How does Frame relay protocol detect link down ?

[arut]

Hello,

I have a general question. How does a higher layer protocol know when
a physical network interface goes down ? Is an interrupt generated
when PHY is down (e.g physical cable is pulled out). How would this
info be propagate to hisher layer protocols ?

Appreciate your answer.

Thanks
asm

[Doug McIntyre]

arut <ar...@mail.com> writes:
>I have a general question. How does a higher layer protocol know when
>a physical network interface goes down ? Is an interrupt generated
>when PHY is down (e.g physical cable is pulled out). How would this
>info be propagate to hisher layer protocols ?

Most WAN protocols have some sort of keepalive protocol talking back
and forth across the link.

In the case of Frame-Relay, its LMI status messages. The Wikipedia
article is decent enough about it.

As to how it gets propegated to higher level protocols, its pretty
dependant on what you have it running on, but in general, it doesn't.
In the context of TCP/IP, the TCP session figures out that its packets
aren't being ACK'd, and eventually drops the pipe. UDP just goes out
to nowhere never to return. On a router, generally, the interface
will go down, and then there won't be a route for whatever you have
going, and things will log that, and the router will send back
whatever is appropriate for that protocol being down.