Summing up the networking grief

Question

I'm apparently having similar issues as many others with internet/network performance following update of my MacBook to 10.5/10.5.1. Possible solutions are spread out over many threads here, so I'd like to try to pull them together and note that they don't fix the problems. I have read something to the effect that "Apple is working on a fix;" this message is basically an alternative to sitting on my hands until that happens.
The problems:
- Very slow download speeds; streaming AV can't keep buffers full (was no problem on same connection with Tiger)
- Large downloads (>15MB) routinely stall, with connection dropping (Activity Monitor incoming data line drops to zero)
- AIM unusable, whether through iChat or Adium: constant bogus "signed on from 2 locations" messages and server request timeout errors.
The environment:
- 2Ghz Intel Core Duo MacBook
- PPPoA DSL via Actiontec GT701 modem/router, both wired and wireless, 1280/640 Kbps
- 1997 iMac running 10.4.9 on same network has no apparent trouble; outperforms MacBook in network speed and stability
The attempted solutions:
- Upgrading router firmware (Actiontec GT701)
- Using Airport Express instead of DSL modem/router's native wireless
- Going from open wireless to WEP to WPA, public to closed SSID
- Disabling router firewall
- Changing AIM port to 443
- Using OpenDNS instead of ISP DNS
- Deleting NetworkInterfaces.plist, preferences.plist, com.apple.airport.preferences.plist, com.apple.network.identification.plist
- Trying Adium instead of iChat
- Trying Firefox instead of Safari
- Changing QuickTime streaming connection speeds to max
- Enabling 802.1X login
- Bypassing airport/wireless altogether and plugging straight into the DSL modem/router.
- sudo ifconfig en1 mtu 1400
Going to OpenDNS and setting the mtu to 1400 both seem to have a small positive impact on the problems, but are far from completely resolving them.
I'm pretty sure the DSL or this particular modem/router is a key part of the problem, because connecting to other networks, wired or wireless, is nowhere near as bad.

Answer 1

MTU 1400 is not going to kill your speeds at any bandwidth home users have. Please do the math before making a statement like that.
The MTU values are not a craps shoot and can be calculated. I believe I made the recommendation to go to 1400 during a troubleshooting session because if it does not work, then the issue is not with packet size since that value would account for PPPoE/A overhead, any sort of IP/GRE tunnel overhead, and any additional TCP options that may be set by the user. If it does work, then the real MTU can be calculated.
If you ONLY have PPPoE in the loop (probably most people), 1492 would be fine since it's an 8 byte header. Taking it any lower than that should be uncommon.
In reality, you should never have to touch the MTU. You should transmit at the size your local media will support. Other protocols, such as PMTU-D, kick in to auto-adjust things down. It can't adjust up. If you have to touch this, it's due to one of those protocols not being able to function currently either due to a router or firewall (possibly your systems built-in firewall). MSS should normally be adjusted before the MTU is. However, that setting doesn't seem to take in Leopard.
Taking your window size down to 65535 would cause a heck of a lot more impact than a 1400 byte MTU. There's a good reason why it is increased.
At 65535 byte window and 100ms of latency, which is quite possible over the Internet, you're going to be limited to about 5 Mbps. It's set high for a reason. To get 30 Mbps, which many of us do have to our homes, you'll need a 375,000 byte buffer. Once you have a buffer that large, the other speed improvements provided by Leopard such as SACK need to be enabled or if you drop a packet you'll be in a world of hurt.
Message was edited by: mreckhof