1 Interplay between routing and forwarding routing algorithm local - PDF document

Transport layer: Summary Application � principles behind transport layer services: � multiplexing/demultiplexing UDP TCP � reliable data transfer � flow control IP � congestion control � instantiation and implementation in the Internet Link � UDP � TCP Physical 26/9-08 Datakommunikation - Jonny Pettersson, UmU Network Layer Overview: Goals: � network layer services � understand principles � virtual circuit and datagram behind network layer networks services: � what’s inside a router? � forwarding � IP: Internet Protocol � routing (path selection) � IPv4 datagram format � IPv4 addressing � dealing with scale � ICMP � how a router works � (IPv6 – later) � advanced topics: IPv6, � next time multicast � routing algorithms � instantiation and � routing in the Internet implementation in the � broadcast and multicast Internet routing 26/9-08 Datakommunikation - Jonny Pettersson, UmU Network layer functions application transport � transport segments from network data link sending to receiving hosts physical � network layer protocols in network network data link data link every host, router network physical physical data link physical network network three important functions: data link data link physical physical � routing: determine route network network taken by packets from source data link data link to dest. Routing algorithms physical physical network data link � forwarding: move packets physical application from router’s input to transport network network appropriate router output data link network physical data link network data link physical � call setup: some network data link physical physical architectures require router call setup along path before data flows 26/9-08 Datakommunikation - Jonny Pettersson, UmU 1

Interplay between routing and forwarding routing algorithm local forwarding table header value output link 0100 3 0101 2 0111 2 1001 1 value in arriving packet’s header 1 0111 2 3 26/9-08 Datakommunikation - Jonny Pettersson, UmU Network service model Q: What service model for “channel” transporting datagrams from sender to receiver? Example services for a Example services for flow of datagrams: individual datagrams: � in-order datagram � guaranteed delivery delivery � guaranteed delivery � guaranteed minimum with less than 40 msec bandwidth to flow delay � restrictions on changes in inter- packet spacing 26/9-08 Datakommunikation - Jonny Pettersson, UmU Network layer service models: Guarantees ? Network Service Congestion Architecture Model Bandwidth Loss Order Timing feedback none no Internet best effort no no no (inferred via loss) ATM CBR constant yes yes yes no rate congestion ATM VBR guaranteed yes yes yes no rate congestion ATM ABR guaranteed no yes no yes minimum none no ATM UBR yes no no 26/9-08 Datakommunikation - Jonny Pettersson, UmU 2

Hur “förmedlar” man data? � Identifierare i headern � Virtual circuit (connection-oriented) � Datagram (connectionless) � Vad krävs � Unika adresser � Identifiera enskilda portar i en router/switch 26/9-08 Datakommunikation - Jonny Pettersson, UmU Virtual Circuit Switching (VCS) � VC - Virtual circuit � Tre steg � Uppkoppling (call setup) • permanent • via “signaler” � Dataöverföring � Nedkoppling � Varje paket har en VC identifierare � Varje router på vägen lagrar tillstånd för varje koppling � Länk- och routerresurser kan allokerars 26/9-08 Datakommunikation - Jonny Pettersson, UmU Virtual circuits: signaling protocols � used to setup, maintain, teardown VC � used in ATM, frame-relay, X.25 � not used in today’s Internet (~) application application 6. Receive data 5. Data flow begins transport transport 4. Call connected 3. Accept call network network 1. Initiate call data link 2. incoming call data link physical physical 26/9-08 Datakommunikation - Jonny Pettersson, UmU 3

Kännetecken för VCS � Minst 1 RTT fördröjning vid uppkoppling � Liten OH för varje datapaket � Vid fel, riv allt och koppla nytt � Hur vet switchen vägen till destinationen? � När kopplingen är uppe � En väg finns � Meddelanden kommer att skickas vidare � Resursallokering 26/9-08 Datakommunikation - Jonny Pettersson, UmU Datagram networks: the Internet model � Varje paket har info om destinationen, ingen call setup � Routers har inga “tillstånd” � “Forwarding” tabell, uppdateras av bakgrundsprocess � Kännetecken � Paket kan skickas när som helst och var som helst � Sändaren vet inte om paketet kan levereras � Paket hanteras oberoende av varandra � Kan hitta vägar runt problem application application transport transport network network 1. Send data data link 2. Receive data data link physical physical 26/9-08 4 billion Forwarding table possible entries Destination Address Range Link Interface 11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111 11001000 00010111 00011000 00000000 through 1 11001000 00010111 00011000 11111111 11001000 00010111 00011001 00000000 through 2 11001000 00010111 00011111 11111111 otherwise 3 26/9-08 Datakommunikation - Jonny Pettersson, UmU 4

Longest prefix matching Prefix Match Link Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 11001000 00010111 00011 2 otherwise 3 Examples DA: 11001000 00010111 00010110 10100001 Which interface? DA: 11001000 00010111 00011000 10101010 Which interface? 26/9-08 Datakommunikation - Jonny Pettersson, UmU Datagram eller VC nätverk � Contention � Flera paket vill till samma länk samtidigt � Congestion � När köerna i routern är fulla och paket börjar slängas � Datagram � Bryr sig inte, kontroll högre upp � Högt utnyttjande � Smarta ändsystem � VCS � “Hop-by-hop” flödeskontroll � Konservativt � QoS � Dumma ändsystem 26/9-08 Datakommunikation - Jonny Pettersson, UmU Router Architecture Overview Two key router functions: � run routing algorithms/protocol (RIP, OSPF, BGP) � forwarding datagrams from incoming to outgoing link 26/9-08 Datakommunikation - Jonny Pettersson, UmU 5

Input Port Functions Physical layer: bit-level reception Decentralized switching : Data link layer: e.g., Ethernet � given datagram dest., lookup output port see chapter 5 using routing table in input port memory � goal: complete input port processing at ‘line speed’ � queuing: if datagrams arrive faster than forwarding rate into switch fabric 26/9-08 Datakommunikation - Jonny Pettersson, UmU Input Port Queuing � Fabric slower than input ports combined -> queueing may occur at input queues � Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward � queueing delay and loss due to input buffer overflow! 26/9-08 Datakommunikation - Jonny Pettersson, UmU Three types of switching fabrics Via memory Via bus First generation � datagram routers: from input � packet copied port memory by system’s to output (single) CPU port memory � speed limited via a shared by memory bandwidth (2 bus bus crossings � bus per datagram) contention: Via an interconection network Modern routers: switching � overcome bus bandwidth � input port speed limitations processor limited by performs � Advanced design: bus lookup, copy fragmenting datagram into bandwidth into memory in fixed length cells, switch output port cells through the fabric 26/9-08 6

Output Ports � Buffering required when datagrams arrive from fabric faster than the transmission rate � Scheduling discipline chooses among queued datagrams for transmission 26/9-08 Datakommunikation - Jonny Pettersson, UmU Output port queueing � buffering when arrival rate via switch exceeds output line speed � queueing (delay) and loss due to output port buffer overflow! 26/9-08 Datakommunikation - Jonny Pettersson, UmU How much buffering? � RFC 3439 rule of thumb: average buffering equal to “typical” RTT (say 250 msec) times link capacity C � e.g., C = 10 Gps link: 2.5 Gbit buffer � Recent recommendation: with N flows, . buffering equal to RTT C N 26/9-08 Datakommunikation - Jonny Pettersson, UmU 7