Simple Internetworking Network 1 (Ethernet) Internetworking Network 2 (Ethernet) H1 H2 H3 H7 R3 H8 Network 4 (point-to-point) R1 R2 Kameswari Chebrolu H4 Network 3 (FDDI) Router Dept. of Electrical Engineering, IIT Kanpur H6 H5 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 10.0.0.1 Class A Network Host 0 � Receive datagrams from hosts and routers on one network 14 16 128.32.12.89 Class B Network Host 1 0 � Forward datagrams to hosts or routers on other networks 21 8 192.43.54.06 Class C Network Host 1 1 0 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 � A host maintains a cache of recently used routes � Resource constraints � If cache lookup fails, use default router
Router Forwarding Example � When a packet arrives at a router A, Network 1 (Ethernet) Network 2 (Ethernet) � If (NetworkNum of destination = NetworkNum of H1 H2 H3 H7 R3 H8 one of my interfaces) then deliver packet to 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 NetworkNum NextHop (FDDI) � B is next hop for the packet 1 R3 2 R1 � Process of selecting next hop host or router is called H6 H5 ROUTING Scalability: Router tables list only set of network numbers Protocol Stack IP Packet Format 0 4 8 16 19 31 TOS Length Version HLen H1 H8 Ident Flags Offset TCP R1 R2 R3 TCP TTL Protocol Checksum SourceAddr IP IP IP IP IP DestinationAddr Pad Options (variable) (variable) ETH ETH FDDI FDDI PPP ETH PPP ETH 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 � Challenges: Heterogeneity and Scale � Manual configuration is tedious � Solution: Internet Protocol (IP) � Dynamical learning of tables achieved by ARP � ARP relies on broadcast � Connectionless datagram service � Hierarchical addressing � Broadcast request if mapping missing � ARP mechanism to translate IP to link-level � Destination machines responds with the mapping addresses � Table entries are discarded is not refreshed
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