CMPE 80N: Introduction to Networking and the Internet. Katia Obraczka Computer Engineering UCSC Baskin Engineering Lecture 20

Transcription

1 CMPE 80N: Introduction to Networking and the Internet Katia Obraczka Computer Engineering UCSC Baskin Engineering Lecture 0

2 Announcements Final exam: June 7 th at 4pm. Comprehensive. Photo id required. 1 page 8.5x11in double-sided handwritten cheat sheet. Need to be turned in with final exam.

3 Announcements Review session tonight! E 180 (E Simularium). Starts at 7:45pm.

4 Network layer. Subnets. DHCP. NAT. IPv6. Tunneling. Last class

5 Today Network layer (cont d). Routing.

6 Interplay between routing and forwarding routing algorithm local forwarding table header value output link value in arriving packet s header

7 Graph Abstraction 5 Graph: G = (N,E) u 1 v x w 1 y 5 z N = set of routers = { u, v, w, x, y, z } E = set of links = { (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }

8 Graph abstraction: costs u 1 5 c(x,x ) = cost of link (x,x ) v 3 w 5 - e.g., c(w,z) = 5 z 3 1 x y cost could always be 1, or 1 inversely related to bandwidth, or inversely related to congestion Cost of path (x 1, x, x 3,, x p ) = c(x 1,x ) + c(x,x 3 ) + + c(x p-1,x p ) Routing algorithm: algorithm that finds least-cost path

9 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

10 Routing Algorithm Classification Static or dynamic? Static: routes change slowly over time Dynamic: routes change more quickly periodic update in response to link cost changes

11 Link-State Routing Dijkstra s algorithm Net topology, link costs known to all nodes accomplished via link state broadcast all nodes have same info Computes least cost paths from one node ( source ) to all other nodes. Results in forwarding table for node. Each node runs the algorithm.

12 Step Dijkstra s algorithm: example N' u ux uxy uxyv uxyvw uxyvwz D(v),p(v),u,u,u D(w),p(w) 5,u 4,x 3,y 3,y D(x),p(x) 1,u D(y),p(y),x D(z),p(z) 4,y 4,y 4,y u 1 5 Computing shortest paths v 3 from u to all other nodes. w 5 z 3 1 x y 1

13 Dijkstra s algorithm: example () Resulting shortest-path tree from u: u v x w y z Resulting forwarding table in u: destination link v x y w z (u,v) (u,x) (u,x) (u,x) (u,x)

14 Distance Vector Routing

15 Distance Vector Routing Bellman-Ford algorithm. Node uses information from its neighbors to compute its routing table. D x (y) = min[c(x,v) + d v (y)] for every y.

16 Bellman-Ford example u 1 5 v x w 1 y 5 z Clearly, d v (z) = 5, d x (z) = 3, d w (z) = 3 B-F equation says: d u (z) = min { c(u,v) + d v (z), c(u,x) + d x (z), c(u,w) + d w (z) } = min { + 5, 1 + 3, 5 + 3} = 4 Node that achieves minimum is next hop in shortest path forwarding table

17 Distance vector algorithm Basic idea: From time-to-time, each node sends its own distance vector estimate to neighbors When a node x receives new DV estimate from neighbor, it updates its own DV using B-F equation: D x (y) min v {c(x,v) + D v (y)} for each node y N

18 Distance Vector Algorithm Iterative, asynchronous: each local iteration caused by: local link cost change DV update message from neighbor Distributed: each node notifies neighbors only when its DV changes neighbors then notify their neighbors if necessary Each node: wait for (change in local link cost or msg from neighbor) recompute estimates if DV to any dest has changed, notify neighbors

19 Comparison of LS and DV algorithms

20 Routing on the Internet

21 Hierarchical Routing Our routing study thus far - idealization all routers identical network flat not true in practice scale: with 00 million destinations: can t store all dest s in routing tables! routing table exchange would swamp links! administrative autonomy Internet = network of networks each network admin may want to control routing in its own network

22 Hierarchical Routing 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 Gateway router Direct link to router in another AS

23 Interconnected ASes 3c 3a 3b AS3 1a 1c 1d 1b Intra-AS Routing algorithm AS1 Forwarding table Inter-AS Routing algorithm a c AS b forwarding table configured by both intra- and inter-as routing algorithm intra-as routing sets entries for internal dests. inter-as & intra-as routing sets entries for external dests.

24 Inter-AS tasks suppose router in AS1 receives datagram destined outside of AS1: router should forward packet to gateway router, but which one? AS1 must: 1. learn which dests are reachable through AS, which through AS3. propagate this reachability info to all routers in AS1 Job of inter-as routing! 3c 3a 3b AS3 1a 1c 1d 1b AS1 a c AS b

25 Intra-AS Routing also known as Interior Gateway Protocols (IGP) most common Intra-AS routing protocols: RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

26 Inter-AS Routing BGP: Border Gateway Protocol.