Ericsson

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China Mobile

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Transport SIPCORE Working Group SIP Via transit realm This specification defines a new Session Initiation Protocol (SIP) Via header field parameter, 'received-realm', which allows a SIP entity acting as an entry point to a transit network to indicate from which adjacent upstream network a SIP request is received by using a network realm value associated with the adjacent network.

When Session Initiation Protocol (SIP) sessions are established between networks belonging to different operators or between interconnected networks belonging to the same operator (or enterprise), the SIP requests associated with the session might traverse transit networks. Such transit networks might provide different kinds of services. In order to provide such services, a transit network often needs to know to which operator (or enterprise) the adjacent upstream network from which the SIP session initiation request is received belongs. This specification defines a new SIP Via header field parameter, 'received-realm', which allows a SIP entity acting as an entry point to a transit network to indicate from which adjacent upstream network a SIP request is received by using a network realm value associated with the adjacent network. NOTE: As the adjacent network can be an enterprise network, an Inter Operator Identifier (IOI) cannot be used to identify the network because IOIs are not defined for enterprise networks. The following sections describe use cases where the information is needed.

The Third Generation Partnership Project (3GPP) TS 23.228 specifies how an IP Multimedia Subsystem (IMS) network can be used to provide transit functionality. An operator can use its IMS network to provide transit functionality, e.g., to non-IMS customers, to enterprise networks, and to other network operators. The transit network operator can provide application services to the networks for which it is providing transit functionality. Transit application services are typically not provided on a per user basis, as the transit network does not have access to the user profiles of the networks for which the application services are provided. Instead, the application services are provided per served network. When a SIP entity that provides application services (e.g., an Application Server) within a transit network receives a SIP request, in order to apply the correct services, it needs to know the adjacent upstream network from which the SIP request is received.

A transit network operator normally interconnects to many different operators, including other transit network operators, and provides transit routing of SIP requests received from one operator network towards the destination. The destination can be within an operator network to which the transit network operator has a direct interconnect or within an operator network that only can be reached via one or more interconnected transit operators. For each customer (i.e., interconnected network operator) for which the transit network operator routes SIP requests towards the requested destination, a set of transit routing policies are defined. These policies are used to determine how a SIP request shall be routed towards the requested destination to meet the agreement the transit network operator has with its customer. When a SIP entity that performs the transit routing functionality receives a SIP request, in order to apply the correct set of transit routing policies, it needs to know from which of its customers (i.e., adjacent upstream network) the SIP request is received.

The mechanism defined in this specification MUST only be used by SIP entities that are able to verify from which adjacent upstream network a SIP request is received. The mechanism for verifying from which adjacent upstream network a SIP request is received is outside the scope of this specification. Such a mechanism might be based on, for instance, receiving the SIP request on an authenticated Virtual Private Network (VPN), on a specific IP address, or on a specific network access.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 .

SIP entity: A SIP User Agent (UA), or SIP proxy, as defined in RFC 3261. Adjacent upstream SIP network: The adjacent SIP network in the direction from which a SIP request is received. Network entry point: A SIP entity on the border of the network, which receives SIP requests from adjacent upstream networks. Inter Operator Identifier (IOI): A globally unique identifier to correlate billing information generated within the IP Multimedia Subsystem (IMS). JWS: A JSON Web Signature, as defined in .

The Via 'received-realm' header field parameter value is represented as a combination of an operator identifier whose value represents the adjacent network and a serialized JSON Web Signature (JWS) . The JWS Payload consists of the operator identifier and other SIP information element values. The procedures for encoding the JWS and calculating the signature are defined in . As the JWS Payload information is found in other SIP information elements, the JWS Payload is detached from the serialized JWS conveyed in the header field parameter, as described in Appendix F of . The operator identifier and the serialized JWS are separated using a colon character.

The operator identifier is a token value that represents the adjacent operator. The scope of the value is only within the network that inserts the value. The operator identifier value is case insensitive.

The following header parameters MUST be included in the JWS. The "typ" parameter MUST have a "JWT" value. The "alg" parameter MUST have the value of the algorithm used to calculate the JWS. NOTE: Operators need to agree on the set of supported algorithms for calculating the JWT signature. NOTE: The "alg" parameter values for specific algorithms are listed in the IANA JSON Web Signature and Encryption Algorithms sub-registry of the JSON Object Signing and Encryption (JOSE) registry. Operators need to use algorithms for which an associated "alg" parameter value has been registered. The procedures for defining new values are defined in .

The following claims MUST be included in the JWS Payload: The "sip_from_tag" claim has the value of the From 'tag' header field parameter of the SIP message. The "sip_date" claim has the value of the Date header field in the SIP message, encoded in JSON NumericDate format . The "sip_callid" claim has the value of the Call-ID header field in the SIP message. The "sip_cseq_num" claim has the numeric value of the CSeq header field in the SIP message. The "sip_via_branch" claim has the value of the Via branch header field parameter of the Via header field, in the SIP message, to which the 'received-realm' header field parameter is attached. The "sip_via_opid" claim has the value of the operator identifier part of the Via 'received-realm' header field parameter of the Via header field, in the SIP message, to which the 'received-realm' header field parameter is attached.

As the JWS Payload is not carried in the 'received-realm' parameter, in order to make sure that the sender and the receiver construct the JWS Payload object in the same way, the JSON representation of the JWS Payload object MUST be computed as follows: All claims MUST be encoded using lowercase characters. The claims MUST be in the same order as listed in . All claims except "sip_date" MUST be encoded as StringOrURI JSON string value . The "sip_date" claim MUST be encoded as a JSON NumericDate value . The JWS Payload MUST follow the rules for the construction of the thumbprint of a JSON Web Key (JWK) as defined in Section 3, Step 1 only, of .

NOTE: Line breaks are for display purposes only.

This section describes the syntax extensions to the ABNF syntax defined in by defining a new Via header field parameter, 'received-realm'. The ABNF defined in this specification is conformant to RFC 5234 . "EQUAL", "LDQUOT", "RDQUOT", and "ALPHA" are defined in . "DIGIT" is defined in .

EQUAL, COLON, token, LDQUOT, RDQUOT, ALPHA, and DIGIT are as defined in . NOTE: The two adjacent dots in the 'jws' part are due to the detached payload being replaced by an empty string .

NOTE: Line breaks are for display purposes only.

This section describes how a SIP entity, acting as an entry point to a network, uses the 'received-realm' Via header field parameter.

When a SIP entity acting as a network entry point forwards a SIP request or initiates a SIP request on its own (e.g., a Public Switched Telephone Network (PSTN) gateway), the SIP entity adds a Via header field to the SIP request, according to the procedures in RFC 3261 . In addition, if the SIP entity is able to assert the adjacent upstream network and if the SIP entity is aware of a network realm value defined for that network, the SIP entity can add a 'received-realm' Via header field parameter conveying the network realm value as the operator identifier () part of the header field parameter, to the Via header field added to the SIP request. In addition, the SIP entity MUST also calculate a JWS () and add the calculated JWS Header and JWS Signature as the 'jws' part of the Via header field parameter.

When a SIP entity receives a Via 'received-realm' header field parameter and intends to perform actions based on the header field parameter value, it MUST first recalculate the JWS and check whether the result matches the JWS received. If there is not a match, the SIP entity MUST discard the received 'received-realm' header field parameter. The SIP entity MAY also take additional actions (e.g., rejecting the SIP request) based on local policy.

This section shows an example of a SIP INVITE request and the associated response, which contains a Via header field (inserted into the request and removed from the response by the T_EP SIP proxy) with a 'received-realm' header field parameter.

Via: SIP/2.0/UDP IP_UA -- INVITE ----------------------------> Via: SIP/2.0/UDP IP_TEP;branch=z9hG4bK776; received-realm="myoperator:eyJ0eXAiOiJKV1QiLA0KICJh bGciOiJIUzI1N..dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW 1gFWFOEjXk" Via: SIP/2.0/UDP IP_UA; received=IP_UA <- 200 OK ---------------------------- Via: SIP/2.0/UDP IP_TEP;branch=z9hG4bK776; received-realm="myoperator:eyJ0eXAiOiJKV1QiLA0KICJh bGciOiJIUzI1N..dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW 1gFWFOEjXk" Via: SIP/2.0/UDP IP_UA; received=IP_UA <- 200 OK------ Via: SIP/2.0/UDP IP_UA; received=IP_UA ]]>

This specification defines a new Via header field parameter called 'received-realm' in the "Header Field Parameters and Parameter Values" sub-registry created by . The syntax is defined in . The required information is:

This specification defines new JSON Web Token claims in the "JSON Web Token Claims" sub-registry created by . Claim Name: sip_from_tag Claim Description: SIP From tag header field parameter value Change Controller: IESG Reference: RFC 8055, RFC 3261 Claim Name: sip_date Claim Description: SIP Date header field value Change Controller: IESG Reference: RFC 8055, RFC 3261 Claim Name: sip_callid Claim Description: SIP Call-Id header field value Change Controller: IESG Reference: RFC 8055, RFC 3261 Claim Name: sip_cseq_num Claim Description: SIP CSeq numeric header field parameter value Change Controller: IESG Reference: RFC 8055, RFC 3261 Claim Name: sip_via_branch Claim Description: SIP Via branch header field parameter value Change Controller: IESG Reference: RFC 8055, RFC 3261

As the 'received-realm' Via header field parameter can be used to trigger applications, it is important to ensure that the parameter has not been added to the SIP message by an unauthorized SIP entity. The 'received-realm' Via header field parameter is inserted, signed, verified, and consumed within an operator network. The operator MUST discard parameters received from another network, and the parameter MUST only be inserted by SIP entities that are able to verify from which adjacent upstream network a SIP request is received. The operator also needs to take great care in ensuring that the key used to calculate the JWS Signature value is only known by the network entities signing and adding the JWS Signature to the 'received-realm' Via header field parameter of a SIP message and to network entities verifying and consuming the parameter value. The operator MUST use a key management policy that protects against unauthorized access to the stored keys within nodes where the keys associated with the JWS Signature are stored and that protects against cryptoanalysis attacks using captured data sent on the wire. A SIP entity MUST NOT use the adjacent network information if there is a mismatch between the JWS Signature received in the SIP header field and the JWS Signature calculated by the receiving entity. Generic security considerations for JWS are defined in .

3GPP

Thanks to Adam Roach and Richard Barnes for providing comments and feedback on the document. Francis Dupoint performed the Gen-ART review.