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Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Informational                             July 13, 2020
Expires: January 14, 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

   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
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   This Internet-Draft will expire on January 14, 2021.

Copyright Notice

   Copyright (c) 2020 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
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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 . . . . . . . . . . . . . . . . . . . .   4
   6.  PASSporT Retrieval  . . . . . . . . . . . . . . . . . . . . .   5
   7.  Gateways  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   10. Security Considerations . . . . . . . . . . . . . . . . . . .   7
   11. Informative References  . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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.

   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

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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 will advertise one or more designated
   CPS instances.  A service provider's CPS serves as a place where
   callers 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.

   The process of locating a destination CPS and submitting a PASSporT
   requires Internet connectivity between the call originator and the
   destination network.  Ordinarily, that network connectivity could be
   leveraged to initiate a SIP session, during which in-band STIR could
   be used.  The applicability of this architecture is therefore to
   those cases where, for whatever reason, SIP calls cannot reliably be
   placed end-to-end, but an HTTP transaction can reliably be sent to
   the destination network from the out-of-band authentication service
   (OOB-AS) in the caller's network.  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

   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.  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

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   PASSporTs should be submitted to that CPS.  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":"https://cps.example.com" }.
   TNAuthList values can be either Service Provider Codes (SPCs) or
   telephone numbers or number ranges.  CPS URIs MUST be HTTPS URIs.
   [More TBD].

   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] credentials issued
   to individual service providers.  As these certificates are
   themselves signed by a CA, the URI would be bound securely to the
   service provider.  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

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

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   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 using its STIR credential
   [RFC8226]the same one it would use to sign calls via mutual TLS; 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.

   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 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 are persisted by 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 [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 occur simultaneously, 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

   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.  Note that a CPS will need to inspect the
   "dest" element of a PASSporT to determine which OOB-VS should receive
   the PASSporT in this case.  [TBD: Which sub/not protocol to use for
   the case where the CPS and OOB-VS are not composed in a single

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   Pulling of PASSporTs from the CPS will follow the basic REST flow
   described in [I-D.ietf-stir-oob] Section 9.  In the push interface
   case, exactly how a CPS determines which PASSporTs to send to an out-
   of-band verification service 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 contacts the CPS
   via its OOB-VS after having received a call in cases when the call
   signaling does not itself carry a STIR signature.  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 call signaling 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.

7.  Gateways

   In some deployment architectures, gateways might perform a function
   that interfaces with a CPS for the retrieval or storage of PASSporTs.
   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 take a PASSporT out of in-band SIP INVITEs 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 functions.

   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.

8.  Acknowledgments

   We would like to thank Alex Fenichel for contributions to this

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9.  IANA Considerations

   This memo includes no request to IANA.

10.  Security Considerations


11.  Informative References

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

              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, <https://www.rfc-editor.org/info/rfc3311>.

   [RFC4916]  Elwell, J., "Connected Identity in the Session Initiation
              Protocol (SIP)", RFC 4916, DOI 10.17487/RFC4916, June
              2007, <https://www.rfc-editor.org/info/rfc4916>.

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://www.rfc-editor.org/info/rfc7159>.

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

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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,

Author's Address

   Jon Peterson
   Neustar, Inc.

   Email: jon.peterson@neustar.biz

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