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Differences from draft-begen-avt-rams-scenarios-00.txt
AVT A. Begen
Internet-Draft Cisco
Intended status: Informational February 15, 2011
Expires: August 19, 2011
Considerations and Guidelines for Deploying the Rapid Acquisition of
Multicast RTP Sessions (RAMS) Method
draft-begen-avt-rams-scenarios-01
Abstract
The Rapid Acquisition of Multicast RTP Sessions (RAMS) solution is a
method based on RTP and 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 offers guidelines.
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/.
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 August 19, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Example Scenarios . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Scenario #1: Two Multicast Groups . . . . . . . . . . . . 4
4.2. Scenario #2: One Multicast Group . . . . . . . . . . . . . 5
4.3. Scenario #3: SSRC Multiplexing . . . . . . . . . . . . . . 6
4.4. Scenario #4: Payload-Type Multiplexing . . . . . . . . . . 7
5. Feedback Target and SSRC Signaling Issues . . . . . . . . . . 7
6. FEC during RAMS and Bandwidth Issues . . . . . . . . . . . . . 7
6.1. Scenario #1 . . . . . . . . . . . . . . . . . . . . . . . 8
6.2. Scenario #2 . . . . . . . . . . . . . . . . . . . . . . . 9
6.3. Scenario #3 . . . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
The Rapid Acquisition of Multicast RTP Sessions (RAMS) solution is a
method based on RTP and 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 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].
The RAMS specification [I-D.ietf-avt-rapid-acquisition-for-rtp] has
provisions for concurrently acquiring multiple streams inside a
multicast RTP session. However, the specification has mostly focused
on the simplest case, which is when the RTP receiver acquires only
one multicast stream at a time, to explain the protocol details.
There are certain deployment models 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 several multicast RTP sessions. In
scenarios where multiple RAMS sessions will be simultaneously run by
an RTP receiver, they need to be coordinated. In this document, we
present scenarios from real-life deployments and provide guidelines.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
Editor's note: I am inclined not use any 2119 keyword in this
document and remove this section altogether.
3. Background
In the following discussion, 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 (that has audio and video
multiplexed together) 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
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(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
[I-D.ietf-avt-rapid-acquisition-for-rtp]).
Two or more multicast RTP streams can be transmitted in the same RTP
session (i.e., 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 (i.e., in different UDP flows),
which is called session multiplexing. In practice, there are
different deployment models for each multiplexing scheme.
Generally, two different media streams with different clock rates are
suggested to use different SSRCs or to be carried in different RTP
sessions to avoid complications in RTCP reports. 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 in the same RTP
session but 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 SDP
descriptions [RFC4566] will be provided. For an example of a full
SDP description, refer to Section 8.3 of
[I-D.ietf-avt-rapid-acquisition-for-rtp].
4. Example Scenarios
4.1. 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.
We run an individual RAMS session for each of these RTP streams that
we want to rapidly acquire. 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.
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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
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].
4.2. Scenario #2: One Multicast Group
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 (Note that the RAMS request
sent by an RTP receiver could indicate the desire to acquire all or a
subset or one of the available RTP streams in an SSM session).
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.
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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 the above SDP 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
respective SSM sessions can use different feedback target transport
addresses, then their SSRC values do not have to be different from
each other.
4.3. 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, 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 always one SSM session and one feedback target.
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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
4.4. Scenario #4: Payload-Type Multiplexing
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 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, since this is not an easy requirement to satisfy,
RAMS specification forbids to have more than one RTP session to be
associated with a specific feedback target.
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).
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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 not only the RTP
receivers that are not FEC capable but also the ones that are FEC
capable but are not willing to receive FEC during the rapid
acquisition.
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. 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.
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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-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
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 SSM session.
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.
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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
a=mid:Channel1
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 can 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 could be
possible to infer the bitrate of this FEC stream and set the Max
Receive Bitrate value accordingly.
7. Security Considerations
There are no security considerations in this document.
8. IANA Considerations
There are no IANA considerations in this document.
9. Acknowledgments
I would like to thank various individuals in the AVT and MMUSIC WGs,
and my friends at Cisco for their comments and feedback.
10. References
10.1. 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-17 (work in
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progress), November 2010.
[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.
[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.
10.2. Informative References
[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.
Author's Address
Ali Begen
Cisco
181 Bay Street
Toronto, ON M5J 2T3
Canada
Email: abegen@cisco.com
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