Firewall, DSL, PPPoE and Path MTU discovery

When configure DSL broadband setup, PPPoE mode is usually the option to choose, not the bridge mode, which do not require username/password and broadcast or share the link with your neighbour. PPPoE will bring 28 byte or more overhead to Ethernet fram underneath, thus the MTU size on the source need to reduce ( 1500 -28) to avoide the fragment along the way.
I remembered when I worked for Truestar 4 years ago, there are many times I need to reduce the MTU size to as low as 1460 because BELL support ask me to do so.
More often than not, there is a Path MTU discovery method built-in to automatically do this for you to automatically detect the smallest MTU size for all intermediate devices. However, many firewall discard the ICMP, which cause the discovery failed.

http://en.wikipedia.org/wiki/Path_MTU_discovery

NAT traversal and UPnP

NAT traversal is not new to me, it is a must feature for many smart Router/NAT and gateway device, especially for use of voice/video and VPN. However, I have not fully understood the benefit and solution it bring to the ever-complex technology world. NAT traversal or NAT-T help peer-to-peer application to traverse a NAT gateway and thus it has been disable generally in enterprise world, while it is widely used by home route/NAT gateway.

I read an article today to learn more about UPnP, and I found that some of functions it provide overlap with NAT traversal. Take a look at this :

"NAT traversal allows network devices or peer-to-peer applications to traverse a NAT gateway by dynamically opening and closings ports for communication with outside services."

That is what UPnP is doing , you can find out this when you look up your home router's UPnP port forwarding list . I admit that I am confused by that, but I will later on to explore the topic of this with the example of following applications:
Windows XP remote assistant - will this feature really work when a private IP XP machine send out help request to remote friend
VoIP ATA - How VoIP ATA open the port 5060 to the outside world without a network admin configure in router? Can you get VoIP ATA or X-lite work in a enterprise firewall? If yes, what is necessary?
Now it is come to Skype - what is the screte of Skype because it works everywhere?
Enable and Disable SSDP service on XP and see what happening
MSN live message - what server port this application open on the NAT device?
Security discussion - When it open the port without your knowledge can be a good thing for the sake of convenience , but can become a vulunrability for hacker to exploit , any report incidents about this?

here is the link that I recommend:

http://www.microsoft.com/windowsxp/using/setup/expert/crawford_02july22.mspx