Control of multipath TCP and optimization of multipath routing in the Internet
Abstract.
There are moves in the Internet architecture community to
add multipath capabilities to TCP, so that end-systems will be able to
shift their traffic away from congested parts of the network. We study
two problems relating to the design of multipath TCP. (i) We investigate
stochastic packet-level behaviour of some proposed multipath congestion
control algorithms, and find that they do not behave how we might expect
from fluid modeling: they tend to flap randomly between their available
paths. We explain why, and propose a congestion control algorithm that
does not flap. (ii) We consider how the path choice offered by the network
affects the ability of end-systems to shift their traffic between a pool of
resources. We define a resource poolability index, which measures for
each resource how easy it is for traffic to be shifted away from that
resource e.g. in the event of a traffic surge or link failure.
Key words.
multipath TCP, congestion control, resource pooling, fluid
model, load balancing
A network with four shared resources and four traffic sources, three of
which have a choice of path. The resources have capacities
C1=1,
C2=2,
C3=3,
C4=5.
Each resource is colour-coded according
to its
resource poolability Ψ:
- If resource 1 experiences a traffic
surge then traffic from sources
s1 & s2 can easily be
rerouted over other paths, which increases load on link 4,
so some traffic from s3 will be rerouted as a knock-on effect.
Traffic from s4 has no other paths available, but
it doesn't suffer as much as it might, because
s1 moves some of its traffic
from resource 1 to resource 3 which has more capacity available.
Mathematically, resource 1 has resource poolability
Ψ1,1=0.91.
-
If resource 4 experiences a traffic surge then
traffic from sources
s1 & s2 will have to use
resource 1 instead. But resource 1 has little capacity (as indicated by its small size), so
congestion will increase significantly. Mathematically, resource 4 has
resource poolability Ψ4,4=0.54.