The Transport Layer
Although Layer 3 protocols provide routers with the various mechanisms required for forwarding packets, there are limits to their capabilities. Layer 3 only provides the mechanisms for internetwork data transfers. Layer 4, the transport layer, can provide the Layer 3 network protocols with end-to-end reliability and integrity.
The transport layer may be required to guarantee error-free delivery of packets and sequencing of delivered packets, as well as provide quality-of-service guarantees. The functions of these two layers are highly interrelated. Consequently, they are usually bundled into protocol stacks.
An example of a Layer 4 protocol is the Transmission Control Protocol (TCP). TCP is almost always referenced in combination ...
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