[gunnar]

I have a Sympatico DSL connection (PPPoE) via Ethernet to a G4 800. The G4 800 runs a software basestation to give access to a PowerBook G4 400. On the PowerBook I can surf the Web but can hardly receive mail. Sometimes it works but 95% of the time I get nothing. This is through 4 different accounts. Sending works fine for some reason. Has anyone had a similar experience? Is there anyway to fix this annoying problem?

[DBursey]

I have an Airport basestation unit, but otherwise run a similar setup. I too am unable to receive any sympatico email using OE, but like you, I can send without any problem. I've talked to sympatico support, and they confirm the server details as set up in my internet control panel. I don't know what the problem is. Please post back if you find out.

[gunnar]

Phew, that's good to know. I was going to go buy a basestation in hopes it would share the connection better. I've received two possible suggestions so far. The first is to lower the MTU. I have no idea what this is as of yet but I suspect it's some kind of ping/timing number. The second is regarding a MAC address (not Mac) that the host computer shows to Sympatico. The other wireless machine apparently needs to use the same MAC to autheticate with the Sympatico mail/network gateway servers. I might be way off on both of these, but that's what people have told me thus far. I'll investigate.

[Camelot]

Originally posted by gunnar:
Phew, that's good to know. I was going to go buy a basestation in hopes it would share the connection better. I've received two possible suggestions so far. The first is to lower the MTU. I have no idea what this is as of yet but I suspect it's some kind of ping/timing number. The second is regarding a MAC address (not Mac) that the host computer shows to Sympatico. The other wireless machine apparently needs to use the same MAC to autheticate with the Sympatico mail/network gateway servers. I might be way off on both of these, but that's what people have told me thus far. I'll investigate.

The MAC address issue is a complete non-starter.

Their mail server will have zero knowledge of the MAC address of either of your systems. MAC addresses live on the physical layer and as such only relate to local networks.

The MTU is a distinct probability, though. It defines the maximum size packet a NIC can handle. In most ethernet networks, the limit is 1500 bytes. However, you say you're running PPPoE which has an overhead of (::thinks hard; 40 bytes? or 8 bytes? I forget which, now. 8, I think - meaning the MTU on a PPPoE network is 1492 bytes.

The problem is that your PowerMac knows it's running PPPoE and therefore sets the MTU on the ethernet line to the lower MTU supported by PPPoE. However, the PowerBook doesn't know anything about the PPPoE link, so it uses the maximum size MTU it can, 1500. A 1500-byte packet can't traverse the 1492-byte limit imposed by PPPoE.

The solution is to force the wireless line to use a lower MTU.

Assuming you're using Mac OS X, run the command:

ifconfig -a

on the PowerMac. In the section related to en0 (ethernet) you should see the MTU setting:

en0: flags=8822<BROADCAST,SMART,SIMPLEX,MULTICAST> mtu 1500

I'm guessing the number is 1492, but you should check it anyway.

Now, make sure that the en1 (airport) link is also running at the same (or lower - just not higher). If it is, you're set here, otherwise, run the command:

sudo ifconfig en1 mtu <size of packet>

where <size of packet> is the MTU of the PPPoE link.

Once set on the base station, the PowerBook should automatically detect the lower MTU (although you might need to sleep/wake to reinitialize the network. If the PowerBook doesn't automatically sync, run the sudo ifconfig en1 mtu command on this machine, too.

[gunnar]

You're absolutely correct about the MTU setting. Although I haven't followed your directions yet, I used a one line command from MacOSHints to test it:

% sudo ifconfig en1 mtu 1492

I gather that your procedure makes the change "stick". The above only works until you reboot. Excellent description by the way.

[Camelot]

Originally posted by gunnar:

I gather that your procedure makes the change "stick". The above only works until you reboot. Excellent description by the way.

No, actually, my command is not persistent.

In order to make the change stick you'll need to add the command to some script that runs at startup.

The other aspect is that if run on the base station system, the remote systems should automatically detect the lower MTU of the base station, meaning you shouldn't need to run the command on the wireless hosts, just the base station.

[gunnar]

I found a persistent solution which is as you said, a startup script. I don't have a basestation, just another computer with an airport card, so I change the client MTU. It works wonderfully now.