Simple Internetworking Network 1 (Ethernet) Internetworking Network 2 (Ethernet) H1 H2 H3 H7 R3 H8 Network 4 R1 (point-to-point) R2 Kameswari Chebrolu H4 Network 3 (FDDI) Dept. of Electrical Engineering, IIT Kanpur Router H5 H6 Problems to address: Solution: Heterogeneity Internet Protocol (IP) Scale Advantages of Best-effort service Service Model Reconfigurable after a failure without concern for state Underlying networks can Provides basic building block from which a variety of Lose packets services can be implemented Reorder packets Minimum network service assumption helps in Deliver same packet more than once heterogeneous network integration Can delay packets arbitrarily IP can “run over anything” Connectionless Datagram service (Best Effort) Pigeon powered Internet: 1hr. 42 minutes to transfer 64 bytes No state maintained in the network Packet switching core No guarantees to data delivery Addressing Datagram Forwarding Globally unique Host Hierarchical (32 bit binary number): Computers that execute application programs on behalf of Consists of two parts: network and host users Network part: Identifies the network of the host Examples: Personal computers, workstations, batch systems Host part: Identifies the host within the network etc IP address space divided into 5 classes (A,B,C,D,E) Routers 7 24 Building blocks that interconnect networks Class A 0 Network Host 10.0.0.1 14 16 Receive datagrams from hosts and routers on one network Class B 1 0 Network Host 128.32.12.89 Forward datagrams to hosts or routers on other networks 21 8 Class C 1 1 0 Network Host 192.43.54.06 Datagram Forwarding Cont.. Host Forwarding On receipt of a packet Forwarding algorithms employed by hosts If (NetworkNum of destination = NetworkNum of my different from those of gateways interface) then deliver packet to destination on that interface Number of hosts far exceed gateways else deliver packet to default router Algorithms change with time Resource constraints A host maintains a cache of recently used routes If cache lookup fails, use default router Router Forwarding Example Network 1 (Ethernet) When a packet arrives at a router A, Network 2 (Ethernet) If (NetworkNum of destination = NetworkNum of H1 H2 H3 one of my interfaces) then deliver packet to H7 R3 H8 destination on that interface Network 4 (point-to-point) else chooses another router B, such that R1 B is closer to the destination address R2 Forwarding Table of R2 B is directly reachable from G H4 Network 3 (FDDI) NetworkNum NextHop B is next hop for the packet 1 R3 2 R1 Process of selecting next hop host or router is called H5 H6 ROUTING Scalability: Router tables list only set of network numbers Protocol Stack IP Packet Format 0 4 8 16 19 31 Version HLen TOS Length H1 H8 Ident Flags Offset TCP R1 R2 R3 TCP TTL Protocol Checksum SourceAddr IP IP IP IP IP DestinationAddr Options (variable) Pad ETH ETH FDDI FDDI PPP PPP ETH ETH (variable) Data Address Resolution Protocol (ARP) Summary Translates IP addresses to link-level addresses Problem: How to build huge networks by interconnecting smaller networks Each host maintains a table of address pairs Manual configuration is tedious Challenges: Heterogeneity and Scale Dynamical learning of tables achieved by ARP Solution: Internet Protocol (IP) ARP relies on broadcast Connectionless datagram service Broadcast request if mapping missing Hierarchical addressing Destination machines responds with the mapping ARP mechanism to translate IP to link-level Table entries are discarded is not refreshed addresses.