What is a data transfer protocol?

Question

PHP

I just don’t need to explain what it is for and how it works, or throw links to Wikipedia.
Everywhere they write a protocol - a set of agreements, a set of rules. But nowhere for some reason does not explain what it is at all? Is it a program or what? Where does the protocol "live"? On my computer or on web servers? Ok, everywhere they write - a set of agreements. And in what form are they presented and where are they described? Someone explain in understandable language.

Answer 1

A protocol is really a set of agreements. It is needed so that two programs can send data or commands to each other. The protocol can only be described in the documentation.
For example, Morse code is also a protocol, and when a man on the street comes up to you and puts two fingers to his lips, because you are wearing headphones and you still won’t hear about a cigarette - this is also a protocol. Therefore, it can "live" only in the minds of those who use it.
Or the URL on the site is also, in general, a protocol. The server says: "Kick at http address What is a data transfer protocol?, I will give you the contents of the page" and indeed, when you go to this address, this is your question. Did you agree with the server ?. A tie on the door in a hostel is also protocol (and I do not advise you to break it).
Wikipedia also has this definition:
And it fits your question very well, actually. When the US Ambassador to Russia, for example, needs some information, he cannot just send a text message to Sergei Lavrov, he must send an official request . Why not a data transfer protocol?
Another protocol can be compared with the alphabet or dictionary. The protocol "lives" in the same place as the alphabet.

Answer 2

This is what describes how two systems should communicate. Because tasks for the exchange between information systems (of the same type or not of the same type) are very different, then they create a description of the exchange - a protocol.
As an example - the protocol for transferring letters between mail servers. Specificity - the transfer of information, mostly text. Therefore, the SMTP PROTOCOL servers communicate like this (briefly):
HELO
MAIL FROM:
RCPT TO:
DATA
RSET
SEND FROM:
SOML FROM:
SAML FROM:
VRFY
EXPN
HELP
NOOP
QUIT
Those. establish a connection, set transmission parameters. You can feel this protocol yourself by connecting to the SMTP server using a banal telnet client:
$ telnet smtp.mail.ru 25
Trying 94.100.177.1...
Connected to smtp.mail.ru.
Escape character is '^]'.
220 mail.ru ESMTP Wed, 18 Nov 2009 20:44:13 +0300
HELO some.host.ru
250 mx30.mail.ru Hello some.host.ru [123.45.67.89]
AUTH LOGIN
334 VXNlcm5hbWU6
dXNlckBtYWlsLnJ1
334 UGFzc3dvcmQ5
centication
X3dvcmQ2 succeeded
Here we connect to the SMTP server, it greets us with the code 220, in response we call our host name HELO some.host.ru
This is actually the data transfer protocol.

Answer 3

Data is transmitted in packets, and the protocol is the order of the fields in each such packet.
Analogue: transportation of cargo by trains with a certain order and the appointment of wagons in them. (these are agreements)
The rest is on Wikipedia.

Answer 4

It's like a file format, a data recording form, a network protocol is used both on the server and on the client, the server writes a packet in this format and transfers it to the client, the client reads it in this format, that's it, I'm tired of chewing, work with any protocol in practice and understand.

Answer 5

Protocols are usually described in RFCs.