Generating Call-ID, From and To tags, Branch-ID and Cseq

The library provides the sip_guid() function to generate unique identifiers for the Call-ID, From, and To tags. The stack generates the identifier by combining the upper 32 bits that are returned by the gethrtime() function with a 32–bit random number from the /dev/urandom pseudo-device. The stack randomly replaces numbers in the result with alphabetic characters. The applicatioon is responsible for freeing the string that is returned by the sip_guid() function.

The library provides the sip_branchid() function to generate Branch-ID fields. If the application invokes the sip_branchid() function without a SIP message, or if the SIP message does not have a VIA header, the stack forms a Branch-ID field by using the sip_guid() function. If the SIP message already has a VIA header, the stack generates the Branch-ID by using the MD5 hash of the following fields:

• To tag

• From tag

• Call-ID

• Request URI

• Topmost VIA header

• Sequence number from the Cseq header

All branch identifiers begin with the string z9hG4bK. This behavior complies with RFC 3261. The application is responsible for freeing the string that is returned by the sip_branchid() function.

An application can use the sip_get_cseq() or sip_get_rseq() functions to obtain the initial sequence number. The sip_get_cseq() and sip_get_rseq() functions use the 31 most significant bits of the value that is returned by the time(2) function call.