MTU maximum transmission unit

Source: Internet
Author: User

Maximum Transmission Unit (MTU) Maximum Transmission Unit

The maximum receiving unit of pppoe is 1492, the maximum load of Ethernet is 1500 ETS, The pppoe header is 6 octets, And the PPP protocol ID is 2 octets. Therefore, the MTU of PPP cannot exceed 1492


The Protocol requires that the MTU value on all clients not exceed 1492 (default value: 1500 ). When some websites ignore ICMP messages and cannot deliver messages, or there is a black hole router between the visited website and the ICS host, ntbug429047


When the computer is connected to the network for data transmission, the data is divided into packets of different sizes and the MTU value is set. MTU is an English Maximum Transmission
Unit stands for "maximum transmission unit ". That is, the maximum number of bytes of data packets transmitted through the TCP/IP protocol,
The larger the MTU value, the larger the packet. Theoretically, the transmission rate can be increased, but the MTU value cannot be too large. Because the packet size is too large, the chance of errors during transmission increases greatly. The default value is pppoe.
The maximum MTU value is 1492, while the maximum MTU value of Ethernet is 1500. on the Internet, the default MTU size is 576 bytes,
Why is there such a difference? In general, let's not talk about the lan. to make full use of the LAN performance, MTU can be slightly larger. Internet is the opposite. For dial-up users,
It's called a slow process. Therefore, small data packets should be used for Internet.
The difference between the MTU value setting and the broadband access method will also affect the Internet access speed. The recommended MTU configuration is: Set to 576 for dial-up users; For pppoe
Set the DSL user to 1400; for other users, set to 1500.

Ping 1480

When packet needs to be fragmented but DF set appears, it means that the size (1480 bytes) of the data packet exceeds the maxmtu of the ISP.
If data conflict fragments are generated, we will reduce the packet size in the ping command, for example, 1460 until the returned information is reply from bytes = 1460 time = 134 Ms TTL = 115 is normal, the value of the 1460 packet size is the ISP's pppoe

In 9x, run = regedit and press enter to enter the registry. Expand hkey_locat_machine in sequence
/, You will see several sub-items that contain 000x, and find the key with the name "driverdesc
", The key value is the 000x primary key of" TCP/IP ", which has the network attribute settings of the computer, such as IP address 192.168.0.xx, Gateway, subnet mask 255.255.0, etc. Create a "DWORD Value" under this key and name it "maxmtu"
", Modify the key value to 1460 in decimal format,

Win95, 98 system:
Open the Registry Editor and find the location "hkey_locat_machine ".
"," X "indicates a number. If multiple communication protocols are installed on the computer, projects such as" 0001 "," 0002 ", and" 0003 "will appear. If you open the project, you will see "driverdesc
"Is" TCP/IP ", that is, the" Registry "setting of the NIC. You can right-click here to add a" maxmtu"
"DWORD Value, modify the key value to 1460 in decimal format, and restart.

One of the following two methods can be used for Win2000, NT, XP, and 2003 systems. We recommend that you use the first method. If not, try again.
1. Enable pmtud's backhole Transmission

/Services/TCPIP/parameters/primary key, find the primary key enablepmtubhdetect
If not, create the DWORD Value, set its key value to 1, save it, restart the computer, and test whether the fault is solved?
2. manually set the MTU value of the NIC

Under/services/TCPIP/parameters/interfaces/primary key, the following folders are displayed, representing each Nic installed on the machine. view each folder in sequence and view IPaddress on the right of the window.
If the value is the IP address of the network adapter used to connect to the Internet, you need to modify this folder.
Go to this folder and find a primary key named MTU. If not, create the DWORD Value, change the key value to 1460 in decimal format, and restart.

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