So to come back to your question, why not just use a two-way handshake? The short answer is because a two way handshake would only allow one party to establish an ISN, and the other party to acknowledge it. Which means only one party can send data. But TCP is a bi-directional communication protocol, which means either end ought to be able to send data reliably. So in effect, what you have is exactly your description of the two-way handshake, but in each direction.
Hence, four events occurring. And again, the middle two flags happen in the same packet. As such three packets are involved in a full TCP connection initiation process. The three-way handshake is necessary because both parties need to syn chronize their segment sequence numbers used during their transmission. In order for the connection to work, each side needs to verify that it can send packets to the other side. The only way to be sure that you got a packet to the other side is by getting a packet from them that, by definition, would not have been sent unless the packet you sent got through.
There's actually a third kind of message, but we'll get to that in a moment. Before the connection starts, neither side really knows anything about the other. The client sends a SYN packet to the server, to request proof that its messages can get through.
That doesn't tell either person anything, but it's the first step of the handshake. If the SYN gets through, then the server knows that the client can send packets to it, because, well, it just happened.
But that doesn't prove that the server can send packets back: clients can send SYNs for lots of reasons. This is the second step of the handshake. So it sends back an ACK: just a plain ACK this time, because it doesn't need proof anymore that its packets can get through. This is the final step of the handshake: the client now knows that packets can go both ways, and that the server is just about to figure this out because it knows the ACK will go through.
Once that ACK gets through, now the server knows that it can send packets to the client. It also knows that the client knows this, so it can start sending data right away. The handshake is complete. We have a good channel.
Well, strictly speaking, we can't be certain we have a good channel. Just because this sequence of packets got through does not strictly guarantee that others will.
We can't prove that without sending an infinite number of SYNs and ACKs, and then nothing else would ever get done, so that's not really a practical option. But in practice, three steps turns out to be good enough for most purposes. Actually, a 3-way handshake isn't the only means of establishing a TCP connection.
TCP connection is bidirectional. What this means is that it actually is a pair of one-way connections. Two simplex connections form one duplex TCP session, agree? So logically there are four steps involved; but because SYN and ACK flags are different "fields" of TCP header, they can be set simultaneously - the second and the third steps of the four are combined, so technically there are three packet exchanges. Each simplex half- connection uses 2-way exchange, as you proposed. It is not necessary at all.
The previous answers just describe the system without discussing the need for random sequence numbers etc. Hosts on the network read all data segments within a session and exchange information about what data is received using the information in the TCP header.
TCP is a full-duplex protocol, where each logical connection represents two one-way communication streams or sessions. To set up the connection, the hosts do a TCP 3-way handshake. A three-way handshake is primarily used to create a TCP socket connection to reliably transmit data between devices. For example, it supports communication between a web browser on the client side and a server every time a user navigates the Internet. As soon as a client requests a communication session with the server, a three-way handshake process initiates TCP traffic by following three steps.
First, a connection between server and client is established, so the target server must have open ports that can accept and initiate new connections. This SYN packet is a random sequence number that the client wants to use for the communication for example, X. This packet includes two sequence numbers. The first one is ACK one, which is set by the server to one more than the sequence number it received from the client e.
The second one is the SYN sent by the server, which is another random sequence number for example, Y. Once again, each side must acknowledge the sequence number received by incrementing it by one.
Upon completion of this process, the connection is created and the host and server can communicate. Skip to content. Here is a graphical representation of the process: As the name implies, the three way handshake process consists of three steps: Host A initiates the connection by sending the TCP SYN packet to the destination host. The packet contains the random sequence number e.
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