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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