> 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 =
> 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
--_000_DB6P190MB0280166D013A47FAE38E1AA8C88A0DB6P190MB0280EURP_ Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable 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@orange.fr> Sent: Tuesday, August 8, 2017 1:37 PM To: Moacir Ferreira Cc: users@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@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@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@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. --_000_DB6P190MB0280166D013A47FAE38E1AA8C88A0DB6P190MB0280EURP_ Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable <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@orange.fr><br> <b>Sent:</b> Tuesday, August 8, 2017 1:37 PM<br> <b>To:</b> Moacir Ferreira<br> <b>Cc:</b> users@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@hotmail.com</a= href=3D"mailto:users@ovirt.org" class=3D"">users@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@hotmail.com</a= 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> --_000_DB6P190MB0280166D013A47FAE38E1AA8C88A0DB6P190MB0280EURP_--