Understanding Linux Network Internals

Chapter 23. Internet Protocol Version 4 (IPv4): Miscellaneous Topics

This chapter wraps up our discussion of the IPv4 layer in the networking code. It covers general topics such as the management of information in the IPv4 layer by the kernel, statistics, and the user interface through /proc. The chapter also includes a brief discussion of the limitations of the IPv4 protocol, which led to the development of IPv6.

Long-Living IP Peer Information

At the IP layer, there is no concept of a stateful connection. Because IP is a stateless protocol, there are no parameters or connection-related data structures to keep, except for statistics. (These are optional and are not required by the protocol itself.) However, to improve performance, the kernel keeps information about some parameters on a per-destination IP address base. We will see an example in a moment.

Any host that has recently carried on an exchange of data with a Linux box is considered an IP peer. The kernel allocates a data structure for each peer to preserve some long-living information. At the moment, not many parameters are kept in the structure. The most important one is the IP packet ID. We saw in Chapter 18 that each IP packet is identified by a 16-bit field called ID. Instead of having a single shared ID, incremented for each IP packet regardless of the destination, one unique instance is kept for each IP peer. (This solution is an implementation choice; it is not imposed by any standard.) We already had a little discussion ...