[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]

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:  January 10, 2011                                T. VanCaenegem
                                                          Alcatel-Lucent
                                                                  Z. Vax
                                                   Microsoft Corporation
                                                            July 9, 2010


       Unicast-Based Rapid Acquisition of Multicast RTP Sessions
              draft-ietf-avt-rapid-acquisition-for-rtp-11

Abstract

   When an RTP receiver joins a 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 (or appearance)
   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 can 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 multicast
   stream.  This unicast RTP flow can be transmitted at a faster than
   natural bitrate 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 in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.




VerSteeg, et al.        Expires January 10, 2011                [Page 1]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 10, 2011.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.


















VerSteeg, et al.        Expires January 10, 2011                [Page 2]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Acquisition of RTP Streams vs. RTP Sessions  . . . . . . .  6
     1.2.  Outline  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  7
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Elements of Delay in Multicast Applications  . . . . . . . . .  9
   5.  Protocol Design Considerations and Their Effect on
       Resource Management for Rapid Acquisition  . . . . . . . . . . 10
   6.  Rapid Acquisition of Multicast RTP Sessions (RAMS) . . . . . . 12
     6.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.2.  Message Flows  . . . . . . . . . . . . . . . . . . . . . . 13
     6.3.  Synchronization of Primary Multicast Streams . . . . . . . 23
     6.4.  Burst Shaping and Congestion Control in RAMS . . . . . . . 23
     6.5.  Failure Cases  . . . . . . . . . . . . . . . . . . . . . . 26
   7.  Encoding of the Signaling Protocol in RTCP . . . . . . . . . . 27
     7.1.  Extensions . . . . . . . . . . . . . . . . . . . . . . . . 28
       7.1.1.  Vendor-Neutral Extensions  . . . . . . . . . . . . . . 29
       7.1.2.  Private Extensions . . . . . . . . . . . . . . . . . . 29
     7.2.  RAMS Request . . . . . . . . . . . . . . . . . . . . . . . 29
     7.3.  RAMS Information . . . . . . . . . . . . . . . . . . . . . 32
     7.4.  RAMS Termination . . . . . . . . . . . . . . . . . . . . . 34
   8.  SDP Signaling  . . . . . . . . . . . . . . . . . . . . . . . . 35
     8.1.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . 35
     8.2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . 36
     8.3.  Example and Discussion . . . . . . . . . . . . . . . . . . 37
   9.  NAT Considerations . . . . . . . . . . . . . . . . . . . . . . 39
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 40
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 42
     11.1. Registration of SDP Attributes . . . . . . . . . . . . . . 42
     11.2. Registration of SDP Attribute Values . . . . . . . . . . . 42
     11.3. Registration of FMT Values . . . . . . . . . . . . . . . . 43
     11.4. SFMT Values for RAMS Messages Registry . . . . . . . . . . 43
     11.5. RAMS TLV Space Registry  . . . . . . . . . . . . . . . . . 44
     11.6. RAMS Response Code Space Registry  . . . . . . . . . . . . 45
   12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 47
   13. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 47
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 47
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 47
     14.2. Informative References . . . . . . . . . . . . . . . . . . 49
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 50









VerSteeg, et al.        Expires January 10, 2011                [Page 3]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


1.  Introduction

   Most multicast flows carry a stream of inter-related data.  The
   receivers need to acquire certain information 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 (although
   its content can change over time) 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
   multicast stream [IC2009].

   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 might 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 might
   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 can 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 might not be the same for different receivers; it
   usually varies depending on the join time, length of the Reference
   Information repetition (or appearance) 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



VerSteeg, et al.        Expires January 10, 2011                [Page 4]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   applications and describe a method that uses the fundamental tools
   offered by the existing RTP and RTCP protocols [RFC3550].  In this
   method, either the multicast source (or the distribution source in a
   source-specific 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
   [RFC5760]) 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 and asks for the Reference
   Information.  The retransmission server starts a new unicast RTP
   (retransmission) session and sends the Reference Information to the
   RTP receiver over that session.  If there is spare bandwidth, the
   retransmission server might burst the Reference Information 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.  A simplified network diagram showing
   this method through an intermediary network element is depicted in
   Figure 1.

   This method potentially reduces the acquisition delay.  We refer to
   this method as Unicast-based Rapid Acquisition of Multicast RTP
   Sessions.  A primary use case for this method is to reduce the
   channel-change times in IPTV networks where compressed video streams
   are multicast in different SSM sessions and viewers randomly join
   these sessions.























VerSteeg, et al.        Expires January 10, 2011                [Page 5]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


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


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

    Figure 1: Rapid acquisition through an intermediary network element

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

1.1.  Acquisition of RTP Streams vs. RTP Sessions

   In this memo we describe a protocol that handles the rapid
   acquisition of a single multicast RTP session (called primary
   multicast RTP session) carrying one or more RTP streams (called
   primary multicast streams).  If desired, multiple instances of this
   protocol may be run in parallel to acquire multiple RTP sessions
   simultaneously.

   When an RTP receiver requests the Reference Information from the
   retransmission server, it can opt to rapidly acquire a specific
   subset of the available RTP streams in the primary multicast RTP
   session.  Alternatively, it can request the rapid acquisition of all
   of the RTP streams in that RTP session.  Regardless of how many RTP
   streams are requested by the RTP receiver or how many will be
   actually sent by the retransmission server, only one unicast RTP



VerSteeg, et al.        Expires January 10, 2011                [Page 6]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   session will be established by the retransmission server.  This
   unicast RTP session is separate from the associated primary multicast
   RTP session.  As a result, there are always two different RTP
   sessions in a single instance of the rapid acquisition protocol:  (i)
   the primary multicast RTP session with its associated unicast
   feedback and (ii) the unicast RTP session.

   If the RTP receiver wants to rapidly acquire multiple RTP sessions
   simultaneously, separate unicast RTP sessions will be established for
   each of them.

1.2.  Outline

   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.  Finally, Section 10
   discusses the security considerations.

   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].


3.  Definitions

   This document uses the following acronyms and definitions frequently:

   (Primary) SSM (or Multicast) Session:  The multicast session to which
   RTP receivers can join at a random point in time.  A primary SSM
   session can carry multiple RTP streams.

   Primary Multicast RTP Session:  The multicast RTP session an RTP
   receiver is interested in acquiring rapidly.  From the RTP receiver's
   viewpoint, the primary multicast RTP session has one associated
   unicast RTCP feedback stream to a Feedback Target, in addition to the
   primary multicast RTP stream(s).

   Primary Multicast (RTP) Streams:  The RTP stream(s) carried in the



VerSteeg, et al.        Expires January 10, 2011                [Page 7]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   primary multicast RTP 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 (FT):  Unicast RTCP feedback target as defined in
   [RFC5760].  FT_Ap denotes a specific feedback target running on a
   particular address and port.

   Retransmission (or Burst) Packet:  An RTP packet that is formatted as
   defined in Section 4 of [RFC4588].  The payload of a retransmission
   or burst packet comprises the retransmission payload header followed
   by the payload of the original RTP packet.

   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.

   Preamble Information:  A more compact form of the whole or a subset
   of the Reference Information transmitted out-of-band.

   (Unicast) Burst (or Retransmission) RTP Session:  The unicast RTP
   session used to send one or more unicast burst RTP streams and their
   associated RTCP messages.  The terms "burst RTP session" and
   "retransmission RTP session" can be used interchangeably.

   (Unicast) Burst (Stream):  A unicast stream of RTP retransmission
   packets that enable an RTP receiver to rapidly acquire the Reference
   Information associated with a primary multicast stream.  Each burst
   stream is identified by its Synchronization Source (SSRC) identifier
   that is unique in the primary multicast RTP session.  Following the
   session-multiplexing guidelines in [RFC4588], each unicast burst
   stream will use the same SSRC and CNAME as its primary multicast RTP
   stream.

   Retransmission Server (RS):  The RTP/RTCP endpoint that can generate
   the retransmission packets and the burst streams.  RS may also
   generate other non-retransmission packets to aid rapid acquisition.




VerSteeg, et al.        Expires January 10, 2011                [Page 8]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


4.  Elements of Delay in Multicast Applications

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

   o  Multicast switching delay

   o  Reference Information latency

   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 can 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 might 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 can become quite large and be a major contributor
   to the overall delay.



VerSteeg, et al.        Expires January 10, 2011                [Page 9]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   The buffering component of the overall acquisition delay is driven by
   the way the application layer processes the payload.  In many
   multicast applications, an unreliable transport protocol such as UDP
   [RFC0768] is often used to transmit the data packets, and the
   reliability, if needed, is usually addressed through other means such
   as Forward Error Correction (e.g.,
   [I-D.ietf-fecframe-interleaved-fec-scheme]) and retransmission.
   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

   This section is for informational purposes and does not contain
   requirements for implementations.

   Rapid acquisition is an optimization of a system that is expected to
   continue to work correctly and properly whether or not the
   optimization is effective, or even fails due to lost control and
   feedback 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 interference with the multicast session that
      might be simultaneously being received by other hosts.  In
      addition, it must also avoid interference with other multicast



VerSteeg, et al.        Expires January 10, 2011               [Page 10]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


      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 might serve only a single subscriber, limiting
   congestion impact to the traffic of only that subscriber, but others
   can be shared links carrying multicast sessions of many subscribers.
   Also note that the state of these links can vary over time.  The
   receiver might have knowledge of a portion of this network, or might
   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 round-trip times (RTT) 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 streams, and e is
      an excess-bandwidth coefficient.  The total duration of the
      unicast burst must have a reasonable bound; long unicast bursts



VerSteeg, et al.        Expires January 10, 2011               [Page 11]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


      devolve to the bandwidth profile of multi-unicast for the whole
      system.

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

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

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

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

6.1.  Overview

   [RFC5760] 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.  One or more entities different
   from the Distribution Source MAY implement the feedback target



VerSteeg, et al.        Expires January 10, 2011               [Page 12]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   functionality, and different RTP receivers MAY use different feedback
   targets.

   This document builds further on these concepts to reduce the
   acquisition delay 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 RTP session are
   continuously stored.  When an RTP receiver wants to receive a primary
   multicast stream, it can first request a unicast burst from the Burst
   Source before it joins the SSM session.  In this burst, the packets
   are formatted as RTP retransmission packets [RFC4588] and carry
   Reference Information.  This information allows the RTP receiver to
   start usefully consuming the RTP packets sent in the primary
   multicast RTP session.

   Using an accelerated bitrate (as compared to the nominal bitrate 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 in the associated multicast
   stream, 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 SSM 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 and
   the message flows.  They are

   o  Multicast Source

   o  Feedback Target (FT)

   o  Burst/Retransmission Source (BRS)

   o  RTP Receiver (RTP_Rx)










VerSteeg, et al.        Expires January 10, 2011               [Page 13]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


    -----------                                       --------------
   |           |------------------------------------>|              |
   |           |.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.->|              |
   |           |                                     |              |
   | Multicast |          ----------------           |              |
   |  Source   |         | Retransmission |          |              |
   |           |-------->|  Server  (RS)  |          |              |
   |           |.-.-.-.->|                |          |              |
   |           |         |  ------------  |          |              |
    -----------          | |  Feedback  | |<.=.=.=.=.|              |
                         | | Target (FT)| |<~~~~~~~~~| RTP Receiver |
   PRIMARY MULTICAST     |  ------------  |          |   (RTP_Rx)   |
   RTP SESSION with      |                |          |              |
   UNICAST FEEDBACK      |                |          |              |
                         |                |          |              |
   - - - - - - - - - - - |- - - - - - - - |- - - - - |- - - - - - - |- -
                         |                |          |              |
   UNICAST BURST         |  ------------  |          |              |
   (or RETRANSMISSION)   | | Burst  and | |<~~~~~~~~>|              |
   RTP SESSION           | |  Retrans.  | |.........>|              |
                         | |Source (BRS)| |<.=.=.=.=>|              |
                         |  ------------  |          |              |
                         |                |          |              |
                          ----------------            --------------


   -------> Multicast RTP Flow
   .-.-.-.> Multicast RTCP Flow
   .=.=.=.> Unicast RTCP Reports
   ~~~~~~~> Unicast RTCP Feedback Messages
   .......> Unicast RTP Flow

        Figure 2: Flow diagram for unicast-based rapid acquisition

   The feedback target (FT) is the entity as defined in [RFC5760], to
   which RTP_Rx sends its RTCP feedback messages indicating packet loss
   by means of an RTCP NACK message or indicating RTP_Rx's desire to
   rapidly acquire the primary multicast RTP session by means of an RTCP
   feedback message defined in this document.  While the Burst/
   Retransmission Source (BRS) 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 BRS) are combined in
   a single physical entity and they share state.  In the remainder of
   the text, the term Retransmission Server (RS) is used whenever
   appropriate, to refer to this single physical entity.

   FT is involved in the primary multicast RTP session and receives



VerSteeg, et al.        Expires January 10, 2011               [Page 14]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   unicast feedback for that session as in [RFC5760].  BRS is involved
   in the unicast burst (or retransmission) RTP session and transmits
   the unicast burst and retransmission packets formatted as RTP
   retransmission packets [RFC4588] in a single separate unicast RTP
   session to each RTP_Rx.  In the unicast burst RTP session, as in any
   other RTP session, the BRS and RTP_Rx regularly send the periodic
   sender and receiver reports, respectively.

   The unicast burst is started by an RTCP feedback message that is
   defined in this document based on the common packet format provided
   in [RFC4585].  An RTP retransmission is triggered by an RTCP NACK
   message defined in [RFC4585].  Both of these messages are sent to FT
   of the primary multicast RTP session, and can start the unicast
   burst/retransmission RTP session.

   In the RTP/AVPF profile, there are certain rules that apply to
   scheduling of both of these messages sent to FT in the primary
   multicast RTP session, and these are detailed in Section 3.5 of
   [RFC4585].  One of the rules states that in a multi-party session
   such as the SSM sessions we are considering in this specification, an
   RTP_Rx cannot send an RTCP feedback message for a minimum of one
   second period after joining the session (i.e., Tmin=1.0 second).
   While this rule has the goal of avoiding problems associated with
   flash crowds in typical multi-party sessions, it defeats the purpose
   of rapid acquisition.  Furthermore, when RTP receivers delay their
   messages requesting a burst by a deterministic or even a random
   amount, it still does not make a difference since such messages are
   not related and their timings are independent from each other.  Thus,
   in this specification we initialize Tmin to zero and allow the RTP
   receivers to send a burst request message right at the beginning.
   For the subsequent messages during rapid acquisition, the timing
   rules of [RFC4585] still apply.

   Figure 3 depicts an example of messaging flow for rapid acquisition.
   The RTCP feedback messages are explained below.  The optional
   messages are indicated in parentheses and they might or might not be
   present during rapid acquisition.  In a scenario where rapid
   acquisition is performed by a feedback target co-located with the
   media sender, the same method (with the identical message flows)
   still applies.


                  -------------------------
                 | Retransmission  Server  |
    -----------  |  ------   ------------  |   --------    ------------
   | Multicast | | |  FT  | | Burst/Ret. | |  |        |  |    RTP     |
   |  Source   | | |      | |   Source   | |  | Router |  |  Receiver  |
   |           | |  ------   ------------  |  |        |  |  (RTP_Rx)  |



VerSteeg, et al.        Expires January 10, 2011               [Page 15]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


    -----------  |      |         |        |   --------    ------------
     |            -------------------------       |                |
     |                  |         |               |                |
     |-- RTP Multicast ---------->--------------->|                |
     |-. RTCP Multicast -.-.-.-.->-.-.-.-.-.-.-.->|                |
     |                  |         |               |                |
     |                  |         |********************************|
     |                  |         |*      PORT MAPPING SETUP      *|
     |                  |         |********************************|
     |                  |         |               |                |
     |                  |<~~~~~~~~~~~~~~~~~~~~~~~~~ RTCP RAMS-R ~~~|
     |                  |         |               |                |
     |                  |         |********************************|
     |                  |         |* UNICAST SESSION  ESTABLISHED *|
     |                  |         |********************************|
     |                  |         |               |                |
     |                  |         |~~~ RTCP RAMS-I ~~~~~~~~~~~~~~~>|
     |                  |         |               |                |
     |                  |         |... Unicast RTP Burst .........>|
     |                  |         |               |                |
     |                  |<~~~~~~~~~~~~~~~~~~~~~~~~ (RTCP RAMS-R) ~~|
     |                  |         |               |                |
     |                  |         |~~ (RTCP RAMS-I) ~~~~~~~~~~~~~~>|
     |                  |         |               |                |
     |                  |         |               |                |
     |                  |         |               |<= SFGMP Join ==|
     |                  |         |               |                |
     |-- RTP Multicast ------------------------------------------->|
     |-. RTCP Multicast -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.>|
     |                  |         |               |                |
     |                  |         |               |                |
     |                  |         |<~~~~~~~~~~~~~~~ RTCP RAMS-T ~~~|
     |                  |         |               |                |
     |                  |         |               |                |
     |                  |         |               |                |
     :                  :         :               :                :
     :                  :         :               :                :
     |                  |         |<.=.= Unicast RTCP Reports .=.=>|
     :                  :         :    (for the unicast session)   :
     :                  :         :               :                :
     |                  |         |               |                |


   -------> Multicast RTP Flow
   .-.-.-.> Multicast RTCP Flow
   .=.=.=.> Unicast RTCP Reports
   ~~~~~~~> Unicast RTCP Feedback Messages
   =======> SFGMP Messages



VerSteeg, et al.        Expires January 10, 2011               [Page 16]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   .......> Unicast RTP Flow

        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 provisions are needed 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.   Port Mapping Setup:  For the primary multicast RTP session, the
        RTP and RTCP destination ports are declaratively specified
        (Refer to Section 8 for examples in SDP).  However, RTP_Rx needs
        to choose its RTP and RTCP receive ports in the unicast burst
        RTP session.  Since this unicast session is established after
        RTP_Rx has sent a RAMS-Request (RAMS-R) message as unicast
        feedback in the primary multicast RTP session, RTP_Rx MUST first
        setup the port mappings between the unicast and multicast
        sessions and send this mapping information to FT along with the
        RAMS-R message so that BRS knows how to communicate with RTP_Rx.

        The details of this setup procedure are explained in
        [I-D.ietf-avt-ports-for-ucast-mcast-rtp].  Other NAT-related
        issues are left to Section 9 to keep the present discussion
        focused on the RAMS message flows.

   2.   Request:  RTP_Rx sends a rapid acquisition request (RAMS-R) for
        the primary multicast RTP session to the unicast feedback target
        of that session.  The request contains the SSRC identifier of
        RTP_Rx (aka SSRC of packet sender) and can contain the media
        sender SSRC identifier(s) of the primary multicast stream(s) of
        interest (aka SSRC of media source).  The RAMS-R message can
        contain parameters that constrain the burst, such as the buffer
        and bandwidth limits.

        Before joining the SSM session, RTP_Rx learns the addresses for
        the multicast source, group and RS by out-of-band means.  If
        RTP_Rx desires to rapidly acquire only a subset of the primary
        multicast streams available in the primary multicast RTP
        session, RTP_Rx MUST also acquire the SSRC identifiers for the
        desired RTP streams out-of-band.  Based on this information,
        RTP_Rx populates the desired SSRC(s) in the RAMS-R message.

        When FT successfully receives the RAMS-R message, BRS responds



VerSteeg, et al.        Expires January 10, 2011               [Page 17]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


        to it by accepting or rejecting the request.  Immediately before
        BRS sends any RTP or RTCP packet(s) described below, it
        establishes the unicast burst RTP session.

   3.   Response:  BRS sends RAMS-Information (RAMS-I) message(s) to
        RTP_Rx to convey the status for the burst(s) requested by
        RTP_Rx.

        If the primary multicast RTP session associated with FT_Ap on
        which the RAMS-R message was received contains only a single
        primary multicast stream, BRS SHALL always use the SSRC of the
        RTP stream associated with FT_Ap in the RAMS-I message(s)
        regardless of the media sender SSRC requested in the RAMS-R
        message.  In such cases the 'ssrc' attribute MAY be omitted from
        the media description.  If the requested SSRC and the actual
        media sender SSRC do not match, BRS MUST explicitly populate the
        correct media sender SSRC in the initial RAMS-I message (See
        Section 7.3).

        FT_Ap could also be associated with an RTP session that carries
        two or more primary multicast streams.  If RTP_Rx will issue a
        collective request to receive the whole primary multicast RTP
        session, it does not need the 'ssrc' attributes to be described
        in the media description.

        If FT_Ap is associated with two or more RTP sessions, RTP_Rx's
        request will be ambiguous.  To avoid any ambiguity, each FT_Ap
        MUST only associate itself with a single RTP session.

        If RTP_Rx is willing to rapidly acquire only a subset of the
        primary multicast streams, RTP_Rx MUST list all the media sender
        SSRC(s) denoting the stream(s) it wishes to acquire in the
        RAMS-R message.  Upon receiving such a message, BRS MAY accept
        the request for all or a subset of the media sender SSRC(s) that
        matched the RTP stream(s) it serves.  BRS MUST reject all other
        requests with an appropriate response code.


        *  Reject Responses:  BRS MUST take into account any limitations
           that may have been specified by RTP_Rx in the RAMS-R message
           when making a decision regarding the request.  If RTP_Rx has
           requested to acquire the whole primary multicast RTP session
           but BRS cannot provide a rapid acquisition service for any of
           the primary multicast streams, BRS MUST reject the request
           via a single RAMS-I message with a collective reject response
           code and whose media sender SSRC field is set to one of SSRCs
           served by this FT_Ap.  Upon receiving this RAMS-I message,
           RTP_Rx abandons the rapid acquisition attempt and can



VerSteeg, et al.        Expires January 10, 2011               [Page 18]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


           immediately join the multicast session by sending an SFGMP
           Join message towards its upstream multicast router.

           In all other cases, BRS MUST send a separate RAMS-I message
           with the appropriate response code for each primary multicast
           stream that has been requested by RTP_Rx but cannot be served
           by BRS.

        *  Accept Responses:  BRS MUST send at least one separate RAMS-I
           message with the appropriate response code for each primary
           multicast stream that has been requested by RTP_Rx and will
           be served by BRS.  Such RAMS-I messages comprise fields that
           can be used to describe the individual unicast burst streams.
           When there is a RAMS-R request for multiple primary multicast
           streams, BRS MUST include all the individual RAMS-I messages
           corresponding to that RAMS-R request in the same compound
           RTCP packet if these messages fit in the same packet.

           The RAMS-I message carries the RTP sequence number of the
           first packet transmitted in the respective RTP stream to
           allow RTP_Rx to detect any missing initial packet(s).  When
           BRS accepts a request for a primary multicast stream, this
           field MUST always be populated in the RAMS-I message(s) sent
           for this particular primary multicast stream.  It is
           RECOMMENDED that BRS sends a RAMS-I message at the start of
           the burst so that RTP_Rx can quickly detect any missing
           initial packet(s).


        It is possible that the RAMS-I message for a primary multicast
        stream can get delayed or lost, and RTP_Rx can start receiving
        RTP packets before receiving a RAMS-I message.  RTP_Rx MUST NOT
        make protocol dependencies on quickly receiving the initial
        RAMS-I message.  For redundancy purposes, it is RECOMMENDED that
        BRS repeats the RAMS-I messages multiple times as long as it
        follows the RTCP timer rules defined in [RFC4585].

   4.   Unicast Burst:  For the primary multicast stream(s) for which
        the request is accepted, BRS starts sending the unicast burst(s)
        that comprises one or more RTP retransmission packets sent in
        the unicast burst RTP session.  In addition, BRS MAY send
        preamble information data to RTP_Rx in addition to the requested
        burst, to prime the media decoder(s).  The format of this
        preamble data is RTP-payload specific and not specified here.

   5.   Updated Request:  RTP_Rx MAY send an updated RAMS-R message (as
        unicast feedback in the primary multicast RTP session) with a
        different value for one or more fields of an earlier RAMS-R



VerSteeg, et al.        Expires January 10, 2011               [Page 19]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


        message.  If there is already a burst planned for or being
        transmitted to a particular RTP_Rx for a particular stream, the
        newly arriving RAMS-R is an updated request; otherwise, it is a
        new request.  BRS determines the identity of the requesting
        RTP_Rx by looking at its canonical name identifier (CNAME item
        in the SDES source description).  Thus, RTP_Rx MUST choose a
        globally unique CNAME identifier.  Different such ways are
        provided in [I-D.ietf-avt-rtp-cnames].  In addition to one or
        more fields with updated values, an updated RAMS-R message may
        also include the fields whose values did not change.

        Upon receiving an updated request, BRS can use the updated
        values for sending/shaping the burst, or refine the values and
        use the refined values for sending/shaping the burst.
        Subsequently, BRS MAY send an updated RAMS-I message in the
        unicast burst RTP session to indicate the changes it made.

        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
        needs or does not need to 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.

   6.   Updated Response:  BRS can send more than one RAMS-I messages in
        the unicast burst RTP session, e.g., to update the value of one
        or more fields in an earlier RAMS-I message.  The updated RAMS-I
        messages might or might not be a direct response to a RAMS-R
        message.  BRS can also send updated RAMS-I messages to signal
        RTP_Rx in real time to join the SSM session, when BRS had
        already sent an initial RAMS-I message, e.g., at the start of
        the burst.  RTP_Rx depends on BRS 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 BRS is not capable of
        determining the join time in the initial RAMS-I message, BRS
        MUST send another RAMS-I message (with the join time
        information) later.

   7.   Multicast Join Signaling:  The RAMS-I message allows BRS to
        signal explicitly when RTP_Rx needs to send the SFGMP Join
        message.  RTP_Rx SHOULD use this information from the most
        recent RAMS-I message unless it has more accurate information.
        If the request is accepted, this information MUST be conveyed in
        at least one RAMS-I message and its value MAY be updated by
        subsequent RAMS-I messages.

        There can be missing packets if RTP_Rx joins the multicast
        session too early or too late.  For example, if RTP_Rx starts



VerSteeg, et al.        Expires January 10, 2011               [Page 20]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


        receiving the primary multicast stream while it is still
        receiving the unicast burst at a high excess bitrate, this can
        result in an increased risk of packet loss.  Or, if RTP_Rx joins
        the multicast session some time after the unicast burst is
        finished, there can be a gap between the burst and multicast
        data (a number of RTP packets might be missing).  In both cases,
        RTP_Rx can issue retransmission requests (via RTCP NACK messages
        sent as unicast feedback in the primary multicast RTP session)
        [RFC4585] to the FT entity of RS to fill the gap.  BRS might or
        might not respond to such requests.  When it responds and the
        response causes significant changes in one or more values
        reported earlier to RTP_Rx, an updated RAMS-I SHOULD be sent to
        RTP_Rx.

   8.   Multicast Receive:  After the join, RTP_Rx starts receiving the
        primary multicast stream(s).

   9.   Terminate:  BRS can know when it needs to ultimately stop the
        unicast burst based on its parameters.  However, RTP_Rx may need
        to ask BRS to terminate the burst prematurely or at a specific
        sequence number.  For this purpose, it uses the RAMS-Termination
        (RAMS-T) message sent as RTCP feedback in the unicast burst RTP
        session.  A separate RAMS-T message is sent for each primary
        multicast stream served by BRS unless an RTCP BYE message has
        been sent in the unicast burst RTP session as described in Step
        10.  For the burst requests that were rejected by BRS, there is
        no need to send a RAMS-T message.

        If RTP_Rx wants to terminate a burst prematurely, it SHALL send
        a plain RAMS-T message for the SSRC of the primary multicast
        stream it wishes to terminate.  This message is sent in the
        unicast burst RTP session.  Upon receiving this message BRS MUST
        terminate the unicast burst.  If RTP_Rx requested to acquire the
        entire primary multicast RTP session but wants to terminate this
        request before it learns the individual media sender SSRC(s) via
        RAMS-I message(s) or RTP packets, RTP_Rx cannot use RAMS-T
        message(s) and thus MUST send an RTCP BYE message in the unicast
        burst RTP session to terminate the request.

        Otherwise, the default behavior for RTP_Rx is to send a RAMS-T
        message in the unicast burst RTP session immediately after it
        joins the multicast session and started receiving multicast
        packets.  In that case, RTP_Rx SHALL send a RAMS-T message with
        the sequence number of the first RTP packet received in the
        primary multicast stream.  Then, BRS MUST terminate the
        respective 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.



VerSteeg, et al.        Expires January 10, 2011               [Page 21]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


        If an RTCP BYE message has not been issued yet as described in
        Step 10, RTP_Rx MUST send at least one RAMS-T message for each
        primary multicast stream served by BRS.  The RAMS-T message(s)
        MUST be addressed to BRS and sent in the unicast burst RTP
        session.  Against the possibility of a message loss, it is
        RECOMMENDED that RTP_Rx repeats the RAMS-T messages multiple
        times as long as it follows the RTCP timer rules defined in
        [RFC4585].

        The binding between RAMS-T and ongoing bursts is achieved
        through RTP_Rx's CNAME identifier, and packet sender and media
        sender SSRCs.  Choosing a globally unique CNAME makes sure that
        the RAMS-T messages are processed correctly.

   10.  Terminate with RTCP BYE:  When RTP_Rx is receiving one or more
        burst streams, if RTP_Rx becomes no longer interested in
        acquiring any of the primary multicast streams, RTP_Rx SHALL
        issue an RTCP BYE message for the unicast burst RTP session and
        another RTCP BYE message for the primary multicast RTP session.
        These RTCP BYE messages are sent to BRS and FT logical entities,
        respectively.

        Upon receiving an RTCP BYE message, BRS MUST terminate the rapid
        acquisition operation, and cease transmitting any further burst
        packets and retransmission packets.  If support for [RFC5506]
        has 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 still included.  With
        the information contained in the receiver report, RS can 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 6.4.

        Since an RTCP BYE message issued for the unicast burst RTP
        session terminates that session and ceases transmitting any
        further packets in that session, there is no need for sending
        explicit RAMS-T messages, which would only terminate their
        respective bursts.

   For the purpose of gathering detailed information about RTP_Rx's
   rapid acquisition experience, [I-D.ietf-avt-multicast-acq-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



VerSteeg, et al.        Expires January 10, 2011               [Page 22]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   application that supports rapid acquisition.  Support for this XR
   report is, however, OPTIONAL.

6.3.  Synchronization of Primary Multicast Streams

   When RTP_Rx acquires multiple primary multicast streams, RTP_Rx can
   need to synchronize them for the playout.  This synchronization is
   traditionally achieved by the help of the RTCP sender reports
   [RFC3550].  If the playout will start before RTP_Rx has joined the
   multicast session, RTP_Rx needs to receive the information reflecting
   the synchronization among the primary multicast streams early enough
   so that it can play out the media in a synchronized fashion.

   The suggested approach is to use the RTP header extension mechanism
   [RFC5285] and convey the synchronization information in a header
   extension as defined in [I-D.ietf-avt-rapid-rtp-sync].  Per [RFC4588]
   "if the original RTP header carried an RTP header extension, the
   retransmission packet SHOULD carry the same header extension."  Thus,
   as long as the multicast source emits a header extension with the
   synchronization information frequently enough, there is no additional
   task that needs to be carried out by BRS.  The synchronization
   information will be sent to RTP_Rx along with the burst packets.  The
   frequent header extensions sent in the primary multicast RTP sessions
   also allow rapid synchronization of the RTP streams for the RTP
   receivers that do not support RAMS or that directly join the
   multicast session without running RAMS.  Thus, in RAMS applications,
   it is RECOMMENDED that the multicast sources frequently send
   synchronization information by using header extensions following the
   rules presented in [I-D.ietf-avt-rapid-rtp-sync].  The regular sender
   reports are still sent in the unicast session by following the rules
   of [RFC3550].

6.4.  Burst Shaping and Congestion Control in RAMS

   This section provides informative guidelines about how BRS can shape
   the transmission of the unicast burst and how congestion can be dealt
   within the RAMS process.  When RTP_Rx detects that the unicast burst
   is causing severe congestion, it can prefer to send a RAMS-T message
   immediately to stop 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 bitrate also represents a better utilization
   of BRS resources.  As the burst may continue until it catches up with
   the primary multicast stream, the higher the bursting bitrate, the
   less data BRS needs to transmit.  However, a higher bitrate for the
   burst also increases the chances for congestion-caused packet loss.



VerSteeg, et al.        Expires January 10, 2011               [Page 23]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   Thus, as discussed in Section 5, there has to be an upper bound on
   the bandwidth used by the burst.

   When BRS transmits the unicast burst, it is expected to take into
   account all available information to prevent any packet loss that
   might 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
   bitrate can be determined by taking into account the following
   information, 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 bitrate
       adaptations for the burst.  When these receiver reports indicate
       packet loss, this can indicate a certain congestion state in the
       path from RS to RTP_Rx.

   c.  Information obtained via RTCP NACKs provided by RTP_Rx in the
       primary multicast RTP session, allowing in-session bitrate
       adaptations for the burst.  Such RTCP NACKs are transmitted by
       RTP_Rx in response to packet loss detection in the burst.  NACKs
       can indicate a certain congestion state on the path from RS to
       RTP_Rx.

   d.  There can be other feedback received from RTP_Rx, e.g., in the
       form of ECN-CE markings [I-D.ietf-avt-ecn-for-rtp] that can
       influence in-session bitrate adaptation.

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

   f.  Transport protocol-specific information.  For example, when DCCP
       is used to transport the RTP burst, the ACKs from the DCCP client
       can be leveraged by the BRS / DCCP server for burst shaping and
       congestion control.

   g.  Pre-configured settings for each RTP_Rx or a set of RTP_Rxs that
       indicate the upper-bound bursting bitrates 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 and where
       the network between RS and this link is provisioned and
       dimensioned to carry the burst streams, the bursting bitrate does
       not exceed the provisioned value.  These settings can also be
       dynamically adapted using application-aware knowledge.



VerSteeg, et al.        Expires January 10, 2011               [Page 24]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   BRS chooses the initial burst bitrate as follows:

   o  When using RAMS in environments as described in (g), BRS MUST
      transmit the burst packets at an initial bitrate higher than the
      nominal bitrate, but within the engineered or reserved bandwidth
      limit.

   o  When BRS cannot determine a reliable bitrate value for the unicast
      burst (using a through g), it is desirable that BRS chooses an
      appropriate initial bitrate not above the nominal bitrate and
      increases it gradually unless a congestion is detected.

   In both cases, during the burst transmission BRS MUST continuously
   monitor for packet losses as a result of congestion by means of one
   or more of the mechanisms described in (b,c,d,e,f).  When BRS relies
   on RTCP receiver reports, sufficient bandwidth needs to be provided
   to RTP Rx for RTCP transmission in the unicast burst RTP session.  To
   achieve a reasonable fast adaptation against congestion, it is
   recommended that RTP_Rx sends a receiver report at least once every
   two RTTs between RS and RTP_Rx.  Although the specific heuristics and
   algorithms that deduce a congestion state and how subsequently BRS
   acts are outside the scope of this specification, the following two
   methods are the best practices:

   o  Upon detection of a significant amount of packet loss, which BRS
      attributes to congestion, BRS decreases the burst bitrate.  The
      rate by which BRS increases and decreases the bitrate for the
      burst can be determined by a TCP-friendly bitrate adaptation
      algorithm for RTP over UDP , or in the case of (f) by the
      congestion control algorithms defined in DCCP [RFC5762].

   o  If the congestion is persistent and BRS has to reduce the burst
      bitrate to a point where the RTP Rx buffer might underrun or the
      burst will consume too many resources, BRS terminates the burst
      and transmits a RAMS-I message to RTP Rx with the appropriate
      response code.  It is then up to RTP Rx to decide when to join the
      multicast session.

   Even though there is no congestion experienced during the burst,
   congestion may anyway arise near the end of the burst as RTP_Rx
   eventually needs to join the multicast session.  During this brief
   period both the burst packets and the multicast packets can be
   simultaneously received by RTP_Rx, thus enhancing the risk of
   congestion.

   Since BRS signals RTP_Rx when BRS expects RTP_Rx to send the SFGMP
   Join message, BRS can have a rough estimate of when RTP_Rx will start
   receiving multicast packets in the SSM session.  BRS can keep on



VerSteeg, et al.        Expires January 10, 2011               [Page 25]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   sending burst packets but reduces the bitrate accordingly at the
   appropriate instant by taking the bitrate of the whole SSM session
   into account.  If BRS ceases transmitting the burst packets before
   the burst catches up, any gap resulting from this imperfect switch-
   over by RTP_Rx can be later repaired by requesting retransmissions
   for the missing packets from RS.  The retransmissions can be shaped
   by BRS to make sure that they do not cause collateral loss in the
   primary multicast RTP session and the unicast burst RTP session.

6.5.  Failure Cases

   In the following, we examine the implications of losing the RAMS-R,
   RAMS-I or RAMS-T messages and other failure cases.

   When RTP_Rx sends a RAMS-R message to initiate a rapid acquisition
   but the message gets lost and FT does not receive it, RTP_Rx will get
   neither a RAMS-I message, nor a unicast burst.  In this case, RTP_Rx
   MAY resend the request when it is eligible to do so based on the RTCP
   timer rules defined in [RFC4585].  Or, after a reasonable amount of
   time, RTP_Rx can time out (based on the previous observed response
   times) and immediately join the SSM session.

   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 can either discard the burst data or decide not to
   interrupt the unicast burst, and be prepared to join the SSM session
   at an appropriate time it determines or as indicated in a subsequent
   RAMS-I message (if available).  If the network is subject to packet
   loss, it is RECOMMENDED that BRS repeats the RAMS-I messages multiple
   times based on the RTCP timer rules defined in [RFC4585].

   In the failure cases where the RAMS-R message is lost and RTP_Rx
   gives up, or the RAMS-I message is lost, RTP_Rx MUST still terminate
   the burst(s) it requested by following the rules described in
   Section 6.2.

   In the case a RAMS-T message sent by RTP_Rx does not reach its
   destination, BRS can continue sending burst packets even though
   RTP_Rx no longer needs them.  In such cases, it is RECOMMENDED that
   RTP_Rx repeats the RAMS-T message multiple times based on the RTCP
   timer rules defined in [RFC4585].  BRS MUST be provisioned to
   deterministically terminate the burst when it can no longer send the
   burst packets faster than it receives the primary multicast stream
   packets.

   Section 6.3.5 of [RFC3550] explains the rules pertaining to timing
   out an SSRC.  When BRS accepts to serve the requested burst(s) and
   establishes the retransmission session, BRS needs to check the



VerSteeg, et al.        Expires January 10, 2011               [Page 26]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   liveness of RTP_Rx via the RTCP messages and reports RTP_Rx sends.
   The default rules explained in [RFC3550] apply in RAMS as well.


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.
   However, 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] and is also sketched in Figure 4.


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

     Figure 4: The common packet format for the RTCP feedback messages

   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 sender 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).  Any Reserved
   field SHALL be set to zero and ignored.

   Depending on the specific scenario and timeliness/importance of a
   RAMS message, it can be desirable to send it in a reduced-size RTCP
   packet [RFC5506].  However, unless support for [RFC5506] has been



VerSteeg, et al.        Expires January 10, 2011               [Page 27]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   signaled, compound RTCP packets MUST be used by following [RFC3550]
   rules.

   Following the rules specified in [RFC3550], all integer fields in the
   messages defined below are carried in network-byte order, that is,
   most significant byte (octet) first, also known as big-endian.
   Unless otherwise stated, numeric constants are in decimal (base 10).

7.1.  Extensions

   To improve the functionality of the RAMS method in certain
   applications, it can be desirable to define new fields in the RAMS
   Request, Information and Termination messages.  Such fields MUST be
   encoded as Type-Length-Value (TLV) elements as described below and
   sketched in Figure 5:

   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 (in octets)
      of the TLV element excluding the Type and Length fields, and the
      8-bit Reserved field between them.  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.

   In the extensions, the Reserved field SHALL be set to zero and
   ignored.  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
   zero.

   In a RAMS message, any vendor-neutral or private extension MUST be
   placed after the mandatory fields and mandatory TLV elements (if
   any).  The extensions MAY be placed in any order.  The support for
   extensions (unless specified otherwise) 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      |   Reserved    |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                             Value                             :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 5: Structure of a TLV element




VerSteeg, et al.        Expires January 10, 2011               [Page 28]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


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 11.5.

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

7.1.2.  Private Extensions

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

   A certain range of TLV Types (between - and including - 128 and 254 )
   is reserved for private extensions (Refer to Section 11.5).  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 6.  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     |   Reserved    |            Length             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Enterprise Number                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                             Value                             :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 6: Structure of a private extension

7.2.  RAMS Request

   The RAMS Request message is identified by SFMT=1.  This message is
   sent as unicast feedback in the primary multicast RTP session by
   RTP_Rx to request rapid acquisition of a primary multicast RTP
   session, or one or more primary multicast streams belonging to the
   same primary multicast RTP session.  In the RAMS-R message, RTP_Rx
   MUST set both the packet sender SSRC and media sender SSRC fields to
   its own SSRC since the media sender SSRC may not be known.  RTP_Rx
   MUST provide explicit signaling as described below to request



VerSteeg, et al.        Expires January 10, 2011               [Page 29]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   stream(s).  This minimizes the chances of accidentally requesting a
   wrong primary multicast stream.

   The FCI field MUST contain only one RAMS Request.  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=1     |                    Reserved                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :      Optional TLV-encoded Fields (and Padding, if needed)     :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 7: FCI field syntax for the RAMS Request message

   o  Requested Media Sender SSRC(s):  Mandatory TLV element that lists
      the media sender SSRC(s) requested by RTP_Rx.  BRS MUST ignore the
      media sender SSRC specified in the header of the RAMS-R message.

      If the Length field is set to zero (i.e., no media sender SSRC is
      listed), it means that RTP_Rx is requesting to rapidly acquire the
      entire primary multicast RTP session.  Otherwise, RTP_Rx lists the
      individual media sender SSRCs in this TLV element and sets the
      Length field of the TLV element to 4*n, where n is the number of
      SSRC entries.

      Type:  1

   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 can have knowledge of its buffering requirements.  These
      requirements can be application and/or device specific.  For
      instance, RTP_Rx might 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 BRS is told the minimum
      buffering requirement of the receiver, BRS can tailor the burst(s)
      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 bursts and any payload-specific information MUST NOT be
      smaller than the specified value.  Otherwise, the backfill will



VerSteeg, et al.        Expires January 10, 2011               [Page 30]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


      not be able to build up the desired level of buffer at RTP_Rx and
      can cause buffer underruns.

      Type:  2

   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 data due to buffer overflow.

      RTP_Rx can have knowledge of its buffering requirements.  These
      requirements can be application or device specific.  For instance,
      one particular RTP_Rx might have more physical memory than another
      RTP_Rx, and thus, can buffer more data.  If BRS knows the
      buffering ability of the receiver, BRS can tailor the burst(s)
      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 bursts and any payload-specific information MUST NOT be
      larger than this value.  Otherwise, the backfill may cause buffer
      overflows at RTP_Rx.

      Type:  3

   o  Max Receive Bitrate (64 bits):  Optional TLV element that denotes
      the maximum bitrate (in bits per second) at which the RTP_Rx can
      process the aggregation of the unicast burst(s) and any payload-
      specific information that will be provided by BRS.  The limits can
      include local receiver limits as well as network limits that are
      known to the receiver.

      If specified, the total bitrate of the unicast burst(s) plus any
      payload-specific information MUST NOT be larger than this value.
      Otherwise, congestion and packet loss may occur.

      Type:  4

   o  Request for Preamble Only (0 bits):  Optional TLV element that
      indicates that RTP_Rx is only requesting the preamble information
      for the desired primary multicast stream(s).  If this TLV element
      exists in the RAMS-R message, BRS MUST NOT send any burst packets
      other than the preamble packets.  Since this TLV element does not
      carry a Value field, the Length field MUST be set to zero.

      Type:  5

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



VerSteeg, et al.        Expires January 10, 2011               [Page 31]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


7.3.  RAMS Information

   The RAMS Information message is identified by SFMT=2.  This message
   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.

   A separate RAMS-I message with the appropriate response code is sent
   in the unicast burst RTP session by BRS for each primary multicast
   stream that has been requested by RTP_Rx.  In each of these RAMS-I
   messages, the media sender SSRC and packet sender SSRC fields are
   both set to the SSRC of BRS, which equals the SSRC of the respective
   primary multicast stream.

   The FCI field MUST contain only one RAMS Information.  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=2     |      MSN      |          Response             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :      Optional TLV-encoded Fields (and Padding, if needed)     :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 8: FCI field syntax for the RAMS Information message

   A RAMS-I message has the following fields:

   o  Message Sequence Number (8 bits) :  Mandatory field that denotes
      the sequence number of the RAMS-I message for the particular media
      sender SSRC specified in the message header.  The MSN value SHALL
      be set to zero only when a new RAMS request is received.  During
      rapid acquisition, the same RAMS-I message MAY be repeated for
      redundancy purposes without incrementing the MSN value.  If an
      updated RAMS-I message will be sent (either with a new information
      or an updated information), the MSN value SHALL be incremented by
      one.  In the MSN field, the regular wrapping rules apply.

   o  Response (16 bits):  Mandatory field that denotes the 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 11.6.  If the new response code is intended to be used
      privately by a vendor, there is no need for IANA management.
      Instead, the vendor MUST use the private extension mechanism



VerSteeg, et al.        Expires January 10, 2011               [Page 32]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


      (Section 7.1.2) to convey its message and MUST indicate this by
      putting zero in the Response field.

   The following TLV elements have been defined for the RAMS-I messages:

   o  Media Sender SSRC (32 bits):  Optional TLV element that specifies
      the media sender SSRC of the unicast burst stream.  While this
      information is already available in the message header, it can be
      useful to repeat it in an explicit field.  If FT_Ap that received
      the RAMS-R message is associated with a single primary multicast
      stream but the requested media sender SSRC does not match the SSRC
      of the RTP stream associated with this FT_Ap, BRS includes this
      TLV element in the initial RAMS-I message to let RTP_Rx know that
      the media sender SSRC has changed.  If the two SSRCs match, there
      is no need to include this TLV element.

      Type:  31

   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 respective RTP stream.  This allows RTP_Rx to know
      whether one or more packets sent by BRS have been dropped at the
      beginning of the stream.  If BRS accepts the RAMS request, this
      element exists.  If BRS rejects the RAMS request, this element
      SHALL NOT exist.

      Type:  32

   o  Earliest Multicast Join Time (32 bits):  TLV element that
      specifies the delta time (in ms) between the arrival of the first
      RTP packet in the RTP stream (which could be a burst packet or a
      payload-specific packet) and the earliest time instant when RTP_Rx
      sends an SFGMP Join message to join the multicast session.  A zero
      value in this field means that RTP_Rx can send the SFGMP Join
      message right away.

      If the RAMS request has been accepted, BRS sends this field at
      least once, so that RTP_Rx knows when to join the multicast
      session.  If the burst request has been rejected as indicated in
      the Response field, this field MUST be set to zero.  In that case,
      it is up to RTP_Rx when or whether to join the multicast session.

      When BRS serves two or more bursts and sends a separate RAMS-I
      message for each burst, the join times specified in these RAMS-I
      messages should correspond to more or less the same time instant,
      and RTP_Rx sends the SFGMP Join message based on the earliest join
      time.




VerSteeg, et al.        Expires January 10, 2011               [Page 33]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


      Type:  33

   o  Burst Duration (32 bits):  Optional TLV element that denotes the
      duration of the burst, i.e., the delta difference between the
      first and the last burst packet, that BRS is planning to send (in
      ms) in the respective RTP stream.  In the absence of additional
      stimulus, BRS will send a burst of this duration.  However, the
      burst duration can be modified by subsequent events, including
      changes in the primary multicast stream and reception of RAMS-T
      messages.

      BRS MUST terminate the flow in a deterministic timeframe, even if
      it does not get a RAMS-T or a BYE from RTP_Rx.  It is OPTIONAL to
      send this field in a 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 zero.

      Type:  34

   o  Max Transmit Bitrate (64 bits):  Optional TLV element that denotes
      the maximum bitrate (in bits per second) that will be used by BRS
      for the RTP stream associated with this RAMS-I message.

      Type:  35

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

7.4.  RAMS Termination

   The RAMS Termination message is identified by SFMT=3.

   The RAMS Termination is used to assist BRS in determining when to
   stop the burst.  A separate RAMS-T message is sent in the unicast
   burst RTP session by RTP_Rx for each primary multicast stream that
   has been served by BRS.  Each of these RAMS-T messages has the
   appropriate media sender SSRC populated in its message header.

   If RTP_Rx wants BRS to stop a burst prematurely, it sends a plain
   RAMS-T message as described below.  Upon receiving this message, BRS
   stops the respective burst immediately.  If RTP_Rx wants BRS to
   terminate all of the bursts, it needs to send all of the respective
   RAMS-T messages in a single compound RTCP packet.

   The default behavior for RTP_Rx is to send a RAMS-T message
   immediately after it joined the multicast session and started
   receiving multicast packets.  In that case, RTP_Rx includes the
   sequence number of the first RTP packet received in the primary



VerSteeg, et al.        Expires January 10, 2011               [Page 34]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   multicast stream in the RAMS-T message.  With this information, BRS
   can decide when to terminate the unicast burst.

   The FCI field MUST contain only one RAMS Termination.  The FCI field
   has the structure depicted in Figure 9.


      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     |                    Reserved                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :      Optional TLV-encoded Fields (and Padding, if needed)     :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 9: 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
      first packet received from the SSM session for a particular
      primary multicast stream.  The low 16 bits contain the sequence
      number of the first packet received from the SSM session, and the
      most significant 16 bits extend that sequence number with the
      corresponding count of sequence number cycles, which can be
      maintained according to the algorithm in Appendix A.1 of
      [RFC3550].

      Type:  61

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


8.  SDP Signaling

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].)








VerSteeg, et al.        Expires January 10, 2011               [Page 35]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


         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 "rai"

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

   o  'rai' stands for Rapid Acquisition Indication and indicates the
      use of RAMS messages as defined in Section 7.

   This document also defines a new media-level SDP attribute ('rams-
   updates') that indicates whether BRS supports updated request
   messages or not.  This attribute is used in a declarative manner and
   no Offer/Answer Model behavior is specified.  If BRS supports updated
   request messages and this attribute is included in the SDP
   description, RTP_Rx can send updated requests.  BRS might or might
   not be able to accept value changes in every field in an updated
   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 still repeat its initial request without
   changes, though.

8.2.  Requirements

   The use of SDP to describe the RAMS entities normatively requires the
   support for:

   o  The SDP grouping framework and flow identification (FID) semantics
      [RFC5888]

   o  The RTP/AVPF profile [RFC4585]

   o  The RTP retransmission payload format [RFC4588]

   o  The RTCP extensions for SSM sessions with unicast feedback
      [RFC5760]

   o  The multicast RTCP port attribute [I-D.ietf-avt-rtcp-port-for-ssm]

   o  Multiplexing RTP and RTCP on a single port on both endpoints in
      the unicast session[RFC5761]

   The support for the source-specific media attributes [RFC5576] can
   also be needed.




VerSteeg, et al.        Expires January 10, 2011               [Page 36]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


8.3.  Example and Discussion

   This section provides a declarative SDP [RFC4566] example for
   enabling rapid acquisition of multicast RTP sessions.

        v=0
        o=ali 1122334455 1122334466 IN IP4 rams.example.com
        s=Rapid Acquisition Example
        t=0 0
        a=group:FID 1 2
        a=rtcp-unicast:rsi
        m=video 41000 RTP/AVPF 98
        i=Primary Multicast Stream
        c=IN IP4 233.252.0.2/255
        a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1
        a=rtpmap:98 MP2T/90000
        a=multicast-rtcp:42000
        a=rtcp:43000 IN IP4 192.0.2.1
        a=rtcp-fb:98 nack
        a=rtcp-fb:98 nack rai
        a=ssrc:123321 cname:iptv-ch32@rams.example.com
        a=rams-updates
        a=mid:1
        m=video 51000 RTP/AVPF 99
        i=Unicast Retransmission Stream (Ret. and Rapid Acq. Support)
        c=IN IP4 192.0.2.1
        a=sendonly
        a=rtpmap:99 rtx/90000
        a=rtcp-mux
        a=fmtp:99 apt=98;rtx-time=5000
        a=mid:2


         Figure 10: Example SDP for a single-channel RAMS scenario

   In this example SDP description, we have a primary multicast (source)
   stream and a unicast retransmission stream.  The source stream is
   multicast from a distribution source (with a source IP address of
   198.51.100.1) to the multicast destination address of 233.252.0.2 and
   port 41000.  The corresponding RTCP traffic is multicast to the same
   multicast destination address at port 42000.  Here, we are assuming
   that the multicast RTP and RTCP ports are carefully chosen so that
   different RTP and RTCP streams do not collide with each other.

   The feedback target (FT_Ap) residing on RS (with an address of
   192.0.2.1) at port 43000 is declared with the "a=rtcp" line
   [RFC3605].  The support for the conventional retransmission is
   indicated through the "a=rtcp-fb:98 nack" line.  The RTP receiver(s)



VerSteeg, et al.        Expires January 10, 2011               [Page 37]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   can report missing packets on the source stream to the feedback
   target and request retransmissions.  The SDP above includes the
   "a=sendonly" line for the media description of the retransmission
   stream since the retransmissions are sent in only one direction.

   The support for rapid acquisition is indicated through the "a=rtcp-
   fb:98 nack rai" line.  The parameter 'rtx-time' applies to both the
   conventional retransmissions and rapid acquisition.  However, when
   rapid acquisition is enabled, the standard definition for the
   parameter 'rtx-time' given in [RFC4588] is not followed.  Instead,
   when rapid acquisition support is enabled, 'rtx-time' specifies the
   time in milliseconds that BRS keeps an RTP packet in its cache
   available for retransmission (measured from the time the packet was
   received by BRS, not from the time indicated in the packet
   timestamp).

   Once an RTP_Rx has acquired an SDP description, it can ask for rapid
   acquisition before it joins a primary multicast RTP session.  To do
   so, it sends a RAMS-R message to the feedback target of that primary
   multicast RTP session.  If FT_Ap is associated with only one RTP
   stream, RTP_Rx does not need to learn the SSRC of that stream via an
   out-of-band method.  If BRS accepts the rapid acquisition request, it
   will send an RAMS-I message with the correct SSRC identifier.  If
   FT_Ap is associated with a multi-stream RTP session and RTP_Rx is
   willing to request rapid acquisition for the entire session, RTP_Rx
   again does not need to learn the SSRCs via an out-of-band method.
   However, if RTP_Rx intends to request a particular subset of the
   primary multicast streams, it must learn their SSRC identifiers and
   list them in the RAMS-R message.  Since this RTP_Rx has not yet
   received any RTP packets for the primary multicast stream(s), RTP_Rx
   must in this case learn the SSRC value(s) from the 'ssrc' attribute
   of the media description [RFC5576].  In addition to the SSRC value,
   the 'cname' source attribute must also be present in the SDP
   description [RFC5576].

   Listing the SSRC values for the primary multicast streams 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_Rx that observed an
   SSRC collision with a media sender must change its own SSRC [RFC5760]
   by following the rules defined in [RFC3550].

   A feedback target that receives a RAMS-R feedback message becomes
   aware that the prediction chain at RTP_Rx side has been broken or
   does not exist anymore.  If the necessary conditions are satisfied
   (as outlined in Section 7 of [RFC4585]) and available resources
   exist, BRS can react to the RAMS-R message by sending any transport-
   layer (and optional payload-specific, when allowed) feedback
   message(s) and starting the unicast burst.



VerSteeg, et al.        Expires January 10, 2011               [Page 38]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


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


9.  NAT Considerations

   For a variety of reasons, one or more NAPT devices (hereafter simply
   called NAT) can exist between RTP_Rx and RS.  NATs have a variety of
   operating characteristics for UDP traffic [RFC4787].  For a NAT to
   permit traffic from BRS 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 BRS (which is on the 'outside' of the NAT).  This traffic
       has 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 BRS on that port.  For
   (a), RTP_Rx MUST send UDP traffic to keep the NAT binding alive, at
   least every 30 seconds [RFC4787].  While (a) is more like an
   automatic/dynamic configuration, (b) is more like a manual/static
   configuration.

   When RTP_Rx sends a request (RAMS-R) message to FT as unicast
   feedback in the primary multicast RTP session, and the request is
   received by FT, a new unicast burst RTP session will be established
   between BRS and RTP_Rx.

   While the FT and BRS ports on RS are already signaled via out-of-band
   means (e.g., SDP), RTP_Rx needs to convey the RTP and RTCP ports it
   wants to use on its side for the new session.  Since there are two
   RTP sessions (one multicast and one unicast) involved during this
   process and one of them is established upon a feedback message sent
   in the other one, this requires an explicit port mapping method.

   Applications using RAMS MUST support the solution presented in
   [I-D.ietf-avt-ports-for-ucast-mcast-rtp] both on the RS and RTP_Rx
   side to allow RTP receivers to use their desired ports and to support



VerSteeg, et al.        Expires January 10, 2011               [Page 39]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   RAMS behind NAT devices.  The port mapping message exchange needs to
   take place whenever RTP_Rx intends to make use of the RAMS protocol
   for rapidly acquiring a specific multicast RTP session, prior to
   joining the associated SSM session.


10.  Security Considerations

   Applications that are using RAMS make heavy use of the unicast
   feedback mechanism described in [RFC5760], the payload format defined
   in [RFC4588] and the port mapping solution specified in
   [I-D.ietf-avt-ports-for-ucast-mcast-rtp].  Thus, these applications
   are subject to the general security considerations discussed in
   [RFC5760], [RFC4588] and [I-D.ietf-avt-ports-for-ucast-mcast-rtp].
   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 applications using RAMS.

   First of all, much of the session description information is
   available in the SDP descriptions that are distributed to the media
   senders, 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
   senders, distribution sources, feedback targets as well as other
   session-specific information can be protected.

   Compared to an RTCP NACK message that triggers one or more
   retransmissions, a RAMS Request (RAMS-R) message can 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 can potentially contain a large number of
   retransmission packets.  Since these packets are sent at a faster
   than the nominal rate, RAMS consumes more resources on the
   retransmission servers, RTP receivers and the network.  In
   particular, this can make 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 can trigger inappropriate burst streams or alter the
   existing burst streams in an inappropriate way.  For example, if the
   Max Receive Bitrate field is altered by a tampered RAMS-R message,
   the updated burst can overflow the buffer at the receiver side, or
   oppositely, can slow down the burst to the point that it becomes
   useless.  Tampered RAMS Termination (RAMS-T) messages can terminate



VerSteeg, et al.        Expires January 10, 2011               [Page 40]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   valid burst streams prematurely resulting in gaps in the received RTP
   packets.  RAMS Information (RAMS-I) messages contain fields that are
   critical for a successful rapid acquisition.  Any tampered
   information in the RAMS-I message can easily cause an RTP receiver to
   make wrong decisions.  Consequently, the RAMS operation can fail.

   While most of the denial-of-service attacks can be prevented by the
   integrity and authenticity checks enabled by Secure RTP (SRTP)
   [RFC3711], 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 can temporarily consume a large amount of
   resources, a series of the RAMS-R messages can temporarily overload
   the retransmission server.  In these circumstances, the
   retransmission server can, 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
   [RFC5760].

   [RFC4588] RECOMMENDS that the cryptography mechanisms are used for
   the retransmission payload format to provide protection against known
   plain-text 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].

   To protect the RTCP messages from man-in-the-middle and replay
   attacks, the RTP receivers and retransmission server SHOULD perform a
   DTLS-SRTP handshake [RFC5764] over the RTCP channel.  Because there
   is no integrity-protected signaling channel between an RTP receiver
   and the retransmission server, the retransmission server MUST
   maintain a list of certificates owned by legitimate RTP receivers, or
   their certificates MUST be signed by a trusted Certificate Authority.
   Once the DTLS-SRTP security is established, non-SRTCP-protected
   messages received from a particular RTP receiver are ignored by the
   retransmission server.  To reduce the impact of DTLS-SRTP overhead
   when communicating with different feedback targets on the same
   retransmission server, it is RECOMMENDED that RTP receivers and the



VerSteeg, et al.        Expires January 10, 2011               [Page 41]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   retransmission server both support TLS Session Resumption without
   Server-side State [RFC5077].  To help scale SRTP to handle many RTP
   receivers asking for retransmissions of identical data, implementors
   may consider using the same SRTP key for SRTP data sent to the
   receivers [I-D.ietf-avt-srtp-ekt] and consider the security of such
   SRTP key sharing.


11.  IANA Considerations

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

   Ali Begen
   abegen@cisco.com


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

11.1.  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

11.2.  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
   the 'rtcp-fb' attribute, refer to Sections 4.2 and 6.2 of [RFC4585].











VerSteeg, et al.        Expires January 10, 2011               [Page 42]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


        Value name:     rai
        Long name:      Rapid Acquisition Indication
        Usable with:    nack
        Reference:      [RFCXXXX]

11.3.  Registration of FMT Values

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


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

11.4.  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
  ----- -------------------------------------------------- -------------
  0     Reserved                                           [RFCXXXX]
  1     RAMS Request                                       [RFCXXXX]
  2     RAMS Information                                   [RFCXXXX]
  3     RAMS Termination                                   [RFCXXXX]
  4-254                          Assignable - Specification Required
  255   Reserved                                           [RFCXXXX]


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

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

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





VerSteeg, et al.        Expires January 10, 2011               [Page 43]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


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

   New RAMS functionality is intended to be introduced by using the
   extension mechanism within the existing RAMS message types not by
   introducing new message types unless it is absolutely necessary.

11.5.  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,
   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
   ---- -------------------------------------------------- -------------
   0    Reserved                                           [RFCXXXX]
   1    Requested Media Sender SSRC(s)                     [RFCXXXX]
   2    Min RAMS Buffer Fill Requirement                   [RFCXXXX]
   3    Max RAMS Buffer Fill Requirement                   [RFCXXXX]
   4    Max Receive Bitrate                                [RFCXXXX]
   5    Request for Preamble Only                          [RFCXXXX]
   6-30                          Assignable - Specification Required
   31   Media Sender SSRC                                  [RFCXXXX]
   32   RTP Seqnum of the First Packet                     [RFCXXXX]
   33   Earliest Multicast Join Time                       [RFCXXXX]
   34   Burst Duration                                     [RFCXXXX]
   35   Max Transmit Bitrate                               [RFCXXXX]
   36-60                         Assignable - Specification Required
   61   Extended RTP Seqnum of First Multicast Packet      [RFCXXXX]
   62-127                        Assignable - Specification Required
   128-254                                       No IANA Maintenance
   255  Reserved                                           [RFCXXXX]


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






VerSteeg, et al.        Expires January 10, 2011               [Page 44]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   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.

11.6.  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 should be classified following the guidelines below:


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


   The Response code 65536 is reserved for future use.

   The registry is initialized with the following entries:




















VerSteeg, et al.        Expires January 10, 2011               [Page 45]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


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

  100   Parameter update for RAMS session                  [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 bitrate requirement in RAMS-R message  [RFCXXXX]

  500   An unspecified BRS internal error has occurred     [RFCXXXX]
  501   BRS has insufficient bandwidth to start RAMS
        session                                            [RFCXXXX]
  502   Burst is terminated due to network congestion      [RFCXXXX]
  503   BRS has insufficient CPU cycles to start RAMS
        session                                            [RFCXXXX]
  504   RAMS functionality is not available on BRS         [RFCXXXX]
  505   RAMS functionality is not available for RTP_Rx     [RFCXXXX]
  506   RAMS functionality is not available for
        the requested multicast stream                     [RFCXXXX]
  507   BRS has no valid starting point available for
        the requested multicast stream                     [RFCXXXX]
  508   BRS has no reference information available for
        the requested multicast stream                     [RFCXXXX]
  509   BRS has no RTP stream matching the requested SSRC  [RFCXXXX]
  510   RAMS request to acquire the entire session
        has been denied                                    [RFCXXXX]
  511   Only the preamble information is sent              [RFCXXXX]
  512   RAMS request has been denied due to a policy       [RFCXXXX]


   Any registration for an unassigned Response code needs to 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.








VerSteeg, et al.        Expires January 10, 2011               [Page 46]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


12.  Contributors

   Dave Oran, Magnus Westerlund and Colin Perkins have contributed
   significantly to this specification by providing text and solutions
   to some of the issues raised during the development of this
   specification.


13.  Acknowledgments

   The following individuals have reviewed the earlier versions of this
   specification and provided helpful comments:  Colin Perkins, Joerg
   Ott, Roni Even, Dan Wing, Tony Faustini, Peilin Yang, Jeff Goldberg,
   Muriel Deschanel, Orit Levin, Guy Hirson, Tom Taylor, Xavier Marjou,
   Ye-Kui Wang, Zixuan Zou, Ingemar Johansson, Haibin Song, Ning Zong,
   Jonathan Lennox, Jose Rey, Sean Sheedy and Keith Drage.


14.  References

14.1.  Normative References

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

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

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

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

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

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

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888, June 2010.

   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,



VerSteeg, et al.        Expires January 10, 2011               [Page 47]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


              "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.

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

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

   [RFC3605]  Huitema, C., "Real Time Control Protocol (RTCP) attribute
              in Session Description Protocol (SDP)", RFC 3605,
              October 2003.

   [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.

   [RFC5285]  Singer, D. and H. Desineni, "A General Mechanism for RTP
              Header Extensions", RFC 5285, July 2008.

   [I-D.ietf-avt-rapid-rtp-sync]
              Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
              Flows", draft-ietf-avt-rapid-rtp-sync-11 (work in
              progress), May 2010.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761, April 2010.

   [I-D.ietf-avt-rtcp-port-for-ssm]
              Begen, A., "RTP Control Protocol (RTCP) Port for Multicast
              Sessions", draft-ietf-avt-rtcp-port-for-ssm-00 (work in
              progress), June 2010.

   [I-D.ietf-avt-ports-for-ucast-mcast-rtp]
              Begen, A. and B. Steeg, "Port Mapping Between Unicast and
              Multicast RTP Sessions",
              draft-ietf-avt-ports-for-ucast-mcast-rtp-02 (work in
              progress), May 2010.



VerSteeg, et al.        Expires January 10, 2011               [Page 48]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


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

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, January 2008.

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

14.2.  Informative References

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

   [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.ietf-avt-rtp-cnames]
              Begen, A., Perkins, C., and D. Wing, "Guidelines for
              Choosing RTP Control Protocol (RTCP) Canonical Names
              (CNAMEs)", draft-ietf-avt-rtp-cnames-00 (work in
              progress), June 2010.

   [I-D.ietf-avt-multicast-acq-rtcp-xr]
              Begen, A. and E. Friedrich, "Multicast Acquisition Report
              Block Type for RTP Control Protocol (RTCP) Extended
              Reports (XRs)", draft-ietf-avt-multicast-acq-rtcp-xr-01
              (work in progress), May 2010.

   [I-D.ietf-avt-ecn-for-rtp]
              Westerlund, M., Johansson, I., Perkins, C., and K.
              Carlberg, "Explicit Congestion Notification (ECN) for RTP
              over UDP", draft-ietf-avt-ecn-for-rtp-01 (work in
              progress), March 2010.

   [I-D.ietf-fecframe-interleaved-fec-scheme]
              Begen, A., "RTP Payload Format for 1-D Interleaved Parity
              FEC", draft-ietf-fecframe-interleaved-fec-scheme-09 (work
              in progress), January 2010.



VerSteeg, et al.        Expires January 10, 2011               [Page 49]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


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

   [RFC5762]  Perkins, C., "RTP and the Datagram Congestion Control
              Protocol (DCCP)", RFC 5762, April 2010.

   [I-D.ietf-avt-srtp-ekt]
              McGrew, D., Andreasen, F., Wing, D., and L. Dondeti,
              "Encrypted Key Transport for Secure RTP",
              draft-ietf-avt-srtp-ekt-00 (work in progress), March 2010.

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

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

   [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
   181 Bay Street
   Toronto, ON  M5J 2T3
   Canada

   Email:  abegen@cisco.com










VerSteeg, et al.        Expires January 10, 2011               [Page 50]

Internet-Draft  Rapid Acquisition of RTP Sessions - RAMS       July 2010


   Tom VanCaenegem
   Alcatel-Lucent
   Copernicuslaan 50
   Antwerpen,   2018
   Belgium

   Email:  Tom.Van_Caenegem@alcatel-lucent.be


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

   Email:  zeevvax@microsoft.com



































VerSteeg, et al.        Expires January 10, 2011               [Page 51]


Html markup produced by rfcmarkup 1.108, available from http://tools.ietf.org/tools/rfcmarkup/