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Versions: (draft-levin-avt-rtcp-burst) 00 01 02 03 draft-ietf-avt-rapid-acquisition-for-rtp

AVT                                                          B. VerSteeg
Internet-Draft                                                  A. Begen
Intended status:  Standards Track                          Cisco Systems
Expires:  October 18, 2009                                T. VanCaenegem
                                                          Alcatel-Lucent
                                                                  Z. Vax
                                                   Microsoft Corporation
                                                          April 16, 2009


       Unicast-Based Rapid Acquisition of Multicast RTP Sessions
          draft-versteeg-avt-rapid-synchronization-for-rtp-03

Status of this Memo

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Copyright Notice




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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   When an RTP receiver joins a primary multicast session, it may need
   to acquire and parse certain Reference Information before it can
   process any data sent in the multicast session.  Depending on the
   join time, length of the Reference Information repetition interval,
   size of the Reference Information as well as the application and
   transport properties, the time lag before an RTP receiver can
   usefully consume the multicast data, which we refer to as the
   Acquisition Delay, varies and may be large.  This is an undesirable
   phenomenon for receivers that frequently switch among different
   multicast sessions, such as video broadcasts.

   In this document, we describe a method using the existing RTP and
   RTCP protocol machinery that reduces the acquisition delay.  In this
   method, an auxiliary unicast RTP session carrying the Reference
   Information to the receiver precedes/accompanies the primary
   multicast stream.  This unicast RTP flow may be transmitted at a
   faster than natural rate to further accelerate the acquisition.  The
   motivating use case for this capability is multicast applications
   that carry real-time compressed audio and video.  However, the
   proposed method can also be used in other types of multicast
   applications where the acquisition delay is long enough to be a
   problem.


















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  6
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Elements of Delay in Multicast Applications  . . . . . . . . .  7
   5.  Protocol Design Considerations and Their Effect on
       Resource Management for Rapid Acquisition  . . . . . . . . . .  9
   6.  Rapid Acquisition of Multicast RTP Sessions  . . . . . . . . . 11
     6.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.2.  Message Flows  . . . . . . . . . . . . . . . . . . . . . . 12
     6.3.  Shaping the Unicast Burst  . . . . . . . . . . . . . . . . 20
     6.4.  Failure Cases  . . . . . . . . . . . . . . . . . . . . . . 20
   7.  Encoding of the Signaling Protocol in RTCP . . . . . . . . . . 21
     7.1.  RAMS Request . . . . . . . . . . . . . . . . . . . . . . . 22
     7.2.  RAMS Information . . . . . . . . . . . . . . . . . . . . . 23
     7.3.  RAMS Termination . . . . . . . . . . . . . . . . . . . . . 26
     7.4.  Extensions . . . . . . . . . . . . . . . . . . . . . . . . 27
   8.  SDP Definitions and Examples . . . . . . . . . . . . . . . . . 28
     8.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . 28
     8.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 28
   9.  NAT Considerations . . . . . . . . . . . . . . . . . . . . . . 31
   10. Known Implementations  . . . . . . . . . . . . . . . . . . . . 31
     10.1. Open Source RTP Receiver Implementation by Cisco . . . . . 31
     10.2. IPTV Commercial Implementation by Microsoft  . . . . . . . 31
   11. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 32
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 32
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 32
     13.1. Registration of SDP Attribute Values . . . . . . . . . . . 32
     13.2. Registration of FMT Values . . . . . . . . . . . . . . . . 33
   14. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 33
   15. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     15.1. draft-versteeg-avt-rapid-synchronization-for-rtp-03  . . . 34
     15.2. draft-versteeg-avt-rapid-synchronization-for-rtp-02  . . . 34
     15.3. draft-versteeg-avt-rapid-synchronization-for-rtp-01  . . . 34
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 35
     16.2. Informative References . . . . . . . . . . . . . . . . . . 36
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37












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1.  Introduction

   Most multicast flows carry a stream of inter-related data.  Certain
   information must first be acquired by the receivers to start
   processing any data sent in the multicast session.  This document
   refers to this information as Reference Information.  The Reference
   Information is conventionally sent periodically in the multicast
   session and usually consists of items such as a description of the
   schema for the rest of the data, references to which data to process
   for the receivers, encryption information including keys, as well as
   any other information required to process the data in the primary
   multicast stream.

   Real-time multicast applications require the receivers to buffer
   data.  The receiver may have to buffer data to smooth out the network
   jitter, to allow loss-repair methods such as Forward Error Correction
   and retransmission to recover the missing packets, and to satisfy the
   data processing requirements of the application layer.

   When a receiver joins a multicast session, it has no control over
   what point in the flow is currently being transmitted.  Sometimes the
   receiver may join the session right before the Reference Information
   is sent in the session.  In this case, the required waiting time is
   usually minimal.  Other times, the receiver may join the session
   right after the Reference Information has been transmitted.  In this
   case, the receiver has to wait for the Reference Information to
   appear again in the flow before it can start processing any multicast
   data.  In some other cases, the Reference Information is not
   contiguous in the flow but dispersed over a large period, which
   forces the receiver to wait for all of the Reference Information to
   arrive before starting to process the rest of the data.

   The net effect of waiting for the Reference Information and waiting
   for various buffers to fill up is that the receivers may experience
   significantly large delays in data processing.  In this document, we
   refer to the difference between the time an RTP receiver joins the
   multicast session and the time the RTP receiver acquires all the
   necessary Reference Information as the Acquisition Delay.  The
   acquisition delay may not be the same for different receivers; it
   usually varies depending on the join time, length of the Reference
   Information repetition interval, size of the Reference Information as
   well as the application and transport properties.

   The varying nature of the acquisition delay adversely affects the
   receivers that frequently switch among multicast sessions.  In this
   specification, we address this problem for RTP-based multicast
   applications and describe a method that uses the fundamental tools
   offered by the existing RTP and RTCP protocols [RFC3550].  In this



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   method, either the multicast source (or the distribution source in a
   single-source multicast (SSM) session) retains the Reference
   Information for a period after its transmission, or an intermediary
   network element joins the multicast session and continuously caches
   the Reference Information as it is sent in the session and acts as a
   feedback target (See [I-D.ietf-avt-rtcpssm]) for the session.  When
   an RTP receiver wishes to join the same multicast session, instead of
   simply issuing a Source Filtering Group Management Protocol (SFGMP)
   Join message, it sends a request to the feedback target for the
   session asking for the Reference Information.  The feedback target
   starts a unicast retransmission RTP session and sends the Reference
   Information to the RTP receiver over that session.  If there is spare
   bandwidth, the feedback target may also burst the Reference
   Information at a faster than its natural rate.  As soon as the
   receiver acquires the Reference Information, it can join the
   multicast session and start processing the multicast data.  This
   method potentially reduces the acquisition delay.  We refer to this
   method as Unicast-based Rapid Acquisition of Multicast RTP Sessions.
   A simplified network diagram showing this method through an
   intermediary network element is depicted in Figure 1.


                                       +-------------------+
                                  +--->|   Intermediary    |
                                  | ...|  Network Element  |
                                  | :  +-------------------+
                                  | :
                                  | v
          +-----------+        +----------+           +----------+
          | Multicast |        |  Router  |---------->| Joining  |
          |  Source   |------->|          |..........>|   RTP    |
          +-----------+        +----------+           | Receiver |
                                    |                 +----------+
                                    |
                                    |                 +----------+
                                    +---------------->| Existing |
                                                      |    RTP   |
                                                      | Receiver |
                                                      +----------+

          ...> Unicast RTP Flow
          ---> Multicast RTP Flow

    Figure 1: Rapid acquisition through an intermediary network element

   A primary design goal in this solution is to use the existing tools
   in the RTP/RTCP protocol family.  This improves the versatility of
   the existing implementations, and promotes faster deployment and



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   better interoperability.  To this effect, we use the unicast
   retransmission support of RTP [RFC4588] and the capabilities of RTCP
   to handle the signaling needed to accomplish the acquisition.  The
   packet(s) carrying the Reference Information are sent by the feedback
   target in the auxiliary unicast RTP session for rapid acquisition.
   These are constructed as retransmission packets that would have been
   sent in a unicast RTP session to recover the missing packets at an
   RTP receiver that has never received any packet.  In fact, a single
   RTP session MAY be used for both rapid acquisition and
   retransmission-based loss repair.  Furthermore, the session can be
   used to simultaneously provide the unicast burst for the rapid
   acquisition and the repair packets requested by the RTP receivers
   when they detect lost burst packets or lost RTP packets in the
   primary multicast stream.  The conventional RTCP feedback message
   that requests the retransmission of the missing packets [RFC4585]
   indicates their sequence numbers.  However, upon joining a new
   session the RTP receiver has never received a packet, and thus, does
   not know the sequence numbers.  Instead, the RTP receiver sends a
   newly defined RTCP feedback message to request the Reference
   Information needed to rapidly get on the track with the primary
   multicast session.  It is also worth noting that in order to issue
   the initial RTCP message to the feedback target, the SSRC of the
   session to be joined must be known prior to any packet reception, and
   hence, needs to be signaled out-of-band (or otherwise communicated to
   the RTP receiver in advance of the initiation of the rapid
   acquisition operation).  In a Session Description Protocol (SDP)
   description, the SSRC MUST be signaled through the 'ssrc' attribute
   [I-D.ietf-avt-rtcpssm].

   In the rest of this specification, we have the following outline:  In
   Section 4, we describe the delay components in generic multicast
   applications.  Section 5 presents an overview of the protocol design
   considerations for rapid acquisition.  We provide the protocol
   details of the rapid acquisition method in Section 6 and Section 7.
   Section 8 and Section 9 discuss the SDP signaling issues with
   examples and NAT-related issues, respectively.

   Note that Section 3 provides a list of the definitions frequently
   used in this document.


2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].





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3.  Definitions

   This document uses the following acronyms and definitions frequently:

   Primary Multicast Session:  The multicast RTP session to which RTP
   receivers can join at a random point in time.

   Primary Multicast Stream:  The RTP stream carried in the primary
   multicast session.

   Source Filtering Group Management Protocol (SFGMP):  Following the
   definition in [RFC4604], SFGMP refers to the Internet Group
   Management Protocol (IGMP) version 3 [RFC3376] and the Multicast
   Listener Discovery Protocol (MLD) version 2 [RFC3810] in the IPv4 and
   IPv6 networks, respectively.

   Feedback Target:  (Unicast RTCP) Feedback target as defined in
   [I-D.ietf-avt-rtcpssm].

   Retransmission Packet:  An RTP packet that is formatted as defined in
   [RFC4588].

   Reference Information:  The set of certain media content and metadata
   information that is sufficient for an RTP receiver to start usefully
   consuming a media stream.  The meaning, format and size of this
   information are specific to the application and are out of scope of
   this document.

   Burst (Stream):  A unicast stream of RTP retransmission packets that
   enable an RTP receiver to rapidly acquire the Reference Information.
   The burst stream is typically transmitted at an accelerated rate.

   Retransmission Server (RS):  The RTP/RTCP endpoint that can generate
   the retransmission packets and the burst stream.


4.  Elements of Delay in Multicast Applications

   In an any-source (ASM) or a single-source (SSM) multicast delivery
   system, there are three major elements that contribute to the overall
   acquisition delay when an RTP receiver switches from one multicast
   session to another one.  These are:

   o  Multicast switching delay

   o  Reference Information latency





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   o  Buffering delays

   Multicast switching delay is the delay that is experienced to leave
   the current multicast session (if any) and join the new multicast
   session.  In typical systems, the multicast join and leave operations
   are handled by a group management protocol.  For example, the
   receivers and routers participating in a multicast session may use
   the Internet Group Management Protocol (IGMP) version 3 [RFC3376] or
   the Multicast Listener Discovery Protocol (MLD) version 2 [RFC3810].
   In either of these protocols, when a receiver wants to join a
   multicast session, it sends a message to its upstream router and the
   routing infrastructure sets up the multicast forwarding state to
   deliver the packets of the multicast session to the new receiver.
   Depending on the proximity of the upstream router, the current state
   of the multicast tree, the load on the system and the protocol
   implementation, the join times vary.  Current systems provide join
   latencies usually less than 200 milliseconds (ms).  If the receiver
   had been participating in another multicast session before joining
   the new session, it needs to send a Leave message to its upstream
   router to leave the session.  In common multicast routing protocols,
   the leave times are usually smaller than the join times, however, it
   is possible that the Leave and Join messages may get lost, in which
   case the multicast switching delay inevitably increases.

   Reference Information latency is the time it takes the receiver to
   acquire the Reference Information.  It is highly dependent on the
   proximity of the actual time the receiver joined the session to the
   next time the Reference Information will be sent to the receivers in
   the session, whether the Reference Information is sent contiguously
   or not, and the size of the Reference Information.  For some
   multicast flows, there is a little or no interdependency in the data,
   in which case the Reference Information latency will be nil or
   negligible.  For other multicast flows, there is a high degree of
   interdependency.  One example of interest is the multicast flows that
   carry compressed audio/video.  For these flows, the Reference
   Information latency may become quite large and be a major contributor
   to the overall delay.

   The buffering component of the overall acquisition delay is driven by
   the way the application layer processes the payload.  In many
   multicast applications, an unreliable transport protocol such as UDP
   [RFC0768] is often used to transmit the data packets, and the
   reliability, if needed, is usually addressed through other means such
   as Forward Error Correction and retransmission
   [I-D.ietf-rmt-pi-norm-revised].  These loss-repair methods require
   buffering at the receiver side to function properly.  In many
   applications, it is also often necessary to de-jitter the incoming
   data packets before feeding them to the application.  The de-



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   jittering process also increases the buffering delays.  Besides these
   network-related buffering delays, there are also specific buffering
   needs that are required by the individual applications.  For example,
   standard video decoders typically require an amount, sometimes a
   significant amount, of coded video data to be available in the pre-
   decoding buffers prior to starting to decode the video bitstream.


5.  Protocol Design Considerations and Their Effect on Resource
    Management for Rapid Acquisition

   Rapid acquisition is an optimization of a system that must continue
   to work correctly whether or not the optimization is effective, or
   even fails due to lost control messages, congestion, or other
   problems.  This is fundamental to the overall design requirements
   surrounding the protocol definition and to the resource management
   schemes to be employed together with the protocol (e.g., QoS
   machinery, server load management, etc).  In particular, the system
   needs to operate within a number of constraints:

   o  First, a rapid acquisition operation must fail gracefully.  The
      user experience must, except perhaps in pathological
      circumstances, be not significantly worse for trying and failing
      to complete rapid acquisition compared to simply joining the
      multicast session.

   o  Second, providing the rapid acquisition optimizations must not
      cause collateral damage to either the multicast session being
      joined, or other multicast sessions sharing resources with the
      rapid acquisition operation.  In particular, the rapid acquisition
      operation must avoid self-interference with the multicast session
      that may be simultaneously being received by other hosts.  In
      addition, it must also avoid interference with other multicast
      sessions sharing the same network resources.  These properties are
      possible, but are usually difficult to achieve.

   One challenge is the existence of multiple bandwidth bottlenecks
   between the receiver and the server(s) in the network providing the
   rapid acquisition service.  In commercial IPTV deployments, for
   example, bottlenecks are often present in the aggregation network
   connecting the IPTV servers to the network edge, the access links
   (e.g., DSL, DOCSIS) and in the home network of the subscribers.  Some
   of these links may serve only a single subscriber, limiting
   congestion impact to the traffic of only that subscriber, but others
   can be shared links carrying multicast sessions of many subscribers.
   Also note that the state of these links may be varying over time.
   The receiver may have knowledge of a portion of this network, or may
   have partial knowledge of the entire network.  The methods employed



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   by the devices to acquire this network state information is out of
   scope for this document.  The receiver should be able to signal the
   server with the bandwidth that it believes it can handle.  The server
   also needs to be able to rate limit the flow in order to stay within
   the performance envelope that it knows about.  Both the server and
   receiver need to be able to inform the other of changes in the
   requested and delivered rates.  However, the protocol must be robust
   in the presence of packet loss, so this signaling must include the
   appropriate default behaviors.

   A second challenge is that for some uses (e.g., high-bitrate video)
   the unicast burst bandwidth is high while the flow duration of the
   unicast burst is short.  This is because the purpose of the unicast
   burst is to allow the RTP receiver to join the multicast quickly and
   thereby limit the overall resources consumed by the burst.  Such
   high-bitrate, short-duration flows are not amenable to conventional
   admission control techniques.  For example, per-flow signaled
   admission control techniques such as RSVP have too much latency and
   control channel overhead to be a good fit for rapid acquisition.
   Similarly, using a TCP (or TCP-like) approach with a 3-way handshake
   and slow-start to avoid inducing congestion would defeat the purpose
   of attempting rapid acquisition in the first place by introducing
   many RTTs of delay.

   These observations lead to certain unavoidable requirements and goals
   for a rapid acquisition protocol.  These are:

   o  The protocol must be designed to allow a deterministic upper bound
      on the extra bandwidth used (compared to just joining the
      multicast session).  A reasonable size bound is e*B, where B is
      the "nominal" bandwidth of the primary multicast stream, and e is
      an "excess-bandwidth" coefficient The total duration of the
      unicast burst must have a reasonable bound; long unicast bursts
      devolve to the bandwidth profile of multi-unicast for the whole
      system.

   o  The scheme should minimize (or better eliminate) the overlap of
      the unicast burst and the primary multicast stream.  This
      minimizes the window during which congestion could be induced on a
      bottleneck link compared to just carrying the multicast or unicast
      packets alone.

   o  The scheme must minimize (or better eliminate) any gap between the
      unicast burst and the primary multicast stream, which has to be
      repaired later, or in the absence of repair, will result in loss
      being experienced by the application.

   In addition to the above, there are some other protocol design issues



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   to be considered.  First, there is at least one RTT of "slop" in the
   control loop.  In starting a rapid acquisition burst, this manifests
   as the time between the client requesting the unicast burst and the
   burst description (and possibly the first unicast burst packets)
   arriving at the receiver.  For managing and terminating the unicast
   burst, there are two possible approaches for the control loop:  The
   receiver can adapt to the unicast burst as received, converge based
   on observation and explicitly terminate the unicast burst with a
   second control loop exchange (which takes a minimum of one RTT, just
   as starting the unicast burst does).  Alternatively, the server
   generating the unicast burst can pre-compute the burst parameters
   based on the information in the initial request and tell the receiver
   the burst duration.

   The protocol described in the next section allows either method of
   controlling the rapid acquisition unicast burst.


6.  Rapid Acquisition of Multicast RTP Sessions

   We start this section with an overview of the rapid acquisition of
   multicast sessions (RAMS) method.

6.1.  Overview

   [I-D.ietf-avt-rtcpssm] specifies an extension to the RTP Control
   Protocol (RTCP) to use unicast feedback in an SSM session.  It
   defines an architecture that introduces the concept of Distribution
   Source, which - in an SSM context - distributes the RTP data and
   redistributes RTCP information to all RTP receivers.  This RTCP
   information is retrieved from the Feedback Target, to which RTCP
   unicast feedback traffic is sent.  The Feedback Target MAY be
   implemented in one or more entities different from the Distribution
   Source, and different RTP receivers MAY use different Feedback
   Targets.

   This document builds further on these concepts to reduce the
   acquisition time when an RTP receiver wants to join a multicast
   session at a random point in time by introducing the concept of the
   Burst Source and new RTCP feedback messages.  The Burst Source has a
   cache where the most recent packets from the primary multicast
   session are continuously stored.  When an RTP receiver wants to
   receive the primary multicast stream, prior to joining the SSM
   session, it will first request a unicast burst from the Burst Source.
   In this burst, the packets are formatted as RTP retransmission
   packets [RFC4588] and carry the Reference Information.  This
   information allows the RTP receiver to start usefully consuming the
   RTP packets sent in the primary multicast session.



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   Using an accelerated rate (as compared to the rate of the primary
   multicast stream) for the unicast burst implies that at a certain
   point in time, the payload transmitted in the unicast burst is going
   to be the same as the payload multicast in the SSM session, i.e., the
   unicast burst will catch up with the primary multicast stream.  At
   this point, the RTP receiver no longer needs to receive the unicast
   burst and can join the primary multicast session.  This method is
   referred to as the Rapid Acquisition of Multicast Sessions (RAMS).

   This document proposes extensions to [RFC4585] for an RTP receiver to
   request a unicast burst as well as for additional control messaging
   that can be leveraged during the acquisition process.

6.2.  Message Flows

   Figure 2 shows the main entities involved in rapid acquisition:

   o  Multicast Source

   o  Feedback Target (FT)

   o  Burst Source

   o  Retransmission Source

   o  RTP Receiver (RR)

























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             +----------------------------------------------+
             |            Retransmission Server             |
             |                    (RS)                      |
             |  +----------+ +--------+ +----------------+  |
             |  | Feedback | | Burst  | | Retransmission |  |
             |  |  Target  | | Source | |     Source     |  |
             |  +----------+ +--------+ +----------------+  |
             +----------------------------------------------+
                                 ^ ^ :
                                 | ' :
                                 | ' :
                                 | ' v
         +-----------+        +----------+            +----------+
         |           |        |          |'''''''''''>|          |
         | Multicast |------->|  Router  |...........>|   RTP    |
         |  Source   |        |          |<~~~~~~~~~~~| Receiver |
         |           |        |          |----------->|   (RR)   |
         +-----------+        +----------+            +----------+


         '''> Unicast RTCP Messages
         ~~~> SFGMP Messages
         ...> Unicast RTP Flow
         ---> Multicast RTP Flow

        Figure 2: Flow diagram for unicast-based rapid acquisition

   A Retransmission Source can equally act as a Burst Source.  The
   Retransmission Source can also incorporate the Feedback Target
   ([I-D.ietf-avt-rtcpssm] permits the feedback target to be a
   retransmission server, since it is a logical function to which RRs
   send their unicast feedback), and we will use the term Retransmission
   Server (RS) in the remainder of the document to refer to a single
   physical entity comprising these three entities.  Note that the same
   method (with the identical message flows) would also apply in a
   scenario where rapid acquisition is performed by a feedback target
   co-located with the media source.

   As the unicast burst packets are formatted as RTP retransmission
   packets [RFC4585], the unicast burst and RTP retransmissions MAY be
   provided in one and the same RTP (retransmission) session.

   The unicast burst is triggered by the RTCP feedback message defined
   in this document, whereas an RTP retransmission is triggered by an
   RTCP NACK message defined in [RFC4585].  Pending on RAMS practices,
   there may be a gap between the end of the burst and the reception of
   the primary multicast stream because of the imperfections in the
   switch-over.  If needed, RR can make use of the RTCP NACK message to



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   request a retransmission for the missing packets in the gap.

   Editor's note:  As stated above, FT, Burst Source and Retransmission
   Source are logical entities.  For efficiency and simplicity, they MAY
   be implemented by a single physical Retransmission Server (RS).  In a
   number of places throughout this document we assume (and explicitly
   state so) that all three are implemented by the same physical entity
   and therefore share the same IP address and the port number.  The
   authors look to the AVT community for recommendations on whether this
   is sufficient or the mechanism has to explicitly address other
   topologies where FT, Burst Source and Retransmission Source are not
   co-located.

   Figure 3 depicts an example of messaging flow for rapid acquisition.
   The RTCP feedback messages are explained below.  Note that the
   messages indicated in parentheses may or may not be present during
   rapid acquisition.


































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     +-----------+   +----------------+   +----------+   +------------+
     | Multicast |   | Retransmission |   |          |   |    RTP     |
     |  Source   |   |     Server     |   |  Router  |   |  Receiver  |
     |           |   |      (RS)      |   |          |   |    (RR)    |
     +-----------+   +----------------+   +----------+   +------------+
         |                   |                 |                |
         |-- RTP Multicast ------------------->|                |
         |                   |                 |                |
         |-- RTP Multicast ->|                 |                |
         |                   |                 |                |
         |                   |<'''''''''''''''''' RTCP RAMS-R ''|
         |                   |                 |                |
         |                   |                 |                |
         |                   |'' (RTCP RAMS-I) ''''''''''''''''>|
         |                   |                 |                |
         |                   |                 |                |
         |                   |.. Unicast RTP Burst ............>|
         |                   |                 |                |
         |                   |<''''''''''''''''''(RTCP RAMS-R)''|
         |                   |                 |                |
         |                   |                 |                |
         |                   |'' (RTCP RAMS-I) ''''''''''''''''>|
         |                   |                 |                |
         |                   |                 |                |
         |                   |                 |<~ SFGMP Join ~~|
         |                   |                 |                |
         |                   |                 |                |
         |-- RTP Multicast ------------------------------------>|
         |                   |                 |                |
         |                   |                 |                |
         |                   |<'''''''''''''''''' RTCP RAMS-T ''|
         |                   |                 |                |
         |                   |                 |                |
         |                   |<''''''''''''''''''' (RTCP NACK)''|
         |                   |                 |                |
         |                   |                 |                |
         |                   |.. (Unicast Retransmissions) ....>|
         |                   |                 |                |
         |                   |                 |                |
         |                   |<''''''''''''''''''' (RTCP BYE) ''|
         |                   |                 |                |
         |                   |                 |                |


     '''> Unicast RTCP Messages
     ~~~> SFGMP Messages
     ...> Unicast RTP Flow
     ---> Multicast RTP Flow



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        Figure 3: Message flows for unicast-based rapid acquisition

   This document defines the expected behaviors of RS and RR.  It is
   instructive to have independently operating implementations on RS and
   RR that request the burst, describe the burst, start the burst, join
   the multicast session and stop the burst.  These implementations send
   messages to each other, but there MUST be provisions for the cases
   where the control messages get lost, or re-ordered, or are not being
   delivered to their destinations.

   The following steps describe rapid acquisition in detail:

   1.  Request:  RR sends a rapid acquisition request for the new
       multicast RTP session to the feedback target address of that
       session.  The request contains the SSRC of RR and the SSRC of the
       media source.  This RTCP feedback message is defined as the RAMS-
       Request (RAMS-R) message and MAY contain parameters, which may
       constrain the burst, such as the bandwidth limit.  Other
       parameters may be related to the amount of buffering capacity
       available at RR, which may be used by RS to prepare a burst that
       conforms with RR's requirements.

       Before joining the primary multicast session, a new joining RR
       learns the addresses associated with the new multicast session
       (addresses for the multicast source, group and retransmission
       server) by out-of-band means.  Also note that since no RTP
       packets have been received yet for this session, the SSRC must be
       obtained out-of-band.  See Section 8 for details.

   2.  Response:  RS receives the RAMS-R message and decides whether to
       accept it or not.  RS MUST send an (at least one) RAMS-
       Information (RAMS-I) message to RR.  The first RAMS-I message MAY
       precede the unicast burst or it MAY be sent during the burst.
       Additional RAMS-I messages MAY be sent during the burst and these
       RAMS-I messages may or may not be a direct response to an RAMS-R
       message.

       Note that RS learns the IP address and port information for RR
       from the RAMS-R message it received.  (This description glosses
       over the NAT details.  Refer to Section 9 for a discussion of
       NAT-related issues.)

       If RS cannot provide a rapid acquisition service, RS rejects the
       request and informs RR immediately via an RAMS-I message.  If RR
       receives a message indicating that its rapid acquisition request
       has been denied, it abandons the rapid acquisition attempt and
       MAY immediately join the multicast session by sending an SFGMP
       Join message to its upstream multicast router for the new primary



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       multicast session.

       If RS accepts the request, it sends an RAMS-I message to RR
       (before commencing the unicast burst or during the unicast burst)
       that comprises fields that can be used to describe the unicast
       burst (e.g., the maximum bitrate and the duration of the unicast
       burst).

       The unicast burst duration MAY be calculated by RS, and its value
       MAY be updated by messages received from RR.  The join time
       information (for the new multicast session) MUST be populated in
       at least one of the RAMS-I messages.  Note that RS MAY send the
       RAMS-I message after a significant delay, so RR SHOULD NOT make
       protocol dependencies on quickly receiving an RAMS-I message.

   3.  Unicast Burst:  If the request is accepted, RS starts sending the
       unicast burst that comprises one or more RTP retransmission
       packets.  In addition, there MAY be optional payload-specific
       information that RS chooses to send to RR.  Such an example is
       discussed in [I-D.begen-avt-rtp-mpeg2ts-preamble] for
       transmitting the payload-specific information for MPEG2 Transport
       Streams.

   4.  Updated Request:  RR MAY send a new RAMS-R message with a
       different value for one or more fields of an earlier RAMS-R
       message.  Upon receiving an updated request, RS MAY use the
       updated values for sending/shaping the burst, or refine the
       values and use the refined values for sending/shaping the burst.
       RS MAY send a new RAMS-I message to indicate the changes it made.
       However, note that RS does not have to send a new RAMS-I, or the
       new RAMS-I message may get lost.  It is also possible that the
       updated RAMS-R message could have been lost.  Thus, RR SHOULD NOT
       make protocol dependencies on quickly (or ever) receiving a new
       RAMS-I message, or assume that RS will honor the requested
       changes.

       RR may be in an environment where the available resources are
       time-varying, which may or may not deserve sending a new updated
       request.  Determining the circumstances where RR should or should
       not send an updated request and the methods that RR can use to
       detect and evaluate the time-varying available resources are not
       specified in this document.

   5.  Updated Response:  RS may send more than one RAMS-I messages,
       e.g., to update the value of one or more fields in an earlier
       RAMS-I message and/or to signal RR in real time to join the
       primary multicast session.  RR usually depends on RS to learn the
       join time, which can be conveyed by the first RAMS-I message, or



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       can be sent/revised in a later RAMS-I message.  If RS is not
       capable of determining the join time in the first RAMS-I message,
       it MUST send another RAMS-I message (with the join time
       information) later.

   6.  Multicast Join Signaling:  In principal, RR can join the primary
       multicast session any time during or after the end of the unicast
       burst via an SFGMP Join message.  However, there may be missing
       packets if RR joins the primary multicast session too early or
       too late.  For example, if RR starts receiving the primary
       multicast stream while it is still receiving the unicast burst at
       a high excess bitrate, this may result in an increased risk of
       packet loss.  Or, if RR joins the primary multicast session some
       time after the unicast burst is finished, there may be a gap
       between the burst and multicast data (a number of RTP packets may
       be missing).  In both cases, RR MAY issue retransmissions
       requests [RFC4585] to fill the gap.

       Yet, there are cases where the remaining available bandwidth may
       limit the number of retransmissions that can be provided within a
       certain time period, causing the retransmission data to arrive
       too late at RR (from an application-layer point of view).  To
       cope with such cases, the RAMS-I message allows RS to signal
       explicitly when RR should send the SFGMP Join message.
       Alternatively, RS may pre-compute the burst duration and the time
       RR should send the SFGMP Join message.  This information may be
       conveyed in the RAMS-I message and can be updated in a subsequent
       RAMS-I message.  While RR MAY use a locally calculated join time,
       it SHOULD use the information from the most recent RAMS-I
       message.

   7.  Multicast Receive:  After the join, RR starts receiving the
       primary multicast stream.

   8.  Terminate:  RS may know when it needs to stop the unicast burst
       based on the burst parameters, or RR MAY explicitly let RS know
       the sequence number of the first RTP packet it received from the
       multicast session, or RR MAY request RS to terminate the burst
       immediately.

       Regardless of whether or not RS knows when it needs to stop the
       burst, RR SHALL use the RAMS-Termination (RAMS-T) message at an
       appropriate time.  RR can choose to send the RAMS-T message
       before or after it starts receiving the multicast data.  In the
       latter case, RR SHALL include the sequence number of the first
       RTP packet received in the primary multicast session in the
       RAMS-T message, and RS SHOULD terminate the burst after it sends
       the unicast burst packet whose Original Sequence Number (OSN)



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       field in the RTP retransmission payload header matches this
       number minus one.

       If RR wants to stop the burst prior to receiving the multicast
       data, it sends an RAMS-T message without an RTP sequence number.

       Note that RR MUST send at least one RAMS-T message.  Against the
       possibility of a message loss, RR MAY repeat the RAMS-T message
       multiple times as long as it follows the RTCP timer rules defined
       in [RFC4585].

   9.  Terminate with RTCP BYE:  When RR is no longer interested in
       receiving the primary multicast stream and the associated unicast
       burst, RR SHALL issue an RTCP BYE message to RS to terminate the
       burst and the RTP retransmission session.  Upon receiving an RTCP
       BYE message, RS MUST terminate the rapid acquisition operation,
       and cease transmitting any further packets of the associated
       unicast burst.  Section 6.1 of [RFC3550] mandates the RTCP BYE
       message always to be sent with a sender or receiver report in a
       compound RTCP packet (If no data has been received, an empty
       receiver report MUST be included).  With the information
       contained in the receiver report, RS can also figure out how many
       duplicate RTP packets have been delivered to RR (Note that this
       will be an upper-bound estimate as one or more packets might have
       been lost during the burst transmission).  The impact of
       duplicate packets and measures that can be taken to minimize the
       impact of receiving duplicate packets will be addressed in
       Section 6.3.

       Note that if RR decides to switch to a new multicast session
       after it already joined a multicast session following a rapid
       acquisition request, RR MUST also send an RTCP BYE message to the
       Feedback Target for the RTP session associated with the current
       primary multicast stream.

       Editor's note:  Is there a benefit for sending an RAMS-T message
       in conjuction with an RTCP BYE message in this case?

   Note that for the purpose of gathering detailed information about
   RR's rapid acquisition experience, [I-D.begen-avt-rapid-sync-rtcp-xr]
   defines an RTCP Extended Report (XR) Block.  This report is designed
   to be payload-independent, thus, it can be used by any multicast
   application that supports rapid acquisition.  Support for this XR
   report is, however, optional.







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6.3.  Shaping the Unicast Burst

   Editor's note:  This section may discuss sizing of the buffers,
   output buffer overload protection, output bandwidth management,
   adjustment of burst rate and duration.

   TBC.

6.4.  Failure Cases

   All RAMS messages MAY be sent several times against the possibility
   of message loss as long as RS/RR follows the RTCP timer rules defined
   in [RFC4585].  In the following, we examine the implications of
   losing the RAMS-R, RAMS-I or RAMS-T messages.

   When RR sends an RAMS-R message to initiate a rapid acquisition but
   the message gets lost and RS does not receive it, RR will not get an
   RAMS-I message, nor a unicast burst.  In this case, RR MAY resend the
   request when it is eligible to do so.  Or, after a reasonable amount
   of time, RR MAY time out (based on the previous observed response
   times) and immediately join the primary multicast session.  In this
   case, RR MUST still send an RAMS-T message.

   In the case RR starts receiving a unicast burst but it does not
   receive a corresponding RAMS-I message within a reasonable amount of
   time, RR MAY either discard the burst data and stop the burst by
   sending an RAMS-T message to RS, or decide not to interrupt the
   unicast burst and be prepared to join the primary multicast session
   at an appropriate time it determines or indicated in a subsequent
   RAMS-I message (if available).  In either case, RR SHALL send an
   RAMS-T message to RS at an appropriate time.

   In the case the RAMS-T message sent by RR does not reach its
   destination, RS may continue sending the unicast burst even though RR
   no longer needs it.  In some cases, RS has not pre-computed the burst
   duration and does not know when to stop the burst.  To cover that
   case, RR MAY repeat the RAMS-T message multiple times as long as it
   follows the RTCP timer rules defined in [RFC4585].  RS MUST be
   provisioned to deterministically terminate the burst at some point,
   even if it never receives an RAMS-T message for an ongoing burst.

   Upon a failure if RR becomes no longer interested in receiving the
   primary multicast stream and the associated unicast burst, RR SHALL
   issue an RTCP BYE message to RS to terminate the burst and the RTP
   retransmission session.  Only after that, RR MAY send a new rapid
   acquisition request for another primary multicast session.





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7.  Encoding of the Signaling Protocol in RTCP

   This section defines the formats of the RTCP transport-layer feedback
   messages that are exchanged between the Retransmission Server (RS)
   and RTP Receiver (RR) during rapid acquisition.  These messages are
   payload-independent and MUST be used by all RTP-based multicast
   applications that support rapid acquisition regardless of the payload
   they carry.

   Specific payload formats are not defined in this document, but a
   framework is presented that allows payload-specific information to be
   included in the exchange.

   The common packet format for the RTCP feedback messages is defined in
   Section 6.1 of [RFC4585].  Each feedback message has a fixed-length
   field for version, padding, feedback message type (FMT), payload type
   (PT), length, SSRC of packet sender, SSRC of media source as well as
   a variable-length field for feedback control information (FCI).  In
   the transport-layer feedback messages, the PT field is set to RTPFB
   (205).

   In the feedback messages defined in this section, optional extensions
   are encoded by using TLV elements as described below and sketched in
   Figure 4:

   o  Type:  A single-octet identifier that defines the type of the
      parameter represented in this TLV element.

   o  Length:  A two-octet field that indicates the length of the Value
      field in octets.

   o  Value:  Variable-size set of octets that contains the specific
      value for the parameter.

   If a TLV element does not fall on a 32-bit boundary, the last word
   must be padded to the boundary using further bits set to 0.


      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             |     Value     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Value contd.                         /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 4: Structure of a TLV element




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   Editor's note:  The optional fields in the RAMS messages (defined
   below) will be converted to TLV elements in a later version of this
   document.

7.1.  RAMS Request

   The RAMS Request message is identified by PT=RTPFB and FMT=6.

   The FCI field MUST contain only one RAMS Request.

   The RAMS Request is used by RR to request rapid acquisition for a new
   multicast RTP session.

   The FCI field has the structure depicted in Figure 5.

   Editor's note:  We have not finalized whether RAMS-R messages need a
   sequence number or not.


      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               Min RAMS Buffer Fill Requirement                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               Max RAMS Buffer Fill Requirement                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Max Receive Bitrate                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                    (TLV-encoded Extensions)                   :
     :                                                               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 5: FCI field syntax for the RAMS Request message

   o  Min RAMS Buffer Fill Requirement (32 bits):  Optional TLV element
      that denotes the minimum number of octets of content the RTP
      receiver desires to have in its buffer as a result of the unicast
      burst.

      The RTP receiver may have knowledge of its buffering requirements.
      These requirements may be application or device specific.  For
      instance, the receiver may need to have a certain amount of data
      in its application buffer to handle interdependency within the
      data.  If RS is told the buffering ability of the receiver, it may
      tailor the burst more precisely.  The methods used by the receiver
      to determine this value are application specific, and thus, out of
      scope.




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      If specified, the amount of backfill that will be provided by the
      unicast burst SHOULD NOT be smaller than this value since it will
      not be able to build up the desired level of buffer at RR and may
      cause buffer underruns.

   o  Max RAMS Buffer Fill Requirement (32 bits):  Optional TLV element
      that denotes the maximum number of octets of content the RTP
      receiver can buffer without losing the burst data due to buffer
      overflow.

      The RTP receiver may have knowledge of its buffering requirements.
      These requirements may be application or device specific.  For
      instance, one receiver may have more physical memory than another
      receiver, and thus, can buffer more data.  If RS knows the
      buffering ability of the receiver, it may tailor the burst more
      precisely.  The methods used by the receiver to determine this
      value are application specific, and thus, out of scope.

      If specified, the amount of backfill that will be provided by the
      unicast burst SHOULD NOT be larger than this value since it may
      cause buffer overflows at RR.

   o  Max Receive Bitrate (32 bits):  Optional TLV element that denotes
      the maximum bitrate (in bits per second) that the RTP receiver can
      process the unicast burst.  This rate should include whatever
      knowledge the receiver has that would provide an upper bound on
      the unicast burst bitrate.  The limits may include local receiver
      limits as well as network limits that are known to the receiver.

      If specified, the unicast burst bitrate SHOULD NOT be larger than
      this value since it may cause congestion and packet loss.

   The semantics of the RAMS-R feedback message is independent of the
   payload type.

7.2.  RAMS Information

   The RAMS Information message is identified by PT=RTPFB and FMT=7.

   The FCI field MUST contain only one RAMS Information.

   The RAMS Information is used to describe the unicast burst that will
   be sent for rapid acquisition.  It also includes other useful
   information for RR as described below.  Optional payload-specific
   information MAY follow RAMS Information.

   The FCI field has the structure depicted in Figure 6.




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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      MSN    |S|   Response    |             Rsvd.             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Extended RTP Seqnum of the First Burst Packet          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Earliest Multicast Join Time                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Burst Duration                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Max Burst Bitrate                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                    (TLV-encoded Extensions)                   :
     :                                                               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 6: FCI field syntax for the RAMS Information message

   o  Message Sequence Number (8 bits) :  Mandatory field that denotes
      the sequence number of this RAMS-I message.  During rapid
      acquisition, multiple RAMS-I messages MAY be sent and/or the same
      RAMS-I message MAY be repeated.  The first RAMS-I message SHALL
      have an MSN value of 0.  This value SHALL NOT be changed if the
      same RAMS-I message is sent to the same RR multiple times for
      redundancy purposes.  If a new information is conveyed in a new
      RAMS-I message, the MSN value SHALL be incremented by one.

   o  Support for Updated Requests (1 bit):  Mandatory field that
      denotes whether RS supports updated request messages or not.  A
      value of zero in this field means that RS does not support updated
      request messages and RS will ignore such requests.  In this case,
      RR SHOULD NOT send updated requests.  However, RR MAY repeat its
      initial request.  A value of one in this field means that RS
      supports updated request messages.  In this case, RR MAY send
      updated requests.

      Note that even if this field is set to one, RS may or may not be
      able to accept value changes in every field in an RAMS-R message.
      Furthermore, RS may or may not honor the requested changes
      depending on the resources available.

      Editor's note:  The use of this flag is still under discussion.

   o  Response (8 bits):  Mandatory field that denotes the RS response
      code for this RAMS-I message.

      Editor's note:  Response codes will be defined and registered with



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      IANA in a later version.  Current proposals include:

      1.  Success

      2.  Bad_Request

      3.  No_Server_Resources_Available

      4.  No_Network_Resources_Available

      5.  No_Buffered_Content_Available

   o  Rsvd (16 bits):  Reserved.  This field SHALL be set to 0.

   o  Extended RTP Seqnum of the First Burst Packet (32 bits):
      Mandatory field that specifies the extended RTP sequence number of
      the first packet that will be sent as part of the burst.  This
      allows RR to know if one or more packets have been dropped at the
      beginning of the burst. 32-bit extended RTP sequence number is
      constructed by putting the 16-bit RTP sequence number in the lower
      two bytes and octet 0's in the higher two bytes.

   o  Earliest Multicast Join Time (32 bits):  Optional TLV element that
      specifies the time difference (i.e., delta time) between the
      arrival of this RAMS-I message and the earliest time instant when
      RR could join the primary multicast session.  A zero value in this
      field means that RR can join the primary multicast session right
      away.

      Editor's note:  We need to decide whether we will use ms or RTP
      ticks in this field.

      Note that if the RAMS request has been accepted, RS MUST send this
      field at least once, so that RR knows when to join the primary
      multicast session.  If the burst request has been rejected as
      indicated in the Response field, this field MAY be omitted or set
      to 0.  In that case, it is up to RR when or whether to join the
      primary multicast session.

   o  Burst Duration (32 bits):  Optional TLV element that denotes the
      duration of the burst that RS is planning to send (in RTP ticks).
      In the absence of additional stimulus, RS will send a burst of
      this duration.  However, the burst duration may be modified by
      subsequent events, including changes in the primary multicast
      stream and reception of RAMS-T messages.

      Note that RS MUST terminate the flow in a deterministic timeframe,
      even if it does not get an RAMS-T or a BYE from RR.  It is



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      optional to send this field in an RAMS-I message when the burst
      request is accepted.  If the burst request has been rejected as
      indicated in the Response field, this field MAY be omitted or set
      to 0.

   o  Max Burst Bitrate (32 bits):  Optional TLV element that denotes
      the maximum bitrate (in bits per second) that will be used by RS
      for the unicast burst.

   The semantics of the RAMS-I feedback message is independent of the
   payload type.

   The RAMS-I message MAY be sent multiple times at the start of, prior
   to, or during the unicast burst.  The subsequent RAMS-I messages MAY
   signal changes in any of the fields.

7.3.  RAMS Termination

   The RAMS Termination message is identified by PT=RTPFB and FMT=8.

   The FCI field MUST contain only one RAMS Termination.

   The RAMS Termination may be used to assist RS in determining when to
   stop the burst.

   If prior to sending the RAMS-T message RR has already joined the
   primary multicast session and received at least one RTP packet from
   the multicast session, RR includes the sequence number of the first
   RTP packet in the RAMS-T message.  With this information, RS can
   decide when to terminate the unicast burst.

   If RR issues the RAMS-T message before it has joined and/or begun
   receiving RTP packets from the primary multicast session, RR does not
   specify any sequence number in the RAMS-T message, which indicates RS
   to stop the burst immediately.  However, the RAMS-T message may get
   lost and RS may not receive this message.

   The FCI field has the structure depicted in Figure 7.













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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Extended RTP Seqnum of First Multicast Packet         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                    (TLV-encoded Extensions)                   :
     :                                                               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 7: FCI field syntax for the RAMS Termination message

   o  Extended RTP Seqnum of First Multicast Packet (32 bits):  Optional
      TLV element that specifies the extended RTP sequence number of the
      of the first multicast packet received by RR.  If no RTP packet
      has been received from the primary multicast session, this field
      does not exist and tells RS to stop the burst immediately.

   The semantics of the RAMS-T feedback message is independent of the
   payload type.

7.4.  Extensions

   To improve the functionality of the RAMS method in certain
   applications, it may be desirable to define new fields in the RAMS
   Request, Information and Termination messages.  Such fields MUST be
   defined as TLV elements.  If the goal in defining these new fields is
   to extend the protocol in a vendor-neutral manner, they MUST be
   registered with IANA through an Informational or a Standards-track
   RFC.  The support for these new fields is OPTIONAL.  In an RAMS
   message, any extension MUST be placed after any existing mandatory
   field for that message.

   Editor's note:  We should specify the requirements for registering
   new TLV elements.

   It is also desirable to allow vendors to use vendor-specific
   extensions (in TLV format) in any of the RAMS messages.  For
   interoperability, such extensions MUST NOT collide with each other.

   Editor's note:  Some approaches are currently being examined for
   vendor-specific extensions.  A potential solution is depicted in
   Figure 8.  In this approach, the enterprise numbers are used from
   http://www.iana.org/assignments/enterprise-numbers.  This will ensure
   the uniqueness of the vendor-specific extensions.







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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :          Mandatory Fields as Defined in This Document         :
     :                                                               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Type = TBD  |            Length             |  Ent. Number  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               Ent. Number contd.              |     Value     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Value contd.                         /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 8: Structure of a vendor-specific extension


8.  SDP Definitions and Examples

8.1.  Definitions

   The syntax of the 'rtcp-fb' attribute has been defined in [RFC4585].
   Here we add the following syntax to the 'rtcp-fb' attribute (the
   feedback type and optional parameters are all case sensitive):

   (In the following ABNF [RFC5234], fmt, SP and CRLF are used as
   defined in [RFC4566].)

         rtcp-fb-syntax = "a=rtcp-fb:" rtcp-fb-pt SP rtcp-fb-val CRLF

         rtcp-fb-pt         = "*"   ; wildcard: applies to all formats
                            / fmt   ; as defined in SDP spec

         rtcp-fb-val        = "nack" SP "ssli"

   The following parameter is defined in this document for use with
   'nack':

   o  'ssli' stands for Stream Synchronization Loss Indication and
      indicates the use of RAMS messages as defined in Section 7.

8.2.  Examples

   This section provides a declarative SDP [RFC4566] example for
   enabling rapid acquisition of multicast RTP sessions.  The following
   example uses the SDP grouping semantics [RFC3388], the RTP/AVPF
   profile [RFC4585], the RTP retransmissions [RFC4588], the RTCP
   extensions for SSM sessions with unicast feedback
   [I-D.ietf-avt-rtcpssm] and the source-specific media attributes



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   [I-D.ietf-mmusic-sdp-source-attributes].

   In the example below, we have two primary source streams and two
   unicast retransmission streams for each of these source streams.  The
   source streams are multicast from a distribution source (with a
   source IP address of 8.166.1.1) in different multicast sessions.  For
   each source stream, a feedback target address (9.30.30.1) is also
   specified with the 'rtcp' attribute.  The RTP receiver(s) can report
   missing packets on the source stream to the feedback target and
   request retransmissions.  The parameter 'rtx-time' specifies the time
   in milliseconds (measured from the time a packet was first sent) that
   the sender (in this case the feedback target) keeps an RTP packet in
   its buffers available for retransmission.

   For the first source stream, only the conventional retransmission
   support is enabled.  For the second source stream, both the
   conventional retransmission and rapid acquisition support are
   enabled.  This is achieved by the "a=rtcp-fb:98 nack ssli" line.

   When an RTP receiver requires rapid acquisition for a new multicast
   session it wants to join, it sends an RAMS-R message to the feedback
   target.  This feedback message has to have the SSRC of the primary
   source session for which rapid acquisition is requested for.
   However, since this RTP receiver has not received any RTP packets
   from this primary source session yet, the RTP receiver MUST learn the
   SSRC value from the 'ssrc' attribute of the media description
   [I-D.ietf-avt-rtcpssm].  In addition to the SSRC value, the 'cname'
   source attribute MUST also be present in the SDP description
   [I-D.ietf-mmusic-sdp-source-attributes].  Note that listing the SSRC
   values for the primary source sessions in the SDP file does not
   create a problem in SSM sessions when an SSRC collision occurs.  This
   is because in SSM sessions, an RTP receiver that observed an SSRC
   collision with a media source MUST change its own SSRC
   [I-D.ietf-avt-rtcpssm] by following the rules defined in [RFC3550].

   A feedback target that receives an RAMS-R feedback message becomes
   aware that the prediction chain at the RTP receiver side has been
   broken or does not exist any more.  If the necessary conditions are
   satisfied (as outlined in Section 7 of [RFC4585]) and available
   resources exist, the feedback target MAY react to the RAMS-R message
   by sending any payload-specific feedback message(s) and starting the
   unicast burst.









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        v=0
        o=ali 1122334455 1122334466 IN IP4 rams.example.com
        s=Rapid Acquisition Examples
        t=0 0
        a=group:FID 1 2
        a=group:FID 3 4
        a=rtcp-unicast:rsi
        m=video 40000 RTP/AVPF 96
        i=Primary Multicast Stream #1
        c=IN IP4 224.1.1.1/255
        a=source-filter: incl IN IP4 224.1.1.1 192.0.2.2
        a=recvonly
        a=rtpmap:96 MP2T/90000
        a=rtcp:40001 IN IP4 192.0.2.1
        a=rtcp-fb:96 nack
        a=mid:1
        m=video 40002 RTP/AVPF 97
        i=Unicast Retransmission Stream #1 (Ret. Support Only)
        c=IN IP4 192.0.2.1
        a=recvonly
        a=rtpmap:97 rtx/90000
        a=rtcp:40003
        a=fmtp:97 apt=96
        a=fmtp:97 rtx-time=3000
        a=mid:2
        m=video 41000 RTP/AVPF 98
        i=Primary Multicast Stream #2
        c=IN IP4 224.1.1.2/255
        a=source-filter: incl IN IP4 224.1.1.2 192.0.2.2
        a=recvonly
        a=rtpmap:98 MP2T/90000
        a=rtcp:41001 IN IP4 192.0.2.1
        a=rtcp-fb:98 nack
        a=rtcp-fb:98 nack ssli
        a=ssrc:123321 cname:iptv-ch32@rams.example.com
        a=mid:3
        m=video 41002 RTP/AVPF 99
        i=Unicast Retransmission Stream #2 (Ret. and Rapid Acq. Support)
        c=IN IP4 192.0.2.1
        a=recvonly
        a=rtpmap:99 rtx/90000
        a=rtcp:41003
        a=fmtp:99 apt=98; rtx-time=5000
        a=mid:4

   The offer/answer model considerations [RFC3264] for the 'rtcp-fb'
   attribute are provided in Section 4.2 of [RFC4585].




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   Editor's note:  We will provide more SDP examples in a later version
   as needed.


9.  NAT Considerations

   Editor's note:  This section will explain the potential issues
   experienced in NAT environments.  Some solution approaches will be
   presented.  This section will also include a recommendation for the
   RTP/RTCP Muxing solution that is discussed in
   [I-D.ietf-avt-rtp-and-rtcp-mux].


10.  Known Implementations

10.1.  Open Source RTP Receiver Implementation by Cisco

   An open source RTP Receiver code that implements the functionalities
   introduced in this document is available.  For documentation, visit
   the following URL:

   http://www.cisco.com/en/US/docs/video/cds/cda/vqe/3_0/user/guide/
   ch1_over.html

   The code is also available at:

   ftp://ftpeng.cisco.com/ftp/vqec/

   Note that this code is under development and may be based on an
   earlier version of this document.  As we make progress in the draft,
   the source code will also be updated to reflect the changes.

   Some preliminary results based on this code are available in [CCNC09]
   and [IC2009].

10.2.  IPTV Commercial Implementation by Microsoft

   Rapid Acquisition of Multicast RTP Sessions is supported as part of
   the Microsoft Mediaroom Internet Protocol Television (IPTV) and
   multimedia software platform.  This system is in wide commercial
   deployment.  More information can be found at:

   http://www.microsoft.com/mediaroom

   http://informitv.com/articles/2008/10/13/channelchangetimes/






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11.  Open Issues

   o  Update message formats to reflect the TLV encapsulations.

   o  Check whether we can register only one FMT value for all RAMS
      messages and use a sub FMT value in the messages to indicate the
      specific RAMS message.  This change seams feasible and will be
      made in a later version.

   o  Define the structure for vendor-specific extensions and check
      whether SDES can be used for this purpose.

   o  The use of extended seqnums in the RAMS messages.

   o  Check whether RFC 5506 should be used/supported.

   o  Update the SDP example (Use correct unicast and multicast
      addresses).

   o  Burst shaping issues and security considerations sections.


12.  Security Considerations

   TBC.


13.  IANA Considerations

   This document registers a new SDP attribute value and several new
   RTCP packets.

   The following contact information shall be used for all registrations
   in this document:

   Ali Begen
   abegen@cisco.com

   170 West Tasman Drive
   San Jose, CA 95134 USA

13.1.  Registration of SDP Attribute Values

   This document registers a new value for the 'nack' attribute to be
   used with the 'rtcp-fb' attribute in SDP.  For more information about
   'rtcp-fb', refer to [RFC4585].





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        Value name:     ssli
        Long name:      Stream Synchronization Loss Indication
        Usable with:    nack
        Reference:      This document

13.2.  Registration of FMT Values

   Within the RTPFB range, the following three format (FMT) values are
   registered:


        Name:           RAMS-R
        Long name:      Rapid Acquisition of Multicast Sessions Request
        Value:          6
        Reference:      This document


     Name:           RAMS-I
     Long name:      Rapid Acquisition of Multicast Sessions Information
     Value:          7
     Reference:      This document


     Name:           RAMS-T
     Long name:      Rapid Acquisition of Multicast Sessions Termination
     Value:          8
     Reference:      This document


14.  Acknowledgments

   The authors would like to specially thank Peilin Yang for his
   contributions to this document and detailed reviews.

   The authors also thank the following individuals for their
   contributions, comments and suggestions to this document:  Dave Oran,
   Tony Faustini, Jeff Goldberg, Muriel Deschanel, Orit Levin, Roni
   Even, Guy Hirson, Tom Taylor, Xavier Marjou, Ye-Kui Wang, Zixuan Zou,
   Ingemar Johansson, Haibin Song, Ning Zong, Jonathan Lennox and Sean
   Sheedy.


15.  Change Log








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15.1.  draft-versteeg-avt-rapid-synchronization-for-rtp-03

   The following are the major changes compared to version 02:

   o  The title and message names have been changed.

   o  RTCP message semantics have been added.  RAMS protocol has been
      revised to handle updated requests and responses.

   o  Definitions have been revised.

   o  RTP/RTCP muxing reference has been added.

15.2.  draft-versteeg-avt-rapid-synchronization-for-rtp-02

   The following are the major changes compared to version 01:

   o  The discussion around MPEG2-TS has been moved to another document.

   o  The RAMS-R, RAMS-I and RAMS-T messages have been extensively
      modified and they have been made mandatory.

   o  IANA Considerations section has been updated.

   o  The discussion of RTCP XR report has been moved to another
      document.

   o  A new section on protocol design considerations has been added.

15.3.  draft-versteeg-avt-rapid-synchronization-for-rtp-01

   The following are the major changes compared to version 00:

   o  The core of the rapid synchronization method is now payload-
      independent.  But, the draft still defines payload-specific
      messages that are required for enabling rapid synch for the RTP
      flows carrying MPEG2-TS.

   o  RTCP APP packets have been removed, new RTCP transport-layer and
      payload-specific feedback messages have been defined.

   o  The step for leaving the current multicast session has been
      removed from Section 6.2.

   o  A new RTCP XR (Multicast Join) report has been defined.

   o  IANA Considerations section have been updated.




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   o  Editorial changes to clarify several points.


16.  References

16.1.  Normative References

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

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

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, October 2002.

   [RFC3810]  Vida, R. and L. Costa, "Multicast Listener Discovery
              Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

   [RFC4604]  Holbrook, H., Cain, B., and B. Haberman, "Using Internet
              Group Management Protocol Version 3 (IGMPv3) and Multicast
              Listener Discovery Protocol Version 2 (MLDv2) for Source-
              Specific Multicast", RFC 4604, August 2006.

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

   [RFC3388]  Camarillo, G., Eriksson, G., Holler, J., and H.
              Schulzrinne, "Grouping of Media Lines in the Session
              Description Protocol (SDP)", RFC 3388, December 2002.

   [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.

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              July 2006.

   [I-D.ietf-avt-rtcpssm]
              Schooler, E., Ott, J., and J. Chesterfield, "RTCP
              Extensions for Single-Source Multicast Sessions with
              Unicast Feedback", draft-ietf-avt-rtcpssm-18 (work in
              progress), March 2009.




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   [I-D.ietf-mmusic-sdp-source-attributes]
              Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", draft-ietf-mmusic-sdp-source-attributes-02 (work
              in progress), October 2008.

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

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

16.2.  Informative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [I-D.ietf-rmt-pi-norm-revised]
              Adamson, B., Bormann, C., London, U., and J. Macker,
              "NACK-Oriented Reliable Multicast Protocol",
              draft-ietf-rmt-pi-norm-revised-09 (work in progress),
              March 2009.

   [I-D.begen-avt-rtp-mpeg2ts-preamble]
              Begen, A., "RTP Payload Format for MPEG2-TS Preamble",
              draft-begen-avt-rtp-mpeg2ts-preamble-00 (work in
              progress), March 2009.

   [I-D.begen-avt-rapid-sync-rtcp-xr]
              Begen, A., "Rapid Multicast Synchronization Report Block
              Type for RTCP XR", draft-begen-avt-rapid-sync-rtcp-xr-00
              (work in progress), March 2009.

   [I-D.ietf-avt-rtp-and-rtcp-mux]
              Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port",
              draft-ietf-avt-rtp-and-rtcp-mux-07 (work in progress),
              August 2007.

   [CCNC09]   Begen, A., Glazebrook, N., and W. VerSteeg, "A Unified
              Approach for Repairing Packet Loss and Accelerating
              Channel Changes in Multicast IPTV (IEEE CCNC)",
              January 2009.

   [IC2009]   Begen, A., Glazebrook, N., and W. VerSteeg, "Reducing
              Channel Change Times in IPTV with Real-Time Transport
              Protocol (IEEE Internet Computing)", May 2009.



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Authors' Addresses

   Bill VerSteeg
   Cisco Systems
   5030 Sugarloaf Parkway
   Lawrenceville, GA  30044
   USA

   Email:  billvs@cisco.com


   Ali Begen
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email:  abegen@cisco.com


   Tom VanCaenegem
   Alcatel-Lucent
   Copernicuslaan 50
   Antwerpen,   2018
   Belgium

   Email:  Tom.Van_Caenegem@alcatel-lucent.be


   Zeev Vax
   Microsoft Corporation
   1065 La Avenida
   Mountain View, CA  94043
   USA

   Email:  zeevvax@microsoft.com















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