transportheader

Transportheader refers to the portion of a network packet that carries information used by the transport layer of the Internet protocol suite. It sits between the network (IP) header and the payload, and is generated by transport-layer protocols such as TCP and UDP. The transport header provides addressing, reliability, sequencing, and flow control functions that enable end-to-end communication between processes on different hosts.

The TCP header contains a rich set of fields. Core fields include source port and destination port (each 16 bits) used to identify the communicating processes, a sequence number and an acknowledgment number for reliable data transfer, and a data offset that indicates the header size. The header also includes flags (such as SYN, ACK, FIN, RST, PSH, URG) that control connection establishment, data transfer, and termination, as well as a window size for flow control, a checksum for error detection, and an urgent pointer. Options may be present to extend functionality. A TCP header is at least 20 bytes long and can grow up to 60 bytes with options.

The UDP header is much simpler and consists of four fields: source port, destination port, length, and checksum, totaling 8 bytes. UDP provides no built-in reliability or ordering, making it faster and more lightweight than TCP for certain applications.

Both TCP and UDP use a pseudo-header as part of the checksum calculation to protect critical addressing information from the IP layer. Port numbers (0–65535) multiplex multiple connections or services on a host, with well-known ports assigned to common services and ephemeral ports used for client-side connections.