Flash crowds and DOS on POTS

Mon May 17 09:15:54 UTC 2004

On Mon, 17 May 2004 10:32:32 +0200
Iljitsch van Beijnum <iljitsch at muada.com> wrote:

| If they knew the difference between a busy signal and a congestion
| signal they probably would...

Er, no.  Congestion signal normally means that there are no circuits
and the phone network has handled that situation without any issues.
But that's not the primary threat that the switches have to handle

When the call demand far exceeds the number of circuits/operators
available, repeated busy or congestion tones will cause callers to
make repeat attempts.  Local (originating) switches handle this just
fine, and then send forward a C7 call set-up request to the switch
that handles inbound for that number range (I guess, that's their
equivalent of an "MX" host).  And that's where things go wrong.

Digital circuit-switches such as AXE10, DMS100/250 etc are far more
vulnerable to high levels of call-set-up traffic, which would cause
their processors to be overloaded.  Again, the IP analogy is obvious.
Multiple-repeat-attempts at call setup to the same number (and same
destination switch) from numerous originating switches, cause the
processor at the destination switch to be overloaded and to crash.
That's doesn't result in busy or congestion signals - that results
in NO signals (not even dialtone in that exchange's local area).

The telcos' priority therefore is to block the call-setup-attempts at
the edges of their network (i.e. originating /early transit switches).
This is known as "call-gapping" and is not without some controversy.

-- 
Richard Cox