Typical Features of OSI Layer

The transport layer (Layer 4) defines several functions, the most important of which are error recovery and flow control. Routers discard packets for many reasons, including bit errors, congestion and instances in which no correct routes are known. As you have read already, most data-link protocols notice errors but then discard frames that have errors. The OSI transport layer might provide for retransmission (error recovery) and help to avoid congestion (flow control)—or it might not. It really just depends on the particular protocol. However, if error recovery or flow control is performed with the more modern protocol suites, the functions typically are performed with a Layer 4 protocol.

OSI Layer 4 includes some other features as well. Table 6-2 summarizes the main features of the OSI transport layer. You will read about the specific implementation of these protocols in the sections about TCP and UDP.

Table 6-2 OSI Transport Layer Features

Feature

Explanation

Connection-oriented or connectionless

Defines whether the protocol establishes some correlation between two endpoints before any user data is allowed to be transferred (connection oriented), or not (connectionless).

Error recovery

The process of noticing errored or lost segments and causing them to be resent.

Reliability

Another term for error recovery.

Flow control

Processes that control the rates at which data is transferred between two endpoints.

Segmenting application data

Application layer protocols may need to send large chunks of data— much larger than can fit inside one IP packet. The transport layer is responsible for segmenting the larger data into pieces, called segments, that can fit inside a packet.

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