Routing IP Routing Protocols 1 Routing Problem Definition

Routing & IP Routing Protocols 1

Routing: Problem Definition • You are a router in a packet switched network and you receive a packet destined to some remote node – E. g. , router A below receives a packet destined to node F • Question: How does A know where to send this packet? – Does A send it to B? C? or D? • Answer: Recall from our earlier discussion in IP that router A consults a forwarding table to make this decision • Routing Problem: How does router A build this forwarding table? – Built by a routing algorithm (protocol): The job of the routing algorithm is to determine the next hop router for ALL destinations in the network Forwarding Table in A Dest. B D C E F Next Hop B D D Cost 2 1 3 2 4 5 2 A B 2 1 D 3 C 3 1 5 F 1 E 2 2

End-to-End Path Determination: Routing principles Routing protocol 5 Goal: determine “good” path (sequence of routers) thru network from source to dest. Graph abstraction for routing algorithms: • graph nodes are routers • graph edges are physical links – link cost: delay, $ cost, or congestion level (amount of traffic carried on the link) 2 A B 2 1 D 3 C 3 1 5 F 1 E 2 • “good” path: – typically means minimum cost path 3

Forwarding vs Routing • Distinction between “Forwarding” and “Routing” – Forwarding consists of taking a packet, looking at its destination address, consulting the forwarding table, and sending the packet in a direction determined by the table • Very easy once the forwarding table has been built – Routing is the process by which the forwarding table is built • Need a routing protocol to dynamically build and maintain the table Forwarding table in A 5 Dest. Next Hop Cost B D C E F B D D 2 1 3 2 4 B 2 A 2 1 D 3 C 3 1 5 F 1 E 2 4

Routing Algorithm classification Global or decentralized information? Global: • all routers have complete topology, link cost info • “link state” algorithms Decentralized: • router knows physically-connected neighbors, link costs to neighbors • iterative process of computation, exchange of info with neighbors • “distance vector” algorithms 5

Link-State Algorithms: General Idea • Have each router build the complete topology of the network • Once the complete topology is built, have each router run an algorithm to compute the shortest path from itself to all other routers (nodes) in the network 2 Issues • How does a router build the complete topology of the network? • How does a router compute the shortest path to all other nodes in the network using this topology information? – Dijkstra’s Shortest Path Algorithm 5 2 A B 2 1 D 3 C 3 1 5 F 1 E 2 6

Building the Network Topology • At the heart of a link state algorithm is the discovery of a node’s links’ states – Each node is assumed to capable of finding out the state of the link to its neighbors (up or down) and the cost of each link – Each node creates an update packet, also called a link-state packet (LSP) and periodically sends this information to all of its neighbors – Node’s neighbors send the packet to their neighbors and so on until the LSP is received by all nodes in the network LSP Contents • ID of the node -- A • List of neighbors and the cost to each neighbor – (B, 2), (C, 5), (D, 1) • A sequence number • A time-to-live (TTL) 5 2 A B 2 1 D 3 C 3 1 5 F 1 E 2 7

Reliable Flooding • Reliable flooding is the process of making sure that all nodes get a copy of LSP from all other nodes in the network • When a node X receives a copy of an LSP that originated at some other node Y, it checks to see if it has already stored a copy of an LSP from Y. • If not, it stores the LSP. • If it already has a copy, it compares the Seq. Nos; if the new LSP has a larger Seq. No, it is assumed to be more recent, and the last LSP is stored replacing the old one. Otherwise the new LSP is discarded • The new LSP is then forwarded on all neighbors of X except the neighbor from which the LSP was just received 5 2 A B 2 1 D 3 C 3 1 5 F 1 E 2 8

Computing the Shortest Path: Dijkstra’s Algorithm Notation: • Cost(i, j): link cost from node i to j. – Cost(A, B) = 2 – Cost(A, C) = 5 – Cost(A, F) = infinity 5 • Distance(v): current value of cost of path from source to destination V – Distance(D) = 1 • Pred(v): predecessor node of v along path from source to v – Pred(D) = A 2 A B 2 1 D 3 C 3 1 5 F 1 2 E • N: set of nodes whose least cost path definitively known 9

Dijsktra’s Algorithm – where A is the source node 1 Initialization: 2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then Distance (v) = cost(A, v) 6 else Distance (v) = infinity 7 8 Loop 9 find w not in N such that Distance(w) is a minimum 10 add w to N 11 update Distance(v) for all v adjacent to w and not in N: 12 Distance(v) = min( Distance(v), Distance(w) + cost(w, v) ) 13 /* new distance to v is either old distance to v or known 14 shortest path distance to w plus cost from w to v */ 15 until all nodes in N 10

Dijkstra’s algorithm: example Step 0 1 2 3 4 5 N A AD ADEBCF D(B), p(B) 2, A D(C), p(C) D(D), p(D) D(E), p(E) 1, A 5, A infinity 4, D 2, D 3, E D(F), p(F) infinity 4, E 5 2 A B 2 1 D 3 C 3 1 5 F 1 E 2 Algorithm complexity: n nodes • each iteration: need to check all nodes, w, not in N • n*(n+1)/2 comparisons: O(n 2) – O(nlogn) algorithm possible 11

Distance Vector Algos: General Idea • We do NOT need to know the complete topology of the network to build the forwarding table! • If a node X simply tells its neighbor Y the cost of reaching another node Z via itself (X), then – neighbor Y can compute the cost of reaching Z via X by simply adding the cost of reaching X from Y and the cost of reaching Z from X, which was advertised by X Example • If D tells A that it can reach E with a cost of 1, then A knows it can reach E via D with a cost of 1+1 = 2 • If D tells A that it can reach F with a cost of 3, then A knows it can reach F via D with a cost of 1+3 = 4 5 2 A B 2 1 D 3 C 3 1 5 F 1 E 2 12

Distance Vector Routing Algorithm • Each node tells its neighbors the cost of reaching every other node via itself • A node computes its cost of reaching a destination via each of its neighbors and picks the best one distributed: • each node communicates only with directlyattached neighbors iterative: • continues until no nodes exchange new information • self-terminating: no “signal” to stop X D (Y, Z) distance from X to = Y via Z as next hop Y = c(X, Z) + D (Z) X X D (Y) = min {D (Y, w)} w X C(X, Y) C( X, T) Y D (Z) Y T Z T D (Z) 13

Distance Table Structure Distance Table Data Structure 7 A B 1 C 2 8 1 E 2 E cost to destination via D () A B D A 1 14 5 B 7 8 5 C 6 9 4 D 4 11 2 destination • each node has its own distance table • row for each possible destination • column for each directly-attached neighbor to node • example: in node X, for dest. Y via neighbor Z: D 14

Distance Table gives Forwarding Table E cost to destination via Outgoing link to use, cost B D A 1 14 5 A A, 1 B 7 8 5 B D, 5 C 6 9 4 C D, 4 D 4 11 2 D D, 4 Distance Table destination A destination D () Forwarding (Routing) Table 15

Distance Vector Routing: overview Each node: Initialize the Distance Table Structure wait for (msg from neighbor) recompute distance table if least cost path to any dest has changed, notify neighbors 16

Initialization At all nodes, X: 1 Initialization: 2 for all adjacent nodes v: 3 DX(*, v) = infinity /* the * operator means "for all rows" */ X 4 D (v, v) = c(X, v) 5 for all destinations, y X 6 send D (y) to each neighbor /* Cost of reaching Y via X */ 17

Distance Vector Algorithm: Example This column shows the Init. results X 2 Y 7 Init Info sent We need to adjust values if necessary 1 Z 18

Distance Vector Algorithm (cont. ): 8 loop 9 wait (until I receive update from neighbor V) 10 if (update received from neighbor V wrt destination Y) 11 /* shortest path from V to some Y has changed */ 12 /* V has sent a new value for its DV(Y) */ 13 /* call this received new value is "newval" */ X 14 for the single destination Y: D (Y, V) = c(X, V) + newval 15 X 16 if we have a new D (Y) for any destination Y 17 send new value of DX(Y) to all neighbors 18 19 forever 19

Distance Vector Algorithm: Example Initial values X 2 Y 7 info sent Adjusted new value 1 Z X Y D (Z, Y) = c(X, Y) + D (Z) = 2+1 = 3 Z X D (Y, Z) = c(X, Z) + D (Y) = 7+1 = 8 20

Distance Vector Algorithm: Example This column Shows the Init. results X 2 Y 7 Init Info sent Adjusted new values New Info sent Adjust values if necessary 1 Z 21

Hierarchical Routing Our routing study thus far assumed • all routers to be “identical” and the network to be “flat” • 2 Problems exist with such a model: 1. scale: with 200 million destinations: – can’t store all destinations in routing tables! – routing table exchange would swamp links! 2. administrative autonomy – Can’t assume that a network with the scale of Internet will be administered by a single authority • Solution? – Internet is NOT flat, but is a network of networks – each network admin controls routing in its own network – next – http: //www. ilkertemir. com/backbone-map/ • Turkey’s past and current Internet backbone map 22

Hierarchical Routing C. b a C B. a A. a b A. c d A a b • aggregate routers into regions, “autonomous systems” (AS) • routers in same AS run same routing protocol – “intra-AS” routing protocol – routers in different AS can run different intra-AS routing protocol a c B b c gateway routers • special routers in AS • run intra-AS routing protocol with all other routers in AS • also responsible for routing to destinations outside AS – run inter-AS routing protocol with other 23 gateway routers

Intra-AS Routing • Also known as Interior Gateway Protocols (IGP) • Most common Intra-AS routing protocols: – OSPF: Open Shortest Path First – RIP: Routing Information Protocol – IGRP: Interior Gateway Routing Protocol (Cisco proprietary) 24

OSPF (Open Shortest Path First) • “open”: publicly available • Uses Link State algorithm – LS advertisement dissemination to entire AS via flooding – Topology map at each node – Route computation using Dijkstra’s algorithm – Carried in OSPF messages directly over IP • OSPF has its own network layer protocol number like IP! 25

RIP (Routing Information Protocol) • Distance vector algorithm • Included in BSD-UNIX Distribution in 1982 • Distance metric: # of hops (max = 15 hops) – 16 is the infinity to eliminate routing loops • Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement) • Each advertisement: list of up to 25 destination nets within AS – If more destinations, send multiple advertisements 26

RIP: Example z w A x D B y C Destination Network w y z x …. Next Router Num. of hops to dest. …. . . A B B -- 2 2 7 1 Routing table in D 27

RIP: Example Dest w x z …. Next C … w hops 4. . . A Advertisement from A to D z x Destination Network w y z x …. D B C y Next Router Num. of hops to dest. …. . . A B B A -- Routing table in D 2 2 7 5 1 28

RIP Table processing • RIP routing tables managed by application-level process called route-d (daemon) • advertisements sent in UDP packets, periodically repeated routed Transprt (UDP) network (IP) link physical Transprt (UDP) forwarding table network (IP) link physical 29

Inter-AS routing in the Internet: BGP • Each BGP router communicates only with its neighbors – R 1 with R 2, R 3 with R 4 – Global info about routes to destination networks propagates in an AS-by-AS manner via the exchange of BGP messages 30

Why different Intra- and Inter-AS routing ? Policy: • Intra-AS: single admin, so no policy decisions needed • Inter-AS: admin wants control over how its traffic routed, who routes through its net. Performance: • Intra-AS: can focus on performance • Inter-AS: policy may dominate over performance 31