CAN'T SCREEN SHARE! TRIED EVERYTHING! PLEASE HELP!

Question

My girlfriend just got a new Macbook and I've been helping her set it up across campus though our network at my university. It worked fine and then one day it didn't. Every time she asks or I ask to share her screen it gives this error to both of us:
"Failed to start Screen Sharing because: Screen sharing could not be established."
I've been reading discussions on how to fix this for 2 weeks now and nothing has seemed to work. What is weird though is that I can screen share PERFECTLY with my Uncle who lives in Wisconsin (I am in Arkansas) from campus. However, I can't seem to connect to my girlfriend's computer who is simply just a few buildings away.
The following is what we've tried:
1. Switch from wired to wireless network
2. *Set "System Prefs>Quicktime>Streaming" to "1.5 Mbps T1/Intranet/LAN"
3. *Set "iChat Prefs>Audio/Video>Bandwidth Limit to both "500" and "None"
4. *Both computers have a firewall set to "Allow all incoming connections"
5. *When #4 didn't work we set firewall to "Set access for specific services and applications > iChat.app set to "Allow incoming connections"
6. Rebooted both Macs
7. Deleted all com.apple.iChat.X.plist files and started over fresh
8. *Enabled "Screen Sharing" in System Prefs>Sharing
*Restarted iChat afterwards
I think that's all. It is just really frustrating because I can screen share with someone clear across the nation but not someone who is on my same network. The only difference is that she is using a Gmail account through AOL and I am using a @me.com address. I thought maybe Gmail wasn't working so I had her switch to an AOL account and still same error.
We cannot port forward for obvious reasons at school but it worked before and nothing on the firewall has changed. I work at the IT department at our university and spoke with the person who runs the Firewall and supposedly there are no blocked ports on the part of the network we use. I know with iChat 4 in Leopard it doesn't use a specific port, it bounces around until it finds one according to article regarding port forwarding and NAT routers and iChat.
The weird part is that when we are both off campus (different houses) and try it, Screen Sharing tries to start, she gets the big words across her screen ("Screen Sharing Started" I think...) and we can hear about 5 seconds of audio and then it errors out again. I really want to get this working can someone please tell me how? This is really frustrating.
Thanks,
-jg

Answer 1

Hi,
First off from reading this it should be an AIM valid name (@mac.com, AIM or @me.com) to AIM valid name or
a Jabber valid name (which includes Googletalk valid Gmail IDs) to another Jabber valid name
(that is both should be using the same type of Buddy list)
As you say the Apple Doc on this is not clear on the Screen Sharing part - listing only the changes to the AV ports compared to the Previous Doc (Now article HT1507). It does not mention the Screen Sharing at all.
Reading around suggests the Screen Sharing is based on Apple Remote Desktop and reading the com.apple.ichatAgent.plist with QuickLook shows a reference to a ARD link/process. (the same can be seen if you install Developers Tools from the Install disk which will give you Property List Editor which will open the .plist files).
The Article TS16929 (Previously Doc 106439 shows that Apple Remote Desktop uses port 5900 and 5988.
Having said that though I run Little Snitch, an application that monitors and reports which IPs and Ports and application is trying to use. This shows me a completely random port in the 15xxx range along side the AV port 16402 for the Audio component. It maybe that Apple Remote Desktop also moves to a random port after it has established a connection.
Most people fail on the Audio part and get the standard Log Pop up screen to send or not to Apple - BUT you clearly have the other message...
"Failed to start Screen Sharing because: Screen sharing could not be established."
.... which is unrelated to the Audio part.
This would clearly put things in to the realms of a firewall or setting issue with possibly modem/router or LAN features getting in the way. (Leaving aside ISP related things for the moment).
You say iChat is in the Allowed Specifics List in the Leopard Firewall.
You say that nothing has changed on the Campus Firewall and can connect Off-site form On-site so it would seem not to be there.
On this last issue, check if they have introduced a Streaming cap on Intra-Site links. They may be trying to prevent other things such as Peer-to-Peer File Sharing or thing like everyone sharing their iTunes Libraries and other similar activities. The ability to Share Optical Drives re MacBook Airs maybe a factor.
I would tend to look towards your girlfriend's Mac.
Since iChat 2 it seems that some failed chats seem to hold on to ports or have some hold over the Network going on.
It seems most common when in System Preferences > Network the drop down at the top listing "Location" is left at the Automatic Default.
There are essentially two fixes for this.
1) Set up another Location and copy the settings across and switch between the two to force the Network to re-establish itself.
2)A longer way of clearing what is going on.
a) Shut any Airport card down if using WiFi
b) Shut down the computer.
c) Disconnect any Ethernet cable from either Computer or routing device (whichever is easier to reach but leave the routing device still connected to the Internet - just isolate the computer from the Internet)
d) Restart the computer (this is with no Internet Connection)
e) Restart iChat.
f) Restart Airport card or plug in Ethernet cable.
g) Using the Buddy List, select Available and make iChat Login (can also be done from the iChat Menu (> Accounts submenu in iChat 4)
On your last paragraph I would ask if you can Video chat at this set of locations ?
This is because in this scenario a feature of one of the modems may be an issue.
Some modems and routers have DOS (Denial of Service) or SPI (Stateful Packet Inspection) protection. These tend to be threshold based and have no setting control in the devices set up pages (other than On or Off).
iChat can stream so much data, particularly in Video chats over what are now quite high Internet speeds that a DOS feature will consider it an Internet attack and cut the port involved. SPI tends to be judging whether the data stream is valid and does not seem to keep up and the result is the same.
In Video chats the end that has the Problem tends to get a Error -7 In the Log.
The Other end gets the "No Data for 10 Secs" message.
Summary.
Essentially though as you have tried a deletion of all the .plists for iChat and you imply both ends it is more likely something about the Network or how the Mac thinks Network resources are already allocated.
There is not an easy .plist deletion for this hence the two methods listed above.
NOTEs: 1) #8 in your list is not related to iChat.
This is what you can now use of Apple Remote Desktop itself. It has it's own Screen Sharing app (without Audio like iChat) that is invoked from the Finder's Go Menu > Go to Serve and entering a VCN:// IP address. (As a System Application, as it were, it is in /System/Core Services/Screen Sharing.app)
It does need both Macs to have the Setting Enabled as you have done it #8 though
2) Some ISPs it is suggested block Screen Sharing. (Verizon was the name I heard mentioned). How they tell what the data steam is specifically I do not know as it would seem that the Audio part of an iChat Screen share would make it look like an A/V chat and that Apple Remote Desktop must be a valid way of working for some people. However iChat is Peer-to-Peer in AV chats and Screen Sharing and some ISPs are known to actively work against these file sharing apps that bear the same generic term.
In your case I do not think this applies as you can Connect Off site via whatever ISP your Campus uses.
I hope this helps.
9:58 PM Wednesday; October 1, 2008