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Re: IPv6 fragmentation and IPv6 header parsing

To: kuznet@xxxxxxxxxxxxx
Subject: Re: IPv6 fragmentation and IPv6 header parsing
From: Brad Chapman <kakadu@xxxxxxxxxxxxx>
Date: Wed, 01 Aug 2001 18:16:56 -0400
Cc: netdev@xxxxxxxxxxx
References: <>
Sender: owner-netdev@xxxxxxxxxxx
User-agent: Mozilla/5.0 (X11; U; Linux 2.4.7 i586; en-US; C-UPD: MaxLinux0301) Gecko/20001107 Netscape6/6.0
Mr. Kuznetsov,

kuznet@xxxxxxxxxxxxx wrote:


      I read somewhere that the correct size for an IPv6 link was 1500.
Is this wrong? Is the correct MTU smaller? If it is, then sorry in
advance ;-)

Yes, it is 1280. And lower MTUs are simply prohibited for IPv6 networks by law,
it is difference of IPv4, where network can have any mtu.

Sorry again :-( I remember reading somewhere a while ago that an IPv6 packet was
sized around 1500, but I also remember reading online that it was 1280....Guess I was
wrong. Sorry :-(

IPv6 is designed specially to avoid such things. If you have some
idea, when this can be useful, right starting point is not to hack something
contradicting to the ideology, but to workaround this at protocol level.

Side note: connection tracking is serious offence even for IPv4. I am puzzled, why the code is so primitive and forces defragmentation even when it is possible just to save fragments and resent them. This prevents f.e. usage of conntrack on routers, which need only to account. In fact, the most rarely happening case is considered as the most common one... It would be good if IPv6 did this right from the very beginning, rather than repeated mistakes of conntrack in IP.

Since I didn't write the original conntrack code, I'm not sure of what Rusty
Russell thought when he wrote it. But, IMHO he made ip_conntrack do fragmentation-
on-the-fly because it would then be easier to track the guts of the packet, and do NAT.
Now that NAT, as we have both said, is not necessary anymore for IPv6; we may not
even need redirection and port forwarding either. Anyway, like I said to Harald,
anything regarding a system in ip6_conntrack where we save fragments and resend them
and/or block them, will have to wait until 2.5. For now, just make your MTU a proper
size and ip6_conntrack should work.
BTW: It will probably be another month or so before ip6_conntrack is stable.



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