When referring to the data structures of a packet, the term Protocol Data Unit (PDU) is used, what is the proper
term to refer to a single unit of TCP data at the transport layer?

A.
TCP segment.

B.
TCP datagram.

C.
TCP frame.

D.
TCP packet.

Explanation:
In the OSI model layer 4 is the transport layer. In the TCP/IP model, Application Layer data is encapsulated in a
Layer 4 TCP segment. That TCP segment is encapsulated in a Layer 3 IP packet. Data, segments, and
packets are examples of Protocol Data Units (PDUs).
Incorrect Answers:
B: TCP datagrams is not a notion that is used in the TCP/IP model.
C: The TCP frame is at the Layer 2 Ethernet layer, not at the transport level which is layer 4.D: A TCP packet is at the application layer, not at the transport level.

Conrad, Eric, Seth Misenar and Joshua Feldman, CISSP Study Guide, 2nd Edition, Syngress, Waltham, 2012,
p. 70