Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Informational                         February 22, 2021
Expires: August 26, 2021

                 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

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   This Internet-Draft will expire on August 26, 2021.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified 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 . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Submitting a PASSporT . . . . . . . . . . . . . . . . . . . .   5
   6.  PASSporT Retrieval  . . . . . . . . . . . . . . . . . . . . .   6
   7.  Gateways  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   7
   11. Informative References  . . . . . . . . . . . . . . . . . . .   7
   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
   [I-D.ietf-stir-oob] 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

   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 who thus will learn about
   the parties to communication independently, so an alternative
   security architecture becomes possible.  These functions may be
   crucial to the adoption of STIR in some environments, like mobile
   networks, where in-band transmission of STIR may not be feasible.

   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.

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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.

   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, that network
   connectivity could 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
   destination network from the 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 simply agree to choose from a

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   list of available CPS providers, say.  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 is a simple JSON
   object containing one or more pairs of TNAuthList values pointing to
   the URIs of CPSs, e.g. { "1234":"" }.
   TNAuthList values can be either Service Provider Codes (SPCs) or
   telephone numbers or number ranges.  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.

   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.

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5.  Submitting a PASSporT

   Submitting a PASSporT to a CPS as specified in the STIR out-of-band
   framework [I-D.ietf-stir-oob] 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 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
   [I-D.ietf-stir-oob] Section 9 [more TBD].  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).

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6.  PASSporT Retrieval

   The STIR out-of-band framework [I-D.ietf-stir-oob] 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 simultaneous, an out-of-band capable
   verification service 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.

   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.

   [TBD: Which sub/not protocol to use for the case where the CPS and
   OOB-VS are not composed in a single function?]

   Pulling of PASSporTs from the CPS will follow the basic REST flow
   described in [I-D.ietf-stir-oob] Section 9.  Exactly how a CPS
   determines which PASSporTs an OOB-VS is eligible to receive is a
   matter of implementation.  An OOB-VS could for example subscribe to a
   range of telephone numbers, which will be directed to that OOB-VS by
   the CPS (provided the OOB-VS is authorized to receive them by the
   CPS).  In the pull model, a terminating service provider polls the
   CPS via its OOB-VS after having received a call, in those cases where
   the call signaling does not itself carry a PASSporT.  In the push
   model, a PASSporT might be sent to the OOB-VS either before or after
   unsigned call signaling has been received by the terminating domain.
   Domains using the push model may therefore need to adopt a model
   where the display of call verification for alerting is held
   momentarily 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 [I-D.ietf-stir-oob] Section 7.4,
   will be the covered by future versions of this specification.

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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

   The simplest way to interface 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
   [I-D.ietf-stir-cert-delegation] 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

   It is the aim of this mechanism to provide

11.  Informative References

              Peterson, J., "STIR Certificate Delegation", draft-ietf-
              stir-cert-delegation-03 (work in progress), July 2020.

              Rescorla, E. and J. Peterson, "STIR Out-of-Band
              Architecture and Use Cases", draft-ietf-stir-oob-07 (work
              in progress), March 2020.

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              Peterson, J., "PASSporT Extension for Diverted Calls",
              draft-ietf-stir-passport-divert-09 (work in progress),
              July 2020.

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

   [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,

   [RFC3311]  Rosenberg, J., "The Session Initiation Protocol (SIP)
              UPDATE Method", RFC 3311, DOI 10.17487/RFC3311, October
              2002, <>.

   [RFC4916]  Elwell, J., "Connected Identity in the Session Initiation
              Protocol (SIP)", RFC 4916, DOI 10.17487/RFC4916, June
              2007, <>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <>.

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

   [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,

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

Author's Address

   Jon Peterson
   Neustar, Inc.


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