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IETF RFC 5760
RTP Control Protocol (RTCP) Extensions for Single-Source Multicast Sessions with Unicast Feedback
Last modified on Saturday, February 27th, 2010
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Internet Engineering Task Force (IETF) J. Ott
Request for Comments: 5760 Aalto University
Category: Standards Track J. Chesterfield
ISSN: 2070-1721 University of Cambridge
E. Schooler
Intel
February 2010
RTP Control Protocol (RTCP) Extensions for
Single-Source Multicast Sessions with Unicast Feedback
 Abstract
This document specifies an extension to the Real-time Transport
Control Protocol (RTCP) to use unicast feedback to a multicast
sender. The proposed extension is useful for single-source multicast
sessions such as Source-Specific Multicast (SSM) communication where
the traditional model of many-to-many group communication is either
not available or not desired. In addition, it can be applied to any
group that might benefit from a sender-controlled summarized
reporting mechanism.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in 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/RFC 5760.
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
Ott, et al. Standards Track PAGE 1 
RFC 5760 RTCP with Unicast Feedback February 2010
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.
Table of Contents
1. Introduction ....................................................3
2. Conventions and Acronyms ........................................4
3. Definitions .....................................................5
4. Basic Operation .................................................6
5. Packet Types ...................................................10
6. Simple Feedback Model ..........................................11
7. Distribution Source Feedback Summary Model .....................13
8. Mixer/Translator Issues ........................................36
9. Transmission Interval Calculation ..............................37
10. SDP Extensions ................................................39
11. Security Considerations .......................................41
12. Backwards Compatibility .......................................51
13. IANA Considerations ...........................................51
14. References ....................................................53
Appendix A. Examples for Sender-Side Configurations ...............57
Appendix B. Distribution Report Processing at the Receiver ........60
Ott, et al. Standards Track PAGE 2
RFC 5760 RTCP with Unicast Feedback February 2010
1. Introduction
The Real-time Transport Protocol (RTP) [1] provides a real-time
transport mechanism suitable for unicast or multicast communication
between multimedia applications. Typical uses of RTP are for real-
time or near real-time group communication of audio and video data
streams. An important component of the RTP protocol is the control
channel, defined as the RTP Control Protocol (RTCP). RTCP involves
the periodic transmission of control packets between group members,
enabling group size estimation and the distribution and calculation
of session-specific information such as packet loss and round-trip
time to other hosts. An additional advantage of providing a control
channel for a session is that a third-party session monitor can
listen to the traffic to establish network conditions and to diagnose
faults based on receiver locations.
RTP was designed to operate in either a unicast or multicast mode.
In multicast mode, it assumes an Any Source Multicast (ASM) group
model, where both one-to-many and many-to-many communication are
supported via a common group address in the range 224.0.0.0 through
239.255.255.255. To enable Internet-wide multicast communication,
intra-domain routing protocols (those that operate only within a
single administrative domain, e.g., the Distance Vector Multicast
Routing Protocol (DVMRP) [16] and Protocol Independent Multicast
(PIM) [17][18]) are used in combination with inter-domain routing
protocols (those that operate across administrative domain borders,
e.g., Multicast BGP (MBGP) [19] and the Multicast Source Discovery
Protocol (MSDP) [20]). Such routing protocols enable a host to join
a single multicast group address and send data to or receive data
from all members in the group with no prior knowledge of the
membership. However, there is a great deal of complexity involved at
the routing level to support such a multicast service in the network.
Many-to-many communication is not always available or desired by all
group applications. For example, with Source-Specific Multicast
(SSM) [8][9] and satellite communication, the multicast distribution
channel only supports source-to-receiver traffic. In other cases,
such as large ASM groups with a single active data source and many
passive receivers, it is sub-optimal to create a full routing-level
mesh of multicast sources just for the distribution of RTCP control
packets. Thus, an alternative solution is preferable.
Although a one-to-many multicast topology may simplify routing and
may be a closer approximation to the requirements of certain RTP
applications, unidirectional communication makes it impossible for
receivers in the group to share RTCP feedback information with other
group members. In this document, we specify a solution to that
problem. We introduce unicast feedback as a new method to distribute
Ott, et al. Standards Track PAGE 3
RFC 5760 RTCP with Unicast Feedback February 2010
RTCP control information amongst all session members. This method is
designed to operate under any group communication model, ASM or SSM.
The RTP data stream protocol itself is unaltered.
Scenarios under which the unicast feedback method can provide benefit
include but are not limited to:
a) SSM groups with a single sender.
The proposed extensions allow SSM groups that do not have many-to-
many communication capability to receive RTP data streams and to
continue to participate in the RTP control protocol (RTCP) by
using multicast in the source-to-receiver direction and unicast to
send receiver feedback to the source on the standard RTCP port.
b) One-to-many broadcast networks.
Unicast feedback may also be beneficial to one-to-many broadcast
networks, such as a satellite network with a terrestrial low-
bandwidth return channel or a broadband cable link. Unlike the
SSM network, these networks may have the ability for a receiver to
multicast return data to the group. However, a unicast feedback
mechanism may be preferable for routing simplicity.
c) ASM with a single sender.
A unicast feedback approach can be used by an ASM application with
a single sender to reduce the load on the multicast routing
infrastructure that does not scale as efficiently as unicast
routing does. Because this is no more efficient than a standard
multicast group RTP communication scenario, it is not expected to
replace the traditional mechanism.
The modifications proposed in this document are intended to
supplement the existing RTCP feedback mechanisms described in Section
6 of [1].
2. Conventions and Acronyms
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 RFC 2119 [13].
The following acronyms are used throughout this document:
ASM Any Source Multicast
SSM Source-Specific Multicast
Ott, et al. Standards Track PAGE 4
RFC 5760 RTCP with Unicast Feedback February 2010
3. Definitions
Distribution Source:
In an SSM context, only one entity distributes RTP data and
redistributes RTCP information to all receivers. This entity is
called the Distribution Source. It is also responsible for
forwarding RTCP feedback to the Media Senders and thus creates a
virtual multicast environment in which RTP and RTCP can be
applied.
Note that heterogeneous networks consisting of ASM multiple-sender
groups, unicast-only clients, and/or SSM single-sender receiver
groups MAY be connected via translators or mixers to create a
single-source group (see Section 8 for details).
Media Sender:
A Media Sender is an entity that originates RTP packets for a
particular media session. In RFC 3550, a Media Sender is simply
called a source. However, as the RTCP SSM system architecture
includes a Distribution Source, to avoid confusion, in this
document a media source is commonly referred to as a Media Sender.
There may often be a single Media Sender that is co-located with
the Distribution Source. But although there MUST be only one
Distribution Source, there MAY be multiple Media Senders on whose
behalf the Distribution Source forwards RTP and RTCP packets.
RTP and RTCP Channels:
The data distributed from the source to the receivers is referred
to as the RTP channel and the control information as the RTCP
channel. With standard RTP/RTCP, these channels typically share
the same multicast address but are differentiated via port numbers
as specified in [1]. In an SSM context, the RTP channel is
multicast from the Distribution Source to the receivers. In
contrast, the RTCP or feedback channel is actually the collection
of unicast channels between the receivers and the Distribution
Source via the Feedback Target(s). Thus, bidirectional
communication is accomplished by using SSM in the direction from
Distribution Source to the receivers and using the unicast
feedback channel in the direction from receivers to Distribution
Source. As discussed in the next section, the nature of the
channels between the Distribution Source and the Media Sender(s)
may vary.
(Unicast RTCP) Feedback Target:
The Feedback Target is a logical function to which RTCP unicast
feedback traffic is addressed. The functions of the Feedback
Target and the Distribution Source MAY be co-located or integrated
in the same entity. In this case, for a session defined as having
Ott, et al. Standards Track PAGE 5
RFC 5760 RTCP with Unicast Feedback February 2010
a Distribution Source A, on ports n for the RTP channel and k for
the RTCP channel, the unicast RTCP Feedback Target is identified
by an IP address of Distribution Source A on port k, unless
otherwise stated in the session description. See Section 10 for
details on how the address is specified. The Feedback Target MAY
also be implemented in one or more entities different from the
Distribution Source, and different RTP receivers MAY use different
Feedback Target instances, e.g., for aggregation purposes. In
this case, the Feedback Target instance(s) MUST convey the
feedback received from the RTP receivers to the Distribution
Source using the RTCP mechanisms specified in this document. If
disjoint, the Feedback Target instances MAY be organized in
arbitrary topological structures: in parallel, hierarchical, or
chained. But the Feedback Target instance(s) and Distribution
Source MUST share, e.g., through configuration, enough information
to be able to provide coherent RTCP information to the RTP
receivers based upon the RTCP feedback collected by the Feedback
Target instance(s) -- as would be done if both functions were part
of the same entity.
In order for unicast feedback to work, each receiver MUST direct
its RTCP reports to a single specific Feedback Target instance.
SSRC:
Synchronization source as defined in [1]. This 32-bit value
uniquely identifies each member in a session.
Report blocks:
Report block is the standard terminology for an RTCP reception
report. RTCP [1] encourages the stacking of multiple report
blocks in Sender Report (SR) and Receiver Report (RR) packets. As
a result, a variable-size feedback packet may be created by one
source that reports on multiple other sources in the group. The
summarized reporting scheme builds upon this model through the
inclusion of multiple summary report blocks in one packet.
However, stacking of reports from multiple receivers is not
permitted in the Simple Feedback Model (see Section 6).
4. Basic Operation
As indicated by the definitions of the preceding section, one or more
Media Senders send RTP packets to the Distribution Source. The
Distribution Source relays the RTP packets to the receivers using a
source-specific multicast arrangement. In the reverse direction, the
receivers transmit RTCP packets via unicast to one or more instances
of the Feedback Target. The Feedback Target sends either the
original RTCP reports (the Simple Feedback Model) or summaries of
these reports (the Summary Feedback Model) to the Distribution
Ott, et al. Standards Track PAGE 6
RFC 5760 RTCP with Unicast Feedback February 2010
Source. The Distribution Source in turn relays the RTCP reports
and/or summaries to the Media Senders. The Distribution Source also
transmits the RTCP Sender Reports and Receiver Reports or summaries
back to the receivers, using source-specific multicast.
When the Feedback Target(s) are co-located (or integrated) with the
Distribution Source, redistribution of original or summarized RTCP
reports is trivial. When the Feedback Target(s) are physically
and/or topologically distinct from the Distribution Source, each
Feedback Target either relays the RTCP packets to the Distribution
Source or summarizes the reports and forwards an RTCP summary report
to the Distribution Source. Coordination between multiple Feedback
Targets is beyond the scope of this specification.
The Distribution Source MUST be able to communicate with all group
members in order for either mechanism to work. The general
architecture is displayed below in Figure 1. There may be a single
Media Sender or multiple Media Senders (Media Sender i, 1<=i<=M) on
whose behalf the Distribution Source disseminates RTP and RTCP
packets. The base case, which is expected to be the most common
case, is that the Distribution Source is co-located with a particular
Media Sender. A basic assumption is that communication is multicast
(either SSM or ASM) in the direction of the Distribution Source to
the receivers (R(j), 1<=j<=N) and unicast in the direction of the
receivers to the Distribution Source.
Communication between Media Sender(s) and the Distribution Source may
be performed in numerous ways:
i. Unicast only: The Media Sender(s) MAY send RTP and RTCP via
unicast to the Distribution Source and receive RTCP via unicast.
ii. Any Source Multicast (ASM): The Media Sender(s) and the
Distribution Source MAY be in the same ASM group, and RTP and
RTCP packets are exchanged via multicast.
iii. Source-Specific Multicast (SSM): The Media Sender(s) and the
Distribution Source MAY be in an SSM group with the source being
the Distribution Source. RTP and RTCP packets from the Media
Senders are sent via unicast to the Distribution Source, while
RTCP packets from the Distribution Source are sent via multicast
to the Media Senders.
Note that this SSM group MAY be identical to the SSM group used
for RTP/RTCP delivery from the Distribution Source to the
receivers or it MAY be a different one.
Ott, et al. Standards Track PAGE 7
RFC 5760 RTCP with Unicast Feedback February 2010
Note that Figure 1 below shows a logical diagram and, therefore, no
details about the above options for the communication between Media
Sender(s), Distribution Source, Feedback Target(s), and receivers are
provided.
Configuration information needs to be supplied so that (among other
reasons):
o Media Sender(s) know the transport address of the Distribution
Source or the transport address of the (ASM or SSM) multicast
group used for the contribution link;
o the Distribution Source knows either the unicast transport
address(es) or the (ASM or SSM) multicast transport address(es) to
reach the Media Sender(s);
o receivers know the addresses of their respectively responsible
Feedback Targets; and
o the Feedback Targets know the transport address of the
Distribution Source.
The precise setup and configuration of the Media Senders and their
interaction with the Distribution Source is beyond the scope of this
document (appropriate Session Description Protocol (SDP) descriptions
MAY be used for this purpose), which only specifies how the various
components interact within an RTP session. Informative examples for
different configurations of the Media Sources and the Distribution
Source are given in Appendix A.
Future specifications may be defined to address these aspects.
Ott, et al. Standards Track PAGE 8
RFC 5760 RTCP with Unicast Feedback February 2010
Source-specific
+--------+ +-----+ Multicast
|Media | | | +----------------> R(1)
|Sender 1|<----->| D S | | |
+--------+ | I O | +--+ |
| S U | | | |
+--------+ | T R | | +-----------> R(2) |
|Media |<----->| R C |->+ +---- : | |
|Sender 2| | I E | | +------> R(n-1) | |
+--------+ | B | | | | | |
: | U | +--+--> R(n) | | |
: | T +-| | | | |
| I | |<---------+ | | |
+--------+ | O |F|<---------------+ | |
|Media | | N |T|<--------------------+ |
|Sender M|<----->| | |<-------------------------+
+--------+ +-----+ Unicast
FT = Feedback Target
Transport from the Feedback Target to the Distribution
Source is via unicast or multicast RTCP if they are not
co-located.
Figure 1: System Architecture
The first method proposed to support unicast RTCP feedback, the
'Simple Feedback Model', is a basic reflection mechanism whereby all
Receiver RTCP packets are unicast to the Feedback Target, which
relays them unmodified to the Distribution Source. Subsequently,
these packets are forwarded by the Distribution Source to all
receivers on the multicast RTCP channel. The advantage of using this
method is that an existing receiver implementation requires little
modification in order to use it. Instead of sending reports to a
multicast address, a receiver uses a unicast address yet still
receives forwarded RTCP traffic on the multicast control channel.
This method also has the advantage of being backwards compatible with
standard RTP/RTCP implementations. The Simple Feedback Model is
specified in Section 6.
The second method, the 'Distribution Source Feedback Summary Model',
is a summarized reporting scheme that provides savings in bandwidth
by consolidating Receiver Reports at the Distribution Source,
optionally with help from the Feedback Target(s), into summary
packets that are then distributed to all the receivers. The
Distribution Source Feedback Summary Model is specified in Section 7.
Ott, et al. Standards Track PAGE 9
RFC 5760 RTCP with Unicast Feedback February 2010
The advantage of the latter scheme is apparent for large group
sessions where the basic reflection mechanism outlined above
generates a large amount of packet forwarding when it replicates all
the information to all the receivers. Clearly, this technique
requires that all session members understand the new summarized
packet format outlined in Section 7.1. Additionally, the summarized
scheme provides an optional mechanism to send distribution
information or histograms about the feedback data reported by the
whole group. Potential uses for the compilation of distribution
information are addressed in Section 7.4.
To differentiate between the two reporting methods, a new SDP
identifier is created and discussed in Section 10. The reporting
method MUST be decided prior to the start of the session. A
Distribution Source MUST NOT change the method during a session.
In a session using SSM, the network SHOULD prevent any multicast data
from the receiver being distributed further than the first hop
router. Additionally, any data heard from a non-unicast-capable
receiver by other hosts on the same subnet SHOULD be filtered out by
the host IP stack so that it does not cause problems with respect to
the calculation of the receiver RTCP bandwidth share.
5. Packet Types
The RTCP packet types defined in [1], [26], and [15] are:
Type Description Payload number
-------------------------------------------------------
SR Sender Report 200
RR Receiver Report 201
SDES Source Description 202
BYE Goodbye 203
APP Application-Defined 204
RTPFB Generic RTP feedback 205
PSFB Payload-specific feedback 206
XR RTCP Extension 207
This document defines one further RTCP packet format:
Type Description Payload number
---------------------------------------------------------
RSI Receiver Summary Information 209
Within the Receiver Summary Information packet, there are various
types of information that may be reported and encapsulated in
optional sub-report blocks:
Ott, et al. Standards Track PAGE 10
RFC 5760 RTCP with Unicast Feedback February 2010
Name Long Name Value
------------------------------------------------------------------
IPv4 Address IPv4 Feedback Target Address 0
IPv6 Address IPv6 Feedback Target Address 1
DNS Name DNS name indicating Feedback Target Address 2
Reserved Reserved for Assignment by Standards Action 3
Loss Loss distribution 4
Jitter Jitter distribution 5
RTT Round-trip time distribution 6
Cumulative loss Cumulative loss distribution 7
Collisions SSRC collision list 8
Reserved Reserved for Assignment by Standards Action 9
Stats General statistics 10
RTCP BW RTCP bandwidth indication 11
Group Info RTCP group and average packet size 12
- Unassigned 13 - 255
As with standard RTP/RTCP, the various reports MAY be combined into a
single RTCP packet, which SHOULD NOT exceed the path MTU. Packets
continue to be sent at a rate that is inversely proportional to the
group size in order to scale the amount of traffic generated.
6. Simple Feedback Model
6.1. Packet Formats
The Simple Feedback Model uses the same packet types as traditional
RTCP feedback described in [1]. Receivers still generate Receiver
Reports with information on the quality of the stream received from
the Distribution Source. The Distribution Source still MUST create
Sender Reports that include timestamp information for stream
synchronization and round-trip time calculation. Both Media Senders
and receivers are required to send SDES packets as outlined in [1].
The rules for generating BYE and APP packets as outlined in [1] also
apply.
6.2. Distribution Source Behavior
For the Simple Feedback Model, the Distribution Source MUST provide a
basic packet-reflection mechanism. It is the default behavior for
any Distribution Source and is the minimum requirement for acting as
a Distribution Source to a group of receivers using unicast RTCP
feedback.
The Distribution Source (unicast Feedback Target) MUST listen for
unicast RTCP data sent to the RTCP port. All valid unicast RTCP
packets received on this port MUST be forwarded by the Distribution
Source to the group on the multicast RTCP channel. The Distribution
Ott, et al. Standards Track PAGE 11
RFC 5760 RTCP with Unicast Feedback February 2010
Source MUST NOT stack report blocks received from different receivers
into one packet for retransmission to the group. Every RTCP packet
from each receiver MUST be reflected individually.
If the Media Sender(s) are not part of the SSM group for RTCP packet
reflection, the Distribution Source MUST also forward the RTCP
packets received from the receivers to the Media Sender(s). If there
is more than one Media Sender and these Media Senders do not
communicate via ASM with the Distribution Source and each other, the
Distribution Source MUST forward each RTCP packet originated by one
Media Sender to all other Media Senders.
The Distribution Source MUST forward RTCP packets originating from
the Media Sender(s) to the receivers.
The reflected or forwarded RTCP traffic SHOULD NOT be counted as its
own traffic in the transmission interval calculation by the
Distribution Source. In other words, the Distribution Source SHOULD
NOT consider reflected packets as part of its own control data
bandwidth allowance. However, reflected packets MUST be processed by
the Distribution Source and the average RTCP packet size, RTCP
transmission rate, and RTCP statistics MUST be calculated. The
algorithm for computing the allowance is explained in Section 9.
6.3. Disjoint Distribution Source and Feedback Target(s)
If the Feedback Target function is disjoint from the Distribution
Source, the Feedback Target(s) MUST forward all RTCP packets from the
receivers or another Feedback Target -- directly or indirectly -- to
the Distribution Source.
6.4. Receiver Behavior
Receivers MUST listen on the RTP channel for data and on the RTCP
channel for control. Each receiver MUST calculate its share of the
control bandwidth R/n, in accordance with the profile in use, so that
a fraction of the RTCP bandwidth, R, allocated to receivers is
divided equally between the number of unique receiver SSRCs in the
session, n. R may be rtcp_bw * 0.75 or rtcp_bw * 0.5 (depending on
the ratio of senders to receivers as per [1]) or may be set
explicitly by means of an SDP attribute [10]. See Section 9 for
further information on the calculation of the bandwidth allowance.
When a receiver is eligible to transmit, it MUST send a unicast
Receiver Report packet to the Feedback Target following the rules
defined in Section 9.
Ott, et al. Standards Track PAGE 12
RFC 5760 RTCP with Unicast Feedback February 2010
When a receiver observes either RTP packets or RTCP Sender Reports
from a Media Sender with an SSRC that collides with its own chosen
SSRC, it MUST change its own SSRC following the procedures of [1].
The receiver MUST do so immediately after noticing and before sending
any (further) RTCP feedback messages.
If a receiver has out-of-band information available about the Media
Sender SSRC(s) used in the media session, it MUST NOT use the same
SSRC for itself. Even if such out-of-band information is available,
a receiver MUST obey the above collision-resolution mechanisms.
Further mechanisms defined in [1] apply for resolving SSRC collisions
between receivers.
6.5. Media Sender Behavior
Media Senders listen on a unicast or multicast transport address for
RTCP reports sent by the receivers (and forwarded by the Distribution
Source) or other Media Senders (forwarded by the Distribution Source
if needed). Processing and general operation follows [1].
A Media Sender that observes an SSRC collision with another entity
that is not also a Media Sender MAY delay its own collision-
resolution actions as per [1], by 5 * 1.5 * Td, with Td being the
deterministic calculated reporting interval, for receivers to see
whether the conflict still exists. SSRC collisions with other Media
Senders MUST be acted upon immediately.
Note: This gives precedence to Media Senders and places the burden
of collision resolution on the RTP receivers.
Sender SSRC information MAY be communicated out-of-band, e.g., by
means of SDP media descriptions. Therefore, senders SHOULD NOT
change their own SSRC aggressively or unnecessarily.
7. Distribution Source Feedback Summary Model
In the Distribution Source Feedback Summary Model, the Distribution
Source is required to summarize the information received from all the
Receiver Reports generated by the receivers and place the information
into summary reports. The Distribution Source Feedback Summary Model
introduces a new report block format, the Receiver Summary
Information (RSI) report, and a number of optional sub-report block
formats, which are enumerated in Section 7.1. As described in
Section 7.3, individual instances of the Feedback Target may provide
preliminary summarization to reduce the processing load at the
Distribution Source.
Ott, et al. Standards Track PAGE 13
RFC 5760 RTCP with Unicast Feedback February 2010
Sub-reports appended to the RSI report block provide more detailed
information on the overall session characteristics reported by all
receivers and can also convey important information such as the
feedback address and reporting bandwidth. Which sub-reports are
mandatory and which ones are optional is defined below.
From an RTP perspective, the Distribution Source is an RTP receiver,
generating its own Receiver Reports and sending them to the receiver
group and to the Media Senders. In the Distribution Source Feedback
Summary Model, an RSI report block MUST be appended to all RRs the
Distribution Source generates.
In addition, the Distribution Source MUST forward the RTCP SR reports
and SDES packets of Media Senders without alteration. If the
Distribution Source is actually a Media Sender, even if it is the
only session sender, it MUST generate its own Sender Report (SR)
packets for its role as a Media Sender and its Receiver Reports in
its role as the Distribution Source.
The Distribution Source MUST use its own SSRC value for transmitting
summarization information and MUST perform proper SSRC collision
detection and resolution.
The Distribution Source MUST send at least one Receiver Summary
Information packet for each reporting interval. The Distribution
Source MAY additionally stack sub-report blocks after the RSI packet.
If it does so, each sub-report block MUST correspond to the RSI
packet and constitutes an enhancement to the basic summary
information required by the receivers to calculate their reporting
time interval. For this reason, additional sub-report blocks are not
required but recommended. The compound RTCP packets containing the
RSI packet and the optional corresponding sub-report blocks MUST be
formed according to the rules defined in [1] for receiver-issued
packets, e.g., they MUST begin with an RR packet, contain at least an
SDES packet with a CNAME, and MAY contain further RTCP packets and
SDES items.
Every RSI packet MUST contain either a Group and Average Packet Size
sub-report or an RTCP Bandwidth sub-report for bandwidth indications
to the receivers.
7.1. Packet Formats
All numeric values comprising multiple (usually two or four) octets
MUST be encoded in network byte order.
Ott, et al. Standards Track PAGE 14
RFC 5760 RTCP with Unicast Feedback February 2010
7.1.1. RSI: Receiver Summary Information Packet
The RSI report block has a fixed header size followed by a variable
length report:
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|reserved | PT=RSI=209 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Summarized SSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP Timestamp (most significant word) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP Timestamp (least significant word) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Sub-report blocks :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The RSI packet includes the following fields:
Length: 16 bits
As defined in [1], the length of the RTCP packet in 32-bit words
minus one, including the header and any padding.
SSRC: 32 bits
The SSRC of the Distribution Source.
Summarized SSRC: 32 bits
The SSRC (of the Media Sender) of which this report contains a
summary.
Timestamp: 64 bits
Indicates the wallclock time when this report was sent. Wallclock
time (absolute date and time) is represented using the timestamp
format of the Network Time Protocol (NTP), which is in seconds
relative to 0h UTC on 1 January 1900 [1]. The wallclock time MAY
(but need not) be NTP-synchronized but it MUST provide a
consistent behavior in the advancement of time (similar to NTP).
The full-resolution NTP timestamp is used, which is a 64-bit,
unsigned, fixed-point number with the integer part in the first 32
bits and the fractional part in the last 32 bits. This format is
similar to RTCP Sender Reports (Section 6.4.1 of [1]). The
timestamp value is used to enable detection of duplicate packets,
reordering, and to provide a chronological profile of the feedback
reports.
Ott, et al. Standards Track PAGE 15
RFC 5760 RTCP with Unicast Feedback February 2010
7.1.2. Sub-Report Block Types
For RSI reports, this document also introduces a sub-report block
format specific to the RSI packet. The sub-report blocks are
appended to the RSI packet using the following generic format. All
sub-report blocks MUST be 32-bit aligned.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRBT | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ SRBT-specific data +
| |
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SRBT: 8 bits
Sub-Report Block Type. The sub-report block type identifier. The
values for the sub-report block types are defined in Section 5.
Length: 8 bits
The length of the sub-report in 32-bit words.
SRBT-specific data: <length * 4 - 2> octets
This field may contain type-specific information based upon the
SRBT value.
7.1.3. Generic Sub-Report Block Fields
For the sub-report blocks that convey distributions of values (Loss,
Jitter, RTT, Cumulative Loss), a flexible 'data bucket'-style report
is used. This format divides the data set into variable-size buckets
that are interpreted according to the guide fields at the head of the
report block.
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
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SRBT | Length | NDB | MF |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Minimum Distribution Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum Distribution Value |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Distribution Buckets |
| ... |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Ott, et al. Standards Track PAGE 16
RFC 5760 RTCP with Unicast Feedback February 2010
The SRBT and length fields are calculated as explained in Section
7.1.2.
Number of distribution buckets (NDB): 12 bits
The number of distribution buckets of data. The size of each
bucket can be calculated using the formula
((length * 4) - 12) * 8 / NDB number of bits. The calculation is
based on the length of the whole sub-report block in octets
(length * 4) minus the header of 12 octets. Providing 12 bits for
the NDB field enables bucket sizes as small as 2 bits for a full-
length packet of MTU 1500 bytes. The bucket size in bits must
always be divisible by 2 to ensure proper byte alignment. A
bucket size of 2 bits is fairly restrictive, however, and it is
expected that larger bucket sizes will be more practical for most
distributions.
Multiplicative Factor (MF): 4 bits
2^MF indicates the multiplicative factor to be applied to each
distribution bucket value. Possible values of MF are 0 - 15,
creating a range of values from MF = 1, 2, 4 ... 32768. Appendix
B gives an example of the use of the multiplicative factor; it is
meant to provide more "bits" without having them -- the bucket
values get scaled up by the MF.
Length: 8 bits
The length field tells the receiver the full length of the sub-
report block in 32-bit words (i.e., length * 4 bytes) and enables
the receiver to identify the bucket size. For example, given no
MTU restrictions, the data portion of a distribution packet may be
only as large as 1008 bytes (255 * 4 - 12), providing up to 4032
data buckets of length 2 bits, or 2016 data buckets of length 4
bits, etc.
Minimum distribution value (min): 32 bits
The minimum distribution value, in combination with the maximum
distribution value, indicates the range covered by the data bucket
values.
Maximum distribution value (max): 32 bits
The maximum distribution value, in combination with the minimum
distribution value, indicates the range covered by the data bucket
values. The significance and range of the distribution values is
defined in the individual subsections for each distribution type
(DT).
Ott, et al. Standards Track PAGE 17
RFC 5760 RTCP with Unicast Feedback February 2010
Distribution buckets: each bucket is ((length * 4) - 12) * 8 / NDB
bits
The size and number of buckets is calculated as outlined above
based upon the value of NDB and the length of the packet. The
values for distribution buckets are equally distributed; according
to the following formula, distribution bucket x (with 0 <= x <
NDB) covers the value range:
x = 0; [min, min + (max - min) / NDB]
x > 0; [min + (x) * (max - min) / NDB,
min + (x + 1) * (max - min) / NDB]
Interpretation of the minimum, maximum, and distribution values in
the sub-report block is sub-report-specific and is addressed
individually in the sections below. The size of the sub-report block
is variable, as indicated by the packet length field.
Note that, for any bucket-based reporting, if the Distribution Source
and the Feedback Target(s) are disjoint entities, out-of-band
agreement on the bucket-reporting granularity is recommended to allow
the Distribution Source to forward accurate information in these
kinds of reports to the receivers.
7.1.4. Loss Sub-Report Block
The Loss sub-report block allows a receiver to determine how its own
reception quality relates to the other recipients. A receiver may
use this information, e.g., to drop out of a session (instead of
sending lots of error repair feedback) if it finds itself isolated at
the lower end of the reception quality scale.
The Loss sub-report block indicates the distribution of losses as
reported by the receivers to the Distribution Source. Values are
expressed as a fixed-point number with the binary point at the left
edge of the field similar to the "fraction lost" field in SR and RR
packets, as defined in [1]. The Loss sub-report block type (SRBT) is
4.
Valid results for the minimum distribution value field are 0 - 254.
Similarly, valid results for the maximum distribution value field are
1 - 255. The minimum distribution value MUST always be less than the
maximum.
For examples on processing summarized loss report sub-blocks, see
Appendix B.
Ott, et al. Standards Track PAGE 18
RFC 5760 RTCP with Unicast Feedback February 2010
7.1.5. Jitter Sub-Report Block
A Jitter sub-report block indicates the distribution of the estimated
statistical variation of the RTP data packet inter-arrival time
reported by the receivers to the Distribution Source. This allows
receivers both to place their own observed jitter values in context
with the rest of the group and to approximate dynamic parameters for
playout buffers. See [1] for details on the calculation of the
values and the relevance of the jitter results. Jitter values are
measured in timestamp units with the rate used by the Media Sender
and expressed as unsigned integers. The minimum distribution value
MUST always be less than the maximum. The Jitter sub-report block
type (SRBT) is 5.
Since timestamp units are payload-type specific, the relevance of a
jitter value is impacted by any change in the payload type during a
session. Therefore, a Distribution Source MUST NOT report jitter
distribution values for at least 2 reporting intervals after a
payload type change occurs.
7.1.6. Round-Trip Time Sub-Report Block
A Round-Trip Time sub-report indicates the distribution of round-trip
times from the Distribution Source to the receivers, providing
receivers with a global view of the range of values in the group.
The Distribution Source is capable of calculating the round-trip time
to any other member since it forwards all the SR packets from the
Media Sender(s) to the group. If the Distribution Source is not
itself a Media Sender, it can calculate the round-trip time from
itself to any of the receivers by maintaining a record of the SR
sender timestamp and the current time as measured from its own system
clock. The Distribution Source consequently calculates the round-
trip time from the Receiver Report by identifying the corresponding
SR timestamp and subtracting the RR advertised holding time as
reported by the receiver from its own time difference measurement, as
calculated by the time the RR packet is received and the recorded
time the SR was sent.
The Distribution Source has the option of distributing these round-
trip time estimations to the whole group, uses of which are described
in Section 7.4. The round-trip time is expressed in units of 1/65536
seconds and indicates an absolute value. This is calculated by the
Distribution Source, based on the Receiver Report responses declaring
the time difference since an original Sender Report and on the
holding time at the receiver. The minimum distribution value MUST
always be less than the maximum. The Round-Trip Time sub-report
block type (SRBT) is 6.
Ott, et al. Standards Track PAGE 19
RFC 5760 RTCP with Unicast Feedback February 2010
Note that if the Distribution Source and the Feedback Target
functions are disjoint, it is only possible for the Distribution
Source to determine RTT if all the Feedback Targets forward all
RTCP reports from the receivers immediately (i.e., do not perform
any preliminary summarization) and include at least the RR packet.
7.1.7. Cumulative Loss Sub-Report Block
The cumulative loss field in a Receiver Report [1], in contrast to
the fraction lost field, is intended to provide some historical
perspective on the session performance, i.e., the total number of
lost packets since the receiver joined the session. The cumulative
loss value provides a longer-term average by summing over a larger
sample set (typically the whole session). The Distribution Source
MAY record the values as reported by the receiver group and report a
distribution of values. Values are expressed as a fixed-point number
with the binary point at the left edge of the field, in the same
manner as the Loss sub-report block. Since the individual Receiver
Reports give the cumulative number of packets lost but not the
cumulative number sent, which is required as a denominator to
calculate the long-term fraction lost, the Distribution Source MUST
maintain a record of the cumulative number lost and extended highest
sequence number received, as reported by each receiver at some point
in the past. Ideally, the recorded values are from the first report
received. Subsequent calculations are then based on (<the new
cumulative loss value> - <the recorded value>) / (<new extended
highest sequence number> - <recorded sequence number>).
Valid results for the minimum distribution value field are 0 - 254.
Similarly, valid results for the maximum distribution value field are
1 - 255. The minimum distribution value MUST always be less than the
maximum. The Cumulative Loss sub-report block type (SRBT) is 7.
7.1.8. Feedback Target Address Sub-Report Block
The Feedback Target Address block provides a dynamic mechanism for
the Distribution Source to signal an alternative unicast RTCP
feedback address to the receivers. If a block of this type is
included, receivers MUST override any static SDP address information
for the session with the Feedback Target address provided in this
sub-report block.
If a Feedback Target Address sub-report block is used, it MUST be
included in every RTCP packet originated by the Distribution Source
to ensure that all receivers understand the information. In this
manner, receiver behavior should remain consistent even in the face
of packet loss or when there are late session arrivals.
Ott, et al. Standards Track PAGE 20
RFC 5760 RTCP with Unicast Feedback February 2010
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRBT={0,1,2} | Length | Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: Address :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SRBT: 8 bits
The type of sub-report block that corresponds to the type of
address is as follows:
0: IPv4 address
1: IPv6 address
2: DNS name
Length: 8 bits
The length of the sub-report block in 32-bit words. For an IPv4
address, this should be 2 (i.e., total length = 4 + 4 = 2 * 4
octets). For an IPv6 address, this should be 5. For a DNS name,
the length field indicates the number of 32-bit words making up
the string plus 1 byte and any additional padding required to
reach the next word boundary.
Port: 2 octets
The port number to which receivers send feedback reports. A port
number of 0 is invalid and MUST NOT be used.
Address: 4 octets (IPv4), 16 octets (IPv6), or n octets (DNS name)
The address to which receivers send feedback reports. For IPv4
and IPv6, fixed-length address fields are used. A DNS name is an
arbitrary-length string that is padded with null bytes to the next
32-bit boundary. The string MAY contain Internationalizing Domain
Names in Applications (IDNA) domain names and MUST be UTF-8
encoded [11].
A Feedback Target Address block for a certain address type (i.e.,
with a certain SRBT of 0, 1, or 2) MUST NOT occur more than once
within a packet. Numerical Feedback Target Address blocks for IPv4
and IPv6 MAY both be present. If so, the resulting transport
addresses MUST point to the same logical entity.
If a Feedback Target address block with an SRBT indicating a DNS name
is present, there SHOULD NOT be any other numerical Feedback Target
Address blocks present.
Ott, et al. Standards Track PAGE 21
RFC 5760 RTCP with Unicast Feedback February 2010
The Feedback Target Address presents a significant security risk if
accepted from unauthenticated RTCP packets. See Sections 11.3 and
11.4 for further discussion.
7.1.9. Collision Sub-Report Block
The Collision SSRC sub-report provides the Distribution Source with a
mechanism to report SSRC collisions amongst the group. In the event
that a non-unique SSRC is discovered based on the tuple [SSRC,
CNAME], the collision is reported in this block and the affected
nodes must reselect their respective SSRC identifiers.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRBT=8 | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: Collision SSRC :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Collision SSRC: n * 32 bits
This field contains a list of SSRCs that are duplicated within the
group. Usually this is handled by the hosts upon detection of the
same SSRC; however, since receivers in an SSM session using the
Distribution Source Feedback Summary Model no longer have a global
view of the session, the collision algorithm is handled by the
Distribution Source. SSRCs that collide are listed in the packet.
Each Collision SSRC is reported only once for each collision
detection. If more Collision SSRCs need to be reported than fit
into an MTU, the reporting is done in a round robin fashion so
that all Collision SSRCs have been reported once before the second
round of reporting starts. On receipt of the packet, receiver(s)
MUST detect the collision and select another SSRC, if the
collision pertains to them.
The Collision sub-report block type (SRBT) is 8.
Collision detection is only possible at the Distribution Source. If
the Distribution Source and Feedback Target functions are disjoint
and collision reporting across RTP receiver SSRCs shall be provided,
the Feedback Targets(s) MUST forward the RTCP reports from the RTP
receivers, including at least the RR and the SDES packets to the
Distribution Source.
Ott, et al. Standards Track PAGE 22
RFC 5760 RTCP with Unicast Feedback February 2010
In system settings in which, by explicit configuration or
implementation, RTP receivers are not going to act as Media Senders
in a session (e.g., for various types of television broadcasting),
SSRC collision detection MAY be omitted for RTP receivers. In system
settings in which an RTP receiver MAY become a Media Sender (e.g.,
any conversational application), SSRC collision detection MUST be
provided for RTP receivers.
Note: The purpose of SSRC collision reporting is to ensure unique
identification of RTCP entities. This is of particular relevance
for Media Senders so that an RTP receiver can properly associate
each of the multiple incoming media streams (via the Distribution
Source) with the correct sender. Collision resolution for Media
Senders is not affected by the Distribution Source's collision
reporting because all SR reports are always forwarded among the
senders and to all receivers. Collision resolution for RTP
receivers is of particular relevance for those receivers capable
of becoming a Media Sender. RTP receivers that will, by
configuration or implementation choice, not act as a sender in a
particular RTP session, do not necessarily need to be aware of
collisions as long as the those entities receiving and processing
RTCP feedback messages from the receivers are capable of
disambiguating the various RTCP receivers (e.g., by CNAME).
Note also that RTP receivers should be able to deal with the
changing SSRCs of a Media Sender (like any RTP receiver has to
do.) and, if possible, be prepared to continuously render a media
stream nevertheless.
7.1.10. General Statistics Sub-Report Block
The General Statistics sub-report block is used if specifying buckets
is deemed too complex. With the General Statistics sub-report block,
only aggregated values are reported back. The rules for the
generation of these values are provided in point b of Section 7.2.1.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRBT=10 | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MFL | HCNL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Median inter-arrival jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Ott, et al. Standards Track PAGE 23
RFC 5760 RTCP with Unicast Feedback February 2010
Median fraction lost (MFL): 8 bits
The median fraction lost indicated by Receiver Reports forwarded
to this Distribution Source, expressed as a fixed-point number
with the binary point at the left edge of the field. A value of
all '1's indicates that the MFL value is not provided.
Highest cumulative number of packets lost (HCNL): 24 bits
Highest 'cumulative number of packets lost' value out of the most
recent RTCP RR packets from any of the receivers. A value of all
'1's indicates that the HCNL value is not provided.
Median inter-arrival jitter: 32 bits
Median 'inter-arrival jitter' value out of the most recent RTCP RR
packets from the receiver set. A value of all '1's indicates that
this value is not provided.
The General Statistics sub-report block type (SRBT) is 10.
Note that, in case the Distribution Source and the Feedback Target
functions are disjoint, it is only possible for the Distribution
Source to determine the median of the inter-arrival jitter if all the
Feedback Targets forward all RTCP reports from the receivers
immediately and include at least the RR packet.
7.1.11. RTCP Bandwidth Indication Sub-Report Block
This sub-report block is used to inform the Media Senders and
receivers about either the maximum RTCP bandwidth that is supposed to
be used by each Media Sender or the maximum bandwidth allowance to be
used by each receiver. The value is applied universally to all Media
Senders or all receivers. Each receiver MUST base its RTCP
transmission interval calculation on the average size of the RTCP
packets sent by itself. Conversely, each Media Sender MUST base its
RTCP transmission interval calculation on the average size of the
RTCP packets sent by itself.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRBT=11 | Length |S|R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTCP Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Ott, et al. Standards Track PAGE 24
RFC 5760 RTCP with Unicast Feedback February 2010
Sender (S): 1 bit
The contained bandwidth value applies to each Media Sender.
Receivers (R): 1 bit
The contained bandwidth value applies to each RTP receiver.
Reserved: 14 bits
MUST be set to zero upon transmission and ignored upon reception.
RTCP Bandwidth: 32 bits
If the S bit is set to 1, this field indicates the maximum
bandwidth allocated to each individual Media Sender. This also
informs the receivers about the RTCP report frequency to expect
from the senders. This is a fixed-point value with the binary
point in between the second and third bytes. The value represents
the bandwidth allocation per receiver in kilobits per second, with
values in the range 0 <= BW < 65536.
If the R bit is set to 1, this field indicates the maximum
bandwidth allocated per receiver for sending RTCP data relating to
the session. This is a fixed-point value with the binary point in
between the second and third bytes. The value represents the
bandwidth allocation per receiver in kilobits per second, with
values in the range 0 <= BW < 65536. Each receiver MUST use this
value for its bandwidth allowance.
This report block SHOULD only be used when the Group and Average
Packet Size sub-report block is NOT included. The RTCP Bandwidth
Indication sub-report block type (SRBT) is 11.
7.1.12. RTCP Group and Average Packet Size Sub-Report Block
This sub-report block is used to inform the Media Senders and
receivers about the size of the group (used for calculating feedback
bandwidth allocation) and the average packet size of the group. This
sub-report MUST always be present, appended to every RSI packet,
unless an RTCP Bandwidth Indication sub-report block is included (in
which case it MAY, but need not, be present).
Note: The RTCP Bandwidth Indication sub-report block allows the
Distribution Source to hide the actual group size from the
receivers while still avoiding feedback implosion.
Ott, et al. Standards Track PAGE 25
RFC 5760 RTCP with Unicast Feedback February 2010
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SRBT=12 | Length | Average Packet Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Group Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Group size: 32 bits
This field provides the Distribution Source's view of the number
of receivers in a session. Note that the number of Media Senders
is not explicitly reported since it can be derived by observing
the RTCP SR packets forwarded by the Distribution Source. The
group size is calculated according to the rules outlined in [1].
When this sub-report block is included, this field MUST always be
present.
Average RTCP packet size: 16 bits
This field provides the Distribution Source's view of the average
RTCP packet size as locally calculated, following the rules
defined in [1]. The value is an unsigned integer, counting
octets. When this sub-report block is included, this field MUST
always be present.
The Group and Average Packet Size sub-report block type (SRBT) is 12.
7.2. Distribution Source Behavior
In the Distribution Source Feedback Summary Model, the Distribution
Source (the unicast Feedback Target) MUST listen for unicast RTCP
packets sent to the RTCP port. All RTCP packets received on this
port MUST be processed by the Distribution Source, as described
below. The processing MUST take place per Media Sender SSRC for
which Receiver Reports are received.
The Distribution Source acts like a regular RTCP receiver. In
particular, it receives an RTP stream from each RTP Media Sender(s)
and MUST calculate the proper reception statistics for these RTP
streams. It MUST transmit the resulting information as report blocks
contained in each RTCP packet it sends (in an RR packet).
Note: This information may help to determine the transmission
characteristics of the feed path from the RTP sender to the
Distribution Source (if these are separate entities).
The Distribution Source is responsible for accepting RTCP packets
from the receivers and for interpreting and storing per-receiver
information, as defined in [1]. The importance of providing these
Ott, et al. Standards Track PAGE 26
RFC 5760 RTCP with Unicast Feedback February 2010
functions is apparent when creating the RSI and sub-report block
reports since incorrect information can have serious implications.
Section 11 addresses the security risks in detail.
As defined in [1], all RTCP reports from the Distribution Source MUST
start with an RR report, followed by any relevant SDES fields. Then
the Distribution Source MUST append an RSI header and sub-reports
containing any summarization-specific data. In addition, either the
Group and Average Packet Size sub-report or the Receiver RTCP
Bandwidth sub-report block MUST be appended to the RSI header.
A Distribution Source has the option of masking the group size by
including only an RTCP Bandwidth sub-report. If both sub-reports are
included, the receiver is expected to give precedence to the
information contained in the Receiver RTCP Bandwidth sub-report.
The Receiver RTCP Bandwidth sub-report block provides the
Distribution Source with the capability to control the amount of
feedback from the receivers, and the bandwidth value MAY be increased
or decreased based upon the requirements of the Distribution Source.
Regardless of the value selected by the Distribution Source for the
Receiver RTCP Bandwidth sub-report block, the Distribution Source
MUST continue to forward Sender Reports and RSI packets at the rate
allowed by the total RTCP bandwidth allocation. See Section 9 for
further details about the frequency of reports.
A Distribution Source MAY start out reporting group size and switch
to Receiver RTCP Bandwidth reporting later on and vice versa. If the
Distribution Source does so, it SHOULD ensure that the
correspondingly indicated values for the Receiver RTCP Bandwidth sub-
report roughly match, i.e., are at least the same order of magnitude.
In order to identify SSRC collisions, the Distribution Source is
responsible for maintaining a record of each SSRC and the
corresponding CNAME within at least one reporting interval by the
group, in order to differentiate between clients. It is RECOMMENDED
that an updated list of more than one interval be maintained to
increase accuracy. This mechanism should prevent the possibility of
collisions since the combination of SSRC and CNAME should be unique,
if the CNAME is generated correctly. If collisions are not detected,
the Distribution Source will have an inaccurate impression of the
group size. Since the statistical probability is very low that
collisions will both occur and be undetectable, this should not be a
significant concern. In particular, the clients would have to
randomly select the same SSRC and have the same username + IP address
(e.g., using private address space behind a NAT router).
Ott, et al. Standards Track PAGE 27
RFC 5760 RTCP with Unicast Feedback February 2010
7.2.1. Receiver Report Aggregation
The Distribution Source is responsible for aggregating reception-
quality information received in RR packets. In doing so, the
Distribution Source MUST consider the report blocks received in every
RR packet and MUST NOT consider the report blocks of an SR packet.
Note: the receivers will get the information contained in the SR
packets anyway by packet forwarding, so duplication of this
information should be avoided.
For the optional sub-report blocks, the Distribution Source MAY
decide which are the most significant feedback values to convey and
MAY choose not to include any. The packet format provides
flexibility in the amount of detail conveyed by the data points.
There is a tradeoff between the granularity of the data and the
accuracy of the data based on the multiplicative factor (MF), the
number of buckets, and the min and max values. In order to focus on
a particular region of a distribution, the Distribution Source can
adjust the minimum and maximum values and either increase the number
of buckets, and possibly the factorization, or decrease the number of
buckets and provide more accurate values. See Appendix B for
detailed examples on how to convey a summary of RTCP Receiver Reports
as RSI sub-report block information.
For each value the Distribution Source decides to include in an RSI
packet, it MUST adhere to the following measurement rules.
a) If the Distribution Source intends to use a sub-report to convey
a distribution of values (Sections 7.1.3 to 7.1.7), it MUST keep
per-receiver state, i.e., remember the last value V reported by
the respective receiver. If a new value is reported by a
receiver, the existing value MUST be replaced by the new one.
The value MUST be deleted (along with the entire entry) if the
receiver is timed out (as per Section 6.3.5 of [1]) or has sent a
BYE packet (as per Section 6.3.7 of [1]).
All the values collected in this way MUST be included in the
creation of the subsequent Distribution sub-report block.
The results should correspond as closely as possible to the
values received during the interval since the last report. The
Distribution Source may stack as many sub-report blocks as
required in order to convey different distributions. If the
distribution size exceeds the largest packet length (1008 bytes
data portion), more packets MAY be stacked with additional
information (but in total SHOULD NOT exceed the path MTU).
Ott, et al. Standards Track PAGE 28
RFC 5760 RTCP with Unicast Feedback February 2010
All stacked sub-reports MUST be internally consistent, i.e.,
generated from the same session data. Overlapping of
distributions is therefore allowed, and variation in values
should only occur as a result of data set granularity, with the
more accurate bucket sizes taking precedence in the event that
values differ. Non-divisible values MUST be rounded up or down
to the closest bucket value, and the number of data buckets must
always be an even number, padding where necessary with an
additional zero bucket value to maintain consistency.
Note: This intentionally provides persistent full coverage of the
entire session membership to avoid oscillations due to changing
membership samples.
Scheduling the transmission of summarization reports is left to
the discretion of the Distribution Source. However, the
Distribution Source MUST adhere to the bandwidth limitations for
Receiver Reports as defined for the respective AV profile in use.
b) If the Distribution Source intends to report a short summary
using the General Statistics sub-report block format, defined in
Section 7.1.10, for EACH of the values included in the report
(MFL, HCNL, average inter-arrival jitter), it MUST keep a timer
T_summary as well as a sufficient set of variables to calculate
the summaries for the last three reporting intervals. This MAY
be achieved by keeping per-receiver state (i.e., remember the
last value V reported by the respective receiver) or by
maintaining summary variables for each of these intervals.
The summary values are included here to reflect the current group
situation. By recording the last three reporting intervals, the
Distribution Source incorporates reports from all members while
allowing for individual RTCP Receiver Report packet losses. The
process of collecting these values also aims to avoid resetting
any of the values and then having to send out an RSI report based
upon just a few values collected. If data is available for less
than three reporting intervals (as will be the case for the first
two reports sent), only those values available are considered.
The timer T_summary MUST be initialized as T_summary = 1.5 * Td,
where Td is the deterministic reporting interval, and MUST be
updated following timer reconsideration whenever the group size
or the average RTCP size ("avg_rtcp_size") changes. This choice
should allow each receiver to report once per interval.
Ott, et al. Standards Track PAGE 29
RFC 5760 RTCP with Unicast Feedback February 2010
Td
__^__
t0/ \ t1 t2 t3 t4 t5
---+---------+---------+---------+---------+---------+------->
\____ ____/ : : : :
: V : : : : :
:T_summary: : : : :
:=1.5 * Td: : : : :
\______________ ______________/ : :
: V : : :
: 3 * T_summary : :
: : : :
\______________ ______________/ :
: V :
: 3 * T_summary :
: :
\______________ ______________/
V
3 * T_summary
Figure 2: Overview of Summarization Reporting
Figure 2 depicts how the summarization reporting shall be performed.
At time t3, the RTCP reports collected from t0 through t3 are
included in the RSI reporting; at time t4, those from t1 through t4;
and so on. The RSI summary report sent MUST NOT include any RTCP
report from more than three reporting intervals ago, e.g., the one
sent at time t5, must not include reports received at the
Distribution Source prior to t2.
7.2.2. Handling Other RTCP Packets from RTP Receivers
When following the Feedback Summary Model, the Distribution Source
MAY reflect any other RTCP packets of potential relevance to the
receivers (such as APP, RTPFB, PSFB) to the receiver group. Also, it
MAY decide not to forward other RTCP packets not needed by the
receivers such as BYE, RR, SDES (because the Distribution Source
performs collision resolution), group size estimation, and RR
aggregation. The Distribution Source MUST NOT forward RR packets to
the receiver group.
If the Distribution Source is able to interpret and aggregate
information contained in any RTCP packets other than RR, it MAY
include the aggregated information along with the RSI packet in its
own compound RTCP packet.
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RFC 5760 RTCP with Unicast Feedback February 2010
Aggregation MAY be a null operation, i.e., the Distribution Source
MAY simply append one or more RTCP packets from receivers to the
compound RTCP packet (containing at least RR, SDES, and RSI from the
Distribution Source).
Note: This is likely to be useful only for a few cases, e.g., to
forward aggregated information from RTPFB Generic NACK packets and
thereby maintain the damping property of [15].
Note: This entire processing rule implies that the flow of
information contained in non-RR RTCP packets may terminate at the
Distribution Source, depending on its capabilities and
configuration.
The configuration of the RTCP SSM media session (expressed in SDP)
MUST specify explicitly which processing the Distribution Source will
apply to which RTCP packets. See Section 10.1 for details.
7.2.3. Receiver Report Forwarding
If the Media Sender(s) are not part of the SSM group for RTCP packet
reflection, the Distribution Source MUST explicitly inform the Media
Senders of the receiver group. To achieve this, the Distribution
Source has two options: 1) it forwards the RTCP RR and SDES packets
received from the receivers to the Media Sender(s); or 2) if the
Media Senders also support the RTCP RSI packet, the Distribution
Source sends the RSI packets not just to the receivers but also to
the Media Senders.
If the Distribution Source decides to forward RR and SDES packets
unchanged, it MAY also forward any other RTCP packets to the senders.
If the Distribution Source decides to forward RSI packets to the
senders, the considerations of Section 7.2.2 apply.
7.2.4. Handling Sender Reports
The Distribution Source also receives RTCP (including SR) packets
from the RTP Media Senders.
The Distribution Source MUST forward all RTCP packets from the Media
Senders to the RTP receivers.
If there is more than one Media Sender and these Media Senders do not
communicate via ASM with the Distribution Source and each other, the
Distribution Source MUST forward each RTCP packet from any one Media
Sender to all other Media Senders.
Ott, et al. Standards Track PAGE 31
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7.2.5. RTCP Data Rate Calculation
As noted above, the Distribution Source is a receiver from an RTP
perspective. The Distribution Sources MUST calculate its
deterministic transmission interval Td as every other receiver;
however, it MAY adjust its available data rate depending on the
destination transport address and its local operation:
1. For sending its own RTCP reports to the SSM group towards the
receivers, the Distribution Source MAY use up to the joint share
of all receivers as it is forwarding summaries on behalf of all of
them. Thus, the Distribution Source MAY send its reports up to
every Td/R time units, with R being the number of receivers.
2. For sending its own RTCP reports to the Media Senders only (i.e.,
if the Media Senders are not part of the SSM group), the allocated
rate depends on the operation of the Distribution Source:
a) If the Distribution Source only sends RSI packets along with
its own RTCP RR packets, the same rate calculation applies as
for #1 above.
b) If the Distribution Source forwards RTCP packets from all other
receivers to the Media Senders, then it MUST adhere to the same
bandwidth share for its own transmissions as all other
receivers and use the calculation as per [1].
7.2.6. Collision Resolution
A Distribution Source observing RTP packets from a Media Sender with
an SSRC that collides with its own chosen SSRC MUST change its own
SSRC following the procedures of [1] and MUST do so immediately after
noticing.
A Distribution Source MAY use out-of-band information about the Media
Sender SSRC(s) used in the media session when available to avoid SSRC
collisions with Media Senders. Nevertheless, the Distribution Source
MUST monitor Sender Report (SR) packets to detect any changes,
observe collisions, and then follow the above collision-resolution
procedure.
For collision resolution between the Distribution Source and
receivers or the Feedback Target(s) (if a separate entity, as
described in the next subsection), the Distribution Source and the
Feedback Target (if separate) operate similar to ordinary receivers.
Ott, et al. Standards Track PAGE 32
RFC 5760 RTCP with Unicast Feedback February 2010
7.3. Disjoint Distribution Source and Feedback Target
If the Distribution Source and the Feedback Target are disjoint, the
processing of the Distribution Source is limited by the amount of
RTCP feedback information made available by the Feedback Target.
The Feedback Target(s) MAY simply forward all RTCP packets incoming
from the RTP receivers to the Distribution Source, in which case the
Distribution Source will have all the necessary information available
to perform all the functions described above.
The Feedback Target(s) MAY also perform aggregation of incoming RTCP
packets and send only aggregated information to the Distribution
Source. In this case, the Feedback Target(s) MUST use correctly
formed RTCP packets to communicate with the Distribution Source and
they MUST operate in concert with the Distribution Source so that the
Distribution Source and the Feedback Target(s) appear to be operating
as a single entity. The Feedback Target(s) MUST report their
observed receiver group size to the Distribution Source, either
explicitly by means of RSI packets or implicitly by forwarding all RR
packets.
Note: For example, for detailed statistics reporting, the
Distribution Source and the Feedback Target(s) may need to agree
on a common reporting granularity so that the Distribution Source
can aggregate the buckets incoming from various Feedback Targets
into a coherent report sent to the receivers.
The joint behavior of the Distribution Source and Feedback Target(s)
MUST be reflected in the (SDP-based) media session description as per
Section 7.2.2.
If the Feedback Target performs summarization functions, it MUST also
act as a receiver and choose a unique SSRC for its own reporting
towards the Distribution Source. The collision-resolution
considerations for receivers apply.
7.4. Receiver Behavior
An RTP receiver MUST process RSI packets and adapt session
parameters, such as the RTCP bandwidth, based on the information
received. The receiver no longer has a global view of the session
and will therefore be unable to receive information from individual
receivers aside from itself. However, the information conveyed by
the Distribution Source can be extremely detailed, providing the
receiver with an accurate view of the session quality overall,
without the processing overhead associated with listening to and
analyzing all Receiver Reports.
Ott, et al. Standards Track PAGE 33
RFC 5760 RTCP with Unicast Feedback February 2010
The RTP receiver MUST process the report blocks contained in any RTP
SR and RR packets to complete its view of the RTP session.
The SSRC collision list MUST be checked against the SSRC selected by
the receiver to ensure there are no collisions as MUST be incoming
RTP packets from the Media Senders. A receiver observing RTP packets
from a Media Sender with an SSRC that collides with its own chosen
SSRC MUST change its own SSRC following the procedures of [1]. The
receiver MUST do so immediately after noticing and before sending any
(further) RTCP feedback messages.
A Group and Average Packet Size sub-report block is most likely to be
appended to the RSI header (either a Group Size sub-report or an RTCP
Bandwidth sub-report MUST be included). The group size n allows a
receiver to calculate its share of the RTCP bandwidth, r. Given R,
the total available RTCP bandwidth share for receivers (in the SSM
RTP session) r = R/(n). For the group size calculation, the RTP
receiver MUST NOT include the Distribution Source, i.e., the only RTP
receiver sending RSI packets.
The receiver RTCP bandwidth field MAY override this value. If the
receiver RTCP bandwidth field is present, the receiver MUST use this
value as its own RTCP reporting bandwidth r.
If the RTCP bandwidth field was used by the Distribution Source in an
RTCP session but this field was not included in the last five RTCP
RSI reports, the receiver MUST revert to calculating its bandwidth
share based upon the group size information.
If the receiver has not received any RTCP RSI packets from the
Distribution Source for a period of five times the sender reporting
interval, it MUST cease transmitting RTCP Receiver Reports until the
next RTCP RSI packet is received.
The receiver can use the summarized data as desired. This data is
most useful in providing the receiver with a more global view of the
conditions experienced by other receivers and enables the client to
place itself within the distribution and establish the extent to
which its reported conditions correspond to the group reports as a
whole. Appendix B provides further information and examples of data
processing at the receiver.
The receiver SHOULD assume that any sub-report blocks in the same
packet correspond to the same data set received by the Distribution
Source during the last reporting time interval. This applies to
packets with multiple blocks, where each block conveys a different
range of values.
Ott, et al. Standards Track PAGE 34
RFC 5760 RTCP with Unicast Feedback February 2010
A receiver MUST NOT rely on all of the RTCP packets it sends reaching
the Media Senders or any other receiver. While RR statistics will be
aggregated, BYE packets will be processed, and SSRC collisions will
usually be announced, processing and/or forwarding of further RTCP
packets is up to the discretion of the Distribution Source and will
be performed as specified in the session description.
If a receiver has out-of-band information available about the Media
Sender SSRC(s) used in the media session, it MUST NOT use the same
SSRC for itself. The receiver MUST be aware that such out-of-band
information may be outdated (i.e., that the sender SSRC(s) may have
changed) and MUST follow the above collision-resolution procedure if
necessary.
A receiver MAY use such Media Sender SSRC information when available
but MUST beware of potential changes to the SSRC (which can only be
learned from Sender Report (SR) packets).
7.5. Media Sender Behavior
Media Senders listen on a unicast or multicast transport address for
RTCP reports sent by the receivers (and forwarded by the Distribution
Source) or other Media Senders (optionally forwarded by the
Distribution Source).
Unlike in the case of the simple forwarding model, Media Senders MUST
be able to process RSI packets from the Distribution Source to
determine the group size and their own RTCP bandwidth share. Media
Senders MUST also be capable of determining the group size (and their
corresponding RTCP bandwidth share) from listening to (forwarded)
RTCP RR and SR packets (as mandated in [1]).
As long as they send RTP packets, they MUST also send RTCP SRs, as
defined in [1].
A Media Sender that observes an SSRC collision with another entity
that is not also a Media Sender MAY delay its own collision-
resolution actions, as per [1], by 5 * 1.5 * Td, with Td being the
deterministic calculated reporting interval, for receivers to see
whether the conflict still exists. SSRC collisions with other Media
Senders MUST be acted upon immediately.
Note: This gives precedence to Media Senders and places the burden
of collision resolution on RTP receivers.
Sender SSRC information MAY be communicated out-of-band, e.g., by
means of SDP media descriptions. Therefore, senders SHOULD NOT
change their own SSRC eagerly or unnecessarily.
Ott, et al. Standards Track PAGE 35
RFC 5760 RTCP with Unicast Feedback February 2010
8. Mixer/Translator Issues
The original RTP specification allows a session to use mixers and
translators to help connect heterogeneous networks into one session.
There are a number of issues, however, which are raised by the
unicast feedback model proposed in this document. The term 'mixer'
refers to devices that provide data stream multiplexing where
multiple sources are combined into one stream. Conversely, a
translator does not multiplex streams but simply acts as a bridge
between two distribution mechanisms, e.g., a unicast-to-multicast
network translator. Since the issues raised by this document apply
equally to either a mixer or translator, the latter are referred to
from this point onwards as mixer-translator devices.
A mixer-translator between distribution networks in a session must
ensure that all members in the session receive all the relevant
traffic to enable the usual operation by the clients. A typical use
may be to connect an older implementation of an RTP client with an
SSM distribution network, where the client is not capable of
unicasting feedback to the source. In this instance, the mixer-
translator must join the session on behalf of the client and send and
receive traffic from the session to the client. Certain hybrid
scenarios may have different requirements.
8.1. Use of a Mixer-Translator
The mixer-translator MUST adhere to the SDP description [5] for the
single-source session (Section 11) and use the feedback mechanism
indicated. Implementers of receivers SHOULD be aware that when a
mixer-translator is present in the session, more than one Media
Sender may be active, since the mixer-translator may be forwarding
traffic to the SSM receivers either from multiple unicast sources or
from an ASM session. Receivers SHOULD still forward unicast RTCP
reports in the usual manner to their assigned Feedback
Target/Distribution Source, which in this case -- by assumption --
would be the mixer-translator itself. It is RECOMMENDED that the
simple packet-reflection mechanism be used under these circumstances,
since attempting to coordinate RSI + summarization reporting between
more than one source may be complicated unless the mixer-translator
is capable of summarization.
8.2. Encryption and Authentication Issues
Encryption and security issues are discussed in detail in Section 11.
A mixer-translator MUST be able to follow the same security policy as
the client in order to unicast RTCP feedback to the source, and it
therefore MUST be able to apply the same authentication and/or
encryption policy required for the session. Transparent bridging and
Ott, et al. Standards Track PAGE 36
RFC 5760 RTCP with Unicast Feedback February 2010
subsequent unicast feedback to the source, where the mixer-translator
is not acting as the Distribution Source, is only allowed if the
mixer-translator can conduct the same source authentication as
required by the receivers. A translator MAY forward unicast packets
on behalf of a client but SHOULD NOT translate between multicast-to-
unicast flows towards the source without authenticating the source of
the feedback address information.
9. Transmission Interval Calculation
The Control Traffic Bandwidth referred to in [1] is an arbitrary
amount that is intended to be supplied by a session-management
application (e.g., SDR [21]) or decided based upon the bandwidth of a
single sender in a session.
The RTCP transmission interval calculation either remains the same as
in the original RTP specification [1] or uses the algorithm in [10]
when bandwidth modifiers have been specified for the session.
9.1. Receiver RTCP Transmission
If the Distribution Source is operating in Simple Feedback Model
(which may be indicated in the corresponding session description for
the media session but which the receiver also notices from the
absence of RTCP RSI packets), a receiver MUST calculate the number of
other members in a session based upon its own SSRC count, derived
from the forwarded Sender and Receiver Reports it receives. The
receiver MUST calculate the average RTCP packet size from all the
RTCP packets it receives.
If the Distribution Source is operating in Distribution Source
Feedback Summary Model, the receiver MUST use either the group size
field and the average RTCP packet size field or the Receiver
Bandwidth field from the respective sub-report blocks appended to the
RSI packet.
A receiver uses these values as input to the calculation of the
deterministic calculated interval as per [1] and [10].
9.2. Distribution Source RTCP Transmission
If operating in Simple Feedback Model, the Distribution Source MUST
calculate the transmission interval for its Receiver Reports and
associated RTCP packets, based upon the above control traffic
bandwidth, and MUST count itself as RTP receiver. Receiver Reports
will be forwarded as they arrive without further consideration. The
Distribution Source MAY choose to validate that all or selected
receivers roughly adhere to the calculated bandwidth constraints and
Ott, et al. Standards Track PAGE 37
RFC 5760 RTCP with Unicast Feedback February 2010
MAY choose to drop excess packets for receivers that do not. In all
cases, the average RTCP packet size is determined from the forwarded
Media Senders' and receivers' RTCP packets and from those originated
by the Distribution Source.
If operating in Distribution Source Feedback Summary Model, the
Distribution Source does not share the forward RTCP bandwidth with
any of the receivers. Therefore, the Distribution Source SHOULD use
the full RTCP bandwidth for its Receiver Reports and associated RTCP
packets, as well as RTCP RSI packets. In this case, the average RTCP
packet size is only determined from the RTCP packets originated by
the Distribution Source.
The Distribution Source uses these values as input to the calculation
of the deterministic calculated interval as per [1] and [10].
9.3. Media Senders RTCP Transmission
In Simple Feedback Model, the Media Senders obtain all RTCP SRs and
RRs as they would in an ASM session, except that the packets are
forwarded by the Distribution Source. They MUST perform their RTCP
group size estimate and calculation of the deterministic transmission
interval as per [1] and [10].
In Distribution Source Feedback Summary Model, the Media Senders
obtain all RTCP SRs as they would in an ASM session. They receive
either RTCP RR reports as in ASM (in case these packets are forwarded
by the Distribution Source) or RSI packets containing summaries. In
the former case, they MUST perform their RTCP group size estimate and
calculation of the deterministic transmission interval as per [1] and
[10]. In the latter case, they MUST combine the information obtained
from the Sender Reports and the RSI packets to create a complete view
of the group size and the average RTCP packet size and perform the
calculation of the deterministic transmission interval, as per [1]
and [10], based upon these input values.
9.4. Operation in Conjunction with AVPF and SAVPF
If the RTP session is an AVPF session [15] or an SAVPF session [28]
(as opposed to a regular AVP [6] session), the receivers MAY modify
their RTCP report scheduling, as defined in [15]. Use of AVPF or
SAVPF does not affect the Distribution Source's RTCP transmission or
forwarding behavior.
It is RECOMMENDED that a Distribution Source and possible separate
Feedback Target(s) be configured to forward AVPF/SAVPF-specific RTCP
packets in order to not counteract the damping mechanism built into
AVPF; optionally, they MAY aggregate the feedback information from
Ott, et al. Standards Track PAGE 38
RFC 5760 RTCP with Unicast Feedback February 2010
the receivers as per Section 7.2.2. If only generic feedback packets
that are understood by the Distribution Source and that can easily be
aggregated are in use, the Distribution MAY combine several incoming
RTCP feedback packets and forward the aggregate along with its next
RTCP RR/RSI packet. In any case, the Distribution Source and
Feedback Target(s) SHOULD minimize the extra delay when forwarding
feedback information, but the Distribution Source MUST stay within
its RTCP bandwidth constraints.
In the event that specific APP packets without a format and
summarization mechanism understood by the Feedback Target(s) and/or
the Distribution Source are to be used, it is RECOMMENDED that such
packets are forwarded with minimal delay. Otherwise, the capability
of the receiver to send timely feedback messages is likely to be
affected.
10. SDP Extensions
The Session Description Protocol (SDP) [5] is used as a means to
describe media sessions in terms of their transport addresses,
codecs, and other attributes. Signaling that RTCP feedback will be
provided via unicast, as specified in this document, requires another
session parameter in the session description. Similarly, other SSM
RTCP feedback parameters need to be provided, such as the
summarization model at the sender and the target unicast address to
which to send feedback information. This section defines the SDP
parameters that are needed by the proposed mechanisms in this
document (and that have been registered with IANA).
10.1. SSM RTCP Session Identification
A new session-level attribute MUST be used to indicate the use of
unicast instead of multicast feedback: "rtcp-unicast".
This attribute uses one parameter to specify the model of operation.
An optional set of parameters specifies the behavior for RTCP packet
types (and subtypes).
rtcp-unicast:reflection
This attribute MUST be used to indicate the "Simple Feedback"
model of operation where packet reflection is used by the
Distribution Source (without further processing).
Ott, et al. Standards Track PAGE 39
RFC 5760 RTCP with Unicast Feedback February 2010
rtcp-unicast:rsi *(SP <processing>:<rtcp-type>])
This attribute MUST be used to indicate the "Distribution Source
Feedback Summary" model of operation. In this model, a list or
parameters may be used to explicitly specify how RTCP packets
originated by receivers are handled. Options for processing a
given RTCP packet type are:
aggr: The Distribution Source has means for aggregating the
contents of the RTCP packets and will do so.
forward: The Distribution Source will forward the RTCP packet
unchanged.
term: The Distribution Source will terminate the RTCP packet.
The default rules applying if no parameters are specified are as
follows:
RR and SDES packets MUST be aggregated and MUST lead to RSI
packets being generated. All other RTP packets MUST be terminated
at the Distribution Source (or Feedback Target(s)).
The SDP description needs only to specify deviations from the
default rules. Aggregation of RR packets and forwarding of SR
packets MUST NOT be changed.
The token for the new SDP attribute is "rtcp-unicast" and the formal
SDP ABNF syntax for the new attribute value is as follows:
att-value = "reflection"
/ "rsi" *(SP rsi-rule)
rsi-rule = processing ":" rtcp-type
processing = "aggr" / "forward" / "term" / token ;keep it extensible
rtcp-type = 3*3DIGIT ;the RTCP type (192, 193, 202--209)
10.2. SSM Source Specification
In a Source-Specific Multicast RTCP session, the address of the
Distribution Source needs to be indicated both for source-specific
joins to the multicast group and for addressing unicast RTCP packets
on the backchannel from receivers to the Distribution Source.
Ott, et al. Standards Track PAGE 40
RFC 5760 RTCP with Unicast Feedback February 2010
This is achieved by following the proposal for SDP source filters
documented in [4]. According to the specification, only the
inclusion model ("a=source-filter:incl") MUST be used for SSM RTCP.
There SHOULD be exactly one "a=source-filter:incl" attribute listing
the address of the Distribution Source. The RTCP port MUST be
derived from the m= line of the media description.
An alternative Feedback Target Address and port MAY be supplied using
the SDP RTCP attribute [7], e.g., a=rtcp:<port> IN IP4 192.0.2.1.
This attribute only defines the transport address of the Feedback
Target and does not affect the SSM group specification for media
stream reception.
Two "source-filter" attributes MAY be present to indicate an IPv4 and
an IPv6 representation of the same Distribution Source.
10.3. RTP Source Identification
The SSRC information for the Media Sender(s) MAY be communicated
explicitly out of band (i.e., outside the RTP session). One option
for doing so is the Session Description Protocol (SDP) [5]. If such
an indication is desired, the "ssrc" attribute [12] MUST be used for
this purpose. As per [12], the "cname" Source Attribute MUST be
present. Further Source Attributes MAY be specified as needed.
If used in an SDP session description of an RTCP-SSM session, the
ssrc MUST contain the SSRC intended to be used by the respective
Media Sender and the cname MUST equal the CNAME for the Media Sender.
If present, the role SHOULD indicate the function of the RTP entity
identified by this line; presently, only the "media-sender" role is
defined.
Example:
a=ssrc:314159 cname:iptv-sender@example.com
In the above example, the Media Sender is identified to use the SSRC
identifier 314159 and the CNAME iptv-sender@example.com.
11. Security Considerations
The level of security provided by the current RTP/RTCP model MUST NOT
be diminished by the introduction of unicast feedback to the source.
This section identifies the security weaknesses introduced by the
feedback mechanism, potential threats, and level of protection that
MUST be adopted. Any suggestions on increasing the level of security
Ott, et al. Standards Track PAGE 41
RFC 5760 RTCP with Unicast Feedback February 2010
provided to RTP sessions above the current standard are RECOMMENDED
but OPTIONAL. The final section outlines some security frameworks
that are suitable to conform to this specification.
11.1. Assumptions
RTP/RTCP is a protocol that carries real-time multimedia traffic, and
therefore a main requirement is for any security framework to
maintain as low overhead as possible. This includes the overhead of
different applications and types of cryptographic operations as well
as the overhead to deploy or to create security infrastructure for
large groups.
Although the distribution of session parameters (typically encoded as
SDP objects) through the Session Announcement Protocol (SAP) [22],
email, or the web is beyond the scope of this document, it is
RECOMMENDED that the distribution method employs adequate security
measures to ensure the integrity and authenticity of the information.
Suitable solutions that meet the security requirements outlined here
are included at the end of this section.
In practice, the multicast and group distribution mechanism, e.g.,
the SSM routing tree, is not immune to source IP address spoofing or
traffic snooping; however, such concerns are not discussed here. In
all the following discussions, security weaknesses are addressed from
the transport level or above.
11.2. Security Threats
Attacks on media distribution and the feedback architecture proposed
in this document may take a variety of forms. A detailed outline of
the types of attacks follows:
a) Denial of Service (DoS)
DoS is a major area of concern. Due to the nature of the
communication architecture, a DoS can be generated in a number of
ways by using the unicast feedback channel to the attacker's
advantage.
b) Packet Forgery
Another potential area for attack is packet forgery. In
particular, it is essential to protect the integrity of certain
influential packets since compromise could directly affect the
transmission characteristics of the whole group.
Ott, et al. Standards Track PAGE 42
RFC 5760 RTCP with Unicast Feedback February 2010
c) Session Replay
The potential for session recording and subsequent replay is an
additional concern. An attacker may not actually need to
understand packet content but simply have the ability to record
the data stream and, at a later time, replay it to any receivers
that are listening.
d) Eavesdropping on a Session
The consequences of an attacker eavesdropping on a session already
constitutes a security weakness; in addition, eavesdropping might
facilitate other types of attacks and is therefore considered a
potential threat. For example, an attacker might be able to use
the eavesdropped information to perform an intelligent DoS attack.
11.3. Architectural Contexts
To better understand the requirements of the solution, the threats
outlined above are addressed for each of the three communication
contexts:
a) Source-to-Receiver Communication
The downstream communication channel, from the source to the
receivers, is critical to protect since it controls the behavior
of the group; it conveys the bandwidth allocation for each
receiver, and hence the rate at which the RTCP traffic is unicast,
directly back to the source. All traffic that is distributed over
the downstream channel is generated by a single source. Both the
RTP data stream and the RTCP control data from the source are
included in this context, with the RTCP data generated by the
source being indirectly influenced by the group feedback.
The downstream channel is vulnerable to the four types of attack
outlined above. The denial of service attack is possible but less
of a concern than the other types. The worst case effect of DoS
would be the transmission of large volumes of traffic over the
distribution channel, with the potential to reach every receiver
but only on a one-to-one basis. Consequently, this threat is no
more pronounced than the current multicast ASM model. The real
danger of denial of service attacks in this context comes
indirectly via compromise of the source RTCP traffic. If
receivers are provided with an incorrect group size estimate or
bandwidth allowance, the return traffic to the source may create a
distributed DoS effect on the source. Similarly, an incorrect
feedback address -- whether as a result of a malicious attack or
Ott, et al. Standards Track PAGE 43
RFC 5760 RTCP with Unicast Feedback February 2010
by mistake, e.g., an IP address configuration error (e.g., typing)
-- could directly create a denial of service attack on another
host.
An additional concern relating to Denial of service attacks would
come indirectly through the generation of fake BYE packets,
causing the source to adjust the advertised group size. A
Distribution Source MUST follow the correct rules for timing out
members in a session prior to reporting a change in the group
size, which allows the authentic SSRC sufficient time to continue
to report and, consequently, cancel the fake BYE report.
The danger of packet forgery in the worst case may be to
maliciously instigate a denial of service attack, e.g., if an
attacker were capable of spoofing the source address and injecting
incorrect packets into the data stream or intercepting the source
RTCP traffic and modifying the fields.
The replay of a session would have the effect of recreating the
receiver feedback to the source address at a time when the source
is not expecting additional traffic from a potentially large
group. The consequence of this type of attack may be less
effective on its own, but in combination with other attacks might
be serious.
Eavesdropping on the session would provide an attacker with
information on the characteristics of the source-to-receiver
traffic, such as the frequency of RTCP traffic. If RTCP traffic
is unencrypted, this might also provide valuable information on
characteristics such as group size, Media Source SSRC(s), and
transmission characteristics of the receivers back to the source.
b) Receiver-to-Distribution-Source Communication
The second context is the return traffic from the group to the
Distribution Source. This traffic should only consist of RTCP
packets and should include Receiver Reports, SDES information, BYE
reports, extended reports (XR), feedback messages (RTPFB, PSFB)
and possibly application-specific packets. The effects of
compromise on a single or subset of receivers are not likely to
have as great an impact as in context (a); however, much of the
responsibility for detecting compromise of the source data stream
relies on the receivers.
The effects of compromise of critical Distribution Source control
information can be seriously amplified in the present context. A
large group of receivers may unwittingly generate a distributed
Ott, et al. Standards Track PAGE 44
RFC 5760 RTCP with Unicast Feedback February 2010
DoS attack on the Distribution Source in the event that the
integrity of the source RTCP channel has been compromised and the
compromise is not detected by the individual receivers.
An attacker capable of instigating a packet forgery attack could
introduce false RTCP traffic and create fake SSRC identifiers.
Such attacks might slow down the overall control channel data rate
since an incorrect perception of the group size may be created.
Similarly, the creation of fake BYE reports by an attacker would
cause some group size instability, but should not be effective as
long as the correct timeout rules are applied by the source in
removing SSRC entries from its database.
A replay attack on receiver return data to the source would have
the same implications as the generation of false SSRC identifiers
and RTCP traffic to the source. Therefore, ensuring authenticity
and freshness of the data source is important.
Eavesdropping in this context potentially provides an attacker
with a great deal of potentially personal information about a
large group of receivers available from SDES packets. It would
also provide an attacker with information on group traffic-
generation characteristics and parameters for calculating
individual receiver bandwidth allowances.
c) Receiver-to-Feedback-Target Communication
The third context is the return traffic from the group to the
Feedback Target. It suffers from the same threats as the
receiver-to-source context, with the difference being that now a
large group of receivers may unwittingly generate a distributed
DoS (DDos) attack on the Feedback Target, where it is impossible
to discern if the DDoS is deliberate or due merely to a
misconfiguration of the Feedback Target Address. While deliberate
attacks can be mitigated by properly authenticating messages that
communicate the Feedback Target Address (i.e., the SDP session
description and the Feedback Target Address sub-report block
carried in RTCP), a misconfigured address will originate from an
authenticated source and hence cannot be prevented using security
mechanisms.
Furthermore, the Feedback Target is unable to communicate its
predicament with either the Distribution Source or the session
receivers. From the feedback packets received, the Feedback
Target cannot tell either which SSM multicast group the feedback
belongs to or the Distribution Source, making further analysis and
suppression difficult. The Feedback Target may not even support
RTCP or listen on the port number in question.
Ott, et al. Standards Track PAGE 45
RFC 5760 RTCP with Unicast Feedback February 2010
Note that because the DDoS occurs inside of the RTCP session and
because the unicast receivers adhere to transmission interval
calculations ([1], [10]), the bandwidth misdirected toward the
Feedback Target in the misconfigured case will be limited to a
percentage of the session bandwidth, i.e., the Control Traffic
Bandwidth established for the session.
11.4. Requirements in Each Context
To address these threats, this section presents the security
requirements for each context.
a) The main threat in the source-to-receiver context concerns denial
of service attacks through possible packet forgery. The forgery
may take the form of interception and modification of packets from
the source, or it may simply inject false packets into the
distribution channel. To combat these attacks, data integrity and
source authenticity MUST be applied to source traffic. Since the
consequences of eavesdropping do not affect the operation of the
protocol, confidentiality is not a requirement in this context.
However, without confidentiality, access to personal and group
characteristics information would be unrestricted to an external
listener. Therefore, confidentiality is RECOMMENDED.
b) The threats in the receiver-to-source context concern the same
kinds of attacks, but are considered less important than the
downstream traffic compromise. All the security weaknesses are
also applicable to the current RTP/RTCP security model, and
therefore only recommendations towards protection from compromise
are made. Data integrity is RECOMMENDED to ensure that
interception and modification of an individual receiver's RTCP
traffic can be detected. This would protect against the false
influence of group control information and the potentially more
serious compromise of future services provided by the distribution
functionality. In order to ensure security, data integrity and
authenticity of receiver traffic is therefore also RECOMMENDED.
With respect to data confidentiality, the same situation applies
as in the first context, and it is RECOMMENDED that precautions be
taken to protect the privacy of the data.
c) The threats to the receiver-to-feedback-target context are similar
to those in the receiver-to-source context, and thus the
recommendations to protect against them are similar.
However, there are a couple situations with broader issues to
solve, which are beyond the scope of this document.
Ott, et al. Standards Track PAGE 46
RFC 5760 RTCP with Unicast Feedback February 2010
1. An endpoint experiencing DDoS or the side effects of a
misconfigured RTCP session may not even be a participant in the
session, i.e., may not be listening on the respective port
number and may even support RTCP, so it will be unable to react
within RTCP. Determining that there is a problem will be up to
network administrators and, possibly, anti-malware software
that can perform correlation across receiver nodes.
2. With misconfiguration, unfortunately the normally desirable
usage of SRTP and SRTCP becomes undesirable. Because the
packet content is encrypted, neither the misconfigured Feedback
Target nor the network administrator have the ability to
determine the root cause of the traffic.
In the case where the misconfigured Feedback Target happens to be
a node participating in the session or is an RTCP-enabled node,
the Feedback Target Address block provides a dynamic mechanism for
the Distribution Source to signal an alternative unicast RTCP
feedback address to the receivers. As this type of packet MUST be
included in every RTCP packet originated by the Distribution
Source, all receivers would be able to obtain the corrected
Feedback Target information. In this manner, receiver behavior
should remain consistent even in the face of packet loss or when
there are late-session arrivals. The only caveat is that the
misconfigured Feedback Target is largely uninvolved in the repair
of this situation and thus relies on others for the detection of
the problem.
An additional security consideration, which is not a component of
this specification but which has a direct influence upon the general
security, is the origin of the session-initiation data. This
involves the SDP parameters that are communicated to the members
prior to the start of the session via channels such as an HTTP
server, email, SAP, or other means. It is beyond the scope of this
document to place any strict requirements on the external
communication of such information; however, suitably secure SDP
communication approaches are outlined in Section 11.7.
11.5. Discussion of Trust Models
As identified in the previous sections, source authenticity is a
fundamental requirement of the protocol. However, it is important to
also clarify the model of trust that would be acceptable to achieve
this requirement. There are two fundamental models that apply in
this instance:
Ott, et al. Standards Track PAGE 47
RFC 5760 RTCP with Unicast Feedback February 2010
a) The shared-key model, where all authorized group members share the
same key and can equally encrypt/decrypt the data. This method
assumes that an out-of-band method is applied to the distribution
of the shared group key, ensuring that every key-holder is
individually authorized to receive the key and, in the event of
member departures from the group, a re-keying exercise can occur.
The advantage of this model is that the costly processing
associated with one-way key-authentication techniques is avoided,
as well as the need to execute additional cipher operations with
alternative key sets on the same data set, e.g., in the event that
data confidentiality is also applied. The disadvantage is that,
for very large groups where the receiver set becomes effectively
untrusted, a shared key does not offer much protection.
b) The public-key authentication model, using cryptosystems such as
RSA-based or PGP authentication, provides a more secure method of
source authentication at the expense of generating higher
processing overhead. This is typically not recommended for real-
time data streams but, in the case of RTCP reports, which are
distributed with a minimum interval of 5 seconds, this may be a
viable option (the processing overhead might still be too great
for small, low-powered devices and should therefore be considered
with caution). Wherever possible, however, the use of public key
source authentication is preferable to the shared key model
identified above.
As concerns requirements for protocol acceptability, either model is
acceptable although it is RECOMMENDED that the more secure public-
key-based options be applied wherever possible.
11.6. Recommended Security Solutions
This section presents some existing security mechanisms that are
RECOMMENDED to suitably address the requirements outlined in Section
11.5. This is only intended as a guideline and it is acknowledged
that there are other solutions that would also be suitable to comply
with the specification.
11.6.1. Secure Distribution of SDP Parameters
a) SAP, HTTPS, Email -- Initial distribution of the SDP parameters
for the session SHOULD use a secure mechanism such as the SAP
authentication framework, which allows an authentication
certificate to be attached to the session announcements. Other
methods might involve HTTPS or signed email content from a trusted
source. However, some more commonly used techniques for
distributing session information and starting media streams are
the Real-Time Streaming Protocol (RTSP) [25] and SIP [14].
Ott, et al. Standards Track PAGE 48
RFC 5760 RTCP with Unicast Feedback February 2010
b) RTSP -- RTSP provides a client- or server-initiated stream control
mechanism for real-time multimedia streams. The session
parameters are conveyed using SDP syntax and may adopt standard
HTTP authentication mechanisms in combination with suitable
network (e.g., IPsec)- or transport (e.g., Transport Layer
Security (TLS))-level security.
c) SIP -- A typical use of SIP involving a unicast feedback
identifier might be a client wishing to dynamically join a multi-
party call on a multicast address using unicast RTCP feedback.
The client would be required to authenticate the SDP session
descriptor information returned by the SIP server. The
recommended method for this, as outlined in the SIP specification
[14], is to use an S/MIME message body containing the session
parameters signed with an acceptable certificate.
For the purposes of this profile, it is acceptable to use any
suitably secure authentication mechanism that establishes the
identity and integrity of the information provided to the client.
11.6.2. Suitable Security Solutions for RTP Sessions with Unicast
Feedback
a) SRTP -- SRTP [3] is the recommended Audio/Video Transport (AVT)
security framework for RTP sessions. It specifies the general
packet formats and cipher operations that are used and provides
the flexibility to select different stream ciphers based on
preference/requirements. It can provide confidentiality of the
RTP and RTCP packets as well as protection against integrity
compromise and replay attacks. It provides authentication of the
data stream using the shared-key trust model. Any suitable key-
distribution mechanism can be used in parallel to the SRTP
streams.
b) IPSEC -- A more general group security profile that might be used
is the Group Domain of Interpretation [23], which defines the
process of applying IPSec mechanisms to multicast groups. This
requires the use of the Encapsulating Security Payload (ESP) in
tunnel mode as the framework and it provides the capability to
authenticate -- either using a shared key or individually through
public-key mechanisms. It should be noted that using IPSec would
break the 'transport-independent' condition of RTP and would
therefore not be useable for anything other than IP-based
communication.
c) TESLA - Timed Efficient Stream Loss-Tolerant Authentication
(TESLA) [24] is a scheme that provides a more flexible approach to
data authentication using time-based key disclosure. The
Ott, et al. Standards Track PAGE 49
RFC 5760 RTCP with Unicast Feedback February 2010
authentication uses one-way, pseudo-random key functions based on
key chain hashes that have a short period of authenticity based on
the key disclosure intervals from the source. As long as the
receiver can ensure that the encrypted packet is received prior to
the key disclosure by the source, which requires loose time
synchronization between source and receivers, it can prove the
authenticity of the packet. The scheme does introduce a delay
into the packet distribution/decryption phase due to the key
disclosure delay; however, the processing overhead is much lower
than other standard public-key mechanisms and therefore may be
more suited to small or energy-restricted devices.
11.6.3. Secure Key Distribution Mechanisms
a) MIKEY -- Multimedia Internet KEYing (MIKEY) [29] is the preferred
solution for SRTP sessions providing a shared group-key
distribution mechanism and intra-session rekeying facilities. If
a partly protected communication channel exists, keys may also be
conveyed using SDP as per [27].
b) GSAKMP -- The Group Secure Association Key Management Protocol
(GSAKMP) is the general solution favored for Multicast Secure
group-key distribution. It is the recommended key distribution
solution for Group Domain of Interpretation (GDOI) [RFC 3547]
sessions.
11.7. Troubleshooting Misconfiguration
As noted above, the security mechanisms in place will not help in
case an authorized source spreads properly authenticated and
integrity-protected yet incorrect information about the Feedback
Target. In this case, the accidentally communicated Feedback Target
will receive RTCP packets from a potentially large group of receivers
-- the RTCP rate fortunately limited by the RTCP timing rules.
Yet, the RTCP packets do not provide much context information and, if
encrypted, do not provide any context, making it difficult for the
entity running (the network with) the Feedback Target to debug and
correct this problem, e.g., by tracking down and informing the origin
of the misconfiguration.
One suitable approach may be to provide explicit context information
in RTCP packets that would allow determining the source. While such
an RTCP packet could be defined in this specification, it would be of
no use when using SRTP/SRTCP and encryption of RTCP reports.
Therefore, and because the extensions in this document may not be the
Ott, et al. Standards Track PAGE 50
RFC 5760 RTCP with Unicast Feedback February 2010
only case that may face such a problem, it is desirable to find a
solution that is applicable to RTP at large. Such mechanisms are for
further study in the AVT WG.
12. Backwards Compatibility
The use of unicast feedback to the source should not present any
serious backwards compatibility issues. The RTP data streams should
remain unaffected, as should the RTCP packets from the Media
Sender(s) that continue to enable inter-stream synchronization in the
case of multiple streams. The unicast transmission of RTCP data to a
source that does not have the ability to redistribute the traffic
either by simple reflection or through summaries could have serious
security implications, as outlined in Section 11, but would not
actually break the operation of RTP. For RTP-compliant receivers
that do not understand the unicast mechanisms, the RTCP traffic may
still reach the group in the event that an ASM distribution network
is used, in which case there may be some duplication of traffic due
to the reflection channel, but this should be ignored. It is
anticipated, however, that typically the distribution network will
not enable the receiver to multicast RTCP traffic, in which case the
data will be lost and the RTCP calculations will not include those
receivers. It is RECOMMENDED that any session that may involve non-
unicast-capable clients should always use the simple packet-
reflection mechanism to ensure that the packets received can be
understood by all clients.
13. IANA Considerations
The following contact information shall be used for all registrations
included here:
Contact: Joerg Ott
mail: jo@acm.org
tel: +358-9-470-22460
Based on the guidelines suggested in [2], a new RTCP packet format
has been registered with the RTCP Control Packet type (PT) Registry:
Name: RSI
Long name: Receiver Summary Information
Value: 209
Reference: This document.
This document defines a substructure for RTCP RSI packets. A new
sub-registry has been set up for the sub-report block type (SRBT)
values for the RSI packet, with the following registrations created
initially:
Ott, et al. Standards Track PAGE 51
RFC 5760 RTCP with Unicast Feedback February 2010
Name: IPv4 Address
Long name: IPv4 Feedback Target Address
Value: 0
Reference: This document.
Name: IPv6 Address
Long name: IPv6 Feedback Target Address
Value: 1
Reference: This document.
Name: DNS Name
Long name: DNS Name indicating Feedback Target Address
Value: 2
Reference: This document.
Name: Loss
Long name: Loss distribution
Value: 4
Reference: This document.
Name: Jitter
Long name: Jitter Distribution
Value: 5
Reference: This document.
Name: RTT
Long name: Round-trip time distribution
Value: 6
Reference: This document.
Name: Cumulative loss
Long name: Cumulative loss distribution
Value: 7
Reference: This document.
Name: Collisions
Long name: SSRC Collision list
Value: 8
Reference: This document.
Name: Stats
Long name: General statistics
Value: 10
Reference: This document.
Ott, et al. Standards Track PAGE 52
RFC 5760 RTCP with Unicast Feedback February 2010
Name: RTCP BW
Long name: RTCP Bandwidth indication
Value: 11
Reference: This document.
Name: Group Info
Long name: RTCP Group and Average Packet size
Value: 12
Reference: This document.
The value 3 shall be reserved as a further way of specifying a
Feedback Target Address. The value 3 MUST only be allocated for a
use defined in an IETF Standards Track document.
Further values may be registered on a first come, first served
basis. For each new registration, it is mandatory that a
permanent, stable, and publicly accessible document exists that
specifies the semantics of the registered parameter as well as the
syntax and semantics of the associated sub-report block. The
general registration procedures of [5] apply.
In the registry for SDP parameters, the attribute named "rtcp-
unicast" has been registered as follows:
SDP Attribute ("att-field"):
Attribute Name: rtcp-unicast
Long form: RTCP Unicast feedback address
Type of name: att-field
Type of attribute: Media level only
Subject to charset: No
Purpose: RFC 5760
Reference: RFC 5760
Values: See this document.
14. References
14.1. Normative References
[1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[2] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
Ott, et al. Standards Track PAGE 53
RFC 5760 RTCP with Unicast Feedback February 2010
[3] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
3711, March 2004.
[4] Quinn, B. and R. Finlayson, "Session Description Protocol (SDP)
Source Filters", RFC 4570, July 2006.
[5] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[6] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
[7] Huitema, C., "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October 2003.
[8] Meyer, D., Rockell, R., and G. Shepherd, "Source-Specific
Protocol Independent Multicast in 232/8", BCP 120, RFC 4608,
August 2006.
[9] 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.
[10] Casner, S., "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556,
July 2003.
[11] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
63, RFC 3629, November 2003.
[12] Lennox, J., Ott, J., and T. Schierl, "Source-Specific Media
Attributes in the Session Description Protocol (SDP)", RFC 5576,
June 2009.
[13] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
14.2. Informative References
[14] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[15] 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.
Ott, et al. Standards Track PAGE 54
RFC 5760 RTCP with Unicast Feedback February 2010
[16] Pusateri, T., "Distance Vector Multicast Routing Protocol", Work
in Progress, October 2003.
[17] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol
Specification (Revised)", RFC 4601, August 2006.
[18] Adams, A., Nicholas, J., and W. Siadak, "Protocol Independent
Multicast - Dense Mode (PIM-DM): Protocol Specification
(Revised)", RFC 3973, January 2005.
[19] Bates, T., Chandra, R., Katz, D., and Y. Rekhter, "Multiprotocol
Extensions for BGP-4", RFC 4760, January 2007.
[20] Fenner, B., Ed., and D. Meyer, Ed., "Multicast Source Discovery
Protocol (MSDP)", RFC 3618, October 2003.
[21] Session Directory Rendez-vous (SDR), developed at University
College London by Mark Handley and the Multimedia Research
Group, http://www-mice.cs.ucl.ac.uk/multimedia/software/sdr/.
[22] Handley, M., Perkins, C., and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
[23] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The Group
Domain of Interpretation", RFC 3547, July 2003.
[24] Perrig, A., Song, D., Canetti, R., Tygar, J., and B. Briscoe,
"Timed Efficient Stream Loss-Tolerant Authentication (TESLA):
Multicast Source Authentication Transform Introduction", RFC
4082, June 2005.
[25] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[26] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed., "RTP
Control Protocol Extended Reports (RTCP XR)", RFC 3611, November
2003.
[27] Andreasen, F., Baugher, M., and D. Wing, "Session Description
Protocol (SDP) Security Descriptions for Media Streams", RFC
4568, July 2006.
[28] Ott, J. and E. Carrara, "Extended Secure RTP Profile for Real-
time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, February 2008.
Ott, et al. Standards Track PAGE 55
RFC 5760 RTCP with Unicast Feedback February 2010
[29] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, August
2004.
Ott, et al. Standards Track PAGE 56
RFC 5760 RTCP with Unicast Feedback February 2010
Appendix A. Examples for Sender-Side Configurations
This appendix describes a few common setups, focusing on the
contribution side, i.e., the communications between the Media
Sender(s) and the Distribution Source. In all cases, the same
session description may be used for the distribution side as defined
in the main part of this document. This is because this
specification defines only the media stream setup between the
Distribution Source and the receivers.
A.1. One Media Sender Identical to the Distribution Source
In the simplest case, the Distribution Source is identical to the
Media Sender as depicted in Figure 3. Obviously, no further
configuration for the interaction between the Media Sender and the
Distribution Source is necessary.
Source-specific
+--------+ Multicast
| | +----------------> R(1)
|M D S | | |
|E I O | +--+ |
|D S U | | | |
|I T R | | +-----------> R(2) |
|A R C |->+----- : | |
| = I E | | +------> R(n-1) | |
|S B | | | | | |
|E U | +--+--> R(n) | | |
|N T | | | | |
|D I |<---------+ | | |
|E O |<---------------+ | |
|R N |<--------------------+ |
| |<-------------------------+
+--------+ Unicast
Figure 3: Media Source == Distribution Source
A.2. One Media Sender
In a slightly more complex scenario, the Media Sender and the
Distribution Source are separate entities running on the same or
different machines. Hence, the Media Sender needs to deliver the
media stream(s) to the Distribution Source. This can be done either
via unicasting the RTP stream, via ASM multicast, or via SSM. In
this case, the Distribution Source is responsible for forwarding the
RTP packets comprising the media stream and the RTCP Sender Reports
towards the receivers and conveying feedback from the receivers, as
well as from itself, to the Media Sender.
Ott, et al. Standards Track PAGE 57
RFC 5760 RTCP with Unicast Feedback February 2010
This scenario is depicted in Figure 4. The communication setup
between the Media Sender and the Distribution Source may be
statically configured or SDP may be used in conjunction with some
signaling protocol to convey the session parameters. Note that it is
a local configuration matter of the Distribution Source how to
associate a session between the Media Sender and itself (the
Contribution session) with the corresponding session between itself
and the receivers (the Distribution session).
Source-specific
+-----+ Multicast
| | +----------------> R(1)
| D S | | |
| I O | +--+ |
| S U | | | |
+--------+ | T R | | +-----------> R(2) |
| Media |<---->| R C |->+----- : | |
| Sender | | I E | | +------> R(n-1) | |
+--------+ | B | | | | | |
| U | +--+--> R(n) | | |
| T | | | | |
| I |<---------+ | | |
| O |<---------------+ | |
| N |<--------------------+ |
| |<-------------------------+
+-----+ Unicast
Contribution Distribution
Session Session
(unicast or ASM) (SSM)
Figure 4: One Media Sender Separate from Distribution Source
A.3. Three Media Senders, Unicast Contribution
Similar considerations apply if three Media Senders transmit to an
SSM multicast group via the Distribution Source and individually send
their media stream RTP packets via unicast to the Distribution
Source.
In this case, the responsibilities of the Distribution Source are a
superset to the previous case; the Distribution Source also needs to
relay media traffic from each Media Sender to the receivers and to
forward (aggregated) feedback from the receivers to the Media
Senders. In addition, the Distribution Source relays RTCP packets
(SRs) from each Media Sender to the other two.
Ott, et al. Standards Track PAGE 58
RFC 5760 RTCP with Unicast Feedback February 2010
The configuration of the Media Senders is identical to the previous
case. It is just the Distribution Source that must be aware that
there are multiple senders and then perform the necessary relaying.
The transport address for the RTP session at the Distribution Source
may be identical for all Media Senders (which may make correlation
easier) or different addresses may be used.
This setup is depicted in Figure 5.
Source-specific
+-----+ Multicast
+--------+ | | +----------------> R(1)
| Media |<---->| D S | | |
|Sender 1| | I O | +--+ |
+--------+ | S U | | | |
| T R | | +-----------> R(2) |
+--------+ | R C |->+----- : | |
| Media |<---->| I E | | +------> R(n-1) | |
|Sender 2| | B | | | | | |
+--------+ | U | +--+--> R(n) | | |
| T | | | | |
+--------+ | I |<---------+ | | |
| Media |<---->| O |<---------------+ | |
|Sender 3| | N |<--------------------+ |
+--------+ | |<-------------------------+
+-----+ Unicast
Contribution Distribution
Session Session
(unicast) (SSM)
Figure 5: Three Media Senders, Unicast Contribution
A.4. Three Media Senders, ASM Contribution Group
In this final example, the individual unicast contribution sessions
between the Media Senders and the Distribution Source are replaced by
a single ASM contribution group (i.e., a single common RTP session).
Consequently, all Media Senders receive each other's traffic by means
of IP-layer multicast and the Distribution Source no longer needs to
perform explicit forwarding between the Media Senders. Of course,
the Distribution Source still forwards feedback information received
from the receivers (optionally as summaries) to the ASM contribution
group.
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RFC 5760 RTCP with Unicast Feedback February 2010
The ASM contribution group may be statically configured or the
necessary information can be communicated using a standard SDP
session description for a multicast session. Again, it is up to the
implementation of the Distribution Source to properly associate the
ASM contribution session and the SSM distribution sessions.
Figure 6 shows this scenario.
_ Source-specific
/ \ +-----+ Multicast
+--------+ | | | | +----------------> R(1)
| Media |<->| A | | D S | | |
|Sender 1| | S | | I O | +--+ |
+--------+ | M | | S U | | | |
| | | T R | | +-----------> R(2) |
+--------+ | G | | R C |->+----- : | |
| Media |<->| r |<->| I E | | +------> R(n-1) | |
|Sender 2| | o | | B | | | | | |
+--------+ | u | | U | +--+--> R(n) | | |
| p | | T | | | | |
+--------+ | | | I |<---------+ | | |
| Media |<->| | | O |<---------------+ | |
|Sender 3| \_/ | N |<--------------------+ |
+--------+ | |<-------------------------+
+-----+ Unicast
Contribution Distribution
Session Session
(ASM) (SSM)
Figure 6: Three Media Senders in ASM Group
Appendix B. Distribution Report Processing at the Receiver
B.1. Algorithm
Example processing of Loss Distribution Values
X values represent the loss percentage.
Y values represent the number of receivers.
Number of x values is the NDB value
xrange = Max Distribution Value(MaDV) - Min Distribution Value(MnDV)
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First data point = MnDV,first ydata
then
For each ydata => xdata += (MnDV + (xrange / NDB))
B.2. Pseudo-Code
Packet Variables -> factor,NDB,MnDVL,MaDV
Code variables -> xrange, ydata[NDB],x,y
xrange = MaDV - MnDV
x = MnDV;
for(i=0; i<NDB; i++) {
y = (ydata[i] * factor);
/*OUTPUT x and y values*/
x += (xrange / NDB);
}
B.3. Application Uses and Scenarios
Providing a distribution function in a feedback message has a number
of uses for different types of applications. Although this appendix
enumerates potential uses for the distribution scheme, it is
anticipated that future applications might benefit from it in ways
not addressed in this document. Due to the flexible nature of the
summarization format, future extensions may easily be added. Some of
the scenarios addressed in this section envisage potential uses
beyond a simple SSM architecture, for example, single-source group
topologies where every receiver may in fact also be capable of
becoming the source. Another example may be multiple SSM topologies,
which, when combined, make up a larger distribution tree.
A distribution of values is useful as input into any algorithm,
multicast or otherwise, that could be optimized or tuned as a result
of having access to the feedback values for all group members.
Following is a list of example areas that might benefit from
distribution information:
- The parameterization of a multicast Forward Error Correction (FEC)
algorithm. Given an accurate estimate of the distribution of
reported losses, a source or other distribution agent that does not
have a global view would be able to tune the degree of redundancy
built into the FEC stream. The distribution might help to identify
whether the majority of the group is experiencing high levels of
loss, or whether in fact the high loss reports are only from a
small subset of the group. Similarly, this data might enable a
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RFC 5760 RTCP with Unicast Feedback February 2010
receiver to make a more informed decision about whether it should
leave a group that includes a very high percentage of the worst-
case reporters.
- The organization of a multicast data stream into useful layers for
layered coding schemes. The distribution of packet losses and
delay would help to identify what percentage of members experience
various loss and delay levels, and thus how the data stream
bandwidth might be partitioned to suit the group conditions. This
would require the same algorithm to be used by both senders and
receivers in order to derive the same results.
- The establishment of a suitable feedback threshold. An application
might be interested to generate feedback values when above (or
below) a particular threshold. However, determining an appropriate
threshold may be difficult when the range and distribution of
feedback values is not known a priori. In a very large group,
knowing the distribution of feedback values would allow a
reasonable threshold value to be established, and in turn would
have the potential to prevent message implosion if many group
members share the same feedback value. A typical application might
include a sensor network that gauges temperature or some other
natural phenomenon. Another example is a network of mobile devices
interested in tracking signal power to assist with hand-off to a
different distribution device when power becomes too low.
- The tuning of Suppression algorithms. Having access to the
distribution of round-trip times, bandwidth, and network loss would
allow optimization of wake-up timers and proper adjustment of the
Suppression interval bounds. In addition, biased wake-up functions
could be created not only to favor the early response from more
capable group members but also to smooth out responses from
subsequent respondents and to avoid bursty response traffic.
- Leader election among a group of processes based on the maximum or
minimum of some attribute value. Knowledge of the distribution of
values would allow a group of processes to select a leader process
or processes to act on behalf of the group. Leader election can
promote scalability when group sizes become extremely large.
B.4. Distribution Sub-Report Creation at the Source
The following example demonstrates two different ways to convey loss
data using the generic format of a Loss sub-report block (Section
7.1.4). The same techniques could also be applied to representing
other distribution types.
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RFC 5760 RTCP with Unicast Feedback February 2010
1) The first method attempts to represent the data in as few bytes as
possible.
2) The second method conveys all values without providing any savings
in bandwidth.
Data Set
X values indicate loss percentage reported; Y values indicate the
number of receivers reporting that loss percentage.
X - 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
Y - 1000| 800 | 6 | 1800 | 2600 | 3120 | 2300 | 1100 | 200 | 103
X - 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19
Y - 74 | 21 | 30 | 65 | 60 | 80 | 6 | 7 | 4 | 5
X - 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29
Y - 2 | 10 | 870 | 2300 | 1162 | 270 | 234 | 211 | 196 | 205
X - 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39
Y - 163 | 174 | 103 | 94 | 76 | 52 | 68 | 79 | 42 | 4
Constant value
Due to the size of the multiplicative factor field being 4 bits, the
maximum multiplicative value is 15.
The distribution type field of this packet would be value 1 since it
represents loss data.
Example: 1st Method
Description
The minimal method of conveying data, i.e., small amount of bytes
used to convey the values.
Algorithm
Attempt to fit the data set into a small sub-report size, selected
length of 8 octets
Can we split the range (0 - 39) into 16 4-bit values? The largest
bucket value would, in this case, be the bucket for X values 5 -
7.5, the sum of which is 5970. An MF value of 9 will generate a
multiplicative factor of 2^9, or 512 -- which, multiplied by the
max bucket value, produces a maximum real value of 7680.
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RFC 5760 RTCP with Unicast Feedback February 2010
The packet fields will contain the values:
Header distribution Block
Distribution Type: 1
Number of Data Buckets: 16
Multiplicative Factor: 9
Packet Length field: 5 (5 * 4 => 20 bytes)
Minimum Data Value: 0
Maximum Data Value: 39
Data Bucket values: (each value is 16-bits)
Results, 4-bit buckets:
X - 0 - 2.5 | 2.5 - 5 | 5 - 7.5 | 7.5 - 10
(Y - 1803 | 4403 | 5970 | 853 ) ACTUAL
Y - 4 | 9 | 12 | 2
X - 10 - 12.5 | 12.5 - 15 | 15 - 17.5 | 17.5 - 20
(Y - 110 | 140 | 89.5 | 12.5) ACTUAL
Y - 0 | 0 | 0 | 0
X - 20 - 22.5 | 22.5 - 25 | 25 - 27.5 | 27.5 - 30
(Y - 447 | 3897 | 609.5 | 506.5) ACTUAL
Y - 1 | 8 | 1 | 1
X - 30 - 32.5 | 32.5 - 35 | 35 - 37.5 | 37.5 - 40
(Y - 388.5 | 221.5 | 159.5 | 85.5) ACTUAL
Y - 1 | 0 | 0 | 0
Example: 2nd Method
Description
This demonstrates the most accurate method for representing the
data set. This method doesn't attempt to optimise any values.
Algorithm
Identify the highest value and select buckets large enough to
convey the exact values, i.e., no multiplicative factor.
The highest value is 3120. This requires 12 bits (closest 2 bit
boundary) to represent, therefore it will use 60 bytes to
represent the entire distribution. This is within the max packet
size; therefore, all data will fit within one sub-report block.
The multiplicative value will be 1.
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The packet fields will contain the values:
Header Distribution Block
Distribution Type: 1
Number of Data Buckets: 40
Multiplicative Factor: 0
Packet Length field: 18 (18 * 4 => 72 bytes)
Minimum Loss Value: 0
Maximum Loss Value: 39
Bucket values are the same as the initial data set.
Result
Selecting one of the three methods outlined above might be done by
a congestion parameter or by user preference. The overhead
associated with processing the packets is likely to differ very
little between the techniques. The savings in bandwidth are
apparent, however, using 20, 52, and 72 octets respectively.
These values would vary more widely for a larger data set with
less correlation between results.
Acknowledgements
The authors would like to thank Magnus Westerlund, Dave Oran, Tom
Taylor, and Colin Perkins for detailed reviews and valuable comments.
Ott, et al. Standards Track PAGE 65
RFC 5760 RTCP with Unicast Feedback February 2010
Authors' Addresses
Joerg Ott
Aalto University
School of Science and Technology
Department of Communications and Networking
PO Box 13000
FIN-00076 Aalto
EMail: jo@acm.org
Julian Chesterfield
University of Cambridge
Computer Laboratory,
15 JJ Thompson Avenue
Cambridge
CB3 0FD
UK
EMail: julianchesterfield@cantab.net
Eve Schooler
Intel Research / CTL
MS RNB6-61
2200 Mission College Blvd.
Santa Clara, CA, USA 95054-1537
EMail: eve_schooler@acm.org
Ott, et al. Standards Track PAGE 66
RTP Control Protocol (RTCP) Extensions for Single-Source Multicast Sessions with Unicast Feedback
RFC TOTAL SIZE: 160368 bytes
PUBLICATION DATE: Saturday, February 27th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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