Multipath TCP Ramin Khalili
INTERNET-DRAFT T-Labs/TU-Berlin
Intended Status: Standard Track Nicolas Gast
Expires: August 21, 2013 Miroslav Popovic
Jean-Yves Le Boudec
EPFL-LCA2
February 17, 2013
draft-khalili-mptcp-congestion-control-00
Abstract
This document describes the mechanism of OLIA, the "opportunistic
linked increases algorithm". OLIA is a congestion control algorithm
for MPTCP. The current congestion control algorithm of MPTCP, LIA,
forces a tradeoff between optimal congestion balancing and
responsiveness. OLIA's design departs from this tradeoff and provide
these properties simultaneously. Hence, it solves the identified
performance problems with LIA while retaining non-flappiness and
responsiveness behavior of LIA [5].
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Copyright and License Notice
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Requirements Language . . . . . . . . . . . . . . . . . . . 4
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
2 The set of best paths and paths with maximum windows . . . . . . 5
3 Opportunistic Linked-Increases Algorithm . . . . . . . . . . . . 6
4 Practical considerations . . . . . . . . . . . . . . . . . . . . 8
5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1 Normative References . . . . . . . . . . . . . . . . . . . . 10
6.2 Informative References . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1 Introduction
The current MPTCP implementation uses a congestion control algorithm
called LIA, the "Linked-Increases" algorithm [4]. However, MPTCP with
LIA suffers from important performance issues (see [5] and [6]): in
some scenarios upgrading TCP users to MPTCP can result in a
significant drop in the aggregate throughput in the network without
any benefit for anybody. We also show in [5] that MPTCP users could
be excessively aggressive toward TCP users.
The design of LIA forces a tradeoff between optimal congestion
balancing and responsiveness. Hence, to provide good responsiveness,
LIA's current implementation must depart from optimal congestion
balancing that leads to the identified problems. In this draft, we
introduce OLIA, the "opportunistic linked increases algorithm", as an
alternative to LIA. Contrary to LIA, OLIA's design is not based on a
trade-off between responsiveness and optimal congestion balancing; it
can provide both simultaneously.
Similarly to LIA, OLIA couples the additive increases and uses
unmodified TCP behavior in the case of a loss. The difference between
LIA and OLIA is in the increase part. OLIA's increase part, Equation
(1), has two terms:
- The first term is an adaptation of the increase term of Kelly and
Voice's algorithm [8]. This term is essential to provide optimal
resource pooling.
- The second term guarantees responsiveness and non-flappiness of
OLIA. By measuring the number of transmitted bytes since the last
loss, it reacts to events within the current window and adapts to
changes faster than the first term.
By adapting the window increases as a function of RTTs, OLIA also
compensates for different RTTs. As OLIA is rooted on the optimal
algorithm of [8], it provides fairness and optimal congestion
balancing. Because of the second term, it is responsive and non-
flappy.
OLIA is implemented in the Linux kernel and is now a part of MPTCP's
implementation. In [5], we study the performance of MPTCP with OLIA
over a testbed, by simulations and by theoretical analysis. We prove
theoretically that OLIA is Pareto-optimal and that it satisfies the
design goals of MPTCP described in [4]. Hence, it can provide optimal
congestion balancing and fairness in the network. Our measurements
and simulations indicate that MPTCP with OLIA is as responsive and
non-flappy as MPTCP with LIA and that it solves the identified
problems with LIA.
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The rest of the document provides a description of OLIA. For an
analysis of its performance, we refer to [5].
1.1 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 [1].
1.2 Terminology
Regular TCP: The standard version of TCP that operates between a
single pair of IP addresses and ports [2].
Multipath TCP: A modified version of the regular TCP that allows a
user to spread its traffic across multiple paths.
MPTCP: The proposal for multipath TCP specified in [3].
LIA: The Linked-Increases Algorithm of MPTCP (the congestion control
of MPTCP) [4].
OLIA: The Opportunistic Linked-Increases Algorithm for MPTCP proposed
in [5].
best_paths: The set of paths that are presumably the best paths for
the MPTCP connection.
max_w_paths: The set of paths with largest congestion windows.
collected_paths: The set of paths that are presumably the best paths
but do not have largest congestion window (i.e. the paths of
best_paths that are not in max_w_paths).
rtt_r: The Round-Trip Time on a path r.
MSS_r: The Maximum Segment Size that specifies the largest amount of
data can be transmitted by a TCP packet on the path r.
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2 The set of best paths and paths with maximum windows
A MPTCP connection has access to one or more paths. Let r be one of
these paths. We denote by l_{1r} the number of bytes that were
successfully transmitted over path r between the last two losses seen
on r, and by l_{2r} the number of bytes that are successfully
transmitted over r after the last loss. We denote by
l_r=max{l_{1r},l_{2r}} the smoothed estimation of number of bytes
transmitted on path r between last two losses.
l_{1r} and l_{2r} can be measured by using information that is
already available to a regular TCP user:
- For each ACK on r: l_{2r} <- l_{2r} + (number of bytes that are
acknowledged by ACK),
- For each loss on r: l_{1r} <- l_{2r} and l_{2r} <- 0.
l_{1r} and l_{2r} are initially set to zero when the connection is
established. If no losses have been observed on r until now, then
l_{1r}=0 and l_{2r} is the total number of bytes transmitted on r.
Let rtt_r be the round-trip time observed on path r (e.g. the
smoothed round-trip time used by regular TCP). We denote by
best_paths the set of paths r that have the maximum value of
l_r*l_r/rtt_r and by max_w_paths the set of paths with largest
congestion window. Also, let collected_paths be the set of paths that
are in best_paths but not in max_w_paths. Note that l_{1r}, l_{2r},
l_r, best_paths, max_w_paths and collected_paths are all functions of
time.
best_paths represents the set of paths that are presumably the best
paths for the user: 1/l_r can be considered as an estimate of byte
loss probability on path r, and hence the rate that path r can
provide to a TCP user can be estimated by (2*l_r)^{1/2}/rtt_r.
collected_paths is the set of "collected" paths: the paths that are
presumably the best paths for the user but that are not yet fully
used.
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3 Opportunistic Linked-Increases Algorithm
In this section, we introduce OLIA. OLIA is a window-based
congestion-control algorithm. It couples the increase of congestion
windows and uses unmodified TCP behavior in the case of a loss. OLIA
is an alternative for LIA, the current congestion control algorithm
of MPTCP.
The algorithm only applies to the increase part of the congestion
avoidance phase. The fast retransmit and fast recovery algorithms, as
well as the multiplicative decrease of the congestion avoidance
phase, are the same as in TCP [2]. We also use a similar slow start
algorithm as in TCP, with the modification that we set the ssthresh
(slow start threshold) to be 1 MSS if multiple paths are established.
In the case of a single path flow, we use the same minimum ssthresh
as in TCP (i.e. 2 MSS). The purpose of this modification is to avoid
transmitting unnecessary traffic over congested paths when multiple
paths are available to a user.
For a path r, we denote by w_r the congestion windows on this path
(also called subflow). We denote by MSS_r be the maximum segment size
on the path r. We assume that w_r is maintained in bytes.
Our proposed "Opportunistic Linked-Increases Algorithm" (OLIA) must:
- For each ACK on path r, increase w_r by
w_r/rtt_r^2 alpha_r
( --------------------- + --------- ) (1)
(SUM (w_p/rtt_p))^2 w_r
multiplied by MSS_r * bytes_acked.
The summation in the denominator of the first term is over all the
paths available to the user.
alpha_r is calculated as follows:
- If r is in collected_paths, then
1/number_of_paths
alpha_r = --------------------
|collected_paths|
- If r is in max_w_paths and if collected_paths is not empty, then
1/number_of_paths
alpha_r = - -----------------
|max_w_paths|
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- Otherwise, alpha_r=0.
|collected_paths| and |max_w_paths| are the number of paths in
collected_paths and in max_w_paths. Note that the sum of all alpha_r
is equal to 0.
The first term in (1) is an adaptation of Kelly and Voice's increase
term [8] and provides the optimal resource pooling (Kelly and
Voice's algorithm is based on scalable TCP; the first term in (1) is
a TCP compatible version of their algorithm that compensates also for
different RTTs). The second term, with alpha_r, guarantees
responsiveness and non-flappiness of our algorithm.
By definition of alpha_r, if all the best paths have the largest
window size, then alpha_r=0 for any r. This is because we already use
the capacity available to the user by using all the best path.
If there is any best path with a small window size, i.e. if
collected_paths is not empty, then alpha_r is positive for all r in
collected_paths and negative for all r in max_w_paths. Hence, our
algorithm increases windows faster on the paths that are presumably
best but that have small windows. The increase will be slower on the
paths with maximum windows. In this case, OLIA re-forwards traffic
from fully used paths (i.e. paths in max_w_paths) to paths that have
free capacity available to the users (i.e. paths in collected_paths).
In [4], three goals have been proposed for the design of a practical
multipath congestion control algorithm : (1) Improve throughput: a
multipath TCP user should perform at least as well as a TCP user that
uses the best path available to it. (2) Do no harm: a multipath TCP
user should never take up more capacity from any of its paths than a
TCP user. And (3) balance congestion: a multipath TCP algorithm
should balance congestion in the network, subject to meeting the
first two goals.
Our theoretical results in [5] show that OLIA fully satisfies these
three goals. LIA, however, fails to fully satisfy the goal (3) as
discussed in [4] and [5]. Moreover, in [5], we show through
measurements and by simulation that our algorithm is as responsive
and non-flappy as LIA and that it can solve the identified problems
with LIA.
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4 Practical considerations
Calculation of alpha requires performing costly floating point
operation whenever an ACK received over path r. In practice, however,
we can integrate calculation of alpha and Equation (1) together. Our
algorithm can be therefore simplified as the following.
For each ACK on the path r:
- If r is in collected_paths, increase w_r by
w_r/rtt_r^2 1
------------------- + ----------------------- (2)
(SUM (w_p/rtt_p))^2 w_r * number_of_paths * |collected_paths|
multiplied by MSS_r * bytes_acked.
- If r is in max_w_paths and if collected_paths is not empty,
increase w_r by
w_r/rtt_r^2 1
-------------------- - ------------------------ (3)
(SUM (w_r/rtt_r))^2 w_r * number_of_paths * |max_w_paths|
multiplied by MSS_r * bytes_acked.
- Otherwise, increase w_r by
(w_r/rtt_r^2)
---------------------------------- (4)
(SUM (w_r/rtt_r))^2
multiplied by MSS_r * bytes_acked.
Therefore, to compute the increase, we only need to determine the
sets collected_paths and max_w_paths when an ACK is received on the
path r. We can further simplify the algorithm by updating the sets
collected_paths and max_w_paths only once per round-trip time or
whenever there is a drop on the path.
We can see from above that in some cases (i.e. when r is max_w_paths
and collected_paths is not empty) the increase could be negative.
This is a property of our algorithm as in this case OLIA re-forwards
traffic from paths in max_w_paths to paths in collected_paths. It is
easy to show that using our algorithm, w_r >= 1 for any path r.
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5 Discussion
Our results in [5] show that the identified problems with current
MPTCP implementation are not due to the nature of a window-based
multipath protocol, but rather to the design of LIA. OLIA shows that
it is possible to build an alternative to LIA that mitigates these
problems and that is as responsive and non-flappy as LIA.
Our proposed algorithm can provide similar resource pooling as Kelly
and Voice's algorithm [8] and fully satisfies the design goals of
MPTCP described in [4]. Hence, it can provide optimal congestion
balancing and fairness in the network. Moreover, it is as responsive
and non-flappy as LIA [5].
We therefore believe that IETF should revisit the congestion control
part of MPTCP and that an alternative algorithm, such as OLIA, should
be considered.
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6 References
6.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009.
[3] Ford, A., Raiciu, C., Greenhalgh, A., and M. Handley,
"Architectural Guidelines for Multipath TCP Development",
RFC 6182, March 2011.
[4] Raiciu, C., Handley, M., and D. Wischik, "Coupled
Congestion Control for Multipath Transport Protocols",
RFC 6356, October 2011.
6.2 Informative References
[5] R. Khalili, N. Gast, M. Popovic, U. Upadhyay, and J.-Y. Le
Boudec. "MPTCP is not Pareto-optimality: Performance
issues and a possible solution", ACM CoNext 2012.
[6] R. Khalili, N. Gast, M. Popovic, and J.-Y. Le Boudec.
"Performance Issues with MPTCP", draft-khalili-mptcp-
performance-issues-02.
[7] M. Handly, D. Wischik, C. Raiciu, "Coupled Congestion
Control for MPTCP", presented at 77th IETF meeting,
Anaheim, California.
.
[8] 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.
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Authors' Addresses
Ramin Khalili
T-Labs/TU-Berlin
TEL 3, Ernst-Reuter-Platz 7
10587 Berlin
Germany
Phone: +49 30 8353 58276
EMail: ramin@net.t-labs.tu-berlin.de
Nicolas Gast
EPFL IC ISC LCA2
Station 14
CH-1015 Lausanne
Switzerland
Phone: +41 21 693 1254
EMail: nicolas.gast@epfl.ch
Miroslav Popovic
EPFL IC ISC LCA2
Station 14
CH-1015 Lausanne
Switzerland
Phone: +41 21 693 6466
EMail: miroslav.popovic@epfl.ch
Jean-Yves Le Boudec
EPFL IC ISC LCA2
Station 14
CH-1015 Lausanne
Switzerland
Phone: +41 21 693 6631
EMail: jean-yves.leboudec@epfl.ch
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