Maximum Transmission Unit
The MTU is the maximum payload length for a particular transmission media. For example, the MTU for Ethernet is typically 1500 bytes. (The maximum packet length for Ethernet is typically 1518 bytes, but that includes 14 bytes of Ethernet header and 4 bytes of CRC, leaving 1500 bytes of payload.) If a host wishes to send packet larger than the MTU for a network, the packet must be broken up into chunks no larger than the MTU.
The smallest MTU between two hosts is known as the path MTU.
[ Need some text on PMTU discovery, MTU-vs-MRU ]
Common MTU Values
PPP max, Hyperchannel
16 Mbps Token Ring
4 Mbps Token Ring
Ethernet, PPP default
Optimal PPPoE-over-DSL MTU (see http://www.mynetwatchman.com/kb/adsl/pppoemtu.htm for details)
PPTP (PPP/GRE/IP) default, Windows XP
X.25, default for many SLIP implementations
Setting MTU Values
To set MTU values, use the ifconfig command on many OSes.
ifconfig <interface> mtu <value>
The ifconfig command might also report the MTU along with other information about the interface.
Getting your MTU value can be done using trial and error using the ping command.
C:\>ping 10.1.1.1 -f -l 1373 Pinging 10.1.1.1 with 1373 bytes of data: Packet needs to be fragmented but DF set. Packet needs to be fragmented but DF set. Packet needs to be fragmented but DF set. Packet needs to be fragmented but DF set. Ping statistics for 10.1.1.1: Packets: Sent = 4, Received = 0, Lost = 4 (100% loss), C:\>ping 10.1.1.1 -f -l 1372 Pinging 10.1.1.1 with 1372 bytes of data: Reply from 10.1.1.1: bytes=1372 time=53ms TTL=254 Reply from 10.1.1.1: bytes=1372 time=56ms TTL=254 Reply from 10.1.1.1: bytes=1372 time=56ms TTL=254 Reply from 10.1.1.1: bytes=1372 time=54ms TTL=254 Ping statistics for 10.1.1.1: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 53ms, Maximum = 56ms, Average = 54ms
Setting your MTU value can be done using the registry. See KB article 158474 for more details.
See also RFC 1191 for a table of common MTUs.