Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 31
Header Length, Service Type, and Total Length Fields

The length of the IP header (all fields except for the IP data field) can vary. Not all the fields in the IP header need to be used. Fields are measured in the amount of 32–bit words. The shortest IP header will be 20 bytes; therefore, this field would contain a 5 (20 bytes = 160 bits; 160 bits/32 bits = 5). This field is necessary, for the header can be variable in length depending on the field called options. IPv6 has a static–length header field.

The service field was a great idea, but it is rarely used and is usually set to 0. This was a entry that would allow applications to indicate the type of routing path they would like (the key point here is that the application chooses this field). For example, a real–time protocol would choose low delay, high throughput, and high reliability—a file transfer does not need this. A TELNET session could choose low delay with normal throughput and reliability. There is another side to this story, however. The router must support this feature as well and this usually means building and maintaining multiple routing tables. The Service type is made up of the following fields: precedence, delay, throughput, and reliability. However, supporting this field caused the router to support multiple routing tables per router, and this complication never progressed with the router vendors. This precedence bits of the service field may have an entry of zero (normal precedence) and up to 7 (network control), which allows the transmitting station’s application to indicate to the IP layer the priority of sending the datagram. This is combined with the D (delay), T (throughput), and R (reliability) bits. This field is known as a Type of Service (TOS) identifier, and these bits indicate to a router which route to take:

Header Length, Service Type, and Total Length Fields

D bit. Request low delay when set to 1

T bit. Request high throughput when set to 1

R bit. Request high reliability when set to 1

For example, if there is more than one route to a destination, the router could read this field to pick a route. This becomes important in the OSPF routing protocol, which is the first IP routing protocol to take advantage of this. If the transaction is a file transfer, you may want to set the bits to 0 0 1 to indicate that you do not need low delay or high throughput, but you would like high reliability. TOS fields are set by applications (i.e., TELNET or FTP) and not routers. Routers only read this field, they do not set this field. Based on the information read, routers will select the optimal path for the datagram. It is up to the TCP/IP application running on a host to set these bits before transmitting the packet on the network. It does require a router to maintain multiple routing tables—one for each type of service.

The total length is the length of the datagram (not packet) measured in bytes (this field allots for 16 bits, meaning the data area of the IP datagram may be 65535 bytes in length). IPv6 allows for a concept known as jumbo datagrams. Remember, TCP may not always run over Ethernet, Token Ring, and so on. It may run as a channel attached to a Cray super–computer that supports much larger data sizes.