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CSE 4213: Computer Networks II

This course covers the principles behind network layer services, such as routing and dealing with scale. It also delves into advanced topics like IPv6 and mobility. The course emphasizes the instantiation and implementation of network layer protocols in the Internet.

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CSE 4213: Computer Networks II

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  1. CSE 4213: Computer Networks II Suprakash Datta datta@cs.yorku.ca Office: CSEB 3043 Phone: 416-736-2100 ext 77875 Course page: http://www.cs.yorku.ca/course/4213 These slides are adapted from Jim Kurose’s slides. COSC 4213 - S.Datta

  2. Chapter 4: Network Layer Chapter goals: • understand principles behind network layer services: • routing (path selection) • dealing with scale • how a router works • advanced topics: IPv6, mobility • instantiation and implementation in the Internet COSC 4213 - S.Datta

  3. transport segment from sending to receiving host on sending side encapsulates segments into datagrams on rcving 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 COSC 4213 - S.Datta

  4. Key Network-Layer Functions • forwarding: move packets from router’s input to appropriate router output • routing: determine route taken by packets from source to dest. • Routing algorithms analogy: • routing: process of planning trip from source to dest • forwarding: process of getting through single interchange COSC 4213 - S.Datta

  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 COSC 4213 - S.Datta

  6. 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 datagrams from sender to rcvr? COSC 4213 - S.Datta

  7. Virtual circuits • Telephone-like switching • Connection establishment • Lots of signaling issues • Resource reservation and call admission possible • QoS easy to implement • Shorter packet headers COSC 4213 - S.Datta

  8. 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 COSC 4213 - S.Datta

  9. Addressing ideas • Systematic address assignment and address aggregation • Routing table compaction • Support for distributed routing algorithm COSC 4213 - S.Datta

  10. 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 COSC 4213 - S.Datta

  11. Longest prefix matching Prefix MatchLink 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? COSC 4213 - S.Datta

  12. Internet 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 evolved from telephony human conversation: strict timing, reliability requirements need for guaranteed service “dumb” end systems telephones complexity inside network Datagram or VC network: why? COSC 4213 - S.Datta

  13. What is a router? What’s inside a router? Next: Router designs COSC 4213 - S.Datta

  14. Router Architecture Overview Two key router functions: • run routing algorithms/protocol (RIP, OSPF, BGP) • forwarding datagrams from incoming to outgoing link COSC 4213 - S.Datta

  15. 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 see chapter 5 COSC 4213 - S.Datta

  16. Three types of switching fabrics COSC 4213 - S.Datta

  17. 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) COSC 4213 - S.Datta

  18. 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) COSC 4213 - S.Datta

  19. 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 COSC 4213 - S.Datta

  20. Output Ports • Buffering required when datagrams arrive from fabric faster than the transmission rate • Scheduling discipline chooses among queued datagrams for transmission COSC 4213 - S.Datta

  21. Output port queueing • buffering when arrival rate via switch exceeds output line speed • queueing (delay) and loss due to output port buffer overflow! COSC 4213 - S.Datta

  22. 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! COSC 4213 - S.Datta

  23. Next: • The TCP/IP network layer COSC 4213 - S.Datta

  24. 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 COSC 4213 - S.Datta

  25. 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 “type” of data 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) IP datagram format how much overhead with TCP? • 20 bytes of TCP • 20 bytes of IP • = 40 bytes + app layer overhead COSC 4213 - S.Datta

  26. 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 COSC 4213 - S.Datta

  27. 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 & Reassembly Example • 4000 byte datagram • MTU = 1500 bytes 1480 bytes in data field offset = 1480/8 COSC 4213 - S.Datta

  28. IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link router’s typically have multiple interfaces host may have multiple interfaces 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 COSC 4213 - S.Datta

  29. 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 LAN 223.1.3.2 223.1.3.1 network consisting of 3 subnets COSC 4213 - S.Datta

  30. Recipe To determine 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 COSC 4213 - S.Datta

  31. 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 COSC 4213 - S.Datta

  32. 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 COSC 4213 - S.Datta

  33. IP addresses: how to get one? Q: 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 as server • “plug-and-play” (more in next chapter) COSC 4213 - S.Datta

  34. 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 COSC 4213 - S.Datta

  35. 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 COSC 4213 - S.Datta

  36. 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 COSC 4213 - S.Datta

  37. 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 COSC 4213 - S.Datta

  38. Next: • NAT • Security • Efficient (re)-use of IP addresses COSC 4213 - S.Datta

  39. 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 COSC 4213 - S.Datta

  40. NAT: Network Address Translation • Motivation: local network uses just one IP address as far as outside word is concerned: • no need to be allocated range of addresses from ISP: - just one IP address is used 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). COSC 4213 - S.Datta

  41. 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 COSC 4213 - S.Datta

  42. 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, 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 COSC 4213 - S.Datta

  43. 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 • 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 COSC 4213 - S.Datta

  44. Keeping the internet running ICMP: Internet Control Message Protocol Next: ICMP COSC 4213 - S.Datta

  45. 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 messages 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 –not used (congestion control) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header COSC 4213 - S.Datta

  46. Traceroute: sample output • eclipse 381 % traceroute cs.umass.edu traceroute to cs.umass.edu (128.119.243.168), 30 hops max, 38 byte packets • 1 gateway-92 (130.63.92.1) 259 ms 0.235 ms 0.191 ms • 2 mosquito.gw.yorku.ca (130.63.31.12) 0.231 ms 0.287 ms 0.201 ms • 3 gladiator.gw.yorku.ca (130.63.27.18) 0.370 ms 0.291 ms 0.325 ms • 4 ORION-YORK-RNE.DIST1-TORO.IP.orion.on.ca (66.97.23.49) 0.372 ms 0.242 ms 0.319 ms • 5 BRDR2-TORO-GE2-2.IP.orion.on.ca (66.97.16.125) 0.922 ms 0.891 ms 0.946 ms • 6 c4-tor01.canet4.net (205.189.32.214) 1.240 ms 1.340 ms 1.191 ms • 7 abilene-chinng.canet4.net (205.189.32.97) 11.477 ms 17.612 ms 18.169 ms • 8 nycmng-chinng.abilene.ucaid.edu (198.32.8.83) 27.826 ms 38.481 ms 27.810 ms • 9 noxgs1-PO-6-0-NoX-NOX.nox.org (192.5.89.9) 33.231 ms 32.847 ms 32.665 ms • 10 nox300gw1-Vl-800-NoX-NOX.nox.org (192.5.89.246) 32.939 ms 32.991 ms 33.015 ms • 11 nox300gw1-PEER-NoX-UMASS-192-5-89-102.nox.org (192.5.89.102) 36.151 ms 35.731 ms 36.346 ms • 12 lgrc-rt-106-8.gw.umass.edu (128.119.2.193) 36.044 ms 35.853 ms 35.656 ms • 13 128.119.3.153 (128.119.3.153) 37.921 ms 37.101 ms 37.004 ms • 14 loki.cs.umass.edu (128.119.243.168) 37.686 ms 37.112 ms 36.635 ms COSC 4213 - S.Datta

  47. 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) 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) When source gets this ICMP, stops. Traceroute and ICMP COSC 4213 - S.Datta

  48. IPv6: the “new” IP • Initial motivation:32-bit address space soon to be completely allocated. • Additional motivation: • header format helps speed processing/forwarding • header changes to facilitate QoS IPv6 datagram format: • fixed-length 40 byte header • no fragmentation allowed COSC 4213 - S.Datta

  49. IPv6 Header Priority: identify priority among datagrams in flow Flow Label: identify datagrams in same “flow.” (concept of“flow” not well defined). Next header: identify upper layer protocol for data COSC 4213 - S.Datta

  50. Other Changes from IPv4 • Checksum:removed entirely to reduce processing time at each hop • Options: allowed, but outside of header, indicated by “Next Header” field • ICMPv6: new version of ICMP • additional message types, e.g. “Packet Too Big” • multicast group management functions COSC 4213 - S.Datta

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