Network Layer Overview: Goals: last time understand principles - PDF document

Network Layer Overview: Goals: ❒ last time ❒ understand principles ❍ network layer services behind network layer ❍ IP addressing services: ❍ routing principle: path ❍ routing (path selection) selection ❍ dealing with scale ❒ today ❍ how a router works ❍ IP ❍ advanced topics: IPv6, ❍ hierarchical routing multicast ❍ Internet routing protocols ❒ instantiation and reliable transfer implementation in the • intra-domain Internet • inter-domain ❍ what’s inside a router? ❍ IPv6 ❍ multicast routing 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 1 The Internet Network layer Host, router network layer functions: Transport layer: TCP, UDP IP protocol Routing protocols •addressing conventions •path selection •datagram format Network •RIP, OSPF, BGP •packet handling conventions layer routing table ICMP protocol •error reporting •router “signaling” Link layer physical layer 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 2 IP datagram format IP protocol version 32 bits total datagram number length (bytes) header length head. type of ver length (32-bit words) len service for “type” of data fragment fragmentation/ 16-bit identifier flgs offset reassembly max number time to upper Internet remaining hops live layer checksum (decremented at 32 bit source IP address each router) 32 bit destination IP address upper layer protocol to deliver payload to E.g. timestamp, Options (if any) record route data taken, specify (variable length, list of routers typically a TCP to visit. or UDP segment) 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 3 1

Fragmentering och hopsättning Ident Flags Offset ❒ MTU varierar i olika nät, två val ❍ Alla paket tillräckligt små ❍ Fragmentering och hopsättning ❒ Alla fragment har samma Ident ❒ Tappas ett fragment slängs alla ❒ Varje fragment fullständigt IP-datagram ❒ Hopsättning hos mottagaren ❒ Designat för ytterligare fragmentering 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 4 IP Fragmentation and Reassembly length ID fragflag offset =4000 =x =0 =0 One large datagram becomes several smaller datagrams length ID fragflag offset =1500 =x =1 =0 length ID fragflag offset =1500 =x =1 =1480 length ID fragflag offset =1040 =x =0 =2960 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 5 ICMP: Internet Control Message Protocol used by hosts, routers, ❒ Type Code description gateways to communicate 0 0 echo reply (ping) network-level information 3 0 dest. network unreachable ❍ error reporting: 3 1 dest host unreachable unreachable host, network, 3 2 dest protocol unreachable port, protocol 3 3 dest port unreachable ❍ echo request/reply (used 3 6 dest network unknown by ping) 3 7 dest host unknown network-layer “above” IP: 4 0 source quench (congestion ❒ ❍ ICMP msgs carried in IP control - not used) 8 0 echo request (ping) datagrams 9 0 route advertisement ICMP message: type, code plus ❒ 10 0 router discovery first 8 bytes of IP datagram 11 0 TTL expired causing error 12 0 bad IP header 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 6 2

Skalbarhet ❒ 2 7 + 2 14 + 2 21 olika nätverksadresser i IP ❒ Det blir stora tabeller om varje router ska hitta alla nätverk ❒ Hur får man routing att skala? 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 7 Forwarding algortim ❒ Om direkt kopplad till destinationens nätverk - skicka direkt till destination ❒ annars om destinationen finns i forwarding tabellen - skicka till NextHop routern Network 1 (Ethernet) ❒ annars -skicka till default routern H7 R3 H8 Forwarding tabell R2 H1 H2 H3 NetworkNum NextHop Network 4 (point-to-point) Network 2 (Ethernet) 1 R3 R1 2 R1 R2 H4 3 interface 1 Network 3 (FDDI) 4 interface 0 H5 H6 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 8 DHCP A new node needs an IP-address: ❒ hard-coded by system admin in a file ❒ DHCP: Dynamic Host Configuration Protocol: dynamically get address: “plug-and-play” ❍ host broadcasts “DHCP discover” msg ❍ DHCP server responds with “DHCP offer” msg ❍ host requests IP address: “DHCP request” msg ❍ DHCP server sends address: “DHCP ack” msg ❒ An example on scaling of network administration 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 9 3

Hierarchical Routing Our routing study thus far - idealization ❒ all routers identical ❒ network “flat” … not true in practice scale: with 50 million administrative autonomy destinations: ❒ internet = network of networks ❒ can’t store all dest’s in routing tables! ❒ each network admin may want to control routing in its ❒ routing table exchange own network would swamp links! 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 10 Hierarchical Routing gateway routers ❒ aggregate routers into regions, “autonomous ❒ special routers in AS systems” (AS) ❒ run intra-AS routing protocol with all other ❒ routers in same AS run routers in AS same routing protocol ❒ also responsible for ❍ “intra-AS” routing routing to destinations protocol outside AS ❍ routers in different AS ❍ run inter-AS routing can run different intra- protocol with other AS routing protocol gateway routers 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 11 Intra-AS and Inter-AS routing C.b Gateways: B.a •perform inter-AS A.a routing amongst b c A.c themselves a a C b •perform intra-AS a B routers with other d c routers in their b A AS network layer inter-AS, intra-AS link layer routing in physical layer gateway A.c 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 12 4

Intra-AS and Inter-AS routing Inter-AS routing C.b between B.a A and B A.a Host b c h2 A.c a a C b a B Host d Intra-AS routing c h1 b A within AS B Intra-AS routing within AS A 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 13 Routing in the Internet ❒ The Global Internet consists of Autonomous Systems (AS) interconnected with each other: ❍ Stub AS : small corporation ❍ Multihomed AS : large corporation (no transit) ❍ Transit AS : provider ❒ Two-level routing: ❍ Intra-AS: administrator is responsible for choice ❍ Inter-AS: unique standard 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 14 Intra-AS Routing ❒ Also known as Interior Gateway Protocols (IGP) ❒ Most common IGPs: ❍ RIP: Routing Information Protocol ❍ OSPF: Open Shortest Path First ❍ IGRP: Interior Gateway Routing Protocol (Cisco propr.) 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 15 5

RIP ( Routing Information Protocol) ❒ Distance vector algorithm ❒ Included in BSD-UNIX Distribution in 1982 ❒ Distance metric: # of hops (max = 15 hops) ❍ Can you guess why? ❒ Distance vectors: exchanged every 30 sec via Response Message (also called advertisement ) ❒ Each advertisement: route to up to 25 destination nets ❒ RIP v1 RFC 1058 ❒ RIP v2 RFC 1723 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 16 RIP: Link Failure and Recovery 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 quickly propagates to entire net ❍ poison reverse used to prevent ping-pong loops (infinite distance = 16 hops) 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 17 RIP Table processing ❒ RIP routing tables managed by a pplication-level process called route-d (daemon) ❒ advertisements sent in UDP packets, periodically repeated 2 October 2001 Data Communications, Jonny Pettersson, UmU Network 2 18 6