Maximum Transmission Unit (MTU)

Maximum Transmission Unit

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Maximum Transmission Unit(Maximum transmission unit, MTU) refers to the maximum data packet size (in bytes) that can be passed over a layer of a communication protocol ). The maximum transmission unit parameter is usually related to the communication interface (network interface card, serial port, etc ).

The Internet Protocol allows IP sharding, so that the datagram can be divided into small enough fragments to pass through the links with the maximum transmission unit smaller than the original size of the datagram. This fragment process occurs on the IP layer (the Layer 3 of the OSI model, namely the network layer). It uses the value of the maximum transmission unit that sends the group to the network interface on the link. All parts of the original group are marked, so that the IP layer of the target host can regroup the original datagram.

In Internet protocols, the "maximum transmission unit of a path" of an Internet transmission path is defined as the minimum value of the maximum transmission unit for all IP addresses on the "path" from the source address to the destination address. Or, from another perspective, the maximum number of transmission units that can pass through this "path" without further sharding.

RFC 1191 describes the path maximum transmission unit discovery method, which is a technology used to determine the maximum transmission unit of a path between two IP hosts. It aims to avoid IP fragmentation. In this technology, the source address sets the DF (don't fragment, do not fragment) Mark bit of the datagram, then, the size of the sent datagram is gradually increased. Any device in the path that needs to partition the group will discard the datagram and return an ICMP response with a "too large datagram" to the source address, the source host "obtains" the largest transmission unit through this path without sharding.

Unfortunately, more and more networks Block ICMP transmission (for example, to prevent DoS attacks)-This makes the path's largest transmission unit discovery method not work properly, A common manifestation is that a connection can work normally when the data traffic is low, but once a large amount of data is sent at the same time, it will be immediately suspended (for example, when using IRC, the customer will find that no response is received after sending a ping to prohibit IP spoofing, because the connection is blocked by a large number of welcome messages ). Moreover, in a network that uses the Internet protocol, the "path" from the source address to the destination address often responds to various events (such as server Load balancer, congestion, and power failure) it is dynamically modified, which may cause the maximum transmission unit of the path to change during transmission, sometimes even repeatedly. As a result, when the host finds a new maximum transmission unit that can work securely, more groups are lost.

For most local networks that use Ethernet nowadays, the maximum transmission unit value is 1,500 bytes. However, a system like pppoe reduces this value, usually 1492 (= 1500-2 (PPP)-6 (pppoe )), this may result in the use of the maximum transmission unit Discovery Method: some sites after the firewall is improperly configured become inaccessible. In this case, you may find a work ing method, but it depends on which part of the network you control. These methods include modifying the MSs (maximum segment size, maximum segment size) of the first group used to establish a TCP connection at one end of the firewall ).

For some IBM systems (such as xseries) that support earlier versions of the Ethernet protocol, it is possible to operate on common LAN only after the maximum transmission unit is set to 1492.

[Edit] MTU table of common media

MTU indicates the maximum IP packet that can be transmitted without segmentation (including the IP header, but not the lower-layer header of the protocol stack ).

Network	MTU
Superpass	65535
16 Mbit/s	17914
4 Mbit/s	4464
FDDI	4352
Ethernet	1500
IEEE 802.3/802.2	1492
X.25	576
Point-to-point (low latency)	296