Page 351 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 337

SIP ‘Register’ message to the Registrar. The Registrar then searches its user database for
the SIP‐ID of the UA and the corresponding authentication information and then
requests the UA to authenticate by responding with a SIP ‘401 Unauthorized’ message.
As described before for other systems, authentication is based on a common key / pass-
word and an algorithm that uses a random number on both sides for generating challenge /
response values. As only the random number and the result of the calculation is
exchanged between the UA and the Registrar, proper authentication is possible over an
insecure and non‐encrypted connection. When the UA receives the random number it
uses its private key to calculate a result and returns it in a second Register message to the
Registrar. If the result from the UA is the same as the result calculated by the Registrar, it
answers with a ‘SIP 200 OK’ message and the subscriber is registered with the system.
The Registrar also saves the IP address and the UDP port used on the UA’s side in the
subscriber database so it can forward incoming session requests to the subscriber.
It should be noted at this point that the result codes in the answer messages above
(401, 200, etc.) are based on the result codes of the Hypertext Transfer Protocol (HTTP)
used to request web pages from a web server.
Figure 5.2 shows a SIP Register message recorded with Wireshark [3] after a ‘401
Unauthorized’ response has been received. In the central part of the figure the SIP‐ID

Figure 5.2 SIP Register message.