SIPCORE E. Burger
Internet-Draft Georgetown University
Intended status: Standards Track B. Nagda
Expires: October 9, 2019 Massachusetts Institute of Technology
April 7, 2019
A Session Initiation Protocol (SIP) Response Code for Rejected Calls
draft-ietf-sipcore-rejected-06
Abstract
This document defines the 608 (Rejected) SIP response code. This
response code enables calling parties to learn that an intermediary
rejected their call attempt. The call will not be answered. As a
6xx code, the caller will be aware that future attempts to contact
the same UAS will likely fail. The present use case driving the need
for the 608 response code is when the intermediary is an analytics
engine. In this case, the rejection is by a machine or other
process. This contrasts with the 607 (Unwanted) SIP response code,
which a human at the target UAS indicated the call was not wanted.
In some jurisdictions this distinction is important. This document
also defines the use of the Call-Info header field in 608 responses
to enable rejected callers to contact entities that blocked their
calls in error. This provides a remediation mechanism for legal
callers that find their calls blocked.
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 https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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 October 9, 2019.
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Copyright Notice
Copyright (c) 2019 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
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 7
3.1. Intermediary Operation . . . . . . . . . . . . . . . . . 8
3.2. jCard Construction . . . . . . . . . . . . . . . . . . . 8
3.2.1. JOSE Header . . . . . . . . . . . . . . . . . . . . . 8
3.2.2. JWT Payload . . . . . . . . . . . . . . . . . . . . . 8
3.2.3. JWS Signature . . . . . . . . . . . . . . . . . . . . 9
3.3. UAC Operation . . . . . . . . . . . . . . . . . . . . . . 9
3.4. Legacy Interoperation . . . . . . . . . . . . . . . . . . 9
3.5. Announcement Requirements . . . . . . . . . . . . . . . . 10
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Full Exchange . . . . . . . . . . . . . . . . . . . . . . 11
4.2. Web Site jCard . . . . . . . . . . . . . . . . . . . . . 14
4.3. Multi-modal jCard . . . . . . . . . . . . . . . . . . . . 15
4.4. Legacy Interoperability . . . . . . . . . . . . . . . . . 15
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
5.1. SIP Response Code . . . . . . . . . . . . . . . . . . . . 17
5.2. SIP Feature-Capability Indicator . . . . . . . . . . . . 17
5.3. JSON Web Token Claim . . . . . . . . . . . . . . . . . . 17
5.4. Call-Info Purpose . . . . . . . . . . . . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.1. Normative References . . . . . . . . . . . . . . . . . . 20
8.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
The IETF has been addressing numerous issues surrounding how to
handle unwanted and, depending on the jurisdiction, illegal calls
[RFC5039]. Technologies such as STIR [RFC7340] and SHAKEN [SHAKEN]
address the cryptographic signing and attestation, respectively, of
signaling to ensure the integrity and authenticity of the asserted
caller identity.
This document describes a new SIP response code, 608, which allows
calling parties to learn that an intermediary rejected their call.
As described below, we need a distinct indicator to differentiate
between a user rejection and an intermediary's rejection of a call.
In some jurisdictions, calls, even if unwanted by the user, may not
be blocked unless there is an explicit user request. Moreover, users
may misidentify the nature of a caller.
For example, a legitimate caller may call a user who finds the call
to be unwanted. However, instead of marking the call as unwanted,
the user may mark the call as illegal. With that information, an
analytics engine may determine that all calls from that source should
be blocked. However, in some jurisdictions blocking calls from that
source for other users may not be legal. Likewise, one can envision
jurisdictions that allow an operator to block such calls, but only if
there is a remediation mechanism in place to address false positives.
Some call blocking services may return responses such as 604 (Does
Not Exist Anywhere). This might be a strategy to try to get a
destination's address removed from a calling database. However,
other network elements might also interpret this to mean the user
truly does not exist and might result in the user not being able to
receive calls from anyone, even if wanted. In many jurisdictions,
providing such false signaling is also illegal.
The 608 response code addresses this need of remediating falsely
blocked calls. Specifically, this code informs the SIP User Agent
Client (UAC) that an intermediary blocked the call and provides a
redress mechanism that allows callers to contact the operator of the
intermediary.
In the current call handling ecosystem, users can explicitly reject a
call or later mark a call as being unwanted by issuing a 607 SIP
response code (Unwanted) [RFC8197]. Figure 1 and Figure 2 show the
operation of the 607 SIP response code. The UAS indicates the call
was unwanted. As RFC8197 explains, not only does the called party
desire to reject that call, they can let their proxy know that they
consider future calls from that source unwanted. Upon receipt of the
607 response from the UAS, the proxy may send call information to a
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call analytics engine. For various reasons described in RFC8197, if
a network operator receives multiple reports of unwanted calls, that
may indicate that the entity placing the calls is likely to be a
source of unwanted calls for many people. As such, other customers
of the service provider may want the service provider to
automatically reject calls on their behalf.
Another value of the 607 rejection is presuming the proxy forwards
the response code to the UAC, the calling UAC or intervening proxies
will also learn the user is not interested in receiving calls from
that sender.
+-----------+
| Call |
| Analytics |
| Engine |
+-----------+
^ | (likely not SIP)
| v
+-----------+
+-----+ 607 | Called | 607 +-----+
| UAC | <--------- | Party | <-------- | UAS |
+-----+ | Proxy | +-----+
+-----------+
Figure 1: Unwanted (607) Call Flow
For calls rejected with a 607 from a legitimate caller, receiving a
607 response code can inform the caller to stop attempting to call
the user. Moreover, if a legitimate caller believes the user is
rejecting their calls in error, they can use other channels to
contact the user. For example, if a pharmacy calls a user to let
them know their prescription is available for pickup and the user
mistakenly thinks the call is unwanted and issues a 607 response
code, the pharmacy, having an existing relationship with the
customer, can send the user an email or push a note to the pharmacist
to ask the customer to consider not rejecting their calls in the
future.
Many systems that allow the user to mark the call unwanted (e.g.,
with the 607 response code) also allow the user to change their mind
and unmark such calls. This mechanism is relatively easy to
implement as the user usually has a direct relationship with the
service provider that is blocking calls.
However, things become more complicated if an intermediary, such as a
third-party provider of call management services that classifies
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calls based on the relative likelihood that the call is unwanted,
misidentifies the call as unwanted. Figure 3 shows this case. Note
that the UAS typically does not receive an INVITE since the called
party proxy rejects the call on behalf of the user. In this
situation, it would be beneficial for the caller to learn who
rejected the call, so they can correct the misidentification.
+--------+ +-----------+
| Called | | Call |
+-----+ | Party | | Analytics | +-----+
| UAC | | Proxy | | Engine | | UAS |
+-----+ +--------+ +-----------+ +-----+
| INVITE | | |
| --------------> | INVITE | |
| | ------------------------------> |
| | | |
| | | 607 |
| | <------------------------------ |
| | | |
| | Unwanted call | |
| 607 | -----------------> | |
| <-------------- | indicator | |
| | | |
Figure 2: Unwanted (607) Ladder Diagram
+-----------+
| Call |
| Analytics |
| Engine |
+-----------+
^ | (likely not SIP)
| v
+-----------+
+-----+ 608 | Called | +-----+
| UAC | <--------- | Party | | UAS |
+-----+ | Proxy | +-----+
+-----------+
Figure 3: Rejected (608) Call Flow
In this situation, one might be tempted to have the intermediary use
the 607 response code. 607 indicates to the caller the subscriber
does not want the call. However, RFC8197 specifies that one of the
uses of 607 is to inform analytics engines that a user (human) has
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rejected a call. The problem here is that network elements
downstream from the intermediary might interpret the 607 as coming
from a user (human) that has marked the call as unwanted, as opposed
to coming from an algorithm using statistics or machine learning to
reject the call. An algorithm can be vulnerable to the base rate
fallacy base rate fallacy [BaseRate] algorithm rejecting the call.
In other words, those downstream entities should not rely on another
entity 'deciding' the call is unwanted. By distinguishing between a
(human) user rejection and an intermediary engine's statistical
rejection, a downstream network element that sees a 607 response code
can weight it as a human rejection in its call analytics.
It is useful for blocked callers to have a redress mechanism. One
can imagine that some jurisdictions will require it. However, we
must be mindful that most of the calls that will be blocked will, in
fact, be illegal and eligible for blocking. Thus, providing
alternate contact information for a user would be counterproductive
to protecting that user from illegal communications. This is another
reason we do not propose to simply allow alternate contact
information in a 607 response message.
One might ask why we cannot use the same mechanism an analytics
service provider offers their customers that lets them correct a call
blocked in error? The reason is while there is an existing
relationship between the customer (called party) and the analytics
service provider, it is unlikely there is a relationship between the
caller and the analytics service provider. Moreover, there are
numerous call blocking providers in the ecosystem. As such, we need
a mechanism for indicating an intermediary rejected a call that also
provides contact information for the operator of that intermediary,
without exposing the target user's contact information.
The protocol described in this document uses existing IETF protocol
mechanisms for specifying the redress mechanism. In the Call-Info
header passed back to the UAC, we send additional information
specifying a redress address. We choose to encode the redress
address using jCard [RFC7095]. Conveniently, we use jCard rather
than vCard [RFC6350] as we have a standard marshaling mechanism for
creating a canonical representation of a JSON [RFC8259] object, such
as a jCard, and a standard presentation format for such an object,
namely JWS [RFC7515]. The SIP community is familiar with this
concept as it is the mechanism used by STIR [RFC8224].
The jCard encoding might seem unnecessary at first, but it is
essential to preventing potential network attacks. Suppose, for
example, that the redress address was simply passed as a header
value. One can imagine an adverse agent that maliciously spoofs a
608 response with the same redress address to a large number of
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active callers, who may then all send redress requests to the
specified address (the basis for a denial-of-service attack). The
process would occur as follows: (1) a malicious agent senses INVITE
requests from a variety UACs and (2) spoofs 608 responses with an
unsigned redress address before the intended receivers can respond,
causing (3) the UACs to all contact the redress address at once. The
jCard encoding allows the UAC to verify the blocking intermediary's
identity before contacting the redress address. This guards against
a malicious agent spoofing 608 responses, preventing the denial-of-
service attack. Thus, if the jCard address is unreachable or
undecipherable, either (1) a malicious agent is lying about the jCard
or (2) the redress mechanism is misconfigured.
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. Protocol Operation
For clarity, this section uses the term 'intermediary' as the entity
that acts as a SIP User Agent Server (UAS) on behalf of the user in
the network, as opposed to the user's UAS (colloquially, but not
necessarily, their phone). The intermediary could be a back-to-back
user agent (B2BUA) or a SIP Proxy.
Figure 4 shows an overview of the call flow for a rejected call.
+--------+ +-----------+
| Called | | Call |
+-----+ | Party | | Analytics | +-----+
| UAC | | Proxy | | Engine | | UAS |
+-----+ +--------+ +-----------+ +-----+
| INVITE | | |
| --------------> | Information from | |
| | -----------------> | |
| | INVITE | |
| | Reject | |
| 608 | <----------------- | |
| <-------------- | call | |
| | | |
Figure 4: Rejected (608) Ladder Diagram
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3.1. Intermediary Operation
An intermediary MAY issue the 608 response code in a failure response
for an INVITE, MESSAGE, SUBSCRIBE, or other out-of-dialog SIP
[RFC3261] request to indicate that an intermediary rejected the
offered communication as unwanted by the user. An intermediary MAY
issue the 608 as the value of the "cause" parameter of a SIP reason-
value in a Reason header field [RFC3326].
If an intermediary issues a 608 code and there are not indicators the
calling party will use the contents of the Call-Info header field for
malicious purposes (see Section 6), the intermediary MUST include a
Call-Info header field in the response.
If there is a Call-Info header field, it MUST have the 'purpose'
parameter of 'jwscard'. The value of the Call-Info header field MUST
refer to a valid JWS [RFC7515] encoding of a jCard [RFC7095] object.
Proxies need to be mindful that a downstream intermediary may reject
the attempt with a 608 while other paths may still be in progress.
In this situation, the requirements stated in Section 16.7 of RFC3261
[RFC3261] apply. Specifically, the proxy should cancel pending
transactions and must not create any new branches. Note this is not
a new requirement but simply pointing out the existing 6xx protocol
mechanism in SIP.
3.2. jCard Construction
The intermediary constructs the JWS as follows.
3.2.1. JOSE Header
The JOSE header MUST include the typ, alg, and x5u parameters from
JWS [RFC7515]. The typ parameter MUST have the value "vcard+json".
Implementations MUST support ES256 as JWA [RFC7518] defines it, and
MAY support other registered signature algorithms. Finally, the x5u
parameter MUST be a URI that resolves to the public key certificate
corresponding to the key used to digitally sign the JWS.
3.2.2. JWT Payload
The payload contains two JSON values. The first JWT claim that MUST
be present is the iat (issued at) claim [RFC7519]. The "iat" MUST be
set to the date and time of the issuance of the 608 response. This
mandatory component protects the response from replay attacks.
The second JWT claim that MUST be present is the jcard claim.
Section 5.3 describes the registration. In the construction of the
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jcard claim, the "jcard" MUST include at least one of the URL, EMAIL,
TEL, or ADR properties. UACs supporting this specification MUST be
prepared to receive a full jCard. Call originators (at the UAC) can
use the information returned by the jCard to contact the intermediary
that rejected the call to appeal the intermediary's blocking of the
call attempt. What the intermediary does if the blocked caller
contacts the intermediary is outside the scope of this document.
3.2.3. JWS Signature
JWS [RFC7515] specifies the procedure for calculating the signature
over the jCard JWT. Section 4 of this document has a detailed
example on constructing the JWS, including the signature.
3.3. UAC Operation
A UAC conforming to this specification MUST include the sip.608
feature capability indicator in the Feature-Caps header field of the
INVITE request.
Upon receiving a 608 response, UACs perform normal SIP processing for
6xx responses.
3.4. Legacy Interoperation
If the UAC indicates support for 608 and the intermediary issues a
608, life is good as the UAC will receive all the information it
needs to remediate an erroneous block by an intermediary. However,
what if the UAC does not understand 608? For example, how can we
support callers from a legacy, non-SIP public switched network
connecting to the SIP network via a media gateway?
We address this situation by having the first network element that
conforms with this specification play an announcement in the media.
See Section 3.5 for requirements on the announcement. The simple
rule is a network element that inserts the sip.608 feature capability
MUST be able to convey at a minimum how to contact the operator of
the intermediary that rejected the call attempt.
The degenerate case is the intermediary is the only element that
understands the semantics of the 608 response code. Obviously, any
SIP device will understand that a 608 response code is a 6xx error.
However, there are no other elements in the call path that understand
the meaning of the value of the Call-Info header field. The
intermediary knows this is the case as the INVITE request will not
have the sip.608 feature capability. In this case, one can consider
the intermediary to be the element 'inserting' a virtual sip.608
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feature capability. If the caveats described in Section 3.5 and
Section 6 do not hold, the intermediary MUST play the announcement.
Now we take the case where a network element that understands the 608
response code receives an INVITE for further processing. A network
element conforming with this specification MUST insert the sip.608
feature capability, per the behaviors described in Section 4.2 of
[RFC6809].
Do note that even if a network element plays an announcement
describing the contents of the 608 response message, the network
element MUST forward the 608 response code message as the final
response to the INVITE.
One aspect of using a feature capability is that only the network
elements that will either consume (UAC) or play an announcement
(media gateway, session border controller (SBC [RFC7092]), or proxy)
need to understand the sip.608 feature capability. The rest of the
infrastructure does not need to be modified, assuming that the other
network elements conform to Section 16.6 of [RFC3261], specifically
they will pass headers such as "Feature-Caps: *;+sip.608" unmodified.
3.5. Announcement Requirements
There are a few requirements on the element that handles the
announcement for legacy interoperation.
As noted above, the element that inserts the sip.608 feature
capability is responsible for conveying the information referenced by
the Call-Info header field in the 608 response message. However,
this specification does not mandate how to convey that information.
Let us take the case where a telecommunications service provider
controls the element inserting the sip.608 feature capability. It
would be reasonable to expect the service provider would play an
announcement in the media path towards the UAC (caller). It is
important to note the network element should be mindful of the media
type requested by the UAC as it formulates the announcement. For
example, it would make sense for an INVITE that only indicated audio
codecs in the SDP [RFC4566] to result in an audio announcement.
However, if the INVITE only indicated a real-time text codec and the
network element is able to render the information in the requested
media format, the network element MUST send the information in a text
format, not an audio format.
It is also possible for the network element inserting the sip.608
feature capability to be under the control of the same entity that
controls the UAC. For example, a large call center might have legacy
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UACs, but have a modern outbound calling proxy that understands the
full semantics of the 608 response code. In this case, it is enough
for the outbound calling proxy to digest the Call-Info information
and handle the information digitally, rather than 'transcoding' the
Call-Info information for presentation to the caller.
4. Examples
These examples are not normative, do not include all protocol
elements, and may have errors. Review the protocol documents for
actual syntax and semantics of the protocol elements.
4.1. Full Exchange
Given an INVITE (shamelessly taken from [SHAKEN]):
INVITE sip:+12155550113@tel.one.example.net SIP/2.0
Max-Forwards: 69
Contact: <sip:+12155550112@2001:db8::12:50207;rinstance=9da3088f36cc>
To: <sip:+12155550113@tel.one.example.net>
From: "Alice" <sip:+12155550112@tel.two.example.net>;tag=614bdb40
Call-ID: 79048YzkxNDA5NTI1MzA0OWFjOTFkMmFlODhiNTI2OWQ1ZTI
P-Asserted-Identity: "Alice"<sip:+12155550112@tel.two.example.net>,
<tel:+12155550112>
CSeq: 2 INVITE
Allow: SUBSCRIBE, NOTIFY, INVITE, ACK, CANCEL, BYE, REFER, INFO,
MESSAGE, OPTIONS
Content-Type: application/sdp
Date: Tue, 16 Aug 2016 19:23:38 GMT
Feature-Caps: *;+sip.608
Identity:
eyJhbGciOiJFUzI1NiIsInR5cCI6InBhc3Nwb3J0IiwicHB0Ijoic2hha2VuIiwieDV1I
joiaHR0cDovL2NlcnQtYXV0aC5wb2Muc3lzLmNvbWNhc3QubmV0L2V4YW1wbGUuY2VydC
J9eyJhdHRlc3QiOiJBIiwiZGVzdCI6eyJ0biI6IisxMjE1NTU1MTIxMyJ9LCJpYXQiOiI
xNDcxMzc1NDE4Iiwib3JpZyI6eyJ0biI64oCdKzEyMTU1NTUxMjEyIn0sIm9yaWdpZCI6
IjEyM2U0NTY3LWU4OWItMTJkMy1hNDU2LTQyNjY1NTQ0MDAwMCJ9._28kAwRWnheXyA6n
Y4MvmK5JKHZH9hSYkWI4g75mnq9Tj2lW4WPm0PlvudoGaj7wM5XujZUTb_3MA4modoDtC
A;info=<http://cert.example2.net/example.cert>;alg=ES256
Content-Length: 153
v=0
o=- 13103070023943130 1 IN IP6 2001:db8::177
c=IN IP6 2001:db8::177
t=0 0
m=audio 54242 RTP/AVP 0
a=sendrecv
An intermediary could reply:
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SIP/2.0 608 Rejected
Via: SIP/2.0/UDP 2001:db8::177:60012;branch=z9hG4bK-524287-1
From: "Alice" <sip:+12155550112@tel.two.example.net>;tag=614bdb40
To: <sip:+12155550113@tel.one.example.net>
Call-ID: 79048YzkxNDA5NTI1MzA0OWFjOTFkMmFlODhiNTI2OWQ1ZTI
CSeq: 2 INVITE
Call-Info: <https://block.example.net/complaint.json>;purpose=jwscard
The location https://block.example.net/complaint.json resolves to a
JWS. The JWS would be constructed as follows.
The JWS header of this example jCard could be:
{ {"alg":"ES256"},
{"typ":"vcard+json"},
{"x5u":"https://certs.example.net/reject_key.cer"} }
Now, let us construct a minimal jCard. For this example, the jCard
refers the caller to an email address, bitbucket@blocker.example.net:
["vcard",
[
["version", {}, "text", "4.0"],
["fn", {}, "text", "Robocall Adjudication"],
["email", {"type":"work"},
"text", "bitbucket@blocker.example.net"]
]
]
With this jCard, we can now construct the JWT:
{
"iat":1546008698,
"jcard":["vcard",
[
["version", {}, "text", "4.0"],
["fn", {}, "text", "Robocall Adjudication"],
["email", {"type":"work"},
"text", "bitbucket@blocker.example.net"]
]
]
}
In order to calculate the signature, we need to encode the JOSE
header and JWT into base64. As an implementation note, one can trim
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whitespace in the JSON objects to save a few bytes. UACs MUST be
prepared to receive pretty printed, compact, or bizarrely formatted
JSON. For the purposes of this example, we leave the objects with
pretty whitespace. Speaking of pretty vs. machine formatting, these
examples have line breaks in the base64 encodings for ease of
publication in the RFC format. The specification of base64 allows
for these line breaks and the decoded text works just fine. However,
those extra line break octets would affect the calculation of the
signature. As such, implementations MUST NOT insert line breaks into
the base64 encodings of the JOSE header or JWT. This also means UACs
MUST be prepared to receive arbitrarily long octet streams from the
URI referenced by the Call-Info SIP header.
base64 of JOSE header:
eyB7ImFsZyI6IkVTMjU2In0sCiAgeyJ0eXAiOiJ2Y2FyZCtqc29uIn0sCiAgeyJ4
NXUiOiJodHRwczovL2NlcnRzLmV4YW1wbGUubmV0L3JlamVjdF9rZXkuY2VyIn0g
fQo=
base64 of JWT:
ewogICJpYXQiOjE1NDYwMDg2OTgsCiAgImpjYXJkIjpbInZjYXJkIiwKICAgIFsK
ICAgICAgWyJ2ZXJzaW9uIiwge30sICJ0ZXh0IiwgIjQuMCJdLAogICAgICBbImZu
Iiwge30sICJ0ZXh0IiwgIlJvYm9jYWxsIEFkanVkaWNhdGlvbiJdLAogICAgICBb
ImVtYWlsIiwgeyJ0eXBlIjoid29yayJ9LCAKICAgICAgICAgICAgICAgICJ0ZXh0
IiwgImJpdGJ1Y2tldEBibG9ja2VyLmV4YW1wbGUubmV0Il0KICAgIF0KICBdCn0K
In this case, the object to be signed (remembering this is just a
single, long line; the line breaks are for ease of review but do not
appear in the actual text being signed is as follows:
eyB7ImFsZyI6IkVTMjU2In0sCiAgeyJ0eXAiOiJ2Y2FyZCtqc29uIn0sCiAgeyJ4
NXUiOiJodHRwczovL2NlcnRzLmV4YW1wbGUubmV0L3JlamVjdF9rZXkuY2VyIn0g
fQo=
.
ewogICJpYXQiOjE1NDYwMDg2OTgsCiAgImpjYXJkIjpbInZjYXJkIiwKICAgIFsK
ICAgICAgWyJ2ZXJzaW9uIiwge30sICJ0ZXh0IiwgIjQuMCJdLAogICAgICBbImZu
Iiwge30sICJ0ZXh0IiwgIlJvYm9jYWxsIEFkanVkaWNhdGlvbiJdLAogICAgICBb
ImVtYWlsIiwgeyJ0eXBlIjoid29yayJ9LCAKICAgICAgICAgICAgICAgICJ0ZXh0
IiwgImJpdGJ1Y2tldEBibG9ja2VyLmV4YW1wbGUubmV0Il0KICAgIF0KICBdCn0K
We use the following X.509 PKCS #8-encoded ECDSA private key, also
shamelessly taken from [SHAKEN]), as an example key for signing the
hash of the above text. Do NOT use this key in real life! It is for
example purposes only. At the very least, we would strongly
recommend the key being encrypted at rest.
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-----BEGIN PRIVATE KEY-----
MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgi7q2TZvN9VDFg8Vy
qCP06bETrR2v8MRvr89rn4i+UAahRANCAAQWfaj1HUETpoNCrOtp9KA8o0V79IuW
ARKt9C1cFPkyd3FBP4SeiNZxQhDrD0tdBHls3/wFe8++K2FrPyQF9vuh
-----END PRIVATE KEY-----
The resulting JWS, using the above key on the above object, renders
the following ECDSA P-256 SHA-256 digital signature.
MEUCIQCF2nv/eKvnGQNELZglQTpWbYtzbEf97xH4zKnkLx7S0QIgIl2f5ehMOwjM
TS+skjf1163ihH5+yIHQS3quklEt/9o=
Thus, the JWS stored at https://blocker.example.net/complaints.json,
would contain:
eyB7ImFsZyI6IkVTMjU2In0sCiAgeyJ0eXAiOiJ2Y2FyZCtqc29uIn0sCiAgeyJ4
NXUiOiJodHRwczovL2NlcnRzLmV4YW1wbGUubmV0L3JlamVjdF9rZXkuY2VyIn0g
fQo=.ewogICJpYXQiOjE1NDYwMDg2OTgsCiAgImpjYXJkIjpbInZjYXJkIiwKICA
gIFsKICAgICAgWyJ2ZXJzaW9uIiwge30sICJ0ZXh0IiwgIjQuMCJdLAogICAgICB
bImZuIiwge30sICJ0ZXh0IiwgIlJvYm9jYWxsIEFkanVkaWNhdGlvbiJdLAogICA
gICBbImVtYWlsIiwgeyJ0eXBlIjoid29yayJ9LCAKICAgICAgICAgICAgICAgICJ
0ZXh0IiwgImJpdGJ1Y2tldEBibG9ja2VyLmV4YW1wbGUubmV0Il0KICAgIF0KICB
dCn0K.MEUCIQCF2nv/eKvnGQNELZglQTpWbYtzbEf97xH4zKnkLx7S0QIgIl2f5e
hMOwjMTS+skjf1163ihH5+yIHQS3quklEt/9o=
4.2. Web Site jCard
For an intermediary that provides a Web site for adjudication, the
jCard could contain the following. Note the calculation of the JWS
is not shown; the URI reference in the Call-Info header field would
be to the JWS of the signed jCard.
["vcard",
[
["version", {}, "text", "4.0"],
["fn", {}, "text", "Robocall Adjudication"],
["url", {"type":"work"},
"text", "https://blocker.example.net/adjudication-form"]
]
]
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4.3. Multi-modal jCard
For an intermediary that provides a telephone number and a postal
address, the jCard could contain the following. Note the calculation
of the JWS is not shown; the URI reference in the Call-Info header
field would be to the JWS of the signed jCard.
["vcard",
[
["version", {}, "text", "4.0"],
["fn", {}, "text", "Robocall Adjudication"],
["adr", {"type":"work"}, "text",
["Argument Clinic",
"12 Main St","Anytown","AP","000000","Somecountry"]
]
["tel", {"type":"work"}, "uri", "tel:+1-555-555-0112"]
]
]
Note that it is up to the UAC to decide which jCard contact modality,
if any, it will use.
4.4. Legacy Interoperability
Figure 5 depicts a call flow illustrating legacy interoperability.
In this non-normative example, we see a UAC that does not support the
full semantics for 608. However, there is an SBC that does support
608. Per RFC6809 [RFC6809], the SBC can insert "*;+sip.608" into the
Feature-Caps header field for the INVITE. When the intermediary,
labeled "Called Party Proxy" in the figure, rejects the call, it
knows it can simply perform the processing described in this
document. Since the intermediary saw the sip.608 feature capability,
it knows it does not need to send any media describing whom to
contact in the event of an erroneous rejection.
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+---------+
| Call |
|Analytics|
| Engine |
+---------+
^ |
| v
+---------+
| Called | +-----+ +-----+ +---+ +-----+ +---+
| Party | <---|Proxy| <---|Proxy| <---|SBC| <---|Proxy| <---|UAC|
| Proxy | +-----+ +-----+ +---+ +-----+ +---+
+---------+ | |
| | INVITE |
| INVITE |<--------------------|
|<-----------------------------------| |
| Feature-Caps: *;+sip.608 | |
| | |
| 608 Rejected | |
|----------------------------------->| 183 |
| Call-Info: <...> |-------------------->|
| [path for Call-Info elided | SDP for media |
| for illustration purposes] | |
| |=== Announcement ===>|
| | |
| | 608 |
| |-------------------->|
| | Call-Info: <...> |
Figure 5: Legacy Operation
When the SBC receives the 608 response code, it correlates that with
the original INVITE from the UAC. The SBC remembers that it inserted
the sip.608 feature capability, which means it is responsible for
somehow alerting the UAC the call failed and whom to contact. At
this point the SBC can play a prompt, either natively or through a
mechanism such as NETANN [RFC4240], that sends the relevant
information in the appropriate media to the UAC.
As an example, the SBC could extract the FN and TEL jCard fields and
play something like a special information tone (see Telcordia SR-2275
[SR-2275] section 6.21.2.1 or ITU-T E.180 [ITU.E.180.1998] section
7), followed by "Your call has been rejected by ...", followed by a
text-to-speech translation of the FN text, followed by "You can reach
them on", followed by a text-to-speech translation of the telephone
number in the TEL field.
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Note the SBC also still sends the full 608 response code, including
the Call-Info header, towards the UAC.
5. IANA Considerations
5.1. SIP Response Code
This document defines a new SIP response code, 608 in the "Response
Codes" subregistry of the "Session Initiation Protocol (SIP)
Parameters" registry defined in [RFC3261].
Response code: 608
Description: Rejected
Reference: [RFCXXXX]
5.2. SIP Feature-Capability Indicator
This document defines the feature capability sip.608 in the "SIP
Feature-Capability Indicator Registration Tree" registry defined in
[RFC6809].
Name: sip.608
Description: This feature capability indicator, when included in a
Feature-Caps header field of an INVITE request, indicates that the
entity associated with the indicator will be responsible for
indicating to the caller any information contained in the 608 SIP
response code, specifically the value referenced by the Call-Info
header.
Reference: [RFCXXXX]
5.3. JSON Web Token Claim
This document defines the new JSON Web Token claim in the "JSON Web
Token Claims" sub-registry created by [RFC7519]. Section 3.2.2
defines the syntax. The required information is:
Claim Name: jcard
Claim Description: jCard data
Change Controller: IESG
Reference: [RFCXXXX], [RFC7095]
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5.4. Call-Info Purpose
This document defines the new predefined value "jwscard" for the
"purpose" header field parameter of the Call-Info header field. This
modifies the "Header Field Parameters and Parameter Values"
subregistry of the "Session Initiation Protocol (SIP) Parameters"
registry by adding this RFC as a reference to the line for the header
field "Call-Info" and parameter name "purpose":
Header Field: Call-Info
Parameter Name: purpose
Predefined Values: Yes
Reference: [RFCXXXX]
6. Security Considerations
Intermediary operators need to be mindful of whom they are sending
the 608 response. There is a risk that a truly malicious caller is
being rejected. This raises two issues. The first is the caller,
now alerted that the call is being automatically rejected, may change
their call behavior to defeat call blocking systems. The second, and
more significant risk, is that by providing a contact in the Call-
Info field, the intermediary may be giving the malicious caller a
vector for attack. In other words, the intermediary will be
publishing an address that a malicious actor may use to launch an
attack on the intermediary. Because of this, intermediary operators
may wish to configure their response to only include a Call-Info
field for INVITE or other initiating methods that are signed and pass
validation by STIR [RFC8224].
Another risk is for an attacker to flood a proxy that supports the
sip.608 feature with INVITE requests that lack the sip.608 feature
capability in order to direct the SDP to a victim's device. Because
the mechanism described here can result in an audio file being sent
to the target of the Contact header field, an attacker could use the
mechanism described by this document as an amplification attack,
given a SIP INVITE can be under 1 kilobyte and an audio file can be
hundreds of kilobytes. One remediation for this is for devices that
insert a sip.608 feature capability only transmit media to what is
highly likely to be the actual source of the call attempt. A method
for this is to only play media in response to an INVITE that is
signed and passed validation by STIR [RFC8224].
Yet another risk is a malicious entity or the intermediary itself can
generate a malicious 608 response with a jCard referring to a
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malicious agent. For example, the recipient of a 608 may receive a
TEL URI in the vCard. When the recipient calls that address, the
malicious agent could ask for personally identifying information.
However, instead of using that information to verify the recipient's
identity, they are phishing the information for nefarious ends. As
such, we strongly recommend the recipient validates to whom they are
communicating with if asking to adjudicate an erroneously rejected
call attempt. Since we may also be concerned about intermediate
nodes modifying contact information, we can address both of these
issues with a single solution. The remediation is to require the
intermediary to sign the jCard. Signing the jCard provides integrity
protection. In addition, one can imagine mechanisms such as used by
SHAKEN [SHAKEN] to use signing certificate issuance as a mechanism
for traceback to the entity issuing the jCard, for example tying the
identity of the subject of the certificate to the To field of the
initial SIP request, as if the intermediary was vouching for the From
field of a SIP request with that identity.
Since the decision of whether to include Call-Info in the 608
response is a matter of policy, one thing to consider is whether a
legitimate caller can ascertain whom to contact without such
information being included in the 608. For example, in some
jurisdictions, if the terminating service provider is the
intermediary, the caller can look up who the terminating service
provider is based on the routing information for the dialed number.
As such, the Call-Info jCard could be redundant information.
However, the factors going into a particular service provider's or
jurisdiction's choice of whether or not to include Call-Info is
outside the scope of this document.
7. Acknowledgements
This document liberally lifts from [RFC8197] in its text and
structure. However, the mechanism and purpose of 608 is quite
different than 607. Any errors are the current editor's and not the
editor of RFC8197. Thanks also go to Ken Carlberg of the FCC, Russ
Housley, Paul Kyzivat, and Tolga Asveren for their suggestions on
improving the draft. Tolga's suggestion to provide a mechanism for
legacy interoperability served to expand the draft by 50%. In
addition, Tolga came up with the jCard attack. Finally, Christer
Holmberg as always provided a close reading and fixed a SIP feature
capability bug found by Yehoshua Gev.
Finally, Bhavik Nagda provided clarifying edits as well and more
especially wrote and tested an implementation of the 608 response
code in Kamailio. Code is available at <https://github.com/
nagdab/608_Implementation> .
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8. References
8.1. 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>.
[RFC3326] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
Header Field for the Session Initiation Protocol (SIP)",
RFC 3326, DOI 10.17487/RFC3326, December 2002,
<https://www.rfc-editor.org/info/rfc3326>.
[RFC6809] Holmberg, C., Sedlacek, I., and H. Kaplan, "Mechanism to
Indicate Support of Features and Capabilities in the
Session Initiation Protocol (SIP)", RFC 6809,
DOI 10.17487/RFC6809, November 2012,
<https://www.rfc-editor.org/info/rfc6809>.
[RFC7095] Kewisch, P., "jCard: The JSON Format for vCard", RFC 7095,
DOI 10.17487/RFC7095, January 2014,
<https://www.rfc-editor.org/info/rfc7095>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/info/rfc7515>.
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
<https://www.rfc-editor.org/info/rfc7518>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[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>.
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8.2. Informative References
[BaseRate]
Bar-Hillel, M., "The Base-Rate Fallacy in Probability
Judgements", 4 1977, <
https://apps.dtic.mil/docs/citations/ADA045772>.
[ITU.E.180.1998]
International Telecommunications Union, "Technical
characteristics of tones for the telephone service",
ITU Recommendation E.180/Q.35, March 1998.
[RFC4240] Burger, E., Ed., Van Dyke, J., and A. Spitzer, "Basic
Network Media Services with SIP", RFC 4240,
DOI 10.17487/RFC4240, December 2005,
<https://www.rfc-editor.org/info/rfc4240>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <https://www.rfc-editor.org/info/rfc4566>.
[RFC5039] Rosenberg, J. and C. Jennings, "The Session Initiation
Protocol (SIP) and Spam", RFC 5039, DOI 10.17487/RFC5039,
January 2008, <https://www.rfc-editor.org/info/rfc5039>.
[RFC6350] Perreault, S., "vCard Format Specification", RFC 6350,
DOI 10.17487/RFC6350, August 2011,
<https://www.rfc-editor.org/info/rfc6350>.
[RFC7092] Kaplan, H. and V. Pascual, "A Taxonomy of Session
Initiation Protocol (SIP) Back-to-Back User Agents",
RFC 7092, DOI 10.17487/RFC7092, December 2013,
<https://www.rfc-editor.org/info/rfc7092>.
[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>.
[RFC8197] Schulzrinne, H., "A SIP Response Code for Unwanted Calls",
RFC 8197, DOI 10.17487/RFC8197, July 2017,
<https://www.rfc-editor.org/info/rfc8197>.
[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>.
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[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[SHAKEN] Alliance for Telecommunications Industry Solutions (ATIS)
and the SIP Forum, "Signature-based Handling of Asserted
information using toKENs (SHAKEN)", ATIS 1000074, 1 2017,
<https://www.sipforum.org/download/sip-forum-twg-10-
signature-based-handling-of-asserted-information-using-
tokens-shaken-pdf/?wpdmdl=2813>.
[SR-2275] Telcordia, "Bellcore Notes on the Networks", Telcordia SR-
2275, October 2000.
Authors' Addresses
Eric W. Burger
Georgetown University
37th & O St, NW
Washington, DC 20057
USA
Email: eburger@standardstrack.com
Bhavik Nagda
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139
USA
Email: nagdab@gmail.com
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