J. Chesterfield
University of Cambridge
J. Ott
Internet Draft Tellitec GmbH
Document: draft-ietf-avt-rtcpssm-10 E. Schooler
Intel
Expires: April 2006 24 October 2005
RTCP Extensions for Single-Source Multicast Sessions
with Unicast Feedback
Status of this Memo
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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.
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Table of Contents
1. Conventions and Acronyms......................................2
2. Introduction..................................................2
3. Basic Operation...............................................4
4. Definitions...................................................6
5. Packet types..................................................7
6. Simple Feedback Model.........................................8
7. Distribution Source Feedback Summary Model..................10
8. Mixer/Translator issues......................................24
9. Transmission interval calculation............................26
10. SDP Extensions..............................................27
11. Security Considerations.....................................29
12. Backwards Compatibility.....................................35
13. IANA Considerations.........................................36
14. Bibliography................................................37
15. Appendix: Distribution Report processing at the receiver....40
16. AUTHORS ADDRESSES...........................................48
17. IPR Notice..................................................48
18. FULL COPYRIGHT STATEMENT....................................49
1. Conventions and Acronyms
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in
this document, are to be interpreted as described in RFC 2119.
2. 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 Real-Time 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
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single administrative domain, e.g., DVMRP [16], PIM [17][18]) are
used in combination with an Inter-domain routing protocol (those
that operate across administrative domain borders, e.g., MBGP [19],
MSDP [20]). Such routing protocols enable a host to join a single
multicast group address and to send to or to 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.
In addition, many-to-many communication is not always available, nor
desired by all group applications. For example, with Source-Specific
Multicast (SSM) 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 of the requirements of certain RTP
applications, unidirectional communication makes it impossible for
receivers in the group to share RTCP feedback information amongst
all other group members. Therefore, in this document, we specify a
solution to this problem. We introduce unicast feedback as a new
method to distribute RTCP control information amongst all session
members. It 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 could 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 still 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 using 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.
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c) ASM with a single sender.
A unicast feedback approach may be used by an ASM application
with a single sender, as it would help to prevent overtaxing
multicast routing infrastructure that does not scale as
efficiently. Because it is not 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 [1],
Section 6.
3. Basic Operation
This document proposes two new methods to enable unicast receiver
feedback. Each involves unicasting RTCP packets to a Distribution
Source whose job it is to re-distribute the information to the
members of the group. The Distribution Source must be able to
communicate with all group members in order for either mechanism to
work. The architecture is displayed below in Figure 1. There may be
a single media sender or multiple media senders, Sender i, 1<=i<=N,
on whose behalf the Distribution Source disseminates RTP and RTCP
packets. The Distribution Source may be one and the same as a
particular sender. The basic assumption is that communication is
multicast (either SSM or ASM) in the direction from the Distribution
Source to receivers, R(i), 1<=i<=N, and unicast in the direction
from receivers to the Distribution Source.
The Distribution Source is responsible for relaying RTCP information
between media sender(s) and receivers in both directions and, of
course, for forwarding RTP packets from the media sender(s) to the
receivers.
Communication between Media Senders (on the left of the figure) and
the Distribution Source may be performed in numerous ways:
i. Unicast only: The Media Senders MAY send RTP and RTCP via
unicast to the Distribution Source and receive RTCP via
unicast.
ii. Any Source Multicast (ASM): the Media Senders 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 Senders 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.
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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 MAY be a different one.
The precise setup and the configuration of the senders and the
Distribution Source is beyond the scope of this document
(appropriate SDP descriptions MAY be used for this purpose) that
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.
Source-specific
+-----+ Multicast
+--------+ | | +----------------> R(1)
|Sender 1|<----->| D S | | |
+--------+ | I O | +--+ |
| S U | | | |
+--------+ | T R | | +-----------> R(2) |
|Sender 2|<----->| R C |->+----- : | |
+--------+ | I E | | +------> R(n-1) | |
| B | | | | | |
: | U | +--+--> Rn | | |
: | T | | | | |
| I |<---------+ | | |
| O |<---------------+ | |
+--------+ | N |<--------------------+ |
|Sender N|<----->| |<-------------------------+
+--------+ +-----+ Unicast
Figure 1. System Architecture.
The first method, the 'Simple Feedback Model', is a basic reflection
mechanism whereby all Receiver RTCP packets are unicast to the
Distribution Source and subsequently 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 to
send reports to the Distribution Source, yet still receives
forwarded RTCP traffic on the multicast control data channel. This
method also has the advantage of being backwards compatible with
standard RTP/RTCP implementations.
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 into
one summary packet that is then distributed to all the receivers.
The advantage of this 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
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information to all the receivers. The basic operation of the scheme
is similar to the first method in that receivers send feedback via
unicast to the Distribution Source. In the second scheme, however
the Distribution Source distributes summaries of the feedback over
the multicast channel. Thus, 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 and therefore should not cause problems with
respect to the calculation of the Receiver RTCP bandwidth share.
4. 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. In order for unicast feedback
to work, there MUST be only one session Distribution Source for
any subset of receivers to which RTCP feedback is directed.
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 the
RTCP channel. With standard RTP/RTCP, these channels typically
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share the same multicast address but are differentiated via
port numbers as specified in [1]. In an SSM context, the RTP
channel is multicast, whereas the RTCP or feedback channel is
actually the collection of unicast channels between each
receiver and the source.
Unicast RTCP Feedback Target:
For a session defined as having a Distribution Source A, on
ports n for the RTP channel and k for the RTCP channel, the
unicast RTCP feedback target is the IP address of Source A on
port k unless otherwise stated in the session description. See
Section 10 for details on how the address is specified.
SSRC:
Synchronization source. A 32-bit value that uniquely identifies
each member in a session. See [1] for further information.
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 scheme.
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 208
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Within the Receiver Summary Information packet there are various
types of information that may be reported and encapsulated in
optional sub-report blocks:
Report Block Type Description Identifier number
------------------------------------------------------------------
IPv4 Address IPv4 Unicast Feedback address 0
IPv6 Address IPv6 Unicast Feedback address 1
DNS name DNS name for Unicast Feedback 2
- - reserved - 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 - 9
Stats General statistics 10
Receiver BW RTCP Receiver Bandwidth 11
Group Info RTCP Group and Avg Packet Size 12
- - reserved - 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 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 behaviour
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 (the unicast feedback target) MUST listen
for unicast RTCP data sent to the RTCP port. All unicast data
received on this port MUST be forwarded by the Distribution Source
to the group on the multicast RTCP channel. The Distribution Source
MUST NOT stack report blocks received from different receivers into
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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 from one Media
Sender to all other Media Senders.
The Distribution Source MUST forward RTCP packets originating from
the Media Senders to the receivers.
The reflected or forwarded RTCP traffic SHOULD NOT be included 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, they 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.
If an application wishes to use APP packets, it is recommended that
the Simple Feedback Model be used since it is likely that all
receivers in the session will need to hear the APP-specific packets.
The same applies for all other RTCP packets that are not defined in
the base RTP specification [1]. The decision to use the Simple
Feedback Model MUST be made in advance of the session and MUST be
indicated in the SDP-based announcement or negotiation [5].
6.3 Receiver behavior
Receivers MUST listen on the RTP channel for data and the RTCP
channel for control. Each receiver MUST calculate its share of the
control bandwidth R/n, based on the standard rule 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. 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 indicated unicast
RTCP transport address of the Distribution Source following the
rules defined in section 9.
6.4 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). Processing and general operation follows
[1].
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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 Report (RSI), and a number of optional sub-
report block formats, which are enumerated in Section 7.1.
Transmission of sub-report types is OPTIONAL. They MAY be appended
to the RSI report block to provide more detailed information on the
overall session characteristics reported by all receivers and also
to convey important information such as the feedback address and
reporting bandwidth.
From an RTP perspective, the Distribution Source acts like an RTP
receiver, generating its own Receiver Reports and sending them to
the receiver group and to the Media Senders. However the
Distribution Source is not counted in the receiver bandwidth
allocation, as it summarizes information provided by the other
receivers. Nevertheless, the Distribution Source's transmission rate
MUST adhere to RTCP bandwidth limitations for receivers. In the
Distribution Source Feedback Summary Model, an RSI report block MUST
be appended to all RRs the Distribution Source generates.
In addition, if there are multiple Media Senders, the Distribution
Source forwards their RTCP SR reports without alteration. If the
Distribution Source is actually a Media Sender, even if it is the
only session sender, it MUST in addition 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 an SSRC value for transmitting
summarization information and MUST perform proper SSRC collision
detection and resolution.
Note: Here, one could have optimized for the presumably common
case (Distribution Source == sender) and reduce the forward bit
rate. However, the generalized way always requiring the
Distribution Source to be a receiver with its own SSRC is much
cleaner, albeit this comes at the cost of some extra bits.
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
initial 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. RSI and the optional
corresponding sub-report blocks MUST be sent in addition to the
standard receiver-issued packets, such as Receiver Reports and SDES
packets outlined in [1].
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There is one exception to this rule regarding the signaling of group
size or bandwidth allocation for receivers. Appended to every RSI
packet MUST be either a Group and Average Packet size sub-report or
an RTCP Bandwidth sub-report.
7.1 Packet Formats
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=208 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC/CSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: optional 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.
Timestamp: 64 bits
Indicates the wallclock time, which is seconds relative to
0:00.00 h UTC on 1 January 1900, when this report was sent. This
value is used to enable detection of duplicate packets,
reordering and to provide a chronological profile of the feedback
reports.
7.1.2 Optional 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.
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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 |
| ... |
| ... |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
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 field * 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
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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 are 0 - 15, creating
a range of values from MF = 1, 2, 4 ... 32768.
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 profiles for each
distribution type (DT).
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) covering the value range:
x=0; [min, min+(x+1)*(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 are profile-specific and are addressed
individually in the sections below. The size of the sub-report block
is variable, as indicated by the packet length field.
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.
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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 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.
7.1.5 Jitter sub-report block
A jitter sub-report block indicates the distribution of the
estimated statistical variance 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 how the
values are calculated and the relevance of the jitter results.
Jitter values are measured in timestamp units and expressed as
unsigned integers. The minimum distribution value MUST always be
less than the maximum. The sub-report block type (SRBT) is 5.
Since timestamp units are payload type specific, the relevance of a
jitter value would be 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 in the
event that 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 members since it forwards all the SR packets from
the Media Sender(s) to the group. If the Distribution Source is not
itself the actual 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.
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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 the holding time at the receiver. The minimum
distribution value MUST always be less than the maximum. The sub-
report block type (SRBT) is 6.
7.1.7 Cumulative Loss sub-report block
The cumulative loss field in a Receiver Report [1], in contrast to
the Average 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 presents a smoothed 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. The sender must
maintain a record of the Cumulative number lost and Extended Highest
Sequence number received as reported by each receiver. Ideally the
recorded values are from the first report received. Future
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 sub-report block type (SRBT) is 7.
7.1.8 Feedback Address Target sub-report block
The feedback address target block provides a dynamic mechanism for
the Distribution Source to signal an alternative unicast RTCP
feedback address to the receivers. If this field is included, it
MUST override any static SDP address information for the session.
If this 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
for late session arrivals.
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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 :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Length: 8 bits
The length of the sub-report block in 32-bit words. For an IPv4
address this should be 2 (e.g., 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 (optional)
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 MUST be UTF-8 encoded [11]. For
IPv4, SRBT=0. For IPv6, SRBT=1. For usage of the DNS name,
SRBT=2.
A feedback target address block with a certain SRBT MUST NOT occur
more than once. Dual 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 (i.e., the
Distribution Source).
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.
The feedback target address presents a significant security risk if
accepted from unauthenticated RTCP packets. See section 11.3 a) and
11.4 a).
7.1.9 Collisions 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.
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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 x 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.
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 section 7.2.1 b).
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 interarrival jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
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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.
7.1.11 RTCP Bandwidth indication sub-report block
This sub-report block is used to inform the Media Senders and
receivers about the maximum RTCP bandwidth that is supposed to be
used by each Media Sender or about 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. Each Media Sender MUST base its RTCP
transmission interval calculation on the average size of the RTCP
packets sent by the Distribution Source to the receivers.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 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
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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.
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).
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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Group Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Average Packet 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 observed
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: 32 bits
This field provides the Distribution Source's view of the average
RTCP packet size as locally calculated following the rules
defined in [1]. When this sub-report block is included, this
field MUST always be present.
7.2 Distribution Source behaviour
In the Distribution Source Feedback Summary Model, the Distribution
Source (the unicast feedback target) MUST listen for unicast RTCP
data sent to the RTCP port. All unicast data received on this port
MUST be processed by the Distribution Source as described below.
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).
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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 interpreting and storing per-receiver
information as defined in [1]. The importance of providing these
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 any summarization specific
data to an RR report since it always generates RR 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 information contained in the Receiver RTCP Bandwidth
sub-report block MUST take precedence.
The Receiver RTCP Bandwidth sub-report block provides the
Distribution Source with the capability to control the amount of
feedback from the receivers and MAY be increased or decreased based
on 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
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).
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7.2.1 Receiver Report Aggregation
The Distribution Source is responsible for aggregating reception
quality information received in SR and 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.
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 up to the link MTU.
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.
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Note: This intentionally provides persistent full coverage of
the entire session membership to avoid oscillations due to
changing membership samples.
b) If the Distribution Source intends to report a short summary
using the format defined in section 7.1.10, for EACH of the
values included in the report (AFL, HCNL, average interarrival
jitter), it MUST keep a timer T_summary as well as three
entries: V, V', and V" (indicated as Vi, with i=0,1,2). The
respective value V (or V0) is included in each report sent.
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.
The timer constant T_summary MUST be initialized as T_summary =
1.5*Td, where Td is the 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.
V, V', and V" are initialized to an "empty" value. Because 0
and -1 are sometimes valid values, here are guidelines on how to
initialize the "empty" value. If Vi indicates a signed maximum,
the empty value is -Infinity, for an unsigned maximum it is 0.
If Vi indicates a (signed or unsigned) minimum, the empty value
is +Infinity. If Vi indicates a median, the empty value is 0
and an associated counter VNi is required that is also
initialized to 0 and that dampens the effects of large
oscillations of newly received values. Furthermore, to capture
the distribution needed for the median calculation, state is
needed for each receiver j as Vi_j.
As RTCP receiver reports are received by the Distribution
Source, the values V, V', and V" are updated as described below.
When T_summary expires, the assignments V = V' (V0=V1) and V' =
V" (V1=V2) are carried out and V" (V2) is initialized to
"empty". This also applies to per-receiver state.
Let VR be the newly received value. For a maximum calculation,
if Vr > Vi then Vi = Vr, otherwise Vi remains unchanged.
For a minimum calculation, if Vr < Vi then Vi = Vr, otherwise Vi
remains unchanged.
For a median calculation, Vi_j is updated as follows when a
report is received for receiver j containing the value Vr_j:
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Vi_j = Vr_j
The median is only calculated upon transmission of the RSI
packet: VN is set to the number of reports V0_j available
(1 <= j <= VN) contained in V (V0). Then, the set
V0 = { V0_j | for each V0_j } is sorted and the median V is
determined using V0 and VN.
7.2.2 Receiver Report Forwarding
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).
Instead of forwarding RTCP RRs from the receivers, the Distribution
Source MAY also send the RSI packets to the senders (if it knows
that the Media Senders understand RTCP RSI packets).
7.2.2 Handling Sender Reports
The Distribution Source also receives SR packets from the RTP Media
Senders.
The Distribution Source MUST forward all RTCP packets from the RTCP
Media Senders to the 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 one Media
Sender to all other Media Senders.
7.3 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.
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.
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 Bandwith 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
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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 obtained 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. The appendix B (section 16) 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.
7.4 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 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].
8. Mixer/Translator issues
The original RTP specification allows a session to use mixers and
translators which help to connect heterogeneous networks into one
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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, they 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. Receivers SHOULD be aware that by introducing a mixer-
translator into the session, more than one Media Sender may be
active in a session since the mixer-translator may be forwarding
traffic from either multiple unicast sources or from an ASM session
to the SSM receivers. Receivers SHOULD still forward unicast RTCP
reports in the usual manner to the Distribution Source, which in
this case 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, where the mixer-translator is not acting as
the Distribution Source, and subsequent unicast feedback to the
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.
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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 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 mode
(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 mode, 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 mode, 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
MAY choose to drop excess packets for receivers that do not. In all
cases, the average RTCP packet size is determined from the Media
Senders' and receivers' RTCP packets forwarded and those originated
by the Distribution Source.
If operating in Distribution Source Feedback Summary mode, 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
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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 Mode, 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 calculation of the
deterministic calculated interval as per [1] and [10].
In Distribution Source Feedback Summary Mode, 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 calculation of the
deterministic calculated 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 calculated interval as per [1] and
[10] based upon these input values.
9.4 Operation in conjunction with AVPF
If the RTP session is an AVPF session [15] (as opposed to a regular
AVP [6] session the receivers MAY modify their RTCP report
scheduling as defined in [15]. Use of AVPF does not affect the
Distribution Source's RTCP transmission or forwarding behavior.
It is RECOMMENDED that a Distribution Source operates in the Simple
Forwarding Mode in order not to counteract the damping mechanism
built into AVPF. If only generic feedback packets are in use that
are understood by the Distribution Source and that can easily be
aggregated, 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 SHOULD NOT
delay forwarding feedback information.
In the event that APP specific packets without a defined
summarization format are to be used, the Simple Forwarding Mode of
operation MUST be adopted from the start.
10. SDP Extensions
The Session Description Protocol (SDP) [5] is used as a means to
describe media sessions in terms of their transport addresses,
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codecs, and other attributes. Providing RTCP feedback via unicast as
specified in this document constitutes another session parameter
needed in the session description. Similarly, other single-source
multicast RTCP feedback parameters need to be provided, such as the
summarisation mode 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 also need to be registered with IANA).
10.1 SSM RTCP Session Identification
A new session level attributes MUST be used to indicate the use of
unicast instead of multicast feedback: "rtcp-unicast".
This attribute uses one parameter to specify the mode of operation.
rtcp-unicast:reflection
This attribute MUST be used to indicate the "Simple Feedback"
mode of operation where packet reflection is used by the RTCP
target (without further processing).
rtcp-unicast:rsi
This attribute MUST be used to indicate the "Distribution
Source Feedback Summary" mode of operation.
10.2 SSM Source Specification
In addition, in a Source-Specific Multicast RTCP session, the
Distribution Source needs to be indicated for both source-specific
joins to the multicast group, as well as for addressing unicast RTCP
packets on the backchannel from receivers to the Distribution
Source.
This is achieved by following the proposal for SDP source filters
documented in [4]. According to the specification, for SSM RTCP only
the inclusion mode ("a=source-filter:incl") MUST be used.
There SHOULD be exactly one "a=source-filter:incl" attribute listing
the address of the sender. The RTCP port MUST be derived from the
m= line of the media description.
An alternative Distribution Source feedback address and port MAY be
supplied using the SDP RTCP attribute [7], e.g., a=rtcp:<port> IN
IP4 192.168.1.1.
Two "source-filter" attributes MAY be present to indicate an IPv4
and an IPv6 representation of the Distribution Source address.
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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 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 SAP [22], e-mail, 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. Therefore, all 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. An outline of the
types of attacks in detail:
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 attackers
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.
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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, it might
benefit other types of attack, 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 two communication
contexts:
a) Source-to-Receiver communication
The downstream communication channel, from the source to the
receivers, is critical to protect as 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 four types of attacks. A
denial of service attack is possible, but less of a concern. The
worst case effect 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 by mistake, e.g., an IP address configuration (e.g.,
typing) error, could directly create a denial of service attack
on another host.
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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 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 is not likely to
have as great an impact as the 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 would be amplified most seriously in this context. A
large group of receivers may unwittingly generate a distributed
DoS attack on the Distribution Source in the event that the
integrity of the source RTCP channel has been compromised and 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
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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.
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 first 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 simply injecting 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 second context also 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 are made towards protection from compromise.
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. The same
situation applies as in the first context with respect to data
confidentiality, and it is RECOMMENDED that precautions should be
taken to protect the privacy of the data.
An additional security consideration that is not a component of this
specification but 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
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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:
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 should 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
12.5. This is only intended as a guideline and it is acknowledged
that there are other solutions that would also be suitable to be
compliant with the specification.
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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
RTSP [13] and SIP [14].
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. 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 which 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 is the recommended 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 which 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 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.
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c) TESLA - TESLA [24] is a scheme that provides a more flexible
approach to data authentication using time-based key disclosure. The
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, this 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 -- MIKEY [12] 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 -- GSAKMP is the general solution favored for Multicast
Secure group key distribution. It is the recommended key
distribution solution for GDOI sessions.
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 are the RTCP packets from the source enabling
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 reflect traffic by either mechanism 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 mechanism, 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.
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13. IANA Considerations
The following contact information shall be used for all
registrations included here:
Contact: Joerg Ott
mailto:jo@acm.org
tel:+358-9-451-2460
Based on the guidelines suggested in [2], this document proposes 1
new RTCP packet format to be registered with the RTCP Control Packet
type (PT) Registry:
Name: RSI
Long name: Receiver Summary Information
Value: 208
Reference: This document.
This document defines a substructure for RTCP RSI packets. A new
sub-registry needs to be set up for the sub-report block type (SRBT)
values for the RSI packet, with the following registrations created
initially:
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.
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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.
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 for 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-serve
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.
14. Bibliography
14.1 Normative References
[1] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP -
A Transport Protocol for Real-time Applications," RFC 3550 (STD
64), July 2003.
[2] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
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[3] M. Baugher, D. McGrew, D. Oran, R. Blom, E. Carrara, M. Naslund,
and K. Norrman, "The Secure Real Time Transport Protocol", RFC
3711, March 2004.
[4] B. Quinn, R. Finlayson, "SDP Source-Filters", Internet Draft
draft-ietf-mmusic-sdp-srcfilter-10.txt, Work in Progress,
September 2005.
[5] M. Handley, V. Jacobson, and C. Perkins, "SDP: Session
Description Protocol", Internet Draft draft-ietf-mmusic-sdp-new-
25.txt, Work in Progress, July 2005.
[6] H. Schulzrinne, S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", RFC 3551 (STD 65), July 2003.
[7] C. Huitema, "RTCP Attribute in SDP", RFC 3605, October 2003.
[8] D. Meyer, R. Rockell, G. Shepherd, "Source-Specific Protocol
Independent Multicast in 232/8", Internet Draft draft-ietf-
mboned-ssm232-08.txt, Work in Progress, March 2004.
[9] H. Holbrook, B. Cain, B. Haberman, "Using IGMPv3 For Source-
Specific Multicast", Internet Draft draft-holbrook-idmr-igmpv3-
ssm-08.txt, Work in Progress, October 2004.
[10] S. Casner, "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556,
July 2003.
[11] F. Yergeau, "UTF-8, a transformation format of ISO 10646", RFC
3629, November 2003.
14.2 Informative References
[12] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K. Norrman,
"MIKEY: Multimedia Internet Keying", RFC 3830, August 2004.
[13] H. Schulzrinne, A. Rao, R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[14] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
Peterson, R. Sparks, M. Handley, E. Schooler, "SIP: Session
Initiation Protocol", RFC 3261, June 2002.
[15] J. Ott, S. Wenger, N. Sato, C. Burmeister, and J. Rey, "
Extended RTP Profile for RTCP-based Feedback (RTP/AVPF)",
Internet Draft draft-ietf-ietf-avt-rtcp-feedback-11.txt, August
2004.
[16] Pusateri, T, "Distance Vector Multicast Routing Protocol",
Internet Draft draft-ietf-idmr-dvmrp-v3-11.txt, Work in
Progress, October 2003.
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[17] B. Fenner, M. Handley, H. Holbrook, I. Kouvelas, "Protocol
Independent Multicast - Sparse Mode (PIM-SM): Protocol
Specification (Revised)", Internet Draft, draft-ietf-pim-sm-v2-
new-11.txt, Work in Progress, October 2004.
[18] Adams, A, Nicholas, J, Siadak, W, "Protocol Independent
Multicast- Dense Mode (PIM-DM)", RFC 3973, January 2005.
[19] Bates, T, Rekhter, Y, Chandra, R, Katz, D, "Multiprotocol
Extension fo BGP-4", RFC 2858, June 2000.
[20] D. Meyer, B. Fenner, "Multicast Source Discovery Protocol
(MSDP)", Experimental 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] M. Handley, C. Perkins, E. Whelan, "Session Announcement
Protocol (SAP)", RFC 2974, October 2000.
[23] M. Baugher, T. Hardjono, H. Harney, and B. Weis, "The Group
Domain of Interpretation", RFC 3547, July 2003.
[24] A. Perrig, D. Song, R. Canetti, D. Tygar, B. Briscoe, "TELSA:
Multicast Source Authentication Transform Introduction", RFC
4082, June 2005.
[25] A. Frier, P. Karlton, and P. Kocher, "The SSL 3.0 Protocol",
Netscape Communications Corp., Nov 18, 1996.
[26] T. Friedman, R. Caceres, and A. Clark (eds), "RTCP Reporting
Extensions", RFC 3611, November 2003.
[27] F. Andreasen, M. Baugher, and D. Wing, "Session Description
Protocol Security Descriptions for Media Streams", Internet
Draft draft-ietf-mmusic-sdescriptions-12.txt, September 2005.
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15. 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
Source(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.
15.1 One Media Sender identical to the Distribution Source
In the simplest case, the Distribution Source is identical to the
Media Source as depicted in figure 2. 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 | +--+--> Rn | | |
|N T | | | | |
|D I |<---------+ | | |
|E O |<---------------+ | |
|R N |<--------------------+ |
| |<-------------------------+
+--------+ Unicast
Figure 2: Media Source == Distribution Source
15.2 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 or via ASM multicast. 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.
This scenario is depicted in figure 3. 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
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is a configuration/local matter of the Distribution Source how to
associate a contribution media session between the Media Sender
and itself with the corresponding distribution media session
between itself and the receivers.
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 | +--+--> Rn | | |
| T | | | | |
| I |<---------+ | | |
| O |<---------------+ | |
| N |<--------------------+ |
| |<-------------------------+
+-----+ Unicast
Contribution Distribution
Session Session
(unicast, ASM) (SSM)
Figure 3: One Media Sender Separate from Distribution Source
15.3 Three Media Senders, Unicast Contribution
Similar considerations apply if three media senders transmit to an
SSM multicast group via the Distribution Source and individually
sent their media stream RTP packets via a unicast to the
Distribution Source.
In this case, the responsibilities of the Distribution Source are a
superset to the previous case: it also needs to perform relaying
media traffic from each Media Sender to the receivers and forwarding
(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.
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 4.
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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 | +--+--> Rn | | |
| T | | | | |
+--------+ | I |<---------+ | | |
| Media |<---->| O |<---------------+ | |
|Sender 3| | N |<--------------------+ |
+--------+ | |<-------------------------+
+-----+ Unicast
Contribution Distribution
Session Session
(unicast) (SSM)
Figure 4: Three Media Senders, unicast contribution
15.4 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.
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 5 shows this scenario.
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_ 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 | +--+--> Rn | | |
| p | | T | | | | |
+--------+ | | | I |<---------+ | | |
| Media |<->| | | O |<---------------+ | |
|Sender 3| \_/ | N |<--------------------+ |
+--------+ | |<-------------------------+
+-----+ Unicast
Contribution Distribution
Session Session
(ASM) (SSM)
Figure 5: Three Media Sender in ASM Group
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16. Appendix B: Distribution Report processing at the receiver
16.1 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)
First data point = MnDV,first ydata
then
Foreach ydata => xdata += (MnDV + (xrange / NDB))
16.2 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);
}
16.3 Application Uses and Scenarios
Providing a distribution function in a feedback message has a number
of uses for different types of applications. Although this section
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
summarisation 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, which does
not have a global view, would be able to tune the degree of
redundancy built in to the FEC stream. The distribution might help
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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 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.
16.4 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.
1) The first method attempts to represent the data in as few bytes
as possible.
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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
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 8 Octets
Can we split the range (0 - 39) into 16 4-bit values?
The maximum value would therefore be 5 - 7.5 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
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
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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 - 20
(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.
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 initial data set.
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Result
The selection of which of the three methods outlined above might be
determined by a congestion parameter, or a 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.
18. Acknowledgements
The authors would like to thank Magnus Westerlund for detailed
reviews and valuable comments.
19. Authors' Addresses
Julian Chesterfield
University of Cambridge
Computer Laboratory,
JJ Thompson Avenue,
Cambridge, CB3 0FD, UK
Julian.chesterfield@cl.cam.ac.uk
Joerg Ott
Tellitec Engineering GmbH
Berliner Str. 26
D-13507 Berlin
GERMANY
jo@acm.org
Eve Schooler
Intel CTL / Research
2200 Mission College Blvd., SC12-303
Santa Clara, CA, USA 95054-1537
eve_(underscore) schooler (at) acm.org
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