More Information on IP Packets
The two main functions of the IP protocol are routing and addressing. To route packets to and from machines on a network, IP (the Internet Protocol) uses IP addresses which are carried along in the packets. The brief descriptions in the picture are meaningful enough to give you an idea of the function of the header elements. However, some might not be clear:
The identification tag helps reassemble the packet from several eventual fragments. Data sent over a network is broken down into small sections enveloped in these packets. IP networks, such as the internet, usually are not secure. Packets can be lost, delayed, and arrive in the wrong order. Once they arrive at the destination, the identification tag helps to identify the packet and to reassemble the data back to its original form. The fragmented flag states whether the packet can be fragmented or not. The fragment offset is a field to identify which fragment this packet is attached to. Time to Live (TTL) is a number that indicates how many hops (router passes) the packet can make before it dies. Usually, at each router, a packet is analyzed, and based on the information present at that router on other neighboring routers, a choice is made as to which route is best. The packet is then forwarded to that next router. In this configuration, a packet may well go round. There is also flooding as another method, which implies sending a copy of the packet to each neighboring router; then, only the target machine consumes the packet. Other packets will keep roaming. TTL is a number, typically 255, which decreases each time a packet passes a router. This way, redundant packets will ultimately die once the TTL reaches zero. The header checksum is a number used for error detection and correction during packet transmission. The data in the packet is fed into a mathematical algorithm. The resulting sum travels with the data in the packet. Upon reception, this sum is calculated again using the same algorithm. If it is the same as the original sum, the data is good. Otherwise, it is considered corrupt, and the packet discarded. The payload is the actual data being carried. Notice that data payload can be up to 64 KiloBytes, which is enormous compared to the totality of the header bits.
