Transmission Control Protocol (TCP) keeps track of different information about each connection. TCP set up a complex data structure known as Transmission Control Block (TCB) to do this, which maintains information about the local and remote socket numbers, the send and receive buffers, security and priority values, and the current segment in the queue. The Transmission Control Block (TCB) also manages send and receive sequence numbers.
The following table shows different variables Transmission Control Block (TCB) uses.
TCB send variables
TCB Variable |
Description |
SND.UNA |
Send Unacknowledged |
SND.NXT |
Send Next |
SND.WND |
Send Window |
SND.UP |
Sequence number of last urgent set |
SND.WL1 |
Sequence number for last window update |
SND.WL2 |
Acknowledgment number for last window update |
SND.PUSH |
Sequence number of last pushed set |
ISS |
Initial send sequence number |
TCB Receive variables
TCB Variable |
Description |
RCV.NXT |
Sequence number of next received set |
RCV.WND |
Number of sets that can be received |
RCV.UP |
Sequence number of last urgent data |
RCV.IRS |
Initial receive sequence number |
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