Authors Renata Teixeira, Aman Shaikh and Jennifer Rexford(AT&T), Tim Griffin(Intel) Presenter : Farrukh Shahzad

SIGMETRICS’04 Internet Routing Architecture 2 UCSD Sprint AT&T Verio AOL interdomain routing (BGP) intradomain routing (OSPF,IS-IS) Changes in one AS may impact traffic and routing in other ASes User Web Server End-to-end performance depends on all ASes along the path

SIGMETRICS’04 Distance-vector &Link state routing  Distance vector routing  It involves two factors: the distance or 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. 3

SIGMETRICS’04  Link-state routing  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 4 Distance-vector &Link state routing

SIGMETRICS’04 5  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. Interaction between IGP and BGP

SIGMETRICS’04 Hot-Potato Routing 6 San Francisco Dallas New York Hot-potato routing = route to closest egress point when there is more than one route to destination ISP network 9 10 dst multiple connections to the same peer

SIGMETRICS’04 Hot-Potato Routing Change 7 San Francisco Dallas New York ISP network dst failure - planned maintenance - traffic engineering 11 Routes to thousands of destinations switch exit point!!! Consequences:  Transient forwarding instability  Traffic shift  Inter-domain routing changes 11

SIGMETRICS’04 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 8

SIGMETRICS’04 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 9

SIGMETRICS’04 Measurement Methodology 10 Replay routing decisions from vantage point A and B to identify hot-potato changes AT&T backbone BGP monitor BGP updates OSPF Monitor OSPF messages A B

SIGMETRICS’04 Algorithm for Correlating Routing Changes  Step 1: Process stream of OSPF messages  Group OSPF messages close in time  Transform OSPF messages into vantage point’s routing changes  Step 2: Process stream of BGP updates from vantage point  Group updates close in time  Classify BGP routing changes by possible OSPF cause  Step 3: Match BGP routing changes to OSPF changes in time  Determine causal relationship 11

SIGMETRICS’04 Measurement Methodology 12

SIGMETRICS’04 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

SIGMETRICS’04 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

SIGMETRICS’04 Classifying BGP Rt Changes 15

SIGMETRICS’04  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

SIGMETRICS’04 BGP Reaction Time to Distance Changes 17 (i)Rerun the IGP shortest-path computation (ii)Apply the BGP decision process to select the best route (iii) Send update messages to BGP neighbors for the routes that have changed.

SIGMETRICS’04 Transfer Delay for Multiple Prefixes 18 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.

SIGMETRICS’04 Temporal and Spatial Variability(distance changes effect) 19

SIGMETRICS’04 Hot-Potato Variation Across Prefixes 20

SIGMETRICS’04 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

SIGMETRICS’04 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

SIGMETRICS’04 …conclusion  Setting IGP link weights without accounting for possible changes in the egress points can lead to routing configurations that cause unnecessary congestion. 23

SIGMETRICS’04 Thanks 24