Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 86 is the Cisco approved textbook to use alongside version of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | Page 819 Tuesday May 20 2003 2 53 PM The TCP IP Protocol Suite 819 User Datagram Protocol UDP A connectionless and unreliable protocol that is responsible for transmitting messages but provides no software checking for segment delivery. The advantage that UDP provides is speed. Because UDP provides no acknowledgments less control traffic is sent across the network making the transfer faster. Figure 19-3 Transport Layer Protocols TCP UDP TCP and UDP Segment Format TCP is known as a connection-oriented protocol. Connection-oriented means that the end stations are aware of each other and are constantly communicating about the connection. A classic example of a non-technical connection-oriented communication is a telephone conversation between two people. A good example of a connectionless conversation is the . postal service where once you place the letter in the mail you hope that the mailing process gets the package delivered. Figure 19-4 illustrates the TCP segment header format the field definitions of which are described in the following list. Figure 19-4 TCP Segment Format B Page 820 Tuesday May 20 2003 2 53 PM 820 Chapter 19 Intermediate TCP Source port The number of the source port for this segment. Destination port The number of the destination port for this segment. Sequence number The number used to ensure correct sequencing of the arriving data. It is the number assigned to the first octet in the user data field. Acknowledgment number The next expected TCP octet. Header length The number of 32-bit words in the header. Reserved Set to 0. Code bits The control functions for example setup and termination of a session . Window The number of octets that the sender is willing to accept. Checksum The calculated checksum of the header and data fields. Urgent Indicator of the end of the urgent data. Options One currently defined maximum TCP segment size. Data Upper layer protocol data. When using UDP application layer protocols must provide .
