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Impact of Hot-Potato Routing Changes in IP Networks

Authors Renata Teixeira , Aman Shaikh and Jennifer Rexford(AT&T), Tim Griffin(Intel) http ://www-cse.ucsd.edu/~ teixeira Presenter : Farrukh Shahzad. Impact of Hot-Potato Routing Changes in IP Networks. UCSD. AT&T . AOL. Verio. Sprint. interdomain routing (BGP).

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Impact of Hot-Potato Routing Changes in IP Networks

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  1. Authors RenataTeixeira , AmanShaikh and Jennifer Rexford(AT&T), Tim Griffin(Intel) http://www-cse.ucsd.edu/~teixeira Presenter : FarrukhShahzad Impact of Hot-Potato Routing Changes in IP Networks

  2. UCSD AT&T AOL Verio Sprint interdomain routing (BGP) intradomain routing (OSPF,IS-IS) End-to-end performance depends on all ASes along the path Internet Routing Architecture Web Server User Changes in one AS may impact traffic and routing in other ASes

  3. Distance-vector &Link state routing • Distance vectorrouting • It involves two factors: the distanceor metric, of a destination, and the vector, or direction to take to get there. • Routing information is only exchanged between directly connected neighbors. • A router knows from which neighbor a route was learned, but it does not know where that neighbor learned the route • A router can't see beyond its own neighbors. This aspect of distance vector routing is sometimes referred to as "routing by rumor." • Measures like split horizon and poison reverse are employed to avoid routing loops.

  4. Distance-vector &Link state routing • Link-staterouting • In contrast, requires that all routers know about the paths reachable by all other routers in the network. • Link-state information is flooded throughout the link-state domain (an area in OSPF ) to ensure all routers possess a synchronized copy of the area's link-state database. • From this common database, each router constructs its own relative shortest-path tree, with itself as the root, for all known routes. • BGP is DV routing protocol • OSPF is LS Routing Protocol

  5. Interaction between IGP and BGP • A router combines the BGP and IGP information to construct a forwarding table. • BGP exchanges route advertisements with neighboring domains, and propagate reachability information within AS. • IGP protocol, such as OSPF, computes shortest paths based on configurable link weights. • The interaction between IGP and BGP • Hot potato routing.

  6. dst multiple connections to the same peer 10 9 Hot-potato routing = route to closest egress point when there is more than one route to destination Hot-Potato Routing New York San Francisco ISP network Dallas

  7. dst - failure - planned maintenance - traffic engineering 10 9 11 11 Hot-Potato Routing Change New York San Francisco ISP network Routes to thousands of destinations switch exit point!!! Dallas • Consequences: • Transient forwarding instability • Traffic shift • Inter-domain routing changes

  8. Approach In Paper • Understanding impact in real networks • How often hot-potato changes happen? • How many destinations do they affect? • What are the convergence delays? • Main contributions • Methodology for measuring hot-potato changes • Characterization on AT&T’s IP backbone

  9. Challenges for Identifying Hot-Potato Changes • Cannot collect data from all routers • OSPF: flooding gives complete view of topology • BGP: multi-hop sessions to several vantage points • A single event may cause multiple messages • Group related routing messages in time • Router implementation affects message timing(PDelay) • Real Time & Controlled experiments of router in the lab • Many BGP updates caused by external events • Classify BGP routing changes by possible causes

  10. BGP updates BGP monitor A B OSPF Monitor OSPF messages Replay routing decisions from vantage point A and B to identify hot-potato changes Measurement Methodology AT&T backbone

  11. Measurement Methodology

  12. Sections Details • Section III- A: presents the measurement infrastructure used to collect BGP updates and OSPF LSAs. • Section III-B : describe how to compute the distance vector from the OSPF LSAs in. • Section III-C : explains the classification of BGP routing changes in terms of the possible causes. This sets the stage for the discussion in next section. • Section III-D : How to associate BGP routing changes with related distance changes that occur close in time

  13. Measurement Infrastructure • iBGP session allows the monitor to see changes in the “egress point” of BGP routes. • The BGP monitor also dumps a snapshot of its routes four times a day to provide an initial view of the best route for each prefix for each vantage point, For later classify the type of BGP change as discussed in Section III-C

  14. Classifying BGP Rt Changes

  15. The large volume of BGP updates shows exploration of multiple alternate routes when a router switches from one best path to another . • IGP distance changes cause a router inside the AS to switch from one stable route to another with a different egress point.

  16. BGP Reaction Time to Distance Changes Rerun the IGP shortest-path computation Apply the BGP decision process to select the best route (iii) Send update messages to BGP neighbors for the routes that have changed.

  17. TransferDelay for Multiple Prefixes 1-The LSA is flooded throughout the network and each router computes new distances. For example, A and B compute new distances of 21 and 11, respectively. 2- After their scan timers elapse, and rerun the BGP decision process. If runs first, selects the egress point with a distance of 20, since this is smaller than 21. Sometime afterwards, A selects egress point C. 3- B sends the new route (with egress point E ) to A, A and selects egress point with a distance of 19.

  18. Temporal and Spatial Variability(distance changes effect)

  19. Hot-Potato Variation Across Prefixes

  20. IMPLICATIONS OF HOT POTATOES • Performance Degradation • Routing and Traffic Shifts • Slow Forwarding-Plane Convergence • Measurement Inaccuracies • Active Probes of the Forwarding Plane • External Analysis of BGP Updates

  21. Conclusion • Hot-potato routing plays an important role in • BGP routing changes, and that BGP updates can lag 60 seconds (or more!) behind the related IGP events • The frequency and impact of hot-potato routing depends on the topology and configuration of the network under study • routing is usually a major contributor to large traffic variations. In particular, hot-potato routing changes are responsible for the largest shifts in the traffic matrix

  22. …conclusion • Setting IGP link weights without accounting for possible changes in the egress points can lead to routing configurations that cause unnecessary congestion.

  23. Thanks

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