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Versions: (draft-versteeg-avt-rapid-synchronization-for-rtp) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 RFC 6285

AVT                                                          B. VerSteeg
Internet-Draft                                                  A. Begen
Intended status:  Standards Track                                  Cisco
Expires:  May 20, 2010                                    T. VanCaenegem
                                                          Alcatel-Lucent
                                                                  Z. Vax
                                                   Microsoft Corporation
                                                       November 16, 2009


       Unicast-Based Rapid Acquisition of Multicast RTP Sessions
              draft-ietf-avt-rapid-acquisition-for-rtp-05

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.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  7
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Elements of Delay in Multicast Applications  . . . . . . . . .  8
   5.  Protocol Design Considerations and Their Effect on
       Resource Management for Rapid Acquisition  . . . . . . . . . . 10
   6.  Rapid Acquisition of Multicast RTP Sessions  . . . . . . . . . 12
     6.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.2.  Message Flows  . . . . . . . . . . . . . . . . . . . . . . 13
     6.3.  Shaping the Unicast Burst  . . . . . . . . . . . . . . . . 21
     6.4.  Failure Cases  . . . . . . . . . . . . . . . . . . . . . . 23
   7.  Encoding of the Signaling Protocol in RTCP . . . . . . . . . . 24
     7.1.  Extensions . . . . . . . . . . . . . . . . . . . . . . . . 25
       7.1.1.  Vendor-Neutral Extensions  . . . . . . . . . . . . . . 25
       7.1.2.  Private Extensions . . . . . . . . . . . . . . . . . . 26
     7.2.  RAMS Request . . . . . . . . . . . . . . . . . . . . . . . 26
     7.3.  RAMS Information . . . . . . . . . . . . . . . . . . . . . 28
     7.4.  RAMS Termination . . . . . . . . . . . . . . . . . . . . . 30
   8.  SDP Definitions and Examples . . . . . . . . . . . . . . . . . 31
     8.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . 31
     8.2.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 31
   9.  NAT Considerations . . . . . . . . . . . . . . . . . . . . . . 35
   10. Known Implementations  . . . . . . . . . . . . . . . . . . . . 35
     10.1. Open Source RTP Receiver Implementation by Cisco . . . . . 35
     10.2. IPTV Commercial Implementation by Microsoft  . . . . . . . 36
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 36
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 37
     12.1. Registration of SDP Attributes . . . . . . . . . . . . . . 38
     12.2. Registration of SDP Attribute Values . . . . . . . . . . . 38
     12.3. Registration of FMT Values . . . . . . . . . . . . . . . . 38
     12.4. SFMT Values for RAMS Messages Registry . . . . . . . . . . 39
     12.5. RAMS TLV Space Registry  . . . . . . . . . . . . . . . . . 39
     12.6. RAMS Response Code Space Registry  . . . . . . . . . . . . 40
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 41
   14. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 42
     14.1. draft-ietf-avt-rapid-acquisition-for-rtp-05  . . . . . . . 42
     14.2. draft-ietf-avt-rapid-acquisition-for-rtp-04  . . . . . . . 42
     14.3. draft-ietf-avt-rapid-acquisition-for-rtp-03  . . . . . . . 42
     14.4. draft-ietf-avt-rapid-acquisition-for-rtp-02  . . . . . . . 42
     14.5. draft-ietf-avt-rapid-acquisition-for-rtp-01  . . . . . . . 43
     14.6. draft-ietf-avt-rapid-acquisition-for-rtp-00  . . . . . . . 43
     14.7. draft-versteeg-avt-rapid-synchronization-for-rtp-03  . . . 43
     14.8. draft-versteeg-avt-rapid-synchronization-for-rtp-02  . . . 43
     14.9. draft-versteeg-avt-rapid-synchronization-for-rtp-01  . . . 44
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 44
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 44



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     15.2. Informative References . . . . . . . . . . . . . . . . . . 45
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 47

















































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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,
   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 (that we refer to as Retransmission Server) 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 Retransmission Server starts a unicast
   retransmission RTP session and sends the Reference Information to the
   RTP receiver over that session.  If there is spare bandwidth, the
   Retransmission Server 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    |
                                  | :  |(Retransmission Server)|
                                  | :  +-----------------------+
                                  | :
                                  | 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



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   in the RTP/RTCP protocol family.  This improves the versatility of
   the existing implementations, and promotes faster deployment and
   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
   Retransmission Server 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 SHOULD be used for both rapid acquisition
   and retransmission-based loss repair.  This 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 (NACK) 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.  However, the rapid acquisition method
   introduced in this document does not depend on a specific version of
   either of these group management protocols.  In the remainder of this
   document, SFGMP will refer to any group management protocol that has
   Join and Leave functionalities.

   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.

   (Unicast) 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.  RS may also
   generate other non-retransmission packets to aid the RAMS process.


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:



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   o  Multicast switching delay

   o  Reference Information latency

   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.  Refer to [I-D.begen-avt-rtp-mpeg2ts-preamble]
   for details.

   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



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   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-
   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



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   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
   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 bitrate 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, end-to-end 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.





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   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
   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/or 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 joins 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



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   primary multicast stream, prior to joining the SSM session, it may
   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.

   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/Retransmission Source

   o  RTP Receiver (RTP_Rx)




















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


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

        Figure 2: Flow diagram for unicast-based rapid acquisition

   The feedback target (FT) is the entity as defined in
   [I-D.ietf-avt-rtcpssm], to which RTP_Rx sends its RTCP feedback
   messages indicating packet loss in the primary multicast stream by
   means of an RTCP NACK message or indicating RTP_Rx's desire to
   rapidly acquire the primary multicast stream by means of an RTCP
   feedback message defined in this document.  While the Burst/
   Retransmission Source is responsible for responding to these messages
   and for further RTCP interaction with RTP_Rx in the case of a rapid
   acquisition process, it is assumed in the remainder of the document
   that these two logical entities (FT and Burst/Retransmission Source)
   are combined in a single physical entity and they share state.  In
   the remainder of the text, the term Retransmission Server will be
   used whenever appropriate, to refer to the combined functionality of
   the FT and Burst/Retransmission Source.

   However, it must be noted that only FT is involved in the primary
   multicast session, whereas the Burst/Retransmission Source transmits
   the unicast burst and retransmission packets both formatted as RTP
   retransmission packets [RFC4588] in a single separate unicast RTP
   retransmission session to each RTP_Rx.  In the retransmission
   session, as in any other RTP session, RS and RTP_Rx regularly send



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   the periodic sender and receiver reports, respectively.

   Note also 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.

   The unicast burst is triggered by an RTCP feedback message that is
   defined in this document, whereas an RTP retransmission is triggered
   by an RTCP NACK message defined in [RFC4585].  Based on its design,
   in an RAMS implementation, 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, RTP_Rx may make use
   of the RTCP NACK message to request a retransmission for the missing
   packets in the gap.

   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)      |   |          |   |  (RTP_Rx)  |
     +-----------+   +----------------+   +----------+   +------------+
         |                   |                 |                |
         |-- 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 RTP_Rx.  It is
   instructive to have independently operating implementations on RS and
   RTP_Rx 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:  RTP_Rx 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 RTP_Rx 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 RTP_Rx, which may be used by RS to prepare a burst
       that conforms with RTP_Rx's requirements.

       Before joining the primary multicast session, a new joining
       RTP_Rx 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 RTP_Rx.  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.  The RAMS-I message is sent by the Burst/Retransmission
       Source logical entity that is part of RS.

       Note that RS learns the IP address information for RTP_Rx 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 RTP_Rx immediately via an RAMS-I message.  If
       RTP_Rx 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



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       an SFGMP Join message towards its upstream multicast router for
       the new primary multicast session.

       If RS accepts the request, it sends an RAMS-I message to RTP_Rx
       (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).  A particularly important field in the RAMS-I message
       carries the RTP sequence number of the first packet transmitted
       in the retransmission session to allow RTP_Rx to detect any
       missing initial packet(s).  Note that the first RTP packet
       transmitted in the retransmission session is not necessarily a
       burst packet.  It could be a payload-specific RTP packet (See
       [I-D.begen-avt-rtp-mpeg2ts-preamble] for an example).  When RS
       accepts the request, this field MUST be populated in the RAMS-I
       message and it is RECOMMENDED that the RAMS-I message is sent
       early enough in the session to be useful.  If RS rejects the
       request, this field SHALL NOT exist in the RAMS-I message.

       It is RECOMMENDED to include a sender report with the RAMS-I
       message in the same compound RTCP packet.  This allows rapid
       synchronization among multiple RTP flows
       [I-D.ietf-avt-rapid-rtp-sync].

       The unicast burst duration MAY be calculated by RS, and its value
       MAY be updated by messages received from RTP_Rx.  If RS accepts
       the request, the join time information (for the new multicast
       session) MUST be populated in at least one of the RAMS-I
       messages.  If RS rejects the request, including the join time
       information in a RAMS-I message is OPTIONAL.

       RS MAY send the RAMS-I message after a significant delay, so
       RTP_Rx 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 (The burst packet(s) are sent by the Burst/Retransmission
       Source logical entity).  In addition, there MAY be optional
       payload-specific information that RS chooses to send to RTP_Rx.
       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:  RTP_Rx MAY send a new RAMS-R message (to the FT
       entity of RS) 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



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       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, RTP_Rx SHOULD
       NOT make protocol dependencies on quickly (or ever) receiving a
       new RAMS-I message, or assume that RS will honor the requested
       changes.

       RTP_Rx 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 RTP_Rx should or
       should not send an updated request and the methods that RTP_Rx
       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 RTP_Rx in real time to join the
       primary multicast session.  RTP_Rx usually depends on RS to learn
       the join time, which can be conveyed by the first RAMS-I message,
       or 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, RTP_Rx 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 RTP_Rx joins the primary multicast session too
       early or too late.  For example, if RTP_Rx 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 RTP_Rx 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, RTP_Rx MAY issue
       retransmissions requests (via RTCP NACK messages) [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 RTP_Rx (from an application-layer point of view).  To
       cope with such cases, the RAMS-I message allows RS to signal
       explicitly when RTP_Rx should send the SFGMP Join message.



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       Alternatively, RS may pre-compute the burst duration and the time
       RTP_Rx 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 RTP_Rx 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, RTP_Rx 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 RTP_Rx MAY explicitly let RS
       know the sequence number of the first RTP packet it received from
       the multicast session, or RTP_Rx MAY request RS to terminate the
       burst immediately.

       Regardless of whether or not RS knows when it needs to stop the
       burst, RTP_Rx SHALL use the RAMS-Termination (RAMS-T) message at
       an appropriate time.  RTP_Rx can choose to send the RAMS-T
       message before or after it starts receiving the multicast data.
       In the latter case, RTP_Rx 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) field in the RTP retransmission payload header matches this
       number minus one.

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

       RTP_Rx MUST send at least one RAMS-T message (if an RTCP BYE
       message has not been issued yet as described in Step 9), and the
       RAMS-T message MUST be addressed to the RTCP port of the
       retransmission session.  Against the possibility of a message
       loss, RTP_Rx 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 RTP_Rx is receiving the burst, if
       RTP_Rx becomes no longer interested in the primary multicast
       stream, RTP_Rx SHALL issue an RTCP BYE message for the RTP
       retransmission session and another RTCP BYE message for the
       primary multicast session.

       Upon receiving an RTCP BYE message, RS MUST terminate the rapid
       acquisition operation, and cease transmitting any further regular
       retransmission packets as well as retransmission packets
       associated with the unicast burst.  If support for [RFC5506] has



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       been signaled, the RTCP BYE message MAY be sent in a reduced-size
       RTCP packet.  Otherwise, 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 RTP_Rx (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 an RTCP BYE message issued for the RTP retransmission
       session terminates the whole session and ceases transmitting any
       further packets in that RTP session.  Thus, in this case there is
       no need for sending an (explicit) RAMS-T message, which would
       only terminate the burst.

   Note that for the purpose of gathering detailed information about
   RTP_Rx'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.

6.3.  Shaping the Unicast Burst

   This section provides informative guidelines about how RS can shape
   the transmission of the unicast burst.

   A higher bitrate for the unicast burst naturally conveys the
   Reference Information and media content to RTP_Rx faster.  This way,
   RTP_Rx can start consuming the data sooner, which results in a faster
   acquisition.

   A higher rate also represents a better utilization of RS resources.
   As the burst may continue until it catches up with the primary
   multicast stream, the higher the bursting rate, the less data RS
   needs to transmit.  However, a higher rate for the burst also
   increases the chances for congestion-caused packet loss.  Thus, as
   discussed in Section 5, there must be an upper bound on the extra
   bandwidth used by the burst.

   When RS transmits the burst, it SHOULD take into account all
   available information to prevent any packet loss that may take place
   during the bursting as a result of buffer overflow on the path
   between RS and RTP_Rx and at RTP_Rx itself.  The bursting rate may be



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   determined by taking into account the following data, when available:

   a.  Information obtained via the RAMS-R message, such as Max RAMS
       Buffer Fill Requirement and/or Max Receive Bitrate (See
       Section 7.2).

   b.  Information obtained via RTCP receiver reports provided by RTP_Rx
       in the retransmission session, allowing in-session rate
       adaptations for the burst.  When these receiver reports indicate
       packet loss, this may indicate a certain congestion state in the
       path from RS to RTP_Rx.  Heuristics or algorithms that deduce
       such congestion state and how subsequently the RS should act, are
       outside the scope of this document.

   c.  Information obtained via RTCP NACKs provided by RTP_Rx in the
       primary multicast session, allowing in-session rate adaptations
       for the burst.  Such RTCP NACKs are transmitted by RTP_Rx in
       response to packet loss detection by RTP_Rx in the burst.  NACKs
       may indicate a certain congestion state on the path from RS to
       RTP_Rx.  Heuristics or algorithms that deduce such congestion
       state and how subsequently the RS should act, are outside the
       scope of this document.

   d.  There may be other feedback received from RTP_Rx, e.g., in the
       form of ECN-CE RTCP feedback messages
       [I-D.westerlund-avt-ecn-for-rtp] that may influence in-session
       rate adaptations.

   e.  Information obtained via updated RAMS-R messages, allowing in-
       session rate adaptations, if supported by RS.

   f.  Pre-configured settings for each RTP_Rx or a set of RTP_Rxs that
       indicate the upper-bound bursting rates for which no packet loss
       will occur as a result of congestion along the path of RS to
       RTP_Rx.  For example, in managed IPTV networks, where the
       bottleneck bandwidth along the end-to-end path is known (which is
       generally the access network link) and where the network between
       RS and this link is provisioned and dimensioned to carry the
       burst streams, the bursting rate does not exceed the provisioned
       value.  These settings may also be dynamically adapted using
       application-aware knowledge.

   The initial bursting rate of the unicast burst to RTP_Rx is
   determined by parameters directly obtained from RTP_Rx (a) or by pre-
   configured settings (f).  If such information is not available, RS
   may choose an appropriate initial bursting rate, and could increase
   or decrease the rate based on the feedback information (b, c, d or
   e).  However, this may not be an easy task as by the time packet loss



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   is reported back to RS triggering a rate reduction, packet loss may
   have occurred.

   A specific situation occurs near the end of the unicast burst, when
   RS has almost no more additional data to sustain the relatively
   higher bursting rate, thus, the upper-bound bursting rate
   automatically gets limited by the nominal rate of the primary
   multicast stream.  During this time frame, RTP_Rx will join the
   primary multicast session because it was instructed to do so via an
   RAMS-I message or based on some heuristics.  This means that both the
   burst packets and the primary multicast stream packets will be
   simultaneously received by RTP_Rx for a period of time.

   In this case, when the unicast burst is close to catch up with the
   primary multicast stream, RS may, for example, keep on sending burst
   packets but should reduce the rate accordingly by taking the nominal
   rate of the primary multicast stream into account.  Alternatively, RS
   may immediately cease transmitting the burst packets, when being
   close to catch-up.  Any gap resulting from an imperfect switch by
   RTP_Rx in receiving first the burst packets and then only primary
   multicast stream packets, can be later repaired by requesting
   retransmissions of the missing packets from RS.  The retransmissions
   may also be shaped by RS to make sure that they do not cause
   collateral loss in the primary multicast and retransmission sessions.

6.4.  Failure Cases

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

   In the case RTP_Rx starts receiving a unicast burst but it does not
   receive a corresponding RAMS-I message within a reasonable amount of
   time, RTP_Rx 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, RTP_Rx SHALL send an
   RAMS-T message to RS at an appropriate time.



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   In the case the RAMS-T message sent by RTP_Rx does not reach its
   destination, RS may continue sending the unicast burst even though
   RTP_Rx 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, RTP_Rx 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.

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


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 (RTP_Rx) during rapid acquisition.  These messages
   are referred to as the RAMS Messages.  They are payload-independent
   and MUST be used by all RTP-based multicast applications that support
   rapid acquisition regardless of the payload they carry.

   Payload-specific feedback messages are not defined in this document,
   but an extension mechanism is provided where further optional
   payload-independent and payload-specific information can 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 RAMS messages, the PT field is set to RTPFB (205) and the FMT
   field is set to RAMS (6).  Individual RAMS messages are identified by
   a sub-field called Sub Feedback Message Type (SFMT).

   Depending on the specific scenario and timeliness/importance of a
   RAMS message, it may be desirable to send it in a reduced-size RTCP
   packet [RFC5506].  However, unless support for [RFC5506] has been
   signaled, compound RTCP packets MUST be used by following [RFC3550]
   rules.





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7.1.  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
   encoded as 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 TLV
      element excluding the Type and Length fields in octets.  Note that
      this length does not include any padding that is required for
      alignment.

   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.

   In an RAMS message any vendor-neutral or private extension MUST be
   placed after the mandatory fields (if any).  The support for
   extensions is OPTIONAL.


      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

7.1.1.  Vendor-Neutral Extensions

   If the goal in defining new TLV elements is to extend the
   functionality in a vendor-neutral manner, they MUST be registered
   with IANA through the guidelines provided in Section 12.5.

   The current document defines several vendor-neutral extensions in the
   following sections.







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7.1.2.  Private Extensions

   It is desirable to allow vendors to use private extensions in TLV
   format.  For interoperability, such extensions MUST NOT collide with
   each other.

   A certain range of TLV Types is reserved for private extensions
   (Refer to Section 12.5).  IANA management for these extensions is
   unnecessary and they are the responsibility of individual vendors.

   The structure that MUST be used for the private extensions is
   depicted in Figure 5.  Here, the enterprise numbers are used from
   http://www.iana.org/assignments/enterprise-numbers.  This will ensure
   the uniqueness of the private extensions and avoid any collision.


      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             |  Ent. Number  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               Ent. Number contd.              |     Value     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Value contd.                         /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 5: Structure of a private extension

7.2.  RAMS Request

   The RAMS Request message is identified by SFMT=1.

   The FCI field MUST contain only one RAMS Request.

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

   The FCI field has the structure depicted in Figure 6.


      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    SFMT=1     |          Optional TLV-encoded Fields          |
     +-+-+-+-+-+-+-+-+                                               |
     :      Optional TLV-encoded Fields (and Padding, if needed)     :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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          Figure 6: FCI field syntax for the RAMS Request message

   o  Min RAMS Buffer Fill Requirement (32 bits):  Optional TLV element
      that denotes the minimum milliseconds of data that RTP_Rx desires
      to have in its buffer before allowing the data to be consumed by
      the application.

      RTP_Rx may have knowledge of its buffering requirements.  These
      requirements may be application and/or device specific.  For
      instance, RTP_Rx may need to have a certain amount of data in its
      application buffer to handle transmission jitter and/or to be able
      to support error-control methods.  If RS is told the minimum
      buffering requirement of the receiver, it may tailor the burst
      more precisely, e.g., by choosing an appropriate starting point.
      The methods used by RTP_Rx to determine this value are application
      specific, and thus, out of the scope of this document.

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

      Type:  1

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

      RTP_Rx may have knowledge of its buffering requirements.  These
      requirements may be application or device specific.  For instance,
      one particular RTP_Rx may have more physical memory than another
      RTP_Rx, 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 RTP_Rx.

      Type:  2

   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.



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

      Type:  3

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

7.3.  RAMS Information

   The RAMS Information message is identified by SFMT=2.

   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 RTP_Rx as described below.  Optional payload-specific
   information MAY follow RAMS Information.

   The FCI field has the structure depicted in Figure 7.


      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    SFMT=2     |      MSN      |          Response             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :      Optional TLV-encoded Fields (and Padding, if needed)     :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 7: 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 RTP_Rx 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  Response (16 bits):  Mandatory field that denotes the RS response
      code for this RAMS-I message.  This document defines several
      initial response codes and registers them with IANA.  If a new
      vendor-neutral response code will be defined, it MUST be
      registered with IANA through the guidelines specified in
      Section 12.6.  If the new response code is intended to be used
      privately by a vendor, there is no need for IANA management.



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      Instead, the vendor MUST use the private extension mechanism
      (Section 7.1.2) to convey its message and MUST indicate this by
      putting zero in the Response field.

   o  RTP Seqnum of the First Packet (16 bits):  TLV element that
      specifies the RTP sequence number of the first packet that will be
      sent in the retransmission session.  This allows RTP_Rx to know
      whether one or more packets sent by RS have been dropped at the
      beginning of the retransmission session.  If RS accepts the RAMS
      request, this element MUST exist.  If RS rejects the RAMS request,
      this element SHALL NOT exist.

      Type:  31

   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
      RTP_Rx could join the primary multicast session in RTP ticks.  A
      zero value in this field means that RTP_Rx can join the primary
      multicast session right away.

      Note that if the RAMS request has been accepted, RS MUST send this
      field at least once, so that RTP_Rx 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 RTP_Rx when or whether to join
      the primary multicast session.

      Type:  32

   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 RTP_Rx.  It is
      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.

      Type:  33

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



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      Type:  34

   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.

7.4.  RAMS Termination

   The RAMS Termination message is identified by SFMT=3.

   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 RTP_Rx has already joined the
   primary multicast session and received at least one RTP packet from
   the multicast session, RTP_Rx 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 RTP_Rx issues the RAMS-T message before it has joined and/or begun
   receiving RTP packets from the primary multicast session, RTP_Rx 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 8.


      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    SFMT=3     |          Optional TLV-encoded Fields          |
     +-+-+-+-+-+-+-+-+                                               |
     :      Optional TLV-encoded Fields (and Padding, if needed)     :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 8: 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 RTP_Rx.  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.



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      Type:  61

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


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.

   This document also defines a new SDP attribute ('rams-updates') that
   indicates whether RS supports updated request messages or not.  This
   attribute is used in a declarative manner.  If RS supports updated
   request messages and this attribute is included in the SDP
   description, RTP_Rx MAY send updated requests.  RS may or may not be
   able to accept value changes in every field in an RAMS-R message.
   However, if the 'rams-updates' attribute is not included in the SDP
   description, RTP_Rx SHALL NOT send updated requests (RTP_Rx MAY
   repeat its initial request without changes, though).

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 [I-D.ietf-mmusic-rfc3388bis],
   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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   [RFC5576].

   In the example shown Figure 9, we have a primary multicast stream and
   a unicast retransmission stream.  The source stream is multicast from
   a distribution source (with a source IP address of 192.0.2.2) to the
   multicast destination address of 233.252.0.2 and port 41000.  A
   Retransmission Server including feedback target functionality (with
   an address of 192.0.2.1 and port of 41001) is specified with the
   'rtcp' attribute.  The RTP receiver(s) can report missing packets on
   the source stream to the feedback target and request retransmissions.
   In the RAMS context, the parameter 'rtx-time' specifies the time in
   milliseconds that the Retransmission Server keeps an RTP packet in
   its buffer available for retransmission (measured from the time the
   packet was received by the Retransmission Server).

   The RTP retransmissions are sent on a unicast session with a
   destination port of 41002.  The RTCP port for the unicast session
   (41003) is specified with the 'rtcp' attribute.  In this example,
   both the conventional retransmission and rapid acquisition support
   are enabled.  This is achieved by the additional "a=rtcp-fb:98 nack
   ssli" line.  Note that this SDP includes the "a=sendonly" line for
   the media description of the retransmission stream and is for the
   Retransmission Server (RS).  Its counterpart for the RTP Receiver
   (RTP_Rx) includes the "a=recvonly" line as shown in Figure 10.

   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 of that primary multicast session.  This feedback message has
   to have the SSRC of the primary multicast stream for which rapid
   acquisition is requested for.  However, since this RTP receiver has
   not received any RTP packets from the primary multicast 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 [RFC5576].

   Note that listing the SSRC values for the primary multicast 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, RS MAY react to the RAMS-R message by sending any



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   transport-layer and payload-specific feedback message(s) and starting
   the unicast burst.

        v=0
        o=ali 1122334455 1122334466 IN IP4 rams.example.com
        s=Rapid Acquisition Example
        t=0 0
        a=group:FID 3 4
        a=rtcp-unicast:rsi
        m=video 41000 RTP/AVPF 98
        i=Primary Multicast Stream #2
        c=IN IP4 233.252.0.2/255
        a=source-filter: incl IN IP4 233.252.0.2 192.0.2.2
        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=rams-updates
        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=sendonly
        a=rtpmap:99 rtx/90000
        a=rtcp:41003
        a=fmtp:99 apt=98; rtx-time=5000
        a=mid:4

         Figure 9: Example SDP for RS when RAMS support is enabled




















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        v=0
        o=ali 1122334455 1122334466 IN IP4 rams.example.com
        s=Rapid Acquisition Example
        t=0 0
        a=group:FID 3 4
        a=rtcp-unicast:rsi
        m=video 41000 RTP/AVPF 98
        i=Primary Multicast Stream #2
        c=IN IP4 233.252.0.2/255
        a=source-filter: incl IN IP4 233.252.0.2 192.0.2.2
        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=rams-updates
        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

      Figure 10: Example SDP for RTP_Rx when RAMS support is enabled

   In the above SDP descriptions, the ports for the unicast session are
   also declaratively specified and the RTP receivers are required to
   use them.  In certain scenarios, however, RTP receivers may want to
   use other desired ports to receive the burst and retransmission
   packets.  A current work considers this feature in more general terms
   and provides the recommended mechanisms
   [I-D.begen-avt-ports-for-ucast-mcast-rtp].

   In this section, we considered the simplest scenario where the
   primary multicast session carried only one multicast stream and the
   RTP receiver wanted to rapidly acquire this stream only.  Discussions
   on scenarios where the primary multicast session carries two or more
   multicast streams or the RTP receiver wants to acquire one or more
   multicast streams from multiple multicast RTP sessions at the same
   time are presented in [I-D.begen-avt-rams-scenarios].







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9.  NAT Considerations

   For a variety of reasons, one or more NAPT devices (hereafter simply
   called NAT) are expected to exist between RTP_Rx and RS.  NATs have a
   variety of operating characteristics for UDP traffic [RFC4787].  For
   a NAT to permit traffic from RS to arrive at RTP_Rx, the NAT(s) must
   first either:

   a.  See UDP traffic sent from RTP_Rx (which is on the 'inside' of the
       NAT) to RS (which is on the 'outside' of the NAT).  This traffic
       is sent to the same transport address as the subsequent response
       traffic, OR;

   b.  Be configured to forward certain ports (e.g., using HTML
       configuration, UPnP IGD [UPnP-IGD], DLNA [DLNA]).  Details of
       this are out of scope of this document.

   For both (a) and (b), RTP_Rx is responsible for maintaining the NAT's
   state if it wants to receive traffic from the RS on that port.  For
   (a), RTP_Rx MUST send UDP traffic to keep the NAT binding alive, at
   least every 30 seconds [RFC4787].  Note that while (a) is more like
   an automatic/dynamic configuration, (b) is more like a manual/static
   configuration.


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_4/user/guide/
   vqe_guide3_4.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].





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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/


11.  Security Considerations

   Applications that are using RAMS make heavy use of the unicast
   feedback mechanism described in [I-D.ietf-avt-rtcpssm] and the
   payload format defined in [RFC4588].  Thus, these applications are
   subject to the general security considerations discussed in
   [I-D.ietf-avt-rtcpssm] and [RFC4588].  In this section, we give an
   overview of the guidelines and suggestions described in these
   specifications from a RAMS perspective.  We also discuss the security
   considerations that explicitly apply to RAMS applications.

   First of all, much of the session description information is
   available in the SDP descriptions that are distributed to the media
   sources, Retransmission Servers and RTP Receivers.  Adequate security
   measures are RECOMMENDED to ensure the integrity and authenticity of
   the SDP descriptions so that transport addresses of the media
   sources, Feedback Targets as well as other session-specific
   information can be authenticated.

   Compared to an RTCP NACK message that triggers one or more
   retransmissions, an RAMS Request (RAMS-R) message may trigger a new
   burst stream to be sent by the Retransmission Server.  Depending on
   the application-specific requirements and conditions existing at the
   time of the RAMS-R reception by the Retransmission Server, the
   resulting burst stream may contain potentially a large number of
   retransmission packets.  Since these packets are sent at a faster
   than the nominal rate of the multicast session, RAMS consumes more
   resources on the Retransmission Server, the RTP Receiver and the
   network.  This particularly makes denial-of-service attacks more
   intense, and hence, more harmful than attacks that target ordinary
   retransmission sessions.

   Following the suggestions given in [RFC4588], counter-measures SHOULD
   be taken to prevent tampered or spoofed RTCP packets.  Tampered
   RAMS-R messages may trigger inappropriate burst streams or alter the
   existing burst streams in an inappropriate way.  For example, if the



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   Max Receive Bitrate field is altered by a tampered RAMS-R message,
   the updated burst may overflow the buffer on the receiver side, or
   oppositely, may slow down the burst to the point that it is useless.
   Tampered RAMS Termination (RAMS-T) messages may terminate valid burst
   streams pre-maturely resulting in gaps in the received RTP packets.
   RAMS Information (RAMS-I) messages contain fields that are critical
   for the success of the RAMS operation.  Any tampered information in
   the RAMS-I message may easily cause the RTP Receiver to make wrong
   decisions.  Consequently, the RAMS operation may fail.

   While most of the denial-of-service attacks can be prevented by the
   integrity and authenticity checks enabled by SRTP, an attack can
   still be started by legitimate endpoints that send several valid
   RAMS-R messages to a particular Feedback Target in a synchronized
   fashion and very short amount of time.  Since a RAMS operation may
   temporarily consume a large amount of resources, a series of the
   RAMS-R messages may temporarily overload the Retransmission Server.
   In these circumstances, the Retransmission Server may, for example,
   reject incoming RAMS requests until its resources become available
   again.  One means to ameliorate this threat is to apply a per-
   endpoint policing mechanism on the incoming RAMS requests.  A
   reasonable policing mechanism should consider application-specific
   requirements and minimize false negatives.

   In addition to the denial-of-service attacks, man-in-the-middle and
   replay attacks can also be harmful.  However, RAMS itself does not
   bring any new risks or threats other than the ones discussed in
   [I-D.ietf-avt-rtcpssm].

   [RFC4588] RECOMMENDS that the cryptography mechanisms are used for
   the retransmission payload format to provide protection against known
   plaintext attacks.  As discussed in [RFC4588], the retransmission
   payload format sets the timestamp field in the RTP header to the
   media timestamp of the original packet and this does not compromise
   the confidentiality.  Furthermore, if cryptography is used to provide
   security services on the original stream, then the same services,
   with equivalent cryptographic strength, MUST be provided on the
   retransmission stream per [RFC4588].


12.  IANA Considerations

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







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   Ali Begen
   abegen@cisco.com

   170 West Tasman Drive
   San Jose, CA 95134 USA

   Note to the RFC Editor:  In the following, please replace "XXXX" with
   the number of this document prior to publication as an RFC.

12.1.  Registration of SDP Attributes

   This document registers a new attribute name in SDP.


        SDP Attribute ("att-field"):
        Attribute name:     rams-updates
        Long form:          Support for Updated RAMS Request Messages
        Type of name:       att-field
        Type of attribute:  Media level
        Subject to charset: No
        Purpose:            See this document
        Reference:          [RFCXXXX]
        Values:             None

12.2.  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].


        Value name:     ssli
        Long name:      Stream Synchronization Loss Indication
        Usable with:    nack
        Reference:      [RFCXXXX]

12.3.  Registration of FMT Values

   Within the RTPFB range, the following format (FMT) value is
   registered:











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        Name:           RAMS
        Long name:      Rapid Acquisition of Multicast Sessions
        Value:          6
        Reference:      [RFCXXXX]

12.4.  SFMT Values for RAMS Messages Registry

   This document creates a new sub-registry for the sub-feedback message
   type (SFMT) values to be used with the FMT value registered for RAMS
   messages.  The registry is called the SFMT Values for RAMS Messages
   Registry.  This registry is to be managed by the IANA according to
   the Specification Required policy of [RFC5226].

   The length of the SFMT field in the RAMS messages is a single octet,
   allowing 256 values.  The registry is initialized with the following
   entries:


  Value Name                                               Reference
  ----- -------------------------------------------------- -------------
  1     RAMS Request                                       [RFCXXXX]
  2     RAMS Information                                   [RFCXXXX]
  3     RAMS Termination                                   [RFCXXXX]


   The SFMT values 0 and 255 are reserved for future use.

   Any registration for an unassigned SFMT value MUST contain the
   following information:

   o  Contact information of the one doing the registration, including
      at least name, address, and email.

   o  A detailed description of what the new SFMT represents and how it
      shall be interpreted.

   Note that new RAMS functionality should be introduced by using the
   extension mechanism within the existing RAMS message types not by
   introducing new message types unless it is absolutely necessary.

12.5.  RAMS TLV Space Registry

   This document creates a new IANA TLV space registry for the RAMS
   extensions.  The registry is called the RAMS TLV Space Registry.
   This registry is to be managed by the IANA according to the
   Specification Required policy of [RFC5226].

   The length of the Type field in the TLV elements is a single octet,



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   allowing 256 values.  The Type values 0 and 255 are reserved for
   future use.  The Type values between (and including) 128 and 254 are
   reserved for private extensions.

   The registry is initialized with the following entries:


   Type Description                                        Reference
   ---- -------------------------------------------------- -------------
   1    Min RAMS Buffer Fill Requirement                   [RFCXXXX]
   2    Max RAMS Buffer Fill Requirement                   [RFCXXXX]
   3    Max Receive Bitrate                                [RFCXXXX]
   31   RTP Seqnum of the First Packet                     [RFCXXXX]
   32   Earliest Multicast Join Time                       [RFCXXXX]
   33   Burst Duration                                     [RFCXXXX]
   34   Max Transmit Bitrate                               [RFCXXXX]
   61   Extended RTP Seqnum of First Multicast Packet      [RFCXXXX]


   Any registration for an unassigned Type value MUST contain the
   following information:

   o  Contact information of the one doing the registration, including
      at least name, address, and email.

   o  A detailed description of what the new TLV element represents and
      how it shall be interpreted.

12.6.  RAMS Response Code Space Registry

   This document creates a new IANA TLV space registry for the RAMS
   response codes.  The registry is called the RAMS Response Code Space
   Registry.  This registry is to be managed by the IANA according to
   the Specification Required policy of [RFC5226].

   The length of the Response field is two octets, allowing 65536 codes.
   However, the response codes have been classified and registered
   following an HTTP-style code numbering in this document.  New
   response codes SHALL follow the guidelines below:


   Code Level Purpose
   ---------- ---------------
   1xx        Informational
   2xx        Success
   3xx        Redirection
   4xx        RTP Receiver Error
   5xx        Retransmission Server Error



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   The Response code 65536 is reserved for future use.

   The registry is initialized with the following entries:


  Code  Description                                        Reference
  ----- -------------------------------------------------- -------------
  0     A private response code is included in the message [RFCXXXX]

  100   Parameter update for RAMS session                  [RFCXXXX]
  101   RAMS session was terminated by RAMS-T              [RFCXXXX]
  102   RAMS session was terminated by RS                  [RFCXXXX]

  200   RAMS request has been accepted                     [RFCXXXX]
  201   Unicast burst has been completed                   [RFCXXXX]

  400   Invalid RAMS-R message syntax
  401   Invalid min buffer requirement in RAMS-R message   [RFCXXXX]
  402   Invalid max buffer requirement in RAMS-R message   [RFCXXXX]
  403   Invalid max bw requirement in RAMS-R message       [RFCXXXX]

  500   An unspecified RS internal error has occurred      [RFCXXXX]
  501   RS has no bandwidth to start RAMS session          [RFCXXXX]
  502   RS has no CPU available to start RAMS session      [RFCXXXX]
  503   RAMS functionality is not available on RS          [RFCXXXX]
  504   RAMS functionality is not available for RTP_Rx     [RFCXXXX]
  505   RAMS functionality is not available for
        the requested multicast stream                     [RFCXXXX]
  506   RS has no a valid starting point available for
        the requested multicast stream                     [RFCXXXX]
  507   RS has no reference information available for
        the requested multicast stream                     [RFCXXXX]


   Any registration for an unassigned Response code MUST contain the
   following information:

   o  Contact information of the one doing the registration, including
      at least name, address, and email.

   o  A detailed description of what the new Response code describes and
      how it shall be interpreted.


13.  Acknowledgments

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



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   The authors also thank the following individuals for their
   contributions, comments and suggestions to this document:  Dave Oran,
   Dan Wing, 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.


14.  Change Log

14.1.  draft-ietf-avt-rapid-acquisition-for-rtp-05

   The following are the major changes compared to version 04:

   o  Editorial changes throughout the document.

14.2.  draft-ietf-avt-rapid-acquisition-for-rtp-04

   The following are the major changes compared to version 03:

   o  Clarifications for the definition of RS.

   o  Response codes have been defined.

14.3.  draft-ietf-avt-rapid-acquisition-for-rtp-03

   The following are the major changes compared to version 02:

   o  Clarifications for the RAMS-I message.

   o  Type values have been assigned.

14.4.  draft-ietf-avt-rapid-acquisition-for-rtp-02

   The following are the major changes compared to version 01:

   o  Port mapping discussion has been removed since it will be
      discussed in a separate draft.

   o  Security considerations section has been added.

   o  Burst shaping section has been completed.

   o  Most of the outstanding open issues have been addressed.







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14.5.  draft-ietf-avt-rapid-acquisition-for-rtp-01

   The following are the major changes compared to version 00:

   o  Formal definitions of vendor-neutral and private extensions and
      their IANA registries have been added.

   o  SDP examples were explained in more detail.

   o  The sub-FMT field has been introduced in the RAMS messages for
      message type identification.

   o  Some terminology has been fixed.

   o  NAT considerations section has been added.

14.6.  draft-ietf-avt-rapid-acquisition-for-rtp-00

   This is a resubmission of version 03 as a WG item.

14.7.  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.

14.8.  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.





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

14.9.  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.

   o  Editorial changes to clarify several points.


15.  References

15.1.  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



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              Description Protocol", RFC 4566, July 2006.

   [I-D.ietf-mmusic-rfc3388bis]
              Camarillo, G. and H. Schulzrinne, "The SDP (Session
              Description Protocol) Grouping Framework",
              draft-ietf-mmusic-rfc3388bis-04 (work in progress),
              November 2009.

   [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]
              Ott, J. and J. Chesterfield, "RTCP Extensions for Single-
              Source Multicast Sessions with Unicast Feedback",
              draft-ietf-avt-rtcpssm-19 (work in progress),
              November 2009.

   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", RFC 5576, June 2009.

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

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

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

15.2.  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., Handley, M., and J. Macker,
              "NACK-Oriented Reliable Multicast Transport Protocol",
              draft-ietf-rmt-pi-norm-revised-14 (work in progress),
              September 2009.



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   [I-D.begen-avt-rams-scenarios]
              Begen, A., "Considerations for RAMS Scenarios",
              draft-begen-avt-rams-scenarios-00 (work in progress),
              October 2009.

   [I-D.begen-avt-rtp-mpeg2ts-preamble]
              Begen, A. and E. Friedrich, "RTP Payload Format for
              MPEG2-TS Preamble",
              draft-begen-avt-rtp-mpeg2ts-preamble-03 (work in
              progress), October 2009.

   [I-D.begen-avt-rapid-sync-rtcp-xr]
              Begen, A. and E. Friedrich, "Multicast Acquisition Report
              Block Type for RTCP XR",
              draft-begen-avt-rapid-sync-rtcp-xr-03 (work in progress),
              October 2009.

   [I-D.ietf-avt-rapid-rtp-sync]
              Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
              Flows", draft-ietf-avt-rapid-rtp-sync-07 (work in
              progress), November 2009.

   [I-D.westerlund-avt-ecn-for-rtp]
              Westerlund, M., Johansson, I., Perkins, C., and K.
              Carlberg, "Explicit Congestion Notification (ECN) for RTP
              over UDP", draft-westerlund-avt-ecn-for-rtp-02 (work in
              progress), October 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.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

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

   [I-D.begen-avt-ports-for-ucast-mcast-rtp]
              Begen, A. and B. Steeg, "Port Mapping Between Unicast and
              Multicast RTP Sessions",
              draft-begen-avt-ports-for-ucast-mcast-rtp-01 (work in
              progress), October 2009.




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   [UPnP-IGD]
              Forum, UPnP., "Universal Plug and Play (UPnP) Internet
              Gateway Device (IGD)", November 2001.

   [DLNA]     , DLNA., "http://www.dlna.org/home".

   [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.


Authors' Addresses

   Bill VerSteeg
   Cisco
   5030 Sugarloaf Parkway
   Lawrenceville, GA  30044
   USA

   Email:  billvs@cisco.com


   Ali Begen
   Cisco
   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








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

   Email:  zeevvax@microsoft.com












































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