Session Initiation Protocol (SIP) Call Control - Transfer
RFC 5589

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Network Working Group                                          R. Sparks
Request for Comments: 5589                                       Tekelec
BCP: 149                                                A. Johnston, Ed.
Category: Best Current Practice                                    Avaya
                                                               D. Petrie
                                                               SIPez LLC
                                                               June 2009

       Session Initiation Protocol (SIP) Call Control - Transfer

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

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Abstract

   This document describes providing Call Transfer capabilities in the
   Session Initiation Protocol (SIP).  SIP extensions such as REFER and
   Replaces are used to provide a number of transfer services including
   blind transfer, consultative transfer, and attended transfer.  This
   work is part of the SIP multiparty call control framework.

Sparks, et al.           Best Current Practice                  [Page 1]
RFC 5589                    SIP CC Transfer                    June 2009

Table of Contents

   1. Overview ........................................................3
   2. Actors and Roles ................................................3
   3. Terminology .....................................................4
   4. Requirements ....................................................4
   5. Using REFER to Achieve Call Transfer ............................5
   6. Basic Transfer ..................................................6
      6.1. Successful Transfer ........................................8
      6.2. Transfer with Dialog Reuse ................................11
      6.3. Failed Transfer ...........................................15
           6.3.1. Target Busy ........................................16
           6.3.2. Transfer Target Does Not Answer ....................17
   7. Transfer with Consultation Hold ................................18
      7.1. Exposing Transfer Target ..................................18
      7.2. Protecting Transfer Target ................................19
      7.3. Attended Transfer .........................................24
      7.4. Recovery When One Party Does Not Support REFER ............28
      7.5. Attended Transfer When Contact URI Is Not Known to
           Route to a User Agent .....................................29
      7.6. Semi-Attended Transfer ....................................37
      7.7. Attended Transfer Fallback to Basic Transfer ..............42
   8. Transfer with Referred-By ......................................45
   9. Transfer as an Ad Hoc Conference ...............................49
   10. Transfer with Multiple Parties ................................52
   11. Gateway Transfer Issues .......................................54
      11.1. Coerce Gateway Hairpins to the Same Gateway ..............54
      11.2. Consultative Turned Blind Gateway Glare ..................55
   12. Security Considerations .......................................55
   13. Acknowledgments ...............................................56
   14. References ....................................................56
      14.1. Normative References .....................................56
      14.2. Informative References ...................................57

Sparks, et al.           Best Current Practice                  [Page 2]
RFC 5589                    SIP CC Transfer                    June 2009

1.  Overview

   This document describes providing Call Transfer capabilities and