i-4 routing scalability

1. i-4 routing scalability Taekyoung Kwon Some slides are from Geoff Huston, MichalisFaloutsos, Paul Barford, Jim Kurose, Paul Francis, and Jennifer Rexford

2. outline • What is routing? • Current Internet routing • Focus on BGP • Routing scalability • A case study in IP routing: ViAggre • What is the design space?

3. Whatis routing routing • How do packets get from A to B in the Internet? Internet B A

4. One example • Connectionless forwarding • Each router (switch) makes a LOCAL decision to forward the packet towards B R1 R4 R7 R6 R2 B A R8 R3 R5

5. Routing is… • How does each router know the correct local forwarding decision for any possible destination address? • Through info of the network topology • This info is maintained by a routing protocol • Information • Table size * update rate

6. Routing taxonomy • Distributed* vs. centralized • Static vs. dynamic* • # of hops vs. traffic load • Intra-domain vs. inter-domain

7. Current Internet routing Goals of internet routing • Inter-connection • Fault-tolerant • Scalability • performance • ….

8. Internet routing: two levels • Autonomous system (AS) level • Inter-domain • BGP • Router level • Intra-domain • RIP, OSPF,…

9. Internet structure Original idea Backbone service provider Consumer ” ISP “ Consumer ISP ” Large corporation “ Small “ ” Consumer ISP “ ” Consumer ISP corporation Small Small Small corporation corporation corporation

10. Internetstructure • The reality is… Source: Arbor Networks * Why peering?

11. Tier 3 ISP local ISP local ISP local ISP local ISP local ISP local ISP local ISP local ISP Tier-2 ISP Tier-2 ISP Tier-2 ISP Tier-2 ISP Tier-2 ISP Internet structure • And many tiers Tier 1 ISP Tier 1 ISP Tier 1 ISP

12. Internet routing • Prefix is advertised across ASs Path: 6, 5, 4, 3, 2, 1 4 3 5 2 6 7 1 SNU 147.46.0.0/16 Client

13. Inter-AS routing: BGP • BGP (Border Gateway Protocol):the de facto standard • BGP provides each AS a means to: • Obtain subnet reachability information from neighboring ASs. • Propagate reachability information to all AS-internal routers. • Determine “good” routes to subnets based on reachability information and policy. • allows subnet to advertise its existence to rest of Internet: “I am here”

14. 2c 2b 3c 1b 1d 1c BGP basics • pairs of routers (BGP peers) exchange routing info over semi-permanent TCP connections: BGP sessions • when AS1 advertises a prefix ofAS2 to AS3: • AS1 promises it will forward datagrams towards that prefix. • AS1 can aggregate prefixes in its advertisement 2.3.4.0/24 eBGP session iBGP session 3a 3b 2a AS3 AS2 1a 2.0.0.0/8 AS1 2.3.0.0/16

15. 2c 2b 1b 1d 1c 3c Distributing reachability info • using eBGP session between 3a and 1c, AS3 sends prefix reachability info to AS1. • 1c can then use iBGP do distribute new prefix info to all routers in AS1 • 1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session • when router learns of new prefix, it creates entry for prefix in its forwarding table. eBGP session iBGP session 3a 3b 2a AS3 AS2 1a AS1

16. Path attributes & BGP routes • advertised prefix includes BGP attributes. • prefix + attributes = “route” • two important attributes: • AS-PATH: contains ASs through which prefix advertisement has passed: e.g. AS 6431, AS 7018 • NEXT-HOP: indicates specific internal-AS router to next-hop AS. (may be multiple links from current AS to next-hop-AS) • when gateway router receives route advertisement, uses import policy to accept/decline.

17. BGP route selection • router may learn about more than 1 route to some prefix. Router must select a route. • elimination rules: • local preference value attribute: policy decision • shortest AS-PATH • closest NEXT-HOP router: hot potato routing • additional criteria Network Layer

18. BGP messages • BGP messages exchanged using TCP. • BGP messages: • OPEN: opens TCP connection to peer and authenticates sender • UPDATE: advertises new path (or withdraws old) • KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request • NOTIFICATION: reports errors in previous msg; also used to close connection Network Layer

19. legend: provider B network X W A customer network: C Y BGP routing policy (1/2) • A,B,C are provider networks • X,W,Y are customers (of provider networks) • X is dual-homed: attached to two networks • X does not want to route from B via X to C • .. so X will not advertise to B a route to C Network Layer

20. legend: provider B network X W A customer network: C Y BGP routing policy (2/2) • A advertises path AW to B • B advertises path BAW to X • Should B advertise path BAW to C? • No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers • B wants to force C to route to w via A • B wants to route only to/from its customers! Network Layer

21. Why Intra- and Inter-AS routing different? Policy: • Inter-AS: admin wants control over how its traffic routed, who routes through its net. • Intra-AS: single admin, so no policy decisions needed Scale: • hierarchical routing saves table size, reduced update traffic Performance: • Intra-AS: can focus on performance • Inter-AS: policy may dominate over performance Network Layer

22. Routing scalability Routing table (RT) growth • Multi-homing • Traffic engineering • Non-aggregatable prefix allocation

23. routing message updates • BGP update messages

24. ViAggre Why routing scalability matters? • FIB is expensive

25. Virtual aggregation (ViAggre)

26. ViAggre: Basic Idea

27. ViAggre: Basic Idea

28. ViAggre: Control Plane

29. More practically,…

30. Data plane operations

31. Route stretch

32. Ingress -> aggregation point

33. Aggregation point -> egress (1/3)

34. Aggregation point -> egress (2/3)

35. Aggregation point -> egress (3/3)

36. Design Space now • We will consider general routing design space • IP is just one of the possibilities • But IP networking environments had better be considered as much as possible

37. Design goal of routing 1. Scalability (memory): e.g. sublinear RT size scaling 2. Quality (stretch): the length of a chosen path by a routing scheme compared to shortest path 3. Reliability: fast convergence upon topology changes while minimizing communication costs to maintain coherent non-local knowledge about network topology 4. Name-independent routing: accommodate node addresses/labels assigned independently of the topology (otherwise need to split locator and ID parts in addressing architecture) 5. Message overhead

38. Issue 1: Addressing and routing • Rekhter’s Law: “Addressing can follow topology or topology can follow addressing. Choose one.” 15 10 00 01 02 2 5 03 13 8 1 11 10 6 12 13 22 21 16 12 23 9 20 3 31 33 11 4 7 32 30 14 Name-dependent routing Name-independent routing

39. We want small state!! We want small stretch!! routing debate path length found by the routing algorithm optimal path length Issue 2: state vs. stretch • State: the routing table size describing the network topology • Stretch: ≥1

40. More general trade-off Triangle of trade-offs: • Adaptation costs = convergence measures (e.g. number of messages per topology change) • Memory space = routing table size • Stretch = path length inflation