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'CONV-URL' ** Obsolete normative reference: RFC 1738 (Obsoleted by RFC 4248, RFC 4266) ** Obsolete normative reference: RFC 1866 (Obsoleted by RFC 2854) ** Obsolete normative reference: RFC 2048 (Obsoleted by RFC 4288, RFC 4289) ** Obsolete normative reference: RFC 2234 (Obsoleted by RFC 4234) ** Obsolete normative reference: RFC 2303 (Obsoleted by RFC 3191) ** Obsolete normative reference: RFC 2304 (Obsoleted by RFC 3192) ** Obsolete normative reference: RFC 2396 (Obsoleted by RFC 3986) ** Obsolete normative reference: RFC 2543 (Obsoleted by RFC 3261, RFC 3262, RFC 3263, RFC 3264, RFC 3265) Summary: 13 errors (**), 0 flaws (~~), 5 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 INTERNET-DRAFT A. Vaha-Sipila 2 Expires 24-Mar-2000 Nokia 3 22-Sep-1999 5 URLs for Telephone Calls 6 8 Status of This Memo 10 This document is an Internet-Draft and is in full conformance with 11 all provisions of Section 10 of RFC2026. 13 This document is an Internet-Draft. Internet-Drafts are working 14 documents of the Internet Engineering Task Force (IETF), its areas, 15 and its working groups. Note that other groups may also distribute 16 working documents as Internet-Drafts. 18 Internet-Drafts are draft documents valid for a maximum of six months 19 and may be updated, replaced, or obsoleted by other documents at any 20 time. It is inappropriate to use Internet-Drafts as reference 21 material or to cite them other than as "work in progress." 23 The list of current Internet-Drafts can be accessed at 24 http://www.ietf.org/ietf/1id-abstracts.txt 26 The list of Internet-Draft Shadow Directories can be accessed at 27 http://www.ietf.org/shadow.html. 29 To view the entire list of current Internet-Drafts, please check the 30 "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow 31 Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern 32 Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific 33 Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). 35 The distribution of this document before its expiry date is 36 unlimited. 38 Abstract 40 This document specifies URL (Uniform Resource Locator) schemes 41 terminal in the phone network and the connection types (modes of 42 operation) that can be used to connect to that entity. This 43 specification covers voice calls (normal phone calls, answering 44 machines and voice messaging systems), facsimile (telefax) calls and 45 data calls, both for POTS and digital/mobile subscribers. 47 Table of Contents 49 1. Introduction ................................................ 3 50 1.1 New URL schemes ............................................ 3 51 1.2 Formal definitions ......................................... 3 52 1.3 Requirements ............................................... 4 53 2. URL schemes for telephone calls ............................. 4 54 2.1 Applicability .............................................. 4 55 2.2 "tel" URL scheme ........................................... 4 56 2.3 "fax" URL scheme ........................................... 5 57 2.4 "modem" URL scheme ......................................... 6 58 2.5 Parsing telephone, fax and modem URLs ...................... 6 59 2.5.1 Call type ................................................ 6 60 2.5.2 Phone numbers and their scope ............................ 7 61 2.5.3 Separators in phone numbers .............................. 8 62 2.5.4 Converting the number to the local numbering scheme ...... 8 63 2.5.5 Sending post-dial sequence after call setup .............. 9 64 2.5.6 Pauses in dialing and post-dial sequence ................. 9 65 2.5.7 ISDN subaddresses ........................................ 9 66 2.5.8 T.33 subaddresses ........................................ 10 67 2.5.9 Data call parameters ..................................... 10 68 2.5.10 Telephony service provider identification ............... 11 69 2.6 Examples of Use ............................................ 12 70 2.7 Rationale behind the syntax ................................ 13 71 2.7.1 Why distinguish between call types? ..................... 13 72 2.7.2 Why "tel" is "tel"? ..................................... 13 73 2.7.3 Why to use E.164 numbering? .............................. 14 74 2.7.4 Not everyone has the same equipment as you ............... 14 75 2.7.5 Do not confuse global and local contexts ................. 15 76 3. Comments on usage ........................................... 15 77 4. References .................................................. 16 78 5. Security Considerations ..................................... 17 79 6. Acknowledgements ............................................ 18 80 7. Authors' Addresses .......................................... 18 81 8. Full Copyright Statement .................................... 19 83 1. Introduction 85 1.1 New URL schemes 87 URLs that designate phone or fax numbers that can be dialed have been 88 brought forward in other Internet-Drafts. However, none of these has 89 reached the RFC status. This document tries to remedy the situation. 90 All interested parties are invited to submit comments on this 91 Internet-Draft. Contact information can be found at the end of this 92 document. 94 See also [CONV-URL] for more discussion on conversational URLs. 96 This specification defines three new URL schemes: "tel", "fax" and 97 "modem". They are intended for describing a terminal that can be 98 contacted using the telephone network. The description includes the 99 subscriber (telephone) number of the terminal and the necessary 100 parameters to be able to successfully connect to that terminal. 102 The "tel" scheme describes a connection to a terminal that handles 103 normal voice telephone calls, a voice mailbox or another voice 104 messaging system or a service that can be operated using DTMF codes. 106 The "fax" scheme describes a connection to a terminal that can handle 107 telefaxes (facsimiles). The name (scheme specifier) for the URL is 108 "fax" as recommended by [E.123]. 110 The "modem" scheme describes a connection to a terminal that can 111 handle incoming data calls. The term "modem" refers to a device that 112 does digital-to-analog and analog-to-digital conversions; in addition 113 to these, a "modem" scheme can describe a fully digital connection. 115 The notation for phone numbers is the same which is specified in 116 [RFC2303] and [RFC2304]. However, the syntax definition is a bit 117 different due to the fact that this document specifies URLs whereas 118 [RFC2303] and [RFC2304] specify electronic mail addresses. For 119 example, "/" (used in URLs to separate parts in a hierarchical URL 120 [RFC2396]) has been replaced by ";". In addition, this URL scheme has 121 been synchronized with [RFC2543]. 123 When these URLs are used, the number of parameters should be kept to 124 minimum. This is especially important if the URL is intended to be 125 shown to the end user, printed, or otherwise distributed so that it 126 is visible. 128 1.2 Formal definitions 129 Formal definitions follow [RFC2234]. This specification uses elements 130 from the 'core' definitions (Appendix A of [RFC2234]). Some elements 131 have been defined in previous RFCs. If this is the case, the RFC in 132 question has been referenced in comments. 134 1.3 Requirements 136 Compliant software MUST follow this specification. Requirements are 137 indicated by capitalized words as specified in [RFC2119]. 139 2. URL schemes for telephone calls 141 2.1 Applicability 143 In this document, "user agent" means software that can detect and 144 parse one or more of these URLs and possibly place a call to the 145 remote terminal using hardware and software at its disposal after it 146 has been properly configured, or otherwise utilize the contents of 147 the URL. 149 These URL schemes are used to direct the user agent to place a call 150 using the telephone network, or as a method to transfer or store a 151 phone number plus other relevant data. The network in question may be 152 a landline or mobile phone network, or a combination of these. If the 153 phone network differentiates between (for example) voice and data 154 calls, or if the user agent has several different telecommunications 155 equipment at its disposal, it is possible to specify which kind of 156 call (voice/fax/data) is requested. The URL can also contain 157 information about the capabilities of the remote entity, so that the 158 connection can be established successfully. 160 None of the URL schemes do have a 'path' in them - they are always 161 absolute. The URLs are always case-insensitive, except for the 162 parameter (see below), whose case-sensitivity is 163 application specific. 165 All unsafe and reserved characters (when not used for their reserved 166 purpose) MUST be URL-encoded as explained in [RFC1738]. All 8-bit 167 characters MUST be URL-encoded. 169 2.2 "tel" URL scheme 171 The URL syntax is formally described as follows. For the basis of 172 this syntax, see [RFC2303]. 174 telephone-url = telephone-scheme ":" 175 telephone-subscriber 176 telephone-scheme = "tel" 177 telephone-subscriber = global-phone-number / local-phone-number 178 global-phone-number = "+" base-phone-number [isdn-subaddress] 179 [post-dial] *(area-specifier / service-provider / 180 future-extension) 181 base-phone-number = 1*phonedigit 182 local-phone-number = 1*(phonedigit / dtmf-digit / 183 pause-character) [isdn-subaddress] 184 [post-dial] *(area-specifier / service-provider / 185 future-extension) 186 isdn-subaddress = ";isub=" 1*phonedigit 187 post-dial = ";postd=" 1*(phonedigit / 188 dtmf-digit / pause-character) 189 area-specifier = ";" phone-context-tag "=" phone-context-ident 190 phone-context-tag = "phone-context" 191 phone-context-ident = network-prefix / private-prefix 192 network-prefix = global-network-prefix / local-network-prefix 193 global-network-prefix = "+" 1*phonedigit 194 local-network-prefix = 1*(phonedigit / dtmf-digit / pause-character) 195 private-prefix = (%x21-22 / %x24-29 / %x2C-2F / %x3A / %x3C-40 / 196 %x45-4F / %x51-56 / %x58-60 / %x65-6F / %x71-76 / 197 %x78-7E) *(%x21-3A / %x3C-7E) 198 ; Unsafe and reserved characters must be encoded 199 ; as explained in [RFC1738] 200 service-provider = ";" provider-tag "=" provider-hostname 201 provider-tag = "tsp" 202 provider-hostname = domain ; is defined in [RFC1035] 203 future-extension = ";" 1*(token-char) ["=" ((1*(token-char) 204 ["?" 1*(token-char)]) / quoted-string )] 205 token-char = (%x21 / %x23-27 / %x2A-2B / %x2D-2E / %x30-39 206 / %x41-5A / %x5E-7A / %x7C / %x7E) 207 ; Unsafe and reserved characters must 208 ; be encoded as explained in [RFC1738] 209 quoted-string = %x22 *( "\" CHAR / (%x20-21 / %x23-7E / 210 %80-FF ) %x22 211 ; Unsafe, reserved, and 8-bit characters must 212 ; be encoded as explained in [RFC1738] 213 phonedigit = DIGIT / visual-separator 214 visual-separator = "-" / "." / "(" / ")" 215 pause-character = one-second-pause / wait-for-dial-tone 216 one-second-pause = "p" 217 wait-for-dial-tone = "w" 218 dtmf-digit = "*" / "#" / "A" / "B" / "C" / "D" 220 2.3 "fax" URL scheme 221 The URL syntax is formally described as follows (the definition 222 reuses nonterminals from the above definition). For the basis of this 223 syntax, see [RFC2303] and [RFC2304]. 225 fax-url = fax-scheme ":" fax-subscriber 226 fax-scheme = "fax" 227 fax-subscriber = fax-global-phone / fax-local-phone 228 fax-global-phone = "+" base-phone-number [isdn-subaddress] 229 [t33-subaddress] [post-dial] 230 *(area-specifier / service-provider / 231 future-extension) 232 fax-local-phone = 1*(phonedigit / dtmf-digit / 233 pause-character) [isdn-subaddress] 234 [t33-subaddress] [post-dial] 235 *(area-specifier / service-provider / 236 future-extension) 237 t33-subaddress = ";tsub=" 1*phonedigit 239 2.4 "modem" URL scheme 241 The URL syntax is formally described as follows (the definition 242 reuses nonterminals from the above definitions). For the basis of 243 this syntax, see [RFC2303]. 245 modem-url = modem-scheme ":" remote-host 246 modem-scheme = "modem" 247 remote-host = telephone-subscriber *modem-params 248 modem-params = ";type=" data-capabilities 249 data-capabilities = accepted-modem ["?" data-bits parity 250 stop-bits] 251 accepted-modem = "V21" / "V22" / "V22b" / 252 "V23" / "V26t" / "V32" / 253 "V32b" / "V34" / "V90" / 254 "V110" / "V120" / "B103" / 255 "B212" / "X75" / 256 "vnd." vendor-name "." modem-type 257 data-bits = "7" / "8" 258 parity = "n" / "e" / "o" / "m" / "s" 259 stop-bits = "1" / "2" 260 vendor-name = 1*(ALPHA / DIGIT / "-" / "+") 261 modem-type = 1*(ALPHA / DIGIT / "-" / "+") 263 2.5 Parsing telephone, fax and modem URLs 265 2.5.1 Call type 266 The type of call is specified by the scheme specifier. "Tel" means 267 that a voice call is opened. "Fax" indicates that the call should be 268 a facsimile (telefax) call. "Modem" means that it should be a data 269 call. Not all networks differentiate between the types of call; in 270 this case, the scheme specifier indicates the telecommunications 271 equipment type to use. 273 2.5.2 Phone numbers and their scope 275 and indicate the phone number 276 to be dialed. The phone number can be written in either international 277 or local notation. All phone numbers SHOULD always be written in the 278 international form if there is no good reason to use the local form. 280 Not all numbers are valid within all numbering areas. An optional 281 parameter is used to indicate the numbering area 282 within which this number is valid. The can take 283 three forms: , or 284 . The interpretation of this field is as follows: the 285 number is valid in the user-agent's numbering area if the user- 286 agent's own number starts with one of the given prefixes. For 287 example, if is "+358", the given number is 288 valid only within Finland (even if it is a ). If 289 is "80", the number is valid in an environment 290 where the user-agent's own number starts with "80" - possibly a 291 company internal phone number. 293 There can be multiple instances of . In this case, 294 the number is valid in all of the given numbering areas. 296 The interpretation of is left for future 297 specification within IETF PINT working group and shall be documented 298 in an RFC. SHOULD start with a token which 299 identifies its syntax. 301 If is present, the user agent MUST NOT call out if 302 the user agent is not located within that numbering area. Also, if 303 is present, SHOULD be used 304 whenever possible. 306 Any telephone number MUST contain at least one or 307 , that is, subscriber numbers consisting only of pause 308 characters are not allowed. 310 International numbers MUST begin with the "+" character. Local 311 numbers MUST NOT contain that character. International numbers MUST 312 be written with the country (CC) and national (NSN) numbers as 313 specified in [E.123] and [E.164]. International numbers have the 314 property of being totally unambiguous everywhere in the world if the 315 user agent is properly configured. 317 Local numbers MAY be used if the number only works from inside a 318 certain geographical area or a network. Note that some numbers may 319 work from several networks but not from the whole world - these 320 SHOULD be written in international form. URLs containing local phone 321 numbers should only appear in an environment where all user agents 322 can get the call successfully set up by passing the number to the 323 dialing entity "as is". An example could be a company intranet, where 324 all user agents are located under a the same private telephone 325 exchange. If local phone numbers are used, the document in which they 326 are present SHOULD contain an indication of the context in which they 327 are intended to be used, and an appropriate SHOULD 328 be present in the URL. 330 In some regions, it is popular to write phone numbers using 331 alphabetic characters which correspond to certain numbers on the 332 telephone keypad. Letters in characters do not have 333 anything to do with this, nor is this method supported by these URL 334 schemes. 336 It should also be noted that implementations MUST NOT assume that 337 telephone numbers have a maximum, minimum or fixed length, or that 338 they would always begin with a certain number. Implementors are 339 encouraged to familiarize themselves with the international standards 340 for telephone number notation. 342 2.5.3 Separators in phone numbers 344 All characters MUST be removed from the phone 345 number by the user agent before using it do dial out. These 346 characters are present only to aid readability: they MUST NOT have 347 any other meaning. Note that although [E.123] recommends the use of 348 space (SP) characters as the separators, spaces MUST NOT be used in 349 phone numbers. 351 2.5.4 Converting the number to the local numbering scheme 353 After the telephone number has been extracted, it can be converted to 354 the local dialing convention. (For example, the "+" character might 355 be replaced by the international call prefix, or the international 356 and trunk prefixes might be removed to place a local call.) Numbers 357 that have been specified using or 358 MUST be used by the user agent "as is", without any conversions. 360 2.5.5 Sending post-dial sequence after call setup 362 The number may contain a sequence, which MUST be dialled 363 using Dual Tone Multifrequency (DTMF) in-band signalling or pulse 364 dialing after the call setup is complete. If the user agent does not 365 support DTMF or pulse dialing after the call has been set up, MUST be ignored. In that case, the user SHOULD be notified. 368 2.5.6 Pauses in dialing and post-dial sequence 370 A local phone number or a post-dial sequence may contain characters which indicate a pause while dialing ("p"), or 372 a wait for dial tone ("w"). 374 User agents MAY support this method of dialing, and the final 375 interpretation of these characters is left to the user agent. 377 If it is not supported, user agents MUST ignore everything in the 378 dial string after the first and the user SHOULD be 379 notified. The user or the user agent MAY opt not to place a call if 380 this feature is not supported and these characters are present in the 381 URL. 383 Any characters and all dial string characters after the 384 first or SHOULD be sent to line using 385 DTMF (Dual Tone Multifrequency) in-band signaling, even if dialing is 386 done using direct network signaling (a digital subscriber loop or a 387 mobile phone). If the local infrastructure does not support DTMF 388 codes, the user agent MAY opt to use pulse dialing. However, it 389 should be noted that certain services which are controlled using DTMF 390 tones cannot be controlled with pulse dialing. If pulse dialing is 391 used, the user SHOULD be notified. 393 2.5.7 ISDN subaddresses 395 A phone number MAY also contain an which indicates 396 an ISDN subaddress. User agent SHOULD support ISDN subaddresses. 397 These addresses are sent to the network by using a method available 398 to the user agent (typically, ISDN subscribers send the address with 399 the call setup signalling). If ISDN subaddressing is not supported by 400 the caller, MUST be ignored and the user SHOULD be 401 notified. The user or the user agent MAY opt not to place a call if 402 this feature is not supported. 404 2.5.8 T.33 subaddresses 406 A fax number MAY also contain a , which indicates the 407 start of a T.33 subaddress [T.33]. User agents SHOULD support this. 408 Otherwise MUST be ignored and the user SHOULD be 409 notified. The user or the user agent MAY opt not to place a call if 410 this feature is not supported. 412 2.5.9 Data call parameters 414 indicate the minimum compliance required from the user 415 agent to be able to connect to the remote entity. The minimum 416 compliance is defined as being equal to or a superset of the 417 capabilities of the listed modem type. 419 The user agent MUST call out using compatible hardware, or request 420 that the network provides such a service. 422 For example, if the user agent only has access to a V.22bis modem and 423 the URL indicates that the minimum acceptable connection is V.32bis, 424 the user agent MUST NOT try to connect to the remote host since 425 V.22bis is a subset of V.32bis. However, if the URL lists V.32 as the 426 minimum acceptable connection, the user agent can use V.32bis to 427 create a connection since V.32bis is a superset of V.32. 429 This feature is present because modem pools often have separate 430 numbers for slow modems and fast modems, or have different numbers 431 for analog and ISDN connections, or may use proprietary modems that 432 are incompatible with standards. It is somewhat analogous to the 433 connection type specifier (typecode) in FTP URLs [RFC1738]: it 434 provides the user agent with information that can not be deduced from 435 the scheme specifier, but is helpful for successful operation. 437 This also means that the number of data and stop bits and parity MUST 438 be set according to the information given in the URL, or to default 439 values given in this document, if the information is not present. 441 The capability tokens are listed below. If capabilities suggest that 442 it is impossible to create a connection, the connection MUST NOT be 443 created. 445 If new modem types are standardized by ITU-T, this list can be 446 extended with those capability tokens. Tokens are formed by taking 447 the number of the standard and joining together the first letter (for 448 example, "V"), number (for example, 22) and the first letter of the 449 postfix (for example "bis" would become "b"). 451 Proprietary modem types MUST be specified using the 'vendor naming 452 tree', which takes the form "vnd.x.y", in which "x" is the name of 453 the entity from which the specifications for the modem type can be 454 acquired and "y" is the type or model of the modem. Vendor names MUST 455 share the same name space with vendor names used in MIME types 456 [RFC2048]. Submitting the modem types to ietf-types list for review 457 is strongly recommended. 459 New capabilities MUST always be documented in an RFC, and they MUST 460 refer to this document or a newer version of it. 462 Capability Explanation 464 V21 ITU-T V.21 465 V22 ITU-T V.22 466 V22b ITU-T V.22bis 467 V23 ITU-T V.23 468 V26t ITU-T V.26ter 469 V32 ITU-T V.32 470 V32b ITU-T V.32bis 471 V34 ITU-T V.34 472 V90 ITU-T V.90 473 V110 ITU-T V.110 474 V120 ITU-T V.120 475 X75 ITU-T X.75 476 B103 Bell 103 477 B212 Bell 212 478 Data bits: "8" or "7" The number of data bits. If not 479 specified, defaults to "8". 480 Parity: "n", "e", "o", Parity. None, even, odd, mark or 481 "m", "s" space parity, respectively. If 482 not specified, defaults to "n". 483 Stop bits: "1" or "2" The number of stop bits. If not 484 specified, defaults to "1". 486 2.5.10 Telephony service provider identification 488 It is possible to indicate the identity of the telephony service 489 provider for the given phone number. MAY be used 490 by the user-agent to enhance the user interface, for billing 491 estimates or to otherwise optimize its functionality. It MAY also be 492 ignored by the user-agent. consists of a fully 493 qualified Internet domain name of the telephony service provider, for 494 example ";tsp=terrifictelecom.com". The syntax of the domain name 495 follows Internet domain name rules and is defined in [RFC1035]. 497 2.5.11 Additional parameters 499 In addition to T.33 and ISDN subaddresses, modem types and area 500 specifiers, future extensions to this URL scheme may add other 501 additional parameters ( in the BNF) to these URLs. 502 These parameters are added to the URL after a semicolon (";"). 503 Implementations MUST be prepared to handle additional and/or unknown 504 parameters gracefully. Implementations MAY opt not to use the URL if 505 it contains unknown parameters. 507 For example, can be used to store application- 508 specific additional data about the phone number, its intended use, or 509 any conversions that have been applied to the number. Whenever a 510 is used in an open environment, its syntax and 511 usage MUST be properly documented in an RFC. 513 nonterminal a rephrased version of, and compatible 514 with the as defined in [RFC2543] (which actually 515 borrows BNF from an earlier version of this specification). 517 2.6 Examples of Use 519 tel:+358-555-1234567 521 This URL points to a phone number in Finland capable of receiving 522 voice calls. The hyphens are included to make the number more human- 523 readable: country and area codes have been separated from the 524 subscriber number. 526 fax:+358.555.1234567 528 The above URL describes a phone number which can receive fax calls. 529 It uses dots instead of hyphens as separators, but they have no 530 effect on the functionality. 532 modem:+3585551234567;type=v32b?7e1;type=v110 534 This phone number belongs to an entity which is able to receive data 535 calls. The user agent may opt to use either a ITU-T V.32bis modem (or 536 a faster one, which is compatible with V.32bis), using settings of 7 537 data bits, even parity and one stop bit, or an ISDN connection using 538 ITU-T V.110 protocol. 540 tel:+358-555-1234567;postd=pp22 542 The above URL instructs the user agent to place a voice call to 543 +358-555-1234567, then wait for an implementation-dependent time (for 544 example, two seconds) and emit two DTMF dialing tones "2" on the line 545 (for example, to choose a particular extension number, or to invoke a 546 particular service). 548 tel:0w003585551234567 550 This URL places a voice call to the given number. The number format 551 is intended for local use: the first zero opens an outside line, the 552 "w" character waits for a second dial tone, and the number already 553 has the international access code appended to it ("00"). This kind of 554 phone number MUST NOT be used in an environment where all users of 555 this URL might not be able to successfully dial out by using this 556 number directly. However, this might be appropriate for pages in a 557 company intranet. 559 tel:+1234567890;phone-context=+1234;vnd.foo.ext=foo 561 The URL describes a phone number which, even if it is written in its 562 international form, is only usable within the numbering area where 563 phone numbers start with +1234. There is also a proprietary extension 564 "vnd.foo.ext", which has the value "foo". The meaning of this 565 extension is application-specific. Note that the order of these 566 parameters (phone-context and vnd.foo.ext) is irrelevant. 568 2.7 Rationale behind the syntax 570 2.7.1 Why distinguish between call types? 572 URLs locate resources, which in this case is some telecommunications 573 equipment at a given phone number. However, it is not necessarily 574 enough to know the subscriber number in order to successfully 575 communicate with that equipment. Digital phone networks distinguish 576 between voice, fax and data calls (and possibly other types of calls, 577 not discussed in this specification). To be able to successfully 578 connect to, say, a fax machine, the caller may have to specify that a 579 fax call is being made. Otherwise the call might be routed to the 580 voice number of the subscriber. In this sense, the call type is an 581 integral part of the 'location' of the target resource. 583 The reason to have the call type in the scheme specifier is to make 584 the URL simple to remember and use. Making it a parameter, much like 585 the way modem parameters are handled now, will substantially reduce 586 the usability of this URL (to the humans). 588 2.7.2 Why "tel" is "tel"? 589 There has been discussion on whether the scheme name "tel" is 590 appropriate. To summarize, these are the points made against the 591 other proposals. 593 callto URL schemes locate a resource and do not specify 594 an action to be taken. 595 telephone Too long. Also, "tel" considered to be a more 596 international form. 597 phone Was countered on the basis that "tel" is more 598 internationally acceptable. 600 2.7.3 Why to use E.164 numbering? 602 It should be noted that phone numbers may have 'hierarchical' 603 characteristics, so that one could build a 'forest' of phone numbers 604 with country codes as roots, area codes as branches and subscriber 605 numbers as leaves. However, this is not always the case. Not all 606 areas have area codes; some areas may have different area codes 607 depending on how one wants to route the call; some numbers must 608 always be dialled "as is", without prepending area or country codes; 609 and area codes can and do change. 611 Usually, if something has a hierarchical structure, the URL syntax 612 should reflect that fact. These URLs are an exception. 614 Phone numbers are written almost always in some form which resembles 615 the E.164 notation. Because of this, the syntax in this specification 616 is intuitively clear to most people. This is the usual way to write 617 phone numbers in business cards, advertisements, telephone books and 618 so on. 620 Also, when writing the phone number in the form described in this 621 specification, the writer does not need to know which part of the 622 number is the country code and which part is the area code. If a 623 hierarchical URL would be used (with a "/" character separating the 624 parts of the phone numbers), the writer of the URL would have to know 625 which parts are which. 627 Finally, when phone numbers are written in the international form as 628 specified here, they are unambiguous and can always be converted to 629 the local dialing convention, given that the user agent has the 630 knowledge of the local country and area codes. 632 2.7.4 Not everyone has the same equipment as you 634 There are several ways for the subscriber to dial a phone number: 636 - By pulse dialing. Typically old telephone exchanges. Usually 637 this dialing method has only to be used to set up the call; after 638 connecting to the remote entity, can be sent to the 639 line using DTMF, because it will typically be processed by the 640 remote entity, not the telephone network. 642 - By DTMF. These are the 'beeps' that you hear when you dial on 643 most phones. 645 - By direct network signalling. ISDN subscribers and mobile phone 646 users usually have this. There is no dial tone (or if there is, it 647 is generated locally by the equipment), and the number of the 648 called party is communicated to the telephone network using some 649 network signalling method. After setting up the call, 650 sequences are usually sent using DTMF codes. 652 2.7.5 Do not confuse global and local contexts 654 As an example, +123456789 will be dialled in many countries as 655 00123456789, where the leading "00" is a prefix for international 656 calls. However, if a URL contains a local phone number 00123456789, 657 the user-agent MUST NOT assume that this number is equal to a global 658 phone number +123456789. If a user-agent received a telephony URL 659 with a local number in it, it must make sure that it knows the 660 context in which the local phone number is to be processed. Equally, 661 anyone sending a telephony URL should take into consideration that 662 the recipient may have insufficient information about the phone 663 number's context. 665 3. Comments on usage 667 These are examples of the recommended usage of this URL in HTML 668 documents. 670 First of all, the number SHOULD be visible to the end user, if it is 671 conceivable that the user might not have a user agent which is able 672 to use these URLs. 674 Telephone: +358-555-1234567 676 Second, on a public HTML page, the telehone number in the URL SHOULD 677 always be in the international form, even if the text of the link 678 uses some local format. 680 Telephone: (0555) 1234567 682 or even 684 For more info, call 1-555-IETF- 685 RULZ-OK. 687 Moreover, if the number is a , and the scope of 688 the number is not clear from the context in which the URL is 689 displayed, a human-readable explanation SHOULD be included. 691 For customer service, dial 1234 (only from 692 Terrific Telecom mobile phones). 694 4. References 696 NOTE. References to Internet-Drafts will be removed from the final 697 document which will be submitted to the RFC-Editor. 699 [CONV-URL] Conversational Multimedia URLs. 1997. Pete Cordell. An 700 Internet-Draft (work in progress). 701 704 [RFC1035] Domain Names - Implementation and Specification. November 705 1987. P. Mockapetris. RFC 1035. 706 708 [RFC1738] Uniform Resource Locators (URL). December 1994. T. 709 Berners-Lee et al. RFC 1738. 710 712 [RFC1866] Hypertext Markup Language - 2.0. November 1995. T. 713 Berners-Lee & D. Connolly. RFC 1866. 714 716 [RFC2048] Multipurpose Internet Mail Extensions (MIME) Part Four: 717 Registration Procedures. November 1996. N. Freed et al. RFC 2048. 718 720 [RFC2119] Key Words for Use in RFCs to Indicate Requirement Levels. 721 March 1997. S. Bradner. RFC 2119. 722 724 [RFC2234] Augmented BNF for Syntax Specifications: ABNF. November 725 1997. D. Crocker et al. RFC 2234. 726 728 [RFC2303] Minimal PSTN Address Format in Internet Mail. March 1998. 729 C. Allocchio. RFC 2303. 731 [RFC2304] Minimal FAX Address Format in Internet Mail. March 1998. 732 C. Allocchio. RFC 2304. 734 [RFC2396] Uniform Resource Identifiers (URI): Generic Syntax. August 735 1998. T. Berners-Lee et al. RFC 2396. 736 738 [RFC2543] SIP: Session Initiation Protocol. March 1999. M. Handley et 739 al. RFC 2543. 741 [E.123] ITU-T Recommendation E.123: Telephone Network and ISDN 742 Operation, Numbering, Routing and Mobile Service: Notation for 743 National and International Telephone Numbers. 1993. 745 [E.164] ITU-T Recommendation E.164: Telephone Network and ISDN 746 Operation, Numbering, Routing and Mobile Service: Numbering Plan for 747 the ISDN Era. 1991. 749 [T.33] ITU-T Recommendation T.33: Facsimile Routing Utilizing the 750 Subaddress. 1996. 752 5. Security Considerations 754 It should be noted that the user agent SHOULD NOT call out without 755 the knowledge of the user because of associated risks, which include 757 - call costs (including long calls, long distance calls, 758 international calls and premium rate calls, or calls which do 759 not terminate due to sequences that have been left 760 out 761 by the user agent) 763 - wrong numbers inserted on web pages by malicious users 765 - making the user's phone line unavailable (off-hook) for a 766 malicious purpose 768 - opening a data call to a remote host, thus possibly opening a 769 back door to the user's computer 771 - revealing the user's (possibly unlisted) phone number to the 772 remote host in the caller identification data 774 All of these risks MUST be taken into consideration when designing 775 the user agent. 777 The user agent SHOULD have some mechanism that the user can use to 778 filter out unwanted numbers. The user agent SHOULD NOT use rapid 779 redialing of the number if it is busy to avoid the congestion of the 780 (signaling) network. Also, the user agent SHOULD detect if the number 781 is unavailable or if the call is terminated before the dialing string 782 has been completely processed (for example, the call is terminated 783 while waiting for user input) and not try to call again, unless 784 instructed by the user. 786 6. Acknowledgements 788 Writing this specification would not have been possible without 789 extensive support from many people. 791 Contributors include numerous people from IETF FAX, PINT, URI and 792 URLREG mailing lists, as well as from World Wide Web Consortium and 793 several companies, plus several individuals. Thanks to all people who 794 offered criticism, corrections and feedback. 796 All phone numbers and company names used in the examples of this 797 specification are fictional. Any similarities to real entities are 798 coincidental. 800 7. Authors' Addresses 802 Contact person and version control responsibility for this 803 specification: 805 Nokia Mobile Phones 806 Antti Vaha-Sipila 807 P. O. Box 68 808 FIN-33721 Tampere 809 Finland 811 Electronic mail: avs@iki.fi 812 antti.vaha-sipila@nokia.com 813 Please include your name and electronic mail address in all 814 communications. If you want to receive the newest version of this 815 specification electronically, send mail to the address above. 817 This document expires on the 24th of February, 2000, or when a new 818 version is released. 820 8. Full Copyright Statement 822 To be added to the final RFC.