TNAuthList Profile of Automated Certificate Management Environment (ACME) Authority TokenSomos Inc.United States of Americachris-ietf@chriswendt.netSomos Inc.United States of Americadavidhancock.ietf@gmail.comNeustar Inc.United States of Americamary.ietf.barnes@gmail.comNeustar Inc.Suite 5701800 Sutter StConcordCA94520United States of Americajon.peterson@neustar.biz
sec
acmeSTIRThis document defines a profile of the Automated Certificate Management Environment (ACME) Authority Token for the automated and authorized creation of certificates for Voice over IP (VoIP) telephone providers to support Secure Telephone Identity (STI) using the TNAuthList defined by STI certificates.Status of This Memo
This is an Internet Standards Track document.
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(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by
the Internet Engineering Steering Group (IESG). Further
information on Internet Standards is available in Section 2 of
RFC 7841.
Information about the current status of this document, any
errata, and how to provide feedback on it may be obtained at
.
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Table of Contents
. Introduction
. Requirements Language
. ACME New-Order Identifiers for TNAuthList
. TNAuthList Identifier Authorization
. TNAuthList Authority Token
. "iss" Claim
. "exp" Claim
. "jti" Claim
. "atc" Claim
. Acquiring the Token from the Token Authority
. Token Authority Responsibilities
. Scope of the TNAuthList
. Validating the TNAuthList Authority Token
. Using ACME-Issued Certificates with JSON Web Signature
. Usage Considerations
. Large Number of Noncontiguous TNAuthList Values
. IANA Considerations
. Security Considerations
. References
. Normative References
. Informative References
Acknowledgements
Authors' Addresses
Introduction describes a mechanism for automating certificate management on the Internet. It enables administrative entities to prove effective control over resources like domain names, and it automates the process of generating and issuing certificates. extends ACME to provide a general method of extending the authority and authorization of entities to control a resource via a third party Token Authority beyond the certification authority (CA).This document is a profile document using the Authority Token mechanism defined in . It is a profile that specifically addresses the Secure Telephone Identity Revisited (STIR) problem statement described in , which identifies the need for Internet credentials that can attest authority for the originator of VoIP calls in order to detect impersonation, which is currently an enabler for common attacks associated with illegal robocalling, voicemail hacking, and swatting. These credentials are used to sign Personal Assertion Tokens (PASSporTs) , which can be carried in using protocols such as SIP . Currently, the only defined credentials for this purpose are the certificates specified in using the TNAuthList. This document defines the use of the TNAuthList Authority Token in the ACME challenge to prove the authoritative use of the contents of the TNAuthList, including a Service Provider Code (SPC), a telephone number, or a set of telephone numbers or telephone number blocks.This document also describes the ability for a telephone authority to authorize the creation of CA types of certificates for delegation, as defined in .Requirements Language
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
when, and only when, they appear in all capitals, as shown here.
ACME New-Order Identifiers for TNAuthList defines the procedure that an ACME client uses to order a new certificate from a CA. The new-order request contains an identifier field that specifies the identifier objects the order corresponds to. This document defines a new type of identifier object called TNAuthList. A TNAuthList identifier contains the identity information to be populated in the TNAuthList of the new certificate. For the TNAuthList identifier, the new-order request includes a type set to the string "TNAuthList". The value of the TNAuthList identifier MUST be set to the details of the TNAuthList requested.The string that represents the TNAuthList MUST be
constructed using base64url encoding, as described in
and as defined in JSON Web Signature.
The base64url encoding MUST NOT include any padding characters, and the TNAuthList ASN.1 object MUST be encoded using DER encoding rules.An example of an ACME order object "identifiers" field containing a TNAuthList certificate is as follows:
"identifiers": [{"type":"TNAuthList","value":"F83n2a...avn27DN3"}]
where the "value" object string represents the arbitrary length of the base64url-encoded string.A full new-order request would look as follows:
POST /acme/new-order HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/evOfKhNU60wg",
"nonce": "5XJ1L3lEkMG7tR6pA00clA",
"url": "https://example.com/acme/new-order"
}),
"payload": base64url({
"identifiers": [{"type":"TNAuthList","value":"F83n...n27DN3"}],
"notBefore": "2021-01-01T00:00:00Z",
"notAfter": "2021-01-08T00:00:00Z"
}),
"signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}
On receiving a valid new-order request, the ACME server creates an authorization object (), containing the challenge that the ACME client must satisfy to demonstrate authority for the identifiers specified by the new order (in this case, the TNAuthList identifier). The CA adds the authorization object URL to the "authorizations" field of the order object and returns the order object to the ACME client in the body of a 201 (Created) response.
HTTP/1.1 201 Created
Content-Type: application/json
Replay-Nonce: MYAuvOpaoIiywTezizk5vw
Location: https://example.com/acme/order/1234
{
"status": "pending",
"expires": "2022-01-08T00:00:00Z",
"notBefore": "2022-01-01T00:00:00Z",
"notAfter": "2022-01-08T00:00:00Z",
"identifiers":[{"type":"TNAuthList",
"value":"F83n2a...avn27DN3"}],
"authorizations": [
"https://example.com/acme/authz/1234"
],
"finalize": "https://example.com/acme/order/1234/finalize"
}
TNAuthList Identifier AuthorizationOn receiving the new-order response, the ACME client queries the referenced authorization object to obtain the challenges for the identifier contained in the new-order request, as shown in the following example request and response.
POST /acme/authz/1234 HTTP/1.1
Host: example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": " https://example.com/acme/acct/evOfKhNU60wg",
"nonce": "uQpSjlRb4vQVCjVYAyyUWg",
"url": "https://example.com/acme/authz/1234"
}),
"payload": "",
"signature": "nuSDISbWG8mMgE7H...QyVUL68yzf3Zawps"
}
HTTP/1.1 200 OK
Content-Type: application/json
Link: <https://example.com/acme/some-directory>;rel="index"
{
"status": "pending",
"expires": "2022-01-08T00:00:00Z",
"identifier": {
"type":"TNAuthList",
"value":"F83n2a...avn27DN3"
},
"challenges": [
{
"type": "tkauth-01",
"tkauth-type": "atc",
"token-authority": "https://authority.example.org",
"url": "https://example.com/acme/chall/prV_B7yEyA4",
"token": "IlirfxKKXAsHtmzK29Pj8A"
}
]
}
When processing a certificate order containing an identifier of type "TNAuthList", a CA uses the Authority Token challenge type of "tkauth-01" with a "tkauth-type" of "atc" in to verify that the requesting ACME client has authenticated and authorized control over the requested resources represented by the "TNAuthList" value.The challenge "token-authority" parameter is only used in cases where the VoIP telephone network requires the CA to identify the Token Authority. This is currently not the case for the Signature-based Handling of Asserted information using toKENs (SHAKEN) certificate framework governance but may be used by other frameworks. If a "token-authority" parameter is present, then the ACME client MAY use the "token-authority" value to identify the URL representing the Token Authority that will provide the TNAuthList Authority Token response to the challenge. If the "token-authority" parameter is not present, then the ACME client MUST identify the Token Authority based on locally configured information or local policies.The ACME client responds to the challenge by posting the TNAuthList Authority Token to the challenge URL identified in the returned ACME authorization object, an example of which follows:
POST /acme/chall/prV_B7yEyA4 HTTP/1.1
Host: boulder.example.com
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://example.com/acme/acct/evOfKhNU60wg",
"nonce": "Q_s3MWoqT05TrdkM2MTDcw",
"url": "https://boulder.example.com/acme/authz/asdf/0"
}),
"payload": base64url({
"tkauth": "DGyRejmCefe7v4N...vb29HhjjLPSggwiE"
}),
"signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ"
}
The "tkauth" field is defined as a new field in the challenge object specific to the tkauth-01 challenge type that should contain the TNAuthList Authority Token defined in the next section.TNAuthList Authority TokenThe TNAuthList Authority Token is a profile instance of the ACME Authority Token defined in .The TNAuthList Authority Token protected header MUST comply with "Request Authentication" ().The TNAuthList Authority Token Payload MUST include the mandatory claims "exp", "jti", and "atc" and MAY include the optional claims defined for the Authority Token detailed in the next subsections."iss" ClaimThe "iss" claim is an optional claim defined in . It can be used as a URL identifying the Token Authority that issued the TNAuthList Authority Token beyond the "x5u" or other header claims that identify the location of the certificate or certificate chain of the Token Authority used to validate the TNAuthList Authority Token."exp" ClaimThe "exp" claim, defined in , MUST be included and contains the DateTime value of the ending date and time that the TNAuthList Authority Token expires."jti" ClaimThe "jti" claim, defined in , MUST be included and contains a unique identifier for this TNAuthList Authority Token transaction."atc" ClaimThe "atc" claim MUST be included and is defined in . It contains a JSON object with the following elements:
a "tktype" key with a string value equal to "TNAuthList" to represent a TNAuthList profile of the Authority Token defined by this document. "tktype" is a required key and MUST be included.
a "tkvalue" key with a string value equal to the base64url encoding of the TNAuthList certificate extension ASN.1 object using DER encoding rules. "tkvalue" is a required key and MUST be included.
a "ca" key with a boolean value set to either true when the requested certificate is allowed to be a CA cert for delegation uses or false when the requested certificate is not intended to be a CA cert, only an end-entity certificate. "ca" is an optional key; if not included, the "ca" value is considered false by default.
a "fingerprint" key constructed as defined in , corresponding to the computation of the "Thumbprint" step using the ACME account key credentials. "fingerprint" is a required key and MUST be included.
An example of the TNAuthList Authority Token is as follows:
{
"protected": base64url({
"typ":"JWT",
"alg":"ES256",
"x5u":"https://authority.example.org/cert"
}),
"payload": base64url({
"iss":"https://authority.example.org",
"exp":1640995200,
"jti":"id6098364921",
"atc":{"tktype":"TNAuthList",
"tkvalue":"F83n2a...avn27DN3",
"ca":false,
"fingerprint":"SHA256 56:3E:CF:AE:83:CA:4D:15:B0:29:FF:1B:71:
D3:BA:B9:19:81:F8:50:9B:DF:4A:D4:39:72:E2:B1:F0:B9:38:E3"}
}),
"signature": "9cbg5JO1Gf5YLjjz...SpkUfcdPai9uVYYQ"
}
Acquiring the Token from the Token AuthorityFollowing , the Authority Token should be acquired using a RESTful HTTP POST transaction as follows:
POST /at/account/:id/token HTTP/1.1
Host: authority.example.org
Content-Type: application/json
The request will pass the account identifier as a string in the request parameter "id". This string will be managed as an identifier specific to the Token Authority's relationship with a Communications Service Provider (CSP). There is assumed to also be a corresponding authentication procedure that can be verified for the success of this transaction, for example, an HTTP authorization header containing valid authorization credentials, as defined in .The body of the POST request MUST contain a JSON object with key value pairs corresponding to values that are requested as the content of the claims in the issued token. As an example, the body SHOULD contain a JSON object as follows:
{
"tktype":"TNAuthList",
"tkvalue":"F83n2a...avn27DN3",
"ca":false,
"fingerprint":"SHA256 56:3E:CF:AE:83:CA:4D:15:B0:29:FF:1B:71:D3
:BA:B9:19:81:F8:50:9B:DF:4A:D4:39:72:E2:B1:F0:B9:38:E3"
}
If successful, the response to the POST request returns a 200 (OK) with a JSON body that contains, at a minimum, the TNAuthList Authority Token as a JSON object with a key of "token" and the base64url-encoded string representing the atc token. JSON is easily extensible, so users of this specification may want to pass other pieces of information relevant to a specific application.An example of a successful response would be as follows:
HTTP/1.1 200 OK
Content-Type: application/json
{"token": "DGyRejmCefe7v4N...vb29HhjjLPSggwiE"}
If the request is not successful, the response should indicate the error condition. Specifically, for the case that the authorization credentials are invalid or if the account identifier provided does not exist, the response code MUST be 403 (Forbidden). Other 4xx and 5xx responses MUST follow standard HTTP error condition conventions .Token Authority ResponsibilitiesWhen creating the TNAuthList Authority Token, the Token Authority MUST validate that the information contained in the ASN.1 TNAuthList accurately represents the service provider code (SPC) or telephone number (TN) resources the requesting party is authorized to represent based on their pre-established, verified, and secure relationship between the Token Authority and the requesting party. Note that the fingerprint in the token request is not meant to be verified by the Token Authority but rather is meant to be signed as part of the token so that the party that requests the token can, as part of the challenge response, allow the ACME server to validate that the token requested and used came from the same party that controls the ACME client.Scope of the TNAuthListBecause this specification specifically involves the TNAuthList defined in , which involves SPC, telephone number ranges, and individual telephone numbers, the client may also request an Authority Token with some subset of its own authority as the TNAuthList provided in the "tkvalue" element in the "atc" JSON object. Generally, the scope of authority representing a CSP is represented by a particular SPC (e.g., in North America, an operating company number (OCN) or service provider identifier (SPID)). Based on number allocations, that provider is also generally associated
with a particular set of different telephone number ranges and/or telephone numbers. The TNAuthList can be constructed to define a limited scope of the TelephoneNumberRanges or TelephoneNumbers () either associated with an SPC or with the scope of telephone number ranges or telephone numbers the client has authority over.As recommended in the Security Considerations section in , an Authority Token can either have a scope that attests all of the resources that a client is eligible to receive certificates for or potentially a more limited scope that is intended to capture only those resources for which a client will receive a certificate from a particular certification authority. Any certification authority that sees an Authority Token can learn information about the resources a client can claim. In cases where this incurs a privacy risk, Authority Token scopes should be limited to only the resources that will be attested by the requested ACME certificate.Validating the TNAuthList Authority TokenUpon receiving a response to the challenge, the ACME server MUST perform the following steps to determine the validity of the response.
Verify that the value of the "atc" claim is a well-formed JSON object containing the mandatory key values.
If there is an "x5u" parameter, verify the "x5u" parameter is an HTTPS URL with a reference to a certificate representing the trusted issuer of Authority Tokens for the ecosystem.
If there is an "x5c" parameter, verify the certificate array contains a certificate representing the trusted issuer of Authority Tokens for the ecosystem.
Verify the TNAuthList Authority Token signature using the public key of the certificate referenced by the token's "x5u" or "x5c" parameter.
Verify that "atc" claim contains a "tktype" identifier with the value "TNAuthList".
Verify that the "atc" claim "tkvalue" identifier contains the equivalent base64url-encoded TNAuthList certificate extension string value as the identifier specified in the original challenge.
Verify that the remaining claims are valid (e.g., verify that token has not expired).
Verify that the "atc" claim "fingerprint" is valid and matches the account key of the client making the request.
Verify that the "atc" claim "ca" identifier boolean corresponds to the CA boolean in the Basic Constraints extension in the Certificate Signing Request (CSR) for either CA certificate or end-entity certificate.
If all steps in the token validation process pass, then the ACME server MUST set the challenge object "status" to "valid". If any step of the validation process fails, the "status" in the challenge object MUST be set to "invalid".Using ACME-Issued Certificates with JSON Web SignatureJSON Web Signature (JWS) objects can include an "x5u" header parameter to refer to a certificate that is used to validate the JWS signature. For example, the STIR PASSporT framework uses "x5u" to indicate the STIR certificate used to validate the PASSporT JWS object. The URLs used in "x5u" are expected to provide the required certificate in response to a GET request, not a POST-as-GET, as required for the "certificate" URL in the ACME order object. Thus, the current mechanism generally requires the ACME client to download the certificate and host it on a public URL to make it accessible to relying parties. This section defines an optional mechanism for the certification authority (CA) to host the certificate directly and provide a URL that the ACME client owner can directly reference in the "x5u" of their signed PASSporTs.As described in , when the certificate is ready for making a "finalize" request, the server will return a 200 (OK) with the updated order object. In this response, an ACME server can add a newly defined field called "x5u" that can pass this URL to the ACME client for usage in generated PASSporTs as a publicly available URL for PASSporT validation.
x5u (optional, string):
a URL that can be used to reference the certificate in the "x5u" parameter of a JWS object
The publishing of the certificates at the new "x5u" URL should follow the GET request requirement as mentioned above and should be consistent with the timely publication according to the durations of the certificate life cycle.The following is an example of the use of "x5u" in the response when the certificate status is "valid".
HTTP/1.1 200 OK
Content-Type: application/json
Replay-Nonce: CGf81JWBsq8QyIgPCi9Q9X
Link: <https://example.com/acme/directory>;rel="index"
Location: https://example.com/acme/order/TOlocE8rfgo
{
"status": "valid",
"expires": "2016-01-20T14:09:07.99Z",
"notBefore": "2016-01-01T00:00:00Z",
"notAfter": "2016-01-08T00:00:00Z",
"identifiers": [
"type":"TNAuthList",
"value":"F83n2a...avn27DN3"
],
"authorizations": ["https://sti-ca.com/acme/authz/1234"],
"finalize": "https://example.com/acme/order/TOlocE8rfgo/finalize",
"certificate": "https://example.com/acme/cert/mAt3xBGaobw",
"x5u": "https://example.com/cert-repo/giJI53km23.pem"
}
Usage ConsiderationsLarge Number of Noncontiguous TNAuthList ValuesThere are many scenarios and reasons to have various combinations of SPCs, TNs, and TN ranges. has provided a somewhat unbounded set of combinations. It's possible that a complex noncontiguous set of telephone numbers are being managed by a CSP. Best practice may be simply to split a set of noncontiguous numbers under management into multiple STI certificates to represent the various contiguous parts of the greater noncontiguous set of TNs, particularly if the length of the set of values in an identifier object grows to be too large.IANA ConsiderationsPer this document, IANA has added a new identifier object type to the "ACME Identifier Types" registry defined in .
Label
Reference
TNAuthList
RFC 9448
Security ConsiderationsThe token represented by this document has the credentials to represent the scope of a telephone number, a block of telephone numbers, or an entire set of telephone numbers represented by an SPC. The creation, transport, and any storage of this token MUST follow the strictest of security best practices beyond the recommendations of the use of encrypted transport protocols in this document to protect it from getting in the hands of bad actors with illegitimate intent to impersonate telephone numbers.This document inherits the security properties of . Implementations should follow the best practices identified in .This document only specifies SHA256 for the fingerprint hash. However, the syntax of the fingerprint object would permit other algorithms if, due to concerns about algorithmic agility, a more robust algorithm were required at a future time. Future specifications can define new algorithms for the fingerprint object as needed.ReferencesNormative ReferencesKey words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.The Base16, Base32, and Base64 Data EncodingsThis document describes the commonly used base 64, base 32, and base 16 encoding schemes. It also discusses the use of line-feeds in encoded data, use of padding in encoded data, use of non-alphabet characters in encoded data, use of different encoding alphabets, and canonical encodings. [STANDARDS-TRACK]JSON Web Signature (JWS)JSON Web Signature (JWS) represents content secured with digital signatures or Message Authentication Codes (MACs) using JSON-based data structures. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and an IANA registry defined by that specification. Related encryption capabilities are described in the separate JSON Web Encryption (JWE) specification.JSON Web Token (JWT)JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted.Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.Secure Telephone Identity Credentials: CertificatesIn order to prevent the impersonation of telephone numbers on the Internet, some kind of credential system needs to exist that cryptographically asserts authority over telephone numbers. This document describes the use of certificates in establishing authority over telephone numbers, as a component of a broader architecture for managing telephone numbers as identities in protocols like SIP.Automatic Certificate Management Environment (ACME)Public Key Infrastructure using X.509 (PKIX) certificates are used for a number of purposes, the most significant of which is the authentication of domain names. Thus, certification authorities (CAs) in the Web PKI are trusted to verify that an applicant for a certificate legitimately represents the domain name(s) in the certificate. As of this writing, this verification is done through a collection of ad hoc mechanisms. This document describes a protocol that a CA and an applicant can use to automate the process of verification and certificate issuance. The protocol also provides facilities for other certificate management functions, such as certificate revocation.JSON Web Token Best Current PracticesJSON Web Tokens, also known as JWTs, are URL-safe JSON-based security tokens that contain a set of claims that can be signed and/or encrypted. JWTs are being widely used and deployed as a simple security token format in numerous protocols and applications, both in the area of digital identity and in other application areas. This Best Current Practices document updates RFC 7519 to provide actionable guidance leading to secure implementation and deployment of JWTs.Secure Telephone Identity Revisited (STIR) Certificate DelegationThe Secure Telephone Identity Revisited (STIR) certificate profile provides a way to attest authority over telephone numbers and related identifiers for the purpose of preventing telephone number spoofing. This specification details how that authority can be delegated from a parent certificate to a subordinate certificate. This supports a number of use cases, including those where service providers grant credentials to enterprises or other customers capable of signing calls with STIR.HTTP SemanticsThe Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes.This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230.Automated Certificate Management Environment (ACME) Challenges Using an Authority TokenInformative ReferencesSignature-based Handling of Asserted information using toKENs (SHAKEN): Governance Model and Certificate ManagementATISATIS-1000080.v005Secure Telephone Identity Problem Statement and RequirementsOver the past decade, Voice over IP (VoIP) systems based on SIP have replaced many traditional telephony deployments. Interworking VoIP systems with the traditional telephone network has reduced the overall level of calling party number and Caller ID assurances by granting attackers new and inexpensive tools to impersonate or obscure calling party numbers when orchestrating bulk commercial calling schemes, hacking voicemail boxes, or even circumventing multi-factor authentication systems trusted by banks. Despite previous attempts to provide a secure assurance of the origin of SIP communications, we still lack effective standards for identifying the calling party in a VoIP session. This document examines the reasons why providing identity for telephone numbers on the Internet has proven so difficult and shows how changes in the last decade may provide us with new strategies for attaching a secure identity to SIP sessions. It also gives high-level requirements for a solution in this space.Authenticated Identity Management in the Session Initiation Protocol (SIP)The baseline security mechanisms in the Session Initiation Protocol (SIP) are inadequate for cryptographically assuring the identity of the end users that originate SIP requests, especially in an interdomain context. This document defines a mechanism for securely identifying originators of SIP requests. It does so by defining a SIP header field for conveying a signature used for validating the identity and for conveying a reference to the credentials of the signer.This document obsoletes RFC 4474.PASSporT: Personal Assertion TokenThis document defines a method for creating and validating a token that cryptographically verifies an originating identity or, more generally, a URI or telephone number representing the originator of personal communications. The Personal Assertion Token, PASSporT, is cryptographically signed to protect the integrity of the identity of the originator and to verify the assertion of the identity information at the destination. The cryptographic signature is defined with the intention that it can confidently verify the originating persona even when the signature is sent to the destination party over an insecure channel. PASSporT is particularly useful for many personal-communications applications over IP networks and other multi-hop interconnection scenarios where the originating and destination parties may not have a direct trusted relationship.AcknowledgementsWe would like to thank and for valuable contributions to this document.Authors' AddressesSomos Inc.United States of Americachris-ietf@chriswendt.netSomos Inc.United States of Americadavidhancock.ietf@gmail.comNeustar Inc.United States of Americamary.ietf.barnes@gmail.comNeustar Inc.Suite 5701800 Sutter StConcordCA94520United States of Americajon.peterson@neustar.biz