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TCP is a connection-oriented protocol.
Prior to data transmission, the two communicating hosts go through a
synchronization process to establish a virtual connection. This
synchronization process insures that both sides are ready for data
transmission and allows the devices to determine the initial sequence
numbers. This process is known as a three-way handshake. This is a
three-step process that establishes the virtual connection between the
two devices.
- First, one host initiates a
connection by sending a synchronization (SYN) packet indicating its
initial sequence number of x with a certain bit in the header set to
indicate a connection request. This bit is set in the acknowledgment
number field of the TCP header (also referred to as the code field).
- Second, the other host receives the
packet, records the sequence number of x, replies with an
acknowledgment of x + 1, and includes its own initial sequence
number of y. The acknowledgment number of x + 1 means the host has
received all octets up to and including x, and is expecting x + 1
next.
- Finally, the initiating host
responds with a simple acknowledgment (ACK) of y + 1 (host B’s
sequence number + 1), indicating it received the previous ACK which
finalizes the connection process.
It is important to understand that
sequence numbers are a part of initiating communication between the
two devices. They act as reference starting numbers between the two
devices. The sequence numbers give each host a way to ACK the SYN so
that the receiver knows the sender is responding to the proper
connection request.
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Interactive Media Activity
Drag and Drop: TCP Synchronization
After completing this activity, the student will be able to understand TCP
synchronization.
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