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Network Layer

Network Layer. Gordon College Adapted from Computer Networking: A Top Down Approach. Network Layer. Goals of Learning: understand principles behind network layer services: network layer service models forwarding versus routing how a router works routing (path selection)

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Network Layer

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  1. Network Layer Gordon College Adapted from Computer Networking: A Top Down Approach Network Layer

  2. Network Layer Goals of Learning: • understand principles behind network layer services: • network layer service models • forwarding versus routing • how a router works • routing (path selection) • dealing with scale • advanced topics: IPv6, mobility • implementation in the Internet Network Layer

  3. transport segment from sending to receiving host on sending side encapsulates segments into datagrams on receiving side, delivers segments to transport layer network layer protocols in every host, router Router examines header fields in all IP datagrams passing through it network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical network data link physical application transport network data link physical application transport network data link physical Network header TCP header datagram HTTP header Data segment message Network layer Network Layer

  4. Two Key Network-Layer Functions • forwarding: move packets from router’s input to appropriate router output >>thru a single router • routing: determine route taken by packets from source to dest. >> Overall route needed to arrive at final destination • routing algorithms analogy: • forwarding: process of getting through single intersection • routing: process of planning trip from source to dest Network Layer

  5. routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 value in arriving packet’s header 1 0111 2 3 Interplay between routing and forwarding What determines the values in a forwarding table? ROUTING ALGORITHM - Centralized - Decentralized * Routing messages passed among routers simplified Network Layer

  6. Connection setup • Another (3rd) important function in some network architectures: • ATM, frame relay, X.25 • before datagrams flow, two end hosts and intervening routers establish virtual connection • routers get involved • network vs transport layer connection service: • network: between two hosts (may also involve intervening routers in case of VCs) • transport: between two processes Network Layer

  7. Possible services for individual datagrams: guaranteed delivery guaranteed delivery with less than 40 msec delay Possible services for a flow of datagrams: in-order datagram delivery guaranteed minimum bandwidth to flow restrictions on changes in inter-packet spacing (guaranteed maximum jitter) Network service model Characteristics of end-to-end transport of data between one edge of the network and the other: Network Layer

  8. Network layer service models: Guarantees ? Network Architecture Internet ATM ATM ATM ATM Service Model best effort CBR VBR ABR UBR Congestion feedback no (inferred via loss) no congestion no congestion yes no Bandwidth none constant rate guaranteed rate guaranteed minimum none Loss no yes yes no no Order no yes yes yes yes Timing no yes yes no no Network Layer

  9. Network layer service models: Guarantees ? Network Architecture Internet ATM ATM ATM ATM Service Model best effort CBR VBR ABR UBR Congestion feedback no (inferred via loss) no congestion no congestion yes no Bandwidth none constant rate guaranteed rate guaranteed minimum none Loss no yes yes no no Order no yes yes yes yes Timing no yes yes no no Best Effort Service Network Layer

  10. Network layer connection and connection-less service • datagram network provides network-layer connectionless service • VC network provides network-layer connection service • Details: • service: host-to-host • no choice: network provides one or the other • implementation: in network core internet Network Layer

  11. call setup for each call before data can flow each packet carries VC identifier (not destination host address) every router on source-dest path maintains “state” for each passing connection link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service) “source-to-dest path behaves much like telephone circuit” performance-wise network actions along source-to-dest path Virtual circuits (VC) Network Layer

  12. call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host address) every router on source-dest path maintains “state” for each passing connection link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service) “source-to-dest path behaves much like telephone circuit” performance-wise network actions along source-to-dest path Virtual circuits (VC) 3 Phases Setup Data Transfer VC teardown Network Layer

  13. VC implementation a VC consists of: • path from source to destination • VC numbers, one number for each link along path • entries in forwarding tables in routers along path • packet belonging to VC carries VC number (rather than dest address) • VC number can be changed on each link. • New VC number comes from forwarding table Network Layer

  14. VC number 22 32 12 3 1 2 interface 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 … … … … Forwarding table ALPHA Forwarding table in Router ALPHA: Routers maintain connection state information! Network Layer

  15. used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today’s Internet application transport network data link physical application transport network data link physical Virtual circuits: signaling protocols 6. Receive data 5. Data flow begins 4. Call connected 3. Accept call 1. Initiate call 2. incoming call Network Layer

  16. no call setup at network layer routers: no state about end-to-end connections no network-level concept of “connection” packets forwarded using destination host address packets between same source-dest pair may take different paths application transport network data link physical application transport network data link physical Datagram networks 1. Send data 2. Receive data Network Layer

  17. no call setup at network layer routers: no state about end-to-end connections no network-level concept of “connection” packets forwarded using destination host address packets between same source-dest pair may take different paths application transport network data link physical application transport network data link physical Datagram networks No state info maintained 1. Send data 2. Receive data Network Layer

  18. Forwarding table 4 billion possible entries Destination Address RangeLink 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 Network Layer

  19. Longest prefix matching Each interface responsible for large blocks of contiguous dest. addresses Prefix MatchLink Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 11001000 00010111 00011 2 otherwise 3 Examples Which interface? DA: 11001000 00010111 00010110 10100001 Which interface? DA: 11001000 00010111 00011000 10101010 Network Layer

  20. Internet (datagram) data exchange among computers “elastic” service, no strict timing req. “smart” end systems (computers) can adapt, perform control, error recovery simple inside network, complexity at “edge” many link types different characteristics uniform service difficult ATM (VC) evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service “dumb” end systems telephones complexity inside network Datagram vs. VC network Network Layer

  21. Inside the Router: Architecture Overview Two key router functions: • run routing algorithms/protocol (RIP, OSPF, BGP) • forwardingdatagrams from incoming to outgoing link Cisco 12000 Series Network Layer

  22. Input Port Functions Decentralized switching: • given datagram dest., lookup output port using forwarding table in input port memory • goal: complete input port processing at ‘line speed’ • queuing: if datagrams arrive faster than forwarding rate into switch fabric Physical layer: bit-level reception Data link layer: e.g., Ethernet Network Layer

  23. Three types of switching fabrics Network Layer

  24. Memory Input Port Output Port System Bus Switching Via Memory First generation routers: • traditional computers with switching under direct control of CPU • packet copied to system’s memory • speed limited by memory bandwidth (2 bus crossings per datagram) Network Layer

  25. Switching Via a Bus • datagram from input port memory to output port memory via a shared bus • bus contention: switching speed limited by bus bandwidth • 1 Gbps bus, Cisco 1900: sufficient speed for access and enterprise routers (not regional or backbone) Network Layer

  26. Switching Via An Interconnection Network • overcome bus bandwidth limitations • Banyan networks, other interconnection nets initially developed to connect processors in multiprocessor • Advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric. • Cisco 12000: switches Gbps through the interconnection network • Up to 40Gbps for each slot Cisco 12000 Series Network Layer

  27. Output Ports Data link layer: e.g., Ethernet Physical layer: bit-level reception • Buffering required when datagrams arrive from fabric faster than the transmission rate • Scheduling discipline chooses among queued datagrams for transmission • FCFS (first come - first serve) • WFQ (weighted fair queuing) How about QoS? Network Layer

  28. Output port queueing • buffering when arrival rate via switch exceeds output line speed • queueing (delay) and loss due to output port buffer overflow! Network Layer

  29. 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! Network Layer

  30. IP protocol version number 32 bits total datagram length (bytes) header length (bytes) type of service head. len ver length fragmentation/ reassembly (see next slide) fragment offset “type” of data flgs 16-bit identifier max number remaining hops (decremented at each router) upper layer time to live header checksum 32 bit source IP address 32 bit destination IP address upper layer protocol to deliver payload to E.g. timestamp, record route taken, specify list of routers to visit. Options (if any) data (variable length, typically a TCP or UDP segment) IP datagram format how much overhead including TCP? • 20 bytes of TCP • 20 bytes of IP • = 40 bytes + app layer overhead Network Layer

  31. 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 IP Fragmentation & Reassembly fragmentation: in: one large datagram out: 3 smaller datagrams reassembly Network Layer

  32. length =1500 length =1040 length =4000 length =1500 ID =x ID =x ID =x ID =x fragflag =0 fragflag =1 fragflag =0 fragflag =1 offset =0 offset =370 offset =185 offset =0 One large datagram becomes several smaller datagrams IP Fragmentation and Reassembly Example • 4000 byte datagram • MTU = 1500 bytes 1480 bytes in data field offset = 1480/8 0 185*8 370*8 1480 2960 Reassembled content Network Layer

  33. IP address: 32-bit identifier for host and router interface interface: connection between host/router and physical link router’s typically have multiple interfaces host typically has one interface IP addresses associated with each interface 223.1.1.2 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 IP Addressing: introduction 223.1.1.1 223.1.2.9 223.1.1.4 223.1.1.3 223.1.1.1 = 11011111 00000001 00000001 00000001 223 1 1 1 Network Layer

  34. IP address: subnet part (high order bits) host part (low order bits) What’s a subnet ? device interfaces with same subnet part of IP address can physically reach each other without intervening router Subnets 223.1.1.1 223.1.2.1 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 223.1.1.3 223.1.3.27 subnet 223.1.3.2 223.1.3.1 network consisting of 3 subnets Network Layer

  35. Determining the subnets detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet. 223.1.1.0/24 223.1.2.0/24 223.1.3.0/24 Subnets Subnet mask: /24 Network Layer

  36. How many? Subnets 223.1.1.2 223.1.1.1 223.1.1.4 223.1.1.3 223.1.7.0 223.1.9.2 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.2.6 223.1.3.27 223.1.2.1 223.1.2.2 223.1.3.1 223.1.3.2 Network Layer

  37. host part subnet part 11001000 0001011100010000 00000000 200.23.16.0/23 IP addressing: CIDR CIDR:Classless InterDomain Routing • subnet portion of address of arbitrary length • address format: a.b.c.d/x, where x is # bits in subnet portion of address Network Layer

  38. IP addresses: how to get one? How does host get IP address? • hard-coded by system admin in a file • Wintel: control-panel->network->configuration->tcp/ip->properties • UNIX: /etc/rc.config • DHCP:Dynamic Host Configuration Protocol: dynamically get address from a server • “plug-and-play” Network Layer

  39. IP addresses: how to get one? How does network get subnet part of IP addr? A:gets allocated portion of its provider ISP’s address space ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20 Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23 ... ….. …. …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23 Each company has 512 addresses available Network Layer

  40. 200.23.16.0/23 200.23.18.0/23 200.23.30.0/23 200.23.20.0/23 . . . . . . Hierarchical addressing: route aggregation Hierarchical addressing allows efficient advertisement of routing information: Organization 0 Organization 1 “Send me anything with addresses beginning 200.23.16.0/20” Organization 2 Fly-By-Night-ISP Internet Organization 7 “Send me anything with addresses beginning 199.31.0.0/16” ISPs-R-Us Network Layer

  41. 200.23.16.0/23 200.23.18.0/23 200.23.30.0/23 200.23.20.0/23 . . . . . . Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 Organization 0 “Send me anything with addresses beginning 200.23.16.0/20” Organization 2 Fly-By-Night-ISP Internet Organization 7 “Send me anything with addresses beginning 199.31.0.0/16 or 200.23.18.0/23” ISPs-R-Us Organization 1 Network Layer

  42. IP address admin. Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers • allocates addresses • manages DNS • assigns domain names, resolves disputes Network Layer

  43. NAT: Network Address Translation rest of Internet local network (e.g., home network) 10.0.0/24 10.0.0.1 10.0.0.4 10.0.0.2 138.76.29.7 10.0.0.3 Datagrams with source or destination in this network have 10.0.0/24 address for source, destination (as usual) All datagrams leaving local network have same single source NAT IP address: 138.76.29.7, different source port numbers Network Layer

  44. NAT: Network Address Translation • Motivation: only one IP address seen by outside world: • only a few IP addresses needed from ISP (money) • can change addresses of devices in local network without notifying outside world (admin) • can change ISP without changing addresses of devices in local network (admin) • devices inside local net not explicitly addressable, visible by outside world (security) Network Layer

  45. 2 4 1 3 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 S: 10.0.0.1, 3345 D: 128.119.40.186, 80 1: host 10.0.0.1 sends datagram to 128.119.40.186, 80 2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table S: 128.119.40.186, 80 D: 10.0.0.1, 3345 S: 128.119.40.186, 80 D: 138.76.29.7, 5001 NAT: Network Address Translation NAT translation table WAN side addr LAN side addr 138.76.29.7, 5001 10.0.0.1, 3345 …… …… 10.0.0.1 10.0.0.4 10.0.0.2 138.76.29.7 10.0.0.3 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345 3: Reply arrives dest. address: 138.76.29.7, 5001 Network Layer

  46. NAT: Network Address Translation • 16-bit port-number field: • 60,000 simultaneous connections with a single LAN-side address! • NAT is controversial: • routers should only process up to layer 3 “Hey, you’re messing with the port number” • violates end-to-end argument • NAT possibility must be taken into account by app designers, eg, P2P applications • address shortage should instead be solved by IPv6 Network Layer

  47. used by hosts & routers to communicate 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 ICMP: Internet Control Message Protocol TypeCodedescription 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 Network Layer

  48. Source sends series of UDP segments to dest First has TTL =1 Second has TTL=2, etc. Unlikely port number When nth datagram arrives to nth router: Router discards datagram And sends to source an ICMP message (type 11, code 0 - TTL expired) Message includes name of router& IP address When ICMP message arrives, source calculates RTT Traceroute does this 3 times Stopping criterion UDP segment eventually arrives at destination host Destination returns ICMP “host unreachable” packet (type 3, code 3) [bad port] When source gets this ICMP, stops. Traceroutevia ICMP Network Layer

  49. routing algorithm local forwarding table header value output link 0100 0101 0111 1001 3 2 2 1 value in arriving packet’s header 1 0111 2 3 Interplay between routing, forwarding What determines the values in a forwarding table? ROUTING ALGORITHM - Centralized - Decentralized * Routing messages passed among routers Network Layer

  50. 5 3 5 2 2 1 3 1 2 1 x z w u y v Graph abstraction models network router interaction Graph: G = (N,E) N = set of routers = { u, v, w, x, y, z } E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) } Network Layer

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