TCP segment header:

Explanation of header fields:

• Source and destination port: These fields recognize the local endpoint of the connection. every host might decide for itself how to allocate its own ports starting at 1024. The source and destination socket numbers together discover the connection.

• Sequence and ACK number: This part is used to give a sequence number to each and every byte transferred. This has an gain over giving the sequence no. to every packet since data of a lot of small packets can be combined into one at the time of retransmission, if desired. The ACK signifies the next byte expected from the source and not the last byte received. The ACKs are cumulative as a substitute of selective. Sequence number space is as large as thirty two bit although seventeen bits would have been sufficient if the packets were delivered in order. If packets(message) reach in order then we have a formula and according to this:

(sender side window size) + (receiver side window size) < (sequence number space)

the sequence number space should be seventeen bits. But packets may take dissimilar routes and reach out of order. As a result, we need a larger sequence number space. And for optimisation, this is thirty two bits.

• Header length: This field tells how many thirty two bit words are contained in the TCP

header. This is required because the options field is of variable length.

• Flags: There are 6 one-bit flags.

1.  URG: This bit indicates whether the urgent pointer field in this packet is being used.

2.  ACK: This bit is set to specify the ACK number field in this packet is valid.

3.  PSH: This bit indicates Pushed data. The receiver is requested to send the data to the application upon arrival and not buffer it until a full buffer has been received.

4.  RST: This flag is used to reset a connection that has become puzzled due to a host crash or some other reason. It is also used to discard an invalid segment or refuse an attempt to open a connection. This causes an sudden end to the link, if it existed.

5.  SYN: This bit is used to set up a connections. The connection request(first packet in 3-way handshake) has SYN=1 and ACK=0. The link reply (second packet in three-way handshake) has SYN=1 and ACK=1.

6.  FIN: This bit is used to release a connection. It shows that the sender has no more new data to transmit. On the other hand, it will retransmit any lost or delayed packet. Also, it will continue to getting data from other side. as SYN and FIN packets have to be acknowledged, they have to have a sequence number even if they do not contain any data.

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

The  Window Size  field  tells  how  many  bytes  may  be  sent  starting  at  the  byte acknowledged. Sender coulk send the bytes with sequence number among (ACK#) to (ACK# + window size - 1) A window size of zero is legal and says that the bytes up to and including ACK# -1 have been received, However the receiver would like no more data for the moment. authorization to send can be granted later on by sending a segment with the same ACK number and a nonzero Window Size field.

• Checksum: This is provided for great reliability. It checksums the header, the data, and the abstract pseudoheader. The pseudoheader contains the thiry two bit IP address of the source and destination systems, the protocol number for TCP(six), and the byte count for the TCP segment (including the header).As well as the pseudoheader in  TCP  checksum  computation  helps  identify  missing  packets,  however  doing  so violates the protocol hierarchy since the IP addresses in it belong to the IP layer, not the TCP layer.

• Urgent Pointer: It shows a byte offset from the present sequence number at which urgent data are to be found. Urgent data continues till the last part of the segment. This is not generally used in practice. The similar effect can be had by using 2 TCP connections, one for transferring imperative data.

• Options: It gives a way to add extra facilities not covered by the regular header. For example, Maximum TCP payload that sender is eager to handle. The maximum size of segment is called MSS (Maximum Segment Size). At the time of handshake, both components inform each other about their capability. Minimum of the two is honored. This information is sent in the options of the SYN packets of the 3- way handshake.
  • Window scale option can be used to increase the window size. It may be specified by telling the receiver end that the window size should be interpreted by shifting it left by specified no. of bits. Now, this header option allows window size up to two hundred thirty.

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

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