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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, it is currently work in progress.
=How big is a packet?=
The simple answer is a maximum of 1,500 bytes. But it is more complex. The internet is actually a combination of routers and links. Each router has links to other routers. Your packets go from one place to another by being passed along from one router to another over these links.
One of the main types of link is [[Ethernet]] which is used for Local Area Networks (LAN). You have probably encountered a LAN as they are used in offices and homes, and connect things together. You probably use [[Ethernet]] to connect your broadband router to your computers in your house. [[Ethernet]] allows 1,500 byte packets to be carried. Internet providers use much faster links such as gigabit and 10 gigabit and these often allow bigger packets up to around 9,000 bytes. Some links on the internet are set up specially for certain traffic and have links that support packet sizes like 1,548 bytes.
In most cases links support 1,500 bytes, so there is not problem. However, where we know there is a specific issue there is another option, an option (C). It is a bit of a bodge, but works. We have added this as a user-settable feature on the line so you can contol if you want us to work-around the issue or not. This is mainly for where the link from us to you is restricted, typically to 1,492 bytes, for PPPoE.
It works by exploiting the fact that at the start of a TCP session each end tells the other the MTU it can handle (actually the MSS which is the TCP payload but one is derived from the other). When we
When do you get small links?
Unfortunately the specifications are not that helpful here. RFC1661 defines PPP and states If smaller packets are requested, an implementation MUST still be able to receive the full 1,500 octet information field. RFC2516 defines PPPoE The Maximum-Receive-Unit (MRU) option MUST NOT be negotiated to a larger size than 1,492. But they are not incompatible statements - negotiating 1,492 does not mean you don't have to accept 1,500 byte packets (as per RFC1661), but you can't send on if PPPoE bridging, for example, so logically you would have to fragment the IP packet. Most routers do not do that! This is one of the reasons we get problems with MTU being smaller than 1,500 bytes.
There are also ways to do
As there is no real way to tell if baby jumbo frames are supported on an [[Ethernet]], RFC4638 defines an extra option 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 10/100Mb/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.▼
▲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/
=What is my MTU?=
For example:
[[File:Firebrick-1500MTU-ping.png|200px|thumb|a 1500 ping on FireBrick]]
On Linux:
ping -c1 -M do -s 1472 81.187.81.187
On OSX (Apple):
ping -c1 -D -s 1472 81.187.81.187▼
On Windows:
^^ That's the ping command to remember. The rest of this section is a bit of info about that command.
Which results in:
PING 81.187.81.187 (81.187.81.187) 1472('''1500''') bytes of data.
1480 bytes from 81.187.81.187: icmp_req=1 ttl=59 time=54.1 ms
--- 81.187.81.187 ping statistics ---
'''1 packets transmitted, 1 received''', 0% packet loss, time 0ms
rtt min/avg/max/mdev = 54.108/54.108/54.108/0.000 ms
The things to note are highlighted in bold
If 1500 bytes cannot pass, then a ping result will look like:
''' -s 1472''' When setting size in ping, the size is the payload size - not the full packet size. The full packet size is payload + ICMP header size (28 bytes). The option for payload size is -s.▼
PING 81.187.81.187 (81.187.81.187) 1472(1500) bytes of data.
From 81.187.81.187 icmp_seq=1 '''Frag needed and DF set''' (mtu = 1492)
--- 81.187.81.187 ping statistics ---
'''0 packets transmitted, 0 received''', +1 errors
▲ ping -c1 -D -s 1472 81.187.81.187
==Explanation of the ping options==
▲ ping -n1 -f -l 1472 81.187.81.187
'''-c1 / -n 1''' This is optional, it simply tells ping just to send a single ping
'''-M do / -f / -D''' The important option! This means don't fragment the ping packet - we don't want the ping split into multiple packets when testing MTU!
▲''' -s 1472 / -l 1472''' The payload size. When setting size in ping, the size is the payload size - not the full packet size. '''The full packet size is''' <tt> payload (1472) +
==Two quick examples
Checking if you really have 1500 MTU:
rtt min/avg/max/mdev = 12.491/12.491/12.491/0.000 ms
So, here we ping with '''1472''' bytes, this is 1472 plus 20 bytes for the IP header and 8 bytes for the ICMP header is a '''1500''' byte MTU!
Checking if you have smaller than 1500 MTU:
If you're unlucky, there may be no error message at all (these are from Debian and Ubuntu through a Netgear router to TTW):
$ ping -c1 -M do -s 1465 81.187.81.187
PING 81.187.81.187 (81.187.81.187) 1465''(1493)'' bytes of data.
--- 81.187.81.187 ping statistics ---
1 packets transmitted, 0 received, 100% [[Packet Loss|packet loss]], time 0ms
$ ping -c1 -M do -s 1464 81.187.81.187
PING 81.187.81.187 (81.187.81.187) 1464''(1492)'' bytes of data.
1472 bytes from 81.187.81.187: icmp_seq=1 ttl=63 time=55.4 ms
--- 81.187.81.187 ping statistics ---
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
=Backhaul networks=
==TalkTalk Wholesale==
TTW's default MTU is 1492 bytes. It is possible to increase this to 1500 bytes by ticking 'TCPFix' with an MTU of '1500' on clueless. At the
time of writing (Dec 2014) TTW does not support an MTU of 1508, so users of PPPoE cannot achieve an MTU of more than 1492 bytes.
[Above is the official position. However, there are some experimental results which indicate one might be able to do better. Using a TG582n as the router running PPPoE
and configuring it to support baby jumbo frames:
:eth ifconfig intf=eth_WAN mtu=1508
:ppp ifdetach intf=Internet
:ppp ifconfig intf=Internet mru=1500
:ppp ifattach intf=Internet
then PPP claims to successfully negotiate a 1500 octet MRU with TTW.
{Administrator}[ppp]=>:ppp iflist intf=Internet
Internet: dest eth_WAN [03:43:04] retry : 10
admin state = up oper state = up link state = connected
flags = echo magic accomp restart mru addr route savepwd ipv4 ipv6 chap
class = 12 echointerval = 10 echofail = 5 echototaltolerance = 50
'''administrative mru = 1500 negotiation mru = 1500'''
auth type = auto
...
Using the ping tests from above, a ping of 1468 octets (i.e. a packet size of 1496) now works
but a ping of 1469 octets (packet size of 1497) doesn't. On further testing, one can receive
1500 octet packets, but can only send 1496 octet packets - i.e. MRU=1500, MTU=1496.
This is despite the bridging modem used for the tests not claiming to support an MTU > 1500 octets
and with PPPoE needing to send 1504 octets and receive 1508.
[[Category:MTU]]
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