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From: Maciej Milewski <milu@dat.pl>
To: Matthias Apitz <guru@unixarea.de>
Date: Wed, 18 Jan 2012 10:05:26 +0100
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Cc: freebsd-net@freebsd.org
Subject: Re: PPP to UMTS provider && incoming traffic (TCP, UDP)
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Dnia =C5=9Broda, 18 stycznia 2012 07:35:49 Matthias Apitz pisze:
> Thanks for your feedback; I think too, that UDP downstream is blocked=
 if
> there is not upstream UDP initialized for the same socket pair; and V=
oIP
> is using different UDP ports for up- and downstream media (audio).
>=20
> What I'm wondering is, how does Skype works (which is fine with my UM=
TS
> provider at the same time), have to check this with TCPDUMP...
>=20
> =09matthias
Skype according to wikipedia[1] uses such scenario:
 1.  start
 2.  send UDP packet(s) to HC
 3.  if no response within 5 seconds then
 4.    attempt TCP connection with HC
 5.    if not connected then
 6.      attempt TCP connection with HC on port 80 (HTTP)
 7.      if not connected then
 8.        attempt TCP connection with HC on port 443 (HTTPS)
 9.        if not connected then
10.          attempts++
11.          if attempts=3D=3D5 then
12.            fail
13.          else
14.            wait 6 seconds
15.            goto step 2
16.  Success

So it uses TCP and to obey blocked ports uses www traffic ports: 80 and=
 443.

[1] http://en.wikipedia.org/wiki/Skype_protocol

--=20
Maciej Milewski