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Computer Networks and Communications

Computer Networks and Communications. Lectured by ： Haibo Hu School of Software Engineering, Chongqing University Email ： oram@cqu.edu.cn Cellular Phone: 13608382932 QQ ： 3260457. Chapter 4: Network Layer. Chapter goals: understand principles behind network layer services:

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Computer Networks and Communications

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  1. Computer Networksand Communications Lectured by：Haibo Hu School of Software Engineering, Chongqing University Email：oram@cqu.edu.cn Cellular Phone: 13608382932 QQ：3260457 Network Layer

  2. Chapter 4: Network Layer Chapter goals: • understand principles behind network layer services: • network layer service models(网路层服务模型) • forwarding versus routing (转发和路由) • how a router works (路由器工作原理) • routing (path selection) (路由，选路) • dealing with scale (处理规模) • 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(封装)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 Position: 4.1 Introduction Network layer Overview 封装 解装 Network Layer

  5. Position: 4.1 Introduction Key Network-Layer Functions • forwarding:(转发) move packets from router’s input to appropriate router output • routing:(路由) 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 Network Layer

  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 Position: 4.1 Introduction Interplay(相互作用) between routing and forwarding VC ID Or IP Address Network Layer

  7. Position: 4.1 Introduction Connection setup • 3rd important function in some network architectures: • ATM, frame relay(帧中继), X.25 • Before datagrams flow, two hosts and intervening(中间的)routers establish virtual connection • Routers get involved • Network and transport layer cnctn service: • Network: between two hosts • Transport: between two processes Network Layer

  8. Example services for individual datagrams: guaranteed delivery 确保分组交付 Guaranteed delivery with less than 40 msec delay 确保分组在有限的时间内交付 Example services for a flow of datagrams: In-order datagram delivery Guaranteed minimum bandwidth to flow Restrictions on changes in inter-packet spacing(时延抖动) Position: 4.1 Introduction Network service model Q: What service model for “channel” transporting datagrams from sender to rcvr? Network Layer

  9. Position: 4.1 Introduction 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. Position: 4.2 Virtual circuit and datagram networks Network layer connection and connection-less service • Datagram network provides network-layer connectionless service • VC( virtual circuit, 虚电路)network provides network-layer connection service • Analogous(类似于)to the transport-layer services, but: • Service: host-to-host • No choice: network provides one or the other • Implementation: in the core (but not the edge) 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 “source-to-dest path behaves much like telephone circuit” performance-wise network actions along source-to-dest path Position: 4.2 Virtual circuit and datagram networks Virtual circuits Network Layer

  13. Position: 4.2 Virtual circuit and datagram networks 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

  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 … … … … Position: 4.2 Virtual circuit and datagram networks Forwarding table Forwarding table in northwest router: 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 Position: 4.2 Virtual circuit and datagram networks 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 Position: 4.2 Virtual circuit and datagram networks Datagram networks 1. Send data 2. Receive data Network Layer

  17. Position: 4.2 Virtual circuit and datagram networks Forwarding table 4 billion possible entries Destination Address RangeLink Interface 11001000 00010111 00010000 00000000 A through 0 11001000 00010111 00010111 11111111 11001000 00010111 00011000 00000000 B through 1 11001000 00010111 00011000 11111111 11001000 00010111 00011001 00000000 C through 2 11001000 00010111 00011111 11111111 D otherwise 3 Network Layer

  18. Position: 4.2 Virtual circuit and datagram networks Forwarding table IP Address Range Link Interface 190.23.16.0-190.23.23.255 0 190.23.24.0-190.23.24.255 1 190.23.25.0-190.23.32.255 2 其他 3 2 3 0 1 Network Layer

  19. Position: 4.2 Virtual circuit and datagram networks Longest prefix matching 最长前缀匹配 Prefix Match（前缀匹配）Link 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 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 Position: 4.2 Virtual circuit and datagram networks Datagram or VC network: why? Network Layer

  21. 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

  22. Position: 4.3 What’s inside a router Router Architecture Overview Two key router functions: • run routing algorithms/protocol (RIP, OSPF, BGP) • forwarding datagrams from incoming to outgoing link 交换结构 Network Layer

  23. Position: 4.3 What’s inside a router Input Port Functions Decentralized(分散)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 Network Layer

  24. Position: 4.3 What’s inside a router Three types of switching fabrics Network Layer

  25. Position: 4.3 What’s inside a router Output Ports • Buffering required when datagrams arrive from fabric faster than the transmission rate • Scheduling discipline chooses among queued datagrams for transmission Network Layer

  26. Position: 4.3 What’s inside a router 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

  27. Position: 4.3 What’s inside a router 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

  28. 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

  29. Host, router network layer functions: • ICMP protocol • error reporting • router “signaling” • IGMP protocol • IP protocol • addressing conventions • datagram format • packet handling conventions • Routing protocols • path selection • RIP, OSPF, BGP forwarding table Position: 4.3 What’s inside a router The Internet Network layer Transport layer: TCP, UDP Network layer ARP RARP Link layer physical layer Network Layer

  30. 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

  31. IP datagram format IP protocol version number 32 bits total datagram length (bytes) type of service head. len header length (bytes) 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) how much overhead with TCP? • 20 bytes of TCP • 20 bytes of IP • = 40 bytes + app layer overhead Network Layer

  32. 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 Position: 4.4.1 Datagram format IP Fragmentation & Reassembly fragmentation: in: one large datagram out: 3 smaller datagrams reassembly Network Layer

  33. length =1500 length =4000 length =1040 length =1500 ID =x ID =x ID =x ID =x fragflag =0 fragflag =0 fragflag =1 fragflag =1 offset =0 offset =0 offset =370 offset =185 One large datagram becomes several smaller datagrams Position: 4.4.1 Datagram format IP Fragmentation and Reassembly Example • 4000 byte datagram • MTU = 1500 bytes 1480 bytes in data field offset = 1480/8 Network Layer

  34. 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

  35. IP address: 32-bit identifier for host, 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 223.1.1.2 223.1.2.1 223.1.3.27 223.1.3.1 223.1.3.2 223.1.2.2 Position: 4.4.2 IPv4 addressing 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

  36. 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 Position: 4.4.2 IPv4 addressing Subnets(子网) 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

  37. Recipe(方法,处方) 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 Position: 4.4.2 IPv4 addressing Subnets Subnet mask: /24 Network Layer

  38. How many? Position: 4.4.2 IPv4 addressing 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

  39. Position: 4.4.2 IPv4 addressing IP addressing: CIDR CIDR:Classless InterDomain Routing 无类别域际路由选择，无分类编址 • subnet portion of address of arbitrary length • address format: a.b.c.d/x, where x is # bits in subnet portion of address host part subnet part 11001000 0001011100010000 00000000 200.23.16.0/23 Network Layer

  40. Position: 4.4.2 IPv4 addressing 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” 即插即用型 (more in next chapter) Network Layer

  41. Position: 4.4.2 IPv4 addressing 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

  42. 200.23.16.0/23 200.23.18.0/23 200.23.30.0/23 200.23.20.0/23 . . . . . . Position: 4.4.2 IPv4 addressing 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

  43. 200.23.16.0/23 200.23.18.0/23 200.23.30.0/23 200.23.20.0/23 . . . . . . Position: 4.4.2 IPv4 addressing 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

  44. Position: 4.4.2 IPv4 addressing 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

  45. Position: 4.4.2 IPv4 addressing IP addressing: in practice 中国教育科研网 重庆大学ABC区：202.202.xxx.xxx 主教 行政楼 研究生楼 计算机学院：202.202.5.0/24 Submask=255.255.255.0 254 hosts 自动化学院：202.202.13.192 /26 Submask=255.255.255.192 62 hosts 软件学院： 202.202.68.128 /25 Submask=255.255.255.128 126 hosts Network Layer

  46. X Position: 4.4.2 IPv4 addressing IP addressing: in practice • 通过IP地址和子网掩码可以得知计算机在哪个网络中（网段） • Host 1： • IP Addr. 11001010 11001010 01000100 10000010 202.202. 68. 130 • Submask 11111111 11111111 11111111 10000000 255 255 255 128 • And Oper.11001010 11001010 01000100 10000000 • Subnet 11001010 11001010 01000100 10000000 • Host 2: • IP Addr. 11001010 11001010 01000100 11000010 202.202. 68. 196 • Submask 11111111 11111111 11111111 11000000 255 255 255 192 • And Oper.11001010 11001010 01000100 11000000 Network Layer

  47. Position: 4.4.2 IPv4 addressing 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

  48. Position: 4.4.2 IPv4 addressing 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). Network Layer

  49. Position: 4.4.2 IPv4 addressing 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

  50. 3 1 2 4 S: 10.0.0.1, 3345 D: 128.119.40.186, 80 S: 138.76.29.7, 5001 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 Position: 4.4.2 IPv4 addressing 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

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