n-oriented reliable communications over connectionless IP network. To do this, it establishes a logical connec- tion, determines if errors have occurred in packets, retransmits packets in error, and rearranges the packets if they arrive out of order. Figure 6.47 shows an overview of multiplexing and addressing in TCP/IP protocol stack. The host is identified by the IP address and if the protocol field in the IP packet has the value 6 (decimal), the payload of the Introduction to Telecommunications Network Engineering Host A processes FTP 1 FTP 2 Port numbers in TCP headers Protocol field in IP header IP address Host B Host C FTP 1 Internet FTP 2 Figure 6.47 Addressing and multiplexing of TCP/IP. IP packet is given to the TCP above. The TCP header contains source and destination ports, which define the processes that exchange information. In the example in Figure 6.47, two FTP and one HTTP application processes run simultaneously on host A and they are distinguished with the help of the port numbers. IP packets carrying FTP 1 data contain IP addresses of hosts A and B for routing through the Internet while others carry IP addresses of hosts A and C. Figure 6.48 shows TCP header fields that follow the IP address in the IP packet. The source and destination ports define the source and destination processes, respectively. Some standard server port numbers are defined, such as 25 (decimal) for SMTP and 23 for Telnet, and a set of them are available for use on demand. The sequence number is the number of the first byte in the data segment carried in the TCP message. The acknowledgment number specifies the number of the first byte in the next segment expected to be received. The data offset reveals the number of 32-bit words in the TCP header; that is, it tells where the data section starts. All 6 bits in the reserved field are set to 0. The following 6 control bits are used as follows: URG is set if urgent pointer is in use. ACK is set if acknowledgment field is in use (always when the con- nection is set up). Data Com