In TCP header there was a field called a data field which stores data, but does this field store the IPv4 header + datagram or just the datagram?

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The data is immediately after, not in, the TCP header; it is the TCP payload, which is application data. TCP segments and encapsulates application data into TCP PDUs that are called segments.

IP, in turn, encapsulates TCP segments into IP PDUs called packets, and the IP packets are encapsulated into data-link PDUs called frames.

The IP header is not in a TCP segment (header and data) because IP encapsulates the TCP segment. The TCP header and data are the payload of IP.

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