Why does TCP require three handshakes instead of two?

Hello everyone, I am the island owner Xiaofeng. Today I will talk about why TCP needs 3 handshakes instead of 2.

Suppose there is a river, and there are two people upstream and downstream of the river. These two people can only communicate with each other through the river:

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There were many leaves on the shore, so they wrote the message on the leaves on the shore:

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And assume that leaves upstream will float downstream, and leaves downstream will float upstream.

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However, due to the effect of water flow, the leaves that set out first in the same direction may arrive later (out of order) or may sink into the water (be lost).

That is to say, if a sends four leaves, each with two words written on it, recording "Coder's desert island survival", a puts [Coder] [desert] [island survival] [survival] into the river in sequence, after these four leaves reach b, they may become [island survival] [Coder] [survival] [desert] (random order), or one leaf may be lost, becoming [desert] [Coder] [survival] (random order + data loss):

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So in this case, how can we let B know that what A actually wants to say is "The Survival of a Coder on a Deserted Island"?

It's very simple, with only two mechanisms: numbering and confirmation.

The numbering mechanism is aimed at the disorder problem, and the confirmation mechanism is aimed at the loss problem.

Each leaf that a puts into the river is given a number, which increases in sequence. After receiving the leaves, b reassembles them according to the numbers. In this way, even if the leaves arrive in a disordered order, b can still restore the information according to the numbers:

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The disorder problem is solved, and the leaf loss problem relies on the confirmation mechanism: after receiving each leaf, b will reply with a received message, and attach the number of the received leaf + 1, which is the number of the next leaf expected to be sent by a:

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In this way, after a receives [3 received], it is sure that b has received [2 荒]. If a does not receive [3 received] within a certain period of time, it will pick up a leaf and send [2 荒] again.

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It can be seen that this retransmission mechanism ensures that the information can be sent to b even if the leaf may sink into the water.

With the numbering and confirmation mechanism, a can reliably transmit information to b even in an unreliable medium like a river.

Of course, b can also use this mechanism to send messages to a reliably.

Since each leaf needs to be numbered, a must tell b the initial number of the leaf before sending a message to b. Because b can also send messages to a, both parties must know the initial number of each other's leaves before communication, so that they can correctly confirm the receipt of the leaves and re-arrange the leaves according to the number.

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So both parties AB agreed that the chat initiator would first tell the chat receiver the number, and the chat receiver would also tell the initiator his own number after receiving the initiator's number.

Still assuming that a initiates the communication first, a first sends a leaf with "SYN X" written on it. SYN means that this is a leaf that tells you my initial number (in TCP, SYN is the abbreviation of synchronization, but its function is the same as here), and X means its own initial number.

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According to the confirmation mechanism mentioned earlier, b needs to confirm the leaf after receiving it, so b sends a leaf: "ACK X+1", which means "I confirm that I have received your leaf and look forward to receiving leaf number X+1" (ACK is the abbreviation of acknowledge in TCP, which means confirmation, and its function is the same as here).

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Don't forget that b also needs to tell a its own number, so b immediately sends another leaf "SYN Y", which means "this is a leaf telling you my initial number, my initial number is Y":

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Similarly, a also needs to confirm after receiving it, so a sends a leaf: "ACK Y+1", which means "I confirm that I have received your leaf and look forward to receiving your leaf number Y+1."

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After at least four leaves, A and B can know each other's initial number. Pay attention here:

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It can be seen that both leaves are sent from b to a, so the information of these two leaves can be merged together, so that one less leaf can be sent:

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It can be seen that both parties need to send at least 3 leaves to know the other party's initial number, and this cannot be guaranteed if only two leaves are sent.