Internet DRAFT - draft-burger-sipcore-rejected
draft-burger-sipcore-rejected
SIPCORE E. Burger
Internet-Draft Georgetown University
Intended status: Standards Track July 23, 2018
Expires: January 24, 2019
A Session Initiation Protocol (SIP) Response Code for Rejected Calls
draft-burger-sipcore-rejected-01
Abstract
This document defines the 608 (Rejected) SIP response code. This
response code enables calling parties to learn their call was
rejected by an intermediary and will not be answered. As a 6xx code,
the caller will be aware that future attempts to contact the same UAS
will be likely to 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 and may have
additional requirements beyond the 607 response code. Specifically,
this document defines the use of the Call-Info header in 608
responses to enable rejected callers to contact entities that blocked
their calls in error.
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
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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 January 24, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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 cryptographic signing and attestation, respectively, of
signaling to ensure the integrity and authenticity of the asserted
identity.
This document describes a new SIP response code, 608, which allows
calling parties to learn 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 attempt to get a
destination's address removed from a calling database. However,
other network elements might interpret this to mean the user truly
does not exist and result in the user not being able to receive calls
from anyone, even if wanted. As well, in many jurisdictions,
providing false signaling is illegal.
The 608 response code addresses this need of remediating falsely
blocked calls. Specifically, this code informs the UAC an
intermediary blocked the call and, to satisfy some jurisdictional
requirements for providing a redress mechanism, how to contact the
operator of the intermediary.
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In the 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 shows 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 may wish to let their proxy know they might
consider future calls from that source unwanted. Upon receipt of the
607 response from the UAS, the proxy may send call information to a
call analytics engine. For various reasons described in RFC8197, if
a network operator receives multiple reports of unwanted calls, that
may indicate the entity placing the calls is likely to be a source of
unwanted calls for many people. As such, other users of the service
provider's service may wish the service provider to automatically
reject calls on their behalf based on that and other analytics.
Another value of the 607 rejection is presuming the proxy forwards
the response code to the UAC, the calling UAC or intervening proxies
know 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 the 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, also noting they might consider
not rejecting their calls in the future.
Moreover, many systems that allow the user to mark the call unwanted
(e.g., with the 607 response code) also allow the user to change
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their mind and unmark such calls. This is relatively easy to
implement as the user usually has a direct relationship with the
provider of the blocking service.
+--------+ +-----------+
| Called | | Call |
+-----+ | Party | | Analytics | +-----+
| UAC | | Proxy | | Engine | | UAS |
+-----+ +--------+ +-----------+ +-----+
| INVITE | | |
| --------------> | INVITE | |
| | ------------------------------> |
| | | |
| | | 607 |
| | <------------------------------ |
| | | |
| | Unwanted call | |
| 607 | -----------------> | |
| <-------------- | indicator | |
| | | |
Figure 2: Unwanted (607) Ladder Diagram
However, things get more complicated if an intermediary, such as a
third-party provider of call management services that classify calls
based on the relative likelihood the call is unwanted, misidentifies
the call as unwanted. Figure 3 shows this case. Note the UAS
typically does not receive an INVITE as the proxy rejects the call on
behalf of the user. In this situation, it would be beneficial for
the caller to be able to learn who rejected the call, so they might
be able to correct the misidentification.
In this situation, one might be tempted to have the intermediary use
the 607 response code. 607 indicates to the caller the subscriber
did not get the call and they do not want the call. However, RFC8197
specifies that one of the uses of 607 is to inform analytics engines
that a user (human) has rejected a call. The problem here is network
elements downstream from the intermediary might interpret the 607 as
a user (human) marking the call as unwanted, as opposed to a
statistical, machine learning, vulnerable to the base rate fallacy
[BaseRate] algorithm rejecting the call. In other words, those
downstream entities should not be relying on another entity
'deciding' the call is unwanted. By distinguishing between a (human)
user rejection and an intermediary's statistical rejection, a
downstream network element that sees a 607 response code can weight
it as a human rejection in its call analytics.
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+-----------+
| Call |
| Analytics |
| Engine |
+-----------+
^ | (likely not SIP)
| v
+-----------+
+-----+ 608 | Called | +-----+
| UAC | <--------- | Party | | UAS |
+-----+ | Proxy | +-----+
+-----------+
Figure 3: Rejected (608) Call Flow
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 whilst 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 while
providing contact information for the operator of the intermediary
that provides call rejection services to the called party, without
exposing the target user's contact information.
2. Terminology
This document uses the terms "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" as described in BCP14 [RFC2119][RFC8174] when, and only
when, they appear in all capitals, as shown here.
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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
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].
Unless there are indicators the calling party will use the contents
of the Call-Info header for malicious purposes (see Section 6), if an
intermediary issues a 608 code, the intermediary MUST include a Call-
Info header in the response.
If there is a Call-Info header, it MUST have the 'purpose' parameter
of 'card'. The value of the Call-Info header MUST refer to a valid
vCard [RFC6350] object.
The vCard referenced in the Call-Info header MUST include at least
one of the URL, EMAIL, TEL, or ADR properties. UACs supporting this
specification MUST be prepared to receive a full vCard. Call
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originators (at the UAC) can use the information returned by the
vCard 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.
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. UAC Operation
A UAC conforming to this specification MUST include the sip.608
feature capability tag in the INVITE request.
Upon receiving a 608 response, UACs perform normal SIP processing for
6xx responses.
3.3. 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? Besides a UAC predating
this specification, the could occur for callers from the 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 3.4 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 whom to contact, ideally 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. 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 feature
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capability. As such, the intermediary MUST play the announcement,
with the caveats described in Section 3.4 and Section 6.
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]. This information will be in the vCard referenced by the
Call-Info header in the 608 response message. Note this
specification does not specify the mechanism for such notification to
the UAC (see Section 3.4).
Do note that even if a network element plays an announcement
describing the contents of the 608 response message, the network
element MUST also send the 608 response code message as the final
response to the INVITE.
One aspect of using a feature capability is only the network elements
that will consume (UAC) or play an announcement (media gateway, SBC,
or proxy) need understand the sip.608 feature capability. All other
(existing) infrastructure can remain without modification, assuming
they are conformant to Section 16.6 of [RFC3261], specifically they
will pass headers such as "Feature-Capability: sip.608" unmodified.
3.4. Announcement Requirements
There are a few requirements on the element that will be doing 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 in the 608 response message. However, this
specification does not mandate the modality for conveying 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
actual 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, for
example, the network element SHOULD send the information in a text
format, not an audio format, unless the network element is unable to
render the information in the requested media format.
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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
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, for clarity do not include all
protocol elements, and may have errors. Review the protocol
documents for actual syntax and semantics of the protocol elements.
Given an INVITE (shamelessly taken from [SHAKEN]):
INVITE sip:+12155551213@tel.example1.net SIP/2.0
Max-Forwards: 69
Contact: <sip:+12155551212@69.241.19.12:50207;rinstance=9da3088f36cc>
To: <sip:+12155551213@tel.example1.net>
From: "Alice" <sip:+12155551212@tel.example2.net>;tag=614bdb40
Call-ID: 79048YzkxNDA5NTI1MzA0OWFjOTFkMmFlODhiNTI2OWQ1ZTI
P-Asserted-Identity: "Alice"<sip:+12155551212@tel.example2.net>,
<tel:+12155551212>
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 IP4 192.0.2.177
c=IN IP4 192.0.2.177
t=0 0
m=audio 54242 RTP/AVP 0
a=sendrecv
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An intermediary could reply:
SIP/2.0 608 Rejected
Via: SIP/2.0/UDP 192.0.2.177:60012;branch=z9hG4bK-524287-1
From: "Alice" <sip:+12155551212@tel.example2.net>;tag=614bdb40
To: <sip:+12155551213@tel.example1.net>
Call-ID: 79048YzkxNDA5NTI1MzA0OWFjOTFkMmFlODhiNTI2OWQ1ZTI
CSeq: 2 INVITE
Call-Info: <https://blocker.example.net/complaints.vcf>;purpose=card
A minimal vCard, in this example at https://blocker.example.net/
complaints.vcf, could contain:
BEGIN:VCARD
VERSION:4.0
FN:Robocall Adjudication
EMAIL;TYPE=work:bitbucket@blocker.example.net
END:VCARD
For an intermediary that provides a Web site for adjudication, the
vCard could contain:
BEGIN:VCARD
VERSION:4.0
FN:Robocall Adjudication
URL;TYPE=work:https://blocker.example.net/adjudication-form
END:VCARD
For an intermediary that provides a telephone number and a postal
address, the vCard could contain:
BEGIN:VCARD
VERSION:4.0
FN:Robocall Adjudication
ADR;TYPE=work;Argument Clinic;12 Main St;Anytown;AP;000000;Somewhere
TEL;VALUE=uri;TYPE=work:tel:+1-555-555-1212
END:VCARD
Note that it is up to the UAC to decide which vCard contact modality,
if any, it will use.
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 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
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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.
+---------+
| 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 vCard 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
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them on", followed by a text-to-speech translation of the telephone
number in the TEL field.
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. Please register
the response code in the "Response Codes" subregistry of the "Session
Initiation Protocol (SIP) Parameters" registry at
<http://www.iana.org/assignments/sip-parameters>.
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 that inserted the sip.608 Feature-Caps value 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]
6. Security Considerations
Intermediary operators need to be mindful of whom they are sending
the 608 response to. There is a risk that a truly malicious caller
is being rejected. This raises two issues. The first is the caller,
being alerted their 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 modality in the
Call-Info field, the intermediary may be giving the malicious caller
a vector for attack. In other words, the intermediary will be
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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 purposely not include the sip.608
feature capability in a flood of INVITE requests, direct those
requests to stateless proxies, and direct the Contact header to a
victim device. Because the mechanism described here can result in an
audio file being sent to the target of the Contact header, 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].
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%.
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>.
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[RFC6350] Perreault, S., "vCard Format Specification", RFC 6350,
DOI 10.17487/RFC6350, August 2011,
<https://www.rfc-editor.org/info/rfc6350>.
[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>.
[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>.
8.2. Informative References
[BaseRate]
Bar-Hillel, M., "The Base-Rate Fallacy in Probability
Judgements", 4 1977,
<http://www.dtic.mil/get-tr-doc/pdf?AD=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>.
[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>.
Burger Expires January 24, 2019 [Page 14]
Internet-Draft Status Rejected July 2018
[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>.
[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.
Author's Address
Eric W. Burger
Georgetown University
37th & O St, NW
Washington, DC 20057
USA
Email: eburger@standardstrack.com
Burger Expires January 24, 2019 [Page 15]
