Subject: Re: IP packet forward from one POS link into another

From: Leo Huber <[email-masked]>
Date: Tue, 08 Jun 1999 09:26:56 PDT

thx, James.

could I forward a PPP session into L2TP over POS in
the following way?

POS in:   UDP2/IP2/PPP2/SDH (POS) -> router ->
POS out:  UDP2/IP2/PPP2     /L2TP/UDP1/IP1/PPP1/SDH/->
          | forwarded |     |<--added L2TP encapsu-|
          |<---PPP2-->|     |lation over POS ----->|

Regards,

Leo

>[email-masked] (Leo Huber) writes:
> >          |<-payload->|  |<------ transport ------->|
> >
> > POS in:     UDP2/IP2/   PPP2/L2TP/UDP1/IP1/PPP1/SDH/-> router ->
> >
> > POS out:    UDP2/IP2/   PPP3/SDH (POS) ->
> >
> > 1. PPP1, PPP2, IP1 and L2TP terminate in the router.
> > 2. PPP3 originates in the router.
>
>I would say that PPP3 also terminates on this router; but it's just a
>semantic thing.
>
> > 3. UDP2/IP2 represent a payload to forward from one POS link
> > into another POS link.
>
>That should be "IP2'" -- any router should be modifying at least the
>TTL and checksum, if nothing else.
>
>Note that "SDH" isn't an encapsulation like any of the others in your
>diagram.  All of the other encapsulations prepend a few bytes to the
>PDU or do some data translation (or both).  SDH has a completely
>separate framing mechanism running and looks like a byte-at-a-time
>(not a packet at a time) pipe to PPP.
>
>--
>James Carlson, Software Architect                   <[email-masked]>
>IronBridge Networks / 55 Hayden Avenue   71.246W   Vox:  +1 781 372 8132
>Lexington MA  02421-7996 / USA           42.423N   Fax:  +1 781 372 8090
>"PPP Design and Debugging" --- http://people.ne.mediaone.net/carlson/ppp
>

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---

From: James Carlson <[email-masked]>
Date: Tue, 8 Jun 1999 12:53:03 -0400 (EDT)

>           |<---PPP2-->|     |lation over POS ----->|

Sure.  But now you're not really behaving as a router -- you're
behaving as a LAC.  Routers do forwarding based on layer 3 data; this
is bridging (layer 2 forwarding).

(Note, of course, that if you did that, you'd have exactly *one* user
session over each SDH channel.  These would have to be some pretty
heavy-duty users.)

(Unless, of course, you're doing PPP over Frame Relay on that "input"
side above.  Then you could terminate the Frame Relay locally, use the
DLCI to select the outbound L2TP tunnel, and forward away.  You still
wouldn't be routing in this case.  You'd still be forwarding on layer
2 data, and you'd still be an L2TP LAC.)