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List:       openvpn-users
Subject:    Re: [Openvpn-users] mssfix max is the guide broken
From:       Lev Stipakov <lstipakov () gmail ! com>
Date:       2020-02-13 7:58:20
Message-ID: CAGyAFMXUy6d5zZ3Qr=zwAP+0Z+Zm2zXQLM5NWTF9RK4jwx-ZPQ () mail ! gmail ! com
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[Attachment #2 (multipart/alternative)]


Hi,

<skip>

and the guide continues with "allows IPv4 packets to be transmitted over
> a link with MTU 1473 or higher"
>
> Is this a typo?


A while ago I have tested this part and turns out that mssfix value
indeed *includes* UDP and IP headers, contrary to what documentation says:

> UDP packet size after encapsulation overhead has been added in, but not
including the UDP header itself

I just gave it another try and observed the same behavior - outgoing IP
packet size
equals mssfix value.

I have a setup where the OpenVPN server is behind a link with MTU 1350.
> The VPN uses UDP.
> What can be the maximal mssfix size then, 1322 bytes?
>

Based on observation above, mssfix 1350 should be a safe option.

-- 
-Lev

[Attachment #5 (text/html)]

<div dir="ltr"><div dir="ltr">Hi,</div><div \
class="gmail_quote"><div><br></div><div>&lt;skip&gt;  \
</div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">and the guide \
continues with &quot;allows IPv4 packets to be transmitted over <br> a link with MTU \
1473 or higher&quot;<br> <br>
Is this a typo?</blockquote><div><br></div><div><div>A while ago I have tested this \
part and turns out that mssfix value</div><div>indeed *includes* UDP and IP headers, \
contrary to what documentation says:</div></div><div><br></div><div>&gt; UDP packet \
size after encapsulation overhead has been added in, but not including the UDP header \
itself<br></div><div><br></div><div>I just gave it another try and observed the same \
behavior - outgoing IP packet size</div><div>equals mssfix \
value.</div><div><br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px \
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"> I have a setup where \
the OpenVPN server is behind a link with MTU 1350. <br> The VPN uses UDP.<br>
What can be the maximal mssfix size then, 1322 \
bytes?<br></blockquote><div><br></div><div>Based on observation above, mssfix 1350 \
should be a safe option.</div></div><div><br></div>-- <br><div dir="ltr" \
class="gmail_signature">-Lev</div></div>





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