Huawei U-SYS MRS6100 Technical Manual page 70

Technical Manual
U-SYS MRS6100 Media Resource Server
Contact
The Contact field is present in INVITE, ACK, and REGISTER requests, success
responses, call process responses, and redirection responses to provide an address
for direct communication with the user.
The Contact field in an INVITE or ACK request indicates the location where the
request is originated. With the field, the callee can directly send a request (BYE for
example) to that address, instead of asking a series of proxy servers to forward the
request by using the Via field.
A success response to INVITE can contain the Contact field, which helps to send the
subsequent SIP requests (ACK for example) directly to that address specified in the
field. That address usually indicates the host of the callee. If the host is behind a
firewall, that address indicates the proxy server.
Call process response message to an INVITE request contains a Contact field that
has the same meaning as the success response message. However, a CANCEL
request cannot be directly sent to that address. Instead, the CANCEL must be
forwarded through the same path of the original request.
The Contact field in a REGISTER request indicates the reachable location of the user.
The request also defines a wildcard Contact field * that can only be used with the
expires field with a value of 0 to remove all registrations of a user. In the Contact field,
the expires parameter (optional) can also be specified to indicate the expiration
interval of the registration. If the parameter is not specified, the expires field value is
taken as its default value. If neither case is adopted, it is considered that the expiration
interval of the SIP URI is one hour.
The Contact field in a success response to the REGISTER request returns all
locations that are currently reachable for the user.
The Contact field in a redirect response such as Moved Temporarily, Moved
Permanently, or Address Ambiguous specifies other alternative addresses for retry,
which can be used for a response to a BYE, INVITE or OPTIONS request.
The Contact field has a generic format:
Contact: address; q; action; expires; extension
The address is expressed in the same form as To and From. The q parameter has a
value range of [0, 1] indicating the relative priority of the given location. The greater
the value is, the higher the priority is. The action parameter is only used in a
REGISTER request, indicating the server is required to perform the proxy service or
redirection service on the subsequent requests to the client. If the Contact field does
not contain the parameter, the action to be performed depends on the configurations
of the server. The expires parameter indicates how long the URI is valid either in
seconds or by SIP date. The extension attribute is actually the extension name.
An example of Contact field:
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Chapter 5 MGCP and SIP