In TCP/IP-based end-to-end communication, only the source and destination hosts process the header information added...

In the standard TCP/IP forwarding model, transport-layer headers such as TCP and UDP headers are added by the source host and are mainly interpreted by the destination host. Routers that forward packets between the source and destination operate primarily at the network layer, using the destination IP address in the IP header to make forwarding decisions. Therefore, under normal routing behavior, routers do not process transport-layer header information when deciding how to forward packets.

This is a key concept in layered communication. The source host encapsulates application data with a transport-layer header, then with an IP header, and finally with a data-link header. Each router along the path removes only the Layer 2 frame header, checks the Layer 3 destination IP information, decrements TTL, recalculates the IP header checksum when required, and forwards the packet. The transport-layer content remains unchanged in normal forwarding. HCIA-Datacom uses this principle to explain end-to-end communication and layer responsibilities. Although advanced devices may inspect higher-layer information for security or policy purposes, standard router forwarding in the basic TCP/IP model does not depend on transport-layer processing.