TCP Analyzer and MTU/MSS results

[xoru]

Hello everyone,

This is my first post on these forums, so please inform me if I'm completely off with the board to post in.

I often use(d) speedguide's TCP Analyzer (https://www.speedguide.net/analyzer.php) to determine the MTU/MSS
for the full route from some location where I'm tweaking network settings. However recently, I've noticed that on
some address where I successfully have configured the MTU/MSS for a PPPoE connection in the past now the tool
reports an MTU of 1492.

When I use the 'ping' trick to determine the MTU to a certain domain, I still get the value of 1500 for known correctly
set network paths.

Did something change about the TCP Analyzer or am I overlooking something?

[Philip]

The Analyzer has not been changed recently, it just reads what your connection to the Analyzer server advertises.
It really depends on the path, most likely some modem/router/node near you may be advertising the smaller max packet size to fit PPPoE encapsulation.

[xoru]

The Analyzer has not been changed recently, it just reads what your connection to the Analyzer server advertises.
It really depends on the path, most likely some modem/router/node near you may be advertising the smaller max packet size to fit PPPoE encapsulation.

I understand, but it seems to only occur on the analyzer tool, not using other tools testing against remote hosts. Even my path to speedguide.net has a 1500 MTU.
My ISP supports baby jumbo specifically for the PPPoE encapsulation.

[Philip]

Not sure exactly.. All I can tell you is the Analyzer reads it from the TCP/IP handshake.

[xoru]

Not sure exactly.. All I can tell you is the Analyzer reads it from the TCP/IP handshake.

Thank you, I might Wireshark this at some point to see what's going on.

[helendam]

Geometry Dash If the TCP Analyzer is reporting an MTU of 1492, it suggests that your connection is configured for PPPoE. This is consistent with the standard MTU for PPPoE connections and does not necessarily indicate an issue. However, if you were expecting an MTU of 1500 bytes, it may be worth verifying your network configuration to ensure it aligns with your expectations.

[xoru]

If the TCP Analyzer is reporting an MTU of 1492, it suggests that your connection is configured for PPPoE. This is consistent with the standard MTU for PPPoE connections and does not necessarily indicate an issue. However, if you were expecting an MTU of 1500 bytes, it may be worth verifying your network configuration to ensure it aligns with your expectations.

Hi helendam,

I agree, but my ISP supports baby jumbo frames for that exact reason. I’ve figured out that it’s only happening with TCP packets and only on very specific routes, it never happens with ICMP for example. I think there is a router somewhere using a few more bytes for TCP options bringing the TCP MSS down.

I’ve verified this in Wireshark, I can see the MTU being 1500 on most TCP connections, but some (including to the analyzer) are a bit smaller.