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Connected Identity for STIR

Document Type Active Internet-Draft (stir WG)
Authors Jon Peterson , Chris Wendt
Last updated 2023-03-13
Replaces draft-peterson-stir-rfc4916-update
RFC stream Internet Engineering Task Force (IETF)
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Additional resources Mailing list discussion
Stream WG state WG Document
On agenda stir at IETF-116
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IESG IESG state I-D Exists
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Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Standards Track                                C. Wendt
Expires: 14 September 2023                                         Somos
                                                           13 March 2023

                      Connected Identity for STIR


   The SIP Identity header conveys cryptographic identity information
   about the originators of SIP requests.  The Secure Telephone Identity
   Revisited (STIR) framework however provides no means for determining
   the identity of the called party in a traditional telephone calling
   scenario.  This document updates prior guidance on the "connected
   identity" problem to reflect the changes to SIP Identity that
   accompanied STIR, and considers a revised problem space for connected
   identity as a means of detecting calls that have been retargeted to a
   party impersonating the intended destination, as well as the spoofing
   of mid-dialog or dialog-terminating events by intermediaries or third

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 14 September 2023.

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 . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Connected Identity Problem Statement for STIR . . . . . . . .   4
   4.  Connected Identity without Diversion  . . . . . . . . . . . .   5
   5.  Connected Identity with Diversion . . . . . . . . . . . . . .   6
     5.1.  Mid-dialog Requests . . . . . . . . . . . . . . . . . . .   8
   6.  Connected Identity in Mid-Dialog and Dialog-Terminating
           Requests  . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Authorization Policy for Callers  . . . . . . . . . . . . . .   9
   8.  Creating Pre-Association with Destinations  . . . . . . . . .   9
     8.1.  Media-less Dialogs for Connected Identity . . . . . . . .  10
   9.  The 'rsp' PASSporT Type . . . . . . . . . . . . . . . . . . .  10
   10. UPDATE Procedures for Provisional Dialogs . . . . . . . . . .  11
   11. Updates to RFC4916  . . . . . . . . . . . . . . . . . . . . .  11
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  12
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     15.1.  Informative References . . . . . . . . . . . . . . . . .  12
     15.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   The Session Initiation Protocol (SIP) [RFC3261] initiates sessions,
   and as a step in establishing sessions, it exchanges information
   about the parties at both ends.  Called users review information
   about the calling party, for example, to determine whether to accept
   communications initiated by a SIP, in the same way that users of the
   telephone network assess "Caller ID" information before picking up
   calls.  This information may sometimes be consumed by automata to
   make authorization decisions.  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]).

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   There also exists a related problem: the identity of the party who
   answers a call can differ from that of the initial called party for
   various innocuous reasons such as call forwarding, but in certain
   network environments, it is possible for attackers to hijack the
   route of a called number and direct it to a resource controlled by
   the attacker.  It can potentially be difficult to determine why a
   call reached a target other than the one originally intended, and
   whether the party ultimately reached by the call is one that the
   caller should trust.  The lack of mutual authentication of parties
   moreover makes it possible for outside attackers to inject forged
   messages (e.g.  BYE) into a SIP session.

   The property of providing identity in the backwards direction of a
   call is here called "connected identity."  Previous work on connected
   identity focused on fixing the core semantics of SIP.  [RFC4916]
   allowed a mid-dialog request, such as an UPDATE [RFC3311], to convey
   identity in either direction within the context of an existing
   INVITE-initiated dialog.  In an update to the original [RFC3261]
   behavior, [RFC4916] allowed that UPDATE to alter the From header
   field value for requests in the backwards direction: previously
   [RFC3261] required that the From header field values sent in requests
   in the backwards direction reflect the To header field value of the
   dialog-forming request, for various backwards-compatibility reasons.
   Under the original [RFC3261] rules, if Alice sent a dialog-forming
   request to Bob, then even if Bob's SIP service forwarded that dialog-
   forming request to Carol, Carol would still be required to put Bob's
   identity in the From header field value in any mid-dialog requests in
   the backwards direction.

   One of the original motivating use cases for [RFC4916] was the use of
   connected identity with the SIP Identity [RFC4474] header field.
   While a mid-dialog request in the backwards direction (e.g.  UPDATE)
   can be signed with Identity like any other SIP request, forwarded
   requests would not be signable without the ability to change the mid-
   dialog From header field value: Carol, say, would not be able to
   furnish a key to sign for Bob's identity, if Carol wanted to sign
   requests in the backwards direction.  Carol would however be able to
   sign for her own identity in the From header field value, if mid-
   dialog requests in the backwards direction were permitted to vary
   from the original To header field value.

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   With the obsolence of [RFC4474] by [RFC8224], this specification
   updates [RFC4916] to reflect the changes to the SIP Identity header
   and the revised problem space of STIR.  It also explores some new
   features that would be enabled by connected identity for STIR,
   including the use of connected identity to prevent route hijacking
   and to notify callers when an expected called party has successfully
   been reached.  This document also addresses concerns about applying
   [RFC4916] connected identity to STIR discussed in the SIPBRANDY
   framework [RFC8862].

   One area of connected identity that is not explored in this document
   is the implications for conferencing, especially meshed conferencing
   systems.  This scope of this mechanism is solely two-party
   communications; any work towards multiparty sharing of connected
   identity is left for future work.

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.  Connected Identity Problem Statement for STIR

   The STIR problem statement [RFC7340] enumerates robocalling,
   voicemail hacking, vishing, and swatting as problems with the modern
   telephone network that are enabled, or abetted, by impersonation: by
   the ability of a calling party to arbitrarily set the telephone
   number that will be rendered to end users to identify the caller.

   Today, sophisticated adversaries can redirect calls on the PSTN to
   destinations other than the intended called party.  For some call
   centers, like those associated with financial institutions,
   healthcare, and emergency services, an attacker could hope to gain
   valuable information about people or to prevent some classes of
   important services.  Moreover, on the Internet, the lack of any
   centralized or even federated routing system for telephone numbers
   has resulted in deployments where the routing of calls is arbitrary:
   calls to telephone numbers might be unceremoniously dumped on a PSTN
   gateway, they might be sent to a default intermediary that makes
   forwarding decisions based on a local flat file, various mechanisms
   like private ENUM [RFC6116] might be consulted, or routing might be
   determined in some other, domain-specific way.  In short, there are
   numerous attack surfaces that an adversary could explore to attempt
   to redirect calls for a particular number to someplace other than the
   intended destination.

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   Another motivating use case for connected identity is mid-dialog
   requests, including BYE.  The potential for an intermediary to
   generate a forged BYE in the backwards direction has always been
   built in to the stateful dialog management of SIP.  For example,
   there is a class of mobile fraud attacks ("call stretching") that
   rely on intermediary networks making it appear as if a call has
   terminated to one side, while maintaining that the call is still
   active to the other, in order to create a billing discrepancy that
   could be pocketed by the intermediary.  If BYE requests in both
   directions of a SIP dialog could be authenticated with STIR, just
   like dialog-forming requests, then another impersonation vector
   leading to fraud in the telephone network could be shut down.

   There are however practical limits to what securing the signaling can
   achieve.  [RFC4916] rightly observed that once a SIP call has been
   answered, the called party can be replaced by a different party (with
   a different identity) due to call transfer, call park and retrieval,
   and so on.  In some cases, due to the presence of a back-to-back user
   agent, it can be effectively impossible for the calling party to know
   that this has happened.  The problem statement considered for STIR
   focuses solely on signaling, not whether media from the connected
   party should be rendered to the caller when a dialog has been
   established.  This specification does not consider further any
   threats that arise from a substitution of media, though [RFC8862]
   contains related guidance.

4.  Connected Identity without Diversion

   In sunny-day uses cases, the address-of-record of the party reached
   by an INVITE corresponds to the "dest" field of the PASSporT in the
   INVITE's Identity header field value.  The calling party will,
   however, have no secure assurance that they have reached the proper
   party if an Identity header cannot be sent to them in the backwards
   direction.  Provided that the terminating side of the dialog is STIR-
   capable, they should have the capacity to sign a PASSporT for the
   address-of-record of the called party.

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   This specification therefore adds provisional and final responses,
   including the 100, 180, 183, and 200 responses, to the set of
   messages that can contain an Identity header.  PASSporTs that appear
   in SIP responses SHOULD use a "ppt" of "rsp", which is defined in
   Section 9 (although "div" MAY additionally appear in responses, per
   Section 5).  At a high level, an "rsp" PASSporT is signed similarly
   to the "div" [RFC8946] PASSporT, in so far as the certificate that
   signs a "rsp" PASSporT is signing the "dest" field, rather than the
   "orig" field.  If the terminating side does not possess an
   appropriate credential to sign for the value of the "dest" element
   value in the PASSporT, it MUST NOT sign and send a "rsp" PASSporT in
   the backwards direction.

   [TBD - Identity in 3xx, 4xx, 6xx responses?]

   It is worth noting as well that at the time [RFC4916] was written,
   the Identity mechanism was far stricter about what counted as
   retargeting than [RFC8224], which has canonicalization processes that
   eliminate minor changes to the URIs, especially when telephone
   numbers are the identifiers used by the caller and callee.  For
   sunny-day use cases, a PASSporT in a 183 or 200 OK should be
   sufficient to secure media keys for the purposes of SIPBRANDY

   The handling of an "rsp" PASSporT differs from the handling of a
   PASSporT received in a SIP request.  Most importantly, note that SIP
   responses cannot be rejected, unlike SIP requests -- there is no way
   for the recipient of a response to report errors to the sender.  The
   only protocol action that the calling party could take upon receiving
   a response carrying a problem PASSporT is to issue a CANCEL (for
   provisional dialogs) or BYE request in order to tear down the dialog
   (see Section 7).  Provisional responses moreover are not reliably
   delivered without using 100rel and PRACK, and provisional responses
   may be consumed (without forwarding) by intermediaries under a
   variety of conditions.  In short, their delivery is not guaranteed.

5.  Connected Identity with Diversion

   Many of the use cases that motivate connected identity are not sunny-
   day cases, but instead involve retargeting: when a call acquires a
   new target (in its Request-URI) during transit, then the destination
   will no longer correspond to the target, the "dest" specified by the
   PASSporT in the dialog-forming request.  If a PASSporT in a response
   came signed by a different destination than the caller intended, why
   should the caller trust it?

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   In STIR, the "div" PASSporT type [RFC8946] was created to securely
   record when a call was retargeted from one destination to another.
   Those "div" PASSporTs can be consumed on the terminating side by
   verification services to determine that a call has reached its
   eventual destination for the right reasons.  As [RFC8946] explains
   the situation, the only way those diversion PASSporTs will be seen by
   the calling party is if redirection is used (SIP 3XX responses)
   instead of retargeting; because some network policies aim to conceal
   service logic from the originating party, sending redirections in the
   backwards direction is the only current defined way for secure
   indications of redirection to be revealed to the calling party.  That
   in turn would allow the calling user agent to have a strong assurance
   that legitimate entities in the call path caused the request to reach
   a party that the caller did not anticipate.

   This specification introduces another alternative.  When sending a
   "rsp" PASSporT type in a SIP response, a UAS MAY also include (in
   Identity header field values) any "div" PASSporTs it received in the
   INVITE that initiated this dialog.  Thus, PASSporTs of type "div" MAY
   also appear in SIP responses.  These "div" PASSporTs can enable the
   originating side to receive a secure assurance that the call is being
   fielded by the proper recipient per the routing of the call.  In this
   case, the "dest" signed in the "rsp" PASSporT will be the address-of-
   record of the party who was reached, rather than the "dest" of the
   PASSporT received in the dialog-initiating INVITE.

   An "rsp" PASSporT that signs a different "dest" than the one that
   appeared in the PASSporT of the dialog-forming request MUST send at
   least one "div" PASSporT with it.  If no "div" PASSporTs were
   received in the dialog-forming request, then "rsp" PASSporTs MUST NOT
   be used in responses. "div" is not universally supported, so calls
   may be retargeted without generating a "div" PASSporT, in which case
   the use of "rsp" PASSporTs will not be possible.

   Note that sending "div" PASSporTs in the backwards direction will
   potentially reveal service logic to the called party.  As presumably
   this service logic is enacted on behalf of the called party, the
   called party can make a policy determination about reflecting those
   "div" PASSporTs back to the caller: connected identity may not be
   compatible with some operator policies.

   This mechanism does not require altering the value of the From header
   field value in requests or responses in the backwards direction.
   While this was a major concern of [RFC4916], in many operating
   environments, the From header field value does not even contain the
   identity of the caller that has been asserted by the network, which
   is instead conveyed by the P-Asserted-Identity header field
   [RFC3325].  The contents of PAID were never used for dialog matching,

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   and so in environments where PAID is used, it can be altered more
   dynamically than the From (moreover, [RFC3261], by introducing tag
   parameters to the To and From header field values, eliminated the
   need for stability in From values for dialog identification some time
   ago).  For retargeting that utilizes the [RFC4916] "from-change"
   option tag, see Section 10.  STIR is in general more flexible in
   constructing the "dest" than the Identity header managed addresses-
   of-record at the time [RFC4916] was written.

5.1.  Mid-dialog Requests

   Implementations compliant with this specification MUST validate the
   "div" chain back to the "rsp" PASSporT on any Identity header field
   values received in responses.  The dialog initiator can then treat
   the certificate that signed that "rsp" PASSporT as the appropriate
   certificate to sign any further mid-dialog or dialog-terminating
   requests received in the backwards direction.  Furthermore, the
   "dest" element value in any requests or responses sent in the
   backwards direction during this dialog MUST be the same as the "dest"
   element value in the first response to the dialog-forming request
   that contains a PASSporT - unless the "from-change" extension is
   used, per Section 10.

6.  Connected Identity in Mid-Dialog and Dialog-Terminating Requests

   The use of the connected identity mechanism here specified is not
   limited to provisional dialog requests.  Once a dialog has been
   established with connected identity, any re-INVITEs from either the
   originating and terminating side, as well as any BYE requests, MUST
   contain Identity headers with valid PASSporTs.  This prevents third-
   parties from spoofing any mid-dialog requests in order to redirect
   media or similarly interfere with communications, as well as
   preventing denial of service teardowns by attackers.

   Theoretically, any SIP requests in a dialog could be signed in this
   fashion, though it is unclear how valuable it would be for some (e.g.
   OPTIONS).  Requests with specialized payloads such as INFO or
   MESSAGE, however, would require additional specification for how
   integrity protection for their bodies could be implemented.  Some
   work has been done toward that for MESSAGE (see
   [I-D.ietf-stir-messaging].  This specification thus does not mandate
   PASSporTs for any requests sent in a dialog other than INVITE,
   UPDATE, and BYE.

   It might seem tempting to require that, if an INVITE has been sent
   with an Identity header containing a PASSporT, any CANCEL request
   received for the dialog initiated by that INVITE must also contain an
   Identity header with a PASSporT.  However, CANCEL requests can also

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   sent be sent by stateful proxy servers engaged in parallel forking;
   for example, when branches need to be canceled because a final
   response has been received from a UAS.  It is however REQUIRED by
   this specification that if a UAC sends a CANCEL for its own PASSporT-
   protected INVITE request, that it include an Identity header with a
   valid PASSporT in the CANCEL.  UAS policy will have to determine the
   instances where it will accept unsigned CANCEL requests for a dialog
   initiated with a signed INVITE.

7.  Authorization Policy for Callers

   In a traditional telephone call, the called party receives an
   alerting signal and can make a decision about whether or not to pick
   up a phone.  They may have access to displayed information, like
   "Caller ID", to help them arrive at an authorization decision.  The
   situation is more complicated for callers, however: callers typically
   expect to be connected to the proper destination and are often
   holding telephones in a position that would not enable them to see
   displayed information, if any were available for them to review --
   and moreover, their most direct response to a security breach would
   be to hang up the call they were in the middle of placing.

   While this specification does not prescribe any user experience
   associated with placing a call, it assumes that callers might have
   some way to a set an authorization posture that will result in the
   right thing happening when the connected identity is not expected.
   This is analogous to a situation where SRTP negotiation fails because
   the keys exchanges at the media layer do not match fingerprints
   exchanged at the signaling layer: when a user requests
   confidentiality services, and they are unavailable, media should not
   be exchanged.  Thus we assume that users have a way in their
   interface to require this criticality, on a per-call basis, or
   perhaps on a per-destination basis.  Users will not always place
   calls where the connected identity is crucial, but when they do, they
   should have a way to tell their devices that the call should not be
   completed if it arrives at an unexpected or insecure party.

8.  Creating Pre-Association with Destinations

   Any connected identity mechanism will work best if the user knows
   before initiating a call that connected identity is supported by the
   destination side.  Not every institution that a user wants to connect
   to securely will support STIR and connected identity out of the gate.
   Some sort of directory service might exist advertising support for
   connected identity which institutions could use to inform potential
   callers that, if connected identity is supported when reaching them
   with SIP, there is a potential security problem.  Similarly, user
   devices might keep some sort of log recording that a destination

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   previously supported connected identity, so that if support is
   unavailable later, calling users could be alerted to a potential
   security problem.

8.1.  Media-less Dialogs for Connected Identity

   The user interface of modern smartphones support an address book from
   which users select telephone numbers to dial.  Even when dialing a
   number manually, the interface frequently checks the address book,
   which will display to users any provisioned name for the target of
   the call if one exists.  Similarly, when clicking on a telephone
   number viewed on a web page, or similar service, smartphones often
   prompt users approve the access to the outbound dialer.  These sorts
   of decision points, when the user is still interacting with the user
   interface before a call is placed, provide an opportunity to probe
   what identity would be reached as a destination, and potentially even
   to exchange STIR PASSporTs in order to validate whether or not the
   expected destination can be reached securely.  Again, this is
   probably most meaningful for contacting financial, government, or
   emergency services, for cases where reaching an unintended
   destination may have serious consequences.

   The establishment of media-less dialogs has long been specified as a
   component of third-party call control in SIP [RFC3375], in which an
   INVITE is sent with no SDP.  Similar media-less dialogs have been
   proposed for certain automata per [RFC5552].  In the STIR context, a
   media-less dialog is established by sending an INVITE with an
   Identity header but no SDP.  STIR-aware UAS's that support this
   specification, upon receiving an INVITE with no SDP, carrying a
   PASSporT, with a 100rel in the Require header field value, SHOULD
   follow the mechanism described in Section 4 to send a provisional
   response carrying a PASSporT in the backwards direction.  The
   PASSporT received in the backwards direction could be rendered to the
   originating user to help them decide if they want to place the call.

9.  The 'rsp' PASSporT Type

   This specification defines a "rsp" PASSporT type that is sent only in
   SIP responses; it MUST NOT be sent in SIP requests.

   The header of a "rsp" PASSporT shows a "ppt" of "rsp":

   { "typ":"passport",
     "x5u":"" }

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   The payload of an "rsp" PASSporT looks entirely like a normal
   PASSporT - the only difference is in semantics, as the certificate
   signs for the "dest" header field rather than the "orig".

      { "orig":{"tn":"12155551212"},
        "iat":1443208345 }

   No restrictions are placed here on additional elements appearing in
   the payload of an "rsp" type PASSporT.

10.  UPDATE Procedures for Provisional Dialogs

   [RFC4916] identified a means of sending Identity header field values
   in the backwards direction before a final response to a dialog has
   been received by the UAC.  It relied on negotiating support for
   "from-change" options tags on both sides, followed by the use of the
   UPDATE method to send the connected identity in the backwards
   direction.  This can only happen after the UAS has received and
   responded to a PRACK [RFC3262] from the UAC, which would in turn have
   been triggered by a provisional 1xx response sent earlier by the UAC.

   However, the complexity of this mechanism makes it impractical to
   deploy for both the "sunny day" use case and the diversion use case
   described above.  It may still have utility for corner cases with
   legacy versions of SIP (dating before To and From header field value
   tags) or more complex call parking scenarios.  As such, this
   specification does not deprecate [RFC4916] "from-change" behavior,
   but nor does it provide an update for it for STIR -- that is left for
   future work.

11.  Updates to RFC4916

   [TBD - ways that UPDATEs in the backwards direction can carry
   additional information in support of the above]

   Added Identity in SIP responses, and "rsp" PASSporT type

   In general, the guidance of RFC4916 remains valid for RFC8224.

   The deprecation of the Identity-Info header has a number of
   implications for RFC4916; all of the protocol examples need to be
   updated to reflect that.

12.  Acknowledgments

   We would like to thank YOU for your contributions to this

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13.  IANA Considerations

   This specification defines a new PASSporT type for the PASSport
   Extensions Registry defined in [RFC8225], which resides at

   "rsp" as defined in [RFCThis] Section 9

14.  Security Considerations

   The security considerations of [RFC8224] and [RFC8225] apply to the
   use of the "rsp" PASSporT.  In general, a PASSporT of type "rsp" has
   similar security properties to a [RFC8946] diversion ("div")
   PASSporT.  Relying parties leverage a "rsp" PASSporT to determine the
   recipient of a request, and as with "div," the "dest" element of an
   "rsp" PASSporT is signed, rather than the "orig" element.

   The major threat that "rsp" addresses is the impersonation of a SIP
   response or mid-dialog/dialog-terminating request.  For the latter,
   this might include forging a BYE for a denial-of-service attack, or
   for example forging a re-INVITE that negotiates media channels
   controlled by an attacker.  For the former, some form of route
   hijacking or similar attack can be mounted by forging a dialog-
   forming response that appears to the caller to initiate a dialog with
   the intended destination.  The "rsp" mechanism uses PASSporTs to
   provide a non-repudiable assurance of the signer of such responses
   and requests.

   The value of a "rsp" PASSporT to relying parties, as with all
   PASSporTs, depends on the relying party trusting the certificate that
   signs the PASSporT, and having a reasonable assurance that the
   certificate in question is eligible to sign responses/requests for
   the number in the "dest" field of the "rsp" PASSporT.  For STIR
   certificates that use Service Provider Codes (SPCs), effectively the
   relying party knows the network operator who is vouching for that
   "rsp."  This in turn enables traceback and similar mitigations.

   As was mentioned in Section 5, the use of "div" along with "rsp" in
   responses may reveal the service logic of diversions to calling
   parties; however, since the called party ultimately invokes the "rsp"
   mechanism, any necessary policy controls can prevent the sending of
   "rsp" when that service logic must be protected.

15.  References

15.1.  Informative 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,

   [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,

   [RFC3262]  Rosenberg, J. and H. Schulzrinne, "Reliability of
              Provisional Responses in Session Initiation Protocol
              (SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002,

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

   [RFC3325]  Jennings, C., Peterson, J., and M. Watson, "Private
              Extensions to the Session Initiation Protocol (SIP) for
              Asserted Identity within Trusted Networks", RFC 3325,
              DOI 10.17487/RFC3325, November 2002,

   [RFC3375]  Hollenbeck, S., "Generic Registry-Registrar Protocol
              Requirements", RFC 3375, DOI 10.17487/RFC3375, September
              2002, <>.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474,
              DOI 10.17487/RFC4474, August 2006,

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

   [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, <>.

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   [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,

   [RFC8862]  Peterson, J., Barnes, R., and R. Housley, "Best Practices
              for Securing RTP Media Signaled with SIP", BCP 228,
              RFC 8862, DOI 10.17487/RFC8862, January 2021,

   [RFC8946]  Peterson, J., "Personal Assertion Token (PASSporT)
              Extension for Diverted Calls", RFC 8946,
              DOI 10.17487/RFC8946, February 2021,

15.2.  Informative References

              Peterson, J. and C. Wendt, "Messaging Use Cases and
              Extensions for STIR", Work in Progress, Internet-Draft,
              draft-ietf-stir-messaging-07, 12 March 2023,

   [RFC5552]  Burke, D. and M. Scott, "SIP Interface to VoiceXML Media
              Services", RFC 5552, DOI 10.17487/RFC5552, May 2009,

   [RFC6116]  Bradner, S., Conroy, L., and K. Fujiwara, "The E.164 to
              Uniform Resource Identifiers (URI) Dynamic Delegation
              Discovery System (DDDS) Application (ENUM)", RFC 6116,
              DOI 10.17487/RFC6116, March 2011,

Authors' Addresses

   Jon Peterson
   Neustar, Inc.

   Chris Wendt

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