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'14') (Obsoleted by RFC 4566) == Outdated reference: A later version (-28) exists of draft-ietf-dhc-dhcpv6-26 == Outdated reference: A later version (-01) exists of draft-ietf-sip-dhcpv6-00 == Outdated reference: A later version (-02) exists of draft-ietf-sipping-basic-call-flows-01 == Outdated reference: A later version (-02) exists of draft-ietf-sipping-pstn-call-flows-00 == Outdated reference: A later version (-15) exists of draft-ietf-sipping-service-examples-02 == Outdated reference: A later version (-07) exists of draft-ietf-sip-refer-06 == Outdated reference: A later version (-05) exists of draft-ietf-sip-replaces-02 Summary: 2 errors (**), 0 flaws (~~), 13 warnings (==), 12 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 SIPPING Working Group M. Garcia-Martin 3 Internet-Draft Ericsson 4 Expires: April 11, 2003 October 11, 2002 6 3rd-Generation Partnership Project (3GPP) Release 5 requirements on 7 the Session Initiation Protocol (SIP) 8 draft-ietf-sipping-3gpp-r5-requirements-00.txt 10 Status of this Memo 12 This document is an Internet-Draft and is in full conformance with 13 all provisions of Section 10 of RFC2026. 15 Internet-Drafts are working documents of the Internet Engineering 16 Task Force (IETF), its areas, and its working groups. Note that 17 other groups may also distribute working documents as Internet- 18 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 April 11, 2003. 33 Copyright Notice 35 Copyright (C) The Internet Society (2002). All Rights Reserved. 37 Abstract 39 The 3rd Generation Partnership Project (3GPP) has selected SIP [2]as 40 the session establishment protocol for the 3GPP IP Multimedia Core 41 Network Subsystem (IMS). IMS is part of the Release 5 of the 3GPP 42 specifications. Although SIP is a protocol that fulfills most of the 43 requirements to establish a session in an IP network, SIP has never 44 been evaluated against the specific 3GPP requirements for operation 45 in a cellular network. In this document we express the requirements 46 identified by 3GPP to support SIP for the Release 5 of the 3GPP IMS 47 in cellular networks. 49 Table of Contents 51 1. Conventions . . . . . . . . . . . . . . . . . . . . . . . 5 52 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . 5 53 3. Overview of the 3GPP IMS . . . . . . . . . . . . . . . . . 5 54 4. 3GPP Requirements on SIP . . . . . . . . . . . . . . . . . 8 55 4.1 General requirements . . . . . . . . . . . . . . . . . . . 8 56 4.1.1 Efficient use of the radio interface . . . . . . . . . . . 8 57 4.1.2 Minimum session setup time . . . . . . . . . . . . . . . . 8 58 4.1.3 Minimum support required in the terminal . . . . . . . . . 8 59 4.1.4 Roaming and non-roaming . . . . . . . . . . . . . . . . . 8 60 4.1.5 Terminal mobility management . . . . . . . . . . . . . . . 8 61 4.1.6 IP version 6 . . . . . . . . . . . . . . . . . . . . . . . 9 62 4.2 SIP outbound proxy . . . . . . . . . . . . . . . . . . . . 9 63 4.2.1 SIP outbound proxy . . . . . . . . . . . . . . . . . . . . 9 64 4.2.2 Discovery of the SIP outbound proxy . . . . . . . . . . . 9 65 4.3 Registration . . . . . . . . . . . . . . . . . . . . . . . 9 66 4.3.1 Registration required . . . . . . . . . . . . . . . . . . 10 67 4.3.2 Location of the SIP Registrar . . . . . . . . . . . . . . 10 68 4.3.3 Efficient registration . . . . . . . . . . . . . . . . . . 10 69 4.3.4 Registration for roaming and non-roaming cases . . . . . . 10 70 4.3.5 Visited domain name . . . . . . . . . . . . . . . . . . . 10 71 4.3.6 De-registration . . . . . . . . . . . . . . . . . . . . . 11 72 4.4 SIP Compression . . . . . . . . . . . . . . . . . . . . . 12 73 4.4.1 Compression algorithm independency . . . . . . . . . . . . 12 74 4.4.2 Extensibility of the SIP compression . . . . . . . . . . . 12 75 4.4.3 Minimal impact of SIP compression on the network . . . . . 12 76 4.4.4 Optionality of SIP compression . . . . . . . . . . . . . . 13 77 4.5 QoS requirements related to SIP . . . . . . . . . . . . . 13 78 4.5.1 Independence between QoS signaling and SIP . . . . . . . . 13 79 4.5.2 Coordination between SIP and QoS/Resource allocation . . . 13 80 4.6 Prevention of theft of service . . . . . . . . . . . . . . 14 81 4.7 Radio resource authorization . . . . . . . . . . . . . . . 14 82 4.8 Prevention of malicious usage . . . . . . . . . . . . . . 14 83 4.9 Prevention of denial of service . . . . . . . . . . . . . 15 84 4.10 Identification of users . . . . . . . . . . . . . . . . . 15 85 4.10.1 Private user identity . . . . . . . . . . . . . . . . . . 15 86 4.10.2 Public user identities . . . . . . . . . . . . . . . . . . 15 87 4.10.3 Delivery of the dialed public user ID . . . . . . . . . . 17 88 4.11 Identifiers used for routing . . . . . . . . . . . . . . . 17 89 4.12 Hiding requirements . . . . . . . . . . . . . . . . . . . 17 90 4.12.1 Hiding of the network structure . . . . . . . . . . . . . 17 91 4.12.2 Hiding of IP addresses . . . . . . . . . . . . . . . . . . 17 92 4.12.3 SIP hiding proxy . . . . . . . . . . . . . . . . . . . . . 18 93 4.13 Cell-ID . . . . . . . . . . . . . . . . . . . . . . . . . 18 94 4.13.1 Cell-ID in signaling from the UA to the visited and home 95 networks . . . . . . . . . . . . . . . . . . . . . . . . . 18 96 4.13.2 Format of the cell-ID . . . . . . . . . . . . . . . . . . 18 97 4.14 Release of sessions . . . . . . . . . . . . . . . . . . . 18 98 4.14.1 Ungraceful session release . . . . . . . . . . . . . . . . 19 99 4.14.2 Graceful session release . . . . . . . . . . . . . . . . . 19 100 4.15 Routing of SIP messages . . . . . . . . . . . . . . . . . 19 101 4.15.1 SIP outbound proxy . . . . . . . . . . . . . . . . . . . . 20 102 4.15.2 SIP serving proxy in the home network . . . . . . . . . . 20 103 4.15.3 INVITE might follow a different path than REGISTER . . . . 20 104 4.15.4 SIP inbound proxy . . . . . . . . . . . . . . . . . . . . 20 105 4.15.5 Distribution of the Source Routing set of proxies . . . . 20 106 4.16 Emergency sessions . . . . . . . . . . . . . . . . . . . . 21 107 4.17 Identities used for session establishment . . . . . . . . 21 108 4.17.1 Remote Party Identification presentation . . . . . . . . . 21 109 4.17.2 Remote Party Identification privacy . . . . . . . . . . . 21 110 4.17.3 Remote Party Identification blocking . . . . . . . . . . . 21 111 4.17.4 Anonymity . . . . . . . . . . . . . . . . . . . . . . . . 22 112 4.17.5 Anonymous session establishment . . . . . . . . . . . . . 22 113 4.18 Charging . . . . . . . . . . . . . . . . . . . . . . . . . 22 114 4.18.1 Support of both prepaid and postpaid models . . . . . . . 22 115 4.18.2 Charging correlation levels . . . . . . . . . . . . . . . 23 116 4.18.3 Charging correlation principles . . . . . . . . . . . . . 23 117 4.18.4 Collection of Session Detailed Information . . . . . . . . 24 118 4.19 General support of additional capabilities . . . . . . . . 24 119 4.19.1 Additional capabilities . . . . . . . . . . . . . . . . . 24 120 4.19.2 DTMF signaling . . . . . . . . . . . . . . . . . . . . . . 24 121 4.19.3 Early Media . . . . . . . . . . . . . . . . . . . . . . . 25 122 4.20 Exchange of session description . . . . . . . . . . . . . 25 123 4.21 Prohibition of certain SDP parameters . . . . . . . . . . 25 124 4.21.1 Prohibition of codecs . . . . . . . . . . . . . . . . . . 25 125 4.21.2 Prohibition of media types . . . . . . . . . . . . . . . . 26 126 4.22 Network initiated re-authentication . . . . . . . . . . . 26 127 4.23 Security model . . . . . . . . . . . . . . . . . . . . . . 26 128 4.24 Access Domain Security . . . . . . . . . . . . . . . . . . 27 129 4.24.1 General requirements . . . . . . . . . . . . . . . . . . . 27 130 4.24.2 Authentication . . . . . . . . . . . . . . . . . . . . . . 29 131 4.24.3 Message Protection . . . . . . . . . . . . . . . . . . . . 29 132 4.24.4 Negotiation of mechanisms . . . . . . . . . . . . . . . . 30 133 4.24.5 Verification of messages . . . . . . . . . . . . . . . . . 31 134 4.25 Network Domain Security . . . . . . . . . . . . . . . . . 31 135 5. Security considerations . . . . . . . . . . . . . . . . . 32 136 6. IANA considerations . . . . . . . . . . . . . . . . . . . 32 137 7. Contributors . . . . . . . . . . . . . . . . . . . . . . . 32 138 Normative References . . . . . . . . . . . . . . . . . . . 32 139 Informational References . . . . . . . . . . . . . . . . . 33 140 Author's Address . . . . . . . . . . . . . . . . . . . . . 35 141 Full Copyright Statement . . . . . . . . . . . . . . . . . 36 143 1. Conventions 145 This document does not specify any protocol of any kind. Therefore, 146 the usage of the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", 147 "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and 148 "OPTIONAL" in this document, as described in RFC-2119 [1], does not 149 apply. 151 2. Introduction 153 3GPP has selected SIP [2]as the protocol to establish and tear down 154 multimedia sessions in the IP Multimedia Subsystem (IMS). A 155 description of the IMS can be found in 3GPP Technical Specification 156 23.228 [31]. A comprehensive set of session flows can be found in 157 3GPP Technical Specification 24.228 [32]. 159 This document is an effort to define the requirements applicable to 160 the usage of the SIP protocol suite in cellular networks, and 161 particularly in the 3GPP IMS, and in particular, to the Release 5 of 162 the 3GPP specifications. Further releases of the 3GPP specifications 163 may contain additional requirements to SIP. This document focuses on 164 the requirements identified for the 3GPP Release 5 IMS. 166 The rest of this document is structured as follows: 168 o Section 3 offers an overview of the 3GPP IMS. Readers who are not 169 familiar with it should carefully read this section. 171 o Section 4 contains the 3GPP requirements to SIP. Requirements are 172 grouped by categories. Some requirements include a statement on 173 possible solutions that would be able to fulfill the requirement. 174 Note also that, as a particular requirement might be fulfilled by 175 different solutions, not all the solutions might have an impact on 176 SIP. 178 3. Overview of the 3GPP IMS 180 This section gives the reader an overview of the 3GPP IM CN Subsystem 181 (IMS). It is not intended to be comprehensive. But it provides 182 enough information to understand the basis of the 3GPP IMS. Readers 183 are encouraged to find a more detailed description in the 3GPP 184 Technical Specifications 23.060 [30], 23.228 [31] and 24.228 [32]. 186 For a particular cellular device, the 3GPP IMS network is further 187 decomposed in a home network and a visited network. 189 An IMS subscriber belongs to his or her home network. Services are 190 triggered and may be executed in the home network. One or more SIP 191 servers are deployed in the SIP home network to support the IP 192 Multimedia Subsystem. Among those SIP servers, there is a SIP 193 serving proxy, which is also acting as a SIP registrar. 194 Authentication/Authorization servers may be part of the home network 195 as well. Users are authenticated in the home network. 197 A SIP outbound proxy is provided to support the UA. The SIP outbound 198 proxy is typically located in the visited network, although it may 199 located in the home network as well. The SIP outbound proxy 200 maintains security associations between itself and the terminals, and 201 interworks with the resource management in the packet network. 203 The SIP outbound proxy is assigned after the mobile device has 204 connected to the access network. Once this proxy is assigned, it 205 does not change while the mobile remains connected to the access 206 network. Thus the mobile can move freely within the access network 207 without SIP outbound proxy reassignment. 209 The home network may support also one or more SIP edge proxies. 210 These nodes may act as the first entry point for SIP signaling to the 211 home network and may decide (with the help of location servers) which 212 SIP registrar server to assign to a particular user. Typically the 213 address of the home network SIP edge proxy is configured in DNS in 214 the form of a DNS NAPTR and SRV records for SIP. 216 Additionally, home and visited networks may deploy, if required, a 217 SIP hiding proxy. The main purpose of the SIP hiding proxy is to 218 hide the network configuration. 220 The 3GPP IM CN Subsystem is designed to be access independent. 221 Access is granted from 3GPP cellular terminals or from other 222 terminals that use other accesses out of the scope of 3GPP. 224 3GPP cellular IP Multimedia terminals use the existing General Packet 225 Radio Service (GPRS) [30] as a transport network for IP datagrams. 226 The terminals first connect to the GPRS network to get an IPv6 227 prefix. In order to do this, the terminals must perform a (GPRS) 228 Attach procedure followed by a (GPRS) PDP Context Activation 229 procedure. These GPRS procedures are required to be completed before 230 any IP Multimedia session can be established. 232 As a result of the above-mentioned GPRS procedures, the terminal has 233 built an IPv6 address. The IPv6 address belongs to the same network 234 address space as the SIP outbound proxy. The address does not change 235 as the mobile terminal moves while still attached to the same network 236 address space. 238 If the terminal moves from a GPRS access to another GPRS access, the 239 above-mentioned GPRS procedures needs to start from the beginning to 240 allocate an IPv6 address to the terminal. 242 Figure 1 shows an overview of the 3GPP architecture for IM CN 243 Subsystem. 245 +-------------+ +----------------+ +----------------+ 246 | | | | | +------+ | 247 | | | | | | SIP | | 248 | | | | | |server| | 249 | | | | | | +------+ | 250 +-|+ | | | | | / | 251 | | | | | +------+ | | +------+ | 252 | | | | | | SIP | | | | SIP | | 253 | | ---|-------------|--|----|server|----|---|-|server| | 254 +--+ | | | +------+ | | +------+ | 255 | | | | | | 256 SIP | GPRS access | | Visited Network| | Home Network | 257 dev. +-------------+ +----------------+ +----------------+ 259 Figure 1: Overview of the 3GPP IMS architecture 261 Another possible future configuration is depicted in Figure 2. In 262 that case, a general-purpose computer (e.g., a laptop computer) is 263 connected to a GPRS terminal. The computer hosts the Multimedia 264 application (comprising SIP, SDP, RTP, etc.). The GPRS terminal 265 handles the radio access and the GPRS connectivity. Note that, for 266 the sake of clarity, in this example the home network has not been 267 depicted in the figure. 269 +-------------+ +----------------+ 270 +-------+ | | | | | 271 | | +-|+ | | | | 272 | | | | | | | +------+ | 273 +-------+ | | | | | | SIP | | 274 / / --------| | ---|-------------|-------|server|------ 275 /-------/ +--+ | | | +------+ | 276 | | | | 277 SIP GPRS | GPRS access | | Visited Network| 278 client terminal +-------------+ +----------------+ 280 Figure 2: A computer connected to a GPRS terminal 282 Services are typically executed in an application server. The 283 interface between the SIP server and the application server is based 284 on SIP. However, certain operators may want to reuse the existing 285 technology, and therefore, they may need to interoperate SIP with 286 protocols like CAMEL/Intelligent-Network or Open services 287 Architecture (OSA). 289 4. 3GPP Requirements on SIP 291 4.1 General requirements 293 This section does not specify any particular requirement to SIP. 294 However, it includes a list of general requirements that must be 295 considered when developing solutions to particular requirements. 297 4.1.1 Efficient use of the radio interface 299 The radio interface is a scarce resource. As such, the exchange of 300 signaling messages between the mobile terminal and the network should 301 be minimized. All the mechanisms developed should make an efficient 302 use of the radio interface. 304 See also the related requirements in Section 4.4 306 4.1.2 Minimum session setup time 308 All the procedures and mechanisms should have a minimum impact on the 309 session setup time as perceived by the user. When there is a choice 310 between performing tasks at session establishment and in transactions 311 prior to session establishment, then the tasks should be performed 312 prior to session establishment. 314 See also the related requirements in Section 4.4 316 4.1.3 Minimum support required in the terminal 318 As terminals could be rather small devices, memory requirements, 319 power consumption, processing power, etc. should be kept to a 320 minimum. Mandating support for additional protocols in the terminal 321 must meet this requirement. 323 4.1.4 Roaming and non-roaming 325 All the requirements must be met for both roaming and non-roaming 326 scenarios. There should not be a significant change in the signaling 327 procedures between roaming and non-roaming scenarios. 329 4.1.5 Terminal mobility management 331 As terminal mobility is managed by the access network, there is no 332 need to support terminal mobility management in SIP. 334 4.1.6 IP version 6 336 3GPP IMS is solely designed to use IP version 6. As a consequence, 337 all protocols must support IPv6 addesses. 339 4.2 SIP outbound proxy 341 4.2.1 SIP outbound proxy 343 A SIP outbound proxy is provided to support both roaming and non- 344 roaming scenarios. The SIP outbound proxy may be located either in 345 the home network or in the visited network. 347 4.2.2 Discovery of the SIP outbound proxy 349 There must be a general mechanism so that the mobile device (UA) 350 learns the SIP outbound proxy address. 352 The Internet Draft DHCPv6 option for SIP servers [22] seems to 353 fulfill the requirement. 355 In addition to the above expressed requirement, the 3GPP access 356 network may provide the SIP outbound proxy address during access 357 network bearer establishment. This is considered a less general 358 mechanism though. 360 4.3 Registration 362 The home network must maintain one or more SIP registrars. The SIP 363 registrar authenticates the user and registers the IP address where 364 the user can be located. 366 Once the terminal is switched on, the mobile device UA reads its 367 configuration data. This data may be stored in a SIM card or any 368 other memory device. The configuration data contains an 369 identification of the home network. The device finds the SIP 370 registrar address from the home network domain name. The terminal 371 sends the registration through the SIP outbound proxy. 373 In order to support the search of the registrar, the home network 374 contains one or more SIP servers that may be configured in DNS with 375 the NAPTR/SRV record of SIP. These are the home network edge 376 proxies. Their mission is to serve as a first point of contact in 377 the home network, and decide (with the help of location servers) 378 which SIP registrar server to assign to a particular user. 380 The procedures specified in RFC 3263 [11] applied to a REGISTER 381 message seems to be sufficient to meet this requirement. 383 4.3.1 Registration required 385 A user must register to the IMS before he/she can receive any 386 invitation to any sessions. In addition, it is desirable for the 387 user to register before initiating any sessions. The rationale 388 behind this is that: 390 1. The SIP serving proxy in the home network needs to know when the 391 user is available and from which terminal, in order to route 392 received SIP requests for sessions and services. 394 2. The user can be pre-authenticated early, so that authentication 395 does not contribute to post-dial delay. The procedure should not 396 have a penalty on the session setup time (see also the 397 requirement stated in Section 4.1.2). 399 3. The user is assigned a particular serving proxy. The serving 400 proxy downloads the service profile for that user to trigger 401 services. 403 Therefore, 3GPP has mandated the mobile device UA to register before 404 the mobile device UA initiates any session. 406 4.3.2 Location of the SIP Registrar 408 4.3.3 Efficient registration 410 Due to the scarce radio interface resource, a single registration 411 must be sufficient to insure that the mobile UA is reachable from 412 both the home and visited networks. 414 A single REGISTER message, addressed to the registrar, may traverse 415 the SIP outbound proxy. This can install, if needed, soft 416 registration states in the SIP outbound proxy. 418 4.3.4 Registration for roaming and non-roaming cases 420 Independently of whether the UA is roaming or not, it is desirable 421 for the registration procedure to be the same. 423 4.3.5 Visited domain name 425 The home network must be able to validate the existence of a roaming 426 agreement between the home and the visited network. The home network 427 needs to validate that the user is allowed to roam to such a visited 428 network. Therefore, there must be a mechanism so that the visited 429 network identity is known at registration time at the home network. 431 It is acceptable to represent the visited network identity either as 432 a visited network domain name or as a string. 434 4.3.6 De-registration 436 4.3.6.1 De-registration of users 438 There must be a procedure for a user to de-register from the network. 439 This procedure may be used, e.g., when the user deactivates the 440 terminal. 442 We believe that a REGISTER with an expiration timer of 0 will meet 443 the requirement. 445 4.3.6.2 Network initiated de-registration or re-registration 447 There are a number of situations where the network needs to de- 448 register or trigger a re-registration of a previously registered UA. 449 Examples of usage are described in Section 4.3.6.3, Section 4.3.6.4 450 and Section 4.3.6.5. 452 This implies a need for a notification mechanism whereby the UA can 453 be notified of the de-registration, or a request for re-registration. 455 We believe this requirement is met by the SIP-specific event 456 notification [13] and a registration event package [16]. 458 4.3.6.3 Network initiated de-registration, network maintenance 460 There might be cases when the SIP serving proxy has to shutdown, 461 e.g., due to maintenance operation. Although this situation is not 462 likely to happen in everyday situations, still it is desirable to 463 have a mechanism to inform the UA that his current registration is 464 being cancelled. The UA may initiate another registration process, 465 that will lead to the selection of a new SIP serving proxy. 467 4.3.6.4 Network initiated de-registration, network/traffic determined 469 The system must support a mechanism to avoid inconsistent information 470 storage and remove any redundant registration information. This case 471 will occur when a subscriber roams to a different network without a 472 prior de-registration. This case occurs in normal mobility 473 procedures when the user roams from one access network to another 474 one, or when imposing new service conditions to roamers. 476 4.3.6.5 Network initiated de-registration, administrative 478 For different reasons (e.g., subscription termination, stolen 479 terminal, etc.) a home network administrative function may determine 480 a need to clear a user's SIP registration. It is desirable to have a 481 mechanism whereby the SIP serving proxy can inform the UA that his 482 registration is being cancelled. 484 There must be a procedure for a the SIP serving proxy to de-register 485 users. The de-registration information must be available at all the 486 proxies that keep registration state and the UA. 488 We believe that a procedure based on SIP-specific event notification 489 [13] and a registration event package [16]. 491 4.4 SIP Compression 493 The radio interface is a scarce resource and typically, the available 494 bandwidth over the radio interface is limited. These two factors 495 seem to limit the transport of possibly large SIP messages over the 496 air interface. Particularly, the session setup time might be 497 extended, due to the time needed to transport SIP messages over a 498 limited bandwidth channel. 500 On the other hand, the number and size of certain SIP header values, 501 such as Via or Record-Route, seems to not be limited. A mobile 502 device UA may present limitations in the available memory to store 503 this kind of information. 505 Therefore, there must be a mechanism to efficiently transport SIP 506 signaling packets over the radio interface, by compressing the SIP 507 messages between the mobile device UA and the SIP outbound proxy, and 508 between the SIP outbound proxy and the mobile device UA. Note that 509 compression of IP and transport layer protocol headers that carry 510 these SIP messages is also a requirement, although we believe that 511 does not have an impact on SIP. 513 4.4.1 Compression algorithm independency 515 The chosen solution(s) must be able to allow the operation under 516 several different compression algorithms. 518 4.4.2 Extensibility of the SIP compression 520 The chosen solution(s) must be extensible to facilitate the 521 incorporation of new and improved compression algorithms in a 522 backward compatible way, as they become available. 524 4.4.3 Minimal impact of SIP compression on the network 526 Application specific compression must minimize impacts on existing 527 3GPP access networks (such as base stations transceivers). On the 528 other hand, the compression mechanism should be independent of the 529 access, e.g., the compression must be defined between the mobile 530 device UA and the outbound SIP proxy. 532 4.4.4 Optionality of SIP compression 534 It must be possible to leave the usage of compression for SIP 535 signaling optional. To facilitate mobile terminal roaming between 536 networks which are using compression, the mobile terminal should 537 always support ability to compress SIP signaling. If compression is 538 not supported, communication may continue without compression, 539 depending on the local policy of the visited network. 541 4.4.4.1 Compression reliability 543 The compression mechanism should be reliable and be able to recover 544 automatically from errors generated during the decompression. 546 4.5 QoS requirements related to SIP 548 4.5.1 Independence between QoS signaling and SIP 550 The selection of QoS signaling and resource allocation schemes must 551 be independent of the selected session control protocols. This 552 allows for independent evolution of QoS control and SIP. 554 4.5.2 Coordination between SIP and QoS/Resource allocation 556 4.5.2.1 Allocation before alerting 558 In establishing a SIP session, it must be possible for an application 559 to request that the resources needed for bearer establishment are 560 successfully allocated before the destination user is alerted. Note, 561 however, that it must be also possible for an SIP application in a 562 terminal to alert the user before the radio resources are established 563 (e.g., if the user wants to participate in the media negotiation). 565 We believe this requirement is met by Integration of Resource 566 Management and SIP [17]. 568 4.5.2.2 Destination user participates in the bearer negotiation 570 In establishing a SIP session, it must be possible for a terminating 571 application to allow the destination user to participate in 572 determining which bearers shall be established. Although it must be 573 possible to establish the SIP session without user intervention. 575 We believe this requirement is met by the standard SDP negotiation 576 described in SIP [2] and the SDP offer/answer model [12] and the 577 extensions described in Integration of Resource Management and SIP 578 [17]. 580 4.5.2.3 Successful bearer establishment 582 Successful bearer establishment must include the completion of any 583 required end-to-end QoS signaling, negotiation and resource 584 allocation. 586 We believe this requirement is met by the procedures described in the 587 Integration of Resource Management and SIP [17] . 589 4.6 Prevention of theft of service 591 Users are typically allocated QoS resources. There is an admission 592 control mechanism that prevents users to exceeds the limits 593 negotiated with the network. The network must prevent unauthorized 594 users to make usage of non-authorized resources. For instance, the 595 network must provide mechanism to prevent a user to use the resources 596 allocated to a second user, and for which this second user may be 597 paying. 599 We believe this requirement may be met by the procedures described in 600 the Private SIP extensions for Media Authorization [18]. 602 4.7 Radio resource authorization 604 As radio resources are very valuable the network must be able to 605 manage these in a controlled manner. The network must be able to 606 identify who is using these resources and be able to authorize their 607 usage. For example, a mobile device terminal could execute an 608 unlimited and uncontrolled resorce reservation procedure if the 609 network does not supervise the usage of radio resources. 611 We believe this requirement is met by the procedures described in the 612 Private SIP extensions for Media Authorization [18].. 614 4.8 Prevention of malicious usage 616 The 3GPP IMS must prevent mobile devices for making a malicious usage 617 of the network. For instance, a malicious UA could not follow the 618 Record-Route procedures, so that subsequent request bypass proxies 619 which recorded route. 621 4.9 Prevention of denial of service 623 The risk of a proxy to receive a denial of service attack shall be 624 minimized. For instance, a malicious mobile device could learn a SIP 625 proxy IP address and port number (e.g., in a Record-Route header 626 value) and establish an attack to that proxy. 628 4.10 Identification of users 630 4.10.1 Private user identity 632 In order to use the 3GPP IMS, a user is assigned a private user 633 identity. The home network operator assigns the private user 634 identity, which is used to uniquely identify the user from a network 635 perspective. The private user identity is used, for example, for 636 authentication, authorization, administration, and possibly 637 accounting purposes. Note that the private user identity is not used 638 for routing of SIP messages. 640 The private user identity is a unique global identity defined by the 641 Home Network Operator. The identity takes the form of a Network 642 Access Identifier (NAI) as defined in RFC 2486 [7]. 644 The end user does not have access to the private user identity. 645 Typically the identity is stored in a Subscriber Identity Module 646 card. 648 The private user identity is permanently allocated to a user (it is 649 not a dynamic identity), and is valid for the duration of the user's 650 business subscription with the home network. 652 4.10.1.1 Private user ID in registrations 654 The mobile UA must deliver the private user identity to the SIP 655 outbound proxy and the registrar at registration time. 657 The private user identity is used as the basis for authentication 658 during registration of the mobile user. The term authentication is 659 used in this document with the same meaning as it is defined in RFC 660 2828 [8]. 662 We believe that this requirement is met by populating the username 663 field of the Authorization: header value of the REGISTER request with 664 the private user identity. 666 4.10.2 Public user identities 668 In order to use the 3GPP IMS, a user is assigned one or more public 669 user identities. The user will make use of the public user identity/ 670 identities when requesting communication to other users. For 671 example, the public user identity might be included on a business 672 card. 674 Different public user identities may be grouped into a user profile. 675 A user may have different profiles, each one containing different 676 public user identities. A public user identity can be part of a 677 single user profile. 679 The user may need to register one or more public user identities 680 prior to receiving communications addressed to that public user 681 identity. 683 We believe this requirement is met by populating the From: and To: 684 header values of a REGISTER message with the public user identity. 686 4.10.2.1 Format of the public user identities 688 The public user identity/identities must take the form of a SIP URI 689 (as defined in RFC 3261 [2] and RFC 2396 [4]) or the form of a E.164 690 [37]number. 692 We believe this requirement is met by using SIP URLs and telephone 693 numbers represented in SIP URLs as described in SIP [3]. In 694 addition, tel: URLs as specified in [13] can be used to fulfill the 695 requirement. 697 4.10.2.2 Registration of public user IDs 699 It must be possible to register globally (i.e., through one single UA 700 request) a user that has more than one public identity that belongs 701 to the same user profile, via a mechanism within the IMS. In this 702 case, the user will be registered with all the public identities 703 associated to a user profile. 705 We believe this requirement may be accomplished by external 706 procedures. For example, the user's profile may contain a list of 707 alias identities that the registrar considers active if the primary 708 identity is registered. The user may get informed of the 709 automatically registered public user IDs by subscribing to its own 710 registration state. 712 4.10.2.3 Authentication of the public user ID 714 Public user identities are not authenticated by the 3GPP IMS. 715 However the network authorizes that the public user identity is 716 associated to the registered private user identity. 718 There is a list of public user identities that are associated with 719 each private user ID within the IMS. IMS will reject attempts to use 720 other public identities with this private user ID. 722 4.10.3 Delivery of the dialed public user ID 724 Typically a UA will be registered under a set of different public 725 user IDs. As such, sessions destined to the user can be placed to 726 any of the registered public user IDs. The serving proxy, 727 application server(s) in the termination network may apply certain 728 filtering rules or services based on the public user ID contained in 729 the Request-URI. The UA may also apply certain filtering rules or 730 services based on the called public user ID. 732 As such, it must be possible, for all sessions, to deliver the dialed 733 public user ID to the terminating entities, such as the serving 734 proxy, application servers and the terminating UA. 736 4.11 Identifiers used for routing 738 Routing of SIP signaling within IMS must use SIP URLs as defined in 739 SIP [2]. E.164 [37] format public user identities must not be used 740 for routing within IMS, and session requests based upon E.164 format 741 public user identities will require conversion into SIP URI format 742 for internal IMS usage. 744 We believe that this requirement is achieved by translating E.164 745 numbers into SIP URIs. A database, such as ENUM [10] might do the 746 job. 748 4.12 Hiding requirements 750 Although the requirements included in this section are not optional, 751 the hiding feature is an optional to use through configuration. This 752 means that a network operator can, at his desire, switch the hiding 753 functionality on or off. 755 4.12.1 Hiding of the network structure 757 A network operator need not be required to reveal the internal 758 network structure to another network (in Via, Route, or other 759 headers) that may contain indication of the number of SIP proxies, 760 domain name of the SIP proxies, capabilities of the SIP proxies or 761 capacity of the network. 763 4.12.2 Hiding of IP addresses 765 A network need not be required to expose the explicit IP addresses of 766 the nodes within the network (excluding firewalls and border 767 gateways). 769 4.12.3 SIP hiding proxy 771 In order to support the hiding requirements, a SIP hiding proxy may 772 be included in the SIP signaling path. Such additional proxy may be 773 used to shield the internal structure of a network from other 774 networks. 776 4.13 Cell-ID 778 The identity of the cell through which the 3GPP UA is accessing the 779 IMS (Cell-ID) may be used by the home network to provide localized 780 services or information on the location of the terminal during an 781 emergency call (when emergency calls are handled in IMS, see also the 782 requirement stated in Section 4.16). 784 4.13.1 Cell-ID in signaling from the UA to the visited and home networks 786 Assuming that the Cell-ID is obtained by the UA by other mechanisms 787 outside the scope or beyond SIP, the Cell-ID must be transported at 788 least in the following procedures: 790 o Registration 792 o Session Establishment (Mobile Originated) 794 o Session Establishment (Mobile Terminated) 796 o Session Release 798 The Cell-ID is private information and only of interest in the UA 799 home network. Therefore, the Cell-ID should be removed prior to 800 sending the SIP signaling beyond the originating home network. 802 4.13.2 Format of the cell-ID 804 The cell-ID must be sent in any of the formats described in the 3GPP 805 Technical Specification 23.003 [29]. 807 4.14 Release of sessions 809 In addition to the normal mechanisms to release a SIP session (e.g., 810 BYE), two cases are considered in this section. The ungraceful 811 release of the session (e.g., the terminal moves to an out-of- 812 coverage zone) and the graceful session release ordered by the 813 network (e.g., pre-paid caller runs out of credit). 815 We believe this requirement is met by a SIP entity acting as a so- 816 called transparent back-to-back User Agent. 818 4.14.1 Ungraceful session release 820 If an ungraceful session termination occurs (e.g., flat battery or 821 mobile leaves coverage), when a call stateful SIP proxy server (such 822 as the SIP serving proxy at home) is involved in a session, memory 823 leaks and eventually server failure can occur due to hanging state 824 machines. To ensure stable server operation and carrier grade 825 service, a mechanism to handle the ungraceful session termination 826 issue must be provided. We assume that there is a mechanism by which 827 the SIP outbound proxy is notified, by a mechanism external to SIP, 828 of the ungraceful session termination. This allows transforming the 829 ungraceful session release into a graceful session release ordered by 830 the network (see next section). For example, the SIP outbound proxy, 831 upon reception of the notification of loss of mobile radio coverage, 832 could send a BYE request on behalf of the terminal, although this BYE 833 cannot be authenticated. 835 4.14.2 Graceful session release 837 There must be a mechanism so that an entity in the network may order 838 the release of resources to other entities. This may be used, e.g., 839 in pre-paid calls when the user runs out of credit. 841 This release must not involve any request to the UA to send out a 842 release request (BYE), as the UA might not follow this request. The 843 receiving entity needs the guarantee that resources are released when 844 requested by the ordering entity. 846 The following objectives must be met: 848 o Accurately report the termination to the charging subsystem. 850 o Release the associated network resources: bearer resources and 851 signaling resources. 853 o Notify other parties to the session, if any, of the session 854 termination. 856 Where feasible, this mechanism should be at the SIP protocol level in 857 order to guarantee access independence for the system. 859 4.15 Routing of SIP messages 860 4.15.1 SIP outbound proxy 862 The 3GPP architecture includes a SIP outbound proxy which is 863 typically located in the visited network (although it may be located 864 in the home network). This outbound proxy provides local services 865 such as compression of SIP messages or security functions. In 866 addition, the outbound proxy may interact with the media reservation 867 mechanism to provide authentication and authorization support for 868 media reservation. 870 All mobile terminal originated session setup attempts must transit 871 the outbound proxy, so that the services provided by the outbound 872 proxy can be delivered to the mobile terminal. 874 4.15.2 SIP serving proxy in the home network 876 The serving proxy in the home network allows triggering of user 877 customized services that are typically executed in an application 878 server. 880 All mobile terminal originated session setup attempts must transit 881 the serving proxy in the home network, so that the proxy can properly 882 trigger the SIP services allocated to the user(e.g., speed dial 883 substitution). This implies a requirement for some sort of source- 884 routing mechanism to assure these proxies are transited correctly. 886 4.15.3 INVITE might follow a different path than REGISTER 888 The path taken by an INVITE request need not be restricted to the 889 specific path taken by a mobile terminal originated REGISTER request, 890 e.g., the INVITE may traverse just the SIP outbound proxy and the SIP 891 serving proxy, without passing through any other proxies. However, 892 the path taken by the INVITE may follow the same path taken by the 893 REGISTER . 895 4.15.4 SIP inbound proxy 897 The visited network may apply certain services and policies to 898 incoming sessions (such as establishment of security services or 899 interaction with the media reservation mechanism). Therefore, the 900 visited network may contain a SIP inbound proxy for terminating 901 sessions. In general, the SIP inbound proxy and the SIP outbound 902 proxy are the same SIP proxy. 904 4.15.5 Distribution of the Source Routing set of proxies 906 Section 4.15.2 and Section 4.15.4 assume that a source routing 907 mechanism is used to effect traversal of the required SIP proxies 908 during session setup. 910 There must be some means of dynamically informing the node which adds 911 the source routing set of proxies that the INVITE has to traverse 912 (e.g., the outbound proxy or serving proxy) of what that set of 913 proxies should be. 915 The hiding requirements expressed in Section 4.12 also apply to the 916 said set of proxies. 918 4.16 Emergency sessions 920 3GPP networks already contain alternative procedures to deliver 921 emergency sessions. The Release 5 of the 3GPP specifications does 922 not add any requirement with respect SIP emergency sessions. 924 4.17 Identities used for session establishment 926 4.17.1 Remote Party Identification presentation 928 It must be possible to present to the caller the identity of the 929 party to which he/she may dial back to return a call. 931 We believe this requirement is met by the procedures described in 932 draft-ietf-sip-asserted-identity [19]. 934 4.17.2 Remote Party Identification privacy 936 In addition to the previous requirement, the called party must be 937 able to request that his/her identity not be revealed to the caller. 939 We believe this requirement is met by the procedures described in 940 draft-ietf-sip-privacy-general [20]. 942 4.17.3 Remote Party Identification blocking 944 Regulatory agencies, as well as subscribers, may require the ability 945 of a caller to block the display of their caller identification. 946 This function may be performed by the destination subscriber's SIP 947 serving proxy. In this way, the destination subscriber is still able 948 to do a session-return, session-trace, transfer, or any other 949 supplementary service. 951 Therefore, it must be possible that the caller requests to block the 952 display of his/her identity at the callee's display. 954 We believe this requirement is met by the procedures described in 955 draft-ietf-sip-privacy-general [20]. 957 4.17.4 Anonymity 959 Procedures are required for an anonymous session establishment. 960 However, sessions are not intended to be anonymous to the originating 961 or terminating network operators. 963 We believe this requirement is met by the procedures described in 964 draft-ietf-sip-privacy-general [20] and draft-ietf-sip-asserted- 965 identity [19]. 967 4.17.5 Anonymous session establishment 969 If the caller requests the session to be anonymous, the UAC must not 970 reveal any identity information to the UAS. 972 If the caller requests the session to be anonymous, the terminating 973 network must not reveal any identity or signaling routing information 974 to the destination endpoint. The terminating network should 975 distinguish at least two cases, first if the caller intended the 976 session to be anonymous, and second if the caller's identity was 977 deleted by a transit network. 979 We believe this requirement is met by the procedures described in 980 draft-ietf-sip-privacy-general [20] and draft-ietf-sip-asserted- 981 identity [19]. 983 4.18 Charging 985 The 3GPP charging implications are described in the 3GPP Technical 986 Specification 32.225 [34]. 988 4.18.1 Support of both prepaid and postpaid models 990 Operators may choose to offer prepaid and/or postpaid services. The 991 prepaid model is accomplished with the support of the on-line 992 charging model. The postpaid model is accomplished by the support of 993 the off-line charging model. 995 On-line charging is the process where charging information can 996 affect, in real-time, the service rendered to the user, such as 997 request for a graceful release of an existing session. On-line 998 charging interacts with the SIP signaling. 1000 Off-line charging is the process where charging information does not 1001 affect, in real-time, the service rendered to the user. 1003 4.18.2 Charging correlation levels 1005 The following levels of correlation for IMS charging are considered: 1007 o Correlation within a session. A session may comprise a number of 1008 media components. It must be possible to correlate the charging 1009 data of the different media components belonging to a session. 1011 o Correlation at media component level. For a session comprising 1012 several media types (such as audio and video), charging data is 1013 generated for each media type and needs to be correlated between 1014 network elements. For this, a media identifier shall be unique 1015 and shall clearly identify to which media type of a session this 1016 charging information belongs to. This component identifier is not 1017 exchanged between network elements and is based on the ordering of 1018 media flows in the SDP. This ordering is the same as the one used 1019 in the binding information passed to the GPRS network. 1021 4.18.3 Charging correlation principles 1023 To support the correlation of charging information, the following 1024 principles apply to both offline and online charging: 1026 o The correlation of charging information for an IMS session is 1027 based on the use of IMS Charging Identifiers (ICID). 1029 o The first IMS network entity within the SIP signaling path is 1030 responsible for assigning an ICID. This ICID shall then be passed 1031 along the whole session path in an end-to-end manner. However, 1032 this shall not preclude further elements (other SIP proxies) along 1033 the session path generating additional identifiers to be passed 1034 along. 1036 o The ICID is passed to all IMS network entities in the session 1037 signaling path. This is performed using SIP signaling. 1039 o The addresses of the charging functions are passed by the serving 1040 SIP proxy to all IMS network entities in the session signaling 1041 path. This is to provide a common destination for all the 1042 charging records generated by each IMS network entity with the 1043 same ICID. 1045 o For the charging correlation between the GPRS network and the IMS, 1046 one or more GPRS Charging IDs, which identify the PDP contexts of 1047 the session, are passed from the GPRS network to the IMS. 1049 o The GPRS Charging IDs are passed by the outbound SIP proxy to the 1050 serving SIP proxy and the Application Servers using SIP signaling. 1051 They are not transferred from one home IMS (e.g., caller's home) 1052 to another home IMS (e.g., callee's home). 1054 o Inter Operator Identifiers (IOI) are shared betweenthe caller's 1055 home IMS and the callee's home IMS to provide identifiers of the 1056 home originating and home terminating networks. 1058 4.18.4 Collection of Session Detailed Information 1060 The SIP serving proxy or another SIP server in the home network must 1061 be able to log details of all sessions, such as the duration, source, 1062 and destination of a session, to provide to the charging subsystem. 1064 4.19 General support of additional capabilities 1066 4.19.1 Additional capabilities 1068 3GPP is interested on applying and using additional services, like 1069 those described in SIP Call Control - Transfer [23], SIP Basic Call 1070 Flow Examples [24], SIP PSTN Call Flows [25] and SIP service examples 1071 [26]. Although 3GPP is not going to standardize additional services, 1072 3GPP may make sure that the capabilities that enable those services 1073 are granted in the network. 1075 As such we believe that the SIP REFER method [27] and the Replaces 1076 header [28] constitute a complement to be used as an enabler in order 1077 to meet the above requirement. 1079 4.19.2 DTMF signaling 1081 Support for voice calls must provide a similar level of service to 1082 the existing circuit based voice service. This includes the ability 1083 to utilize DTMF signaling e.g., for control of interactive voice 1084 response systems such as ticket sales lines, timetable information 1085 etc. 1087 The transport of DTMF tones from the mobile terminal to target 1088 systems that may be in the PSTN, or to SIP based solutions (i.e., no 1089 PSTN connection) must be supported. 1091 The transport of DTMF signals may be required for the whole call, 1092 just for the first part, or from some later point in the call, i.e., 1093 the start time and duration of such signaling is unpredictable. 1095 We believe that the mechanisms specified in RFC 2833 [9] meet the 1096 requirement without impacting SIP. 1098 4.19.3 Early Media 1100 As mobile terminals will frequently interoperate with the PSTN, 1101 support for early media is required. 1103 4.20 Exchange of session description 1105 Typically a session description protocol like SDP is used in SIP to 1106 describe the media streams and codecs needed to establish the 1107 session. SIP uses an offer/answer model of the session description, 1108 as described in RFC 3264 [12] where one of the parties offers his 1109 session description and the other answers to that offer. 1111 In the 3GPP IMS, the mobile terminals might have restrictions with 1112 the memory, DSP capacity, etc. As such, it is required a mechanism 1113 by which the Session Description negotiation may conclude with one 1114 out of many codecs per media stream. Both UAC and UAS must know, 1115 prior to any media is sent or received, which codec is used for each 1116 media stream. 1118 In the 3GPP IMS, an efficient use of the network and radio resources 1119 is an important requirement. As such, the network should know in 1120 advance which codecs is used for a particular media stream. The 1121 network access control may use this information to grant access to 1122 the network and control the resource utilization. 1124 Additionally, it is required that the party who pays for the resource 1125 utilization has the opportunity to decide the codecs to use, once 1126 both end parties are aware of the capabilities supported at the 1127 remote UA. 1129 Therefore, it is required a mechanism by which both UAC and UAS have 1130 the ability to negotiate and trim down the number of codecs used per 1131 media stream, so that at the end of the negotiation there can be a 1132 reduced set of agreed codecs per media stream. 1134 We believe that the mechanism specified in RFC 3264 [12] meet the 1135 requirement. 1137 4.21 Prohibition of certain SDP parameters 1139 4.21.1 Prohibition of codecs 1141 The SIP outbound proxy may contain local policy rules with respect 1142 the codecs allowed in the network. For instance, certain networks 1143 may disallow high bandwidth consuming audio codecs. There has to be 1144 a mechanism whereby the SIP outbound proxy can reject a session 1145 establishment attempt when a codec is prohibited in the network due 1146 to local policy. 1148 4.21.2 Prohibition of media types 1150 Certain user's subscription may include restrictions to use certain 1151 media types. For instance, a user may not be allowed to establish a 1152 video session. The SIP serving proxy in the home network downloads 1153 the user profile, which contains the rules of this kind of 1154 restrictions. 1156 As the establishment of sessions traverse the SIP serving proxy in 1157 the home network, this proxy can prohibit an attempt to establish a 1158 session that includes a non-allowed media type for the user. 1159 Therefore, there has to be a mechanism whereby the SIP serving proxy 1160 can reject a session establishment attempt when the session includes 1161 a forbidden media type. 1163 4.22 Network initiated re-authentication 1165 Network operators need to authenticate users to ensure that they are 1166 charged appropriately for the services they use. The re- 1167 authentication done when the user initiates a message will not 1168 suffice for this purpose, as described below. 1170 If the duration of the authentication period is set to a relatively 1171 low value to ensure that the user cannot incur a high amount of 1172 charges between two authentications, it may create a lot of 1173 unnecessary authentications of users which have remained largely 1174 inactive, and therefore utilize unnecessary air interface resources. 1176 If the duration of the authentication period is set to a relatively 1177 high value to avoid these unnecessary authentications the risk is 1178 then that some users may incur high charges between authentications. 1180 A user's authentication is automatically invalidated when a certain 1181 threshold for charges (or number, or duration of sessions) is reached 1182 without giving the user a chance to re-authenticate, even if a valid 1183 registration exists. This would not provide an adequate level of 1184 service. 1186 Consequently it must be possible for the network to initiate a re- 1187 authentication process at any time. The triggers must be set within 1188 the network and may include charging thresholds, number of events, 1189 session duration etc. 1191 4.23 Security model 1193 Section 4.23, Section 4.24 and Section 4.25 have been based on the 1194 3GPP Technical Specifications 33.203 [35], 23.228 [31] and 33.210 1195 [36]. 1197 The scope for security of the 3GPP IMS is securing the SIP signaling 1198 between the various SIP entities. Protecting the end-to-end media 1199 streams may be a future extension but is not considered in the 1200 Release 5 version of the IMS specifications. 1202 Each operator providing IMS services acts as its own domain of trust, 1203 and shares a long-term security association with its subscribers 1204 (e.g., pre-shared keys). Operators may enter into roaming agreements 1205 with other operators, in which case a certain level of trust exists 1206 between their respective domains. 1208 SIP user agents must authenticate to their home network before the 1209 use of IMS resources is authorized. In the Release 5 of the 3GPP IMS 1210 specifications, authentication is performed during registration and 1211 re-registrations. 1213 Portions of the SIP signaling must be protected hop-by-hop. Looking 1214 at Figure 1 in Section 3, we can distinguish two distinct zones where 1215 the required security is unique: 1217 o Access Domain: Between the SIP user device and the visited 1218 network. 1220 o Network Domain: Between the visited and the home networks, or 1221 inside the home network. 1223 Characteristics needed in the Access Domain are quite different from 1224 those of the Network Domain because the terminal's requirements on 1225 mobility, computation restriction, battery limit, bandwidth 1226 conservation and radio interface. SIP entities in the access domain 1227 should be able to maintain security contexts with a large group of 1228 users in parallel. Furthermore, Access Domain provides user specific 1229 security associations while Network Domain provides security 1230 associations between network nodes. Therefore the weight of 1231 protocols and algorithms and the compliance of them with compression 1232 mechanisms are very important to Access Domain Security. It is 1233 therefore required that the security solutions must allow different 1234 mechanisms in these two domains. 1236 4.24 Access Domain Security 1238 4.24.1 General requirements 1239 4.24.1.1 Scalability and Efficiency 1241 3GPP IMS is characterized by a large subscriber base of up to a 1242 billion users, all of which must be treated in a secure manner. 1244 The security solutions must allow global roaming among a large number 1245 of administrative domains. 1247 4.24.1.2 Bandwidth and Roundtrips 1249 The wireless interface in 3GPP terminals is an expensive resource 1250 both in terms of power consumption and maximum utilization of scarce 1251 spectrum. Furthermore, cellular networks have typically long round- 1252 trip time delays, which must be taken in account in the design of the 1253 security solutions. 1255 Any security mechanism that involves 3GPP terminals should not 1256 unnecessarily increase the bandwidth needs. 1258 All security mechanisms that involve 3GPP terminals should minimize 1259 the number of necessary extra roundtrips. In particular, during 1260 normal call signaling there should not be any additional security 1261 related messages. 1263 For example, once an IPsec security association or a TLS connection 1264 is established, no additional round trips are required during session 1265 setup. However, the requirement of minimizing the number of round 1266 trips is hard to satisfy with IKE or TLS. It seems that IKE [6] adds 1267 a number of roundtrips, particularly if run together with legacy 1268 authentication extensions developed in the IPSRA WG. TLS [3] uses 1269 fewer roundtrips, but on the other hand doesn't support UDP. 1271 4.24.1.3 Computation 1273 It must be possible for mobile device terminals to provide security 1274 without requiring public key cryptography and/or certificates. 3GPP 1275 IMS may, however, include optional security schemes that employ these 1276 techniques. 1278 Current HTTP authentication methods use only symmetric cryptography 1279 as required here. Lower-layer mechanisms (ex: IKE, TLS) require 1280 implementation of public-key cryptography and/or Diffie-Helman. If 1281 these lower-layer mechanisms were used, the mobile terminal would 1282 authenticate and negotiate session keys with the visited network 1283 using only symmetric methods. 1285 4.24.1.4 Independence of the transport protocol 1287 The selected security mechanism should work with any transport 1288 protocol allowed by SIP (e.g., TCP, UDP). 1290 4.24.2 Authentication 1292 Authentication, as used in this context, means entity authentication 1293 that enables two entities to verify the identity of the respective 1294 peer. 1296 4.24.2.1 Authentication method 1298 A strong, mutual authentication must be provided. 1300 The authentication method must be able to work when there are zero or 1301 more SIP proxies in the SIP path between the authenticator and the 1302 authenticated user. 1304 It must be possible to support extensible authentication methods. 1305 Therefore authentication using an extensible authentication framework 1306 is strongly recommended. 1308 Authentication methods based on the secure storage of long-term keys 1309 used for authentication and the secure execution of authentication 1310 algorithms must be supported. 1312 The SIP client's credentials must not be transferred as plain text. 1314 3GPP intends to reuse UMTS AKA [15]. UMTS AKA applies a symmetric 1315 cryptographic scheme, provides mutual authentication, and is 1316 typically implemented on a so-called SIM card that provides secure 1317 storage on the user's side. 1319 Additional requirements related to message protection that apply to 1320 the authentication method are stated in Section 4.24.3. 1322 4.24.3 Message Protection 1324 4.24.3.1 Message Protection Mechanisms 1326 SIP entities (typically a SIP client and a SIP proxy) must be able to 1327 communicate using integrity and replay protection. By integrity, we 1328 mean the ability for receiver of a message to verify that the message 1329 has not been modified in transit. SIP entities should be able to 1330 communicate confidentially. In 3GPP IMS, these protection modes must 1331 be based on initial authentication. Integrity protection and 1332 confidentiality must be possible using symmetric cryptographic keys. 1334 It must be possible to handle also error conditions in a satisfactory 1335 manner as to allow recovery (see also Section 4.3.6.3 and Section 1336 4.14). 1338 It must be possible to provide this protection between two adjacent 1339 SIP entities. In future network scenarios it may also be necessary 1340 to provide this protection through proxies, though the 3GPP Release 5 1341 IMS does not require this. 1343 The security mechanism must be able to protect a complete SIP 1344 message. 1346 If header compression/removal or SIP compression is applied to SIP 1347 messages, it must be compatible with message protection. 1349 4.24.3.2 Delegation 1351 3GPP IMS implements distributed security functions responsible for 1352 authentication and message protection. 1354 It must be possible to perform an initial authentication based on 1355 long-term authentication credentials, followed by subsequent 1356 protected signaling that uses short-term authentication credentials, 1357 such as session keys created during initial authentication. The used 1358 authentication mechanism is able to provide such session keys. It 1359 must be possible to apply subsequent message protection as soon as 1360 possible, even during the initial authentication period. 1362 Initial authentication is performed between the SIP UA and the 1363 authenticating SIP serving proxy in the home network. However, the 1364 authentication mechanism must not require access to the long-term 1365 authentication credentials in these nodes. In the home network, the 1366 authenticating SIP serving proxy must support interaction with a 1367 dedicated authentication server in order to accomplish the 1368 authentication task. At the client side a secured (tamper-resistant) 1369 device storing the long-term credentials of the user must perform the 1370 authentication. 1372 Additionally, the SIP serving proxy that performed the initial 1373 authentication must be able to securely delegate subsequent SIP 1374 signaling protection (e.g., session keys for integrity or encryption) 1375 to an authorized SIP proxy further downstream. The tamper-resistant 1376 device at the client side must be able to securely delegate the 1377 session keys to the SIP user agent. 1379 4.24.4 Negotiation of mechanisms 1381 A method must be provided to securely negotiate the security 1382 mechanisms to be used in the access domain. 1384 This method must at least support the negotiation of different 1385 security mechanisms providing integrity protection and encryption, 1386 algorithms used within these mechanisms and additional parameters 1387 they require to be exchanged. 1389 The negotiation mechanism must protect against attackers who do not 1390 have access to authentication credentials. In particular, the 1391 negotiation mechanism must be able to detect a possible man-in-the- 1392 middle attacker who could influence the negotiation result such that 1393 services with weaker or no security are negotiated. 1395 A negotiation mechanism is generally required in all secure protocols 1396 to decide which security services to use and when they should be 1397 started. This security mechanism serves algorithm and protocol 1398 development as well as interoperability. Often, the negotiation is 1399 handled within a security service. For example, the HTTP 1400 authentication scheme includes a selection mechanism for choosing 1401 among appropriate algorithms. Note that when referring to 1402 negotiation we mean just the negotiation, not all functions in 1403 protocols like IKE. For instance, we expect the session key 1404 generation is to be a part of the initial authentication. 1406 SIP entities must be able to use the same security mode parameters to 1407 protect several SIP sessions without re-negotiation. For example, 1408 security mode parameters may be assumed to be valid within the 1409 lifetime of a registration. Note that it is necessary to amortize 1410 the cost of security association setup and parameter negotiation over 1411 several INVITEs. 1413 4.24.5 Verification of messages 1415 4.24.5.1 Verification at the SIP outbound proxy 1417 The SIP outbound proxy must be able to guarantee the message origin 1418 and verify that the message has not been changed (e.g., it is 1419 integrity protected). 1421 4.24.5.2 Verification at the SIP serving proxy 1423 The serving SIP proxy needs to receive an indication if the outbound 1424 proxy was able to verify the message origin and, in the case of a 1425 REGISTER request, whether it was integrity protected or not. 1427 4.25 Network Domain Security 1429 Message authentication, key agreement, integrity and replay 1430 protection, and confidentiality must be provided for communications 1431 between SIP network entities such as proxy servers. 1433 Network domain security mechanisms must be scalable up to a large 1434 number of network elements. 1436 3GPP intends to make it mandatory to have the protection discussed 1437 above at least between two operators, and optional within an 1438 operator's own network. Security gateways exist between operator's 1439 networks. 1441 We believe the above requirements to be fulfilled by applying 1442 security mechanisms as specified in the current IP Security standards 1443 specified in RFC 2401 [5]. 1445 5. Security considerations 1447 This document does not define a protocol, but still presents some 1448 security requirements to protocols. The main security requirements 1449 are stated in Section 4.23, Section 4.24 and Section 4.25. 1450 Additional security-related issues are discussed under Section 4.6, 1451 Section 4.7, Section 4.8, Section 4.9, Section 4.12 and Section 4.10. 1453 6. IANA considerations 1455 This document introduces no new IANA considerations. 1457 7. Contributors 1459 The following people contributed to this document: 1461 Duncan Mills (Vodafone), Gabor Bajko (Nokia), Georg Mayer (Siemens), 1462 Francois-Xerome Derome (Alcatel), Hugh Shieh (AWS), Andrew Allen 1463 (dynamicsoft), Sunil Chotai (mmO2), Keith Drage (Lucent), Jayshree 1464 Bharatia (Nortel), Kevan Hobbis (Huthison 3G UK), Dean Willis 1465 (dynamicsoft), Krisztian Kiss (Nokia), Vesa Torvinen (Ericsson), Jari 1466 Arkko (Ericsson), Sonia Garapaty (Nortel). 1468 Normative References 1470 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 1471 Levels", BCP 14, RFC 2119, March 1997. 1473 [2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., 1474 Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: 1475 Session Initiation Protocol", RFC 3261, June 2002. 1477 Informational References 1479 [3] Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and 1480 P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January 1481 1999. 1483 [4] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform 1484 Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1485 1998. 1487 [5] Kent, S. and R. Atkinson, "Security Architecture for the 1488 Internet Protocol", RFC 2401, November 1998. 1490 [6] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", 1491 RFC 2409, November 1998. 1493 [7] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC 1494 2486, January 1999. 1496 [8] Shirey, R., "Internet Security Glossary", RFC 2828, May 2000. 1498 [9] Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF Digits, 1499 Telephony Tones and Telephony Signals", RFC 2833, May 2000. 1501 [10] Faltstrom, P., "E.164 number and DNS", RFC 2916, September 1502 2000. 1504 [11] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol 1505 (SIP): Locating SIP Servers", RFC 3263, June 2002. 1507 [12] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with 1508 Session Description Protocol (SDP)", RFC 3264, June 2002. 1510 [13] Roach, A., "Session Initiation Protocol (SIP)-Specific Event 1511 Notification", RFC 3265, June 2002. 1513 [14] Olson, S., Camarillo, G. and A. Roach, "Support for IPv6 in 1514 Session Description Protocol (SDP)", RFC 3266, June 2002. 1516 [15] Niemi, A., Arkko, J. and V. Torvinen, "Hypertext Transfer 1517 Protocol (HTTP) Digest Authentication Using Authentication and 1518 Key Agreement (AKA)", RFC 3310, September 2002. 1520 [16] Rosenberg, J., "A Session Initiation Protocol (SIP) Event 1521 Package for Registrations", draft-rosenberg-sip-reg-00 (work in 1522 progress), May 2002. 1524 [17] Rosenberg, J., Camarillo, G. and F. Andreasen, "Integration of 1525 Resource Management and SIP", draft-ietf-sip-manyfolks- 1526 resource-07 (work in progress), April 2002. 1528 [18] Evans, D., Marshall, W. and F. Andreasen, "SIP Extensions for 1529 Media Authorization", draft-ietf-sip-call-auth-06 (work in 1530 progress), May 2002. 1532 [19] Watson, M., Peterson, J. and C. Jennings, "Private Extensions 1533 to the Session Initiation Protocol (SIP) for Asserted Identity 1534 within Trusted Networks", draft-ietf-sip-asserted-identity-02 1535 (work in progress), August 2002. 1537 [20] Peterson, J., "A Privacy Mechanism for the Session Initiation 1538 Protocol (SIP)", draft-ietf-sip-privacy-general-01 (work in 1539 progress), June 2002. 1541 [21] Droms, R., Perkins, C., Bound, J., Volz, B., Carney, M. and T. 1542 Lemon, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", 1543 draft-ietf-dhc-dhcpv6-26 (work in progress), June 2002. 1545 [22] Volz, B. and H. Schulzrinne, "DHCPv6 Options for SIP Servers", 1546 draft-ietf-sip-dhcpv6-00 (work in progress), April 2002. 1548 [23] Sparks, R., "SIP Call Control - Transfer", draft-ietf-sip-cc- 1549 transfer-05 (work in progress), May 2002. 1551 [24] Johnston, A., "Session Initiation Protocol Basic Call Flow 1552 Examples", draft-ietf-sipping-basic-call-flows-01 (work in 1553 progress), October 2002. 1555 [25] Johnston, A., "Session Initiation Protocol PSTN Call Flows", 1556 draft-ietf-sipping-pstn-call-flows-00 (work in progress), 1557 August 2002. 1559 [26] Johnston, A., "SIP Service Examples", draft-ietf-sipping- 1560 service-examples-02 (work in progress), July 2002. 1562 [27] Sparks, R., "The SIP Refer Method", draft-ietf-sip-refer-06 1563 (work in progress), July 2002. 1565 [28] Dean, R., Biggs, B. and R. Mahy, "The Session Inititation 1566 Protocol (SIP) 'Replaces' Header", draft-ietf-sip-replaces-02 1567 (work in progress), May 2002. 1569 [29] 3GPP, "TS 23.003 Numbering, addressing and identification 1570 (Release 5)", September 2002, . 1573 [30] 3GPP, "TS 23.060:General Packet Radio Service (GRPS); Service 1574 Description; Stage 2", September 2002, . 1577 [31] 3GPP, "TS 23.228: IP Multimedia Subsystem (IMS) (Stage 2) - 1578 Release 5", September 2002, . 1581 [32] 3GPP, "TS 24.228: Signaling flows for the IP Multimedia call 1582 control based on SIP and SDP", September 2002, . 1585 [33] 3GPP, "TS 24.229: IP Multimedia Subsystem (IMS) (Stage 3) - 1586 Release 5", September 2002, . 1589 [34] 3GPP, "TS 32.225: Telecommunication Management; Charging 1590 Management; Charging Data Description for IP Multimedia 1591 Subsystem; (Release 5)", September 2002, . 1594 [35] 3GPP, "TS 32.203: 3G Security; Access security for IP based 1595 services; (Release 5)", September 2002, . 1598 [37] ITU-T, "Recommendation E.164 (05/97): The international public 1599 telecommunication numbering plan", May 1997, . 1603 Author's Address 1605 Miguel A. Garcia Martin 1606 Ericsson 1607 Hirsalantie 11 1608 Jorvas FIN-02420 1609 Finland 1611 EMail: miguel.a.garcia@ericsson.com 1613 Full Copyright Statement 1615 Copyright (C) The Internet Society (2002). All Rights Reserved. 1617 This document and translations of it may be copied and furnished to 1618 others, and derivative works that comment on or otherwise explain it 1619 or assist in its implementation may be prepared, copied, published 1620 and distributed, in whole or in part, without restriction of any 1621 kind, provided that the above copyright notice and this paragraph are 1622 included on all such copies and derivative works. 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