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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 2141 (Obsoleted by RFC 8141) ** Obsolete normative reference: RFC 3023 (Obsoleted by RFC 7303) == Outdated reference: A later version (-15) exists of draft-ietf-soc-overload-control-13 Summary: 2 errors (**), 0 flaws (~~), 2 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 IETF SOC Working Group C. Shen 3 Internet-Draft H. Schulzrinne 4 Intended status: Standards Track Columbia U. 5 Expires: May 09, 2014 A. Koike 6 NTT 7 November 05, 2013 9 A Session Initiation Protocol (SIP) Load Control Event Package 10 draft-ietf-soc-load-control-event-package-10.txt 12 Abstract 14 We define a load control event package for the Session Initiation 15 Protocol (SIP). It allows SIP entities to distribute load filtering 16 policies to other SIP entities in the network. The load filtering 17 policies contain rules to throttle calls based on their source or 18 destination domain, telephone number prefix or for a specific user. 19 The mechanism helps to prevent signaling overload and complements 20 feedback-based SIP overload control efforts. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at http://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on May 09, 2014. 39 Copyright Notice 41 Copyright (c) 2013 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (http://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 57 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5 58 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5 59 4. Design Requirements . . . . . . . . . . . . . . . . . . . . . 6 60 5. SIP Load Filtering Overview . . . . . . . . . . . . . . . . . 6 61 5.1. Load Filtering Policy Format . . . . . . . . . . . . . . 6 62 5.2. Load Filtering Policy Computation . . . . . . . . . . . . 7 63 5.3. Load Filtering Policy Distribution . . . . . . . . . . . 7 64 5.4. Applicable Network Domains . . . . . . . . . . . . . . . 10 65 6. Load Control Event Package . . . . . . . . . . . . . . . . . 11 66 6.1. Event Package Name . . . . . . . . . . . . . . . . . . . 11 67 6.2. Event Package Parameters . . . . . . . . . . . . . . . . 12 68 6.3. SUBSCRIBE Bodies . . . . . . . . . . . . . . . . . . . . 12 69 6.4. SUBSCRIBE Duration . . . . . . . . . . . . . . . . . . . 12 70 6.5. NOTIFY Bodies . . . . . . . . . . . . . . . . . . . . . . 12 71 6.6. Notifier Processing of SUBSCRIBE Requests . . . . . . . . 12 72 6.7. Notifier Generation of NOTIFY Requests . . . . . . . . . 13 73 6.8. Subscriber Processing of NOTIFY Requests . . . . . . . . 13 74 6.9. Handling of Forked Requests . . . . . . . . . . . . . . . 14 75 6.10. Rate of Notifications . . . . . . . . . . . . . . . . . . 15 76 6.11. State Delta . . . . . . . . . . . . . . . . . . . . . . . 15 77 7. Load Control Document . . . . . . . . . . . . . . . . . . . . 15 78 7.1. Format . . . . . . . . . . . . . . . . . . . . . . . . . 15 79 7.2. Namespace . . . . . . . . . . . . . . . . . . . . . . . . 16 80 7.3. Conditions . . . . . . . . . . . . . . . . . . . . . . . 16 81 7.3.1. Call Identity . . . . . . . . . . . . . . . . . . . . 16 82 7.3.2. Method . . . . . . . . . . . . . . . . . . . . . . . 19 83 7.3.3. Target SIP Entity . . . . . . . . . . . . . . . . . . 19 84 7.3.4. Validity . . . . . . . . . . . . . . . . . . . . . . 20 85 7.4. Actions . . . . . . . . . . . . . . . . . . . . . . . . . 21 86 7.5. Complete Examples . . . . . . . . . . . . . . . . . . . . 22 87 7.5.1. Load Control Document Examples . . . . . . . . . . . 22 88 7.5.2. Message Flow Examples . . . . . . . . . . . . . . . . 25 89 8. XML Schema Definition for Load Control . . . . . . . . . . . 27 90 9. Related Work . . . . . . . . . . . . . . . . . . . . . . . . 30 91 9.1. Relationship with Load Filtering in PSTN . . . . . . . . 30 92 9.2. Relationship with Other IETF SIP Overload Control Efforts 31 93 10. Discussion of this specification meeting the requirements of 94 RFC5390 . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 95 11. Security Considerations . . . . . . . . . . . . . . . . . . . 37 96 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38 97 12.1. Load Control Event Package Registration . . . . . . . . 38 98 12.2. application/load-control+xml MIME Registration . . . . . 38 99 12.3. Load Control Schema Registration . . . . . . . . . . . . 39 100 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39 101 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 40 102 14.1. Normative References . . . . . . . . . . . . . . . . . . 40 103 14.2. Informative References . . . . . . . . . . . . . . . . . 41 104 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41 106 1. Introduction 108 Proper functioning of Session Initiation Protocol (SIP) [RFC3261] 109 signaling servers is critical in SIP-based communications networks. 110 The performance of SIP servers can be severely degraded when the 111 server is overloaded with excessive number of signaling requests. 112 Both legitimate and malicious traffic can overload SIP servers, 113 despite appropriate capacity planning. 115 There are three common examples of legitimate short-term increases in 116 call volumes. Viewer-voting TV shows or ticket giveaways may 117 generate millions of calls within a few minutes. Call volume may 118 also spike during special holidays such as New Year's Day and 119 Mother's Day. Finally, callers may want to reach friends and family 120 in natural disaster areas such as those affected by hurricanes. When 121 possible, only calls traversing overloaded servers should be 122 throttled under those conditions. 124 SIP load control mechanisms are needed to prevent congestion collapse 125 in these cases [RFC5390]. There are two types of load control 126 approaches. In the first approach, feedback control, SIP servers 127 provide load limits to upstream servers, to reduce the incoming rate 128 of all SIP requests [I-D.ietf-soc-overload-control]. These upstream 129 servers then drop or delay incoming SIP requests. Feedback control 130 is reactive and affects signaling messages that have already been 131 issued by user agent clients. They work well when SIP proxy servers 132 in the core networks (core proxy servers) or destination-specific SIP 133 proxy servers in the edge networks (edge proxy servers) are 134 overloaded. By their nature, they need to distribute rate, drop or 135 window information to all upstream SIP proxy servers and normally 136 affect all calls equally, regardless of destination. For example, in 137 the ticket giveaway case, almost all calls to the hotline will fail 138 at the core proxy servers; if the edge proxy servers leading to the 139 core proxy servers are also overloaded, calls to other destinations 140 will also be rejected or dropped. 142 Here, we propose an additional, complementary load control mechanism, 143 called load filtering. Network operators create load filtering 144 policies that indicate calls to specific destinations or from 145 specific sources should be rate-limited or randomly dropped. These 146 load filtering policies are then distributed to SIP servers and 147 possibly SIP user agents that are likely to generate calls to the 148 affected destinations or from the affected sources. Load filtering 149 works best if it prevents calls as close to the originating user 150 agent clients as possible. 152 The load filtering approach is most applicable for situations where a 153 traffic surge and its source/destination distribution can be 154 predicted in advance. For instance, it is appropriate for a mass- 155 phone-voting event, Mother's Day, New Year's Day, and even a 156 hurricane. However, it is less likely to be effective for the 157 initial phase of unpredicted/unpredictable mass calling events, such 158 as earthquakes or terrorist attacks. In these latter cases, the 159 local traffic load may peak by more than an order of magnitude in 160 minutes, if not seconds. This does not allow time to either 161 effectively identify the load filtering policies needed, nor 162 distribute them to the appropriate servers soon enough to prevent 163 server congestion. Once other, more immediate, techniques (such as 164 the loss-based or rate-based load feedback control methods) have 165 prevented the initial congestion collapse, the load filtering 166 approach can be used to effectively control the continuing overload. 168 Performing SIP load filtering involves the following components of 169 load filtering policies: format definition, computation, distribution 170 and enforcement. This specification defines the load filtering 171 policy, distribution and enforcement in the SIP load control event 172 package built upon existing SIP event notification framework. 173 However, load filtering policy computation is out of scope of this 174 specification, because it depends heavily on the actual network 175 topology and other service provider policies. 177 It should be noted that although the SIP load filtering mechanism is 178 motivated by the SIP overload control problem, which is why this 179 specification refers extensively to parallel SIP overload control 180 related efforts, the applicability of SIP load filtering extends 181 beyond the overload control purpose. For example, it can also be 182 used to implement quality of service or other service level agreement 183 commitments. Therefore, we use the term "load control event 184 package", instead of a narrower term "overload control event 185 package". 187 The rest of this specification is structured as follows: we begin by 188 listing the design requirements for this work in Section 4. We then 189 give an overview of load filtering operation in Section 5. The load 190 control event package for load filtering policy distribution is 191 detailed in Section 6. The load filtering policy format is defined 192 in the two sections that follow, with Section 7 introducing the XML 193 document for load filtering policies and Section 8 listing the 194 associated schema. Section 9 relates this work to corresponding 195 mechanisms in PSTN and other IETF efforts addressing SIP overload 196 control. Section 10 evaluates whether this specification meets the 197 SIP overload control requirements set forth by RFC5390 [RFC5390]. 198 Finally, Section 11 presents security considerations and Section 12 199 provides IANA considerations. 201 2. Conventions 203 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 204 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 205 document are to be interpreted as described in [RFC2119]. 207 3. Definitions 209 This specification reuses the definitions for "Event Package", 210 "Notification", "Notifier", "Subscriber", "Subscription" as in 211 [RFC6665]. The following additional definitions are also used. 213 Load Filtering: A load control mechanism which applies specific 214 actions to selected loads (e.g., SIP requests) matching specific 215 conditions. 217 Load Filtering Policy: A set of zero or more load filtering rules, 218 also known as load filtering rule set. 220 Load Filtering Rule: Conditions and actions to be applied for load 221 filtering. 223 Load Filtering Condition: Elements that describe how to select loads 224 to apply load filtering actions. This specification defines the 225 "call identity", "method", "target SIP identity", and "validity" 226 condition elements (Section 7.3). 228 Load Filtering Action: An operation to be taken by a load filtering 229 server on loads that match the load filtering conditions. This 230 specification allows actions such as accept, reject and redirect 231 of loads (Section 7.4). 233 Load Filtering Server: A server which performs load filtering. In 234 the context of this specification, the load filtering server is 235 the subscriber, which receives load filtering policies from the 236 notifier and enforces those policies during load filtering. 238 Load Control Document: An XML document that describes the load 239 filtering policies (Section 7). It inherits and enhances the 240 common policy document defined in [RFC4745]. 242 4. Design Requirements 244 The SIP load filtering mechanism needs to satisfy the following 245 requirements: 247 o To simplify the solution, we focus on a method for controlling SIP 248 load, rather than a generic application-layer mechanism. 250 o The load filtering policy needs to be distributed efficiently to 251 possibly a large subset of all SIP elements. 253 o The solution should re-use existing SIP protocol mechanisms to 254 reduce implementation and deployment complexity. 256 o For predictable overload situations, such as holidays and mass 257 calling events, the load filtering policy should specify during 258 what time it is to be applied, so that the information can be 259 distributed ahead of time. 261 o For destination-specific overload situations, the load filtering 262 policy should be able to describe the destination domain or the 263 callee. 265 o To address accidental and intentional high-volume call generators, 266 the load filtering policy should be able to specify the caller. 268 o Caller and callee need to be specified as both SIP URIs and 'tel' 269 URIs [RFC3966] in load filtering policies. 271 o It should be possible to specify particular information in the SIP 272 headers (e.g., prefixes in telephone numbers) which allow load 273 filtering over limited regionally-focused overloads. 275 o The solution should draw upon experiences from related PSTN 276 mechanisms where applicable. 278 o The solution should be extensible to meet future needs. 280 5. SIP Load Filtering Overview 282 5.1. Load Filtering Policy Format 284 Load filtering policies are specified by sets of rules. Each rule 285 contains both load filtering conditions and actions. The load 286 filtering conditions define identities of the targets to be 287 filtered(Section 7.3.1). For example, there are two typical resource 288 limits in a possible overload situation, i.e., human destination 289 limits (N number of call takers) and node capacity limits. The load 290 filtering targets in these two cases can be the specific callee 291 numbers or the destination domain corresponding to the overload. 292 Load filtering conditions also indicate the specific message type to 293 be matched (Section 7.3.2), with which target SIP entity the 294 filtering policy is associated (Section 7.3.3) and the period of time 295 when the filtering policy should be activated and deactivated 296 (Section 7.3.4). Load filtering actions describe the desired control 297 functions such as limiting the request rate below a certain level 298 (Section 7.4). 300 5.2. Load Filtering Policy Computation 302 Computing the load filtering policies needs to take into 303 consideration information such as overload time, scope and network 304 topology, as well as service policies. It is also important to make 305 sure that there is no resource allocation loop, and that server 306 capacity is allocated in a way which both prevents overload and 307 maximizes effective throughput (aka goodput). In some cases, in 308 order to better utilize system resources, it may be preferable to 309 employ an algorithm which dynamically computes the load filtering 310 policies based on currently observed server load status, rather than 311 using a purely static filtering policy assignment. The computation 312 algorithm for load filtering policies is out of scope of this 313 specification. 315 5.3. Load Filtering Policy Distribution 317 For load filtering policy distribution, this specification defines 318 the SIP event package for load control, which is an "instantiation" 319 of the generic SIP event notification framework [RFC6665]. The SIP 320 event notification framework provides an existing method for SIP 321 entities to subscribe to and receive notifications when certain 322 events occur. Such a framework forms a scalable event distribution 323 architecture that suits our needs. This specification also defines 324 XML schema of a load control document (Section 7), which is used to 325 encode load filtering policies. 327 In order for load filtering policies to be properly distributed, each 328 capable SIP entity in the network SHOULD subscribe to the SIP load 329 control event package from all its outgoing signaling neighbors, 330 known as notifiers (Section 6.6). Subscription is initiated and 331 maintained during normal server operation. Signaling neighbors are 332 discovered by sending signaling messages. For instance, if A sends 333 signaling requests to B, B is an outgoing signaling neighbor of A. A 334 needs to subscribe to the load control event package of B in case B 335 wants to curb requests from A. On the other hand, if B also sends 336 signaling requests to A, then B also needs to subscribe to A. The 337 subscription of neighboring SIP entities needs to be persistent so 338 that it is in place independently of any specific events requiring 339 load filtering. Key to this is the fact that following initial 340 subscription, the notifier sends a notification without a body if no 341 load filtering policy is defined (Section 6.7), and that the 342 subscription needs to be refreshed periodically to make it 343 persistent, as described in Section 4.1 and Section 4.2 of [RFC6665]. 344 The notifier will send a notification to its subscribers each time a 345 new subscription or a subscription refresh is accepted (Section 6.7). 346 The notification request includes in its body the current load 347 filtering policies (Section 7.1) from the notifier. If no such load 348 filtering policy exists, the notification request is sent without a 349 body. The subscribers MAY terminate the subscription if it no longer 350 considers the notifiers as its signaling neighbor, e.g., after an 351 extended period of absence of signaling message exchange. However, 352 if after un-subscribing, the subscriber determines that signaling 353 with the notifier becomes active again, it MUST immediately subscribe 354 to that notifier again. 356 We use the example architecture shown in Figure 1 to illustrate load 357 filtering policy distribution based on the SIP load control event 358 package mechanism. This scenario consists of two networks belonging 359 to Service Provider A and Service Provider B, respectively. Each 360 provider's network is made up of two SIP core proxy servers and four 361 SIP edge proxy servers. The core proxy servers and edge proxy 362 servers of Service Provider A are denoted as CPa1 to CPa2 and EPa1 to 363 EPa4; the core proxy servers and edge proxy servers of Service 364 Provider B are denoted as CPb1 to CPb2 and EPb1 to EPb4. 366 +-----------+ +-----------+ +-----------+ +-----------+ 367 | | | | | | | | 368 | EPa1 | | EPa2 | | EPa3 | | EPa4 | 369 | | | | | | | | 370 +-----------+ +-----------+ +-----------+ +-----------+ 371 \ / \ / 372 \ / \ / 373 \ / \ / 374 +-----------+ +-----------+ 375 | | | | 376 | CPa1 |------------------| CPa2 | 377 | | | | 378 +-----------+ +-----------+ 379 | | 380 Service | | 381 Provider A | | 382 | | 383 ================================================================= 384 | | 386 Service | | 387 Provider B | | 388 | | 389 +-----------+ +-----------+ 390 | | | | 391 | CPb1 |------------------| CPb2 | 392 | | | | 393 +-----------+ +-----------+ 394 / \ / \ 395 / \ / \ 396 / \ / \ 397 +-----------+ +-----------+ +-----------+ +-----------+ 398 | | | | | | | | 399 | EPb1 | | EPb2 | | EPb3 | | EPb4 | 400 | | | | | | | | 401 +-----------+ +-----------+ +-----------+ +-----------+ 403 Figure 1: Example Network Scenario Using SIP Load Control Event 404 Package Mechanism 406 At initialization stage, the proxy servers first identify all their 407 outgoing signaling neighbors and subscribe to them. The neighbor 408 identification process can be performed by service providers through 409 direct provisioning, or by the proxy servers themselves via 410 progressive learning from the signaling messages sent and received. 411 Assuming all signaling relationships in Figure 1 are bi-directional, 412 after this initialization stage, each proxy server will be subscribed 413 to all its neighbors. That is, EPa1 subscribes to CPa1; CPa1 414 subscribes to EPa1, EPa2, CPa2 and CPb1, so on and so forth. The 415 following cases then show two examples of how load filtering policy 416 distribution in this network works. 418 Case I: EPa1 serves a TV program hotline and decides to limit the 419 total number of incoming calls to the hotline to prevent an overload. 420 To do so, EPa1 sends a notification to CPa1 with the specific hotline 421 number, time of activation and total acceptable call rate. Depending 422 on the load filtering policy computation algorithm, CPa1 may allocate 423 the received total acceptable call rate among its neighbors, namely, 424 EPa2, CPa2, and CPb1, and notify them about the resulting allocation 425 along with the hotline number and the activation time. CPa2 and CPb1 426 may perform further allocation among their own neighbors and notify 427 the corresponding proxy servers. This process continues until all 428 edge proxy servers in the network have been informed about the event 429 and have proper load filtering policy configured. 431 In the above case, the network entity where load filtering policy is 432 first introduced is the SIP server providing access to the resource 433 that creates the overload situation. In other cases, the network 434 entry point of introducing load filtering policy could also be an 435 entity that hosts this resource. For example, an operator may host 436 an application server that performs 800 number translation services. 437 The application server may itself be a SIP proxy server or a SIP 438 Back-to-Back User Agent (B2BUA). If one of the 800 numbers hosted at 439 the application server creates the overload condition, the load 440 filtering policies can be introduced from the application server and 441 then propagated to other SIP proxy servers in the network. 443 Case II: a hurricane affects the region covered by CPb2, EPb3 and 444 EPb4. All these three SIP proxy servers are overloaded. The rescue 445 team determines that outbound calls are more valuable than inbound 446 calls in this specific situation. Therefore, EPb3 and EPb4 are 447 configured with load filtering policies to accept more outbound calls 448 than inbound calls. CPb2 may be configured the same way or receive 449 dynamically computed load filtering policies from EPb3 and EPb4. 450 Depending on the load filtering policy computation algorithm, CPb2 451 may also send out notifications to its outside neighbors, namely CPb1 452 and CPa2, specifying a limit on the acceptable rate of inbound calls 453 to CPb2's responsible domain. CPb1 and CPa2 may subsequently notify 454 their neighbors about limiting the calls to CPb2's area. The same 455 process could continue until all edge proxy servers are notified and 456 have load filtering policies configured. 458 Note that this specification does not define the provisioning 459 interface between the party who determines the load filtering policy 460 and the network entry point where the policy is introduced. One of 461 the options for the provisioning interface is the Extensible Markup 462 Language (XML) Configuration Access Protocol (XCAP) [RFC4825]. 464 5.4. Applicable Network Domains 466 This specification MUST be applied inside a 'Trust Domain'. The 467 concept of a Trust Domain is similar to that defined in [RFC3324]. A 468 Trust Domain for the purpose of SIP load filtering is a set of SIP 469 entities such as SIP proxy servers that are trusted to exchange load 470 filtering policies defined in this specification. In the simplest 471 case, a Trust Domain is a network of SIP entities belonging to a 472 single service provider who deploys it and accurately knows the 473 behaviour of those SIP entities. Such simple Trust Domains may be 474 joined to form larger Trust Domains by bi-lateral agreements between 475 the service providers of the SIP entities. 477 The key requirement of a Trust Domain for the purpose of SIP load 478 filtering is that the behavior of all SIP entities within a given 479 Trust Domain is known to comply to the following set of 480 specifications. 482 o The mechanisms used to secure the communication among SIP entities 483 within the Trust Domain. 485 o The manner used to determine which SIP entities are part of the 486 Trust Domain. 488 o That SIP entities in the Trust Domain are compliant to SIP 489 [RFC3261] 491 o That SIP entities in the Trust Domain are compliant to this 492 document. 494 o The agreement on what types of calls can be affected by this SIP 495 load filtering mechanism. For example, call identity condition 496 elements (Section 7.3.1) and might be limited to 497 describe specific domains; and might be 498 limited to describe within certain prefixes. 500 o The agreement on the destinations to which calls may be redirected 501 when the "redirect" action (Section 7.4) is used. For example, 502 the URI might have to match a given set of domains. 504 It is important to note that effectiveness of SIP load filtering 505 requires that all neighbors that are possible signaling sources 506 participate and enforce the designated load filtering policies. 507 Otherwise, a single non-conforming neighbor could make the whole 508 filtering efforts useless by pumping in excessive traffic to overload 509 the server. Therefore, the SIP server that distributes load 510 filtering policies needs to take counter-measures towards any non- 511 conforming neighbors. A simple method is to reject excessive 512 requests with 503 (Service Unavailable) response messages as if they 513 were obeying the rate. Considering the rejection costs, a more 514 complicated but fairer method would be to allocate at the overloaded 515 server the same amount of processing to the combination of both 516 normal processing and rejection as the overloaded server would devote 517 to processing requests for a conforming upstream SIP server. These 518 approaches work as long as the total rejection cost does not 519 overwhelm the entire server resources. In addition, SIP servers need 520 to handle message prioritization properly while performing load 521 filtering, which is described in Section 6.8. 523 6. Load Control Event Package 525 The SIP load filtering mechanism defines a load control event package 526 for SIP based on [RFC6665]. 528 6.1. Event Package Name 530 The name of this event package is "load-control". This name is 531 carried in the Event and Allow-Events header, as specified in 532 [RFC6665]. 534 6.2. Event Package Parameters 536 No package specific event header field parameters are defined for 537 this event package. 539 6.3. SUBSCRIBE Bodies 541 This document does not define the content of SUBSCRIBE bodies. 542 Future specifications could define bodies for SUBSCRIBE messages, for 543 example to request specific types of load control event 544 notifications. 546 A SUBSCRIBE request sent without a body implies the default 547 subscription behavior as specified in Section 6.7. 549 6.4. SUBSCRIBE Duration 551 The default expiration time for a subscription to load filtering 552 policy is one hour. Since the desired expiration time may vary 553 significantly for subscriptions among SIP entities with different 554 signaling relationships, the subscribers and notifiers are 555 RECOMMENDED to explicitly negotiate appropriate subscription duration 556 when knowledge about the mutual signaling relationship is available. 558 6.5. NOTIFY Bodies 560 The body of a NOTIFY request in this event package contains load 561 filtering policies. The format of the NOTIFY request body MUST be in 562 one of the formats defined in the Accept header field of the 563 SUBSCRIBE request or be the default format, as specified in 564 [RFC6665]. The default data format for the NOTIFY request body of 565 this event package is "application/load-control+xml" (defined in 566 Section 7). This means that when NOTIFY request body exists but no 567 Accept header field is specified in a SUBSCRIBE request, the NOTIFY 568 request body MUST contain "application/load-control+xml" format. 570 6.6. Notifier Processing of SUBSCRIBE Requests 572 The notifier accepts a new subscription or updates an existing 573 subscription upon receiving a valid SUBSCRIBE request. 575 If the identity of the subscriber sending the SUBSCRIBE request is 576 not allowed to receive load filtering policy, the notifier MUST 577 return a 403 "Forbidden" response. 579 If none of MIME types specified in the Accept header of the SUBSCRIBE 580 request is supported, the notifier SHOULD return 406 "Not Acceptable" 581 response. 583 6.7. Notifier Generation of NOTIFY Requests 585 A notifier MUST send a NOTIFY request with its current load filtering 586 policy to the subscriber upon successfully accepting or refreshing a 587 subscription. If no load filtering policy needs to be distributed 588 when the subscription is received, the notifier SHOULD sent a NOTIFY 589 request without body to the subscriber. The content-type header 590 field of this NOTIFY request MUST indicate the correct body format as 591 if the body were present (e.g., "application/load-control+xml"). 592 Sending this NOTIFY request without body is often the case when a 593 subscription is initiated for the first time, e.g., when a SIP entity 594 is just introduced, because there may be no planned events that 595 require load filtering at that time. A notifier SHOULD generate 596 NOTIFY requests each time the load filtering policy changes, with the 597 maximum notification rate not exceeding values defined in 598 Section 6.10. 600 6.8. Subscriber Processing of NOTIFY Requests 602 The subscriber is the load filtering server which enforces load 603 filtering policies received from the notifier. The way subscribers 604 process NOTIFY requests depends on the load filtering policies 605 conveyed in the notifications. Typically, load filtering policies 606 consist of rules specifying actions to be applied to requests 607 matching certain conditions. A subscriber receiving a notification 608 first installs these rules and then enforce corresponding actions on 609 requests matching those conditions, for example, limiting the sending 610 rate of call requests destined for a specific callee. 612 In the case when load filtering policies specify a future validity, 613 it is possible that when the validity time comes, the subscription to 614 the specific notifier that conveyed the rules has expired. In this 615 case, it is RECOMMENDED that the subscriber re-activate its 616 subscription with the corresponding notifier. Regardless of whether 617 this re-activation of subscription is successful or not, when the 618 validity time is reached, the subscriber SHOULD enforce the 619 corresponding rules. 621 Upon receipt of a NOTIFY request with a Subscription-State header 622 field containing the value "terminated", the subscription status with 623 the particular notifier will be terminated. Meanwhile, subscribers 624 MUST also terminate previously received load filtering policies from 625 that notifier. 627 The subscriber SHOULD discard unknown bodies. If the NOTIFY request 628 contains several bodies, none of them being supported, it SHOULD 629 unsubscribe. A NOTIFY request without a body indicates that no load 630 filtering policies need to be updated. 632 When the subscriber enforces load filtering policies, it needs to 633 prioritize requests and select those requests that need to be 634 rejected or redirected. This selection is largely a matter of local 635 policy. It is expected that the subscriber will follow local policy 636 as long as the result in reduction of traffic is consistent with the 637 overload algorithm in effect at that node. Accordingly, the 638 normative behavior in the next three paragraphs should be interpreted 639 with the understanding that the subscriber will aim to preserve local 640 policy to the fullest extent possible. 642 o The subscriber SHOULD honor the local policy for prioritizing SIP 643 requests such as policies based on message type, e.g., INVITEs 644 versus requests associated with existing sessions. 646 o The subscriber SHOULD honor the local policy for prioritizing SIP 647 requests based on the content of the Resource-Priority header 648 (RPH, [RFC4412]). Specific (namespace.value) RPH contents may 649 indicate high priority requests that should be preserved as much 650 as possible during overload. The RPH contents can also indicate a 651 low-priority request that is eligible to be dropped during times 652 of overload. 654 o The subscriber SHOULD honor the local policy for prioritizing SIP 655 requests relating to emergency calls as identified by the SOS URN 656 [RFC5031] indicating an emergency request. 658 A local policy can be expected to combine both the SIP request type 659 and the prioritization markings, and SHOULD be honored when overload 660 conditions prevail. 662 6.9. Handling of Forked Requests 664 Forking is not applicable when this load control event package 665 mechanism is used within a single-hop distance between neighboring 666 SIP entities. If communication scope of the load control event 667 package mechanism is among multiple hops, forking is not expected to 668 happen either because the subscription request is addressed to a 669 clearly defined SIP entity. However, in the unlikely case when 670 forking does happen, the load control event package only allows the 671 first potential dialog-establishing message to create a dialog, as 672 specified in Section 5.9 of [RFC6665]. 674 6.10. Rate of Notifications 676 Rate of notifications is likely not a concern for this local control 677 event package mechanism when it is used in a non-real-time mode for 678 relatively static load filtering policies. Nevertheless, if 679 situation does arise that a rather frequent load filtering policy 680 update is needed, it is RECOMMENDED that the notifier do not generate 681 notifications at a rate higher than once per-second in all cases, in 682 order to avoid the NOTIFY request itself overloading the system. 684 6.11. State Delta 686 It is likely that updates to specific load filtering policies are 687 made by changing only part of the policy parameters only (e.g. 688 acceptable request rate or percentage, but not matching identities). 689 This will typically be because the utilization of a resource subject 690 to overload depends upon dynamic unknowns such as holding time and 691 the relative distribution of offered loads over subscribing SIP 692 entities. The updates could originate manually or be determined 693 automatically by an algorithm that dynamically computes the load 694 filtering policies (Section 5.2). Another factor that is usually not 695 known precisely or needs to be computed automatically is the duration 696 of the event requiring load filtering. Therefore it would also be 697 common for the validity to change frequently. 699 This event package allows the use of state delta as in [RFC6665] to 700 accommodate frequent updates of partial policy parameters. For each 701 NOTIFY transaction in a subscription, a version number that increases 702 by exactly one MUST be included in the NOTIFY request body when the 703 body is present. When the subscriber receives a state delta, it 704 associates the partial updates to the particular policy by matching 705 the appropriate rule id (Section 7.5). If the subscriber receives a 706 NOTIFY request with a version number that is increased by more than 707 one, it knows that it has missed a state delta and needs to ask for a 708 full state snapshot. Therefore, the subscriber ignores that NOTIFY 709 request containing the state delta, and re-sends a SUBSCRIBE request 710 to force a NOTIFY request containing a complete state snapshot. 712 7. Load Control Document 714 7.1. Format 716 A load control document is an XML document that describes the load 717 filtering policies. It inherits and enhances the common policy 718 document defined in [RFC4745]. A common policy document contains a 719 set of rules. Each rule consists of three parts: conditions, actions 720 and transformations. The conditions part is a set of expressions 721 containing attributes such as identity, domain, and validity time 722 information. Each expression evaluates to TRUE or FALSE. Conditions 723 are matched on "equality" or "greater than" style comparison. There 724 is no regular expression matching. Conditions are evaluated on 725 receipt of an initial SIP request for a dialog or standalone 726 transaction. If a request matches all conditions in a rule set, the 727 action part and the transformation part are consulted to determine 728 the "permission" on how to handle the request. Each action or 729 transformation specifies a positive grant to the policy server to 730 perform the resulting actions. Well-defined mechanism are available 731 for combining actions and transformations obtained from more than one 732 sources. 734 7.2. Namespace 736 The namespace URI for elements defined by this specification is a 737 Uniform Resource Namespace (URN) ([RFC2141]), using the namespace 738 identifier 'ietf' defined by [RFC2648] and extended by [RFC3688]. 739 The URN is as follows: 741 urn:ietf:params:xml:ns:load-control 743 7.3. Conditions 745 [RFC4745] defines three condition elements: , and 746 . In this specification, we define new condition elements 747 and reuse the element. The element is not used. 749 7.3.1. Call Identity 751 Since the problem space of this specification is different from that 752 of [RFC4745], the [RFC4745] element is not sufficient for 753 use with load filtering. First, load filtering may be applied to 754 different identities contained in a request, including identities of 755 both the receiving entity and the sending entity. Second, the 756 importance of authentication varies when different identities of a 757 request are concerned. This specification defines new identity 758 conditions that can accommodate the granularity of specific SIP 759 identity header fields. The requirement for authentication depends 760 on which field is to be matched. 762 The identity condition for load filtering is specified by the element and its sub-element . The element 764 itself contains sub-elements representing SIP sending and receiving 765 identity header fields: , , and . All those sub-elements are of an extended form of the 767 [RFC4745] element. In addition to the sub-elements 768 including , , and in the [RFC4745] 769 element, the extended form adds two new sub-elements, namely, and , which will be explained later in this section. 772 The [RFC4745] and elements may contain an "id" 773 attribute, which is the URI of a single entity to be included or 774 excluded in the condition. When used in the , , and elements, this "id" value is the URI 776 contained in the corresponding SIP header field, i.e., From, To, 777 Request-URI, and P-Asserted-Identity. 779 When the element contains multiple sub- 780 elements, the result is combined using logical OR. When the , 781 , and elements contain 782 multiple or or sub-elements, the result is 783 also combined using logical OR. When the sub-element further 784 contains one or more sub-elements, or when the 785 sub-element further contains one or more sub-elements, 786 the result of each or sub-element is combined 787 using a logical OR, similar to that of the [RFC4745] 788 element. However, when the element contains multiple of the 789 , , and sub-elements, 790 the result is combined using logical AND. This allows the call 791 identity to be specified by multiple fields of a SIP request 792 simultaneously, e.g., both the From and the To header fields. 794 The following shows an example of the element, which 795 matches call requests whose To header field contains the SIP URI 796 "sip:alice@hotline.example.com", or the 'tel' URI 797 "tel:+1-212-555-1234". 799 800 801 802 803 804 805 806 808 Before evaluating call-identity conditions, the subscriber shall 809 convert URIs received in SIP header fields in canonical form as per 810 [RFC3261], except that the phone-context parameter shall not be 811 removed, if present. 813 The [RFC4745] and elements may take a "domain" 814 attribute. The "domain" attribute specifies a domain name to be 815 matched by the domain part of the candidate identity. Thus, it 816 allows matching a large and possibly unknown number of entities 817 within a domain. The "domain" attribute works well for SIP URIs. 819 A URI identifying a SIP user, however, can also be a 'tel' URI. We 820 therefore need a similar way to match a group of 'tel' URIs. 821 According to [RFC3966], there are two forms of 'tel' URIs for global 822 numbers and local numbers, respectively. All phone numbers must be 823 expressed in global form when possible. The global number 'tel' URIs 824 start with a "+". The rest of the numbers are expressed as local 825 numbers, which must be qualified by a "phone-context" parameter. The 826 "phone-context" parameter may be labelled as a global number or any 827 number of its leading digits, or a domain name. Both forms of the 828 'tel' URI make the resulting URI globally unique. 830 'Tel' URIs of global numbers can be grouped by prefixes consisting of 831 any number of common leading digits. For example, a prefix formed by 832 a country code or both the country and area code identifies telephone 833 numbers within a country or an area. Since the length of the country 834 and area code for different regions are different, the length of the 835 number prefix also varies. This allows further flexibility such as 836 grouping the numbers into sub-areas within the same area code. 'Tel' 837 URIs of local numbers can be grouped by the value of the "phone- 838 context" parameter. 840 The and sub-elements in the [RFC4745] 841 element do not allow additional attributes to be added directly. 842 Redefining behavior of their existing attribute creates 843 backward-compatibility issues. Therefore, this specification defines 844 the and sub-elements that extend the 845 [RFC4745] element. Both of them have a "prefix" attribute 846 for grouping 'tel' URIs, similar to the "domain" attribute for 847 grouping SIP URIs in existing and sub-elements. For 848 global numbers, the "prefix" attribute value holds any number of 849 common leading digits, for example, "+1-212" for U.S. phone numbers 850 within area code "212" or "+1-212-854" for the organization with U.S. 851 area code "212" and local prefix "854". For local numbers, the 852 "prefix" attribute value contains the "phone-context" parameter 853 value. It should be noted that visual separators (such as the "-" 854 sign) in 'tel' URIs are not used for URI comparison as per [RFC3966]. 856 The following example shows the use of the "prefix" attribute along 857 with the "domain" attribute. It matches those requests calling to 858 the number "+1-202-999-1234" but are not calling from a "+1-212" 859 prefix or a SIP From URI domain of "manhattan.example.com". 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 877 7.3.2. Method 879 The load created on a SIP server depends on the type of initial SIP 880 requests for dialogs or standalone transactions. The 881 element specifies the SIP method to which the load filtering action 882 applies. When this element is not included, the load filtering 883 actions are applicable to all applicable initial requests. These 884 requests include INVITE, MESSAGE, REGISTER, SUBSCRIBE, OPTIONS, and 885 PUBLISH. Non-initial requests, such as ACK, BYE and CANCEL MUST NOT 886 be subjected to load filtering. In addition, SUBSCRIBE requests are 887 not filtered if the event-type header field indicates the event 888 package defined in this specification. 890 The following example shows the use of the element in the 891 case the filtering actions should be applied to INVITE requests. 893 INVITE 895 7.3.3. Target SIP Entity 897 A SIP server that performs load filtering may have multiple paths to 898 route call requests matching the same set of call identity elements. 899 In those situations, the SIP load filtering server may desire to take 900 advantage of alternative paths and only apply load filtering actions 901 to matching requests for the next hop SIP entity that originated the 902 corresponding load filtering policy. To achieve that, the SIP load 903 filtering server needs to associate every load filtering policy with 904 its originating SIP entity. The element is 905 defined for that purpose and it contains the URI of the entity that 906 initiated the load filtering policy, which is generally the 907 corresponding notifier. A notifier MAY include this element as part 908 of the condition of its filtering policy being sent to the 909 subscriber, as below. 911 sip:biloxi.example.com 913 When a SIP load filtering server receives a policy with a element, it SHOULD record it and take it into 915 consideration when making load filtering decisions. If the load 916 filtering server receives a load filtering policy that does not 917 contain a element, it MAY still record the URI of 918 the load filtering policy's originator as the 919 information and consider it when making load filtering decisions. 921 The following are two examples of using the 922 element. 924 Use case I: the network has user A connected to SIP Proxy 1 (SP1), 925 user B connected to SIP Proxy 3 (SP3), SP1 and SP3 connected via 926 SIP Proxy 2 (SP2), and SP2 connected to an Application Server 927 (AS). Under normal load conditions, a call from A to B is routed 928 along the following path: A-SP1-SP2-AS-SP3-B. The AS provides a 929 non-essential service and can be bypassed in case of overload. 930 Now let's assume that AS is overloaded and sends to SP2 a load 931 filtering policy requesting that 50% of all INVITE requests be 932 dropped. SP2 can maintain AS as the for that 933 policy so that it knows the 50% drop action is only applicable to 934 call requests that must go through AS, without affecting those 935 calls directly routed through SP3 to B. 937 Use case II: An 800 translation service is installed on two 938 Application Servers, AS1 and AS2. User A is connected to SP1 and 939 calls 800-1234-4529, which is translated by AS1 and AS2 into a 940 regular E.164 number depending on, e.g., the caller's location. 941 SP1 forwards INVITE requests with Request-URI = "800 number" to 942 AS1 or AS2 based on a load balancing strategy. As calls to 943 800-1234-4529 creates a pre-overload condition in AS1, AS1 sends 944 to SP1 a load filtering policy requesting that 50% of calls 945 towards 800-1234-4529 be rejected. In this case, SP1 can maintain 946 AS1 as the for the rule, and only apply the 947 load filtering policy on incoming requests that are intended to be 948 sent to AS1. Those requests that are sent to AS2, although 949 matching the of the filter, will not be affected. 951 7.3.4. Validity 953 A filtering policy is usually associated with a validity period 954 condition. This specification reuses the element of 955 [RFC4745], which specifies a period of validity time by pairs of 956 and sub-elements. When multiple time periods are 957 defined, the validity condition is evaluated to TRUE if the current 958 time falls into any of the specified time periods. i.e., it 959 represents a logical OR operation across all validity time periods. 961 The following example shows a element specifying a valid 962 period from 12:00 to 15:00 US Eastern Standard Time on 2008-05-31. 964 965 2008-05-31T12:00:00-05:00 966 2008-05-31T15:00:00-05:00 967 969 7.4. Actions 971 The actions a load filtering server takes on loads matching the load 972 filtering conditions are defined by the element in the load 973 filtering policy, which includes any one of the three sub-elements 974 , , and . The element denotes an absolute 975 value of the maximum acceptable request rate in requests per second; 976 the element specifies the relative percentage of incoming 977 requests that should be accepted; the element describes the 978 acceptable window size supplied by the receiver, which is applicable 979 in window-based load filtering. In static load filtering policy 980 configuration scenarios, using the sub-element is RECOMMENDED 981 because it is hard to enforce the percentage rate or window-based 982 load filtering when incoming load from upstream or reactions from 983 downstream are uncertain. (See [I-D.ietf-soc-overload-control] 984 [RFC6357] for more details on rate-based, loss-based and window-based 985 load control.) 987 In addition, the element takes an optional "alt-action" 988 attribute which can be used to explicitly specify the desired action 989 in case a request cannot be processed. The "alt-action" can take one 990 of the following three values: "reject", "redirect" and "drop". 992 o The "reject" action is the default value for "alt-action". It 993 means that the load filtering server will reject the request with 994 a 503 (Service Unavailable) response message. 996 o The "redirect" action means redirecting the request to another 997 target. When it is used, an "alt-target" attribute MUST be 998 defined. The "alt-target" specifies one URI or a list of URIs 999 where the request should be redirected. The server sends out the 1000 redirect URIs in a 300-class response message. 1002 o The "drop" action means simply ignoring the request without doing 1003 anything, which can in certain cases help save processing 1004 capability during overload. For example, when SIP is running over 1005 a reliable transport such as TCP, the "drop" action does not send 1006 out the rejection response, neither does it close the transport 1007 connection. The "drop" action is generally also good at dealing 1008 with telephony DOS attacks. However, when running SIP over an 1009 unreliable transport such as UDP, using the "drop" action will 1010 create message retransmissions that further worsen the possible 1011 overload situation. Therefore, any "drop" action applied to an 1012 unreliable transport MUST be treated as if it were "reject". 1014 In the following element example, the server accepts 1015 maximum of 100 call requests per second. The remaining calls are 1016 redirected to an answering machine. 1018 1019 1021 100 1022 1023 1025 7.5. Complete Examples 1027 7.5.1. Load Control Document Examples 1029 This section presents two complete examples of load control documents 1030 valid with respect to the XML schema defined in Section 8. 1032 The first example assumes that a set of hotlines are set up at 1033 "sip:alice@hotline.example.com" and "tel:+1-212-555-1234". The 1034 hotlines are activated from 12:00 to 15:00 US Eastern Standard Time 1035 on 2008-05-31. The goal is to limit the incoming calls to the 1036 hotlines to 100 requests per second. Calls that exceed the rate 1037 limit are explicitly rejected. 1039 1040 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 INVITE 1055 1056 2008-05-31T12:00:00-05:00 1057 2008-05-31T15:00:00-05:00 1058 1059 1060 1061 1062 100 1063 1064 1066 1067 1069 The second example considers optimizing server resource usage of a 1070 three-day period during the aftermath of a hurricane. Incoming calls 1071 to the domain "sandy.example.com" or to call destinations with prefix 1072 "+1-212" will be limited to a rate of 100 requests per second, except 1073 for those calls originating from a particular rescue team domain 1074 "rescue.example.com". Outgoing calls from the hurricane domain or 1075 calls within the local domain are never limited. All calls that are 1076 throttled due to the rate limit will be forwarded to an answering 1077 machine with updated hurricane rescue information. 1079 1080 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 INVITE 1101 1102 2012-10-25T09:00:00+01:00 1103 2012-10-28T09:00:00+01:00 1104 1105 1106 1107 1109 100 1110 1111 1113 1114 1116 Sometimes it may occur that multiple rules in a ruleset define 1117 actions that match the same methods, call identity and validity. In 1118 those cases, the "first-match-wins" principle is used. For example, 1119 in the following ruleset, the first rule requires all calls from the 1120 "example.com" domain to be rejected. Even though the rule following 1121 that one specifies that calls from "sip:alice@example.com" be 1122 redirected to a specific target "sip:eve@example.com", the calls from 1123 "sip:alice@example.com" will still be rejected because they have 1124 already been matched by the earlier rule. 1126 1127 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 INVITE 1141 1142 2013-7-2T09:00:00+01:00 1143 2013-7-3T09:00:00+01:00 1144 1145 1146 1147 1148 0 1149 1150 1151 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 INVITE 1163 1164 2013-7-2T09:00:00+01:00 1165 2013-7-3T09:00:00+01:00 1166 1167 1168 1169 1171 0 1172 1173 1174 1176 1178 7.5.2. Message Flow Examples 1180 This section presents an example message flow of using the load 1181 control event package mechanism defined in this specification. 1183 atlanta biloxi 1184 | F1 SUBSCRIBE | 1185 |------------------>| 1186 | F2 200 OK | 1187 |<------------------| 1188 | F3 NOTIFY | 1189 |<------------------| 1190 | F4 200 OK | 1191 |------------------>| 1193 F1 SUBSCRIBE atlanta.example.com -> biloxi.example.com 1195 SUBSCRIBE sip:biloxi.example.com SIP/2.0 1196 Via: SIP/2.0/TCP atlanta.example.com;branch=z9hG4bKy7cjbu3 1197 From: sip:atlanta.example.com;tag=162ab5 1198 To: sip:biloxi.example.com 1199 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com 1200 CSeq: 2012 SUBSCRIBE 1201 Contact: sip:atlanta.example.com 1202 Event: load-control 1203 Max-Forwards: 70 1204 Accept: application/load-control+xml 1205 Expires: 3600 1206 Content-Length: 0 1208 F2 200 OK biloxi.example.com -> atlanta.example.com 1210 SIP/2.0 200 OK 1211 Via: SIP/2.0/TCP biloxi.example.com;branch=z9hG4bKy7cjbu3 1212 ;received=192.0.2.1 1213 To: ;tag=331dc8 1214 From: ;tag=162ab5 1215 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com 1216 CSeq: 2012 SUBSCRIBE 1217 Expires: 3600 1218 Contact: sip:biloxi.example.com 1219 Content-Length: 0 1221 F3 NOTIFY biloxi.example.com -> atlanta.example.com 1223 NOTIFY sip:atlanta.example.com SIP/2.0 1224 Via: SIP/2.0/TCP biloxi.example.com;branch=z9hG4bKy71g2ks 1225 From: ;tag=331dc8 1226 To: ;tag=162ab5 1227 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com 1228 Event: load-control 1229 Subscription-State: active;expires=3599 1230 Max-Forwards: 70 1231 CSeq: 1775 NOTIFY 1232 Contact: sip:biloxi.example.com 1233 Content-Type: application/load-control+xml 1234 Content-Length: ... 1236 [Load Control Document] 1238 F4 200 OK atlanta.example.com -> biloxi.example.com 1240 SIP/2.0 200 OK 1241 Via: SIP/2.0/TCP atlanta.example.com;branch=z9hG4bKy71g2ks 1242 ;received=192.0.2.2 1243 From: ;tag=331dc8 1244 To: ;tag=162ab5 1245 Call-ID: 2xTb9vxSit55XU7p8@atlanta.example.com 1246 CSeq: 1775 NOTIFY 1247 Content-Length: 0 1249 8. XML Schema Definition for Load Control 1251 This section defines the XML schema for the load control document. 1252 It extends the Common Policy schema in [RFC4745] in two ways. 1253 Firstly, it defines two mandatory attributes for the 1254 element: version and state. The version attribute allows the 1255 recipient of the notification to properly order them. Versions start 1256 at 0, and increase by one for each new document sent to a subscriber 1257 within the same subscription. Versions MUST be representable using a 1258 non-negative 32 bit integer. The state attribute indicates whether 1259 the document contains a full load filtering policy update, or whether 1260 it contains only state delta as partial update. Secondly, it defines 1261 new members of the and elements. 1263 1264 1271 1273 1275 1276 1277 1278 1279 1280 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1297 1299 1300 1302 1303 1304 1305 1306 1308 1309 1310 1312 1313 1314 1315 1316 1317 1319 1321 1323 1324 1325 1326 1327 1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1340 1341 1342 1343 1344 1345 1346 1348 1349 1351 1352 1353 1355 1356 1357 1358 1359 1361 1362 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1375 1377 1378 1380 1381 1382 1383 1384 1385 1386 1388 1389 1390 1392 1393 1395 1396 1397 1398 1400 1402 9. Related Work 1404 9.1. Relationship with Load Filtering in PSTN 1406 It is known that existing PSTN network also uses a load filtering 1407 mechanism to prevent overload and the filtering policy configuration 1408 is done manually except in specific cases when the Intelligent 1409 Network architecture is used [Q.1248.2][E.412]. This specification 1410 defines a load filtering mechanism based on the SIP event 1411 notification framework that allows automated filtering policy 1412 distribution in suitable environments. 1414 There are control messages associated with PSTN overload control 1415 which would specify an outgoing control list, call gap duration and 1416 control duration [Q.1248.2][E.412]. These items could be roughly 1417 correlated to the identity, action and time fields of the SIP load 1418 filtering policy defined in this specification. However, the load 1419 filtering policy defined in this specification is much more generic 1420 and flexible as opposed to its PSTN counterpart. 1422 Firstly, PSTN load filtering only applies to telephone numbers. The 1423 identity element of SIP load filtering policy allows both SIP URI and 1424 telephone numbers (through 'tel' URI) to be specified. These 1425 identities can be arbitrarily grouped by SIP domains or any number of 1426 leading prefix of the telephone numbers. 1428 Secondly, the PSTN load filtering action is usually limited to call 1429 gapping. The action field in SIP load filtering policy allows more 1430 flexible possibilities such as rate throttle and others. 1432 Thirdly, the duration field in PSTN load filtering specifies a value 1433 in seconds for the load filtering duration only, and the allowed 1434 values are mapped into a value set. The time field in SIP load 1435 filtering policy may specify not only a duration, but also a future 1436 activation time which could be especially useful for automating load 1437 filtering for predictable overloads. 1439 PSTN load filtering can be performed in both edge switches and 1440 transit switches; SIP load filtering can also be applied in both edge 1441 proxy servers and core proxy servers, and even in capable user 1442 agents. 1444 PSTN load filtering also has special accommodation for High 1445 Probability of Completion (HPC) calls, which would be similar to 1446 calls designated by the SIP Resource Priority Headers [RFC4412]. SIP 1447 load filtering mechanism also allows prioritizing the treatment of 1448 these calls by specifying favorable actions for them. 1450 PSTN load filtering also provides administrative option for routing 1451 failed call attempts to either a reorder tone [E.300SerSup3] 1452 indicating overload conditions, or a special recorded announcement. 1453 Similar capability can be provided in SIP load filtering mechanism by 1454 specifying appropriate "alt-action" attribute in the SIP load 1455 filtering action field. 1457 9.2. Relationship with Other IETF SIP Overload Control Efforts 1459 The load filtering policies in this specification consist of 1460 identity, action and time. The identity can range from a single 1461 specific user to an arbitrary user aggregate, domains or areas. The 1462 user can be identified by either the source or the destination. When 1463 the user is identified by the source and a favorable action is 1464 specified, the result is to some extent similar to identifying a 1465 priority user based on authorized Resource Priority Headers [RFC4412] 1466 in the requests. Specifying a source user identity with an 1467 unfavorable action would cause an effect to some extent similar to an 1468 inverse SIP resource priority mechanism. 1470 The load filtering policy defined in this specification is generic 1471 and expected to be applicable not only to the load filtering 1472 mechanism but also to the feedback overload control mechanism in 1473 [I-D.ietf-soc-overload-control]. In particular, both mechanisms 1474 could use specific or wildcard identities for load control and could 1475 share well-known load control actions. The time duration field in 1476 the load filtering policy could also be used in both mechanisms. As 1477 mentioned in Section 1, the load filtering policy distribution 1478 mechanism and the feedback overload control mechanism address 1479 complementary areas in the overload control problem space. Load 1480 filtering is more proactive and focuses on distributing filtering 1481 policies towards the source of the traffic; the hop-by-hop feedback- 1482 based approach is reactive and targets more at traffic already 1483 accepted in the network. Therefore, they could also make different 1484 use of the generic load filtering policy components. For example, 1485 the load filtering mechanism may use the time field in the filtering 1486 policy to specify not only a control duration but also a future 1487 activation time to accommodate a predicable overload such as the one 1488 caused by Mother's Day greetings or a viewer-voting program; the 1489 feedback-based control might not need to use the time field or might 1490 use the time field to specify an immediate load control duration. 1492 10. Discussion of this specification meeting the requirements of 1493 RFC5390 1495 This section evaluates whether the load control event package 1496 mechanism defined in this specification satisfies various SIP 1497 overload control requirements set forth by RFC5390 [RFC5390]. Not 1498 all RFC5390 requirements are found applicable due to the scope of 1499 this specification. Therefore, we categorize the assessment results 1500 into Yes (meet the requirement), P/A (Partially Applicable), No (must 1501 be used in conjunction with another mechanism to meet the 1502 requirement), and N/A (Not Applicable). 1504 REQ 1: The overload mechanism shall strive to maintain the overall 1505 useful throughput (taking into consideration the quality-of- 1506 service needs of the using applications) of a SIP server at 1507 reasonable levels, even when the incoming load on the network is 1508 far in excess of its capacity. The overall throughput under load 1509 is the ultimate measure of the value of an overload control 1510 mechanism. 1512 P/A. The goal of the load filtering is to prevent overload or 1513 maintain overall goodput during the time of overload, but it is 1514 dependent on the advance predictions of the load. If the predictions 1515 are incorrect, in either direction, the effectiveness of the 1516 mechanism will be affected. 1518 REQ 2: When a single network element fails, goes into overload, or 1519 suffers from reduced processing capacity, the mechanism should 1520 strive to limit the impact of this on other elements in the 1521 network. This helps to prevent a small-scale failure from 1522 becoming a widespread outage. 1524 N/A if load filtering policies are installed in advance and do not 1525 change during the potential overload period. P/A if load filtering 1526 policies are dynamically adjusted. The algorithm to dynamically 1527 compute load filtering policies is outside the scope of this 1528 specification, while the distribution of the updated filtering 1529 policies uses the event package mechanism of this specification. 1531 REQ 3: The mechanism should seek to minimize the amount of 1532 configuration required in order to work. For example, it is 1533 better to avoid needing to configure a server with its SIP message 1534 throughput, as these kinds of quantities are hard to determine. 1536 No. This mechanism is entirely dependent on advance configuration, 1537 based on advance knowledge. In order to satisfy Req 3, it should be 1538 used in conjunction with other mechanisms which are not based on 1539 advance configuration. 1541 REQ 4: The mechanism must be capable of dealing with elements that 1542 do not support it, so that a network can consist of a mix of 1543 elements that do and don't support it. In other words, the 1544 mechanism should not work only in environments where all elements 1545 support it. It is reasonable to assume that it works better in 1546 such environments, of course. Ideally, there should be 1547 incremental improvements in overall network throughput as 1548 increasing numbers of elements in the network support the 1549 mechanism. 1551 No. This mechanism is entirely dependent on the participation of all 1552 possible neighbors. In order to satisfy Req 4, it should be used in 1553 conjunction with other mechanisms, some of which are described in 1554 Section 5.4. 1556 REQ 5: The mechanism should not assume that it will only be 1557 deployed in environments with completely trusted elements. It 1558 should seek to operate as effectively as possible in environments 1559 where other elements are malicious; this includes preventing 1560 malicious elements from obtaining more than a fair share of 1561 service. 1563 No. This mechanism is entirely dependent on the non-malicious 1564 participation of all possible neighbors. In order to satisfy Req 5, 1565 it should be used in conjunction with other mechanisms, some of which 1566 are described in Section 5.4. 1568 REQ 6: When overload is signaled by means of a specific message, 1569 the message must clearly indicate that it is being sent because of 1570 overload, as opposed to other, non overload-based failure 1571 conditions. This requirement is meant to avoid some of the 1572 problems that have arisen from the reuse of the 503 response code 1573 for multiple purposes. Of course, overload is also signaled by 1574 lack of response to requests. This requirement applies only to 1575 explicit overload signals. 1577 N/A. This mechanism signals anticipated overload, not actual 1578 overload. However the signals in this mechanism are not used for any 1579 other purpose. 1581 REQ 7: The mechanism shall provide a way for an element to 1582 throttle the amount of traffic it receives from an upstream 1583 element. This throttling shall be graded so that it is not all- 1584 or-nothing as with the current 503 mechanism. This recognizes the 1585 fact that "overload" is not a binary state and that there are 1586 degrees of overload. 1588 Yes. This event package allows rate/loss/window-based overload 1589 control options as discussed in Section 7.4. 1591 REQ 8: The mechanism shall ensure that, when a request was not 1592 processed successfully due to overload (or failure) of a 1593 downstream element, the request will not be retried on another 1594 element that is also overloaded or whose status is unknown. This 1595 requirement derives from REQ 1. 1597 N/A to the load control event package mechanism itself. 1599 REQ 9: That a request has been rejected from an overloaded element 1600 shall not unduly restrict the ability of that request to be 1601 submitted to and processed by an element that is not overloaded. 1602 This requirement derives from REQ 1. 1604 Yes. For example, load filtering policy [Section 5.1] allows the 1605 inclusion of alternative forwarding destinations for rejected 1606 requests. 1608 REQ 10: The mechanism should support servers that receive requests 1609 from a large number of different upstream elements, where the set 1610 of upstream elements is not enumerable. 1612 No. Because this mechanism requires advance configuration of 1613 specifically identified neighbors, it does not support environments 1614 where the number and identity of the upstream neighbors are not known 1615 in advance. In order to satisfy Req 10, it should be used in 1616 conjunction with other mechanisms. 1618 REQ 11: The mechanism should support servers that receive requests 1619 from a finite set of upstream elements, where the set of upstream 1620 elements is enumerable. 1622 Yes. See also answer to REQ 10. 1624 REQ 12: The mechanism should work between servers in different 1625 domains. 1627 Yes. The load control event package mechanism is not limited by 1628 domain boundaries. However, it is likely more applicable in intra- 1629 domain scenarios than in inter-domain scenarios due to security and 1630 other concerns (See also Section 5.4). 1632 REQ 13: The mechanism must not dictate a specific algorithm for 1633 prioritizing the processing of work within a proxy during times of 1634 overload. It must permit a proxy to prioritize requests based on 1635 any local policy, so that certain ones (such as a call for 1636 emergency services or a call with a specific value of the 1637 Resource-Priority header field [RFC4412]) are given preferential 1638 treatment, such as not being dropped, being given additional 1639 retransmission, or being processed ahead of others. 1641 P/A. This mechanism does not specifically address the prioritizing of 1642 work during times of overload. But it does not preclude any 1643 particular local policy. 1645 REQ 14: The mechanism should provide unambiguous directions to 1646 clients on when they should retry a request and when they should 1647 not. This especially applies to TCP connection establishment and 1648 SIP registrations, in order to mitigate against avalanche restart. 1650 N/A to the load control event package mechanism itself. 1652 REQ 15: In cases where a network element fails, is so overloaded 1653 that it cannot process messages, or cannot communicate due to a 1654 network failure or network partition, it will not be able to 1655 provide explicit indications of the nature of the failure or its 1656 levels of congestion. The mechanism must properly function in 1657 these cases. 1659 P/A. Because the load filtering policies are provisioned in advance, 1660 they are not affected by the overload or failure of other network 1661 elements. But, on the other hand, they may not, in those cases, be 1662 able to protect the overloaded network elements (see Req 1). 1664 REQ 16: The mechanism should attempt to minimize the overhead of 1665 the overload control messaging. 1667 Yes. The standardized SIP event package mechanism [RFC6665] is used. 1669 REQ 17: The overload mechanism must not provide an avenue for 1670 malicious attack, including DoS and DDoS attacks. 1672 P/A. This mechanism does provide a potential avenue for malicious 1673 attacks. Therefore the security mechanisms for SIP event packages in 1674 general [RFC6665] and of section 10 of this specification should be 1675 used. 1677 REQ 18: The overload mechanism should be unambiguous about whether 1678 a load indication applies to a specific IP address, host, or URI, 1679 so that an upstream element can determine the load of the entity 1680 to which a request is to be sent. 1682 Yes. The identity of load indication is covered in the load filtering 1683 policy format definition in Section 5.1. 1685 REQ 19: The specification for the overload mechanism should give 1686 guidance on which message types might be desirable to process over 1687 others during times of overload, based on SIP-specific 1688 considerations. For example, it may be more beneficial to process 1689 a SUBSCRIBE refresh with Expires of zero than a SUBSCRIBE refresh 1690 with a non-zero expiration (since the former reduces the overall 1691 amount of load on the element), or to process re-INVITEs over new 1692 INVITEs. 1694 N/A to the load control event package mechanism itself. 1696 REQ 20: In a mixed environment of elements that do and do not 1697 implement the overload mechanism, no disproportionate benefit 1698 shall accrue to the users or operators of the elements that do not 1699 implement the mechanism. 1701 No. This mechanism is entirely dependent on the participation of all 1702 possible neighbors. In order to satisfy Req 20, it should be used in 1703 conjunction with other mechanisms, some of which are described in 1704 Section 5.4. 1706 REQ 21: The overload mechanism should ensure that the system 1707 remains stable. When the offered load drops from above the 1708 overall capacity of the network to below the overall capacity, the 1709 throughput should stabilize and become equal to the offered load. 1711 N/A to the load control event package mechanism itself. 1713 REQ 22: It must be possible to disable the reporting of load 1714 information towards upstream targets based on the identity of 1715 those targets. This allows a domain administrator who considers 1716 the load of their elements to be sensitive information, to 1717 restrict access to that information. Of course, in such cases, 1718 there is no expectation that the overload mechanism itself will 1719 help prevent overload from that upstream target. 1721 N/A to the load control event package mechanism itself. 1723 REQ 23: It must be possible for the overload mechanism to work in 1724 cases where there is a load balancer in front of a farm of 1725 proxies. 1727 Yes. The load control event package mechanism does not preclude its 1728 use in a scenario with server farms. 1730 11. Security Considerations 1732 Two aspects of security considerations arise from this specification. 1733 One is the SIP event notification framework-based load filtering 1734 policy distribution mechanism, the other is the load filtering policy 1735 enforcement mechanism. 1737 Security considerations for SIP event package mechanisms are covered 1738 in Section 6 of [RFC6665]. A particularly relevant security concern 1739 for this event package is that if the notifiers can be spoofed, 1740 attackers can send fake notifications asking subscribers to throttle 1741 all traffic, leading to Denial-of-Service attacks. Therefore, this 1742 SIP load filtering mechanism MUST be used in a Trust Domain 1743 (Section 5.4). But if a legitimate notifier in the Trust Domain is 1744 itself compromised, additional mechanisms will be needed to detect 1745 the attack. 1747 Security considerations for load filtering policy enforcement depends 1748 very much on the contents of the policy. This specification defines 1749 possible match of the following SIP header fields in a load filtering 1750 policy: , , and . The 1751 exact requirement to authenticate and authorize these fields is up to 1752 the service provider. In general, if the identity field represents 1753 the source of the request, it SHOULD be authenticated and authorized; 1754 if the identity field represents the destination of the request, the 1755 authentication and authorization is optional. 1757 In addition, there could be one specific type of attack around the 1758 use the "redirect" action (Section 7.4). Assuming a number of SIP 1759 proxy servers in a Trust Domain are using UDP, and configured to get 1760 their policies from a central server. An attacker spoofs the central 1761 server's address to send a number of NOTIFY bodies telling the proxy 1762 servers to redirect all calls to victim@outside-of-trust-domain.com. 1763 The proxy servers then redirect all calls to victim, who is then 1764 DoSed off of the Internet. To address this type of threat, this 1765 document requires that a Trust Domain agrees on what types of calls 1766 can be affected as well as on the destinations to which calls may be 1767 redirected, as in Section 5.4. 1769 12. IANA Considerations 1771 This specification registers a SIP event package, a new MIME type, a 1772 new XML namespace, and a new XML schema. 1774 12.1. Load Control Event Package Registration 1776 This section registers an event package based on the registration 1777 procedures defined in [RFC6665]. 1779 Package name: load-control 1781 Type: package 1783 Published specification: This specification 1785 Person to contact: Charles Shen, charles@cs.columbia.edu 1787 12.2. application/load-control+xml MIME Registration 1789 This section registers a new MIME type based on the procedures 1790 defined in [RFC6838] and guidelines in [RFC3023]. 1792 MIME media type name: application 1794 MIME subtype name: load-control+xml 1796 Mandatory parameters: none 1798 Optional parameters: Same as charset parameter application/xml in 1799 [RFC3023] 1801 Encoding considerations: Same as encoding considerations of 1802 application/xml in [RFC3023] 1804 Security considerations: See Section 10 of [RFC3023] and Section 11 1805 of this specification 1807 Interoperability considerations: None 1809 Published Specification: This specification 1811 Applications which use this media type: load control of SIP entities 1813 Additional information: 1815 Magic number: None 1817 File extension: .xml 1819 Macintosh file type code: 'TEXT' 1821 Personal and email address for further information: 1823 Charles Shen, charles@cs.columbia.edu 1825 Intended usage: COMMON 1827 Author/Change Controller: IETF SOC Working Group , as designated by the IESG 1830 12.3. Load Control Schema Registration 1832 URI: urn:ietf:params:xml:schema:load-control 1834 Registrant Contact: IETF SOC working group, Charles Shen 1835 (charles@cs.columbia.edu). 1837 XML: the XML schema to be registered is contained in Section 8. 1839 Its first line is 1841 1843 and its last line is 1845 1847 13. Acknowledgements 1849 The authors would like to thank Richard Barnes, Bruno Chatras, Martin 1850 Dolly, Keith Drage, Ashutosh Dutta, Janet Gunn, Vijay Gurbani, Volker 1851 Hilt, Geoff Hunt, Carolyn Johnson, Hadriel Kaplan, Paul Kyzivat, 1852 Salvatore Loreto, Timothy Moran, Eric Noel, Parthasarathi R, Adam 1853 Roach, Shida Schubert, Robert Sparks, Phil Williams and other members 1854 of the SOC and SIPPING working group for many helpful comments. In 1855 particular, Bruno Chatras proposed the and condition elements along with many other text improvements. 1857 Janet Gunn provided detailed text suggestions including Section 10. 1858 Eric Noel suggested clarification on load filtering policy 1859 distribution initialization process. Shida Schubert made many 1860 suggestions such as terminology usage. Phil Williams suggested 1861 adding support for delta updates. Ashutosh Dutta gave pointers to 1862 PSTN references. Adam Roach suggested RFC6665-related and other 1863 helpful clarifications. Richard Barnes made many suggestions such as 1864 referencing the Trust Domain concept of RFC3324, the use of a 1865 separate element for 'tel' URI grouping and addressing the "redirect" 1866 action security threat. 1868 14. References 1870 14.1. Normative References 1872 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1873 Requirement Levels", BCP 14, RFC 2119, March 1997. 1875 [RFC2141] Moats, R., "URN Syntax", RFC 2141, May 1997. 1877 [RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media 1878 Types", RFC 3023, January 2001. 1880 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1881 A., Peterson, J., Sparks, R., Handley, M., and E. 1882 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1883 June 2002. 1885 [RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, 1886 January 2004. 1888 [RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers", RFC 1889 3966, December 2004. 1891 [RFC4745] Schulzrinne, H., Tschofenig, H., Morris, J., Cuellar, J., 1892 Polk, J., and J. Rosenberg, "Common Policy: A Document 1893 Format for Expressing Privacy Preferences", RFC 4745, 1894 February 2007. 1896 [RFC6665] Roach, A., "SIP-Specific Event Notification", RFC 6665, 1897 July 2012. 1899 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 1900 Specifications and Registration Procedures", BCP 13, RFC 1901 6838, January 2013. 1903 14.2. Informative References 1905 [E.300SerSup3] 1906 ITU-T, ., "North American Precise Audible Tone Plan", 1907 E.300 Series Supplement 3 , November 1988. 1909 [E.412] ITU-T, ., "Network Management Controls", E.412-2003 , 1910 January 2003. 1912 [I-D.ietf-soc-overload-control] 1913 Gurbani, V., Hilt, V., and H. Schulzrinne, "Session 1914 Initiation Protocol (SIP) Overload Control", draft-ietf- 1915 soc-overload-control-13 (work in progress), May 2013. 1917 [Q.1248.2] 1918 ITU-T, ., "Interface Recommendation for Intelligent 1919 Network Capability Set4:SCF-SSF interface", Q.1248.2 , 1920 July 2001. 1922 [RFC2648] Moats, R., "A URN Namespace for IETF Documents", RFC 2648, 1923 August 1999. 1925 [RFC3324] Watson, M., "Short Term Requirements for Network Asserted 1926 Identity", RFC 3324, November 2002. 1928 [RFC4412] Schulzrinne, H. and J. Polk, "Communications Resource 1929 Priority for the Session Initiation Protocol (SIP)", RFC 1930 4412, February 2006. 1932 [RFC4825] Rosenberg, J., "The Extensible Markup Language (XML) 1933 Configuration Access Protocol (XCAP)", RFC 4825, May 2007. 1935 [RFC5031] Schulzrinne, H., "A Uniform Resource Name (URN) for 1936 Emergency and Other Well-Known Services", RFC 5031, 1937 January 2008. 1939 [RFC5390] Rosenberg, J., "Requirements for Management of Overload in 1940 the Session Initiation Protocol", RFC 5390, December 2008. 1942 [RFC6357] Hilt, V., Noel, E., Shen, C., and A. Abdelal, "Design 1943 Considerations for Session Initiation Protocol (SIP) 1944 Overload Control", RFC 6357, August 2011. 1946 Authors' Addresses 1947 Charles Shen 1948 Columbia University 1949 Department of Computer Science 1950 1214 Amsterdam Avenue, MC 0401 1951 New York, NY 10027 1952 USA 1954 Phone: +1 212 854 3109 1955 Email: charles@cs.columbia.edu 1957 Henning Schulzrinne 1958 Columbia University 1959 Department of Computer Science 1960 1214 Amsterdam Avenue, MC 0401 1961 New York, NY 10027 1962 USA 1964 Phone: +1 212 939 7004 1965 Email: schulzrinne@cs.columbia.edu 1967 Arata Koike 1968 NTT Service Integration Labs 1969 3-9-11 Midori-cho Musashino-shi 1970 Tokyo 184-0013 1971 Japan 1973 Phone: +81 422 59 6099 1974 Email: koike.arata@lab.ntt.co.jp