Linux Kernel Architecture

Chapter 12: Networks

Comparing TCP and UDP is like comparing the postal service with the telephone network. TCP corre-

sponds to a telephone call. The calling party must set up a connection (which must be accepted by the

person called) before information can be passed. During the call, all information sent is received in the

same order in which it was sent.

UDP can be likened to the postal service. Packets (or letters in this analogy) can be sent to recipients

without contacting them in advance for permission to do so. There is no guarantee that letters will be

delivered (although both the postal service and the network will do their best). Similarly, there is no

guarantee that letters will be sent or received in a particular sequence.

Those interested in further examples of the use ofUDP sockets are referred to the many textbooks on

network and system programming.

12.4 The Layer Model of Network

Implementation

The kernel implements the network layer very similarly to the TCP/IP reference model introduced at the

beginning of this chapter.

The C code is split into levels with clearly defined tasks, and each level is able to communicate only with

the level immediately above and below via clearly defined interfaces. This has the advantage that various

devices, transmission mechanisms, and protocols can be combined. For example, normal Ethernet cards

can be used not only to set up Internet (IP) connections but also to transmit other protocols such as

Appletalk or IPX without the need for any kind of modification to the device driver of the card.

The Layer Model of Network Implementation

Application

C Standard library

Protocol

specific

Hardware Specific

Physical

Transmission

Host to Host Layer

Network Layer

Transport Layer

Application Layer

Kernel

Userspace

struct socket

struct sock

struct proto

dev.c

driver.c

struct net_device

struct

packet_

type

Figure 12-3: Implementation of the layer model in the kernel.

The network subsystem is one of the most comprehensive and demanding parts of the kernel. Why is

this so? The answer is that it deals with a very large number of protocol-specific details and subtleties,

and the code path through the layer is riddled with excessive function pointers in place of direct function