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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DOTS A. Mortensen 3 Internet-Draft Arbor Networks 4 Intended status: Informational F. Andreasen 5 Expires: April 28, 2018 Cisco 6 T. Reddy 7 McAfee, Inc. 8 C. Gray 9 Comcast 10 R. Compton 11 Charter 12 N. Teague 13 Verisign 14 October 25, 2017 16 Distributed-Denial-of-Service Open Threat Signaling (DOTS) Architecture 17 draft-ietf-dots-architecture-05 19 Abstract 21 This document describes an architecture for establishing and 22 maintaining Distributed Denial of Service (DDoS) Open Threat 23 Signaling (DOTS) within and between domains. The document does not 24 specify protocols or protocol extensions, instead focusing on 25 defining architectural relationships, components and concepts used in 26 a DOTS deployment. 28 Status of This Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at https://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on April 28, 2018. 45 Copyright Notice 47 Copyright (c) 2017 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (https://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Context and Motivation . . . . . . . . . . . . . . . . . . . 3 63 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 64 1.1.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 3 65 1.1.2. Definition of Terms . . . . . . . . . . . . . . . . . 3 66 1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 3 67 1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 4 68 2. DOTS Architecture . . . . . . . . . . . . . . . . . . . . . . 5 69 2.1. DOTS Operations . . . . . . . . . . . . . . . . . . . . . 8 70 2.2. Components . . . . . . . . . . . . . . . . . . . . . . . 9 71 2.2.1. DOTS Client . . . . . . . . . . . . . . . . . . . . . 9 72 2.2.2. DOTS Server . . . . . . . . . . . . . . . . . . . . . 10 73 2.2.3. DOTS Gateway . . . . . . . . . . . . . . . . . . . . 11 74 2.3. DOTS Agent Relationships . . . . . . . . . . . . . . . . 12 75 2.3.1. Gatewayed Signaling . . . . . . . . . . . . . . . . . 14 76 3. Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . 16 77 3.1. DOTS Sessions . . . . . . . . . . . . . . . . . . . . . . 16 78 3.1.1. Preconditions . . . . . . . . . . . . . . . . . . . . 16 79 3.1.2. Establishing the DOTS Session . . . . . . . . . . . . 16 80 3.1.3. Maintaining the DOTS Session . . . . . . . . . . . . 17 81 3.2. Modes of Signaling . . . . . . . . . . . . . . . . . . . 18 82 3.2.1. Direct Signaling . . . . . . . . . . . . . . . . . . 18 83 3.2.2. Redirected Signaling . . . . . . . . . . . . . . . . 18 84 3.2.3. Recursive Signaling . . . . . . . . . . . . . . . . . 19 85 3.2.4. Anycast Signaling . . . . . . . . . . . . . . . . . . 22 86 3.3. Triggering Requests for Mitigation . . . . . . . . . . . 23 87 3.3.1. Manual Mitigation Request . . . . . . . . . . . . . . 24 88 3.3.2. Automated Conditional Mitigation Request . . . . . . 25 89 3.3.3. Automated Mitigation on Loss of Signal . . . . . . . 26 90 4. Security Considerations . . . . . . . . . . . . . . . . . . . 26 91 5. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 27 92 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27 93 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 27 94 7.1. Normative References . . . . . . . . . . . . . . . . . . 27 95 7.2. Informative References . . . . . . . . . . . . . . . . . 27 96 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29 98 1. Context and Motivation 100 Signaling the need for help defending against an active distributed 101 denial of service (DDoS) attack requires a common understanding of 102 mechanisms and roles among the parties coordinating defensive 103 response. The signaling layer and supplementary messaging is the 104 focus of DDoS Open Threat Signaling (DOTS). DOTS defines a method of 105 coordinating defensive measures among willing peers to mitigate 106 attacks quickly and efficiently, enabling hybrid attack responses 107 coordinated locally at or near the target of an active attack, or 108 anywhere in-path between attack sources and target. Sample DOTS use 109 cases are elaborated in [I-D.ietf-dots-use-cases]. 111 This document describes an architecture used in establishing, 112 maintaining or terminating a DOTS relationship within a domain or 113 between domains. 115 1.1. Terminology 117 1.1.1. Key Words 119 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 120 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 121 document are to be interpreted as described in [RFC2119]. 123 1.1.2. Definition of Terms 125 This document uses the terms defined in [I-D.ietf-dots-requirements]. 127 1.2. Scope 129 In this architecture, DOTS clients and servers communicate using DOTS 130 signaling. As a result of signals from a DOTS client, the DOTS 131 server may modify the forwarding path of traffic destined for the 132 attack target(s), for example by diverting traffic to a mitigator or 133 pool of mitigators, where policy may be applied to distinguish and 134 discard attack traffic. Any such policy is deployment-specific. 136 The DOTS architecture presented here is applicable across network 137 administrative domains - for example, between an enterprise domain 138 and the domain of a third-party attack mitigation service - as well 139 as to a single administrative domain. DOTS is generally assumed to 140 be most effective when aiding coordination of attack response between 141 two or more participating networks, but single domain scenarios are 142 valuable in their own right, as when aggregating intra-domain DOTS 143 client signals for inter-domain coordinated attack response. 145 This document does not address any administrative or business 146 agreements that may be established between involved DOTS parties. 147 Those considerations are out of scope. Regardless, this document 148 assumes necessary authentication and authorization mechanisms are put 149 in place so that only authorized clients can invoke the DOTS service. 151 A detailed set of DOTS requirements are discussed in 152 [I-D.ietf-dots-requirements], and the DOTS architecture is designed 153 to follow those requirements. Only new behavioral requirements are 154 described in this document. 156 1.3. Assumptions 158 This document makes the following assumptions: 160 o All domains in which DOTS is deployed are assumed to offer the 161 required connectivity between DOTS agents and any intermediary 162 network elements, but the architecture imposes no additional 163 limitations on the form of connectivity. 165 o Congestion and resource exhaustion are intended outcomes of a DDoS 166 attack [RFC4732]. Some operators may utilize non-impacted paths 167 or networks for DOTS, but in general conditions should be assumed 168 to be hostile and DOTS must be able to function in all 169 circumstances, including when the signaling path is significantly 170 impaired. 172 o There is no universal DDoS attack scale threshold triggering a 173 coordinated response across administrative domains. A network 174 domain administrator, or service or application owner may 175 arbitrarily set attack scale threshold triggers, or manually send 176 requests for mitigation. 178 o Mitigation requests may be sent to one or more upstream DOTS 179 servers based on criteria determined by DOTS client administrators 180 and the underlying network configuration. The number of DOTS 181 servers with which a given DOTS client has established 182 communications is determined by local policy and is deployment- 183 specific. For example, a DOTS client of a multi-homed network may 184 support built-in policies to establish DOTS relationships with 185 DOTS servers located upstream of each interconnection link. 187 o The mitigation capacity and/or capability of domains receiving 188 requests for coordinated attack response is opaque to the domains 189 sending the request. The domain receiving the DOTS client signal 190 may or may not have sufficient capacity or capability to filter 191 any or all DDoS attack traffic directed at a target. In either 192 case, the upstream DOTS server may redirect a request to another 193 DOTS server. Redirection may be local to the redirecting DOTS 194 server's domain, or may involve a third-party domain. 196 o DOTS client and server signals, as well as messages sent through 197 the data channel, are sent across any transit networks with the 198 same probability of delivery as any other traffic between the DOTS 199 client domain and the DOTS server domain. Any encapsulation 200 required for successful delivery is left untouched by transit 201 network elements. DOTS server and DOTS client cannot assume any 202 preferential treatment of DOTS signals. Such preferential 203 treatment may be available in some deployments (e.g., intra-domain 204 scenarios), and the DOTS architecture does not preclude its use 205 when available. However, DOTS itself does not address how that 206 may be done. 208 o The architecture allows for, but does not assume, the presence of 209 Quality of Service (QoS) policy agreements between DOTS-enabled 210 peer networks or local QoS prioritization aimed at ensuring 211 delivery of DOTS messages between DOTS agents. QoS is an 212 operational consideration only, not a functional part of the DOTS 213 architecture. 215 o The signal and data channels are loosely coupled, and may not 216 terminate on the same DOTS server. 218 2. DOTS Architecture 220 The basic high-level DOTS architecture is illustrated in Figure 1: 222 +-----------+ +-------------+ 223 | Mitigator | ~~~~~~~~~~ | DOTS Server | 224 +-----------+ +-------------+ 225 | 226 | 227 | 228 +---------------+ +-------------+ 229 | Attack Target | ~~~~~~ | DOTS Client | 230 +---------------+ +-------------+ 232 Figure 1: Basic DOTS Architecture 234 A simple example instantiation of the DOTS architecture could be an 235 enterprise as the attack target for a volumetric DDoS attack, and an 236 upstream DDoS mitigation service as the mitigator. The enterprise 237 (attack target) is connected to the Internet via a link that is 238 getting saturated, and the enterprise suspects it is under DDoS 239 attack. The enterprise has a DOTS client, which obtains information 240 about the DDoS attack, and signals the DOTS server for help in 241 mitigating the attack. The DOTS server in turn invokes one or more 242 mitigators, which are tasked with mitigating the actual DDoS attack, 243 and hence aim to suppress the attack traffic while allowing valid 244 traffic to reach the attack target. 246 The scope of the DOTS specifications is the interfaces between the 247 DOTS client and DOTS server. The interfaces to the attack target and 248 the mitigator are out of scope of DOTS. Similarly, the operation of 249 both the attack target and the mitigator is out of scope of DOTS. 250 Thus, DOTS neither specifies how an attack target decides it is under 251 DDoS attack, nor does DOTS specify how a mitigator may actually 252 mitigate such an attack. A DOTS client's request for mitigation is 253 advisory in nature, and may not lead to any mitigation at all, 254 depending on the DOTS server domain's capacity and willingness to 255 mitigate on behalf of the DOTS client's domain. 257 The DOTS client may be provided with a list of DOTS servers, each 258 associated with one or more IP addresses. These addresses may or may 259 not be of the same address family. The DOTS client establishes one 260 or more sessions by connecting to the provided DOTS server addresses. 262 As illustrated in Figure 2, there are two interfaces between a DOTS 263 server and a DOTS client; a signal channel and (optionally) a data 264 channel. 266 +---------------+ +---------------+ 267 | | <------- Signal Channel ------> | | 268 | DOTS Client | | DOTS Server | 269 | | <======= Data Channel ======> | | 270 +---------------+ +---------------+ 272 Figure 2: DOTS Interfaces 274 The primary purpose of the signal channel is for a DOTS client to ask 275 a DOTS server for help in mitigating an attack, and for the DOTS 276 server to inform the DOTS client about the status of such mitigation. 277 The DOTS client does this by sending a client signal, which contains 278 information about the attack target(s). The client signal may also 279 include telemetry information about the attack, if the DOTS client 280 has such information available. The DOTS server in turn sends a 281 server signal to inform the DOTS client of whether it will honor the 282 mitigation request. Assuming it will, the DOTS server initiates 283 attack mitigation, and periodically informs the DOTS client about the 284 status of the mitigation. Similarly, the DOTS client periodically 285 informs the DOTS server about the client's status, which at a minimum 286 provides client (attack target) health information, but it should 287 also include efficacy information about the attack mitigation as it 288 is now seen by the client. At some point, the DOTS client may decide 289 to terminate the server-side attack mitigation, which it indicates to 290 the DOTS server over the signal channel. A mitigation may also be 291 terminated if a DOTS client-specified mitigation lifetime is 292 exceeded. Note that the signal channel may need to operate over a 293 link that is experiencing a DDoS attack and hence is subject to 294 severe packet loss and high latency. 296 While DOTS is able to request mitigation with just the signal 297 channel, the addition of the DOTS data channel provides for 298 additional and more efficient capabilities. The primary purpose of 299 the data channel is to support DOTS related configuration and policy 300 information exchange between the DOTS client and the DOTS server. 301 Examples of such information include, but are not limited to: 303 o Creating identifiers, such as names or aliases, for resources for 304 which mitigation may be requested. Such identifiers may then be 305 used in subsequent signal channel exchanges to refer more 306 efficiently to the resources under attack, as seen in Figure 3, 307 using JSON to serialize the data: 309 { 310 "https1": [ 311 "192.0.2.1:443", 312 "198.51.100.2:443", 313 ], 314 "proxies": [ 315 "203.0.113.3:3128", 316 "[2001:db8:ac10::1]:3128" 317 ], 318 "api_urls": "https://apiserver.example.com/api/v1", 319 } 321 Figure 3: Protected resource identifiers 323 o Black-list management, which enables a DOTS client to inform the 324 DOTS server about sources to suppress. 326 o White-list management, which enables a DOTS client to inform the 327 DOTS server about sources from which traffic is always accepted. 329 o Filter management, which enables a DOTS client to install or 330 remove traffic filters dropping or rate-limiting unwanted traffic. 332 o DOTS client provisioning. 334 Note that while it is possible to exchange the above information 335 before, during or after a DDoS attack, DOTS requires reliable 336 delivery of this information and does not provide any special means 337 for ensuring timely delivery of it during an attack. In practice, 338 this means that DOTS deployments should not rely on such information 339 being exchanged during a DDoS attack. 341 2.1. DOTS Operations 343 DOTS does not prescribe any specific deployment models, however DOTS 344 is designed with some specific requirements around the different DOTS 345 agents and their relationships. 347 First of all, a DOTS agent belongs to a domain that has an identity 348 which can be authenticated and authorized. DOTS agents communicate 349 with each other over a mutually authenticated signal channel and 350 (optionally) data channel. However, before they can do so, a service 351 relationship needs to be established between them. The details and 352 means by which this is done is outside the scope of DOTS, however an 353 example would be for an enterprise A (DOTS client) to sign up for 354 DDoS service from provider B (DOTS server). This would establish a 355 (service) relationship between the two that enables enterprise A's 356 DOTS client to establish a signal channel with provider B's DOTS 357 server. A and B will authenticate each other, and B can verify that 358 A is authorized for its service. 360 From an operational and design point of view, DOTS assumes that the 361 above relationship is established prior to a request for DDoS attack 362 mitigation. In particular, it is assumed that bi-directional 363 communication is possible at this time between the DOTS client and 364 DOTS server. Furthermore, it is assumed that additional service 365 provisioning, configuration and information exchange can be performed 366 by use of the data channel, if operationally required. It is not 367 until this point that the mitigation service is available for use. 369 Once the mutually authenticated signal channel has been established, 370 it will remain active. This is done to increase the likelihood that 371 the DOTS client can signal the DOTS server for help when the attack 372 target is being flooded, and similarly raise the probability that 373 DOTS server signals reach the client regardless of inbound link 374 congestion. This does not necessarily imply that the attack target 375 and the DOTS client have to be co-located in the same administrative 376 domain, but it is expected to be a common scenario. 378 DDoS mitigation with the help of an upstream mitigator may involve 379 some form of traffic redirection whereby traffic destined for the 380 attack target is steered towards the mitigator. Common mechanisms to 381 achieve this redirection depend on BGP [RFC4271] and DNS [RFC1035]. 383 The mitigator in turn inspects and scrubs the traffic, and forwards 384 the resulting (hopefully non-attack) traffic to the attack target. 385 Thus, when a DOTS server receives an attack mitigation request from a 386 DOTS client, it can be viewed as a way of causing traffic redirection 387 for the attack target indicated. 389 DOTS relies on mutual authentication and the pre-established service 390 relationship between the DOTS client's domain and the DOTS server's 391 domain to provide basic authorization. The DOTS server should 392 enforce additional authorization mechanisms to restrict the 393 mitigation scope a DOTS client can request, but such authorization 394 mechanisms are deployment-specific. 396 Although co-location of DOTS server and mitigator within the same 397 domain is expected to be a common deployment model, it is assumed 398 that operators may require alternative models. Nothing in this 399 document precludes such alternatives. 401 2.2. Components 403 2.2.1. DOTS Client 405 A DOTS client is a DOTS agent from which requests for help 406 coordinating attack response originate. The requests may be in 407 response to an active, ongoing attack against a target in the DOTS 408 client's domain, but no active attack is required for a DOTS client 409 to request help. Operators may wish to have upstream mitigators in 410 the network path for an indefinite period, and are restricted only by 411 business relationships when it comes to duration and scope of 412 requested mitigation. 414 The DOTS client requests attack response coordination from a DOTS 415 server over the signal channel, including in the request the DOTS 416 client's desired mitigation scoping, as described in 417 [I-D.ietf-dots-requirements]. The actual mitigation scope and 418 countermeasures used in response to the attack are up to the DOTS 419 server and mitigator operators, as the DOTS client may have a narrow 420 perspective on the ongoing attack. As such, the DOTS client's 421 request for mitigation should be considered advisory: guarantees of 422 DOTS server availability or mitigation capacity constitute service 423 level agreements and are out of scope for this document. 425 The DOTS client adjusts mitigation scope and provides available 426 mitigation feedback (e.g. mitigation efficacy) at the direction of 427 its local administrator. Such direction may involve manual or 428 automated adjustments in response to updates from the DOTS server. 430 To provide a metric of signal health and distinguish an idle signal 431 channel from a disconnected or defunct session, the DOTS client sends 432 a heartbeat over the signal channel to maintain its half of the 433 channel. The DOTS client similarly expects a heartbeat from the DOTS 434 server, and may consider a session terminated in the extended absence 435 of a DOTS server heartbeat. 437 2.2.2. DOTS Server 439 A DOTS server is a DOTS agent capable of receiving, processing and 440 possibly acting on requests for help coordinating attack response 441 from DOTS clients. The DOTS server authenticates and authorizes DOTS 442 clients as described in Section 3.1, and maintains session state, 443 tracking requests for mitigation, reporting on the status of active 444 mitigations, and terminating sessions in the extended absence of a 445 client heartbeat or when a session times out. 447 Assuming the preconditions discussed below exist, a DOTS client 448 maintaining an active session with a DOTS server may reasonably 449 expect some level of mitigation in response to a request for 450 coordinated attack response. 452 The DOTS server enforces authorization of DOTS clients' signals for 453 mitigation. The mechanism of enforcement is not in scope for this 454 document, but is expected to restrict requested mitigation scope to 455 addresses, prefixes, and/or services owned by the DOTS client's 456 administrative domain, such that a DOTS client from one domain is not 457 able to influence the network path to another domain. A DOTS server 458 MUST reject requests for mitigation of resources not owned by the 459 requesting DOTS client's administrative domain. A DOTS server MAY 460 also refuse a DOTS client's mitigation request for arbitrary reasons, 461 within any limits imposed by business or service level agreements 462 between client and server domains. If a DOTS server refuses a DOTS 463 client's request for mitigation, the DOTS server SHOULD include the 464 refusal reason in the server signal sent to the client. 466 A DOTS server is in regular contact with one or more mitigators. If 467 a DOTS server accepts a DOTS client's request for help, the DOTS 468 server forwards a translated form of that request to the mitigator(s) 469 responsible for scrubbing attack traffic. Note that the form of the 470 translated request passed from the DOTS server to the mitigator is 471 not in scope: it may be as simple as an alert to mitigator operators, 472 or highly automated using vendor or open application programming 473 interfaces supported by the mitigator. The DOTS server MUST report 474 the actual scope of any mitigation enabled on behalf of a client. 476 The DOTS server SHOULD retrieve available metrics for any mitigations 477 activated on behalf of a DOTS client, and SHOULD include them in 478 server signals sent to the DOTS client originating the request for 479 mitigation. 481 To provide a metric of signal health and distinguish an idle signal 482 channel from a disconnected or defunct channel, the DOTS server MUST 483 send a heartbeat over the signal channel to maintain its half of the 484 channel. The DOTS server similarly expects a heartbeat from the DOTS 485 client, and MAY consider a session terminated in the extended absence 486 of a DOTS client heartbeat. 488 2.2.3. DOTS Gateway 490 Traditional client/server relationships may be expanded by chaining 491 DOTS sessions. This chaining is enabled through "logical 492 concatenation" of a DOTS server and a DOTS client, resulting in an 493 application analogous to the Session Initiation Protocol (SIP) 494 [RFC3261] logical entity of a Back-to-Back User Agent (B2BUA) 495 [RFC7092]. The term DOTS gateway is used here in the descriptions of 496 selected scenarios involving this application. 498 A DOTS gateway may be deployed client-side, server-side or both. The 499 gateway may terminate multiple discrete client connections and may 500 aggregate these into a single or multiple DOTS sessions. 502 The DOTS gateway will appear as a server to its downstream agents and 503 as a client to its upstream agents, a functional concatenation of the 504 DOTS client and server roles, as depicted in Figure 4: 506 +-------------+ 507 | | D | | 508 +----+ | | O | | +----+ 509 | c1 |----------| s1 | T | c2 |---------| s2 | 510 +----+ | | S | | +----+ 511 | | G | | 512 +-------------+ 514 Figure 4: DOTS gateway 516 The DOTS gateway MUST perform full stack DOTS session termination and 517 reorigination between its client and server side. The details of how 518 this is achieved are implementation specific. The DOTS protocol does 519 not include any special features related to DOTS gateways, and hence 520 from a DOTS perspective, whenever a DOTS gateway is present, the DOTS 521 session simply terminates/originates there. 523 2.3. DOTS Agent Relationships 525 So far, we have only considered a relatively simple scenario of a 526 single DOTS client associated with a single DOTS server, however DOTS 527 supports more advanced relationships. 529 A DOTS server may be associated with one or more DOTS clients, and 530 those DOTS clients may belong to different domains. An example 531 scenario is a mitigation provider serving multiple attack targets 532 (Figure 5). 534 DOTS clients DOTS server 535 +---+ 536 | c |----------- 537 +---+ \ 538 c1.example.org \ 539 \ 540 +---+ \ +---+ 541 | c |----------------| S | 542 +---+ / +---+ 543 c1.example.com / dots1.example.net 544 / 545 +---+ / 546 | c |----------- 547 +---+ 548 c2.example.com 550 Figure 5: DOTS server with multiple clients 552 A DOTS client may be associated with one or more DOTS servers, and 553 those DOTS servers may belong to different domains. This may be to 554 ensure high availability or co-ordinate mitigation with more than one 555 directly connected ISP. An example scenario is for an enterprise to 556 have DDoS mitigation service from multiple providers, as shown in 557 Figure 6. 559 DOTS client DOTS servers 560 +---+ 561 -----------| S | 562 / +---+ 563 / dots1.example.net 564 / 565 +---+/ +---+ 566 | c |---------------| S | 567 +---+\ +---+ 568 \ dots.example.org 569 \ 570 \ +---+ 571 -----------| S | 572 +---+ 573 c.example.com dots2.example.net 575 Figure 6: Multi-Homed DOTS Client 577 Deploying a multi-homed client requires extra care and planning, as 578 the DOTS servers with which the multi-homed client communicates may 579 not be affiliated. Should the multi-homed client simultaneously 580 request for mitigation from all servers with which it has established 581 signal channels, the client may unintentionally inflict additional 582 network disruption on the resources it intends to protect. In one of 583 the worst cases, a multi-homed DOTS client could cause a permanent 584 routing loop of traffic destined for the client's protected services, 585 as the uncoordinated DOTS servers' mitigators all try to divert that 586 traffic to their own scrubbing centers. 588 The DOTS protocol itself provides no fool-proof method to prevent 589 such self-inflicted harms as a result deploying multi-homed DOTS 590 clients. If DOTS client implementations nevertheless include support 591 for multi-homing, they are expected to be aware of the risks, and 592 consequently to include measures aimed at reducing the likelihood of 593 negative outcomes. Simple measures might include: 595 o Requesting mitigation serially, ensuring only one mitigation 596 request for a given address space is active at any given time; 598 o Dividing the protected resources among the DOTS servers, such that 599 no two mitigators will be attempting to divert and scrub the same 600 traffic; 602 o Restricting multi-homing to deployments in which all DOTS servers 603 are coordinating management of a shared pool of mitigation 604 resources. 606 2.3.1. Gatewayed Signaling 608 As discussed in Section 2.2.3, a DOTS gateway is a logical function 609 chaining DOTS sessions through concatenation of a DOTS server and 610 DOTS client. 612 An example scenario, as shown in Figure 7 and Figure 8, is for an 613 enterprise to have deployed multiple DOTS capable devices which are 614 able to signal intra-domain using TCP [RFC0793] on un-congested links 615 to a DOTS gateway which may then transform these to a UDP [RFC0768] 616 transport inter-domain where connection oriented transports may 617 degrade; this applies to the signal channel only, as the data channel 618 requires a connection-oriented transport. The relationship between 619 the gateway and its upstream agents is opaque to the initial clients. 621 +---+ 622 | c |\ 623 +---+ \ +---+ 624 \-----TCP-----| D | +---+ 625 +---+ | O | | | 626 | c |--------TCP-----| T |------UDP------| S | 627 +---+ | S | | | 628 /-----TCP-----| G | +---+ 629 +---+ / +---+ 630 | c |/ 631 +---+ 632 example.com example.com example.net 633 DOTS clients DOTS gateway (DOTSG) DOTS server 635 Figure 7: Client-Side Gateway with Aggregation 637 +---+ 638 | c |\ 639 +---+ \ +---+ 640 \-----TCP-----| D |------UDP------+---+ 641 +---+ | O | | | 642 | c |--------TCP-----| T |------UDP------| S | 643 +---+ | S | | | 644 /-----TCP-----| G |------UDP------+---+ 645 +---+ / +---+ 646 | c |/ 647 +---+ 648 example.com example.com example.net 649 DOTS clients DOTS gateway (DOTSG) DOTS server 651 Figure 8: Client-Side Gateway without Aggregation 653 This may similarly be deployed in the inverse scenario where the 654 gateway resides in the server-side domain and may be used to 655 terminate and/or aggregate multiple clients to single transport as 656 shown in figures Figure 9 and Figure 10. 658 +---+ 659 | c |\ 660 +---+ \ +---+ 661 \-----UDP-----| D | +---+ 662 +---+ | O | | | 663 | c |--------TCP-----| T |------TCP------| S | 664 +---+ | S | | | 665 /-----TCP-----| G | +---+ 666 +---+ / +---+ 667 | c |/ 668 +---+ 669 example.com example.net example.net 670 DOTS clients DOTS gateway (DOTSG) DOTS server 672 Figure 9: Server-Side Gateway with Aggregation 674 +---+ 675 | c |\ 676 +---+ \ +---+ 677 \-----UDP-----| D |------TCP------+---+ 678 +---+ | O | | | 679 | c |--------TCP-----| T |------TCP------| S | 680 +---+ | S | | | 681 /-----UDP-----| G |------TCP------+---+ 682 +---+ / +---+ 683 | c |/ 684 +---+ 685 example.com example.net example.net 686 DOTS clients DOTS gateway (DOTSG) DOTS server 688 Figure 10: Server-Side Gateway without Aggregation 690 This document anticipates scenarios involving multiple DOTS gateways. 691 An example is a DOTS gateway at the network client's side, and 692 another one at the server side. The first gateway can be located at 693 a CPE to aggregate requests from multiple DOTS clients enabled in an 694 enterprise network. The second DOTS gateway is deployed on the 695 provider side. This scenario can be seen as a combination of the 696 client-side and server-side scenarios. 698 3. Concepts 700 3.1. DOTS Sessions 702 In order for DOTS to be effective as a vehicle for DDoS mitigation 703 requests, one or more DOTS clients must establish ongoing 704 communication with one or more DOTS servers. While the preconditions 705 for enabling DOTS in or among network domains may also involve 706 business relationships, service level agreements, or other formal or 707 informal understandings between network operators, such 708 considerations are out of scope for this document. 710 A DOTS session is established to support bilateral exchange of data 711 between an associated DOTS client and a DOTS server. In the DOTS 712 architecture, data is exchanged between DOTS agents over signal and 713 data channels. Regardless, a DOTS session is characterized by the 714 presence of an established signal channel. A data channel may be 715 established as well, however it is not a prerequisite. Conversely, a 716 DOTS session cannot exist without an established signal channel: when 717 an established signal channel is terminated for any reason, the DOTS 718 session is also said to be terminated. 720 3.1.1. Preconditions 722 Prior to establishing a DOTS session between agents, the owners of 723 the networks, domains, services or applications involved are assumed 724 to have agreed upon the terms of the relationship involved. Such 725 agreements are out of scope for this document, but must be in place 726 for a functional DOTS architecture. 728 It is assumed that as part of any DOTS service agreement, the DOTS 729 client is provided with all data and metadata required to establish 730 communication with the DOTS server. Such data and metadata would 731 include any cryptographic information necessary to meet the message 732 confidentiality, integrity and authenticity requirement in 733 [I-D.ietf-dots-requirements], and might also include the pool of DOTS 734 server addresses and ports the DOTS client should use for signal and 735 data channel messaging. 737 3.1.2. Establishing the DOTS Session 739 With the required business agreements in place, the DOTS client 740 initiates a signal session by contacting its DOTS server(s) over the 741 signal channel and (possibly) the data channel. To allow for DOTS 742 service flexibility, neither the order of contact nor the time 743 interval between channel creations is specified. A DOTS client MAY 744 establish signal channel first, and then data channel, or vice versa. 746 The methods by which a DOTS client receives the address and 747 associated service details of the DOTS server are not prescribed by 748 this document. For example, a DOTS client may be directly configured 749 to use a specific DOTS server IP address and port, and directly 750 provided with any data necessary to satisfy the Peer Mutual 751 Authentication requirement in [I-D.ietf-dots-requirements], such as 752 symmetric or asymmetric keys, usernames and passwords, etc. All 753 configuration and authentication information in this scenario is 754 provided out-of-band by the domain operating the DOTS server. 756 At the other extreme, the architecture in this document allows for a 757 form of DOTS client auto-provisioning. For example, the domain 758 operating the DOTS server or servers might provide the client domain 759 only with symmetric or asymmetric keys to authenticate the 760 provisioned DOTS clients. Only the keys would then be directly 761 configured on DOTS clients, but the remaining configuration required 762 to provision the DOTS clients could be learned through mechanisms 763 similar to DNS SRV [RFC2782] or DNS Service Discovery [RFC6763]. 765 The DOTS client SHOULD successfully authenticate and exchange 766 messages with the DOTS server over both signal and (if used) data 767 channel as soon as possible to confirm that both channels are 768 operational. 770 As described in [I-D.ietf-dots-requirements], the DOTS client can 771 configure preferred values for acceptable signal loss, mitigation 772 lifetime, and heartbeat intervals when establishing the DOTS session. 773 A DOTS session is not active until DOTS agents have agreed on the 774 values for these DOTS session parameters, a process defined by the 775 protocol. 777 Once the DOTS client begins receiving DOTS server signals, the DOTS 778 session is active. At any time during the DOTS session, the DOTS 779 client may use the data channel to manage aliases, manage black- and 780 white-listed prefixes or addresses, leverage vendor-specific 781 extensions, and so on. Note that unlike the signal channel, there is 782 no requirement that the data channel remains operational in attack 783 conditions (See Data Channel Requirements, 784 [I-D.ietf-dots-requirements]). 786 3.1.3. Maintaining the DOTS Session 788 DOTS clients and servers periodically send heartbeats to each other 789 over the signal channel, per Operational Requirements discussed in 790 [I-D.ietf-dots-requirements]. DOTS agent operators SHOULD configure 791 the heartbeat interval such that the frequency does not lead to 792 accidental denials of service due to the overwhelming number of 793 heartbeats a DOTS agent must field. 795 Either DOTS agent may consider a DOTS session terminated in the 796 extended absence of a heartbeat from its peer agent. The period of 797 that absence will be established in the protocol definition. 799 3.2. Modes of Signaling 801 This section examines the modes of signaling between agents in a DOTS 802 architecture. 804 3.2.1. Direct Signaling 806 A DOTS session may take the form of direct signaling between the DOTS 807 clients and servers, as shown in Figure 11. 809 +-------------+ +-------------+ 810 | DOTS client |<------signal session------>| DOTS server | 811 +-------------+ +-------------+ 813 Figure 11: Direct Signaling 815 In a direct DOTS session, the DOTS client and server are 816 communicating directly. Direct signaling may exist inter- or intra- 817 domain. The DOTS session is abstracted from the underlying networks 818 or network elements the signals traverse: in direct signaling, the 819 DOTS client and server are logically adjacent. 821 3.2.2. Redirected Signaling 823 In certain circumstances, a DOTS server may want to redirect a DOTS 824 client to an alternative DOTS server for a DOTS session. Such 825 circumstances include but are not limited to: 827 o Maximum number of DOTS sessions with clients has been reached; 829 o Mitigation capacity exhaustion in the mitigator with which the 830 specific DOTS server is communicating; 832 o Mitigator outage or other downtime, such as scheduled maintenance; 834 o Scheduled DOTS server maintenance; 836 o Scheduled modifications to the network path between DOTS server 837 and DOTS client. 839 A basic redirected DOTS session resembles the following, as shown in 840 Figure 12. 842 +-------------+ +---------------+ 843 | |<-(1)--- DOTS session 1 --->| | 844 | | | | 845 | |<=(2)== redirect to B ======| | 846 | DOTS client | | DOTS server A | 847 | |X-(4)--- DOTS session 1 ---X| | 848 | | | | 849 | | | | 850 +-------------+ +---------------+ 851 ^ 852 | 853 (3) DOTS session 2 854 | 855 v 856 +---------------+ 857 | DOTS server B | 858 +---------------+ 860 Figure 12: Redirected Signaling 862 1. Previously established DOTS session 1 exists between a DOTS 863 client and DOTS server A. 865 2. DOTS server A sends a server signal redirecting the client to 866 DOTS server B. 868 3. If the DOTS client does not already have a separate DOTS session 869 with the redirection target, the DOTS client initiates and 870 establishes DOTS session 2 with DOTS server B. 872 4. Having redirected the DOTS client, DOTS server A ceases sending 873 server signals. The DOTS client likewise stops sending client 874 signals to DOTS server A. DOTS session 1 is terminated. 876 3.2.3. Recursive Signaling 878 DOTS is centered around improving the speed and efficiency of 879 coordinated response to DDoS attacks. One scenario not yet discussed 880 involves coordination among federated domains operating DOTS servers 881 and mitigators. 883 In the course of normal DOTS operations, a DOTS client communicates 884 the need for mitigation to a DOTS server, and that server initiates 885 mitigation on a mitigator with which the server has an established 886 service relationship. The operator of the mitigator may in turn 887 monitor mitigation performance and capacity, as the attack being 888 mitigated may grow in severity beyond the mitigating domain's 889 capabilities. 891 The operator of the mitigator has limited options in the event a DOTS 892 client-requested mitigation is being overwhelmed by the severity of 893 the attack. Out-of-scope business or service level agreements may 894 permit the mitigating domain to drop the mitigation and let attack 895 traffic flow unchecked to the target, but this only encourages attack 896 escalation. In the case where the mitigating domain is the upstream 897 service provider for the attack target, this may mean the mitigating 898 domain and its other services and users continue to suffer the 899 incidental effects of the attack. 901 A recursive signaling model as shown in Figure 13 offers an 902 alternative. In a variation of the use case "End-customer with a 903 single upstream transit provider offering DDoS mitigation services" 904 described in [I-D.ietf-dots-use-cases], a domain operating a DOTS 905 server and mitigator also operates a DOTS client. This DOTS client 906 has an established DOTS session with a DOTS server belonging to a 907 separate administrative domain. 909 With these preconditions in place, the operator of the mitigator 910 being overwhelmed or otherwise performing inadequately may request 911 mitigation for the attack target from this separate DOTS-aware 912 domain. Such a request recurses the originating mitigation request 913 to the secondary DOTS server, in the hope of building a cumulative 914 mitigation against the attack. 916 example.net domain 917 . . . . . . . . . . . . . . . . . 918 . Gn . 919 +----+ 1 . +----+ +-----------+ . 920 | Cc |<--------->| Sn |~~~~~~~| Mitigator | . 921 +----+ . +====+ | Mn | . 922 . | Cn | +-----------+ . 923 example.com . +----+ . 924 client . ^ . 925 . . .|. . . . . . . . . . . . . . 926 | 927 1 | 928 | 929 . . .|. . . . . . . . . . . . . . 930 . v . 931 . +----+ +-----------+ . 932 . | So |~~~~~~~| Mitigator | . 933 . +----+ | Mo | . 934 . +-----------+ . 935 . . 936 . . . . . . . . . . . . . . . . . 937 example.org domain 939 Figure 13: Recursive Signaling 941 In Figure 13, client Cc signals a request for mitigation across 942 inter-domain DOTS session 1 to the DOTS server Sn belonging to the 943 example.net domain. DOTS server Sn enables mitigation on mitigator 944 Mn. DOTS server Sn is half of DOTS gateway Gn, being deployed 945 logically back-to-back with DOTS client Cn, which has pre-existing 946 inter-domain DOTS session 2 with the DOTS server So belonging to the 947 example.org domain. At any point, DOTS server Sn MAY recurse an on- 948 going mitigation request through DOTS client Cn to DOTS server So, in 949 the expectation that mitigator Mo will be activated to aid in the 950 defense of the attack target. 952 Recursive signaling is opaque to the DOTS client. To maximize 953 mitigation visibility to the DOTS client, however, the recursing 954 domain SHOULD provide recursed mitigation feedback in signals 955 reporting on mitigation status to the DOTS client. For example, the 956 recursing domain's mitigator should incorporate into mitigation 957 status messages available metrics such as dropped packet or byte 958 counts from the recursed mitigation. 960 DOTS clients involved in recursive signaling must be able to withdraw 961 requests for mitigation without warning or justification, per 962 [I-D.ietf-dots-requirements]. 964 Operators recursing mitigation requests MAY maintain the recursed 965 mitigation for a brief, protocol-defined period in the event the DOTS 966 client originating the mitigation withdraws its request for help, as 967 per the discussion of managing mitigation toggling in the operational 968 requirements ([I-D.ietf-dots-requirements]). 970 Deployment of recursive signaling may result in traffic redirection, 971 examination and mitigation extending beyond the initial bilateral 972 relationship between DOTS client and DOTS server. As such, client 973 control over the network path of mitigated traffic may be reduced. 974 DOTS client operators should be aware of any privacy concerns, and 975 work with DOTS server operators employing recursive signaling to 976 ensure shared sensitive material is suitably protected. 978 3.2.4. Anycast Signaling 980 The DOTS architecture does not assume the availability of anycast 981 within a DOTS deployment, but neither does the architecture exclude 982 it. Domains operating DOTS servers MAY deploy DOTS servers with an 983 anycast Service Address as described in BCP 126 [RFC4786]. In such a 984 deployment, DOTS clients connecting to the DOTS Service Address may 985 be communicating with distinct DOTS servers, depending on the network 986 configuration at the time the DOTS clients connect. Among other 987 benefits, anycast signaling potentially offers the following: 989 o Simplified DOTS client configuration, including service discovery 990 through the methods described in [RFC7094]. In this scenario, the 991 "instance discovery" message would be a DOTS client initiating a 992 DOTS session to the DOTS server anycast Service Address, to which 993 the DOTS server would reply with a redirection to the DOTS server 994 unicast address the client should use for DOTS. 996 o Region- or customer-specific deployments, in which the DOTS 997 Service Addresses route to distinct DOTS servers depending on the 998 client region or the customer network in which a DOTS client 999 resides. 1001 o Operational resiliency, spreading DOTS signaling traffic across 1002 the DOTS server domain's networks, and thereby also reducing the 1003 potential attack surface, as described in BCP 126 [RFC4786]. 1005 3.2.4.1. Anycast Signaling Considerations 1007 As long as network configuration remains stable, anycast DOTS 1008 signaling is to the individual DOTS client indistinct from direct 1009 signaling. However, the operational challenges inherent in anycast 1010 signaling are anything but negligible, and DOTS server operators must 1011 carefully weigh the risks against the benefits before deploying. 1013 While the DOTS signal channel primarily operates over UDP per 1014 [I-D.ietf-dots-requirements], the signal channel also requires mutual 1015 authentication between DOTS agents, with associated security state on 1016 both ends. 1018 Network instability is of particular concern with anycast signaling, 1019 as DOTS signal channels are expected to be long-lived, and 1020 potentially operating under congested network conditions caused by a 1021 volumetric DDoS attack. 1023 For example, a network configuration altering the route to the DOTS 1024 server during active anycast signaling may cause the DOTS client to 1025 send messages to a DOTS server other than the one with which it 1026 initially established a signaling session. That second DOTS server 1027 may not have the security state of the existing session, forcing the 1028 DOTS client to initialize a new DOTS session. This challenge might 1029 in part be mitigated by use of pre-shared keys and session resumption 1030 [RFC5246][RFC6347], but keying material must be available to all DOTS 1031 servers sharing the anycast Service Address in that case. 1033 While the DOTS client will try to establish a new DOTS session with 1034 the DOTS server now acting as the anycast DOTS Service Address, the 1035 link between DOTS client and server may be congested with attack 1036 traffic, making signal session establishment difficult. In such a 1037 scenario, anycast Service Address instability becomes a sort of 1038 signal session flapping, with obvious negative consequences for the 1039 DOTS deployment. 1041 Anycast signaling deployments similarly must also take into account 1042 active mitigations. Active mitigations initiated through a DOTS 1043 session may involve diverting traffic to a scrubbing center. If the 1044 DOTS session flaps due to anycast changes as described above, 1045 mitigation may also flap as the DOTS servers sharing the anycast DOTS 1046 service address toggles mitigation on detecting DOTS session loss, 1047 depending on whether the client has configured mitigation on loss of 1048 signal. 1050 3.3. Triggering Requests for Mitigation 1052 [I-D.ietf-dots-requirements] places no limitation on the 1053 circumstances in which a DOTS client operator may request mitigation, 1054 nor does it demand justification for any mitigation request, thereby 1055 reserving operational control over DDoS defense for the domain 1056 requesting mitigation. This architecture likewise does not prescribe 1057 the network conditions and mechanisms triggering a mitigation request 1058 from a DOTS client. 1060 However, considering selected possible mitigation triggers from an 1061 architectural perspective offers a model for alternative or 1062 unanticipated triggers for DOTS deployments. In all cases, what 1063 network conditions merit a mitigation request are at the discretion 1064 of the DOTS client operator. 1066 The mitigation request itself is defined by DOTS, however the 1067 interfaces required to trigger the mitigation request in the 1068 following scenarios are implementation-specific. 1070 3.3.1. Manual Mitigation Request 1072 A DOTS client operator may manually prepare a request for mitigation, 1073 including scope and duration, and manually instruct the DOTS client 1074 to send the mitigation request to the DOTS server. In context, a 1075 manual request is a request directly issued by the operator without 1076 automated decision-making performed by a device interacting with the 1077 DOTS client. Modes of manual mitigation requests include an operator 1078 entering a command into a text interface, or directly interacting 1079 with a graphical interface to send the request. 1081 An operator might do this, for example, in response to notice of an 1082 attack delivered by attack detection equipment or software, and the 1083 alerting detector lacks interfaces or is not configured to use 1084 available interfaces to translate the alert to a mitigation request 1085 automatically. 1087 In a variation of the above scenario, the operator may have 1088 preconfigured on the DOTS client mitigation requests for various 1089 resources in the operator's domain. When notified of an attack, the 1090 DOTS client operator manually instructs the DOTS client to send the 1091 relevant preconfigured mitigation request for the resources under 1092 attack. 1094 A further variant involves recursive signaling, as described in 1095 Section 3.2.3. The DOTS client in this case is the second half of a 1096 DOTS gateway (back-to-back DOTS server and client). As in the 1097 previous scenario, the scope and duration of the mitigation request 1098 are pre-existing, but in this case are derived from the mitigation 1099 request received from a downstream DOTS client by the DOTS server. 1100 Assuming the preconditions required by Section 3.2.3 are in place, 1101 the DOTS gateway operator may at any time manually request mitigation 1102 from an upstream DOTS server, sending a mitigation request derived 1103 from the downstream DOTS client's request. 1105 The motivations for a DOTS client operator to request mitigation 1106 manually are not prescribed by this architecture, but are expected to 1107 include some of the following: 1109 o Notice of an attack delivered via e-mail or alternative messaging 1111 o Notice of an attack delivered via phone call 1113 o Notice of an attack delivered through the interface(s) of 1114 networking monitoring software deployed in the operator's domain 1116 o Manual monitoring of network behavior through network monitoring 1117 software 1119 3.3.2. Automated Conditional Mitigation Request 1121 Unlike manual mitigation requests, which depend entirely on the DOTS 1122 client operator's capacity to react with speed and accuracy to every 1123 detected or detectable attack, mitigation requests triggered by 1124 detected attack conditions reduce the operational burden on the DOTS 1125 client operator, and minimize the latency between attack detection 1126 and the start of mitigation. 1128 Mitigation requests are triggered in this scenario by operator- 1129 specified network conditions. Attack detection is deployment- 1130 specific, and not constrained by this architecture. Similarly the 1131 specifics of a condition are left to the discretion of the operator, 1132 though common conditions meriting mitigation include the following: 1134 o Detected attack exceeding a rate in packets per second (pps). 1136 o Detected attack exceeding a rate in bytes per second (bps). 1138 o Detected resource exhaustion in an attack target. 1140 o Detected resource exhaustion in the local domain's mitigator. 1142 o Number of open connections to an attack target. 1144 o Number of attack sources in a given attack. 1146 o Number of active attacks against targets in the operator's domain. 1148 o Conditional detection developed through arbitrary statistical 1149 analysis or deep learning techniques. 1151 o Any combination of the above. 1153 When automated conditional mitigation requests are enabled, 1154 violations of any of the above conditions, or any additional 1155 operator-defined conditions, will trigger a mitigation request from 1156 the DOTS client to the DOTS server. The interfaces between the 1157 application detecting the condition violation and the DOTS client are 1158 implementation-specific. 1160 3.3.3. Automated Mitigation on Loss of Signal 1162 To maintain a DOTS session, the DOTS client and the DOTS server 1163 exchange regular but infrequent messages across the signal channel. 1164 In the absence of an attack, the probability of message loss in the 1165 signaling channel should be extremely low. Under attack conditions, 1166 however, some signal loss may be anticipated as attack traffic 1167 congests the link, depending on the attack type. 1169 While [I-D.ietf-dots-requirements] specifies the DOTS protocol be 1170 robust when signaling under attack conditions, there are nevertheless 1171 scenarios in which the DOTS signal is lost in spite of protocol best 1172 efforts. To handle such scenarios, a DOTS operator may configure the 1173 DOTS session to trigger mitigation when the DOTS server ceases 1174 receiving DOTS client signals (or vice versa) beyond the miss count 1175 or period permitted by the protocol. 1177 The impact of mitigating due to loss of signal in either direction 1178 must be considered carefully before enabling it. Signal loss is not 1179 caused by links congested with attack traffic alone, and as such 1180 mitigation requests triggered by signal channel degradation in either 1181 direction may incur unnecessary costs, in network performance and 1182 operational expense alike. 1184 4. Security Considerations 1186 This section describes identified security considerations for the 1187 DOTS architecture. 1189 DOTS is at risk from three primary attack vectors: agent 1190 impersonation, traffic injection and signal blocking. These vectors 1191 may be exploited individually or in concert by an attacker to 1192 confuse, disable, take information from, or otherwise inhibit DOTS 1193 agents. 1195 Any attacker with the ability to impersonate a legitimate DOTS client 1196 or server or, indeed, inject false messages into the stream may 1197 potentially trigger/withdraw traffic redirection, trigger/cancel 1198 mitigation activities or subvert black/whitelists. From an 1199 architectural standpoint, operators SHOULD ensure best current 1200 practices for secure communication are observed for data and signal 1201 channel confidentiality, integrity and authenticity. Care must be 1202 taken to ensure transmission is protected by appropriately secure 1203 means, reducing attack surface by exposing only the minimal required 1204 services or interfaces. Similarly, received data at rest SHOULD be 1205 stored with a satisfactory degree of security. 1207 As many mitigation systems employ diversion to scrub attack traffic, 1208 operators of DOTS agents SHOULD ensure DOTS sessions are resistant to 1209 Man-in-the-Middle (MitM) attacks. An attacker with control of a DOTS 1210 client may negatively influence network traffic by requesting and 1211 withdrawing requests for mitigation for particular prefixes, leading 1212 to route or DNS flapping. 1214 Any attack targeting the availability of DOTS servers may disrupt the 1215 ability of the system to receive and process DOTS signals resulting 1216 in failure to fulfill a mitigation request. DOTS agents SHOULD be 1217 given adequate protections, again in accordance with best current 1218 practices for network and host security. 1220 5. Contributors 1222 Mohamed Boucadair 1223 Orange 1225 mohamed.boucadair@orange.com 1227 6. Acknowledgments 1229 Thanks to Matt Richardson and Med Boucadair for their comments and 1230 suggestions. 1232 7. References 1234 7.1. Normative References 1236 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1237 Requirement Levels", BCP 14, RFC 2119, 1238 DOI 10.17487/RFC2119, March 1997, 1239 . 1241 7.2. Informative References 1243 [I-D.ietf-dots-requirements] 1244 Mortensen, A., Moskowitz, R., and T. Reddy, "Distributed 1245 Denial of Service (DDoS) Open Threat Signaling 1246 Requirements", draft-ietf-dots-requirements-06 (work in 1247 progress), July 2017. 1249 [I-D.ietf-dots-use-cases] 1250 Dobbins, R., Migault, D., Fouant, S., Moskowitz, R., 1251 Teague, N., Xia, L., and K. Nishizuka, "Use cases for DDoS 1252 Open Threat Signaling", draft-ietf-dots-use-cases-07 (work 1253 in progress), July 2017. 1255 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 1256 DOI 10.17487/RFC0768, August 1980, 1257 . 1259 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 1260 RFC 793, DOI 10.17487/RFC0793, September 1981, 1261 . 1263 [RFC1035] Mockapetris, P., "Domain names - implementation and 1264 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1265 November 1987, . 1267 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 1268 specifying the location of services (DNS SRV)", RFC 2782, 1269 DOI 10.17487/RFC2782, February 2000, 1270 . 1272 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 1273 A., Peterson, J., Sparks, R., Handley, M., and E. 1274 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 1275 DOI 10.17487/RFC3261, June 2002, 1276 . 1278 [RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A 1279 Border Gateway Protocol 4 (BGP-4)", RFC 4271, 1280 DOI 10.17487/RFC4271, January 2006, 1281 . 1283 [RFC4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet 1284 Denial-of-Service Considerations", RFC 4732, 1285 DOI 10.17487/RFC4732, December 2006, 1286 . 1288 [RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast 1289 Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786, 1290 December 2006, . 1292 [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 1293 (TLS) Protocol Version 1.2", RFC 5246, 1294 DOI 10.17487/RFC5246, August 2008, 1295 . 1297 [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer 1298 Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, 1299 January 2012, . 1301 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service 1302 Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, 1303 . 1305 [RFC7092] Kaplan, H. and V. Pascual, "A Taxonomy of Session 1306 Initiation Protocol (SIP) Back-to-Back User Agents", 1307 RFC 7092, DOI 10.17487/RFC7092, December 2013, 1308 . 1310 [RFC7094] McPherson, D., Oran, D., Thaler, D., and E. Osterweil, 1311 "Architectural Considerations of IP Anycast", RFC 7094, 1312 DOI 10.17487/RFC7094, January 2014, 1313 . 1315 Authors' Addresses 1317 Andrew Mortensen 1318 Arbor Networks 1319 2727 S. State St 1320 Ann Arbor, MI 48104 1321 United States 1323 EMail: amortensen@arbor.net 1325 Flemming Andreasen 1326 Cisco 1327 United States 1329 EMail: fandreas@cisco.com 1331 Tirumaleswar Reddy 1332 McAfee, Inc. 1333 Embassy Golf Link Business Park 1334 Bangalore, Karnataka 560071 1335 India 1337 EMail: tireddy@cisco.com 1338 Christopher Gray 1339 Comcast 1340 United States 1342 EMail: Christopher_Gray3@cable.comcast.com 1344 Rich Compton 1345 Charter 1347 EMail: Rich.Compton@charter.com 1349 Nik Teague 1350 Verisign 1351 United States 1353 EMail: nteague@verisign.com