Network Routing algorithms protocols Goal find good path

Network Routing: algorithms & protocols Goal: find “good” path to each destination r Graph abstraction of a network v v 5 Nodes: routers Edges: physical links (with assigned cost) route computation algorithms r link-state (Dijkstra) v v each router knows complete topology & link cost information Run routing algorithm to calculate shortest path to each destination r distance-vector (Bellman-Ford) v Each router knows direct neighbors & link costs to neighbors v Calculate the shortest path to each destination through an iterative process based on the neighbors distances to each destination 5/9/05 1 2 A B 2 1 D 3 C 3 1 5 F 1 E 2 Routing protocols qdefine the format of routing information exchanges q define the computation upon receiving routing updates q network topology changes over time, routing protocol must continuously update the routers with latest changes CS 118/Spring 05

Graph abstraction: costs 5 2 u v 2 1 x • c(x, x’) = cost of link (x, x’) 3 w 3 1 5 z 1 y - e. g. , c(w, z) = 5 • cost could always be 1, or inversely related to bandwidth, or inversely related to congestion 2 Cost of path (x 1, x 2, x 3, …, xp) = c(x 1, x 2) + c(x 2, x 3) + … + c(xp-1, xp) Question: What’s the least-cost path between u and z ? Routing algorithm: algorithm that finds least-cost path 5/9/05 2 CS 118/Spring 05

Dijkstra’s algorithm r Assume net topology, link costs is known r computes least cost paths from one node to all other nodes r Create forwarding table for that node Notation: r c(i, j): link cost from node i to j (∞ if not known) r D(v): current value of cost of path from source to dest. V r p(v): predecessor node along path from source to v, (neighbor of v) r N': set of nodes whose least cost path already known 5/9/05 1 Initialization: 2 N' = {A} 3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(A, v) 6 else D(v) = 7 8 Loop 9 find w not in N' such that D(w) is 10 minimum 10 add w to N' 11 update D(v) for all v adjacent to w and not in N': 12 D(v) = min( D(v), D(w) + c(w, v) ) 13 /* new cost to v is either the old cost, or known shortest path cost to w plus cost from w to v */ 14 until all nodes in N' 3 CS 118/Spring 05

Dijkstra’s algorithm: example Step 0 1 2 3 4 5 D(B), p(B) D(C), p(C) D(D), p(D) D(E), p(E) D(F), p(F) 2, A 1, A 5, A infinity 2, A 4, D 2, D infinity 2, A 3, E 4, E start N' A AD ADEBCF 5 A 2 1 5/9/05 B 2 D 3 C 3 5 F 1 1 E 2 4 CS 118/Spring 05

Dijkstra’s algorithm: example Step 0 1 2 3 4 5 D(B), p(B) D(C), p(C) D(D), p(D) D(E), p(E) D(F), p(F) 2, A 1, A 5, A infinity 2, A 4, D 2, D infinity 3, E 4, E start N A AD ADBEC ADEBCF Resulting forwarding table at A: Resulting shortest-path tree for A: destination link B (A, B) D (A, D) E (A, D) C (A, D) F (A, D) 5 2 A 2 1 5/9/05 B D 3 C 3 1 5 F 1 E 2 5 CS 118/Spring 05

Dijkstra’s algorithm, discussion Algorithm complexity: n nodes r each iteration: need to check all nodes, w, not in N r n(n+1)/2 comparisons: O(n 2) r more efficient implementations possible: O(nlogn) Oscillations possible: r e. g. , link cost = amount of carried traffic D 1 1 0 A 0 0 C e 1+e e initially 5/9/05 B 1 2+e A 0 0 D 1+e 1 B 0 0 C D … recompute routing 1 A 0 0 C 2+e B 1+e … recompute 6 2+e A 0 D 1+e 1 B e 0 C … recompute CS 118/Spring 05

Bellman-Ford Equation Define: Dx(y) : = cost of least-cost path from x to y Then Dx(y) = min {c(x, v) + Dv(y) } v where min is taken over all neighbors v of x 5 u 2 2 1 5/9/05 v x 3 w 3 5 z 1 1 y 2 Du(z) = min {c(u, v) + Dv(z), c(u, x) + Dx(z), c(u, w) + Dw(z) } = min {2 + 5, 1 + 3, 5 + 3} = 4 7 Node leading to shortest path is next hop ➜ forwarding table CS 118/Spring 05

Distance vector protocl (1) Basic idea: r Each node periodically sends its own distance vector estimate to neighbors r When a node x receives new DV estimate from neighbor v, it updates its own DV using B-F equation: Dx(y) ← minv{c(x, v) + Dv(y)} for each node y ∊ N In normal cases, the estimate Dx(y) converge to the actual least cost dx(y) 5/9/05 8 CS 118/Spring 05

Distance Table: example A D() A B D A 1 14 5 2 B 7 8 5 C 6 9 4 D 4 11 2 D destination E cost to destination via 2 8 1 C E E Outgoing D link destination 7 B 1 A A, 1 B D, 5 C D, 4 D D, 2 forwarding table 5/9/05 9 CS 118/Spring 05

Distance Vector Protocol (2) Each node: Iterative, asynchronous: each local iteration caused by: r local link cost change r DV update message from neighbor wait for (change in local link cost of msg from neighbor) recompute estimates Distributed: r each node notifies neighbors if DV to any dest has changed, notify neighbors only when its DV changes v 5/9/05 neighbors then notify their neighbors if necessary 10 CS 118/Spring 05

Distance Vector: an example X 2 Y 7 1 Z Z X D (Y, Z) = c(X, Z) + minw{D (Y, w)} = 7+1 = 8 Y X D (Z, Y) = c(X, Y) + minw {D (Z, w)} = 2+1 = 3 5/9/05 11 CS 118/Spring 05

Distance Vector: link cost changes Link cost changes: node detects local link cost change updates distance table (line 15) if cost change in least cost path, notify neighbors (lines 23, 24) X 4 Y 50 1 Z algorithm terminates “good news travels fast” 5/9/05 1 12 CS 118/Spring 05

Distance Vector: link cost changes (2) Link cost changes: bad news travels slow - “count to infinity” problem! 60 X 4 Y 50 1 Z algorithm continues on! 5/9/05 13 CS 118/Spring 05

Distance Vector: poisoned reverse r If Z routes through Y to get to X : v Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z) 60 X 4 Y 50 1 Z algorithm terminates Will this completely solve count to infinity problem? 5/9/05 14 CS 118/Spring 05

An example for Distance Vector routing with Poisson reverse (PR) A's routing table Dst Dis Nex B C D E F G H 1 3 4 4 7 6 2 B B B H H B's routing table A's update to B w/o PR B C D E F G H 1 3 4 4 7 6 2 Dst Dis Nex A C D E F G H 1 2 3 3 6 5 3 A C D E F G H A 1 A C C C H 1 4 5 5 8 7 3 A A A H A's update to B with PR: B C D E F G H 5/9/05 1 3 2 H 6 2 2 B 2 4 G 15 C 1 E 4 1 3 D 4 F CS 118/Spring 05

Comparison of LS and DV algorithms r distance vector: v distribute one’s own routing table to neighbors • routing update can be large in size, but travels only one link v each node only knows distances to other destinations r link state v broadcast raw topology information to entire net • routing update is small in size, but travels over all links in the net v each node knows entire topology r Performance measure: Message complexity, Time to convergence Robustness: what happens if router malfunctions? LS: v v node can advertise incorrect link cost each node computes only its own table DV: v v 5/9/05 DV node can advertise incorrect path cost each node’s table used by others 16 CS 118/Spring 05

What we have talked about routing r Dijkstra routing algorithm v Given a topology map, compute the shortest paths to all the other nodes r Bellman-Ford routing algorithm v Given the lists of distance to all destinations from all the neighbors, compute the shortest path to destination v Known problem: count-to-infinity v A simple (partial) solution: poison-reverse 5/9/05 17 CS 118/Spring 05

Routing in the Internet r The Global Internet: a large number of Autonomous Systems (AS) interconnected with each other: v Stub AS: end user networks (corporations, campuses) • Multihomed AS: stub ASes that are connected to multiple service providers v Transit AS: Internet service provider r Two-level routing hierarchy: v Intra-AS v Inter-AS 5/9/05 18 CS 118/Spring 05

Internet Hierarchical Routing Inter-AS border (exterior gateway) routers Intra-AS (interior gateway) routers r autonomous system (AS): a set of routers under the same administrative domain r Each AS makes its own decision on internal routing protocol (IGP) to use v All routers in one AS run the same IGP r border routers also run BGP 5/9/05 19 CS 118/Spring 05

Intra-AS and Inter-AS routing Border routers: C. b a C B. a A. a b A. c d A a b inter-AS, intra-AS routing in gateway A. c 5/9/05 a c B c inter-AS routing protocol intra-AS routing protocol b • perform inter-AS routing across AS boundaries • perform intra-AS routing with other routers in each's own AS network layer link layer physical layer 20 CS 118/Spring 05

Intra-AS and Inter-AS routing C. b a Host-1 C A. a b A. c d A a b Intra-AS routing within AS A 5/9/05 Inter-AS routing between A and B c B. a a c Host 18. 2. 4. 157 b B Intra-AS routing within AS B Forwarding table 131. 179. 0. 0 outf-1 18. 0. 0. 0 outf-2 23. 0. 0. 0 outf-2 157. 34. 128. 0 outf-3 222. 8. 192. 0 outf-4 21 CS 118/Spring 05

Intra-AS Routing: Interior Gateway Protocols (IGP) r Most commonly used IGPs: v IS-IS: Intermediate System to Intermediate System Routing protocol v OSPF: Open Shortest Path First v IGRP: Interior Gateway Routing Protocol (Cisco proprietary) v RIP: Routing Information Protocol 5/9/05 22 CS 118/Spring 05

RIP ( Routing Information Protocol) r Distance vector algorithm v Distance metric: # of hops (max = 15 hops) r Neighbor routers exchanged routing advertisement every 30 seconds u v A z C w B D x y destination hops u 1 v 2 w 2 x 3 y 3 z 2 r Failure and Recovery: If no update from neighbor N heard after 180 sec neighbor/link declared dead v v v 5/9/05 All routes via N invalidated; updates sent to neighbors in turn may send out new advertisements (if tables changed) Use poison reverse to prevent ping-pong loops (16 hops = ) 23 CS 118/Spring 05

RIP (Routing Information Protocol) z w A x y D B C Destination Network Next Router w A y B z B x -…. …. Num. of hops to dest. 2 2 7 1. . Routing table in D 5/9/05 24 CS 118/Spring 05

RIP: Example Dest. w x z …. distance 1 1 4 Advertisement from A to D . . . z w A y x Destination Network D B C Next Router Num. of hops to dest. w y z x A B BA -- 2 2 75 1 …. . . Routing table in D 5/9/05 25 CS 118/Spring 05

RIP Implementation r route-d (daemon): an application-level process that manages RIP routing table and generates periodic RIP routing updates Process updates from neighbors v send updates periodically to neighbors (if detect a failure, send right away) v r Keeps the resulting routing table only (not all the updates) routed Transport (UDP) network (IP) Transport (UDP) forwarding table link physical 5/9/05 network (IP) physical 26 CS 118/Spring 05

OSPF (Open Shortest Path First) r A Link State protocol v each node knows its directly connected neighbors & the link distance to each (link-state) v each node periodically broadcasts its link-state to the entire network r Link-State Packet: one entry per neighbor router v ID of the node that created the LSP v a list of direct neighbors, with link cost to each v sequence number for this LSP message (SEQ) v time-to-live (TTL) for information carried in this LSP v Use raw IP packet (protocol ID = 89) 5/9/05 27 CS 118/Spring 05

Building a complete map using Link State r Everyone broadcasts a piece of the topology r Put all the pieces together, you get the complete map Then each node carries out its own routing calculation independently 5/9/05 28 CS 118/Spring 05

Link-State Routing Protocol r The routing daemon running at each node: Builds and maintains topology map at each node v Stores and forwards most recent LSP from all other nodes • decrement TTL of stored LSP; discard info when TTL=0 v Compute routes using Dijkstra’s algorithm v generates its own LSP periodically with increasing SEQ 5/9/05 29 CS 118/Spring 05

Reliable Flooding of LSP rforward each received LSP to all neighbor nodes but the one that sent it veach ISP is reliably delivered over each link vuse the source-ID and SEQ in a LSP to detect duplicates r. LSPs sent both periodically and event-driven X A C B 5/9/05 D X A C B D 30 X A C B D CS 118/Spring 05

Advanced features supported by OSPF r Security: all OSPF messages authenticated r Multiple same-cost paths allowed r For each link, multiple cost metrics for different TOS (eg, satellite link cost set “low” for best effort; high for real time) r Integrated uni- and multicast support: v Multicast OSPF (MOSPF) uses same topology data base as OSPF r Hierarchical OSPF in large domains. 5/9/05 31 CS 118/Spring 05

Hierarchical OSPF 5/9/05 32 CS 118/Spring 05

Hierarchical OSPF r Two-level hierarchy: local area, backbone. Link-state advertisements only in area v each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. r Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers. r Backbone routers: run OSPF routing limited to backbone. r Boundary routers: connect to other AS’s. v 5/9/05 33 CS 118/Spring 05

Inter-AS routing x r BGP (Border Gateway Protocol): the de facto standard r Path Vector protocol: similar to Distance Vector protocol v each Border router broadcast to neighbors (peers) entire path (I. e, sequence of ASs) to destination v E. g. , Path (X, Z) = X, Y 1, Y 2, Y 3, …, Z v 5/9/05 34 CS 118/Spring 05

Example: Forwarding Table in Router d of AS A r Suppose AS A learns from the inter-AS protocol that subnet x is reachable from AS B (gateway A. c) but not from AS C. r Inter-AS protocol propagates reachability info to all internal routers. r Router d determines from intra-AS routing info that its interface I is on the least cost path to c. r Puts in forwarding table entry (x, I). 5/9/05 35 CS 118/Spring 05

Choosing among multiple ASes r Now suppose AS 1 learns from the inter-AS protocol that subnet x is reachable from AS 3 and from AS 2. r To configure forwarding table, router 1 d must determine towards which gateway it should forward packets for dest x. r This is also the job on inter-AS routing protocol! r Hot potato routing: send packet towards closest of two routers. Learn from inter-AS protocol that subnet x is reachable via multiple gateways 5/9/05 Use routing info from intra-AS protocol to determine costs of least-cost paths to each of the gateways Hot potato routing: Choose the gateway that has the smallest least cost 36 Determine from forwarding table the interface I that leads to least-cost gateway. Enter (x, I) in forwarding table CS 118/Spring 05

Internet inter-AS routing: BGP r BGP (Border Gateway Protocol): the de facto standard r BGP provides each AS a means to: 1. Obtain subnet reachability information from neighboring ASs. 2. Propagate the reachability information to all routers internal to the AS. 3. Determine “good” routes to subnets based on reachability information and policy. r Allows a subnet to advertise its existence to rest of the Internet: “I am here” 5/9/05 37 CS 118/Spring 05

BGP basics r Pairs of routers (BGP peers) exchange routing info over a TCP connection: BGP sessions v BGP sessions do not necessarily correspond to physical links. r When AS 2 advertises a prefix to AS 1, AS 2 is promising it will forward any datagrams destined to that prefix towards the prefix. 3 c 3 a 3 b AS 3 2 a 1 c 1 a AS 1 1 d 2 c AS 2 2 b 1 b e. BGP session i. BGP session 5/9/05 38 CS 118/Spring 05

Distributing reachability info r With e. BGP session between 3 a and 1 c, AS 3 sends prefix reachability info to AS 1. r 1 c can then use i. BGP to distribute this new prefix reach info to all routers in AS 1 r 1 b can then re-advertise the new reach info to AS 2 over the 1 b-to 2 a e. BGP session r When router learns about a new prefix, it creates an entry for the prefix in its forwarding table. 3 c 3 a 3 b AS 3 P 1 c 1 a AS 1 1 d 5/9/05 2 a 2 c AS 2 1 b 39 2 b e. BGP session i. BGP session CS 118/Spring 05

Path attributes & BGP routes r When advertising a prefix, advert includes BGP attributes. prefix + attributes = “route” v most important attribute: AS-PATH: contains the ASs through which the advert for the prefix passed: AS 67 AS 17 v r When an e. BGP router receives route advert, uses import policy to accept/decline. r e. BGP router also applies export policy to decide which routers to tell which neighbor e. BGP router 5/9/05 40 CS 118/Spring 05

BGP route selection Router may learn about more than 1 route to some prefix. Router must select route. r Elimination rules: r 1. 2. 3. 4. 5/9/05 Local preference value attribute: policy decision Shortest AS-PATH Closest NEXT-HOP router: hot potato routing Additional criteria 41 CS 118/Spring 05

BGP messages r BGP messages exchanged using TCP. r BGP messages: v OPEN: opens TCP connection to peer and authenticates sender v UPDATE: advertises new path (or withdraws old) v KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request v NOTIFICATION: reports errors in previous msg; also used to close connection 5/9/05 42 CS 118/Spring 05

BGP routing policy 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 5/9/05 43 CS 118/Spring 05

BGP routing policy (2) A advertises to B the path AW B advertises to X the path BAW Should B advertise to C the path BAW? 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! 5/9/05 44 CS 118/Spring 05

Why different Intra- and Inter-AS routing ? Policy: r Inter-AS: admin wants control over how its traffic routed, who routes through its net. r Intra-AS: single admin, so no policy decisions needed Scale: r hierarchical routing saves table size, reduced update traffic Performance: r Intra-AS: can focus on performance r Inter-AS: policy may dominate over performance 5/9/05 45 CS 118/Spring 05