TCP Packet Field Descriptions

The following descriptions summarize the TCP packet fields illustrated in Figure 30-10:
• Source Port and Destination Port—Identifies points at which upper-layer source and destination
processes receive TCP services.
• Sequence Number—Usually specifies the number assigned to the first byte of data in the current
message. In the connection-establishment phase, this field also can be used to identify an initial
sequence number to be used in an upcoming transmission.
• Acknowledgment Number—Contains the sequence number of the next byte of data the sender of
the packet expects to receive.
• Data Offset—Indicates the number of 32-bit words in the TCP header.
• Reserved—Remains reserved for future use.
• Flags—Carries a variety of control information, including the SYN and ACK bits used for
connection establishment, and the FIN bit used for connection termination.
• Window—Specifies the size of the sender’s receive window (that is, the buffer space available for
incoming data).
• Checksum—Indicates whether the header was damaged in transit.
• Urgent Pointer—Points to the first urgent data byte in the packet.
• Options—Specifies various TCP options.

• Data—Contains upper-layer information.