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Date:      Fri, 24 Oct 2003 09:19:02 -0700
From:      Michael Sierchio <kudzu@tenebras.com>
To:        Mailing List FreeBSD Network <freebsd-net@FreeBSD.org>
Subject:   Re: ipsec tunnels & packet length issues
Message-ID:  <3F9950F6.6000208@tenebras.com>
In-Reply-To: <8665iehd1i.fsf@t39bsdems.interne.kisoft-services.com>
References:  <8665iehd1i.fsf@t39bsdems.interne.kisoft-services.com>

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Eric Masson wrote:

> If i reduce  lan interface mtu on "Host" to approximately 1450, the
> tunnel works fine, so it seems that "Tunnel Endpoint" can't process
> correctly packets with a size of 1500 bytes.

You should allow for an IP header with options and the ESP header,
which is smaller than 1450.  For SKIP I use 1366 as the advertised
MTU,  and for IPsec usually 1436, unless I need to accomodate ESP
and AH, in which case it's smaller.

> If more information regarding this issue is needed, just ask.
> 
> Is this a known issue ?

It's a known feature of any sort of IP encapsulation.



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