 a protocol identification field, information and pad fields, and a frame-check sequence for error control. The Internet Layer The Internet Protocol is the heart and soul of the TCP/IP protocol suite and Internet itself- and perhaps the most talked about protocol in his- tory. IP provides a connectionless service across the network, which is sometimes referred to as an unreliable service because the network does not guarantee delivery or packet sequencing. IP packets typically contain an entire message or a piece (fragment) of a message that can be as large as 65,535 bytes in length. The protocol does not provide a flow-control mechanism. IP packets, like all packets, have a header that contains routing and content information (see Figure 7-37). The bits in the packet are num- bered from left to right starting at 0, and each row represents a single 32- bit word. An IP header must contain a minimum of five words. Figure 7-36 PPP frame Address Control Protocol format Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Chapter 7 Header Figure 7-37 Version Length Length IP header Datagram ID Flags Offset Protocol Checksum Source IP Address Destination IP Address IP Options IP Header Fields The IP header contains approximately 15 unique fields. The Version field identifies the version of IP that is being used to encode the packet (IPv4 VS. IP v6, for example). The Internet Header Length (IHL) field identifies the length of the header in 32-bit words. The maximum value of this field is 15, which means that the IP header has a maximum length of 60 octets. The Type of Service (TOS) field gives the transmitting system the abil- ity to request different classes of service for the packets that it transmits into the network. The TOS field is not typically supported in IPv4 but can be used to specify a service priority (0-7) or route o