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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

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   http://www.ietf.org/shadow.html


Copyright and License Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the



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   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document. Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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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              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





































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