Network layer transport segment from sending to receiving host - PDF document

Network layer transport segment from sending ❒ to receiving host application on sending side encapsulates transport ❒ network segments into datagrams data link network physical network data link network on rcving side, delivers ❒ data link physical data link Network Layer Overview and IP segments to transport layer physical physical network network layer protocols in every data link ❒ network physical host, router data link physical Router examines header fields ❒ network network in all IP datagrams passing data link data link physical physical through it network application data link transport physical network data link physical Network Layer 4-1 Network Layer 4-2 Interplay between routing and forwarding Key Network-Layer Functions routing algorithm ❒ forwarding: move packets from router’s input to appropriate router output local forwarding table header value output link 0100 3 0101 2 ❒ routing: determine route taken by packets 0111 2 1001 1 from source to dest. value in arriving packet’s header ❍ Routing algorithms 1 0111 2 3 Network Layer 4-3 Network Layer 4-4 Network service model Virtual circuits “source-to-dest path behaves much like telephone circuit” Example services for individual datagrams: ❍ performance-wise ❒ guaranteed delivery ❍ network actions along source-to-dest path ❒ Guaranteed delivery with less than 40 msec delay ❒ call setup, teardown for each call before data can flow Example services for a flow of datagrams: ❒ each packet carries VC identifier (not destination host address) ❒ In-order datagram delivery ❒ every router on source-dest path maintains “state” for ❒ Guaranteed minimum bandwidth to flow each passing connection ❒ link, router resources (bandwidth, buffers) may be ❒ Restrictions on changes in inter-packet allocated to VC spacing Network Layer 4-5 Network Layer 4-6 1

Datagram networks Forwarding table 4 billion possible entries no call setup at network layer ❒ routers: no state about end-to-end connections ❒ Destination Address Range Link Interface ❍ no network-level concept of “connection” packets forwarded using destination host address ❒ 11001000 00010111 00010000 00000000 ❍ packets between same source-dest pair may take different paths through 0 Why is this OK for the Internet? ❒ 11001000 00010111 00010111 11111111 11001000 00010111 00011000 00000000 through 1 application 11001000 00010111 00011000 11111111 application transport transport network 11001000 00010111 00011001 00000000 network 1. Send data 2. Receive data through 2 data link data link 11001000 00010111 00011111 11111111 physical physical otherwise 3 Network Layer 4-7 Network Layer 4-8 Longest prefix matching Datagram or VC network: why? Internet ATM Prefix Match Link Interface data exchange among computers ❒ evolved from telephony ❒ 11001000 00010111 00010 0 ❍ “elastic” service, no strict human conversation: 11001000 00010111 00011000 1 ❒ timing req. 11001000 00010111 00011 2 ❍ strict timing, reliability “smart” end systems ❒ otherwise 3 requirements (computers) ❍ can adapt, perform control, ❍ need for guaranteed Examples error recovery service ❍ simple inside network, “dumb” end systems DA: 11001000 00010111 00010110 10100001 Which interface? ❒ complexity at “edge” ❍ telephones many link types ❒ DA: 11001000 00010111 00011000 10101010 ❍ complexity inside network Which interface? ❍ different characteristics ❍ uniform service difficult Network Layer 4-9 Network Layer 4-10 IP datagram format The Internet Network layer IP protocol version 32 bits total datagram Host, router network layer functions: number length (bytes) header length type of head. ver length (bytes) len service for fragment Transport layer: TCP, UDP “type” of data 16-bit identifier flgs fragmentation/ offset max number time to upper reassembly Internet IP protocol remaining hops Routing protocols live layer checksum •addressing conventions •path selection (decremented at 32 bit source IP address •datagram format •RIP, OSPF, BGP Network each router) •packet handling conventions 32 bit destination IP address upper layer protocol layer forwarding ICMP protocol E.g. timestamp, to deliver payload to Options (if any) table •error reporting record route •router “signaling” how much overhead data taken, specify with TCP? (variable length, list of routers Link layer 20 bytes of TCP typically a TCP ❒ to visit. physical layer or UDP segment) 20 bytes of IP ❒ = 40 bytes + app ❒ layer overhead Network Layer 4-11 Network Layer 4-12 2

IP Fragmentation & Reassembly IP Fragmentation and Reassembly network links have MTU ❒ length ID fragflag offset (max.transfer size) - largest Example =4000 =x =0 =0 possible link-level frame. fragmentation: ❒ 4000 byte ❍ different link types, One large datagram becomes in: one large datagram datagram different MTUs several smaller datagrams out: 3 smaller datagrams large IP datagram divided ❒ MTU = 1500 bytes ❒ length ID fragflag offset (“fragmented”) within net =1500 =x =1 =0 ❍ one datagram becomes 1480 bytes in reassembly several datagrams length ID fragflag offset data field =1500 =x =1 =185 ❍ “reassembled” only at offset = final destination 1480/8 length ID fragflag offset ❍ IP header bits used to =1040 =x =0 =370 identify, order related fragments Network Layer 4-13 Network Layer 4-14 IP Addressing: introduction Subnets ❒ IP address: 32-bit 223.1.1.1 ❒ IP address: 223.1.1.1 identifier for host, 223.1.2.1 223.1.2.1 ❍ subnet part (high 223.1.1.2 223.1.1.2 router interface order bits) 223.1.1.4 223.1.2.9 223.1.1.4 223.1.2.9 ❍ host part (low order ❒ interface: connection 223.1.2.2 223.1.2.2 223.1.1.3 223.1.3.27 223.1.1.3 223.1.3.27 bits) between host/router ❒ What’s a subnet ? LAN and physical link ❍ device interfaces with 223.1.3.2 223.1.3.1 223.1.3.2 223.1.3.1 ❍ router’s typically have same subnet part of multiple interfaces IP address ❍ host may have multiple ❍ can physically reach network consisting of 3 subnets interfaces 223.1.1.1 = 11011111 00000001 00000001 00000001 each other without ❍ IP addresses intervening router 223 1 1 1 associated with each interface Network Layer 4-15 Network Layer 4-16 Subnets Subnets 223.1.1.0/24 223.1.1.2 223.1.2.0/24 How many? 223.1.1.1 223.1.1.4 Recipe 223.1.1.3 ❒ To determine the subnets, detach each 223.1.9.2 223.1.7.0 interface from its host or router, creating islands of 223.1.9.1 223.1.7.1 isolated networks. 223.1.8.1 223.1.8.0 223.1.3.0/24 Each isolated network 223.1.2.6 223.1.3.27 is called a subnet. Subnet mask: /24 223.1.2.1 223.1.2.2 223.1.3.1 223.1.3.2 Network Layer 4-17 Network Layer 4-18 3