Maximum Transmission Unit (MTU)

Maximum Transmission Unit (MTU) is a communications protocol in a layer above the maximum possible data reported by size (in bytes). Maximum transmission unit of this parameter is usually related with the communication interface (network interface card, serial, etc.).
Internet Protocol allows IP fragmentation so that datagrams can be divided into fragments small enough to pass the maximum transmission unit that the packet is less than the link of the original size. The fragmentation process occurred in the transport layer (OSI model is the fourth layer, transport layer: OSI model, the most important layer. Transport protocol flow control at the same time in order to receive or receiving data based on how quickly to send the appropriate provisions of rate. In addition, the transport layer in accordance with the maximum size of the network can handle long packets will force the split. For example, Ethernet can not receive more than 1500 bytes of the packet. sending the transmission node layer data into smaller pieces of data, while the arrangements of each piece of data for a serial number to reach the receiver node in the data transport layer, can re-correct order. The ordering process to be known.), It uses the packet sent to the network interface on the link maximum transmission unit value. The original fragment are grouped with the tag, so the destination host IP layer will be able to re-form the original data packet overstated.
Internet protocol, an Internet transmission path of the "Path Maximum Transmission Unit" is defined as the source address to the destination address through the "path" on the jump all the IP maximum transmission unit minimum. Or from another perspective, that is, without further fragmentation will be able to pass through this "path" the maximum value of the maximum transmission unit.
RFC 1191 describes "Path MTU discovery method", which is a two IP hosts determine the Path MTU between the technology, its purpose is to avoid IP fragmentation. In this technique, the source address of the data reported to the DF (Don't Fragment) bit, and then gradually increase the size of packets sent - the path that needs to fragment the packet to the device will be this data Report discarded and returns a "datagram too big" ICMP response to the source address - this way, the source host to "learn" to not be able to slice through this path had the largest maximum transmission unit.
Unfortunately, the increasing number of network blocked ICMP traffic (for example to prevent DDOS attacks) - which makes the path MTU discovery does not work, its performance is a common connection in the case of low data traffic can work, but once there the same time large amounts of data to send, it will immediately suspend (for example, when using the IRC, the customer will find that sending a ping against IP spoofing not get any response after, this is because the the connection is blocked a lot of welcome message.) And, in a network using Internet Protocol, from the source address to the destination address of the "path" often in response to various events (load balancing, congestion, power, etc.) and be dynamically modified - it Path maximum transmission unit may result in the transmission process change - sometimes repeatedly change. As a result, you can find a new host security, while the maximum transmission unit, more packets are missing out.
Nowadays most of the use of Ethernet for the LAN, the maximum transmission unit value is 1500 bytes. However, such a system, like PPPoE will reduce this value, which makes the maximum transmission unit in the use of methods that may produce such a result: some are poorly configured firewall is not up to the site becomes. In this case, is likely to find alternative methods, but it depends on your control which part of the network. These include changes to established TCP connection in firewall end of the first group of the MSS (Maximum Segment Size, the maximum segment size).
 For some older versions of the Ethernet protocol to support IBM systems (such as XSeries), may only set the maximum transmission unit only after 1492, in the moment to operate on a common LAN.