Question about Siemens SpeedStream Modem (4100) (SS4100)

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M t u

What the **** is mtu

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In computer networking, the term Maximum Transmission Unit (MTU) refers to the size (in bytes) of the largest packet that a given layer of a communications protocol can pass onwards.
MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.).
The MTU may be fixed by standards (as is the case with Ethernet) or decided at connect time (as is usually the case with point-to-point serial links).
A higher MTU brings higher bandwidth efficiency. However, large packets can block a slow interface for some time, increasing the lag on other packets. For example, a 1,500-byte packet, the largest allowed by Ethernet (and most of the Internet), would block up a 14.4k modem for about one second.
The default MTU on a PPPoE connection is 1,492. The default MTU for a DHCP or PPPoA connection is 1,500.
Increasing the MTU size beyond these levels is possible on a client computer, but not recommended, because the MTU size on the outbound connection to the ISP will nullify any potential gains from a larger MTU.

Posted on Apr 21, 2008

  • Van Estella
    Van Estella Apr 21, 2008

    Finding the Correct MTU
    To find the correct MTU for your configuration you must run a simple
    DOS Ping test. You will simply send out ping requests and progressively
    lower your packet size until the packet no longer needs to be
    fragmented. Please reference the following steps:

    The command for this ping test is ping -f -l xxxx.
    You can use any well known, pingable domain like ping -f -l xxxx in place of for the test.
    •There is a single space between each command.
    •"-l" is a lower case letter L, not the number one.
    •The xxxx are the test packet size. (like 1472)
    At the DOS Prompt type in ping -f -l 1472 and hit Enter. Notice that the packet needs to be fragmented. (you get a message "packet needs to be fragmented but DF set")
    Drop the test packet size down (10 or 12 bytes) and test again. If the packet still needs to be fragmented, drop the test packet size down more and test again until your reach a packet size that does not fragment.Once you have a test packet that is not fragmented increase your packet
    size in small increments and retest until you find the largest possible
    packet that doesn't fragment. Take the maximum packet size from the ping test and add 28. You add 28 bytes because 20 bytes are reserved for the IP header and 8 bytes must be allocated for the ICMP Echo Request header. Remember: You must add 28 to your results from the ping test!
    An example:
    1440 Max packet size from Ping Test
    + 28 IP and ICMP headers
    1468 your optimum MTU setting


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