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School of Computing Science Simon Fraser University

This chapter explores the principles behind network layer services, network layer service models, forwarding vs. routing, and advanced topics such as IPv6 and mobility. It also delves into the implementation and instantiation of the network layer in the Internet.

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School of Computing Science Simon Fraser University

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  1. School of Computing Science Simon Fraser University CMPT 371: Data Communications and Networking Chapter 4: Network Layer Network Layer

  2. Chapter 4: Network Layer Chapter goals: • 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 • instantiation, implementation in the Internet Network Layer

  3. 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Chapter 4: Network Layer Network Layer

  4. 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 layer Network Layer

  5. network link physical message application transport network link physical segment M M Ht Ht M M Hn Hn Hn Hn Ht Ht Ht Ht M M M M Hl Hl Hl Hl Hn Hn Hn Hn Ht Ht Ht Ht M M M M application transport network link physical Recall from Ch1: Encapsulation source datagram frame destination router Network Layer

  6. Key Network-Layer Functions • routing: determine route taken by packets from source to destination • Routing algorithms • forwarding: move packets from router’s input to appropriate output • Uses forwarding table populated by the routing algorithm Network Layer

  7. 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 Network Layer

  8. Example services for individual datagrams: guaranteed delivery Guaranteed delivery with less than 40 msec delay Example services for a flow of datagrams: In-order datagram delivery Guaranteed minimum bandwidth to flow Restrictions on changes in inter-packet spacing Network service model Q: What service model for “channel” transporting packets from sender to receiver? 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 Network Layer

  10. 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Chapter 4: Network Layer Network Layer

  11. Telecommunication networks Packet-switched networks Circuit-switched networks FDM TDM Datagram Networks Networks with VCs Recall from Ch1: Network Taxonomy Network Layer

  12. Network layer connection and connection-less service • Datagram network • provides network-layer connectionless service • Example: Internet • VC network • provides network-layer connection-oriented service • Examples: ATM (Asynchronous Transfer Mode), frame relay, X.25 Network Layer

  13. Network layer connection and connection-less service (cont’d) • Similar to transport-layer services, but: • Service: host-to-host (not process-to-process) • No choice: network provides one service (not both) • Implementation: in the core (not on end systems) Network Layer

  14. 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 “source-to-dest path behaves much like telephone circuit” performance-wise network actions along source-to-dest path Virtual Circuit Networks Network Layer

  15. 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 a VC number • VC number must be changed on each link • New VC number comes from forwarding table Network Layer

  16. 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 Forwarding table in northwest router: Routers maintain connection state information! Network Layer

  17. 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

  18. 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

  19. Forwarding table 32-bit addr  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

  20. Longest prefix matching Prefix MatchLink Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 otherwise 2 Example DA: 11001000 00010111 00011001 10100001 Which interface? Matches 0 and 1, but 1 with longer prefix. Choose interface 1 Network Layer

  21. Internet data exchanged among computers “elastic” service, no strict timing requirements “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 evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service “dumb” end systems telephones complexity has to be inside network Datagram or VC network: why? Network Layer

  22. 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Chapter 4: Network Layer Network Layer

  23. Router Architecture Overview Two key router functions: • run routing algorithms/protocol (RIP, OSPF, BGP) • forward datagrams from incoming to outgoing link Network Layer

  24. Input Port Functions Decentralized switching: • given datagram dst addr, 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 see chapter 5 Network Layer

  25. Three types of switching fabrics Network Layer

  26. 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

  27. 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

  28. Switching Via An Interconnection Network • To overcome bus bandwidth limitations • Use Crossbar,Banyannetworks, or other interconnection nets • initially developed to connect processors in multiprocessor computers • Cisco 12000: switches Gbps through the interconnection network • Advanced design: fragment datagram into fixed length cells, switch cells through the fabric  faster and simpler switching Network Layer

  29. Output Ports • Buffering required when datagrams arrive from fabric faster than the transmission rate • Scheduling discipline chooses among queued datagrams for transmission Network Layer

  30. 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

  31. 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

  32. 4. 1 Introduction 4.2 Virtual circuit and datagram networks 4.3 What’s inside a router 4.4 IP: Internet Protocol Datagram format IPv4 addressing ICMP IPv6 4.5 Routing algorithms Link state Distance Vector Hierarchical routing 4.6 Routing in the Internet RIP OSPF BGP 4.7 Broadcast and multicast routing Chapter 4: Network Layer Network Layer

  33. Host, router network layer functions: • ICMP protocol • error reporting • router “signaling” • IP protocol • addressing conventions • datagram format • packet handling conventions • Routing protocols • path selection • RIP, OSPF, BGP forwarding table The Internet Network layer Transport layer: TCP, UDP Network layer Link layer physical layer Network Layer

  34. IP datagram format IP protocol version number 32 bits total datagram length (bytes) header length (bytes) type of service head. len ver length for fragmentation/ reassembly fragment offset Provides some QoS flgs 16-bit identifier max number remaining hops (decremented at each router) upper layer time to live Internet 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) how much overhead with TCP? • 20 bytes of TCP • 20 bytes of IP • = 40 bytes + app layer overhead Network Layer

  35. network links have MTU (max. transmission unit) - 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 fragmentation: in: one large datagram out: 3 smaller datagrams reassembly IP Fragmentation & Reassembly Network Layer

  36. length =1500 length =1040 length =1500 length =4000 ID =x ID =x ID =x ID =x fragflag =0 fragflag =0 fragflag =1 fragflag =1 offset =0 offset =185 offset =0 offset =370 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 Network Layer

  37. IP address: 32-bit identifier for each host, router networkinterface Represented in Dotted-decimal notation 223 1 1 1 IP Addressing: introduction 11011111 00000001 00000001 00000001 223.1.1.1 Network Layer

  38. Network interface: connection between host/router and physical link routers typically have multiple interfaces host typically has one interface Unique 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 223.1.1.1 How do we assign IPs? 223.1.2.9 223.1.1.4 223.1.1.3 Divide network into subnets, each has a common ID Network Layer

  39. Subnet is: a group of devices that can reach each other without intervening router identified by high order bits of IP addresses 223.1.1.0/24 223.1.2.0/24 223.1.3.0/24 Subnets 11011111 00000001 00000001 00000001 HostID Subnet ID 223.1.1.0/24 /24: # bits in subnet portion of address, subnet mask Network Layer

  40. How many subnets? 6 subnets Recipe: detach each interface from its host or router, creating isolated networks Each isolated network is a subnet 223.1.1.2 223.1.1.1 223.1.1.4 223.1.1.3 223.1.7.0 223.1.9.2 223.1.9.1 223.1.7.1 223.1.8.1 223.1.8.0 223.1.3.27 223.1.2.6 223.1.2.1 223.1.2.2 223.1.3.1 223.1.3.2 Subnets Network Layer

  41. 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 • Old Classful Addressing: • Subnet length had to be /8 (class A), /16 (class B), or /24 (class C) • Why CIDR? • Finer control over address allocation  reduce waste of addresses • Ex: company with 2000 machines would have to get class B, wasting 63,000+ addresses Network Layer

  42. IP addresses: how to get one? Q: How does host get IP address? • hard-coded by system admin in a file • WIN: 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” (more in next chapter) Network Layer

  43. IP addresses: how to get one? Q: 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 Network Layer

  44. 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

  45. 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

  46. IP addressing: the last word... 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

  47. NAT: Network Address Translation • Motivation: local network uses just oneIP address as far as outside world is concerned: • range of addresses not needed from ISP: just one IP address for all devices • can change addresses of devices in local network without notifying outside world • can change ISP without changing addresses of devices in local network • devices inside local net not explicitly addressable, visible by outside world (a security plus). Network Layer

  48. 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

  49. 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

  50. NAT: Network Address Translation Implementation: NAT router must: • outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #) . . . remote clients/servers will respond using (NAT IP address, new port #) as destination addr. • remember (in NAT translation table)every (source IP address, port #) to (NAT IP address, new port #) translation pair • incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table Network Layer

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