PPPoE and MTU



  • I am running the latest pfSense nano version of pfSense using a 15/1 Mb/s DSL (ADSL2+) WAN connection via a DSL modem in bridge mode.  Some time ago I set both my WAN and LAN interface tab settings with MTU/MSS of 1492/1452 to optimize for my PPPoE connection.  I, however, recently ran an online configuration test which told me that my MTU/MSS was set for 1452/1412, so I changed my pfSense WAN and LAN settings back to blank entries.  Running the test again then told me my MTU/MSS was correctly set for 1492/1452.  Since my computer is set for an MTU of 1500, I must conclude–tell me if I am wrong--that setting the pfSense WAN interface for PPPoE operation automatically optimizes the router's MTU/MSS for 1492/1452 and further changing the WAN and LAN MTU/MSS is both unnecessary and results in lower MTU/MSS figures???

    The instructions on entering MTU/MSS on the pfSense WAN are ambiguous--(i.e., if you leave the MTU field blank and it defaults to 1492 will it also clamp the MSS or do you have to enter MTU as 1492 to have MSS clamped at 1452--and if so, then why even have an entry box for MSS?).  So should I leave the WAN MTU/MSS fields blank in pfSense and leave my computer(s) at an MTU of 1500 or should I lower my computer(s)'s MTU to 1492 also?

    The other issue I observe is that my (Windows 8.1) computer has a default MTU of 1500 and RWIN in the neighborhood of 251110 (I can't remember the exact number right now, as I am not at home).  I downloaded a simple configuration utility to set my computer to an MTU of 1492 and an RWIN ca. twice the default value (and a multiple of the MSS) so as to try to improve the efficiency of my internet connection, however, when I then ran the online settings test again it still showed that I am running at the lower (ca. 251110) RWIN value!  Is pfSense setting RWIN via the PPPoE WAN configuration, thus, keeping the connection at the lower setting???  If so, is there a way to increase the RWIN value via pfSense?

    I am speculating because I don't know how pfSense handles PPPoE (i.e., if it assumes everyone who uses it has slow DSL which would not benefit from a larger RWIN value).

    Any insights will be appreciated, thanks.



  • I stumbled upon an interesting article about Windows RWIN auto-tuning that may have answered my question (router does not impact RWIN).  Everyone who uses Windows 7/8 should read these observations:

    https://www.duckware.com/blog/how-windows-is-killing-internet-download-speeds/index.html


Log in to reply