Transport Layer

Layer 4 of the OSI model. Operates in end stations only; not in network nodes. Primary interface to the network for application developers.

Provides end-to-end data delivery between end stations. Shields upper layers from network peculiarities. Improves on network layer services (e.g. adds reliability).

Functions

Connection management

In connection-oriented data communication, transport layer establishes, manages and deletes the connections.

Multiplexing and Demultiplexing

Transport layer manages multiple connections on a single host using $16$-bit port numbers to distinguish services.

• Outgoing connections: multiplexing.
• Incoming connections: demultiplexing.

Segmentation

Applications may send large amounts of data. Transport layer breaks them into smaller segments and adds transport layer headers. The segments are sent individually and reassembled at destination.

Segmentation is done to work around Maximum Transmission Unit (MTU) limitation.

Flow Control and Buffering

Sender transmits as fast as possible without causing data loss. Receiving end has a buffer for incoming data. Sender should not send data beyond buffer space. Sender retains copies of sent-but-unacknowledged data.

Service Primitives

• Listen
  Wait for an incoming connection.
• Connect
  Establish a connection.
• Send
  Transmit data.
• Receive
  Wait for incoming data.
• Disconnect
  Release connection.

Addressing

TSAP

Aka. Transport Service Access Point. Transport layer address. Each host may contain one or more TSAPs.

Port

16-bit numeric component of a TSAP. Distinguishes multiple connections on one host.

Port identification methods:

• Pre-arrangement (e.g., specified in a URL).
• Well-known ports
  Such as 25 for SMTP and 80 for HTTP. First 1024 port numbers are assigned to well-known services by IANA.
• Portmapper — maps service names to port numbers.
• Application-defined.