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To: Mike Tancsa <mike@sentex.net>
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In-reply-to: mike's message of Wed, 08 Aug 2001 16:32:01 -0400.
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Subject: Re: gif MTU of 1280 ? 
From: Jun-ichiro itojun Hagino <itojun@iijlab.net>
Date: Thu, 09 Aug 2001 05:47:09 +0900
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>172.16.1.1/24 <--> 10.0.0.1/30 - - - 10.0.0.2/30<--->192.168.1.1
>
>With 192.168.1.1 and 172.16.1.1 being non RFC 1918 addresses in this 
>example and the 10.0.0.1 IPs being the internal addresses.
>
>Lets say a machine (216.136.204.21 wants to get to 192.168.1.1, and it 
>comes in via the public gateway 172.16.1.1.  To get to 192.168.1.1, it must 
>be fragmented. The question is, when the ICMP message is sent out, with 
>what source address does the packet leave ? e.g. if 216.136.204.21 is 
>configured to ignore all traffic from the RFC 1918 space, and the ICMP 
>message goes out with a source address of 10.0.0.1 or 10.0.0.2, then 
>216.136.204.21 will never hear it.

	i don't understand what is 10.0.0.1 and 10.0.0.2.  is it a tunnel
	outer address like configured with gifconfig(8), or tunnel inner
	address like configured with ifconfig(8)?  i assume it as the latter.

	when an ICMP need fragmented message is emitted from 172.16.1.1
	device to 216.136.204.21, normal routing decisions take place
	for source address determination.  that is, 172.16.1.1 in the normal
	case.

	original packet: IP[216.136.204.21 -> 192.168.1.1] payload
	ICMP message: IP[172.16.1.1 -> 216.136.204.21] ICMP [original packet]

	successfully-encapsulated packet

itojun

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