multipath: a new control
architecture for the internet
By Damon Wischik
mULtIPath tRaNSmISSIoN foR the Internet—that is, allowing users to send
some of their packets along one path
and others along different paths—is
an elegant solution still looking for the
The most obvious benefit of multipath transmission is greater reliability. For example, I’d like my phone to
use WiFi when it can, but seamlessly
switch to cellular when needed, without disrupting my flow of data. In general, the only way to create a reliable
network out of unreliable components
is through redundancy, and multipath
transmission is an obvious solution.
The second benefit of multipath
transmission is that it gives an extra degree of flexibility in sharing the
network. Just as packet switching removed the artificial constraints imposed by splitting links into circuits,
so too multipath removes the artificial
constraints imposed by ‘splitting’ the
network’s total capacity into separate
links (see the accompanying figure).
Flexibility comes with dangers.
By building the Internet with packet
switching, we no longer had the control over congestion that circuit switching provides (crude though it may be),
and this led in 1988 to Internet congestion collapse. Van Jacobson3
realized there needed to be a new system
for controlling congestion, and he had
the remarkable insight that it could be
achieved by end systems on their own.
The Internet has been using his transmission control protocol (TCP) largely
unchanged until recently.
The flexibility offered by multipath
transport also brings dangers. The
claim of the following paper is that,
once we do away with the crude control
of “each flow may use only one path,”
there should be some new control put in
place—and, in fact, the proper control
can be achieved by end systems on their
own. That is to say, if multipath is packet
switching 2.0, then it needs TCP 2.0.
Internet congestion collapse in
if flows have access to multiple paths, then
spikes in traffic on one link can make use of
spare capacity on other links.
1988 was a powerful motivator for the
quick deployment of Jacobson’s TCP.
There is not yet a killer problem for
which multipath congestion control is
the only good solution. Perhaps we will
be unlucky enough to find one. (It has
been shown1 that simple greedy route
choice by end users, combined with intelligent routing by network operators,
can in theory lead to arbitrarily inefficient outcomes, but this has not been
seen in practice.)
Lacking a killer problem, the authors present four vignettes that illustrate the inefficiency and unfairness
of a naïve approach to multipath, and
that showcase the benefit of clever
multipath congestion control.
The niggling problems of naïve approaches to multipath could probably
all be mitigated by special-case fixes
such as “only use paths whose round
trip times are within a factor of two of
each other” or “no flow may use more
than four paths at a time,” perhaps enforced by deep packet inspection. So,
in effect, the authors present a choice
between a single clean control architecture for multipath transmission,
and a series of special-case fixes.
The naïve approach to multipath,
as studied in this paper, is to simply
run separate TCP congestion control
on each path. The clever alternative is
to couple the congestion control on
different paths, with the overall effect
of shifting traffic away from more-
congested paths onto less-congested
paths; two research groups2, 4 have in-
dependently devised an appropriate
form of coupling. This is the approach
under exploration in the mptcp work-
ing group at the IETF, although with
some concessions to graceful coexis-
tence with existing TCP.
1. Acemoglu, D., Johari, R. and Ozdaglar, A.E. Partially
optimal routing. IEEE Journal of Selected Areas in
Communication 25 (2007), 1148–1160.
2. Han, H., Shakkottai, S., Hollot, C.V., Srikant, R. and
Towsley, D. F. Multi-path TCP: A joint congestion
control and routing scheme to exploit path diversity in
the Internet. IEEE/ACM Transactions on Networking
16 (2006), 1260–1271.
3. Jacobson, V. Congestion avoidance and control. In
Proceedings of SIGCOMM 1988 Conference.
4. Kelly, F.P. and Voice, T. Stability of end-to-end algorithms
for joint routing and rate control. ACM/SIGCOMM
Computer Communication Review 35 (2005), 5–12.
Damon Wischik ( firstname.lastname@example.org) is a Royal
Society university research fellow in the Networks
Research Group in the Department of Computer Science
at University College London.