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Internet Engineering Task Force (IETF) A. Begen Request for Comments: 6659 Cisco Category: Informational July 2012 ISSN: 2070-1721

       Considerations for Deploying the Rapid Acquisition of
                Multicast RTP Sessions (RAMS) Method

Abstract

 The Rapid Acquisition of Multicast RTP Sessions (RAMS) solution is a
 method based on RTP and the RTP Control Protocol (RTCP) that enables
 an RTP receiver to rapidly acquire and start consuming the RTP
 multicast data.  Upon a request from the RTP receiver, an auxiliary
 unicast RTP retransmission session is set up between a retransmission
 server and the RTP receiver, over which the reference information
 about the new multicast stream the RTP receiver is about to join is
 transmitted at an accelerated rate.  This often precedes, but may
 also accompany, the multicast stream itself.  When there is only one
 multicast stream to be acquired, the RAMS solution works in a
 straightforward manner.  However, when there are two or more
 multicast streams to be acquired from the same or different multicast
 RTP sessions, care should be taken to configure each RAMS session
 appropriately.  This document provides example scenarios and
 discusses how the RAMS solution could be used in such scenarios.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6659.

Begen Informational [Page 1] RFC 6659 RAMS Considerations July 2012

Copyright Notice

 Copyright (c) 2012 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.

Table of Contents

 1. Introduction ....................................................2
 2. Background ......................................................3
 3. Example Scenarios ...............................................4
    3.1. Scenario #1: Two Multicast Groups ..........................4
    3.2. Scenario #2: One Multicast Group ...........................5
    3.3. Scenario #3: SSRC Multiplexing .............................6
    3.4. Scenario #4: Payload-Type Multiplexing .....................6
 4. Feedback Target and SSRC Signaling Issues .......................7
 5. FEC during RAMS and Bandwidth Issues ............................7
    5.1. Scenario #1 ................................................7
    5.2. Scenario #2 ................................................9
    5.3. Scenario #3 ...............................................10
 6. Security Considerations ........................................10
 7. Acknowledgments ................................................10
 8. References .....................................................11
    8.1. Normative References ......................................11
    8.2. Informative References ....................................11

1. Introduction

 The Rapid Acquisition of Multicast RTP Sessions (RAMS) solution is a
 method based on RTP and the RTP Control Protocol (RTCP) that enables
 an RTP receiver to rapidly acquire and start consuming the RTP
 multicast data.  Through an auxiliary unicast RTP retransmission
 session [RFC4588], the RTP receiver receives reference information
 about the new multicast stream it is about to join.  This often
 precedes, but may also accompany, the multicast stream itself.  The
 RAMS solution is documented in detail in [RFC6285].

Begen Informational [Page 2] RFC 6659 RAMS Considerations July 2012

 The RAMS specification [RFC6285] has provisions for concurrently
 acquiring multiple streams inside a multicast RTP session.  However,
 the RAMS specification does not discuss scenarios where an RTP
 receiver makes use of the RAMS method to rapidly acquire multiple and
 associated multicast streams in parallel, or where different RTP
 sessions are part of the same Source-Specific Multicast (SSM)
 session.  The example presented in Section 8.3 of [RFC6285] addresses
 only the simple case of an RTP receiver rapidly acquiring only one
 multicast stream to explain the protocol details.
 There are certain deployment models where a multicast RTP session
 might have two or more multicast streams associated with it.  There
 are also cases where an RTP receiver might be interested in acquiring
 one or more multicast streams from several multicast RTP sessions.
 Close coordination is required for multiple RAMS sessions
 simultaneously started by an RTP server, where each session is
 initiated with an individual RAMS Request message to a different
 feedback target.  In this document, we present scenarios from real-
 life deployments and discuss how the RAMS solution could be used in
 such scenarios.

2. Background

 In the following discussion, we assume that there are two RTP streams
 (1 and 2) that are in some manner associated with each other.  These
 could be audio and video elementary streams for the same TV channel,
 or they could be an MPEG2 Transport Stream (that has audio and video
 multiplexed together) and its Forward Error Correction (FEC) stream.
 An SSM session is defined by its (distribution) source address and
 (destination) multicast group, and there can be only one feedback
 target per SSM session [RFC5760].  So, if the RTP streams are
 distributed by different sources or over different multicast groups,
 they are considered different SSM sessions.  While different SSM
 sessions can normally share the same feedback target address and/or
 port, RAMS requires each unique feedback target (i.e., the
 combination of address and port) to be associated with at most one
 RTP session (See Section 6.2 of [RFC6285]).
 Two or more multicast RTP streams can be transmitted in the same RTP
 session (e.g., in a single UDP flow).  This is called Synchronization
 Source (SSRC) multiplexing.  In this case, (de)multiplexing is done
 at the SSRC level.  Alternatively, the multicast RTP streams can be
 transmitted in different RTP sessions (e.g., in different UDP flows),
 which is called session multiplexing.  In practice, there are
 different deployment models for each multiplexing scheme.

Begen Informational [Page 3] RFC 6659 RAMS Considerations July 2012

 Generally, to avoid complications in RTCP reports, it is suggested
 that two different media streams with different clock rates use
 different SSRCs or be carried in different RTP sessions.  Some of the
 fields in RAMS messages might depend on the clock rate.  Thus, in a
 single RTP session, RTP streams carrying payloads with different
 clock rates need to have different SSRCs.  Since RTP streams with
 different SSRCs do not share the sequence numbering, each stream
 needs to be acquired individually.
 In the remaining sections, only the relevant portions of the Session
 Description Protocol (SDP) descriptions [RFC4566] will be provided.
 For an example of a full SDP description, refer to Section 8.3 of
 [RFC6285].

3. Example Scenarios

3.1. Scenario #1: Two Multicast Groups

 This is the scenario for session multiplexing where RTP streams 1 and
 2 are transmitted over different multicast groups.  A practical use
 case is where the first and second SSM sessions carry the primary
 video stream and its associated FEC stream, respectively.
 An individual RAMS session is run for each of the RTP streams that
 require rapid acquisition.  Each requires a separate RAMS Request
 message to be sent.  These RAMS sessions can be run in parallel.  If
 they are, the RTP receiver needs to pay attention to using the shared
 bandwidth appropriately among the two unicast bursts.  As explained
 earlier, there has to be a different feedback target for these two
 SSM sessions.
      v=0
      o=ali 1122334455 1122334466 IN IP4 rams.example.com
      s=RAMS Scenarios
      t=0 0
      a=group:FEC-FR Channel1_Video Channel1_FEC
      m=video 40000 RTP/AVPF 96
      c=IN IP4 233.252.0.1/127
      a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
      a=rtcp:41000 IN IP4 192.0.2.1
      a=ssrc:1 cname:ch1_video@example.com
      a=mid:Channel1_Video
      m=application 40000 RTP/AVPF 97
      c=IN IP4 233.252.0.2/127
      a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1
      a=rtcp:42000 IN IP4 192.0.2.1
      a=ssrc:2 cname:ch1_fec@example.com
      a=mid:Channel1_FEC

Begen Informational [Page 4] RFC 6659 RAMS Considerations July 2012

 Note that the multicast destination ports in the above SDP do not
 matter, and they could be the same or different.  The "FEC-FR"
 grouping semantics are defined in [RFC5956].

3.2. Scenario #2: One Multicast Group

 Here, RTP streams 1 and 2 are transmitted over the same multicast
 group with different destination ports.  A practical use case is
 where the SSM session carries the primary video and audio streams,
 each destined to a different port.
 The RAMS Request message sent by an RTP receiver to the feedback
 target could indicate the desire to acquire all or a subset or one of
 the available RTP streams.  Thus, both the primary video and audio
 streams can be acquired rapidly in parallel.  Or, the RTP receiver
 can acquire only the primary video or audio stream, if desired, by
 indicating the specific SSRC in the request.  Compared to the
 previous scenario, the only difference is that in this case the join
 times for both streams need to be coordinated as they are delivered
 in the same multicast session.
      v=0
      o=ali 1122334455 1122334466 IN IP4 rams.example.com
      s=RAMS Scenarios
      t=0 0
      m=video 40000 RTP/AVPF 96
      c=IN IP4 233.252.0.1/127
      a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
      a=rtcp:41000 IN IP4 192.0.2.1
      a=ssrc:1 cname:ch1_video@example.com
      a=mid:Channel1_Video
      m=audio 40001 RTP/AVPF 97
      c=IN IP4 233.252.0.1/127
      a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
      a=rtcp:41000 IN IP4 192.0.2.1
      a=ssrc:2 cname:ch1_audio@example.com
      a=mid:Channel1_Audio
 Note that the destination ports in "m" lines need to be distinct per
 [RFC5888].
 If RTP streams 1 and 2 share the same distribution source, then there
 is only one SSM session, which means that there can be only one
 feedback target (as shown in the SDP description above).  This
 requires RTP streams 1 and 2 to have their own unique SSRC values
 (also as shown in the SDP description above).  If RTP streams 1 and 2
 do not share the same distribution source, meaning that their

Begen Informational [Page 5] RFC 6659 RAMS Considerations July 2012

 respective SSM sessions can use different feedback target transport
 addresses, then their SSRC values do not have to be different from
 each other.

3.3. Scenario #3: SSRC Multiplexing

 This is the scenario for SSRC multiplexing where both RTP streams are
 transmitted over the same multicast group to the same destination
 port.  This is a less practical scenario, but it could be used where
 the SSM session carries both the primary video and audio stream,
 destined to the same port.
 Similar to scenario #2, both the primary video and audio streams can
 be acquired rapidly in parallel.  Or, the RTP receiver can acquire
 only the primary video or audio stream, if desired, by indicating the
 specific SSRC in the request.  In this case, there is only one
 distribution source and the destination multicast address is shared.
 Thus, there is always one SSM session and one feedback target.
      v=0
      o=ali 1122334455 1122334466 IN IP4 rams.example.com
      s=RAMS Scenarios
      t=0 0
      m=video 40000 RTP/AVPF 96 97
      c=IN IP4 233.252.0.1/127
      a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
      a=rtcp:41000 IN IP4 192.0.2.1
      a=ssrc:1 cname:ch1_video@example.com
      a=ssrc:2 cname:ch1_audio@example.com
      a=mid:Channel1

3.4. Scenario #4: Payload-Type Multiplexing

 This is the scenario for payload-type multiplexing.
 In this case, instead of two, there is only one RTP stream (and one
 RTP session) carrying both payload types (e.g., media payload and its
 FEC data).  While this scheme is permissible per [RFC3550], it has
 several drawbacks.  For example, RTP packets carrying different
 payload formats will share the same sequence numbering space, and the
 RAMS operations will not be able to be applied based on the payload
 type.  For other drawbacks and details, see Section 5.2 of [RFC3550].

Begen Informational [Page 6] RFC 6659 RAMS Considerations July 2012

4. Feedback Target and SSRC Signaling Issues

 The RAMS protocol uses the common packet format from [RFC4585], which
 has a field to signal the media sender SSRC.  The SSRCs for the RTP
 streams can be signaled out-of-band in the SDP or could be learned
 from the RTP packets once the transmission starts.  In RAMS, the
 latter cannot be used.
 Signaling the media sender SSRC value helps the feedback target
 correctly identify the RTP stream to be acquired.  If a feedback
 target is serving multiple SSM sessions on a particular port, all the
 RTP streams in these SSM sessions are supposed to have a unique SSRC
 value.  However, this is not an easy requirement to satisfy.  Thus,
 the RAMS specification forbids having more than one RTP session
 associated with a specific feedback target on a specific port.

5. FEC during RAMS and Bandwidth Issues

 Suppose that RTP stream 1 denotes the primary video stream that has a
 bitrate of 10 Mbps and RTP stream 2 denotes the associated FEC stream
 that has a bitrate of 1 Mbps.  Also assume that the RTP receiver
 knows that it can receive data at a maximum bitrate of 22 Mbps.  SDP
 can specify the bitrate ("b=" line in kbps) of each media session
 (per "m" line).
 Note that RAMS can potentially congest the network temporarily.
 Refer to [RFC6285] for a detailed discussion.

5.1. Scenario #1

 This is the scenario for session multiplexing where RTP streams 1 and
 2 are transmitted over different multicast groups.
 This is the preferred deployment model for FEC [RFC6363].  Having FEC
 in a different multicast group provides two flexibility points: RTP
 receivers that are not FEC capable can receive the primary video
 stream without FEC, and RTP receivers that are FEC capable can decide
 to not receive FEC during the rapid acquisition (but still start
 receiving the FEC stream after the acquisition of the primary video
 stream has been completed).

Begen Informational [Page 7] RFC 6659 RAMS Considerations July 2012

      v=0
      o=ali 1122334455 1122334466 IN IP4 rams.example.com
      s=RAMS Scenarios
      t=0 0
      a=group:FEC-FR Channel1_Video Channel1_FEC
      m=video 40000 RTP/AVPF 96
      c=IN IP4 233.252.0.1/127
      a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
      a=rtcp:41000 IN IP4 192.0.2.1
      a=rtpmap:96 MP2T/90000
      b=TIAS:10000
      a=ssrc:1 cname:ch1_video@example.com
      a=mid:Channel1_Video
      m=application 40000 RTP/AVPF 97
      c=IN IP4 233.252.0.2/127
      a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1
      a=rtcp:42000 IN IP4 192.0.2.1
      a=rtpmap:97 1d-interleaved-parityfec/90000
      b=TIAS:1000
      a=ssrc:2 cname:ch1_fec@example.com
      a=mid:Channel1_FEC
 If the RTP receiver does not want to receive FEC until the
 acquisition of the primary video stream is completed, the total
 available bandwidth can be used for faster acquisition of the primary
 video stream.  In this case, the RTP receiver can request a Max
 Receive Bitrate of 22 Mbps in the RAMS Request message for the
 primary video stream.  Once RAMS has been completed, the RTP receiver
 can join the FEC multicast session, if desired.
 If the RTP receiver wants to rapidly acquire both primary and FEC
 streams, it needs to allocate the total bandwidth among the two RAMS
 sessions and indicate individual Max Receive Bitrate values in each
 respective RAMS Request message.  Since less bandwidth will be used
 to acquire the primary video stream, the acquisition of the primary
 video session will take a longer time on the average.
 While the RTP receiver can update the Max Receive Bitrate values
 during the course of the RAMS session, this approach is more error-
 prone, due to the possibility of losing the update messages.

Begen Informational [Page 8] RFC 6659 RAMS Considerations July 2012

5.2. Scenario #2

 Here, RTP streams 1 (primary video) and 2 (FEC) are transmitted over
 the same multicast group with different destination ports.  This is
 not a preferred deployment model.
      v=0
      o=ali 1122334455 1122334466 IN IP4 rams.example.com
      s=RAMS Scenarios
      t=0 0
      a=group:FEC-FR Channel1_Video Channel1_FEC
      m=video 40000 RTP/AVPF 96
      c=IN IP4 233.252.0.1/127
      a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
      a=rtcp:41000 IN IP4 192.0.2.1
      a=rtpmap:96 MP2T/90000
      b=TIAS:10000
      a=ssrc:1 cname:ch1_video@example.com
      a=mid:Channel1_Video
      m=application 40001 RTP/AVPF 97
      c=IN IP4 233.252.0.1/127
      a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
      a=rtcp:41000 IN IP4 192.0.2.1
      a=rtpmap:97 1d-interleaved-parityfec/90000
      b=TIAS:1000
      a=ssrc:2 cname:ch1_fec@example.com
      a=mid:Channel1_FEC
 The RAMS Request message sent by an RTP receiver to the feedback
 target could indicate the desire to acquire all or a subset or one of
 the available RTP streams.  Thus, both the primary video and FEC
 streams can be acquired rapidly in parallel sharing the same
 available bandwidth.  Or, the RTP receiver can acquire only the
 primary video stream by indicating its specific SSRC in the request.
 In this case, the RTP receiver can first acquire the primary video
 stream at the full receive bitrate.  But, upon the multicast join,
 the available bandwidth for the burst drops to 11 Mbps instead of
 12 Mbps.  Regardless of whether FEC is desired or not by the RTP
 receiver, its bitrate needs to be taken into account once the RTP
 receiver joins the SSM session.

Begen Informational [Page 9] RFC 6659 RAMS Considerations July 2012

5.3. Scenario #3

 This is the scenario for SSRC multiplexing where both RTP streams are
 transmitted over the same multicast group to the same destination
 port.
      v=0
      o=ali 1122334455 1122334466 IN IP4 rams.example.com
      s=RAMS Scenarios
      t=0 0
      m=video 40000 RTP/AVPF 96 97
      c=IN IP4 233.252.0.1/127
      a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
      a=rtcp:41000 IN IP4 192.0.2.1
      a=rtpmap:96 MP2T/90000
      a=rtpmap:97 1d-interleaved-parityfec/90000
      a=fmtp:97 L=10; D=10; repair-window=200000
      a=ssrc:1 cname:ch1_video@example.com
      a=ssrc:2 cname:ch1_fec@example.com
      b=TIAS:11000
      a=mid:Channel1
 Similar to scenario #2, both the primary video and audio streams can
 be acquired rapidly in parallel.  Or, the RTP receiver can acquire
 only the primary video stream, if desired, by indicating its specific
 SSRC in the request.
 Note that based on the "a=fmtp" line for the FEC stream, it could be
 possible to infer the bitrate of this FEC stream and set the Max
 Receive Bitrate value accordingly.

6. Security Considerations

 Because this document describes deployment scenarios for RAMS, all
 security considerations are specified in the RAMS specification
 [RFC6285].

7. Acknowledgments

 I would like to thank various individuals in the AVTEXT and MMUSIC
 WGs, and my friends at Cisco for their comments and feedback.

Begen Informational [Page 10] RFC 6659 RAMS Considerations July 2012

8. References

8.1. Normative References

 [RFC6285]  Ver Steeg, B., Begen, A., Van Caenegem, T., and Z. Vax,
            "Unicast-Based Rapid Acquisition of Multicast RTP
            Sessions", RFC 6285, June 2011.

8.2. Informative 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.
 [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
            Description Protocol", RFC 4566, July 2006.
 [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
            "Extended RTP Profile for Real-time Transport Control
            Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
            July 2006.
 [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.
 [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
            Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
 [RFC5956]  Begen, A., "Forward Error Correction Grouping Semantics in
            the Session Description Protocol", RFC 5956,
            September 2010.
 [RFC6363]  Watson, M., Begen, A., and V. Roca, "Forward Error
            Correction (FEC) Framework", RFC 6363, October 2011.

Begen Informational [Page 11] RFC 6659 RAMS Considerations July 2012

Author's Address

 Ali Begen
 Cisco
 181 Bay Street
 Toronto, ON  M5J 2T3
 Canada
 EMail: abegen@cisco.com

Begen Informational [Page 12]

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