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 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 http://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 May 4, 2017. Copyright Notice Copyright (c) 2016 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include 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. Rescorla & Peterson Expires May 4, 2017 [Page 1] Internet-Draft STIR Fallback October 2016 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: Rescorla & Peterson Expires May 4, 2017 [Page 2] Internet-Draft STIR Fallback October 2016 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]. Rescorla & Peterson Expires May 4, 2017 [Page 3] Internet-Draft STIR Fallback October 2016 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. Rescorla & Peterson Expires May 4, 2017 [Page 4] Internet-Draft STIR Fallback October 2016 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. Rescorla & Peterson Expires May 4, 2017 [Page 5] Internet-Draft STIR Fallback October 2016 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 Rescorla & Peterson Expires May 4, 2017 [Page 6] Internet-Draft STIR Fallback October 2016 [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 Rescorla & Peterson Expires May 4, 2017 [Page 7] Internet-Draft STIR Fallback October 2016 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 Rescorla & Peterson Expires May 4, 2017 [Page 8] Internet-Draft STIR Fallback October 2016 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: Rescorla & Peterson Expires May 4, 2017 [Page 9] Internet-Draft STIR Fallback October 2016 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 Rescorla & Peterson Expires May 4, 2017 [Page 10] Internet-Draft STIR Fallback October 2016 [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. Rescorla & Peterson Expires May 4, 2017 [Page 11] Internet-Draft STIR Fallback October 2016 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. Rescorla & Peterson Expires May 4, 2017 [Page 12] Internet-Draft STIR Fallback October 2016 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. Rescorla & Peterson Expires May 4, 2017 [Page 13] Internet-Draft STIR Fallback October 2016 [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, . [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, . [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, . [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, . [RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure Telephone Identity Problem Statement and Requirements", RFC 7340, DOI 10.17487/RFC7340, September 2014, . 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]