Chapter 4: Network Layer Chapter goals: understand principles - PDF document

Chapter 4: Network Layer Chapter goals:  understand principles behind network layer services: services:  network layer service models  forwarding versus routing  how a router works  addressing and routing (path selection)  dealing with scale  advanced topics: IPv6, multicast d d t i IP 6 lti t  instantiation, implementation in the Internet 11/4/2013 Network Layer (SSL) 4-1 Chapter 4: Network Layer  4.5 Routing algorithms  4. 1 Introduction  Link state  4.2 Virtual circuit and  Distance Vector datagram networks datagram networks  Hierarchical routing  4.3 What’s inside a  4.6 Routing in the router Internet  4.4 IP: Internet  RIP Protocol  OSPF  Datagram format  BGP  IPv4 addressing  IPv4 addressing  4.7 Broadcast and  ICMP multicast routing  IPv6 11/4/2013 Network Layer (SSL) 4-2 1

Network layer  delivers segments from sending to receiving host application transport network  sender encapsulates segments data link physical into datagrams network network data link data link data link data link network  Receiver de-encapsulates and physical physical data link physical delivers segments to network network data link data link transport layer physical physical  network layer in every host, network network data link data link every router physical physical network data link physical  router examines IP header application transport network field in every passing fi ld i i network network data link d li k network data link physical network data link datagram (exception: routers physical data link physical physical running MPLS) 11/4/2013 Network Layer (SSL) 4-3 Key Network-Layer Functions  forwarding: move a packet from router’s input interface to an appropriate output input interface to an appropriate output interface  routing: determine route taken by packets from source to destination  global address space and a packet format  global address space and a packet format  routing protocols (intra-network and inter- network ) 11/4/2013 Network Layer (SSL) 4-4 2

Interplay between routing and forwarding  A routing protocol is a routing protocols distributed algorithm  Recent development: a Software Defined local forwarding table header value output link header value output link Network ( SDN ) uses a N t k ( SDN ) 0100 3 central controller to 0101 2 compute routes 0111 2 1001 1 value in arriving packet’s header 1 0111 2 3 11/4/2013 Network Layer (SSL) 4-5 Virtual circuit networks need 3 rd function  Before datagrams can flow, end hosts and routers between them establish a virtual circuit circuit  Routers maintain state info  Following networks designed initially to compete with IP: ATM, frame relay, X.25  Designed more recently to enhance IP: MPLS  Virtual circuits of these networks serve as virtual links in Internet 11/4/2013 Network Layer (SSL) 4-6 3

Network layer service models Q: What service model for delivering datagrams from sender to receiver? Desired services for a Desired services for a Desired services for Desired services for flow of datagrams individual datagrams (connection-oriented): (connectionless):  reliable, in-order  reliable delivery datagram delivery  guaranteed delivery in  guaranteed minimum less than some bound, bandwidth to flow bandwidth to flow for example, 40 msec f l 40 delay  bound on fluctuations in inter-packet spacing (jitter) 11/4/2013 Network Layer (SSL) 4-7 Network layer service models: Guarantees ? Network Service Congestion Loss Order Timing Architecture Model Bandwidth feedback no no Internet best effort none 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 g no no y yes yes y minimum ATM UBR none no no yes no 11/4/2013 Network Layer (SSL) 4-8 4

Chapter 4: Network Layer  4. 1 Introduction  4.5 Routing algorithms  4 2 Virtual circuit and  4.2 Virtual circuit and  Link state  Link state datagram networks  Distance Vector  Hierarchical routing  4.3 What’s inside a  4.6 Routing in the router Internet  4.4 IP: Internet  RIP Protocol  OSPF  OSPF  Datagram format D t f t  BGP  IPv4 addressing  4.7 Broadcast and  ICMP multicast routing  IPv6 11/4/2013 Network Layer (SSL) 4-9 Virtual circuits: signaling protocols  used to set up, maintain, tear down VC  not used in today’s Internet (but may be used underneath the IP layer to provide a virtual link) underneath the IP layer to provide a virtual link) application application 6. Receive data transport 5. Data flow begins transport network 4. Call connected 3. Accept call network data link 1. Initiate call 2. incoming call data link data link physical h l physical 11/4/2013 Network Layer (SSL) 4-10 5

Virtual circuit (VC)  call setup, teardown for each call before data can flow  each packet carries a VC identifier which  each packet carries a VC identifier which  is fixed length and short  only needs to be unique for a link  every router on source-dest path maintains state information for each passing VC  incoming and outgoing VC identifiers, i i d t i VC id tifi  resources allocated to VC (bandwidth, buffers) 11/4/2013 Network Layer (SSL) 4-11 VC Forwarding table VC number 22 32 12 3 1 2 Forwarding table in Forwarding table in interface interface northwest router: number Incoming interface Incoming VC # Outgoing interface Outgoing VC # 1 12 3 22 2 63 1 18 3 7 2 17 1 97 3 87 1 97 3 87 … … … … Forwarding is fast because short fixed-length VC numbers are used vs. IP forwarding table with variable-length prefixes 11/4/2013 Network Layer (SSL) 4-12 6

Datagram networks  no network-level concept of “connection”  each packet forwarded independently using destination host address  packets between same source-dest pair may take  packets between same source dest pair may take different paths application application transport transport network network t k data link 1. Send data 2. Receive data data link physical physical 11/4/2013 Network Layer (SSL) 4-13 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 00011000 00000000 11001000 00010111 00011000 00000000 through 2 11001000 00010111 00011111 11111111 otherwise 3 11/4/2013 Network Layer (SSL) 4-14 7

Longest prefix match Prefix Match Link Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 11001000 00010111 00011 2 otherwise 3 Examples Which interface? DA: 11001000 00010111 00010110 10100001 DA: 11001000 00010111 00011000 10101010 Which interface? A forwarding table in an Internet core router has about 300,000 IP prefixes 11/4/2013 Network Layer (SSL) 4-15 Origins of datagram and VC Internet (datagram) ATM (VC)  data exchange between  evolved from telephony computers  human conversation:  “elastic” service, no strict “ l ti ” i t i t  strict timing, reliability timing requirement requirements  many link types  need for guaranteed  different characteristics services  uniform service difficult  “dumb” end systems  “smart” end systems  telephones (computers) ( p )  complexity inside  complexity inside  can adapt, perform network control, error recovery  simplicity inside network, complexity at “edge” 11/4/2013 Network Layer (SSL) 4-16 8

Chapter 4: Network Layer  4. 1 Introduction  4.5 Routing algorithms  4 2 Virtual circuit and  4.2 Virtual circuit and  Link state  Link state datagram networks  Distance Vector  Hierarchical routing  4.3 What’s inside a  4.6 Routing in the router Internet  4.4 IP: Internet  RIP Protocol  OSPF  OSPF  Datagram format D t f t  BGP  IPv4 addressing  4.7 Broadcast and  ICMP multicast routing  IPv6 11/4/2013 Network Layer (SSL) 4-17 Router architecture overview Two key router functions:  run routing protocols (RIP, OSPF, BGP)  forward datagrams from incoming to outgoing link g g g g forwarding tables routing, management computed,then pushed to routing control plane (software) input ports processor forwarding data plane (hardware) (hardware) hi h high-speed d switching fabric router input ports router output ports 4-18 Network Layer 9