20 - Internet Control Message Protocol

Abstract

This chapter focuses on Internet Control Message Protocol (ICMP) messages, which comprise a protocol running side by side with internet protocol (IP) at the network layer. Routers use ICMP to notify hosts and other routers that a route is unreachable, that there is a problem with a particular path, or that a router is being overloaded. ICMP is a part of the STD-5 specification that includes the IP and is documented in RFC 792. All IP hosts must implement ICMP so that they can receive and send error messages about unreachable destinations, error and status messages about routes and gateways, echo requests and replies to indicate status of reachable hosts, and error messages about the traffic that has timed out. ICMP provides an important adjunct to IP as a channel through which nodes can exchange error and other types of messages about packet exchanges. ICMP messages have a simple structure: a one-octet type field, which indicates what function a message is fulfilling, and a one-octet code field, which may be used to further clarify the contents of the message. ICMP unreachability messages indicate that there is a failure somewhere in the process of addressing the datagram that triggers the message. The most obvious uses for ICMP routing messages are requests for lists of available routers and the replies that include lists of other available routers.