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The MTU is the "Maximum Transmission Unit" used by the TCP protocol. TCP stands for Transmission Control Prototcol. The MTU determines the size of packets used by TCP for each transmission of data. Too large of an MTU size may mean retransmissions if the packet encounters a router along its route that can't handle that large a packet. Too small of an MTU size means relatively more overhead and more acknowledgements that have to be sent and handled. The MTU is rated in "octets" or groups of 8 bits. The so-called "official" internet standard MTU is 576, but the standard rating for ethernet is an MTU of 1500. When trying to decide what MTU is appropriate for your line, you must consider the type of connection you are using. For the purpose of this FAQ, we are going to consider only common cable and DSL MTU settings. Most cable and some DSL ISPs allow a standard 1500 octet MTU. In general, a 1500 MTU is what you would like to have since it works harmoniously with ethernet, but not all ISPs support an MTU of 1500.

Logically, you would also like to get as much data on each transfer as they are willing to send, so you would want select an MTU of 1500 (if your ISP supports it). This would ensure the best possible transfers. If you are using router PPPoE, then your max MTU as allowed by your ISPs and the PPPoE protocol is 1492. Other versions of PPPoE have maximum MTUs of 1400-1492(1438 max for AOL Plus, but 1400 is a better setting for AOL.) You may need to check with your ISP to find out what the maximum MTU is for your network. Setting an MTU that is too small or too great can have extremely deleterious effects on your broadband preformance. Altering your MTU will not affect your latency or TTL. Adjusting the MTU to its ideal setting creates more efficient transfers and thus better overall performance. A well-tweaked system can have high speed, fewer errors and better transfer efficiency.

Feedback received on this FAQ entry:
  • http://www.mynetwatchman.com/kb/ADSL/pppoemtu.htm An MTU of 1454 may provide better throughput than 1492 with PPPoE.

    2012-05-15 13:24:46

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by Mortis See Profile edited by KeysCapt See Profile
last modified: 2003-08-16 20:48:11

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