AVTCORE J. Lennox
Internet-Draft Vidyo
Updates: 3550, 4585 (if approved) M. Westerlund
Intended status: Standards Track Ericsson
Expires: January 7, 2016 Q. Wu
Huawei
C. Perkins
University of Glasgow
July 6, 2015
Sending Multiple Media Streams in a Single RTP Session
draft-ietf-avtcore-rtp-multi-stream-08
Abstract
This 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
media 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.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 7, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Use Cases For Multi-Stream Endpoints . . . . . . . . . . . . 3
3.1. Endpoints with Multiple Capture Devices . . . . . . . . . 3
3.2. Multiple Media Types in a Single RTP Session . . . . . . 4
3.3. Multiple Stream Mixers . . . . . . . . . . . . . . . . . 4
3.4. Multiple SSRCs for a Single Media Source . . . . . . . . 4
4. Use of RTP by endpoints that send multiple media streams . . 5
5. Use of RTCP by Endpoints that send multiple media streams . . 5
5.1. RTCP Reporting Requirement . . . . . . . . . . . . . . . 5
5.2. Initial Reporting Interval . . . . . . . . . . . . . . . 6
5.3. Aggregation of Reports into Compound RTCP Packets . . . . 7
5.3.1. Maintaining AVG_RTCP_SIZE . . . . . . . . . . . . . . 7
5.3.2. Scheduling RTCP when Aggregating Multiple SSRCs . . . 9
5.4. Use of RTP/AVPF or RTP/SAVPF Feedback . . . . . . . . . . 11
5.4.1. Choice of SSRC for Feedback Packets . . . . . . . . . 11
5.4.2. Scheduling an RTCP Feedback Packet . . . . . . . . . 12
6. Adding and Removing SSRCs . . . . . . . . . . . . . . . . . . 14
6.1. Adding RTP Streams . . . . . . . . . . . . . . . . . . . 14
6.2. Removing RTP Streams . . . . . . . . . . . . . . . . . . 15
7. RTCP Considerations for Streams with Disparate Rates . . . . 16
7.1. Timing out SSRCs . . . . . . . . . . . . . . . . . . . . 17
7.1.1. Problems with the RTP/AVPF T_rr_interval Parameter . 18
7.1.2. Avoiding Premature Timeout . . . . . . . . . . . . . 19
7.1.3. Interoperability Between RTP/AVP and RTP/AVPF . . . . 19
7.1.4. Updated SSRC Timeout Rules . . . . . . . . . . . . . 20
7.2. Tuning RTCP transmissions . . . . . . . . . . . . . . . . 20
7.2.1. RTP/AVP and RTP/SAVP . . . . . . . . . . . . . . . . 21
7.2.2. RTP/AVPF and RTP/SAVPF . . . . . . . . . . . . . . . 22
8. Security Considerations . . . . . . . . . . . . . . . . . . . 24
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
11.1. Normative References . . . . . . . . . . . . . . . . . . 24
11.2. Informative References . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
At the time the Real-Time Transport Protocol (RTP) [RFC3550] was
originally designed, and for quite some time after, endpoints in RTP
sessions typically only transmitted a single media stream, and thus
used a single synchronization source (SSRC) per RTP session, where
separate RTP sessions were typically used for each distinct media
type. Recently, however, a number of scenarios have emerged in which
endpoints wish to send multiple RTP media streams, distinguished by
distinct RTP synchronization source (SSRC) identifiers, in a single
RTP session. These are outlined in Section 3. Although the initial
design of RTP did consider such scenarios, the specification was not
consistently written with such use cases in mind. The specification
is thus somewhat unclear in places.
This memo updates [RFC3550] to clarify behaviour in use cases where
endpoints use multiple SSRCs. It also updates [RFC4585] to resolve
problems with regards to timeout of inactive SSRCs, and to clarify
behaviour around inclusion of feedback messages.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119] and indicate requirement levels for compliant
implementations.
3. Use Cases For Multi-Stream Endpoints
This section discusses several use cases that have motivated the
development of endpoints that sends RTP data using multiple SSRCs in
a single RTP session.
3.1. Endpoints with Multiple Capture Devices
The most straightforward motivation for an endpoint to send multiple
simultaneous RTP streams in a single RTP session is when an endpoint
has multiple capture devices, and hence can generate multiple media
sources, of the same media type and characteristics. For example,
telepresence systems of the type described by the CLUE Telepresence
Framework [I-D.ietf-clue-framework] often have multiple cameras or
microphones covering various areas of a room, and hence send several
RTP streams of each type within a single RTP session.
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3.2. Multiple Media Types in a Single RTP Session
Recent work has updated RTP
[I-D.ietf-avtcore-multi-media-rtp-session] and SDP
[I-D.ietf-mmusic-sdp-bundle-negotiation] to remove the historical
assumption in RTP that media sources of different media types would
always be sent on different RTP sessions. In this work, a single
endpoint's audio and video RTP media streams (for example) are
instead sent in a single RTP session to reduce the number of
transport layer flows used.
3.3. Multiple Stream Mixers
There are several RTP topologies which can involve a central device
that itself generates multiple RTP media streams in a session. An
example is a mixer providing centralized compositing for a multi-
capture scenario like that described in Section 3.1. In this case,
the centralized node is behaving much like a multi-capturer endpoint,
generating several similar and related sources.
A more complex example is the selective forwarding middlebox,
described in Section 3.7 of [I-D.ietf-avtcore-rtp-topologies-update].
This is a middlebox that receives media streams from several
endpoints, and then selectively forwards modified versions of some
RTP streams toward the other endpoints to which it is connected. For
each connected endpoint, a separate media source appears in the
session for every other source connected to the middlebox,
"projected" from the original streams, but at any given time many of
them can appear to be inactive (and thus are receivers, not senders,
in RTP). This sort of device is closer to being an RTP mixer than an
RTP translator, in that it terminates RTCP reporting about the mixed
streams, and it can re-write SSRCs, timestamps, and sequence numbers,
as well as the contents of the RTP payloads, and can turn sources on
and off at will without appearing to be generating packet loss. Each
projected stream will typically preserve its original RTCP source
description (SDES) information.
3.4. Multiple SSRCs for a Single Media Source
There are also several cases where multiple SSRCs can be used to send
data from a single media source within a single RTP session. These
include, but are not limited to, transport robustness tools, such as
the RTP retransmission payload format [RFC4588], that require one
SSRC to be used for the media data and another SSRC for the repair
data. Similarly, some layered media encoding schemes, for example
H.264 SVC [RFC6190], can be used in a configuration where each layer
is sent using a different SSRC within a single RTP session.
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4. Use of RTP by endpoints that send multiple media streams
RTP is inherently a group communication protocol. Each endpoint in
an RTP session will use one or more SSRCs, as will some types of RTP
level middlebox. Accordingly, unless restrictions on the number of
SSRCs have been signalled, RTP endpoints can expect to receive RTP
data packets sent using with a number of different SSRCs, within a
single RTP session. This can occur irrespective of whether the RTP
session is running over a point-to-point connection or a multicast
group, since middleboxes can be used to connect multiple transport
connections together into a single RTP session (the RTP session is
defined by the shared SSRC space, not by the transport connections).
Furthermore, if RTP mixers are used, some SSRCs might only be visible
in the contributing source (CSRC) list of an RTP packet and in RTCP,
and might not appear directly as the SSRC of an RTP data packet.
Every RTP endpoint will have an allocated share of the available
session bandwidth, as determined by signalling and congestion
control. The endpoint MUST keep its total media sending rate within
this share. However, endpoints that send multiple media streams do
not necessarily need to subdivide their share of the available
bandwidth independently or uniformly to each media stream and its
SSRCs. In particular, an endpoint can vary the bandwidth allocation
to different streams depending on their needs, and can dynamically
change the bandwidth allocated to different SSRCs (for example, by
using a variable rate codec), provided the total sending rate does
not exceed its allocated share. This includes enabling or disabling
media streams, or their redundancy streams, as more or less bandwidth
becomes available.
5. Use of RTCP by Endpoints that send multiple media streams
The RTP Control Protocol (RTCP) is defined in Section 6 of [RFC3550].
The description of the protocol is phrased in terms of the behaviour
of "participants" in an RTP session, under the assumption that each
endpoint is a participant with a single SSRC. However, for correct
operation in cases where endpoints have multiple SSRC values, the
specification MUST be interpreted as each SSRC counting as a separate
participant in the RTP session, so that an endpoint that has multiple
SSRCs counts as multiple participants.
5.1. RTCP Reporting Requirement
An RTP endpoint that has multiple SSRCs MUST treat each SSRC as a
separate participant in the RTP session. Each SSRC will maintain its
own RTCP-related state information, and hence will have its own RTCP
reporting interval that determines when it sends RTCP reports. If
the mechanism in [I-D.ietf-avtcore-rtp-multi-stream-optimisation] is
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not used, then each SSRC will send RTCP reports for all other SSRCs,
including those co-located at the same endpoint.
If the endpoint has some SSRCs that are sending data and some that
are only receivers, then they will receive different shares of the
RTCP bandwidth and calculate different base RTCP reporting intervals.
Otherwise, all SSRCs at an endpoint will calculate the same base RTCP
reporting interval. The actual reporting intervals for each SSRC are
randomised in the usual way, but reports can be aggregated as
described in Section 5.3.
5.2. Initial Reporting Interval
When a participant joins a unicast session, the following text from
Section 6.2 of [RFC3550] is relevant: "For unicast sessions... the
delay before sending the initial compound RTCP packet MAY be zero."
The basic assumption is that this also ought to apply in the case of
multiple SSRCs. Caution has to be exercised, however, when an
endpoint (or middlebox) with a large number of SSRCs joins a unicast
session, since immediate transmission of many RTCP reports can create
a significant burst of traffic, leading to transient congestion and
packet loss due to queue overflows.
To ensure that the initial burst of traffic generated by an RTP
endpoint is no larger than would be generated by a TCP connection, an
RTP endpoint MUST NOT send more than four compound RTCP packets with
zero initial delay when it joins an RTP session, independently of the
number of SSRCs used by the endpoint. Each of those initial compound
RTCP packets MAY include aggregated reports from multiple SSRCs,
provided the total compound RTCP packet size does not exceed the MTU,
and the avg_rtcp_size is maintained as in Section 5.3.1. Aggregating
reports from several SSRCs in the initial compound RTCP packets
allows a substantial number of SSRCs to report immediately.
Endpoints SHOULD prioritize reports on SSRCs that are likely to be
most immediately useful, e.g., for SSRCs that are initially senders.
An endpoint that needs to report on more SSRCs than will fit into the
four compound RTCP reports that can be sent immediately MUST send the
other reports later, following the usual RTCP timing rules including
timer reconsideration. Those reports MAY be aggregated as described
in Section 5.3.
Note: The above is based on an TCP initial window of 4 packets,
not the larger TCP initial windows for which there is an ongoing
experiment. The reason for this is a desire to be conservative,
since an RTP endpoint will also in many cases start sending RTP
data packets at the same time as these initial RTCP packets are
sent.
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5.3. Aggregation of Reports into Compound RTCP Packets
As outlined in Section 5.1, an endpoint with multiple SSRCs has to
treat each SSRC as a separate participant when it comes to sending
RTCP reports. This will lead to each SSRC sending a compound RTCP
packet in each reporting interval. Since these packets are coming
from the same endpoint, it might reasonably be expected that they can
be aggregated to reduce overheads. Indeed, Section 6.1 of [RFC3550]
allows RTP translators and mixers to aggregate packets in similar
circumstances:
"It is RECOMMENDED that translators and mixers combine individual
RTCP packets from the multiple sources they are forwarding into
one compound packet whenever feasible in order to amortize the
packet overhead (see Section 7). An example RTCP compound packet
as might be produced by a mixer is shown in Fig. 1. If the
overall length of a compound packet would exceed the MTU of the
network path, it SHOULD be segmented into multiple shorter
compound packets to be transmitted in separate packets of the
underlying protocol. This does not impair the RTCP bandwidth
estimation because each compound packet represents at least one
distinct participant. Note that each of the compound packets MUST
begin with an SR or RR packet."
This allows RTP translators and mixers to generate compound RTCP
packets that contain multiple SR or RR packets from different SSRCs,
as well as any of the other packet types. There are no restrictions
on the order in which the RTCP packets can occur within the compound
packet, except the regular rule that the compound RTCP packet starts
with an SR or RR packet. Due to this rule, correctly implemented RTP
endpoints will be able to handle compound RTCP packets that contain
RTCP packets relating to multiple SSRCs.
Accordingly, endpoints that use multiple SSRCs can aggregate the RTCP
packets sent by their different SSRCs into compound RTCP packets,
provided 1) the resulting compound RTCP packets begin with an SR or
RR packet; 2) they maintain the average RTCP packet size as described
in Section 5.3.1; and 3) they schedule packet transmission and manage
aggregation as described in Section 5.3.2.
5.3.1. Maintaining AVG_RTCP_SIZE
The RTCP scheduling algorithm in [RFC3550] works on a per-SSRC basis.
Each SSRC sends a single compound RTCP packet in each RTCP reporting
interval. When an endpoint uses multiple SSRCs, it is desirable to
aggregate the compound RTCP packets sent by its SSRCs, reducing the
overhead by forming a larger compound RTCP packet. This aggregation
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can be done as described in Section 5.3.2, provided the average RTCP
packet size calculation is updated as follows.
Participants in an RTP session update their estimate of the average
RTCP packet size (avg_rtcp_size) each time they send or receive an
RTCP packet (see Section 6.3.3 of [RFC3550]). When a compound RTCP
packet that contains RTCP packets from several SSRCs is sent or
received, the avg_rtcp_size estimate for each SSRC that is reported
upon is updated using div_packet_size rather than the actual packet
size:
avg_rtcp_size = (1/16) * div_packet_size + (15/16) * avg_rtcp_size
where div_packet_size is packet_size divided by the number of SSRCs
reporting in that compound packet. The number of SSRCs reporting in
a compound packet is determined by counting the number of different
SSRCs that are the source of Sender Report (SR) or Receiver Report
(RR) RTCP packets within the compound RTCP packet. Non-compound RTCP
packets (i.e., RTCP packets that do not contain an SR or RR packet
[RFC5506]) are considered to report on a single SSRC.
An SSRC that doesn't follow the above rule, and instead uses the full
RTCP compound packet size to calculate avg_rtcp_size, will derive an
RTCP reporting interval that is overly large by a factor that is
proportional to the number of SSRCs aggregated into compound RTCP
packets and the size of set of SSRCs being aggregated relative to the
total number of participants. This increased RTCP reporting interval
can cause premature timeouts if it is more than five times the
interval chosen by the SSRCs that understand compound RTCP that
aggregate reports from many SSRCs. A 1500 octet MTU can fit five
typical size reports into a compound RTCP packet, so this is a real
concern if endpoints aggregate RTCP reports from multiple SSRCs.
The issue raised in the previous paragraph is mitigated by the
modification in timeout behaviour specified in Section 7.1.2 of this
memo. This mitigation is in place in those cases where the RTCP
bandwidth is sufficiently high that an endpoint, using avg_rtcp_size
calculated without taking into account the number of reporting SSRCs,
can transmit more frequently than approximately every 5 seconds.
Note, however, that the non-modified endpoint's RTCP reporting is
still negatively impacted even if the premature timeout of its SSRCs
are avoided. If compatibility with non-updated endpoints is a
concern, the number of reports from different SSRCs aggregated into a
single compound RTCP packet SHOULD either be limited to two reports,
or aggregation ought not used at all. This will limit the non-
updated endpoint's RTCP reporting interval to be no larger than twice
the RTCP reporting interval that would be chosen by an endpoint
following this specification.
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5.3.2. Scheduling RTCP when Aggregating Multiple SSRCs
This section revises and extends the behaviour defined in Section 6.3
of [RFC3550], and in Section 3.5.3 of [RFC4585] if the RTP/AVPF
profile or the RTP/SAVPF profile is used, regarding actions to take
when scheduling and sending RTCP packets where multiple reporting
SSRCs are aggregating their RTCP packets into the same compound RTCP
packet. These changes to the RTCP scheduling rules are needed to
maintain important RTCP timing properties, including the inter-packet
distribution, and the behaviour during flash joins and other changes
in session membership.
The variables tn, tp, tc, T, and Td used in the following are defined
in Section 6.3 of [RFC3550]. The variable T_rr_last is defined in
[RFC4585].
Each endpoint MUST schedule RTCP transmission independently for each
of its SSRCs using the regular calculation of tn for the RTP profile
being used. Each time the timer tn expires for an SSRC, the endpoint
MUST perform RTCP timer reconsideration and, if applicable, T_rr_int
based suppression. If the result indicates that a compound RTCP
packet is to be sent by that SSRC, and the transmission is not an
early RTCP packet [RFC4585], then the endpoint SHOULD try to
aggregate RTCP packets of additional SSRCs that are scheduled in the
future into the compound RTCP packet before it is sent. The reason
to limit or not aggregate at due to backwards compatibility reasons
was discussed earlier in Section 5.3.1.
Aggregation proceeds as follows. The endpoint selects the SSRC that
has the smallest tn value after the current time, tc, and prepares
the RTCP packets that SSRC would send if its timer tn expired at tc.
If those RTCP packets will fit into the compound RTCP packet that is
being generated, taking into account the path MTU and the previously
added RTCP packets, then they are added to the compound RTCP packet;
otherwise they are discarded. This process is repeated for each
SSRC, in order of increasing tn, until the compound RTCP packet is
full, or all SSRCs have been aggregated. At that point, the compound
RTCP packet is sent.
When the compound RTCP packet is sent, the endpoint MUST update tp,
tn, and T_rr_last (if applicable) for each SSRC that was included.
These variables are updated as follows:
a. For the first SSRC that reported in the compound RTCP packet, set
the effective transmission time, tt, of that SSRC to tc.
b. For each additional SSRC that reported in the compound RTCP
packet, calculate the transmission time that SSRC would have had
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if it had not been aggregated into the compound RTCP packet.
This is derived by taking tn for that SSRC, then performing
reconsideration and updating tn until tp + T <= tn. Once this is
done, set the effective transmission time, tt, for that SSRC to
the calculated value of tn. If the RTP/AVPF profile or the RTP/
SAVPF profile is being used, then T_rr_int based suppression MUST
NOT be used in this calculation.
c. Calculate average effective transmission time, tt_avg, for the
compound RTCP packet based on the tt values for all SSRCs sent in
the compound RTCP packet. Set tp for each of the SSRCs sent in
the compound RTCP packet to tt_avg. If the RTP/AVPF profile or
the RTP/SAVPF profile is being used, set T_tt_last for each SSRC
sent in the compound RTCP packet to tt_avg.
d. For each of the SSRCs sent in the compound RTCP packet, calculate
new tn values based on the updated parameters and the usual RTCP
timing rules, and reschedule the timers.
When using the RTP/AVPF profile or the RTP/SAVPF profile, the above
mechanism only attempts to aggregate RTCP packets when the compound
RTCP packet to be sent is not an early RTCP packet, and hence the
algorithm in Section 3.5.3 of [RFC4585] will control RTCP scheduling.
If T_rr_interval == 0, or if T_rr_interval != 0 and option 1, 2a, or
2b of the algorithm are chosen, then the above mechanism updates the
necessary variables. However, if the transmission is suppressed per
option 2c of the algorithm, then tp is updated to tc as aggregation
has not taken place.
Reverse reconsideration MUST be performed following Section 6.3.4 of
[RFC3550]. In some cases, this can lead to the value of tp after
reverse reconsideration being larger than tc. This is not a problem,
and has the desired effect of proportionally pulling the tp value
towards tc (as well as tn) as the group size shrinks in direct
proportion the reduced group size.
The above algorithm has been shown in simulations to maintain the
inter-RTCP packet transmission time distribution for each SSRC, and
to consume the same amount of bandwidth as non-aggregated RTCP
packets. With this algorithm the actual transmission interval for an
SSRC triggering an RTCP compound packet transmission is following the
regular transmission rules. The value tp is set to somewhere in the
interval [0,1.5/1.21828*Td] ahead of tc. The actual value is average
of one instance of tc and the randomized transmission times of the
additional SSRCs, thus the lower range of the interval is more
probable. This compensates for the bias that is otherwise introduced
by picking the shortest tn value out of the N SSRCs included in
aggregate.
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The algorithm also handles the cases where the number of SSRCs that
can be included in an aggregated packet varies. An SSRC that
previously was aggregated and fails to fit in a packet still has its
own transmission scheduled according to normal rules. Thus, it will
trigger a transmission in due time, or the SSRC will be included in
another aggregate. The algorithm's behaviour under SSRC group size
changes is as follows:
RTP sessions where the number of SSRC are growing: When the group
size is growing, Td grows in proportion to the number of new SSRCs
in the group. When reconsideration is performed due to expiry of
the tn timer, that SSRC will reconsider the transmission and with
a certain probability reschedule the tn timer. This part of the
reconsideration algorithm is only impacted by the above algorithm
by having tp values that were in the future instead of set to the
time of the actual last transmission at the time of updating tp.
RTP sessions where the number of SSRC are shrinking: When the group
shrinks, reverse reconsideration moves the tp and tn values
towards tc proportionally to the number of SSRCs that leave the
session compared to the total number of participants when they
left. The setting of the tp value forward in time related to the
tc could be believed to have negative effect. However, the reason
for this setting is to compensate for bias caused by picking the
shortest tn out of the N aggregated. This bias remains over a
reduction in the number of SSRCs. The reverse reconsideration
compensates the reduction independently of aggregation being used
or not. The negative effect that can occur on removing an SSRC is
that the most favourable tn belonged to the removed SSRC. The
impact of this is limited to delaying the transmission, in the
worst case, one reporting interval.
In conclusion the investigations performed has found no significant
negative impact on the scheduling algorithm.
5.4. Use of RTP/AVPF or RTP/SAVPF Feedback
This section discusses the transmission of RTP/AVPF feedback packets
when the transmitting endpoint has multiple SSRCs. The guidelines in
this section also apply to endpoints using the RTP/SAVPF profile.
5.4.1. Choice of SSRC for Feedback Packets
When an RTP/AVPF endpoint has multiple SSRCs, it can choose what SSRC
to use as the source for the RTCP feedback packets it sends. Several
factors can affect that choice:
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o RTCP feedback packets relating to a particular media type SHOULD
be sent by an SSRC that receives that media type. For example,
when audio and video are multiplexed onto a single RTP session,
endpoints will use their audio SSRC to send feedback on the audio
received from other participants.
o RTCP feedback packets and RTCP codec control messages that are
notifications or indications regarding RTP data processed by an
endpoint MUST be sent from the SSRC used for that RTP data. This
includes notifications that relate to a previously received
request or command [RFC4585][RFC5104].
o If separate SSRCs are used to send and receive media, then the
corresponding SSRC SHOULD be used for feedback, since they have
differing RTCP bandwidth fractions. This can also affect the
consideration if the SSRC can be used in immediate mode or not.
o Some RTCP feedback packet types require consistency in the SSRC
used. For example, if a TMMBR limitation [RFC5104] is set by an
SSRC, the same SSRC needs to be used to remove the limitation.
o If several SSRCs are suitable for sending feedback, if might be
desirable to use an SSRC that allows the sending of feedback as an
early RTCP packet.
When an RTCP feedback packet is sent as part of a compound RTCP
packet that aggregates reports from multiple SSRCs, there is no
requirement that the compound packet contains an SR or RR packet
generated by the sender of the RTCP feedback packet. For reduced-
size RTCP packets, aggregation of RTCP feedback packets from multiple
sources is not limited further than Section 4.2.2 of [RFC5506].
5.4.2. Scheduling an RTCP Feedback Packet
When an SSRC has a need to transmit a feedback packet in early mode
it MUST schedule that packet following the algorithm in Section 3.5
of [RFC4585] modified as follows:
o To determine whether an RTP session is considered to be a point-
to-point session or a multiparty session, an endpoint MUST count
the number of distinct RTCP SDES CNAME values used by the SSRCs
listed in the SSRC field of RTP data packets it receives and in
the "SSRC of sender" field of RTCP SR, RR, RTPFB, or PSFB packets
it receives. An RTP session is considered to be a multiparty
session if more than one CNAME is used by those SSRCs, unless
signalling indicates that the session is to be handled as point to
point, or RTCP reporting groups
[I-D.ietf-avtcore-rtp-multi-stream-optimisation] are used. If
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RTCP reporting groups are used, an RTP session is considered to be
a point-to-point session if the endpoint receives only a single
reporting group, and considered to be a multiparty session if
multiple reporting groups are received, or if a combination of
reporting groups and SSRCs that are not part of a reporting group
are received. Endpoints MUST NOT determine whether an RTP session
is multiparty or point-to-point based on the type of connection
(unicast or multicast) used, or on the number of SSRCs received.
o When checking if there is already a scheduled compound RTCP packet
containing feedback messages (Step 2 in Section 3.5.2 of
[RFC4585]), that check MUST be done considering all local SSRCs.
o If an SSRC is not allowed to send an early RTCP packet, then the
feedback message MAY be queued for transmission as part of any
early or regular scheduled transmission that can occur within the
maximum useful lifetime of the feedback message (T_max_fb_delay).
This modifies the behaviour in bullet 4a) in Section 3.5.2 of
[RFC4585].
The first bullet point above specifies a rule to determine if an RTP
session is to be considered a point-to-point session or a multiparty
session. This rule is straightforward to implement, but is known to
incorrectly classify some sessions as multiparty sessions. The known
problems are as follows:
Endpoint with multiple synchronization contexts: An endpoint that is
part of a point-to-point session can have multiple synchronization
contexts, for example due to forwarding an external media source
into a interactive real-time conversation. In this case the
classification will consider the peer as two endpoints, while the
actual RTP/RTCP transmission will be under the control of one
endpoint.
Selective Forwarding Middlebox: The SFM as defined in Section 3.7 of
[I-D.ietf-avtcore-rtp-topologies-update] has control over the
transmission and configurations between itself and each peer
endpoint individually. It also fully controls the RTCP packets
being forwarded between the individual legs. Thus, this type of
middlebox can be compared to the RTP mixer, which uses its own
SSRCs to mix or select the media it forwards, that will be
classified as a point-to-point RTP session by the above rule.
In the above cases it is very reasonable to use RTCP reporting groups
[I-D.ietf-avtcore-rtp-multi-stream-optimisation]. If that extension
is used, an endpoint can indicate that the multitude of CNAMEs are in
fact under a single endpoint or middlebox control by using only a
single reporting group.
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The above rules will also classify some sessions where the endpoint
is connected to an RTP mixer as being point to point. For example
the mixer could act as gateway to an Any Source Multicast based RTP
session for the discussed endpoint. However, this will in most cases
be okay, as the RTP mixer provides separation between the two parts
of the session. The responsibility falls on the mixer to act
accordingly in each domain.
Finally, we note that signalling mechanisms could be defined to
override the rules when it would result in the wrong classification.
6. Adding and Removing SSRCs
The set of SSRCs present in a single RTP session can vary over time
due to changes in the number of endpoints in the session, or due to
changes in the number or type of media streams being sent.
Every endpoint in an RTP session will have at least one SSRC that it
uses for RTCP reporting, and for sending media if desired. It can
also have additional SSRCs, for sending extra media streams or for
additional RTCP reporting. If the set of media streams being sent
changes, then the set of SSRCs being sent will change. Changes in
the media format or clock rate might also require changes in the set
of SSRCs used. An endpoint can also have more active SSRCs than it
has active RTP media streams, and send RTCP relating to SSRCs that
are not currently sending RTP data packets so that its peers are
aware of the SSRCs, and have the associated context (e.g., clock
synchronisation and an SDES CNAME) in place to be able to play out
media as soon as they becomes active.
In the following, we describe some considerations around adding and
removing RTP streams and their associated SSRCs.
6.1. Adding RTP Streams
When an endpoint joins an RTP session it can have zero, one, or more
RTP streams it will send, or that it is prepared to send. If it has
no RTP stream it plans to send, it still needs an SSRC that will be
used to send RTCP feedback. If it will send one or more RTP streams,
it will need the corresponding number of SSRC values. The SSRCs used
by an endpoint are made known to other endpoints in the RTP session
by sending RTP and RTCP packets. SSRCs can also be signalled using
non-RTP means (e.g., [RFC5576]). Unless restricted by signalling, an
endpoint can, at any time, send an additional RTP stream, identified
by a new SSRC (this might be associated with a signalling event, but
that is outside the scope of this memo). This makes the new SSRC
visible to the other endpoints in the session, since they share the
single SSRC space inherent in the definition of an RTP session.
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An endpoint that has never sent an RTP stream will have an SSRC that
it uses for RTCP reporting. If that endpoint wants to start sending
an RTP stream, it is RECOMMENDED that it use its existing SSRC for
that stream, since otherwise the participant count in the RTP session
will be unnecessary increased, leading to a longer RTCP reporting
interval and larger RTCP reports due to cross reporting. If the
endpoint wants to start sending more than one RTP stream, it will
need to generate a new SSRC for the second and any subsequent RTP
streams.
An endpoint that has previously stopped sending an RTP stream, and
that wants to start sending a new RTP stream, cannot generally re-use
the existing SSRC, and often needs to generate a new SSRC, because an
SSRC cannot change media type (e.g., audio to video) or RTP timestamp
clock rate [RFC7160], and because the SSRC might be associated with a
particular semantic by the application (note: an RTP stream can pause
and restart using the same SSRC, provided RTCP is sent for that SSRC
during the pause; these rules only apply to new RTP streams reusing
an existing SSRC).
6.2. Removing RTP Streams
An SSRC is removed from an RTP session in one of two ways. When an
endpoint stops sending RTP and RTCP packets using an SSRC, then that
SSRC will eventually time out as described in Section 6.3.5 of
[RFC3550]. Alternatively, an SSRC can be explicitly removed from use
by sending an RTCP BYE packet as described in Section 6.3.7 of
[RFC3550]. It is RECOMMENDED that SSRCs are removed from use by
sending an RTCP BYE packet. Note that [RFC3550] requires that the
RTCP BYE SHOULD be the last RTP/RTCP packet sent in the RTP session
for an SSRC. If an endpoint needs to restart an RTP stream after
sending an RTCP BYE for its SSRC, it needs to generate a new SSRC
value for that stream.
The finality of sending RTCP BYE, means that endpoints needs to
consider if the ceasing of transmission of an RTP stream is temporary
or more permanent. Temporary suspension of media transmission using
a particular RTP stream (SSRC) needs to maintain that SSRC as an
active participant, by continuing RTCP transmission for it. That way
the media sending can be resume immediately, knowing that the context
is in place. Permanent transmission halting needs to send RTCP BYE
to allow the other participants to use the RTCP bandwidth resources
and clean up their state databases.
An endpoint that ceases transmission of all its RTP streams but
remains in the RTP session MUST maintain at least one SSRC that is to
be used for RTCP reporting and feedback (i.e., it cannot send a BYE
for all SSRCs, but needs to retain at least one active SSRC). As
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some Feedback packets can be bound to media type there might be need
to maintain one SSRC per media type within an RTP session. An
alternative can be to create a new SSRC to use for RTCP reporting and
feedback. However, to avoid the perception that an endpoint drops
completely out of an RTP session such a new SSRC ought to be first
established before terminating all the existing SSRCs.
7. RTCP Considerations for Streams with Disparate Rates
An RTP session has a single set of parameters that configure the
session bandwidth. These are the RTCP sender and receiver fractions
(e.g., the SDP "b=RR:" and "b=RS:" lines [RFC3556]), and the
parameters of the RTP/AVPF profile [RFC4585] (e.g., trr-int) if that
profile (or its secure extension, RTP/SAVPF [RFC5124]) is used. As a
consequence, the base RTCP reporting interval, before randomisation,
will be the same for every sending SSRC in an RTP session.
Similarly, every receiving SSRC in an RTP session will have the same
base reporting interval, although this can differ from the reporting
interval chosen by sending SSRCs. This uniform RTCP reporting
interval for all SSRCs can result in RTCP reports being sent more
often, or too seldom, than is considered desirable for a RTP stream.
For example, consider a scenario when an audio flow sending at tens
of kilobits per second is multiplexed into an RTP session with a
multi-megabit high quality video flow. If the session bandwidth is
configured based on the video sending rate, and the default RTCP
bandwidth fraction of 5% of the session bandwidth is used, it is
likely that the RTCP bandwidth will exceed the audio sending rate.
If the reduced minimum RTCP interval described in Section 6.2 of
[RFC3550] is then used in the session, as appropriate for video where
rapid feedback on damaged I-frames is wanted, the uniform reporting
interval for all senders could mean that audio sources are expected
to send RTCP packets more often than they send audio data packets.
This bandwidth mismatch can be reduced by careful tuning of the RTCP
parameters, especially trr_int when the RTP/AVPF profile is used, but
cannot be avoided entirely as it is inherent in the design of the
RTCP timing rules, and affects all RTP sessions that contain flows
with greatly mismatched bandwidth.
Different media rates or desired RTCP behaviours can also occur with
SSRCs carrying the same media type. A common case in multiparty
conferencing is when a small number of video streams are shown in
high resolution, while the others are shown as low resolution
thumbnails, with the choice of which is shown in high resolution
being voice activity controlled. Here the differences are both in
actual media rate and in choices for what feedback messages might be
needed. Other examples of differences that can exist are due to the
intended usage of a media source. A media source carrying the video
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of the speaker in a conference is different from a document camera.
Basic parameters that can differ in this case are frame-rate,
acceptable end-to-end delay, and the SNR fidelity of the image.
These differences affect not only the needed bit-rates, but also
possible transmission behaviours, usable repair mechanisms, what
feedback messages the control and repair requires, the transmission
requirements on those feedback messages, and monitoring of the RTP
stream delivery. Other similar scenarios can also exist.
Sending multiple media types in a single RTP session causes that
session to contain more SSRCs than if each media type was sent in a
separate RTP session. For example, if two participants each send an
audio and a video flow in a single RTP session, that session will
comprise four SSRCs, but if separate RTP sessions had been used for
audio and video, each of those two RTP sessions would comprise only
two SSRCs. Sending multiple media streams in an RTP session hence
increases the amount of cross reporting between the SSRCs, as each
SSRC reports on all other SSRCs in the session. This increases the
size of the RTCP reports, causing them to be sent less often than
would be the case if separate RTP sessions where used for a given
RTCP bandwidth.
Finally, when an RTP session contains multiple media types, it is
important to note that the RTCP reception quality reports, feedback
messages, and extended report blocks used might not be applicable to
all media types. Endpoints will need to consider the media type of
each SSRC only send or process reports and feedback that apply to
that particular SSRC and its media type. Signalling solutions might
have shortcomings when it comes to indicating that a particular set
of RTCP reports or feedback messages only apply to a particular media
type within an RTP session.
From an RTCP perspective, therefore, it can be seen that there are
advantages to using separate RTP sessions for each media stream,
rather than sending multiple media streams in a single RTP session.
However, these are frequently offset by the need to reduce port use,
to ease NAT/firewall traversal, achieved by combining media streams
into a single RTP session. The following sections consider some of
the issues with using RTCP in sessions with multiple media streams in
more detail.
7.1. Timing out SSRCs
Various issues have been identified with timing out SSRC values when
sending multiple media streams in an RTP session.
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7.1.1. Problems with the RTP/AVPF T_rr_interval Parameter
The RTP/AVPF profile includes a method to prevent regular RTCP
reports from being sent too often. This mechanism is described in
Section 3.5.3 of [RFC4585], and is controlled by the T_rr_interval
parameter. It works as follows. When a regular RTCP report is sent,
a new random value, T_rr_current_interval, is generated, drawn evenly
in the range 0.5 to 1.5 times T_rr_interval. If a regular RTCP
packet is to be sent earlier then T_rr_current_interval seconds after
the previous regular RTCP packet, and there are no feedback messages
to be sent, then that regular RTCP packet is suppressed, and the next
regular RTCP packet is scheduled. The T_rr_current_interval is
recalculated each time a regular RTCP packet is sent. The benefit of
suppression is that it avoids wasting bandwidth when there is nothing
requiring frequent RTCP transmissions, but still allows utilization
of the configured bandwidth when feedback is needed.
Unfortunately this suppression mechanism skews the distribution of
the RTCP sending intervals compared to the regular RTCP reporting
intervals. The standard RTCP timing rules, including reconsideration
and the compensation factor, result in the intervals between sending
RTCP packets having a distribution that is skewed towards the upper
end of the range [0.5/1.21828, 1.5/1.21828]*Td, where Td is the
deterministic calculated RTCP reporting interval. With Td = 5s this
distribution covers the range [2.052s, 6.156s]. In comparison, the
RTP/AVPF suppression rules act in an interval that is 0.5 to 1.5
times T_rr_interval; for T_rr_interval = 5s this is [2.5s, 7.5s].
The effect of this is that the time between consecutive RTCP packets
when using T_rr_interval suppression can become large. The maximum
time interval between sending one regular RTCP packet and the next,
when T_rr_interval is being used, occurs when T_rr_current_interval
takes its maximum value and a regular RTCP packet is suppressed at
the end of the suppression period, then the next regular RTCP packet
is scheduled after its largest possible reporting interval. Taking
the worst case of the two intervals gives a maximum time between two
RTCP reports of 1.5*T_rr_interval + 1.5/1.21828*Td.
This behaviour can be surprising when Td and T_rr_interval have the
same value. That is, when T_rr_interval is configured to match the
regular RTCP reporting interval. In this case, one might expect that
regular RTCP packets are sent according to their usual schedule, but
feedback packets can be sent early. However, the above-mentioned
issue results in the RTCP packets actually being sent in the range
[0.5*Td, 2.731*Td] with a highly non-uniform distribution, rather
than the range [0.41*Td, 1.23*Td]. This is perhaps unexpected, but
is not a problem in itself. However, when coupled with packet loss,
it raises the issue of premature timeout.
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7.1.2. Avoiding Premature Timeout
In RTP/AVP [RFC3550] the timeout behaviour is simple, and is 5 times
Td, where Td is calculated with a Tmin value of 5 seconds. In other
words, if the configured RTCP bandwidth allows for an average RTCP
reporting interval shorter than 5 seconds, the timeout is 25 seconds
of no activity from the SSRC (RTP or RTCP), otherwise the timeout is
5 average reporting intervals.
RTP/AVPF [RFC4585] introduces different timeout behaviours depending
on the value of T_rr_interval. When T_rr_interval is 0, it uses the
same timeout calculation as RTP/AVP. However, when T_rr_interval is
non-zero, it replaces Tmin in the timeout calculation, most likely to
speed up detection of timed out SSRCs. However, using a non-zero
T_rr_interval has two consequences for RTP behaviour.
First, due to suppression, the number of RTP and RTCP packets sent by
an SSRC that is not an active RTP sender can become very low, because
of the issue discussed in Section 7.1.1. As the RTCP packet interval
can be as long as 2.73*Td, then during a 5*Td time period an endpoint
might in fact transmit only a single RTCP packet. The long intervals
result in fewer RTCP packets, to a point where a single RTCP packet
loss can sometimes result in timing out an SSRC.
Second, the RTP/AVPF changes to the timeout rules reduce robustness
to misconfiguration. It is common to use RTP/AVPF configured such
that RTCP packets can be sent frequently, to allow rapid feedback,
however this makes timeouts very sensitive to T_rr_interval. For
example, if two SSRCs are configured one with T_rr_interval = 0.1s
and the other with T_rr_interval = 0.6s, then this small difference
will result in the SSRC with the shorter T_rr_interval timing out the
other if it stops sending RTP packets, since the other RTCP reporting
interval is more than five times its own. When RTP/AVP is used, or
RTP/AVPF with T_rr_interval = 0, this is a non-issue, as the timeout
period will be 25s, and differences between configured RTCP bandwidth
can only cause premature timeouts when the reporting intervals are
greater than 5s and differ by a factor of five. To limit the scope
for such problematic misconfiguration, we propose an update to the
RTP/AVPF timeout rules in Section 7.1.4.
7.1.3. Interoperability Between RTP/AVP and RTP/AVPF
If endpoints implementing the RTP/AVP and RTP/AVPF profiles (or their
secure variants) are combined within a single RTP session, and the
RTP/AVPF endpoints use a non-zero T_rr_interval that is significantly
below 5 seconds, there is a risk that the RTP/AVPF endpoints will
prematurely timeout the SSRCs of the RTP/AVP endpoints, due to their
different RTCP timeout rules. Conversely, if the RTP/AVPF endpoints
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use a T_rr_interval that is significant larger than 5 seconds, there
is a risk that the RTP/AVP endpoints will timeout the SSRCs of the
RTP/AVPF endpoints.
Mixing endpoints using two different RTP profiles within a single RTP
session is NOT RECOMMENDED. However, if mixed RTP profiles are used,
and the RTP/AVPF endpoints are not updated to follow Section 7.1.4 of
this memo, then the RTP/AVPF session SHOULD be configured to use
T_rr_interval = 4 seconds to avoid premature timeouts.
The choice of T_rr_interval = 4 seconds for interoperability might
appear strange. Intuitively, this value ought to be 5 seconds, to
make both the RTP/AVP and RTP/AVPF use the same timeout period.
However, the behaviour outlined in Section 7.1.1 shows that actual
RTP/AVPF reporting intervals can be longer than expected. Setting
T_rr_interval = 4 seconds gives actual RTCP intervals near to those
expected by RTP/AVP, ensuring interoperability.
7.1.4. Updated SSRC Timeout Rules
To ensure interoperability and avoid premature timeouts, all SSRCs in
an RTP session MUST use the same timeout behaviour. However,
previous specification are inconsistent in this regard. To avoid
interoperability issues, this memo updates the timeout rules as
follows:
o For the RTP/AVP, RTP/SAVP, RTP/AVPF, and RTP/SAVPF profiles, the
timeout interval SHALL be calculated using a multiplier of five
times the deterministic RTCP reporting interval. That is, the
timeout interval SHALL be 5*Td.
o For the RTP/AVP, RTP/SAVP, RTP/AVPF, and RTP/SAVPF profiles,
calculation of Td, for the purpose of calculating the participant
timeout only, SHALL be done using a Tmin value of 5 seconds and
not the reduced minimal interval, even if the reduced minimum
interval is used to calculate RTCP packet transmission intervals.
This changes the behaviour for the RTP/AVPF or RTP/SAVPF profiles
when T_rr_interval != 0. Specifically, the first paragraph of
Section 3.5.4 of [RFC4585] is updated to use Tmin instead of
T_rr_interval in the timeout calculation for RTP/AVPF entities.
7.2. Tuning RTCP transmissions
This sub-section discusses what tuning can be done to reduce the
downsides of the shared RTCP packet intervals. First, it is
considered what possibilities exist for the RTP/AVP [RFC3551]
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profile, then what additional tools are provided by RTP/AVPF
[RFC4585].
7.2.1. RTP/AVP and RTP/SAVP
When using the RTP/AVP or RTP/SAVP profiles, the options for tuning
the RTCP reporting intervals are limited to the RTCP sender and
receiver bandwidth, and whether the minimum RTCP interval is scaled
according to the bandwidth. As the scheduling algorithm includes
both randomisation and reconsideration, one cannot simply calculate
the expected average transmission interval using the formula for Td
given in Section 6.3.1 of [RFC3550]. However, by considering the
inputs to that expression, and the randomisation and reconsideration
rules, we can begin to understand the behaviour of the RTCP
transmission interval.
Let's start with some basic observations:
a. Unless the scaled minimum RTCP interval is used, then Td prior to
randomization and reconsideration can never be less than Tmin.
The default value of Tmin is 5 seconds.
b. If the scaled minimum RTCP interval is used, Td can become as low
as 360 divided by RTP Session bandwidth in kilobits per second.
In SDP the RTP session bandwidth is signalled using a "b=AS"
line. An RTP Session bandwidth of 72kbps results in Tmin being 5
seconds. An RTP session bandwidth of 360kbps of course gives a
Tmin of 1 second, and to achieve a Tmin equal to once every frame
for a 25 frame-per-second video stream requires an RTP session
bandwidth of 9Mbps. Use of the RTP/AVPF or RTP/SAVPF profile
allows more frequent RTCP reports for the same bandwidth, as
discussed below.
c. The value of Td scales with the number of SSRCs and the average
size of the RTCP reports, to keep the overall RTCP bandwidth
constant.
d. The actual transmission interval for a Td value is in the range
[0.5*Td/1.21828,1.5*Td/1.21828], and the distribution is skewed,
due to reconsideration, with the majority of the probability mass
being above Td. This means, for example, that for Td = 5s, the
actual transmission interval will be distributed in the range
[2.052s, 6.156s], and tending towards the upper half of the
interval. Note that Tmin parameter limits the value of Td before
randomisation and reconsideration are applied, so the actual
transmission interval will cover a range extending below Tmin.
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Given the above, we can calculate the number of SSRCs, n, that an RTP
session with 5% of the session bandwidth assigned to RTCP can support
while maintaining Td equal to Tmin. This will tell us how many media
streams we can report on, keeping the RTCP overhead within acceptable
bounds. We make two assumptions that simplify the calculation: that
all SSRCs are senders, and that they all send compound RTCP packets
comprising an SR packet with n-1 report blocks, followed by an SDES
packet containing a 16 octet CNAME value [RFC7022] (such RTCP packets
will vary in size between 54 and 798 octets depending on n, up to the
maximum of 31 report blocks that can be included in an SR packet).
If we put this packet size, and a 5% RTCP bandwidth fraction into the
RTCP interval calculation in Section 6.3.1 of [RFC3550], and
calculate the value of n needed to give Td = Tmin for the scaled
minimum interval, we find n=9 SSRCs can be supported (irrespective of
the interval, due to the way the reporting interval scales with the
session bandwidth). We see that to support more SSRCs without
changing the scaled minimum interval, we need to increase the RTCP
bandwidth fraction from 5%; changing the session bandwidth to a
higher value would reduce the Tmin. However, if using the default 5%
allocation of RTCP bandwidth, an increase will result in more SSRCs
being supported given a fixed Td target.
Based on the above, when using the RTP/AVP profile or the RTP/SAVP
profile, the key limitation for rapid RTCP reporting in small unicast
sessions is going to be the Tmin value. The RTP session bandwidth
configured in RTCP has to be sufficiently high to reach the reporting
goals the application has following the rules for the scaled minimal
RTCP interval.
7.2.2. RTP/AVPF and RTP/SAVPF
When using RTP/AVPF or RTP/SAVPF, we have a powerful additional tool
for tuning RTCP transmissions: the T_rr_interval parameter. Use of
this parameter allows short RTCP reporting intervals; alternatively
it gives the ability to sent frequent RTCP feedback without sending
frequent regular RTCP reports.
The use of the RTP/AVPF or RTP/SAVPF profile with T_rr_interval set
to a value greater than zero but smaller than Tmin allows more
frequent RTCP feedback than the RTP/AVP or RTP/SAVP profiles, for a
given RTCP bandwidth. This happens because Tmin is set to zero after
the transmission of the initial RTCP report, causing the reporting
interval for later packet to be determined by the usual RTCP
bandwidth-based calculation, with Tmin=0, and the T_rr_interval.
This has the effect that we are no longer restricted by the minimal
interval (whether the default 5 second minimum, or the reduced
minimum interval). Rather, the RTCP bandwidth and the T_rr_interval
are the governing factors, allowing faster feedback. Applications
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that care about rapid regular RTCP feedback ought to consider using
the RTP/AVPF or RTP/SAVPF profile, even if they don't use the
feedback features of that profile.
The use of the RTP/AVPF or RTP/SAVPF profile allows RTCP feedback
packets to be sent frequently, without also requiring regular RTCP
reports to be sent frequently, since T_rr_interval limits the rate at
which regular RTCP packets can be sent, while still permitting RTCP
feedback packets to be sent. Applications that can use feedback
packets for some media streams, e.g., video streams, but don't want
frequent regular reporting for other media streams, can configure the
T_rr_interval to a value so that the regular reporting for both audio
and video is at a level that is considered acceptable for the audio.
They could then use feedback packets, which will include RTCP SR/RR
packets unless reduced size RTCP feedback packets [RFC5506] are used,
for the video reporting. This allows the available RTCP bandwidth to
be devoted on the feedback that provides the most utility for the
application.
Using T_rr_interval still requires one to determine suitable values
for the RTCP bandwidth value. Indeed, it might make this choice even
more important, as this is more likely to affect the RTCP behaviour
and performance than when using the RTP/AVP or RTP/SAVP profile, as
there are fewer limitations affecting the RTCP transmission.
When T_rr_interval is non-zero, there are configurations that need to
be avoided. If the RTCP bandwidth chosen is such that the Td value
is smaller than, but close to, T_rr_interval, then the actual regular
RTCP packet transmission interval can become very large, as discussed
in Section 7.1.1. Therefore, for configuration where one intends to
have Td smaller than T_rr_interval, then Td is RECOMMENDED to be
targeted at values less than 1/4th of T_rr_interval which results in
that the range becomes [0.5*T_rr_interval, 1.81*T_rr_interval].
With the RTP/AVPF or RTP/SAVPF profiles, using T_rr_interval = 0 has
utility, and results in a behaviour where the RTCP transmission is
only limited by the bandwidth, i.e., no Tmin limitations at all.
This allows more frequent regular RTCP reporting than can be achieved
using the RTP/AVP profile. Many configurations of RTCP will not
consume all the bandwidth that they have been configured to use, but
this configuration will consume what it has been given. Note that
the same behaviour will be achieved as long as T_rr_interval is
smaller than 1/3 of Td as that prevents T_rr_interval from affecting
the transmission.
There exists no method for using different regular RTCP reporting
intervals depending on the media type or individual media stream,
other than using a separate RTP session for each type or stream.
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8. Security Considerations
When using the secure RTP protocol (RTP/SAVP) [RFC3711], or the
secure variant of the feedback profile (RTP/SAVPF) [RFC5124], the
cryptographic context of a compound secure RTCP packet is the SSRC of
the sender of the first RTCP (sub-)packet. This could matter in some
cases, especially for keying mechanisms such as Mikey [RFC3830] which
allow use of per-SSRC keying.
Otherwise, the standard security considerations of RTP apply; sending
multiple media streams from a single endpoint in a single RTP session
does not appear to have different security consequences than sending
the same number of media streams spread across different RTP
sessions.
9. IANA Considerations
No IANA actions are needed.
10. Acknowledgments
The authors like to thank Harald Alvestrand and everyone else who has
been involved in the development of this document.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
2006.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, February 2008.
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[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, April 2009.
11.2. Informative References
[I-D.ietf-avtcore-multi-media-rtp-session]
Westerlund, M., Perkins, C., and J. Lennox, "Sending
Multiple Types of Media in a Single RTP Session", draft-
ietf-avtcore-multi-media-rtp-session-07 (work in
progress), March 2015.
[I-D.ietf-avtcore-rtp-multi-stream-optimisation]
Lennox, J., Westerlund, M., Wu, W., and C. Perkins,
"Sending Multiple Media Streams in a Single RTP Session:
Grouping RTCP Reception Statistics and Other Feedback",
draft-ietf-avtcore-rtp-multi-stream-optimisation-06 (work
in progress), July 2015.
[I-D.ietf-avtcore-rtp-topologies-update]
Westerlund, M. and S. Wenger, "RTP Topologies", draft-
ietf-avtcore-rtp-topologies-update-10 (work in progress),
July 2015.
[I-D.ietf-clue-framework]
Duckworth, M., Pepperell, A., and S. Wenger, "Framework
for Telepresence Multi-Streams", draft-ietf-clue-
framework-22 (work in progress), April 2015.
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-22 (work in progress), June 2015.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
[RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC
3556, July 2003.
[RFC3830] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
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[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, February 2008.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
[RFC6190] Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
"RTP Payload Format for Scalable Video Coding", RFC 6190,
May 2011.
[RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla,
"Guidelines for Choosing RTP Control Protocol (RTCP)
Canonical Names (CNAMEs)", RFC 7022, September 2013.
[RFC7160] Petit-Huguenin, M. and G. Zorn, "Support for Multiple
Clock Rates in an RTP Session", RFC 7160, April 2014.
Authors' Addresses
Jonathan Lennox
Vidyo, Inc.
433 Hackensack Avenue
Seventh Floor
Hackensack, NJ 07601
USA
Email: jonathan@vidyo.com
Magnus Westerlund
Ericsson
Farogatan 6
SE-164 80 Kista
Sweden
Phone: +46 10 714 82 87
Email: magnus.westerlund@ericsson.com
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Qin Wu
Huawei
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
Email: sunseawq@huawei.com
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
United Kingdom
Email: csp@csperkins.org
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