iptel B. Rosen
Internet-Draft Emergicom
Expires: August 15, 2005 February 11, 2005
Dialstring parameter for the sip URI
draft-rosen-iptel-dialstring-01.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
RFC3966 explicitly states that tel uris may not represent a dial
string. That leaves no way specify a dialstring 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 dialstring as a SIP URI.
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Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
2. Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 5
Intellectual Property and Copyright Statements . . . . . . . . 6
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1. Requirements notation
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].
2. Problem
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). One enters a series of digits that invoke some
kind of function. The entered sequence, called a "dialstring" might
be translated to a telephone number, or it may invoke a special
service. In many newer designs, the mapping between a dialstring 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
dialstring to an intermediary that can transform the dialstring to a
phone number or a service invocation.
At some point, a dialstring is translated to a phone number. After
that point, it is no longer a dialstring. However, there is no
current way for any entity to determine if translation has already
been accomplished.
Use of DTMF detectors post dial is not uncommon. A common functions
some systems have is to express a string that incorporates fixed time
delays, or in some cases, 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 DTMF 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.
3. Requirements
A mechanism to express a dialstring is required. A dialstring
consists of a sequence of
* The digits 0-9
* The special characters # and *
* The MF digits A-D
A dialstring always exists within a context. The context MUST be
specified when expressing a dialstring.
It MUST be possible to distinguish between a dialstring and a phone
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number.
It MUST be possible to express a short pause, and a "Wait for call
completion" in a dialstring.
4. Solution
A new value for the user parameter is defined, "dialstring". This
value may be used in a sip or sips URI when the userpart is a
dialstring. The userpart is a sequence of the characters 0-9, A-F, P
and X. E is represent *, F represents #, P is a pause (short wait,
like a comma in a modem string) and X represents call completion.
When the "user=dialstring" is used, a context parameter as defined in
[RFC3966] MUST be specified.
A proxy server or B2BUA which is authoratative for the context may
translate the dialstring to a telephone number or service invocation
URI. If such a translation is performed, the proxy server MUST
change the URI to specify user=phone.
5. Security Considerations
Dialstrings 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. Dialstrings normally should not be sent over the open
Internet without some kind of protection against eavesdropping.
6. 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.
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Author's Address
Brian Rosen
Emergicom
470 Conrad Dr
Mars, PA 16046
US
Phone: +1 724 382 1051
Email: br@brianrosen.net
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