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packets on port 80 (the HTTP port) should be forwarded to it; this
needs to be explicitly configured on the NAT engine on the IGD.
10.2.3 NAT Traversal
The problems described above would be solved if the applications
that need to embed IP address information in their payload would
be able to find out the external IP address and immediately embed
this address in the packet’s payload.
This is exactly one of the services that is provided by a UPnP
Internet Gateway Device. IGD’s typically contain a NAT engine to
provide IP address multiplexing: thus multiple PC’s can benefit from
a single ISP account and all access the Internet using the same
public IP address. If the IGD is UPnP enabled, a UPnP Control
Point can retrieve the external IP address in use from the IGD.
Applications residing on the Control Point can then embed this IP
address in their payload if needed.
This will render the complex ALG
mechanism superfluous.
For example MSN Messenger (the application) can use the UPnP
API in Windows XP (the Control Point) to retrieve the public IP
address from the 715PVW (the IGD) and embed it in the SIP
messages to set up a video session.
The second benefit of UPnP NAT Traversal is about configuring
services on your LAN. For example, if you would want to deploy a
small web or FTP server on your LAN, the appropriate ports (either
80 or 21 resp.) need to be configured on the NAT router: only by
making an explicit mapping to the internal host that is running the
web or FTP server, the NAT box can know where to forward the
packets coming from the Internet to.
UPnP makes entering this mapping on the UPnP NAT box easy.
You can see how to do this in the next section.
10.3 UPNP SUPPORT IN OPERATING SYSTEMS
10.3.1 Microsoft Windows
Both Windows ME and Windows XP support UPnP natively. UPnP
support in Windows ME is restricted to IGD discovery and
presentation. Windows XP UPnP support is more elaborate and
includes NAT Traversal functionality. By default previous versions of
Windows don’t have native UPnP support. However, using the
“Network Setup Disk”
from Windows XP, similar UPnP control point
support as in Windows XP can be enabled on Windows
98/98SE/ME.
10.3.2 Macintosh OS
So far, Macintosh has not announced support for Universal Plug
and Play.
10.3.3 Linux
Currently there are no distributions of Linux that come with UPnP.
However, a free SDK for UPnP under Linux is available from Intel.
Some other companies also offer a commercial Linux UPnP
package.
