Internet DRAFT - draft-gwock-rmcat-ccfs
draft-gwock-rmcat-ccfs
RMCAT WG J. Gwock
Internet-Draft S. Lee
Intended Status: Experimental Line Plus
Expires: February 28, 2020 August 29, 2019
Congestion Control based on Forward path Status
draft-gwock-rmcat-ccfs-02
Abstract
This document describes CCFS(Congestion Control based on Forward path
Status), a rate adaptation scheme for interactive real-time media
applications, such as video conferencing. CCFS classifies the forward
paths's status as throttled, competing, probing and default which is
managed based on estimated parameters - bottleneck bandwidth, queue
delay and loss ratio. Considering only forward path status minimizes
influence of backward path's network events such as congestion. It is
also free from compatibility issues because it defines generalized
feedback message.
Status of this Memo
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Copyright and License Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
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document authors. All rights reserved.
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4 Detailed Description of CCFS . . . . . . . . . . . . . . . . . . 5
4.1 CCFS Negotiation . . . . . . . . . . . . . . . . . . . . . . 5
4.2 Rxer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.2.1 Feedback message format . . . . . . . . . . . . . . . . 6
4.2.2 CCFS feedback message size . . . . . . . . . . . . . . . 11
4.2.3 Handle CCFS control messages . . . . . . . . . . . . . . 11
4.3 Txer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.3.1 Constants . . . . . . . . . . . . . . . . . . . . . . . 12
4.3.2 Monitoring time on txer . . . . . . . . . . . . . . . . 13
4.3.3 Forward path bandwidth estimation . . . . . . . . . . . 14
4.3.4 Queue delay estimation and find increase pattern . . . . 15
4.3.5 Handle by status . . . . . . . . . . . . . . . . . . . . 16
4.3.5.1 Control event . . . . . . . . . . . . . . . . . . . 17
4.3.5.2 Handle control event by status . . . . . . . . . . . 18
4.4 CCFS Control messages . . . . . . . . . . . . . . . . . . . 21
4.4.1 Update preferred feedback interval . . . . . . . . . . . 21
4.4.2 Update preferred monitoring time . . . . . . . . . . . . 22
5 Implement . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
6 Security Considerations . . . . . . . . . . . . . . . . . . . . 22
7 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 22
8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8.1 Normative References . . . . . . . . . . . . . . . . . . . . 22
8.2 Informative References . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
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1 Introduction
Interactive real-time multi-media applications have a requirement
that controls their transmitting rate to adapt to bandwidth changes
and maintains low queuing delay over the network [RFC2914]. To solve
this challenge, many metrics such as RTT, packet loss and ECN are
used to reason the current network condition.
These real-time communication applications can have two streaming
paths - forward path to send media and backward path to receive
media. Moreover, each path is independent in most of the cases. For
example, if congestion occurs in the backward path, their is no need
to lower the transmitting rate on the forward path. However, if RTT
is used for congestion control, careful approaching is required
because RTT could be affected by not only the forward path's latency
but the backward path's latency. Although it is used to control the
transmitting rate, a metric or behavior such as RTT could be affected
by backward path's status and lead to a wrong decision.
CCFS uses metrics reflecting only the forward path's characteristic
in its algorithm to remove the influence of backward path's
conditions.
CCFS is a sender-based method to be free from compatibility issues.
That is, coexistence of multiple CCFS sender versions are available
because of algorithm variations or any other issues. To achieve this,
passive behavior of CCFS receiver and generalized feedback mechanisms
are defined in this memo.
2 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3 Overview
There are two modules to carry out a CCFS - Txer and Rxer. Txer is an
abbreviation for transmitter of CCFS and rxer means a receiver of
CCFS. Txer has a main active role to control the transmitting bitrate
by examining CCFS feedback messages from an associated rxer. Rxer
operates passively except when sending periodic CCFS feedback
messages. Txer and rxer manage multiple RTP streams if they are able
to share network paths. For example, multiple RTP streams using same
4 tuple - source ip, source port, destination ip and destination port
- would be associated with one txer and rxer.
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To start CCFS, Txer and rxer must complete CCFS negotiation.
Negotiation should be accomplished on an external channel before
associating RTP session is established.
Rxer sends a periodic CCFS feedback message if CCFS feature is
negotiated. Txer estimates various metrics, mainly 3 metrics -
bottlenecked bandwidth, queue latency and loss ratio. Then, it makes
a decision on the forward path's status, which is one of throttled,
probing, competing and default. Txer operates target transmitting
rate based on the forward path's status.
Throttled status: detected the network queue is piling up. The
current transmitting rate should be lowered to empty the queue.
Competing status: detected cross traffics. The current
transmitting rate should be controlled to keep the queue latency
within targeting queue latency.
Probing status: The forward path's bottlenecked bandwidth may be
larger than the estimated bandwidth. The current transmitting rate
should be increased to probe the bottlenecked bandwidth.
Default status: does not belong to above 3 statuses. The current
transmitting rate should be controlled to keep the queue latency
within targeting queue latency.
While the probing status, the transmitting rate should be increased
to probe available bandwidth. However, it can lead to congestion and
this can harm the media quality. To minimize the side effects,
sending redundant packets like FEC packets is recommended[I-D.ietf-
dt-rmcat-adaptive-fec].
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4 Detailed Description of CCFS
4.1 CCFS Negotiation
CCFS must be negotiated before run. Defining the way of negotiation
is beyond the scope of this document. It may use SDP negotiation[RFC
4566] or an application defined protocol. However, parameters that
should be decided from negotiation are described here.
1. txer id (4 bytes): CCFS txer's ID should be decided. This is used
as SSRC in RTCP messages used by CCFS. So this value should unique
among transmitting RTP stream's SSRC.
2. rxer id (4 bytes): Also, the rxer associated with the txer must
have its own ID to use as SSRC in RTCP messages.
3. preferred feedback interval time: Rxer should know initial
feedback interval time. This interval may be changed by the txer in
the session.
4. preferred monitoring time: A feedback message has information of
received packets for this recent monitored time.
5. lower-layer protocol: RTP packets are sent on the UDP stack in
most cases. However it may be sent on other transport layers
dependding on the application requirement. A different congestion
control mechanism for different lower-layer protocol stack is
reasonable. To decide which congestion control mechanism should be
used, both rxer's transport layer and txer's transport layer is
needed. CCFS described in this memo supposes only UDP. However CCFS
txer may be modified for other transport layers.
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4.2 Rxer
Rxer must know the preferred feedback interval time and its default
is 100 msec. This feedback interval time can be changed by RTCP
message sent by a txer. Rxer starts a periodic timer with that
interval when a session starts. When the timer fires an event rxer
determines whether sends a feedback or not. Rxer does not send
feedbacks if it has not received any packets and has not sent a
feedback before, even when the feedback interval time is passed. So
the first feedback message must include reports about one or more
received RTP packets. And the rxer should periodically send feedbacks
once it has sent the first feedbacks message. Even when there is no
received packets for the last feedback interval time, the rxer must
send a feedback because it could be an important signal.
One rxer is able to report multiple receiving RTP streams that seem
to use the same network path into one feedback message. This may make
a larger feedback message. If a feedback message size can be bigger
than MTU size, immediately rxer must sends the feedback even before
arriving the next feedback interval time. This feedback message's
monitored time value should be smaller than preferred feedback
interval time. Rxer must restart periodic timer without changing the
used interval right after sending the large feedback message.
4.2.1 Feedback message format
CCFS feedback message has a similar design goal as the [I-D.ietf-dt-
rmcat-feedback-message]. However, CCFS feedback message needs more
specific information for the CCFS algorithm.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| FMT | PT = 205 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC (Rxer Id) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC (Txer Id) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ CCFS Feedback Message Header +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. Report-Block of 1st Media Source .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. Report-Block of Nth Media Source .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
CCFS Feedback Message Format
The first 4 octets are the RTCP header, with PT=205 and FMT=CCFSFB,
and next 4 octets are the SSRC of the packet sender. CCFS replaces
SSRC with rxer id which should be obtained from the prior
negotiation.
Section 6.1 of [RFC4585] requires the RTCP header to be followed by
the SSRC of the media source being reported upon. Txer id is located
here instead of a specific RTP SSRC. And SSRC of the media source to
be reported is located within its Report-Block.
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And next 8 octets are a CCFS Feedback Message Header that is
formatted as below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Report Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Feedback Sequence | Monitored Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
CCFS Feedback Message Header
Report Time(4 octets) : The time instant when this feedback message
is generated. The value of the report time is derived from the same
wall clock used to generate the NTP timestamp field in RTCP Sender
Report (SR) packets. It is formatted as the middle 32 bits of an NTP
format timestamp, as described in Section 4 of [RFC3550]. If the rxer
does not use NTP, it can be replaced with other measures such as
system up time, but the corresponding txer should be informed.
Feedback Sequence(2 octets) : Feedback message's own sequence number.
Txer finds out the feedback message was lost or not by watching this
feedback sequence's increasement. If a feedback message was lost,
Txer must ignore the period that is monitored by the lost feedback
message. Also Txer can figure out the forward path's packet loss
event if the reported packet sequence number is not continued even
though the feedback sequence is increase by one.
Monitored time(2 octets) : Actually observed millisecond duration to
generate a feedback message. Normally this is the time by txer
preferred and equal or bigger than the current feedback interval
time. However if the building feedback information causes bigger
message size than MTU size, rxer must immediately send the feedback
message with the real monitored time. So that monitored time can be
smaller than the current txer preferred monitor time.
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Then multiple report blocks are followed. One report block describes
received packets from one media source that is identified by SSRC.
Report block size is not fixed and format is here:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of a Media Source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Report Packet Count | Start Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pkt-Element_1 | Pkt-Element_2 | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pkt-Element_n | Zero Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
CCFS Report-Block
SSRC of a Media Source(4 octets) : RTP SSRC to report.
Report Packet Count(2 octets) : The reporting rtp packet count. If
this value is zero remain fields of this Report-Block should be
ignored.
Start Sequence Number(2 octets) : Start RTP sequence number of
reported packets. This is the sequence number of the following first
Pkt-Elements.
Pkt-Element(1 or 2 octets) : Describes for each packet. The count of
Pkt-Element is the total report packet count and these are sorted by
RTP sequence number order.
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Pkt-Element format is here:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|L|X| Abs-Delta |
+-+-+-+-+-+-+-+-+
One octet Pkt-Element. X=0.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|L|X|E|N| Abs-Delta |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Two octet Pkt-Element. L=0 and X=1.
L(1 bit) : Set for a lost packet. If set, Pkt-Element size is one
octet and remain 7 bits must be ignored.
X(1 bit) : Set if Pkt-Element size is two octets.
E(1 bit) : Set if received packet and ECN of IP header of RTP packet
is 0x3.
N(1 bit) : Set if packet interval of the received packet is negative.
This means the packet was out of ordered.
Abs-Delta(6 or 12 bits) : Packet arrival interval millisecond that is
presented in absolute value. The interval is the time between the
received time of the lower sequenced RTP packet and the received time
of the Pkt-Element's RTP packet.
If the Pkt-Element is the first, the Abs-Delta is an interval from a
monitoring start time and it is always positive. And the monitoring
start time is the subtracted monitored time from report time.
The Abs-Delta of remain Pkt-Elements is the absolute interval arrival
time between two RTP packets. In the most cases, the sequence number
of two RTP packets will be successive but it is not always true
because of various network conditions. When S sequenced RTP
packet(hear in after "S RTP") is lost, Abs-Delta for S+1 RTP means
interval time between S-1 RTP and S+1 RTP. Also, S RTP can be arrived
before S+1 RTP. In this case, Abs-Delta for S+1 RTP means for
interval time between S RTP and S+1 RTP. And N flag for S+1 RTP must
be set.
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If an absolute value of packet arrival interval is bigger than 63,
Pkt-Element size must be 2 octets with X set.
When an absolute value of packet arrival interval is bigger than
4094(2^12 - 2) milliseconds, Abs-Delta must be 4095(2^12 - 1). This
means the packet arrival interval is 4095 milliseconds or more.
4.2.2 CCFS feedback message size
Rxer builds a feedback message based on recent received RTP packets.
One rxer aggregates multiple RTP streams. And sometimes, by network
condition, many packets should be arrived in a short time. These
facts make CCFS feedback message packet big. If the CCFS feedback
message size is bigger than an accepted packet size by MTU, which
will cause exception cases. So CCFS rxer should consider feedback
message size. That is, the CCFS feedback message packet size should
be smaller than MTU size.
If the feedback message size approaches an available size by MTU,
rxer must immediately send the feedback message. The monitored time
value within the feedback message will be shorter than a txer
preferred monitoring time. After sending the instant feedback
message, rxer must re-start monitoring for the next feedback.
CCFS feedback message size can be estimated as:
FBM-Size = 20 + 8R + 1.5T
R means an associated RTP stream count. And T means a total report
packet count. 1.5 is the assumed average size of Pkt-Element and this
can not be exact. However recommend the constant 1.5 for the
simplicity of the implementation.
4.2.3 Handle CCFS control messages
CCFS control message is a type of RTCP message sent by a txer. This
RTCP messages should be defined if the txer requires for a specific
feature. If the rxer receives understandable control messages, it
should respond accordingly. If not, it should ignore and discard
them.
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4.3 Txer
Txer keeps sent rtp packet array(txed_rtps) about rtp streams. The
txed_rtps contains sent local timestamp, packet size, RTP sequence
number and ssrc.
Txer estimates variable metrics when the feedback message is sent for
each time txer receives a feedback message. This means that all the
estimated metrics are the past of backward one way latency ago but
remove the effect of the backward path that is the feedback message's
network path.
It estimates forward path bandwidth, queue latency and loss ratio
using the feedback message and txed_rtps in monitored time.
Feedback messages have its own sequence number. Txer can examine
there is forward path packet losses between successive feedback
messages. Also when the increase of the feedback message's sequence
number is over one, which means backward path packet losses, txer
must take consider only reported periods to remove affect of the
backward path's network impairments.
And than it decides forward path status and targeting send rate based
on the metrics and current status.
The forward path status has four status and described in Section 3.
Actually this status affects txer's logic in globally.
4.3.1 Constants
Txer logic is described using pseudo code. For the simplicity, all
the constants used are listed up here.
FwdBwEstWei = 0.9
I_FwdBwEstWei = (1.0 - FwdBwEstWei)
TargetQDelay = 50msec
WndBrFract = 1sec
TrigProbQDFractMax = 0.8
TrigProbBrFractMin = 1.0
TrigProbQMRangeMax = 10msec
TrigProbLossRtMax = 0.02
TrigProbECNRtMax = 0.0
TrigStopProbQDFractMin = 1.3
TrigStopProbBrFractMin = 1.2
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TrigStopProbLossRtMax = 0.0
TrigStopProbECNRtMax = 0.0
TrigCompQDelayMin = 200msec
TrigCompQMRangeMi = 100msec
TrigStopCompQDelayMax = 100msec
TrigStopCompQMRangeMax = 20msec
TrigThroQDFractMin = 1.5
TrigThroBrFractMax = 0.9
Thro2CompQIncrTime = 1sec
DfltQDFractLow = 0.5
DfltQDFractHi = 1.1
DfltBrIncrRate = 1.01
DfltBrDecrRate = 0.95
CompTargetTxbwRate = 1.3
ThroTargetTxbwRate = 0.5
ProbingTime = 4sec
CompQDFractLow = 0.5
CompQDFractHi = 1.0
CompBrIncrRate = 1.02
4.3.2 Monitoring time on txer
Whenever txer receives a feedback message, txer calculates the
monitored time that is matched with the monitored time on rxer.
The latest end sequence in the feedback message is the base point of
time to find out monitored time on txer. However total reported
packet could be zero that means there was no received rtp packet for
the last monitored time for all receiving rtp streams.
In this case, monitoring time on txer must be shifted for monitored
time on rxer.
--------------------------------------------------------------------
shift_time = fbm.report_time - last_fbm_report_time
end_tstmp = last_end_tstmp + shift_time
bgn_tstmp = end_tstmp - fbm.monitored_time
--------------------------------------------------------------------
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The fbm stands for feedback message and tstmp is timestamp of txer.
This instance has results from parsed a feedback message.
bgn_tstmp is begin timestamp and end_tstamp is end timestamp. They
are points of time for a monitored period on txer.
For the more general cases, total reported packet count would be more
than zero. Txer determines a monitored period on txer using the
latest rtp packet in the reported packets.
--------------------------------------------------------------------
pkt_key = (ssrc, latest_end_seq) = find_out(fbm)
latest_sent_tstmp = txed_rtps.get_tstmp(pkt_key)
offset = fbm.report_time - fbm.get_rcvd_time(pkt_key)
end_tstmp = latest_sent_tstmp + offset
bgn_tstmp = end_tstmp - fbm.monitored_time
--------------------------------------------------------------------
First of all, find out the sent local time stamp(latest_sent_tstmp)
of the latest rtp packet among the reported.
And offset is a time between feedback message report time and the
received time of the latest rtp packet.
Each received time for reported rtp packets is calculated as follow:
--------------------------------------------------------------------
seq = fbm.start_sequence_number
sum_received_time = fbm.report_time - fbm.monitored_time
for all seq in a SSRC
received_time[seq] = sum_received_time + fbm.get_delta(seq)
sum_received_time = received_time[seq]
--------------------------------------------------------------------
4.3.3 Forward path bandwidth estimation
The forward path's bandwidth is estimated based on received rate on
the rxer.
fwd_bw = tot_rxed_size / fbm.monitored_time
The tot_rxed_size is sum of sent packet size that is found from
txed_rtps with the reported ssrc and seq. If there are the lost
packets, they should be excluded. CCFS updates esti_bw with the
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fwd_bw using moving average to remove outlier. Unfortunately the
moving average calculation causes time penalty. Moreover, if wrong
estimated value - too small or too large is used, it would affect
esti_bw negatively. So, CCFS checks its validation to minimize the
noise.
--------------------------------------------------------------------
if( status != Throttled &&
(status == Competing && target_bw < fwd_bw && lost == 0) &&
(sent_size > tot_rxed_size)
|| (sent_size == tot_rxed_size && target_bw < fwd_bw) )
esti_bw = FwdBwEstWei*esti_bw + I_FwdBwEstWei*fwd_bw
--------------------------------------------------------------------
First of all, the current status must not be throttled because target
bandwidth shrinks during throttled status to empty queue. And if the
current status is competing update esti_bw only if it fulfils the
certain condition. After status check, sent_size should be larger
than tot_rxed_size because it means the sent bitrate is about
bottlenecked bandwidth or greater for the period. And if the current
target bandwidth is underestimated, it updates esti_bw.
4.3.4 Queue delay estimation and find increase pattern
CCFS uses LEDBAT[RFC6817]'s queue delay estimation to estimate
forward path's queue delay. To do that, received timestamp have to be
calculated for each packets using ATO field in the feedback message.
And the queue delay is the minimum queue delay among the reported
packet's estimated queue delays.
The queue delay can not be exact but its relative values and pattern
can be used as an important signal. CCFS finds out increasing pattern
and its duration as follows.
--------------------------------------------------------------------
if( last_qdelay < qdelay )
incr_count++
if(incr_count == 1)
incr_start_tstmp = end_tstmp
incr_duration = end_tstmp - incr_start_tstmp;
if(incr_count >= 3 && duration >= IncrMinDuration)
is_increasing = true
else
incr_count = 0
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incr_start_tstmp = 0
is_increasing = false
last_qdelay = qdelay
--------------------------------------------------------------------
Above logic can be replaced by others if it shows good performance.
4.3.5 Handle by status
Update transmitting rate (target_txbw) based on the calculated
parameters.
--------------------------------------------------------------------
qd_fract = qdelay / target_qdelay
br_fract = recved_size_wnd / sent_size_wnd
--------------------------------------------------------------------
Above two fractions are used directly to check status and control
send rate. The recved_size_wnd means that total received packet size
for the last window time(WndBrFract) and the sent_size_wnd is the
total sent packet size for the same time.
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4.3.5.1 Control event
Before controlling transmitting rate, a certain condition makes
status change and CCFS defines this conditions as control event. The
control event list and required condition are presented here.
--------------------------------------------------------------------
Control Event | Conditions
--------------------------------------------------------------------
CTRL_NOTHING | * default value
--------------------------------------------------------------------
|1. qdelay > TrigCompQDelayMin
CTRL_START_COMPETE | && qmrange > TrigCompQMRangeMin
|2. Throttled status
| && incr_duration > Thro2CompQIncrTime
--------------------------------------------------------------------
|status is not Probing
| && is_increasing == false
| && qd_fract < TrigProbQDFractMax
CTRL_START_PROBING | && br_fract >= TrigProbBrFractMin
| && qmrange < TrigProbQMRangeMax
| && ecn_rate < TrigProbECNRtMax
| && loss_rate < TrigProbLossRtMax
--------------------------------------------------------------------
|is_increasing
CTRL_DETECT_THROTTLE | && qd_fract > QDFractMinTrigThro
| && br_fract < BrFractMaxTrigThro
--------------------------------------------------------------------
|Competing status
| && comp_duration > CompMaintainTime
CTRL_STOP_COMPETE | && qdelay < QDelayMaxTrigCompStop
| && qmrange < QMRangeTrigCompStop
--------------------------------------------------------------------
|1. Probing status
| && is_increasing
|2. Probing status
| && qd_fract > TrigStopProbQDFractMin
|3. Probing status
CTRL_STOP_PROBING | && br_fract > TrigStopProbBrFractMin
|4. Probing status
| && ecn_rate >= TrigStopProbECNRtMax
|5. Probing status
| && loss_rate >= TrigStopProbLossRtMax
--------------------------------------------------------------------
CTRL_RESOLV_THROTTLE | Throttled status
| && qdFract < 1.0
--------------------------------------------------------------------
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4.3.5.2 Handle control event by status
CTRL_START_COMPETE and CTRL_DETECT_THROTTLE are important signals to
react promptly irrelevant the forward status. So, extracted handlers
are described as follows.
--------------------------------------------------------------------
do_start_compete():
target_qdelay = TargetQDelay + TargetXQDelay
target_txbw = esti_bw * CompTargetTxbwRate
status = Competing
return
do_detect_throttle():
thro_backup_target_txbw = esti_bw
target_txbw = esti_bw * ThroTargetTxbwRate
status = Throttled
return
--------------------------------------------------------------------
Comprehensive handling for each status may seem to be complicated.
So, this document supplements the pseudo code with simple available
status change diagram.
+-------------------+ +---------+
| Default |<====================>| Probing |
+-------------------+ +---------+
| ^ | ^ |
| | | | |
| | | | |
| | | | |
| | V | |
| | +-----------+ |
| | | Competing |<--------------------------+
| | +-----------+ |
| | | ^ |
| | | | |
| | | | |
| | | | |
V | V | |
+-------------------+ |
| Throttled |<--------------------------+
+-------------------+
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+----------------+
| Default status |
+----------------+
Event: CTRL_NOTHING
if(qd_fract < DfltQDFractLow && target_txbw < esti_bw)
target_txbw *= DfltBrIncrRate
else if(qd_fract > DfltQDFractHi)
target_txbw *= DfltBrDecrRate
Event: CTRL_START_PROBING
prob_backup_target_txrt = esti_bw
target_txbw = esti_bw + prob_bw
prob_start_tstmp = curr_tstmp
status = Probing
Event: CTRL_START_COMPETE
do_start_compete()
Event: CTRL_DETECT_THROTTLE
do_detect_throttle()
+------------------+
| Competing status |
+------------------+
Event: CTRL_NOTHING
if( qd_fract < CompQDFractLow || qd_fract < CompQDFractHi )
target_txrt *= CompBrIncrRate
Event: CTRL_SOTP_COMPETE
target_qdelay = TargetQDelay
status = Default
Event: CTRL_DETECT_THROTTLE
do_detect_throttle()
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+----------------+
| Probing status |
+----------------+
Event: CTRL_NOTHING
if(curr_tstmp - prob_start_tstmp > ProbingTime)
status = Default
target_txnw = prob_backup_target_txbw + prob_bw
Event: CTRL_STOP_PROBING
target_txbw = esti_bw
status = Default
Event: CTRL_START_COMPETE
do_start_compete()
Event: CTRL_DETECT_THROTTLE
do_detect_throttle()
+------------------+
| Throttled status |
+------------------+
Event: CTRL_RESOLV_THROTTLE
target_txbw = thro_backup_target_txbw
status = Default
Event: CTRL_START_COMPETE
do_start_compete()
Event: CTRL_DETECT_THROTTLE
thro_backup_target_txbw = esti_bw
target_txbw = esti_bw * ThroTargetTxbwRate
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4.4 CCFS Control messages
If txer wants to implement a specific feature that needs rxer's help,
it can send CCFS control messages. CCFS control message is a RTCP
message with FMT=CCFSCTRL value.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| FMT | PT = 205 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC (Txer Id) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC (Rxer Id) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|X| CMT | Length | . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
. Control Message Value .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SSRC for media source is replaced with Rxer Id.
Control message block is followed and it can be multiple. The X bit
indicates there is a control message block following the current
block.
CMT(5 bits) is the control message type to inform rxer which control
message value type should be used. If rxer does not understand the
CMT, it can discard the message.
Length(10 bits) is the octet size of the control message value.
Control Message Value is different depending on the CMT value.
4.4.1 Update preferred feedback interval
Txer can change the preferred feedback interval if need. This message
doesn't need to be guaranteed so rxer won't send response message.
CMT: 1
Length: 2
Control Message Value: Interval time(msec)
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4.4.2 Update preferred monitoring time
Txer can change the monitoring time if need. This message doesn't
need to be guaranteed so rxer won't send response message but applied
feedback message have the updated monitored field value.
CMT: 2
Length: 2
Control Message Value: Monitoring time(msec)
5 Implement
<TBD>
6 Security Considerations
<Security considerations text>
7 IANA Considerations
<IANA considerations text>
8 References
8.1 Normative References
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[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, DOI
10.17487/RFC4585, July 2006, <http://www.rfc-
editor.org/info/rfc4585>.
[RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
"Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
DOI 10.17487/RFC6817, December 2012, <http://www.rfc-
editor.org/info/rfc6817>.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI
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Internet Draft CCFS February 2019
10.17487/RFC2119, March 1997, <http://www.rfc-
editor.org/info/rfc2119>.
[RFC1776] Crocker, S., "The Address is the Message", RFC 1776, DOI
10.17487/RFC1776, April 1 1995, <http://www.rfc-
editor.org/info/rfc1776>.
[TRUTHS] Callon, R., "The Twelve Networking Truths", RFC 1925, DOI
10.17487/RFC1925, April 1 1996, <http://www.rfc-
editor.org/info/rfc1925>.
8.2 Informative References
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000,
<http://www.rfc-editor.org/info/rfc2914>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC5513] Farrel, A., "IANA Considerations for Three Letter
Acronyms", RFC 5513, DOI 10.17487/RFC5513, April 1 2009,
<http://www.rfc-editor.org/info/rfc5513>.
[I-D.ietf-dt-rmcat-feedback-message]
"RTP Control Protocol (RTCP) Feedback for Congestion
Control", <https://tools.ietf.org/html/draft-ietf-avtcore-
cc-feedback-message-02>
[I-D.ietf-dt-rmcat-adaptive-fec]
"Congestion Control Using FEC for Conversational Media",
<https://datatracker.ietf.org/doc/draft-singh-rmcat-
adaptive-fec/>
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Authors' Addresses
Jungnam Gwock
Line Plus
South Korea
EMail: jungnam.gwock@linecorp.com
Sanghyun Lee
Line Plus
South Korea
EMail: sanghyun.lee@linecorp.com
Gwock, et al. Expires February 28, 2020 [Page 24]
