Transport Layer

The transport layer's responsibilities include end-to-end message transfer capabilities independent of the underlying network, along with error control, fragmentation and flow control. End to end message transmission or connecting applications at the transport layer can be categorized as either:

1. connection-oriented e.g. TCP
2. connectionless e.g UDP

The transport layer can be thought of literally as a transport mechanism e.g. a vehicle whose responsibility is to make sure that its contents (passengers/goods) reach its destination safely and soundly, unless a higher or lower layer is responsible for safe delivery.

The transport layer provides this service of connecting applications together through the use of ports. Since IP provides only a best effort delivery, the transport layer is the first layer of the TCP/IP stack to offer reliability. Note that IP can run over a reliable data link protocol such as the High-Level Data Link Control (HDLC). Protocols above transport, such as RPC, also can provide reliability.

For example, TCP is a connection-oriented protocol that addresses numerous reliability issues to provide a reliable byte stream:

• data arrives in-order
• data has minimal error (i.e correctness)
• duplicate data is discarded
• lost/discarded packets are resent
• includes traffic congestion control

The newer SCTP is also a "reliable", connection-oriented, transport mechanism. It is Message-stream-oriented — not byte-stream-oriented like TCP — and provides multiple streams multiplexed over a single connection. It also provides multi-homing support, in which a connection end can be represented by multiple IP addresses (representing multiple physical interfaces), such that if one fails, the connection is not interrupted. It was developed initially for telephony applications (to transport SS7 over IP), but can also be used for other applications.

UDP is a connectionless datagram protocol. Like IP, it is a best effort or "unreliable" protocol. Reliability is addressed through error detection using a weak checksum algorithm. UDP is typically used for applications such as streaming media (audio, video, Voice over IP etc) where on-time arrival is more important than reliability, or for simple query/response applications like DNS lookups, where the overhead of setting up a reliable connection is disproportionately large. RTP is a datagram protocol that is designed for real-time data such as streaming audio and video.

TCP and UDP are used to carry an assortment of higher-level applications. The appropriate transport protocol is chosen based on the higher-layer protocol application. For example, the File Transfer Protocol expects a reliable, but the Network File System assumes that the subordinate Remote Procedure Call protocol, not transport, will guarantee reliable transfer. Other applications, such as VoIP, can tolerate some loss of packets, but not the reordering or delay that could be caused by retransmission.