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Versions: (draft-welzl-rmcat-coupled-cc) 00 01 02 03 04 05 06 07

RTP Media Congestion Avoidance Techniques (rmcat)               S. Islam
Internet-Draft                                                  M. Welzl
Intended status: Experimental                                S. Gjessing
Expires: January 29, 2017                             University of Oslo
                                                           July 28, 2016


                Coupled congestion control for RTP media
                     draft-ietf-rmcat-coupled-cc-03

Abstract

   When multiple congestion controlled RTP sessions traverse the same
   network bottleneck, it can be beneficial to combine their controls
   such that the total on-the-wire behavior is improved.  This document
   describes such a method for flows that have the same sender, in a way
   that is as flexible and simple as possible while minimizing the
   amount of changes needed to existing RTP applications.  It specifies
   how to apply the method for both the NADA and Google congestion
   control algorithms.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 29, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   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



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   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.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Limitations . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Architectural overview  . . . . . . . . . . . . . . . . . . .   4
   5.  Roles . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  SBD . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  FSE . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.3.  Flows . . . . . . . . . . . . . . . . . . . . . . . . . .   7
       5.3.1.  Example algorithm 1 - Active FSE  . . . . . . . . . .   7
       5.3.2.  Example algorithm 2 - Conservative Active FSE . . . .   8
   6.  Application . . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  NADA  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.2.  GCC . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.3.  General recommendations . . . . . . . . . . . . . . . . .  10
   7.  Expected feedback from experiments  . . . . . . . . . . . . .  11
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     11.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Appendix A.  Scheduling . . . . . . . . . . . . . . . . . . . . .  14
   Appendix B.  Example algorithm - Passive FSE  . . . . . . . . . .  14
     B.1.  Example operation (passive) . . . . . . . . . . . . . . .  17
   Appendix C.  Change log . . . . . . . . . . . . . . . . . . . . .  21
     C.1.  draft-welzl-rmcat-coupled-cc  . . . . . . . . . . . . . .  21
       C.1.1.  Changes from -00 to -01 . . . . . . . . . . . . . . .  21
       C.1.2.  Changes from -01 to -02 . . . . . . . . . . . . . . .  21
       C.1.3.  Changes from -02 to -03 . . . . . . . . . . . . . . .  21
       C.1.4.  Changes from -03 to -04 . . . . . . . . . . . . . . .  22
       C.1.5.  Changes from -04 to -05 . . . . . . . . . . . . . . .  22
     C.2.  draft-ietf-rmcat-coupled-cc . . . . . . . . . . . . . . .  22
       C.2.1.  Changes from draft-welzl-rmcat-coupled-cc-05  . . . .  22
       C.2.2.  Changes from -00 to -01 . . . . . . . . . . . . . . .  22
       C.2.3.  Changes from -01 to -02 . . . . . . . . . . . . . . .  22
       C.2.4.  Changes from -02 to -03 . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22







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1.  Introduction

   When there is enough data to send, a congestion controller must
   increase its sending rate until the path's capacity has been reached;
   depending on the controller, sometimes the rate is increased further,
   until packets are ECN-marked or dropped.  This process inevitably
   creates undesirable queuing delay -- an effect that is amplified when
   multiple congestion controlled connections traverse the same network
   bottleneck.

   The Congestion Manager (CM) [RFC3124] couples flows by providing a
   single congestion controller.  It is hard to implement because it
   requires an additional congestion controller and removes all per-
   connection congestion control functionality, which is quite a
   significant change to existing RTP based applications.  This document
   presents a method to combine the behavior of congestion control
   mechanisms that is easier to implement than the Congestion Manager
   [RFC3124] and also requires less significant changes to existing RTP
   based applications.  It attempts to roughly approximate the CM
   behavior by sharing information between existing congestion
   controllers.  It is able to honor user-specified priorities, which is
   required by rtcweb [RFC7478].

2.  Definitions

   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].

   Available Bandwidth:
         The available bandwidth is the nominal link capacity minus the
         amount of traffic that traversed the link during a certain time
         interval, divided by that time interval.

   Bottleneck:
         The first link with the smallest available bandwidth along the
         path between a sender and receiver.

   Flow:
         A flow is the entity that congestion control is operating on.
         It could, for example, be a transport layer connection, an RTP
         session, or a subsession that is multiplexed onto a single RTP
         session together with other subsessions.

   Flow Group Identifier (FGI):
         A unique identifier for each subset of flows that is limited by
         a common bottleneck.




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   Flow State Exchange (FSE):
         The entity that maintains information that is exchanged between
         flows.

   Flow Group (FG):
         A group of flows having the same FGI.

   Shared Bottleneck Detection (SBD):
         The entity that determines which flows traverse the same
         bottleneck in the network, or the process of doing so.

3.  Limitations

   Sender-side only:
         Coupled congestion control as described here only operates
         inside a single host on the sender side.  This is because,
         irrespective of where the major decisions for congestion
         control are taken, the sender of a flow needs to eventually
         decide the transmission rate.  Additionally, the necessary
         information about how much data an application can currently
         send on a flow is often only available at the sender side,
         making the sender an obvious choice for placement of the
         elements and mechanisms described here.

   Shared bottlenecks do not change quickly:
         As per the definition above, a bottleneck depends on cross
         traffic, and since such traffic can heavily fluctuate,
         bottlenecks can change at a high frequency (e.g., there can be
         oscillation between two or more links).  This means that, when
         flows are partially routed along different paths, they may
         quickly change between sharing and not sharing a bottleneck.
         For simplicity, here it is assumed that a shared bottleneck is
         valid for a time interval that is significantly longer than the
         interval at which congestion controllers operate.  Note that,
         for the only SBD mechanism defined in this document
         (multiplexing on the same five-tuple), the notion of a shared
         bottleneck stays correct even in the presence of fast traffic
         fluctuations: since all flows that are assumed to share a
         bottleneck are routed in the same way, if the bottleneck
         changes, it will still be shared.

4.  Architectural overview

   Figure 1 shows the elements of the architecture for coupled
   congestion control: the Flow State Exchange (FSE), Shared Bottleneck
   Detection (SBD) and Flows.  The FSE is a storage element that can be
   implemented in two ways: active and passive.  In the active version,
   it initiates communication with flows and SBD.  However, in the



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   passive version, it does not actively initiate communication with
   flows and SBD; its only active role is internal state maintenance
   (e.g., an implementation could use soft state to remove a flow's data
   after long periods of inactivity).  Every time a flow's congestion
   control mechanism would normally update its sending rate, the flow
   instead updates information in the FSE and performs a query on the
   FSE, leading to a sending rate that can be different from what the
   congestion controller originally determined.  Using information
   about/from the currently active flows, SBD updates the FSE with the
   correct Flow State Identifiers (FSIs).  This document describes both
   active and passive versions, however the passive version is put into
   the appendix as it is extremely experimental.


                         -------  <---  Flow 1
                         | FSE |  <---  Flow 2 ..
                         -------  <---  .. Flow N
                            ^
                            |             |
                         -------          |
                         | SBD |  <-------|
                         -------


             Figure 1: Coupled congestion control architecture

   Since everything shown in Figure 1 is assumed to operate on a single
   host (the sender) only, this document only describes aspects that
   have an influence on the resulting on-the-wire behavior.  It does,
   for instance, not define how many bits must be used to represent
   FSIs, or in which way the entities communicate.  Implementations can
   take various forms: for instance, all the elements in the figure
   could be implemented within a single application, thereby operating
   on flows generated by that application only.  Another alternative
   could be to implement both the FSE and SBD together in a separate
   process which different applications communicate with via some form
   of Inter-Process Communication (IPC).  Such an implementation would
   extend the scope to flows generated by multiple applications.  The
   FSE and SBD could also be included in the Operating System kernel.

5.  Roles

   This section gives an overview of the roles of the elements of
   coupled congestion control, and provides an example of how coupled
   congestion control can operate.






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5.1.  SBD

   SBD uses knowledge about the flows to determine which flows belong in
   the same Flow Group (FG), and assigns FGIs accordingly.  This
   knowledge can be derived in three basic ways:

   1.  From multiplexing: it can be based on the simple assumption that
       packets sharing the same five-tuple (IP source and destination
       address, protocol, and transport layer port number pair) and
       having the same Differentiated Services Code Point (DSCP) in the
       IP header are typically treated in the same way along the path.
       The latter method is the only one specified in this document: SBD
       MAY consider all flows that use the same five-tuple and DSCP to
       belong to the same FG.  This classification applies to certain
       tunnels, or RTP flows that are multiplexed over one transport
       (cf. [transport-multiplex]).  Such multiplexing is also a
       recommended usage of RTP in rtcweb [rtcweb-rtp-usage].

   2.  Via configuration: e.g. by assuming that a common wireless uplink
       is also a shared bottleneck.

   3.  From measurements: e.g. by considering correlations among
       measured delay and loss as an indication of a shared bottleneck.

   The methods above have some essential trade-offs: e.g., multiplexing
   is a completely reliable measure, however it is limited in scope to
   two end points (i.e., it cannot be applied to couple congestion
   controllers of one sender talking to multiple receivers).  A
   measurement-based SBD mechanism is described in [I-D.ietf-rmcat-sbd].
   Measurements can never be 100% reliable, in particular because they
   are based on the past but applying coupled congestion control means
   to make an assumption about the future; it is therefore recommended
   to implement cautionary measures, e.g. by disabling coupled
   congestion control if enabling it causes a significant increase in
   delay and/or packet loss.  Measurements also take time, which entails
   a certain delay for turning on coupling (refer to
   [I-D.ietf-rmcat-sbd] for details).

5.2.  FSE

   The FSE contains a list of all flows that have registered with it.
   For each flow, it stores the following:

   o  a unique flow number to identify the flow

   o  the FGI of the FG that it belongs to (based on the definitions in
      this document, a flow has only one bottleneck, and can therefore
      be in only one FG)



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   o  a priority P, which here is assumed to be represented as a
      floating point number in the range from 0.1 (unimportant) to 1
      (very important).

   o  The rate used by the flow in bits per second, FSE_R.

   Note that the priority does not need to be a floating point value and
   its value range does not matter for this algorithm: the algorithm
   works with a flow's priority portion of the sum of all priority
   values.  Priorities can therefore be mapped to the "very-low", "low",
   "medium" or "high" priority levels described in
   [I-D.ietf-rtcweb-transports] using the values 1, 2, 4 and 8,
   respectively.

   In the FSE, each FG contains one static variable S_CR which is the
   sum of the calculated rates of all flows in the same FG.  This value
   is used to calculate the sending rate.

   The information listed here is enough to implement the sample flow
   algorithm given below.  FSE implementations could easily be extended
   to store, e.g., a flow's current sending rate for statistics
   gathering or future potential optimizations.

5.3.  Flows

   Flows register themselves with SBD and FSE when they start,
   deregister from the FSE when they stop, and carry out an UPDATE
   function call every time their congestion controller calculates a new
   sending rate.  Via UPDATE, they provide the newly calculated rate and
   optionally (if the algorithm supports it) the desired rate.  The
   desired rate is less than the calculated rate in case of application-
   limited flows; otherwise, it is the same as the calculated rate.

   Below, two example algorithms are described.  While other algorithms
   could be used instead, the same algorithm must be applied to all
   flows.

5.3.1.  Example algorithm 1 - Active FSE

   This algorithm was designed to be the simplest possible method to
   assign rates according to the priorities of flows.  Simulations
   results in [fse] indicate that it does however not significantly
   reduce queuing delay and packet loss.

   (1)  When a flow f starts, it registers itself with SBD and the FSE.
        FSE_R is initialized with the congestion controller's initial
        rate.  SBD will assign the correct FGI.  When a flow is assigned
        an FGI, it adds its FSE_R to S_CR.



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   (2)  When a flow f stops or pauses, its entry is removed from the
        list.

   (3)  Every time the congestion controller of the flow f determines a
        new sending rate CC_R, the flow calls UPDATE, which carries out
        the tasks listed below to derive the new sending rates for all
        the flows in the FG.  A flow's UPDATE function uses a local
        (i.e. per-flow) temporary variable S_P, which is the sum of all
        the priorities.

        (a)  It updates S_CR.


               S_CR = S_CR + CC_R - FSE_R(f)

        (b)  It calculates the sum of all the priorities, S_P.


               S_P = 0
               for all flows i in FG do
                   S_P = S_P + P(i)
               end for

        (c)  It calculates the sending rates for all the flows in an FG
             and distributes them.


               for all flows i in FG do
                   FSE_R(i) = (P(i)*S_CR)/S_P
                   send FSE_R(i) to the flow i
               end for

5.3.2.  Example algorithm 2 - Conservative Active FSE

   This algorithm extends algorithm 1 to conservatively emulate the
   behavior of a single flow by proportionally reducing the aggregate
   rate on congestion.  Simulations results in [fse] indicate that it
   can significantly reduce queuing delay and packet loss.

   (1)  When a flow f starts, it registers itself with SBD and the FSE.
        FSE_R is initialized with the congestion controller's initial
        rate.  SBD will assign the correct FGI.  When a flow is assigned
        an FGI, it adds its FSE_R to S_CR.

   (2)  When a flow f stops or pauses, its entry is removed from the
        list.





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   (3)  Every time the congestion controller of the flow f determines a
        new sending rate CC_R, the flow calls UPDATE, which carries out
        the tasks listed below to derive the new sending rates for all
        the flows in the FG.  A flow's UPDATE function uses a local
        (i.e. per-flow) temporary variable S_P, which is the sum of all
        the priorities, and a local variable DELTA, which is used to
        calculate the difference between CC_R and the previously stored
        FSE_R.  To prevent flows from either ignoring congestion or
        overreacting, a timer keeps them from changing their rates
        immediately after the common rate reduction that follows a
        congestion event.  This timer is set to 2 RTTs of the flow that
        experienced congestion because it is assumed that a congestion
        event can persist for up to one RTT of that flow, with another
        RTT added to compensate for fluctuations in the measured RTT
        value.

        (a)  It updates S_CR based on DELTA.


               if Timer has expired or not set then
                 DELTA = CC_R - FSE_R(f)
                 if DELTA < 0 then  // Reduce S_CR proportionally
                   S_CR = S_CR * CC_R / FSE_R(f)
                   Set Timer for 2 RTTs
                 else
                   S_CR = S_CR + DELTA
                 end if
                end if

        (b)  It calculates the sum of all the priorities, S_P.


               S_P = 0
               for all flows i in FG do
                   S_P = S_P + P(i)
               end for

        (c)  It calculates the sending rates for all the flows in an FG
             and distributes them.


               for all flows i in FG do
                   FSE_R(i) = (P(i)*S_CR)/S_P
                   send FSE_R(i) to the flow i
               end for






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6.  Application

   This section specifies how the FSE can be applied to specific
   congestion control mechanisms and makes general recommendations that
   facilitate applying the FSE to future congestion controls.

6.1.  NADA

   Network-Assisted Dynamic Adapation (NADA) [I-D.ietf-rmcat-nada] is a
   congestion control scheme for rtcweb.  It calculates a reference rate
   r_ref upon receiving an acknowledgment, and then, based on the
   reference rate, it calculates a video target rate r_vin and a sending
   rate for the flows, r_send.

   When applying the FSE to NADA, the UPDATE function call described in
   Section 5.3 gives the FSE NADA's reference rate r_ref.  The
   recommended algorithm for NADA is the Active FSE in Section 5.3.1.
   In step 3 (c), when the FSE_R(i) is "sent" to the flow i, this means
   updating r_ref(r_vin and r_send) of flow i with the value of
   FSE_R(i).

6.2.  GCC

   Google Congestion Control (GCC) [I-D.ietf-rmcat-gcc] is another
   congestion control scheme for rtcweb.  The rate control of GCC
   employs two parts: controlling the bandwidth estimate based on delay,
   and controlling the bandwidth estimate based on loss.  Both are
   designed to estimate the available bandwidth, A_hat.

   When applying the FSE to GCC, the UPDATE function call described in
   Section 5.3 gives the FSE GCC's estimate of available bandwidth
   A_hat.  The recommended algorithm for GCC is the Active FSE in
   Section 5.3.1.  In step 3 (c), when the FSE_R(i) is "sent" to the
   flow i, this means updating A_hat of flow i with the value of
   FSE_R(i).

6.3.  General recommendations

   This section provides general advice for applying the FSE to
   congestion control mechanisms.

   Receiver-side calculations:
         When receiver-side calculations make assumptions about the rate
         of the sender, the calculations need to be synchronized or the
         receiver needs to be updated accordingly.  This applies to TFRC
         [RFC5348], for example, where simulations showed somewhat less
         favorable results when using the FSE without a receiver-side
         change [fse].



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7.  Expected feedback from experiments

   The algorithm described in this memo has so far been evaluated using
   simulations covering all the tests for more than one flow from
   [I-D.ietf-rmcat-eval-test] (see [IETF-93], [IETF-94]).  Experiments
   should confirm these results using at least one of the same
   congestion control algorithms (GCC or NADA) with real-life code
   (e.g., browsers communicating over an emulated network covering the
   conditions in [I-D.ietf-rmcat-eval-test].  The tests with real-life
   code should be repeated afterwards in real network environments and
   monitored.  Implementers and testers are invited to document their
   findings in an Internet draft.

8.  Acknowledgements

   This document has benefitted from discussions with and feedback from
   David Hayes, Mirja Kuehlewind, Karen Nielsen, Andreas Petlund, David
   Ros (who also gave the FSE its name), Zaheduzzaman Sarker, Varun
   Singh, Anna Brunstrom, Martin Stiemerling, and Kristian Hiorth.  The
   authors would like to thank Xiaoqing Zhu and Stefan Holmer for
   helping with NADA and GCC.

   This work was partially funded by the European Community under its
   Seventh Framework Programme through the Reducing Internet Transport
   Latency (RITE) project (ICT-317700).

9.  IANA Considerations

   This memo includes no request to IANA.

10.  Security Considerations

   In scenarios where the architecture described in this document is
   applied across applications, various cheating possibilities arise:
   e.g., supporting wrong values for the calculated rate, the desired
   rate, or the priority of a flow.  In the worst case, such cheating
   could either prevent other flows from sending or make them send at a
   rate that is unreasonably large.  The end result would be unfair
   behavior at the network bottleneck, akin to what could be achieved
   with any UDP based application.  Hence, since this is no worse than
   UDP in general, there seems to be no significant harm in using this
   in the absence of UDP rate limiters.

   In the case of a single-user system, it should also be in the
   interest of any application programmer to give the user the best
   possible experience by using reasonable flow priorities or even
   letting the user choose them.  In a multi-user system, this interest
   may not be given, and one could imagine the worst case of an "arms



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   race" situation, where applications end up setting their priorities
   to the maximum value.  If all applications do this, the end result is
   a fair allocation in which the priority mechanism is implicitly
   eliminated, and no major harm is done.

11.  References

11.1.  Normative References

   [I-D.ietf-rmcat-gcc]
              Holmer, S., Lundin, H., Carlucci, G., Cicco, L., and S.
              Mascolo, "A Google Congestion Control Algorithm for Real-
              Time Communication", draft-ietf-rmcat-gcc-02 (work in
              progress), July 2016.

   [I-D.ietf-rmcat-nada]
              Zhu, X., Pan, R., Ramalho, D., Cruz, S., Jones, P., Fu,
              J., D'Aronco, S., and C. Ganzhorn, "NADA: A Unified
              Congestion Control Scheme for Real-Time Media", draft-
              ietf-rmcat-nada-02 (work in progress), March 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3124]  Balakrishnan, H. and S. Seshan, "The Congestion Manager",
              RFC 3124, DOI 10.17487/RFC3124, June 2001,
              <http://www.rfc-editor.org/info/rfc3124>.

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 5348, DOI 10.17487/RFC5348, September 2008,
              <http://www.rfc-editor.org/info/rfc5348>.

11.2.  Informative References

   [fse]      Islam, S., Welzl, M., Gjessing, S., and N. Khademi,
              "Coupled Congestion Control for RTP Media", ACM SIGCOMM
              Capacity Sharing Workshop (CSWS 2014) and ACM SIGCOMM CCR
              44(4) 2014; extended version available as a technical
              report from
              http://safiquli.at.ifi.uio.no/paper/fse-tech-report.pdf ,
              2014.







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   [fse-noms]
              Islam, S., Welzl, M., Hayes, D., and S. Gjessing,
              "Managing Real-Time Media Flows through a Flow State
              Exchange", IEEE NOMS 2016, Istanbul, Turkey , 2016.

   [I-D.ietf-rmcat-eval-test]
              Sarker, Z., Singh, V., Zhu, X., and D. Ramalho, "Test
              Cases for Evaluating RMCAT Proposals", draft-ietf-rmcat-
              eval-test-03 (work in progress), March 2016.

   [I-D.ietf-rmcat-sbd]
              Hayes, D., Ferlin, S., Welzl, M., and K. Hiorth, "Shared
              Bottleneck Detection for Coupled Congestion Control for
              RTP Media.", draft-ietf-rmcat-sbd-04 (work in progress),
              March 2016.

   [I-D.ietf-rtcweb-transports]
              Alvestrand, H., "Transports for WebRTC", Internet-draft
              draft-ietf-rtcweb-transports-11.txt, January 2016.

   [IETF-93]  Islam, S., Welzl, M., and S. Gjessing, "Updates on Coupled
              Congestion Control for RTP Media", July 2015,
              <http://www.ietf.org/proceedings/93/slides/
              slides-93-rmcat-3.pptx>.

   [IETF-94]  Islam, S., Welzl, M., and S. Gjessing, "Updates on Coupled
              Congestion Control for RTP Media", November 2015,
              <https://www.ietf.org/proceedings/94/slides/slides-94-
              rmcat-5.pdf>.

   [RFC7478]  Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
              Time Communication Use Cases and Requirements", RFC 7478,
              DOI 10.17487/RFC7478, March 2015,
              <http://www.rfc-editor.org/info/rfc7478>.

   [rtcweb-rtp-usage]
              Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time
              Communication (WebRTC): Media Transport and Use of RTP",
              Internet-draft draft-ietf-rtcweb-rtp-usage-26.txt, March
              2016.

   [transport-multiplex]
              Westerlund, M. and C. Perkins, "Multiple RTP Sessions on a
              Single Lower-Layer Transport", Internet-draft draft-
              westerlund-avtcore-transport-multiplexing-07.txt, October
              2013.





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Appendix A.  Scheduling

   When connections originate from the same host, it would be possible
   to use only one single sender-side congestion controller which
   determines the overall allowed sending rate, and then use a local
   scheduler to assign a proportion of this rate to each RTP session.
   This way, priorities could also be implemented as a function of the
   scheduler.  The Congestion Manager (CM) [RFC3124] also uses such a
   scheduling function.

Appendix B.  Example algorithm - Passive FSE

   Active algorithms calculate the rates for all the flows in the FG and
   actively distribute them.  In a passive algorithm, UPDATE returns a
   rate that should be used instead of the rate that the congestion
   controller has determined.  This can make a passive algorithm easier
   to implement; however, when round-trip times of flows are unequal,
   shorter-RTT flows will update and react to the overall FSE state more
   often than longer-RTT flows, which can produce unwanted side effects.
   This problem is more significant when the congestion control
   convergence depends on the RTT.  While the passive algorithm works
   better for congestion controls with RTT-independent convergence, it
   can still produce oscillations on short time scales.  The algorithm
   described below is therefore considered as highly experimental.
   Results of a simplified passive FSE algorithm with both NADA and GCC
   can be found in [fse-noms].

   This passive version of the FSE stores the following information in
   addition to the variables described in Section 5.2:

   o  The desired rate DR.  This can be smaller than the calculated rate
      if the application feeding into the flow has less data to send
      than the congestion controller would allow.  In case of a bulk
      transfer, DR must be set to CC_R received from the flow's
      congestion module.

   The passive version of the FSE contains one static variable per FG
   called TLO (Total Leftover Rate -- used to let a flow 'take'
   bandwidth from application-limited or terminated flows) which is
   initialized to 0.  For the passive version, S_CR is limited to
   increase or decrease as conservatively as a flow's congestion
   controller decides in order to prohibit sudden rate jumps.

   (1)  When a flow f starts, it registers itself with SBD and the FSE.
        FSE_R and DR are initialized with the congestion controller's
        initial rate.  SBD will assign the correct FGI.  When a flow is
        assigned an FGI, it adds its FSE_R to S_CR.




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   (2)  When a flow f stops or pauses, it sets its DR to 0 and sets P to
        -1.

   (3)  Every time the congestion controller of the flow f determines a
        new sending rate CC_R, assuming the flow's new desired rate
        new_DR to be "infinity" in case of a bulk data transfer with an
        unknown maximum rate, the flow calls UPDATE, which carries out
        the tasks listed below to derive the flow's new sending rate,
        Rate.  A flow's UPDATE function uses a few local (i.e. per-flow)
        temporary variables, which are all initialized to 0: DELTA,
        new_S_CR and S_P.

        (a)  For all the flows in its FG (including itself), it
             calculates the sum of all the calculated rates, new_S_CR.
             Then it calculates the difference between FSE_R(f) and
             CC_R, DELTA.


               for all flows i in FG do
                   new_S_CR = new_S_CR + FSE_R(i)
               end for
               DELTA =  CC_R - FSE_R(f)

        (b)  It updates S_CR, FSE_R(f) and DR(f).


               FSE_R(f) = CC_R
               if DELTA > 0 then  // the flow's rate has increased
                   S_CR = S_CR + DELTA
               else if DELTA < 0 then
                   S_CR = new_S_CR + DELTA
               end if
               DR(f) = min(new_DR,FSE_R(f))

        (c)  It calculates the leftover rate TLO, removes the terminated
             flows from the FSE and calculates the sum of all the
             priorities, S_P.














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               for all flows i in FG do
                  if P(i)<0 then
                     delete flow
                  else
                     S_P = S_P + P(i)
                  end if
               end for
               if DR(f) < FSE_R(f) then
                  TLO = TLO + (P(f)/S_P) * S_CR - DR(f))
               end if

        (d)  It calculates the sending rate, Rate.


               Rate = min(new_DR, (P(f)*S_CR)/S_P + TLO)

               if Rate != new_DR and TLO > 0 then
                   TLO = 0  // f has 'taken' TLO
               end if

        (e)  It updates DR(f) and FSE_R(f) with Rate.


               if Rate > DR(f) then
                   DR(f) = Rate
               end if
               FSE_R(f)  = Rate

   The goals of the flow algorithm are to achieve prioritization,
   improve network utilization in the face of application-limited flows,
   and impose limits on the increase behavior such that the negative
   impact of multiple flows trying to increase their rate together is
   minimized.  It does that by assigning a flow a sending rate that may
   not be what the flow's congestion controller expected.  It therefore
   builds on the assumption that no significant inefficiencies arise
   from temporary application-limited behavior or from quickly jumping
   to a rate that is higher than the congestion controller intended.
   How problematic these issues really are depends on the controllers in
   use and requires careful per-controller experimentation.  The coupled
   congestion control mechanism described here also does not require all
   controllers to be equal; effects of heterogeneous controllers, or
   homogeneous controllers being in different states, are also subject
   to experimentation.

   This algorithm gives all the leftover rate of application-limited
   flows to the first flow that updates its sending rate, provided that
   this flow needs it all (otherwise, its own leftover rate can be taken
   by the next flow that updates its rate).  Other policies could be



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   applied, e.g. to divide the leftover rate of a flow equally among all
   other flows in the FGI.

B.1.  Example operation (passive)

   In order to illustrate the operation of the passive coupled
   congestion control algorithm, this section presents a toy example of
   two flows that use it.  Let us assume that both flows traverse a
   common 10 Mbit/s bottleneck and use a simplistic congestion
   controller that starts out with 1 Mbit/s, increases its rate by 1
   Mbit/s in the absence of congestion and decreases it by 2 Mbit/s in
   the presence of congestion.  For simplicity, flows are assumed to
   always operate in a round-robin fashion.  Rate numbers below without
   units are assumed to be in Mbit/s.  For illustration purposes, the
   actual sending rate is also shown for every flow in FSE diagrams even
   though it is not really stored in the FSE.

   Flow #1 begins.  It is a bulk data transfer and considers itself to
   have top priority.  This is the FSE after the flow algorithm's step
   1:


   ----------------------------------------
   | # | FGI |  P  | FSE_R  |  DR  | Rate |
   |   |     |     |        |      |      |
   | 1 |  1  |  1  |   1    |   1  |   1  |
   ----------------------------------------
   S_CR = 1, TLO = 0



   Its congestion controller gradually increases its rate.  Eventually,
   at some point, the FSE should look like this:


   -----------------------------------------
   | # | FGI |  P  |  FSE_R  |  DR  | Rate |
   |   |     |     |         |      |      |
   | 1 |  1  |  1  |   10    |  10  |  10  |
   -----------------------------------------
   S_CR = 10, TLO = 0



   Now another flow joins.  It is also a bulk data transfer, and has a
   lower priority (0.5):





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   ------------------------------------------
   | # | FGI |   P   | FSE_R  |  DR  | Rate |
   |   |     |       |        |      |      |
   | 1 |  1  |   1   |   10   |  10  |  10  |
   | 2 |  1  |  0.5  |    1   |   1  |   1  |
   ------------------------------------------
   S_CR = 11, TLO = 0



   Now assume that the first flow updates its rate to 8, because the
   total sending rate of 11 exceeds the total capacity.  Let us take a
   closer look at what happens in step 3 of the flow algorithm.


   CC_R = 8. new_DR = infinity.
   3 a) new_S_CR = 11; DELTA = 8 - 10 = -2.
   3 b) FSE_Rf) = 8. DELTA is negative, hence S_CR = 9;
        DR(f) = 8.
   3 c) S_P = 1.5.
   3 d) new sending rate = min(infinity, 1/1.5 * 9 + 0) = 6.
   3 e) FSE_R(f) = 6.

   The resulting FSE looks as follows:
   -------------------------------------------
   | # | FGI |   P   |  FSE_R  |  DR  | Rate |
   |   |     |       |         |      |      |
   | 1 |  1  |   1   |    6    |   8  |   6  |
   | 2 |  1  |  0.5  |    1    |   1  |   1  |
   -------------------------------------------
   S_CR = 9, TLO = 0



   The effect is that flow #1 is sending with 6 Mbit/s instead of the 8
   Mbit/s that the congestion controller derived.  Let us now assume
   that flow #2 updates its rate.  Its congestion controller detects
   that the network is not fully saturated (the actual total sending
   rate is 6+1=7) and increases its rate.












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   CC_R=2. new_DR = infinity.
   3 a) new_S_CR = 7; DELTA = 2 - 1 = 1.
   3 b) FSE_R(f) = 2. DELTA is positive, hence S_CR = 9 + 1 = 10;
        DR(f) = 2.
   3 c) S_P = 1.5.
   3 d) new sending rate = min(infinity, 0.5/1.5 * 10 + 0) = 3.33.
   3 e) DR(f) = FSE_R(f) = 3.33.

   The resulting FSE looks as follows:
   -------------------------------------------
   | # | FGI |   P   |  FSE_R  |  DR  | Rate |
   |   |     |       |         |      |      |
   | 1 |  1  |   1   |    6    |   8  |   6  |
   | 2 |  1  |  0.5  |   3.33  | 3.33 | 3.33 |
   -------------------------------------------
   S_CR = 10, TLO = 0



   The effect is that flow #2 is now sending with 3.33 Mbit/s, which is
   close to half of the rate of flow #1 and leads to a total utilization
   of 6(#1) + 3.33(#2) = 9.33 Mbit/s.  Flow #2's congestion controller
   has increased its rate faster than the controller actually expected.
   Now, flow #1 updates its rate.  Its congestion controller detects
   that the network is not fully saturated and increases its rate.
   Additionally, the application feeding into flow #1 limits the flow's
   sending rate to at most 2 Mbit/s.


   CC_R=7. new_DR=2.
   3 a) new_S_CR = 9.33; DELTA = 1.
   3 b) FSE_R(f) = 7, DELTA is positive, hence S_CR = 10 + 1 = 11;
        DR = min(2, 7) = 2.
   3 c) S_P = 1.5; DR(f) < FSE_R(f), hence TLO = 1/1.5 * 11 - 2 = 5.33.
   3 d) new sending rate = min(2, 1/1.5 * 11 + 5.33) = 2.
   3 e) FSE_R(f) = 2.

   The resulting FSE looks as follows:
   -------------------------------------------
   | # | FGI |   P   |  FSE_R  |  DR  | Rate |
   |   |     |       |         |      |      |
   | 1 |  1  |   1   |    2    |   2  |   2  |
   | 2 |  1  |  0.5  |   3.33  | 3.33 | 3.33 |
   -------------------------------------------
   S_CR = 11, TLO = 5.33






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   Now, the total rate of the two flows is 2 + 3.33 = 5.33 Mbit/s, i.e.
   the network is significantly underutilized due to the limitation of
   flow #1.  Flow #2 updates its rate.  Its congestion controller
   detects that the network is not fully saturated and increases its
   rate.


   CC_R=4.33. new_DR = infinity.
   3 a) new_S_CR = 5.33; DELTA = 1.
   3 b) FSE_R(f) = 4.33. DELTA is positive, hence S_CR = 12;
        DR(f) = 4.33.
   3 c) S_P = 1.5.
   3 d) new sending rate: min(infinity, 0.5/1.5 * 12 + 5.33 ) = 9.33.
   3 e) FSE_R(f) = 9.33, DR(f) = 9.33.

   The resulting FSE looks as follows:
   -------------------------------------------
   | # | FGI |   P   |  FSE_R  |  DR  | Rate |
   |   |     |       |         |      |      |
   | 1 |  1  |   1   |    2    |   2  |   2  |
   | 2 |  1  |  0.5  |   9.33  | 9.33 | 9.33 |
   -------------------------------------------
   S_CR = 12, TLO = 0



   Now, the total rate of the two flows is 2 + 9.33 = 11.33 Mbit/s.
   Finally, flow #1 terminates.  It sets P to -1 and DR to 0.  Let us
   assume that it terminated late enough for flow #2 to still experience
   the network in a congested state, i.e. flow #2 decreases its rate in
   the next iteration.




















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   CC_R = 7.33. new_DR = infinity.
   3 a) new_S_CR = 11.33; DELTA = -2.
   3 b) FSE_R(f) = 7.33. DELTA is negative, hence S_CR = 9.33;
        DR(f) = 7.33.
   3 c) Flow 1 has P = -1, hence it is deleted from the FSE.
        S_P = 0.5.
   3 d) new sending rate: min(infinity, 0.5/0.5*9.33 + 0) = 9.33.
   3 e) FSE_R(f) = DR(f) = 9.33.

   The resulting FSE looks as follows:
   -------------------------------------------
   | # | FGI |   P   |  FSE_R  |  DR  | Rate |
   |   |     |       |         |      |      |
   | 2 |  1  |  0.5  |   9.33  | 9.33 | 9.33 |
   -------------------------------------------
   S_CR = 9.33, TLO = 0



Appendix C.  Change log

C.1.  draft-welzl-rmcat-coupled-cc

C.1.1.  Changes from -00 to -01

   o  Added change log.

   o  Updated the example algorithm and its operation.

C.1.2.  Changes from -01 to -02

   o  Included an active version of the algorithm which is simpler.

   o  Replaced "greedy flow" with "bulk data transfer" and "non-greedy"
      with "application-limited".

   o  Updated new_CR to CC_R, and CR to FSE_R for better understanding.

C.1.3.  Changes from -02 to -03

   o  Included an active conservative version of the algorithm which
      reduces queue growth and packet loss; added a reference to a
      technical report that shows these benefits with simulations.

   o  Moved the passive variant of the algorithm to appendix.






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C.1.4.  Changes from -03 to -04

   o  Extended SBD section.

   o  Added a note about window-based controllers.

C.1.5.  Changes from -04 to -05

   o  Added a section about applying the FSE to specific congestion
      control algorithms, with a subsection specifying its use with
      NADA.

C.2.  draft-ietf-rmcat-coupled-cc

C.2.1.  Changes from draft-welzl-rmcat-coupled-cc-05

   o  Moved scheduling section to the appendix.

C.2.2.  Changes from -00 to -01

   o  Included how to apply the algorithm to GCC.

   o  Updated variable names of NADA to be in line with the latest
      version.

   o  Added a reference to [I-D.ietf-rtcweb-transports] to make a
      connection to the prioritization text there.

C.2.3.  Changes from -01 to -02

   o  Minor changes.

   o  Moved references of NADA and GCC from informative to normative.

   o  Added a reference for the passive variant of the algorithm.

C.2.4.  Changes from -02 to -03

   o  Minor changes.

   o  Added a section about expected feedback from experiments.

Authors' Addresses








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   Safiqul Islam
   University of Oslo
   PO Box 1080 Blindern
   Oslo  N-0316
   Norway

   Phone: +47 22 84 08 37
   Email: safiquli@ifi.uio.no


   Michael Welzl
   University of Oslo
   PO Box 1080 Blindern
   Oslo  N-0316
   Norway

   Phone: +47 22 85 24 20
   Email: michawe@ifi.uio.no


   Stein Gjessing
   University of Oslo
   PO Box 1080 Blindern
   Oslo  N-0316
   Norway

   Phone: +47 22 85 24 44
   Email: steing@ifi.uio.no























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