This is used in the TCP flow control mechanism described later. The window field contains a 16-bit number that indicates how many bytes a TCP receiver is able to buffer. The URG field, which is rarely used, indicates that an “urgent” data point will occur later in the byte stream. The ACK flag is set in TCP segments where the acknowledgment sequence number field holds the next sequence number to be expected. The PSH flag is used to indicate that a TCP segment is the last in a sequence of segments sent by the application and that the receiving TCP should deliver these data directly to the application. The RST flag terminates a connection, and is used both to abort an active TCP connection when, for example, the controlling process has crashed, and to indicate that a TCP port is closed. The FIN flag is set in the final segment on a TCP connection and the SYN flag is set in the first segment. The flags field contains six flags: FIN, SYN, RST, PSH, ACK, and URG. Since the size of the TCP header is always divisible by 4 to allow for operation on processor architectures that require 32-bit alignment of 32-bit memory accesses, the header length field can be efficiently represented using only 4 bits. The hlen field holds the length of the header, including options and padding, counted in 4-byte words. In practice, most TCP segments, except for the initial SYN segment, have the ACK flag set. The acknowledgment number field is defined only if the ACK flag is set. The acknowledgment number field holds the sequence number of the next byte the receiver is expecting on this connection. If the segment contains a SYN or a FIN flag, which both occupy a position in the TCP byte sequence, the sequence number refers to the SYN or FIN. The sequence number field contains the sequence number of the first byte of data in the TCP segment. The sequence and acknowledgment number fields are both 32-bit fields that hold TCP sequence numbers. Unlike UDP, where the source port number is optional, both the source and destination ports must be present in the TCP header. The source and destination port numbers hold the TCP port numbers of the sending and receiving process for the TCP segment. Urgent pointer: If the URG flag is set, this 16-bit field points to a place in the byte stream that contains data that the application has defined to be urgent. Ĭhecksum: The 16-bit checksum is the Internet checksum of the data, the TCP header, and the IP destination and source addresses.Window: The 16-bit window field holds the amount of bytes that the receiver is able to receive. ![]()
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