is encoded using the 16-
bit chunks as defined by Section 4.1.1, 4.1.2, and 4.1.3 in
[RFC3611]. The Terminating Null Chunk MUST be used as padding in
cases the total number of chunks would otherwise be odd and thus the
report block wouldn't reach a 32-bit boundary.
The ECN Nonce report block structure is the following:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT |R|R|R|R|INV|RNV| Block Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of Media Sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Begin_seq | End_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| chunk 1 | chunk 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| chunk n-1 | chunk n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
BT: Block Type, the value identifying this block is [TBA1].
R: Bits are reserved and MUST be set to 0 on transmission and MUST be
ignored on reception.
Block Length: The block length of this full report block in 32-bit
words minus one. The minimal report block size is 3, i.e. fixed
parts (12 bytes) plus 2 chunks (4 bytes) expressed as 32-bit words
(3+1) minus 1.
SSRC of Media Sender SSRC of Media Sender that this report concerns
INV: Initial Nonce Value. Which is the value of Nonce prior to the
XOR addition of the ECN field value for the packet that start the
nonce reporting interval. This first included sequence number is
given by the "begin_seq" value. 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 up to the packet indicated by the
"end_seq" sequence number value.
begin_seq: First Sequence number this report covers.
end_seq: Last RTP sequence number included in this report.
chunk i: A chunk reporting on a part of bit-vector indicating if the
packet was excluded from the ECN Nonce due to being lost or ECN CE
marked.
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The Nonce sum initial value for a new media sender (new SSRC) SHALL
be 00b. Otherwise the Initial value is the Nonce value calculated
for the RTP packet with sequence number begin_seq -1. The initial
value for the expressed reporting interval is included in the INV
field. The receiver calculate the 2-bit Nonce XOR sum over all
received RTP packets in the reporting interval including the one with
end_seq sequence number. We note that the RTCP participant doing the
Nonce sum MUST perform suppression of packet duplicates. The nonce
sum will become incorrect if any duplicates are included in the sum.
All packets not received or received as ECN-CE marked when
constructing the ECN Nonce report MUST be explicitly marked in the
bitvector.
The Nonce reporting interval is RECOMMENDED to cover all the RTP
packets received during the three last regular reporting intervals.
This is to ensure that the sender will receive a report over all RTP
packets. Failure to deliver reports that cover all the packets may
be interpreted as an attempt to cheat. Two additional considerations
must be made when selecting the reporting interval. First, are the
MTU considerations. The packet vector and its encoding into chunks
results in a variable sized report. The size depends on two main
factors, the number of packets to report on and the frequency of bit-
value changes in the vector. The reporting interval may need to be
shortened to two or even one reporting interval if the resulting ECN
nonce report becomes too big to fit into the RTCP packet.
Secondly, the RTP sequence number can easily wrap and that needs to
be considered when they are handed. 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 an extended sequence number which is
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. If an application is
having these issues, the frequency of regular RTCP reporting should
be modified by ensuring that the application chooses appropriate
settings for the minimum RTCP reporting interval parameters.
Both the ECN-CE and packet loss information is structured as bit
vectors 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 "end_seq" 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-values
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representing packets with higher sequence number (modulo 16) than
"end_seq" are not valid and SHALL be ignored.
The produced bit-vector is 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]). Where the
Terminating Null Chunk may only appear as the last chunk, and only in
cases where the number of chunks otherwise would be odd.
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.
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
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[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)
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 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.
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
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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. RTCP XR Report blocks
Two new RTCP XR report blocks needs to be assigned block type codes.
8.4. STUN attribute
A new STUN attribute in the Comprehension-optional range needs to be
assigned...
8.5. 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 changing the
counter fields. The Issue, 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.
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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
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
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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
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
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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. Examples of SDP Signalling
(tbd)
11. Open Issues
As this draft is under development some known open issues exist and
are collected here. Please consider them and provide input.
1. Packet duplication. Packet duplication results in uncertainties
in the ECN summary counters. At the same time suppressing
duplicates and ignoring their ECN marks may also be problematic.
Consider the case when a packet get duplicated prior to a
congestion point and one version arrives with a ECT mark, and the
other with CE mark. What to report?
2. The negotiation and directionality attribute is going to need
some consideration for multi-party sessions when readonly
capability might be sufficient to enable ECN for all incomming
streams. However, it would beneficial to know if no potential
sender support setting ECN.
3. Consider initiation optimizations that allows for multi SSRC
sender nodes to still have rapid usage of ECN.
4. Feedback suppression for ECN-CE, both for groups, and in case an
additional CE mark arrives within a RTT at the receiver.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[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.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[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.
12.2. 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
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Notification", draft-ietf-tsvwg-ecn-tunnel-03 (work in
progress), July 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),
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.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
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.
[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
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Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 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
and Consequences", RFC 5506, April 2009.
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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Piers O'Hanlon
University College London
Computer Science Department
Gower Street
London WC1E 6BT
United Kingdom
Email: p.ohanlon@cs.ucl.ac.uk
Ken Carlberg
G11
1600 Clarendon Blvd
Arlington VA
USA
Email: carlberg@g11.org.uk
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