Packets

However, the protocol field is important. It identifies which higher- level TCP/ IP protocol sent the data. When data arrives at its destination (either the packet’s destination address equals the host’s own IP address, or it is a broadcast address) this field tells IP which protocol module to pass it on to. 

One control field, the time-to-live (TTL) field, is interesting. It is initialised by the sender to a particular value, usually 64, and decremented by one (or the number of seconds it is held on to) by every router that the packet passes through. When it reaches zero the packet is discarded and the sender notified using the Internet Control Message Protocol (ICMP), a network layer protocol for sending network- related messages. 

The TTL field is a safety mechanism which prevents packets from travelling the Internet forever in routing loops. It is exploited in a novel way by the Traceroute diagnostic tool. 

Although the total field length in the IP protocol header is 16 bits, IP packets are usually much smaller than the 64 KB maximum this implies. For one thing, the link layer will have to split this into smaller chunks anyway, so most of the efficiency advantages of sending data in large blocks is lost. For another, IP standards did not historically require a host to accept a packet of more than 576 bytes in length. Many TCP/ IP applications limit themselves to using 512- byte blocks for this reason, though today most implementations of the protocol aren’t so restricted.