icient routing Time exceeded messages to inform sources that a datagram has exceeded its allocated time to exist within the internetwork Router advertisement and router solicitation messages to determine the addresses of routers on directly attached subnetworks A more recent addition to ICMP provides a way for new nodes to discover the subnet mask currently used in an internetwork. All in all, ICMP is an integral part of any IP implementation, particularly those that run in routers. The ICMP Router Discovery Protocol (IRDP) uses router advertisement and router solicitation messages to discover addresses of routers on directly attached subnets. In IRDP, each router periodically multicasts router advertisement messages from each of its interfaces. Hosts discover the addresses of routers on the directly attached subnet by listening for these messages. Hosts can use router solicitation messages to request immediate advertisements, rather than waiting for unsolicited messages. IRDP offers several advantages over other methods of discovering addresses of neigh- boring routers. Primarily, it does not require hosts to recognize routing pro- tocols, nor does not it require manual configuration by an administrator. Router advertisement messages enable hosts to discover the existence of neighboring routers, but not which router is best to reach a particular Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25, Internets, Intranets, and Extranets Chapter 7 destination. If a host uses a poor first-hop router to reach a particular des- tination, it receives a redirect message identifying a better choice. Transport Layer TCP and the UDP implement the Internet transport layer. TCP provides connection-oriented data transport, whereas UDP operation is connectionless. Transmission Control Protocol (TCP) TCP provides full-duplex, acknowledged, and flow-controll