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The transport layer implements reliable internetwork data transport services that are transparent to upper layers. Transport-layer functions typically include flow control, multiplexing, virtual circuit management, and error checking and recovery.  Services located in Transport layer both segment and reassemble data from upper layer applications and unite it onto the same data stream.  They provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an Internetwork.  It also hides details of any network dependent information from the higher layer by providing transparent data transfer. 

Flow Control

-Data integrity is ensured by maintaining flow control and allowing users the option to request reliable data transport between systems.  Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process.  Reliable data transport employs a connection-oriented communication session between systems.  The protocols ensure that the following are achieved: 

-segments delivered are acknowledged to sender upon delivery.

-non acknowledged segments are re-sent.

-segments are put back in sequence upon arrival at their destination.

-a manageable data flow is maintained to avoid congestion, overloading, and data loss.

Multiplexing

The Transport layer is responsible for providing mechanisms for multiplexing upper layer applications.  Multiplexing enables data from several applications to be transmitted onto a single physical link.

Virtual Circuits

Virtual circuits are established, maintained, and terminated by the transport layer.

Error Checking and Recovery

Error checking involves creating various mechanisms for detecting transmission errors, while error recovery involves taking an action, such as requesting that data be retransmitted, to resolve any errors that occur.

Acknowledgments

-Positive acknowledgement with retransmission ensures that reliable data delivery by requiring a receiving machine to send an acknowledgment message to the sender when it receives data.  The sending machine documents each segment sent and waits for an acknowledgment before sending the next segment.  Using windowing, the machine will transfer an agreed upon number of segments.  If the receiving machine receives all the segments intact, it will request the next segment of the next window.  If it misses a segment, it will request the missing segment and will transmit a request for the next segment of the next window, when the first window's segments are all received.

-During a transfer, congestion can occur because high speed computers can generate data faster that the network can transfer it or because many computers are using the network and sending datagrams through a single gateway.  When a machine receives a flood of datagrams, it stores them in a buffer.  If the buffer fills, all additional datagrams are discarded.  Transport can issue a "not ready" signal to stop a device from transmitting additional segments.  Once the buffer is emptied, it sends a "ready" transport indicator.  When the waiting machine receives this "go" signal, it continues where it left off.  To avoid failures in data transfers, the receiving host acknowledges every segment it receives.

Connection-Oriented Communications

-In reliable transport operations

• One device first establishes a connection oriented session with is peer (Initiation)
• Both host's application programs begin by notifying their individual Operating Systems that a connection is about to be initiated. (Synchronization)
• The two Operating Systems communicate by sending messages over the network confirming that the transfer is approved and both sides are ready for it to take place. (Negotiating)
• Once the synchronization is complete, a connection is fully established and data transfer begins.  (Established)
• The data transfers.  While the information is being transferred between hosts, the two machines periodically check in with each other, communicating through their protocol software to ensure that all is going well and that data is being received properly.

Windowing

-A window is the number of segments that can be sent without receiving an acknowledgement.  Windowing can increase the throughput for data exchanges by limiting the number of acknowledgments needed for total segments transferred.  Example: if the window size is three then an acknowledgment is required after the third segment is transferred.