One document matched: draft-ietf-avtcore-ecn-for-rtp-01.xml
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<rfc category="std" docName="draft-ietf-avtcore-ecn-for-rtp-01"
ipr="trust200902">
<front>
<title abbrev="ECN for RTP over UDP/IP">Explicit Congestion Notification
(ECN) for RTP over UDP</title>
<author fullname="Magnus Westerlund" initials="M." surname="Westerlund">
<organization>Ericsson</organization>
<address>
<postal>
<street>Farogatan 6</street>
<city>SE-164 80 Kista</city>
<country>Sweden</country>
</postal>
<phone>+46 10 714 82 87</phone>
<email>magnus.westerlund@ericsson.com</email>
</address>
</author>
<author fullname="Ingemar Johansson" initials="I." surname="Johansson">
<organization>Ericsson</organization>
<address>
<postal>
<street>Laboratoriegrand 11</street>
<city>SE-971 28 Lulea</city>
<country>SWEDEN</country>
</postal>
<phone>+46 73 0783289</phone>
<email>ingemar.s.johansson@ericsson.com</email>
</address>
</author>
<author fullname="Colin Perkins" initials="C. " surname="Perkins">
<organization>University of Glasgow</organization>
<address>
<postal>
<street>School of Computing Science</street>
<city>Glasgow</city>
<code>G12 8QQ</code>
<country>United Kingdom</country>
</postal>
<email>csp@csperkins.org</email>
</address>
</author>
<author fullname="Piers O'Hanlon" initials="P." surname="O'Hanlon">
<organization abbrev="UCL">University College London</organization>
<address>
<postal>
<street>Computer Science Department</street>
<street>Gower Street</street>
<city>London</city>
<code>WC1E 6BT</code>
<country>United Kingdom</country>
</postal>
<email>p.ohanlon@cs.ucl.ac.uk</email>
</address>
</author>
<author fullname="Ken Carlberg" initials="K." surname="Carlberg">
<organization>G11</organization>
<address>
<postal>
<street>1600 Clarendon Blvd</street>
<city>Arlington</city>
<code>VA</code>
<country>USA</country>
</postal>
<email>carlberg@g11.org.uk</email>
</address>
</author>
<date day="14" month="March" year="2011" />
<abstract>
<t>This document specifies how explicit congestion notification (ECN)
can be used with Real-time Transport Protocol (RTP) over UDP flows that
use RTP Control Protocol (RTCP) as feedback mechanism. It defines one
RTP Control Protocol Extended Reports (RTCP XR) extension for ECN
summary, a RTCP transport feedback format for timely reporting of
congestion events, and an Session Traversal Utilities for NAT (STUN)
extension used in the optional initilization method using Interactive
Connectivity Establishment (ICE). Signalling and procedures for
negotiation of capabilities and initilization methods are also
defined.</t>
</abstract>
</front>
<middle>
<section anchor="sec-intro" title="Introduction">
<t>This document outlines how Explicit Congestion Notification (ECN)
<xref target="RFC3168"></xref> can be used for <xref
target="RFC3550">Real-time Transport Protocol (RTP)</xref> flows running
over UDP/IP which use RTP Control Protocol (RTCP) as a feedback
mechanism. The solution consists of feedback of ECN congestion
experienced markings to the sender using RTCP, verification of ECN
functionality end-to-end, and 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 signalling mechanisms
using <xref target="RFC4566">Session Description Protocol (SDP)</xref>
is included.</t>
<t>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.</t>
<t>ECN provides a way for networks to send congestion control signals to
a media transport without having to impair the media. Unlike losses, the
signals unambiguously indicate congestion to the transport as quickly as
feedback delays allow, and without confusing congestion with losses that
might have occurred for other reasons such as transmission errors,
packet-size errors, routing errors, badly implemented middleboxes,
policy violations and so forth.</t>
<t>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 <xref target="RFC3168"></xref>.
In addition, transport protocols have to be modified to inform the
sender that ECN marked packets are being received, so it can respond to
the congestion. <xref target="RFC3168">TCP</xref>, <xref
target="RFC4960">SCTP</xref> and <xref target="RFC4340">DCCP</xref> have
been updated to support ECN, but to date there is no specification how
UDP-based transports, such as <xref target="RFC3550"> RTP</xref>, can
use ECN. This is due to the lack of feedback mechanisms directly in UDP.
Instead the signaling control protocol on top of UDP needs to provide
that feedback, which for RTP is RTCP.</t>
<t>The remainder of this memo is structured as follows. We start by
describing the conventions, definitions and acronyms used in this memo
in <xref target="sec-2119"></xref>, and the design rationale and
applicability in <xref target="sec-rationale"></xref>. <xref
target="sec-overview"></xref> provides an overview of how ECN is used
with RTP over UDP. Then the definition of the RTCP extensions for ECN
feedback in <xref target="sec-rtcp-ecn"></xref>. Then the SDP signalling
extensions required are specified <xref
target="sec-sdp-ext"></xref>.Then the full details of how ECN is used
with RTP over UDP is defined in <xref target="sec-definition"></xref>.
In <xref target="sec-rtcp-translator-mixer"></xref> we discuss how RTCP
ECN feedback is handled in RTP translators and mixers. <xref
target="sec-impl"></xref> discusses some implementation considerations,
<xref target="sec-iana"></xref> lists IANA considerations, and <xref
target="sec-security"></xref> discusses the security considerations.</t>
</section>
<section anchor="sec-2119" title="Conventions, Definitions and Acronyms">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in <xref
target="RFC2119"> RFC 2119</xref>.</t>
<t>Abbreviations <list style="symbols">
<t>ECN: Explicit Congestion Notification</t>
<t>ECT: ECN Capable Transport</t>
<t>ECN-CE: ECN Congestion Experienced</t>
<t>not-ECT: Not ECN Capable Transport</t>
</list></t>
<t> This document uses the terms sender and receiver according to the
following definition: <list style="hanging">
<t hangText="Sender:">Sender of RTP packets carrying an encoded
media stream. The sender has the possibility to effect how this
transmission is performed. It is one end-point of the ECN control
loop.</t>
<t hangText="Receiver:">A receiver of RTP packets with the intention
to consume the media stream in some form. It sends RTCP feedback on
the received stream. It is the other end-point of the ECN control
loop.</t>
</list></t>
<t>Note: RTP mixers or translators that operate in such a manner that
they terminate or split the ECN control loop will take on the role of
receivers or senders. This is further discussed in <xref
target="sec-applicability"></xref>.</t>
<t>The meaning of the term ECN support depends on which entity between
the sender and receiver (inclusive) that is considered. We distinguish
between:<list style="symbols">
<t>ECN-Capable Host: Sender or receiver of media.</t>
<t>ECN-Capable Transport: ECT = all ends are ECN capable hosts.</t>
<t>ECN-Capable Packets: Packets are either ECT or CE.</t>
<t>ECN-Oblivious Relay: Router or middlebox that treats ECN-Capable
Packets no differently from Not-ECT.</t>
<t>ECN-Capable Queue: Supports ECN marking of ECN-Capable
Packets.</t>
<t>ECN-Blocking Middlebox: Discards ECN-Capable Packets.</t>
<t>ECN-Reverting Middlebox: Changes ECN-Capable Packets to
Not-ECT.</t>
</list></t>
<!---->
</section>
<section anchor="sec-rationale"
title="Discussion, Requirements, and Design Rationale">
<t>ECN has been specified for use with <xref
target="RFC3168">TCP</xref>, <xref target="RFC4960">SCTP</xref>, and
<xref target="RFC4340">DCCP</xref> 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.</t>
<t>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: <list style="hanging">
<t hangText="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).</t>
<t hangText="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 <xref target="RFC4585"></xref> allows more rapid feedback in
most cases).</t>
<t hangText="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 <xref
target="RFC3550"></xref>, the dynamics of the congestion response
may be quite different to those of TCP or other transport
protocols.</t>
<t hangText="Middleboxes:">The RTP framework explicitly supports the
concept of mixers and translators, which are middleboxes that are
involved in media transport functions.</t>
<t hangText="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 <xref target="RFC5760"></xref>).</t>
<t hangText="Application Awareness:">ECN support via TCP, DCCP, and
SCTP constrain the awareness and reaction to packet loss within
those protocols. By adding support of ECN through RTCP, the
application is made aware of packet loss and may choose one or more
approaches in response to that loss.</t>
<t hangText="Counting vs Detecting Congestion:">TCP and the
protocols derived from it are mainly designed to respond the same
whether they experience a burst of congestion indications within one
RTT or just one. Whereas real-time applications may be concerned
with the amount of congestion experienced, whether it is distributed
smoothly or in bursts. When feedback of ECN was added to TCP <xref
target="RFC3168"></xref>, the receiver was designed to flip the echo
congestion experienced (ECE) flag to 1 for a whole RTT then flop it
back to zero. Whereas ECN feedback in RTCP will need to report a
count of how much congestion has been experienced within an RTCP
reporting period, irrespective of round trip times.</t>
</list>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.</t>
<t>ECN support is more important for RTP sessions than, for instance, is
the case for TCP. This is because 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 as voice telephony.</t>
<section title="Requirements">
<t>Considering ECN, transport protocols supporting ECN, and RTP based
applications one can create a set of requirements that must be
satisfied to at least some degree if ECN is to used by RTP over UDP.
<list style="symbols">
<t>REQ 1: A mechanism MUST negotiate and initiate the usage of ECN
for RTP/UDP/IP sessions so that an RTP sender will not send
packets with ECT in the IP header unless it knows all potential
receivers will understand any CE indications they might
receive.</t>
<t>REQ 2: A mechanism MUST feedback the reception of any packets
that are ECN-CE marked to the packet sender</t>
<t>REQ 3: Provided mechanism SHOULD minimise the possibility for
cheating</t>
<t>REQ 4: Some detection and fallback mechanism SHOULD exist to
avoid loss of communication due to the attempted usage of ECN in
case an intermediate node clears ECT or drops packets that are ECT
marked.</t>
<t>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).</t>
<t>REQ 6: Negotiation of ECN SHOULD NOT cause media clipping at
the start of a session.</t>
</list></t>
<t>The following sections describes how these requirements can be meet
for RTP over UDP.</t>
</section>
<section anchor="sec-applicability" title="Applicability">
<t>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.</t>
<t>Due to the need for each RTP sender that intended to use ECN with
RTP to track all participants in the RTP session the sub-sampling of
the group membership as specified by <xref target="RFC2762">"Sampling
of the Group Membership in RTP"</xref> MUST NOT be used. </t>
<t>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 in various forms and at various nodes. As an
example, the sender can change the media encoding, or the receiver can
change the subscription to a layered encoding, or either reaction can
be accomplished by a transcoding middlebox. RFC 5117 identifies seven
topologies in which RTP sessions may be configured, and which may
affect the ability to use ECN: <list style="hanging">
<t hangText="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.</t>
<t hangText="Topo-Multicast:">This is either an any source
multicast (ASM) group <xref target="RFC3569"></xref> with
potentially several active senders and multicast RTCP feedback, or
a source specific multicast (SSM) group <xref
target="RFC4607"></xref> 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 <xref
target="RFC3550"></xref>, <xref target="RFC4585"></xref>, and
<xref target="RFC5760"></xref>), and can be used by a sender to
determine if all its receivers, and the network paths to those
receivers, support ECN (see <xref
target="sec-initiation"></xref>). 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 known receivers support ECN, and irrespective of
whether the paths from other senders to their receivers support
ECN. "all its known receivers" are all the SSRCs that the RTP
sender has received RTP or RTCP from the last five reporting
intervals, i.e. they are not timed out. Note that group membership
may change during the lifetime of a multicast RTP session,
potentially introducing new receivers that are not ECN capable or
have a path that doesn't support ECN. Senders must use the
mechanisms described in <xref target="sec-ecn-failure"></xref> to
monitor that all receivers continue to support ECN, and they need
to fallback to non-ECN use if any senders do not.</t>
<t hangText="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 to set the ECN bits to not-ECT
when sending packets, so causing ECN negotiation on the path
containing the translator to fail (any new RTP translator that
does not wish to support ECN may do so similarly). An ECN aware
RTP translator may act in one of three ways: <list style="symbols">
<t>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.</t>
<t>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 <xref target="RFC3168"></xref>: if
an ECN marked packet is split into two, then both the outgoing
packets must be ECN marked identically to the original; if
several ECN marked packets are combined into one, the outgoing
packet must be either ECN-CE marked or dropped if any of the
incoming packets are ECN-CE marked. If the outgoing combined
packet is not ECN-CE marked, then it MUST be ECT marked if any
of the incoming packets were ECT marked. When RTCP ECN
feedback packets (<xref target="sec-rtcp-ecn"></xref>) are
received, they must be rewritten to match the modifications
made to the media stream (see <xref
target="sec-rtcp-ecn-translator"></xref>).</t>
<t>If the translator is a media transcoder, the output RTP
media stream may have radically different characteristics than
the input RTP media stream. Each side of the translator must
then be considered as a separate transport connection, with
its own ECN processing. This requires the translator interpose
itself into the ECN negotiation process, effectively splitting
the connection into two parts with their own negotiation. Once
negotiation has been completed, the translator must generate
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 <xref
target="sec-rtcp-ecn-synthetic"></xref>).</t>
</list> It is recognised that ECN and RTCP processing in an RTP
translator that modifies the media stream is non-trivial.</t>
<t hangText="Topo-Mixer:">A mixer is an RTP-level middlebox that
aggregates multiple RTP streams, mixing them together to generate
a new RTP stream. The mixer is visible to the other participants
in the RTP session, and is also usually visible in the associated
signalling session. The RTP flows on each side of the mixer are
treated independently for ECN purposes, with the mixer generating
its own RTCP ECN feedback, and responding to ECN feedback for data
it sends. Since 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.</t>
<t hangText="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.</t>
<t hangText="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.</t>
<t hangText="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."</t>
</list> These topologies may be combined within a single RTP
session.</t>
<t>The ECN mechanism defined in this memo is applicable to both sender
and receiver controlled congestion algorithms. The mechanism ensures
that both senders and receivers will know about ECN-CE markings and
any packet losses. Thus the actual decision point for the congestion
control is not relevant. This is a great benefit as the rate of an RTP
session can be varied in a number of ways, for example a unicast media
sender might use TFRC <xref target="RFC5348"></xref> or some other
algorithm, while a multicast session could use a sender based scheme
adapting to the lowest common supported rate, or a receiver driven
mechanism using layered coding to support more heterogeneous
paths.</t>
<t>To ensure timely feedback of CE marked packets when needed, this
mechanism requires support for the RTP/AVPF profile <xref
target="RFC4585"></xref> or any of its derivatives, such as RTP/SAVPF
<xref target="RFC5124"></xref>. The standard RTP/AVP profile <xref
target="RFC3551"></xref> 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.</t>
</section>
<section title="Interoperability">
<t>The interoperability requirements for this specification are that
there is at least one common interoperability point for all
implementations. Since initialization using RTP and RTCP is the one
method that works in all cases, although is not optimal for all
usages, it is selected as mandatory to implement this initialisation
method. This method requires both the RTCP XR extension and the ECN
feedback format, which requires the RTP AVPF profile to ensure timely
feedback.</t>
<t>When one considers all the uses of ECN for RTP it is clear that
congestion control mechanisms that are receiver driven only (<xref
target="sec-congestion"></xref>) do not require timely feedback of
congestion events. If such a congestion control mechanism is combined
with an initialization method that also doesn't require timely
feedback using RTCP, like the leap of faith or the ICE based method
then neither the ECN feedback format nor AVPF is strictly needed.
However, we would like to point out that fault detection can be
improved by using receiver side detection (<xref
target="sec-fallback"></xref>) and early reporting of such cases using
the ECN feedback mechanism.</t>
<t>For interoperability we do mandate the implementation of AVPF, with
both RTCP extensions and the necessary signalling to support a common
operations mode. This specification will still recommend the usage of
AVPF in all cases as negotiation of the common interoperability point
requires AVPF, and mixed negotiation of AVP and AVPF depending on
other SDP attributes in the same media block are difficult and the
fact that fault detection can be improved when using AVPF. The use of
the ECN feedback format is also recommended but cases where there is
no requirement for timely feedback will be noted. The term "no timely
feedback required" will be used to indicate usage that employs this
specification in combination with receiver driven congestion control,
and initialization methods that do not require timely feedback, i.e.
currently leap of faith and ICE based. We also note that any receiver
driven congestion control solution that still requires RTCP for
signalling of any adaptation information to the sender will still
require AVPF.</t>
</section>
</section>
<section anchor="sec-overview"
title="Overview of Use of ECN with RTP/UDP/IP">
<t>The solution for using ECN with RTP over UDP/IP consists of four
different pieces that together make the solution work:</t>
<t><list style="numbers">
<t>Negotiation of the capability to use ECN with RTP/UDP/IP</t>
<t>Initiation and initial verification of ECN capable transport</t>
<t>Ongoing use of ECN within an RTP session</t>
<t>Handling of dynamic groups through failure detection,
verification and fallback</t>
</list></t>
<t>The solution includes a new SDP attribute (<xref
target="sec-sdp-ecn"></xref>), the definition of new extensions to RTCP
(<xref target="sec-rtcp-ecn"></xref>) and STUN (<xref
target="sec-stun-init-ecn"></xref>).</t>
<t>Before an RTP session can be created, a signalling protocol is often
used to discover the other participants and negotiate session parameters
(see <xref target="sec-signalling"></xref>). At the minimum a signalling
protocol is used to configure RTP session participants through a
declarative method. 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 pass
ECN-marked packets (for example, a firewall that blocks traffic with the
ECN bits set). This document defines the information that needs to be
negotiated, and provides a mapping to SDP for use in both declarative
and offer/answer contexts.</t>
<t>When a sender joins a session for which all participants claim ECN
capability, it must verify if that capability is usable. There are three
ways in which this verification may be done (<xref
target="sec-initiation"></xref>): <list style="symbols">
<t>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 will reveal
whether or not it supports ECN when it sends its first RTCP report
to each source. If the RTCP report includes ECN information,
verification will have succeeded and sources can continue to send
ECT packets. If not, verification fails and each sender MUST stop
using ECN.</t>
<t>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 STUN
request was received (see <xref target="sec-stun-init-ecn"></xref>),
along with an ICE signalling option (<xref
target="sec-ice-ecn"></xref>).</t>
<t>Thirdly, the sender may make a leap of faith that ECN will work.
This is only recommended for applications that know they are running
in controlled environments where ECN functionality has been verified
through other means. In this mode it is assumed that ECN works, and
the system reacts to failure indicators if the assumption proved
wrong. The use of this method relies on a high confidence that ECN
operation will be successful, or an application where failure is not
serious. The impact on the network and other users must be
considered when making a leap of faith, so there are limitations on
when this method is allowed.</t>
</list>The first mechanism, using RTP with RTCP feedback, has the
advantage of working for all RTP sessions, but the disadvantages of
potential clipping if ECN marked RTP packets are discarded by
middleboxes, and slow verification of ECN support. The STUN-based
mechanism is faster to verify ECN support, but only works in those
scenarios supported by end-to-end STUN, such as within an ICE exchange.
The third one, leap-of-faith, has the advantage of avoiding additional
tests or complexities and enabling ECN usage from the first media
packet. The downside is that if the end-to-end path contains middleboxes
that do not pass ECN, the impact on the application can be severe: in
the worst case, all media could be lost if a middlebox that discards ECN
marked packets is present. A less severe effect, but still requiring
reaction, is the presence of a middlebox that re-marks ECT marked
packets to non-ECT, possibly marking packets with a CE mark as non-ECT.
This can force the network into heavy congestion due to
non-responsiveness, and seriously impact media quality.</t>
<t>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, unless no timely feedback is
required. An RTCP feedback report is sent as soon as possible according
to the transmission rules for feedback that are in place. This feedback
report indicates the receipt of new CE marks since the last ECN feedback
packet, and also counts the total number of CE marked packets through a
cumulative 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.
<xref target="sec-ongoing"></xref> describes the ongoing use of ECN
within an RTP session.</t>
<t>This rapid feedback is not optimised for reliability, therefore an
additional procedure, the RTCP ECN summary reports, is used to ensure
more reliable, but less timely, reporting of the ECN information. The
ECN summary report contains the same information as the ECN feedback
format, only packed differently for better efficiency with reports for
many sources. It is sent in a compound RTCP packet, along with regular
RTCP reception reports. By using cumulative counters for seen CE, ECT,
not-ECT, and packet loss the sender can determine what events have
happened since the last report, independently of any RTCP packets having
been lost.</t>
<t>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.</t>
<t>There are numerous reasons why the path the RTP packets take from the
sender to the receiver may change, e.g., mobility, link failure followed
by re-routing around it. Such an event may result in the packet being
sent through a node that is ECN non-compliant, thus re-marking or
dropping packets with ECT set. To prevent this from impacting the
application for longer than necessary, the operation of ECN is
constantly monitored by all senders. 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 packets received with the number that
were sent, while also reporting CE marked and lost packets. If these
numbers do not agree, it can be inferred that the path does not reliably
pass ECN-marked packets (<xref target="sec-interpret"></xref> discusses
how to interpret the different cases). A sender detecting a possible ECN
non-compliance issue should then stop sending ECT marked packets to
determine if that allows the packets to be correctly delivered. If the
issues can be connected to ECN, then ECN usage is suspended and possibly
also re-negotiated.</t>
</section>
<section anchor="sec-rtcp-ecn" title="RTCP Extensions for ECN feedback">
<t>This documents defines two different RTCP extensions: one RTP/AVPF
<xref target="RFC4585"></xref> transport layer feedback format for
urgent ECN information, and one RTCP XR <xref target="RFC3611"></xref>
ECN summary report block type for regular reporting of the ECN marking
information. The full definition of these extensions usage as part of
the complete solution is laid out in <xref
target="sec-definition"></xref>.</t>
<section anchor="sec-rtcp-ecn-fb"
title="RTP/AVPF Transport Layer ECN Feedback packet">
<t>This RTP/AVPF transport layer feedback format is intended for usage
in AVPF early or immediate feedback modes when information needs to
urgently reach the sender. Thus its main use is to report on reception
of an ECN-CE marked RTP packet so that the sender may perform
congestion control, or to speed up the initiation procedures by
rapidly reporting that the path can support ECN-marked traffic. The
feedback format is also defined with <xref target="RFC5506">reduced
size RTCP</xref> in mind, where RTCP feedback packets may be sent
without accompanying Sender or Receiver Reports that would contain the
Extended Highest Sequence number and the accumulated number of packet
losses. Both are important for ECN to verify functionality and keep
track of when CE marking does occur.</t>
<t>The RTP/AVPF transport layer feedback packet starts with the common
header defined by the <xref target="RFC4585">RTP/AVPF profile</xref>
which is reproduced here for the reader's information:</t>
<figure anchor="fig-avpf-common"
title="RTP/AVPF Common Packet Format for Feedback Messages">
<artwork><![CDATA[
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=RTPFB=205 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Feedback Control Information (FCI) :
: :
]]></artwork>
</figure>
<t>From <xref target="fig-avpf-common"></xref> 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].</t>
<figure anchor="fig-ecn-feedback" title="ECN Feedback Format">
<artwork><![CDATA[
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>The FCI information for the <xref target="fig-ecn-feedback">ECN
Feedback format</xref> are the following:</t>
<t><list style="hanging">
<t hangText="Extended Highest Sequence Number:">The least
significant 20-bits from an Extended highest sequence number
received value as defined by <xref target="RFC3550"></xref>. Used
to indicate for which packet this report is valid up to.</t>
<t hangText="Lost Packets Counter:">The cumulative number of RTP
packets that the receiver expected to receive from this SSRC,
minus the number of packets it actually received. This is the same
as the cumulative number of packets lost defined in Section 6.4.1
of <xref target="RFC3550"></xref> except represented in 12-bit
signed format, compared to 24-bit in RTCP SR or RR packets. As
with the equivalent value in RTCP SR or RR packets, note that
packets that arrive late are not counted as lost, and the loss may
be negative if there are duplicates.</t>
<!--We appear to have a saturation issue with the packet loss counter if more than 11 bits
of losses-duplication or duplications-loss happens in a session. Maybe we should change
the format to a sign bit and a wrapping counter?-->
<t hangText="CE Counter:">The cumulative number of RTP packets
received from this SSRC since the receiver joined the RTP session
that were ECN-CE marked. 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.</t>
<t hangText="ECT(0) Counter:">The cumulative number of RTP packets
received from this SSRC since the receiver joined the RTP session
that had an ECN field value of 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 have been received.</t>
<t hangText="ECT(1) Counter:">The cumulative number of RTP packets
received from this SSRC since the receiver joined the RTP session
that had an ECN field value of 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 have been received.</t>
<t hangText="not-ECT Counter:">The cumulative number of RTP
packets received from this SSRC since the receiver joined the RTP
session that had an ECN field value of not-ECT. The receiver
should keep track of this value using a local representation that
is 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 have been received.</t>
</list>Each FCI block reports on a single source (SSRC). 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.</t>
<t>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.</t>
<t>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 fact regular RTCP
reporting will include the longer versions of the fields and there
will be less of an issue with wrapping unless the packet rate of the
application is so high that the fields will wrap within a regular RTCP
reporting interval. In that case the feedback packet will need to be
sent in a compound packet together with the SR or RR packet.</t>
<t>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.</t>
<t>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.</t>
</section>
<section anchor="sec-ecn-summary-report"
title="RTCP XR Report block for ECN summary information">
<t>This unilateral XR 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.</t>
<t>The ECN Summary report block consists of one report block
header:<figure>
<artwork><![CDATA[ 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure></t>
<t>and then followed of one or more of the following report data
blocks:</t>
<t><figure>
<artwork><![CDATA[ 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t><list style="hanging">
<t hangText="BT:">Block Type identifying the ECN summary report
block. Value is [TBA2].</t>
<t hangText="Reserved:">All bits SHALL be set to 0 on transmission
and ignored on reception.</t>
<t hangText="Block Length:">The length of the report block. Used
to indicate the number of report data blocks present in the ECN
summary report. This length will be 3*n, where n is the number of
ECN summary report blocks, since blocks are a fixed size.</t>
<t hangText="SSRC of Media Sender:">The SSRC identifying the media
sender this report is for.</t>
<t hangText="CE Counter:">as in <xref
target="sec-rtcp-ecn-fb"></xref>.</t>
<t hangText="ECT(0) Counter:">as in <xref
target="sec-rtcp-ecn-fb"></xref>.</t>
<t hangText="ECT(1) Counter:">as in <xref
target="sec-rtcp-ecn-fb"></xref>.</t>
<t hangText="not-ECT Counter:">as in <xref
target="sec-rtcp-ecn-fb"></xref>.</t>
</list></t>
<t>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 rely on
the information sent in the Sender Report (SR) or Receiver Report (RR)
blocks part of the same RTCP compound packet. The information
available in SR or RR are the Extended Highest Sequence number and the
accumulated number of packet losses.</t>
<t>All the SSRCs that are present in the SR or RR SHALL also be
included in the RTCP XR ECN summary report. In cases where the number
of senders are so large that the combination of SR/RR and the ECN
summary for all the senders exceed the MTU, then only a subset of the
senders SHOULD be included so that the reports for the subset fits
within the MTU. The subsets SHOULD be selected round-robin across
multiple intervals so that all sources are reported.</t>
</section>
</section>
<section anchor="sec-sdp-ext" title="SDP Signalling Extensions for ECN">
<t>This section defines a number of SDP signalling extensions used in
the negotiation of the ECN for RTP support when using SDP. This include
one SDP attribute "ecn-capable-rtp" that negotiates the actual operation
of ECN for RTP. Two SDP signalling parameters are defined to indicate
the usage of the RTCP XR ECN summary block and the AVPF feedback format
for ECN. One ICE option SDP reprensenation is also defined.</t>
<section anchor="sec-sdp-ecn"
title="Signalling ECN Capability using SDP">
<t>One new SDP attribute, "a=ecn-capable-rtp", is defined. This is a
media level attribute, thus it is normally included as part of the
media description, but if present at session level the same
configuration applies to all media descriptions. It is not subject to
the character set chosen. The aim of this signalling is to indicate
the capability of the sender and receivers to support ECN, and to
negotiate the method of ECN initiation to be used in the session. The
attribute takes a list of initiation methods, ordered in decreasing
preference. The defined values for the initiation method are:</t>
<t><list style="hanging">
<t hangText="rtp:">Using RTP and RTCP as defined in <xref
target="sec-rtp-init-ecn"></xref>.</t>
<t hangText="ice:">Using STUN within ICE as defined in <xref
target="sec-stun-init-ecn"></xref>.</t>
<t hangText="leap:">Using the leap of faith method as defined in
<xref target="sec-leap-init-ecn"></xref>.</t>
</list></t>
<t>Further methods may be specified in the future, so unknown methods
MUST be ignored upon reception.</t>
<t>In addition, a number of OPTIONAL parameters may be included in the
"a=ecn-capable-rtp" attribute as follows:</t>
<t><list style="hanging">
<t hangText="mode:">This parameter signals the endpoint's
capability to set and read ECN marks in UDP packets. An
examination of various operating systems has shown that end-system
support for ECN marking of UDP packets may be symmetric or
asymmetric. By this we mean that some systems may allow end points
to set the ECN bits in an outgoing UDP packet but not read them,
while others may allow applications to read the ECN bits but not
set them. This either/or case may produce an asymmetric support
for ECN and thus should be conveyed in the SDP signalling. The
"mode=setread" state is the ideal condition where an endpoint can
both set and read ECN bits in UDP packets. The "mode=setonly"
state indicates that an endpoint can set the ECT bit, but cannot
read the ECN bits from received UDP packets to determine if
upstream congestion occurred. The "mode=readonly" state indicates
that the endpoint can read the ECN bits to determine if congestion
has occurred for incomming packet, 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".</t>
<t hangText="ect:">This parameter makes it possible to express the
preferred ECT marking. This is either "random", "0", or "1", with
"0" being implied if not specified. The "ect" parameter describes
a receiver preference, and is useful in the case where the
receiver knows it is behind a link using IP header compression,
the efficiency of which would be seriously disrupted if it were to
receive packets with randomly chosen ECT marks. It is RECOMMENDED
that ECT(0) marking be used.</t>
</list></t>
<t>The <xref target="RFC5234">ABNF</xref> grammar for the
"a=ecn-capable-rtp" attribute is as follows:</t>
<t><figure>
<artwork><![CDATA[ ecn-attribute = "a=ecn-capable-rtp:" SP init-list [SP parm-list]
init-list = init-value *("," init-value)
init-value = "rtp" / "ice" / "leap" / init-ext
init-ext = token
parm-list = parm-value *(";" SP parm-value)
parm-value = mode / ect / parm-ext
mode = "mode=" ("setonly" / "setread" / "readonly")
ect = "ect=" ("0" / "1" / "random")
parm-ext = parm-name "=" parm-value-ext
parm-name = token
parm-value-ext = token / quoted-string
quoted-string = DQUOTE *qdtext DQUOTE
qdtext = %x20-21 / %x23-7E / %x80-FF
; any 8-bit ascii except <">
; external references:
; token: from RFC 4566
; SP and DQUOTE from RFC 5234]]></artwork>
</figure></t>
<t>When SDP is used with the offer/answer model <xref
target="RFC3264"></xref>, 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 any number of
optional parameters following. The answering party compares the list
of initiation methods in the offer with those it supports in order of
preference. If there is a match, and if the receiver wishes to attempt
to use ECN in the session, it includes an "a=ecn-capable-rtp"
attribute containing its single preferred choice of initiation method
in the media sections of the answer. If there is no matching
initiation method capability, or if the receiver does not wish to
attempt to use ECN in the session, it does not include an
"a=ecn-capable-rtp" attribute in its answer. If the attribute is
removed in the answer then ECN MUST NOT be used in any direction for
that media flow. If there are initilization methods that are unknown,
they MUST be ignored on reception and MUST NOT be included in an
answer. The answer may also include optional parameters, as discussed
below.</t>
<t>If the "mode=setonly" parameter is present in the
"a=ecn-capable-rtp" attribute of the offer and the answering party is
also "mode=setonly", then there is no common ECN capability, and the
answer MUST NOT include the "a=ecn-capable-rtp" attribute. Otherwise,
if the offer is "mode=setonly" then ECN may only be initiated in the
direction from the offering party to the answering party.</t>
<t>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.</t>
<t>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 or the
answer.</t>
<t>In an RTP session using multicast all participants intending to
send RTP packets needs support setting ECT in the RTP packets, and all
participants receiving needs to have the capability to read ECN values
on incoming packets. Especially the later is important, otherwise no
sender in the multicast session will be able to enable ECN. If a
session is negotiated using offer/answer it is preferable that
intended session participant would be aware of the signalling
attributes and if not capable but ECN for RTP aware SHOULD refuse to
join the session. For intended session participants that are not aware
of the ECN for RTP signalling and simple ignore the signalling
attribute the other party in the offer/answer exchange SHOULD
terminate the SIP dialog so that the participant leaves the session.
</t>
<t>The "ect=" parameter in the "a=ecn-capable-rtp" attribute is set
independently in the offer and the answer. Its value in the offer
indicates a preference for the sending behaviour of the answering
party, and its value in the answer indicates a sending preference for
the behaviour of the offering party. It will be the senders choice to
honour the receivers preference for what to receive or not. In
multicast sessions, any sender SHOULD send using the value declared in
the ect parameter.</t>
<t>Unknown optional parameters MUST be ignored on reception, and MUST
NOT be included in the answer. That way new parameters may be
introduced and verified to be supported by the other end-point by
having them include it in any answer.</t>
<t>When SDP is used in a declarative manner, for example in a
multicast session using the Session Announcement Protocol (SAP, <xref
target="RFC2974"></xref>), negotiation of session description
parameters is not possible. The "a=ecn-capable-rtp" attribute MAY be
added to the session description to indicate that the sender will use
ECN in the RTP session. The attribute MUST include a single method of
initiation. Participants MUST NOT join such a session unless they have
the capability to receive 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). The
mode parameter MAY be included also in declarative usage, to indicate
the minimal capability is required by the consumer of the SDP. So for
example in a SSM session the participants configured with a particular
SDP will all be in a media receive only mode, thus mode=readonly will
work as the capability of reporting on the ECN markings in the
received is what is required. However, using "mode=readonly" also in
ASM sessions is reasonable, unless all senders are required to attempt
to use ECN for their outgoing RTP data traffic, in which case the mode
needs to be set to "setread".</t>
<t>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.</t>
<t>As described in <xref target="sec-congestion"></xref>, RTP sessions
using ECN require rapid RTCP ECN feedback, unless timely feedback is
not required due to a receiver driven congestion control. To ensure
that the sender can react to ECN-CE marked packets timely feedback is
usually required. Thus, the use of the Extended RTP Profile for
RTCP-Based Feedback (RTP/AVPF) <xref target="RFC4585"></xref> or other
profile that inherits AVPF's signalling rules, MUST be signalled
unless timely feedback is not required. If timely feedback is not
required it is still RECOMMENDED to used AVPF. The signalling of an
AVPF based profile is likely to be required even if the preferred
method of initialization and the congestion control does not require
timely feedback, as the common interoperable method is likely to be
signalled or the improved fault reaction is desired.</t>
</section>
<section anchor="sec-fb-sdp-par" title="RTCP Feedback SDP Parameter">
<t>A new "nack" feedback parameter "ecn" is defined to indicate the
usage of the RTCP ECN feedback packet <xref target="sec-rtcp-ecn-fb">
format</xref>. The ABNF <xref target="RFC5234"></xref> definition of
the SDP parameter extension is:</t>
<t><figure>
<artwork><![CDATA[rtcp-fb-nack-param = <See section 4.2 of RFC 4585>
rtcp-fb-nack-param /= ecn-fb-par
ecn-fb-par = SP "ecn"
]]></artwork>
</figure>The offer/answer rules for this SDP feedback parameters are
specified in <xref target="RFC4585">AVPF</xref>.</t>
</section>
<section anchor="sec-xr-sdp-par" title="XR Block SDP Parameter">
<t>A new unilateral RTCP XR block for ECN summary information is
specified, thus the XR block SDP signalling also needs to be extended
with a parameter. This is done in the same way as for the other XR
blocks. The XR block SDP attribute as defined in Section 5.1 of the
<xref target="RFC3611">RTCP XR specification</xref> is defined to be
extendible. As no parameter values are needed for this ECN summary
block, this parameter extension consistis of a simple parameter name
used to indicate support and intent to use the XR block.</t>
<t><figure>
<artwork><![CDATA[xr-format = <See Section 5.1 of [RFC3611]>
xr-format /= ecn-summary-par
ecn-summary-par = "ecn-sum"
]]></artwork>
</figure></t>
<t>For SDP declarative and offer/answer usage, see the RTCP XR
specification<xref target="RFC3611"> </xref> and its specifciation of
how to handle unilateral parameters.</t>
</section>
<section anchor="sec-ice-ecn"
title="ICE Parameter to Signal ECN Capability">
<t>One new ICE <xref target="RFC5245"></xref> option, "rtp+ecn", is
defined. This is used with the SDP session level "a=ice-options"
attribute in an SDP offer to indicate that the initiator of the ICE
exchange has the capability to support ECN for RTP-over-UDP flows (via
"a=ice-options: rtp+ecn"). The answering party includes this same
attribute at the session level in the SDP answer if it also has the
capability, and removes the attribute if it does not wish to use ECN,
or doesn't have the capability to use ECN. If the ICE initiation
method (<xref target="sec-stun-init-ecn"></xref>) actually is going to
be used, it is also needs to be explicitly negotiated using the
"a=ecn-capable-rtp" attribute. This ICE option SHALL be included when
the ICE initiation method is offered or declared in the SDP. </t>
<t><list style="empty">
<t>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.</t>
</list></t>
</section>
</section>
<section anchor="sec-definition" title="Use of ECN with RTP/UDP/IP">
<t>In the detailed specification of the behaviour below, the different
functions in the general case will first be discussed. In case special
considerations are needed for middleboxes, multicast usage etc, those
will be specially discussed in related subsections.</t>
<section anchor="sec-signalling" title="Negotiation of ECN Capability">
<t>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 and the method for initial ECT verification. This
memo defines the mappings of this information onto SDP for both
declarative and offer/answer usage. There is one SDP extension to
indicate if ECN support should be used, and the method for <xref
target="sec-sdp-ecn">initiation</xref>. Further parameters to indicate
support for the <xref target="sec-fb-sdp-par">AVPF ECN feedback
format</xref> and the <xref target="sec-xr-sdp-par">ECN XR summary
report</xref>. In addition an <xref target="sec-ice-ecn">ICE parameter
is defined</xref> to indicate that ECN initiation using STUN is
supported as part of an ICE exchange.</t>
<t>An RTP system that supports ECN and uses SDP in the signalling MUST
implement the SDP extension to signal ECN capability as described in
<xref target="sec-sdp-ecn"></xref>, the ECN feedback SDP parameter
<xref target="sec-fb-sdp-par"></xref>, and the ECN XR SDP parameter
<xref target="sec-xr-sdp-par"></xref>. It MAY also implement
alternative ECN capability negotiation schemes, such as the ICE
extension described in <xref target="sec-ice-ecn"></xref>.</t>
<t>The "ecn-capable-rtp" SDP attribute MUST always be used when
employing ECN for RTP according to this specification. As the XR ECN
summary report is required independently of the initialization method,
or congestion control scheme the "rtcp-xr" attribute with the
"ecn-sum" parameter MUST also be used. The "rtcp-fb" attribute with
the "nack" parameter "ecn" MUST be used whenever the initialization
method or a congestion control algorithm requiring timely sender side
knowledge of received CE markings. If the congestion control scheme
uses additional signalling they should be indicated as appropriate for
those signalling methods. </t>
</section>
<section anchor="sec-initiation"
title="Initiation of ECN Use in an RTP Session">
<t>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.</t>
<t>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 their destination(s). There is some
risk that the use of ECN will result in either reset of the ECN field,
or loss of all packets with ECT or ECN-CE markings. If the path
between the sender and the receivers exhibits either of these
behaviours one needs to stop using ECN immediately to protect both the
network and the application.</t>
<t>The RTP senders and receivers SHALL NOT ECT mark their RTCP traffic
at any time. This is to ensure that packet loss due to ECN marking
will not effect the RTCP traffic and the necessary feedback
information it carries.</t>
<t>An RTP system that supports ECN MUST implement the initiation of
ECN using in-band RTP and RTCP described in <xref
target="sec-rtp-init-ecn"></xref>. It MAY also implement other
mechanisms to initiate ECN support, for example the STUN-based
mechanism described in <xref target="sec-stun-init-ecn"></xref> or use
the leap of faith option if the session supports the limitations
provided in <xref target="sec-leap-init-ecn"></xref>. If support for
both in-band and out-of-band 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.</t>
<t>No matter how ECN usage is initiated, the sender MUST continually
monitor the ability of the network, and all its receivers, to support
ECN, following the mechanisms described in <xref
target="sec-ecn-failure"></xref>. This is necessary because path
changes or changes in the receiver population may invalidate the
ability of the system to use ECN.</t>
<section anchor="sec-rtp-init-ecn"
title="Detection of ECT using RTP and RTCP">
<t>The ECN initiation phase using RTP and RTCP to detect if the
network path supports ECN comprises three stages. Firstly, the RTP
sender generates some small fraction of its traffic with ECT marks
to act 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.</t>
<t><list style="hanging">
<t hangText="Generating ECN Probe Packets:">During the ECN
initiation phase, an RTP sender SHALL mark a small fraction of
its RTP traffic as ECT, while leaving the reminder of the
packets unmarked. The main reason for only marking some packets
is to maintain usable media delivery during the ECN initiation
phase in those cases where ECN is not supported by the network
path. A secondary reason to send some not-ECT packets are to
ensure that the receivers will send RTCP reports on this sender,
even if all ECT marked packets are lost in transit. The not-ECT
packets also provide a base-line to compare performance
parameters against. A fourth reason for only probing with a
small number of packets is to reduce the risk that significant
numbers of congestion markings might be lost if ECT is cleared
to Not-ECT by an ECN-Reverting Meddlebox. Then any resulting
lack of congestion response is likely to have little damaging
affect on others. An RTP sender is RECOMMENDED to send a minimum
of two packets with ECT markings per RTCP reporting interval. In
case an random ECT pattern is intended to be used, at least one
with ECT(0) and one with ECT(1) per reporting interval, in case
a single ECT marking is to be used, only that ECT value SHOULD
be sent. The RTP sender 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.</t>
<t>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 <xref target="I-D.ietf-avt-rtp-no-op">NO-OP
payloads</xref>, 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 sent in duplicate with and without
ECT, since their RTP sequence number is used to identify packets
that are received with ECN markings.</t>
<t hangText="Generating RTCP ECN Feedback:">If ECN capability
has been negotiated in an RTP session, the receivers in the
session MUST listen for ECT or ECN-CE marked RTP packets, and
generate RTCP ECN feedback packets (<xref
target="sec-rtcp-ecn-fb"></xref>) to mark their receipt. An
immediate or early (depending on the RTP/AVPF mode) ECN feedback
packet SHOULD be generated on receipt of the first ECT or ECN-CE
marked packet from a sender that has not previously sent any ECT
traffic. Each regular RTCP report MUST also contain an ECN
summary report (<xref target="sec-ecn-summary-report"></xref>).
Reception of subsequent ECN-CE marked packets MUST result in
additional early or immediate ECN feedback packets being sent
unless no timely feedback is required.</t>
<t hangText="Determination of ECN Support:">RTP is a group
communication protocol, where members can join and leave the
group at any time. This complicates the ECN initiation phase,
since the sender must wait until it believes the group
membership has stabilised before it can determine if the paths
to all receivers support ECN (group membership changes after the
ECN initiation phase has completed are discussed in <xref
target="sec-ongoing"></xref>).</t>
<t>An RTP sender shall consider the group membership to be
stable after it has been in the session and sending ECT-marked
probe packets for at least three RTCP reporting intervals (i.e.,
after sending its third regularly scheduled RTCP packet), and
when a complete RTCP reporting interval has passed without
changes to the group membership. ECN initiation is considered
successful when the group membership is stable, and all known
participants have sent one or more RTCP ECN feedback packets
indicating correct receipt of the ECT-marked RTP packets
generated by the sender.</t>
<t>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 <xref
target="sec-ongoing"></xref>, 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. <list
style="empty">
<t>Note: One use case that requires further consideration is
a unicast connection with several SSRCs multiplexed onto the
same flow (e.g., an 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.</t>
<!-- We'll likely need signalling knowledge to be able to
determine if multiple SSRCs belong to a single end-point.
(csp) -->
</list></t>
<t>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.</t>
</list></t>
<t>If the ECN negotiation succeeds, this indicates that the path can
pass some ECN-marked traffic, and that the receivers support ECN
feedback. This does not necessarily imply that the path can robustly
convey ECN feedback; <xref target="sec-ongoing"></xref> describes
the ongoing monitoring that must be performed to ensure the path
continues to robustly support ECN.</t>
<t>When a sender or receiver detects ECN failures on paths they
should log these to enable follow up and statistics gathering
regarding broken paths. The logging mechanism used is implementation
dependent.</t>
</section>
<section anchor="sec-stun-init-ecn"
title="Detection of ECT using STUN with ICE">
<t>This section describes an OPTIONAL method that can be used to
avoid media impact and also ensure an ECN capable path prior to
media transmission. This method is considered in the context where
the session participants are using <xref target="RFC5245">ICE</xref>
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.</t>
<t>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, an additional
connectivity check is performed, sending the "ECT Check" attribute
in a STUN packet that is ECT marked. On reception of the packet, a
STUN server supporting this extension will note the received ECN
field value, and send a STUN/UDP/IP packet in reply, with the ECN
field set to not-ECT, and including an ECN check attribute. A STUN
server that doesn't understand the extension or is incapable of
reading the ECN values on incoming STUN packets should follow the
STUN specifications rule for unknown comprehension-optional
attributes, i.e. ignore the attribute. Which will result in the
sender receiving a STUN response but without the ECN Check STUN
attribute.</t>
<t>The STUN ECN check attribute contains one field and a flag. The
flag indicates whether the echo field contains a valid value or not.
The field is the ECN echo field, and when valid contains the two ECN
bits from the packet it echoes back. The ECN check attribute is a
comprehension optional attribute.</t>
<t><figure anchor="fig-ECN-Check" title="ECN Check STUN Attribute">
<artwork><![CDATA[ 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|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t><list style="hanging">
<t hangText="V:">Valid (1 bit) ECN Echo value field is valid
when set to 1, and invalid when set 0.</t>
<t hangText="ECF:">ECN Echo value field (2 bits) contains the
ECN field value of the STUN packet it echoes back when field is
valid. If invalid the content is arbitrary.</t>
<t hangText="Reserved:">Reserved bits (29 bits) SHALL be set to
0 on transmission, and SHALL be ignored on reception.</t>
</list>This attribute MAY be included in any STUN request to
request the ECN field to be echoed back. In STUN requests the V bit
SHALL be set to 0. A compliant STUN server receiving a request with
the ECN Check attribute SHALL read the ECN field value of the IP/UDP
packet the request was received in. Upon forming the response the
server SHALL include the ECN Check attribute setting the V bit to
valid and include the read value of the ECN field into the ECF
field. If the STUN responder was unable to ascertain, due to
temporary errors, the ECN value of the STUN request, it SHALL set
the V bit in the response to 0. The STUN client may retry
immediately.</t>
</section>
<section anchor="sec-leap-init-ecn"
title="Leap of Faith ECT initiation method">
<t>This method for initiating ECN usage is a leap of faith that
assumes that ECN will work on the used path(s). The method is to go
directly to "ongoing use of ECN" as defined in <xref
target="sec-ongoing"></xref>. Thus all RTP packets MAY be marked as
ECT and the failure detection MUST be used to detect any case when
the assumption that the path was ECT capable is wrong. This method
is only recommended for controlled environments where the whole
path(s) between sender and receiver(s) has been built and verified
to be ECT.</t>
<t>If the sender marks all packets as ECT while transmitting on a
path that contains an ECN-blocking middlebox, then receivers
downstream of that middlebox will not receive any RTP data packets
from the sender, and hence will not consider it to be an active RTP
SSRC. The sender can detect this and revert to sending packets
without ECT marks, since RTCP SR/RR packets from such receivers will
either not include a report for sender's SSRC, or will report that
no packets have been received, but this takes at least one RTCP
reporting interval. It should be noted that a receiver might
generate its first RTCP packet immediately on joining a unicast
session, or very shortly after joining a RTP/AVPF session, before it
has had chance to receive any data packets. A sender that receives
RTCP SR/RR packet indicating lack of reception by a receiver SHOULD
therefore wait for a second RTCP report from that receiver to be
sure that the lack of reception is due to ECT-marking. Since this
recovery process can take several tens of seconds, during which time
the RTP session is unusable for media, it is NOT RECOMMENDED that
the leap-of-faith ECT initiation method be used in environments
where ECN-blocking middleboxes are likely to be present.</t>
</section>
</section>
<section anchor="sec-ongoing"
title="Ongoing Use of ECN Within an RTP Session">
<t>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.</t>
<section title="Transmission of ECT-marked RTP Packets">
<t>After a sender has successfully initiated ECN usage, it SHOULD
mark all the RTP data packets it sends as ECT. The sender SHOULD
mark packets as ECT(0) unless the receiver expresses a preference
for ECT(1) or random using the "ect" parameter in the
"a=ecn-capable-rtp" attribute.</t>
<t>The sender SHALL NOT include ECT marks on outgoing RTCP packets,
and SHOULD NOT include ECT marks on any other outgoing control
messages (e.g. <xref target="RFC5389">STUN</xref> packets, <xref
target="RFC4347">DTLS</xref> handshake packets, or <xref
target="I-D.zimmermann-avt-zrtp">ZRTP</xref> control packets) that
are multiplexed on the same UDP port. For control packets there
might be exceptions, like the STUN based ECN check defined in <xref
target="sec-stun-init-ecn"></xref>.</t>
</section>
<section title="Reporting ECN Feedback via RTCP">
<t>An RTP receiver that receives a packet with an ECN-CE mark, or
that detects a packet loss, MUST schedule the transmission of an
RTCP ECN feedback packet as soon as possible (subject to the
constraints of <xref target="RFC4585"></xref> and <xref
target="RFC3550"></xref>) to report this back to the sender unless
no timely feedback required. There should be no difference in
behavior if ECN-CE marks or packet drops are detected. The feedback
RTCP packet sent SHALL consist of at least one ECN feedback packet
(<xref target="sec-rtcp-ecn"></xref>) 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 <xref target="RFC5506"></xref> 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.</t>
<t>Every time a regular compound RTCP packet is to be transmitted,
an ECN-capable RTP receiver MUST include an RTCP XR ECN summary
report as described in <xref target="sec-ecn-summary-report"></xref>
as part of the compound packet.</t>
<t>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 in a reasonably timely manner,
allowing the sender to react on a single or a few reports. <list
style="empty">
<t>A possible future optimisation might be to define some form
of feedback suppression mechanism to reduce the RTCP reporting
overhead for group communication using ECN.</t>
</list></t>
</section>
<section anchor="sec-congestion"
title="Response to Congestion Notifications">
<t>The reception of RTP packets with ECN-CE marks in the IP header
are a notification that congestion is being experience. The default
reaction on the reception of these ECN-CE marked packets MUST be to
provide the congestion control algorithm with notification and that
it is treated as a packet loss would when it comes to indicating
congestion. </t>
<t>We note that there MAY be other reactions to ECN-CE specified in
the future. Such an alternative reaction MUST be specified and
considered to be safe for deployment under any restrictions
specified. A potential example for an alternative reaction could be
emergency communications (such as that generated by first
responders, as opposed to the general public) in networks where the
user has been authorized. A more detailed description of these other
reactions, as well as the types of congestion control algorithms
used by end-nodes, is outside of the scope of this document. </t>
<t>Depending on the media format, type of session, and RTP topology
used, there are several different types of congestion control that
can be used. </t>
<t><list style="hanging">
<t hangText="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 <xref
target="RFC5348"></xref> is one example that might be used.</t>
<t hangText="Receiver-Driven Congestion Control:">If a 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 XR ECN
Summary to the sender, even if it can adapt without contact with
the sender, so that the sender can determine if ECN is supported
on the network path. The timeliness of RTCP feedback is less of
a concern with receiver driven congestion control, and regular
RTCP reporting of ECN summary information is sufficient (without
using RTP/AVPF immediate or early feedback).</t>
<t hangText="Hybrid:">There might be mechanisms that utilize
both some receiver behaviors and some sender side monitoring,
thus requiring both feedback of congestion events to the sender
and taking receiver decisions and possible signalling to the
sender. From this solution the congestion control algorithm
needs to use the signalling to indicate which functions of ECN
that is needed to be used.</t>
</list></t>
<t>Responding to congestion indication in the case of multicast
traffic is a more complex problem than for unicast traffic. The
fundamental problem is diverse paths, i.e. when different receivers
don't see the same path, and thus have different bottlenecks, so the
receivers may get ECN-CE marked packets due to congestion at
different points in the network. This is problematic for sender
driven congestion control, since when receivers are heterogeneous in
regards to capacity the sender is limited to transmitting at the
rate the slowest receiver can support. This often becomes a
significant limitation as group size grows. Also, as group size
increases the frequency of reports from each receiver decreases,
which further reduces the responsiveness of the mechanism.
Receiver-driven congestion control has the advantage that each
receiver can choose the appropriate rate for its network path,
rather than all having to settle for the lowest common rate.</t>
<t>We note that ECN support is not a silver bullet to improving
performance. The use of ECN gives the chance to respond to
congestion before packets are dropped in the network, improving the
user experience by allowing the RTP application to control how the
quality is reduced. An application which ignores ECN congestion
experienced feedback is not immune to congestion: the network will
eventually begin to discard packets if traffic doesn't respond. It
is in the best interest of an application to respond to ECN
congestion feedback promptly, to avoid packet loss.</t>
</section>
</section>
<section anchor="sec-ecn-failure" title="Detecting Failures">
<t>Senders and receivers can deliberately ignore ECN-CE and thus get a
benefit over behaving flows (cheating). Nonce <xref
target="RFC3540"></xref> is an addition to TCP that solves this issue
as long as the sender 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 use of RTP. Still a significant
portion of RTP senders are infrastructure devices (for example,
streaming media servers) that do have an interest in protecting both
service quality and the network. Even though there may be cases where
nonce can be applicable also for RTP, it is not included in this
specification. This as a receiver interested in cheating would simple
claim to not support Nonce. It is however worth mention that, as
real-time media is commonly 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 they will
affect media quality.</t>
<t>RTP sessions can also suffer from path changes resulting in a
non-ECN compliant node becoming part of the path. That node may
perform either of two actions that has 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 sender
has sent 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 re-mark a packet from
ECT or CE to not-ECT. That has less dire results, however, it should
be detected so that ECN usage can be suspended to prevent misusing the
network.</t>
<t>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 unless there is duplication in the network. 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
re-marks the packets. In both cases the usage of ECN is broken on the
path. By tracking all the different possible ECN field values a sender
can quickly detect if some non-compliant behavior is happing on the
path.</t>
<t>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.</t>
<section anchor="sec-fallback" title="Fallback mechanisms">
<t>Upon the detection of a potential failure both the sender and the
receiver can react to mitigate the situation.</t>
<t>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.</t>
<t>A sender that detects high packet loss rates for ECT-marked
packets SHOULD immediately switch to sending packets as not-ECT to
determine if the losses potentially are due to the ECT markings. If
the losses disappear when the ECT-marking is discontinued, the RTP
sender should go back to initiation procedures to attempt to verify
the apparent loss of ECN capability of the used path. If a
re-initiation fails then the two possible actions exist:</t>
<t><list style="numbers">
<t>Periodically retry the ECN initiation to detect if a path
change occurs to a path that is ECN capable.</t>
<t>Renegotiating the session to disable ECN support. This is 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.</t>
</list>We foresee the possibility of flapping ECN capability due
to several reasons: video switching MCU or similar middleboxes that
selects to deliver media from the sender only intermittently; load
balancing devices may in worst case result in that some packets take
a different network path then the others; mobility solutions that
switch underlying network path in a transparent way for the sender
or receiver; and membership changes in a multicast group. It is
however appropriate to mention that there are also issues such as
re-routing of traffic due to a flappy route table or excessive
reordering and other issues that are not directly ECN related but
nevertheless may cause problems for ECN.</t>
</section>
<section anchor="sec-interpret"
title="Interpretation of ECN Summary information">
<t>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:</t>
<t><list style="hanging">
<t hangText="CE Counter:">The number of RTP packets received so
far in the session with an ECN field set to CE (11b).</t>
<t hangText="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).</t>
<t hangText="not-ECT Counter:">The number of RTP packets
received so far in the session with an ECN field set to not-ECT
(00b)</t>
<t hangText="Lost Packets counter:">The number of RTP packets
that are expected minus the number received.</t>
<t hangText="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.</t>
</list>The counters will be initiated to zero to 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, since if one report is missing, the
reporting interval becomes longer, but is otherwise equally
valid.</t>
<t>In a perfect world the number of not-ECT packets received should
be equal to the number sent minus the lost packets counter, and the
sum of the ECT(0), ECT(1), and CE counters should be equal to the
number of ECT marked packet sent. Two issues may cause a mismatch in
these statistics: severe network congestion or unresponsive
congestion control might cause some ECT-marked packets to be lost,
and packet duplication might result in some packets being received,
and counted in the statistics, multiple times (potentially with a
different ECN-mark on each copy of the duplicate).</t>
<t>The level of packet duplication included in the report can be
estimated from the sum over all of fields counting received packets
compared to the number of packets sent. A high level of packet
duplication increases the uncertainty in the statistics, making it
more difficult to draw firm conclusions about the behaviour of the
network. This issue is also present with standard RTCP reception
reports.</t>
<t>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.</t>
<t>Dropping of ECT packets: To determine if the packet drop ratio is
different between not-ECT and ECT marked transmission requires a mix
of transmitted traffic. The sender should compare if the delivery
percentage (delivered / transmitted) between ECT and not-ECT is
significantly different. Care must be taken if the number of packets
are low in either of the categories. One must also take into account
the level of CE marking. A CE marked packet would have been dropped
unless it was ECT marked. Thus, the packet loss level for not-ECT
should be aprroximately equal to the loss rate for ECT when counting
the CE marked packets as lost ones. A sender performing this
calculation needs to ensure that the difference is statistcally
significant.</t>
<t>If erronous behavior is detected, it should be logged to enable
follow up and statistics gathering.</t>
</section>
</section>
</section>
<section anchor="sec-rtcp-translator-mixer"
title="Processing RTCP ECN Feedback in RTP Translators and Mixers">
<t>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. This includes both downstream RTCP
reports generated by the media sender, and also reports generated by the
receivers, flowing upstream back towards the sender.</t>
<section anchor="sec-rtcp-ecn-translator"
title="Fragmentation and Reassembly in Translators">
<t>An RTP translator may fragment or reassemble RTP data packets
without changing the media encoding, and without reference to the
congestion state of the networks it bridges. An example of this might
be to combine packets of a voice-over-IP stream coded with one 20ms
frame per RTP packet into new RTP packets with two 20ms frames per
packet, thereby reducing the header overheads and so stream bandwidth,
at the expense of an increase in latency. If multiple data packets are
re-encoded into one, or vice versa, the RTP translator MUST assign new
sequence numbers to the outgoing packets. Losses in the incoming RTP
packet stream may also induce corresponding gaps in the outgoing RTP
sequence numbers. An RTP translator MUST rewrite RTCP packets to make
the corresponding changes to their sequence numbers, and to reflect
the impact of the fragmentation or reassembly. This section describes
how that rewriting is to be done for RTCP ECN feedback packets.
Section 7.2 of <xref target="RFC3550"></xref> describes general
procedures for other RTCP packet types.</t>
<t>RTCP ECN feedback packets (<xref target="sec-rtcp-ecn-fb"></xref>)
contain six fields that are rewritten in an RTP translator that
fragments or reassembles packets: the extended highest sequence
number, the lost packets counter, the CE counter, and not-ECT counter,
the ECT(0) counter, and the ECT(1) counter. The RTCP XR report block
for ECN summary information (<xref
target="sec-ecn-summary-report"></xref>) includes a subset of these
fields excluding the extended highest sequence number and lost packets
counter. The procedures for rewriting these fields are the same for
both types of RTCP ECN feedback packet.</t>
<t>When receiving an RTCP ECN feedback packet for the translated
stream, an RTP translator first determines the range of packets to
which the report corresponds. The extended highest sequence number in
the RTCP ECN feedback packet (or in the RTCP SR/RR packet contained
within the compound packet, in the case of RTCP XR ECN summary
reports) specifies the end sequence number of the range. For the first
RTCP ECN feedback packet received, the initial extended sequence
number of the range may be determined by subtracting the sum of the
lost packets counter, the CE counter, the not-ECT counter, the ECT(0)
counter and the ECT(1) counter from the extended highest sequence
number (this will be inaccurate if there is packet duplication). For
subsequent RTCP ECN feedback packets, the starting sequence number may
be determined as being one after the extended highest sequence number
of the previous RTCP ECN feedback packet received from the same SSRC.
These values are in the sequence number space of the translated
packets.</t>
<t>Based on its knowledge of the translation process, the translator
determines the sequence number range for the corresponding original,
pre-translation, packets. The extended highest sequence number in the
RTCP ECN feedback packet is rewritten to match the final sequence
number in the pre-translation sequence number range.</t>
<t>The translator then determines the ratio, R, of the number of
packets in the translated sequence number space (numTrans) to the
number of packets in the pre-translation sequence number space
(numOrig) such that R = numTrans / numOrig. The counter values in the
RTCP ECN feedback report are then scaled by dividing each of them by
R. For example, if the translation process combines two RTP packets
into one, then numOrig will be twice numTrans, giving R=0.5, and the
counters in the translated RTCP ECN feedback packet will be twice
those in the original.</t>
<!--MW: Do we need to discuss the need for keeping base sequence number and pick new ones
at the front of already translated range every time the R factor changes? -->
<t>The ratio, R, may have a value that leads to non-integer multiples
of the counters when translating the RTCP packet. For example, a VoIP
translator that combines two adjacent RTP packets into one if they
contain active speech data, but passes comfort noise packets
unchanged, would have an R values of between 0.5 and 1.0 depending on
the amount of active speech. Since the counter values in the
translated RTCP report are integer values, rounding will be necessary
in this case.</t>
<t>When rounding counter values in the translated RTCP packet, the
translator should try to ensure that they sum to the number of RTP
packets in the pre-translation sequence number space (numOrig). The
translator should also try to ensure that no non-zero counter is
rounded to a zero value, since that will lose information that a
particular type of event has occurred. It is recognised that it may be
impossible to satisfy both of these constraints; in such cases, it is
better to ensure that no non-zero counter is mapped to a zero value,
since this preserves congestion adaptation and helps the RTCP-based
ECN initiation process.</t>
<t>It should be noted that scaling the RTCP counter values in this way
is meaningful only on the assumption that the level of congestion in
the network is related to the number of packets being sent. This is
likely to be a reasonable assumption in the type of environment where
RTP translators that fragment or reassemble packets are deployed, as
their entire purpose is to change the number of packets being sent to
adapt to known limitations of the network, but is not necessarily
valid in general.</t>
<t>The rewritten RTCP ECN feedback report is sent from the other side
of the translator to that which it arrived (as part of a compound RTCP
packet containing other translated RTCP packets, where
appropriate).</t>
</section>
<section anchor="sec-rtcp-ecn-synthetic"
title="Generating RTCP ECN Feedback in Media Transcoders">
<t>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 <xref
target="RFC3550"></xref> describes general procedures for other RTCP
packet types.</t>
<t>An RTP translator acting as a media transcoder in this manner does
not have its own SSRC, and hence is not visible to other entities at
the RTP layer. RTCP ECN feedback packets and RTCP XR report blocks for
ECN summary information that are received from downstream relate to
the translated stream, and so must be processed by the translator as
if it were the original media source. These reports drive the
congestion control loop and media adaptation between the translator
and the downstream receiver. If there are multiple downstream
receivers, a logically separate transcoder instance must be used for
each receiver, and must process RTCP ECN feedback and summary reports
independently to the other transcoder instances. An RTP translator
acting as a media transcoder in this manner MUST NOT forward RTCP ECN
feedback packets or RTCP XR ECN summary reports from downstream
receivers in the upstream direction.</t>
<t>An RTP translator acting as a media transcoder will generate RTCP
reports upstream towards the original media sender, based on the
reception quality of the original media stream at the translator. The
translator will run a separate congestion control loop and media
adaptation between itself and the media sender for each of its
downstream receivers, and must generate RTCP ECN feedback packets and
RTCP XR ECN summary reports for that congestion control loop using the
SSRC of that downstream receiver.</t>
</section>
<section title="Generating RTCP ECN Feedback in Mixers">
<t>An RTP mixer terminates one-or-more RTP flows, combines them into a
single outgoing media stream, and transmits that new stream as a
separate RTP flow. A mixer has its own SSRC, and is visible to other
participants in the session at the RTP layer.</t>
<t>An ECN-aware RTP mixer must generate RTCP ECN feedback packets and
RTCP XR report blocks for ECN summary information relating to the RTP
flows it terminates, in exactly the same way it would if it were an
RTP receiver. These reports form part of the congestion control loop
between the mixer and the media senders generating the streams it is
mixing. A separate control loop runs between each sender and the
mixer.</t>
<t>An ECN-aware RTP mixer will negotiate and initiate the use of ECN
on the mixed flows it generates, and will accept and process RTCP ECN
feedback reports and RTCP XR report blocks for ECN relating to those
mixed flows as if it were a standard media sender. A congestion
control loop runs between the mixer and its receivers, driven in part
by the ECN reports received.</t>
<t>An RTP mixer MUST NOT forward RTCP ECN feedback packets or RTCP XR
ECN summary reports reports from downstream receivers in the upstream
direction.</t>
</section>
</section>
<section anchor="sec-impl" title="Implementation considerations">
<t>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.</t>
</section>
<section anchor="sec-iana" title="IANA Considerations">
<t>Note to RFC Editor: please replace "RFC XXXX" below with the RFC
number of this memo, and remove this note.</t>
<section title="SDP Attribute Registration">
<t>Following the guidelines in <xref target="RFC4566"></xref>, the
IANA is requested to register one new SDP attribute:<list
style="symbols">
<t>Contact name, email address and telephone number: Authors of
RFCXXXX</t>
<t>Attribute-name: ecn-capable-rtp</t>
<t>Type of attribute: media-level</t>
<t>Subject to charset: no</t>
</list></t>
<t>This attribute defines the ability to negotiate the use of ECT (ECN
capable transport). This attribute should be put in the SDP offer if
the offering party wishes to receive an ECT flow. The answering party
should include the attribute in the answer if it wish to receive an
ECT flow. If the answerer does not include the attribute then ECT MUST
be disabled in both directions.</t>
</section>
<section title="RTP/AVPF Transport Layer Feedback Message">
<t>The IANA is requested to register one new RTP/AVPF Transport Layer
Feedback Message in the table of FMT values for RTPFB Payload Types
<xref target="RFC4585"></xref> as defined in <xref
target="sec-rtcp-ecn-fb"></xref>:</t>
<figure>
<artwork><![CDATA[
Name: RTCP-ECN-FB
Long name: RTCP ECN Feedback
Value: TBA1
Reference: RFC XXXX
]]></artwork>
</figure>
</section>
<section title="RTCP Feedback SDP Parameter">
<t>The IANA is requested to register one new SDP "rtcp-fb" attribute
"nack" parameter "ecn" in the SDP ("ack" and "nack" Attribute Values)
registry.</t>
<t><figure>
<artwork><![CDATA[ Value name: ecn
Long name: Explicit Congestion Notification
Usable with: nack
Reference: RFC XXXX]]></artwork>
</figure></t>
</section>
<section title="RTCP XR Report blocks">
<t>The IANA is requested to register one new RTCP XR Block Type as
defined in <xref target="sec-ecn-summary-report"></xref>:</t>
<figure>
<artwork><![CDATA[
Block Type: TBA2
Name: ECN Summary Report
Reference: RFC XXXX
]]></artwork>
</figure>
</section>
<section title="RTCP XR SDP Parameter">
<t>The IANA is requested to register one new RTCP XR SDP Parameter
"ecn-sum" in the "RTCP XR SDP Parameters" registry.</t>
<t><figure>
<artwork><![CDATA[ Parameter name XR block (block type and name)
-------------- ------------------------------------
ecn-sum TBA2 ECN Summary Report Block]]></artwork>
</figure></t>
</section>
<section anchor="stun-attr" title="STUN attribute">
<t>A new STUN <xref target="RFC5389"></xref> attribute in the
Comprehension-optional range under IETF Review (0x0000 - 0x3FFF) is
request to be assigned to the STUN attribute defined in <xref
target="sec-stun-init-ecn"></xref>. The STUN attribute registry can
currently be found at:
http://www.iana.org/assignments/stun-parameters/stun-parameters.xhtml.</t>
</section>
<section anchor="ice-opt" title="ICE Option">
<t>A new ICE option "rtp+ecn" is registered in the registry that <xref
target="I-D.ietf-mmusic-ice-options-registry">"IANA Registry for
Interactive Connectivity Establishment (ICE) Options"</xref>
creates.</t>
</section>
</section>
<section anchor="sec-security" title="Security Considerations">
<t>The usage of ECN with RTP over UDP as specified in this document has
the following known security issues that needs to be considered.</t>
<t>External threats to the RTP and RTCP traffic:</t>
<t><list style="hanging">
<t hangText="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.</t>
<t hangText="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.</t>
<t hangText="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 re-marking. It will be
mitigated by turning off ECN, however, if the attacker can modify
its response to drop packets the same vulnerability exist.</t>
<t
hangText="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.</t>
</list></t>
<t>The following are threats that exist from misbehaving senders or
receivers:</t>
<t><list style="hanging">
<t hangText="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.</t>
<t hangText="Receivers misbehaving:">A receiver may prevent the
usage of ECN in an RTP session by reporting itself as non ECN
capable. Thus forcing the sender to turn off usage of ECN. In a
point-to-point scenario there is little incentive to do this as it
will only affect the receiver. Thus failing to utilise an
optimisation. For multi-party session there exist some motivation
why a receiver would misbehave as it can prevent also the other
receivers from using ECN. As an insider into the session it is
difficult to determine if a receiver is misbehaving or simply
incapable, making it basically impossible in the incremental
deployment phase of ECN for RTP usage to determine this. If
additional information about the receivers and the network is known
it might be possible to deduce that a receiver is misbehaving. If it
can be determined that a receiver is misbehaving, the only response
is to exclude it from the RTP session and ensure that is doesn't any
longer have any valid security context to affect the session.</t>
<t hangText="Misbehaving Senders:">The enabling of ECN gives the
media packets a higher degree of probability to reach the receiver
compared to not-ECT marked ones on a ECN capable path. However, this
is no magic bullet and failure to react to congestion will most
likely only slightly delay a 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
<xref target="RFC3168"></xref> for more discussion of this
issue.</t>
</list></t>
<t>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. <xref target="RFC3711">Secure RTP (SRTP)</xref>
does satisfy the requirement to protect this mechanism despite only
providing authentication if a entity is within the security context or
not. <xref target="RFC4301">IPsec</xref> and <xref
target="RFC4347">DTLS</xref> can also provide the necessary security
functions.</t>
<t>The signalling protocols used to initiate an RTP session also needs
to be source authenticated and integrity protected to prevent an
external attacker from modifying any signalling. Here an appropriate
mechanism to protect the used signalling needs to be used. For SIP/SDP
ideally <xref target="RFC5751">S/MIME</xref> would be used. However,
with the limited deployment a minimal mitigation strategy is to require
use of <xref target="RFC3261">SIPS (SIP over TLS)</xref> <xref
target="RFC5630"></xref> to at least accomplish hop-by-hop
protection.</t>
<t>We do note that certain mitigation methods will require network
functions.</t>
</section>
<section anchor="sec-examples" title="Examples of SDP Signalling">
<t>This section contain a few different examples of the signalling
mechanism defined in this specification in an SDP context. If there is
discrepancies between these examples and the specification text, the
specification text is what is correct.</t>
<section title="Basic SDP Offer/Answer">
<t>This example is a basic offer/answer SDP exchange, assumed done by
SIP (not shown). The intention is to establish a basic audio session
point to point between two users.</t>
<t>The Offer:</t>
<figure>
<artwork><![CDATA[ v=0
o=jdoe 3502844782 3502844782 IN IP4 10.0.1.4
s=VoIP call
i=SDP offer for VoIP call with ICE and ECN for RTP
b=AS:128
b=RR:2000
b=RS:2500
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6
a=ice-options:rtp+ecn
t=0 0
m=audio 45664 RTP/AVPF 97 98 99
c=IN IP4 192.0.2.3
a=rtpmap:97 G719/48000/1
a=fmtp:97 maxred=160
a=rtpmap:98 AMR-WB/16000/1
a=fmtp:98 octet-align=1; mode-change-capability=2
a=rtpmap:99 PCMA/8000/1
a=maxptime:160
a=ptime:20
a=ecn-capable-rtp: ice rtp ect=0 mode=setread
a=rtcp-fb:* nack ecn
a=rtcp-fb:* trr-int 1000
a=rtcp-xr:ecn-sum
a=candidate:1 1 UDP 2130706431 10.0.1.4 8998 typ host
a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr
10.0.1.4 rport 8998
]]></artwork>
</figure>
<t>This SDP offer offers a single media stream with 3 media payload
types. It proposes to use ECN with RTP, with the ICE based
initilziation as being prefered over the RTP/RTCP one. Leap of faith
is not suggested to be used. The offerer is capable of both setting
and reading the ECN bits. In addition the RTCP ECN feedback packet is
configured and the RTCP XR ECN summary report. ICE is also proposed
with two candidates.</t>
<t>The Answer:</t>
<figure>
<artwork><![CDATA[ v=0
o=jdoe 3502844783 3502844783 IN IP4 198.51.100.235
s=VoIP call
i=SDP offer for VoIP call with ICE and ECN for RTP
b=AS:128
b=RR:2000
b=RS:2500
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
a=ice-options:rtp+ecn
t=0 0
m=audio 53879 RTP/AVPF 97 99
c=IN IP4 198.51.100.235
a=rtpmap:97 G719/48000/1
a=fmtp:97 maxred=160
a=rtpmap:99 PCMA/8000/1
a=maxptime:160
a=ptime:20
a=ecn-capable-rtp: ice ect=0 mode=readonly
a=rtcp-fb:* nack ecn
a=rtcp-fb:* trr-int 1000
a=rtcp-xr:ecn-sum
a=candidate:1 1 UDP 2130706431 198.51.100.235 53879 typ host ]]></artwork>
</figure>
<t>The answer confirms that only one media stream will be used. One
RTP Payload type was removed. ECN capability was confirmed, and the
initilization method will be ICE. However, the answerer is only
capable of reading the ECN bits, which means that ECN can only be used
for RTP flowing from the offerer to the answerer. ECT always set to 0
will be used in both directions. Both the RTCP ECN feedback packet and
the RTCP XR ECN summary report will be used.</t>
</section>
<section title="Declarative Multicast SDP">
<t>The below session describes an any source multicast using session
with a single media stream.</t>
<figure>
<artwork><![CDATA[ v=0
o=jdoe 3502844782 3502844782 IN IP4 198.51.100.235
s=Multicast SDP session using ECN for RTP
i=Multicasted audio chat using ECN for RTP
b=AS:128
t=3502892703 3502910700
m=audio 56144 RTP/AVPF 97
c=IN IP4 233.252.0.212/127
a=rtpmap:97 g719/48000/1
a=fmtp:97 maxred=160
a=maxptime:160
a=ptime:20
a=ecn-capable-rtp: rtp mode=readonly; ect=0
a=rtcp-fb:* nack ecn
a=rtcp-fb:* trr-int 1500
a=rtcp-xr:ecn-sum
]]></artwork>
</figure>
<t>In the above example, as this is declarative we need to require
certain functionality. As it is ASM the initliziation method that can
work here is the RTP/RTCP based one. So that is indicated. The ECN
setting and reading capability to take part of this session is at
least read. If one is capable of setting that is good, but not
required as one can skip using ECN for anything one send oneself. The
ECT value is recommended to be set to 0 always. The ECN usage in this
session requires both ECN feedback and the XR ECN summary report, so
their usage are also indicated.</t>
</section>
</section>
<section title="Open Issues">
<t>As this draft is under development some known open issues exist and
are collected here. Please consider them and provide input.</t>
<t><list style="numbers">
<t>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 incoming streams. However,
it would beneficial to know if no potential sender support setting
ECN.</t>
<t>Consider initiation optimizations that allows for multi SSRC
sender nodes to still have rapid usage of ECN.</t>
<t>Should we report congestion in bytes or packets? RTCP usually
does this in terms of packets, but there may be an argument that we
want to report bytes for ECN. draft-ietf-tsvwg-byte-pkt-congest is
extremely unclear on what is the right approach.</t>
<t>We have a saturation problem with the packet loss counters. They
do need to continue working even if saturation happens due to long
sessions where more lost packets than the counters can handle.</t>
</list></t>
</section>
<section title="Acknowledgments">
<t>The authors wish to thank the following persons for their reviews and
comments: Thomas Belling, Bob Briscoe, Roni Even, Thomas Frankkila,
Christian Groves, Cullen Jennings Tom Van Caenegem, Simo Veikkolainen,
Lei Zhu, Christer Holmgren.</t>
</section>
</middle>
<back>
<references title="Normative References">
&rfc2119;
<?rfc include='reference.RFC.2762'?>
&rfc3168;
&rfc3550;
&rfc3611;
&rfc5234;
&rfc5245;
&rfc5348;
&rfc5389;
&icereg;
</references>
<references title="Informative References">
&rfc2974;
&rfc3261;
&rfc3264;
&rfc3540;
&rfc3551;
&rfc3569;
&rfc3711;
&rfc5751;
&rfc4301;
&rfc4340;
&rfc4347;
&rfc4566;
&rfc4585;
&rfc4607;
&rfc4960;
&rfc5124;
&rfc5506;
&rfc5630;
&rfc5760;
&no-op;
&zrtp;
</references>
</back>
</rfc>| PAFTECH AB 2003-2026 | 2026-04-23 16:27:00 |