The handshake, in networking, telecommunication, and computing, is an automated communication process between 2 devices/systems, occuring prior normal data exchange starts, (often) allowing devices to identify each other, verify security credentials, and agree on communication rules, but not all handshakes do all 3 roles. [1] A handshaking process example is between a hypervisor and an application in a guest virtual machine.
The exact role of handshaking slightly differs by fields:
Networking: A handshake establishes a connection/negotiates communication parameters. E.g., TCP 3-way handshake establishes a connection between a client and server.
Telecommunications: A handshake allows devices to coordinate communication settings or establish a link. E.g., A modem handshake negotiates connection parameters between 2 modems.
Computing: A handshake is used between software components, devices, or systems to initialize communication, authenticate, or agree on protocols. E.g., A TLS handshake between a browser and a web server establishes encryption settings.
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TCP (transmission control protocol) 3-way handshake is a connection-oriented transport protocole establishing a session across IP networks, like the Internet, ensuring data is delivered accurately. Many applications like web, email, FTP, use TCP.
It involves 3 separate messages exchanged between a client and a server to establish network connections, known as SYN, SYN-ACK, and ACK, and uses sequence numbers to track datas.
SYN (synchronize): The client sends a Synchronize packet to the server to initiate the connection. "Synchronize" refers that both sides agree on starting points for communication by synchronizing the sequencer numbers for both device to understand how to track the conversation. Its main role is to:
Request a new TCP connection.
Share the sender's initial sequence number.
Prepare both sides to track data reliably.
Note: They synchronize (agree on) the knowledge of the number. It does NOT mean to synchronize (have) the same numbers.
SYN-ACK (synchronize-acknowledge): The server acknowledges the request and sends its own Synchronize packet back.
ACK (acknowledge): The client acknowledges the server's response, confirming the connection is ready for data transmission. Its main role is to:
Confirm received data.
Tell the sender what data is expected next.
Help TCP detect missing packets and resend them if needed .
During the process, both sides must confirm:
The other device is reachable
Both sides agree on initial sequence numbers (e.g., 103, 300, 100)
Both sides are ready to communicate
In short, the 3 steps are as follows:
SYN (Synchronize): The client sends a message to the server asking to start a connection.
SYN-ACK (Synchronize-Acknowledge): The server replies, confirming it received the client message and asking to start its own connection.
ACK (Acknowledge): The client replies to the server's confirmation, finalizing the connection and opening the door for data transfer.
TCP 3-way handshake analogy Imagine a client (the computer) wants to talk to a server (a website).
Step 1: SYN — "Can we connect?"
The client sends a SYN (synchronize) packet to server, saying "I want to start a connection. Here is my starting sequence number." E.g., SYN, Seq=100 (introduction)
Step 2: SYN-ACK — "Yes, I can connect"
The server replies to client via SYN + ACK, "I received your request, and I agree. Here is my sequence number." E.g.,
Server: SYN-ACK, Seq=300, Ack=101
SYN=300 → server's starting sequence number
ACK=101 → "I received your Seq=100, send me the next number"
Step 3: ACK — "Great, we're connected"
The client sends an ACK back to server. E.g., Client: ACK, Ack=301 to confirm "I received your sequence number. The connection is ready."
SSL/TLS Handshake is used to establish a secure, encrypted connection (like when we browse a HTTPS website).
Hello: The client and server exchange random byte strings and agree on the cryptographic cipher suite to use.Authentication: The server sends its digital certificate to the client, which the client validates.Key Exchange: The client and server exchange or compute a premaster secret to generate shared session keys for secure, symmetric encryption.
TCP sequence numbers are used to keep track of every byte of data sent over a connection, counting bytes, NOT packet numbers. It labels data packets.
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Note: Actual such sequence numbers are much larger.
SYN: [1] E.g., The sequencer number generates its sequence number 9001, where ACK is set 0 (not acknowledging previous data) and SYN is set 1 (indicates request to synchronize sequence number and establish new connection).
SYN-ACK: The server replies with SYN set 1 and ACK set as 9002, with its own sequence number set as 5001.
Sequence Numbers increment based on data transmission, which happens after the handshake:
The client started at 9001 in Step 1.
In Step 3, since the client has "used" that first sequence number to initiate the handshake, thus its next sequence number being 9002.