Multipath TCP Ramin Khalili
INTERNET-DRAFT T-Labs/TU-Berlin
Intended Status: Standard Track Nicolas Gast
Expires: April 24, 2014 Miroslav Popovic
Jean-Yves Le Boudec
EPFL-LCA2
October 21, 2013
Opportunistic Linked-Increases Congestion Control Algorithm for MPTCP
draft-khalili-mptcp-congestion-control-02
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
[4], 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, as shown by different studies [5, 6,
7, 8]. OLIA is now part of the UCLouvain's MPTCP implementation [9].
Status of this Memo
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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, paths with maximum windows, and
collected paths . . . . . . . . . . . . . . . . . . . . . . . . 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]. 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. This
leads to important performance issues (refer to [5] and [6]): (i) in
some scenarios upgrading TCP users to MPTCP results in a significant
drop in the aggregate throughput in the network without any benefit
for anybody; and (ii) MPTCP users can be excessively aggressive
toward TCP users.
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 [5].
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 [10]. 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 [10], 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
UCLouvain's MPTCP 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. Recent studies show that MPTCP with
OLIA always outperforms MPTCP with LIA and is very responsive to the
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changes in the environment [7, 8].
The rest of the document provides a description of OLIA. For an
analysis of its performance, we refer to [5, 7, 8].
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].
all_paths: The set of all the paths established by a MPTCP
connection.
best_paths: The set of paths in all_paths that are presumably the
best paths for the MPTCP connection.
max_w_paths: The set of paths in all_paths with largest congestion
windows.
collected_paths: The set of paths in all_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).
w_r: The congestion windows on a path r.
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, paths with maximum windows, and collected paths
A MPTCP connection has access to one or more paths (subflows). Let
all_paths be the set of these paths and r be one of them. We denote
by l_{1r} the number of bytes that were successfully transmitted over
path r between the last two losses seen on r, by l_{2r} the number of
bytes that are successfully transmitted over r after the last loss,
and by l_r=max{l_{1r},l_{2r}} the smoothed estimation of number of
bytes transmitted on 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) and w_r be the
congestion windows on path r. We denote by best_paths the set of
paths r in all_paths that have the maximum value of l_r*l_r/rtt_r, by
max_w_paths the set of paths r in all_paths with largest w_r, and by
collected_paths the set of best paths that do not have maximum window
size, i.e.:
- best_paths = { r | r = arg max_{p in all_paths} (l_p*l_p/rtt_p) }
- max_w_paths = { r | r = arg max_{p in all_paths} (w_p) }
- collected_paths = { r | r in best_paths and not in max_w_paths }.
where arg max is the argument of maximum, the set of points of the
given argument for which the given function is maximum. arg max is
applied over all paths p in all_paths.
best_paths represents the set of paths that are presumably the best
paths (in term of transmission rate) 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. A collected path is a path that is presumably
good but is not fully used. The set collected_paths can be empty.
Note that l_{1r}, l_{2r}, l_r, rtt_r, w_r, best_paths, max_w_paths
and collected_paths are all functions of time.
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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 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. Note that this modification will not
affect the single path TCP.
As defined before, we denote by w_r the congestion windows on the
path r and by MSS_r the maximum segment size on this path. 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_{p in all_paths} (w_p/rtt_p))^2 w_r
multiplied by MSS_r * bytes_acked.
The summation in the denominator is over all paths p in all_paths.
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 [10] 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 [5] and [6]. 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. In [7], Chen et al. study how MPTCP with LIA and OLIA
performs in the wild with a common wireless environment, namely using
both WiFi and Cellular simultaneously. Their results show that MPTCP
with OLIA is very responsive to the changes in the environment and
always outperforms MPTCP with LIA. Furthermore, using Experimental
Design, Paasch et al. [8] show that MPTCP with OLIA satisfy the
design goal of MPTCP in a very wide range of scenarios and always
outperform MPTCP 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_p (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_p (w_p/rtt_p))^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_p (w_p/rtt_p))^2
multiplied by MSS_r * bytes_acked.
The summation in the dominator of equations (2), (3), and (4) is over
the path p in all_paths. 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 [10] and fully satisfies the design goals of
MPTCP described in [4]. Hence, it can provide optimal congestion
balancing and fairness in the network [5]. Moreover, it is as
responsive and non-flappy as LIA and outperforms LIA in realistic
scenarios such as wireless networks (refer to [5, 7, 8]).
We therefore believe that mptcp working group 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.
6.2 Informative References
[4] Raiciu, C., Handley, M., and D. Wischik, "Coupled
Congestion Control for Multipath Transport Protocols",
RFC 6356, October 2011.
[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 (The
extended version of this paper will be appeared at
IEEE/ACM Transaction of Networking).
[6] R. Khalili, N. Gast, M. Popovic, and J.-Y. Le Boudec.
"Performance Issues with MPTCP", draft-khalili-mptcp-
performance-issues-04.
[7] Chen Y.-C, Y.-S. Lim, R. J. Gibbens, E. M. Nahum, R.
Khalili, and D. Towsley, "A Measurement-based Study of
Multipath TCP Performance over Wireless Networks." ACM IMC
2013.
[8] C. Paasch, R. Khalili, and O. Bonaventure, "On the
Benefits of Applying Experimental Design to Improve
Multipath TCP." ACM CoNEXT 2013.
[9] UCL, Louvain-la-Neuve, Belgium, "MultiPath TCP-Linux
kernel implementation," 2013 [Online]. Available:
http://mptcp.info.ucl.ac.be/.
[10] 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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