SIP Dial String: Dial String Parameter for the Session Initiation Protocol Uniform Resource Identifier
A new user=dialstring value that lets a SIP URI say its user part is keypad input, not a phone number.
Rosen, B., "Dial String Parameter for the Session Initiation Protocol Uniform Resource Identifier", RFC 4967, DOI 10.17487/RFC4967, July 2007, <https://www.rfc-editor.org/info/rfc4967>.
@misc{rfc4967,
series = {Request for Comments},
number = 4967,
howpublished = {RFC 4967},
publisher = {RFC Editor},
doi = {10.17487/RFC4967},
url = {https://www.rfc-editor.org/info/rfc4967},
author = {B. Rosen},
title = {{Dial String Parameter for the Session Initiation Protocol Uniform Resource Identifier}},
pagetotal = 6,
year = 2007,
month = jul,
} RFC text
Contents
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).
Abstract
RFC 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
7. Normative References . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
A user at a phone often has a limited User Interface, and in some
cases, is limited to a 10 key pad (and sometimes a "flash" function
with the switchhook). The user enters a series of digits that invoke
some kind of function. The entered sequence, called a "dial string",
may be translated to a telephone number, or it may invoke a special
service. In many newer designs, the mapping between a dial string
and a phone number or service URI is contained within the phone
(digitmap). However, there are many phones and terminal adapters
that do not have internal translation mechanisms. Without a
translation mechanism in the phone, the phone must send the dial
string in a 'sip:' or 'sips:' URI [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.
2. Terminology
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 [RFC2119].
It is assumed that the reader is familiar with the terminology and
acronyms defined in [RFC3261].
3. Requirements
A mechanism to express a dial string in a 'sip:' or 'sips:' URI is
required. A dial string consists of a sequence of
* the digits 0-9
* the special characters # and *
* the DTMF digits A-D
* characters representing a short pause, and a "Wait for call
completion" in a dial string
Note: DTMF = dual tone multi-frequency. Each "tone:" is actually two
frequencies superimposed. DTMF is a 4 x 4 matrix with four row
frequencies (697, 770, 852, 941 Hz) and four column frequencies
(1209, 1336, 1477, 1633). Most telephones only implement 3 of the 4
columns, which are used just as the telephone dial pad implies.
Thus, the digit 2 is the first row, second column, and consists of
770Hz and 1209Hz frequencies mixed together. The fourth column is
not used in the PSTN (Public Switched Telephone Network). The
"digits" for the fourth column are usually expressed using the
letters A through D. Thus, "C" is 852/1633Hz. Some systems do use
these digits, so we include them in the definition of the dial
string.
A dial string always exists within a context. The context MUST be
specified when expressing a dial string.
It MUST be possible to distinguish between a dial string and a user
part that happens to consist of the same characters.
4. Solution
A new alternative value for the "userinfo" parameter of the 'sip:' or
'sips:' URI schemes is defined, "dialstring". This value may be used
in a 'sip:' or 'sips:' URI when the user part is a dial string. The
dial string is a sequence of the characters 0-9, A-F, P, X, '*' and
'#'. E represents *, F represents #, P is a pause (short wait, like
a comma in a modem string) and X represents "wait for call
completion".
When the "user=dialstring" is used, a context parameter, as defined
in [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;phone-context=atlanta.example.com@example.com;user=dialstring
;existing voicemail systems have a local access extension,
;then expect to see the extension number as DTMF for the mailbox
sip:450X123;phone-context=biloxi.example.com@example.com;user=dialstring
5. IANA Considerations
[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.
6. Security Considerations
Dial strings exposed to the Internet may reveal information about
internal network details or service invocations that could allow
attackers to use the PSTN or the Internet to attack such internal
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].
7. Normative References
[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.
Author's Address
Brian Rosen
NeuStar
470 Conrad Dr
Mars, PA 16046
USA
Phone: +1 724 382 1051
EMail: br@brianrosen.net
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AI ELI5
On this page
§1The Dial String Problem
A user at a phone often has a limited interface, sometimes only a 10 key pad and a flash function. The digits the user enters are called a dial string. It may be translated to a telephone number, or it may invoke a special service.
Many newer designs hold the mapping from dial string to number or service URI inside the phone (a digitmap). Phones and terminal adapters without such a mechanism must send the dial string in a sip: or sips: URI to an intermediary that knows the dialing plan and can transform it.
The catch is that the intermediary can only do this if it recognizes the user part as a dial string. Today nothing distinguishes a dial string from a user part that merely uses the same characters.
Imagine handing someone a note with just numbers on it. They cannot tell if it is a phone number or the buttons you pressed on a machine. This memo adds a label to the note.
§2DTMF Tones and Voicemail Strings
The RFC also points to DTMF (dual tone multi-frequency) detectors that are used after the first number has been dialed. Some systems express a string that includes fixed time delays, or an actual wait for call completion, after which more DTMF signals are sent.
Many voicemail systems work this way: they use a common phone number and then expect the desired mailbox number as a series of DTMF digits. Many gateways can interpret such strings, but there is no standardized way to write them down, which leads to interoperability problems between endpoints. Being able to say that a dial string is being presented helps here too.
Some phone systems want you to dial a number, wait, and then press more buttons. This gives everyone one agreed way to write that down.
§3Requirements for Expressing a Dial String
The RFC requires a way to express a dial string in a sip: or sips: URI. A dial string is a sequence of the digits 0-9, the special characters pound and star, the DTMF digits A-D, and characters for a short pause and for a wait for call completion.
The A-D digits come from the fourth column of the DTMF 4 x 4 matrix, which is not used in the PSTN (Public Switched Telephone Network). Some systems do use them, so they are included.
Two more requirements apply. A dial string always exists within a context, and the context MUST be specified. It also MUST be possible to tell a dial string from a user part made of the same characters.
A dial string can hold number buttons, star, pound, and a few extra tones, plus a pause. It must always say which phone system it belongs to.
§4The user=dialstring Parameter
The solution is a new alternative value, dialstring, for the user parameter of the sip: and sips: URI schemes. The dial string uses the characters 0-9, A-F, P, X, star and pound. Here E stands for star, F stands for pound, P is a pause (a short wait, like a comma in a modem string) and X means wait for call completion.
When user=dialstring is used, a phone-context parameter as defined in RFC 3966 MUST be given. It would normally be a domain name. That name does not have to resolve to a host, but it MUST be under the administrative control of the entity managing the local phone context. The value is normally configured in the user agent. It sits between the digits and the @ in the userinfo.
A dial string SHOULD NOT be used for an address of record in a REGISTER. Parameters are ignored in registration, so two registrations with different phone contexts would look equivalent, which is probably not desirable.
The label is called user=dialstring. Letters stand in for the special buttons, and a note says whose phone system the numbers belong to.
§5Translation and Examples
A proxy server or Back to Back User Agent that is authoritative for the context may translate the dial string into a telephone number or a service invocation URI. The number MAY be written as a global or local tel: URI, or it MAY stay a sip: or sips: URI with the parameter changed to user=phone.
The RFC gives two examples. The first is what a SIP phone might emit when a user dials extension 123. The second is a voicemail case, where a local access extension is followed by the mailbox number sent as DTMF using X.
A helper at the phone company reads the button presses and turns them into a real phone number or a service.
sip:123;phone-context=atlanta.example.com@example.com;user=dialstring
sip:450X123;phone-context=biloxi.example.com@example.com;user=dialstring§6Security and IANA Considerations
Dial strings exposed to the Internet may reveal internal network details or service invocations. That could let attackers use the PSTN or the Internet to attack internal 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) as described in RFC 3261.
On the IANA side, RFC 3969 defines a SIP URI parameter sub-registry in which the user parameter has predefined values. This RFC adds the value dialstring and is added to the references for that parameter.
Button presses can give away secrets about a company's phone system, so they should stay inside the building or travel in a locked box.