Subject: Re: [L2tpext] Ns in ZLB ACK

From: Motonori Shindo <[email-masked]>
Date: Thu, 13 Mar 2003 18:05:26 +0900 (JST)

Venkatesan,

What was the Nr of the HELLO sent by LNS? If it was 10, then LAC
sending ZLB with Ns = 40 looks violating the spec. Even if it is the
case, packet with Ns = 10 will be retransmitted by LAC and make it to
the LNS eventually, and tere will be no significant effect; it is just
one waste packet sent by LAC. If it ended up with something more
catastrophic, then implementation at either side must be broken.

Regards,

From: Venkatesan Pradeep <[email-masked]>
Subject: [L2tpext] Ns in ZLB ACK
Date: Fri, 14 Feb 2003 18:11:51 -0800

> I was testing an LNS using a Cisco LAC (IOS 12.2(T)) and observed a
> problem with the Ns in ZLB ACKs sent by the LAC. The LNS was being
> stress tested and we were seeing packet loss on it. The LNS had
> advertised a window of 20 and the next expected seq was 10. The LAC
> had sent packets with Ns = 10 thru Ns = 29 and 10 more were queued
> for transmission. The first packet (Ns = 10) didn't make it to the
> LNS. The LNS sent a HELLO packet and the LAC responded with
> a ZLB ACK with Ns = 40.
> 
> The LNS discarded the ZLB because it was outside its window. It looks
> like the LAC increments the sequence number right after queueing the
> packets for transmission and is using that for the ZLB.
> 
> I think that the LNS was right in rejecting the ZLB. Am I correct?.
> If so, what should the Ns in the ZLB be in this scenario?
>  Ns = 29: This will make sure that it is within the LNS' window but
>           given that a non-ZLB packet with Ns=29 had been transmitted,
>           the next packet (ZLB or not) should have Ns=30
>  Ns = 30: This will satisfy the Ns increment rule but the packet will
>           be outside the LNS' window.
> 
> Thanks,
> 
> Pradeep.
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From: Motonori Shindo <[email-masked]>
Date: Thu, 13 Mar 2003 18:05:26 +0900 (JST)

Venkatesan,

Regards,