Transport layer bertanggung jawab untuk proses-ke-proses pengiriman. pengiriman paket, bagian dari pesan, dari satu proses ke proses lainnya.
The User Datagram Protocol (UDP) is called a connectionless, unreliable transport protocol. It does not add anything to the services of IP except to provide process-to-process communication instead of host-to-host communication.
UDP length = IP length – IP header’s length
TCP is a connection-oriented protocol; it creates a virtual connection between two TCPs to send data. In addition, TCP uses flow and error control mechanisms at the transport level.
The bytes of data being transferred in each connection are numbered by TCP. The numbering starts with a randomly generated number.
The value in the sequence number field of a segment defines the number of the first data byte contained in that segment.
The value of the acknowledgment field in a segment defines the number of the next byte a party expects to receive. The acknowledgment number is cumulative.
A SYN segment cannot carry data, but it consumes one sequence number. A SYN + ACK segment cannot carry data, but does consume one sequence number. An ACK segment, if carrying no data, consumes no sequence number.
The FIN segment consumes one sequence number if it does not carry data. The FIN + ACK segment consumes one sequence number if it does not carry data.
A sliding window is used to make transmission more efficient as well as to control the flow of data so that the destination does not become overwhelmed with data. TCP sliding windows are byte-oriented.
Some points about TCP sliding windows:
❏ The size of the window is the lesser of rwnd and cwnd.
❏ The source does not have to send a full window’s worth of data.
❏ The window can be opened or closed by the receiver, but should not be shrunk.
❏ The destination can send an acknowledgment at any time as long as it does not result in a shrinking window.
❏ The receiver can temporarily shut down the window; the sender, however, can always send a segment of 1 byte after the window is shut down.
ACK segments do not consume sequence numbers and are not acknowledged. In modern implementations, a retransmission occurs if the retransmission timer expires or three duplicate ACK segments have arrived. No retransmission timer is set for an ACK segment. Data may arrive out of order and be temporarily stored by the receiving TCP, but TCP guarantees that no out-of-order segment is delivered to the process. The receiver TCP delivers only ordered data to the process.