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Advanced Networks 1. Delayed Internet Routing Convergence 2. The Impact of Internet Policy and Topology on Delayed Routing Convergence

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The Problem How to Recover from Failure Quickly? Phone systems recover, failover, in milliseconds Internet takes an order of minutes Loss of Connectivity Packet Loss Latency

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The Problem (cont) Failure over on the internet not very good Sluggish Backup systems Internet has to adjust to the failure Path must be restored to back up

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The Questions Why does convergence take so long? What is the upper bound for convergence? What causes this delayed convergence? What can we do about it?

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Theory Unexpected Interaction of: Protocol timers Router Implementation Policies (Safe/Unsafe)

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Theory (cont) Distance vector algorithm has issues Lack of sufficient info to determine if next hop choice will cause loops

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Convergence Accelerators Use of Path Vector Split Horizon Triggered updates Diffusion Timers

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Policies Admins can implement unsafe policies Policies can cause route oscillations Routers default to Shortest Path Even if constrained upper-bound might be as high factorial

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Point of Paper Measure the convergence behavior of BGP 4 Done for Bellman-Ford O(n 3 ) Convergence in BGP is NOT much better than RIP Give an upper and lower bounds to convergence

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The Work Done 2 year study 250,000 routing fault injections 25 Internet providers End to End performance measurements

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Terminology Tup: (New) Route Announcement Tdown: Route Withdrawal Tshort: Shorter Route Replaces Current Current Route is Withdrawn Implicitly Tlong: Shorter Route Replaced with longer one Represents a failure and failover Current Route is Withdrawn Implicitly

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Latency

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Latency (cont) Oscillation greater than 3 minutes 20% of Tlong 40% of Tdown Equivalence Latency Classes Tlong,Tdown Tshort,Tup

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Latency per ISP

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BGP Update Volume Average Message Per Event Type Tup: Route Announcement Tdown: Route Withdrawal Tshort: Shorter Route Replacement Tlong: Longer Route Replacement

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Questions Why do Tlong and Tdown cause 2 times the amout of updates? Why do certain ISP produce more updates per event? Relationship between number of updates and convergence latency?

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Questions (cont) What makes an ISP have a higher latency? Interesting Points ISP3: Japan’s National Backbone ISP5 Canadian ISP Latency NOT Dependant Geographic Distance or Network Distance (aka hop count)

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Graph Analysis No relationship between day of the week and Latency! Independent of Network load and congestion

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End to End Measurements Route Oscillation effects performance Drop Packets, Buffering of Packets Out of order delivery

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Failover from end to end view Time after ICMP echo arrived after Tup Simulates a failover 80% of test sites began returning after 30 seconds 100% after one minute

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BGP Convergence Model IBGP ignored Full Mesh Ignore ingress and egress filters Exclude MinRouteAdver Updates messages follow FIFO ordering

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BGP Convergence Example Start: 0(*R, 1R, 2R) 1(0R, *R, 2R) 2(0R, 1R, *R) R Withdraws routes R -> 0 W R -> 1 W R -> 2 W

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BGP Convergence Example 0(-, -, *2R) 1(-, -, *2R) 2(*01R, 10R, -) 0(-, *1R, 2R) 1(*0R, -, 2R) 2(*0R, 1R, -) 1 and 2 receive new announcement from 0 0 -> 1 01R (loop) 0 -> 2 01R 0(-, *1R, 2R) 1(-, -, *2R) 2(01R, *1R, -) 0 and 2 receive new announcement from 1 1 -> 0 10R (loop) 1 -> 2 10R

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BGP Convergence Example 0 and 1 receive new announcement from 2 2 -> 0 20R 2 -> 1 20R 0(-, -, -) 1(-, -, *20R) 2(*01R, 10R, -) 0 and 2 receive new announcement from 1 1 -> 0 12R 1 -> 2 12R 0(-, *12R, -) 1(-, -, *20R) 2(*01R, -, -) … 48 steps later 0(-, -, -) 1(-, -, -) 2(-, -, -)

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Upper Bound For n nodes there exist 0((n-1)!) distinct paths When a route is withdrawn, a new route is found of equal or increasing length Message count could be a bad as (n-1)O((n-1)!) until convergence Not really possible on the internet

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Lower Bound Made possible by MinRouteAdver timers (n-1) Rounds to convergence

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MinRouteAdver Minimum time between route advertisements Gives a AS time to pick a good route before announcing it In standard BGP, timer only applied to announcements Does Not apply to explicit withdrawls

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Example Reloaded Instead of 48 rounds only took 13 rounds

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Example Reloaded

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Question Reloaded Why do Tup/Tshort converge quicker than Tdown/Tlong? Answer: Tup/Tshort are decreasing while Tdown/Tlong are increasing One a path is selected a longer one will not be picked While on Tdown/Tlong you pick the next best one until you are out of choices O(1) for Tup while O(n) for Tdown

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Question Reloaded Why is there different latencies between the five ISPs? Answer: The topological factors, length and number of possible paths (peering relationships, policies and agreements) are the answer. Longer routes announced, longer latencies Longer routes the more MinRouteAdver rounds

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Loop Detection Loop Detection done at receiver side If done, at sender you can get more out of MinRouteAdver round MinRouteAdver is good but causes a 30 second delay in end to end communication at best

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Convergence Delay Due to Policies and Topology 2 nd study of convergence 20 unique advertisement between 200 pairs of ISPs, 6 months Measure the impact of Policies Measure the impact of Topology Analysis

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Multi-home Networks One network, two ISPs Better connectivity + backup Failover = New route convergence Work done in this Paper Convergence Analysis of Tdown event

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Work Done Fault injection announcements Logged table snapshot to disk Survey of backbone providers Routing and peering policies Used data to discuss impact on convergence

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Policy How policy impacts number and length of ASPaths with a given route Limited inbound acceptance by all ISP

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Inbound Filtering Example ISP D filters peering session with ISPG D only accept G’s backbone and customers routes ISP A filters peering session with D A only accept D’s backbone and customers routes ISP A will accepts G’s routes by chaining

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Outbound Filters A will advertise routes with paths “D G” and “D” but not “C D G” Done by 13% of ISPs Combinations of ASPath and prefix filters create unintentional back-up transit paths

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Topological Effect Interaction of MinRouteAdver timers MinRouteAdver is per peer not prefix MinRouteAdver interference delays convergence

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Backup Path Selection

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Convergence Latency

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Convergence Latency (cont) ISP1 explored one backup path of length 2 ISP2 explored backup paths of length 2 and 3 ISP 3 explored backup paths of length 5

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Convergence Latency (cont)

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