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Internet Engineering Task Force (IETF)                          E. WildeRequest for Comments: 5724                                   UC BerkeleyCategory: Standards Track                                 A. Vaha-SipilaISSN: 2070-1721                                                    Nokia                                                            January 2010URI 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 2010Table of Contents1. Introduction ....................................................31.1. What is GSM? ...............................................31.2. What is SMS? ...............................................31.2.1. SMS Content .........................................31.2.2. SMS Infrastructure ..................................41.2.2.1. SMS Centers ................................41.2.3. Uniform Resource Identifiers ........................41.2.4. SMS Messages and the Internet .......................51.2.4.1. SMS Messages and the Web ...................61.2.4.2. SMS Messages and Forms .....................61.3. Requirements Language ......................................62. The "sms" URI Scheme ............................................72.1. Applicability ..............................................72.2. Formal Definition ..........................................72.3. Processing an "sms" URI ....................................92.4. Comparing "sms" URIs .......................................92.5. Examples of Use ...........................................102.6. Using "sms" URIs in HTML Forms ............................103. URI Scheme Registration ........................................113.1. URI Scheme Name ...........................................113.2. Status ....................................................113.3. URI Scheme Syntax .........................................113.4. URI Scheme Semantics ......................................113.5. Encoding Considerations ...................................123.6. Applications/Protocols That Use This URI Scheme Name ......123.7. Interoperability Considerations ...........................123.8. Security Considerations ...................................123.9. Contact ...................................................124. Security Considerations ........................................125. IANA Considerations ............................................146. Acknowledgements ...............................................147. References .....................................................147.1. Normative References ......................................147.2. Informative References ....................................15Appendix A.  Syntax of telephone-subscriber .......................17Wilde & Vaha-Sipila         Standards Track                     [Page 2]

RFC 5724                    sms" URI Scheme                 January 20101.  Introduction1.1.  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 thisWilde & 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 accessWilde & Vaha-Sipila         Standards Track                     [Page 4]

RFC 5724                    sms" URI Scheme                 January 2010   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.Wilde & Vaha-Sipila         Standards Track                     [Page 5]

RFC 5724                    sms" URI Scheme                 January 20101.2.4.1.  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; seeSection 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].Wilde & Vaha-Sipila         Standards Track                     [Page 6]

RFC 5724                    sms" URI Scheme                 January 20102.  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 inRFC 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 inRFC 3986Wilde & Vaha-Sipila         Standards Track                     [Page 7]

RFC 5724                    sms" URI Scheme                 January 2010   Some illustrative examples using this syntax are given inSection 2.5.   The syntax definition for <telephone-subscriber> is taken fromSection 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, butRFC 3966 (plus all applicable errata) is the   normative reference.  The description of phone numbers inRFC 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.Wilde & Vaha-Sipila         Standards Track                     [Page 8]

RFC 5724                    sms" URI Scheme                 January 20102.3.  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" inRFC 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 inSection 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.Wilde & Vaha-Sipila         Standards Track                     [Page 9]

RFC 5724                    sms" URI Scheme                 January 2010   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 NOTWilde & Vaha-Sipila         Standards Track                    [Page 10]

RFC 5724                    sms" URI Scheme                 January 2010   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   sms3.2.  Status   Permanent3.3.  URI Scheme Syntax   SeeSection 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].  SeeSection 2.3 for the details of operation.Wilde & Vaha-Sipila         Standards Track                    [Page 11]

RFC 5724                    sms" URI Scheme                 January 20103.5.  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.  SeeSection 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   SeeSection 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.edu4.  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 betweenWilde & Vaha-Sipila         Standards Track                    [Page 12]

RFC 5724                    sms" URI Scheme                 January 2010   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 toWilde & Vaha-Sipila         Standards Track                    [Page 13]

RFC 5724                    sms" URI Scheme                 January 2010   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 inSection 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 inSection 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.  References7.1.  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 2010Appendix A.  Syntax of 'telephone-subscriber'   The following syntax is reproduced fromSection 3 of [RFC3966].  It   defines the <telephone-subscriber> part used in the "sms" URI scheme   syntax.  Please note that it includes Erratum 203 [Err203] forRFC3966, 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 / DIGITWilde & Vaha-Sipila         Standards Track                    [Page 17]

RFC 5724                    sms" URI Scheme                 January 2010Authors' 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]