Network Working Group                                      M. Westerlund
Internet-Draft                                              I. Johansson
Intended status: Standards Track                                Ericsson
Expires: January 11, 2011                                     C. Perkins
                                                   University of Glasgow
                                                             P. O'Hanlon
                                                             K. Carlberg
                                                           July 10, 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 January 11, 2011.

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   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions, Definitions and Acronyms  . . . . . . . . . . . .  4
   3.  Discussion, Requirements, and Design Rationale . . . . . . . .  4
     3.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Use of ECN with RTP/UDP/IP . . . . . . . . . . . . . . . . . .  9
     4.1.  Negotiation of ECN Capability  . . . . . . . . . . . . . . 12
     4.2.  Initiation of ECN Use in an RTP Session  . . . . . . . . . 17
     4.3.  Ongoing Use of ECN Within an RTP Session . . . . . . . . . 22
     4.4.  Detecting Failures and Receiver Misbehaviour . . . . . . . 26
   5.  RTCP Extensions for ECN feedback . . . . . . . . . . . . . . . 29
     5.1.  ECN Feedback packet  . . . . . . . . . . . . . . . . . . . 29
     5.2.  RTCP XR Report block for ECN summary information . . . . . 32
     5.3.  RTCP XR Report Block for ECN Nonce . . . . . . . . . . . . 33
   6.  Processing RTCP ECN Feedback in RTP Translators and Mixers . . 36
     6.1.  Fragmentation and Reassembly in Translators  . . . . . . . 36
     6.2.  Generating RTCP ECN Feedback in Media Transcoders  . . . . 38
     6.3.  Generating RTCP ECN Feedback in Mixers . . . . . . . . . . 39
   7.  Implementation considerations  . . . . . . . . . . . . . . . . 40
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 40
     8.1.  SDP Attribute Registration . . . . . . . . . . . . . . . . 40
     8.2.  AVPF Transport Feedback Message  . . . . . . . . . . . . . 40
     8.3.  RTCP XR Report blocks  . . . . . . . . . . . . . . . . . . 40
     8.4.  STUN attribute . . . . . . . . . . . . . . . . . . . . . . 41
     8.5.  ICE Option . . . . . . . . . . . . . . . . . . . . . . . . 41
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 41
   10. Examples of SDP Signalling . . . . . . . . . . . . . . . . . . 43
   11. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 44
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 44
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 44
     12.2. Informative References . . . . . . . . . . . . . . . . . . 45
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 46

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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.
   This congestion signal allows applications to react in a controlled
   manner, rather than responding to uncontrolled packet loss, and so
   improves the user experience while benefiting the network.  By
   default, this reaction can be expected to be in the form of reducing
   the transmission rate.  In addition, the use of ECN support outlined
   in this document helps minimize the disruption of the flow (and the
   user experience) by rapidly conveying congested conditions without
   packet loss.

   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.  The means by which ECN is used with RTP over UDP is
   defined in Section 4, along with RTCP extensions for ECN feedback in
   Section 5.  In Section 6 we discuss how RTCP ECN feedback is handled
   in RTP translators and mixers.  Section 7 discusses some
   implementation considerations, Section 8 lists IANA considerations,
   and Section 9 discusses the security considerations.

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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].


   ECN:  Explicit Congestion Notification

   ECT:  ECN Capable Transport

   ECN-CE:  ECN Congestion Experienced

   not-ECT:  Not ECN Capable Transport

3.  Discussion, Requirements, and Design Rationale

   ECN has been specified for use with TCP [RFC3168], SCTP [RFC4960],
   and DCCP [RFC4340] transports.  These are all unicast protocols which
   negotiate the use of ECN during the initial connection establishment
   handshake (supporting incremental deployment, and checking if ECN
   marked packets pass all middleboxes on the path).  ECN Congestion
   Experienced (ECN-CE) marks are immediately echoed back to the sender
   by the receiving end-point using an additional bit in feedback
   messages, and the sender then interprets the mark as equivalent to a
   packet loss for congestion control purposes.

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

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

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

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

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

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

   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.

   These differences will significantly alter the shape of ECN support
   in RTP-over-UDP compared to ECN support in TCP, SCTP, and DCCP, but
   do not invalidate the need for ECN support.  Indeed, in many ways,
   ECN support is more important for RTP sessions, since the impact of
   packet loss in real-time audio-visual media flows is highly visible
   to users.  Effective ECN support for RTP flows running over UDP will
   allow real-time audio-visual applications to respond to the onset of
   congestion before routers are forced to drop packets, allowing those
   applications to control how they reduce their transmission rate, and
   hence media quality, rather than responding to, and trying to conceal
   the effects of, unpredictable packet loss.  Furthermore, widespread
   deployment for ECN and active queue management in routers, should it
   occur, can potentially reduce unnecessary queueing delays in routers,
   lowering the round-trip time and benefiting interactive applications
   of RTP, such as voice telephony.

3.1.  Requirements

   Considering ECN and these protocols one can create a set of
   requirements that must be satisfied to at least some degree if ECN is
   used by an other protocol (such as RTP over UDP)

   o  REQ 1: A mechanism MUST negotiate and initiate the usage of ECN
      for RTP/UDP/IP sessions

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

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   o  REQ 3: Provide 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.

   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 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

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      be used by a sender to determine if all its receivers, and the
      network paths to those receivers, support ECN (see Section 4.2).
      It is somewhat more difficult to determine if all network paths
      from all senders to all receivers support ECN.  Accordingly, we
      allow ECN to be used by an RTP sender using multicast UDP provided
      the sender has verified that the paths to all its known receivers
      support ECN, and irrespective of whether the paths from other
      senders to their receivers support ECN.  Note that group
      membership may change during the lifetime of a multicast RTP
      session, potentially introducing new receivers that are not ECN
      capable.  Senders must use the mechanisms described in Section 4.4
      to monitor that all receivers continue to support ECN, and 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, and should be ECT marked if any of the incoming packets
         are ECT marked.  When RTCP ECN feedback packets (Section 5) are
         received, they must be rewritten to match the modifications

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         made to the media stream (see Section 6.1).

      *  If the translator is a media transcoder, the output RTP media
         stream may have radically different characteristics than the
         input RTP media stream.  Each side of the translator must then
         be considered as a separate transport connection, with its own
         ECN processing.  This requires the translator interpose itself
         into the ECN negotiation process, effectively splitting the
         connection into two parts with their own negotiation.  Once
         negotiation has been completed, the translator must generate
         RTCP ECN feedback back to the source based on its own
         reception, and must respond to RTCP ECN feedback received from
         the receiver(s) (see Section 6.2).

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

   Topo-Mixer:  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.

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

   These topologies may be combined within a single RTP session.

   The ECN mechanism defined in this memo is applicable to both sender
   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.  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.  Failure detection, verification and fallback

   Before an RTP session can be created, a signalling protocol is used
   to discover the other participants and negotiate session parameters
   (see Section 4.1).  One of the parameters that can be negotiated is
   the capability of a participant to support ECN functionality, or

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

   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 4.2):

   o  The sender may generate a (small) subset of its RTP data packets
      with the ECN field set to ECT(0) or ECT(1).  Each receiver will
      then send an RTCP feedback packet indicating the reception of the
      ECT marked RTP packets.  Upon reception of this feedback from each
      receiver it knows of, the sender can consider ECN functional for
      its traffic.  Each sender does this verification independently of
      each other.  If a new receiver joins an existing session it also
      needs to verify ECN support.  If verification fails the sender
      needs to stop using ECN.  As the sender will not know of the
      receiver prior to it sending RTP or RTCP packets, the sender will
      wait for the first RTCP packet from the new receiver to determine
      if that contains ECN feedback or not.

   o  Alternatively, ECN support can be verified during an initial end-
      to-end STUN exchange (for example, as part of ICE connection
      establishment).  After having verified connectivity without ECN
      capability an extra STUN exchange, this time with the ECN field
      set to ECT(0) or ECT(1), is performed.  If successful the path's
      capability to convey ECN marked packets is verified.  A new STUN
      attribute is defined to convey feedback that the ECT marked STUN
      request was received (see Section 8.4), along with an ICE
      signalling option (Section 8.5).

   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

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   clipping if ECN marked RTP packets are discarded by middleboxes, and
   slow verification of ECN support.  The STUN-based mechanism is faster
   to verify ECN support, but only works in those scenarios supported by
   end-to-end STUN, such as within an ICE exchange.  The third one,
   leap-of-faith, has the advantage of avoiding additional tests or
   complexities and enabling ECN usage from the first media packet.  The
   downside is that if the end-to-end path contains middleboxes that do
   not pass ECN, the impact on the application can be severe: in the
   worst case, all media could be lost if a middlebox that discards ECN
   marked packets is present.  A less severe effect, but still requiring
   reaction, is the presence of a middlebox that re-marks ECT marked
   packets to non-ECT, possibly marking packets with a CE mark as non-
   ECT.  This can force the network into heavy congestion due to non-
   responsiveness, and seriously impact media quality.

   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 4.3
   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

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   packet being sent through a node that is ECN non-compliant, thus re-
   marking or dropping packets with ECT set.  To prevent this from
   impacting the application for longer than necessary, the operation of
   ECN is constantly monitored by all senders.  Both the RTCP ECN
   summary reports and the ECN feedback packets allow the sender to
   compare the number of ECT(0), ECT(1), and non-ECT marked packets
   received with the number that were sent, while also reporting CE
   marked and lost packets.  If these numbers do not agree, it can be
   inferred that the path does not reliably pass ECN-marked packets
   (Section 4.4.2 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.

   This specification offers an option of computing and reporting an ECN
   nonce over all received packets that were not ECN-CE marked or
   reported explicitly lost.  This provides an additional means to
   detect any packet re-marking that happens in the network, and can
   also be used by a sender to detect receivers that lie about reception
   of CE-marked packets (it is to be noted that the incentive for
   receivers to lie in their ECN reports is low for RTP/UDP/IP sessions,
   since increased congestion levels are likely to cause unpredictable
   packet losses that decrease the media quality more than would
   reducing the data rate).  To enable the sender to verify the ECN
   nonce, the sender must learn the sequence number of all packets that
   was either CE marked or lost, otherwise it can't correctly exclude
   these packet from the ECN nonce sum.  This is done using a new RTCP
   XR report type, the Nonce Report, that contains the nonce sums and
   indicating the lost or ECN-CE marked packets using a run length
   encoded bit-vector (see Section 5.3).  Due to the size of ECN Nonce
   Reports, and as most RTP-based applications have little incentive to
   lie about ECN marks, the use of the ECN nonce is OPTIONAL.

   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.

4.1.  Negotiation of ECN Capability

   The first stage of ECN negotiation for RTP-over-UDP is to signal the
   capability to use ECN.  This includes negotiating if ECN is to be
   used symmetrically, the method for initial ECT verification, and
   whether the ECN nonce is to be used.  This memo defines the mappings
   of this information onto SDP 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

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   is defined to indicate that ECN initiation using STUN is supported as
   part of an ICE exchange.

   An RTP system that supports ECN and uses SDP in the signalling MUST
   implement the SDP extension to signal ECN capability as described in
   Section 4.1.1.  It MAY also implement alternative ECN capability
   negotiation schemes, such as the ICE extension described in
   Section 4.1.2.

4.1.1.  Signalling ECN Capability using SDP

   One new SDP attribute, "a=ecn-capable-rtp", is defined.  This is a
   media level attribute, which MUST NOT be used at the session level.
   It is not subject to the character set chosen.  The aim of this
   signalling is to indicate the capability of the sender and receivers
   to support ECN, and to negotiate the method 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 4.2.1.

   ice:  Using STUN within ICE as defined in Section 4.2.2.

   leap:  Using the leap of faith method as defined in Section 4.2.3.

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

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

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      case when both the offerer and answerer set the "mode=" parameter
      to "setonly" or "readonly", or when an RTP sender entity considers
      offering "readonly".

   o  The "nonce" parameter may be used to signal whether the ECN nonce
      is to be used in the session.  This parameter takes two values;
      "nonce=1" for nonce proposed or shall be used, and "nonce=0" for
      no nonce.  If this parameter is not specified, the default is no

   o  The "ect" 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.  If the ECN nonce
      is used then this parameter MUST be ignored, and random ECT is
      implied; if the ECN nonce is not used, it 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     = nonce / mode / ect / parm-ext
      mode           = "mode=" ("setonly" / "setread" / "readonly")
      nonce          = "nonce=" ("0" / "1")
      ect            = "ect=" ("random" / "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

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   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.

   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.

   If the "nonce=1" parameter is present in the "a=ecn-capable-rtp"
   attribute of the offer, the answer MUST explicitly include the
   "nonce=" parameter in the "a=ecn-capable-rtp" attribute of the answer
   to indicate if it supports the ECN nonce.  If the answer indicates
   support ("nonce=1") then ECN nonce SHALL be used in the session; if
   the answer does not include the "nonce=" parameter, or includes
   "nonce=0", then the ECN nonce SHALL NOT be used.  The answer MAY
   include a "nonce=0" parameter in an answer even if not included in

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   the offer.  This indicates that the answerer supports and is
   interested in using ECN-nonce in this session, but it is not
   currently enabled.  If the offerer supports use of the nonce then it
   SHOULD run a second round of offer/answer to enable use of the ECN

   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).  If the nonce
   parameter is included then the ECN nonce SHALL be used in the
   session.  The mode parameter MAY be included also in declarative
   usage, to indicate which capability is required by the consumer of
   the SDP.  So for example in a SSM session the participants configured
   with a particular SDP will all be in a media receive only mode, thus
   mode=readonly will work as the capability of reporting on the ECN
   markings in the received is what is required.

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

   As described in Section 4.3.3, RTP sessions using ECN require rapid
   RTCP ECN feedback, in order that the sender can react to ECN-CE
   marked packets.  Thus, the use of the Extended RTP Profile for RTCP-
   Based Feedback (RTP/AVPF) [RFC4585] MUST be signalled.

   When using ECN nonce, the RTCP XR signalling indicating the ECN Nonce
   report MUST also be included in the SDP [RFC3611].

4.1.2.  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

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   to indicate that the initiator of the ICE exchange has the capability
   to support ECN for RTP-over-UDP flows (via "a=ice-options: rtp+ecn").
   The answering party includes this same attribute at the session level
   in the SDP answer if it also has the capability, and removes the
   attribute if it does not wish to use ECN, or doesn't have the
   capability to use ECN.  If this initiation method (Section 4.2.2)
   actually is going to be used, it is explicitly negotiated using the
   "a=ecn-capable-rtp" attribute.

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

4.2.  Initiation of ECN Use in an RTP Session

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

   At the start of the RTP session, when the first packets with ECT are
   sent, it is important to verify that IP packets with ECN field values
   of ECT or ECN-CE will reach 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
   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 4.2.1.  It MAY also
   implement other mechanisms to initiate ECN support, for example the
   STUN-based mechanism described in Section 4.2.2 or use the leap of
   faith option if the session supports the limitations provided in
   Section 4.2.3.  If support for both 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 4.4.  This is
   necessary because path changes or changes in the receiver population

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   may invalidate the ability of the network to support ECN.

4.2.1.  Detection of ECT using RTP and RTCP

   The ECN initiation phase using RTP and RTCP to detect if the network
   path supports ECN comprises three stages.  Firstly, the RTP sender
   generates some 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.  An RTP sender is
      RECOMMENDED to send a minimum of two packets with ECT markings per
      RTCP reporting interval, one with ECT(0) and one with ECT(1), and
      will continue to send some ECT marked traffic as long as the ECN
      initiation phase continues.  The sender SHOULD NOT mark all RTP
      packets as ECT during the ECN initiation phase.

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

   Generating RTCP ECN Feedback:  If ECN capability has been negotiated
      in an RTP session, the 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

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      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 4.3).

      An RTP sender shall consider the group membership to be stable
      after it has been in the session and sending ECT-marked probe
      packets for at least three RTCP reporting intervals (i.e., after
      sending its third regularly scheduled RTCP packet), and when a
      complete RTCP reporting interval has passed without changes to the
      group membership.  ECN initiation is considered successful when
      the group membership 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.

      As an optimisation, if an RTP sender is initiating ECN usage
      towards a unicast address, then it MAY treat the ECN initiation as
      provisionally successful if it receives a single RTCP ECN feedback
      report indicating successful receipt of the ECT-marked packets,
      with no negative indications, from a single RTP receiver.  After
      declaring provisional success, the sender MAY generate ECT-marked
      packets as described in Section 4.3, provided it continues to
      monitor the RTCP reports for a period of three RTCP reporting
      intervals from the time the ECN initiation started, to check if
      there is any other participants in the session.  If other
      participants are detected, the sender MUST fallback to only ECT-
      marking a small fraction of its RTP packets, while it determines
      if ECN can be supported following the full procedure described

         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

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         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 4.3 describes the ongoing monitoring
   that must be performed to ensure the path continues to robustly
   support ECN.

4.2.2.  Detection of ECT using STUN with ICE

   This section describes an OPTIONAL method that can be used to avoid
   media impact and also ensure 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, the STUN server 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

   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.

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

                    Figure 1: ECN Check STUN Attribute

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

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

   Reserved:  Reserved bits (29 bits) SHOULD 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.

4.2.3.  Leap of Faith ECT initiation method

   This method for initiating ECN usage is a leap of faith that assumes
   that ECN will work on the used path(s).  It is not generally
   recommended as the impact on both the application and the network may
   be substantial if the path is not ECN capable.  Applications may
   experience high packet loss rates, this is both from dropped ECT
   marked packets, and as a result of driving the network into higher
   degrees of congestion by not being responsive to ECN marks.  The
   network may experience higher degrees of congestion due to the
   unresponsiveness of the sender due to lost ECN-CE marks from non-
   compliant re-marking.

   The method is to go directly to "ongoing use of ECN" as defined in
   Section 4.3.  Thus all RTP packets MAY be marked as ECT and the
   failure detection MUST be used to detect any case when the assumption
   that the path was ECT capable is wrong.

   If the sender marks all packets as ECT while transmitting on a path
   that contains a middlebox that drops all ECT-marked packets, then a

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   receiver downstream of that middlebox will not receive any RTP data
   packets from that sender, and hence will not consider it to be an
   active RTP SSRC.  The sender can detect this, since SR/RR packets
   from such receivers will either not include a report for the sender's
   SSRC, or will include a report claiming that no packets have been
   received.  The sender should be aware that a receiver may 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 may therefore
   have to wait for a second RTCP report from that receiver to be sure
   that the lack of reception is due to ECT-marking.

   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.  It is NOT RECOMMENDED that the leap-of-faith ECT
   initiation method is used on unmanaged public Internet paths.

4.2.4.  ECN Nonce during initiation

   If the ECN Nonce was enabled in the signalling, it SHALL be used
   during the initiation phase as described in Section

4.3.  Ongoing Use of ECN Within an RTP Session

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

4.3.1.  Transmission of ECT-marked RTP Packets

   After a sender has successfully initiated ECN usage, it SHOULD mark
   all the RTP data packets it sends as ECT.  The sender SHOULD mark
   packets as ECT(0) unless the receiver expresses a preference for
   ECT(1) or random choice using the "ect" parameter in the "a=ecn-
   capable-rtp" attribute; or unless the ECN nonce is in use, in which
   case random ECT marks MUST be used.  If the sender selects a random
   choice of ECT marking, the sender MUST record the statistics for the
   different ECN values sent.  If ECN nonce is activated the sender must
   record the value and calculate the ECN-nonce sum for outgoing packets
   [RFC3540] to allow the use of the ECN-nonce to detect receiver
   misbehaviour (see Section 4.4).  Guidelines on the random choice of
   ECT values are provided in Section 8 of [RFC3540].

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   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.

4.3.2.  Reporting ECN Feedback via RTCP

   An RTP receiver that receives a packet with an ECN-CE mark, or that
   detects a packet loss, MUST schedule the transmission of an RTCP ECN
   feedback packet as soon as possible (subject to the constraints of
   [RFC4585] and [RFC3550]) to report this back to the sender.  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.  ECN Nonce information SHOULD NOT
   be included in early or immediate reports, only when regular reports
   are sent.

   Every time a regular compound RTCP packet is to be transmitted, 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.  If ECN-
   nonce is enabled the receiver MUST also include an RTCP XR Nonce
   report packet as described in Section 5.3.  It is important to
   configure the RTCP bandwidth (e.g. using an SDP "b=" line) such that
   the bit-rate is sufficient for a usage that includes these regular
   summary and nonce reports, and feedback on ECN-CE events.

   The multicast feedback implosion problem, that occurs when many
   receivers simultaneously send feedback to a single sender, must also
   be considered.  The RTP/AVPF transmission rules will limit the amount
   of feedback that can be sent, avoiding the implosion problem but also
   delaying feedback by varying degrees from nothing up to a full RTCP
   reporting interval.  As a result, the full extent of a congestion
   situation may take some time to reach the sender, although some
   feedback should arrive 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

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   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 4.2.1.  ECN Nonce Reporting

   When ECN Nonce reporting is used, it requires both the ECN nonce sum
   and the sequence numbers for packets where the ECN marking has been
   lost to be reported.  This information is variable size as it depends
   on both the total number of packet sent per reporting interval and
   the CE and Packet loss pattern how many bits are required for

   The RTCP packets may be lost, and to avoid the possibility for
   cheating by "losing" the Nonce information for where one is cheating
   the nonce coverage needs to be basically complete.  Thus the Nonce
   reporting SHOULD cover at least the 3 regular reporting intervals.
   The only exception allowed is if the reporting information becomes
   too heavy and makes the RTCP report packet become larger than the
   MTU.  In that case a receiver MAY reduce the coverage for the ECN
   nonce to only the last or two last reporting intervals.  A sender
   should consider the received size report for cases where the coverage
   is not at least three reporting intervals and determine if this may
   be done to cheat or not.  Failure to have reported on all intervals
   MAY be punished by reducing the congestion safe rate.

   The ECN nonce information in the ECN feedback packet consists of both
   a start value for the nonce prior to the first packet in the
   reporting interval and the final 2-bit XOR sum over all the received
   ECN values, both not-ECT and ECT for the report interval.  The report
   interval is explicitly signalled in the RTCP XR Nonce report packet.
   The initial value for the Nonce is 00b.

4.3.3.  Response to Congestion Notifications

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

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   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
      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

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   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.

4.4.  Detecting Failures and Receiver Misbehaviour

   ECN-nonce is defined in RFC3540 as a means to ensure that a TCP
   clients does not mask ECN-CE marks, this assumes that the sending
   endpoint (server) acts on behalf of the network.

   The assumption about the senders acting on the behalf of the network
   may be reduced due to the nature of peer-to-peer use of RTP.  Still a
   significant portion of RTP senders are infrastructure devices (for
   example, streaming media servers) that do have an interest in
   protecting both service quality and the network.  In addition, 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 to not-ECT.  That
   has less dire results, however, it should be detected so that ECN
   usage can be suspended to prevent misusing the network.

   The ECN feedback packet allows the sender to compare the number of
   ECT marked packets of different type with the number it actually
   sent.  The number of ECT packets received plus the number of CE
   marked and lost packets should correspond to the number of sent ECT
   marked packets.  If this number doesn't agree there are two likely
   reasons, a translator changing the stream or not carrying the ECN
   markings forward, or that some node re-marks the packets.  In both
   cases the usage of ECN is broken on the path.  By tracking all the

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   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.

4.4.1.  Fallback mechanisms

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

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

   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.

4.4.2.  Interpretation of ECN Summary information

   This section contains discussion on how you can use the ECN summary
   report information in detecting various types of ECN path issues.

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   Lets start to review the information the reports provide on a per
   source (SSRC) basis:

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

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

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

   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

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   changed towards not-ECT then this is clearly indication that the path
   results in clearing of the ECT field.

   Dropping of ECT packets: To determine if the packet drop ratio is
   different between not-ECT and ECT marked transmission requires a mix
   of transmitted traffic.  The sender should compare if the delivery
   percentage (delivered / transmitted) between ECT and not-ECT is
   significantly different.  Care must be taken if the number of packets
   are low in either of the categories.

4.4.3.  Using ECN-nonce

   This document offers ECN Nonce as a method of strengthening the
   detection of failures, and to allow senders to verify the receiver
   behavior.  We note that it appears counter-productive for a receiver
   to attempt to cheat as it most likely will have negative impact on
   its media quality.  However, certain usages of RTP may result in a
   situation that is more similar to TCP, i.e. where packet losses are
   repaired and a higher bit-rate is desirable.  Thus RTP sessions that
   use repair mechanisms as FEC or retransmission may consider the usage
   of the ECN nonce to prevent cheating.

5.  RTCP Extensions for ECN feedback

   This documents defines three different RTCP extensions: one RTCP AVPF
   NACK Transport feedback format for urgent ECN information; one RTCP
   XR ECN summary report block type for regular reporting of the ECN
   marking information; and one additional RTCP XR report block type for
   ECN nonce.

5.1.  ECN Feedback packet

   This AVPF NACK feedback format is intended for usage in AVPF early or
   immediate feedback modes when information needs to urgently reach the
   sender.  Thus its main 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 RTCP AVPF NACK 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      |          length               |
   |                  SSRC of packet sender                        |
   |                  SSRC of media source                         |
   :            Feedback Control Information (FCI)                 :
   :                                                               :

                   Figure 2: AVPF Feedback common header

   From Figure 2 it can be determined the identity of the feedback
   provider and for which RTP packet sender it applies.  Below is the
   feedback information format defined that is inserted as FCI for this
   particular feedback messages that is identified with an FMT
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   | Extended Highest Sequence Number      | Lost packets counter  |
   |     CE Counter                | not-ECT Counter               |
   | ECT (0) Counter               | ECT (1) Counter               |

                       Figure 3: ECN Feedback Format

   The FCI information for the ECN Feedback format (Figure 3) are the

   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

   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 always equal 3, 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.

5.3.  RTCP XR Report Block for ECN Nonce

   This RTCP XR block is for ECN Nonce reporting.  It consists of an
   initial part that contains the ECN nonce XOR sum, followed by a
   series of bit-vector chunks that indicate which RTP sequence numbers
   were lost or CE-marked, and so weren't included in the ECN nonce sum.
   The bit-vector uses 1 to indicate that the packet wasn't included in
   the ECN nonce sum and 0 for packets that where.

   The bit-vector is expressed using either Run-Length Encoding or 15-
   bit explicit bit-vectors.  The whole vector is encoded using the 16-
   bit chunks as defined by Section 4.1.1, 4.1.2, and 4.1.3 in
   [RFC3611].  The Terminating Null Chunk MUST be used as padding in
   cases the total number of chunks would otherwise be odd and thus the
   report block wouldn't reach a 32-bit boundary.

   The ECN Nonce report block structure is the following:

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |      BT       |R|R|R|R|INV|RNV|         Block Length          |
   |                        SSRC of Media Sender                   |
   |          Begin_seq            |             End_seq           |
   |          chunk 1              |             chunk 2           |
   :                              ...                              :
   |          chunk n-1            |             chunk n           |

   BT:  Block Type, the value identifying this block is [TBA3].

   R: Bits are reserved and MUST be set to 0 on transmission and MUST be
      ignored on reception.

   Block Length:  The block length of this full report block in 32-bit
      words minus one.  The minimal report block size is 3, i.e. fixed
      parts (12 bytes) plus 2 chunks (4 bytes) expressed as 32-bit words
      (3+1) minus 1.

   SSRC of Media Sender  SSRC of Media Sender that this report concerns

   INV:  Initial Nonce Value.  Which is the value of Nonce prior to the
      XOR addition of the ECN field value for the packet that start the
      nonce reporting interval.  This first included sequence number is
      given by the "begin_seq" value.  This to allow running
      calculations and only need to save nonce values at reporting

   RNV:  Resulting Nonce Value.  The Nonce sum value resulting after
      having XOR the ECN field value for all packets received and not
      ECN-CE marked with the INV value up to the packet indicated by the
      "end_seq" sequence number value.

   begin_seq:  First Sequence number this report covers.

   end_seq:  Last RTP sequence number included in this report.

   chunk i:  A chunk reporting on a part of bit-vector indicating if the
      packet was excluded from the ECN Nonce due to being lost or ECN CE

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   The Nonce sum initial value for a new media sender (new SSRC) SHALL
   be 00b.  Otherwise the Initial value is the Nonce value calculated
   for the RTP packet with sequence number begin_seq -1.  The initial
   value for the expressed reporting interval is included in the INV
   field.  The receiver calculates the 2-bit Nonce XOR sum over all
   received RTP packets in the reporting interval including the one with
   end_seq sequence number.  We note that the RTCP participant doing the
   Nonce sum MUST perform suppression of packet duplicates.  The nonce
   sum will become incorrect if any duplicates are included in the sum.
   All packets not received or received as ECN-CE marked when
   constructing the ECN Nonce report MUST be explicitly marked in the

   The Nonce reporting interval is RECOMMENDED to cover all the RTP
   packets received during the three last regular reporting intervals.
   This is to ensure that the sender will receive a report over all RTP
   packets.  Failure to deliver reports that cover all the packets may
   be interpreted as an attempt to cheat.

   Two additional considerations must be made when selecting the
   reporting interval.  First, are the MTU considerations.  The packet
   vector and its encoding into chunks results in a variable sized
   report.  The size depends on two main factors, the number of packets
   to report on and the frequency of bit-value changes in the vector.
   The reporting interval may need to be shortened to two or even one
   reporting interval if the resulting ECN nonce report becomes too big
   to fit into the RTCP packet.

   Secondly, the RTP sequence number can easily wrap and that needs to
   be considered when they are handled.  The report SHALL NOT report on
   more than 32768 consecutive packets.  The last sequence number is the
   extended sequence number that is equal too or smaller (less than
   65535 packets) than the value present in the Receiver Reports
   "extended highest sequence number received" field.  The "first
   sequence number" value is thus an extended sequence number which is
   smaller than the "last sequence number".  If there is a wrap between
   the first sequence number and the last, i.e. if the first sequence
   number is greater than the last sequence number (when seen as 16-bit
   unsigned integers), this needs to included in the calculation.  If an
   application is having these issues, the frequency of the regular RTCP
   reporting should be modified by ensuring that the application chooses
   appropriate settings for the minimum RTCP reporting interval

   Both the ECN-CE and packet loss information is structured as bit
   vectors where the first bit represents the RTP packet with the
   sequence number equal to the First Sequence number.  The bit-vector
   will contain values representing all packets up to and including the

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   one in the "end_seq" field.  The chunk mechanism used to represent
   the bit-vector in an efficient way may appear longer upon reception
   if an explicit bit-vector is used as the last chunk.  Bit-values
   representing packets with higher sequence number (modulo 16) than
   "end_seq" are not valid and SHALL be ignored.

   The produced bit-vector is encoded using chunks.  The chunks are any
   of the three types defined in [RFC3611], Run Length Chunk (Section
   4.1.1 of [RFC3611]), Bit Vector Chunk (Section 4.1.2 of [RFC3611]),
   or Terminating Null Chunk (Section 4.1.3 of [RFC3611]).  Where the
   Terminating Null Chunk may only appear as the last chunk, and only in
   cases where the number of chunks otherwise would be odd.

6.  Processing RTCP ECN Feedback in RTP Translators and Mixers

   RTP translators and mixers that support ECN feedback are required to
   process, and potentially modify or generate, RTCP packets for the
   translated and/or mixed streams.

6.1.  Fragmentation and Reassembly in Translators

   An RTP translator may fragment or reassemble RTP data packets without
   changing the media encoding, 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.

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   When receiving an RTCP ECN feedback packet, 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).

   The RTCP XR Report Block for the ECN nonce is used to convey the ECN
   nonce and an explicit bit vector of which packets were ECN marked.
   It is not meaningful to translate this report block, since it relates
   to particular packets that only exist on one side of the translator.
   An RTP translator MAY silently drop ECN nonce report blocks when
   translating RTCP packets, or it MAY consume ECN nonce report blocks
   received from downstream, and generate its own ECN nonce reports to
   send upstream, based on its reception of the media stream.  If the
   RTP translator is a party to the signalling exchange, ECN nonce
   SHOULD NOT be negotiated.

6.2.  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, RTCP XR report blocks for
   ECN summary information, and RTCP XR report blocks for the ECN nonce
   that are received from downstream relate to the translated stream,
   and so must be processed by the translator as if it were the original

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   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, RTCP XR
   ECN summary reports, or RTCP XR ECN nonce reports from downstream
   receivers in the upstream direction.

   An RTP translator acting as a media transcoder will generate RTCP
   reports upstream towards the original media sender, based on the
   reception quality of the original media stream at the translator.
   The translator will run a separate congestion control loop and media
   adaptation between itself and the media sender for each of its
   downstream receivers, and must generate RTCP ECN feedback packets and
   RTCP XR ECN summary reports (and RTCP XR ECN nonce reports, if
   desired) for that congestion control loop using the SSRC of that
   downstream receiver.

6.3.  Generating RTCP ECN Feedback in Mixers

   An RTP mixer terminates one-or-more RTP flows, combines them into a
   single outgoing media stream, and transmits that new stream as a
   separate RTP flow.  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.  An ECN-aware RTP mixer can optionally generate RTCP XR
   report blocks containing ECN nonce information.  These reports form
   part of the congestion control loop between the mixer and the media
   senders generating the streams it is mixing.  A separate control loop
   runs between each sender and the mixer.

   An ECN-aware RTP mixer will negotiate and initiate the use of ECN on
   the mixed flows it generates, and will accept and process RTCP ECN
   feedback reports, RTCP XR report blocks for ECN, and RTCP XR report
   blocks for the ECN nonce 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, RTCP XR ECN
   summary reports, or RTCP XR ECN nonce reports from downstream
   receivers in the upstream direction.

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7.  Implementation considerations

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

8.  IANA Considerations

   Note to RFC Editor: please replace "RFC XXXX" below with the RFC
   number of this memo, and remove this note.

8.1.  SDP Attribute Registration

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

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

   o  Attribute-name: ecn-capable-rtp

   o  Type of attribute: media-level

   o  Subject to charset: no

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

8.2.  AVPF Transport Feedback Message

   A new RTCP Transport feedback message needs a FMT code point
   assigned. ...

8.3.  RTCP XR Report blocks

   Two new RTCP XR report blocks needs to be assigned block type codes.

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8.4.  STUN attribute

   A new STUN attribute in the Comprehension-optional range needs to be

8.5.  ICE Option

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

9.  Security Considerations

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

   External threats to the RTP and RTCP traffic:

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

   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.  To mitigate the risk of cheating receivers
      the solution include ECN-Nonce that makes it probabilistically
      unlikely that a receiver can cheat for more than a few packets
      before being found out.  See [RFC3168] and [RFC3540] for more

   Receivers misbehaving:  A receiver may prevent the usage of ECN in an
      RTP session by reporting itself as non ECN capable or simple
      provide invalid ECN-nonce values.  Thus forcing the sender to turn
      off usage of ECN.  In a point-to-point scenario there is little
      incentive to do this as it will only affect the receiver.  Thus
      failing to utilise an optimisation.  For multi-party session there
      exist some motivation why a receiver would misbehave as it can
      prevent also the other receivers from using ECN.  As an insider
      into the session it is difficult to determine if a receiver is
      misbehaving or simply incapable, making it basically impossible in
      the incremental deployment phase of ECN for RTP usage to determine
      this.  If additional information about the receivers and the
      network is known it might be possible to deduce that a receiver is
      misbehaving.  If it can be determined that a receiver is
      misbehaving, the only response is to exclude it from the RTP
      session and ensure that is doesn't any longer have any valid
      security context to affect the session.

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   Misbehaving Senders:  The enabling of ECN gives the media packets a
      higher degree of probability to reach the receiver compared to
      not-ECT marked ones.  However, this is no magic bullet and failure
      to react to congestion will most likely only slightly delay a
      buffer under-run, in which its session also will experience packet
      loss and increased delay.  There are some chance that the media
      senders traffic will push other traffic out of the way without
      being effected to negatively.  However, we do note that a media
      sender still needs to implement congestion control functions to
      prevent the media from being badly affected by congestion events.
      Thus the misbehaving sender is getting a unfair share.  This can
      only be detected and potentially prevented by network monitoring
      and administrative entities.  See Section 7 of [RFC3168] for more
      discussion of this issue.

   ECN as covert channel:  As the ECN fields two bits can be set to two
      different values for ECT, it is possible to use ECN as a covert
      channel with a possible bit-rate of one or two bits per packet.
      For more discussion of this issue please see

   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

10.  Examples of SDP Signalling


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11.  Open Issues

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

   1.  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.

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

12.  References

12.1.  Normative References

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

   [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,

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              October 2008.

12.2.  Informative References

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

              Briscoe, B., "Tunnelling of Explicit Congestion
              Notification", draft-ietf-tsvwg-ecn-tunnel-08 (work in
              progress), March 2010.

              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.

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

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

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

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

   [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
   Department 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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