Subject: MTU Maximum Packet Size Test Application
Author: Kevin Ottalini (qUiCkSiLvEr)
Category: Network Diagnostic
Keywords: MTU, packet, Ethernet, TCP/IP, fragmentation, ping
You can download the latest tool (v1.0d, 17Apr2011) from here: mtutest.zip
Put the tool in a folder by itself and make a shortcut on your desktop for easy use.
Note: This program requires Microsoft Dot Net 1.0, 1.1 or 2.0 to be installed on your system (see bottom of page for more info).
This tool works under Windows XP, WinXP 64 and Server 2003, it has not been tested under Vista or Vista 64 or other Windows Operating Systems.
Maximum Transmission Unit or Maximum Transfer Unit the largest physical packet size measured in bytes that a network can transmit.
For example, an Ethernet MTU is 1500 bytes.
Packets that are longer than the MTU must be divided or fragmented into smaller frames. The layer 3 network protocol, typically IP, extracts the MTU from the layer 2 data link protocol such as Ethernet and fragments the messages into that frame size so that they can travel over the lower layer.
Every network has a different MTU, which is typically set by the network administrator or has a default Value set when the operating system was installed.
On Windows 95/98/XP/Vista you can set the MTU of your machine for best performance although typically the default of 1500 is usually the best.
Changing network properties for VISTA is not recommended at this time since the defaults appear to do a pretty good job already.
Ideally, you want the MTU to be the same as the smallest MTU of all the networks between your machine and a message’s final destination, otherwise, if your messages are larger than one of the intervening MTUs, they will get broken up (fragmented), which slows down transmission speeds.
Trial and error is the only sure way of finding the optimal MTU, but there are some guidelines that can help.
For example, the MTU of many PPP connections is 576, so if you connect to the Internet via PPP, you might want to set your machine’s MTU to 576 too.
Most Ethernet networks, on the other hand, have an MTU of 1500, which is the default MTU setting for Windows 95, 98, XP, Server and Vista.
Some Current and Historical MTU sizes:
16 Mbit/Sec Token Ring
4 Mbits/Sec Token Ring
The largest Ethernet packet size recommended. This is the typical setting for non-PPPoE, non-VPN connections.
The maximum MTU recommended for Internet PPPoE implementations.
The maximum ping data payload before fragmentation errors are received on non-PPPoE, non-VPN connections.
The size DHCP prefers.
The maximum ping data payload before fragmentation errors are received when using a PPPoE-connected machine.
Usable by AOL if you don’t have large email attachments, etc.
TCP Data size (MSS) when MTU is 1500 and not using PPPoE
TCP Data size (MSS) when MTU is 1492 and using PPPoE
The size VPN and PPTP prefer.
Maximum size for AOL DSL.
Typical value to connect to dial-up ISPs.
NOTE: 28 bytes need to be added to the Discovery value since this tool discovers the ping (ICMP) packet size which doesn’t include the 28-byte TCP/IP header.
IE: 28 is the sum of IP and ICMP headers, 48 is the sum of IP, TCP and PPPoE headers.
Normally you should never need to change the MTU size but it doesn’t hurt to check since some application may have tweaked it.
This tool is useful to solved this problem: MTU pint test, solved SMTP, HTTPS protocol problem in Firewall