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Out-of-Band STIR for Service Providers

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Author Jon Peterson
Last updated 2024-04-08 (Latest revision 2023-10-23)
Replaces draft-peterson-stir-servprovider-oob
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Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Standards Track                         23 October 2023
Expires: 25 April 2024

                 Out-of-Band STIR for Service Providers


   The Secure Telephone Identity Revisited (STIR) framework defines
   means of carrying its Persona Assertion Tokens (PASSporTs) either in-
   band, within the headers of a SIP request, or out-of-band, through a
   service that stores PASSporTs for retrieval by relying parties.  This
   specification defines a way that the out-of-band conveyance of
   PASSporTs can be used to support large service providers, for cases
   in which in-band STIR conveyance is not universally available.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

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   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 25 April 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   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.  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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Service Provider Deployment Architecture for Out-of-Band
           STIR  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Advertising a CPS . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Submitting a PASSporT . . . . . . . . . . . . . . . . . . . .   5
   6.  PASSporT Retrieval  . . . . . . . . . . . . . . . . . . . . .   6
   7.  Gateways  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   8
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     11.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   STIR [RFC8224] provides a cryptographic assurance of the identity of
   calling parties in order to prevent impersonation, which is a key
   enabler of unwanted robocalls, swatting, vishing, voicemail hacking,
   and similar attacks (see [RFC7340]).  The STIR out-of-band [RFC8816]
   framework enables the delivery of PASSporT [RFC8225] objects through
   a Call Placement Service (CPS), rather than carrying them within a
   signaling protocol such as SIP.  Out-of-band conveyance is valuable
   when end-to-end SIP delivery of calls is partly or entirely
   unavailable due to network border policies, calls routinely
   transitting a gateway to the PSTN, or similar circumstances.

   While out-of-band STIR can be implemented as an open Internet
   service, it then requires complex security measures to enable the CPS
   function without allowing the CPS to collect data about the parties
   placing calls.  This specification describes CPS implementations that
   act specifically on behalf of service providers who will be
   processing the calls that STIR secures, and thus who will necessarily
   know the parties communicating, so an alternative security
   architecture becomes possible.  These functions may be crucial to the

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   adoption of STIR in some environments, like legacy non-IP telephone
   networks, where in-band transmission of PASSporTs may not be

   Environments that might support this flavor of STIR out-of-band
   include carriers, large enterprises, call centers, or any Internet
   service that aggregates on behalf of a large number of telephone
   endpoints.  That last case may include certain classes of gateway or
   transit providers.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Service Provider Deployment Architecture for Out-of-Band STIR

   The architecture in this specification assumes that every
   participating service provider is associated with one or more
   designated CPS instances.  A service provider's CPS serves as a place
   where callers, or in some cases gateways, can deposit a PASSporT when
   attempting to place a call to a subscriber of the destination service
   provider; if the caller's domain supports in-band STIR, this can be
   done at the same time as an in-band STIR call is placed.  The
   terminating service provider could operate the CPS themselves, or a
   third party could operate the CPS on the destination's behalf.  This
   model does not assume a monolithic CPS that acts on behalf of all
   service providers, but nor does it prohibit multiple service
   providers from sharing a CPS provider.  Moreover, a particular CPS
   can be a logically distributed entity compromised of several
   geographically distant entities that flood PASSporTs among themselves
   to support an anycast-like service.

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   The process of locating a destination CPS and submitting a PASSporT
   naturally requires Internet connectivity to the CPS.  If the CPS is
   deployed in the terminating service provider network, any such
   network connectivity could instead be leveraged by a caller to
   initiate a SIP session, during which in-band STIR could be used
   normally.  The applicability of this architecture is therefore to
   those cases where, for whatever reason, SIP requests cannot reliably
   convey PASSporTs end-to-end, but an HTTP transaction can reliably be
   sent to the CPS from an out-of-band authentication service (OOB-AS).
   It is hoped that as IP connectivity between telephone providers
   increases, there will be less need for an out-of-band mechanism, but
   it can serve as a fallback mechanism in cases where service providers
   cannot predict whether end-to-end delivery of SIP calls will occur.

4.  Advertising a CPS

   If more than one CPS exists for a given deployment, there will need
   to be some means of discovering CPSs, either administratively or
   programmatically.  Many services providers have bilateral agreements
   to peer with one another, and in those environments, identifying
   their respective CPS's could be a simple matter of provisioning.  A
   consortium of service providers could agree to choose from a list of
   available CPS providers, say.  But in more pluralist environments,
   some mechanism is needed to discover the CPS associated with the
   target of a call.

   In order to allow the CPS chosen by a service provider to be
   discovered securely, this specification defines a CPS advertisement.
   Effectively, a CPS advertisement is a document which contains the URL
   of a CPS, as well as any information needed to determine which
   PASSporTs should be submitted to that CPS (e.g., Service Provider
   Codes (SPCs) or telephone number ranges).  An advertisement may be
   signed with a STIR [RFC8226] credential, or another credential that
   is trusted by the participants in a given STIR environment.  The
   advantage to signing with STIR certificates is that they contain a
   "TNAuthList" value indicating the telephone network resources that a
   service provider controls.  This information can be matched with a
   TNAuthList value in the CPS advertisement to determine whether the
   signer has the authority to advertise a particular CPS as the proper
   destination for PASSporTs.

   The format of a service provider CPS advertisement consists of a
   simple JSON object containing one or more pairs of TNAuthList values
   pointing to the URIs of CPSs, e.g. {
   "0-1234":"" }. The format of this is a hyphen-
   separated concatenation of the [RFC8226] TNAuthList TNEntry values
   ("0" for SPC, "1" for telephone number range, "2" for individual
   telephone numnber) with the TNAuthList value.  Note for in case "1",

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   telephone number ranges are expressed by a starting telephone number
   followed by a count, and the count itself is here also by hyphen-
   separated from the TN (e.g., "1-15714341000-99").  An advertisement
   can contain multiple such ranges by adding more pairs.  CPS URIs MUST
   be HTTPS URIs.  These CPS URIs SHOULD be publicly reachable, as
   service providers cannot usually anticipate all of the potential
   callers that might want to connect with them, but in more constrained
   environments, they MAY be only reachable over a closed network.

   Advertising an SPC may be inappropriate in environments where an
   originating domains has no ready means to determine whether a given
   called telephone number falls within a scope of an SPC (such as a
   national routing database that maps telephone numbers to SPCs).  In
   such environments, TN based advertisements could enable discovery
   instead.  Also, note that PASSporTs can be used to sign communication
   where the "orig" and/or "dest" are not telephone numbers as such, but
   instead URI-based identifiers; these PASSporTs typically would not be
   signed by an [RFC8226] certificate, and future specification would be
   required to identify URI-based prefixes for CPS advertisements.

   CPS advertisements could be made available through existing or new
   databases, potentially aggregated across multiple service providers
   and distributed to call originators as necessary.  They could be
   discovered during the call routing process, including through a DNS
   lookup.  They could be shared through a distributed database among
   the participants in a multilateral peering arrangement.

   An alternative to CPS advertisements that may be usable in some
   environments is adding a field to STIR [RFC8226] certificates
   identifying the CPS URI issued to individual service providers.  As
   these certificates are themselves signed by a CA, and contain their
   own TNAuthList, the URI would be bound securely to the proper
   telephone network identifiers.  As STIR assumes a community of
   relying parties who trust these credentials, this method perhaps best
   mirrors the trust model required to allow a CPS to authorize PASSporT
   submission and retrieval.

5.  Submitting a PASSporT

   Submitting a PASSporT to a CPS as specified in the STIR out-of-band
   framework [RFC8816] requires security measures which are intended to
   prevent the CPS from learning the identity of the caller (or callee),
   to the degree possible.  In this service provider case, however, the
   CPS is operated by the service provider of the callee (or an entity
   operating on their behalf), and as such the information that appears
   in the PASSporT is redundant with call signaling that the terminating
   party will receive anyway.  Therefore, the service provider out-of-
   band framework does not attempt to conceal the identity of the

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   originating or terminating party from the CPS.

   An out-of-band authentication service (OOB-AS) forms a secure
   connection with the target CPS.  This may happen at the time a call
   is being placed, or it may be a persistent connection, if there is a
   significant volume of traffic sent over this interface.  The OOB-AS
   SHOULD authenticate itself to the CPS via mutual TLS using its STIR
   credential [RFC8226], the same one it would use to sign calls; this
   helps mitigate the risk of flooding that more open OOB
   implementations may face.  Furthermore, use of mutual TLS prevents
   attackers from replaying captured PASSporTs to the CPS.  A CPS makes
   its own policy decision as to whether it will accept calls from a
   particular OOB-AS, and at what volumes.  A CPS can use this mechanism
   can authorize service providers who already hold STIR credentials to
   submit PASSporTs to a CPS, but alternative mechanisms would be
   required for any entities that do not hold a STIR credential,
   including gateway or transit providers who want to submit PASSporTs.
   See Section 7 below for more on their behavior.

   Service provider out-of-band PASSporTs do not need to be encrypted
   for storage at the CPS, although use of transport-layer security to
   prevent eavesdropping on the connection between the CPS and OOB-ASs
   is REQUIRED.  PASSporTs will typically be submitted to the CPS at the
   time they are created by an AS; if the PASSporT is also being used
   for in-band transit within a SIP request, the PASSporT can be
   submitted to the CPS before or after the SIP request is sent, at the
   discretion of the originating domain.  An OOB-AS will use a REST
   interface to submit PASSporTs to the CPS as described in [RFC8816]
   Section 9.  PASSporTs persist at the CPS for as long as is required
   for them to be retrieved (see the next section), but in any event for
   no longer than the freshness interval of the PASSporT itself (a
   maximum of sixty seconds).

6.  PASSporT Retrieval

   The STIR out-of-band framework [RFC8816] proposes two means that
   called parties can acquire PASSporTs out-of-band: through a retrieval
   interface, or through a subscription interface.  In the service
   provider context, where many calls to or from the same number may
   pass through a CPS simultaneously, an out-of-band capable
   verification service (OOB-VS) may therefore operate in one of two
   modes: it can either pull PASSporTs from the CPS after calls arrive,
   or receive push notifications from the CPS for incoming calls.

   Pulling of PASSporTs from the CPS will follow the basic REST flow
   described in [RFC8816] Section 9.  In the pull model, a terminating
   service provider polls the CPS via its OOB-VS after having received a
   call for which the call signaling does not itself carry a PASSporT.

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   Exactly how a CPS determines which PASSporTs an OOB-VS is eligible to
   receive over this interface is a matter of local policy.  If a CPS
   serves only one service provider, then all PASSporTs submitted to the
   CPS are made available to the OOB-VS of that provider; indeed, the
   CPS and OOB-VS may be colocated or effectively operated as a
   consolidated system.  In a multi-provider environment, the STIR
   credential of the terminating domain can be used by the CPS to
   determine the range of TNAuthLists for which an OOB-VS is entitled to
   receive PASSporTs; this may be through a mechanism like mutual TLS,
   or through using the STIR credential to sign a token that is
   submitted to the CPS by the retrieving OOB-VS.  Note that a multi-
   provider CPS will need to inspect the "dest" element of a PASSporT to
   determine which OOB-VS should receive the PASSporT.

   In a push model, an OOB-VS could for example subscribe to a range of
   telephone numbers or SPCs, which will be directed to that OOB-VS by
   the CPS (provided the OOB-VS is authorized to receive them by the
   CPS).  PASSporT might be sent to the OOB-VS either before or after
   unsigned call signaling has been received by the terminating domain.
   In either model, the terminating side may need to delay rendering a
   call verification indicator when alerting, in order to await the
   potential arrival of a PASSporT at the OOB-VS.  The exact timing of
   this, and its interaction with the substitution attack described in
   [RFC8816] Section 7.4, is left for future work.

7.  Gateways

   In some deployment architectures, gateways might perform a function
   that interfaces with a CPS for the retrieval or storage of PASSporTs,
   especially in cases when in-band STIR service providers need to
   exchange secure calls with providers that can only be reached by STIR
   out-of-band.  For example, a closed network of in-band STIR providers
   may send SIP INVITEs to a gateway in front of a traditional PSTN
   tandem that services a set of legacy service providers.  In that
   environment, a gateway might extract a PASSporT from an in-band SIP
   INVITE and store it in a CPS that was established to handle requests
   for one or more legacy providers, who in turn consume those PASSporTs
   through an OOB-VS to assist in robocall mitigation and similar

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   The simplest way to implement a gateway performing this sort of
   function for a service provider CPS system is to issue credentials to
   the gateway that allow it to act on behalf of the legacy service
   providers it supports: this would allow it to both add PASSporTs to
   the CPS acting on behalf of the legacy providers, and also to create
   PASSporTs for in-band STIR conveyance from the legacy-providers to
   terminating service providers in the closed STIR network.  For
   example, a service provider could issue a delegate certificate
   [RFC9060] for this purpose.

8.  Acknowledgments

   We would like to thank Alex Fenichel for contributions to this

9.  IANA Considerations

   This memo includes no request to IANA.

10.  Security Considerations

   The Security Considerations of [RFC8816] apply to this document as
   well, including concerns about potential denial-of-service vectors
   and traffic analysis.  However, that specification's model focused a
   great deal on the privacy implications of uploading PASSporTs to a
   third-party web service.  This draft mitigates those concerns by
   making the CPS one of the parties to call setup (or an entity
   contractual acting on their behalf).  That said, any architecture in
   which PASSporTs are shared with a federated or centralized CPS raises
   potential concerns about data collection [RFC7258].

   Unlike [RFC8816], this document proposes the use of STIR certificates
   to authenticate transactions with a CPS as well as signatures for CPS
   advertisements.  This presumes an environment where STIR certificates
   are issued by trust anchors which are already trusted by the CPS,
   potentially to gateways and similar services.  Common STIR
   deployments use Service Provider Codes (SPCs) instead of telephone
   numbers ranges to identify service providers today; determining
   whether a given SPC entitles a service provider to access PASSporTs
   for a given telephone number is not trivial, but is a necessary
   component of this CPS architecture.  If anyone with a STIR
   certificate is able to publish or access PASSporTs for any telephone
   number, this would create an intolerable security and privacy

11.  References

11.1.  Normative References

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8224]  Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 8224,
              DOI 10.17487/RFC8224, February 2018,

   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,

   [RFC8816]  Rescorla, E. and J. Peterson, "Secure Telephone Identity
              Revisited (STIR) Out-of-Band Architecture and Use Cases",
              RFC 8816, DOI 10.17487/RFC8816, February 2021,

11.2.  Informative References

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,

   [RFC9060]  Peterson, J., "Secure Telephone Identity Revisited (STIR)
              Certificate Delegation", RFC 9060, DOI 10.17487/RFC9060,
              September 2021, <>.

Author's Address

   Jon Peterson
   Neustar, Inc.

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