Understanding Linux Network Internals

IP Options

Because of the overhead associated with the time needed to process IP options , they have never been used much. In the next sections, we will see one by one the IP options handled by the Linux kernel and how they are processed.

Here are the main APIs involved with IP option management, all of them defined in net/ipv4/ip_options.c. To understand some of them, remember that not all of the IP options of a packet need to be replicated in all of its fragments.

ip_options_compile

Parses a block of options from an IP header and initializes an instance of an ip_options structure accordingly. This structure will be used later to process the options; it includes flags and pointers that tell the part of the routing subsystem that handles forwarding what has to be written into the IP header options space, and where. ip_options_compile is described in detail in the section "Option Parsing."

ip_options_build

Initializes the portion of an IP header dedicated to the options, based on an input ip_options structure. This function is used when transmitting locally generated packets. Thanks to an input parameter, it can distinguish fragments and treat them accordingly: it omits from the header of each fragment those options that do not have to be copied into that fragment (see the section "IP options" in Chapter 18), and overwrites them with null options instead. It also clears the flags of the ip_options structure (such as opt->rr_needaddr) that are used to signal the need to add a timestamp ...