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rfc:rfc5724

Internet Engineering Task Force (IETF) E. Wilde Request for Comments: 5724 UC Berkeley Category: Standards Track A. Vaha-Sipila ISSN: 2070-1721 Nokia

                                                          January 2010
   URI Scheme for Global System for Mobile Communications (GSM)
                    Short Message Service (SMS)

Abstract

 This memo specifies the Uniform Resource Identifier (URI) scheme
 "sms" for specifying one or more recipients for an SMS message.  SMS
 messages are two-way paging messages that can be sent from and
 received by a mobile phone or a suitably equipped networked device.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc5724.

Copyright Notice

 Copyright (c) 2010 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Wilde & Vaha-Sipila Standards Track [Page 1] RFC 5724 sms" URI Scheme January 2010

Table of Contents

 1. Introduction ....................................................3
    1.1. What is GSM? ...............................................3
    1.2. What is SMS? ...............................................3
         1.2.1. SMS Content .........................................3
         1.2.2. SMS Infrastructure ..................................4
                1.2.2.1. SMS Centers ................................4
         1.2.3. Uniform Resource Identifiers ........................4
         1.2.4. SMS Messages and the Internet .......................5
                1.2.4.1. SMS Messages and the Web ...................6
                1.2.4.2. SMS Messages and Forms .....................6
    1.3. Requirements Language ......................................6
 2. The "sms" URI Scheme ............................................7
    2.1. Applicability ..............................................7
    2.2. Formal Definition ..........................................7
    2.3. Processing an "sms" URI ....................................9
    2.4. Comparing "sms" URIs .......................................9
    2.5. Examples of Use ...........................................10
    2.6. Using "sms" URIs in HTML Forms ............................10
 3. URI Scheme Registration ........................................11
    3.1. URI Scheme Name ...........................................11
    3.2. Status ....................................................11
    3.3. URI Scheme Syntax .........................................11
    3.4. URI Scheme Semantics ......................................11
    3.5. Encoding Considerations ...................................12
    3.6. Applications/Protocols That Use This URI Scheme Name ......12
    3.7. Interoperability Considerations ...........................12
    3.8. Security Considerations ...................................12
    3.9. Contact ...................................................12
 4. Security Considerations ........................................12
 5. IANA Considerations ............................................14
 6. Acknowledgements ...............................................14
 7. References .....................................................14
    7.1. Normative References ......................................14
    7.2. Informative References ....................................15
 Appendix A.  Syntax of telephone-subscriber .......................17

Wilde & Vaha-Sipila Standards Track [Page 2] RFC 5724 sms" URI Scheme January 2010

1. Introduction

1.1. What is GSM?

 GSM (Global System for Mobile Communications) is a digital mobile
 phone standard that is used extensively in many parts of the world.
 First named after its frequency band around 900 MHz, GSM-900 has
 provided the basis for several other networks utilizing GSM
 technology, in particular, GSM networks operating in the frequency
 bands around 1800 MHz and 1900 MHz.  When referring to "GSM" in this
 document, we mean any of these GSM-based networks that operate a
 short message service.

1.2. What is SMS?

 The Short Message Service (SMS) [SMS] is an integral part of the GSM
 network technology.  It has been very successful and currently is a
 major source of revenue for many GSM operators.  SMS as a service is
 so successful that other Global Switched Telephone Network (GSTN)
 technologies have adopted it as well, in particular, the Integrated
 Services Digital Network (ISDN).  Because of this development, this
 memo uses the term "SMS client" to refer to user agents that are able
 to send and/or receive SMS messages.

1.2.1. SMS Content

 GSM SMS messages are alphanumeric paging messages that can be sent to
 and from SMS clients.  SMS messages have a maximum length of 160
 characters (7-bit characters from the GSM character set [SMS-CHAR]),
 or 140 octets.  Other character sets (such as UCS-2 16-bit
 characters, resulting in 70-character messages) MAY also be supported
 [SMS-CHAR], but are defined as being optional by the SMS
 specification.  Consequently, applications handling SMS messages as
 part of a chain of character-processing applications MUST make sure
 that character sets are correctly mapped to and from the character
 set used for SMS messages.
 While the 160-character content type for SMS messages is by far the
 one most widely used, there are numerous other content types for SMS
 messages, such as small bitmaps ("operator logos") and simple formats
 for musical notes ("ring tones").  However, these formats are
 proprietary and are not considered in this memo.
 SMS messages are limited in length (140 octets), and the first
 versions of the SMS specification did not specify any standardized
 methods for concatenating SMS messages.  As a consequence, several
 proprietary methods were invented, but the current SMS specification
 does specify message concatenation.  In order to deal with this

Wilde & Vaha-Sipila Standards Track [Page 3] RFC 5724 sms" URI Scheme January 2010

 situation, SMS clients composing messages SHOULD use the standard
 concatenation method based on the header in the TP-User Data field as
 specified in the SMS specification [SMS].  When sending a message to
 an SMS recipient whose support for concatenated messages is unknown,
 the SMS client MAY opt to use the backwards-compatible (text-based)
 concatenation method defined in the SMS specification [SMS].
 Proprietary concatenation methods SHOULD NOT be used except in closed
 systems, where the capabilities of the recipient(s) are always known.

1.2.2. SMS Infrastructure

 SMS messages can be transmitted over an SMS client's network
 interface using the signaling channels of the underlying GSTN
 infrastructure, so there is no delay for call setup.  Alternatively,
 SMS messages may be submitted through other front-ends (for example,
 Web-based services), which makes it possible for SMS clients to run
 on computers that are not directly connected to a GSTN network
 supporting SMS.
 SMS messages sent with the GSTN SMS service MUST be sent as class 1
 SMS messages, if the client is able to specify the message class.

1.2.2.1. SMS Centers

 For delivery within GSTN networks, SMS messages are stored by an
 entity called SMS Center (SMSC) and sent to the recipient when the
 subscriber connects to the network.  The number of a cooperative SMSC
 must be known to the SMS sender (i.e., the entity submitting the SMS
 message to a GSTN infrastructure) when sending the message (usually
 the SMSC's number is configured in the SMS client and specific for
 the network operator to which the sender has subscribed).  In most
 situations, the SMSC number is part of the sending SMS client's
 configuration.  However, in some special cases (such as when the SMS
 recipient only accepts messages from a certain SMSC), it may be
 necessary to send the SMS message over a specific SMSC.  The scheme
 specified in this memo does not support the specification of SMSC
 numbers, so in case of scenarios where messages have to be sent
 through a certain SMSC, there must be some other context establishing
 this requirement or message delivery may fail.

1.2.3. Uniform Resource Identifiers

 One of the core specifications for identifying resources on the
 Internet is [RFC3986], specifying the syntax and semantics of a
 Uniform Resource Identifier (URI).  The most important notion of URIs
 are "schemes", which define a framework within which resources can be
 uniquely identified and addressed.  URIs enable users to access
 resources and are used for very diverse schemes, such as access

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 protocols (HTTP, FTP), broadcast media (TV channels [RFC2838]),
 messaging (email [RFC2368]), and even telephone numbers (voice
 [RFC3966]).
 URIs often are mentioned together with Uniform Resource Names (URNs)
 and/or Uniform Resource Locators (URLs), and it often is unclear how
 to separate these concepts.  For the purpose of this memo, only the
 term URI will be used, referring to the most fundamental concept.
 The World Wide Web Consortium (W3C) has issued a note
 [uri-clarification] discussing the topic of URIs, URNs, and URLs in
 detail.

1.2.4. SMS Messages and the Internet

 One of the important reasons for the universal access of the Web is
 the ability to access all information through a unique interface.
 This kind of integration makes it easy to provide information as well
 as to consume it.  One aspect of this integration is the support of
 user agents (in the case of the Web, commonly referred to as
 browsers) for multiple content formats (such as HTML, GIF, JPEG) and
 access schemes (such as HTTP, HTTPS, FTP).
 The "mailto" scheme has proven to be very useful and popular because
 most user agents support it by providing an email composition
 facility when the user selects (e.g., clicks on) the URI.  Similarly,
 the "sms" scheme can be supported by user agents by providing an SMS
 message composition facility when the user selects the URI.  In cases
 where the user agent does not provide a built-in SMS message
 composition facility, the scheme could still be supported by opening
 a Web page that provides such a service.  The specific Web page to be
 used could be configured by the user, so that each user could use the
 SMS message composition service of his choice.
 The goal of this memo is to specify the "sms" URI scheme so that user
 agents (such as Web browsers and email clients) can start to support
 it.  The "sms" URI scheme identifies SMS message endpoints as
 resources.  When "sms" URIs are dereferenced, implementations MAY
 create a message and present it to be edited before being sent, or
 they MAY invoke additional services to provide the functionality
 necessary for composing a message and sending it to the SMS message
 endpoint.  In either case, simply activating a link with an "sms" URI
 SHOULD NOT cause a message to be sent without prior user
 confirmation.

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1.2.4.1. SMS Messages and the Web

 SMS messages can provide an alternative to "mailto" URIs [RFC2368],
 or "tel" or "fax" URIs [RFC3966].  When an "sms" URI is activated,
 the user agent MAY start a program for sending an SMS message, just
 as "mailto" may open a mail client.  Unfortunately, most browsers do
 not support the external handling of internally unsupported URI
 schemes in the same generalized way as most of them support external
 handling of content for media types that they do not support
 internally.  Ideally, user agents should implement generic URI
 parsers and provide a way to associate unsupported schemes with
 external applications (or Web-based services).
 The recipient of an SMS message need not be a mobile phone.  It can
 be a server that can process SMS messages, either by gatewaying them
 to another messaging system (such as regular electronic mail), or by
 parsing them for supplementary services.
 SMS messages can be used to transport almost any kind of data (even
 though there is a very tight size limit), but the only standardized
 data formats are character-based messages in different character
 encodings.  SMS messages have a maximum length of 160 characters
 (when using 7-bit characters from the SMS character set), or 140
 octets.  However, SMS messages can be concatenated to form longer
 messages.  It is up to the user agent to decide whether to limit the
 length of the message, and how to indicate this limit in its user
 interface if necessary.  There is one exception to this; see
 Section 2.6.

1.2.4.2. SMS Messages and Forms

 The Hypertext Markup Language (HTML) [HTML401] provides a way to
 collect information from a user and pass it to a server for
 processing.  This functionality is known as "HTML forms".  A
 filled-in form is usually sent to the destination using the Hypertext
 Transfer Protocol (HTTP) or email.  However, SMS messages can also be
 used as the transport mechanism for these forms.  Depending on the
 network configuration, the sender's telephone number may be included
 in the SMS message, thus providing a weak form of authentication.

1.3. Requirements Language

 The capitalized 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].

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2. The "sms" URI Scheme

 Syntax definitions are given using the Augmented BNF (ABNF) for
 syntax specifications [RFC5234].

2.1. Applicability

 This URI scheme provides information that can be used for sending SMS
 message(s) to specified recipient(s).  The functionality is
 comparable to that of the "mailto" URI, which (as per [RFC2368]) can
 also be used with a comma-separated list of email addresses.
 The notation for phone numbers is taken from [RFC3966] and its
 Erratum 203 [Err203].  Appendix A provides a corrected syntax of the
 telephone number.  Refer to that document for information on why this
 particular format was chosen.
 How SMS messages are sent to the SMSC or other intermediaries is
 outside the scope of this specification.  SMS messages can be sent
 over the GSM air interface by using a modem and a suitable protocol
 or by accessing services over other protocols, such as a Web-based
 service for sending SMS messages.  Also, SMS message service options
 like deferred delivery and delivery notification requests are not
 within the scope of this document.  Such services MAY be requested
 from the network by the user agent if necessary.
 SMS messages sent as a result of this URI MUST be sent as class 1 SMS
 messages, if the user agent is able to specify the message class.

2.2. Formal Definition

 The URI scheme's keywords specified in the following syntax
 description are case-insensitive.  The syntax of an "sms" URI is
 formally described as follows, where the URI base syntax is taken
 from [RFC3986]:
sms-uri        = scheme ":" sms-hier-part [ "?" sms-fields ]
scheme         = "sms"
sms-hier-part  = sms-recipient *( "," sms-recipient )
sms-recipient  = telephone-subscriber ; defined in RFC 3966
sms-fields     = sms-field *( "&" sms-field )
sms-field      = sms-field-name "=" escaped-value
sms-field-name = "body" / sms-field-ext ; "body" MUST only appear once
sms-field-ext  = 1*( unreserved )
escaped-value  = *( unreserved / pct-encoded ) ; defined in RFC 3986

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 Some illustrative examples using this syntax are given in
 Section 2.5.
 The syntax definition for <telephone-subscriber> is taken from
 Section 5.1 of [RFC3966].  Please consider Erratum 203 [Err203] in
 that specification.  For the reader's convenience, Appendix A
 contains a fixed syntax of the telephone number URI scheme, including
 Erratum 203, but RFC 3966 (plus all applicable errata) is the
 normative reference.  The description of phone numbers in RFC 3966
 (Section 5.1) states: "The 'telephone-subscriber' part of the URI
 indicates the number.  The phone number can be represented in either
 global (E.164) or local notation.  All phone numbers MUST use the
 global form unless they cannot be represented as such.  Numbers from
 private numbering plans, emergency ("911", "112"), and some
 directory-assistance numbers (e.g., "411") and other "service codes"
 (numbers of the form N11 in the United States) cannot be represented
 in global (E.164) form and need to be represented as a local number
 with a context.  Local numbers MUST be tagged with a 'phone-
 context'."
 This specification defines a single <sms-field>: "body".  Extensions
 to this specification MAY define additional fields.  Extensions MUST
 NOT change the semantics of the specifications they are extending.
 Unknown fields encountered in "sms" URIs MUST be ignored by
 implementations.
 The "body" <sms-field> is used to define the body of the SMS message
 to be composed.  It MUST not appear more than once in an "sms" URI.
 It consists of percent-encoded UTF-8 characters.  Implementations
 MUST make sure that the "body" <sms-field> characters are converted
 to a suitable character encoding before sending, the most popular
 being the 7-bit SMS character encoding, another variant (though not
 as universally supported as 7-bit SMS) is the UCS-2 character
 encoding (both specified in [SMS-CHAR]).  Implementations MAY choose
 to discard (or convert) characters in the <sms-body> that are not
 supported by the SMS character set they are using to send the SMS
 message.  If they do discard or convert characters, they MUST notify
 the user.
 The syntax definition for <escaped-value> refers to the text of an
 SMS where all <reserved> (as per [RFC3986]) characters in the SMS
 text are percent-encoded, please refer to [RFC3986] for the
 definitions of <unreserved> and <pct-encoded> and for details about
 percent-encoding.
 User agents SHOULD support multiple recipients and SHOULD make it
 clear to users what the entire list of recipients is before
 committing the user to sending all the messages.

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2.3. Processing an "sms" URI

 The following list describes the steps for processing an "sms" URI:
 1.  The phone number of the first <sms-recipient> is extracted.  It
     is the phone number of the final recipient and it MUST be written
     in international form with country code, unless the number only
     works from inside a certain geographical area or a network.  Note
     that some numbers may work from several networks but not from the
     whole world -- these SHOULD be written in international form.
     According to [RFC3966], all international numbers MUST begin with
     a "+" character.  Hyphens, dots, and parentheses (referred to as
     "visual separators" in RFC 3966) are used only to improve
     readability and MUST NOT convey any other meaning.
 2.  The "body" <sms-field> is extracted, if present.
 3.  The user agent SHOULD provide some means for message composition,
     either by implementing this itself or by accessing a service that
     provides it.  Message composition SHOULD start with the body
     extracted from the "body" <sms-field>, if present.
 4.  After message composition, a user agent SHOULD try to send the
     message first using the default delivery method employed by that
     user agent.  If that fails, the user agent MAY try another
     delivery method.
 5.  If the URI contains a comma-separated list of recipients (i.e.,
     it contains multiple <sms-recipient> parts), all of them are
     processed in this manner.  Exactly the same message SHOULD be
     sent to all of the listed recipients, which means that the
     message resulting from the message composition step for the first
     recipient is used unaltered for all other recipients as well.

2.4. Comparing "sms" URIs

 Two "sms" URIs are equivalent according to the following rules.
 Since the definition of the <telephone-subscriber> is taken from
 [RFC3966], equivalence of individual values of <telephone-subscriber>
 is based on the rules defined in Section 4 of [RFC3966], repeated
 here for convenience:
 o  Both must be either a <local-number> or a <global-number<, i.e.,
    start with a "+".
 o  The <global-number-digits> and the <local-number-digits> must be
    equal, after removing all visual separators.

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 o  For mandatory additional parameters and the <phone-context> and
    <extension> parameters defined in [RFC3966], the <phone-context>
    parameter value is compared either as a host name if it is a
    <domainname> or digit-by-digit if it is <global-number-digits>.
    The latter is compared after removing all <visual-separator>
    characters.
 o  Parameters are compared according to <pname>, regardless of the
    order they appeared in the URI.  If one URI has a parameter name
    not found in the other, the two URIs are not equal.
 o  URI comparisons are case-insensitive.
 Since "sms" URIs can contain multiple <telephone-subscriber>s as well
 as <sms-fields>, in addition to adopting the rules defined for
 comparing <telephone-subscriber>s as defined by [RFC3966], two "sms"
 URIs are only equivalent if their <sms-fields> are identical, and if
 all <telephone-subscriber>s, compared pairwise as a set (i.e.,
 without taking sequence into consideration), are equivalent.

2.5. Examples of Use

 sms:+15105550101
 This indicates an SMS-message-capable recipient at the given
 telephone number.  The message is sent using the user agent's default
 SMS delivery method.
 sms:+15105550101,+15105550102
 This indicates SMS-message-capable recipients at the given telephone
 numbers.  The identical message should be sent to both recipients
 using the user agent's default SMS delivery method.
 sms:+15105550101?body=hello%20there
 In this case, a message (initially being set to "hello there", which
 may be modified by the user before sending) will be sent via SMS
 using the user agent's default SMS delivery method.

2.6. Using "sms" URIs in HTML Forms

 When using an "sms" type URI as an action URI for HTML form
 submission [HTML401], the form contents MUST be packaged in the SMS
 message just as they are packaged when using a "mailto" URI
 [RFC2368], using the "application/x-www-form-urlencoded" media type
 (as defined by HTML [HTML401]), effectively packaging all form data
 into URI-compliant syntax [RFC3986].  The SMS message MUST NOT

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 contain any HTTP header fields, only the form data.  The media type
 is implicit.  It MUST NOT be transferred in the SMS message.  Since
 the SMS message contains the form field values, the body <sms-field>
 of an "sms" type URI used for an HTML form will be ignored.
 The character encoding used for form submissions MUST be UTF-8
 [RFC3629].  It should be noted, however, that user agents MUST
 percent-encode form submissions before sending them (this encoding is
 specified by the URI syntax [RFC3986]).
 The user agent SHOULD inform the user about the possible security
 hazards involved when submitting the form (it is probably being sent
 as plain text over an air interface).
 If the form submission is longer than the maximum SMS message size,
 the user agent MAY either concatenate SMS messages, if it is able to
 do so, or it MAY refuse to send the message.  The user agent MUST NOT
 send out partial form submissions.

3. URI Scheme Registration

 This memo requests the registration of the Uniform Resource
 Identifier (URI) scheme "sms" for specifying one or more recipients
 for an SMS message.  The registration request complies with
 [RFC4395].

3.1. URI Scheme Name

 sms

3.2. Status

 Permanent

3.3. URI Scheme Syntax

 See Section 2.

3.4. URI Scheme Semantics

 The "sms" URI scheme defines a way for a message to be composed and
 then transmitted using the SMS message transmission method.  This
 scheme can thus be compared to be "mailto" URI scheme [RFC2368].  See
 Section 2.3 for the details of operation.

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3.5. Encoding Considerations

 The optional body field of "sms" URIs may contain a message text, but
 this text uses percent-encoded UTF-8 characters and thus can always
 be represented using URI characters.  See Section 2 for the details
 of encoding.

3.6. Applications/Protocols That Use This URI Scheme Name

 The "sms" URI scheme is intended to be used in a similar way as the
 "mailto" URI scheme [RFC2368].  By using "sms" URIs, authors can
 embed information into documents that can be used as a starting point
 for initiating message composition.  Whether the client is sending
 the message itself (for example, over a GSM air interface) or
 redirecting the user to a third party for message composition (such
 as a Web service for sending SMS messages) is outside of the scope of
 the URI scheme definition.

3.7. Interoperability Considerations

 No interoperability issues have been identified.

3.8. Security Considerations

 See Section 4.

3.9. Contact

 Erik Wilde
 School of Information
 UC Berkeley
 Berkeley, CA 94720-4600
 U.S.A.
 tel:+1-510-6432252
 mailto:dret@berkeley.edu

4. Security Considerations

 SMS messages are transported without any provisions for privacy or
 integrity, so SMS users should be aware of these inherent security
 problems of SMS messages.  Unlike electronic mail, where additional
 mechanisms exist to layer security features on top of the basic
 infrastructure, there currently is no such framework for SMS
 messages.
 SMS messages very often are delivered almost instantaneously (if the
 receiving SMS client is online), but there is no guarantee for when
 SMS messages will be delivered.  In particular, SMS messages between

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 different network operators sometimes take a long time to be
 delivered (hours or even days) or are not delivered at all, so
 applications SHOULD NOT make any assumptions about the reliability
 and performance of SMS message transmission.
 In most networks, sending SMS messages is not a free service.
 Therefore, SMS clients MUST make sure that any action that incurs
 costs is acknowledged by the end user, unless explicitly instructed
 otherwise by the end user.  If an SMS client has different ways of
 submitting an SMS message (such as a Web service and a phone line),
 then the end user MUST have a way to control which way is chosen.
 SMS clients often are limited devices (typically mobile phones), and
 the sending SMS client SHOULD NOT make any assumptions about the
 receiving SMS client supporting any non-standard services, such as
 proprietary message concatenation or proprietary content types.
 However, if the sending SMS client has prior knowledge about the
 receiving SMS client, then he MAY use this knowledge to compose non-
 standard SMS messages.
 There are certain special SMS messages defined in the SMS
 specification [SMS] that can be used, for example, to turn on
 indicators on the phone display or to send data to certain
 communication ports (comparable to UDP ports) on the device.  Certain
 proprietary systems (for example, the Wireless Application Protocol
 [WAP]) define configuration messages that may be used to reconfigure
 the devices remotely.  Any SMS client SHOULD make sure that malicious
 use of such messages is not possible, for example, by filtering out
 certain SMS User Data header fields.  Gateways that accept SMS
 messages (e.g., in email messages or Web forms) and pass them on to
 an SMSC SHOULD implement this kind of "firewalling" approach as well.
 Because of the narrow bandwidth of the SMS communications channel,
 there should also be checks in place for excessively long
 concatenated messages.  As an example, it may take two minutes to
 transfer thirty concatenated text messages.
 Unchecked input from a user MUST NOT be used to populate any other
 fields in an SMS message other than the User Data field (not
 including the User Data header field).  All other parts, including
 the User Data header, of the short message should only be generated
 by trusted means.
 By including "sms" URIs in unsolicited messages (a.k.a. "spam") or
 other types of advertising, the originator of the "sms" URIs may
 attempt to reveal an individual's phone number and/or to link the
 identity (i.e., email address) used for messaging with the identity
 (i.e., phone number) used for the mobile phone.  This attempt to

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 collect information may be a privacy issue, and user agents may make
 users aware of that risk before composing or sending SMS messages.
 Users agents that do not provide any feedback about this privacy
 issue make users more vulnerable to this kind of attack.
 A user agent SHOULD NOT send out SMS messages without the knowledge
 of the user because of associated risks, which include sending masses
 of SMS messages to a subscriber without his consent, and the costs
 involved in sending an SMS message.
 As suggested functionality, the user agent MAY offer a possibility
 for the user to filter out those phone numbers that are expressed in
 local format, as most premium-rate numbers are expressed in local
 format, and because determining the correct local context (and hence
 the validity of the number to this specific user) may be very
 difficult.
 When using "sms" URIs as targets of forms (as described in
 Section 2.6), the user agent SHOULD inform the user about the
 possible security hazards involved when submitting the form (it is
 probably being sent as plain text over an air interface).

5. IANA Considerations

 IANA has registered the "sms" URI scheme, using the template in
 Section 3, in accordance with [RFC4395].

6. Acknowledgements

 This document has been prepared using the IETF document DTD described
 in [RFC2629].
 Thanks to (listed alphabetically) Claudio Allocchio, Derek Atkins,
 Nevil Brownlee, John Cowan, Leslie Daigle, Lisa Dusseault, Miguel
 Garcia, Vijay Gurbani, Alfred Hoenes, Cullen Jennings, Graham Klyne,
 Larry Masinter, Alexey Melnikov, Michael Patton, Robert Sparks, and
 Magnus Westerlund for their comments.

7. References

7.1. Normative References

 [Err203]   RFC Errata, "Errata ID 203", RFC 3629,
            <http://www.rfc-editor.org>.
 [HTML401]  Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01
            Specification", W3C REC-html401, December 1999,
            <http://www.w3.org/TR/html401/>.

Wilde & Vaha-Sipila Standards Track [Page 14] RFC 5724 sms" URI Scheme January 2010

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
            10646", STD 63, RFC 3629, November 2003.
 [RFC3966]  Schulzrinne, H., "The tel URI for Telephone Numbers",
            RFC 3966, December 2004.
 [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
            Resource Identifier (URI): Generic Syntax", STD 66,
            RFC 3986, January 2005.
 [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
            Registration Procedures for New URI Schemes", BCP 35,
            RFC 4395, February 2006.
 [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [SMS]      European Telecommunications Standards Institute, "3GPP TS
            23.040 V7.0.1 (2007-03): 3rd Generation Partnership
            Project; Technical Specification Group Core Network and
            Terminals; Technical realization of the Short Message
            Service (SMS) (Release 7)", March 2007, <http://
            www.3gpp.org/ftp/Specs/archive/23_series/23.040/
            23040-701.zip>.
 [SMS-CHAR]
            European Telecommunications Standards Institute, "TS 100
            900 (GSM 03.38 version 7.2.0 Release 1998): Digital
            Cellular Telecommunications System (Phase 2+); Alphabets
            and language-specific information", July 1999, <http://
            www.3gpp.org/ftp/Specs/archive/03_series/03.38/
            0338-720.zip>.

7.2. Informative References

 [RFC2368]  Hoffmann, P., Masinter, L., and J. Zawinski, "The mailto
            URL scheme", RFC 2368, June 1998.
 [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
            June 1999.
 [RFC2838]  Zigmond, D. and M. Vickers, "Uniform Resource Identifiers
            for Television Broadcasts", RFC 2838, May 2000.

Wilde & Vaha-Sipila Standards Track [Page 15] RFC 5724 sms" URI Scheme January 2010

 [WAP]      WAP Forum, "Wireless Application Protocol - Architecture
            Specification (WAP-210-WAPArch-20010712)", July 2001.
 [uri-clarification]
            World Wide Web Consortium, "URIs, URLs, and URNs:
            Clarifications and Recommendations 1.0", W3C uri-
            clarification , September 2001,
            <http://www.w3.org/TR/uri-clarification/>.

Wilde & Vaha-Sipila Standards Track [Page 16] RFC 5724 sms" URI Scheme January 2010

Appendix A. Syntax of 'telephone-subscriber'

 The following syntax is reproduced from Section 3 of [RFC3966].  It
 defines the <telephone-subscriber> part used in the "sms" URI scheme
 syntax.  Please note that it includes Erratum 203 [Err203] for RFC
 3966, which changes the definition of <isdn-subaddress>.
 telephone-subscriber = global-number / local-number
 global-number        = global-number-digits *par
 local-number         = local-number-digits *par context *par
 par                  = parameter / extension / isdn-subaddress
 isdn-subaddress      = ";isub=" 1*paramchar
 extension            = ";ext=" 1*phonedigit
 context              = ";phone-context=" descriptor
 descriptor           = domainname / global-number-digits
 global-number-digits = "+" *phonedigit DIGIT *phonedigit
 local-number-digits  =
    *phonedigit-hex (HEXDIG / "*" / "#")*phonedigit-hex
 domainname           = *( domainlabel "." ) toplabel [ "." ]
 domainlabel          = alphanum
                        / alphanum *( alphanum / "-" ) alphanum
 toplabel             = ALPHA / ALPHA *( alphanum / "-" ) alphanum
 parameter            = ";" pname ["=" pvalue ]
 pname                = 1*( alphanum / "-" )
 pvalue               = 1*paramchar
 paramchar            = param-unreserved / unreserved / pct-encoded
 unreserved           = alphanum / mark
 mark                 = "-" / "_" / "." / "!" / "~" / "*" /
                        "'" / "(" / ")"
 pct-encoded          = "%" HEXDIG HEXDIG
 param-unreserved     = "[" / "]" / "/" / ":" / "&" / "+" / "$"
 phonedigit           = DIGIT / [ visual-separator ]
 phonedigit-hex       = HEXDIG / "*" / "#" / [ visual-separator ]
 visual-separator     = "-" / "." / "(" / ")"
 alphanum             = ALPHA / DIGIT

Wilde & Vaha-Sipila Standards Track [Page 17] RFC 5724 sms" URI Scheme January 2010

Authors' Addresses

 Erik Wilde
 UC Berkeley
 School of Information
 Berkeley, CA 94720-4600
 U.S.A.
 Phone: +1-510-6432253
 EMail: dret@berkeley.edu
 URI:   http://dret.net/netdret/
 Antti Vaha-Sipila
 Nokia
 EMail: antti.vaha-sipila@nokia.com
 URI:   http://www.iki.fi/avs/

Wilde & Vaha-Sipila Standards Track [Page 18]

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