CMPE 150/L : Introduction to Computer Networks. Chen Qian Computer Engineering UCSC Baskin Engineering Lecture 14

Transcription

1 CMPE 150/L : Introduction to Computer Networks Chen Qian Computer Engineering UCSC Baskin Engineering Lecture 14 1

2 Two notes on routing algorithm Do not believe ou can understand an routing protocol, e.g., link-state or distance vector, just b attending the lectures. I didn t understand them when I was an undergrad. Best approach to learn: Tr to write our own program to simulate the protocols. 2

3 Dijkstra s algorithm: eample D(v) p(v) D(w) p(w) D() p() D() p() D() p() Step N' 0 u 7,u 3,u 5,u 1 uw 6,w 5,u 11,w 2 uw 6,w 11,w 14, 3 uwv 10,v 14, 4 uwv 12, 5 uwv notes: construct shortest path tree b tracing predecessor nodes ties can eist (can be broken arbitraril) u 5 3 w v Network Laer 3

4 Network Laer 4 Dijkstra s algorithm: another eample Step N' u u u uv uvw uvw D(v),p(v) 2,u 2,u 2,u D(w),p(w) 5,u 4, 3, 3, D(),p() 1,u D(),p() 2, D(),p() 4, 4, 4, u w v

5 Dijkstra s algorithm: eample (2) resulting shortest-path tree from u: v w u resulting forwarding table in u: destination v w link (u,v) (u,) (u,) (u,) (u,) Network Laer 5

6 Dijkstra s algorithm, discussion algorithm compleit: n nodes each iteration: need to check all nodes, w, not in N n(n+1)/2 comparisons: O(n 2 ) more efficient implementations possible: O(nlogn) oscillations possible: e.g., support link cost equals amount of carried traffic: 1 D A 1 1+e C e e initiall B 1 A 2+e 0 D 0 1+e 1 C B given these costs, find new routing. resulting in new costs 0 D A 0 2+e C 1+e B given these costs, find new routing. resulting in new costs A 2+e 0 D 0 1+e 1 C B given these costs, find new routing. resulting in new costs 0 Network Laer 6

7 Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms link state distance vector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Laer 7

8 Distance vector algorithm Bellman-Ford equation (dnamic programming) let d () := cost of least-cost path from to then d () = min {c(,v) + d v () } v cost from neighbor v to destination cost to neighbor v min taken over all neighbors v of Network Laer 8

9 Bellman-Ford eample u v w clearl, d v () = 5, d () = 3, d w () = 3 B-F equation sas: d u () = min { c(u,v) + d v (), c(u,) + d (), c(u,w) + d w () } = min {2 + 5, 1 + 3, 5 + 3} = 4 node achieving minimum is net hop in shortest path, used in forwarding table Network Laer 9

10 Distance vector algorithm D () = estimate of least cost from to maintains distance vector D = [D (): є N ] node : knows cost to each neighbor v: c(,v) maintains its neighbors distance vectors. For each neighbor v, maintains D v = [D v (): є N ] Network Laer 10

11 Distance vector algorithm ke idea: from time-to-time, each node sends its own distance vector estimate to neighbors when receives new DV estimate from neighbor, it updates its own DV using B-F equation: D () min v {c(,v) + D v ()} for each node N under minor, natural conditions, the estimate D () converge to the actual least cost d () Network Laer 11

12 Distance vector algorithm iterative, asnchronous: each local iteration caused b: local link cost change DV update message from neighbor distributed: each node notifies neighbors onl when its DV changes neighbors then notif their neighbors if necessar each node: wait for (change in local link cost or msg from neighbor) recompute estimates if DV to an dest has changed, notif neighbors Network Laer 12

13 node table from cost to D () = min{c(,) + D (), c(,) + D ()} = min{2+0, 7+1} = 2 from 0 cost to D () = min{c(,) + D (), c(,) + D ()} = min{2+1, 7+0} = 3 node table from cost to node table from cost to time Network Laer 13

14 node table from node table from cost to D () = min{c(,) + D (), c(,) + D ()} = min{2+0, 7+1} = 2 cost to from from 0 cost to 2 cost to from from cost to cost to D () = min{c(,) + D (), c(,) + D ()} = min{2+1, 7+0} = node table from cost to from cost to from cost to time Network Laer 14

15 Distance vector: link cost changes link cost changes: node detects local link cost change updates routing info, recalculates distance vector if DV changes, notif neighbors good news travels fast t 0 : detects link-cost change, updates its DV, informs its neighbors. t 1 : receives update from, updates its table, computes new least cost to, sends its neighbors its DV. t 2 : receives s update, updates its distance table. s least costs do not change, so does not send a message to. Network Laer 15

16 Distance vector: link cost changes link cost changes: node detects local link cost change bad news travels slow - count to infinit problem! 44 iterations before algorithm stabilies: see tet poisoned reverse: If Z routes through Y to get to X : Z tells Y its (Z s) distance to X is infinite (so Y won t route to X via Z) Network Laer 16

17 Comparison of LS and DV algorithms message compleit LS: with n nodes, E links, O(nE) msgs sent DV: echange between neighbors onl convergence time varies speed of convergence LS: O(n 2 ) algorithm requires O(nE) msgs ma have oscillations DV: convergence time varies ma be routing loops count-to-infinit problem robustness: what happens if router malfunctions? LS: node can advertise incorrect link cost each node computes onl its own table DV: DV node can advertise incorrect path cost each node s table used b others error propagate thru network Network Laer 4-17

18 Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms link state distance vector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Laer 4-18

19 Hierarchical routing our routing stud thus far - idealiation all routers identical network flat not true in practice scale: with 600 million destinations: can t store all dest s in routing tables! routing table echange would swamp links! administrative autonom internet = network of networks each network admin ma want to control routing in its own network Network Laer 4-19

20 Hierarchical routing aggregate routers into regions, autonomous sstems (AS) routers in same AS run same routing protocol intra-as routing protocol routers in different AS can run different intra- AS routing protocol gatewa router: at edge of its own AS has link to router in another AS Network Laer 4-20

21 Interconnected ASes 3c 3a 3b AS3 1a 1c 1d 1b Intra-AS Routing algorithm AS1 Forwarding table Inter-AS Routing algorithm 2a 2c AS2 2b forwarding table configured b both intraand inter-as routing algorithm intra-as sets entries for internal dests inter-as & intra-as sets entries for eternal dests Network Laer 4-21

22 Inter-AS tasks suppose router in AS1 receives datagram destined outside of AS1: router should forward packet to gatewa router, but which one? AS1 must: 1. learn which dests are reachable through AS2, which through AS3 2. propagate this reachabilit info to all routers in AS1 job of inter-as routing! 3c other networks 3b 3a AS3 1a AS1 1c 1d 1b 2a 2c AS2 2b other networks Network Laer 4-22

23 Eample: setting forwarding table in router 1d suppose AS1 learns (via inter-as protocol) that subnet reachable via AS3 (gatewa 1c), but not via AS2 inter-as protocol propagates reachabilit info to all internal routers router 1d determines from intra-as routing info that its interface I is on the least cost path to 1c installs forwarding table entr (,I) other networks 3c 3a 3b AS3 1a AS1 1c 1d 1b 2a 2c AS2 2b other networks Network Laer 4-23

24 Eample: choosing among multiple ASes now suppose AS1 learns from inter-as protocol that subnet is reachable from AS3 and from AS2. to configure forwarding table, router 1d must determine which gatewa it should forward packets towards for dest this is also job of inter-as routing protocol! other networks 3c 3a 3b AS3 1a AS1 1c 1d? 1b 2a 2c AS2 2b other networks Network Laer 4-24

25 Eample: choosing among multiple ASes now suppose AS1 learns from inter-as protocol that subnet is reachable from AS3 and from AS2. to configure forwarding table, router 1d must determine towards which gatewa it should forward packets for dest this is also job of inter-as routing protocol! hot potato routing: send packet towards closest of two routers. Network Laer 25

26 Traditional Computer Networks Data plane: Packet streaming Forward, filter, buffer, mark, rate-limit, and measure packets

27 Traditional Computer Networks Control plane: Distributed algorithms Track topolog changes, compute routes, install forwarding rules

28 Traditional Computer Networks Management plane: Human time scale Collect measurements and configure the equipment

29 Software Defined Networking (SDN) Logicall-centralied control Smart, slow API to the data plane (e.g., OpenFlow) Switches Dumb, fast

30 OpenFlow Networks 30

31 Data-Plane: Simple Packet Handling Simple packet-handling rules Pattern: match packet header bits Actions: drop, forward, modif, send to controller Priorit: disambiguate overlapping patterns Counters: #btes and #packets 1. src=1.2.*.*, dest=3.4.5.* drop 2. src = *.*.*.*, dest=3.4.*.* forward(2) 3. src= , dest=*.*.*.* send to controller

32 32 Unifies Different Kinds of Boes Router Match: longest destination IP prefi Action: forward out a link Switch Match: destination MAC address Action: forward or flood Firewall Match: IP addresses and TCP/UDP port numbers Action: permit or den NAT Match: IP address and port Action: rewrite address and port

33 E.g.: Dnamic Access Control Inspect first packet of a connection Consult the access control polic Install rules to block or route traffic

34 34 E.g.: Server Load Balancing Pre-install load-balancing polic Split traffic based on source IP src=0* src=1*

35 Chapter 4: outline 4.1 introduction 4.2 virtual circuit and datagram networks 4.3 what s inside a router 4.4 IP: Internet Protocol datagram format IPv4 addressing ICMP IPv6 4.5 routing algorithms link state distance vector hierarchical routing 4.6 routing in the Internet RIP OSPF BGP 4.7 broadcast and multicast routing Network Laer 4-35

36 Intra-AS Routing also known as interior gatewa protocols (IGP) most common intra-as routing protocols: RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gatewa Routing Protocol (Cisco proprietar) Network Laer 4-36

37 RIP ( Routing Information Protocol) included in BSD-UNIX distribution in 1982 distance vector algorithm distance metric: # hops (ma = 15 hops), each link has cost 1 DVs echanged with neighbors ever 30 sec in response message (aka advertisement) each advertisement: list of up to 25 destination subnets (in IP addressing sense) from router A to destination subnets: u A C B D v w subnet hops u 1 v 2 w Network Laer 4-37

38 RIP: eample w A D B C routing table in router D destination subnet net router # hops to dest w A 2 B 2 B Network Laer 4-38

39 RIP: eample A-to-D advertisement net hops dest w C w A D B C routing table in router D destination subnet net router # hops to dest w A 2 B 2 A B Network Laer 4-39

40 RIP: link failure, recover if no advertisement heard after 180 sec --> neighbor/link declared dead routes via neighbor invalidated new advertisements sent to neighbors neighbors in turn send out new advertisements (if tables changed) link failure info quickl (?) propagates to entire net Network Laer 4-40

41 RIP table processing RIP routing tables managed b application-level process called route-d advertisements sent in UDP packets, periodicall repeated routed routed transport (UDP) transprt (UDP) network (IP) forwarding table forwarding table network (IP) link link phsical phsical Network Laer 4-41