Subject: RE: [L2tpext] Ns in ZLB ACK

From: Venkatesan Pradeep <[email-masked]>
Date: Thu, 13 Mar 2003 11:49:40 -0800

The Nr was 10. And yes, if the LAC retransmits got thru to the LNS then
then we were ok. However, if they didn't reach in time the tunnel was
brought down (after the LNS' max retransmission period expired).

Pradeep.

-----Original Message-----
From: Motonori Shindo [mailto:[email-masked]]
Sent: Thursday, March 13, 2003 1:05 AM
To: Venkatesan Pradeep
Cc: [email-masked]
Subject: Re: [L2tpext] Ns in ZLB ACK

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.