Subject: [L2tpext] Size packet in L2TP standard

From: "Francisco Javier Munnoz Calle (JVT)" <[email-masked]>
Date: Wed, 12 Jun 2002 19:21:12 +0200

Dear sirs/madams,

In our work team, we are researching about the standard L2TP (RFC 2661)
and we have observed a possible weakness about it we would like to
comment you about, if it could result interesting as an improvement to
deal with in the standard.

After some investigation, we have a doubt about the implementation of
the L2TP packet size management mechanism, as it seems not to be
included in the proposed drafts nowadays. In a more detalied way we
haven't found any L2TP mechanism which lets two L2TP extremes (LNS and
LAC) to interact to carry out the following:

- Negotiation about packet size sent through the tunnel.

- To indicate MTU or MRU at the user's side. We have only found in "L2TP
Link Extension" draft the possibility of negociating LCP options from
both extremes LAC and LNS; including the possibility that: firstly LAC
provides a maximum MRU value; secondly, LNS starts the negotiation with
that value and finally it could be reduced if it is necessary.

This function must be useful to avoid problems like the ones presented
when IP fragmentation (gathered in RFC 2661) is employed.

Trying to be helpful, best regards.

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From: "W. Mark Townsley" <[email-masked]>
Date: Wed, 12 Jun 2002 15:57:48 -0400

> - Negotiation about packet size sent through the tunnel.

There are a number of ways to negotiate or configure MTU sizes throughout a
network or at the end hosts, depending on where you are trying to make the
adjustment. For L2TP, the most general and straightforward is via PPP. If the
MTU (with appended L2TP/PPP headers) is known between the LAC and LNS, there are
two chances to negotiate this correctly. The PPP MRU option may be negotiated
first at the LAC, then again if LCP is renegotiated at the LNS. This is part of
RFC 2661, the "Link Extensions" draft you mentioned is not required.

When the MTU is not managed across the network in an ideal manner, a large PPP
MRU is expected by SLA at the client, or the PPP client is broken (sending PPP
packets larger than that which was negotiated), alternatives must be sought. In
these cases, either the connectivity between the LAC and LNS will need to
support a larger MTU than the PPP MRU + tunnel overhead (perhaps via Ethernet
Jumbo or Baby-Jumbo Frames), or IP fragmentation and reassembly will need to be
performed at the LAC and LNS (with serious affects on performance likely).

Hope this helps,

- Mark