Understanding Linux Network Internals

IP Options

As described earlier in this chapter, network stacks are required to implement a number of IP options that applications can use if they choose to. To accommodate information related to options, the basic 20-byte IP header is extended up to another 40 bytes.

Most IP options are used very rarely, and in particular contexts. Different options can be combined into the same IP packet. However, with the exception of the "End of Option List" and "No Operation" options, there can be at most one instance of each option in a header. The presence of options also influences the fragmentation/defragmentation process, as we will see in the section "Packet Fragmentation/Defragmentation."

Some options are very simple and can be specified by a single byte; more complex options require a more flexible format and are called multibyte options.

Figure 18-4 shows the format of both kinds of options. Note that the option data in a multibyte option does not start at a 32-bit boundary.

Figure 18-4. (a) Single IP option format; (b) multibyte IP option format

Each option has an 8-bit field named type that can be further decomposed into three subfields, shown in Figure 18-5. The most common values for type are listed in Table 18-1.^[*] It shows the symbols used for options by the Linux kernel and how the value of the symbol breaks down into the three fields in Figure 18-5.

Figure 18-5. Format of the type ...