14 Configuring SIP Infrastructure Applications

This chapter describes Proxy Registrar and STUN Service. Topics include:

• Section 14.1, "Proxy Registrar"

• Section 14.2, "STUN Service"

14.1 Proxy Registrar

The Proxy Registrar is a user agent server (UAS) that implements the proxy and registrar functions described in RFC 3261. This SIP entity is a router of messages. The Proxy Registrar's registrar function processes the REGISTER requests from User Agent clients and uses a Location Service to store a binding (that is, an association) between a user's address of record (AOR) and the user's SIP or SIPS URIs that are located in a CONTACT field. Upon receiving requests to the AOR, the proxy function locates the mapped URIs through a Location Service lookup and then proxies the request using the location information retrieved by this lookup. Table 14-1 describes the attributes of the Proxy Registrar.

Table 14-1 Attributes of the Proxy Registrar

Attributes	Description
CurrentRegDevices	A read-only attribute that displays the number of currently registered devices.
DefaultExpires	Sets the expiration value for the REGISTER request if the client has not indicated a preferred value itself. The default value for this attribute is 3600 seconds.
MaxExpires	Sets the maximum expiration value for the REGISTER request accepted by the server. Although a client can request any expiration value in the REGISTER request, the server can set a maximum amount of time that it accepts for expiration. If the client requests a time greater than the value set for MaxEpires, then the server sets the expiration time for that particular REGISTER request to the value set for MaxExpires. The default value for this attribute is 7200 seconds.
MinExpires	Specifies the minimum expiration value for a REGISTER request accepted by server. While clients can request any expiration time, they can also specify a very low value for the expiration of the REGISTER request. Such low values require clients to update registration information frequently, which creates traffic on the network. If a client requests a value that is below this minimum expiration time, then the server does not accept the REGISTER request and responds with a 423 (Interval Too Brief) error response per RFC 3261. This response message specifies the lowest expiration time allowed, which is set by the MinExpires attribute. The server is allowed to shorten an expiration time, but can never lengthen one. The default value for this attribute is 60 seconds.
SipRegAllowThirdParty	Specifies whether the Proxy Registrar allows third-party registrations. In a third-party registration, the entity issuing the request (in the From header) is different from the entity being registered (in the To header) to whom the provided Contact information applies. If set to true, the Proxy Registrar allows third party registrations. If set to false (the default value), then third-party registrations are rejected (the requestor receives a 403 Forbidden status code). This is a read-only attribute that is always set to false.
SipRegMaxUsers	A read-only attribute that specifies the maximum number of users supported by the Proxy Registrar.

14.2 STUN Service

The OWLCS STUN Service implements STUN -- Simple Traversal of User Datagram Protocol (UDP) Through Network Address Translators (NATs). As described in RFC 3489, STUN enables STUN clients behind a NAT (that is, clients behind a router) to discover the presence of a NAT, the type of NAT, and then to learn the address bindings (including IP addresses) allocated by the NAT.

STUN is a client-server protocol in which a STUN client sends a request (a Binding Request) to a server, which in turn sends a response. OWLCS supports the receipt of Binding Requests from a client, which are sent over UDP and are used to both discover the presence of a NAT and discover the public IP address and the port mappings that it generates. When a STUN client sends a Binding Request to the STUN server, the STUN Server examines the request's source IP address and port and copies them into a response that it sends back to the client. When the STUN client receives the Binding Response, it compares the IP address and port in the packet with the local IP address and port to which it bound itself when it sent the Binding Request to the STUN Server.

The attributes of the STUN Service MBean (described in Table 14-2) enable you to set the STUN Server's primary and secondary IP addresses and ports that form the four RFC 3489-dictated address-port combinations used by the STUN server to receive client Binding Requests. Per RFC 3489, the combinations are as follows:

• A1, P1 -- The Primary Address and Primary Port

• A2, P1 -- The Secondary Address and the Primary Port

• A1, P2 -- The Primary Address and the Secondary Port

• A2, P2 -- The Secondary Address and the Secondary Port

Typically, the STUN server's Primary Port (P1) is set to UDP port 3478. The Stun server uses the Secondary Address and Secondary Port values (A2, P2) in the CHANGED-ADDRESS attribute included in its Binding Response.

Table 14-2 Attributes of the STUNService MBean

Attribute	Value
Autostart	Set to true for the Stun Server to start automatically when OWLCS starts.
PrimaryAddress	The primary STUN address on which to listen for incoming Binding Requests. The default value is 127.0.0.1.
PrimaryPort	The primary STUN port on which to listen for incoming Binding Requests. The value is UDP port 3478, the default STUN Port as described in RFC 3489.
SecondaryAddress	The secondary STUN address on which to listen for incoming Binding Requests. This cannot be the same value as PrimaryAddress.
SecondaryPort	The secondary STUN port to which to listen for incoming Binding Requests. The default value is UDP port 3479.