《计算机网络与通讯》课程教学资源（PPT课件讲稿，英文版）Network Layer

IP datagram format IP protocol version number 32 bits total datagram header leng th head. type of length(bytes) (bytes ver length fragment- or type" of data 16-bit identifier Ifgs offset fragmentation/ max number time toupper Internet reassembly remaining hops live ayer checksum (decremented at each router) 32 bit source ip address upper layer protoco 32 bit destination ip address to deliver payload to Options(if any) E. g. timestamp record route data taken, pecify (variable length list of routers pically a TCp to visit or UDP segment) 4: Network Layer 4b-1

4: Network Layer 4b-1 IP datagram format ver length 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier Internet checksum time to live 32 bit source IP address IP protocol version number header length (bytes) max number remaining hops (decremented at each router) for fragmentation/ reassembly total datagram length (bytes) upper layer protocol to deliver payload to head. len type of service “type” of data flgs fragment offset upper layer 32 bit destination IP address Options (if any) E.g. timestamp, record route taken, pecify list of routers to visit

IP Fragmentation reassembly g network links have mTU max transfer size)-largest possible link-level frame o different link types fragmentation different MTUs in: one large datagram o large ip datagram divided out: 3 smaller datagrams Fragmented")within net o one datagram becomes several datagrams reassembly o"reassembled"only at final destination o Ip header bits used to identify, order related fragments 4: Network layer 4b-2

4: Network Layer 4b-2 IP Fragmentation & Reassembly  network links have MTU (max.transfer size) - largest possible link-level frame.  different link types, different MTUs  large IP datagram divided (“fragmented”) within net  one datagram becomes several datagrams  “reassembled” only at final destination  IP header bits used to identify, order related fragments fragmentation: in: one large datagram out: 3 smaller datagrams reassembly

IP Fragmentation and Reassembly length ID fragflag loffse 4000|=X 〓0 One large datagram becomes several smaller datagrams length id fragflag offset 1500= 0 length ID fragflag offset =1500|=x 1480 length ID frago lag offset 1040=X=01=2960 4: Network Layer 4b-3

4: Network Layer 4b-3 IP Fragmentation and Reassembly ID =x offset =0 fragflag =0 length =4000 ID =x offset =0 fragflag =1 length =1500 ID =x offset =1480 fragflag =1 length =1500 ID =x offset =2960 fragflag =0 length =1040 One large datagram becomes several smaller datagrams

ICMP: Internet Control Message protocol o used by hosts, routers gateways to communication Type Code description network-level information 0 0 echo reply(ping 3 dest network unreachable o error reporting: unreachable host, network 3 1 dest host unreachable 3 port, protoco 3 o echo request/reply(used 3 2367 dest protocol unreachable dest port unreachable dest network unknown y ping dest host un known g network-layer above"IP: 4 source quench(congestion o ICMP msas carried in IP control -not used datagrams 8 echo request(ping o ICMP message: type, code plus route advertisement first 8 bytes of ip datagram 10 000000 router dIscovery 11 TTL expired causing error bad ip header 4: Network layer 4b-4

4: Network Layer 4b-4 ICMP: Internet Control Message Protocol  used by hosts, routers, gateways to communication network-level information  error reporting: unreachable host, network, port, protocol  echo request/reply (used by ping)  network-layer “above” IP:  ICMP msgs carried in IP datagrams  ICMP message: type, code plus first 8 bytes of IP datagram causing error Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header

Routing in the Internet o The global Internet consists of autonomous Systems (AS) interconnected with each other o Stub AS: small corporation o Multihomed AS: large corporation(no transit) o Transit AS: provider 口TWo-| evel routing o Intra-AS: administrator is responsible for choice o Inter-AS: unique standard 4: Network layer 4b-5

4: Network Layer 4b-5 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

Internet AS Hierarchy Intra-AS border(exterior gateway)routers C b B a Aa b Ac c a b Inter-AS interior(gateway)routers 4: Network Layer 4b-6

4: Network Layer 4b-6 Internet AS Hierarchy Intra-AS border (exterior gateway) routers Inter-AS interior (gateway) routers

Intra-AS Routing D Also known as Interior Gateway Protocols(IGP 口 Most common IGPs: o RIP: Routing information Protocol o OSPF: Open shortest path first o IGRP: Interior Gateway routing Protocol ( Cisco propr 4: Network Layer 4b-7

4: Network Layer 4b-7 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.)

RIP(Routing Information Protocol) o Distance vector algorithm D Included in BSD-UNIX Distribution in 1982 o Distance metric: t of hops(max 15 hops) o Can you guess why? o Distance vectors: exchanged every 30 sec via Response Message(also called advertisement) D Each advertisement: route to up to 25 destination nets 4: Network layer 4b-8

4: Network Layer 4b-8 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(Routing Information Protocol) A D B Destination Network Next Router Num. of hops to dest wyzx Routing table in d 4: Network layer 4b-9

4: Network Layer 4b-9 RIP (Routing Information Protocol) Destination Network Next Router Num. of hops to dest. w A 2 y B 2 z B 7 x -- 1 …. …. .... w x y z A C D B Routing table in D

RIP: Link Failure and Recovery If no advertisement heard after 180 sec neighbor/link declared dead o routes via neighbor invalidated o new advertisements sent to neighbors o neighbors in turn send out new advertisements(if tables changed) o link failure info quickly propagates to entire net o poison reverse used to prevent ping- pong loops (infinite distance 16 hops) 4: Network Layer 4b-10

4: Network Layer 4b-10 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)