The IPv4 protocol field identifies TCP with protocol number 6.
In an IPv4 packet header, the 8-bit Protocol field indicates which next-level protocol should receive the packet’s payload. The Internet Assigned Numbers Authority assigns decimal 6 to TCP, the Transmission Control Protocol. This lets a host pass the payload to the correct transport-layer handler after processing the IP header.
TCP provides connection-oriented, reliable delivery. It establishes a connection, numbers bytes, acknowledges received data, retransmits missing segments, and regulates sending with flow and congestion control. IPv4 itself does not guarantee delivery; those reliability features belong to TCP.
The nearby values are easy to confuse. Protocol 17 identifies UDP, protocol 1 identifies ICMP, and protocol 89 identifies OSPF. These numbers are not port numbers: TCP port 6 would identify an application endpoint, while IPv4 protocol number 6 identifies TCP as the payload protocol. In IPv6, the corresponding field is called Next Header, although TCP retains number 6.