Network Protocols: Design and Analysis

1. Network Protocols: Design and Analysis Polly Huang EE NTU http://cc.ee.ntu.edu.tw/~phuang phuang@cc.ee.ntu.edu.tw

2. Internet Routing III [Tsuchiya88a] [Labovitz00a]

3. Landmark Routing [Tsuchiya88a]

4. Context • fairly early in the Internet life • before BGP-3 • before CIDR • example of SIGCOMM “wild idea” paper

5. Key Idea • Self-configuring hierarchy for routing with many routers

6. Why Landmark Routing? • area routing requires knowledge of topology, maybe doesn’t get best aggregration possible • LM knows about internal structure of nearby nodes, even if in different AS • dynamic address assignment—easier to manage • reduce size of routing table… because address are automatic, and reassigned on-demand, can get better aggregation than area hierarchy • could be more reliable if congestion because supports multiple (?) • different approach than area routing

7. Landmark Routing Disadvantages • don’t always get shortest path [but true about all routing protocols that have aggregation/policy] • admin control? (paper hints at approaches, but not fully explored) • performance not fully explored? • less info further away from destination, therefore more likely to get poor quality routes to it [but no different from area routing] • performance of LM placement/config algorithms? • combines routing and address (but so does area routing) • addressing • address may not be stable • LM uses variable length address

8. Landmark hierarchy • Details about things nearby and less information about things far away • Not defined by arbitrary boundaries • thus, not well suited to the real world that does have administrative boundaries • (although he says something about adding admin boundaries)

9. A Landmark 9 8 6 7 5 11 3 10 4 Router 1 is a landmark of radius 2 1 2

10. Landmark Overview • Landmark routers have “height” which determines how far away they can be seen (visibility) • Routers within Radius n can see a landmark router LM(n) • See means that those routers have LM(n)’s address and know next hop to reach it. • Router x as an entry for router y if x is within radius of y • Distance vector style routing with simple metric • Routing table: Landmark (LM2(d)), Level(2), Next hop

11. LM Hierarchy Definition • Each LM (Li) associated with level (i) and radius (ri) • Every node is an L0 landmark • Recursion: some Li are also Li+1 • Every Li is seen by at least one Li+1 • Terminating state when all level j LMs see entire network

12. y a X b LM addresses • LM(2).LM(1).LM(0) (x.a.b and y.a.b) • LM level maps to radius (part of configuration), e.g.: • LM level 0: radius 2 • LM level 1: radius 4 • LM level 2: radius 8 • If destination is more than two hops away, will not have complete routing information, refer to LM(1) portion of address, if not then refer to LM(2)..(c would forward based on y in y.a.b) c

13. LM Routing • LM does not imply hierarchical forwarding • It is not a source route • En route to LM(1) may encounter router that is within LM(0) radius of destination address (like longest match) • Paths may be asymmetric

14. LM self-configuration • Bottom-up hierarchy construction algorithm • goal to bound number of children • Every router is L0 landmark • All routers advertise themselves over a distance • All Li landmarks run election to self-promote one or more Li+1 landmarks • Dynamic algorithm to adapt to topology changes--Efficient hierarchy

15. Landmark Routing: Basic Idea - Not shortest path - Packet does not necessarily follow specified landmarks • Source wants to reach LM0[a], whose address is c.b.a: • Source can see LM2[c], so sends packet towards c • Entering LM1[b] area, first router diverts packet to b • Entering LM0[a] area, packet delivered to a

16. Landmark Routing: Example

17. Landmark Level Next hop 2 LM2[d] f LM1[i] 1 k LM0[e] 0 f LM0[k] 0 k LM0[f] 0 f Routing table for Router g Router g r0 = 2, r1 = 4, r2 = 8 hops Router t How to go from d.i.g to d.n.t? How does path length compare to shortest path?

18. Evaluation • analytic results • but bounds not very helpful • simulation • routing table size (R) • mean path length • distance to nearby landmark • (seems weak) rtg table size r/d = radius/distance mean path len [Figure 6 from Tsuchiya88a]

19. Questions?

20. BGP Routing Convergence Times [Labovitz00a]

21. Context • BGP widely deployed in the Internet • but poorly understood

22. Key Idea • convergence time takes longer we expected • observes 2-3 minute convergence times (6x longer than expected!) • bounds on BGP convergence: O(n!) worst case, O((n-3)*30s) [n is number of ASes]

23. Why is Convergence Important? • robustness • PSTN (telephone) failover times are in milliseconds • Internet failover times are in 10s of seconds • open research question: how can Internet routing do much better?

24. Methodology • experiments over Internet: manually injected faults propagate across net • simulation to study worst case behavior • theoretical analysis—helps understand worst case bounds • traces of 2 years of convergence times

25. Methodology Picture ([Labovitz00a] Figure 1) Internet-scale experimentation. What kinds of complexities arise? Have to be careful with real routes;

26. Observed Convergence Latency New Route, Long->Short Fail-over (Tup and Tshort) Short->Long Fail-Over (Tlong) Failure (Tdown) Labovitz00a Figure 2a Long tailed distribution (up to 15 minutes); more msgs in longer waits; long absolute times

27. Other Observations • No correlation between network distance (latency, router, or AS hops) and convergence times • Why is long convergence bad?…

28. Affects on Traffic ([Labovitz00a] figure 4a) Why does loss go up? There’s always a direct path? some people use old paths, routing loops

29. How To Tell What’s Going On? • Simulate BGP • model one router per AS • assume full routing mesh • ignore latency • synchronous processing via global queue • simple model that captures key details

30. What’s going on? • there are many possible routes (indirect through other ASes) and it takes a long time w/BGP to figure out that none work • BGP can try all paths of length 2, then 3, then 4 => O(n!) steps • even with min-route-adver it still can take O(n) steps

31. R AS2 AS3 AS0 AS1 *B R via 3 B R via 13 B R via 23 *B R via 3 B R via 03 B R via 23 *B R via 3 B R via 03 B R via 13 * * * *B R via 013 B R via 103 *B R via 203 AS0 AS1 AS2 BGP Convergence Example

32. What about MinRouteAdver? • BGP hasa minimum advertisement interval timer • designed to limit updates • and to encourage aggregation • How does it affect convergence? • by delaying announcements, routers figure out the pain sooner • see section 5.2 • result: n-3 rounds rather than n!

33. Does this explain measurements? • Tup/Tshort converge quickly because they shorten path length and therefore are quickly accepted • Tdown/Tlong converge slowly because BGP tries hard to find all alternatives • Tlong actually sometimes goes quicker if it’s “not long enough” and can preempt some of the thrashing

34. Other Observations • Could do loop detection at sender side and not just receiver side • xxx

35. Questions?