Which protocol is NOT part of the Transport Layer?

The choice of ICMP as the correct answer is grounded in the fundamental architecture of the OSI and TCP/IP models. The Transport Layer is primarily responsible for end-to-end communication, reliability, and flow control between host systems. It encompasses protocols such as TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). Both of these protocols are integral to facilitating data transmission and ensuring that data packets are delivered correctly and in order.

ICMP (Internet Control Message Protocol), on the other hand, operates at the Network Layer, not the Transport Layer. Its primary function is to send error messages and operational information relevant to network conditions, rather than to manage data flow and reliability between two endpoints, which is the key role of the Transport Layer protocols. This distinction in function and layer placement clearly identifies ICMP as separate from areas of responsibility covered by Transport Layer protocols like TCP and UDP. In summary, recognizing the specific roles and layers of these protocols helps clarify why ICMP is not included in the Transport Layer.