Which of the following functions are performed by the OSI Transport layer

Data segments in the transport layer are transported in a reliable, transparent manner. As a result, it provides end-to-end error checking and recovery, multiplexing, virtual circuit management, and flow control. Each message is assigned a sequence number at the time of transmission. Upon receipt, the packets are reassembled, errors are checked, and acknowledgement is provided. It ensures that the transmitting device does not send more data than the receiving device can handle.Transport layer specifications include TCP(Transmission Control Protocol), UDP(User Datagram Protocol) or RTP (Realtime Transport Protocol). Protocols such as TCP, or Transmission Control Protocol, are connection-oriented. In connection-oriented transport protocols, if a segment is dropped, the sender has the ability to detect and retransmit the segment. Receivers acknowledge segments that they receive. Using these acknowledgments, a sender can decide which segments have been successfully received and which segments need to be retransmitted. The UDP protocol is a connectionless protocol. Unreliable transport is provided by connectionless protocols, since if a segment is dropped, the sender is unaware of the drop, and no retransmission is attempted. In the same way that Layers 2 and 3 offer flow control services, Layer 4 also offers flow control services. At Layer 4, windowing and buffering are the most commonly used flow control methodologies. TCP communication uses windowing, which means one or more segments are sent at once, and the receiver can acknowledge receipt of all the segments in a window. TCP sometimes uses a sliding window.

Upon acknowledgment of the first segment (the receiver sends an acknowledgment requesting the next segment), the window size doubles. If the first two segments are successfully received, the next window contains four segments. There is an exponential increase in window size until the receiver acknowledges the successful receipt of all segments within a certain period of time (called the round-trip time or RTT) or until a configured maximum window size has been reached.Using buffering, a device (for example, a router) stores segments in a piece of memory (sometimes called a buffer or a queue) if the bandwidth is not available to transmit those segments. There is a finite capacity for a queue, however, and it can overflow (that is, drop segments) in the event of prolonged network congestion.

Written by pramod