Sending Multiple RTP
Streams in a Single RTP Session: Grouping RTCP Reception Statistics and
Other FeedbackVidyo, Inc.433 Hackensack AvenueSeventh FloorHackensackNJ07601USjonathan@vidyo.comEricssonFarogatan 2Kista164 80Sweden+46 10 714 82 87magnus.westerlund@ericsson.comHuawei101 Software Avenue, Yuhua DistrictNanjing,Jiangsu210012Chinabill.wu@huawei.comUniversity of GlasgowSchool of Computing ScienceGlasgowG12 8QQUnited Kingdomcsp@csperkins.orgAVTCORE WGRTP allows multiple RTP streams to be sent in a single session, but
requires each Synchronisation Source (SSRC) to send RTCP reception
quality reports for every other SSRC visible in the session. This causes
the number of RTCP reception reports to grow with the number of SSRCs,
rather than the number of endpoints. In many cases most of these RTCP
reception reports are unnecessary, since all SSRCs of an endpoint are
normally co-located and see the same reception quality. This memo
defines a Reporting Group extension to RTCP to reduce the reporting
overhead in such scenarios.IntroductionThe Real-time Transport Protocol (RTP) is a
protocol for group communication, supporting multiparty multimedia
sessions. A single RTP session can support multiple participants sending
at once, and can also support participants sending multiple simultaneous
RTP streams. Examples of the latter might include a participant with
multiple cameras who chooses to send multiple views of a scene, or a
participant that sends audio and video flows multiplexed in a single RTP
session. Rules for handling RTP sessions containing multiple RTP streams
are described in with some clarifications in
.An RTP endpoint will have one or more synchronisation sources
(SSRCs). It will have at least one RTP Stream, and thus SSRC, for each
media source it sends, and might use multiple SSRCs per media source
when using media scalability features,
forward error correction, RTP
retransmission, or similar mechanisms. An endpoint that is not
sending any RTP stream, will have at least one SSRC to use for reporting
and any feedback messages. Each SSRC has to send RTCP sender reports
corresponding to the RTP packets it sends, and receiver reports for
traffic it receives. That is, every SSRC will send RTCP packets to
report on every other SSRC. This rule is simple, but can be quite
inefficient for endpoints that send large numbers of RTP streams in a
single RTP session. Consider a session comprising ten participants, each
sending three media sources, each with their own RTP stream. There will
be 30 SSRCs in such an RTP session, and each of those 30 SSRCs will send
an RTCP Sender Report/Receiver Report packet (containing several report
blocks) per reporting interval as each SSRC reports on all the others.
However, the three SSRCs comprising each participant are commonly
co-located such that they see identical reception quality. If there was
a way to indicate that several SSRCs are co-located, and see the same
reception quality, then two-thirds of those RTCP reports could be
suppressed. This would allow the remaining RTCP reports to be sent more
often, while keeping within the same RTCP bandwidth fraction.This memo defines such an RTCP extension, RTCP Reporting Groups. This
extension is used to indicate the SSRCs that originate from the same
endpoint, and therefore have identical reception quality, hence allowing
the endpoints to suppress unnecessary RTCP reception quality
reports.TerminologyThe key words "MUST", "MUST NOT",
"REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are
to be interpreted as described in BCP 14 when, and only when, they appear in all capitals,
as shown here.RTCP Reporting GroupsAn RTCP Reporting Group is a set of synchronization sources (SSRCs)
that are co-located at a single endpoint (which could be an end host or
a middlebox) in an RTP session. Since they are co-located, every SSRC in
the RTCP reporting group will have an identical view of the network
conditions, and see the same lost packets, jitter, etc. This allows a
single representative to send RTCP reception quality reports on behalf
of the rest of the reporting group, reducing the number of RTCP packets
that need to be sent without loss of information.Semantics and Behaviour of RTCP Reporting GroupsA group of co-located SSRCs that see identical network conditions
can form an RTCP reporting group. If reporting groups are in use, an
RTP endpoint with multiple SSRCs MAY put those SSRCs into a reporting
group if their view of the network is identical; i.e., if they report
on traffic received at the same interface of an RTP endpoint. SSRCs
with different views of the network MUST NOT be put into the same
reporting group.An endpoint that has combined its SSRCs into an RTCP reporting
group will choose one (or a subset) of those SSRCs to act as
"reporting source(s)" for that RTCP reporting group. A reporting
source will send RTCP SR/RR reception quality reports on behalf of the
other members of the RTCP reporting group. A reporting source MUST
suppress the RTCP SR/RR reports that relate to other members of the
reporting group, and only report on remote SSRCs. The other members
(non reporting sources) of the RTCP reporting group will suppress
their RTCP reception quality reports, and instead send an RTCP RGRS
packet (see ) to indicate that they are part
of an RTCP reporting group and give the SSRCs of the reporting
sources.If there are large numbers of remote SSRCs in the RTP session, then
the reception quality reports generated by the reporting source might
grow too large to fit into a single compound RTCP packet, forcing the
reporting source to use a round-robin policy to determine what remote
SSRCs it includes in each compound RTCP packet, and so reducing the
frequency of reports on each SSRC. To avoid this, in sessions with
large numbers of remote SSRCs, an RTCP reporting group MAY use more
than one reporting source. If several SSRCs are acting as reporting
sources for an RTCP reporting group, then each reporting source MUST
have non-overlapping sets of remote SSRCs it reports on.An endpoint MUST NOT create an RTCP reporting group that comprises
only a single local SSRC (i.e., an RTCP reporting group where the
reporting source is the only member of the group), unless it is
anticipated that the group might have additional SSRCs added to it in
the future.If a reporting source leaves the RTP session (i.e., if it sends a
RTCP BYE packet, or leaves the session without sending BYE under the
rules of ), the remaining
members of the RTCP reporting group MUST either (a) have another
reporting source, if one exists, report on the remote SSRCs the
leaving SSRC reported on, (b) choose a new reporting source, or (c)
disband the RTCP reporting group and begin sending reception quality
reports following and .The RTCP timing rules assign different bandwidth fractions to
senders and receivers. This lets senders transmit RTCP reception
quality reports more often than receivers. If a reporting source in an
RTCP reporting group is a receiver, but one or more non-reporting
SSRCs in the RTCP reporting group are senders, then the endpoint MAY
treat the reporting source as a sender for the purpose of RTCP
bandwidth allocation, increasing its RTCP bandwidth allocation,
provided it also treats one of the senders as if it were a receiver
and makes the corresponding reduction in RTCP bandwidth for that SSRC.
However, the application needs to consider the impact on the frequency
of transmitting of the synchronization information included in RTCP
Sender Reports.Identifying Members of an RTCP Reporting GroupWhen RTCP Reporting Groups are in use, the other SSRCs in the RTP
session need to be able to identify which SSRCs are members of an RTCP
reporting group. Two RTCP extensions are defined to support this: the
RTCP RGRP SDES item is used by the reporting source(s) to identify an
RTCP reporting group, and the RTCP RGRS packet is used by other
members of an RTCP reporting group to identify the reporting
source(s).Definition and Use of the RTCP RGRP SDES ItemThis document defines a new RTCP SDES item to identify an RTCP
reporting group. The motivation for giving a reporting group an
identify is to ensure that the RTCP reporting group and its member
SSRCs can be correctly associated when there are multiple reporting
sources, and to ensure that a reporting SSRC can be associated with
the correct reporting group if an SSRC collision occurs.This document defines the RTCP Source Description (SDES) RGRP
item. The RTCP SDES RGRP item MUST be sent by the reporting sources
in a reporting group, and MUST NOT be sent by other members of the
reporting group or by SSRCs that are not members of any RTCP
reporting group. Specifically, every reporting source in an RTCP
reporting group MUST include an RTCP SDES packet containing an RGRP
item in every compound RTCP packet in which it sends an RR or SR
packet (i.e., in every RTCP packet it sends, unless Reduced-Size RTCP is in use).Syntactically, the format of the RTCP SDES RGRP item is identical
to that of the RTCP SDES CNAME item,
except that the SDES item type field MUST have value RGRP=(TBA)
instead of CNAME=1. The value of the RTCP SDES RGRP item MUST be
chosen with the same concerns about global uniqueness and the same
privacy considerations as the RTCP SDES CNAME. The value of the RTCP
SDES RGRP item MUST be stable throughout the lifetime of the
reporting group, even if some or all of the reporting sources change
their SSRC due to collisions, or if the set of reporting sources
changes.
Note to RFC Editor: please replace (TBA) in the above
paragraph with the RTCP SDES item type number assigned to the
RGRP item, then delete this note.
An RTP mixer or translator that forwards RTCP SR or RR packets
from members of a reporting group MUST forward the corresponding
RTCP SDES RGRP items as well, even if it otherwise strips SDES items
other than the CNAME item.Definition and Use of the RTCP RGRS PacketA new RTCP packet type is defined to allow the members of an RTCP
reporting group to identify the reporting sources for that group.
This allows participants in an RTP session to distinguish an SSRC
that is sending empty RTCP reception reports because it is a member
of an RTCP reporting group, from an SSRC that is sending empty RTCP
reception reports because it is not receiving any traffic. It also
explicitly identifies the reporting sources, allowing other members
of the RTP session to know which SSRCs are acting as the reporting
sources for an RTCP reporting group, and allowing them to detect if
RTCP packets from any of the reporting sources are being lost.The format of the RTCP RGRS packet is defined below. It comprises
the fixed RTCP header that indicates the packet type and length, the
SSRC of the packet sender, and a list of reporting sources for the
RTCP reporting group of which the packet sender is a member.The fields in the RTCP RGRS packet have the following definition:
version (V):
2 bits unsigned integer. This field
identifies the RTP version. The current RTP version is 2.
padding (P):
1 bit. If set, the padding bit
indicates that the RTCP packet contains additional padding
octets at the end that are not part of the control information
but are included in the length field. See .
Source Count (SC):
5 bits unsigned integer.
Indicates the number of reporting source SSRCs that are included
in this RTCP packet. As the RTCP RGRS packet MUST NOT be not
sent by reporting sources, all the SSRCs in the list of
reporting sources will be different from the SSRC of the packet
sender. Every RTCP RGRS packet MUST contain at least one
reporting source SSRC.
Payload type (PT):
8 bits unsigned integer. The
RTCP packet type number that identifies the packet as being an
RTCP RGRS packet. The RGRS RTCP packet has the value [TBA].
Note to RFC Editor: please replace [TBA] here, and in the
packet format diagram above, with the RTCP packet type that
IANA assigns to the RTCP RGRS packet.
Length:
16 bits unsigned integer. The length of
this packet in 32-bit words minus one, including the header and
any padding. This is in line with the definition of the length
field used in RTCP sender and receiver reports . Since all RTCP RGRS packets include at least
one reporting source SSRC, the length will always be 2 or
greater.
SSRC of packet sender:
32 bits. The SSRC of the
sender of this packet.
List of SSRCs for the Reporting Source(s):
A
variable length size (as indicated by SC header field) of the 32
bit SSRC values of the reporting sources for the RTCP Reporting
Group of which the packet sender is a member.
Every source that belongs to an RTCP reporting group but is not a
reporting source MUST include an RTCP RGRS packet in every compound
RTCP packet in which it sends an RR or SR packet (i.e., in every
RTCP packet it sends, unless Reduced-Size
RTCP is in use). Each RTCP RGRS packet MUST contain the SSRC
identifier of at least one reporting source. If there are more
reporting sources in an RTCP reporting group than can fit into an
RTCP RGRS packet, the members of that reporting group MUST send the
SSRCs of the reporting sources in a round-robin fashion in
consecutive RTCP RGRS packets, such that all the SSRCs of the
reporting sources are included over the course of several RTCP
reporting intervals.An RTP mixer or translator that forwards RTCP SR or RR packets
from members of a reporting group MUST also forward the
corresponding RGRS RTCP packets. If the RTP mixer or translator
rewrites SSRC values of the packets it forwards, it MUST make the
corresponding changes to the RTCP RGRS packets.Interactions with the RTP/AVPF Feedback ProfileUse of the RTP/AVPF Feedback Profile
allows SSRCs to send rapid RTCP feedback requests and codec control
messages. If use of the RTP/AVPF profile has been negotiated in an RTP
session, members of an RTCP reporting group can send rapid RTCP
feedback and codec control messages following
and , as updated by , and by the following
considerations.The members of an RTCP reporting group will all see identical
network conditions. Accordingly, one might therefore think that it
doesn't matter which SSRC in the reporting group sends the RTP/AVPF
feedback or codec control messages. There might be, however, cases
where the sender of the feedback/codec control message has semantic
importance, or when only a subset of the members of an RTCP reporting
group might want to send RTP/AVPF feedback or a codec control message
in response to a particular event. For example, an RTP video sender
might choose to treat packet loss feedback received from SSRCs known
to be audio receivers with less urgency than feedback that it receives
from video receivers when deciding what packets to retransmit, and a
multimedia receiver using reporting groups might want to choose the
outgoing SSRC for feedback packets to reflect this.Each member of an RTCP reporting group SHOULD therefore send
RTP/AVPF feedback/codec control messages independently of the other
members of the reporting group, to respect the semantic meaning of the
message sender. The suppression rules of will
ensure that only a single copy of each feedback packet is (typically)
generated, even if several members of a reporting group send the same
feedback. When an endpoint knows that several members of its RTCP
reporting group will be sending identical feedback, and that the
sender of the feedback is not semantically important, then that
endpoint MAY choose to send all its feedback from the reporting source
and deterministically suppress feedback packets generated by the other
sources in the reporting group.It is important to note that the RTP/AVPF timing rules operate on a
per-SSRC basis. Using a single reporting source to send all feedback
for a reporting group will hence limit the amount of feedback that can
be sent to that which can be sent by one SSRC. If this limit is a
problem, then the reporting group can allow each of its members to
send its own feedback, using its own SSRC.If the RTP/AVPF feedback messages or codec control requests are
sent as compound RTCP packets, then those compound RTCP packets MUST
include either an RTCP RGRS packet or an RTCP SDES RGRP item,
depending on whether they are sent by the reporting source or a
non-reporting source in the RTCP reporting group respectively. The
contents of non-compound RTCP feedback or codec control messages are
not affected by the use of RTCP reporting groups.Interactions with RTCP Extended Report (XR) PacketsWhen using RTCP Extended Reports (XR) with
RTCP reporting groups, it is RECOMMENDED that the reporting source is
used to send the RTCP XR packets. If multiple reporting sources are in
use, the reporting source that sends the SR/RR packets that relate to
a particular remote SSRC SHOULD send the RTCP XR reports about that
SSRC. This is motivated as one commonly combine the RTCP XR metrics
with the regular report block to more fully understand the situation.
Receiving these blocks in different compound packets reduces their
value as the measuring intervals are not synchronized in those
cases.Some RTCP XR report blocks are specific to particular types of
media, and might be relevant to only some members of a reporting
group. For example, it would make no sense for an SSRC that is
receiving video to send a VoIP metric RTCP XR report block. Such media
specific RTCP XR report blocks MUST be sent by the SSRC to which they
are relevant, and MUST NOT be included in the common report sent by
the reporting source. This might mean that some SSRCs send RTCP XR
packets in compound RTCP packets that contain an empty RTCP SR/RR
packet, and that the time period covered by the RTCP XR packet is
different to that covered by the RTCP SR/RR packet. If it is important
that the RTCP XR packet and RTCP SR/RR packet cover the same time
period, then that source SHOULD be removed from the RTCP reporting
group, and send standard RTCP packets instead.Middlebox ConsiderationsMany different types of middlebox are used with RTP. RTCP reporting
groups are potentially relevant to those types of RTP middlebox that
have their own SSRCs and generate RTCP reports for the traffic they
receive. RTP middleboxes that do not have their own SSRC, and that
don't send RTCP reports on the traffic they receive, cannot use the
RTCP reporting groups extension, since they generate no RTCP reports
to group.An RTP middlebox that has several SSRCs of its own can use the RTCP
reporting groups extension to group the RTCP reports it generates.
This can occur, for example, if a middlebox is acting as an RTP mixer
for both audio and video flows that are multiplexed onto a single RTP
session, where the middlebox has one SSRC for the audio mixer and one
for the video mixer part, and when the middlebox wants to avoid cross
reporting between audio and video.A middlebox cannot use the RTCP reporting groups extension to group
RTCP packets from the SSRCs that it is forwarding. It can, however,
group the RTCP packets from the SSRCs it is forwarding into compound
RTCP packets following the rules in and . If the middlebox is
using RTCP reporting groups for its own SSRCs, it MAY include RTCP
packets from the SSRCs that it is forwarding as part of the compound
RTCP packets its reporting source generates.A middlebox that forwards RTCP SR or RR packets sent by members of
a reporting group MUST forward the corresponding RTCP SDES RGRP items,
as described in . A middlebox that forwards
RTCP SR or RR packets sent by member of a reporting group MUST also
forward the corresponding RTCP RGRS packets, as described in . Failure to forward these packets can cause
compatibility problems, as described in .If a middlebox rewrites SSRC values in the RTP and RTCP packets
that it is forwarding, then it MUST make the corresponding changes in
RTCP SDES packets containing RGRP items and in RTCP RGRS packets, to
allow them to be associated with the rewritten SSRCs.SDP Signalling for Reporting GroupsThis document defines the "a=rtcp-rgrp" Session Description Protocol (SDP) attribute
to indicate if the session participant is capable of supporting RTCP
Reporting Groups for applications that use SDP for configuration of
RTP sessions. It is a property attribute, and hence takes no value.
TESTING:
See
and
.
A participant that proposes the use of RTCP Reporting
Groups SHALL itself support the reception of RTCP Reporting Groups.
The formal definition of this attribute is:
Name:
rtcp-rgrp
Value:
Usage Level:
session, media
Type of attribute:
Media or session level
Charset Dependent:
no
Example:
a=rtcp-rgrp
When using SDP Offer/Answer , the following
procedures are to be used:
Generating the initial SDP offer: If the offerer supports the
RTCP reporting group extensions, and is willing to accept RTCP
packets containing those extensions, then it MUST include an
"a=rtcp-rgrp" attribute in the initial offer. If the offerer does
not support RTCP reporting groups extensions, or is not willing to
accept RTCP packets containing those extensions, then it MUST NOT
include the "a=rtcp-rgrp" attribute in the offer.
Generating the SDP answer: If the SDP offer contains an
"a=rtcp-rgrp" attribute, and if the answerer supports RTCP
reporting groups and is willing to receive RTCP packets using the
RTCP reporting groups extensions, then the answerer MAY include an
"a=rtcp-rgrp" attribute in the answer and MAY send RTCP packets
containing the RTCP reporting groups extensions. If the offer does
not contain an "a=rtcp-rgrp" attribute, or if the offer does
contain such an attribute but the answerer does not wish to accept
RTCP packets using the RTCP reporting groups extensions, then the
answer MUST NOT include an "a=rtcp-rgrp" attribute.
Offerer Processing of the SDP Answer: If the SDP answer
contains an "a=rtcp-rgrp" attribute, and the corresponding offer
also contained an "a=rtcp-rgrp" attribute, then the offerer MUST
be prepared to accept and process RTCP packets that contain the
reporting groups extension, and MAY send RTCP packets that contain
the reporting groups extension. If the SDP answer contains an
"a=rtcp-rgrp" attribute, but the corresponding offer did not
contain the "a=rtcp-rgrp" attribute, then the offerer MUST reject
the call. If the SDP answer does not contain an "a=rtcp-rgrp"
attribute, then the offerer MUST NOT send packets containing the
RTCP reporting groups extensions, and does not need to process
packet containing the RTCP reporting groups extensions.
In declarative usage of SDP, such as the Real Time Streaming Protocol (RTSP) and the
Session Announcement Protocol (SAP), the
presence of the attribute indicates that the session participant MAY
use RTCP Reporting Groups in its RTCP transmissions. An implementation
that doesn't explicitly support RTCP Reporting Groups MAY join a RTP
session as long as it has been verified that the implementation
doesn't suffer from the problems discussed in .Properties of RTCP Reporting GroupsThis section provides additional information on what the resulting
properties are with the design specified in . The content of this section is
non-normative.Bandwidth Benefits of RTCP Reporting GroupsTo understand the benefits of RTCP reporting groups, consider a
scenario in which the two endpoints in a session each have a hundred
sources, of which eight each are sending within any given reporting
interval.For ease of analysis, we can make the simplifying approximation
that the duration of the RTCP reporting interval is equal to the total
size of the RTCP packets sent during an RTCP interval, divided by the
RTCP bandwidth. (This will be approximately true in scenarios where
the bandwidth is not so high that the minimum RTCP interval is
reached.) For further simplification, we can assume RTCP senders are
following the recommendations regarding Compound RTCP Packets in ; thus, the per-packet
transport-layer overhead will be small relative to the RTCP data.
Thus, only the actual RTCP data itself need be considered.In a report interval in this scenario, there will, as a baseline,
be 200 SDES packets, 184 RR packets, and 16 SR packets. This amounts
to approximately 6.5 kB of RTCP per report interval, assuming 16-byte
CNAMEs and no other SDES information.Using the original
everyone-reports-on-every-sender feedback rules, each of the 184
receivers will send 16 report blocks, and each of the 16 senders will
send 15. This amounts to approximately 76 kB of report block traffic
per interval; 92% of RTCP traffic consists of report blocks.If reporting groups are used, however, there is only 0.4 kB of
reports per interval, with no loss of useful information.
Additionally, there will be (assuming 16-byte RGRPs, and a single
reporting source per reporting group) an additional 2.4 kB per cycle
of RGRP SDES items and RGRS packets. Put another way, the unmodified
reporting interval is approximately 9 times
longer than if reporting groups are in use.Compatibility of RTCP Reporting GroupsThe RTCP traffic generated by receivers using RTCP Reporting Groups
might appear, to observers unaware of these semantics, to be generated
by receivers who are experiencing a network disconnection, as the
non-reporting sources appear not to be receiving a given sender at
all.This could be a potentially critical problem for such a sender
using RTCP for congestion control, as such a sender might think that
it is sending so much traffic that it is causing complete congestion
collapse.However, such an interpretation of the session statistics would
require a fairly sophisticated RTCP analysis. Any receiver of RTCP
statistics which is just interested in information about itself needs
to be prepared that any given reception report might not contain
information about a specific media source, because reception reports
in large conferences can be round-robined.Thus, it is unclear to what extent such backward compatibility
issues would actually cause trouble in practice.Security ConsiderationsThe security considerations of and apply. If the RTP/AVPF
profile is in use, then the security considerations of (and , if used) also apply.
If RTCP XR is used, the security consideration of and any XR report blocks used also apply.The RTCP SDES RGRP item is vulnerable to malicious modifications
unless integrity protected is used. A modification of this item's length
field cause the parsing of the RTCP packet in which it is contained to
fail. Depending on the implementation, parsing of the full compound RTCP
packet can also fail causing the whole packet to be discarded. A
modification to the value of this SDES item would make the receiver of
the report think that the sender of the report was a member of a
different RTCP reporting group. This will potentially create an
inconsistency, when the RGRS reports the source as being in the same
reporting group as another source with another reporting group
identifier. What impact on a receiver implementation such
inconsistencies would have are difficult to fully predict. One case is
when congestion control or other adaptation mechanisms are used, an
inconsistent report can result in a media sender to reduce its bit-rate.
However, a direct modification of the receiver report or a feedback
message itself would be a more efficient attack, and equally costly to
perform.The new RGRS RTCP Packet type is very simple. The common RTCP packet
type header shares the security risks with previous RTCP packet types.
Errors or modification of the length field can cause the full compound
packet to fail header validation (see ) resulting in the whole compound RTCP packet being
discarded. Modification of the SC or P fields would cause inconsistency
when processing the RTCP packet, likely resulting it being classified as
invalid. A modification of the PT field would cause the packet being
interpreted under some other packet type's rules. In such case the
result might be more or less predictable but packet type specific.
Modification of the SSRC of packet sender would attribute this packet to
another sender. Resulting in a receiver believing the reporting group
applies also for this SSRC, if it exists. If it doesn't exist, unless
also corresponding modifications are done on a SR/RR packet and a SDES
packet the RTCP packet SHOULD be discarded. If consistent changes are
done, that could be part of a resource exhaustion attack on a receiver
implementation. Modification of the "List of SSRCs for the Reporting
Source(s)" would change the SSRC the receiver expect to report on behalf
of this SSRC. If that SSRC exist, that could potentially change the
report group used for this SSRC. A change to another reporting group
belonging to another endpoint is likely detectable as there would be a
mismatch between the SSRC of the packet sender's endpoint information,
transport addresses, SDES CNAME etc and the corresponding information
from the reporting group indicated.In general the reporting group is providing limited impacts attacks.
The most significant result from an deliberate attack would be to cause
the information to be discarded or be inconsistent, including discard of
all RTCP packets that are modified. This causes a lack of information at
any receiver entity, possibly disregarding the endpoints participation
in the session.To protect against this type of attacks from external non trusted
entities, integrity and source authentication SHOULD be applied. This
can be done, for example, by using SRTP
with appropriate key-management, other options exist as discussed in
RTP Security Options.The Report Group Identifier has a potential privacy impacting
properties. If this would be generated by an implementation in such a
way that is long term stable or predictable, it could be used for
tracking a particular end-point. Therefore it is RECOMMENDED that it be
generated as a short-term persistent RGRP, following the rules for
short-term persistent CNAMEs in . The rest of
the information revealed, i.e. the SSRCs, the size of reporting group
and the number of reporting sources in a reporting group is of less
sensitive nature, considering that the SSRCs and the communication would
anyway be revealed without this extension. By encrypting the report
group extensions the SSRC values would preserved confidential, but can
still be revealed if SRTP is used. The
size of the reporting groups and number of reporting sources are likely
determinable from analysis of the packet pattern and sizes. However,
this information appears to have limited value.IANA Considerations(Note to the RFC-Editor: in the following, please replace "TBA" with
the IANA-assigned value, and "XXXX" with the number of this document,
then delete this note)The IANA is requested to register one new RTCP SDES items in the
"RTCP SDES Item Types" registry, as follows:
IANA "RTCP SDES Item Types" Registry
Value
Abbrev
Name
Reference
TBA
RGRP
Reporting Group Identifier
RFC XXXX
The definition of the RTCP SDES RGRP item is given in of this memo.The IANA is also requested to register one new RTCP packet type in
the "RTCP Control Packet Types (PT)" Registry as follows:
IANA "RTCP Control Packet Types (PT)" Registry
Value
Abbrev
Name
Reference
TBA
RGRS
Reporting Group Reporting Sources
RFC XXXX
The definition of the RTCP RGRS packet type is given in of this memo.The IANA is also requested to register one new SDP attribute:
SDP Attribute ("att-field"):
Attribute name:
rtcp-rgrp
Long form:
RTCP Reporting Groups
Long form:
RTCP Reporting Groups
Type of name:
att-field
Type of attribute:
Media or session level
Subject to charset:
No
Purpose:
Negotiate or configure the use of the RTCP
Reporting Group Extension.
Reference:
RFC XXXX
Values:
None
The definition of the "a=rtcp-rgrp" SDES attribute is given in of this memo.ReferencesNormative ReferencesSending Multiple RTP Streams in a Single RTP SessionThis memo expands and clarifies the behaviour of Real-time
Transport Protocol (RTP) endpoints that use multiple synchronization sources
(SSRCs). This occurs, for example, when an endpoint sends multiple RTP
streams in a single RTP session. This memo updates RFC 3550 with regards to
handling multiple SSRCs per endpoint in RTP sessions, with a particular focus
on RTCP behaviour. It also updates RFC 4585 to update and clarify the
calculation of the timeout of SSRCs and the inclusion of feedback
messages.A Framework for SDP Attributes when MultiplexingThe purpose of this specification is to provide a framework for
analyzing the multiplexing characteristics of Session Description Protocol
(SDP) attributes when SDP is used to negotiate the usage of single 5-tuple for
sending and receiving media associated with multiple media descriptions. This
specification also categorizes the existing SDP attributes based on the
framework described herein.Session Description Protocol (SDP) Offer/Answer procedures for
Interactive Connectivity Establishment (ICE)This document describes Session Description Protocol (SDP) Offer/
Answer procedures for carrying out Interactive Connectivity Establishment
(ICE) between the agents. This document obsoletes RFC 5245.Informative References