I understand the meaning of this field, and how it is used.

But I can't find an example for when it is needed, I read that it is used in Telnet, why?
When do I need this flag?


It is essentially obsolete. But Jeremy Stretch has a good write-up about it.


I found the answer:

Because telnet is connecting to a remote computer, we want to be able to have the commands we type executed right after we typed them. That is why they are flagged with URG flag.

  • 3
    Nope, it says URG is used when transmitting a break. That's a special input on a terminal, which is not transmitted as a normal character by telnet (or by the serial lines where it was originally used). en.wikipedia.org/wiki/Break_key#Teleprinter – sourcejedi May 25 '15 at 14:20

Let’s say telnet is initiated from a client. If the server,for some reason cannot pass requested data to client ( since client is advertising window 0) - the command just hangs.

To avoid this, the server sends a segment with urg flag set along with the urg pointer set too.

The client once reading this will have to accept data from the server.

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