MTU and MSS as it Relates to DSL, ATM, Ethernet, and PPPoE



  • I have a question that has perplexed me for some time.  I am not sure if this is the correct part of the pfSense forum in which to post the question as I am not setting up my pfSense box as a PPPoE server, rather, I am trying to correctly adjust my MSS and MTU settings to use my pfSense router efficiently with my WAN DSL connection.  I am hoping that someone who really knows their stuff can provide me some enlightenment.

    I am informed that MTU is standardized at 1500, but PPPoE reduces max MTU to 1492 due to its overhead and MSS should be set to 1452–that is the typical arrangement for DSL.

    My ISP, however, layers PPPoE on Ethernet, on ATM before it is layered on DSL (actually ADSL2+).  Various articles I have read, therefore, suggest modifying MTU down to 1454 so as to maximumize the capacity of the ATM cells so they have no padding (one article, for some reason I have yet to figure out, suggests an MTU of 1452 which results in 2 bytes of padding).

    Since TCP/IP takes 40 bytes from the MTU figure (20 bytes for the TCP header and 20 bytes for the IP header), I understand that the MSS should be 40 bytes lower than the MTU figure.

    It would follow logically, then, with my ISP's use of an ATM layer, I should reduce MTU/MSS for my DSL connection from 1492/1452 to 1454/1414 (or 1452/1412 as the one article on MTU I read suggests.

    Searching the internet for answers, I found one (CISCO) page that rather suggests a MTU/MSS ratio of 1492/1440, another (CISCO) page that suggests 1492/1412, and some other (unsupported) suggestions.

    My question: are some people writing these online articles confusing MTU and MSS?  What is the right figure for my need (i.e. to maximize throughput and avoid retransmissions)?  Do my Windows (8) computers on my LAN automatically adjust MTU/MSS for the values I specify in my pfSense router?  Some suggest that the PPPoE protocol itself is hopelessly flawed and "MSS clamping" should be implemented.

    Any information that would enlighten me in this complicated matter would be greatly appreciated!

    Can anyone offer me some enlightenment in this matter.

    Thanks.



  • any idea anyone ?



  • me too looking for this .



  • Google is your best friend. Having said that here's some general info

    • There's both PPPoE and PPPoA. The "pppoe using atm" (ppp actually) is actually called pppoa, not pppoe.

    • PPPoA will be found as an option in your ISP's modem, and adsl wifi routers you can buy in the market.

    • You will not find PPPoA option in pfSense and other open source routers like dd-wrt, openwrt, tomato, ipfire, untangle etc. Good news is most likely if you select PPPoE, it will work as well, but it won't use ATM.

    • The only way to use both pfSense and also have PPPoA is use the adsl modem in bridge mode and use it to connect using pppoa w/ username password.

    If you're connecting to the internet directly using pfSense without any modem in between, then the MTU is best left to be auto.



  • i have adsl modem running in bridge mode..

    ppoe is configured on ddwrt.
    after some reading i found that my packets wont fragment if payload is set to 1464 bytes so add 28 bytes to it my max and optimum mtu is 1492.


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