Primary note: Leopard (10.5.x) has an auto-tuning TCP/IP stack. There is no longer a need for user intervention.
First of all, if you've run the Tweak Tester it likely advised you to turn on 'selective ACKs;' unfortunately selective ACKs are not supported by OS X.
Update: As of Mac OS X 10.4.8 released 9/29/06, selective ACKs are now on by default.
- A brief note on MTU values: the default MTU under OS X is 1500, and this will work fine for most users. Some ISP/router combinations require a user to change the MTU to a different value, usually 1450. In these cases the change to the MTU isn't made to improve performance; it ensures functionality. Usually, if not always, the DSLR Tweak Tester will report the MTU of an OS X machine as 1488 - this is a misreading on the part of the Tweak Tester caused by OS X having timestamps enabled (see below).
To change timestamps to off, and eliminate the '1488' (i.e. the erroneous MTU of 1488 reported by the Broadband Reports Tweak Tester), this value: "net.inet.tcp.rfc1323" should be set to zero.
To restore timestamps, the value should be set to 1 (note: this the system default.)
There is now a GUI utility called RMAC * that will modify the MTU value, timestamps, TCP receive window, etc. The documentation is in Japanese, but its use is quite self-explanatory. If you want your modifications to survive a reboot, make sure to have RMAC install a Startup Item.
* Note about RMAC:
A note on how RMAC works: when you launch it, it presents you with a number of networking parameters you can change (e.g. MTU, RWIN, timestamps, etc.). When you change the values and click 'apply' RMAC changes the settings just as if you were typing the commands (e.g. sudo ifconfig en0 mtu 1450) at the comand line in Terminal. Thus, you can play with these various settings to optimize your connection. However, when you restart your machine these settings will be lost, as OS X will load the default network parameters at boot time.
For this reason RMAC gives you the option to "Install RMAC to StartUp Item" - do not confuse a StartupItem with a login item, i.e. an application which is automatically launched at login. What this does it create a script which is placed in '/Library/StartupItems' (a directory which is created if it doesn't already exist). This script overwrites the OS X networking defaults at boot time with current modifications you have made to the networking parameters via RMAC. This procedure is very similar to the one described in the Apple KB article, except the process is automatic - RMAC does all the work. If you wish to add commands that aren't included in the RMAC GUI (e.g. ethernet speed or duplex. even ipfw rules), you can add them yourself by clicking the 'Edit Script File' button. After RMAC has installed the StartUp item there is no need to run the application again, unless you want to make further changes.
The previous link to this utility no longer works. You can currently find it on this Japanese SourceForge page
(Google translated page). Thanks to mirage3d
for the heads up.
For 10.4.x and earlier I use Platypus... It allows setting the parameters either on demand or permanently (just put the Platypus script in the Startup Items preference pane.
If you'd rather set the parameters by 'hand' (10.4.x and earlier), all it takes is a terminal command:
sudo sysctl -w net.inet.tcp.recvspace=x, where x is the desired RWIN. This will not survive a reboot, though.