wdiff rfc9970.original rfc9970.txt

Network Working Group

Internet Engineering Task Force (IETF) J. Peterson
Internet-Draft
Request for Comments: 9970 TransUnion
Intended status:
Category: Standards Track C. Wendt
Expires: 8 January 2026
ISSN: 2070-1721 Somos
7 July 2025
May 2026

Connected Identity for STIR
draft-ietf-stir-rfc4916-update-07 Secure Telephone Identity Revisited (STIR)

Abstract

The Session Initiation Protocol (SIP) Identity header field conveys
cryptographic identity information about the originators of SIP
requests. The However, the Secure Telephone Identity Revisited (STIR) framework,
however,
framework 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 STIR. It
also 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 dialog-terminating events by intermediaries or third
parties.

Status of This Memo

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Internet-Drafts are draft documents valid the IETF community. It has
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Internet Standards is available in Section 2 of six months RFC 7841.

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This Internet-Draft will expire on 8 January 2026.
https://www.rfc-editor.org/info/rfc9970.

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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 . . . . . . . . . . . . . . 7
6. Connected Identity in Mid-Dialog and Dialog-Terminating
Requests . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Authorization Policy for Callers . . . . . . . . . . . . . . 9
8. Creating Pre-Association with Destinations . . . . . . . . . 10
9. The 'rsp' "rsp" PASSporT Type . . . . . . . . . . . . . . . . . . . 11
10. UPDATE Procedures for Provisional Dialogs . . . . . . . . . . 11
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
12. Privacy Considerations . . . . . . . . . . . . . . . . . . . 12
13. Security Considerations . . . . . . . . . . . . . . . . . . . 12
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
14.1. Normative References . . . . . . . . . . . . . . . . . . 13
14.2. Informative References . . . . . . . . . . . . . . . . . 15
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15

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 SIP, 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 automated
systems 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 [RFC7375]).

There also exists a

A related problem: the problem also exists: 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. In certain network
environments, however, 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, intended and
whether the party ultimately reached by the call is one that the
caller should trust. The Moreover, 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 the identity of the called party to the
calling party is called "connected identity". Previous work on
connected identity focused on fixing the core semantics of SIP.
[RFC4916] allowed allows 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 also allows that UPDATE
to alter the From header field value for requests in the backwards direction:
previously [RFC3261] required
direction; this is an update to the behavior described in [RFC3261],
which requires that the From header field values sent in requests in
the backwards direction reflect the To header field value of the
dialog-forming request. Under the original [RFC3261]
rules, rules in [RFC3261], 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. field [RFC4474].
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 properly signed without the ability to change
the mid-dialog From header field value: Carol, say, value. Thus, Carol 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
mid-dialog requests in the backwards direction were permitted to vary
from the original To header field value.

With the obsolescence of [RFC4474] by [RFC8224], this specification
supersedes the guidance of [RFC4916] to reflect the changes to the
SIP Identity header field 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 [RFC4916] 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. The scope of this mechanism is solely two-party
communications; multiparty sharing of connected identity is left for
future work.

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.

This document assumes familiarity with common messages, response
codes, and header fields used in SIP
[RFC3261], [RFC3261] and the elements
present in the PASSporT Personal Assertion Token (PASSporT) [RFC8225] token format.

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 impersonation,
i.e., 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 Public
Switched Telephone Network (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 Calls to telephone numbers might be
dumped on a PSTN gateway, they might be sent to a default
intermediary that makes forwarding decisions based on a local
configuration file, potentially using various mechanisms such as
consulting a private ENUM [RFC6116], 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.

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 to one side as if a
call has terminated, while maintaining that the call is still active
to the other side, 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, in the
same way as dialog-forming requests, then another impersonation
vector leading to fraud in the telephone network could be shut down.

Finally, telephone numbers are widely used today for two-factor
authentication (TFA) prior to accessing web resources, which
typically rely on sharing some sort of one-time password or similar
unique link to validate control of a telephone number. These systems
are often capable of using either telephone calls or messages for
TFA. Connected identity is very valuable for these use cases because
it gives a strong assurance to the calling party that they have in
fact reached the telephone for the called telephone number.

There

However, there are however practical limits to what securing the signaling
can achieve. [RFC4916] rightly observed observes 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 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 straightforward call setup, the address-of-record (AoR) 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 field 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 AoR of the called party.

This specification therefore adds provisional and final SIP
responses, including the 100, 180, 183, and 200 responses, to the set
of messages that may contain an Identity header field. PASSporTs
that appear in SIP responses SHOULD use a "ppt" of "rsp", which is
defined in Section 9 (although "div" [RFC8946] may additionally
appear in responses, per Section 5). PASSporTs of the "rsp" type
will be referred to throughout this specification as "rsp" PASSporTs.
At a high level, an "rsp" PASSporT is signed similarly to the "div"
[RFC8946] PASSporT, in so far
PASSporT [RFC8946], insofar as the certificate that signs a "rsp"
PASSporT is signing the "dest" field, 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.

While it might seem attractive to provide identity for SIP failure
response codes (4XX, 5XX, 6XX), those explicitly do not form dialogs
or connections, connections and are thus outside the scope of this specification.
The same applies to SIP redirect (3XX) response codes, though see
[RFC8946],
Section 7 of [RFC8946] for guidance on authentication redirection.

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 is described in [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 basic
use cases, a PASSporT in a 183 or 200 OK should be sufficient to
secure media keys for the purposes of SIPBRANDY [RFC8862].

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). Moreover, provisional responses are not reliably
delivered without using 100rel and PRACK Provisional Response
Acknowledgement (PRACK) [RFC3262], 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

Use cases involving authorized retargeting motivate connected
identity: when
identity. 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?

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 currently 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 User Agent Server (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 MUST be the address-of-record of the party who was
reached,
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 a dialog-forming request with a different "dest" value
than the original PASSporT claimed, 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 the decision
to trust any "div" or "rsp" PASSporT is, as always in STIR, a matter
of local policy of the relying parties: some Some stricter systems may not
want to trust any "rsp" that differs from the "dest" in the PASSporT
of the original request.

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: 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 (PAID) header field
[RFC3325]. The contents of PAID were never used for dialog matching,
and so in environments where PAID is used, it can be altered more
dynamically than the From header field (moreover, [RFC3261], by introducing tag
parameters to the To and From header field values, eliminated [RFC3261]
eliminates the need for stability in From values for dialog
identification some time ago). For retargeting that utilizes the [RFC4916]
"from-change" option tag, tag in [RFC4916], see Section 10. STIR is, in
general, more flexible in constructing the "dest" than the Identity
header field managed addresses-of-record at the time [RFC4916] was
written.

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

The use of the connected identity mechanism here specified in this
document 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, SHOULD contain Identity header fields with valid PASSporTs.
If only the terminating side supports connected identity, obviously
the originator cannot be expected to know that it needs to send
PASSporTs for subsequent requests like BYE. Doing so 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 and it also
prevents 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 [RFC9475]). This
specification thus does not recommend 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 field containing a PASSporT, any CANCEL
request received for the dialog initiated by that INVITE must also
contain an Identity header field with a PASSporT. However, CANCEL
requests can also be sent by stateful proxy servers engaged in
parallel forking; forking, for example, when branches need to be canceled
because a final response has been received from a UAS. This
specification does not forbid a User Agent Client (UAC) from sending
a CANCEL for its own PASSporT-protected INVITE requests, as there may
be limited use cases where it would be useful to relying parties, but
recipients of a CANCEL should not expect PASSporTs to be present in
connected identity cases.

Mid-dialog requests also require special handling in diversion cases.
Relying parties who intended to trust an "rsp" PASSporT MUST validate
any "div" chain back to the "rsp" PASSporT on any Identity header
field values received in responses (per [RFC8946]). 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.

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
However, the situation is more complicated for callers, however: callers because they
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 --
moreover, review.
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 as expected.
This is analogous to a situation where Secure Real-time Transport
Protocol (SRTP) negotiation fails because the keys exchanged at the
media layer do not match the fingerprints exchanged at the signaling
layer:
when When a user requests confidentiality services, services and they are
unavailable, media should not be exchanged. Thus Thus, we assume that
users have a way in their interface to require this criticality, on a
per-call basis, 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
unauthenticated 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 that advertises support
for connected identity, which institutions then could use to inform
potential callers that, if connected identity is not 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 previously supported connected identity, so that if
support is unavailable later, calling users could be alerted to a
potential security problem.

The user interface of modern smartphones support supports 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, page or similar service, smartphones often
prompt users to 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, 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 [RFC3725], in which an
INVITE is sent with no SDP. Similar media-less dialogs have been
proposed for certain automated systems per [RFC5552]. In the STIR
context, a media-less dialog is established by sending an INVITE with
an Identity header field but no SDP. STIR-aware UASes 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' "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. Any "rsp"
PASSporTs received in requests MUST be ignored.

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

{ "typ":"passport",
"ppt":"rsp",
"alg":"ES256",
"x5u":"https://www.example.com/cert.cer"
"x5u":"https://www.example.com/cert.pem" }

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

{ "orig":{"tn":"12155551212"},
"dest":{"tn":["12155551214"]},
"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 identifies 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 relies 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 primary use case and the diversion use case
described above. It may still have utility for corner cases with
legacy versions of SIP (that date before the addition of the To and
From header field value tags) or more complex call parking scenarios.
As such, this specification does not deprecate [RFC4916] "from-
change" behavior, the "from-change"
behavior in [RFC4916], nor does it provide an update for it for STIR
-- that is left for future work.

11. IANA Considerations

This specification defines a new PASSporT type for the "Personal
Assertion Token (PASSporT) Extensions" registry defined in [RFC8225], which
resides at https://www.iana.org/assignments/passport/: <https://www.iana.org/assignments/passport/>:

ppt value "rsp"

Reference [RFCThis], value: rsp

Reference: RFC 9970, Section 9

12. Privacy Considerations

Note that sending connected identity can reveal information about the
called party. If a called party does not wish to be identified, it
is especially important not to share rich call data (RCD) in the
backwards direction, particular in business-to-consumer calling
cases. From a user experience perspective, this would likely work
similarly to current systems for sharing numbers, names, and even
pictures from calling parties to called parties -- users have
considerable control over that experience, and similarly for
connected identity, this must be an opt-in choice for users. In
general, RCD is more commonly used by enterprises than by individual
users.

13. 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. PASSporT
[RFC8946]. Relying parties leverage a "rsp" PASSporT to determine
the recipient of a request, and as with "div," "div", the "dest" element of
an "rsp" PASSporT is signed, 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, 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, PASSporT and having a reasonable assurance that the
certificate in question is eligible to sign responses/requests for
the number in the "dest" field claim 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.

The use of PASSporTs within responses creates a novel potential
vector for amplification attacks, as many responses may be sent in
response to a single SIP request, and the presence of a PASSporT
meaningfully increases the size of SIP responses. However, given
that PASSporTs can only be present in responses to requests carrying
a PASSporT, and thus requests with strong sender authentication,
called parties have adequate means to authorize the source of
requests and disregard spoofs intended to trigger amplification
attacks.

14. References

14.1. Normative References

[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>.

[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,
<https://www.rfc-editor.org/info/rfc3261>.

[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002,
<https://www.rfc-editor.org/info/rfc3262>.

[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>.

[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,
<https://www.rfc-editor.org/info/rfc3325>.

[RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
Camarillo, "Best Current Practices for Third Party Call
Control (3pcc) in the Session Initiation Protocol (SIP)",
BCP 85, RFC 3725, DOI 10.17487/RFC3725, April 2004,
<https://www.rfc-editor.org/info/rfc3725>.

[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>.

[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>.

[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,
<https://www.rfc-editor.org/info/rfc8862>.

[RFC8946] Peterson, J., "Personal Assertion Token (PASSporT)
Extension for Diverted Calls", RFC 8946,
DOI 10.17487/RFC8946, February 2021,
<https://www.rfc-editor.org/info/rfc8946>.

14.2. Informative References

[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,
<https://www.rfc-editor.org/info/rfc4474>.

[RFC5552] Burke, D. and M. Scott, "SIP Interface to VoiceXML Media
Services", RFC 5552, DOI 10.17487/RFC5552, May 2009,
<https://www.rfc-editor.org/info/rfc5552>.

[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,
<https://www.rfc-editor.org/info/rfc6116>.

[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>.

[RFC7375] Peterson, J., "Secure Telephone Identity Threat Model",
RFC 7375, DOI 10.17487/RFC7375, October 2014,
<https://www.rfc-editor.org/info/rfc7375>.

[RFC9475] Peterson, J. and C. Wendt, "Messaging Use Cases and
Extensions for Secure Telephone Identity Revisited
(STIR)", RFC 9475, DOI 10.17487/RFC9475, December 2023,
<https://www.rfc-editor.org/info/rfc9475>.

Acknowledgments

We would like to thank Russ Housley, Jonathan Rosenberg, and Orie
Steele for their contributions to this specification.

Authors' Addresses

Jon Peterson
TransUnion
Email: jon.peterson@transunion.com

Chris Wendt
Somos
Email: chris-ietf@chriswendt.net chris@appliedbits.com