Internet Engineering Task Force C. Raiciu
Internet-Draft M. Handley
Intended status: Experimental D. Wischik
Expires: April 22, 2010 University College London
October 19, 2009
Coupled Multipath-Aware Congestion Control
draft-raiciu-mptcp-congestion-00
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Abstract
Often endpoints are connected by multiple paths, but communications
are usually restricted to a single path per socket. Resource usage
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within the network would be more efficient were it possible for these
multiple paths to be used concurrently.
The use of multiple paths simultaneously, specifically within a
Multipath TCP protocol, necessitates the development of new
congestion control algorithms. If existing algorithms such as TCP
New Reno were run independently on each path, the multipath flow
would take more than its fair share if there was a common bottleneck.
Further, it is desirable that a source with multiple paths available
will transfer more traffic using the least congested of the paths,
hence achieving resource pooling. This would increase the overall
utilization of the network and also its robustness to failure.
This document presents a congestion control algorithm which couples
the congestion control algorithms running on different subflows by
linking their increase functions, and dynamically controls the
overall aggresiveness of the multipath flow. The result is a
practical algorithm that is fair to TCP at bottlenecks while moving
traffic away from congested links.
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Table of Contents
1. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Coupled Congestion Control Algorithm . . . . . . . . . . . . . 5
4. Implementation Optimizations . . . . . . . . . . . . . . . . . 7
4.1. Implementation Considerations when CWND is Expressed
in Packets . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Requirements Language
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].
2. Introduction
Multipath TCP (MPTCP, [I-D.ford-mptcp-multiaddressed]) is a set of
extensions to regular TCP [RFC0793] that allow one TCP connection to
be spread across multiple paths. MPTCP distributes load through the
creation of separate "subflows" across potentially disjoint paths.
How should congestion control be performed for multipath TCP? First,
each subflow must have its own congestion control state (i.e. cwnd)
so that capacity on that path is matched by offered load. The
simplest way to achieve this goal is to simply run TCP New Reno
congestion control [RFC5681] on each subflow. However this solution
is unsatisfactory as it gives the multipath flow an unfair share when
the paths taken by its different subflows share a common bottleneck.
Bottleneck fairness is just one requirement multipath congestion
control should meet. The following three goals capture the desirable
properties of a practical multipath congestion control algorithm:
o Goal 1 (Improve Throughput) A multipath flow should perform at
least as well as a single path flow would on the best of the paths
available to it.
o Goal 2 (Do no harm) A multipath flow should not take up more
capacity on any one of its paths than if it was a single path flow
using only that route. This guarantees it will not unduly harm
other flows.
o Goal 3 (Balance congestion) A multipath flow should move as much
traffic as possible off its most congested paths, subject to
meeting the first two goals.
Goals 1 and 2 together ensure fairness at the bottleneck. Goal 3
captures the concept of resource pooling [WISCHIK]: if each multipath
flow sends more data through its least congested path, the traffic in
the network will move away from congested areas. This improves
robustness and overall throughput, among other things. The way to
achieve resource pooling is to effectively "couple" the congestion
control loops for the different subflows.
We propose an algorithm that couples only the additive increase
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function of the subflows, and uses unmodified TCP New Reno behavior
in case of a drop. The algorithm relies on the traditional TCP
mechanisms to detect drops, to retransmit data, etc.
Detecting shared bottlenecks reliably is quite difficult, so our
proposal always assumes there is a shared bottleneck and throttles
the aggressiveness of the multipath flow such that its total
throughput is no more than that of a regular TCP running on the best
path available.
It is intended that the algorithm presented here can be applied to
other multipath transport protocols, such as alternative multipath
extensions to TCP, or indeed any other congestion-aware transport
protocols. However, for the purposes of example this document will,
where appropriate, refer to the MPTCP protocol.
It is foreseeable that different congestion controllers will be
implemented for Multipath transport, each aiming to achieve different
properties in the resource pooling/fairness/stability design space.
In particular, solutions that give better resource pooling may be
proposed. This algorithm is conservative from this point of view,
sacrificing resource pooling for stability.
3. Coupled Congestion Control Algorithm
The algorithm we present only applies to the congestion avoidance
state. The slow start, fast retransmit, and fast recovery algorithms
are the same as [RFC5681].
Let cwnd_i be the congestion window on the subflow i, and assume
there is always data to send. Let tot_cwnd be the sum of the
congestion windows of all subflows in the connection. Let p_i, rtt_i
and mss_i be the drop probability, round trip time and maximum
segment size on subflow i.
We assume throughout this document that the congestion window is
maintained in bytes, unless otherwise specified. We briefly describe
the algorithm for packet-based implementations of cwnd in section
Section 4.1.
Our proposed "Linked Increases" algorithm is:
o For each ack received on subflow i, increase cwnd_i by min (
ceil(alfa*bytes_acked*mss_i/tot_cwnd) , bytes_acked*mss_i/cwnd_i )
o For each drop event on subflow i, decrease set cwnd_i to
max(cwnd_i/2,2). A drop event is one or more packet drops
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experienced by a subflows in the same round trip time.
The decrease function is the same as in TCP New Reno, so we will not
discuss it further in the remainder of this document.
The increase formula takes the minimum between the computed increase
for the multipath subflow (first argument to min), and the increase
TCP would get in the same scenario (the second argument). In this
way, we ensure that a multipath subflow is NEVER more aggressive than
a TCP flow, hence achieving goal 2 (do no harm)
ceil returns the smallest integer greater than or equal to its real-
valued input. As long as bytes_acked is non-zero, the increase is
non-zero. The increase rounds up the computed value; and it ensures
that multipath does not suffer more from rounding errors than TCP
(which it might, as tot_cwnd is always greater than individual
cwnds). We discuss how to implement this formula in practice in the
next section.
We assume appropriate byte counting (ABC, [RFC3465]) is used, hence
the bytes_acked variable records the number of bytes newly
acknowledged. If ABC is not used, bytes_acked SHOULD be set to
mss_i.
To compute tot_cwnd, it is an easy mistake to sum up cwnd_i across
all subflows: when a flow is in fast retransmit, its cwnd is
typically inflated and no longer represents the real congestion
window. The correct behavior is to use the ssthresh value for flows
in fast retransmit whe computing tot_cwnd.
"alfa" is a parameter of the algorithm that describes the
aggresiveness of the multipath flow. To meet Goal 1 (improve
throughput), the value of alfa is chosen such that the aggregate
throughput of the multipath flow is equal to the rate a TCP flow
would get if it ran on the best path.
The total throughput of a multipath flow depends on the value of alfa
and the drop probabilities, maximum segment sizes and round trip
times of its paths. Since we require that the total throughput is no
worse than the throughput a single TCP would get on the fastest path,
it is impossible to choose a-priori a single value of alfa that
achieves the desired throughput in every ocasion. Hence, alfa must
be computed for each multipath flow, based on the observed properties
of the paths.
The formula to compute alfa is:
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2
cwnd_i * mss_i
max ---------------
i 2
rtt_i
alfa = tot_cwnd * -------------------------
/ cwnd_i * mss_i \ 2
| sum ----------------|
\ i rtt_i /
The formula is derived by equalizing the rate of the multipath flow
with the rate of a TCP running on the fastest path, and solving for
alfa.
4. Implementation Optimizations
It is possible to implement our algorithm by calculating tot_cwnd on
each ack, however this would be costly especially when the number of
subflows is large. To avoid this overhead the implementation SHOULD
maintain tot_cwnd per connection, and MUST update its value when the
individual subflows' windows are updated. Updating only requires one
more addition or subtraction operation compared to the regular, per
subflow congestion control code, so its performance impact should be
minimal.
Computing alfa per ack is also costly. Simply maintaining alfa per
connection does not help, as its value would still need to be updated
per ack. If we assume RTTs are constant, it is sufficient to compute
a once per drop, as a does not change between drops (the insight here
is that cwnd_i/cwnd_j = constant as long as both windows increase).
Experimental results show that even if round trip times are not
constant, using average round trip time instead of instantaneous
round trip time gives good precision for computing alfa. Hence, we
recommend that alfa SHOULD be computed once per drop according to the
formula above, by replacing rtt_i with rtt_avg_i.
rtt_avg_i is computed by sampling the srtt_i whenever the window can
accomodate one more packet, i.e. when cwnd / mss < (cwnd+increase)/
mss. The samples are averaged once per sawtooth into rtt_avg_i.
This sampling ensures that there is no sampling bias for larger
windows
Given tot_cwnd and alfa, the congestion control algorithm is run for
each subflow independently. The window increase per ack is alfa *
mss * bytes_acked / tot_cwnd. Compared to traditional increase code,
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this would require floating point operations to be performed on each
ack.
To avoid such costly operations, implementors SHOULD add a state
variable to each subflow incr_i = mss_i * mss_i * alfa / tot_cwnd.
When alfa or tot_cwnd changes, incr_i MUST be updated. Since the
change is rare (once per sawtooth) the performance impact should be
minimal. With incr_i properly set, the increase per ack becomes
bytes_acked * incr_i / mss_i. As this requires only integer
multiplication, the overhead is comparable to existing
implementations of TCP.
4.1. Implementation Considerations when CWND is Expressed in Packets
When the congestion control algorithm maintains cwnd in packets rates
than bytes, the code to compute tot_cwnd remains unchanged.
To compute the rtt_avg_i, srtt will be sampled when cwnd_i is
incremented.
To compute the increase when an ack is received, the implementation
for multipath congestion control is a simple extension of the TCP New
Reno code. In TCP New Reno cwnd_cnt is an additional state variable
records the number of bytes acked since the last cwnd increment. cwnd
is incremented again only when cwnd_cnt > cwnd.
In the multipath case, cwnd_cnt_i is maintained for each subflow as
above, and cwnd_i is increased by 1 when cwnd_cnt_i > tot_cwnd / alfa
. To avoid costly floating point operations, the right hand side of
the inequality can be stored as a per connection state variable that
is updated only when tot_cwnd or alfa change.
5. Discussion
To achieve perfect resource pooling, one must couple both increase
and decrease of congestion windows across subflows, as in [KELLY].
Yet this tends to exhibit "flappiness": when the paths have similar
levels of congestion, the congestion controller will tend to allocate
all the window to one random subflow, and allocate zero window to the
other subflows. The controller will perform random flips between
these stable points. points. This seems not desirable in general,
and is particularly bad when the achieved rates depend on the RTT (as
in the current Internet): in such a case, the resulting rate with
fluctuate unpredictably depending on which state the controller is
in, hence violating Goal 1.
By only coupling increases our proposal removes flappiness but also
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reduces the extent of resource pooling the protocol achieves. The
algorithm will allocate window to the subflows such that p_i * cwnd_i
= constant, for all i. Thus, when the drop probabilities of the
subflows are equals, each subflow will get an equal window, removing
flappiness. when they are different, progressively more window will
be allocated to the flow with the lower drop probability. In
contrast, perfect resource pooling requires that all the window
should be allocated on the path with the lowest drop probability.
6. Security Considerations
None.
Detailed security analysis for the Multipath TCP protocol itself is
included in [I-D.ford-mptcp-multiaddressed] and [REF]
7. Acknowledgements
The authors are supported by Trilogy
(http://www.trilogy-project.org), a research project (ICT-216372)
partially funded by the European Community under its Seventh
Framework Program. The views expressed here are those of the
author(s) only. The European Commission is not liable for any use
that may be made of the information in this document.
8. IANA Considerations
None.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[I-D.ford-mptcp-multiaddressed]
Ford, A., Raiciu, C., Handley, M., and S. Barre, "TCP
Extensions for Multipath Operation with Multiple
Addresses", draft-ford-mptcp-multiaddressed-01 (work in
progress), July 2009.
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[KELLY] Kelly, F. and T. Voice, "Stability of end-to-end
algorithms for joint routing and rate control", ACM
SIGCOMM CCR vol. 35 num. 2, pp. 5-12, 2005,
<http://portal.acm.org/citation.cfm?id=1064415>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC3465] Allman, M., "TCP Congestion Control with Appropriate Byte
Counting (ABC)", RFC 3465, February 2003.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
[WISCHIK] Wischik, D., Handley, M., and M. Bagnulo Braun, "The
Resource Pooling Principle", ACM SIGCOMM CCR vol. 38 num.
5, pp. 47-52, October 2008,
<http://ccr.sigcomm.org/online/files/p47-handleyA4.pdf>.
Authors' Addresses
Costin Raiciu
University College London
Gower Street
London WC1E 6BT
UK
Email: c.raiciu@cs.ucl.ac.uk
Mark Handley
University College London
Gower Street
London WC1E 6BT
UK
Email: m.handley@cs.ucl.ac.uk
Damon Wischik
University College London
Gower Street
London WC1E 6BT
UK
Email: d.wischik@cs.ucl.ac.uk
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