Network Working Group                                      M. Westerlund
Internet-Draft                                              I. Johansson
Intended status: Standards Track                                Ericsson
Expires: April 28, 2011                                       C. Perkins
                                                   University of Glasgow
                                                             P. O'Hanlon
                                                             K. Carlberg
                                                        October 25, 2010

        Explicit Congestion Notification (ECN) for RTP over UDP


   This document specifies how explicit congestion notification (ECN)
   can be used with RTP/UDP flows that use RTCP as feedback mechanism.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 28, 2011.

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   Copyright (c) 2010 IETF Trust and the persons identified as the
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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions, Definitions and Acronyms  . . . . . . . . . . . .  4
   3.  Discussion, Requirements, and Design Rationale . . . . . . . .  4
     3.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Overview of Use of ECN with RTP/UDP/IP . . . . . . . . . . . . 10
   5.  RTCP Extensions for ECN feedback . . . . . . . . . . . . . . . 13
     5.1.  RTP/AVPF Transport Layer ECN Feedback packet . . . . . . . 13
     5.2.  RTCP XR Report block for ECN summary information . . . . . 16
   6.  Use of ECN with RTP/UDP/IP . . . . . . . . . . . . . . . . . . 17
     6.1.  Negotiation of ECN Capability  . . . . . . . . . . . . . . 17
     6.2.  Initiation of ECN Use in an RTP Session  . . . . . . . . . 21
     6.3.  Ongoing Use of ECN Within an RTP Session . . . . . . . . . 27
     6.4.  Detecting Failures . . . . . . . . . . . . . . . . . . . . 29
   7.  Processing RTCP ECN Feedback in RTP Translators and Mixers . . 33
     7.1.  Fragmentation and Reassembly in Translators  . . . . . . . 33
     7.2.  Generating RTCP ECN Feedback in Media Transcoders  . . . . 35
     7.3.  Generating RTCP ECN Feedback in Mixers . . . . . . . . . . 36
   8.  Implementation considerations  . . . . . . . . . . . . . . . . 36
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 36
     9.1.  SDP Attribute Registration . . . . . . . . . . . . . . . . 37
     9.2.  RTP/AVPF Transport Layer Feedback Message  . . . . . . . . 37
     9.3.  RTCP XR Report blocks  . . . . . . . . . . . . . . . . . . 37
     9.4.  STUN attribute . . . . . . . . . . . . . . . . . . . . . . 37
     9.5.  ICE Option . . . . . . . . . . . . . . . . . . . . . . . . 38
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 38
   11. Examples of SDP Signalling . . . . . . . . . . . . . . . . . . 40
   12. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 40
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 41
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 41
     13.2. Informative References . . . . . . . . . . . . . . . . . . 42
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 43

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1.  Introduction

   This document outlines how Explicit Congestion Notification (ECN)
   [RFC3168] can be used for RTP [RFC3550] flows running over UDP/IP
   which use RTCP as 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 SDP is

   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.

   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.

   The introduction of ECN into the Internet requires changes to both
   the network and transport layers.  At the network layer, IP
   forwarding has to be updated to allow routers to mark packets, rather
   than discarding them in times of congestion [RFC3168].  In addition,
   transport protocols have to be modified to inform the sender that ECN
   marked packets are being received, so it can respond to the
   congestion.  TCP [RFC3168], SCTP [RFC4960] and DCCP [RFC4340] have
   been updated to support ECN, but to date there is no specification
   how UDP-based transports, such as RTP [RFC3550], can use ECN.  This
   is due to the lack of feedback mechanisms directly in UDP.  Instead
   the signaling control protocol on top of UDP needs to provide that
   feedback, which for RTP is RTCP.

   The remainder of this memo is structured as follows.  We start by
   describing the conventions, definitions and acronyms used in this
   memo in Section 2, and the design rationale and applicability in
   Section 3.  Section 4 provides an overview of how ECN is used with
   RTP over UDP.  Then the definition of the RTCP extensions for ECN
   feedback in Section 5.  Then the full details of how ECN is used with
   RTP over UDP is defined in Section 6.  In Section 7 we discuss how
   RTCP ECN feedback is handled in RTP translators and mixers.
   Section 8 discusses some implementation considerations, Section 9

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   lists IANA considerations, and Section 10 discusses the security

2.  Conventions, Definitions and Acronyms

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].


   o  ECN: Explicit Congestion Notification

   o  ECT: ECN Capable Transport

   o  ECN-CE: ECN Congestion Experienced

   o  not-ECT: Not ECN Capable Transport

   The meaning of the term ECN support depends on which entity between
   the sender and receiver (inclusive) that is considered.  We
   distinguish between:

   o  ECN-Capable Host: Sender or receiver of media.

   o  ECN-Capable Transport: ECT = all ends are ECN capable hosts.

   o  ECN-Capable Packets: Packets are either ECT or CE.

   o  ECN-Oblivious Relay: Router or middlebox that treats ECN-Capable
      Packets no differently from Not-ECT.

   o  ECN-Capable Queue: Supports ECN marking of ECN-Capable Packets.

   o  ECN-Blocking Middlebox: Discards ECN-Capable Packets.

   o  ECN-Reverting Middlebox: Changes ECN-Capable Packets to Not-ECT.

3.  Discussion, Requirements, and Design Rationale

   ECN has been specified for use with TCP [RFC3168], SCTP [RFC4960],
   and DCCP [RFC4340] transports.  These are all unicast protocols which
   negotiate the use of ECN during the initial connection establishment
   handshake (supporting incremental deployment, and checking if ECN
   marked packets pass all middleboxes on the path).  ECN Congestion
   Experienced (ECN-CE) marks are immediately echoed back to the sender

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   by the receiving end-point using an additional bit in feedback
   messages, and the sender then interprets the mark as equivalent to a
   packet loss for congestion control purposes.

   If RTP is run over TCP, SCTP, or DCCP, it can use the native ECN
   support provided by those protocols.  This memo does not concern
   itself further with these use cases.  However, RTP is more commonly
   run over UDP.  This combination does not currently support ECN, and
   we observe that it has significant differences from the other
   transport protocols for which ECN has been specified.  These include:

   Signalling:  RTP relies on separate signalling protocols to negotiate
      parameters before a session can be created, and doesn't include an
      in-band handshake or negotiation at session set-up time (i.e.
      there is no equivalent to the TCP three-way handshake in RTP).

   Feedback:  RTP does not explicitly acknowledge receipt of datagrams.
      Instead, the RTP Control Protocol (RTCP) provides reception
      quality feedback, and other back channel communication, for RTP
      sessions.  The feedback interval is generally on the order of
      seconds, rather than once per network RTT (although the RTP/AVPF
      profile [RFC4585] allows more rapid feedback in most cases).

   Congestion Response:  While it is possible to adapt the transmission
      of many audio/visual streams in response to network congestion,
      and such adaptation is required by [RFC3550], the dynamics of the
      congestion response may be quite different to those of TCP or
      other transport protocols.

   Middleboxes:  The RTP framework explicitly supports the concept of
      mixers and translators, which are middleboxes that are involved in
      media transport functions.

   Multicast:  RTP is explicitly a group communication protocol, and was
      designed from the start to support IP multicast (primarily ASM,
      although a recent extension supports SSM with unicast feedback

   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.

   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

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      congestion experienced, whether it is distributed smoothly or in
      bursts.  When feedback of ECN was added to TCP [RFC3168], the
      receiver was designed to flip the echo congestion experienced
      (ECE) flag to 1 for a whole RTT then flop it back to zero.
      Whereas ECN feedback in RTCP will need to report a count of how
      much congestion has been experienced within an RTCP reporting
      period, irrespective of round trip times.

   These differences 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.

   ECN support is more important for RTP sessions that for instance is
   the case for TCP- This 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.

3.1.  Requirements

   Considering ECN, transport protocols supporting ECN, and RTP based
   applications one can create a set of requirements that must be
   satisfied to at least some degree if ECN is to used by RTP over UDP.

   o  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.

   o  REQ 2: A mechanism MUST feedback the reception of any packets that
      are ECN-CE marked to the packet sender

   o  REQ 3: Provided mechanism SHOULD minimise the possibility for

   o  REQ 4: Some detection and fallback mechanism SHOULD exist to avoid
      loss of communication due to the attempted usage of ECN in case an
      intermediate node clears ECT or drops packets that are ECT marked.

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   o  REQ 5: Negotiation of ECN SHOULD NOT significantly increase the
      time taken to negotiate and set-up the RTP session (an extra RTT
      before the media can flow is unlikely to be acceptable for some
      use cases).

   o  REQ 6: Negotiation of ECN SHOULD NOT cause media clipping at the
      start of a session.

   The following sections describes how these requirements can be meet
   for RTP over UDP.

3.2.  Applicability

   The use of ECN with RTP over UDP is dependent on negotiation of ECN
   capability between the sender and receiver(s), and validation of ECN
   support in all elements of the network path(s) traversed.  RTP is
   used in a heterogeneous range of network environments and topologies,
   with various different signalling protocols, all of which need to be
   verified to support ECN before it can be used.

   The usage of ECN is further dependent on a capability of the RTP
   media flow to react to congestion signalled by ECN marked packets.
   Depending on the application, media codec, and network topology, this
   adaptation can occur 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:

   Topo-Point-to-Point:  This is a standard unicast flow.  ECN may be
      used with RTP in this topology in an analogous manner to its use
      with other unicast transport protocols, with RTCP conveying ECN
      feedback messages.

   Topo-Multicast:  This is either an any source multicast (ASM) group
      with potentially several active senders and multicast RTCP
      feedback, or a source specific multicast (SSM) group with a single
      sender and unicast RTCP feedback from receivers.  RTCP is designed
      to scale to large group sizes while avoiding feedback implosion
      (see Section 6.2 of [RFC3550], [RFC4585], and [RFC5760]), and can
      be used by a sender to determine if all its receivers, and the
      network paths to those receivers, support ECN (see Section 6.2).
      It is somewhat more difficult to determine if all network paths
      from all senders to all receivers support ECN.  Accordingly, we
      allow ECN to be used by an RTP sender using multicast UDP provided
      the sender has verified that the paths to all its known receivers
      support ECN, and irrespective of whether the paths from other

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      senders to their receivers support ECN.  Note that group
      membership may change during the lifetime of a multicast RTP
      session, potentially introducing new receivers that are not ECN
      capable.  Senders must use the mechanisms described in Section 6.4
      to monitor that all receivers continue to support ECN, and they
      need to fallback to non-ECN use if any senders do not.

   Topo-Translator:  An RTP translator is an RTP-level middlebox that is
      invisible to the other participants in the RTP session (although
      it is usually visible in the associated signalling session).
      There are two types of RTP translator: those do not modify the
      media stream, and are concerned with transport parameters, for
      example a multicast to unicast gateway; and those that do modify
      the media stream, for example transcoding between different media
      codecs.  A single RTP session traverses the translator, and the
      translator must rewrite RTCP messages passing through it to match
      the changes it makes to the RTP data packets.  A legacy, ECN-
      unaware, RTP translator is expected to ignore the ECN bits on
      received packets, and to set the ECN bits to not-ECT when sending
      packets, so causing ECN negotiation on the path containing the
      translator to fail (any new RTP translator that does not wish to
      support ECN may do so similarly).  An ECN aware RTP translator may
      act in one of three ways:

      *  If the translator does not modify the media stream, it should
         copy the ECN bits unchanged from the incoming to the outgoing
         datagrams, unless it is overloaded and experiencing congestion,
         in which case it may mark the outgoing datagrams with an ECN-CE
         mark.  Such a translator passes RTCP feedback unchanged.

      *  If the translator modifies the media stream to combine or split
         RTP packets, but does not otherwise transcode the media, it
         must manage the ECN bits in a way analogous to that described
         in Section 5.3 of [RFC3168]: if an ECN marked packet is split
         into two, then both the outgoing packets must be ECN marked
         identically to the original; if several ECN marked packets are
         combined into one, the outgoing packet must be either ECN-CE
         marked or dropped if any of the incoming packets are ECN-CE
         marked.  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 (Section 5) are
         received, they must be rewritten to match the modifications
         made to the media stream (see Section 7.1).

      *  If the translator is a media transcoder, the output RTP media
         stream may have radically different characteristics than the
         input RTP media stream.  Each side of the translator must then
         be considered as a separate transport connection, with its own

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         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 Section 7.2).

      It is recognised that ECN and RTCP processing in an RTP translator
      that modifies the media stream is non-trivial.

   Topo-Mixer:  A mixer is an RTP-level middlebox that aggregates
      multiple RTP streams, mixing them together to generate a new RTP
      stream.  The mixer is visible to the other participants in the RTP
      session, and is also usually visible in the associated signalling
      session.  The RTP flows on each side of the mixer are treated
      independently for ECN purposes, with the mixer generating its own
      RTCP ECN feedback, and responding to ECN feedback for data it
      sends.  Since connections are treated independently, it would seem
      reasonable to allow the transport on one side of the mixer to use
      ECN, while the transport on the other side of the mixer is not ECN
      capable, if this is desired.

   Topo-Video-switch-MCU:  A video switching MCU receives several RTP
      flows, but forwards only one of those flows onwards to the other
      participants at a time.  The flow that is forwarded changes during
      the session, often based on voice activity.  Since only a subset
      of the RTP packets generated by a sender are forwarded to the
      receivers, a video switching MCU can break ECN negotiation (the
      success of the ECN negotiation may depend on the voice activity of
      the participant at the instant the negotiation takes place - shout
      if you want ECN).  It also breaks congestion feedback and
      response, since RTP packets are dropped by the MCU depending on
      voice activity rather than network congestion.  This topology is
      widely used in legacy products, but is NOT RECOMMENDED for new
      implementations and cannot be used with ECN.

   Topo-RTCP-terminating-MCU:  In this scenario, each participant runs
      an RTP point-to-point session between itself and the MCU.  Each of
      these sessions is treated independently for the purposes of ECN
      and RTCP feedback, potentially with some using ECN and some not.

   Topo-Asymmetric:  It is theoretically possible to build a middlebox
      that is a combination of an RTP mixer in one direction and an RTP
      translator in the other.  To quote RFC 5117 "This topology is so
      problematic and it is so easy to get the RTCP processing wrong,
      that it is NOT RECOMMENDED to implement this topology."

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   These topologies may be combined within a single RTP session.

   The ECN mechanism defined in this memo is applicable to both sender
   and receiver controlled congestion algorithms.  The mechanism ensures
   that both senders and receivers will know about ECN-CE markings and
   any packet losses.  Thus the actual decision point for the congestion
   control is not relevant.  This is a great benefit as the rate of an
   RTP session can be varied in a number of ways, for example a unicast
   media sender might use TFRC [RFC5348] or some other algorithm, while
   a multicast session could use a sender based scheme adapting to the
   lowest common supported rate, or a receiver driven mechanism using
   layered coding to support more heterogeneous paths.

   To ensure timely feedback of CE marked packets, this mechanism
   requires support for the RTP/AVPF profile [RFC4585] or any of its
   derivatives, such as RTP/SAVPF [RFC5124].  The standard RTP/AVP
   profile [RFC3551] does not allow any early or immediate transmission
   of RTCP feedback, and has a minimal RTCP interval whose default value
   (5 seconds) is many times the normal RTT between sender and receiver.

   The control of which RTP data packets are marked as ECT, and whether
   ECT(0) or ECT(1) is used, is due to the sender.  RTCP packets MUST
   NOT be ECT marked, whether generated by sender or receivers.

4.  Overview of Use of ECN with RTP/UDP/IP

   The solution for using ECN with RTP over UDP/IP consists of four
   different pieces that together make the solution work:

   1.  Negotiation of the capability to use ECN with RTP/UDP/IP

   2.  Initiation and initial verification of ECN capable transport

   3.  Ongoing use of ECN within an RTP session

   4.  Handling of dynamic groups through failure detection,
       verification and fallback

   The solution includes a new SDP attribute (Section 6.1.1), the
   definition of new extensions to RTCP (Section 5) and STUN
   (Section 6.2.2).

   Before an RTP session can be created, a signalling protocol is used
   to discover the other participants and negotiate session parameters
   (see Section 6.1).  One of the parameters that can be negotiated is
   the capability of a participant to support ECN functionality, or
   otherwise.  Note that all participants having the capability of

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   supporting ECN does not necessarily imply that ECN is usable in an
   RTP session, since there may be middleboxes on the path between the
   participants which don't pass ECN-marked packets (for example, a
   firewall that blocks traffic with the ECN bits set).  This document
   defines the information that needs to be negotiated, and provides a
   mapping to SDP for use in both declarative and offer/answer contexts.

   When a sender joins a session for which all participants claim ECN
   capability, it must verify if that capability is usable.  There are
   three ways in which this verification may be done (Section 6.2):

   o  The sender may generate a (small) subset of its RTP data packets
      with the ECN field set to ECT(0) or ECT(1).  Each receiver will
      then send an RTCP feedback packet indicating the reception of the
      ECT marked RTP packets.  Upon reception of this feedback from each
      receiver it knows of, the sender can consider ECN functional for
      its traffic.  Each sender does this verification independently of
      each other.  If a new receiver joins an existing session it 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 succeeeded and sources can
      continue to send ECT packets.  If not, verification fails and each
      sender MUST stop using ECN.

   o  Alternatively, ECN support can be verified during an initial end-
      to-end STUN exchange (for example, as part of ICE connection
      establishment).  After having verified connectivity without ECN
      capability an extra STUN exchange, this time with the ECN field
      set to ECT(0) or ECT(1), is performed.  If successful the path's
      capability to convey ECN marked packets is verified.  A new STUN
      attribute is defined to convey feedback that the ECT marked STUN
      request was received (see Section 9.4), along with an ICE
      signalling option (Section 9.5).

   o  Thirdly, the sender may make a leap of faith that ECN will work.
      This is only recommended for applications that know they are
      running in controlled environments where ECN functionality has
      been verified through other means.  In this mode it is assumed
      that ECN works, and the system reacts to failure indicators if the
      assumption proved wrong.  The use of this method relies on a high
      confidence that ECN operation will be successful, or an
      application where failure is not serious.  The impact on the
      network and other users must be considered when making a leap of
      faith, so there are limitations on when this method is allowed.

   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

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   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.

   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.  An RTCP feedback report
   is sent as soon as possible by 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.  Section 6.3
   describes the ongoing use of ECN with an RTP session.

   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
   large reports.  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.

   RTCP traffic MUST NOT be ECT marked for the following reason.  ECT
   marked traffic may be dropped if the path is not ECN compliant.  As
   RTCP is used to provide feedback about what has been transmitted and
   what ECN markings that are received, it is important that these are
   received in cases when ECT marked traffic is not getting through.

   There are numerous reasons why the path the RTP packets take from the
   sender to the receiver may change, e.g., mobility, link failure
   followed by re-routing around it.  Such an event may result in the
   packet being sent through a node that is ECN non-compliant, thus re-

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   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
   (Section 6.4.2 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.

5.  RTCP Extensions for ECN feedback

   This documents defines two different RTCP extensions: one RTP/AVPF
   [RFC4585] transport layer feedback format for urgent ECN information,
   and one RTCP XR [RFC3611] 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 Section 6.

5.1.  RTP/AVPF Transport Layer ECN Feedback packet

   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 reduced size RTCP [RFC5506]
   in mind, where RTCP feedback packets may be sent without accompanying
   Sender or Receiver Reports that would contain the Extended Highest
   Sequence number and the accumulated number of packet losses.  Both
   are important for ECN to verify functionality and keep track of when
   CE marking does occur.

   The RTP/AVPF transport layer feedback packet starts with the common
   header defined by the RTP/AVPF profile [RFC4585] which is reproduced
   here for the reader's information:

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |V=2|P|   FMT   |  PT=RTPFB=205 |          length               |
   |                  SSRC of packet sender                        |
   |                  SSRC of media source                         |
   :            Feedback Control Information (FCI)                 :
   :                                                               :

       Figure 1: RTP/AVPF Common Packet Format for Feedback Messages

   From Figure 1 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 =
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   | Extended Highest Sequence Number      | Lost packets counter  |
   |     CE Counter                | not-ECT Counter               |
   | ECT (0) Counter               | ECT (1) Counter               |

                       Figure 2: ECN Feedback Format

   The FCI information for the ECN Feedback format (Figure 2) are the

   Extended Highest Sequence Number:  The least significant 20-bit from
      an Extended highest sequence number received value as defined by
      [RFC3550].  Used to indicate for which packet this report is valid
      up to.

   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 [RFC3550]
      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.

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   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 to 0 if more than 65535 packets has been received.

   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

   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

   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.

   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.

   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.

   The Extended Highest sequence number and packet loss fields are both
   truncated in comparison to the RTCP SR or RR versions.  This is to
   save bits as the representation is redundant unless reduced size RTCP
   is used in such a way that only feedback packets are transmitted,
   with no SR or RR in the compound RTCP packet.  Due to that regular
   RTCP reporting will include the longer versions of the fields the
   wrapping issue will be less unless the packet rate of the application
   is so high that the fields will wrap within a regular RTCP reporting

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   interval.  In those case the feedback packet need to be sent in a
   compound packet together with the SR or RR packet.

   There is an issue with packet duplication in relation to the packet
   loss counter.  If one avoids holding state for which sequence number
   has been received then the way one can count loss is to count the
   number of received packets and compare that to the number of packets
   expected.  As a result a packet duplication can hide a packet loss.
   If a receiver is tracking the sequence numbers actually received and
   suppresses duplicates it provides for a more reliable packet loss
   indication.  Reordering may also result in that packet loss is
   reported in one report and then removed in the next.

   The CE counter is actually more robust for packet duplication.
   Adding each received CE marked packet to the counter is not an issue.
   If one of the clones was CE marked that is still a indication of
   congestion.  Packet duplication has potential impact on the ECN
   verification.  Thus the sum of packets reported may be higher than
   the number sent.  However, most detections are still applicable.

5.2.  RTCP XR Report block for ECN summary information

   This report block combined with RTCP SR or RR report blocks carries
   the same information as the ECN Feedback Packet and shall be based on
   the same underlying information.  However, there is a difference in
   semantics between the feedback format and this XR version.  Where the
   feedback format is intended to report on a CE mark as soon as
   possible, this extended report is for the regular RTCP report and
   continuous verification of the ECN functionality end-to-end.

   The ECN Summary report block consists of one report block header:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |      BT       | Reserved      |         Block Length          |

   and then followed of one or more of the following report data blocks:

    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               |

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   BT:  Block Type identifying the ECN summary report block.  Value is

   Reserved:  All bits SHALL be set to 0 on transmission and ignored on

   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.

   SSRC of Media Sender:  The SSRC identifying the media sender this
      report is for.

   CE Counter:  as in Section 5.1.

   ECT(0) Counter:  as in Section 5.1.

   ECT(1) Counter:  as in Section 5.1.

   not-ECT Counter:  as in Section 5.1.

   The Extended Highest Sequence number and the packet loss counter for
   each SSRC is not present in RTCP XR report, in contrast to the
   feedback version.  The reason is that this summary report will always
   be sent in a RTCP compound packet where the Extended Highest Sequence
   number and the accumulated number of packet losses are present in the
   RTCP Sender Report or Receiver Report packet's report block.

6.  Use of ECN with RTP/UDP/IP

   In the detailed specification of the behaviour below, the different
   functions in the general case will first be discussed.  In case
   special considerations are needed for middleboxes, multicast usage
   etc, those will be specially discussed in related subsections.

6.1.  Negotiation of ECN Capability

   The first stage of ECN negotiation for RTP-over-UDP is to signal the
   capability to use ECN.  This includes negotiating if ECN is to be
   used symmetrically 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
   initiation.  In addition an ICE parameter is defined to indicate that
   ECN initiation using STUN is supported as part of an ICE exchange.

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   An RTP system that supports ECN and uses SDP in the signalling MUST
   implement the SDP extension to signal ECN capability as described in
   Section 6.1.1.  It MAY also implement alternative ECN capability
   negotiation schemes, such as the ICE extension described in
   Section 6.1.2.

6.1.1.  Signalling ECN Capability using SDP

   One new SDP attribute, "a=ecn-capable-rtp", is defined.  This is a
   media level attribute, which MUST NOT be used at the session level.
   It is not subject to the character set chosen.  The aim of this
   signalling is to indicate the capability of the sender and receivers
   to support ECN, and to negotiate the method of ECN initiation to be
   used in the session.  The attribute takes a list of initiation
   methods, ordered in decreasing preference.  The defined values for
   the initiation method are:

   rtp:  Using RTP and RTCP as defined in Section 6.2.1.

   ice:  Using STUN within ICE as defined in Section 6.2.2.

   leap:  Using the leap of faith method as defined in Section 6.2.3.

   In addition, a number of OPTIONAL parameters may be included in the
   "a=ecn-capable-rtp" attribute as follows:

   mode:  This parameter signals the endpoint's capability to set and
      read ECN marks in UDP packets.  An examination of various
      operating systems has shown that end-system support for ECN
      marking of UDP packets may be symmetric or asymmetric.  By this we
      mean that some systems may allow end points to set the ECN bits in
      an outgoing UDP packet but not read them, while others may allow
      applications to read the ECN bits but not set them.  This
      either/or case may produce an asymmetric support for ECN and thus
      should be conveyed in the SDP signalling.  The "mode=setread"
      state is the ideal condition where an endpoint can both set and
      read ECN bits in UDP packets.  The "mode=setonly" state indicates
      that an endpoint can set the ECT bit, but cannot read the ECN bits
      from received UDP packets to determine if upstream congestion
      occurred.  The "mode=readonly" state indicates that the endpoint
      can read the ECN bits to determine if downstream congestion has
      occurred, but it cannot set the ECT bits in outgoing UDP packets.
      When the "mode=" parameter is omitted it is assumed that the node
      has "setread" capabilities.  This option can provide for an early
      indication that ECN cannot be used in a session.  This would be
      case when both the offerer and answerer set the "mode=" parameter
      to "setonly" or "readonly", or when an RTP sender entity considers
      offering "readonly".

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   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.  Iit is
      RECOMMENDED that ECT(0) marking be used.

   The ABNF [RFC5234] grammar for the "a=ecn-capable-rtp" attribute is
   as follows:

      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")
      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

   When SDP is used with the offer/answer model [RFC3264], the party
   generating the SDP offer MUST insert an "a=ecn-capable-rtp" attribute
   into the media section of the SDP offer of each RTP flow for which it
   wishes to use ECN.  The attribute includes one or more ECN initiation
   methods in a comma separated list in decreasing order of preference,
   with some number of optional parameters following.  The answering
   party compares the list of initiation methods in the offer with those
   it supports in order of preference.  If there is a match, and if the
   receiver wishes to attempt to use ECN in the session, it includes an
   "a=ecn-capable-rtp" attribute containing its single preferred choice
   of initiation method in the media sections of the answer.  If there
   is no matching initiation method capability, or if the receiver does
   not wish to attempt to use ECN in the session, it does not include an
   "a=ecn-capable-rtp" attribute in its answer.  If the attribute is
   removed in the answer then ECN MUST NOT be used in any direction for
   that media flow.  The answer may also include optional parameters, as
   discussed below.

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   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

   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

   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.

   The "ect=" parameter in the "a=ecn-capable-rtp" attribute is set
   independently in the offer and the answer.  Its value in the offer
   indicates a preference for the behaviour of the answering party, and
   its value in the answer indicates a preference for the behaviour of
   the offering party.  It will be the senders choice if to honor the
   receivers preference or not.

   When SDP is used in a declarative manner, for example in a multicast
   session using the Session Announcement Protocol (SAP, [RFC2974]),
   negotiation of session description parameters is not possible.  The
   "a=ecn-capable-rtp" attribute MAY be added to the session description
   to indicate that the sender will use ECN in the RTP session.  The
   attribute MUST include a single method of initiation.  Participants
   MUST NOT join such a session unless they have the capability to
   understand ECN-marked UDP packets, implement the method of
   initiation, and can generate RTCP ECN feedback (note that having the
   capability to use ECN doesn't necessarily imply that the underlying
   network path between sender and receiver supports ECN).  The mode
   parameter MAY be included also in declarative usage, to indicate
   which capability is required by the consumer of the SDP.  So for
   example in a SSM session the participants configured with a
   particular SDP will all be in a media receive only mode, thus

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   mode=readonly will work as the capability of reporting on the ECN
   markings in the received is what is required.

   The "a=ecn-capable-rtp" attribute MAY be used with RTP media sessions
   using UDP/IP transport.  It MUST NOT be used for RTP sessions using
   TCP, SCTP, or DCCP transport, or for non-RTP sessions.

   As described in Section 6.3.3, RTP sessions using ECN require rapid
   RTCP ECN feedback, in order that the sender can react to ECN-CE
   marked packets.  Thus, the use of the Extended RTP Profile for RTCP-
   Based Feedback (RTP/AVPF) [RFC4585] or other profile that inherits
   AVPF's signalling rules, MUST be signalled.

6.1.2.  ICE Parameter to Signal ECN Capability

   One new ICE [RFC5245] option, "rtp+ecn", is defined.  This is used
   with the SDP session level "a=ice-options" attribute in an SDP offer
   to indicate that the initiator of the ICE exchange has the capability
   to support ECN for RTP-over-UDP flows (via "a=ice-options: rtp+ecn").
   The answering party includes this same attribute at the session level
   in the SDP answer if it also has the capability, and removes the
   attribute if it does not wish to use ECN, or doesn't have the
   capability to use ECN.  If this initiation method (Section 6.2.2)
   actually is going to be used, it is explicitly negotiated using the
   "a=ecn-capable-rtp" attribute.

      Note: This signalling mechanism is not strictly needed as long as
      the STUN ECN testing capability is used within the context of this
      document.  It may however be useful if the ECN verification
      capability is used in additional contexts.

6.2.  Initiation of ECN Use in an RTP Session

   Once the sender and the receiver(s) have agreed that they have the
   capability to use ECN within a session, they may attempt to initiate
   ECN use.

   At the start of the RTP session, when the first packets with ECT are
   sent, it is important to verify that IP packets with ECN field values
   of ECT or ECN-CE will reach 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.

   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

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   will not effect the RTCP traffic and the necessary feedback
   information it carries.

   An RTP system that supports ECN MUST implement the initiation of ECN
   using in-band RTP and RTCP described in Section 6.2.1.  It MAY also
   implement other mechanisms to initiate ECN support, for example the
   STUN-based mechanism described in Section 6.2.2 or use the leap of
   faith option if the session supports the limitations provided in
   Section 6.2.3.  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.

   No matter how ECN usage is initiated, the sender MUST continually
   monitor the ability of the network, and all its receivers, to support
   ECN, following the mechanisms described in Section 6.4.  This is
   necessary because path changes or changes in the receiver population
   may invalidate the ability of the system to use ECN.

6.2.1.  Detection of ECT using RTP and RTCP

   The ECN initiation phase using RTP and RTCP to detect if the network
   path supports ECN comprises three stages.  Firstly, the RTP sender
   generates some small fraction of its traffic with ECT marks to act 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.

   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, one with ECT(0) and one with ECT(1), and will continue
      to send some ECT marked traffic as long as the ECN initiation

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      phase continues.  The sender SHOULD NOT mark all RTP packets as
      ECT during the ECN initiation phase.

      This memo does not mandate which RTP packets are marked with ECT
      during the ECN initiation phase.  An implementation should insert
      ECT marks in RTP packets in a way that minimises the impact on
      media quality if those packets are lost.  The choice of packets to
      mark is clearly very media dependent, but the usage of RTP NO-OP
      payloads [I-D.ietf-avt-rtp-no-op], if supported, would be an
      appropriate choice.  For audio formats, if would make sense for
      the sender to mark comfort noise packets or similar.  For video
      formats, packets containing P- or B-frames, rather than I-frames,
      would be an appropriate choice.  No matter which RTP packets are
      marked, those packets MUST NOT be duplicated in transmission,
      since their RTP sequence number is used to identify packets that
      are received with ECN markings.

   Generating RTCP ECN Feedback:  If ECN capability has been negotiated
      in an RTP session, the receivers in the session MUST listen for
      ECT or ECN-CE marked RTP packets, and generate RTCP ECN feedback
      packets (Section 5.1) to mark their receipt.  An immediate or
      early (depending on the RTP/AVPF mode) ECN feedback packet SHOULD
      be generated on receipt of the first ECT or ECN-CE marked packet
      from a sender that has not previously sent any ECT traffic.  Each
      regular RTCP report MUST also contain an ECN summary report
      (Section 5.2).  Reception of subsequent ECN-CE marked packets
      SHOULD result in additional early or immediate ECN feedback
      packets being sent.

   Determination of ECN Support:  RTP is a group communication protocol,
      where members can join and leave the group at any time.  This
      complicates the ECN initiation phase, since the sender must wait
      until it believes the group membership has stabilised before it
      can determine if the paths to all receivers support ECN (group
      membership changes after the ECN initiation phase has completed
      are discussed in Section 6.3).

      An RTP sender shall consider the group membership to be stable
      after it has been in the session and sending ECT-marked probe
      packets for at least three RTCP reporting intervals (i.e., after
      sending its third regularly scheduled RTCP packet), and when a
      complete RTCP reporting interval has passed without changes to the
      group membership.  ECN initiation is considered successful when
      the group membership is stable, and all known participants have
      sent one or more RTCP ECN feedback packets indicating correct
      receipt of the ECT-marked RTP packets generated by the sender.

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      As an optimisation, if an RTP sender is initiating ECN usage
      towards a unicast address, then it MAY treat the ECN initiation as
      provisionally successful if it receives a single RTCP ECN feedback
      report indicating successful receipt of the ECT-marked packets,
      with no negative indications, from a single RTP receiver.  After
      declaring provisional success, the sender MAY generate ECT-marked
      packets as described in Section 6.3, provided it continues to
      monitor the RTCP reports for a period of three RTCP reporting
      intervals from the time the ECN initiation started, to check if
      there is any other participants in the session.  If other
      participants are detected, the sender MUST fallback to only ECT-
      marking a small fraction of its RTP packets, while it determines
      if ECN can be supported following the full procedure described

         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

      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.

   If the ECN negotiation succeeds, this indicates that the path can
   pass some ECN-marked traffic, and that the receivers support ECN
   feedback.  This does not necessarily imply that the path can robustly
   convey ECN feedback; Section 6.3 describes the ongoing monitoring
   that must be performed to ensure the path continues to robustly
   support ECN.

   When a sender or receiver detects ECN failures on paths they should
   log these to enable follow up and statistics gathering regarding
   broken paths.  The logging mechanism used is implementation

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6.2.2.  Detection of ECT using STUN with ICE

   This section describes an OPTIONAL method that can be used to avoid
   media impact and also ensure an ECN capable path prior to media
   transmission.  This method is considered in the context where the
   session participants are using ICE [RFC5245] to find working
   connectivity.  We need to use ICE rather than STUN only, as the
   verification needs to happen from the media sender to the address and
   port on which the receiver is listening.

   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 are incapable of reading the ECN values
   on incomming STUN packets SHALL follow the STUN specifications rule
   for unknown comprehension-required attributes, i.e. send a 420
   (Unknown Attribute) response back.

   The STUN ECN check attribute contains one field and a flag.  The flag
   indicates if the echo field contains a valid value or not.  The field
   is the ECN echo field, and when valid contains the two ECN bits from
   the packet it echoes back.  The ECN check attribute is a
   comprehension optional attribute.

    0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |         Type                  |            Length             |
   |           Reserved                                      |ECF|V|

                    Figure 3: ECN Check STUN Attribute

   V: Valid (1 bit) ECN Echo value field is valid when set to 1, and
      invalid when set 0.

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   ECF:  ECN Echo value field (2 bits) contains the ECN field value of
      the STUN packet it echoes back when field is valid.  If invalid
      the content is arbitrary.

   Reserved:  Reserved bits (29 bits) SHALL be set to 0 on transmission,
      and SHALL be ignored on reception.

   This attribute MAY be included in any STUN request to request the ECN
   field to be echoed back.  In STUN requests the V bit SHALL be set to
   0.  A STUN server receiving a request with the ECN Check attribute
   which understand it SHALL read the ECN field value of the IP/UDP
   packet the request was received in.  Upon forming the response the
   server SHALL include the ECN Check attribute setting the V bit to
   valid and include the read value of the ECN field into the ECF field.
   If the STUN responder was unable to assertain 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.

6.2.3.  Leap of Faith ECT initiation method

   This method for initiating ECN usage is a leap of faith that assumes
   that ECN will work on the used path(s).  The method is to go directly
   to "ongoing use of ECN" as defined in Section 6.3.  Thus all RTP
   packets MAY be marked as ECT and the failure detection MUST be used
   to detect any case when the assumption that the path was ECT capable
   is wrong.  This method is only recommended for controlled
   environments where the whole path(s) between sender and receiver(s)
   has been built and verified to be ECT.

   If the sender marks all packets as ECT while transmitting on a path
   that contains an ECN-blocking middlebox, then receivers downstream of
   that middlebox will not receive any RTP data packets from the sender,
   and hence will not consider it to be an active RTP SSRC.  The sender
   can detect this and revert to sending packets without ECT marks,
   since RTCP SR/RR packets from such receivers will either not include
   a report for sender's SSRC, or will report that no packets have been
   received, but this takes at least one RTCP reporting interval.  It
   should be noted that a receiver might generate its first RTCP packet
   immediately on joining a unicast session, or very shortly after
   joining a RTP/AVPF session, before it has had chance to receive any
   data packets.  A sender that receives RTCP SR/RR packet indicating
   lack of reception by a receiver SHOULD therefore wait for a second
   RTCP report from that receiver to be sure that the lack of reception
   is due to ECT-marking.  Since this recovery process can take several
   tens of seconds, during which time the RTP session is unusable for
   media, it is NOT RECOMMENDED that the leap-of-faith ECT initiation
   method be used in environments where ECN-blocking middleboxes are
   likely to be present.

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6.3.  Ongoing Use of ECN Within an RTP Session

   Once ECN usage has been successfully initiated for an RTP sender,
   that sender begins sending all RTP data packets as ECT-marked, and
   its receivers continue sending ECN feedback information via RTCP
   packets.  This section describes procedures for sending ECT-marked
   data, providing ECN feedback information via RTCP, responding to ECN
   feedback information, and detecting failures and misbehaving

6.3.1.  Transmission of ECT-marked RTP Packets

   After a sender has successfully initiated ECN usage, it SHOULD mark
   all the RTP data packets it sends as ECT.  The sender SHOULD mark
   packets as ECT(0) unless the receiver expresses a preference for
   ECT(1) using the "ect" parameter in the "a=ecn-capable-rtp"

   The sender SHALL NOT include ECT marks on outgoing RTCP packets, and
   SHOULD NOT include ECT marks on any other outgoing control messages
   (e.g.  STUN [RFC5389] packets, DTLS [RFC4347] handshake packets, or
   ZRTP [I-D.zimmermann-avt-zrtp] control packets) that are multiplexed
   on the same UDP port.  For control packets there might be exceptions,
   like the STUN based ECN check defined in Section 6.2.2.

6.3.2.  Reporting ECN Feedback via RTCP

   An RTP receiver that receives a packet with an ECN-CE mark, or that
   detects a packet loss, MUST schedule the transmission of an RTCP ECN
   feedback packet as soon as possible (subject to the constraints of
   [RFC4585] and [RFC3550]) to report this back to the sender.  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 (Section 5) reporting on the packets
   received since the last ECN feedback packet, and SHOULD contain an
   RTCP SR or RR packet.  The RTP/AVPF profile in early or immediate
   feedback mode SHOULD be used where possible, to reduce the interval
   before feedback can be sent.  To reduce the size of the feedback
   message, reduced size RTCP [RFC5506] MAY be used if supported by the
   end-points.  Both RTP/AVPF and reduced size RTCP MUST be negotiated
   in the session set-up signalling before they can be used.

   Every time a regular compound RTCP packet is to be transmitted, an
   ECN-capable RTP receiver MUST include an RTCP XR ECN summary report
   as described in Section 5.2 as part of the compound packet.

   The multicast feedback implosion problem, that occurs when many
   receivers simultaneously send feedback to a single sender, must also

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   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.

      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.

   In case a receiver driven congestion control algorithm is to be used
   and has been agreed upon through signalling, the algorithm MAY
   specify that the immediate scheduling (and later transmission) of
   ECN-CE feedback of any received ECN-CE mark is not required and shall
   not be done (since it is not necessary for congestion control
   purposes in such cases).  In that case ECN feedback is only sent
   using regular RTCP reports for verification purpose and in response
   to the initiation process ("rtp") of any new media senders as
   specified in Section 6.2.1.

6.3.3.  Response to Congestion Notifications

   When RTP packets are received with ECN-CE marks, the sender and/or
   receivers MUST react with congestion control as-if those packets had
   been lost.  Depending on the media format, type of session, and RTP
   topology used, there are several different types of congestion
   control that can be used.

   Sender-Driven Congestion Control:  The sender may be responsible for
      adapting the transmitted bit-rate in response to RTCP ECN
      feedback.  When the sender receives the ECN feedback data it feeds
      this information into its congestion control or bit-rate
      adaptation mechanism so that it can react on it as if it was
      packet losses that was reported.  The congestion control algorithm
      to be used is not specified here, although TFRC [RFC5348] is one
      example that might be used.

   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

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      current number of layers.  If congestion is experienced, the
      receiver drops one layer, so reducing the resource consumption on
      the path towards itself.  For example, if a layered media encoding
      scheme such as H.264 SVC is used, the receiver may change its
      layer subscription, and so reduce the bit rate it receives.  The
      receiver MUST still send RTCP ECN feedback to the sender, even if
      it can adapt without contact with the sender, so that the sender
      can determine if ECN is supported on the network path.  The
      timeliness of RTCP feedback is less of a concern with receiver
      driven congestion control, and regular RTCP reporting of ECN
      feedback is sufficient (without using RTP/AVPF immediate or early

   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.

   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.

6.4.  Detecting Failures

   Senders and receivers can deliberately ignore ECN-CE and thus get a
   benefit over behaving flows (cheating).  Nonce [RFC3540] 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

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   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.  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.

   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.

   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 their 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.

   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.

6.4.1.  Fallback mechanisms

   Upon the detection of a potential failure both the sender and the
   receiver can react to mitigate the situation.

   A receiver that detects a packet loss burst MAY schedule an early
   feedback packet to report this to the sender that includes at least
   the RTCP RR and the ECN feedback message.  Thus speeding up the
   detection at the sender of the losses and thus triggering sender side

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   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:

   1.  Periodically retry the ECN initiation to detect if a path change
       occurs to a path that is ECN capable.

   2.  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

   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
   switches 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 ecxessive
   reordering and other issues that are not directly ECN related but
   nevertheless cause problems in receivers.

6.4.2.  Interpretation of ECN Summary information

   This section contains discussion on how you can use the ECN summary
   report information in detecting various types of ECN path issues.
   Lets start to review the information the reports provide on a per
   source (SSRC) basis:

   CE Counter:  The number of RTP packets received so far in the session
      with an ECN field set to CE (11b).

   ECT (0/1) Counters:  The number of RTP packets received so far in the
      session with an ECN field set to ECT (0) and ECT (1) respectively
      (10b / 01b).

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   not-ECT Counter:  The number of RTP packets received so far in the
      session with an ECN field set to not-ECT (00b)

   Lost Packets counter:  The number of RTP packets that are expected
      minus the number received.

   Extended Highest Sequence number:  The highest sequence number seen
      when sending this report, but with additional bits, to handle
      disambiguation when wrapping the RTP sequence number field.

   The counters will be initiated to zero 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.

   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).

   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

   Detecting clearing of ECN field: If the ratio between ECT and not-ECT
   transmitted in the reports has become all not-ECT or substantially
   changed towards not-ECT then this is clearly indication that the path
   results in clearing of the ECT field.

   Dropping of ECT packets: To determine if the packet drop ratio is
   different between not-ECT and ECT marked transmission requires a mix
   of transmitted traffic.  The sender should compare if the delivery
   percentage (delivered / transmitted) between ECT and not-ECT is
   significantly different.  Care must be taken if the number of packets
   are low in either of the categories.  One must also take into account
   the level of CE marking.  A CE marked packet would have been dropped

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   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

   If erronous behavior is detected, it should be logged to enable
   follow up and statistics gathering.

7.  Processing RTCP ECN Feedback in RTP Translators and Mixers

   RTP translators and mixers that support ECN feedback are required to
   process, and potentially modify or generate, RTCP packets for the
   translated and/or mixed streams.  This includes both downstream RTCP
   reports generated by the media sender, and also reports generated by
   the receivers, flowing upstream back towards the sender.

7.1.  Fragmentation and Reassembly in Translators

   An RTP translator may fragment or reassemble RTP data packets without
   changing the media encoding, and without reference to the congestion
   state of the networks it bridges.  An example of this might be to
   combine packets of a voice-over-IP stream coded with one 20ms frame
   per RTP packet into new RTP packets with two 20ms frames per packet,
   thereby reducing the header overheads and so stream bandwidth, at the
   expense of an increase in latency.  If multiple data packets are re-
   encoded into one, or vice versa, the RTP translator MUST assign new
   sequence numbers to the outgoing packets.  Losses in the incoming RTP
   packet stream may also induce corresponding gaps in the outgoing RTP
   sequence numbers.  An RTP translator MUST rewrite RTCP packets to
   make the corresponding changes to their sequence numbers, and to
   reflect the impact of the fragmentation or reassembly.  This section
   describes how that rewriting is to be done for RTCP ECN feedback
   packets.  Section 7.2 of [RFC3550] describes general procedures for
   other RTCP packet types.

   RTCP ECN feedback packets (Section 5.1) 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
   (Section 5.2) 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.

   When receiving an RTCP ECN feedback packet for the translated stream,

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   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.

   Based on its knowledge of the translation process, the translator
   determines the sequence number range for the corresponding original,
   pre-translation, packets.  The extended highest sequence number in
   the RTCP ECN feedback packet is rewritten to match the final sequence
   number in the pre-translation sequence number range.

   The translator then determines the ratio, R, of the number of packets
   in the translated sequence number space (numTrans) to the number of
   packets in the pre-translation sequence number space (numOrig) such
   that R = numTrans / numOrig.  The counter values in the RTCP ECN
   feedback report are then scaled by dividing each of them by R. For
   example, if the translation process combines two RTP packets into
   one, then numOrig will be twice numTrans, giving R=0.5, and the
   counters in the translated RTCP ECN feedback packet will be twice
   those in the original.

   The ratio, R, may have a value that leads to non-integer multiples of
   the counters when translating the RTCP packet.  For example, a VoIP
   translator that combines two adjacent RTP packets into one if they
   contain active speech data, but passes comfort noise packets
   unchanged, would have an R values of between 0.5 and 1.0 depending on
   the amount of active speech.  Since the counter values in the
   translated RTCP report are integer values, rounding will be necessary
   in this case.

   When rounding counter values in the translated RTCP packet, the
   translator should try to ensure that they sum to the number of RTP
   packets in the pre-translation sequence number space (numOrig).  The
   translator should also try to ensure that no non-zero counter is
   rounded to a zero value, 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

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   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.

   It should be noted that scaling the RTCP counter values in this way
   is meaningful only on the assumption that the level of congestion in
   the network is related to the number of packets being sent.  This is
   likely to be a reasonable assumption in the type of environment where
   RTP translators that fragment or reassemble packets are deployed, as
   their entire purpose is to change the number of packets being sent to
   adapt to known limitations of the network, but is not necessarily
   valid in general.

   The rewritten RTCP ECN feedback report is sent from the other side of
   the translator to that which it arrived (as part of a compound RTCP
   packet containing other translated RTCP packets, where appropriate).

7.2.  Generating RTCP ECN Feedback in Media Transcoders

   An RTP translator that acts as a media transcoder cannot directly
   forward RTCP packets corresponding to the transcoded stream, since
   those packets will relate to the non-transcoded stream, and will not
   be useful in relation to the transcoded RTP flow.  Such a transcoder
   will need to interpose itself into the RTCP flow, acting as a proxy
   for the receiver to generate RTCP feedback in the direction of the
   sender relating to the pre-transcoded stream, and acting in place of
   the sender to generate RTCP relating to the transcoded stream, to be
   sent towards the receiver.  This section describes how this proxying
   is to be done for RTCP ECN feedback packets.  Section 7.2 of
   [RFC3550] describes general procedures for other RTCP packet types.

   An RTP translator acting as a media transcoder in this manner does
   not have its own SSRC, and hence is not visible to other entities at
   the RTP layer.  RTCP ECN feedback packets and RTCP XR report blocks
   for ECN summary information that are received from downstream relate
   to the translated stream, and so must be processed by the translator
   as if it were the original media source.  These reports drive the
   congestion control loop and media adaptation between the translator
   and the downstream receiver.  If there are multiple downstream
   receivers, a logically separate transcoder instance must be used for
   each receiver, and must process RTCP ECN feedback and summary reports
   independently to the other transcoder instances.  An RTP translator
   acting as a media transcoder in this manner MUST NOT forward RTCP ECN
   feedback packets or RTCP XR ECN summary reports from downstream
   receivers in the upstream direction.

   An RTP translator acting as a media transcoder will generate RTCP
   reports upstream towards the original media sender, based on the

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   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.

7.3.  Generating RTCP ECN Feedback in Mixers

   An RTP mixer terminates one-or-more RTP flows, combines them into a
   single outgoing media stream, and transmits that new stream as a
   separate RTP flow.  A mixer has its own SSRC, and is visible to other
   participants in the session at the RTP layer.

   An ECN-aware RTP mixer must generate RTCP ECN feedback packets and
   RTCP XR report blocks for ECN summary information relating to the RTP
   flows it terminates, in exactly the same way it would if it were an
   RTP receiver.  These reports form part of the congestion control loop
   between the mixer and the media senders generating the streams it is
   mixing.  A separate control loop runs between each sender and the

   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.

   An RTP mixer MUST NOT forward RTCP ECN feedback packets or RTCP XR
   ECN summary reports reports from downstream receivers in the upstream

8.  Implementation considerations

   To allow the use of ECN with RTP over UDP, the RTP implementation
   must be able to set the ECT bits in outgoing UDP datagrams, and must
   be able to read the value of the ECT bits on received UDP datagrams.
   The standard Berkeley sockets API pre-dates the specification of ECN,
   and does not provide the functionality which is required for this
   mechanism to be used with UDP flows, making this specification
   difficult to implement portably.

9.  IANA Considerations

   Note to RFC Editor: please replace "RFC XXXX" below with the RFC

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   number of this memo, and remove this note.

9.1.  SDP Attribute Registration

   Following the guidelines in [RFC4566], the IANA is requested to
   register one new SDP attribute:

   o  Contact name, email address and telephone number: Authors of

   o  Attribute-name: ecn-capable-rtp

   o  Type of attribute: media-level

   o  Subject to charset: no

   This attribute defines the ability to negotiate the use of ECT (ECN
   capable transport).  This attribute should be put in the SDP offer if
   the offering party wishes to receive an ECT flow.  The answering
   party should include the attribute in the answer if it wish to
   receive an ECT flow.  If the answerer does not include the attribute
   then ECT MUST be disabled in both directions.

9.2.  RTP/AVPF Transport Layer Feedback Message

   The IANA is requested to register one new RTP/AVPF Transport Layer
   Feedback Message in the table of FMT values for RTPFB Payload Types
   [RFC4585] as defined in Section 5.1:

      Name:          RTCP-ECN-FB
      Long name:     RTCP ECN Feedback
      Value:         6
      Reference:     RFC XXXX

9.3.  RTCP XR Report blocks

   The IANA is requested to register one new RTCP XR Block Type as
   defined in Section 5.2:

      Block Type: 13
      Name:       ECN Summary Report
      Reference:  RFC XXXX

9.4.  STUN attribute

   A new STUN [RFC5389] attribute in the Comprehension-optional range
   under IETF Review (0x0000 - 0x3FFF) is request to be assigned to the
   STUN attribute defined in Section 6.2.2.  The STUN attribute registry

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   can currently be found at:

9.5.  ICE Option

   A new ICE option "rtp+ecn" is registered in the non-existing registry
   which needs to be created.

10.  Security Considerations

   The usage of ECN with RTP over UDP as specified in this document has
   the following known security issues that needs to be considered.

   External threats to the RTP and RTCP traffic:

   Denial of Service affecting RTCP:  For an attacker that can modify
      the traffic between the media sender and a receiver can achieve
      either of two things. 1.  Report a lot of packets as being
      Congestion Experience marked, thus forcing the sender into a
      congestion response. 2.  Ensure that the sender disable the usage
      of ECN by reporting failures to receive ECN by changing the
      counter fields.  The Issue, can also be accomplished by injecting
      false RTCP packets to the media sender.  Reporting a lot of CE
      marked traffic is likely the more efficient denial of service tool
      as that may likely force the application to use lowest possible
      bit-rates.  The prevention against an external threat is to
      integrity protect the RTCP feedback information and authenticate
      the sender of it.

   Information leakage:  The ECN feedback mechanism exposes the
      receivers perceived packet loss, what packets it considers to be
      ECN-CE marked and its calculation of the ECN-none.  This is mostly
      not considered sensitive information.  If considered sensitive the
      RTCP feedback shall be encrypted.

   Changing the ECN bits  An on-path attacker that see the RTP packet
      flow from sender to receiver and who has the capability to change
      the packets can rewrite ECT into ECN-CE thus forcing the sender or
      receiver to take congestion control response.  This denial of
      service against the media quality in the RTP session is impossible
      for en end-point to protect itself against.  Only network
      infrastructure nodes can detect this illicit 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.

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   Denial of Service affecting the session set-up signalling:  If an
      attacker can modify the session signalling it can prevent the
      usage of ECN by removing the signalling attributes used to
      indicate that the initiator is capable and willing to use ECN with
      RTP/UDP.  This attack can be prevented by authentication and
      integrity protection of the signalling.  We do note that any
      attacker that can modify the signalling has more interesting
      attacks they can perform than prevent the usage of ECN, like
      inserting itself as a middleman in the media flows enabling wire-
      tapping also for an off-path attacker.

   The following are threats that exist from misbehaving senders or

   Receivers 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.

   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.

   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

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      of the way without being effected to negatively.  However, we do
      note that a media sender still needs to implement congestion
      control functions to prevent the media from being badly affected
      by congestion events.  Thus the misbehaving sender is getting a
      unfair share.  This can only be detected and potentially prevented
      by network monitoring and administrative entities.  See Section 7
      of [RFC3168] for more discussion of this issue.

   We note that the end-point security functions needs to prevent an
   external attacker from affecting the solution easily are source
   authentication and integrity protection.  To prevent what information
   leakage there can be from the feedback encryption of the RTCP is also
   needed.  For RTP there exist multiple solutions possible depending on
   the application context.  Secure RTP (SRTP) [RFC3711] does satisfy
   the requirement to protect this mechanism despite only providing
   authentication if a entity is within the security context or not.
   IPsec [RFC4301] and DTLS [RFC4347] can also provide the necessary
   security functions.

   The signalling protocols used to initiate an RTP session also needs
   to be source authenticated and integrity protected to prevent an
   external attacker from modifying any signalling.  Here an appropriate
   mechanism to protect the used signalling needs to be used.  For SIP/
   SDP ideally S/MIME [RFC5751] would be used.  However, with the
   limited deployment a minimal mitigation strategy is to require use of
   SIPS (SIP over TLS) [RFC3261] [RFC5630] to at least accomplish hop-
   by-hop protection.

   We do note that certain mitigation methods will require network

11.  Examples of SDP Signalling


12.  Open Issues

   As this draft is under development some known open issues exist and
   are collected here.  Please consider them and provide input.

   1.  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.

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   2.  Consider initiation optimizations that allows for multi SSRC
       sender nodes to still have rapid usage of ECN.

   3.  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. (csp)

   4.  Add examples of SDP signalling

13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, September 2001.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3611]  Friedman, T., Caceres, R., and A. Clark, "RTP Control
              Protocol Extended Reports (RTCP XR)", RFC 3611,
              November 2003.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 5348, September 2008.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

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13.2.  Informative References

              Andreasen, F., "A No-Op Payload Format for RTP",
              draft-ietf-avt-rtp-no-op-04 (work in progress), May 2007.

              Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media
              Path Key Agreement for Unicast Secure RTP",
              draft-zimmermann-avt-zrtp-22 (work in progress),
              June 2010.

   [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", RFC 2974, October 2000.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
              Congestion Notification (ECN) Signaling with Nonces",
              RFC 3540, June 2003.

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              July 2003.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

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   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              July 2006.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
              Real-time Transport Control Protocol (RTCP)-Based Feedback
              (RTP/SAVPF)", RFC 5124, February 2008.

   [RFC5506]  Johansson, I. and M. Westerlund, "Support for Reduced-Size
              Real-Time Transport Control Protocol (RTCP): Opportunities
              and Consequences", RFC 5506, April 2009.

   [RFC5630]  Audet, F., "The Use of the SIPS URI Scheme in the Session
              Initiation Protocol (SIP)", RFC 5630, October 2009.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, January 2010.

   [RFC5760]  Ott, J., Chesterfield, J., and E. Schooler, "RTP Control
              Protocol (RTCP) Extensions for Single-Source Multicast
              Sessions with Unicast Feedback", RFC 5760, February 2010.

Authors' Addresses

   Magnus Westerlund
   Farogatan 6
   SE-164 80 Kista

   Phone: +46 10 714 82 87

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   Ingemar Johansson
   Laboratoriegrand 11
   SE-971 28 Lulea

   Phone: +46 73 0783289

   Colin Perkins
   University of Glasgow
   School of Computing Science
   Glasgow  G12 8QQ
   United Kingdom


   Piers O'Hanlon
   University College London
   Computer Science Department
   Gower Street
   London  WC1E 6BT
   United Kingdom


   Ken Carlberg
   1600 Clarendon Blvd
   Arlington  VA


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