Nat Types; Simple Traversal Of Udp Through Nat - Linksys SPA921-NA Administrator's Manual

Technology Background
This is the preferred option because it eliminates the need for managing NAT on the 900 Series
phone. If this is not available, you need to discuss with the ITSP how to use the NAT Support
Parameters provided by the 900 Series phone, such as <Outbound Proxy> and <STUN Server
Enable>.
A typical application of a NAT is to allow all the devices in a subscriber home network to
access the Internet through a router with a single public IP address assigned by an ISP. The IP
header of the packets sent from the private network to the public network is substituted by NAT
with the public IP address and a port assigned by the router. The receiver of the packets on the
public network sees the packets as coming from the external address instead of the private
address of the device.

NAT Types

The ways that NAT is implemented can be divided into the following categories:
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With symmetric NAT, all requests from the same internal IP address and port to a specific
destination IP address and port are mapped to a unique external source IP address and port. If
the same internal host sends a packet with the same source address and port to a different
destination, a different mapping is used. Only an external host that receives a packet can send
a UDP packet back to the internal host.

Simple Traversal of UDP Through NAT

Simple Traversal of UDP through NATs (STUN) is a protocol defined by RFC 3489, that allows
a client behind a NAT device to find out its public address, the type of NAT it is behind, and
the port associated on the Internet connection with a particular local port. This information is
used to set up UDP communication between two hosts that are both behind NAT routers. Open
source STUN software can be obtained at the following website:
http://www.voip-info.org/wiki-Open+Source+VOIP+Software
STUN does not work with a symmetric NAT router. To determine the type of NAT your router
uses, complete the following steps:
Step 1
Enable debugging on the 900 Series phone:
1.
Linksys 900 Series IP Phone Administrator Guide
1-12
Full cone NAT—Also known as one-to-one NAT. All requests from the same internal IP address and
port are mapped to the same external IP address and port. An external host can send a packet to the
internal host, by sending a packet to the mapped external address
Restricted cone NAT—All requests from the same internal IP address and port are mapped to the
same external IP address and port. Unlike a full cone NAT, an external host can send a packet to the
internal host only if the internal host had previously sent a packet to it.
Port restricted cone NAT/symmetric NAT—Port restricted cone NAT or symmetric NAT is like a
restricted cone NAT, but the restriction includes port numbers. Specifically, an external host can
send a packet to a particular port on the internal host only if the internal host had previously sent a
packet from that port to the external host.
Make sure you do not have firewall running on your PC that could block the syslog port (by default
this is 514).
Chapter 1 Introducing Linksys 900 Series IP Phones
Document Version 3.0