Internet DRAFT - draft-ietf-stir-servprovider-oob
draft-ietf-stir-servprovider-oob
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
draft-ietf-stir-servprovider-oob-05
Abstract
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 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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Please review these documents carefully, as they describe your rights
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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
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.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"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":"https://cps.example.com" }. 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
functions.
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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
specification.
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
vulnerability.
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,
<https://www.rfc-editor.org/info/rfc2119>.
[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,
<https://www.rfc-editor.org/info/rfc8224>.
[RFC8225] Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
<https://www.rfc-editor.org/info/rfc8225>.
[RFC8226] Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", RFC 8226,
DOI 10.17487/RFC8226, February 2018,
<https://www.rfc-editor.org/info/rfc8226>.
[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,
<https://www.rfc-editor.org/info/rfc8816>.
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, <https://www.rfc-editor.org/info/rfc7258>.
[RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement and Requirements",
RFC 7340, DOI 10.17487/RFC7340, September 2014,
<https://www.rfc-editor.org/info/rfc7340>.
[RFC9060] Peterson, J., "Secure Telephone Identity Revisited (STIR)
Certificate Delegation", RFC 9060, DOI 10.17487/RFC9060,
September 2021, <https://www.rfc-editor.org/info/rfc9060>.
Author's Address
Jon Peterson
Neustar, Inc.
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Email: jon.peterson@team.neustar
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