Network Architecture (R02)
IP Multipath – Path Selection&CC
Jon Crowcroft,
http://www.cl.cam.ac.uk/~jac22
http://www.cl.cam.ac.uk/teaching/1213/R02/
Multipath
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Could be useful load balancing
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When Traffic Matrix deviates from
expected
How to assign rates to alternate paths
IP or Application Layer
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CDN, especially P2P (Torrent or Storm)
already effectively multipath at App
Current IP routing mainly only corner cases
Multipath IP Routing
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Simplest case is equal-cost multipath
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Can be seen as simple “bonding” technique
Combines with multihoming/resilience
For any metric, in an interdomain
protocol, can do k-shortest paths
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Problem #1 is path metric – bottleneck link
capacity and round trip time are both
important
Problem #2 is BGP
This paper concentrates on rate/path
problem
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Sidesteps the question of route computation
for now….
Starts off from the BitTorrent example
Looks at a MPTCP/MPIP model in contrast
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Builds an convex optimisation style framework (as
per previous Frank Kelly et al) – F. P. Kelly and T.
Voice. Stability of end-to-end algorithms for joint
routing and rate control. ACM SIGCOMM
Computer Communication Review, 35(2):5{12,
2005.see
So max utility subject to path constraints
BitTorrent behaviour
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Currently, Swarms choose a number of
neighbours to fetch blocks of a file from,
monitor the TCP rate achieved, drop the
slowest and pick a new neighbour at “random”
SeeM. Mitzenmacher, A. Richa, and R.
Sitaraman.The power of two random choices:
A survey of the techniques and results. In P.
Pardalos, S. Rajasekaran, , and J. Rolim,
editors, Handbook of Randomized Computing,
pages 255{312. 2001
IP versus Torrent models
Load balancing…c.f. Valiant/data
centers
Two different rate assignments
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TCP is well known to have a 1/RTT dependence
in the long term throughput of a given
(unipath) flow.
So do they allow for this or not in the
multipath framework? Choice
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Coordinate rates, don’t factor in rtt
Uncoordinated rates, factor in rtt
See also TCP Friendly rate controlled
transport protocol work by Handley et al
Capacity regions
Note on this version of paper
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This is the shorter, CACM version – there’s a MSR
tech report and an Infocom version.
In Cisco manuals, you can do Multipath BGP, but be
aware this is mainly just for multihomeing an ISP on
another (same motive as OSPF-ECM).
The general problem is very hard, see
Loop-freeness in multipath BGP through
propagating the longest path, Van Beijnum,
Iljitsch (2008) Loop-freeness in multipath BGP
through propagating the longest path. Masters thesis,
University Carlos III of Madrid, Madrid, Spain
Other missing architectural pieces
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How to indicate at a sender a packet
from a coordinated flow belongs on a
particular sub-path, in general (if the
end system isn’t multihomed)?
How to tell at a receiver which subpath
a packet arrived over?
What about short lived flows?
Obvious deployment scenarios
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Smart phone with wifi & 3G
Data center networks
…
Reference/credit for diagrams
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@article{Key:2011:PSM:1866739.1866762,
author = {Key, Peter and Massouli{\'e},
Laurent and Towsley, Don}, title = {Path
selection and multipath congestion control},
journal = {Commun. ACM}, issue_date =
{January 2011}, volume = {54}, number = {1},
month = jan, year = {2011}, issn = {0001-0782},
pages = {109--116}, numpages = {8}, url =
{http://doi.acm.org/10.1145/1866739.186676
2}, doi = {10.1145/1866739.1866762}, acmid =
{1866762}, publisher = {ACM}, address = {New
York, NY, USA}, }