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MTU tweak: a fix for upload pain

While traveling and staying at Hostel Tyn in Prague's city center, I ran into a strange problem with my laptop on their wireless network.

When many people were using the network (either on the hostel's public computers or on the wireless network), sometimes things bogged down a bit. That wasn't a big deal and required merely a little patience.

But after a while I noticed that absolutely no "uploads" worked. Not via ssh, not via browser POST, nothing. They always hung. Even when only a file upload of 10 KB or so was involved. So I started to wonder what was going on.

As I considered trying some kind of rate limiting via iptables, I remembered somewhere hearing that occasionally you can run into mismatched MTU settings between the Ethernet LAN you're on and your operating system's network settings.

I checked my setup and saw something like this:

ifconfig wlan0
wlan0     Link encap:Ethernet  HWaddr xx:xx:xx:xx:xx:xx
          inet addr:10.x.x.x  Bcast:10.x.x.x  Mask:
          inet6 addr: fe80::xxx:xxxx:xxxx:xxxx/64 Scope:Link
          RX packets:1239 errors:0 dropped:0 overruns:0 frame:0
          TX packets:20 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:191529 (191.5 KB)  TX bytes:4543 (4.5 KB)

The MTU 1500 stood out as being worthy of tweaking. So I tried a completely unscientific change:

sudo ifconfig wlan0 mtu 1400

Then tried the same HTTP POST that had been consistently failing, and poof! It worked fine. Every time.

I think mostly likely something more than 1400 bytes would've been possible, perhaps just a few short of 1500. The number 1492 rings familiar. I'll be old-fashioned and not look it up on the web. But this 1400-byte MTU worked fine and solved the problem. To my delight.

As an interesting aside, before making the change, I found one web application where uploads did work fine anyway: Google's Picasa. I'm not sure why, but maybe it sliced & diced the upload stream into smaller chunks on its own? A mystery for another day.


Levelhead said...

Yes, I also found that altering the MTU was the only thing that finally gave me reliable upload capability and well beyond. What a relief. This after many months of tweaking and scanning this and that, connecting directly to PC instead of modem etc. to no avail. Would like to briefly share my path to success in case it can help someone else struggling with this.
Not a very fluent computer person I finally stumbled upon instructions of how to determine the optimal MTU settings at
I did it manually via DOS command as it outlines further down page because for some reason the automatic DrTCP program did not work in arriving at the optimal MTU settings for me at all.

Anyone having this problem just be sure to also change the MTU setting on your modem to match the new MTU setting on your PC. I found that leaving the modem set on "Auto" for MTU did not work.
This has made uploading batches of pictures to Flickr etc. a breeze. Happy days.

Roman said...

Thanks a lot! It saved a great chunk of my life. Ubuntu 10.04 netbook edition. Same problem - no uploads at all, but only picasa. After changing mtu on wireless to 1400 it's working again