Subject: PPP Payload in L2TP

From: "Walter Klausberger" <[email-masked]>
Date: Tue, 10 Feb 1998 17:37:14 +0100

Hi
I have a question about PPP over L2TP.

In L2TP Draft, Page 8 (end of page) is written:
Frames from the remote user are received at the POP, stripped of CRC, link
framing, and transparency bytes,...

Question: If the PPP connection works over ISDN (synch. HDLC Framing)
does the address and control field belong to the link framing, or are they
also encapsulated into L2TP.

This question arises, because we plan to transport PPP over AAL5 and Frame
Relay. This PPP packets will be forwarded into a L2TP tunnel. The framing of
AAl5 or Frame Relay is different to the framing of ISDN/HDLC

with best regards
Walter Klausberger
Siemens Austria

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From: Richard Shea <[email-masked]>
Date: Tue, 10 Feb 1998 12:19:56 -0500

> also encapsulated into L2TP.

Address and control field are encapsulated into L2TP as necessary (i.e.
when present).  In the case of HDLC I believe "link framing" would
consist of the "flag sequence" which would of course not be encapsulated
in L2TP.

Richard.

-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
Richard Shea                                  [email-masked]
Bay Networks, Extranet Access                     978-266-1011 x210

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From: Rene Tio <[email-masked]>
Date: Tue, 10 Feb 1998 11:05:10 -0800 (PST)

Rich,

Perhaps I am misunderstanding your remark, but PPP in Frame Relay says the
frame format is:

   +-+-+-+-+-+-+-+-+
   |  Flag (0x7e)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Q.922 Address         |    Control    |  NLPID(0xcf)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         PPP Protocol          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

If link framing is only the flag sequence, and the LAC only strips this
portion, the LNS would see the Q.922 header in full.  Is this the intent of
the L2TP text?

Thanks
--
Rene Tio <[email-masked]>

> From [email-masked]  Tue Feb 10 09:25:40 1998
>

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From: Richard Shea <[email-masked]>
Date: Tue, 10 Feb 1998 14:15:06 -0500

> Perhaps I am misunderstanding your remark, but PPP in Frame Relay says

For HDLC (ala rfc 1662) the flag sequence is the link framing which is
removed.  I honestly don't know what the intent is for FR or AAL5
encapsulated PPP over L2TP.  IF (big if) I had to guess, I would guess
that everything above except "PPP Protocol" would be removed.  I know
for a fact that my LNS implementation wouldn't know what to do with the
rest of it!

> > > Frames from the remote user are received at the POP, stripped of
> CRC,
> > > link
> > > does the address and control field belong to the link framing, or
> are
> > > they