What is a function of Layer 4 of the OSI model?

1. Segmentation.

2. End-to-end delivery.

3. Port addressing for upper layer protocol.

4. Flow Control via Windowing and Buffering.

5. Error Check.

TRANSPORT LAYER

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers. The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery. The transport layer provides:

• Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.
• Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.
• Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.
• Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).

Typically, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame. The transport layer header information must then include control information, such as message start and message end flags, to enable the transport layer on the other end to recognize message boundaries. In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above.

to describe the ordered and reliable delivery of data between source and destination

 

The transport layer controls the reliability of communications through flow control, segmentation, and error control. Two great examples of transport protocols are TCP (as in TCP/IP) and UDP.

Transport layer is the layer that transport data from source to destination use tcp or udp in the case of tcp it check the date if there is any problem data down and ask to resend and it used in the important data like download and upload(ftp) in the case of udp if there is any error in data it down without ask to resend it almost used in online videos and voice streaming

its job is to manage transportation of data, that is how data is transport between devices

this answer for tcp/ip model 

layer 4 is "transport "transfer data to segments

Include this layer tcp/udp protocol