Requesting Answering Modes for the Session Initiation Protocol (SIP)
RFC 5373

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• 07

Network Working Group                                     D. Willis, Ed.
Request for Comments: 5373                             Softarmor Systems
Category: Standards Track                                       A. Allen
                                                Research in Motion (RIM)
                                                           November 2008

  Requesting Answering Modes for 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" (STD1) for the standardization state and
   status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2008 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (http://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

   This document extends SIP with two header fields and associated
   option tags that can be used in INVITE requests to convey the
   requester's preference for user-interface handling related to
   answering of that request.  The first header, "Answer-Mode",
   expresses a preference as to whether the target node's user interface
   waits for user input before accepting the request or, instead,
   accepts the request without waiting on user input.  The second
   header, "Priv-Answer-Mode", is similar to the first, except that it
   requests administrative-level access and has consequent additional
   authentication and authorization requirements.  These behaviors have
   applicability to applications such as push-to-talk and to diagnostics
   like loop-back.  Usage of each header field in a response to indicate
   how the request was handled is also defined.

Willis & Allen              Standards Track                     [Page 1]
RFC 5373                  SIP Answering Modes              November 2008

Table of Contents

   1.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  5
   2.  Syntax of Header Fields and Option Tags  . . . . . . . . . . .  5
   3.  Usage of the Answer-Mode and Priv-Answer-Mode Header Fields  .  6
   4.  Usage of the Answer-Mode and Priv-Answer-Mode Header
       Fields in Requests . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  The Difference Between Answer-Mode and Priv-Answer-Mode  .  7
     4.2.  The "require" Modifier . . . . . . . . . . . . . . . . . .  9
     4.3.  Procedures at User Agent Clients (UAC) . . . . . . . . . .  9
       4.3.1.  All Requests . . . . . . . . . . . . . . . . . . . . .  9
       4.3.2.  REGISTER Transactions  . . . . . . . . . . . . . . . .  9
       4.3.3.  INVITE Transactions  . . . . . . . . . . . . . . . . . 10
     4.4.  Procedures at Intermediate Proxies . . . . . . . . . . . . 12
       4.4.1.  General Proxy Behavior . . . . . . . . . . . . . . . . 12
       4.4.2.  Issues with Automatic Answering and Forking  . . . . . 12
     4.5.  Procedures at User Agent Servers (UAS) . . . . . . . . . . 13
       4.5.1.  INVITE Transactions  . . . . . . . . . . . . . . . . . 13
   5.  Usage of the Answer-Mode and Priv-Answer-Mode Header
       Fields in Responses  . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Procedures at the UAS  . . . . . . . . . . . . . . . . . . 14
     5.2.  Procedures at the UAC  . . . . . . . . . . . . . . . . . . 15
   6.  Examples of Usage  . . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  REGISTER Request . . . . . . . . . . . . . . . . . . . . . 15
     6.2.  INVITE Request . . . . . . . . . . . . . . . . . . . . . . 16
     6.3.  200 (OK) Response  . . . . . . . . . . . . . . . . . . . . 16
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
     7.1.  Attack Sensitivity Depends on Media Characteristics  . . . 17
     7.2.  Application Design Affects Attack Opportunity  . . . . . . 19
     7.3.  Applying the Analysis  . . . . . . . . . . . . . . . . . . 19
     7.4.  Minimal Policy Requirement . . . . . . . . . . . . . . . . 21
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
     8.1.  Registration of Header Fields  . . . . . . . . . . . . . . 22
     8.2.  Registration of Header Field Parameters  . . . . . . . . . 22
     8.3.  Registration of SIP Option Tags  . . . . . . . . . . . . . 22
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 23