linux-kernel - Re: batman-adv: design suggestions

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Message-Id: <201008141659.14868.lindner_marek@yahoo.de>
Date:	Sat, 14 Aug 2010 16:59:13 +0200
From:	Marek Lindner <lindner_marek@...oo.de>
To:	Vasiliy Kulikov <segooon@...il.com>,
	Linus Lüssing <linus.luessing@....de>
Cc:	"Greg Kroah-Hartman" <gregkh@...e.de>,
	Andrew Lunn <andrew@...n.ch>,
	Sven Eckelmann <sven.eckelmann@....de>,
	Simon Wunderlich <siwu@....tu-chemnitz.de>,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: batman-adv: design suggestions

On Friday 13 August 2010 20:18:33 Vasiliy Kulikov wrote:
> d) Why do you send icmp TTL exceeded for the icmp itself? E.g. in case
> of loop or/and small default TTL you'll probably get a storm of icmps.
> Exactly in this case IP silently drops TTL exceeded icmps ;)

These layer2 icmp packets are not ordinary icmp packets. We needed to provide 
a mechanism to make the network topology visible to debug tools like ping or 
traceroute which normally "see" no more than one hop as they operate on 
layer3. Hence, batman-adv does not send an icmp packet for each payload TTL 
exceeded but for traceroute only. I recommend reviewing the traceroute code to 
understand how this is supposed to work:
http://www.open-mesh.org/browser/trunk/batctl/traceroute.c

I'd be interested to learn about a problematic scenario in which this 
mechanism breaks.

Regards,
Marek
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