31 Days Before Your CCNA Exam

Day 29 25Flow ControlFlow control is handled by TCP through a process called windowing. The two end devices negotiatethe window size when initially establishing the connection; then they dynamically renegotiatewindow size during the life of the connection, increasing its size until it reaches the maximumwindow size of 65,535 bytes or until errors occur. Window size is specified in the window field ofthe TCP header. After sending the amount of data specified in the window size, the source mustreceive an acknowledgment before sending the next window size of data.Connection Establishment and TerminationConnection establishment is the process of initializing sequence and acknowledgment fields andagreeing on port numbers and window size. The three-way connection establishment phase shownin Figure 29-4 must occur before data transfer can proceed.Figure 29-4TCP Connection EstablishmentWebBrowserSEQ=200SYN, DPORT=80, SPORT=1027SEQ=1450, ACK=201SYN, ACK, DPORT=1027, SPORT=80SEQ=201, ACK=1451ACK, DPORT=80, SPORT=1027WebServerIn the figure, DPORT and SPORT are the destination and source ports. SEQ is the sequence number.In bold are SYN and ACK, which each represent a 1-bit flag in the TCP header used to signalconnection establishment. TCP initializes the Sequence Number and Acknowledgment Numberfields to any number that fits into the 4-byte fields.After data transfer is complete, a four-way termination sequence occurs that uses an additionalflag, called the FIN bit, as shown in Figure 29-5.Figure 29-5TCP Connection TerminationACK, FIN SEQ=1000PCPCACK ACK=1001ACK, FIN ACK=1001SEQ=1470ACK ACK=1471