In TCP/IP stack, which layer is equivalent to transport layer of OSI model?

Concept

A protocol stack layer is identified by the SERVICE it provides, not only by its name. The OSI Transport layer (Layer 4) provides end-to-end delivery between two hosts: segmentation, end-to-end flow control, and error control, using port-based connections.

Mapping to the TCP/IP model

In the classic four-layer TCP/IP (DoD/ARPANET) reference model, the layer that performs exactly this end-to-end host-to-host job is the Host-to-Host layer (also written as the Transport layer in modern descriptions). It is where TCP and UDP live, providing port-addressed, end-to-end communication between processes on two hosts.

Why the other layers do not match

• The Internet layer carries IP packets and does logical addressing and routing — that is the OSI Network layer (Layer 3), not Transport.

• The Application layer of TCP/IP rolls together the OSI Application, Presentation and Session layers (Layers 5-7) — well above Transport.

Conclusion

The end-to-end, TCP/UDP-bearing layer is the Host-to-Host layer, so it is the TCP/IP equivalent of the OSI Transport layer. Note: "Host-to-Host" and "Transport" are two names for the SAME layer; since the option set offers "Host-to-Host Layer" and not a literal "Transport layer" choice, "None of these" is not required — Host-to-Host is the matching layer.