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The 'sip-trunking-capability' Link Relation Type
RFC 9409

Document Type RFC - Informational (July 2023)
Authors Kaustubh Inamdar , Sreekanth Narayanan , Derek Engi , Gonzalo Salgueiro
Last updated 2023-07-14
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Murray Kucherawy
Send notices to (None)
RFC 9409

Internet Engineering Task Force (IETF)                        K. Inamdar
Request for Comments: 9409                                  Unaffiliated
Category: Informational                                     S. Narayanan
ISSN: 2070-1721                                                  D. Engi
                                                            G. Salgueiro
                                                               July 2023

            The 'sip-trunking-capability' Link Relation Type


   This Informational document defines the 'sip-trunking-capability'
   link relation type that may be used by an enterprise telephony
   Session Initiation Protocol (SIP) network to retrieve a SIP trunking
   capability set document, which contains the capabilities and
   configuration requirements of an Internet Telephony Service Provider
   (ITSP).  These technical requirements allow for seamless peering
   between SIP-based enterprise telephony networks and the ITSP.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2023 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
   ( 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.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  The 'sip-trunking-capability' Link Relation Type
   3.  Example Usage
   4.  IANA Considerations
   5.  Security Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Authors' Addresses

1.  Introduction

   RFC 8288 [RFC8288] defines a way to indicate relationships between
   resources on the Web. This document specifies the 'sip-trunking-
   capability' link relation type according to the rules of RFC 8288.
   Links with this relationship type can be used to exchange capability
   information between potential peer devices.  In the event that
   systems require additional parameters and configuration to negotiate
   communication, a well-known URI can be utilized to deliver
   information to potential peers, including machine-readable
   instructions and parameters needed for peering.

   The 'sip-trunking-capability' link relation type may be used on web
   resources hosted by ITSPs to provide a structured and detailed
   capability set document.  The capability set document [SIP-AUTO-PEER]
   encapsulates a set of characteristics of an ITSP, which when
   retrieved by enterprise telephony network devices allows for
   automated establishment of SIP [RFC3261] trunking between the two
   telephony networks.

2.  The 'sip-trunking-capability' Link Relation Type

   A capability set document is hosted via web resources by the ITSP.  A
   unique location of the document can be preconfigured and provided to
   each peer by the ITSP, or a centrally published resource can be used
   that dynamically generates the capability set document based on one
   or more Uniform Resource Identifiers (URIs) [RFC3986] determined by
   the peering device.  The capability set document describes the
   configuration parameters required to successfully establish SIP
   trunking between an enterprise and an ITSP network.  The capability
   set document is composed of structured and machine-readable
   parameters that can be converted into configuration data to meet the
   communication requirements of the ITSP.  The need for an enterprise
   telephony network to obtain a capability set document from an ITSP is
   documented in "Automatic Peering for SIP Trunks" [SIP-AUTO-PEER].

3.  Example Usage

   This section provides an example of possible use of the 'sip-
   trunking-capability' relation type.  The enterprise network device
   solicits the location of the capability set document from the well-
   known URI hosted by the ITSP using the WebFinger protocol [RFC7033].
   The following examples include line breaks and indentation for

      GET /.well-known/webfinger?

   The location of the capability set document is returned to the
   network device in the "href" attribute.

      HTTP/1.1 200 OK
      Access-Control-Allow-Origin: *
      Content-Type: application/jrd+json
         "subject" : "",
         "links" :
               "rel" : "sip-trunking-capability",
               "href" : ""

   The ITSP may use an authentication framework such as OAuth 2.0
   [RFC6749] to determine the identity of the enterprise telephony
   network to provide the appropriate capability set document.

4.  IANA Considerations

   IANA has registered the 'sip-trunking-capability' link relation under
   the "Link Relation Types" registry as follows:

   Relation Name:  sip-trunking-capability

   Description:  Refers to a capability set document that defines
      parameters or configuration requirements for automated peering and
      communication-channel negotiation of the Session Initiation
      Protocol (SIP).

   Reference:  RFC 9409

5.  Security Considerations

   The 'sip-trunking-capability' relation type is not known to introduce
   any new security issues not already discussed in RFC 8288 for generic
   use of web-linking mechanisms.  However, it is recommended to
   exercise caution when publishing potentially sensitive capability
   information over unencrypted or unauthenticated channels.  Additional
   security recommendations are outlined in the capability set document
   definition.  See the Security Considerations section in "Automatic
   Peering for SIP Trunks" [SIP-AUTO-PEER].

6.  References

6.1.  Normative References

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,

6.2.  Informative References

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,

   [RFC7033]  Jones, P., Salgueiro, G., Jones, M., and J. Smarr,
              "WebFinger", RFC 7033, DOI 10.17487/RFC7033, September
              2013, <>.

              Inamdar, K., Narayanan, S., and C. F. Jennings, "Automatic
              Peering for SIP Trunks", Work in Progress, Internet-Draft,
              draft-ietf-asap-sip-auto-peer-07, 13 January 2023,


   This document resulted from the discussions in the ASAP Working
   Group, especially the detailed and thoughtful comments of Paul Jones,
   Marc Petit-Huguenin, Mark Nottingham, Cullen Jennings, Jonathan
   Rosenberg, Jon Peterson, Chris Wendt, Jean Mahoney, and Murray
   Kucherawy.  Additional thanks to Joe Clarke, Tim Bray, Christopher
   Wood, Dan Romascanu, David Dong, Éric Vyncke, Robert Wilton, and Lars
   Eggert for their reviews and feedback.

Authors' Addresses

   Kaustubh Inamdar

   Sreekanth Narayanan

   Derek Engi
   Ann Arbor, MI
   United States of America
   Phone: +1 919 392 7966

   Gonzalo Salgueiro
   7200-12 Kit Creek Rd.
   Research Triangle Park, NC 27709
   United States of America
   Phone: +1 919 392 3266