netdev - Re: Re: IPv4 / IPv6 over IPv4 IPsec tunnel: setting the DF bit

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Date:	Thu, 30 Jan 2014 16:59:45 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Simon Schneider <simon-schneider@....net>
Cc:	netdev@...r.kernel.org
Subject: Re: Re: IPv4 / IPv6 over IPv4 IPsec tunnel: setting the DF bit

On Thu, Jan 30, 2014 at 04:26:24PM +0100, Simon Schneider wrote:
> Hi Hannes,
> thanks once again for the quick reply.
> 
> Quickly checked the ip manpage. I'm clear about the case where pmtudisc is in effect (default) - the DF bit must be TRUE in this case, for PMTUD to work.
> 
> Not sure what you meant by:
> 
> "but DF bit should get copied from inner packet up to tunnel header in every
> case"
> 
> Do you mean the nopmtudisc case?

Exactly. In nopmtudisc mode the flag is set based on the inner protocols df
bit, default cleared. In pmtudisc mode the DF-flag is always set.

> Also, IPv6 must be different then - there's no DF bit to be copied.

If packet cannot traverse a router frag_needed is returned, tunnel
endpoint relays the icmp info to the original sender and it should update
its pmtu. There is no way to fragment the packet mid-path.

Also IPv6 tunnel endpoint do not fragment the tunnel packets while
encapsulating.

ipsec mode tunnel is allowed to fragment the packets while encapsulation.

> Could you please clarify?

Hope I did. ;)

Greetings,

  Hannes

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