 as shown in Figure 6.41. Version specifies the IP protocol version being used, in this case, version 4. The Internet header length (IHL) specifies header length as a number of 32-bit words. The mini- mum value of IHL is 5 and the maximum is 15. With the type of service field the host may specify the datagram priority. It also contains flag bits D (Delay), T (Throughput), and R (Reliability), which the host can set to 1 to indicate about which feature it cares most. In practice, most routers ignore the type of service field altogether. The total length field tells the length of the IP packet including the header and user data. It gives the total number of bytes or octets and its maxi- mum value is 65,535 bytes. The IP layer may divide long datagrams into shorter fragments, which is necessary, for example, when the data are Introduction to Telecommunications Network Engineering 32 bits, 4 bytes 12345678 12345678 12345678 12345678 Version Type of service Total length Identification Flags Fragment offset Time-to-live Protocol Header checksum Source address Destination address Options Padding User data User data Figure 6.41 IP packet and header (IPv4). transmitted over Ethernet, where the maximum size of a data field is 1,500 bytes. An Ethernet section almost always exists between end hosts, SO long datagrams need to be split into segments with a maximum size of 1,500 bytes. Then we can say that the typical size of an IP datagram is not longer than around 1,500 bytes. The identification field has the same value for all fragments that belong to the same original IP datagram. Flags contain one unused bit, a don't fragment (DF) bit, and a more fragments (MF) bit. By setting the DF bit the host can request the network to use a route where a datagram need not to be fragmented. When a datagram is fragmented all of its fragments, except the last one, have the MF bit set. All machines are required to accept fragments of length 576 bytes or less. The fragment offset tells where in the current datagram this fragment belongs. T