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MTU network?

Posted on 2007-11-29
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Last Modified: 2010-04-21
Guys I have had this question come up regarding the network but have not idea what it could be,  is MTU a setting on the ADSL router or is it some we need to change on the PIX firewall.  Not got a clue what this is.

As discussed, pMTU (Maximum Transmission Unit) is required to find the optimum packet size across a WAN network. In order for pMTU to work correctly, ICMP type 3 code 4 needs to be enabled on the client side. Not having the MTU sized correctly can significantly slow down the performance of the system.

 e.g. If we send a packet size of 1500 and the clients MTU size is 500; when the packet reaches the clients end it would need to be fragmented into 3 packets of 500. This can increase the latency on the network and therefore produce poor performance.
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Question by:ncomper
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by:Johnjces
ID: 20379072
You are correct!

MTU can usually be set both in routers, firewalls and in ADSL and cable modems. PCs too.

There is a tweak "out there" (Google) for an MTU sizer that will adjust your Windows MTU size in your registry and allow you to experiment with best settings.

The default standard for most network devices is 1500 but have seen some WAN MTUs set at 1492.

Setting the MTU too high or too low can severely impact your LAN.

John
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by:omarfarid
ID: 20380310
Hi,

It is nice to find what is the best MTU size between two m/c on the network.

But you need to know that if your m/cs are connected over WAN then any router in between that doesn't support your MTU then the result will be the other way.

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markdwilson1234 earned 500 total points
ID: 20434104
Hi,

MTU ("Maximum Transmission Unit") is a parameter that relates to the underlying network.  Simply put, it is the maximum size of packet that the network can carry before the packet needs to be broken up into smaller ones ("fragmented").

For example, with Ethernet, the typical MTU size is 1500 bytes.  On a T1 interface, it may be higher (e.g. 2500 bytes).  Some Ethernet cards, routers and switches support "jumbo frames" of 3,000 bytes.  To know what the maxmum MTU your network equipment can support is a matter of either trying to adjust the MTU size to its maximum, or reading the technical specifications of your equipment.

When people use "Virtual LANs" in their Ethernet network, that will often decrease the available MTU size, as the Virtual LAN Ethernet packet has an additional header of 4 bytes (the VLAN tag).  Some people have VLANs of VLANs reducing the size even futher.  This is assuming that they are unable to increase the overall MTU size because some equipment that is required to see the VLANs is unable to deal with packets of more than 1500 bytes.  Also, if GRE tunnels are used (or other types of tunnel) the overhead of these protocols will also decrease MTU size.

I agree, when traffic is travelling from one interface with a high MTU to one with a low MTU the traffic will need to be fragmented, which can slow performance as the router has to do the work of breaking up the packet (which takes up the router's CPU capacity).

But the biggest problems I have seen are when people are transmitting large packets (e.g. 1500 bytes) and they have the "Don't Fragment" flag turned on on the packet.  This means that when a router has to send the packet across a link where the MTU is not big enough, it instead of fragmenting it, drops it altogether.  This can cause connections to freeze, as any retransmitted packets will also be dropped.

To prevent this kind of problem, many computers / TCP/IP implementations supoprt pMTUd (path MTU detection), where the two ends communicate and test the network to see what their MTU size is.  This of course only works only, as you rightly say that ICMP packets with type codes 3 and 4 are permitted between both ends.

Another way to figure out what your MTU size is, is to peform a ping test, with a specific packet size:
e.g. on Windows, you can try:
ping -f -l 1400 <hostname>

The '-f' flag tells Ping to not fragment the packets.
The '-l' flag sets the size of the data portion of the packet ot 1400 bytes.  This, plus the ping and TCP/IP header is the size of the packet.
For the packet to go through the network successfully, (the size of the data portion + the ping and TCP header ) must be <= to the minimum MTU of any network link between the sender and the receiver.

You can see the actual size of the packet using a network monitoring tool like WireShark.

Please note however, this only tests the MTU in one direction.


Hope this helps.

Mark
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by:ncomper
ID: 31411827
Amaxing answer sorry i took so long to respond
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