topology formed by pathlets and vnodes Highly flexible supports both local

Topology formed by pathlets and vnodes highly

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topology formed by pathlets and vnodes Highly flexible; supports both “local” policies with small forwarding tables and many paths, and complex BGP policies Challenges for source routing: Incentives to provide multiple paths; selecting paths; security; ... 25 CSE 222A – Lecture 12: WAN Routing Alternatives
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Do IP Networks Manage Themselves? In some sense, yes: u TCP senders send less traffic during congestion u Routing protocols adapt to topology changes But, does the network run efficiently ? u Congested link when idle paths exist? u High-delay path when a low-delay path exists? How should routing adapt to the traffic? u Avoiding congested links in the network u Satisfying application requirements (e.g., delay) … essential questions of traffic engineering 26 CSE 222A – Lecture 12: WAN Routing Alternatives
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ARPAnet Routing (1969) Routing u Shortest-path routing based on link metrics; distance vector Metrics u Instantaneous queue length plus a constant u Each node updates distance computation periodically 3 2 2 1 1 3 1 5 20 congested link 27 CSE 222A – Lecture 12: WAN Routing Alternatives
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Problems With the Algorithm Instantaneous queue length u Poor indicator of expected delay u Fluctuates widely, even at low traffic levels u Leading to routing oscillations Distance-vector routing u Transient loops during (slow) convergence u Triggered by link weight changes, not just failures Protocol overhead u Frequent dissemination of link metric changes u Leading to high overhead in larger topologies 28 CSE 222A – Lecture 12: WAN Routing Alternatives
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New ARPAnet Routing (1979) Averaging of the link metric over time u Old: Instantaneous delay fluctuates a lot u New: Averaging reduces the fluctuations Link-state protocol u Old: Distance-vector path computation leads to loops u New: Link-state protocol where each router computes shortest paths based on the complete topology Reduce frequency of updates u Old: Sending updates on each change is too much u New: Send updates if change passes a threshold 29 CSE 222A – Lecture 12: WAN Routing Alternatives
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Performance of New Algorithm Light load u Delay dominated by the constant part (transmission delay and propagation delay) Medium load u Queuing delay is no longer negligible on all links u Moderate traffic shifts to avoid congestion Heavy load u Very high metrics on congested links u Busy links look bad to all
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  • Fall '08
  • Vahdat,A
  • routing protocol

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