Differentiating between requests and responses

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#SoftwareDesign I guess

Why? To avoid an infinite loop in server-server (or peer-to-peer) communications.

Approaches

HTTP

Using the message structure.
An HTTP message consists of two main parts: the request line (for requests) or the status line (for responses), the headers, and the optional body.

HTTP Request:
GET /path/to/resource HTTP/1.1
Host: example.com
[Other Headers]

HTTP Response:
HTTP/1.1 200 OK
Content-Type: text/html
[Other Headers]
[Response Body]

Related:

MQTT

Using message types. There is generally a type for request and a type for response, e.g. SUBSCRIBE and SUBACK, and even PINGREQ and PINGRESP.

Check the section 2.1.2 MQTT Control Packet type. https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html (opens in a new tab)

IRC is similar.
There are success replies (RPL_*) and errors (ERR_*) for most commands, e.g. WHOIS, RPL_WHOISUSER, RPL_ENDOFWHOIS, ERR_NOSUCHNICK, ERR_NONICKNAMEGIVEN, just to list a few.

Related:

END.