Date: Wed, 2 Aug 2000 09:52:46 -0700 (PDT) From: Archie Cobbs <archie@whistle.com> To: Ruslan Ermilov <ru@FreeBSD.ORG> Cc: Archie Cobbs <archie@whistle.com>, Charles Mott <cmott@scientech.com>, Erik Salander <erik@whistle.com>, net@FreeBSD.ORG, Julian Elischer <julian@elischer.org>, Brian Somers <brian@Awfulhak.org>, Eivind Eklund <perhaps@yes.no> Subject: Re: Improved PPTP support for libalias(3) Message-ID: <200008021652.JAA81974@bubba.whistle.com> In-Reply-To: <20000802194901.D36141@sunbay.com> from Ruslan Ermilov at "Aug 2, 2000 07:49:01 pm"
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Ruslan Ermilov writes:
> > > > > > Is this a limitation of a specific server implementation, or
> > > > > > a limitation of the PPTP standard?
> > > > >
> > > > > It's simply a limitation in our address translation module for PPTP.
> > > > > There's nothing implied wrong with the standard itself or the server
> > > > > implementation.
> > > > >
> > > > > This limitation could be eliminated with more coding, but it's
> > > > > somewhat ugly (you have to make two TCP streams appear as one).
> > > >
> > > > Sorry, I may have misinterpreted your question...
> > > >
> > > > It is inherent in the PPTP standard that there be at most ONE
> > > > PPTP TCP control connection between any two IP addresses.
> > > >
> > > > If you think about it for a second you can see why: when a machine
> > > > receives a GRE packet, it identifies the call using the pair
> > > > <sourceIP, CallID>. That means that there can be at most ONE entity
> > > > living at sourceIP doling out CallID's for calls to the local
> > > > machine/IP address.. otherwise CallID's would not be guaranteed
> > > > to be unique.
^^^^^^^^^^^^^^^^^ this is why ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > > This is all right, except libalias(3) is supposed to intercept
> > > outgoing PPTP call requests messages and alias CallID to be unique,
> > > see AliasHandlePptpOut().
> >
> > Sorry, I don't understand your point... libalias already does this fine,
> > that's not the problem.
> >
> > The problem is that two internal clients connecting to the same
> > external server at the same time will result in two TCP connections
> > to the same server seeming to come from the same IP address, which
> > violates the protocol.
>
> Sorry, I misinterpreted your explanation. But I still do not understand
> why there is such a limitation in PPTP, can you see any sense here?
Because there can be only one "controlling authority" for each
IP address.. see above.
-Archie
___________________________________________________________________________
Archie Cobbs * Whistle Communications, Inc. * http://www.whistle.com
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