Rejecting Anonymous Requests in the Session Initiation Protocol (SIP)
RFC 5079

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Network Working Group                                       J. Rosenberg
Request for Comments: 5079                                         Cisco
Category: Standards Track                                  December 2007

 Rejecting Anonymous Requests in the Session Initiation Protocol (SIP)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   The Session Initiation Protocol (SIP) allows for users to make
   anonymous calls.  However, users receiving such calls have the right
   to reject them because they are anonymous.  SIP has no way to
   indicate to the caller that the reason for call rejection was that
   the call was anonymous.  Such an indication is useful to allow the
   call to be retried without anonymity.  This specification defines a
   new SIP response code for this purpose.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 2
   3.  Server Behavior . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  UAC Behavior  . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  433 (Anonymity Disallowed) Definition . . . . . . . . . . . . . 4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 6

Rosenberg                   Standards Track                     [Page 1]
RFC 5079                   ACR Response Code               December 2007

1.  Introduction

   The Session Initiation Protocol (SIP) [RFC3261] allows for users to
   make anonymous calls.  In RFC 3261, this is done by including a From
   header field whose display name has the value of "Anonymous".
   Greater levels of anonymity were subsequently defined in [RFC3323],
   which introduces the Privacy header field.  The Privacy header field
   allows a requesting User Agent (UA) to ask for various levels of
   anonymity, including user level anonymity, header level anonymity,
   and session level anonymity.  [RFC3325] additionally defined the
   P-Asserted-Identity header field, used to contain an asserted
   identity.  RFC 3325 also defined the 'id' value for the Privacy
   header field, which is used to request the network to remove the
   P-Asserted-Identity header field.

   Though users need to be able to make anonymous calls, users that
   receive such calls retain the right to reject the call because it is
   anonymous.  SIP does not provide a response code that allows the User
   Agent Server (UAS), or a proxy acting on its behalf, to explicitly
   indicate that the request was rejected because it was anonymous.  The
   closest response code is 403 (Forbidden), which doesn't convey a
   specific reason.  While it is possible to include a reason phrase in
   a 403 response that indicates to the human user that the call was
   rejected because it was anonymous, that reason phrase is not useful
   for automata and cannot be interpreted by callers that speak a
   different language.  An indication that can be understood by an
   automaton would allow for programmatic handling, including user
   interface prompts, or conversion to equivalent error codes in the
   Public Switched Telephone Network (PSTN) when the client is a
   gateway.

   To remedy this, this specification defines the 433 (Anonymity
   Disallowed) response code.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

Rosenberg                   Standards Track                     [Page 2]
RFC 5079                   ACR Response Code               December 2007

3.  Server Behavior

   A server (generally acting on behalf of the called party, though this
   need not be the case) MAY generate a 433 (Anonymity Disallowed)
   response when it receives an anonymous request, and the server
   refuses to fulfill the request because the requestor is anonymous.  A
   request SHOULD be considered anonymous when the identity of the
   originator of the request has been explicitly withheld by the
   originator.  This occurs in any one of the following cases:

   o  The From header field contains a URI within the anonymous.invalid
      domain.

   o  The From header field contains a display name whose value is