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Network Layer CS 3516 – Computer Networks PowerPoint Presentation
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Network Layer CS 3516 – Computer Networks

Network Layer CS 3516 – Computer Networks

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Network Layer CS 3516 – Computer Networks

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  1. Network LayerCS 3516 – Computer Networks

  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 • Instantiation, implementation in the Internet

  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

  4. 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 everyhost and 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 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

  5. Two Key Network-Layer Functions analogy: • routing: process of planning trip from source to destination • forwarding: process of getting through single interchange • forwarding: move packets from router’s input to appropriate router output • routing: determine route taken by packets from source to destination • routing algorithms

  6. 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

  7. Connection Setup • 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 vsTransport Layer connection service: • network: between two hosts (may also involve intervening routers in case of Virtual Circuits (VCs)) • transport: between two processes

  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 datagrams from sender to receiver?

  9. Example 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

  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

  11. Network Layer Connection and Connection-less Service • Datagram network provides network-layer connectionless service • VC network provides network-layer connection service • Analogous to the transport-layer services, but: • service: host-to-host • no choice: network provides one or the other • implementation: in network core

  12. Call setup, teardown for each call before data can flow Each packet carries VC identifier (not destination host address) Everyrouter 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-destpath behaves much like telephone circuit Performance-wise (predictable service) Network actions along source-to-dest path Virtual Circuits (VCs)

  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

  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 (Forwarding table in northwest router) Routers maintain connection state information!

  15. Used to setup, maintain and teardown VC Used in ATM, frame-relay, X.25 Notused 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

  16. Must do 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. 4 billion possible entries Forwarding Table 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

  18. Longest Prefix Matching 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 DA: 11001000 00010111 00011000 10101010 Which interface?

  19. 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 or VC network: Why?

  20. 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

  21. Router Architecture Overview Two key router functions: • Run routing algorithms/protocol (RIP, OSPF, BGP) • Forwarding datagrams from incoming to outgoing link

  22. Input Port Functions Decentralized switching: • Given datagram destination, 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)

  23. Output Ports • Buffering required when datagrams arrive from fabric faster than the transmission rate • Scheduling discipline chooses among queued datagrams for transmission (More on queueing next slides…)

  24. Output Port Queueing • Buffering when arrival rate via switch exceeds output line speed • Queueing(delay) and loss due to output port buffer overflow!

  25. . RTT C N How Much Buffering? • RFC 3439 rule of thumb: average buffering equal to “typical” RTT (say 250 msec) times link capacity C (so, RTT•C) • e.g., C = 10 Gpslink 2.5 Gbit buffer • Recent recommendation: with N flows, buffering equal to

  26. 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 • Queueingdelay and loss due to input buffer overflow!

  27. 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

  28. 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

  29. 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

  30. 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 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 with TCP? • 20 bytes of TCP • 20 bytes of IP • = 40 bytes + app layer overhead

  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

  32. length =1500 length =1500 length =4000 length =1040 ID =x ID =x ID =x ID =x fragflag =0 fragflag =1 fragflag =1 fragflag =0 offset =0 offset =0 offset =370 offset =185 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

  33. 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

  34. IP address: 32-bit identifier for host, router interface Interface: connection between host/router and physical link routers typically have multiple interfaces hosts typically have one interface IP addresses associated with each interface 223.1.1.2 223.1.2.1 223.1.3.27 223.1.3.1 223.1.3.2 223.1.2.2 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

  35. 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

  36. Recipe To determine 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

  37. 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

  38. host part subnet part 11001000 0001011100010000 00000000 200.23.16.0/23 IP addressing: CIDR CIDR:Classless InterDomainRouting • Subnet portion of address of arbitrary length • Address format: a.b.c.d/x, where x is # bits in subnet portion of address

  39. IP addresses: How to Get One? Q: How does host get IP address? • Hard-codedby system admin in a file • Windows: 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” • (next slide)

  40. DHCP: Dynamic Host Configuration Protocol Goal: allow host to dynamically obtain its IP address from network server when it joins network • Can renew its lease on address in use • Allows reuse of addresses (only hold address while connected/”on”) • Support for mobile users who want to join network (more shortly) • DHCP overview: • Host broadcasts “DHCP discover” msg [optional] • DHCP server responds with “DHCP offer” msg [optional] • Host requests IP address: “DHCP request” msg • DHCP server sends address: “DHCP ack” msg

  41. E B A DHCP Client-Server Scenario 223.1.2.1 DHCP 223.1.1.1 server 223.1.1.2 223.1.2.9 223.1.1.4 223.1.2.2 arriving DHCP client needs address in this network 223.1.1.3 223.1.3.27 223.1.3.2 223.1.3.1

  42. DHCP discover src : 0.0.0.0, 68 dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654 DHCP client-server scenario arriving client DHCP server: 223.1.2.5 DHCP offer src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 654 Lifetime: 3600 secs DHCP request src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddrr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs time DHCP ACK src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs Network Layer

  43. DHCP: more than IP address DHCP can return more than just allocated IP address on subnet: • address of first-hop router for client • name and IP address of DNS sever • network mask (indicating network versus host portion of address)

  44. DHCP UDP IP Eth Phy DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP: Example • Connecting laptop needs its IP address, addr of first-hop router, addr of DNS server use DHCP • DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.1 Ethernet 168.1.1.1 • Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server router (runs DHCP) • Ethernet demux’ed to IP demux’ed, UDP demux’ed to DHCP

  45. DHCP UDP IP Eth Phy DHCP UDP IP Eth Phy DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP DHCP: Example • DCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server • Encapsulation of DHCP server, frame forwarded to client, demux’ing up to DHCP at client • Client now knows its IP address, name and IP address of DSN server, IP address of its first-hop router router (runs DHCP)

  46. DHCP: Wireshark Output (home LAN) reply Message type: Boot Reply (2) Hardware type: Ethernet Hardware address length: 6 Hops: 0 Transaction ID: 0x6b3a11b7 Seconds elapsed: 0 Bootp flags: 0x0000 (Unicast) Client IP address: 192.168.1.101 (192.168.1.101) Your (client) IP address: 0.0.0.0 (0.0.0.0) Next server IP address: 192.168.1.1 (192.168.1.1) Relay agent IP address: 0.0.0.0 (0.0.0.0) Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Server host name not given Boot file name not given Magic cookie: (OK) Option: (t=53,l=1) DHCP Message Type = DHCP ACK Option: (t=54,l=4) Server Identifier = 192.168.1.1 Option: (t=1,l=4) Subnet Mask = 255.255.255.0 Option: (t=3,l=4) Router = 192.168.1.1 Option: (6) Domain Name Server Length: 12; Value: 445747E2445749F244574092; IP Address: 68.87.71.226; IP Address: 68.87.73.242; IP Address: 68.87.64.146 Option: (t=15,l=20) Domain Name = "hsd1.ma.comcast.net." Message type: Boot Request (1) Hardware type: Ethernet Hardware address length: 6 Hops: 0 Transaction ID: 0x6b3a11b7 Seconds elapsed: 0 Bootp flags: 0x0000 (Unicast) Client IP address: 0.0.0.0 (0.0.0.0) Your (client) IP address: 0.0.0.0 (0.0.0.0) Next server IP address: 0.0.0.0 (0.0.0.0) Relay agent IP address: 0.0.0.0 (0.0.0.0) Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Server host name not given Boot file name not given Magic cookie: (OK) Option: (t=53,l=1) DHCP Message Type = DHCP Request Option: (61) Client identifier Length: 7; Value: 010016D323688A; Hardware type: Ethernet Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a) Option: (t=50,l=4) Requested IP Address = 192.168.1.101 Option: (t=12,l=5) Host Name = "nomad" Option: (55) Parameter Request List Length: 11; Value: 010F03062C2E2F1F21F92B 1 = Subnet Mask; 15 = Domain Name 3 = Router; 6 = Domain Name Server 44 = NetBIOS over TCP/IP Name Server …… request

  47. 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

  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) Alldatagramsleaving local network have same single source NAT IP address: 138.76.29.7, different source port numbers

  49. NAT: Network Address Translation • Motivation: local network uses just one IP 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)

  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