MTU: Difference between revisions

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<indicator name="Front">[[File:Menu-packet.svg|link=:Category:MTU|30px|Back up to the MTU Page]]</indicator>

This page will be about MTU,

Most of our fixed line services (eg broadband) support an MTU of 1500, but it may depend on your equipment and you may be limited to 1492.

Our L2TP service would be lower still due to overheads see: [[L2TP_MTU]].

=tl;dr=

To simply test if you have 1500 MTU:

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===What is my MTU?===

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AAISP customers can ping 81.187.81.187, which is the next hop from your DSL line.

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=What is my MTU?=

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~~If you can ping a box, you can check what the largest packet size you can get to it is.~~ AAISP customers can ping 81.187.81.187, which is the next hop from your DSL line.

For example:

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[[File:Firebrick-1500MTU-ping.png|200px|thumb|a 1500 ping on FireBrick]]

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'''On Linux:'''

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On Linux:

ping -c1 -M do -s 1472 81.187.81.187

On OSX (Apple):

'''On OSX (Apple):'''

ping -c1 -D -s 1472 81.187.81.187

On Windows:

'''On Windows:'''

ping -n 1 -f -l 1472 81.187.81.187

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'''0 packets transmitted, 0 received''', +1 errors

[[File:Mtu-osx-ani.gif|800px|frameless]]

==Explanation of the ping options==

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1 packets transmitted, 1 received, 0% [[Packet Loss|packet loss]], time 0ms

rtt min/avg/max/mdev = 55.448/55.448/55.448/0.000 ms

=MTU Overview=

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As there is no real way to tell if baby jumbo frames are supported on an [[Ethernet]], RFC4638 defines an extra option for PPPoE to negotiate this at the [[Ethernet]] level. Of course two ends could simply agree to handle slightly larger [[Ethernet]] frames by configuration as well. Sadly this is not always the same level of operation or the same equipment that does the MRU negotiation at the PPP level, and if that knows PPPoE is involved it will not negotiate more than 1,492 MRU as per RFC2516. So typically some configuration is needed.

The upshot of all this? It is possible to get BT FTTC (Fibre to the Cabinet) circuits (which use PPPoE) working on full 1,500 byte PPP by using modified pppd on the customer end, a suitable network card that will handle 1,508 byte frames at 10/100Mbit/s. We have done this! (Thanks to TonyHoyle and Simon, customers on irc, for tweaking pppd and testing this for us). The new FireBrick does, of course, support PPPoE with baby jumbo frames to handle 1,500 byte MTU and even bonds multiple lines. Using the right modem (and the DLINK 320B in bridge mode do this) you can negotiate and use 1508 bytes over ADSL as well.

The upshot of all this? It is possible to get BT FTTC (Fibre to the Cabinet) and BT FTTP (Fibre to the Premises) circuits (which use PPPoE) working on full 1,500 byte PPP by using modified pppd on the customer end, a suitable network card that will handle 1,508 byte frames at 10/100Mbit/s. We have done this! (Thanks to TonyHoyle and Simon, customers on irc, for tweaking pppd and testing this for us). The new FireBrick does, of course, support PPPoE with baby jumbo frames to handle 1,500 byte MTU and even bonds multiple lines. Using the right modem (and the DLINK 320B in bridge mode do this) you can negotiate and use 1508 bytes over ADSL as well. In addition, BTs FTTP ONT units support baby jumbo frames and can support a 1,500 byte MTU.