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
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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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