5 Network Layer Network Layer Network Layer Network Layer Example: - PDF document

Network Layer Network Layer Network Layer Network Layer Network Layer Comparison of LS and DV algorithms Message complexity Robustness: what happens  1 Introduction  5 Routing algorithms if router malfunctions?  LS: with n nodes, E links, O(nE) msgs sent  2 Virtual circuit and  Link state LS:  DV: exchange between datagram networks  Distance Vector  node can advertise neighbors only  Hierarchical routing incorrect link cost  3 What’s inside a  convergence time varies  6 Routing in the  each node computes only router its own table Internet Speed of convergence  4 IP: Internet DV:  RIP  LS: O(n 2 ) algorithm requires Protocol O(nE) msgs  DV node can advertise  OSPF  Datagram format  may have oscillations incorrect path cost  BGP  IPv4 addressing  each node’s table used by  DV: convergence time varies  ICMP others  may be routing loops  IPv6 • error propagate thru  count-to-infinity problem network Network Layer Network Layer 25 26 Network Layer Network Layer Network Layer Network Layer Hierarchical Routing Hierarchical Routing Our routing study thus far - idealization  aggregate routers into Gateway router  all routers identical regions, “autonomous  Direct link to router in systems” (AS)  network “flat” another AS  routers in same AS run … not true in practice same routing protocol scale: with 200 million administrative autonomy  “intra-AS” routing protocol destinations:  internet = network of  routers in different AS networks  can’t store all dest’s in can run different intra- routing tables!  each network admin may AS routing protocol want to control routing in its  routing table exchange own network would swamp links! Network Layer Network Layer 27 28 Network Layer Network Layer Network Layer Network Layer Inter-AS tasks Interconnected ASes AS1 must:  suppose router in AS1 learn which dests are 1. receives datagram reachable through AS2, which through destined outside of 3c AS3 AS1: 3a 2c 3b 2a 2. propagate this  router should AS3 2b 1c forward packet to reachability info to all AS2 1a gateway router, but routers in AS1 1b AS1  forwarding table 1d which one? Job of inter-AS routing! configured by both intra- and inter-AS routing algorithm Intra-AS Inter-AS Routing Routing algorithm algorithm  intra-AS sets entries 3c 3a for internal dests 2c Forwarding 3b 2a AS3 2b table  inter-AS & intra-As 1c AS2 sets entries for 1a 1b AS1 external dests 1d Network Layer Network Layer 29 30 5

Network Layer Network Layer Network Layer Network Layer Example: Choosing among multiple ASes Example: Setting forwarding table in router 1d  now suppose AS1 learns from inter-AS protocol that  suppose AS1 learns (via inter-AS protocol) that subnet subnet x is reachable from AS3 and from AS2. x reachable via AS3 (gateway 1c) but not via AS2.  to configure forwarding table, router 1d must determine towards which gateway it should forward  inter-AS protocol propagates reachability info to all packets for dest x. internal routers.  this is also job of inter-AS routing protocol!  router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c.  installs forwarding table entry (x,I) x 3c x 3a 2c 3b 2a 3c AS3 2b 1c 3a 2c AS2 3b 2a 1a AS3 2b 1b 1c AS1 1d AS2 1a 1b AS1 Given x , which interface to forward? 1d Given x , which interface to forward? Network Layer Network Layer 31 32 Network Layer Network Layer Network Layer Network Layer Example: Choosing among multiple ASes Network Layer  now suppose AS1 learns from inter-AS protocol that subnet x is reachable from AS3 and from AS2.  1 Introduction  5 Routing algorithms  to configure forwarding table, router 1d must  2 Virtual circuit and  Link state determine towards which gateway it should forward packets for dest x. datagram networks  Distance Vector  this is also job of inter-AS routing protocol!  Hierarchical routing  3 What’s inside a  6 Routing in the  hot potato routing: send packet towards closest of router two routers. Internet  4 IP: Internet  RIP Protocol  OSPF  Datagram format Determine from Learn from inter-AS Use routing info  BGP from intra-AS Hot potato routing: forwarding table the  IPv4 addressing protocol that subnet Choose the gateway interface I that leads x is reachable via protocol to determine  ICMP to least-cost gateway. multiple gateways costs of least-cost that has the paths to each smallest least cost Enter (x,I) in  IPv6 forwarding table of the gateways Network Layer Network Layer 33 34 Network Layer Network Layer Network Layer Network Layer Intra-AS Routing Network Layer  also known as Interior Gateway Protocols (IGP)  1 Introduction  5 Routing algorithms  most common Intra-AS routing protocols:  2 Virtual circuit and  Link state datagram networks  Distance Vector  RIP: Routing Information Protocol  Hierarchical routing  3 What’s inside a  6 Routing in the  OSPF: Open Shortest Path First router Internet  4 IP: Internet  IGRP: Interior Gateway Routing Protocol (Cisco  RIP Protocol proprietary)  OSPF  Datagram format  BGP  IPv4 addressing  ICMP  IPv6 Network Layer Network Layer 35 36 6

Network Layer Network Layer Network Layer Network Layer RIP ( Routing Information Protocol) RIP advertisements  distance vectors: exchanged among  distance vector algorithm neighbors every 30 sec via Response  included in BSD-UNIX Distribution in 1982 Message (also called advertisement)  distance metric: # of hops (max = 15 hops)  each advertisement: list of up to 25 From router A to subnets: destination hops destination subnets within AS u v u 1 w A B v 2 w 2 x x 3 C D z y 3 y z 2 Network Layer Network Layer 37 38 Network Layer Network Layer Network Layer Network Layer RIP: Example RIP: Example Dest Next hops Advertisement w - 1 from A to D D z x - 1 z C 4 w x y …. … ... z A B w x y A D B C Destination Network Next Router Num. of hops to dest. C w A 2 Destination Network Next Router Num. of hops to dest. y B 2 w A 2 z B 7 y B 2 x -- 1 z B A 7 5 …. …. .... x -- 1 Routing/Forwarding table in D …. …. .... Routing/Forwarding table in D Network Layer Network Layer 39 40 Network Layer Network Layer Network Layer Network Layer RIP: Link Failure and Recovery RIP Table processing  If no advertisement heard after 180 sec -->  RIP routing tables managed by application-level neighbor/link declared dead process called route-d (daemon)  routes via neighbor invalidated  advertisements sent in UDP packets, periodically repeated  new advertisements sent to neighbors  neighbors in turn send out new advertisements (if routed routed tables changed)  link failure info quickly (?) propagates to Transprt Transprt (UDP) (UDP) entire net network forwarding forwarding network  poison reverse used to prevent ping-pong loops (IP) table table (IP) (infinite distance = 16 hops) link link physical physical Network Layer Network Layer 41 42 7

Network Layer Network Layer Network Layer Network Layer Network Layer OSPF (Open Shortest Path First)  “open”: publicly available  1 Introduction  5 Routing algorithms  uses Link State algorithm  2 Virtual circuit and  Link state  LS packet dissemination datagram networks  Distance Vector  topology map at each node  Hierarchical routing  3 What’s inside a  route computation using Dijkstra’s algorithm  6 Routing in the router Internet  4 IP: Internet  OSPF advertisement carries one entry per neighbor  RIP Protocol router  OSPF  Datagram format  advertisements disseminated to entire AS (via  BGP  IPv4 addressing flooding)  ICMP  carried in OSPF messages directly over IP (rather than TCP  IPv6 or UDP Network Layer Network Layer 43 44 Network Layer Network Layer Network Layer Network Layer OSPF “advanced” features (not in RIP) Hierarchical OSPF  security: all OSPF messages authenticated (to prevent malicious intrusion)  multiple same-cost paths allowed (only one path in RIP)  For each link, multiple cost metrics for different ToS (e.g., satellite link cost set “low” for best effort; high for real time)  integrated uni- and multicast support:  Multicast OSPF (MOSPF) uses same topology data base as OSPF  hierarchical OSPF in large domains. Network Layer Network Layer 45 46 Network Layer Network Layer Network Layer Network Layer Hierarchical OSPF Network Layer  two-level hierarchy: local area, backbone.  1 Introduction  5 Routing algorithms  Link-state advertisements only in area  2 Virtual circuit and  Link state  each node has detailed area topology; only know datagram networks  Distance Vector direction (shortest path) to nets in other areas.  Hierarchical routing  3 What’s inside a  area border routers: “summarize” distances to nets  6 Routing in the router in own area, advertise to other Area Border routers. Internet  4 IP: Internet  backbone routers: run OSPF routing limited to  RIP Protocol backbone.  OSPF  Datagram format  BGP  boundary routers: connect to other AS’s.  IPv4 addressing  ICMP  IPv6 Network Layer Network Layer 47 48 8