Network Working Group B. Rosen Request for Comments: 4967 NeuStar Category: Standards Track July 2007 Dial String Parameter for the Session Initiation Protocol Uniform Resource Identifier Status of This Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The IETF Trust (2007).
AbstractRFC 3966 explicitly states that 'tel' URIs may not represent a dial string. That leaves no way specify a dial string in a standardized way. Great confusion exists with the SIP URI parameter "user=phone", and specifically, if it can represent a dial string. This memo creates a new value for the user parameter "dialstring", so that one may specify "user=dialstring" to encode a dial string as a 'sip:' or 'sips:' URI. 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 4 7. Normative References . . . . . . . . . . . . . . . . . . . . . 5
RFC3261] to an intermediary that can transform the dial string to a phone number or a service invocation. The intermediary is able to perform this transform provided that it knows the context (i.e., dialing plan) within which the number was dialed. There is a problem here. The intermediary can apply its transformation only if it recognizes that the user part of the SIP URI is a dial string. However, there is currently no way to distinguish a user part consisting of a dial string from a user part that happens to be composed of characters that would appear in a dial string. Use of DTMF (dual tone multi-frequency) detectors after the initial number has been dialed is not uncommon. A common function some systems have is to express a string that incorporates fixed time delays, or in some cases, an actual "wait for call completion" after which additional DTMF signals are emitted. For example, many voicemail systems use a common phone number, after which the system expects the desired mailbox number as a series of DTMF digits to deposit a message for. Many gateways have the ability to interpret such strings, but there is no standardized way to express them, leading to interoperability problems between endpoints. This is another case where the ability to indicate that a dial string is being presented would be useful. RFC2119]. It is assumed that the reader is familiar with the terminology and acronyms defined in [RFC3261].
RFC3966], MUST be specified. The context parameter would normally be a domain name. The domain name does not have to resolve to any actual host but MUST be under the administrative control of the entity managing the local phone context. The context parameter
value is normally configured in the user agent. The syntax of the context parameter follows the same conventions as the same parameter in [RFC3966], that is, it appears between the digits and the "@" in the userinfo [RFC3261] of the URI: dialstring = dialstring-digits context; context from RFC 3966 dialstring-digits = *dialstring-element dialstring-digit *dialstring-element dialstring-digit = HEXDIG / "*" / "#"; HEXDIG from RFC 3966 dialstring-element = dialstring-digit / "P" / "X" / visual-separator; visual-separator from RFC 3966 A dial string SHOULD NOT be used for an AoR (Address of Record) in a REGISTER. Parameters are ignored in registration. Thus, two registrations with different phone-contexts would be considered equivalent, which is probably not desirable. A proxy server or Back to Back User Agent (B2BUA) [RFC3261], which is authoritative for the context, may translate the dial string to a telephone number or service invocation URI. The telephone number MAY be expressed as a global or local tel: URI, or it MAY be left as a sip: or sips: URI with the URI parameter value changed from "user= " to "user=phone". Examples of dial string use include: ;what a SIP Phone might emit when a user dials extension 123 sip:123;firstname.lastname@example.org;user=dialstring ;existing voicemail systems have a local access extension, ;then expect to see the extension number as DTMF for the mailbox sip:450X123;email@example.com;user=dialstring RFC3969] defines a 'sip:' or 'sips:' URI Parameter sub registry. The "user" parameter is specified as having predefined values. This RFC defines a new value for the "user" parameter, "dialstring". This RFC has been added to the references listed for the "user" parameter.
systems. Dial strings normally SHOULD NOT be sent beyond the domain of the UAC (User Agent Client). If they are sent across the Internet, they SHOULD be protected against eavesdropping with TLS (Transport Layer Security) per the procedures in [RFC3261]. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, 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, June 2002. [RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 3966, December 2004. [RFC3969] Camarillo, G., "The Internet Assigned Number Authority (IANA) Uniform Resource Identifier (URI) Parameter Registry for the Session Initiation Protocol (SIP)", BCP 99, RFC 3969, December 2004.
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