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Checking references for intended status: Informational ---------------------------------------------------------------------------- No issues found here. Summary: 0 errors (**), 0 flaws (~~), 3 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 TICTOC Working Group Tal Mizrahi 2 Internet Draft Marvell 3 Intended status: Informational 4 Expires: October 2013 April 25, 2013 6 Security Requirements of Time Protocols 7 in Packet Switched Networks 8 draft-ietf-tictoc-security-requirements-05.txt 10 Abstract 12 As time and frequency distribution protocols are becoming 13 increasingly common and widely deployed, concern about their exposure 14 to various security threats is increasing. This document defines a 15 set of security requirements for time protocols, focusing on the 16 Precision Time Protocol (PTP) and the Network Time Protocol (NTP). 17 This document also discusses the security impacts of time protocol 18 practices, the performance implications of external security 19 practices on time protocols and the dependencies between other 20 security services and time synchronization. 22 Status of this Memo 24 This Internet-Draft is submitted to IETF 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), its areas, and its working groups. Note that 29 other groups may also distribute working documents as Internet- 30 Drafts. 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 The list of current Internet-Drafts can be accessed at 38 http://www.ietf.org/ietf/1id-abstracts.txt. 40 The list of Internet-Draft Shadow Directories can be accessed at 41 http://www.ietf.org/shadow.html. 43 This Internet-Draft will expire on October 25, 2013. 45 Copyright Notice 47 Copyright (c) 2013 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 (http://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. Introduction ................................................. 3 63 2. Conventions Used in this Document ............................ 5 64 2.1. Terminology ............................................. 5 65 2.2. Abbreviations ........................................... 5 66 2.3. Common Terminology for PTP and NTP ...................... 5 67 2.4. Terms used in this Document ............................. 6 68 3. Security Threats ............................................. 7 69 3.1. Threat Model ............................................ 7 70 3.1.1. Internal vs. External Attackers .................... 7 71 3.1.2. Man in the Middle (MITM) vs. Packet Injector ....... 8 72 3.2. Threat Analysis.......................................... 8 73 3.2.1. Packet Interception and Manipulation ............... 8 74 3.2.2. Spoofing ........................................... 8 75 3.2.3. Replay Attack ...................................... 8 76 3.2.4. Rogue Master Attack ................................ 8 77 3.2.5. Packet Interception and Removal .................... 9 78 3.2.6. Packet Delay Manipulation .......................... 9 79 3.2.7. L2/L3 DoS Attacks .................................. 9 80 3.2.8. Cryptographic Performance Attacks .................. 9 81 3.2.9. DoS Attacks against the Time Protocol ............. 10 82 3.2.10. Grandmaster Time Source Attack (e.g., GPS fraud) . 10 83 3.3. Threat Analysis Summary ................................ 10 84 4. Requirement Levels .......................................... 12 85 5. Security Requirements ....................................... 12 86 5.1. Clock Identity Authentication and Authorization ........ 13 87 5.1.1. Authentication and Authorization of Masters ....... 14 88 5.1.2. Recursive Authentication and Authorization of Masters 89 (Chain of Trust) ......................................... 14 90 5.1.3. Authentication and Authorization of Slaves ........ 15 91 5.1.4. PTP: Authentication and Authorization of PTP TCs by the 92 Master ................................................... 16 93 5.1.5. PTP: Authentication and Authorization of Control 94 Messages ................................................. 17 95 5.2. Protocol Packet Integrity .............................. 18 96 5.2.1. PTP: Hop-by-hop vs. End-to-end Integrity Protection 18 97 5.2.1.1. Hop-by-Hop Integrity Protection .............. 19 98 5.2.1.2. End-to-End Integrity Protection .............. 19 99 5.3. Availability ........................................... 20 100 5.4. Replay Protection ...................................... 20 101 5.5. Cryptographic Keys and Security Associations ........... 21 102 5.5.1. Key Freshness ..................................... 21 103 5.5.2. Security Association .............................. 21 104 5.5.3. Unicast and Multicast Associations ................ 22 105 5.6. Performance ............................................ 22 106 5.7. Confidentiality......................................... 23 107 5.8. Protection against Packet Delay and Interception Attacks 24 108 5.9. Combining Secured with Unsecured Nodes ................. 25 109 5.9.1. Secure Mode ....................................... 25 110 5.9.2. Hybrid Mode ....................................... 25 111 6. Summary of Requirements ..................................... 26 112 7. Additional security implications ............................ 28 113 7.1. Security and on-the-fly Timestamping ................... 28 114 7.2. PTP: Security and Two-Step Timestamping ................ 28 115 7.3. Intermediate Clocks .................................... 29 116 7.4. External Security Protocols and Time Protocols.......... 30 117 7.5. External Security Services Requiring Time .............. 30 118 7.5.1. Timestamped Certificates .......................... 30 119 7.5.2. Time Changes and Replay Attacks ................... 31 120 8. Issues for Further Discussion ............................... 31 121 9. Security Considerations ..................................... 31 122 10. IANA Considerations......................................... 31 123 11. Acknowledgments ............................................ 31 124 12. References ................................................. 31 125 12.1. Normative References .................................. 31 126 12.2. Informative References ................................ 32 127 13. Contributing Authors ....................................... 33 129 1. Introduction 131 As time protocols are becoming increasingly common and widely 132 deployed, concern about the resulting exposure to various security 133 threats is increasing. If a time protocol is compromised, the 134 applications it serves are prone to a range of possible attacks 135 including Denial-of-Service (DoS) or incorrect behavior. 137 This document focuses on the security aspects of the Precision Time 138 Protocol (PTP) [IEEE1588] and the Network Time Protocol [NTPv4]. The 139 Network Time Protocol was defined with an inherent security protocol, 140 defined in [NTPv4] and in [AutoKey]. [IEEE1588] includes an 141 experimental security protocol, defined in Annex K of the standard, 142 but this Annex was never formalized into a fully defined security 143 protocol. 145 While NTP includes an inherent security protocol, the absence of a 146 standard security solution for PTP undoubtedly contributed to the 147 wide deployment of unsecured time synchronization solutions. However, 148 in some cases security mechanisms may not be strictly necessary, 149 e.g., due to other security practices in place, or due to the 150 architecture of the network. A time synchronization security 151 solution, much like any security solution, is comprised of various 152 building blocks, and must be carefully tailored for the specific 153 system it is deployed in. Based on a system-specific threat 154 assessment, the benefits of a security solution must be weighed 155 against the potential risks, and based on this tradeoff an optimal 156 security solution can be selected. 158 The target audience of this document includes: 160 o Timing and networking equipment vendors - can benefit from this 161 document by deriving the security features that should be 162 supported in the time/networking equipment. 164 o Standard development organizations - can use the requirements 165 defined in this document when specifying security mechanisms for a 166 time protocol. 168 o Network operators - can use this document as a reference when 169 designing the network and its security architecture. As stated 170 above, the requirements in this document may be deployed 171 selectively based on a careful per-system threat analysis. 173 This document attempts to add clarity to the time protocol security 174 requirements discussion by addressing a series of questions: 176 (1) What are the threats that need to be addressed for the time 177 protocol, and thus what security services need to be provided? (e.g. 178 a malicious NTP server or PTP master) 180 (2) What external security practices impact the security and 181 performance of time keeping, and what can be done to mitigate these 182 impacts? (e.g. an IPsec tunnel in the time protocol traffic path) 183 (3) What are the security impacts of time protocol practices? (e.g. 184 on-the-fly modification of timestamps) 186 (4) What are the dependencies between other security services and 187 time protocols? (e.g. which comes first - the certificate or the 188 timestamp?) 190 In light of the questions above, this document defines a set of 191 requirements for security solutions for time protocols, focusing on 192 PTP and NTP. 194 2. Conventions Used in this Document 196 2.1. Terminology 198 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 199 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 200 document are to be interpreted as described in [KEYWORDS]. 202 This document describes security requirements, and thus requirements 203 are phrased in the document in the form "the security mechanism 204 MUST/SHOULD/...". Note, that the phrasing does not imply that this 205 document defines a specific security mechanism, but defines the 206 requirements with which every security mechanism should comply. 208 2.2. Abbreviations 210 BC Boundary Clock 212 DoS Denial of Service 214 MITM Man In The Middle 216 NTP Network Time Protocol 218 OC Ordinary Clock 220 PTP Precision Time Protocol 222 TC Transparent Clock 224 2.3. Common Terminology for PTP and NTP 226 This document refers to both PTP and NTP. For the sake of 227 consistency, throughout the document the term "master" applies to 228 both a PTP master and an NTP server. Similarly, the term "slave" 229 applies to both PTP slaves and NTP clients. The term "protocol 230 packets" refers generically to PTP and NTP messages. 232 2.4. Terms used in this Document 234 o Clock - A node participating in the protocol (either PTP or NTP). 235 A clock can be a master, a slave, or an intermediate clock (see 236 corresponding definitions below). 238 o Control packets - Packets used by the protocol to exchange 239 information between clocks that is not strictly related to the 240 time. NTP uses NTP Control Messages. PTP uses Announce, Signaling 241 and Management messages. 243 o End-to-end security - A security approach where secured packets 244 sent from a source to a destination is not modified by 245 intermediate nodes. 247 o Grandmaster - A master that receives time information from a 248 locally attached clock device, and not through the network. A 249 grandmaster distributes its time to other clocks in the network. 251 o Hop-by-hop security - A security approach where secured packets 252 sent from a source to a destination may be modified by 253 intermediate nodes. In this approach intermediate nodes share the 254 encryption key with the source and destination, allowing them to 255 re-encrypt or re-authenticate modified packets before relaying 256 them to the destination. 258 o Intermediate clock - A clock that receives timing information from 259 a master, and sends timing information to other clocks. 260 In NTP this term refers to an NTP server that is not a Stratum 1 261 server. In PTP this term refers to a BC or a TC. 263 o Master - A clock that generates timing information to other clocks 264 in the network. 265 In NTP 'master' refers to an NTP server. In PTP 'master' refers to 266 a master OC (aka grandmaster) or to a port of a BC that is in the 267 master state. 269 o Protocol packets - Packets used by the time protocol. The 270 terminology used in this document distinguishes between time 271 packets and control packets. 273 o Secured clock - A clock that supports a security mechanism that 274 complies to the requirements in this document. 276 o Slave - A clock that receives timing information from a master. In 277 NTP 'slave' refers to an NTP client. In PTP 'slave' refers to a 278 slave OC, or to a port of a BC that is in the slave state. 280 o Time packets - Protocol packets carrying time information. 282 o Unsecured clock - A clock that does not support a security 283 mechanism according to the requirements in this document. 285 3. Security Threats 287 This section discusses the possible attacker types and analyzes 288 various attacks against time protocols. 290 The literature is rich with security threats of time protocols, e.g., 291 [Traps], [AutoKey], [TM], [SecPTP], and [SecSen]. The threat analysis 292 in this document is mostly based on [TM]. 294 3.1. Threat Model 296 A time protocol can be attacked by various types of attackers. 298 The analysis in this document classifies attackers according to 2 299 criteria, as described in Section 3.1.1. and Section 3.1.2. 301 3.1.1. Internal vs. External Attackers 303 In the context of internal and external attackers, the underlying 304 assumption is that the time protocol is secured either by an 305 encryption or an authentication mechanism, or both. 307 Internal attackers either have access to a trusted segment of the 308 network, or possess the encryption or authentication keys. An 309 internal attack can also be performed by exploiting vulnerabilities 310 in devices; for example, by installing malware, or obtaining 311 credentials to reconfigure the device. Thus, an internal attacker can 312 maliciously tamper with legitimate traffic in the network, as well as 313 generate its own traffic and make it appear legitimate to its 314 attacked nodes. 316 External attackers, on the other hand, do not have the keys, and have 317 access only to the encrypted or authenticated traffic. 319 Obviously, in the absence of a security mechanism there is no 320 distinction between internal and external attackers, since all 321 attackers are internal in practice. 323 3.1.2. Man in the Middle (MITM) vs. Packet Injector 325 MITM attackers are located in a position that allows interception and 326 modification of in-flight protocol packets. It is assumed that an 327 MITM attacker has physical access to a segment of the network, or has 328 gained control of one of the nodes in the network. 330 A traffic injector is not located in an MITM position, but can attack 331 by generating protocol packets. An injector can reside either within 332 the attacked network, or on an external network that is connected to 333 the attacked network. An injector can also potentially eavesdrop on 334 protocol packets sent as multicast, record them and replay them 335 later. 337 3.2. Threat Analysis 339 3.2.1. Packet Interception and Manipulation 341 A packet interception and manipulation attack results when an MITM 342 attacker intercepts timing protocol packets, alters them and relays 343 them to their destination, allowing the attacker to maliciously 344 tamper with the protocol. This can result in a situation where the 345 time protocol is apparently operational but providing intentionally 346 inaccurate information. 348 3.2.2. Spoofing 350 In spoofing, an injector masquerades as a legitimate node in the 351 network by generating and transmitting protocol packets or control 352 packets. For example, an attacker can impersonate the master, 353 allowing malicious distribution of false timing information. As with 354 packet interception and manipulation, this can result in a situation 355 where the time protocol is apparently operational but providing 356 intentionally inaccurate information. 358 3.2.3. Replay Attack 360 In a replay attack, an attacker records protocol packets and replays 361 them at a later time without any modification. This can also result 362 in a situation where the time protocol is apparently operational but 363 providing intentionally inaccurate information. 365 3.2.4. Rogue Master Attack 367 In a rogue master attack, an attacker causes other nodes in the 368 network to believe it is a legitimate master. As opposed to the 369 spoofing attack, in the Rogue Master attack the attacker does not 370 fake its identity, but rather manipulates the master election process 371 using malicious control packets. For example, in PTP, an attacker can 372 manipulate the Best Master Clock Algorithm (BMCA), and cause other 373 nodes in the network to believe it is the most eligible candidate to 374 be a grandmaster. 376 In PTP, a possible variant of this attack is the rogue TC/BC attack. 377 Similar to the rogue master attack, an attacker can cause victims to 378 believe it is a legitimate TC or BC, allowing the attacker to 379 manipulate the time information forwarded to the victims. 381 3.2.5. Packet Interception and Removal 383 A packet interception and removal attack results when an MITM 384 attacker intercepts and drops protocol packets, preventing the 385 destination node from receiving some or all of the protocol packets. 387 3.2.6. Packet Delay Manipulation 389 In a packet delay manipulation scenario, an MITM attacker intercepts 390 protocol packets, and relays them to their destination after adding a 391 maliciously computed delay. The attacker can use various delay attack 392 strategies; the added delay can be constant, jittered, or slowly 393 wandering. Each of these strategies has a different impact, but they 394 all effectively manipulate the attacked clock. 396 Note that the victim still receives one copy of each packet, contrary 397 to the replay attack, where some or all of the packets may be 398 received by the victim more than once. 400 3.2.7. L2/L3 DoS Attacks 402 There are many possible Layer 2 and Layer 3 DoS attacks. As the 403 target's availability is compromised, the timing protocol is affected 404 accordingly. 406 3.2.8. Cryptographic Performance Attacks 408 In cryptographic performance attacks, an attacker transmits fake 409 protocol packets, causing high utilization of the cryptographic 410 engine at the receiver, which attempts to verify the integrity of 411 these fake packets. 413 This DoS attack is applicable to all encryption and authentication 414 protocols. However, when the time protocol uses a dedicated security 415 mechanism implemented in a dedicated cryptographic engine, this 416 attack can be applied to cause DoS specifically to the time protocol 418 3.2.9. DoS Attacks against the Time Protocol 420 An attacker can attack a clock by sending an excessive number of time 421 protocol packets, thus degrading the victim's performance. This 422 attack can be implemented, for example, using the attacks described 423 in Section 3.2.2. and Section 3.2.4. 425 3.2.10. Grandmaster Time Source Attack (e.g., GPS fraud) 427 Grandmasters receive their time from an external accurate time 428 source, such as an atomic clock or a GPS clock, and then distribute 429 this time to the slaves using the time protocol. 431 Time source attack are aimed at the accurate time source of the 432 grandmaster. For example, if the grandmaster uses a GPS based clock 433 as its reference source, an attacker can jam the reception of the GPS 434 signal, or transmit a signal similar to one from a GPS satellite, 435 causing the grandmaster to use a false reference time. 437 Note that this attack is outside the scope of the time protocol. 438 While various security measures can be taken to mitigate this attack, 439 these measures are outside the scope of the security requirements 440 defined in this document. 442 3.3. Threat Analysis Summary 444 The two key factors to a threat analysis are the impact and the 445 likelihood of each of the analyzed attacks. 447 Table 1 summarizes the security attacks presented in Section 3.2. 448 For each attack, the table specifies its impact, and its 449 applicability to each of the attacker types presented in Section 3.1. 451 Table 1 clearly shows the distinction between external and internal 452 attackers, and motivates the usage of authentication and integrity 453 protection, significantly reducing the impact of external attackers. 455 The Impact column provides an intuitive measure of the severity of 456 each attack, and the relevant Attacker Type columns provide an 457 intuition about how difficult each attack is to implement, and hence 458 about the likelihood of each attack. 460 The impact column in Table 1 can have one of 3 values: 462 o DoS - the attack causes denial of service to the attacked node, 463 the impact of which is not restricted to the time protocol. 465 o Accuracy degradation - the attack yields a degradation in the 466 slave accuracy, but does not completely compromise the slaves' 467 time and frequency. 469 o False time - slaves align to a false time or frequency value due 470 to the attack. Note that if the time protocol aligns to a false 471 time, it may cause DoS to other applications that rely on accurate 472 time. However, for the purpose of the analysis in this section we 473 distinguish this implication from 'DoS', which refers to a DoS 474 attack that is not necessarily aimed at the time protocol. 475 All attacks that have a '+' for 'False Time' implicitly have a '+' 476 for 'Accuracy Degradation'. 478 The Attacker Type columns refer to the 4 possible combinations of the 479 attacker types defined in Section 3.1. 481 +-----------------------------+-------------------++-------------------+ 482 | Attack | Impact || Attacker Type | 483 | +-----+--------+----++---------+---------+ 484 | |False|Accuracy| ||Internal |External | 485 | |Time |Degrad. |DoS ||MITM|Inj.|MITM|Inj.| 486 +-----------------------------+-----+--------+----++----+----+----+----+ 487 |Interception and manipulation| + | | || + | | | | 488 +-----------------------------+-----+--------+----++----+----+----+----+ 489 |Spoofing | + | | || + | + | | | 490 +-----------------------------+-----+--------+----++----+----+----+----+ 491 |Replay attack | + | | || + | + | | | 492 +-----------------------------+-----+--------+----++----+----+----+----+ 493 |Rogue master attack | + | | || + | + | | | 494 +-----------------------------+-----+--------+----++----+----+----+----+ 495 |Interception and removal | | + | + || + | | + | | 496 +-----------------------------+-----+--------+----++----+----+----+----+ 497 |Packet delay manipulation | + | | || + | | + | | 498 +-----------------------------+-----+--------+----++----+----+----+----+ 499 |L2/L3 DoS attacks | | | + || + | + | + | + | 500 +-----------------------------+-----+--------+----++----+----+----+----+ 501 |Crypt. performance attacks | | | + || + | + | + | + | 502 +-----------------------------+-----+--------+----++----+----+----+----+ 503 |Time protocol DoS attacks | | | + || + | + | | | 504 +-----------------------------+-----+--------+----++----+----+----+----+ 505 |Master time source attack | + | | || + | + | + | + | 506 |(e.g., GPS spoofing) | | | || | | | | 507 +-----------------------------+-----+--------+----++----+----+----+----+ 508 Table 1 Threat Analysis - Summary 510 The threats discussed in this section provide the background for the 511 security requirements presented in Section 5. 513 4. Requirement Levels 515 The security requirements are presented in Section 5. Each 516 requirement is defined with a requirement level, in accordance with 517 the requirement levels defined in Section 2.1. 519 The requirement levels in this document are affected by the following 520 factors: 522 o Impact: 523 The possible impact of not implementing the requirement, as 524 illustrated in the 'impact' column of Table 1. 525 For example, a requirement that addresses a threat that can be 526 implemented by an external injector is typically a 'MUST', since 527 the threat can be implemented by all the attacker types analyzed 528 in Section 3.1. 530 o Difficulty of the corresponding attack: 531 The level of difficulty of the possible attacks that become 532 possible by not implementing the requirement. The level of 533 difficulty is reflected in the 'Attacker Type' column of Table 1. 534 For example, a requirement that addresses a threat that only 535 compromises the availability of the protocol is typically no more 536 than a 'SHOULD'. 538 o Practical considerations: 539 Various practical factors that may affect the requirement. 540 For example, if a requirement is very difficult to implement, or 541 is applicable to very specific scenarios, these factors may reduce 542 the requirement level. 544 Section 5. lists the requirements. For each requirement there is a 545 short explanation detailing the reason for its requirement level. 547 5. Security Requirements 549 This section defines a set of security requirements. These 550 requirements are phrased in the form "the security mechanism 551 MUST/SHOULD/MAY...". However, this document does not specify how 552 these requirements can be met. While these requirements can be 553 satisfied by defining explicit security mechanisms for time 554 protocols, at least a subset of the requirements can be met by 555 applying common security practices to the network or by using 556 existing security protocols, such as [IPsec] or [MACsec]. Thus, 557 security solutions that address these requirements are outside the 558 scope of this document. 560 5.1. Clock Identity Authentication and Authorization 562 Requirement 564 The security mechanism MUST support authentication. 566 Requirement 568 The security mechanism MUST support authorization. 570 Requirement Level 572 The requirements in this subsection address the spoofing attack 573 (Section 3.2.2.), and the rogue master attack (Section 3.2.4.). 575 The requirement level of these requirements is 'MUST' since in the 576 absence of these requirements the protocol is exposed to attacks that 577 are easy to implement and have a high impact. 579 Discussion 581 Authentication refers to verifying the identity of the peer clock. 582 Authorization, on the other hand, refers to verifying that the peer 583 clock is permitted to play the role that it plays in the protocol. 584 For example, some nodes may be permitted to be masters, while other 585 nodes are only permitted to be slaves or TCs. 587 Authorization requires clocks to maintain a list of authorized 588 clocks, or a "black list" of clocks that should be denied service or 589 revoked. 591 It is noted that while the security mechanism is required to provide 592 an authorization mechanism, the deployment of such a mechanism 593 depends on the nature of the network. For example, a network that 594 deploys PTP may consist of a set of identical OCs, where all clocks 595 are equally permitted to be a master. In such a network an 596 authorization mechanism may not be necessary. 598 The following subsections describe 4 distinct cases of clock 599 authentication. 601 5.1.1. Authentication and Authorization of Masters 603 Requirement 605 The security mechanism MUST support an authentication mechanism, 606 allowing slaves to authenticate the identity of masters. 608 Requirement 610 The authentication mechanism MUST allow slaves to verify that the 611 authenticated master is authorized to be a master. 613 Requirement Level 615 The requirements in this subsection address the spoofing attack 616 (Section 3.2.2.), and the rogue master attack (Section 3.2.4.). 618 The requirement level of these requirements is 'MUST' since in the 619 absence of these requirements the protocol is exposed to attacks that 620 are easy to implement and have a high impact. 622 Discussion 624 Clocks authenticate masters in order to ensure the authenticity of 625 the time source. It is important for a slave to verify the identity 626 of the master, as well as to verify that the master is indeed 627 authorized to be a master. 629 5.1.2. Recursive Authentication and Authorization of Masters (Chain of 630 Trust) 632 Requirement 634 The security mechanism MUST support recursive authentication and 635 authorization of the master, to be used in cases where time 636 information is conveyed through intermediate clocks. 638 Requirement Level 640 The requirement in this subsection addresses the spoofing attack 641 (Section 3.2.2.), and the rogue master attack (Section 3.2.4.). 643 The requirement level of this requirement is 'MUST' since in the 644 absence of this requirement the protocol is exposed to attacks that 645 are easy to implement and have a high impact. 647 Discussion 648 In some cases a slave is connected to an intermediate clock, that is 649 not the primary time source. For example, in PTP a slave can be 650 connected to a Boundary Clock (BC) or a Transparent Clock (TC), which 651 in turn is connected to a grandmaster. A similar example in NTP is 652 when a client is connected to a stratum 2 server, which is connected 653 to a stratum 1 server. In both the PTP and the NTP cases, the slave 654 authenticates the intermediate clock, and the intermediate clock 655 authenticates the grandmaster. This recursive authentication process 656 is referred to in [AutoKey] as proventication. 658 Specifically in PTP, this requirement implies that if a slave is 659 receives time information through a TC, it must authenticate the TC 660 it is attached to, as well as authenticate the master it receives the 661 time information from, as per Section 5.1.1. Similarly, if a TC 662 receives time information through an attached TC, it must 663 authenticate the attached TC. 665 5.1.3. Authentication and Authorization of Slaves 667 Requirement 669 The security mechanism MAY provide a means for a master to 670 authenticate its slaves. 672 Requirement 674 The security mechanism MAY provide a means for a master to verify 675 that the sender of a protocol packet is authorized to send a packet 676 of this type. 678 Requirement Level 680 The requirement in this subsection prevents DoS attacks against the 681 master (Section 3.2.9.). 683 The requirement level of this requirement is 'MAY' since: 685 o Its low impact, i.e., in the absence of this requirement the 686 protocol is only exposed to DoS. 688 o Practical considerations: requiring an NTP server to authenticate 689 its clients may significantly impose on the server's performance. 691 Note that while the requirement level of this requirement is 'MAY', 692 the requirement in Section 5.1.1. is 'MUST'; the security mechanism 693 must provide a means for authentication and authorization, with an 694 emphasis on the master. Authentication and authorization of slaves is 695 specified in this subsection as 'MAY'. 697 Discussion 699 Slaves and intermediate clocks are authenticated by masters in order 700 to verify that they are authorized to receive timing services from 701 the master. 703 Authentication of slaves prevents unauthorized clocks from receiving 704 time services. Preventing the master from serving unauthorized clocks 705 can help in mitigating DoS attacks against the master. Note that the 706 authentication of slaves might put a higher load on the master than 707 serving the unauthorized clock, and hence this requirement is a 708 SHOULD. 710 5.1.4. PTP: Authentication and Authorization of PTP TCs by the Master 712 Requirement 714 The security mechanism for PTP MAY provide a means for a master to 715 authenticate the identity of the P2P TCs directly connected to it. 717 Requirement 719 The security mechanism for PTP MAY provide a means for a master to 720 verify that P2P TCs directly connected to it are authorized to be 721 TCs. 723 Requirement Level 725 The requirement in this subsection prevents DoS attacks against the 726 master (Section 3.2.9.). 728 The requirement level of this requirement is 'MAY' for the same 729 reasons specified in Section 5.1.3. 731 Discussion 733 P2P TCs that are one hop from the master use the PDelay_Req and 734 PDelay_Resp handshake to compute the link delay between the master 735 and TC. These TCs are authenticated by the master. 737 Authentication of TCs, much like authentication of slaves, reduces 738 unnecessary load on the master and peer TCs, by preventing the master 739 from serving unauthorized clocks. 741 5.1.5. PTP: Authentication and Authorization of Control Messages 743 Requirement 745 The security mechanism for PTP MUST support authentication of 746 Announce messages. The authentication mechanism MUST also verify that 747 the sender is authorized to be a master. 749 Requirement 751 The security mechanism for PTP MUST support authentication and 752 authorization of Management messages. 754 Requirement 756 The security mechanism MAY support authentication and authorization 757 of Signaling messages. 759 Requirement Level 761 The requirements in this subsection address the spoofing attack 762 (Section 3.2.2.), and the rogue master attack (Section 3.2.4.). 764 The requirement level of the first two requirements is 'MUST' since 765 in the absence of these requirements the protocol is exposed to 766 attacks that are easy to implement and have a high impact. 768 The requirement level of the third requirement is 'MAY' since its 769 impact greatly depends on the application for which the Signaling 770 messages are used for. 772 Discussion 774 Master election is performed in PTP using the Best Master Clock 775 Algorithm (BMCA). Each Ordinary Clock (OC) announces its clock 776 attributes using Announce messages, and the best master is elected 777 based on the information gathered from all the candidates. Announce 778 messages must be authenticated in order to prevent rogue master 779 attacks (Section 3.2.4.). Note, that this subsection specifies a 780 requirement that is not necessarily included in Section 5.1.1. or in 781 Section 5.1.3. , since the BMCA is initiated before clocks have been 782 defined as masters or slaves. 784 Management messages are used to monitor or configure PTP clocks. 785 Malicious usage of Management messages enables various attacks, such 786 as the rogue master attack, or DoS attack. 788 Signaling messages are used by PTP clocks to exchange information 789 that is not strictly related to time information or to master 790 selection, such as unicast negotiation. Authentication and 791 authorization of Signaling message may be required in some systems, 792 depending on the application these messages are used for. 794 5.2. Protocol Packet Integrity 796 Requirement 798 The security mechanism MUST protect the integrity of protocol 799 packets. 801 Requirement Level 803 The requirement in this subsection addresses the packet interception 804 and manipulation attack (Section 3.2.1.). 806 The requirement level of this requirement is 'MUST' since in the 807 absence of this requirement the protocol is exposed to attacks that 808 are easy to implement and have high impact. 810 Discussion 812 While Section 5.1. refers to ensuring the identity an authorization 813 of the source of a protocol packet, this subsection refers to 814 ensuring that the packet arrived intact. The integrity protection 815 mechanism ensures the authenticity and completeness of data from the 816 data originator. 818 5.2.1. PTP: Hop-by-hop vs. End-to-end Integrity Protection 820 Requirement 822 A security mechanism for PTP MUST support hop-by-hop integrity 823 protection. 825 Requirement 827 A security mechanism for PTP SHOULD support end-to-end integrity 828 protection. 830 Requirement Level 832 The requirement in this subsection addresses the packet interception 833 and manipulation attack (Section 3.2.1.). 835 The requirement level of the first requirement is 'MUST' since in the 836 absence of this requirement the protocol is exposed to attacks that 837 are easy to implement and have a high impact. 839 The requirement level of the first requirement is 'SHOULD' since in 840 the presence of recursive authentication (Section 5.1.2.) this 841 requirement may be redundant. 843 Discussion 845 Specifically in PTP, when protocol packets are subject to 846 modification by TCs, the integrity protection can be enforced in one 847 of two approaches, end-to-end or hop-by-hop. 849 5.2.1.1. Hop-by-Hop Integrity Protection 851 Each hop that needs to modify a protocol packet: 853 o Verifies its integrity. 855 o Modifies the packet, i.e., modifies the correctionField. 856 Note: Transparent Clocks (TCs) improve the end-to-end accuracy by 857 updating a "correctionField" (clause 6.5 in [IEEE1588]) in the PTP 858 packet by adding the latency caused by the current TC. 860 o Re-generates the integrity protection, e.g., re-computes a Message 861 Authentication Code. 863 In the hop-by-hop approach, the integrity of protocol packets is 864 protected by induction on the path from the originator to the 865 receiver. 867 This approach is simple, but allows rogue TCs to modify protocol 868 packets. 870 5.2.1.2. End-to-End Integrity Protection 872 In this approach, the integrity protection is maintained on the path 873 from the originator of a protocol packet to the receiver. This allows 874 the receiver to directly validate the protocol packet without the 875 ability of intermediate TCs to manipulate the packet. 877 Since TCs need to modify the correctionField, a separate integrity 878 protection mechanism is used specifically for the correctionField. 880 The end-to-end approach limits the TC's impact to the correctionField 881 alone, while the rest of the protocol packet is protected on an end- 882 to-end basis. It should be noted that this approach is more difficult 883 to implement than the hop-by-hop approach, as it requires the 884 correctionField to be protected separately from the other fields of 885 the packet, possibly using different cryptographic mechanisms and 886 keys. 888 5.3. Availability 890 Requirement 892 The security mechanism SHOULD include measures to mitigate DoS 893 attacks against the time protocol. 895 Requirement Level 897 The requirement in this subsection prevents DoS attacks against the 898 protocol (Section 3.2.9.). 900 The requirement level of this requirement is 'SHOULD' due to its low 901 impact, i.e., in the absence of this requirement the protocol is only 902 exposed to DoS. 904 Discussion 906 The protocol availability can be compromised by several different 907 attacks. 909 An attacker can inject protocol messages to implement the spoofing 910 attack (Section 3.2.2.) or the rogue master attack (Section 3.2.4. 911 ), causing DoS to the victim (Section 3.2.9.). An authentication 912 mechanism (Section 5.1.) limits these attacks strictly to internal 913 attackers, and thus prevents external attackers from performing them. 915 The DoS attacks described in Section 3.2.7. are performed at lower 916 layers than the time protocol layer, and are thus outside the scope 917 of the security requirements defined in this document. 919 5.4. Replay Protection 921 Requirement 923 The security mechanism MUST include a replay prevention mechanism. 925 Requirement Level 927 The requirement in this subsection prevents replay attacks (Section 928 3.2.3.). 930 The requirement level of this requirement is 'MUST' since in the 931 absence of this requirement the protocol is exposed to attacks that 932 are easy to implement and have a high impact. 934 Discussion 936 The replay attack (Section 3.2.3.) can compromise both the integrity 937 and availability of the protocol. Common encryption and 938 authentication mechanisms include replay prevention mechanisms that 939 typically use a monotonously increasing packet sequence number. 941 5.5. Cryptographic Keys and Security Associations 943 5.5.1. Key Freshness 945 Requirement 947 The cryptographic keys MUST be refreshed frequently. 949 Requirement Level 951 The requirement level of this requirement is 'MUST' since key 952 freshness is an essential property for cryptographic algorithms, as 953 discussed below. 955 Discussion 957 Key freshness guarantees that both sides share a common updated 958 secret key. It also helps in preventing replay and playback attacks. 959 Thus, it is important for keys to be refreshed frequently. 961 5.5.2. Security Association 963 Requirement 965 The security protocol SHOULD support an association protocol where: 967 o Two or more clocks authenticate each other. 969 o The clocks generate and agree on a cryptographic session key. 971 Requirement 973 Each instance of the association protocol SHOULD produce a different 974 session key. 976 Requirement Level 977 The requirement level of this requirement is 'SHOULD' since it may be 978 expensive in terms of performance, especially in low-cost clocks. 980 Discussion 982 The security requirements in Section 5.1. and Section 5.2. require 983 usage of cryptographic mechanisms, deploying cryptographic keys. A 984 security association is an important building block in these 985 mechanisms. 987 5.5.3. Unicast and Multicast Associations 989 Requirement 991 The security mechanism SHOULD support security association protocols 992 for unicast and for multicast associations. 994 Requirement Level 996 The requirement level of this requirement is 'SHOULD' since it may be 997 expensive in terms of performance, especially for low-cost clocks. 999 Discussion 1001 A unicast protocol requires an association protocol between two 1002 clocks, whereas a multicast protocol requires an association protocol 1003 among two or more clocks, where one of the clocks is a master. 1005 5.6. Performance 1007 Requirement 1009 The security mechanism MUST be designed in such a way that it does 1010 not significantly degrade the quality of the time transfer. 1012 Requirement 1014 The mechanism SHOULD minimize computational load. 1016 Requirement 1018 The mechanism SHOULD minimize storage requirements of client state in 1019 the master. 1021 Requirement 1022 The mechanism SHOULD minimize the bandwidth overhead required by the 1023 security protocol. 1025 Requirement Level 1027 While the quality of the time transfer is clearly a 'MUST', the other 1028 3 performance requirements are 'SHOULD', since some systems may be 1029 more sensitive to resource consumption than others, and hence these 1030 requirements should be considered on a per-system basis. 1032 Discussion 1034 Performance efficiency is important since client restrictions often 1035 dictate a low processing and memory footprint, and because the server 1036 may have extensive fan-out. 1038 Note that the performance requirements refer to a time-protocol- 1039 specific security mechanism. In systems where a security protocol is 1040 used for other types of traffic as well, this document does not place 1041 any performance requirements on the security protocol performance. 1042 For example, if IPsec encryption is used for securing all information 1043 between the master and slave node, including information that is not 1044 part of the time protocol, the requirements in this subsection are 1045 not necessarily applicable. 1047 5.7. Confidentiality 1049 Requirement 1051 The security mechanism MAY provide confidentiality protection of the 1052 protocol packets. 1054 Requirement Level 1056 The requirement level of this requirement is 'MAY' since it does not 1057 prevent severe threats, as discussed below. 1059 Discussion 1061 In the context of time protocols, confidentiality is typically of low 1062 importance, since timing information is typically not considered 1063 secret information. 1065 Confidentiality can play an important role when service providers 1066 charge their customers for time synchronization services, and thus an 1067 encryption mechanism can prevent eavesdroppers from obtaining the 1068 service without payment. Note that these cases are, for now, rather 1069 esoteric. 1071 Confidentiality can also prevent an MITM attacker from identifying 1072 protocol packets. Thus, confidentiality can assist in protecting the 1073 timing protocol against MITM attacks such as packet delay (Section 1074 3.2.6.), manipulation and interception and removal attacks. Note, 1075 that time protocols have predictable behavior even after encryption, 1076 such as packet transmission rates and packet lengths. Additional 1077 measure can be taken to mitigate encrypted traffic analysis by random 1078 padding of encrypted packets and by adding random dummy packets. 1079 Nevertheless, encryption does not prevent such MITM attacks, but 1080 rather makes these attacks more difficult to implement. 1082 5.8. Protection against Packet Delay and Interception Attacks 1084 Requirement 1086 The security mechanism SHOULD include means to protect the protocol 1087 from MITM attacks that degrade the clock accuracy. 1089 Requirement Level 1091 The requirements in this subsection address MITM attacks such as the 1092 3.2.1.). 1094 The requirement level of this requirement is 'SHOULD'. In the absence 1095 of this requirement the protocol is exposed to attacks that are easy 1096 to implement and have a high impact. On the other hand, the 1097 implementation of this requirement depends on the topology and 1098 properties of the system, and is thus not necessarily applicable to 1099 all deployments. 1101 Discussion 1103 While this document does not define specific security solutions, we 1104 note that common practices for protection against MITM attacks use 1105 redundant masters (e.g. [NTPv4]), or redundant paths between the 1106 master and slave (e.g. [DelayAtt]). If one of the time sources 1107 indicates a time value that is significantly different than the other 1108 sources, it is assumed to be erroneous or under attack, and is 1109 therefore ignored. 1111 Thus, MITM attack prevention derives a requirement from the security 1112 mechanism, and a requirement from the network topology. While the 1113 security mechanism should support the ability to detect delay 1114 attacks, it is noted that in some networks it is not necessarily 1115 possible to provide the redundancy needed for such a detection 1116 mechanism. 1118 5.9. Combining Secured with Unsecured Nodes 1120 Integrating a security mechanism into a time synchronized system is a 1121 complex and expensive process, and hence in some cases may require 1122 incremental deployment, where new equipment supports the security 1123 mechanism, and is required to interoperate with legacy equipment 1124 without the security features. 1126 5.9.1. Secure Mode 1128 Requirement 1130 The security mechanism MUST support a secure mode, where only secured 1131 clocks are permitted to take part in the time protocol. In this mode 1132 every protocol packet received from an unsecured clock MUST be 1133 discarded. 1135 Requirement Level 1137 The requirement level of this requirement is 'MUST' since the full 1138 capacity of the security requirements defined in this document can 1139 only be achieved in secure mode. 1141 Discussion 1143 While the requirement in this subsection is similar to the one in 1144 5.1. , it refers to the secure mode, as opposed to the hybrid mode 1145 presented in the next subsection. 1147 5.9.2. Hybrid Mode 1149 Requirement 1151 The security protocol MAY support a hybrid mode, where both secured 1152 and unsecured clocks are permitted to take part in the protocol. 1154 Requirement Level 1156 The requirement level of this requirement is a 'MAY', since it is not 1157 necessarily required in all systems. This document recommends to 1158 deploy the 'Secure Mode' described in Section 5.9.1. where possible. 1160 Discussion 1161 The hybrid mode allows both secured and unsecured clocks to take part 1162 in the time protocol. NTP, for example, allows a mixture of secured 1163 and unsecured nodes. 1165 Requirement 1167 A master in the hybrid mode SHOULD be a secured clock. 1169 A secured slave in the hybrid mode SHOULD discard all protocol 1170 packets received from unsecured clocks. 1172 Requirement Level 1174 The requirement level of this requirement is a 'SHOULD', since it may 1175 not be applicable to all deployments. For example, a hybrid network 1176 may require the usage of unsecured masters or TCs. 1178 Discussion 1180 This requirement ensures that the existence of unsecured clocks does 1181 not compromise the security provided to secured clocks. Hence, 1182 secured slaves only "trust" protocol packets received from a secured 1183 clock. 1185 An unsecured slave can receive protocol packets either from unsecured 1186 clocks, or from secured clocks. Note that the latter does not apply 1187 when encryption is used. When integrity protection is used, the 1188 unsecured slave can receive secured packets ignoring the integrity 1189 protection. 1191 Note that the security scheme in [NTPv4] with [AutoKey] does not 1192 satisfy this requirement, since nodes prefer the server with the most 1193 accurate clock, which is not necessarily the server that supports 1194 authentication. For example, a stratum 2 server is connected to two 1195 stratum 1 servers, Server A, supporting authentication, and server B, 1196 without authentication. If server B has a more accurate clock than A, 1197 the stratum 2 server chooses server B, in spite of the fact it does 1198 not support authentication. 1200 6. Summary of Requirements 1202 +-----------+---------------------------------------------+--------+ 1203 | Section | Requirement | Type | 1204 +-----------+---------------------------------------------+--------+ 1205 | 5.1. | Authentication & authorization of sender. | MUST | 1206 | +---------------------------------------------+--------+ 1207 | | Authentication & authorization of master. | MUST | 1208 | +---------------------------------------------+--------+ 1209 | | Recursive authentication & authorization. | MUST | 1210 | +---------------------------------------------+--------+ 1211 | | Authentication of slaves. | MAY | 1212 | +---------------------------------------------+--------+ 1213 | | PTP: Authentication of TCs by master. | MAY | 1214 | +---------------------------------------------+--------+ 1215 | | PTP: Authentication & authorization of | MUST | 1216 | | Announce messages. | | 1217 | +---------------------------------------------+--------+ 1218 | | PTP: Authentication & authorization of | MUST | 1219 | | Management messages. | | 1220 | +---------------------------------------------+--------+ 1221 | | PTP: Authentication & authorization of | MAY | 1222 | | Signaling messages. | | 1223 +-----------+---------------------------------------------+--------+ 1224 | 5.2. | Integrity protection. | MUST | 1225 | +---------------------------------------------+--------+ 1226 | | PTP: hop-by-hop integrity protection. | MUST | 1227 | +---------------------------------------------+--------+ 1228 | | PTP: end-to-end integrity protection. | SHOULD | 1229 +-----------+---------------------------------------------+--------+ 1230 | 5.3. | Protection against DoS attacks. | SHOULD | 1231 +-----------+---------------------------------------------+--------+ 1232 | 5.4. | Replay protection. | MUST | 1233 +-----------+---------------------------------------------+--------+ 1234 | 5.5. | Key freshness. | MUST | 1235 | +---------------------------------------------+--------+ 1236 | | Security association. | SHOULD | 1237 | +---------------------------------------------+--------+ 1238 | | Unicast and multicast associations. | SHOULD | 1239 +-----------+---------------------------------------------+--------+ 1240 | 5.6. | Performance: no degradation in quality of | MUST | 1241 | | time transfer. | | 1242 | +---------------------------------------------+--------+ 1243 | | Performance: computation load. | SHOULD | 1244 | +---------------------------------------------+--------+ 1245 | | Performance: storage. | SHOULD | 1246 | +---------------------------------------------+--------+ 1247 | | Performance: bandwidth. | SHOULD | 1248 +-----------+---------------------------------------------+--------+ 1249 | 5.7. | Confidentiality protection. | MAY | 1250 +-----------+---------------------------------------------+--------+ 1251 | 5.8. | Protection against delay and interception | SHOULD | 1252 | | attacks. | | 1253 +-----------+---------------------------------------------+--------+ 1254 | 5.9. | Secure mode. | MUST | 1255 | +---------------------------------------------+--------+ 1256 | | Hybrid mode. | MAY | 1257 +-----------+---------------------------------------------+--------+ 1258 Table 2 Summary of Security Requirements 1260 7. Additional security implications 1262 This section discusses additional implications of the interaction 1263 between time protocols and security mechanisms. 1265 This section refers to time protocol security mechanisms, as well as 1266 to "external" security mechanisms, i.e., security mechanisms that are 1267 not strictly related to the time protocol. 1269 7.1. Security and on-the-fly Timestamping 1271 Time protocols often require that protocol packets be modified during 1272 transmission. Both NTP and PTP in one-step mode require clocks to 1273 modify protocol packets based on the time of transmission and/or 1274 reception. 1276 In the presence of a security mechanism, whether encryption or 1277 integrity protection: 1279 o During transmission the encryption and/or integrity protection 1280 MUST be applied after integrating the timestamp into the packet. 1282 To allow high accuracy, timestamping is typically performed as close 1283 to the transmission or reception time as possible. However, since the 1284 security engine must be placed between the timestamping function and 1285 the physical interface, it may introduce non-deterministic latency 1286 that causes accuracy degradation. These performance aspects have been 1287 analyzed in the literature, e.g., in [1588IPsec] and [Tunnel]. 1289 7.2. PTP: Security and Two-Step Timestamping 1291 PTP supports a two-step mode of operation, where the time of 1292 transmission of protocol packets is communicated without modifying 1293 the packets. As opposed to one-step mode, two-step timestamping can 1294 be performed without the requirement to encrypt after timestamping. 1296 Note that if an encryption mechanism such as IPsec is used, it 1297 presents a challenge to the timestamping mechanism, since time 1298 protocol packets are encrypted when traversing the physical 1299 interface, and are thus impossible to identify. A possible solution 1300 to this problem [IPsecSync] is to include an indication in the 1301 encryption header that identifies time protocol packets. 1303 7.3. Intermediate Clocks 1305 A time protocol allows slaves to receive time information from an 1306 accurate time source. Time information is sent over a path that often 1307 traverses one or more intermediate clocks. 1309 o In NTP, time information originated from a stratum 1 server can be 1310 distributed to stratum 2 servers, and in turn distributed from the 1311 stratum 2 servers to NTP clients. In this case, the stratum 2 1312 servers are a layer of intermediate clocks. These intermediate 1313 clocks are referred to as "secondary servers" in [NTPv4]. 1315 o In PTP, BCs and TCs are intermediate nodes used to improve the 1316 accuracy of time information conveyed between the grandmaster and 1317 the slaves. 1319 A common rule of thumb in network security is that end-to-end 1320 security is the best policy, as it secures the entire path between 1321 the data originator and its receiver. The usage of intermediate nodes 1322 implies that if a security mechanism is deployed in the network, all 1323 intermediate nodes MUST possess the security key (hop-by-hop 1324 security) since they must be able to send time information to the 1325 slaves, or to modify time information sent through them. 1327 This inherent property of using intermediate clocks increases the 1328 system's exposure to internal threats, as there is a large number of 1329 nodes that possess the security keys. 1331 Thus, there is a tradeoff between the achievable clock accuracy of a 1332 system, and the robustness of its security solution. On one hand high 1333 clock accuracy calls for hop-by-hop involvement in the protocol, also 1334 known as on-path support. On the other hand, a robust security 1335 solution calls for end-to-end data protection. 1337 7.4. External Security Protocols and Time Protocols 1339 Time protocols are often deployed in systems that use security 1340 mechanisms and protocols. 1342 A typical example is the 3GPP Femtocell network [3GPP], where IPsec 1343 is used for securing traffic between a Femtocell and the Femto 1344 Gateway. In some cases, all traffic between these two nodes may be 1345 secured by IPsec, including the time protocol traffic. This use-case 1346 is thoroughly discussed in [IPsecSync]. 1348 Another typical example is the usage of MACsec encryption ([MACsec]) 1349 in L2 networks that deploy time synchronization [AvbAssum]. 1351 The usage of external security mechanisms may affect time protocols 1352 as follows: 1354 o Timestamping accuracy can be affected, as described in 7.1. 1356 o If traffic is secured between two nodes in the network, no 1357 intermediate clocks can be used between these two nodes. In the 1358 [3GPP] example, if traffic between the Femtocell and the Femto 1359 Gateway is encrypted, then time protocol packets are necessarily 1360 transported over the underlying network without modification, and 1361 thus cannot enjoy the improved accuracy provided by intermediate 1362 clock nodes. 1364 7.5. External Security Services Requiring Time 1366 Cryptographic protocols often use time as an important factor in the 1367 cryptographic algorithm. If a time protocol is compromised, it may 1368 consequently expose the security protocols that rely on it to various 1369 attacks. Two examples are presented in this section. 1371 7.5.1. Timestamped Certificates 1373 Certificate validation requires the sender and receiver to be roughly 1374 time synchronized. Thus, synchronization is required for establishing 1375 security protocols such as IKEv2 and TLS. 1377 An even stronger interdependence between a time protocol and a 1378 security mechanism is defined in [AutoKey], which defines mutual 1379 dependence between the acquired time information, and the 1380 authentication protocol that secures it. This bootstrapping behavior 1381 results from the fact that trusting the received time information 1382 requires a valid certificate, and validating a certificate requires 1383 knowledge of the time. 1385 7.5.2. Time Changes and Replay Attacks 1387 A successful attack on a time protocol may cause the attacked clocks 1388 to go back in time. The erroneous time may expose cryptographic 1389 algorithms that rely on time to prevent replay attacks. 1391 8. Issues for Further Discussion 1393 The Key distribution is outside the scope of this document. Although 1394 this is an essential element of any security system, it is outside 1395 the scope of this document. 1397 9. Security Considerations 1399 The security considerations of network timing protocols are presented 1400 throughout this document. 1402 10. IANA Considerations 1404 There are no new IANA considerations implied by this document. 1406 11. Acknowledgments 1408 The authors gratefully acknowledge Stefano Ruffini, Doug Arnold, 1409 Kevin Gross, Dieter Sibold, Dan Grossman and Laurent Montini for 1410 their thorough review and helpful comments. The authors would also 1411 like to thank members of the TICTOC WG for providing feedback on the 1412 TICTOC mailing list. 1414 This document was prepared using 2-Word-v2.0.template.dot. 1416 12. References 1418 12.1. Normative References 1420 [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate 1421 Requirement Levels", BCP 14, RFC 2119, March 1997. 1423 [NTPv4] Mills, D., Martin, J., Burbank, J., Kasch, W., 1424 "Network Time Protocol Version 4: Protocol and 1425 Algorithms Specification", RFC 5905, June 2010. 1427 [AutoKey] Haberman, B., Mills, D., "Network Time Protocol 1428 Version 4: Autokey Specification", RFC 5906, June 1429 2010. 1431 [IEEE1588] IEEE TC 9 Instrumentation and Measurement Society, 1432 "1588 IEEE Standard for a Precision Clock 1433 Synchronization Protocol for Networked Measurement and 1434 Control Systems Version 2", IEEE Standard, 2008. 1436 12.2. Informative References 1438 [Traps] Treytl, A., Gaderer, G., Hirschler, B., Cohen, R., 1439 "Traps and pitfalls in secure clock synchronization" 1440 in Proceedings of 2007 International Symposium for 1441 Precision Clock Synchronization for Measurement, 1442 Control and Communication, ISPCS 2007, pp. 18-24, 1443 2007. 1445 [TM] T. Mizrahi, "Time synchronization security using IPsec 1446 and MACsec", ISPCS 2011, pp. 38-43, 2011. 1448 [SecPTP] J. Tsang, K. Beznosov, "A security analysis of the 1449 precise time protocol (short paper)," 8th 1450 International Conference on Information and 1451 Communication Security (ICICS 2006), pp. 50-59, 2006. 1453 [SecSen] S. Ganeriwal, C. Popper, S. Capkun, M. B. Srivastava, 1454 "Secure Time Synchronization in Sensor Networks", ACM 1455 Trans. Info. and Sys. Sec., Volume 11, Issue 4, July 1456 2008. 1458 [AvbAssum] D. Pannell, "Audio Video Bridging Gen 2 Assumptions", 1459 IEEE 802.1 AVB Plenary, work in progress, May 2012. 1461 [IPsecSync] Y. Xu, "IPsec security for packet based 1462 synchronization", IETF, draft-xu-tictoc-ipsec- 1463 security-for-synchronization (work in progress), 2011. 1465 [3GPP] 3GPP, "Security of Home Node B (HNB) / Home evolved 1466 Node B (HeNB)", 3GPP TS 33.320 10.4.0 (work in 1467 progress), 2011. 1469 [1588IPsec] A. Treytl, B. Hirschler, "Securing IEEE 1588 by IPsec 1470 tunnels - An analysis", in Proceedings of 2010 1471 International Symposium for Precision Clock 1472 Synchronization for Measurement, Control and 1473 Communication, ISPCS 2010, pp. 83-90, 2010. 1475 [Tunnel] A. Treytl, B. Hirschler, and T. Sauter, "Secure 1476 tunneling of high precision clock synchronisation 1477 protocols and other timestamped data", in Proceedings 1478 of the 8th IEEE International Workshop on Factory 1479 Communication Systems (WFCS), vol. ISBN 978-1-4244- 1480 5461-7, pp. 303-313, 2010. 1482 [DelayAtt] T. Mizrahi, "A Game Theoretic Analysis of Delay 1483 Attacks against Time Synchronization Protocols", 1484 accepted, to appear in Proceedings of the 1485 International IEEE Symposium on Precision Clock 1486 Synchronization for Measurement, Control and 1487 Communication, ISPCS, 2012. 1489 [MACsec] IEEE 802.1AE-2006, "IEEE Standard for Local and 1490 Metropolitan Area Networks - Media Access Control 1491 (MAC) Security", 2006. 1493 [IPsec] S. Kent, K. Seo, "Security Architecture for the 1494 Internet Protocol", IETF, RFC 4301, 2005. 1496 13. Contributing Authors 1498 Karen O'Donoghue 1499 ISOC 1501 Email: odonoghue@isoc.org 1503 Authors' Addresses 1505 Tal Mizrahi 1506 Marvell 1507 6 Hamada St. 1508 Yokneam, 20692 Israel 1510 Email: talmi@marvell.com