Скачать или смотреть TCP segment format | TCP header | computer network

TCP
UDP
transmission control protocol
user datagram protocol
source port address
destination port address
sequence number
acknowledgment number
header length
flags
window size


Transmission Control Protocol

Source and destination port :

These fields identify the local endpoint of the connection.

Sequence and ACK number :
This field is used to give a sequence number to each and every byte transferred.
The ACK signifies the next byte expected from the source and not the last byte received.

Header length :
This field tells how many 32-bit words are contained in the TCP header. This is needed because the options field is of variable length.

Flags : There are six one-bit flags.

URG : This bit indicates whether the urgent pointer field in this packet is being used.
ACK : This bit is set to indicate the ACK number field in this packet is valid.
PSH : This bit indicates PUSHed data.
RST : This flag is used to reset a connection that has become confused due to a host crash or some other reason.
SYN : This bit is used to establish connections. The connection request(1st packet in 3-way handshake) has SYN=1 and ACK=0. The connection reply (2nd packet in 3-way handshake) has SYN=1 and ACK=1.

FIN : This bit is used to release a connection. It specifies that the sender has no more fresh data to transmit.

Window Size :
Flow control in TCP is handled using a variable-size sliding window.

Checksum :
This is provided for extreme reliability. It checksums the header, the data, and the conceptual pseudoheader.

Urgent Pointer :
Indicates a byte offset from the current sequence number at which urgent data are to be found. Urgent data continues till the end of the segment. This is not used in practice.

Options :
Provides a way to add extra facilities not covered by the regular header.

Data :
This can be of variable size. TCP knows its size by looking at the IP size header.