Subject: MRU Renegotiation Problem

From: Srikrishnan <[email-masked]>
Date: Mon, 03 Apr 2000 05:20:22 -0700

Hello All,

   I've one question regarding implementing LAC.

   Following is the scenario I've my doubt,

Step 1) PPP user negotiates LCP with LAC ( say  ISP) and MRU negotiated
is 1500   ( let us assume, 1500 is maximum driver capability at the LAC
physical port to PPP)

Step 2)  LAC established tunnel/call to LNS and tunnels all layer 2
(PPP) data from now on to LNS

Step 3) PPP renegotiates LCP with LNS ( which is possible according to
RFC),
and at this time PPP negotiates 1524 MRU, which is accepted by LNS.

Step 4) After negotiation, LNS sends data packets to PPP User through
LAC( which  should ideal be transparent ).

My question is if the packet size is 1524 ( >1500) what will happen?

Issue here is,

* PPP in LAC side has updated its MRU as 1500 after initial negoitation
and not
change after renegotiation.
* Therez no way for LNS to update LAC about the changes in MRU.
   ( therez a mechanism to update change about ACCM not MRU)

What should be solution?

Is the packets dropped at LAC - PPP physical port?
Then therez no way for notification?

Has anyone have any solution to this?

I will appreciate any response. Thanks

Regards
Srikrishnan.

---

From: Srinivasan Chakravarthy <[email-masked]>
Date: Mon, 03 Apr 2000 18:23:59 -0700

As long as the LAC has the capability (like driver buffer size and other
parameters) to transmit 1524 bytes of the PPP packets (which the user sends
after renegotiating with LNS), there should be no problem in the LAC. My view is
LAC should not remember (or even bother) the negotiated PPP parameters if the
remote user and LNS renegotiates. How the LAC to know if the link is
renegotiated between LNS and remote user is an implementation specific detail.

Is my view valid ? Any better solution ?

Thanks & Regards,

Srinivasan.

---

From: "W. Mark Townsley" <[email-masked]>
Date: Tue, 04 Apr 2000 09:30:34 -0400

> renegotiated between LNS and remote user is an implementation specific detail.

The L2TP RFC states that the LAC MUST be able to tunnel PPP packets up
to the default MRU for PPP. This happens to be 1500 bytes (before the
IP/UDP/L2TP header). As long as the LNS does not allow an MRU larger
than this, the LAC should be OK. Incidently, it is probably never a good
idea to try and negotiate a larger MRU than 1500 anyway given the
likelihood of fragmentation on the intervening IP link. In fact, it is
better to ensure that the PPP MRU is some value lower than 1500 to
accomodate the additional headers. There is some discussion of this in
draft-ietf-l2tpext-l2tp-security-00.txt.

There have been proposals to communicate a negotiated MRU from the LNS
to the LAC, as well as suggest parameters from the LAC for the LNS to
negotiate. See draft-ietf-l2tpext-link-00.txt for more info. 

- Mark