User's Manual

If a device is behind a NAT, transport addresses obtained are not publicly routable, and
therefore, not useful in a number of multimedia applications. The limited lifetime of the NAT
port mapping can also cause the SIP signaling to fail. If a port mapping is idle, it can be released
by the NAT and reassigned to other applications.
The STUN protocol lets an IP Deskphone discover the presence and type of NATs between
the Avaya 1100 Series IP Deskphone and the public Internet. In addition, an IP Deskphone
can discover the mapping between the private IP address and port number and the public IP
address and port number. Typically, a service provider operates a STUN server in the public
Internet, with STUN-enabled IP Deskphones embedded in end-devices, which are possibly
behind a NAT.
A STUN server can be located using DNS SRV records using the domain of the service provider
as the lookup. STUN typically uses the well-known port number 3478. STUN is a binary
encoded protocol with a 20-octet header field and possibly additional attributes. The STUN
protocol learns the public IP addresses, and therefore, some security is necessary.
To initiate a STUN lookup, the IP Deskphone sends one or more Binding Request packets
using UDP to the STUN server. These packets must be sent from the same IP address that
the IP Deskphone uses for the other protocol, because this is the address translation
information that the IP Deskphone tries to discover.
The server returns Binding Response packets, which tell the IP Deskphone the public IP
address and port number from which it received the Binding Request. The IP Deskphone
knows the private IP address and port number it used to send the Binding Request, and
therefore, it learns the mapping between the private and public address space being performed
by the NAT. If the Binding Response packets indicate the same address and port number as
the request, the IP Deskphone knows no NATs are present.
The IP Deskphone supports two methods for NAT traversal of the signaling path:
• SIP_PING
• STUN
The NAT traversal method can be selected manually through the Device Settings menu or
configured through the device configuration file. The default NAT traversal method is NONE.
The IP Deskphone can conduct SIP dialogs through a Symmetric NAT using UDP. This allows
the IP Deskphone to work from behind and/or in front of a symmetrical NAT with servers and/
or clients that support RFC3581. For this feature to work properly, the receiving end device
must support RFC3581. This feature is enabled or disabled through the USE_RPORT
parameter in the device configuration file.
Note:
RFC3581 does not address NAT traversal for media or voice.
Features
194 SIP Software for Avaya 1100 Series IP Deskphones-Administration November 2012
Comments? [e-mail]