IChat-initiated Screen Sharing across Internet

Question

For those of us supporting Aged Parental Units and their Macs, iChat Screensharing is the Killer App to end all killer apps. And when it doesn't work, it's really frustrating.
One thing that makes iChat Screensharing preferable to Timbuktu or generic VNC shareware is that I don't have to walk the "computer illiterate" (can they retire that cute phrase in 2009?) through configuring port-mapping on his NAT router. "All" I have to do is talk the user through:
1. getting an AIM i.d.
2. configuring iChat to use that AIM i.d.
3. finding System Preferences > Sharing, and enabling Screen Sharing
+(have I forgotten anything?)+
At that point I should be able to ask to share the other person's screen. The really big piece of magic, *contacting someone who is behind a NAT router*, is handled; I presume by some clever sleight-of-hand by the AOL server which the AIM client is logged into.
Okay: I can't get it to work for my father. He's running 10.5.6. I can screen-share using the iChat interface when I'm on the same LAN as he, but not from outside.
His firewall is off. His router is an Airport Extreme, to which he's connected by ethernet cable. The Airport Extreme configuration is about as vanilla as it gets.
Can anyone spot something I've forgotten to do, or ask a followup, or suggest the next troubleshooting strategy?
Thanks,
Chap

Answer 1

Hi,
It is somewhat of a feature that Getting iChat to work depends on the devices used at each end.
iChat has to be able to identify very clearly where it is in relation to the Internet for it work/connect in Video and Audio chats.
As far as we can tell Video and Audio chats will work if you Port Forward certain ports Or allow them via other methods such as Port Triggering, DMZ or UPnP.
We know the ports through using Application like Little Snitch and also Apple have published them.
For Tiger and Earlier http://support.apple.com/kb/HT1507?viewlocale=en_US
For iChat 4 in Leopard http://support.apple.com/kb/HT2282?viewlocale=en_US
What the second one does not tell you is that you need some of the Info from the first if using Bonjour or Jabber IDs as well as AIM.
It also fails to say anything about the Screen Sharing.
AS I have said before the Testing that some of the Regular posters have done show that Screen Sharing seems to be a Random port, separate from the Audio component of that connection.
As you will see from the two links the title includes the mention of NAT (Network Address Translation)
Port Forwarding, Port Triggering and DMZ all rely heavily on NAT being done in the routing device.
UPnP seem less reliant on NAT to get through any routing device.
If you have a modem that routes it is likely that it is issuing IP addresses to the computers (DHCP)
IF you then add a router that also does DHCP then the computers get two IPs (Although they can only display one)
iChat can not cope with this.
It is possible to get a router as well as a non routing modem to pass the Public IP to the computer which would exclude the need to open any ports (As they would All be open)
An Base Station can be behind a Routing Modem and set to Not be a router (Just a wireless access point) which would also preclude the need to Set Port Mapping Protocol in it.
We are in the land of "Ifs, Buts and Maybes" when we try and cover all the possibilities that people may have when dealing with a certain lack of information.
7:23 PM Tuesday; December 30, 2008