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Networking: MTU leads to madness
Sun, 08 Jan 2006

Once in awhile my ISP tells me to alter my DSL router's MTU; sometimes it helps, sometimes it kills the connection. MTU stands for "maximum transmission unit" and is the threshold at which the routing hardware decides to fragment a data 'packet' into smaller packets for transmission. ie: For DSL the normal MTU is 1492 (bytes), meaning if an application sends a larger packet it'll be subdivided and then automagicly re-assembled at the 'other end' somewhere. To my mind, this suggests altering MTU can impact performance depending on the characteristics of the network... however, stranger things occur.

In the past I've had some oddities, but I don't recall specifics; todays oddities are just plain weird -- if you're a network administrator type who can make sense of this, please let me know :)

Right now, I've found a couple files on my ISP's webserver (in my virtual host) that I cannot download.. they'll hang indefinately. I have a third file that can download, but that always hangs for 10 seconds in the middle. Curious. Other files are fine.. its just these specific files. They can be moved elsewhere in the directory structure and the same problem occurs.. yet zipping them up and the problem vanishes.

Another data point is that I have a file I cannot upload anywhere; until I altered the MTU it would stall across FTP half way through; after altering the MTU it stalls for 10s, but then completes. It is zipped.. if I unzip it, or zip it up again, then it will complete no problem.

In these cases, other files are not a problem, and big transfers are fine.. I can move a 16MB file at 400K/s, and torrenting seems to work fine. All of this suggests that some characteristics of the files in question (or more to point, their content) seems to take odds with the network. Perhaps some specific sequence of bytes in these files fights with some compression algorithym or the packet-splitting algorithym in some manner, or any number of networking components. Certainly, these files were transferred just fine _in the past_.. so something is _up_. Mind you, much of the equipment in my house is new due to the previous lightning damage, and god only knows what Ma Bell has done with our DSL line of late..

I can tell you this -- when I first started having problems, it made me think the virtual web host was having disk issues (always hanging at the same point... hmm..) and made me panic briefly. Now I just know the network is fracked. Oh fine :/

Ah well. There are always many somethings to look into. Who has the time?

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