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M2M Cellular Gateway
3.9.5 Special AP & ALG
As a pure NAT gateway, it doesn't allow an active connection request from outside world. All this
kind of requests will be ignored by the NAT gateway. But at the client hosts in the Intranet, users
may use applications that need more service ports to be allowed for passing through the NAT
gateway. The "Special AP" feature in the gateway can solve this problem. That is, some
applications require multiple connections, like Internet games, Video conferencing, Internet
telephony, etc. Because of the firewall function, these applications cannot work with a pure NAT
router. The Special Applications feature allows some of these applications to work with this
product.
Besides, application‐level gateway (ALG) allows customized NAT traversal filters to be plugged into
the gateway to support address and port translation for certain application layer "control/data"
protocols such as FTP, BitTorrent, SIP, RTSP, file transfer in IM applications, etc. In order for these
protocols to work through NAT or a firewall, either the application has to know about an address/port
number combination that allows incoming packets, or the NAT has to monitor the control traffic and
open up port mappings (firewall pinhole) dynamically as required. Legitimate application data can thus
be passed through the security checks of the firewall or NAT that would have otherwise restricted the
traffic for not meeting its limited filter criteria.
In "Special AP & ALG" page, there is one configuration window for "ALG" feature. The gateway
provides "SIP ALG" here. In addition, there also be one "Special AP List" window to list all your defined
special applications. Using "Add" or "Edit" button to add and create one new special application or to
Index skipping is used to reserve slots for new function insertion, when required.
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