8:53 a.m.
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But if you receive a 9000 MTU frame on an "input" interface that results se=
nding it out on an interface of a 1500 MTU, then if you set DF bit the fram=
e will just be dropped by the router. If you want your data to "cross" your=
frame over a different MTU path, then you can not set DF to 1. This is a q=
uite simple and easy thing to demonstrate. Just create a simple virtual lab=
with 3 Linux doing routing and test it. So, if your goal is to communicate=
over paths that may have a MTU lower than 9000 you better make sure your s=
erver sends out a frame that the path can support.
Moacir
________________________________
From: Fabrice Bacchella <fabrice.bacchella(a)orange.fr>
Sent: Tuesday, August 8, 2017 1:37 PM
To: Moacir Ferreira
Cc: users(a)ovirt.org
Subject: Re: [ovirt-users] Users Digest, Vol 71, Issue 37
The border router will do like any other router on the world. If the DF bit=
is set (common case) or if it's IPv6, it will not fragment but send an ICM=
P.
Le 8 ao=FBt 2017 =E0 13:34, Moacir Ferreira <moacirferreira(a)hotmail.com<mai=
lto:moacirferreira@hotmail.com>> a =E9crit :
True! But in some point of the network it may be necessary to make the MTU =
1500. For example, if your data need to cross the Internet. The border rout=
er in between your LAN and the Internet will have to fragment a large frame=
back to a normal one to send it over the Internet. This router will just "=
die" if you have a heavy load.
Moacir
________________________________
From: Fabrice Bacchella <fabrice.bacchella@orange.fr<mailto:fabrice.bacchel=
la(a)orange.fr>>
Sent: Tuesday, August 8, 2017 12:23 PM
To: Moacir Ferreira
Cc: users@ovirt.org<mailto:users@ovirt.org>
Subject: Re: [ovirt-users] Users Digest, Vol 71, Issue 37
Le 8 ao=FBt 2017 =E0 11:49, Moacir Ferreira <moacirferreira(a)hotmail.com<mai=
lto:moacirferreira@hotmail.com>> a =E9crit :
This is by far more complex. A good NIC will have an offload engine (LSO - =
Large Segment Offload) and, if so, the NIC driver will report a MTU of 64K =
to the IP stack. The IP stack will then send data to the NIC as if the MTU =
were 64K and the NIC will fragment it to the size of the "declared" MTU on =
the interface so PMTUD will not be efficient in such scenario. If all this =
takes place in the server, then you get no problem. But if a standard route=
r is configured to support 9K jumbo frame in one interface (i.e.: LAN conne=
ction) and 1500 in another (i.e.: WAN connection) then the router will be r=
esponsible for the fragmentation.
That's happen only if the bit don't fragment is not set, otherwise router a=
re not allowed to do that and send back a "packet to big" ICMP, it's
called=
path mtu discovery. To my knowledge, it's usually set, and even mandatory =
on IPv6.
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<html>
<head>
<meta http-equiv=3D"Content-Type" content=3D"text/html;
charset=3Diso-8859-=
1">
<style type=3D"text/css" style=3D"display:none;"><!-- P
{margin-top:0;margi=
n-bottom:0;} --></style>
</head>
<body dir=3D"ltr">
<div id=3D"divtagdefaultwrapper"
style=3D"font-size:12pt;color:#000000;font=
-family:Calibri,Helvetica,sans-serif;" dir=3D"ltr">
<p>But if you receive a 9000 MTU frame on an "input" interface
th=
at results sending it out on an interface of a 1500 MTU, then if you s=
et DF bit the frame will just be dropped by the router. If you want your da=
ta to "cross" your frame over a different MTU path,
then you can not set DF to 1. This is a quite simple and easy thing to dem=
onstrate. Just create a simple virtual lab with 3 Linux doing routing and t=
est it. So, if your goal is to communicate over paths that may have a MTU l=
ower than 9000 you better make sure
your server sends out a frame that the path can support.<br>
</p>
<br>
Moacir<br>
<br>
<div style=3D"color: rgb(49, 55, 57);">
<hr tabindex=3D"-1" style=3D"display:inline-block; width:98%">
<div id=3D"divRplyFwdMsg" dir=3D"ltr"><font
style=3D"font-size:11pt" face=
=3D"Calibri, sans-serif" color=3D"#000000"><b>From:</b>
Fabrice Bacchella &=
lt;fabrice.bacchella(a)orange.fr&gt;<br>
<b>Sent:</b> Tuesday, August 8, 2017 1:37 PM<br>
<b>To:</b> Moacir Ferreira<br>
<b>Cc:</b> users(a)ovirt.org<br>
<b>Subject:</b> Re: [ovirt-users] Users Digest, Vol 71, Issue 37</font>
<div> </div>
</div>
<div>The border router will do like any other router on the world. If the D=
F bit is set (common case) or if it's IPv6, it will not fragment but send a=
n ICMP.
<div class=3D""><br class=3D"">
<div>
<blockquote type=3D"cite" class=3D"">
<div class=3D"">Le 8 ao=FBt 2017 =E0 13:34, Moacir Ferreira <<a
href=3D"=
mailto:moacirferreira@hotmail.com"
class=3D"">moacirferreira(a)hotmail.com</a=
> a =E9crit :</div>
<br
class=3D"Apple-interchange-newline">
<div class=3D"">
<div id=3D"divtagdefaultwrapper" dir=3D"ltr" class=3D""
style=3D"font-style=
:normal; font-weight:normal; letter-spacing:normal; orphans:auto; text-alig=
n:start; text-indent:0px; text-transform:none; white-space:normal; widows:a=
uto; word-spacing:0px; font-size:12pt; font-family:Calibri,Helvetica,sans-s=
erif">
<div class=3D"" style=3D"margin-top:0px;
margin-bottom:0px">True! But in so=
me point of the network it may be necessary to make the MTU 1500. For examp=
le, if your data need to cross the Internet. The border router in between y=
our LAN and the Internet will have to
fragment a large frame back to a normal one to send it over the Internet. =
This router will just "die" if you have a heavy load.<span class=
=3D"Apple-converted-space"> </span><br
class=3D"">
</div>
<br class=3D"">
Moacir<br class=3D"">
<br class=3D"">
<div class=3D"" style=3D"color:rgb(49,55,57)">
<hr tabindex=3D"-1" class=3D"" style=3D"display:inline-block;
width:919.234=
375px">
<div id=3D"divRplyFwdMsg" dir=3D"ltr"
class=3D""><font class=3D"" style=3D"=
font-size:11pt" face=3D"Calibri, sans-serif"><b
class=3D"">From:</b><span c=
lass=3D"Apple-converted-space"> </span>Fabrice Bacchella
<<a href=
=3D"mailto:fabrice.bacchella@orange.fr"
class=3D"">fabrice.bacchella@orange=
.fr</a>><br class=3D"">
<b class=3D"">Sent:</b><span
class=3D"Apple-converted-space"> </span>T=
uesday, August 8, 2017 12:23 PM<br class=3D"">
<b class=3D"">To:</b><span
class=3D"Apple-converted-space"> </span>Moa=
cir Ferreira<br class=3D"">
<b class=3D"">Cc:</b><span
class=3D"Apple-converted-space"> </span><a =
href=3D"mailto:users@ovirt.org"
class=3D"">users(a)ovirt.org</a><br class=3D"=
">
<b class=3D"">Subject:</b><span
class=3D"Apple-converted-space"> </spa=
n>Re: [ovirt-users] Users Digest, Vol 71, Issue 37</font>
<div class=3D""> </div>
</div>
<div class=3D""><br class=3D"">
<div class=3D"">
<blockquote type=3D"cite" class=3D"">
<div class=3D"">Le 8 ao=FBt 2017 =E0 11:49, Moacir Ferreira <<a
href=3D"=
mailto:moacirferreira@hotmail.com"
class=3D"">moacirferreira(a)hotmail.com</a=
> a =E9crit :</div>
<br
class=3D"Apple-interchange-newline">
<div class=3D"">
<div id=3D"divtagdefaultwrapper" dir=3D"ltr" class=3D""
style=3D"font-style=
:normal; font-weight:normal; letter-spacing:normal; orphans:auto; text-alig=
n:start; text-indent:0px; text-transform:none; white-space:normal; widows:a=
uto; word-spacing:0px; font-size:12pt; font-family:Calibri,Helvetica,sans-s=
erif">
<div class=3D"" style=3D"margin-top:0px; margin-bottom:0px">This
is by far =
more complex. A good NIC will have an offload engine (LSO - Large Segment O=
ffload) and, if so, the NIC driver will report a MTU of 64K to the IP stack=
. The IP stack will then send data to
the NIC as if the MTU were 64K and the NIC will fragment it to the size of=
the "declared" MTU on the interface so PMTUD will not be efficie=
nt in such scenario. If all this takes place in the server, then you get no=
problem. But if a standard router is configured
to support 9K jumbo frame in one interface (i.e.: LAN connection) and 1500=
in another (i.e.: WAN connection) then the router will be responsible for =
the fragmentation.</div>
</div>
</div>
</blockquote>
<br class=3D"">
</div>
<div class=3D"">That's happen only if the bit don't fragment is
not set, ot=
herwise router are not allowed to do that and send back a "packet to b=
ig" ICMP, it's called path mtu discovery. To my knowledge, it's usuall=
y set, and even mandatory on IPv6.</div>
</div>
</div>
</div>
</div>
</blockquote>
</div>
<br class=3D"">
</div>
</div>
</div>
</div>
</body>
</html>
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