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.

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 https://www.rfc-editor.org/info/rfc9409.

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RFC 8288 [RFC 8288] 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 [RFC 3261] trunking between the two telephony networks.

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) [RFC 3986] 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].

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 [RFC 7033]. The following examples include line breaks and indentation for clarity. The location of the capability set document is returned to the network device in the "href" attribute. The ITSP may use an authentication framework such as OAuth 2.0 [RFC 6749] to determine the identity of the enterprise telephony network to provide the appropriate capability set document.

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

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 [SIP-AUTO-PEER] section in "Automatic Peering for SIP Trunks" [SIP-AUTO-PEER].

[RFC8288]

M. Nottingham, "Web Linking", RFC 8288, DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.

[RFC3261]

J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J. Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.

[RFC3986]

T. Berners-Lee, R. Fielding, and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.

[RFC6749]

D. Hardt, "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.

[RFC7033]

P. Jones, G. Salgueiro, M. Jones, and J. Smarr, "WebFinger", RFC 7033, DOI 10.17487/RFC7033, September 2013,
<https://www.rfc-editor.org/info/rfc7033>.

[SIP-AUTO-PEER]

Cisco Systems, K. Inamdar, S. Narayanan, and C. F. Jennings, "Automatic Peering for SIP Trunks", Internet-Draft draft-ietf-asap-sip-auto-peer-07, January 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-asap-sip-auto-peer-07>.

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.

Kaustubh Inamdar

Sreekanth Narayanan

Derek Engi

Gonzalo Salgueiro