3.1 Introduction

IP is the workhorse protocol of the TCP/IP protocol suite. All TCP, UDP, ICMP, and IGMP data gets transmitted as IP datagrams (Figure 1.4). A fact that amazes many newcomers to TCP/IP, especially those from an X.25 or SNA background, is that IP provides an unreliable, connectionless datagram delivery service.

By unreliable we mean there are no guarantees that an IP datagram successfully gets to its destination. IP provides a best effort service. When something goes wrong, such as a router temporarily running out of buffers, IP has a simple error handling algorithm: throw away the datagram and try to send an ICMP message back to the source. Any required reliability must be provided by the upper layers (e.g., TCP).

The term connectionless means that IP does not maintain any state information about successive datagrams. Each datagram is handled independently from all other datagrams. This also means that IP datagrams can get delivered out of order. If a source sends two consecutive datagrams (first A, then B) to the same destination, each is routed independently and can take different routes, with B arriving before A.

In this chapter we take a brief look at the fields in the IP header, describe IP routing, and cover subnetting. We also look at two useful commands: ifconfig and netstat. We leave a detailed discussion of some of the fields in the IP header for later when we can see exactly how the fields are used. RFC 791 [Postel 1981a] is the official specification of IP.