TCP Specification



Transmission is made reliable via the use of sequence numbers and acknowledgments.


Conceptually, each octet of data is assigned a sequence
number.


The sequence number of the first octet of data in a segment is transmitted with that segment and is called the segment sequence number.


Segments also carry an acknowledgment number which is the sequence number of the next expected data octet of transmissions in the reverse direction.


When the TCP transmits a segment containing data, it puts a copy on a retransmission queue and starts a timer; when the acknowledgment for that data is received, the segment is deleted from the queue.


If the acknowledgment is not received before the timer runs out, the segment is retransmitted.