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i-4 routing scalability Taekyoung Kwon Some slides are from Geoff Huston, Michalis Faloutsos, Paul Barford, Jim Kurose, Paul Francis, and Jennifer Rexford outline • What is routing? • Current Internet routing – Focus on BGP • Routing scalability • A case study in IP routing: ViAggre • What is the design space? What is routing routing • How do packets get from A to B in the Internet? Internet A B One example • Connectionless forwarding – Each router (switch) makes a LOCAL decision to forward the packet towards B R1 R4 R7 R6 A R2 B R8 R3 R5 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 Routing taxonomy • Distributed* vs. centralized • Static vs. dynamic* – # of hops vs. traffic load • Intra-domain vs. inter-domain Current Internet routing Goals of internet routing • • • • • Inter-connection Fault-tolerant Scalability performance …. Internet routing: two levels • Autonomous system (AS) level – Inter-domain – BGP • Router level – Intra-domain – RIP, OSPF,… Internet structure Original idea Backbone service provider “ Consumer” ISP Small corporation Large corporation “Consumer”ISP “Consumer ” ISP Small corporation Small corporation “Consumer ” ISP Small corporation Internet structure • The reality is… * Why peering? Source: Arbor Networks Internet structure • And many tiers local ISP Tier 3 ISP Tier-2 ISP local ISP local ISP local ISP Tier-2 ISP Tier 1 ISP Tier 1 ISP Tier-2 ISP local local ISP ISP Tier 1 ISP Tier-2 ISP local ISP Tier-2 ISP local ISP Internet routing • Prefix is advertised across ASs Path: 6, 5, 4, 3, 2, 1 4 3 5 2 1 Client 7 6 SNU 147.46.0.0/16 Inter-AS routing: BGP • BGP (Border Gateway Protocol): the de facto standard • BGP provides each AS a means to: 1. Obtain subnet reachability information from neighboring ASs. 2. Propagate reachability information to all ASinternal routers. 3. Determine “good” routes to subnets based on reachability information and policy. • allows subnet to advertise its existence to rest of Internet: “I am here” BGP basics • pairs of routers (BGP peers) exchange routing info over semi-permanent TCP connections: BGP sessions • when AS1 advertises a prefix of AS2 to AS3: – AS1 promises it will forward datagrams towards that prefix. – AS1 can aggregate prefixes in its advertisement eBGP session 3c 3a 3b AS3 2.0.0.0/8 2.3.4.0/24 1a AS1 iBGP session 2a 1c 1d 2c AS2 1b 2.3.0.0/16 2b 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 3c 3a 3b AS3 1a AS1 iBGP session 2a 1c 1d 2c AS2 1b 2b 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. BGP route selection • • router may learn about more than 1 route to some prefix. Router must select a route. elimination rules: 1. 2. 3. 4. local preference value attribute: policy decision shortest AS-PATH closest NEXT-HOP router: hot potato routing additional criteria Network Layer 4-17 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 4-18 BGP routing policy (1/2) legend: B W X A provider network customer network: C Y 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 4-19 BGP routing policy (2/2) legend: B W X A provider network customer network: C Y 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 4-20 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 4-21 Routing scalability Routing table (RT) growth • Multi-homing • Traffic engineering • Non-aggregatable prefix allocation routing message updates • BGP update messages ViAggre Why routing scalability matters? • FIB is expensive Virtual aggregation (ViAggre) ViAggre: Basic Idea ViAggre: Basic Idea ViAggre: Control Plane More practically,… Data plane operations Route stretch Ingress -> aggregation point Aggregation point -> egress (1/3) Aggregation point -> egress (2/3) Aggregation point -> egress (3/3) 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 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 Issue 1: Addressing and routing • Rekhter’s Law: “Addressing can follow topology or topology can follow addressing. Choose one.” 00 10 20 30 01 11 21 31 02 12 13 22 23 32 2 03 6 3 33 10 5 13 8 16 11 15 1 12 4 9 7 14 Name-dependent routing Name-independent routing Issue 2: state vs. stretch We want small state!! We want small stretch!! routing debate • State: the routing table size describing the network topology • Stretch: path length found by the routing algorithm optimal path length ≥1 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