Network Working Group E. Rescorla
Internet-Draft Mozilla
Intended status: Standards Track J. Peterson
Expires: May 4, 2017 Neustar
October 31, 2016
STIR Out of Band Architecture and Use Cases
draft-rescorla-stir-fallback-01.txt
Abstract
Existing work has defined ways to secure the identity of SIP calls,
but the in-band mechanisms defined in that work do not properly
transit the PSTN. This document defines out-of-band mechanisms which
do not require modifying the messages that pass over the PSTN.
Status of This Memo
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This Internet-Draft will expire on May 4, 2017.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Operating Environments . . . . . . . . . . . . . . . . . . . 4
4. Dataflows . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Case 1: VoIP to PSTN Call . . . . . . . . . . . . . . . . 6
5.2. Case 2: Two Smart PSTN endpoints . . . . . . . . . . . . 6
5.3. Case 3: PSTN to VoIP Call . . . . . . . . . . . . . . . . 6
5.4. Case 4: Gateway Out-of-band . . . . . . . . . . . . . . . 7
6. Solution Architecture . . . . . . . . . . . . . . . . . . . . 7
6.1. Credentials and Phone Numbers . . . . . . . . . . . . . . 7
6.2. Solution Architecture . . . . . . . . . . . . . . . . . . 8
6.3. Security Analysis . . . . . . . . . . . . . . . . . . . . 9
6.4. Substitution Attacks . . . . . . . . . . . . . . . . . . 9
7. Call Placement Service Discovery and Interface . . . . . . . 10
8. Some Potential Enhancements . . . . . . . . . . . . . . . . . 11
8.1. Encrypted PASSporTs . . . . . . . . . . . . . . . . . . . 11
8.1.1. Credential Lookup . . . . . . . . . . . . . . . . . . 12
8.2. Federated Call Placement Services . . . . . . . . . . . . 12
8.3. Escalation to VoIP . . . . . . . . . . . . . . . . . . . 13
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
11. Security Considerations . . . . . . . . . . . . . . . . . . . 13
12. Informative References . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
The STIR problem statement [RFC7340] describes widespread problems
enabled by impersonation in the telephone network, including illegal
robocalling, voicemail hacking, and swatting. As telephone services
are increasingly migrating onto the Internet, and using Voice over IP
(VoIP) protocols such as SIP [RFC3261], it is necessary for these
protocols to support stronger identity mechanisms to prevent
impersonation. For example, [I-D.ietf-stir-rfc4474bis] defines an
Identity header of SIP requests capable of carrying a PASSporT
[I-D.ietf-stir-passport] object in SIP as a means to
cryptographically attest that the originator of a telephone call is
authorized to use the calling party number (or, for native SIP cases,
SIP URI) associated with the originator of the call. of the request.
Not all telephone calls use SIP today, however; and even those that
do use SIP do not always carry SIP signaling end-to-end. Most calls
from telephone numbers still traverse the Public Switched Telephone
Network (PSTN) at some point. Broadly, calls fall into one of three
categories:
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1. One or both of the endpoints is actually a PSTN endpoint.
2. Both of the endpoints are non-PSTN (SIP, Jingle, ...) but the
call transits the PSTN at some point.
3. Non-PSTN calls which do not transit the PSTN at all (such as
native SIP end-to-end calls).
The first two categories represent the majority of telephone calls
associated with problems like illegal robocalling: many robocalls
today originate on the Internet but terminate at PSTN endpoints.
However, the core network elements that operate the PSTN are legacy
devices that are unlikely to be upgradable at this point to support
an in-band authentication system. As such, those devices largely
cannot be modified to pass signatures originating on the Internet--or
indeed any inband signaling data--intact. In some cases they will
strip the signatures from PSTN signaling; in others, they might
damage them to the point where they cannot be verified. For those
first two categories above, any in-band authentication scheme does
not seem practical in the current environment.
But while the core network of the PSTN remains fixed, the endpoints
of the telephone network are becoming increasingly programmable and
sophisticated. Landline "plain old telephone service" deployments,
especially in the developed world, are shrinking, and increasingly
being replaced by three classes of intelligent devices: smart phones,
IP PBXs, and terminal adapters. All three are general purpose
computers, and typically all three have Internet access as well as
access to the PSTN. This provides a potential avenue for building an
authentication system that changes only the endpoints while leaving
the PSTN intact.
This specification therefore builds on the PASSporT
[I-D.ietf-stir-passport] mechanism and the work of
[I-D.ietf-stir-rfc4474bis] to define a way that a PASSporT object
created in the originating network of the call can reach the
terminating network even when it cannot be carried end-to-end in-band
in the call signaling. This relies on a new service defined in this
document that permits the PASSporT object to be stored during call
processing and retrieved for verification purposes.
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 RFC
2119 [RFC2119].
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3. Operating Environments
This section describes the environment in which the proposed
mechanism is intended to operate. In the simplest setting, Alice is
calling Bob through some set of gateways and/or the PSTN. Both Alice
and Bob have smart devices which can be modified, but they do not
have a clear connection between them: Alice cannot inject any data
into signaling which Bob can read, with the exception of the asserted
destination and origination E.164 numbers. The calling party number
might originate from her own device or from the network. These
numbers are effectively the only data that can be used for
coordination between the endpoints.
+---------+
/ \
+--- +---+
+----------+ / \ +----------+
| | | Gateways | | |
| Alice |<----->| and/or |<----->| Bob |
| (caller) | | PSTN | | (callee) |
+----------+ \ / +----------+
+--- +---+
\ /
+---------+
In a more complicated setting, Alice and/or Bob may not have a smart
or programmable device, but one or both of them are behind a STIR-
aware gateway that can participate in out-of-band coordination, as
shown below:
+---------+
/ \
+--- +---+
+----------+ +--+ / \ +--+ +----------+
| | | | | Gateways | | | | |
| Alice |<-|GW|->| and/or |<-|GW|->| Bob |
| (caller) | | | | PSTN | | | | (callee) |
+----------+ +--+ \ / +--+ +----------+
+--- +---+
\ /
+---------+
In such a case, Alice might have an analog connection to her gateway/
switch which is responsible for her identity. Similarly, the gateway
would verify Alice's identity, generate the right calling party
number information and provide that number to Bob using ordinary POTS
mechanisms.
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4. Dataflows
Because in these operating environments endpoints cannot pass
cryptographic information to one another directly through signaling,
any solution must involve some rendezvous mechanism to allow
endpoints to communicate. We call this rendezvous service a "call
placement service" (CPS), a service where a record of call placement,
in this case a PASSporT, can be stored for future retrieval. In
principle this service could communicate any information, but
minimally we expect it to include a full-form PASSporT that attests
the caller, callee, and the time of the call. The callee can use the
existence of a PASSporT for a given incoming call as rough validation
of the asserted origin of that call. (See Section 8.1.1 for
limitations of this design.)
There are roughly two plausible dataflow architectures for the CPS:
The callee registers with the CPS. When the caller wishes to
place a call to the callee, it sends the PASSporT to the CPS which
forwards it to the callee.
The caller stores the PASSporT with the CPS at the time of call
placement. When the callee receives the call, it contacts the CPS
and retrieves the CDR.
While the first architecture is roughly isomorphic to current VoIP
protocols, it shares their drawbacks. Specifically, the callee must
maintain a full-time connection to the CPS to serve as a notification
channel. This comes with the usual networking costs to the callee
and is especially problematic for mobile endpoints. Thus, we focus
on the second architecture in which the PSTN incoming call serves as
the notification channel and the callee can then contact the CPS to
retrieve the PASSporT.
5. Use Cases
The following are the motivating use cases for this mechanism. Bear
in mind that just as in [I-D.ietf-stir-rfc4474bis] there may be
multiple Identity headers in a single SIP INVITE, so there may be
multiple PASSporTs in this out-of-band mechanism associated with a
single call. For example, a SIP user agent might create a PASSporT
for a call with an end user credential, and as the call exits the
originating administrative domain the network authentication service
might create its own PASSporT for the same call. As such, these use
cases may overlap in the processing of a single call.
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5.1. Case 1: VoIP to PSTN Call
A call originates in the SIP world in a STIR-aware administrative
domain. The local authentication service for that administrative
domain creates a PASSporT which is carried in band in the call per
[I-D.ietf-stir-rfc4474bis]. The call is routed out of the
originating administrative domain and eventually reaches a gateway to
the PSTN. Eventually, the call will terminate on a mobile smartphone
that supports this out-of-band mechanism.
In this use case, the originating authentication service can store
the PASSporT with the appropriate CPS for the target telephone number
as a fallback in case SIP signaling will not reach end-to-end. When
the destination mobile smartphone receives the call over the PSTN, it
consults the CPS and discovers a PASSporT from the originating
telephone number waiting for it. It uses this PASSporT to verify the
calling party number.
5.2. Case 2: Two Smart PSTN endpoints
A call originates with an enterprise PBX that has both Internet
access and a built-in gateway to the PSTN. It will immediately drop
its call to the PSTN, but before it does, it provisions a PASSporT on
the CPS associated with the target telephone number.
After normal PSTN routing, the call lands on a smart mobile handset
that supports the STIR out-of-band mechanism. It queries the
appropriate CPS over the Internet to determine if a call has been
placed to it by a STIR-aware device. It finds the PASSporT
provisioned by the enterprise PBX and uses it to verify the calling
party number.
5.3. Case 3: PSTN to VoIP Call
A call originates with an enterprise PBX that has both Internet
access and a built-in gateway to the PSTN. It will immediate drop
the call to the PSTN, but before it does, it provisions a PASSporT
with the CPS associated with the target telephone number. However,
it turns out that the call will eventually route through the PSTN to
an Internet gateway, which will translate this into a SIP call and
deliver it to an administrative domain with a STIR verification
service.
In this case, the Internet gateway that receives the call from the
PSTN can query the appropriate CPS to determine if the original
caller created and provisioned a PASSporT for this call. If so, it
can retrieve the PASSporT and, when it creates a SIP INVITE for this
call, add a corresponding Identity header per
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[I-D.ietf-stir-rfc4474bis]. When the SIP INVITE reaches the
destination administrative domain, it will be able to verify the
PASSporT normally. Note that to avoid discrepancies with the Date
header field value, only full-form PASSporT should be used for this
purpose.
5.4. Case 4: Gateway Out-of-band
A call originates in the SIP world in a STIR-aware administrative
domain. The local authentication service for that administrative
domain creates a PASSporT which is carried in band in the call per
[I-D.ietf-stir-rfc4474bis]. The call is routed out of the
originating administrative domain and eventually reaches a gateway to
the PSTN.
In this case, the originating authentication service does not support
the out-of-band mechanism, so instead the gateway to the PSTN
extracts the PASSporT from the SIP request and provisions it to the
CPS. (When the call reaches the gateway to the PSTN, the gateway
might first check the CPS to see if a PASSporT object had already
been provisioned for this call, and only provision a PASSporT if none
is present).
Ultimately, the call may terminate on the PSTN, or be routed back to
the IP world. In the former case, perhaps the destination endpoints
queries the CPS to retrieve the PASSporT provisioned by the first
gateway. Or if the call ultimately returns to the IP world, it might
be the gateway from the PSTN back to the Internet that retrieves the
PASSporT from the CPS and attaches it to the new SIP INVITE it
creates, or it might be the terminating administrative domain's
verification service that checks the CPS when an INVITE arrives with
no Identity header field. Either way the PASSporT can survive the
gap in SIP coverage caused by the PSTN leg of the call.
6. Solution Architecture
In this section, we discuss a strawman architecture along the lines
described in the previous section. This discussion is deliberately
sketchy, focusing on broad concepts and skipping over details. The
intent here is merely to provide a rough concept, not a complete
solution.
6.1. Credentials and Phone Numbers
We start from the premise of the STIR problem statement [RFC7340]
that phone numbers can be associated with credentials which can be
used to attest ownership of numbers. For purposes of exposition, we
will assume that ownership is associated with the endpoint (e.g., a
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smartphone) but it might well be associated with a provider or
gateway acting for the endpoint instead. It might be the case that
multiple entities are able to act for a given number, provided that
they have the appropriate authority. [I-D.ietf-stir-certificates]
describes a credentials system suitable for this purpose; the
question of how an entity is determined to have control of a given
number is out of scope for the current document.
6.2. Solution Architecture
An overview of the basic calling and verification process is shown
below. In this diagram, we assume that Alice has the number
+1.111.111.1111 and Bob has the number +2.222.222.2222.
Alice Call Placement Service Bob
-----------------------------------------------------------------------
<- Authenticate as 1.111.111.1111 ---->
Store PASSporT ->
Call from 1.111.111.1111 ---------------------------------------------->
<- Authenticate as 1.222.222.2222 ---->
<-------------- Retrieve call record
from 1.111.111.1111?
(1.222.222.2222,1.111.111.1111) -->
[Ring phone with callerid
= 1.111.111.1111]
When Alice wishes to make a call to Bob, she contacts the CPS and
authenticates to prove her ownership of her E.164 number. Once she
has authenticated, she then stores a PASSporT on the CPS. The
PASSpoRT is stored under Bob's number.
Once Alice has stored the PASSporT, she then places the call to Bob
as usual. At this point, Bob's phone would usually ring and display
Alice's number (+1.111.111.1111), which is informed by the existing
caller-id mechanisms (i.e., the CIN field of the IAM). Instead,
Bob's phone transparently contacts the CPS and requests any current
PASSporTs for calls to Bob. The CPS responds with any such PASSporTs
(assuming they exists). If such a PASSpoRT exists, Bob's phone can
then present the callerid information as valid. Otherwise, the call
is unverifiable. Note that this does not necessarily mean that the
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call is bogus; because we expect incremental deployment many
legitimate calls will be unverifiable.
6.3. Security Analysis
The primary attack we seek to prevent is an attacker convincing the
callee that a given call is from some other caller C. There are two
scenarios to be concerned with:
The attacker wishes to simulate a call when none exists.
The attacker wishes to substitute himself for an existing call as
described in Section 6.4.
If an attacker can inject fake PASSporT into the CPS or in the
communication from the CPS to the callee, he can mount either attack.
As PASSporTs should be digitally signed by an appropriate authority
for the number and verified by the callee (see Section 6.1), this
should not arise in ordinary operations. For privacy and robustness
reasons, using TLS on the originating side when storing the PASSporT
at the CPS is recommended.
The entire system depends on the security of the credential
infrastructure. If the authentication credentials for a given number
are compromised, then an attacker can impersonate calls from that
number.
6.4. Substitution Attacks
All that receipt of the PASSporT from the CPS proves to the called
party is that Alice is trying to call Bob (or at least was as of very
recently). It does not prove that any particular incoming call is
from Alice. Consider the scenario in which we have a service which
provides an automatic callback to a user-provided number. In that
case, the attacker try to arrange for a false caller-id value, as
shown below:
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Attacker Callback Service CPS Bob
-----------------------------------------------------------------------
Place call to Bob ---------->
Store PASSporT for
CS:Bob -------------->
Call from CS (forged caller-id info) -------------------------------->
Call from CS ---------------------------> X
<----- Retrieve PASSporT
for CS:Bob
PASSporT for CS:Bob --------------------------->
[Ring phone with callerid = CS]
In order to mount this attack, the attacker contacts the Callback
Service (CS) and provides it with Bob's number. This causes the CS
to initiate a call to Bob. As before, the CS contacts the CPS to
insert an appropriate PASSporT and then initiates a call to Bob.
Because it is a valid CS injecting the PASSporT, none of the security
checks mentioned above help. However, the attacker simultaneously
initiates a call to Bob using forged caller-id information
corresponding to the CS. If he wins the race with the CS, then Bob's
phone will attempt to verify the attacker's call (and succeed since
they are indistinguishable) and the CS's call will go to busy/voice
mail/call waiting. Note: in a SIP environment, the callee might
notice that there were multiple INVITEs and thus detect this attack.
7. Call Placement Service Discovery and Interface
In order for the two ends of the out-of-band dataflow to coordinate,
they must agree on a way to discover a CPS and retrieve PASSporT
objects from it based solely on the rendezvous information available:
the calling party number and the called number. There are a number
of potential service discovery mechanisms that could be used for this
purpose. The means of service discovery may vary by use case.
There exist a number of common directory systems that might be used
to translate telephone numbers into the URIs of a CPS. ENUM
[RFC6116] is commonly implemented, though no "golden root" central
ENUM administration exists that could be easily reused today to help
the endpoints discover a common CPS. Other protocols associated with
queries for telephone numbers, such as the TeRI
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[I-D.peterson-modern-teri] protocol, could also serve for this
application.
Another possibility is to use a single distributed service for this
function. VIPR [I-D.rosenberg-dispatch-vipr-overview] proposed a
RELOAD [RFC6940] usage for telephone numbers to help direct calls to
enterprises on the Internet. It would be possible to describe a
similar RELOAD usage to identify the CPS where calls for a particular
telephone number should be stored. One advantage that the STIR
architecture has over VIPR is that it assumes a credential system
that proves authority over telephone numbers; those credentials could
be used to determine whether or not a CPS could legitimately claim to
be the proper store for a given telephone number.
Future versions of this specification will identify suitable service
discovery mechanisms for out-of-band STIR.
8. Some Potential Enhancements
Section 4 provides a broad sketch of an approach. In this section,
we consider some potential enhancements. Readers can feel free to
skip this section, as it is not necessary to get the flavor of the
document.
8.1. Encrypted PASSporTs
In the system described in Section 4, the CPS learns the PASSporT for
every call, which is undesirable from a privacy perspective. The
situation can be improved by having the caller store encrypted
PASSporTs. A number of schemes are possible, but for concreteness we
sketch one possibility.
The general idea is that each user's credentials are not just
suitable for authentication to the CPS but also are an asymmetric key
pair suitable for use in an encryption mode. When Alice wants to
store a PASSporT for Bob she retrieves Bob's credentials (see
Section 8.1.1) and then encrypts the PASSporT under Bob's public key.
[The encryption needs to be done in such a way that if you don't have
Bob's key, the message is indistinguishable from random. This is
straightforward, but not compatible with typical secure message
formats, which tend to indicate the recipient's identity.] The
PASSporT is then stored with the CPS under Alice's identity. When
Bob receives a call, he just asks the CPS (anonymously) for any calls
from Alice to anyone. He then trial-decrypts each and if any of them
is for him, he proceeds as before. In this way, the CPS learns
Alice's call velocity but not who she is calling. This mechanism is
suitable for cases where credentials are issued to end-user devices,
rather than large operators.
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8.1.1. Credential Lookup
In order to encrypt a PASSporT, the caller needs access to the
callee's credentials (specifically their public key). This requires
some sort of directory/lookup system. This document does not specify
any particular scheme, but a list of requirements would be something
like:
Obviously, if there is a single central database and the caller and
callee each contact it in real time to determine the other's
credentials, then this represents a real privacy risk, as the central
database learns about each call. A number of mechanisms are
potentially available to mitigate this:
Have endpoints pre-fetch credentials for potential counterparties
(e.g., their address book or the entire database).
Have caching servers in the user's network that proxy their
fetches and thus conceal the relationship between the user and the
credentials they are fetching.
Clearly, there is a privacy/timeliness tradeoff in that getting
really up-to-date knowledge about credential validity requires
contacting the credential directory in real-time (e.g., via OCSP).
This is somewhat mitigated for the caller's credentials in that he
can get short-term credentials right before placing a call which only
reveals his calling rate, but not who he is calling. Alternately,
the CPS can verify the caller's credentials via OCSP, though of
course this requires the callee to trust the CPS's verification.
This approach does not work as well for the callee's credentials, but
the risk there is more modest since an attacker would need to both
have the callee's credentials and regularly poll the database for
every potential caller.
We consider the exact best point in the tradeoff space to be an open
issue.
8.2. Federated Call Placement Services
The discussion above is written in terms of a single CPS, but this
potentially has scaling problems, as well as allowing the CPS to
learn about every call. These issues can be alleviated by having a
federated CPS. If a credential lookup service is already available,
the CPS location can also be stored in the callee's credentials.
A service discovery mechanism for out-of-band STIR should ideally
enable federation of the CPS function.
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8.3. Escalation to VoIP
If the call is to be carried over the PSTN, then the security
properties described above are about the best we can do. However, if
Alice and Bob are both VoIP capable, then there is an opportunity to
provide a higher quality of service and security. The basic idea is
that the PASSporT contains an addition claim containing rendezvous
information for Alice (e.g., Alice's SIP URI). Once Bob has verified
Alice's PASSporT, he can initiate a VoIP connection directly to
Alice, thus bypassing the PSTN. Mechanisms of this type are out of
scope of this document.
9. Acknowledgments
The ideas in this document come out of discussions with Richard
Barnes and Cullen Jennings.
10. IANA Considerations
This memo includes no request to IANA.
11. Security Considerations
This entire document is about security, but the detailed security
properties depend on having a single concrete scheme to analyze.
12. Informative References
[I-D.ietf-stir-certificates]
Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", draft-ietf-stir-
certificates-10 (work in progress), October 2016.
[I-D.ietf-stir-passport]
Wendt, C. and J. Peterson, "Personal Assertion Token
(PASSporT)", draft-ietf-stir-passport-10 (work in
progress), October 2016.
[I-D.ietf-stir-rfc4474bis]
Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-14
(work in progress), October 2016.
[I-D.peterson-modern-teri]
Peterson, J., "An Architecture and Information Model for
Telephone-Related Information (TeRI)", draft-peterson-
modern-teri-01 (work in progress), July 2016.
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[I-D.rosenberg-dispatch-vipr-overview]
Rosenberg, J., Jennings, C., and M. Petit-Huguenin,
"Verification Involving PSTN Reachability: Requirements
and Architecture Overview", draft-rosenberg-dispatch-vipr-
overview-04 (work in progress), October 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://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,
<http://www.rfc-editor.org/info/rfc3261>.
[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,
<http://www.rfc-editor.org/info/rfc6116>.
[RFC6940] Jennings, C., Lowekamp, B., Ed., Rescorla, E., Baset, S.,
and H. Schulzrinne, "REsource LOcation And Discovery
(RELOAD) Base Protocol", RFC 6940, DOI 10.17487/RFC6940,
January 2014, <http://www.rfc-editor.org/info/rfc6940>.
[RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement and Requirements",
RFC 7340, DOI 10.17487/RFC7340, September 2014,
<http://www.rfc-editor.org/info/rfc7340>.
Authors' Addresses
Eric Rescorla
Mozilla
Email: ekr@rtfm.com
Jon Peterson
Neustar, Inc.
1800 Sutter St Suite 570
Concord, CA 94520
US
Email: jon.peterson@neustar.biz
Rescorla & Peterson Expires May 4, 2017 [Page 14]