Sequence Number, Data Offset, and Window are the fields found in a TCP header. TCP hosts create a connection-oriented session with one another. The following are the fields found in a TCP header:

* Sequence Number: Refers to the first byte of data in the current message. This field helps TCP to reassemble the packets in the correct order. For example, when data is transferred between an FTP server and FTP client, the receiver uses this field to eassemble the packets into the original file.
* Data Offset: Refers to the number of 32-bit words in the TCP header.
* Window: Refers to the size of the available space for the incoming data.
* Source Port and Destination Port: Refer to the point where upper-layer source and destination processes receive TCP services. Both TCP and UDP packets contain these fields.
* Acknowledgment Number: Refers to the sequence number of the next byte of data which the sender will receive.
* Reserved: Reserved for future use.
* Flags: Contains control information, such as the SYN and ACK bits which are used to establish and acknowledge communication, and the FIN bit which is used to terminate the connection.
* Checksum: An indicator of any damage to the header while being in transit. Both TCP and UDP packets contain this field.
* Urgent Pointer: Refers to the first urgent data byte in the packet.
* Options: Used to specify TCP options. Only TCP packets contain this field.
* Data: Has upper-layer information.

TCP is used for unicast transmissions and provides connection -oriented services for upper layer protocols. It will establish a state of connection between two devices before any data is transferred; for example, before a workstation can exchange HTTP packets with Web server, a TCP connection must be established between the workstation and the Web server.

The Length field is found in a User Datagram Protocol (UDP) header, where it specifies the length of the UDP header and data. UDP headers contain the Source Port, Destination Port, Length, and Checksum fields.

Sequence number, acknowledgment number, and windows size are fields not found in a UDP header because UDP provides none of the services that require use of these fields. That is, UDP cannot re-sequence packets that arrive out of order, nor does UDP acknowledge receipt (thus the term non-guaranteed to describe UDP). Furthermore, since UDP does not acknowledge packets, there is no need to manage the window size, which refers to the number of packets that can be received without an acknowledgment.

The Type-of-Service field is found in an Internet Protocol (IP) header, where it specifies the handling of a current datagram by an upper-layer protocol.