AT&T

md3135@att.com

Comcast

Comcast Technology Center Philadelphia, PA 19103 USA chris-ietf@chriswendt.net

ART STIR Internet-Draft This document adds new assertion values for a Resource Priority Header (“rph”) claim and a new SIP Priority Header claim (“sph”) for protection of the “psap-callback” value as part of the “rph” PASSporT extension, in support of the security of Emergency Services Networks for emergency call origination and callback.

Personal Assertion Token (PASSporT) Extension for Resource Priority Authorization extended the Personal Assertion Token (PASSporT) specification defined in to allow the inclusion of cryptographically signed assertions of authorization for the values populated in the Session Initiation Protocol (SIP) “Resource-Priority” header field . introduces the need and justification for the protection of both the SIP “Resource-Priority” and “Priority” header fields, used for categorizing the priority use of the call in the telephone network, specifically for emergency calls. Compromise of the SIP “Resource-Priority” or “Priority” header fields could lead to misuse of network resources (i.e., during congestion scenarios), impacting the application services supported using the SIP “Resource-Priority” header field and the handling of Public Saftey Answering Point (PSAP) callbacks. allows extensions by which an authority on the originating side verifying the authorization of a particular communication for the SIP “Resource-Priority” header field or the SIP “Priority” header field can use PASSPorT claims to cryptographically sign the information associated with either the SIP “Resource-Priority” or “Priority” header field and convey assertion of those values by the signing party authorization. A signed SIP “Resource-Priority” or “Priority” header field will allow a receiving entity (including entities located in different network domains/boundaries) to verify the validity of assertions to act on the information with confidence that the information has not been spoofed or compromised. This document adds new “auth” array key values for a Resource Priority Header (“rph”) claim defined in , in support of Emergency Services Networks for emergency call origination and callback. This document additionally defines a new PASSporT claim, “sph”, including protection of the SIP Priority header for the indication of an emergency service call-back assigned the value “psap-callback” as defined in . The use of the newly defined claim and key values corresponding to the SIP ‘Resource-Priority’ and ‘Priority’ header fields for emergency services is introduced in but otherwise out-of-scope of this document. In addition, the PASSPorT claims and values defined in this document are intended for use in environments where there are means to verify that the signer of the SIP ‘Resource-Priority’ and ‘Priority’ header fields is authoritative.

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.

This specification defines the ability to sign the SIP Resource-Priority Header field namespace for local emergency communications defined in and represented by the string “esnet.x” where x is the priority-level allowed in the esnet namespace. As of the writing of this specification the priority-level is between 0 and 4, but may be extended by future specifications. Similar to the values allowed by for the “auth” JSON object key inside the “rph” claim, the string “esnet.x” with the appropriate value should be used when resource priority is required for local emergency communications corresponding and exactly matching the SIP Resource-Priority header string representing the namespace invoked in the call. When using “esnet.x” as the “auth” assertion value in emergency service destined calls, the “orig” claim of the PASSporT MUST represent the calling party number that initiates the call to emergency services. The “dest” claim MUST either be a country or region specific dial string (e.g., “911” for North America or “112” GSM defined string used in Europe and other countries) or “urn:service:sos” as defined in , representing the emergency services destination of the call. The following is an example of an “rph” claim for SIP ‘Resource-Priority’ header field with an “esnet.1” assertion:

For emergency services callbacks, the “orig” claim of the “rph” PASSporT MUST represent the Public Saftey Answering Point (PSAP) telephone number. The “dest” claim MUST be the telephone number representing the original calling party of the emergency service call that is being called back. The following is an example of an “rph” claim for SIP ‘Resource-Priority’ header field with a “esnet.0” assertion:

After the header and claims PASSporT objects have been constructed, their signature is generated normally per the guidance in using the full form of PASSPorT. The credentials (i.e., Certificate) used to create the signature must have authority over the namespace of the “rph” claim, and there is only one authority per claim. The authority MUST use its credentials associated with the specific service supported by the resource priority namespace in the claim. If r-values are added or dropped by the intermediaries along the path, the intermediaries must generate a new “rph” header and sign the claim with their own authority.

As defined in the SIP Priority header may be set to the value “psap-callback” for emergency services callback calls. Because some SIP networks may act on this value and provide priority or other special routing based on this value, it is important to protect and validate the authoritative use associated with it. Therefore, we define a new claim key as part of the “rph” PASSporT, “sph”, which MUST be used only for authorized emergency callbacks and correspond to a SIP Priority header with the value “psap-callback”. The value of the “sph” claim key should only be “psap-callback” to match the SIP Priority header field value for authorized emergency services callbacks. The following is an example of an “sph” claim for SIP ‘Priority’ header field with the value “psap-callback”:

The order of the claim keys MUST follow the rules of Section 9; the claim keys MUST appear in lexicographic order. Therefore, the claim keys discussed in this document appear in the PASSporT Payload in the following order, dest iat orig rph sph

The use of the compact form of PASSporT is not specified in this document or recommended for ‘rph’ PASSporTs.

The authors would like to thank Brian Rosen, Terry Reese, and Jon Peterson for helpful suggestions, comments, and corrections.

This specification requests that the IANA add one new claim to the JSON Web Token Claims registry as defined in . Claim Name: “sph” Claim Description: SIP Priority header field Change Controller: IESG Specification Document(s): [RFCThis]

The security considerations discussed in , Section 12, are applicable here.

Emergency calls from citizens to authorities, and call back of such emergency calls by authorities to citizens need assurances that headers intended to get appropriate priority from the networks they traverse, and in some cases, appropriate routing. Protection of the SIP Resource Priority Header and the SIP Priority header is needed for such calls. This document describes the environment for placing emergency calls and call backs which motivate the need and use of the mechanisms described in other documents This document describes Session Initiation Protocol (SIP), an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences. [STANDARDS-TRACK] This document defines two new Session Initiation Protocol (SIP) header fields for communicating resource priority, namely, "Resource-Priority" and "Accept-Resource-Priority". The "Resource-Priority" header field can influence the behavior of SIP user agents (such as telephone gateways and IP telephones) and SIP proxies. It does not directly influence the forwarding behavior of IP routers. [STANDARDS-TRACK] The content of many communication services depends on the context, such as the user's location. We describe a 'service' URN that allows well-known context-dependent services that can be resolved in a distributed manner to be identified. Examples include emergency services, directory assistance, and call-before-you-dig hot lines. [STANDARDS-TRACK] After an emergency call is completed (terminated either prematurely by the emergency caller or normally by the call taker), the call taker may feel the need for further communication. For example, the call may have been dropped by accident without the call taker having sufficient information about the current state of an accident victim. A call taker may trigger a callback to the emergency caller using the contact information provided with the initial emergency call. This callback could, under certain circumstances, be treated like any other call and, as a consequence, it may get blocked by authorization policies or may get forwarded to an answering machine.The IETF emergency services architecture specification already offers a solution approach for allowing Public Safety Answering Point (PSAP) callbacks to bypass authorization policies in order to reach the caller without unnecessary delays. Unfortunately, the specified mechanism only supports limited scenarios. This document discusses shortcomings of the current mechanisms and illustrates additional scenarios where better-than-normal call treatment behavior would be desirable. We describe a solution based on a new header field value for the SIP Priority header field, called "psap-callback", to mark PSAP callbacks. This document creates the new Session Initiation Protocol (SIP) Resource Priority header field namespace 'esnet' and registers this namespace with IANA. The new header field namespace allows for local emergency session establishment to a public safety answering point (PSAP), between PSAPs, and between a PSAP and first responders and their organizations. 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. This 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. In 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. 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. This document extends the Personal Assertion Token (PASSporT) specification defined in RFC 8225 to allow the inclusion of cryptographically signed assertions of authorization for the values populated in the Session Initiation Protocol (SIP) 'Resource-Priority' header field, which is used for communications resource prioritization. In 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. Over 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. As the Internet and the telephone network have become increasingly interconnected and interdependent, attackers can 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. This document analyzes threats in the resulting system, enumerating actors, reviewing the capabilities available to and used by attackers, and describing scenarios in which attacks are launched. Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA).To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry.This is the third edition of this document; it obsoletes RFC 5226. RFC 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.