* More reader feedback
The recent discussion of Point-to-Point Protocol over Ethernet (PPPoE) and associated issues related to MTU (Maximum Transmission Unit) size, led to a lot of reader feedback, mostly related to the wrong acronym translation being inserted. So far as that’s concerned, it’s been addressed, and we ask for an XOFF (which is a decimal 19, hex 13, or “control S” in the ASCII code set).
The recent discussion of Point-to-Point Protocol over Ethernet (PPPoE) and associated issues related to Maximum Transmission Unit (MTU) size, led to a lot of reader feedback, mostly related to the wrong acronym translation being inserted. So far as that’s concerned, it’s been addressed, and we ask for an XOFF (which is a decimal 19, hex 13, or “control S” in the ASCII code set).
In the midst of the responses, though there were a couple of e-mails that were particularly useful. First of all a couple of readers pointed out that it’s the addition of PPP in addition to Ethernet framing that caused the problem. And in many ways, the addition of PPP adds very little. You need some lower-layer protocol to provide framing around the IP PDUs (protocol data units, otherwise generically called “packets”), but either PPP – which is historically used on dial-up links – or Ethernet is sufficient. PPPoE is a bit redundant, but adding additional protocol layers that may or may not be needed is not a big issue for most folks now that we have such an abundance of bandwidth.
One reader did point out “RFC 2516 defines PPPoE and is dated February 1999. It states: ‘Since Ethernet has a maximum payload size of 1500 octets, the PPPoE header is 6 octets and the PPP Protocol ID is 2 octets, the PPP MTU MUST NOT be greater than 1492.’ So this problem was identified when PPPoE was born. It’s fairly inexcusable that Tier 1 support couldn’t provide the answer and a fix on your first call.”
But, as it turns out, the answer is – for a number of reasons – a bit more complex than simply looking for an MTU size of less than 1492. And, depending on the number of intermediate protocol layers that get heaped on your data, as mandated by the Department of Redundancy Department, your MTU may need to be considerably smaller than 1492 bytes.
Another reader, Mike Iversen of Martin Marietta Materials, wrote that “www.speedguide.net has been invaluable to me for helping with MTU issues. They have a utility called TCPOptomize that will figure out Max MTU and then update you network card setting with the appropriate value, with no need for manually editing the registry.” He continued “MTU is important when using a site to site VPN over PPPoE you have to reduce the MTU again because of the overhead of IPsec etc.”
Steve tried the utility and, while he has not tested it extensively, the test for the recommended MTU size was 1300 bytes – exactly as recommended by the DSL service provider.
(If you’re either old enough or you’ve studied telecommunications history to have recognized the XOFF reference, XON (decimal 17, hex 11, or “control Q”) for all other issues.)
