RFC 3515, "The Session Initiation Protocol (SIP) Refer Method", April 2003Source of RFC: sip (rai)
Errata ID: 4652
Reported By: Brett Tate
Date Reported: 2016-03-30
Section 2.1 says:
The Refer-To header field MAY be encrypted as part of end-to-end encryption.
It should say:
If the URI contains a comma, question mark or semicolon, the URI MUST be enclosed in angle brackets (< and >). The Refer-To header field MAY be encrypted as part of end-to-end encryption.
If addr-spec is used when there are parameters, it is ambiguous if the parameters are URI parameters or header parameters. For consistency with RFC 3261 section 20, the same bracket rule is indicated even if comma and question mark do not cause an issue.