Subject: L2TP Hello Message Semantics

From: "Vic Diloreto" <[email-masked]>
Date: Sun, 20 Feb 2000 16:33 -0500

A question regarding the hello message exchange between a LAC and an LNS.
We have noticed that when a LAC sends a Hello, and the LNS responds, that
sometimes the ack gets lost due to network congestion. The LAC retries, but the
LNS does not respond as it thinks it has already responded. Is this appropriate 
behavior for the LNS ? Should the LAC be doing something different to make 
sure the Hello gets a response ?

Thanks,

Vic Diloreto

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From: Motonori Shindo <[email-masked]>
Date: Mon, 21 Feb 2000 11:58:59 +0900

Vic,

Here's an exerpt from rfc-2661.

                                                       Such a message
   would be considered a duplicate of a message already received and
   ignored from processing. However, in order to ensure that all
   messages are acknowledged properly (particularly in the case of a
   lost ZLB ACK message), receipt of duplicate messages MUST be
   acknowledged by the reliable transport. This acknowledgement may
   either piggybacked on a message in queue, or explicitly via a ZLB
   ACK.

My view to L2TP is that L2TP protocol stack consists of two distinct
layers: message delivery and protocol processing. From this
perspective, the paragraph above can be elaborated as "ignored by the
protocol processsing layer, however it must be always acknowledged by
the message delivery layer because it must guarantee the reliable
transport.

There used to be an implementation that never acknowledges the
duplicted messages. I know of at least one implementation that had
this problem. Newer version of this software fixed this problem, but
in a little bit weird way.

Regards,

From: "Vic Diloreto" <[email-masked]>
Subject: L2TP Hello Message Semantics
Date: Sun, 20 Feb 2000 16:33 -0500

> sure the Hello gets a response ?

=====================================================
Motonori Shindo
   Systems Engineer     
   Ascend Communications Japan K.K.
   A wholly owned subsidiary of Lucent Technologies.
   email: [email-masked] or [email-masked]
   TEL: +81-3-5325-7306 
=====================================================

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From: Tatsuya Yanagibashi <[email-masked]>
Date: Mon, 21 Feb 2000 12:18:24 +0900

It happens to lost ZLB Ack from LNS because of network congestion.

From l2tp RFC, a reciever of contorol message MUST acknowledge by piggybacked 
on a message in queue, or explicitly via a ZLB ACK even if that control message 
seem to be duplicated.

Regards,
Tatsuya Yanagibashi