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<concept id="GUID-CADC7578-5E96-58B2-9208-F7E53CA5EA32" xml:lang="en"><title>Overview</title><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>SIP NAT Traversal Controller API is an ECOM plug-in API that provides the

connection between the SIP stack and an external UNSAF protocol module. The

Network Address Translator (NAT) devices are found between the SIP User Agent

(SIP UA) and SIP proxy, and have the UA as an outbound proxy. This creates

the problems to maintain SIP connectivity between the UA and the remaining

of the SIP network that are described as follows: </p>

<ul>

<li id="GUID-54670533-2436-5214-A2D8-FFE336B1433F"><p>The SIP

UA registers its contact to SIP registrar using the private IP address &

UDP/TCP port pair within the contact URI. These addresses are not accessible

from the proxy. The NAT maps the private addresses to some other addresses

which can be public or private, in another address range. The proxy can access

the UA when you send the SIP requests and responses to addresses that the

NAT allocates for the UA. You must have the Contact header of the REGISTER

request to contain addresses allocated by the NAT for the UA, instead of the

private addresses of the UA. </p> </li>

<li id="GUID-7EF2F69F-C01F-5F41-8348-323580473E23"><p>The NAT allocates IP

addresses to the UA when the UA sends IP packets to a destination through

the NAT. The NAT type determines if the address-port pairs are either applicable

or dependent on the IP address and UDP or TCP port. The UA sends the IP packets

to the UDP or TCP port. </p> <p>Depending on the NAT type, the addresses within

the contact URI must be those that any device outside the NAT can see and

use or those that only the specific destination (SIP proxy in this case) can

see. To detect addresses the UA uses Unilateral Self-Address Fixing (UNSAF)

protocols defined within IETF. These protocols include the Simple Traversal

of UDP through NATs (STUN) or Traversal Using Relay NAT (TURN). </p><p>The

allocation created by NAT is not permanent. It expires if the UDP flow or

TCP connection between the end-points remains idle. To prevent expiration

the UA send keepalive messages within the flow to refresh the mapping. Different

types of keepalive messages are used like Carriage-Return/Line-Feed (CRLF)

sequences, OPTIONS requests or STUN binding requests</p></li>

</ul>

<p>To enable all these processes the SIP stack contains an ECOM plug-in API,

SIP NAT Traversal Controller API. The stack uses these plug-ins to access

and use a separate UNSAF protocol implementation. This implementation supports

protocols like STUN and TURN. These protocols enable the SIP stack to detect

the server reflexive and relayed addresses for the UA The protocols also implements

the keepalive messages for the flow toward the outbound proxy. </p>

</conbody></concept>