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Network Layer Lecture 15 Imran Ahmed University of Management & Technology

Network Layer Lecture 15 Imran Ahmed University of Management & Technology. Agenda. Introduction & Network layer functions Routing principles Hierarchical routing The Internet protocol (IP) Routing in the Internet What’s inside a router.

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Network Layer Lecture 15 Imran Ahmed University of Management & Technology

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  1. Network Layer Lecture 15 Imran Ahmed University of Management & Technology

  2. Agenda • Introduction & Network layer functions • Routing principles • Hierarchical routing • The Internet protocol (IP) • Routing in the Internet • What’s inside a router

  3. 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

  4. 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

  5. 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

  6. IP Addressing • must include full address in each packet • addresses must be unique for entire network 10000000 00001010 00000010 00011101 128.10.2.29 IP Address : 32-bit identifier for host/routerinterface (decimal notation)

  7. 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 223.1.1.2 223.1.2.1 223.1.3.27 223.1.3.1 223.1.3.2 223.1.2.2 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

  8. IP Addressing Token ring 192.5.64.3 Q: Every host has one unique IP-address (true/false)? A: False: one address per interface, not one address per host e.g., a host with multiple connections, routers • host may also have multiple interfaces • IP addresses associated with interface, not host • what about routers? Ethernet 128.10.4.92 A router’s typically have multiple interfaces (so multiple IP addresses)

  9. IP Address Classes “class-full” addressing: Bit position: 0 1 2 3 8 16 31 1.0.0.0 to 126.255.255.255 Class A 0 Net ID Host ID 128.0.0.0 to 191.255.255.255 Class B 1 0 Net ID Host ID 192.0.0.0 to 223.255.255.255 Class C 1 1 0 Net ID Host ID 224.0.0.0 to 239.255.255.255 Class D 1 1 1 0 Multicast address Class E Reserved for future use, experiment 1 1 1 1 Reserved for experiments Determining the class: if first bit is 0 then class A else if second bit 0 then class B else if ...

  10. Special Addresses Q: How many different IP addresses can be formed within a class A: Not all possible IP addresses (defined by the number of bits for the class) are used to identify a specific interface: NetId HostId Special Address * All 0’s Network address * All 1’s Direct broadcast address All 0’s * Specific host on this network All 0’s All 0’s This host on this network All 1’s All 1’s Limited broadcast address 127 any Loopback address

  11. 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

  12. 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

  13. 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

  14. 10101000 11110000 00010100 00000000 00000000 Subnet Masking IP address: 10101000 11110000 00010100 00100101 Mask: 11111111 11111111 11111111 00000000 Applying Mask:

  15. Example Subnet Masks • host’s address is 140.134.6.5 • subnet mask is 255.255.0.0 • host address is 140.134.6.5 • subnet mask is 255.255.255.0 • host address is 140.134.6.5 • subnet mask is 255.255.255.192 No subnetting Subnetting with 8 bits Subnetting with 10 bits

  16. 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

  17. IP addresses: how to get one? Q: How does host get IP address? • Hard-coded by system admin in a file • /etc/hosts • DHCP:Dynamic Host Configuration Protocol: dynamically get address from as server • “plug-and-play”

  18. 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

  19. 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

  20. 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

  21. 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

  22. 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 • It can change addresses of devices in local network without notifying outside world • It can change ISP without changing addresses of devices in local network • The devices inside local net not explicitly addressable, visible by outside world (a security plus).

  23. 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

  24. 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, 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

  25. 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

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