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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group H. Schulzrinne 3 Internet-Draft Columbia U. 4 Expires: November 21, 2004 May 23, 2004 6 The tel URI for Telephone Numbers 7 draft-ietf-iptel-tel-rfc2806bis-08 9 Status of this Memo 11 By submitting this Internet-Draft, I certify that any applicable 12 patent or other IPR claims of which I am aware have been disclosed, 13 and any of which I become aware will be disclosed, in accordance with 14 RFC 3668. 16 Internet-Drafts are working documents of the Internet Engineering 17 Task Force (IETF), its areas, and its working groups. Note that other 18 groups may also distribute working documents as Internet-Drafts. 20 Internet-Drafts are draft documents valid for a maximum of six months 21 and may be updated, replaced, or obsoleted by other documents at any 22 time. It is inappropriate to use Internet-Drafts as reference 23 material or to cite them other than as "work in progress." 25 The list of current Internet-Drafts can be accessed at http:// 26 www.ietf.org/ietf/1id-abstracts.txt. 28 The list of Internet-Draft Shadow Directories can be accessed at 29 http://www.ietf.org/shadow.html. 31 This Internet-Draft will expire on November 21, 2004. 33 Copyright Notice 35 Copyright (C) The Internet Society (2004). All Rights Reserved. 37 Abstract 39 This document specifies the URI (Uniform Resource Identifier) scheme 40 "tel". The ``tel'' URI describes resources identified by telephone 41 numbers. 43 Table of Contents 45 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 46 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 47 3. URI Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 5 48 4. URI Comparisons . . . . . . . . . . . . . . . . . . . . . . . 6 49 5. Phone Numbers and Their Context . . . . . . . . . . . . . . . 7 50 5.1 Phone Numbers . . . . . . . . . . . . . . . . . . . . . . 7 51 5.1.1 Separators in Phone Numbers . . . . . . . . . . . . . 7 52 5.1.2 Alphabetic Characters Corresponding to Digits . . . . 8 53 5.1.3 Alphabetic, * and \\# Characters as Identifiers . . . 8 54 5.1.4 Global Numbers . . . . . . . . . . . . . . . . . . . . 8 55 5.1.5 Local Numbers . . . . . . . . . . . . . . . . . . . . 8 56 5.2 ISDN Subaddresses . . . . . . . . . . . . . . . . . . . . 10 57 5.3 Phone Extensions . . . . . . . . . . . . . . . . . . . . . 10 58 5.4 Other Parameters . . . . . . . . . . . . . . . . . . . . . 10 59 6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 60 7. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 11 61 7.1 Why Not Just Put Telephone Numbers in SIP URIs? . . . . . 11 62 7.2 Why Not Distinguish Between Call Types? . . . . . . . . . 12 63 7.3 Why tel? . . . . . . . . . . . . . . . . . . . . . . . . . 12 64 7.4 Do Not Confuse Numbers with How They Are Dialed . . . . . 12 65 8. Usage of Telephone URIs in HTML . . . . . . . . . . . . . . . 12 66 9. Use of tel URIs with SIP (Informative) . . . . . . . . . . . . 13 67 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 14 68 11. Security Considerations . . . . . . . . . . . . . . . . . . 14 69 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . 15 70 13. Changes Since RFC 2806 . . . . . . . . . . . . . . . . . . . 15 71 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 72 14.1 Normative References . . . . . . . . . . . . . . . . . . . . 15 73 14.2 Informative References . . . . . . . . . . . . . . . . . . . 16 74 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 17 75 Intellectual Property and Copyright Statements . . . . . . . . 18 77 1. Introduction 79 This document defines the URI scheme "tel". The "tel" URI describes 80 resources identified by telephone numbers. A telephone number is a 81 string of decimal digits that uniquely indicates the network 82 termination point. The number contains the information necessary to 83 route the call to this termination point. (This definition is 84 derived from [E.164], but encompasses both public and private 85 numbers.) 87 The "tel" URI telephone number is not restricted in the type of 88 termination point it refers to. The termination point can be in the 89 public telephone network, a private telephone network or the 90 Internet. The termination point can be fixed or wireless and address 91 a fixed wired, mobile or nomadic terminal. The terminal addressed 92 can support any electronic communication service (ECS) including 93 voice, data and fax. The URI can refer to resources identified by a 94 telephone number, including but not limited to originators or targets 95 of a telephone call. 97 The "tel" URI is a globally unique identifier ("name") only; it does 98 not describe the steps necessary to reach a particular number and 99 does not imply dialing semantics. Furthermore, it does not refer to a 100 specific physical device, only to a telephone number. 102 Telephone numbers as commonly understood actually comprise two 103 related, but distinct concepts: as a canonical address-of-record and 104 as a dial string. We define the concepts below: 106 Address-of-record or identifier: The telephone number is understood 107 here as the canonical address-of-record or identifier for a 108 termination point within a specific network. For the public 109 network, these numbers follow the rules in E.164 [E.164], while 110 private numbers follow the rules of the owner of the private 111 numbering plan. Subscribers publish such identifiers as a 112 universal means of being reached, independent of the location of 113 the caller. (Naturally, not all numbers are reachable from 114 everywhere, for a variety of technical and local policy reasons. 115 Also, a single termination point may be reachable from different 116 networks and may have multiple such identifiers.) 117 Dial string: "Dial strings" are the actual numbers, symbols and 118 pauses entered by a user to place a phone call. A dial-string is 119 consumed by one or more network entities, and understood in the 120 context of the configuration of these entities. It is used to 121 generate an address-of-record or identifier in the sense of the 122 previous paragraph so that a call can be routed. Dial-strings may 123 require pre-pended digits to egress the private branch exchange 124 (PBX) the end system is connected to, and they may include 125 post-dial dual-tone multi-frequency (DTMF) signaling that could 126 control an IVR or reach an extension. Dial strings are beyond the 127 scope of this document. 129 Both approaches can be encoded into a URI. For dial strings, this 130 URI is passed to an entity that can reproduce the actions specified 131 in the dial string. For example, in an analog phone system, a dialer 132 translates the dial string into a sequence of actions such as waiting 133 for dial tone, sending DTMF digits, pausing and generating post-dial 134 DTMF digits after the callee picks up. In an integrated services 135 digital network (ISDN) or ISDN user part (ISUP) environment, the 136 signaling elements receiving protocol messages containing the dial 137 string perform the appropriate protocol actions. As noted, this 138 approach is beyond the scope of this specification. 140 The approach described here has the URI specify the telephone number 141 as an identifier, which can be either globally unique or only be 142 valid within a local context. The dialing application is aware of 143 the local context, knowing, for example, whether special digits need 144 to be dialed to seize an outside line, whether network, pulse or tone 145 dialing is needed and what tones indicate call progress. The dialing 146 application then converts the telephone number into a dial sequence 147 and performs the necessary signaling actions. The document below 148 assumes the second model. The dialer does not have to be a user 149 application as found in traditional desktop operating systems, but 150 could well be part of an IP-to-PSTN gateway. 152 To reach a telephone number from a phone on a PBX, for example, the 153 user of that phone has to know how to convert the telephone number 154 identifier into a dial string appropriate for that phone. The 155 telephone number itself does not convey what needs to be done for a 156 particular terminal. Instructions may include dialing "9" before 157 placing a call or prepending a "00" to reach a number in a foreign 158 country. The phone may also need to strip area and country codes. 160 The identifier approach described in this document has the 161 disadvantage that certain services, such as electronic banking or 162 voicemail, cannot be specified in a "tel" URI. 164 The notation for phone numbers in this document is similar to that in 165 RFC 3191 [RFC3191] and RFC 3192 [RFC3192]. However, the syntax 166 differs since this document describes URIs whereas RFC 3191 and RFC 167 3192 specify electronic mail addresses. RFC 3191 and RFC 3192 use "/" 168 to indicate parameters (qualifiers). Since URI use the forward slash 169 to describe path hierarchy, the URI scheme described here uses the 170 semicolon, in keeping with Session Initiation Protocol (SIP) URI 171 conventions [RFC3261]. 173 The 'tel' URI can be used as a request URI in SIP [RFC3261] requests. 174 The SIP specification also inherits the 'subscriber' part of the 175 syntax as part of the 'user element' in the SIP URI. Other protocols 176 may use this URI for both query-based and prefix-based applications. 178 The "tel" URI does not specify the call type such as voice, fax, or 179 data call and does not provide the connection parameters for a data 180 call. The type and parameters are assumed to be negotiated either 181 in-band by the telephone device or through a signaling protocol such 182 as SIP. 184 2. Terminology 186 In this document, the key words "MUST", "MUST NOT", "REQUIRED", 187 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", 188 and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 189 [RFC2119] and indicate requirement levels for compliant 190 implementations. 192 3. URI Syntax 194 The URI is defined using the ABNF (augmented Backus-Naur form) 195 described in RFC 2234 [RFC2234] and uses elements from the core 196 definitions (Appendix A of RFC 2234). 198 The syntax definition follows RFC 2396 [RFC2396], indicating the 199 actual characters contained in the URI. Note that the reserved 200 characters "+", ";", "=", and "?" MUST NOT be escaped as they are 201 delimiters for the "tel" URI scheme. These reserved characters MUST 202 be escaped if they appear in parameter values. 204 Characters other than those in the "reserved" and "unsafe" sets (see 205 RFC 2396 [RFC2396]) are equivalent to their "% HEX HEX" encoding. 207 The "tel" URI has the following syntax: 209 telephone-uri = "tel:" telephone-subscriber 210 telephone-subscriber = global-number / local-number 211 global-number = global-number-digits *par 212 local-number = local-number-digits *par context *par 213 par = parameter / extension / isdn-subaddress 214 isdn-subaddress = ";isub=" 1*uric 215 extension = ";ext=" 1*phonedigit 216 context = ";phone-context=" descriptor 217 descriptor = domainname / global-number-digits 218 global-number-digits = "+" 1*phonedigit 219 local-number-digits = 1*phonedigit-hex 220 domainname = *( domainlabel "." ) toplabel [ "." ] 221 domainlabel = alphanum 222 / alphanum *( alphanum / "-" ) alphanum 223 toplabel = ALPHA / ALPHA *( alphanum / "-" ) alphanum 224 parameter = ";" pname ["=" pvalue ] 225 pname = 1*( alphanum / "-" ) 226 pvalue = 1*paramchar 227 paramchar = param-unreserved / unreserved / escaped 228 unreserved = alphanum / mark 229 mark = "-" / "_" / "." / "!" / "~" / "*" / 230 "'" / "(" / ")" 231 escaped = "%" HEXDIG HEXDIG 232 param-unreserved = "[" / "]" / "/" / ":" / "&" / "+" / "$" 233 phonedigit = DIGIT / [ visual-separator ] 234 phonedigit-hex = HEXDIG / "*" / "#" / [ visual-separator ] 235 visual-separator = "-" / "." / "(" / ")" 236 alphanum = ALPHA / DIGIT 237 reserved = ";" | "/" | "?" | ":" | "@" | "&" | 238 "=" | "+" | "$" | "," 239 uric = reserved | unreserved | escaped 241 Each parameter name ("pname"), the ISDN subaddress, the 'extension' 242 and the 'context' MUST NOT appear more than once. The 243 'isdn-subaddress' or 'extension' MUST appear first, if present, 244 followed by the 'context' parameter, if present, followed by any 245 other parameters in lexicographical order. 247 This simplifies comparison when the "tel" URI is compared 248 character-by-character, such as in SIP URIs [RFC3261]. 250 4. URI Comparisons 252 Two "tel" URIs are equivalent according to the following rules: 254 o Both must be either a 'local-number' or both must be a 255 'global-number', i.e., start with a '+'. 257 o The 'global-number-digits' and the 'local-number-digits' must be 258 equal, after removing all visual separators. 259 o For mandatory additional parameters (Section 5.4) and the 260 'phone-context' and 'extension' parameters defined in this 261 document, The 'phone-context' parameter value is compared as a 262 host name if it is a 'domainname' or digit-by-digit if it is 263 'global-number-digits'. The latter is compared after removing all 264 'visual-separator' characters. 265 o Parameters are compared according to 'pname', regardless of the 266 order they appeared in the URI. If one URI has a parameter name 267 not found in the other, the two URIs are not equal. 268 o URI comparisons are case-insensitive. 270 All parameter names and values SHOULD use lower-case characters since 271 tel URIs may be used within contexts where comparisons are 272 case-sensitive. 274 Section 19.1.4 in the SIP specification [RFC3261] discusses one 275 such case. 277 5. Phone Numbers and Their Context 279 5.1 Phone Numbers 281 The 'telephone-subscriber' part of the URI indicates the number. The 282 phone number can be represented in either global (E.164) or local 283 notation. All phone numbers MUST use the global form unless they 284 cannot be represented as such. Numbers from private numbering plans, 285 emergency ("911", "112") and some directory assistance numbers (e.g., 286 "411") and other "service codes" (numbers of the form N11 in the 287 United States) cannot be represented in global (E.164) form, and need 288 to be represented as a local number with a context. Local numbers 289 MUST be tagged with a 'phone-context' (Section 5.1.5). 291 Implementations MUST NOT assume that telephone numbers have a 292 maximum, minimum or fixed length, or that they always begin with a 293 certain number. 295 5.1.1 Separators in Phone Numbers 297 Phone numbers MAY contain visual separators. Visual separators 298 ('visual-separator') merely aid readability and are not used for URI 299 comparison or placing a call. 301 Despite complicating comparisons, this specification retains the 302 visual separators to follow the spirit of RFC 2396 [RFC2396], 303 which remarks that "A URI often needs to be remembered by people, 304 and it is easier for people to remember a URI when it consists of 305 meaningful components." Also, ISBN URNs documented in RFC 3187 306 [RFC3187] use visual separators in a manner similar to this 307 specification. 309 Even though ITU-T E.123 [E.123] recommends the use of space 310 characters as visual separators in printed telephone numbers, 311 "tel" URIs cannot use spaces in visual separators to avoid 312 excessive escaping. 314 5.1.2 Alphabetic Characters Corresponding to Digits 316 In some countries, it is popular to write phone numbers using 317 alphabetic characters which correspond to certain numbers on the 318 telephone keypad. The URI format does not support this notation 319 since the mapping from alphabetic characters to digits is not 320 completely uniform internationally, although there are standards 321 [E.161][T1.703] addressing this issue. 323 5.1.3 Alphabetic, * and \\# Characters as Identifiers 325 Since called and calling terminal numbers (TNs) are encoded in BCD in 326 ISUP, six additional values per digit can be encoded, sometimes 327 represented as the hexadecimal characters A through F. Similarly, 328 DTMF allows for the encoding of the symbols *, \# and A through D. 329 However, in accordance with E.164, they may not be included in global 330 numbers. Their meaning in local numbers is not defined here, but they 331 are not prohibited. 333 5.1.4 Global Numbers 335 Globally unique numbers are identified by the leading "+" character. 336 Global numbers MUST be composed with the country (CC) and national 337 (NSN) numbers as specified in E.123 [E.123] and E.164 [E.164]. 338 Globally unique numbers have the property of being unambiguous 339 everywhere in the world and SHOULD be used. 341 5.1.5 Local Numbers 343 Local numbers are unique only within a certain geographical area or a 344 certain part of the telephone network, e.g., a private branch 345 exchange (PBX), a state or province, a particular local exchange 346 carrier or a particular country. URIs with local phone numbers 347 should only appear in environments where all local entities can 348 successfully set up the call by passing the number to the dialing 349 software. Digits needed for accessing an outside line, for example, 350 are not included in local numbers. Local numbers SHOULD NOT be used 351 unless there is no way to represent the number as a global number. 353 Local numbers require that the originator and recipient are 354 configured appropriately, so that they can insert and recognize 355 the correct descriptors. Since there is no algorithm to 356 independently pick the same descriptor, labeling numbers with 357 their context increases the chances of mis-configuration, so that 358 valid identifiers are rejected by mistake. The algorithm to 359 select descriptors was chosen that accidental collisions should be 360 rare, but they cannot be ruled out. 362 Local numbers MUST have a 'phone-context' parameter that identifies 363 the scope of their validity. The parameter MUST be chosen to 364 unambiguously identify the local context within which the number is 365 unique. Thus, the combination of the descriptor in the 366 'phone-context' parameter and local number is again globally unique. 367 The parameter value is defined by the assignee of the local number. 368 It does NOT indicate a prefix that turns the local number into a 369 global (E.164) number. 371 There are two ways to label the context: via a global number or any 372 number of its leading digits (e.g., "+33") and via a domain name, 373 e.g., "houston.example.com". The choice between the two is left to 374 the "owner" of the local number and is governed by whether there is a 375 global number or domain name that is a valid identifier for a 376 particular local number. 378 The domain name does not have to resolve to any actual host, but MUST 379 be under the administrative control of the entity managing the local 380 phone context. 382 A global number context consists of the initial digits of a valid 383 global number. All global numbers matching these initial digits must 384 be assigned to the same organization that is describing the context 385 and no such matching number can be used by any other organization. 386 For example, +49-6151-16 would be a suitable context for the 387 Technical University of Darmstadt, as it uses all numbers starting 388 with those digits. If such an initial string of digits does not 389 exist, the organization SHOULD use the lowest number of the global 390 number range assigned to it. (This can occur if two organizations 391 share the same decimal block of numbers. For example, assume an 392 organization owns the number range +1-212-939-7000 through 393 +1-212-939-7199. +1-212-939-7 would not be a valid global number 394 context, but +1-212-939-7000 would work.) It is not required that 395 local numbers within the context actually begin with the chosen set 396 of initial numbers. 398 A context consisting of the initial digits of a global number does 399 not imply that adding these to the local number will generate a valid 400 E.164 number. It might do so by coincidence, but this cannot be 401 relied upon. (For example, "911" should be labeled with the context 402 "+1", but "+1-911" is not a valid E.164 number.) 404 National freephone numbers do not need a context, even though they 405 are not necessarily reachable from outside a particular country code 406 or numbering plan. Recall that "tel" URIs are identifiers; it is 407 sufficient that a global number is unique, but it is not required 408 that it be reachable from everywhere. 410 Even non-freephone numbers may be out of date or not be reachable 411 from a particular location. For example, premium services such as 412 "900" numbers in the North American numbering plan are often not 413 dialable from outside the particular country code. 415 The two label types were chosen so that, in almost all cases, a 416 local administrator can pick an identifier that is reasonably 417 descriptive and does not require a new IANA-managed assigned 418 number. It is up to the administrator to assign an appropriate 419 identifier and to use it consistently. Often, an organization can 420 choose among several different identifiers. 422 If the recipient of a "tel" URI uses the URI simply for 423 identification, the receiver does not need to know anything about the 424 context descriptor. It simply treats it as one part of a globally 425 unique identifier, with the other being the local number. If a 426 recipient of the URI intends to place a call to the local number, it 427 MUST understand the context and be able to place calls within that 428 context. 430 5.2 ISDN Subaddresses 432 A phone number MAY also contain an 'isdn-subaddress' parameter which 433 indicates an ISDN subaddress. 435 ISDN subaddresses typically contain International Alphabet 5 (IA5 436 [T.50]) characters, but may contain any octet value. 438 5.3 Phone Extensions 440 Phone extensions identify stations behind a non-ISDN PBX and are 441 roughly equivalent in functionality to ISDN subaddresses. They are 442 identified with the 'extension' parameter. At most one of the 443 'isdn-subaddress' and 'extension' parameters can appear in a tel URI, 444 i.e., they cannot appear both at the same time. 446 5.4 Other Parameters 448 Future protocol extensions to this URI scheme may add other 449 parameters ('parameter' in the ABNF). Such parameters can be either 450 mandatory or optional. Mandatory parameters start with "m-". An 451 implementation MAY ignore optional parameters. An implementation 452 MUST NOT use the URI if it contains unknown mandatory parameters. 453 The "m-" prefix cannot be added to parameters that were already 454 registered (except to create a new, logically distinct parameter). 455 The "phone-context" parameter in this document is mandatory, "isub" 456 and "ext" are optional. 458 New mandatory parameters must be described in a standards-track RFC, 459 while an informational RFC is sufficient for optional parameters. 461 For example, 'parameter' parameters can be used to store 462 application-specific additional data about the phone number, its 463 intended use, or any conversions that have been applied to the 464 number. 466 Entities that forward protocol requests containing tel URIs with 467 optional parameters MUST NOT delete or modify parameters they do not 468 understand. 470 6. Examples 472 tel:+1-201-555-0123: This URI points to a phone number in the United 473 States. The hyphens are included to make the number more 474 human-readable; they separate country, area codes and subscriber 475 number. 476 tel:7042;phone-context=cs.columbia.edu: The URI describes a local 477 phone number valid within the context "cs.columbia.edu". 478 tel:863-1234;phone-context=+1-914-555: The URI describes a local 479 phone number that is valid within a particular phone prefix. 481 7. Rationale 483 7.1 Why Not Just Put Telephone Numbers in SIP URIs? 485 The "tel" URI describes a service, reaching a telephone number, that 486 is independent of the means of doing so, be it via a SIP-to-PSTN 487 gateway, a direct SIP call via E.164 number ("ENUM") translation 488 [RFC2916], some other signaling protocols such as H.323 or a 489 traditional circuit-switched call initiated on the client side via, 490 say, the Telephony Application Programming Interface (TAPI). It is 491 thus, in spirit, closer to the URN schemes that also leave the 492 resolution to an external mechanism. The same "tel" URI may get 493 translated to any number of other URIs in the process of setting up 494 the call. 496 7.2 Why Not Distinguish Between Call Types? 498 Signaling protocols such as SIP allow to negotiate the call type and 499 parameters, making the very basic indication within the URI scheme 500 moot. Also, since the call type can change frequently, any such 501 indication in a URI is likely to be out of date. If such designation 502 is desired for a device that directly places calls without a 503 signaling protocol such as SIP, mechanisms such as the "type" 504 attribute for the "A" element in HTML may be more appropriate. 506 7.3 Why tel? 508 "Tel" was chosen since it is widely recognized none of the other 509 suggestions appeared appropriate. "Callto" was discarded since URI 510 schemes locate a resource and do not specify an action to be taken. 511 "Telephone" and "phone" were considered too long and not as 512 internationally recognized. 514 7.4 Do Not Confuse Numbers with How They Are Dialed 516 As an example, the E.164 number "+1-212-555-3141" will be dialed in 517 many countries as 00-1-212-555-3141, where the leading "00" is a 518 prefix for international calls. (In general, a "+" symbol in E.164 519 indicates that an international prefix is required.) 521 8. Usage of Telephone URIs in HTML 523 Links using the "tel" URI SHOULD enclose the telephone number, so 524 that users can easily predict the action taken when following the 525 link. 527 Dial +1-212-555-0101 528 for assistance. 530 instead of 532 Dial this number 533 for assistance. 535 On a public HTML page, the telephone number in the URI SHOULD always 536 be in the global form, even if the text of the link uses some local 537 format. 539 Telephone (if dialing in the United States): 540 (201) 555-0111 542 or even 543 For having RFCs read aloud, call 544 1-555-IETF-RFC. 546 9. Use of tel URIs with SIP (Informative) 548 SIP can use the "tel" URI anywhere a URI is allowed, for example as a 549 Request-URI, along with "sip" and "sips" URIs. For brevity, we will 550 imply "sips" URIs when talking about SIP URIs. Both "tel" and SIP 551 URIs can contain telephone numbers. In SIP URIs, they appear as the 552 user part, i.e., before the @ symbol (Section 19.1.6 in [RFC3261]). 554 Unless a SIP UA connects directly to a PSTN gateway, one of the SIP 555 proxy servers has to translate the tel URI to a SIP URI, with the 556 host part of that URI pointing to a gateway. Typically, the outbound 557 proxy server, as the first proxy server visited by a call request, 558 performs this translation. A proxy server can translate all tel URIs 559 to the same SIP host name or select a different gateway for different 560 tel prefixes, based, for example, on information learned from TRIP 561 [RFC3219]. However, a proxy server could also delegate this 562 translation task to any other proxy server since proxy servers are 563 free to apply whatever routing logic they desire. For local numbers, 564 the proxy MUST NOT translate "tel" URIs whose context it does not 565 understand. 567 As noted earlier, all phone numbers MUST use the global form unless 568 they cannot be represented as such. If the local-number format is 569 used, it MUST be qualified by the 'phone-context' parameter. 570 Effectively, the combination of local number and phone context makes 571 the tel URI globally unique. 573 While web pages, vCard business cards, address books and directories 574 can easily contain global tel URIs, users on twelve-button (IP) 575 phones cannot dial such numbers directly and are typically accustomed 576 to dialing shorter strings, e.g., for PBX extensions or local 577 numbers. These so-called dial-strings (Section 1) are not directly 578 represented by tel URIs, as noted. We refer to the rules that govern 579 the translation of dial strings into SIP and tel URIs, global or 580 local, as the dial plan. Currently, translations from dial strings 581 to tel URIs have to take place in end systems. Future efforts may 582 provide means to carry dial strings in a SIP URI, for example, but no 583 such mechanisms exist at the time of writing. 585 A SIP UA can use a dial plan to translate dial strings into SIP or 586 "tel" URIs. The dial plan can be manually configured or, preferably, 587 be downloaded as part of a device configuration mechanism. (At this 588 time, there is no standardized mechanism for this.) 589 A mobile user can use at least two dial plans, namely the dial plan 590 for the network that he is currently visiting and the dial plan for 591 his home network. Generally, dialed numbers that are meant to 592 represent global numbers will look the same after the translation 593 regardless of the dial plan, even if, say, the visited network uses 594 '0' for dialing an 'outside' number and the user's home network uses 595 '9', as long as the user is aware of the current dial plan. However, 596 local extensions that do not have a direct global equivalent may well 597 behave differently. To avoid any ambiguity, the dial plan MUST 598 insert a suitable 'phone-context' string when performing the 599 translation. If the 'phone-context' is a domain name, there are 600 three cases: 601 1. The outbound proxy recognizes the domain name in the tel or SIP 602 URI as its local context and can route the request to a gateway 603 that understands the local number. 604 2. The outbound proxy does not use the same phone context, but can 605 route to a proxy that handles this phone context. This routing 606 can be done via a lookup table or the domain name of the phone 607 context might be set up to reflect the SIP domain name of a 608 suitable proxy. For example, a proxy may always route calls with 609 tel URIs like 611 tel:1234;phone-context=munich.example.com 613 to the SIP proxy located at munich.example.com. (Proxies that 614 receive a tel URI with a context they do not understand are 615 obligated to return a 404 (Not Found) status resonse, so that an 616 outbound proxy may decide to attempt such a heuristic.) 617 3. The outbound proxy does not recognize the phone context and 618 cannot find the appropriate proxy cognizant of that phone 619 context. In that case, the translation fails and the outbound 620 proxy returns a 404 (Not Found) error response. 622 10. Acknowledgments 624 This document is derived from RFC 2806 [RFC2806], written by Antti 625 Vaehae-Sipilae. Flemming Andreasen, Francois Audet, Lawrence Conroy, 626 Cullen Jennings, Michael Hammer, Paul Kyzivat, Andrew Main, Xavier 627 Marjou, Jon Peterson, Mike Pierce, Jonathan Rosenberg and James Yu 628 provided extensive comments. 630 11. Security Considerations 632 The security considerations parallel those for the mailto URL 633 [RFC2368]. 635 Web clients and similar tools MUST NOT use the tel URI to place 636 telephone calls without the explicit consent of the user of that 637 client. Placing calls automatically without appropriate user 638 confirmation may incur a number of risks, such as those described 639 below: 640 o Calls may incur costs. 641 o The URI may be used to place malicious or annoying calls. 642 o A call will take the user's phone line off-hook, thus preventing 643 its use. 644 o A call may reveal the user's, possibly unlisted, phone number to 645 the remote host in the caller identification data, and may allow 646 the attacker to correlate the user's phone number with other 647 information such as the e-mail or IP address. 649 This is particularly important for tel URIs embedded in HTML links as 650 a malicious party may hide the true nature of the URI in the link 651 text, as in 653 Find free information here 654 tel:+1-800-555-1234 656 Tel URIs may reveal private information, similar to including phone 657 numbers as text. However, the presence of the tel: schema identifier 658 may make it easier for an adversary using a search engine to discover 659 such numbers. 661 12. IANA Considerations 663 This document requires no IANA actions. 665 13. Changes Since RFC 2806 667 The specification is syntactically backwards-compatible with the 668 "tel" URI defined in RFC 2806 [RFC2806], but has been completely 669 rewritten. This document more clearly distinguishes telephone 670 numbers as identifiers of network termination points from dial 671 strings and removes the latter from the purview of "tel" URIs. 672 Compared to RFC 2806, references to carrier selection, dial context, 673 fax and modem URIs, post-dial strings and pause characters have been 674 removed. The URI syntax now conforms to RFC 2396 [RFC2396]. 676 A section on using tel URIs in SIP was added. 678 14. References 680 14.1 Normative References 682 [E.123] International Telecommunications Union, "Notation for 683 national and international telephone numbers, e-mail 684 addresses and web addresses", Recommendation E.123, 685 February 2001. 687 [E.161] International Telecommunications Union, "Arrangement of 688 digits, letters and symbols on telephones and other 689 devices that can be used for gaining access to a telephone 690 network", Recommendation E.161, May 1995. 692 [E.164] International Telecommunications Union, "The international 693 public telecommunication numbering plan", Recommendation 694 E.164, May 1997. 696 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 697 Requirement Levels", BCP 14, RFC 2119, March 1997. 699 [RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 700 Specifications: ABNF", RFC 2234, November 1997. 702 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 703 A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, 704 "SIP: Session Initiation Protocol", RFC 3261, June 2002. 706 [T1.703] ANSI, "Allocation of Letters to the Keys of Numeric 707 Keypads for Telecommunications", Standard T1.703-1995 708 (R1999), 1999. 710 14.2 Informative References 712 [I-D.yu-tel-url] 713 Yu, J., "New Parameters for the 'tel' URL to Support 714 Number Portability and Freephone Service", 715 draft-yu-tel-url-08 (work in progress), November 2003. 717 [RFC2368] Hoffman, P., Masinter, L. and J. Zawinski, "The mailto URL 718 scheme", RFC 2368, July 1998. 720 [RFC2396] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform 721 Resource Identifiers (URI): Generic Syntax", RFC 2396, 722 August 1998. 724 [RFC2806] Vaha-Sipila, A., "URLs for Telephone Calls", RFC 2806, 725 April 2000. 727 [RFC2916] Faltstrom, P., "E.164 number and DNS", RFC 2916, September 728 2000. 730 [RFC3187] Hakala, J. and H. Walravens, "Using International Standard 731 Book Numbers as Uniform Resource Names", RFC 3187, October 732 2001. 734 [RFC3191] Allocchio, C., "Minimal GSTN address format in Internet 735 Mail", RFC 3191, October 2001. 737 [RFC3192] Allocchio, C., "Minimal FAX address format in Internet 738 Mail", RFC 3192, October 2001. 740 [RFC3219] Rosenberg, J., Salama, H. and M. Squire, "Telephony 741 Routing over IP (TRIP)", RFC 3219, January 2002. 743 [T.50] International Telecommunications Union, "International 744 Reference Alphabet (IRA) (Formerly International Alphabet 745 No. 5 or IA5) - Information technology - 7-bit coded 746 character set for information interchange", Recommendation 747 T.50, 1992. 749 Author's Address 751 Henning Schulzrinne 752 Columbia University 753 Department of Computer Science 754 450 Computer Science Building 755 New York, NY 10027 756 US 758 Phone: +1 212 939 7042 759 EMail: hgs@cs.columbia.edu 760 URI: http://www.cs.columbia.edu 762 Intellectual Property Statement 764 The IETF takes no position regarding the validity or scope of any 765 Intellectual Property Rights or other rights that might be claimed to 766 pertain to the implementation or use of the technology described in 767 this document or the extent to which any license under such rights 768 might or might not be available; nor does it represent that it has 769 made any independent effort to identify any such rights. 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