AVTCORE WG J. Lennox
Internet-Draft Vidyo
Updates: 3550 (if approved) M. Westerlund
Intended status: Standards Track Ericsson
Expires: January 12, 2014 Q. Wu
Huawei
C. Perkins
University of Glasgow
July 11, 2013
Sending Multiple Media Streams in a Single RTP Session: Grouping RTCP
Reception Statistics and Other Feedback
draft-ietf-avtcore-rtp-multi-stream-optimisation-00
Abstract
RTP allows multiple media streams to be sent in a single session, but
requires each Synchronisation Source (SSRC) to send RTCP reception
quality reports for every other SSRC visible in the session. This
causes the number of RTCP reception reports to grow with the number
of SSRCs, rather than the number of endpoints. In many cases most of
these RTCP reception reports are unnecessary, since all SSRCs of an
endpoint are co-located and see the same reception quality. This
memo defines a Reporting Group extension to RTCP to reduce the
reporting overhead in such scenarios.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 12, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Grouping of RTCP Reception Statistics and Other Feedback . . 3
3.1. Semantics and Behavior of Reporting Groups . . . . . . . 3
3.2. Determine the Report Group . . . . . . . . . . . . . . . 4
3.3. RTCP Packet Reporting Group's Reporting Sources . . . . . 5
3.4. RTCP Source Description (SDES) item for Reporting Groups 6
3.5. Middlebox Considerations . . . . . . . . . . . . . . . . 6
3.6. SDP signaling for Reporting Groups . . . . . . . . . . . 6
3.7. Bandwidth Benefits of RTCP Reporting Groups . . . . . . . 6
3.8. Consequences of RTCP Reporting Groups . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The Real-time Transport Protocol (RTP) [RFC3550] is a protocol for
group communication, supporting multiparty multimedia sessions. A
single RTP session can support multiple participants sending at once,
and can also support participants sending multiple simultaneous media
streams. Examples of the latter might include a participant with
multiple cameras who chooses to send multiple views of a scene, or a
participant that sends audio and video flows multiplexed in a single
RTP session. Rules for handling RTP sessions containing multiple
media streams are described in [RFC3550] with some clarifications in
[I-D.ietf-avtcore-rtp-multi-stream].
An RTP endpoint will have one or more synchronisation sources (SSRCs)
that send and receive media streams (it will have one SSRC for each
media stream it sends). Each SSRC has to send RTCP sender reports
corresponding to the RTP packets it sends, and receiver reports for
traffic it receives. That is, every SSRC will send RTCP packets to
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report on every other SSRC. This rule is simple, but can be quite
inefficient for endpoints that send large numbers of media streams in
a single RTP session. Consider a session comprising ten
participants, each sending three media streams with their own SSRC.
There will be 30 SSRCs in such an RTP session, and 30 RTCP reception
reports will be sent per reporting interval as each SSRC reports on
all the others. However, the three SSRCs comprising each participant
will almost certainly see identical reception quality, since they are
co-located. If there was a way to indicate that several SSRCs are
co-located, and see the same reception quality, then two-thirds of
those RTCP reports could be suppressed.
This memo defines such an RTCP extension, Reporting Groups. This
extension is used to indicate the SSRCs that originate from the same
endpoint, and therefore have identical reception quality, allowing
the endpoint to suppress unnecessary RTCP reception reports.
2. Terminology
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. Grouping of RTCP Reception Statistics and Other Feedback
3.1. Semantics and Behavior of Reporting Groups
An RTCP Reporting Group indicates that a set of sources (SSRCs) that
originate from a single entity (endpoint or middlebox) in an RTP
session, and therefore all the sources in the group have an identical
view of the network. If reporting groups are in use, two sources
SHOULD be put into the same reporting group if their view of the
network is identical; i.e., if they report on traffic received at the
same interface of an RTP endpoint. Sources with different views of
the network MUST NOT be put into the same reporting group.
If reporting groups are in use, an endpoint MUST NOT send reception
reports from one source in a reporting group about another one in the
same group ("self-reports"). Similarly, an endpoint MUST NOT send
reception reports about a remote media source from more than one
source in a reporting group ("cross-reports"). Instead, it MUST pick
one of its local media sources as the "reporting" source for each
remote media source, and use it to send reception reports about that
remote source; all the other media sources in the reporting group
MUST NOT send any reception reports about that remote media source.
This reporting source MUST also be the source for any RTP/AVPF
Feedback [RFC4585] or Extended Report (XR) [RFC3611] packets about
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the corresponding remote sources as well. If a reporting source
leaves the session (i.e., if it sends a BYE, or leaves the group
without sending BYE under the rules of [RFC3550] section 6.3.7),
another reporting source MUST be chosen if any members of the group
still exist.
An endpoint or middlebox MAY use multiple sources as reporting
sources; however, each reporting source MUST have non-overlapping
sets of remote SSRCs it reports on. This is primarily to be done
when the reporting source's number of reception report blocks is so
large that it would be forced to round-robin around the sources.
Thus, by splitting the reports among several reporting SSRCs, more
consistent reporting can be achieved.
If RTP/AVPF feedback is in use, a reporting source MAY send immediate
or early feedback at any point when any member of the reporting group
could validly do so.
An endpoint SHOULD NOT create single-source reporting groups, unless
it is anticipated that the group might have additional sources added
to it in the future.
3.2. Determine the Report Group
A remote RTP entity, such as an endpoint or a middlebox needs to be
able to determine the report group used by another RTP entity. To
achieve this goal two RTCP extensions have been defined. For the
SSRCs that are reporting on behalf of the reporting group, an SDES
item RGRP has been defined for providing the report group with an
identifier. For SSRCs that aren't reporting on any peer SSRC a new
RTCP packet type is defined. This RTCP packet type "Reporting
Sources" lists the SSRC that are reporting on this SSRC's behalf.
This divided approach has been selected for the following reasons:
1. To enable an explicit indication of who reports on this SSRC's
behalf. Being explicit prevents the remote entity from detecting
that is missing the reports if there issues with the reporting
SSRC's RTCP packets.
2. To enable explicit identification of the SSRCs that are actively
reporting as one entity.
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3.3. RTCP Packet Reporting Group's Reporting Sources
This section defines a new RTCP packet type called "Reporting Group's
Reporting Sources" (RGRS). It identifies the SSRC(s) that report on
behalf of the SSRC that is the sender of the RGRS packet.
This packet consists of the fixed RTCP packet header which indicates
the packet type, the number of reporting sources included and the
SSRC which behalf the reporting SSRCs report on. This is followed by
the list of reporting SSRCs.
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| SC | PT=RGRS(TBA) | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
: SSRC for Reporting Source :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The RTCP Packets field has the following definition.
version (V): This field identifies the RTP version. The current
version is 2.
padding (P): 1 bit If set, the padding bit indicates that the packet
contains additional padding octets at the end that are not part of
the control information but are included in the length field. See
[RFC3550].
Source Count (SC): 5 bits Indicating the number of reporting SSRCs
(1-31) that are included in this RTCP packet type.
Payload type (PT): 8 bits This is the RTCP packet type that
identifies the packet as being an RTCP FB message. The RGRS RTCP
packet has the value [TBA].
Length: 16 bits The length of this packet in 32-bit words minus one,
including the header and any padding. This is in line with the
definition of the length field used in RTCP sender and receiver
reports [RFC3550].
SSRC of packet sender: 32 bits. The SSRC of the sender of this
packet which indicates which SSRCs that reports on its behalf,
instead of reporting itself.
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SSRC for Reporting Source: A variable number (as indicated by Source
Count) of 32-bit SSRC values. Each SSRC is an reporting SSRC
belonging to the same Report Group.
Each RGRS packet MUST contain at least one reporting SSRC. In case
the reporting SSRC field is insufficient to list all the SSRCs that
are reporting in this report group, the SSRC SHALL round robin around
the reporting sources.
Any RTP mixer or translator which forwards SR or RR packets from
members of a reporting group MUST forward the corresponding RGRS RTCP
packet as well.
3.4. RTCP Source Description (SDES) item for Reporting Groups
A new RTCP Source Description (SDES) item is defined for the purpose
of identifying reporting groups.
The Source Description (SDES) item "RGRP" is sent by a reporting
group's reporting SSRC. Syntactically, its format is the same as the
RTCP SDES CNAME item [RFC6222], and MUST be chosen with the same
global-uniqueness and privacy considerations as CNAME. This name
MUST be stable across the lifetime of the reporting group, even if
the SSRC of a reporting source changes.
Every source which belongs to a reporting group MUST either include
an RGRP SDES item in an SDES packet (if it is a reporting source), or
an RGRS packet (if it is not), in every compound RTCP packet in which
it sends an RR or SR packet (i.e., in every RTCP packet it sends,
unless Reduced-Size RTCP [RFC5506] is in use).
Any RTP mixer or translator which forwards SR or RR packets from
members of a reporting group MUST forward the corresponding RGRP SDES
items as well, even if it otherwise strips SDES items other than
CNAME.
3.5. Middlebox Considerations
This section discusses middlebox considerations for Reporting groups.
To be expanded.
3.6. SDP signaling for Reporting Groups
TBD
3.7. Bandwidth Benefits of RTCP Reporting Groups
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To understand the benefits of RTCP reporting groups, consider a
scenario in which the two endpoints in a session each have a hundred
sources, of which eight each are sending within any given reporting
interval.
For ease of analysis, we can make the simplifying approximation that
the duration of the RTCP reporting interval is equal to the total
size of the RTCP packets sent during an RTCP interval, divided by the
RTCP bandwidth. (This will be approximately true in scenarios where
the bandwidth is not so high that the minimum RTCP interval is
reached.) For further simplification, we can assume RTCP senders are
following the recommendations regarding Compound RTCP Packets in
[I-D.ietf-avtcore-rtp-multi-stream]; thus, the per-packet transport-
layer overhead will be small relative to the RTCP data. Thus, only
the actual RTCP data itself need be considered.
In a report interval in this scenario, there will, as a baseline, be
200 SDES packets, 184 RR packets, and 16 SR packets. This amounts to
approximately 6.5 kB of RTCP per report interval, assuming 16-byte
CNAMEs and no other SDES information.
Using the original [RFC3550] everyone-reports-on-every-sender
feedback rules, each of the 184 receivers will send 16 report blocks,
and each of the 16 senders will send 15. This amounts to
approximately 76 kB of report block traffic per interval; 92% of RTCP
traffic consists of report blocks.
If reporting groups are used, however, there is only 0.4 kB of
reports per interval, with no loss of useful information.
Additionally, there will be (assuming 16-byte RGRPs, and a single
reporting source per reporting group) an additional 2.4 kB per cycle
of RGRP SDES items and RGRS packets. Put another way, the unmodified
[RFC3550] reporting interval is approximately 8.9 times longer than
if reporting groups are in use.
3.8. Consequences of RTCP Reporting Groups
The RTCP traffic generated by receivers using RTCP Reporting Groups
might appear, to observers unaware of these semantics, to be
generated by receivers who are experiencing a network disconnection,
as the non-reporting sources appear not to be receiving a given
sender at all.
This could be a potentially critical problem for such a sender using
RTCP for congestion control, as such a sender might think that it is
sending so much traffic that it is causing complete congestion
collapse.
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However, such an interpretation of the session statistics would
require a fairly sophisticated RTCP analysis. Any receiver of RTCP
statistics which is just interested in information about itself needs
to be prepared that any given reception report might not contain
information about a specific media source, because reception reports
in large conferences can be round-robined.
Thus, it is unclear to what extent such backward compatibility issues
would actually cause trouble in practice.
4. Security Considerations
The security considerations of [RFC3550] and
[I-D.ietf-avtcore-rtp-multi-stream] apply.
(tbd: any security considerations due to these extensions?)
5. IANA Considerations
(Note to the RFC-Editor: please replace "TBA" with the IANA-assigned
value, and "XXXX" with the number of this document, prior to
publication as an RFC.)
The IANA is requested to register one new RTCP SDES items in the
"RTCP SDES Item Types" registry, as follows:
Value Abbrev Name Reference
TBA RGRP Reporting Group [RFCXXXX]
Figure 1: Item for the IANA Source Attribute Registry
The IANA is also requested to register one new RTCP packet type as
follows:
Value Abbrev Name Reference
TBA RGRR Reporting Group Reporting Sources [RFCXXXX]
Figure 2: Item for the IANA RTCP Control Packet Types (PT) Registry
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC6222] Begen, A., Perkins, C., and D. Wing, "Guidelines for
Choosing RTP Control Protocol (RTCP) Canonical Names
(CNAMEs)", RFC 6222, April 2011.
6.2. Informative References
[I-D.ietf-avtcore-rtp-multi-stream]
Lennox, J., Westerlund, M., Wu, W., and C. Perkins, "RTP
Considerations for Endpoints Sending Multiple Media
Streams", draft-ietf-avtcore-rtp-multi-stream-00 (work in
progress), April 2013.
[RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611, November
2003.
[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.
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, April 2009.
Authors' Addresses
Jonathan Lennox
Vidyo, Inc.
433 Hackensack Avenue
Seventh Floor
Hackensack, NJ 07601
US
Email: jonathan@vidyo.com
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Magnus Westerlund
Ericsson
Farogatan 6
SE-164 80 Kista
Sweden
Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com
Qin Wu
Huawei
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
Email: sunseawq@huawei.com
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
United Kingdom
Email: csp@csperkins.org
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