Are SIP authentication credentials typically cached across multiple
dialogs?  Seems (to me anyway) that RFC 3261 is vague on the subject.
It is talked about in Section 22.2:
 
   Once authentication credentials have been supplied
   (either directly by the user, or discovered in an internal keyring),
   UAs SHOULD cache the credentials for a given value of the To header
   field and "realm" and attempt to re-use these values on the next
   request for that destination.  UAs MAY cache credentials in any way
   they would like.
 
 
Seems the benefit to cache credentials across multiple dialogs is to
reduce traffic (can avoid the challenge/response messaging).  Seems a
drawback is that the "copy attack" risk associated with digest
authentication is increased however.  In other words, the longer cached
credentials are allowed to be used, the greater the availaibility for an
attacker to use them.  
 
What is the "industry" standard implementation?  How do some of the more
popular user agent toolkits handle this?  
 
Thanks

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