Network Architecture (R02) IP Multipath – Path Selection&CC

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 • Could be useful load balancing • When Traffic Matrix deviates from expected • How to assign rates to alternate paths • IP or Application Layer • CDN, especially P2P (Torrent or Storm) already effectively multipath at App • Current IP routing mainly only corner cases

Multipath IP Routing • Simplest case is equal-cost multipath • Can be seen as simple “bonding” technique • Combines with multihoming/resilience • For any metric, in an interdomain protocol, can do k-shortest paths • 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 • Sidesteps the question of route computation for now…. • Starts off from the BitTorrent example • Looks at a MPTCP/MPIP model in contrast • 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 • 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 • 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 • 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 • 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 • 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 • Smart phone with wifi & 3G • Data center networks • …

Reference/credit for diagrams • @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/10.1145/1866739.1866762}, doi = {10.1145/1866739.1866762}, acmid = {1866762}, publisher = {ACM}, address = {New York, NY, USA}, }

Network Architecture (R02) IP Multipath – Path Selection&CC