Understanding Linux Network Internals

Chapter 24. Layer Four Protocol and Raw IP Handling

This chapter describes the interface between L3 and L4 protocols. The only L3 protocol considered here is IP. The L4 protocols include the familiar TCP, UDP, and ICMP, along with several other ones. The L4 protocols are not covered in this book for reasons of space and complexity. However, this chapter explains what happens when applications handle their own L4 (and sometimes L3) processing through raw IP.

In particular, this chapter explains:

• How L4 protocols register with the kernel and tell the kernel what kind of traffic they are interested in

• How ingress packets are passed to the correct L4 protocol handler

• How applications tell the kernel to let the application process headers

We saw in Chapter 21 the functions that L4 protocols use to transmit an IP datagram. Since this book focuses on IP, this chapter covers only those L4 protocols that sit on top of IP. The chapter describes the IPv4 interface and then briefly shows where IPv6 differs.

Available L4 Protocols

A few key L4 protocols are statically compiled into the kernel. Several less-common protocols can be compiled as modules. Table 24-1 shows the protocols that are statically compiled in.

Table 24-1. Protocols statically compiled into the kernel

Protocol	RFC# (Year)
UDP	768(1980)
ICMP	792(1981)
TCP	793(1981)

Table 24-2 lists some of the protocols in the second category. They can be added to the kernel from the section “Networking Support → Networking Options” in the ...