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    10 <!DOCTYPE concept

    11   PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

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

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

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

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

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

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

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

    19 <ul>

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    37 protocols defined within IETF. These protocols include the Simple Traversal

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

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

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

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

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

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

    44 </ul>

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

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

    47 and use a separate UNSAF protocol implementation. This implementation supports

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

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

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

    51 </conbody></concept>