Challenge-Responses in Requests not in the Dialog

A digest authentication session starts from the client response to a 
www-authenticate/proxy-authenticate challenge and lasts until the client receives another challenge in the protection space defined by the auth-realm. Credentials are not cached across dialogs; however, if a User Agent (UA) is configured with the auth-realm of its outbound proxy, when one exists, the UA may cache credentials for that auth-realm across dialogs.

Note:

Existing Oracle Communications Session Border Controller behavior with surrogate-agents is that they cache credentials from REGISTER for INVITE sessions only if the Oracle Communications Session Border Controller is considered a UA sending to its outbound proxy.