The transport layer

 the transport layer segments and reassembles data into a data stream. services located in the transport layer segment and reassemble data from the upper-layer application and unite it into the same data stream. they provide end to end transport services and can establish a logical connection between the sending host and destination host on an internetwork some of you are probably familiar with TCP and UDP already if so you know that both work at the transport layer and that TCP is a reliable service and UDP is not this means that application developers have more options because they have a choice between the tow protocols when working with TCP/IP protocols.

NOTE - the term reliable networking can be used at the transport layer it means that acknowledgments, sequencing, and flow control will be used. 

Flow Control:-

# the segments delivered are acknowledged back to the sender upon their reception.

# any segments not acknowledged are retransmitted.

# segments are sequenced back into their proper order upon arrival at their destination.

# a manageable data flow is maintained in order to avoid congestion overloading and data loss.

CONNECTION - ORIENTED COMMUNICATION 

transport operation a device that wants to transmit sets up a connection-oriented communication is then transferred with a remote device by creating a session the transmitting device first establishes a connection-oriented session with its peer system which is called a call setup or a three-way handshake. data is then transferred when the transfer is finished a call termination takes place to tear down the virtual circuit.

#the first connection agreement segment is a request for synchronization.

# the next segments acknowledge the requst and establish connection parameters the rules between hosts. these segments request that the receiver sequencing is synchronized here as well so that a bidirectional connection is formed.

the final segment is also an acknowledgment. it notifies the destination host that the connection agreement has been accepted and that the actual connection has been established. data transfer can now begin.

a service is considered conneciton oriented if it has the following characteristics :

# a virtual circuit is set up 

# it uses sequncing

# it uses acknowledgments.

#it uses flow control 

NOTE :- the types of flow control are buffering windowing and congestion avoidance.

windowing :- data throughput happens quickly and efficiently and as you can imagine it would be slow if the transmitting machine had to wait for an acknowledgment after sending each segment. the quantity of data segments that the transmitting machine is allowd to send without receiving and acknowledgment for them is called a window.

windows are used to control the amount of outstanding unacknowledged data segments.

ACKNOWLEDGMENTS :-

data delivery ensures the intefrity of a stream of data sent from one machine to the other through a fully functional data link. it guarantees that the data wont be duplicated or lost this is achieved through something called positive acknowledgment with retransmission a technique that requires a receiving machine to communicate with the receives data the sender documents each segment measured in bytes it then sends and waits for this acknowledgment before sending the next segment round of bytes when it sends a segment the transmitting machine starts a timer and retransmits if it expires before and acknowledgment is returned from the receiving end.