Information Technology Reference
In-Depth Information
Connection Establishment
TCP connections are established between two systems by synchronizing with each other's
initial sequence numbers. This is accomplished by an exchange of connection-establishing
segments with the SYN bit set and initial sequence numbers. The TCP connection is full-
duplex; each side sends its own initial sequence number and must receive an acknowledgment
from the other side.
Figure 6-6 shows the three-way handshake for connection establishment.
Three-Way Handshake for Connection Establishment
Figure 6-6
Host A
Host B
SYN(X)
Steps for
connection
establishment
ACK(X)
SYN(Y)
ACK(Y)
Host A
Host B
SYN(X)
Three-way
handshake
ACK(X), SYN(Y)
ACK(Y)
The following steps must occur to have an established connection:
Host A sends a SYN with sequence number X to Host B.
Step 1
Host B must ACK that Host A's sequence number is X.
Step 2
Host B sends a SYN with sequence number Y to Host A.
Step 3
Host A must ACK that Host B's sequence number is Y.
Step 4
This operation is implemented in three steps because Steps 2 and 3 can be sent in one segment.
Therefore, this is called a three-way handshake .
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