datagram does not follow the same virtual circuit concept. Every packet is sent across the network and can take a different route to get from point A to B. Also, a datagram is not checked at every node along the route; only the header (or address) information is checked for the location of the des- tination. As each packet is received, it is scanned for the destination address and then immediately routed along to the next node. No CRCs are performed on the data. It is not the network's responsibility to guarantee the integrity of the data; that is left to the receiving device using a higher- level protocol. With this concept of a different path for each packet, the idea of sequentially numbering the packets is lost. The network treats every packet as its own entity (or as packet #1). If a packet gets lost or some- thing else happens, the network does not care. The sequencing and reassembly of the data back into its original form is handled by the receiv- ing end's higher-layer protocols. An example of this is Transmission Con- trol Protocol/Internet Protocols (TCP/IP). IP packets are datagrams that are segments of the original data stream. IP also works at Layer 3 of the OSI model. As a network layer protocol, its only concern is to break the data down into packets and make its best effort to deliver the packets. Fig- ure 7-12 shows the OSI model comparison to TCP/IP. TCP, at the receiving end, is responsible for reassembling the data back into the serial data stream and ensuring the integrity and sequencing of the 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 X.25, Internets, Intranets, and Extranets Figure 7-12 OSI Reference Model Internet Protocol Suite OSI and TCP/IP protocol stacks. Application FTP, Telnet Presentation SMTP, SNMP, RTP Session Transport TCP, UDP Routing Protocols Network ARP, R