20.3 INTERNET PROTOCOL (IP)

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Internet Protocol (IP) is the protocol that enables various networks to talk to each other. IP defines the data formats for transferring data between various networks, and it also specifies the addressing and routing mechanisms. The service delivered by IP is unreliable connectionless packet service. The service is unreliable because there is no guarantee that the packets will be delivered—packets may be lost if there is congestion, though a best effort is made for the delivery. The packets may not be received in sequence, packets may be duplicated, and packets may arrive at the destination with variable delay. The service is connectionless because each packet is handled independently. IP defines the rules for discarding packets, generating error messages, and how hosts and routers should process the packets.

The main functions of the IP layer are addressing and routing. Each machine is given an IP address that is unique on the network. The destination address in the IP datagram is used to route the packet from the source to the destination.

IP is implemented as software. This software must run on every end system and on every router in any internet using the TCP/IP protocol suite.

In Figure 20.4, the router X may deliver the packet to network Q directly or it may deliver it to router Y, which in turn delivers to network Q. So, the packets may take different routes and arrive at the end system B out of sequence. It is the TCP layer that takes care of presenting the data in proper format to the application layer.

Note

It is important to note that the IP layer does not provide a reliable service. The packets may be lost on the route from the source to the destination if there is congestion in the network. It is the responsibility of the TCP layer to ask for retransmissions and ensure that all the packets are received at the destination.

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