ams will be and how many datagrams will be used in a segment. Figure 7-16 Source Port Destination Port The TCP header. (byte Sequence Number count) Acknowledge Number (buffer Reserved Flags Window Offset space Checksum Urgent Pointer urgent Options (+padding) Data (variable) needed 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 TCP ensures the integrity of the data when it arrives, as well as reorder- ing the data back into sequence. If data is corrupted, TCP asks for a retransmission. If data does not arrive, or if it does not arrive within a spec- ified period of time (usually one second), TCP requests a retransmission. TCP does not number its segments. Instead, it numbers the octets to send. It performs the ACK/NAK function based on octet counts expected versus received. Address Resolution Protocols (ARPs) On some media (such as IEEE 802 LANs), media addresses and IP addresses are dynamically discovered through the use of two other mem- bers of the IP suite: Address Resolution Protocol (ARP) Reverse Address Resolution Protocol (RARP) ARP uses broadcast messages to determine the hardware Media Access Control (MAC)-layer address corresponding to a particular internetwork address. ARP is sufficiently generic to enable the use of IP with virtually any type of underlying media-access mechanism. When the MAC address is not known, ARP generates a message across the LAN, as shown in Fig- ure 7-17. The message is that the sender has an IP address to send, but it does not know the MAC address. This broadcast asks for the MAC layer address to use. All devices on the network hear the broadcast, but only the A broadcast Figure 7-17 to all The ARP query. Who owns to this IP address? Address Resolution Protocol (ARP) 1 Downloaded from Digital Engineering Library @ McGra