Chapter 4: Network Layer Chapter goals: Network Layer • Understand principles behind network layer services: – network layer service models CS 3516 – Computer Networks CS 3516 Computer Networks – forwarding versus routing – how a router works – routing (path selection) – dealing with scale • Instantiation, implementation in the Internet Network Layer Chapter 4: Network Layer • Transport segment from application transport sending to receiving host network • 4. 1 Introduction • 4.5 Routing algorithms data link • On sending side physical network network • 4.2 Virtual circuit and data link – Link state data link network encapsulates segments physical physical data link physical datagram networks – Distance Vector network into datagrams network data link data link • 4.3 What’s inside a • On rcving side delivers physical – Hierarchical routing physical On rcving side, delivers • 4.6 Routing in the network t k network t k router segments to transport data link data link • 4.4 IP: Internet Internet physical physical network layer data link • Network layer protocols physical – RIP Protocol application network transport – OSPF network data link network – Datagram format in every host and router physical data link network data link physical – BGP • Router examines header data link physical – IPv4 addressing • 4.7 Broadcast and physical – ICMP fields in all IP datagrams multicast routing – IPv6 passing through it Interplay Between Routing and Forwarding Two Key Network-Layer Functions • forwarding: move routing algorithm analogy: packets from local forwarding table • routing: process of router’s input to header value output link 0100 3 planning trip from appropriate router 0101 2 0111 2 source to destination source to destination 1001 1 output • routing: determine • forwarding: process value in arriving packet’s header of getting through route taken by 1 0111 single interchange packets from source 2 3 to destination – routing algorithms 1
Network Service Model Connection Setup • 3 rd important function in some network architectures: Q: What service model for “channel” transporting datagrams from sender to receiver? – ATM, frame relay, X.25 • Before datagrams flow, two end hosts and intervening Example services for Example services for a flow individual datagrams: of datagrams: routers establish virtual connection • Guaranteed delivery • Guaranteed delivery • In-order datagram • In order datagram – routers get involved l • Guaranteed delivery • Network vs Transport Layer connection service: delivery • Guaranteed minimum with less than 40 – network: between two hosts (may also involve msec delay bandwidth to flow intervening routers in case of Virtual Circuits • Restrictions on changes (VCs)) in inter-packet spacing – transport: between two processes Example Network Layer Service Chapter 4: Network Layer Models • 4. 1 Introduction • 4.5 Routing algorithms Guarantees ? Network Service Congestion • 4.2 Virtual circuit and – Link state Architecture Model Bandwidth Loss Order Timing feedback datagram networks – Distance Vector • 4.3 What’s inside a Internet best effort none no no no – Hierarchical routing no (inferred • 4.6 Routing in the via loss) via loss) router ATM CBR constant yes yes yes no • 4.4 IP: Internet Internet rate congestion – RIP ATM VBR guaranteed yes yes yes no Protocol rate congestion – OSPF – Datagram format ATM ABR guaranteed no yes no yes – BGP – IPv4 addressing minimum • 4.7 Broadcast and ATM UBR none no yes no – ICMP no multicast routing – IPv6 Network Layer Connection and Virtual Circuits (VCs) Connection-less Service source-to-dest path behaves much like telephone • Datagram network provides network-layer circuit connectionless service – Performance-wise (predictable service) • VC network provides network-layer – Network actions along source-to-dest path connection service connection service • Call setup, teardown for each call before data can flow • Analogous to the transport-layer services, • Each packet carries VC identifier (not destination host but: address) • Every router on source-dest path maintains “state” for – service: host-to-host each passing connection • Link, router resources (bandwidth, buffers) may be – no choice: network provides one or the other allocated to VC (dedicated resources = predictable service) – implementation: in network core 2
Forwarding Table VC Implementation VC number A VC consists of: 22 32 12 1 3 1. Path from source to destination 2 (Forwarding table in 2. VC numbers, one number for each link along interface northwest router) path number 3. Entries in forwarding tables in routers along 3 Entries in forwarding tables in routers along I Incoming interface Incoming VC # Outgoing interface Outgoing VC # i i f I i VC # O i i f O i VC # path • 1 12 3 22 Packet belonging to VC carries VC number 2 63 1 18 (rather than dest address) 3 7 2 17 • 1 97 3 87 VC number can be changed on each link. … … … … – New VC number comes from forwarding Routers maintain connection state information! table Datagram Networks Virtual Circuits: Signaling Protocols • Must do call setup at network layer • Routers: no state about end-to-end connections • Used to setup, maintain and teardown VC – No network-level concept of “connection” • Used in ATM, frame-relay, X.25 • Packets forwarded using destination host address • Not used in today’s Internet – Packets between same source-dest pair may take different paths application application 6. Receive data 5. Data flow begins transport application transport application 4. Call connected 3. Accept call network transport network transport 1. Initiate call data link 2. incoming call network 1. Send data 2. Receive data data link network physical data link physical data link physical physical Network Layer 4-16 4 billion Forwarding Table possible entries Longest Prefix Matching Destination Address Range Link Interface Prefix Match Link Interface 11001000 00010111 00010 0 11001000 00010111 00010000 00000000 11001000 00010111 00011000 1 through 0 11001000 00010111 00011 2 11001000 00010111 00010111 11111111 otherwise 3 11001000 00010111 00011000 00000000 through 1 Examples 11001000 00010111 00011000 11111111 DA: 11001000 00010111 00010110 10100001 Which interface? 11001000 00010111 00011001 00000000 through 2 DA: 11001000 00010111 00011000 10101010 Which interface? 11001000 00010111 00011111 11111111 otherwise 3 3
Datagram or VC network: Why? Chapter 4: Network Layer Internet (datagram) ATM (VC) • 4. 1 Introduction • 4.5 Routing algorithms • Data exchange among • Evolved from telephony • 4.2 Virtual circuit and computers • Human conversation: – Link state – “Elastic” service, no datagram networks – Distance Vector – strict timing, reliability strict timing req. • 4.3 What’s inside a requirements – Hierarchical routing • “Smart” end systems • 4.6 Routing in the Smart end systems – need for guaranteed router (computers) service • 4.4 IP: Internet Internet – Can adapt, perform • “Dumb” end systems – RIP control, error recovery Protocol – telephones – OSPF – Simple inside network, – Datagram format – complexity inside complexity at “edge” – BGP – IPv4 addressing network • 4.7 Broadcast and • Many link types – ICMP – Different characteristics multicast routing – IPv6 – Uniform service difficult Router Architecture Overview Input Port Functions Two key router functions: • Run routing algorithms/protocol (RIP, OSPF, BGP) • Forwarding datagrams from incoming to outgoing link Physical layer: bit-level reception b l l Decentralized switching : Data link layer: • Given datagram destination, lookup e.g., Ethernet output port using forwarding table in (see chapter 5) input port memory • Goal: complete input port processing at ‘line speed’ • Queuing: if datagrams arrive faster than forwarding rate into switch fabric Output Ports Output Port Queueing • Buffering required when datagrams arrive from fabric faster than the transmission rate • Buffering when arrival rate via switch exceeds • Scheduling discipline chooses among queued output line speed datagrams for transmission • Queueing (delay) and loss due to output port (More on queueing next slides…) buffer overflow! 4
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