Transport Layers 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 (Ex: TCP)
2. Connection less (Ex: UDP)
The transport layer can be thought of literally as a transport mechanism whose responsibility is to make sure that its contents 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 use of ports.
TCP is connection oriented protocol that addresses numerous reliability issues to provide a reliable byte stream. TCP features are:
1. Data arrives in order
2. Data has minimal error
3. Duplicate data is discarded
4. Lost packets are resent
5. Includes traffic congestion control.
The new transport layer protocol SCTP is a reliable connection oriented, message-stream oriented transport mechanism. It 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. Such that if one fails, the connection is not interrupted.
UDP is a connection-less data-gram protocol. It is a best effort protocol. Reliability is addressed through error detection using a weak checksum algorithm. It is used for applications such as streaming media (audio, video, VOIP etc.) In these applications timely arrival of data is more important than reliability.
The appropriate transport protocol (TCP, UDP, and SCTP) is chosen based on the higher layer protocol application. For example FTP (file transfer protocol) expects a reliable transfer whereas Network File System assumes non reliable data transfer. Applications like VOIP can tolerate some loss of packets but not reordering or delay that could be caused by re-transmission.
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