Connection-oriented transport protocols can be either stream-oriented or message-oriented. In a message-oriented transport protocol, each block of data is sent and received as a single block of data. An individual message is never broken down when received, although a transport protocol may be forced to break down a message if it is larger than the physical network's maximum transmission unit (MTU). However, this is transparent to the application.
In a stream-oriented transport, data is sent and received as a continuous stream of data without any message boundaries. The transport protocol is free to coalesce data from discrete send() calls and deliver it to a single recv() call as well as to break down the data buffer given to a single send() call and deliver it to several recv() calls. An application has no control over how these breakdowns occur, and an application gets no notification of how data was sent. If an application needs to communicate in discrete messages, it is the responsibility of the application to do message framing, for example by proceeding message data by the length of the message on send and receiving until the specified length has arrived.
Some message-oriented protocols support stream semantics in addition to message semantics. If an application opens a socket with SOCK_STREAM on such a transport, then the transport ignores all message boundaries when transmitting and receiving data. It simply delivers data as it becomes available without regard to whether the data comprises a complete message. This feature is handy for maximizing application portability. Note, however, that stream-oriented transport protocols cannot implement message-oriented semantics due to lack of information.