Practical TCP/IP and Ethernet Networking- P23: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | 92 Practical TCP IP and Ethernet Networking Fragment offset 13 bits This field indicates where in the original datagram this fragment belongs. The fragment offset is measured in units of 8 bytes 64 bits . The first fragment has offset zero. In other words the transmitted offset value is equal to the actual offset divided by eight. This constraint necessitates fragmentation in such a way that the offset is always exactly divisible by eight. The 13-bit offset also limits the maximum sized datagram that can be fragmented to 64 kb. Time to live 8 bits The purpose of this field is to cause undeliverable datagrams to be discarded. Every router that processes a datagram must decrease the TTL by one and if this field contains the value zero then the datagram must be destroyed. The original design called for TTL to be decremented not only for the time it passed a datagram but also for each second the datagram is held up at a router hence the time to live . Currently all routers simply decrement it every time they pass a datagram. Protocol 8 bits This field indicates the next higher level protocol used in the data portion of the Internet datagram in other words the protocol that resides above IP in the protocol stack and which has passed the datagram on to IP. Typical values are 0x0806 for ARP and 0x8035 for RARP. 0x meaning hex . Header checksum 16 bits This is a checksum on the header only referred to as a standard Internet checksum . Since some header fields change . TTL this is recomputed and verified at each point that the IP header is processed. It is not necessary to cover the data portion of the datagram as the protocols making use of IP such as ICMP IGMP UDP and TCP all have a checksum in their headers to cover their own header and data. To calculate it the header is divided up into 16-bit words. These words are then added together normal binary addition with carry one by one and the interim sum stored in a 32-bit accumulator. When done the upper 16 bits of the .
