Network Layer and Routing

1. Network Layer and Routing

2. NL responsibility • Main responsibilities • Host to Host delivery. • Finding path to destination. • Services provided to TL, receive from DL • Connectionless and connection oriented services • Identifying source and destination uniquely and thereby use NL address. Fragments TL data if necessary. Uses packet switching (store and forward) with datagram approach. • In the router NL finds the appropriate interface from which it will reach the destination. • At the destination matches the address and check corrupt packets, and assembly fragmented units and then sent to TL

3. Switching • How is a packet sent from one node to another node. • Circuit switching • Packet switching • VC approach – call set up before message transfer • Datagram approach – independent packet, does not need call setup phase.

4. Transport Layer:TCP, UDP • Routing Protocols: • path selection • RIP, OSPF, BGP • IP Protocols: • addressing conventions • Datagram format • Packet handling conventions Routing Table • ICMP protocol: • error reporting • Router “signaling” Link Layer Internet Protocol • Network layer of datagram-oriented network such as internet has three major components: • Network protocol, e.g IP • Path determination • Error reporting

5. Internet structure • At the NL internet can be viewed as a collection of subnets or Autonomous systems (AS) that are interconnected.

6. Internet working • TL takes data streams and breaks them up into datagrams. Datagrams cab be up to 64Kbytes each, but usually not more than 1500 bytes. If necessary the fragments them and sent to internet. When all pieces finally reaches the destination they are reassembled by NL to original datagram. It is then handed over to TL which inserts it into receiving process.

7. Distance Vector Algorithm • It is iterative, asynchronous and distrbuted. • Each node keeps a distance table. Node X’s distance table entry, DX(Y,Z) is the sum of the cost of the direct one hop link between X and Z, plus neighbor Z’s currently known least cost path from itself to Y. DX(Y,Z) = c(X,Z)+minw{DZ(Y,W)}

8. After the algorithm converged Cost to destination via 1 B C 7 Destination 2 8 A 1 E D 2

9. Y 1 2 7 X Z

10. Link cost changes and failure Y 1 1 4 • When link-cost decreases 50 X Z t0 t2 t1

11. Y 60 1 4 50 X Z • When link-cost increases • Routing loop • Count to infinity problem t0 t2 t3 t1

12. IPv4 addressing • IPv4 is 32 bits long • Usually written in dotted decimal notation, eg. 193.32.216.9 • In the globak internet each interface must have a unique IP address. • Network address and host address. • Subnet mask • Meaning of 223.1.1.0/24

13. IPv4 header 16 bits 16 bits • header has 20bytes fixed part and a variable length optional part. • IHL – header length in 32 bit words. minimum IHL = 5 words = 20 bytes. maximum is 15 words. • types of service – 6 bit field. voice, text etc. different classes of service. • total length – 65,535 bytes IHL Version Total length Type of Service Identification Fragment offset D F MF TTL protocol Header checksum Source Add Destination Add options ( 0 or more words)  

14. Identification – identifies the fragment which datagram it belongs to. • unused bit fields. • DF stands for don’t fragment. • MF stands for more fragment – all fragments except the last one has this bit set. • Fragment offset – where in the current datagram this fragment belongs. there is a maximum 213 fragments per datagram. • TTL – counter to limit packet lifetimes in seconds. max. value 255. it must be decremented on each hop, and supposed to be decremented when queued for long time in a router. when it hits 0 the packet is discarded and a warning message is sent back to source. • Protocol – TCP or UDP etc. • checksum – for header only. • source and destination address – IP addresses. • Options – security, strict source routing, loose source routing, record routes, timestamp etc.

15. Classful addressing 1.0.0.0 – 127.255.255.255 0 network Host 128.0.0.0 – 191.255.255.255 10 network Host 192.0.0.0 – 223.255.255.255 110 network Host 224.0.0.0 – 239.255.255.255 1110 network Mutlicast address 240.0.0.0 – 255.255.255.255 11110 network For future use 192.68.1.20/255.255.255.0 or 192.168.1.20/24 network number or address – 192.68.1.0 host number of address - 20

16. Special IP • 0.0.0.0 – this host; used when the machine is booting. • IP address with 0 as the network number refer to the current network. • IP with all 1s allows broadcasting on local network, typically a LAN. • IP address with all 1s in host field allows broadcasting on the remote network that matches the network number. • 127.x.x.x - loopback

18. Addressing and routing Key fields of IP datagram Misc fields Source IP address Desti.IP address Data Transporting a datagram from src to dest A E 223.1.1.1 223.1.2.2 223.1.2.9 223.1.1.4 223.1.1.2 Hub 223.1.2.1 223.1.3.27 223.1.1.3 Routing table in A B

19. Routing table in router • How is this table created in the router?

20. A campus network hosts routers APE CSE main router

21. Subnets • Splitting a network into several subnetworks. • Reduced network traffic • Routers create broadcast domains. The smaller broadcast domains you create, the less network traffic on that network segment. • Optimized network performance • This is a result of reduced network traffic • Simplified management • It’s easier to identify and isolate network problems in a group of smaller connected networks than within one gigantic network. • Facilitated spanning of large geographical distances

22. Internet corporation for Assigned Names and Numbers (ICANN) – manages IP numbers and DNS root servers • ICANN appointed some regional authority called Internet registry who are now responsible for IP address assignment for a particular region • North america: American registry for internet • Reseaux IP Europeans • Asia pacific Network Information center (APNIC)

23. CIDR • Classless InterDomain Routing • IP block: • network address/subnet mask • 192.168.10.0/24 • the range of Ip address of this block is; • 192.168.10.1 to 192.168.10.254 • 192.168.10.0 cannot be assigned to any host • 192.168.10.255 cannot be assigned to any host