Designing a Predictable Backbone Network Rui Zhang-Shen [email protected] http://www.stanford.edu/~rzhang Clean Slate Seminar Feb 13, 2006 US Backbone Networks: Observations
A few tens of core nodes, Each aggregating traffic for a region, Interconnected by an increasingly rich mesh of high-capacity long-haul optical trunks Robustness Load-balancing Low utilizationlinks over-provisioned for Uncertainty in traffic matrix the network is designed for Headroom for future growth Granularity of link capacity Prepare to take over when links or routers fail Minimize congestion and delay variation Efficiency sacrificed for robustness and flexibility How flexible are networks today? What fraction of allowable traffic matrices can they support?
Abilene 25% Over Prov.: 0.025% 50% Over Prov.: 0.66% AT&T 25% Over Prov.: 0.0006% 50% Over Prov.: 0.15% Verio 25% Over Prov.: 0.0004% 50% Over Prov.: 1.15% Sprint 25% Over Prov.: 0.0003% 50% Over Prov.: 0.06%
Verio, AT&T and Sprint topologies courtesy of RocketFuel Our Approach Assume we know or estimate the traffic entering and leaving each Regional Network Requires only local knowledge of users and market estimates Connect regional nodes by a logical full-mesh Use Valiant Load Balancing (VLB) over whole network
Valiant Load-Balancing r 1 42 2r/N 2 3 N r
4 r r VLB: Flow View 2 1 3 4 N
Characteristics of VLB Flexible Simple Each packet needs look up only once in the backbone
Each flow is evenly split over N paths Routing decisions are local Robust Can support all traffic matrices Can recover quickly from failures Efficient Proof: Requires minimum total capacity for supporting all
traffic matrices To tolerate k failures, each link needs 2r /(N-k) Implications of VLB Network Max. utilization under normal condition = Max. utilization under up to k failures = Output: capacity required on each link Routers
design made simpler can be simpler One IP lookup per packet in network No dynamic rerouting requirement What About Delay? Routing can be adaptive Only load-balance when needed There
are express paths Delay is bounded Delay variations are reduced Delay is less important than delay variations Questions & Comments? VLB: A Network Design Framework No need to split traffic evenly Associate pi with node i, where pi 0, i pi=1 Load
balance pi of each flow to node i Previous example: pi =1/N, 8 i Can formulate optimization problems to determine pi Can be applied to heterogeneous networks Can introduce constraints (capacity, etc.) Heterogeneous Network Minimize
Answer: maxi j; j i cij /ri Future Directions Interconnection of multiple VLB networks Relationship of physical topology and logical topology
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