draft-ietf-rmcat-rtp-cc-feedback-03.txt   draft-ietf-rmcat-rtp-cc-feedback-04.txt 
Network Working Group C. Perkins Network Working Group C. Perkins
Internet-Draft University of Glasgow Internet-Draft University of Glasgow
Intended status: Informational November 14, 2016 Intended status: Informational July 1, 2018
Expires: May 18, 2017 Expires: January 2, 2019
RTP Control Protocol (RTCP) Feedback for Congestion Control in RTP Control Protocol (RTCP) Feedback for Congestion Control in
Interactive Multimedia Conferences Interactive Multimedia Conferences
draft-ietf-rmcat-rtp-cc-feedback-03 draft-ietf-rmcat-rtp-cc-feedback-04
Abstract Abstract
This memo discusses the types of congestion control feedback that it This memo discusses the types of congestion control feedback that it
is possible to send using the RTP Control Protocol (RTCP), and their is possible to send using the RTP Control Protocol (RTCP), and their
suitability of use in implementing congestion control for unicast suitability of use in implementing congestion control for unicast
multimedia applications. multimedia applications.
Status of This Memo Status of This Memo
skipping to change at page 1, line 34 skipping to change at page 1, line 34
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This Internet-Draft will expire on May 18, 2017. This Internet-Draft will expire on January 2, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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skipping to change at page 4, line 34 skipping to change at page 4, line 34
send speech data or comfort noise with sufficient frequency that they send speech data or comfort noise with sufficient frequency that they
are counted as senders for the purpose of the RTCP reporting interval are counted as senders for the purpose of the RTCP reporting interval
calculation. calculation.
RTCP feedback packets can be full, compound, RTCP feedback packets, RTCP feedback packets can be full, compound, RTCP feedback packets,
or non-compound RTCP packets. A compound RTCP packet is sent once or non-compound RTCP packets. A compound RTCP packet is sent once
for every Nnc non-compound RTCP packets. for every Nnc non-compound RTCP packets.
Compound RTCP packets contain a Sender Report (SR) packet and a Compound RTCP packets contain a Sender Report (SR) packet and a
Source Description (SDES) packet, and an RTP Congestion Control Source Description (SDES) packet, and an RTP Congestion Control
Feedback (RC2F) packet [I-D.dt-rmcat-feedback-message]. Non-compound Feedback (CCFB) packet [I-D.ietf-avtcore-cc-feedback-message]. Non-
RTCP packets contain only the RC2F packet. Since each participant compound RTCP packets contain only the CCFB packet. Since each
sends only a single media stream, the extensions for RTCP report participant sends only a single media stream, the extensions for RTCP
aggregation [I-D.ietf-avtcore-rtp-multi-stream] and reporting group report aggregation [RFC8108] and reporting group optimisation
optimisation [I-D.ietf-avtcore-rtp-multi-stream-optimisation] are not [I-D.ietf-avtcore-rtp-multi-stream-optimisation] are not used.
used.
Within each compound RTCP packet, the SR packet will contain a sender Within each compound RTCP packet, the SR packet will contain a sender
information block (28 octets) and a single reception report block (24 information block (28 octets) and a single reception report block (24
octets), for a total of 52 octets. A minimal SDES packet will octets), for a total of 52 octets. A minimal SDES packet will
contain a header (4 octets) and a single chunk containing an SSRC (4 contain a header (4 octets) and a single chunk containing an SSRC (4
octets) and a CNAME item, and if the recommendations for choosing the octets) and a CNAME item, and if the recommendations for choosing the
CNAME [RFC7022] are followed, the CNAME item will comprise a 2 octet CNAME [RFC7022] are followed, the CNAME item will comprise a 2 octet
header, 16 octets of data, and 2 octets of padding, for a total SDES header, 16 octets of data, and 2 octets of padding, for a total SDES
packet size of 28 octets. The RC2F packets contains an XR block packet size of 28 octets. The CCFB packets contains an RTCP header
header and SSRC (8 octets), a block type and timestamp (8 octets), and SSRC (8 octets), a report timestamp (4 octets), the SSRC,
the SSRC, beginning and ending sequence numbers (8 octets), and 2*Nr beginning and ending sequence numbers (8 octets), and 2*Nr octets of
octets of reports, for a total of 24 + 2*Nr octets. If IPv4 is used, reports, for a total of 20 + 2*Nr octets. If IPv4 is used, with no
with no IP options, the UDP/IP header will be 28 octets in size. IP options, the UDP/IP header will be 28 octets in size. This gives
This gives a total compound RTCP packet size of Sc = 132 + 2*Nr a total compound RTCP packet size of Sc = 128 + 2*Nr octets.
octets.
The non-compound RTCP packets will comprise just the RC2F packet with The non-compound RTCP packets will comprise just the CCFB packet with
a UDP/IP header. It can be seen that these packets will be Snc = 48 a UDP/IP header. It can be seen that these packets will be Snc = 48
+ 2*Nr octets in size. + 2*Nr octets in size.
The RTCP reporting interval calculation ([RFC3550], Section 6.2) for The RTCP reporting interval calculation ([RFC3550], Section 6.2) for
a two-party session where both participants are senders, reduces to a two-party session where both participants are senders, reduces to
Trtcp = n * Srtcp/Brtcp where Srtcp = (Sc + Nnc * Snc)/(1 + Nnc) is Trtcp = n * Srtcp/Brtcp where Srtcp = (Sc + Nnc * Snc)/(1 + Nnc) is
the average RTCP packet size in octets, Brtcp is the bandwidth the average RTCP packet size in octets, Brtcp is the bandwidth
allocated to RTCP in octets per second, and n is the number of allocated to RTCP in octets per second, and n is the number of
participants (n=2 in this scenario). participants (n=2 in this scenario).
To ensure a report is sent every Nr frames, it is necessary to set To ensure a report is sent every Nr frames, it is necessary to set
the RTCP reporting interval Trtcp = Nr * Tf, which when substituted the RTCP reporting interval Trtcp = Nr * Tf, which when substituted
into the previous gives Nr * Tf = n * Srtcp/Brtcp. into the previous gives Nr * Tf = n * Srtcp/Brtcp.
Solving for the RTCP bandwidth, Brtcp, and expanding the definition Solving for the RTCP bandwidth, Brtcp, and expanding the definition
of Srtcp gives Brtcp = (n * (Sc + Nnc * Snc))/(Nr * Tf * (1 + Nnc)). of Srtcp gives Brtcp = (n * (Sc + Nnc * Snc))/(Nr * Tf * (1 + Nnc)).
If we assume every report is a compound RTCP packet (i.e., Nnc = 0), If we assume every report is a compound RTCP packet (i.e., Nnc = 0),
the frame duration Tf = 20ms, and an RTCP report is sent for every the frame duration Tf = 20ms, and an RTCP report is sent for every
second frame (i.e., 25 RTCP reports per second), this expression second frame (i.e., 25 RTCP reports per second), this expression
gives the needed RTCP bandwidth Brtcp = 53.1kbps. Increasing the gives the needed RTCP bandwidth Brtcp = 51.6kbps. Increasing the
frame duration, or reducing the frequency of reports, reduces the frame duration, or reducing the frequency of reports, reduces the
RTCP bandwidth, as shown below: RTCP bandwidth, as shown below:
+--------------+-------------+----------------+ +--------------+-------------+----------------+
| Tf (seconds) | Nr (frames) | rtcp_bw (kbps) | | Tf (seconds) | Nr (frames) | rtcp_bw (kbps) |
+--------------+-------------+----------------+ +--------------+-------------+----------------+
| 20ms | 2 | 53.1 | | 20ms | 2 | 51.6 |
| 20ms | 4 | 27.3 | | 20ms | 4 | 26.6 |
| 20ms | 8 | 14.5 | | 20ms | 8 | 14.1 |
| 20ms | 16 | 8.01 | | 20ms | 16 | 7.8 |
| 60ms | 2 | 17.7 | | 60ms | 2 | 17.2 |
| 60ms | 4 | 9.1 | | 60ms | 4 | 8.9 |
| 60ms | 8 | 4.8 | | 60ms | 8 | 4.7 |
| 60ms | 16 | 2.66 | | 60ms | 16 | 2.6 |
+--------------+-------------+----------------+ +--------------+-------------+----------------+
Table 1: Required RTCP bandwidth for VoIP feedback Table 1: Required RTCP bandwidth for VoIP feedback
The final row of the table (60ms frames, report every 16 frames) The final row of the table (60ms frames, report every 16 frames)
sends RTCP reports once per second, giving an RTCP bandwidth of sends RTCP reports once per second, giving an RTCP bandwidth of
2.66kbps. 2.6kbps.
The overhead can be reduced by sending some reports in non-compound The overhead can be reduced by sending some reports in non-compound
RTCP packets [RFC5506]. For example, if we alternate compound and RTCP packets [RFC5506]. For example, if we alternate compound and
non-compound RTCP packets, i.e., Nnc = 1, the calculation gives: non-compound RTCP packets, i.e., Nnc = 1, the calculation gives:
+--------------+-------------+----------------+ +--------------+-------------+----------------+
| Tf (seconds) | Nr (frames) | rtcp_bw (kbps) | | Tf (seconds) | Nr (frames) | rtcp_bw (kbps) |
+--------------+-------------+----------------+ +--------------+-------------+----------------+
| 20ms | 2 | 36.7 | | 20ms | 2 | 35.9 |
| 20ms | 4 | 19.1 | | 20ms | 4 | 18.8 |
| 20ms | 8 | 10.4 | | 20ms | 8 | 10.2 |
| 20ms | 16 | 6.0 | | 20ms | 16 | 5.9 |
| 60ms | 2 | 12.2 | | 60ms | 2 | 12.0 |
| 60ms | 4 | 6.4 | | 60ms | 4 | 6.2 |
| 60ms | 8 | 3.5 | | 60ms | 8 | 3.4 |
| 60ms | 16 | 2.0 | | 60ms | 16 | 2.0 |
+--------------+-------------+----------------+ +--------------+-------------+----------------+
Table 2: Required RTCP bandwidth for VoIP feedback (alternating Table 2: Required RTCP bandwidth for VoIP feedback (alternating
compound and non-compound reports) compound and non-compound reports)
The RTCP bandwidth needed for 60ms frames, reporting every 16 frames The RTCP bandwidth needed for 60ms frames, reporting every 16 frames
(once per second), can be seen to drop to 2.01kbps. This calculation (once per second), can be seen to drop to 2.0kbps. This calculation
can be repeated for other patterns of compound and non-compound RTCP can be repeated for other patterns of compound and non-compound RTCP
packets, feedback frequency, and frame duration, as needed. packets, feedback frequency, and frame duration, as needed.
Note: To achieve the RTCP transmission intervals above the RTP/SAVPF Note: To achieve the RTCP transmission intervals above the RTP/SAVPF
profile with T_rr_interval=0 is used, since even when using the profile with T_rr_interval=0 is used, since even when using the
reduced minimal transmission interval, the RTP/SAVP profile would reduced minimal transmission interval, the RTP/SAVP profile would
only allow sending RTCP at most every 0.11s (every third frame of only allow sending RTCP at most every 0.11s (every third frame of
video). Using RTP/SAVPF with T_rr_interval=0 however is capable of video). Using RTP/SAVPF with T_rr_interval=0 however is capable of
fully utilizing the configured 5% RTCP bandwidth fraction. fully utilizing the configured 5% RTCP bandwidth fraction.
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Consider a point to point video call between two end systems. There Consider a point to point video call between two end systems. There
will be four RTP flows in this scenario, two audio and two video, will be four RTP flows in this scenario, two audio and two video,
with all four flows being active for essentially all the time (the with all four flows being active for essentially all the time (the
audio flows will likely use voice activity detection and comfort audio flows will likely use voice activity detection and comfort
noise to reduce the packet rate during silent periods, and does not noise to reduce the packet rate during silent periods, and does not
cause the transmissions to stop). cause the transmissions to stop).
Assume all four flows are sent in a single RTP session, each using a Assume all four flows are sent in a single RTP session, each using a
separate SSRC; the RTCP reports from co-located audio and video SSRCs separate SSRC; the RTCP reports from co-located audio and video SSRCs
at each end point are aggregated [I-D.ietf-avtcore-rtp-multi-stream]; at each end point are aggregated [RFC8108]; the optimisations in
the optimisations in [I-D.ietf-avtcore-rtp-multi-stream-optimisation] [I-D.ietf-avtcore-rtp-multi-stream-optimisation] are used; and
are used; and congestion control feedback is sent congestion control feedback is sent
[I-D.dt-rmcat-feedback-message]. [I-D.ietf-avtcore-cc-feedback-message].
When all members are senders, the RTCP timing rules in Section 6.2 When all members are senders, the RTCP timing rules in Section 6.2
and 6.3 of [RFC3550] and [RFC4585] reduce to: and 6.3 of [RFC3550] and [RFC4585] reduce to:
rtcp_interval = avg_rtcp_size * n / rtcp_bw rtcp_interval = avg_rtcp_size * n / rtcp_bw
where n is the number of members in the session, the avg_rtcp_size is where n is the number of members in the session, the avg_rtcp_size is
measured in octets, and the rtcp_bw is the bandwidth available for measured in octets, and the rtcp_bw is the bandwidth available for
RTCP, measured in octets per second (this will typically be 5% of the RTCP, measured in octets per second (this will typically be 5% of the
session bandwidth). session bandwidth).
skipping to change at page 7, line 39 skipping to change at page 7, line 39
packet. The RTCP SR packet contains the 28 octet header and sender packet. The RTCP SR packet contains the 28 octet header and sender
information, but no report blocks (since the reporting is delegated). information, but no report blocks (since the reporting is delegated).
The RTCP SDES packet will comprise a header (4 octets), originating The RTCP SDES packet will comprise a header (4 octets), originating
SSRC (4 octets), a CNAME chunk, a terminating chunk, and any padding. SSRC (4 octets), a CNAME chunk, a terminating chunk, and any padding.
If the CNAME follows [RFC7022] and [I-D.ietf-rtcweb-rtp-usage] it If the CNAME follows [RFC7022] and [I-D.ietf-rtcweb-rtp-usage] it
will be 18 octets in size, and will need 1 octet of padding, making will be 18 octets in size, and will need 1 octet of padding, making
the SDES packet 28 octets in size. The RTCP RGRS packet will be 12 the SDES packet 28 octets in size. The RTCP RGRS packet will be 12
octets in size. This gives a total of 28 + 28 + 12 = 68 octets. octets in size. This gives a total of 28 + 28 + 12 = 68 octets.
The aggregated compound RTCP packet from the reporting SSRC will The aggregated compound RTCP packet from the reporting SSRC will
contain an RTCP SR packet, an RTCP SDES packet, and an RTCP XR contain an RTCP SR packet, an RTCP SDES packet, and an RTCP
congestion control feedback packet. The RTCP SR packet will contain congestion control feedback packet. The RTCP SR packet will contain
two report blocks, one for each of the remote SSRCs (the report for two report blocks, one for each of the remote SSRCs (the report for
the other local SSRC is suppressed by the reporting group extension), the other local SSRC is suppressed by the reporting group extension),
for a total of 28 + (2 * 24) = 76 octets. The RTCP SDES packet will for a total of 28 + (2 * 24) = 76 octets. The RTCP SDES packet will
comprise a header (4 octets), originating SSRC (4 octets), a CNAME comprise a header (4 octets), originating SSRC (4 octets), a CNAME
chunk, an RGRP chunk, a terminating chunk, and any padding. If the chunk, an RGRP chunk, a terminating chunk, and any padding. If the
CNAME follows [RFC7022] and [I-D.ietf-rtcweb-rtp-usage] it will be 18 CNAME follows [RFC7022] and [I-D.ietf-rtcweb-rtp-usage] it will be 18
octets in size. The RGRP chunk similarly comprises 18 octets, and 3 octets in size. The RGRP chunk similarly comprises 18 octets, and 3
octets of padding are needed, for a total of 48 octets. The RTCP XR octets of padding are needed, for a total of 48 octets. The RTCP
congestion control feedback report comprises an 8 octet XR header, an congestion control feedback (CCFB) report comprises an 8 octet RTCP
8 octet RC2F header, then for each of the remote audio and video header and SSRC,an 4 report timestamp, then for each of the remote
SSRCs, an 8 octet report header, and 2 octets per packet reported audio and video SSRCs, an 8 octet report header, and 2 octets per
upon, and padding to a 4 octet boundary, if needed; that is 8 + 8 + 8 packet reported upon, and padding to a 4 octet boundary if needed;
+ (2 * Nv) + 8 + (2 * Na) where Nv is the number of video packets per that is 8 + 4 + 8 + (2 * Nv) + 8 + (2 * Na) where Nv is the number of
report, and Na is the number of audio packets per report. video packets per report, and Na is the number of audio packets per
report.
The complete compound RTCP packet contains the RTCP packets from both The complete compound RTCP packet contains the RTCP packets from both
the reporting and non-reporting SSRCs, an SRTP authentication tag, the reporting and non-reporting SSRCs, an SRTP authentication tag,
and a UDP/IPv4 header. The size of this RTCP packet is therefore: and a UDP/IPv4 header. The size of this RTCP packet is therefore:
252 + (2 * Nv) + (2 * Na) octets. Since the aggregate RTCP packet 248 + (2 * Nv) + (2 * Na) octets. Since the aggregate RTCP packet
contains reports from two SSRCs, the RTCP packet size is halved contains reports from two SSRCs, the RTCP packet size is halved
before use [I-D.ietf-avtcore-rtp-multi-stream]. Accordingly, we before use [RFC8108]. Accordingly, we define Sc = (248 + (2 * Nv) +
define Sc = (252 + (2 * Nv) + (2 * Na))/2 for this scenario. (2 * Na))/2 for this scenario.
How many packets does the RTCP XR congestion control feedback packet How many packets does the RTCP XR congestion control feedback packet
report on? This is obviously highly dependent on the choice of codec report on? This is obviously highly dependent on the choice of codec
and encoding parameters, and might be quite bursty if the codec sends and encoding parameters, and might be quite bursty if the codec sends
I-frames from which later frames are predicted. For now though, I-frames from which later frames are predicted. For now though,
assume constant rate media with an MTU around 1500 octets, with assume constant rate media with an MTU around 1500 octets, with
reports for both audio and video being aggregated and sent to align reports for both audio and video being aggregated and sent to align
with video frames. This gives the following, assuming Nr =1 and Nnc with video frames. This gives the following, assuming Nr =1 and Nnc
= 0 (i.e., send a compound RTCP packet for each video frame, and no = 0 (i.e., send a compound RTCP packet for each video frame, and no
non-compound packets), and using the calculation from Scenario 1: non-compound packets), and using the calculation from Scenario 1:
Brtcp = (n * (Sc + Nnc * Snc))/(Nr * Tf * (1 + Nnc)) Brtcp = (n * (Sc + Nnc * Snc))/(Nr * Tf * (1 + Nnc))
+---------+---------+--------------+--------------+-----------------+ +---------+---------+--------------+--------------+-----------------+
| Data | Video | Video | Audio | Required RTCP | | Data | Video | Video | Audio | Required RTCP |
| Rate | Frame | Packets per | Packets per | bandwidth: | | Rate | Frame | Packets per | Packets per | bandwidth: |
| (kbps) | Rate | Report: Nv | Report: Na | Brtcp (kbps) | | (kbps) | Rate | Report: Nv | Report: Na | Brtcp (kbps) |
+---------+---------+--------------+--------------+-----------------+ +---------+---------+--------------+--------------+-----------------+
| 100 | 8 | 1 | 6 | 33.3 (33%) | | 100 | 8 | 1 | 6 | 33.2 (33%) |
| 200 | 16 | 1 | 3 | 65.0 (33%) | | 200 | 16 | 1 | 3 | 65.0 (32%) |
| 350 | 30 | 1 | 2 | 120.1 (35%) | | 350 | 30 | 1 | 2 | 120.9 (34%) |
| 700 | 30 | 2 | 2 | 121.9 (17%) | | 700 | 30 | 2 | 2 | 121.9 (17%) |
| 700 | 60 | 1 | 1 | 240.0 (34%) | | 700 | 60 | 1 | 1 | 240.0 (34%) |
| 1024 | 30 | 3 | 2 | 122.8 (12%) | | 1024 | 30 | 3 | 2 | 122.8 (11%) |
| 1400 | 60 | 2 | 1 | 241.8 (17%) | | 1400 | 60 | 2 | 1 | 241.9 (17%) |
| 2048 | 30 | 6 | 2 | 125.6 ( 6%) | | 2048 | 30 | 6 | 2 | 125.6 ( 6%) |
| 2048 | 60 | 3 | 1 | 243.8 (12%) | | 2048 | 60 | 3 | 1 | 243.8 (11%) |
| 4096 | 30 | 12 | 2 | 131.3 ( 3%) | | 4096 | 30 | 12 | 2 | 131.2 ( 3%) |
| 4096 | 60 | 6 | 1 | 294.4 ( 6%) | | 4096 | 60 | 6 | 1 | 249.4 ( 6%) |
+---------+---------+--------------+--------------+-----------------+ +---------+---------+--------------+--------------+-----------------+
Table 3: Required RTCP bandwidth, reporting on every frame Table 3: Required RTCP bandwidth, reporting on every frame
The RTCP bandwidth needed scales inversely with Nr. That is, it is The RTCP bandwidth needed scales inversely with Nr. That is, it is
halved if Nr=2 (report on every second packet), is reduced to one- halved if Nr=2 (report on every second packet), is reduced to one-
third if Nr=3 (report on every third packet), and so on. third if Nr=3 (report on every third packet), and so on.
The needed RTCP bandwidth scales as a percentage of the data rate The needed RTCP bandwidth scales as a percentage of the data rate
following the ratio of the frame rate to the data rate. As can be following the ratio of the frame rate to the data rate. As can be
skipping to change at page 9, line 27 skipping to change at page 9, line 30
octets, plus UDP/IP header. That is, Snc = (96 + 2*Nv + 2*Na)/2. octets, plus UDP/IP header. That is, Snc = (96 + 2*Nv + 2*Na)/2.
Repeating the analysis above, but alternating compound and non- Repeating the analysis above, but alternating compound and non-
compound reports, i.e., setting Nnc = 1, gives: compound reports, i.e., setting Nnc = 1, gives:
+---------+---------+--------------+--------------+-----------------+ +---------+---------+--------------+--------------+-----------------+
| Data | Video | Video | Audio | Required RTCP | | Data | Video | Video | Audio | Required RTCP |
| Rate | Frame | Packets per | Packets per | bandwidth: | | Rate | Frame | Packets per | Packets per | bandwidth: |
| (kbps) | Rate | Report: Nv | Report: Na | Brtcp (kbps) | | (kbps) | Rate | Report: Nv | Report: Na | Brtcp (kbps) |
+---------+---------+--------------+--------------+-----------------+ +---------+---------+--------------+--------------+-----------------+
| 100 | 8 | 1 | 6 | 23.5 (23%) | | 100 | 8 | 1 | 6 | 23.5 (23%) |
| 200 | 16 | 1 | 3 | 45.5 (23%) | | 200 | 16 | 1 | 3 | 45.5 (22%) |
| 350 | 30 | 1 | 2 | 84.4 (24%) | | 350 | 30 | 1 | 2 | 84.4 (24%) |
| 700 | 30 | 2 | 2 | 85.3 (12%) | | 700 | 30 | 2 | 2 | 85.3 (12%) |
| 700 | 60 | 1 | 1 | 166.9 (24%) | | 700 | 60 | 1 | 1 | 166.9 (23%) |
| 1024 | 30 | 3 | 2 | 86.2 ( 8%) | | 1024 | 30 | 3 | 2 | 86.2 ( 8%) |
| 1400 | 60 | 2 | 1 | 168.8 (12%) | | 1400 | 60 | 2 | 1 | 168.8 (12%) |
| 2048 | 30 | 6 | 2 | 89.1 ( 4%) | | 2048 | 30 | 6 | 2 | 89.1 ( 4%) |
| 2048 | 60 | 3 | 1 | 170.6 ( 8%) | | 2048 | 60 | 3 | 1 | 170.6 ( 8%) |
| 4096 | 30 | 12 | 2 | 94.7 ( 2%) | | 4096 | 30 | 12 | 2 | 94.7 ( 2%) |
| 4096 | 60 | 6 | 1 | 176.3 ( 4%) | | 4096 | 60 | 6 | 1 | 176.2 ( 4%) |
+---------+---------+--------------+--------------+-----------------+ +---------+---------+--------------+--------------+-----------------+
Table 4: Required RTCP bandwidth, reporting on every frame, with Table 4: Required RTCP bandwidth, reporting on every frame, with
reduced-size reports reduced-size reports
The use of reduced-size RTCP gives a noticeable reduction in the The use of reduced-size RTCP gives a noticeable reduction in the
needed RTCP bandwidth, and can be combined with reporting every few needed RTCP bandwidth, and can be combined with reporting every few
frames rather than every frames. Overall, it is clear that the RTCP frames rather than every frames. Overall, it is clear that the RTCP
overhead can be reasonable across the range of data and frame rates, overhead can be reasonable across the range of data and frame rates,
if RTCP is configured carefully. if RTCP is configured carefully.
skipping to change at page 10, line 36 skipping to change at page 10, line 36
If it is desired to use RTCP in something close to it's current form If it is desired to use RTCP in something close to it's current form
for congestion feedback in WebRTC, the multimedia congestion control for congestion feedback in WebRTC, the multimedia congestion control
algorithm needs be designed to work with feedback sent every few algorithm needs be designed to work with feedback sent every few
frames, since that fits within the limitations of RTCP. That frames, since that fits within the limitations of RTCP. That
feedback can be a little more complex than just an acknowledgement, feedback can be a little more complex than just an acknowledgement,
provided care is taken to consider the impact of the extra feedback provided care is taken to consider the impact of the extra feedback
on the overhead, possibly allowing for a degree of semantic feedback, on the overhead, possibly allowing for a degree of semantic feedback,
meaningful to the codec layer as well as the congestion control meaningful to the codec layer as well as the congestion control
algorithm. algorithm.
The format described in [I-D.dt-rmcat-feedback-message] seems The format described in [I-D.ietf-avtcore-cc-feedback-message] seems
sufficient for the needs of congestion control feedback. There is sufficient for the needs of congestion control feedback. There is
little point optimising this format: the main overhead comes from the little point optimising this format: the main overhead comes from the
UDP/IP headers and the other RTCP packets included in the compound UDP/IP headers and the other RTCP packets included in the compound
packets, and can be lowered by using the [RFC5506] extensions and packets, and can be lowered by using the [RFC5506] extensions and
sending reports less frequently. sending reports less frequently.
Further study of the scenarios of interest is needed, to ensure that Further study of the scenarios of interest is needed, to ensure that
the analysis presented is applicable to other media topologies, and the analysis presented is applicable to other media topologies, and
to sessions with different data rates and sizes of membership. to sessions with different data rates and sizes of membership.
skipping to change at page 11, line 18 skipping to change at page 11, line 18
There are no actions for IANA. There are no actions for IANA.
7. Acknowledgements 7. Acknowledgements
Thanks to Magnus Westerlund and the members of the RMCAT feedback Thanks to Magnus Westerlund and the members of the RMCAT feedback
design team for their feedback. design team for their feedback.
8. Informative References 8. Informative References
[I-D.dt-rmcat-feedback-message] [I-D.ietf-avtcore-cc-feedback-message]
Sarker, Z., Perkins, C., Singh, V., and M. Ramalho, "RTP Sarker, Z., Perkins, C., Singh, V., and M. Ramalho, "RTP
Control Protocol (RTCP) Feedback for Congestion Control", Control Protocol (RTCP) Feedback for Congestion Control",
draft-dt-rmcat-feedback-message-01 (work in progress), draft-ietf-avtcore-cc-feedback-message-01 (work in
October 2016. progress), March 2018.
[I-D.ietf-avtcore-rtp-multi-stream]
Lennox, J., Westerlund, M., Wu, Q., and C. Perkins,
"Sending Multiple RTP Streams in a Single RTP Session",
draft-ietf-avtcore-rtp-multi-stream-11 (work in progress),
December 2015.
[I-D.ietf-avtcore-rtp-multi-stream-optimisation] [I-D.ietf-avtcore-rtp-multi-stream-optimisation]
Lennox, J., Westerlund, M., Wu, Q., and C. Perkins, Lennox, J., Westerlund, M., Wu, Q., and C. Perkins,
"Sending Multiple RTP Streams in a Single RTP Session: "Sending Multiple RTP Streams in a Single RTP Session:
Grouping RTCP Reception Statistics and Other Feedback", Grouping RTCP Reception Statistics and Other Feedback",
draft-ietf-avtcore-rtp-multi-stream-optimisation-12 (work draft-ietf-avtcore-rtp-multi-stream-optimisation-12 (work
in progress), March 2016. in progress), March 2016.
[I-D.ietf-rtcweb-rtp-usage] [I-D.ietf-rtcweb-rtp-usage]
Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time
Communication (WebRTC): Media Transport and Use of RTP", Communication (WebRTC): Media Transport and Use of RTP",
draft-ietf-rtcweb-rtp-usage-26 (work in progress), March draft-ietf-rtcweb-rtp-usage-26 (work in progress), March
2016. 2016.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>. July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed., [RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
"RTP Control Protocol Extended Reports (RTCP XR)", "RTP Control Protocol Extended Reports (RTCP XR)",
RFC 3611, DOI 10.17487/RFC3611, November 2003, RFC 3611, DOI 10.17487/RFC3611, November 2003,
<http://www.rfc-editor.org/info/rfc3611>. <https://www.rfc-editor.org/info/rfc3611>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004, RFC 3711, DOI 10.17487/RFC3711, March 2004,
<http://www.rfc-editor.org/info/rfc3711>. <https://www.rfc-editor.org/info/rfc3711>.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, [RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control "Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
DOI 10.17487/RFC4585, July 2006, DOI 10.17487/RFC4585, July 2006, <https://www.rfc-
<http://www.rfc-editor.org/info/rfc4585>. editor.org/info/rfc4585>.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for [RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
2008, <http://www.rfc-editor.org/info/rfc5124>. 2008, <https://www.rfc-editor.org/info/rfc5124>.
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size [RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, DOI 10.17487/RFC5506, April and Consequences", RFC 5506, DOI 10.17487/RFC5506, April
2009, <http://www.rfc-editor.org/info/rfc5506>. 2009, <https://www.rfc-editor.org/info/rfc5506>.
[RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla, [RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla,
"Guidelines for Choosing RTP Control Protocol (RTCP) "Guidelines for Choosing RTP Control Protocol (RTCP)
Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022, Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022,
September 2013, <http://www.rfc-editor.org/info/rfc7022>. September 2013, <https://www.rfc-editor.org/info/rfc7022>.
[RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP [RFC7201] Westerlund, M. and C. Perkins, "Options for Securing RTP
Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014, Sessions", RFC 7201, DOI 10.17487/RFC7201, April 2014,
<http://www.rfc-editor.org/info/rfc7201>. <https://www.rfc-editor.org/info/rfc7201>.
[RFC8108] Lennox, J., Westerlund, M., Wu, Q., and C. Perkins,
"Sending Multiple RTP Streams in a Single RTP Session",
RFC 8108, DOI 10.17487/RFC8108, March 2017,
<https://www.rfc-editor.org/info/rfc8108>.
Author's Address Author's Address
Colin Perkins Colin Perkins
University of Glasgow University of Glasgow
School of Computing Science School of Computing Science
Glasgow G12 8QQ Glasgow G12 8QQ
United Kingdom United Kingdom
Email: csp@csperkins.org Email: csp@csperkins.org
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