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

Value	Link Type
65535	PPP max, Hyperchannel
17914	16 Mbps Token Ring
4464	4 Mbps Token Ring
1500	Ethernet, PPP default
1492	IEEE 802.3/802.2
1480	PPPoE
1460	L2TP
1454	Optimal PPPoE-over-DSL MTU (see http://www.mynetwatchman.com/kb/adsl/pppoemtu.htm for details)
1372	PPTP (PPP/GRE/IP) default, Windows XP
576	X.25, default for many SLIP implementations

Setting MTU Values

UNIX/Linux

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.

```
 ifconfig <interface>
```

Windows

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.