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Versions: 00 01 draft-begen-avtext-rams-scenarios

AVT                                                             A. Begen
Internet-Draft                                             Cisco Systems
Intended status:  Informational                          October 1, 2009
Expires:  April 4, 2010


                   Considerations for RAMS Scenarios
                   draft-begen-avt-rams-scenarios-00

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   This Internet-Draft will expire on April 4, 2010.

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Abstract

   The Rapid Acquisition of Multicast RTP Sessions (RAMS) solution is a
   method based on RTP and RTCP protocol family that enables an RTP
   receiver to rapidly acquire and start usefully consuming the RTP



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   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 pace.  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
   appropriately.  This document provides example scenarios and make
   practical recommendations.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements Notation  . . . . . . . . . . . . . . . . . . . .  3
   3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Illustrative Scenarios . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Scenario #1  . . . . . . . . . . . . . . . . . . . . . . .  4
     4.2.  Scenario #2  . . . . . . . . . . . . . . . . . . . . . . .  5
     4.3.  Scenario #3  . . . . . . . . . . . . . . . . . . . . . . .  6
     4.4.  Scenario #4  . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Feedback Target and SSRC Signaling Issues  . . . . . . . . . .  6
   6.  FEC during RAMS and Bandwidth Issues . . . . . . . . . . . . .  7
     6.1.  Scenario #1  . . . . . . . . . . . . . . . . . . . . . . .  7
     6.2.  Scenario #2  . . . . . . . . . . . . . . . . . . . . . . .  8
     6.3.  Scenario #3  . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
















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1.  Introduction

   The Rapid Acquisition of Multicast RTP Sessions (RAMS) solution is a
   method based on RTP and RTCP protocol family that enables an RTP
   receiver to rapidly acquire and start usefully consuming the RTP
   multicast data.  Through an auxiliary unicast RTP retransmission
   session [RFC4588], the RTP receiver receives a 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
   [I-D.ietf-avt-rapid-acquisition-for-rtp].

   To focus on the protocol details, the RAMS specification
   [I-D.ietf-avt-rapid-acquisition-for-rtp] has only considered the
   simplest case, which is that the RTP receiver acquires only one
   multicast stream at a time.  However, there are many applications
   where a multicast RTP session may have two or more multicast streams
   associated with it.  There are also cases, where an RTP receiver may
   be interested in acquiring one or more multicast streams from
   multiple multicast RTP sessions.  In scenarios where multiple RAMS
   sessions will be simultaneously run by the RTP receiver, care should
   be taken to coordinate them.  In this document, we provide scenarios
   from real-life deployments and make recommendations.


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.  Background

   In the following, we assume that there are two RTP streams (1 and 2)
   that are somehow associated with each other.  These could be audio
   and video elementary streams for the same TV channel, or they could
   be an MPEG2-TS stream and its Forward Error Correction (FEC) stream.

   It is important to note that a source-specific multicast (SSM)
   session is defined by its (distribution) source address and
   (destination) multicast group and there can be only one feedback
   target per SSM session [I-D.ietf-avt-rtcpssm].  So, if the RTP
   streams are distributed by different sources or over different
   multicast groups, they have to be in different SSM sessions.
   Different SSM sessions may share the same feedback target address
   and/or port.




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   Different multicast RTP streams can be transmitted in the same RTP
   session (i.e., in a single UDP flow).  This is called SSRC
   multiplexing.  In this case, (de)multiplexing is done at the SSRC
   level.  Alternatively, different multicast RTP streams can be
   transmitted in different RTP sessions (i.e., in different UDP flows),
   which is called session multiplexing.  In practice, there are
   different deployment models for each multiplexing scheme.

   It is also important to note that two different media streams with
   different clock rates should use different SSRCs or RTP sessions to
   avoid complications in RTCP reports.  Some of the fields in RAMS
   messages depend on the clock rate.  Thus, in a single RTP session,
   RTP streams carrying payloads with different clock rates should have
   different SSRCs.  Since RTP streams in the same RTP session but with
   different SSRCs do not share the sequence numbering, each stream
   needs to be acquired individually.

   In the following, only the relevant portions of the SDP descriptions
   [RFC4566] will be provided.


4.  Illustrative Scenarios

4.1.  Scenario #1

   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 session carries the primary
   video stream and its associated FEC stream, respectively.

   We run an individual RAMS session for each RTP stream that we want to
   rapidly acquire.  These RAMS sessions MAY run in parallel.  If they
   are, the RTP receiver needs to pay attention to using the shared
   bandwidth appropriately among different RAMS sessions.  Note that
   there may be different feedback targets for these two SSM sessions.
   If that is the case, RTP streams 1 and 2 may have the same SSRC
   value.  However, if both SSM sessions use the same transport address
   (IP address and port) for their feedback targets (as shown in the SDP
   below), the SSRCs of the RTP streams 1 and 2 MUST be different from
   each other to avoid any ambiguity in the RAMS requests.











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        a=group:FEC-XR RTP1 RTP2
        m=video 40000 RTP/AVP 96
        c=IN IP4 233.252.0.1/127
        a=rtcp:41001 IN IP4 192.0.2.1
        a=ssrc:1 cname:rtp1@example.com
        a=mid:RTP1
        m=application 40000 RTP/AVP 97
        c=IN IP4 233.252.0.2/127
        a=rtcp:41001 IN IP4 192.0.2.1
        a=ssrc:2 cname:rtp2@example.com
        a=mid:RTP2

   Note that the destination ports in the above SDP do not matter, and
   they could be the same or different.

4.2.  Scenario #2

   This is the scenario for session multiplexing where 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.

   Similar to scenario #1, we run individual RAMS sessions for each RTP
   stream that we want to rapidly acquire.  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 on the same multicast
   session.

        m=video 40000 RTP/AVP 96
        c=IN IP4 233.252.0.1/127
        a=rtcp:41001 IN IP4 192.0.2.1
        a=ssrc:1 cname:rtp1@example.com
        a=mid:RTP1
        m=audio 40001 RTP/AVP 97
        c=IN IP4 233.252.0.1/127
        a=rtcp:41001 IN IP4 192.0.2.1
        a=ssrc:2 cname:rtp2@example.com
        a=mid:RTP2

   Note that the destination ports in the above SDP MUST be distinct per
   [I-D.ietf-mmusic-rfc3388bis].

   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.  This requires RTP streams 1 and 2 to have their own
   unique SSRC values (as shown in the SDP above).  If RTP streams 1 and
   2 do not share the same distribution source, meaning that their



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

4.3.  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.  This is a less practical scenario but it could be used where
   the SSM session carries the primary video and audio streams, destined
   to the same port.

   Similar to scenario #2, we run individual RAMS sessions and the join
   time needs to be coordinated.  In this case, there is only one
   distribution source and the destination multicast address is shared.
   Thus, there is only one SSM session and one feedback target.

        m=video 40000 RTP/AVP 96 97
        c=IN IP4 233.252.0.1/127
        a=rtcp:41001 IN IP4 192.0.2.1
        a=ssrc:1 cname:rtp1@example.com
        a=ssrc:2 cname:rtp2@example.com
        a=mid:Channel1

4.4.  Scenario #4

   This is the scenario for payload-type multiplexing.

   In this case, instead of two, we have 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
   retransmission and 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].


5.  Feedback Target and SSRC Signaling Issues

   The RAMS protocol uses the common packet format from [RFC4585], which
   has a field to signal the media source SSRC.  Thus, currently we
   require the RAMS Request messages to have this field properly filled.
   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 our scenario, the latter cannot be used.

   Signaling the media source SSRC value will help the feedback target



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   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 MUST have a unique SSRC value.
   Otherwise, the feedback target cannot discern the incoming RTCP
   feedback messages.

   If there are no provisions to assign unique SSRC values to the RTP
   streams in a deployment, the feedback target transport addresses MUST
   be assigned appropriately.  Unique feedback target addresses can be
   used without any issues if the deployment only covers scenario #1.
   Using unique feedback target transport addresses may or may not be
   sufficient in scenario #2.  In scenario #3, there is one feedback
   target.  Thus, SSRCs must be unique among the RTP streams that a
   particular feedback target (IP address and port) is responsible for.


6.    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 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).

6.1.  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.  Having FEC in a
   different multicast group provides flexibility for both RTP receivers
   that are not FEC capable or the ones that are not willing to receive
   FEC during the RAMS session.

















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        a=group:FEC-XR RTP1 RTP2
        m=video 40000 RTP/AVP 96
        c=IN IP4 233.252.0.1/127
        a=rtpmap:96 MP2T/90000
        b=TIAS:10000
        a=mid:RTP1
        m=application 40000 RTP/AVP 97
        c=IN IP4 233.252.0.2/127
        a=rtpmap:97 1d-interleaved-parityfec/90000
        b=TIAS:1000
        a=mid:RTP2

   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 may request a Max
   Receive Bitrate of 22 Mbps in the RAMS Request message.  Once RAMS
   has been completed, the RTP receiver MAY 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.

6.2.  Scenario #2

   This is the scenario for session multiplexing where RTP streams 1 and
   2 are transmitted over the same multicast group with different
   destination ports.

        a=group:FEC-XR RTP1 RTP2
        m=video 40000 RTP/AVP 96
        c=IN IP4 233.252.0.1/127
        a=rtpmap:96 MP2T/90000
        b=TIAS:10000
        a=mid:RTP1
        m=application 40001 RTP/AVP 97
        c=IN IP4 233.252.0.1/127
        a=rtpmap:97 1d-interleaved-parityfec/90000
        b=TIAS:1000
        a=mid:RTP2



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   Similar to scenario #1, the RTP receiver can first ask for RAMS for
   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 multicast.

   If the RTP receiver wants to rapidly acquire both primary and FEC
   streams, the same conditions explained for scenario #1 apply.  The
   only difference from scenario #1 is that here the join time is the
   same for both the primary video and FEC streams.

6.3.  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.

        m=video 40000 RTP/AVP 96 97
        c=IN IP4 233.252.0.1/127
        a=rtpmap:96 MP2T/90000
        a=rtpmap:97 1d-interleaved-parityfec/90000
        b=TIAS:11000
        a=mid:Channel1

   This is similar to scenario #2.  However, since we cannot explicitly
   specify the bitrates for the primary and FEC streams, the RTP
   receiver may request to rapidly acquire both streams in parallel.  In
   this case, two separate RAMS Request messages have to be sent by the
   RTP receiver to the feedback target.

   Note that based on the "a=fmtp" line for the FEC stream, it may be
   possible to infer the bitrate of this FEC stream and set the Max
   Receive Bitrate value accordingly.


7.  Security Considerations

   TBD.


8.  IANA Considerations

   There are no IANA considerations in this document.


9.  References




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9.1.  Normative References

   [I-D.ietf-avt-rapid-acquisition-for-rtp]
              Steeg, B., Begen, A., Caenegem, T., and Z. Vax, "Unicast-
              Based Rapid Acquisition of Multicast RTP Sessions",
              draft-ietf-avt-rapid-acquisition-for-rtp-03 (work in
              progress), September 2009.

   [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.

   [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.

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

9.2.  Informative References

   [I-D.ietf-mmusic-rfc3388bis]
              Camarillo, G., "The SDP (Session Description Protocol)
              Grouping Framework", draft-ietf-mmusic-rfc3388bis-03 (work
              in progress), July 2009.












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Author's Address

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

   Email:  abegen@cisco.com










































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