Understanding Linux Network Internals

IP Header

Readers might be familiar with the basic fields of the IP header, but there are a few parameters that are not well known and some others that have changed in meaning over time. Figure 18-2 shows the header, and the text that follows summarizes their purposes:

Version

Version of the protocol. Currently only versions 4 and 6 are implemented. Version 4 is described in this chapter. Version 6 is not covered in this book, although we will sometimes mention how IPv6 differs from IPv4 when this is useful in context.

Figure 18-2. IP header

Header Length (IHL)

Length of the header, expressed in units of 32 bits.

Type of Service (TOS)

This 8-bit field is composed of three subfields. I will not go into detail about them because their use is very limited, for many reasons. Originally this field was meant to facilitate Quality of Service (QoS) features by telling routers which criteria were considered most important by the packet's sender: minimum delay, maximum throughput, and so on. The TOS field can still be used in this way, but Internet researchers found it too vague and have decided to implement QoS differently. Therefore, they introduced the Differentiated Services^[*] (diffserv) model, changing the structure and meaning of the field. The new meaning associated with the diffserv model is shown in Figure 18-3(b). DSCP stands for DiffServ Code Point. Each possible value has a unique ...