GATE CS 2003 Q28 — Concept of Layering: OSI and TCP/IP Protocol Stacks

The correct option is D: End to end connectivity.

Explanation:

• Network Layer vs. Transport Layer: The primary responsibility of the Network Layer is host-to-host delivery (routing packets from the source host to the destination host). The Transport Layer extends this service to provide process-to-process (end-to-end) communication, ensuring that data is delivered to the correct application process on the destination host.
• Why other options are incorrect: A transport protocol is not strictly required to be reliable. For example, UDP (User Datagram Protocol) is a widely used transport layer protocol that does not guarantee:
  • Recovery from packet losses (Option A)
  • Detection of duplicate packets (Option B)
  • In-order delivery of packets (Option C)
  Since UDP is still a transport protocol despite lacking these features, they are not mandatory functionalities for all transport protocols.

Therefore, the only functionality that must be implemented by any transport protocol over and above the network protocol is end-to-end (process-to-process) connectivity.