1. Routing Jinyang Li

2. Administravia Hand in PS1 to Hui Zhang before you leave! Email me project teamlist by Oct-1

3. Routing basics Model the network as a graph Goal: Find a (best) path from A to B You favorite path finding algorithm? –Breadth first search –Bellman-Ford –Dijkstra –Floyd-Warshall Routing protocols must be decentralized

4. Challenges Network topology is dynamic –Links go up and down –Nodes go up and down –Link costs (metrics) change Nodes might have stale information Nodes might have different information

5. Basic decentralized routing algorithms Distance-vector (DV) Link state (LS)

6. Distance vector routing Based on Bellman-Ford Nodes only keep path metrics to all destinations Neighboring nodes exchange path metrics

7. Distance vector routing a b c d 1 1 10 1 a: a, 0 b: b, 1 c: c, 10 a: a, 1 b: b, 0 c: c, 1 a: a, 10 b: b, 1 c: c, 0 d: d, 1 c: c, 1 d: d, 0

8. DV: routing table update a a: a, 0 b: b, 1 c: c, 10 b c d a: a, 1 b: b, 0 c: c, 1 1 1 10 1 a: a, 10 b: b, 1 c: c, 0 d: d, 1 c: c, 1 d: d, 0 a: 1 b: 0 c: 1 += a: a, 0 b: b, 1 c: b, 2 a: 10 b: 1 c: 0 d: 1 += a: a, 1 b: b, 0 c: c, 1 d: c, 2

9. DV: routing table update When does DV find best paths? Static topology, synchronous exchange –A node learns the best path ≤ x hops after x rounds of exchange –DV converges after n rounds if longest short path is n hops

10. DV under dynamics DV update rule –reduce path metric if get a better one from nbr Always correct w/ static topology Might be wrong when topology changes –My path metric is based on new topology –Neighbor’s path metric could be for old topology

11. DV: count-to-infinity a a: a, 0 b: b, 1 c: b, 2 d: b, 3 b c d a: a, 1 b: b, 0 c: c, 1 d: c, 2 1 1 10 1 a: b, 2 b: b, 1 c: c, 0 d: d, 1 a: c, 1 b: c, 2 c: c, 3 d: d, 0 a: a, 1 b: b, 0 c: inf d: inf a: 10 b: inf c: c, 0 d: d, 1 a: 0 b: 1 c: 2 d: 3 + a: a, 1 b: b, 0 c: a, 3 d: a, 4 = Incorrect update of path metric based on old topology a: 1 b: 0 c: 3 d: 4 + = a: a, 0 b: b, 1 c: b, 4 d: b, 5

12. (Partial) solutions to count-to-infinity Make infinity a finite number (e.g. 64) Split-horizon –Do not advertise routes you learnt from neighbor N back to N Split-horizon with poison reverse –Advertise route you learnt from neighbor N with an infinity metric

13. Path vector: no more count-to-infinity a a: a, 0 b: b, 1 c: b,c, 2 d: b,c,d, 3 b c d a: a, 1 b: b, 0 c: c, 1 d: c,d, 2 1 1 10 1 a: b,a, 2 b: b, 1 c: c, 0 d: d, 1 a: c, 1 b: c,b, 2 c: c,b,a, 3 d: d, 0 a: a, 1 b: b, 0 c: inf d: inf a: a, 10 b: inf c: c, 0 d: d, 1 a: a, 0 b: b,1 c: b,c 2 d: b,c,d 3 + a: a, 1 b: b, 0 c: inf d: inf = Discard old info based on path vector

14. DV Summary Periodic exchange among neighbors Each update has O(N) size, N is the # of nodes (routable prefixes) Convergence delays –Explicit path vector info speeds up convergence

15. Link State Routing In DV, topology is implicit in the routing tables –Convergence is delayed when using old topology for updates LSR: make topology explicit!

16. Link State Routing Each nodes keeps link state information (complete topology) Each node computes paths based on topology using a centralized algorithm a b c d 1 1 10 1 a: a, 0 b: b, 1 c: b, 2 d: b, 3 Dijkstra

17. Link state updates Both ends of a link floods link state to the entire network –Immediately upon change –Periodically with a long period LS seq # distinguishes old LS from new ones Old LS times out eventually

18. Link State vs. DV Routing state –LS: O(E) to keep complete topology –DV: O(N) to keep path metrics to all nodes Routing message overhead –LS: O(E*E) floods each LS to entire network –DV: O(E*N) to exchange routing tables on all links LS converges faster than DV Does LS guarantee loop-free forwarding?

19. Common link metrics What’s the “cost” of different links? –1 –Latency –Bandwidth –Queue length –…

20. Other routing algorithms? LS/DV find optimal paths Both incur substantial message overhead Trade off path optimality for lower overhead? –Compact routing: O( ) state, 3 times longer paths in the worst case –Geographic routing: constant state

21. Routing on the Internet -- from algorithms to protocols

22. Intra-domain and Inter-domain routing

23. Intra-domain routing Goal: –Find best paths between all intra-AS networks –Traffic engineering to load balance different paths Popular IGP (interior gateway) protocols: –OSPF (LS) –IS-IS (LS) –RIP (DV)

24. Inter-domain routing Goal: –Provide reachability for different ASes –Comply to polices of different ASes BGP: path vector based on ASes

25. BGP Routing policies Protocol operations Disseminating BGP routes within an AS BGP challenges –Policy interactions –Multihoming –security

26. Inter-AS topology is not simply a graph AT&T Another ISP Small ISP pays $$ to AT&T NYU pays $ to small ISP Free for traffic between customers of two peering ISPs only Small ISP NYUCustomer

27. BGP export policy: what to reveal to neighbors? If you tell N about A --> you agree to forward traffic from N to A –If you do not want to forward traffic to A, don’t tell others about it Always advertise customer routes –Carrying traffic for customer brings $$$ Advertise non-customer routes to customers only –If you advertise non-customer routes to another provider/peer, you are carrying traffic for nothing!

28. BGP import policies: which route to use? Not simply shortest path! Different preferences for routes from different ASes Customer > peer > provider Customers pay for their traffic Avoid paying providers by using peer routes

29. Example BGP routes >show ip bgp 216.165.108.8 BGP routing table entry for 216.165.0.0/17, version 221058 Paths: (41 available, best #39, table Default-IP-Routing-Table) Not advertised to any peer 4513 701 7018 12 12, (aggregated by 12 192.76.177.66) 209.10.12.125 from 209.10.12.125 (209.10.12.125) Origin IGP, metric 4103, localpref 100, valid, external, atomic- aggregate …. AS Path Longest matching prefix Next hop High values are better

30. Route selection based on attributes Local Pref Used to prefer customer > peer > provider high values are better ASPATH Prefer paths with lowest # of ASes MED Tell others to choose one exit point over another low values are better IGP path cost Lower values are better leads to “hot potato” routing Router ID

31. Hot potato routing All ASes want to get rid of external traffic asap Hot potato routing causes asymmetric traffic MED=100 MED=500 Blue AS’ preferred route

32. BGP operations A router establishes a BGP session with its neighbors over TCP Neighbors might be many hops away Two neighbors exchange –UPDATE (announcements, withdrawal) –KEEPALIVE

33. Disseminating routes within an AS Routers establish eBGP sessions between different ASes Routers inside an AS establish iBGP session to learn external routes

34. Challenges of route dissemination Loop free –Routers should not disagree on how to route Complete –Each router chooses route as if it knows all external routes from all eBGP sessions Scalable

35. A strawman that works: full mesh dissemination Each router establishes an iBGP session with all eBGP speaking routers. Complete All routers know all routes. Loop free All routers know the same set of routes.  Not scalable Requires e(e-1)/2 + ei iBGP sessions among e eBGP routers and i non-eBGP routers

36. A simple route reflector setup Requires e+i BGP sessions Clients and the reflector exchange less traffic  All loads are on one router  Not all clients get best routes if there are multiple egress routers Route Reflector client RR tells clients best route for each prefix over iBGP RR learns routes from eBGP sessions

37. A problematic RR topology setup RR1 Reflector client RR2 tells clients its best route to D, next hop RR2 RR2 Reflector client RR2 learns equally good route to prefix D from eBGP RR1 learns best route to prefix D from eBGP 3 1 3 2 1 RR1 tells clients its best route to D, next hop RR1

38. BGP Routing policies Protocol operations Disseminating BGP routes within an AS BGP challenges –Policy interactions –Multi-homing –Security

39. When policy goes against shortest path… Each AS prefers two-hop route via its clock-wise neighbor AS1 AS3 AS2 AS0

40. Shortest path routing always converges Why? Shortest paths form a DAG (directed acyclic graph) from all nodes to a destination. When polices override shortest path, there’s danger…

41. Ensuring convergence Global policy check –Each AS submits its policy & neighbors to a global registry –Centrally check for bad policy interactions 8Checking is NP-complete 8Topology might change Gao/Rexford (today’s paper) –AS graphs are hierarchical –Restrict the set of allowed policies

42. Gao’s observation AS graphs are not just any graph Provider-subscriber relationships form a DAG Publisher- subscriber link Peering link

43. Gao’s rule for convergence Do not go against DAG edges –Customer route > provider  peer routes If peering links do not cause cycles… –Customer  peer routes > provider routes A peering link that will not cause cycle A peering link that might cause cycle

44. Gao’s rule for convergence Gao’s rule matches ISPs’ incentives –ISP Incentives: customer > peer > provider –Gao’s: customer > peer  provider

45. BGP Routing policies Protocol operations Disseminating BGP routes within an AS BGP challenges –Policy interactions –Multi-homing –Security

46. BGP and multi-homing “stub” AS uses  2 links to the same ISP “stub” AS uses  2 links to different ISPs Transit AS uses  2 providers & peers

47. Stub AS with a single ISP Resilient to a single link failure –announce d/19 on both links Balance load between two links –split prefix, announce sub-prefix on different links No need for a public AS number for stub Stub AS, d/19 Announce one route to d/19 Announce d1/20Announce d2/20

48. AS 5 AS 6 Stub AS with multiple ISPs Resilient to one ISP failure –announce prefix over both links in primary/backup setup Balance load between two ISPs –split prefix and announce sub-prefix on each link Need a public AS number Stub AS 12, d/19 Announce d/19 with ASPATH 12 Announce d/19 with ASPATH 12 12 12 Announce d/19 with 5 12 Announce d/19 with 6 12 12 12

49. Service providers multi-home Load balance transit traffic on many prefixes (inter-domain traffic engineering) –Control both outbound and inbound traffic Redundancy –Primary/backup etc. Challenge: scalability and predictability