TICTOC Working Group Tal Mizrahi Internet Draft Marvell Intended status: Informational Expires: March 2013 September 14, 2012 TICTOC Security Requirements draft-ietf-tictoc-security-requirements-03.txt Abstract As time synchronization protocols are becoming increasingly common and widely deployed, concern about their exposure to various security threats is increasing. This document defines a set of security requirements for time synchronization protocols, focusing on the Precision Time Protocol (PTP) and the Network Time Protocol (NTP). This document also discusses the security impacts of time synchronization protocol practices, the time synchronization performance implications of external security practices, the dependencies between other security services and time synchronization. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on March 14, 2013. Tal Mizrahi Expires March 14, 2013 [Page 1] Internet-Draft TICTOC Security Requirements September 2012 Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction ................................................. 3 2. Conventions Used in this Document ............................ 4 2.1. Terminology ............................................. 4 2.2. Terms & Abbreviations ................................... 5 3. Security Threats ............................................. 5 3.1. Threat Model ............................................ 5 3.1.1. Internal vs. External Attackers .................... 6 3.1.2. Man in the Middle (MITM) vs. Packet Injector ....... 6 3.2. Threat Analysis.......................................... 6 3.2.1. Packet Interception and Manipulation ............... 6 3.2.2. Spoofing ........................................... 6 3.2.3. Replay Attack ...................................... 7 3.2.4. Rogue Master Attack ................................ 7 3.2.5. Packet Interception and Removal .................... 7 3.2.6. Packet Delay Manipulation .......................... 7 3.2.7. Cryptographic Performance Attacks .................. 7 3.2.8. L2/L3 DoS Attacks .................................. 8 3.2.9. Master Time Source Spoofing (e.g. GPS fraud) ....... 8 3.3. Threat Analysis Summary ................................. 8 4. Security Requirements ........................................ 9 4.1. Clock Identity Authentication ........................... 9 4.1.1. Authentication of Masters ......................... 10 4.1.2. Recursive Authentication of Masters (Chain of Trust)10 4.1.3. Authentication of Slaves .......................... 11 4.1.4. PTP: Authentication of Transparent Clocks.......... 11 4.1.5. PTP: Authentication of Announce Messages .......... 11 4.2. Data integrity ......................................... 12 4.2.1. PTP: Hop-by-hop vs. End-to-end Integrity Protection 12 4.2.1.1. Hop by Hop Integrity Protection .............. 12 4.2.1.2. End to End Integrity Protection .............. 13 Tal Mizrahi Expires March 14, 2013 [Page 2] Internet-Draft TICTOC Security Requirements September 2012 4.3. Availability ........................................... 13 4.4. Replay Protection ...................................... 14 4.5. Cryptographic Keys & Security Associations ............. 14 4.5.1. Security Association .............................. 14 4.5.2. Unicast and Multicast ............................. 14 4.5.3. Key Freshness ..................................... 14 4.6. Performance ............................................ 15 4.7. Confidentiality......................................... 15 4.8. Protection against packet delay attacks ................ 16 4.9. Combining Secured with Unsecured Nodes ................. 16 4.9.1. Secure Mode ....................................... 17 4.9.2. Hybrid Mode ....................................... 17 5. Summary of Requirements ..................................... 18 6. Additional security implications ............................ 19 6.1. Security and on-the-fly Timestamping ................... 19 6.2. Security and Two-Step Timestamping ..................... 20 6.3. Intermediate Clocks .................................... 20 6.4. The Effect of External Security Protocols on Time Synchronization ............................................. 21 6.5. External Security Services Requiring Time Synchronization21 7. Issues for Further Discussion ............................... 21 8. Security Considerations ..................................... 21 9. IANA Considerations ......................................... 22 10. Acknowledgments ............................................ 22 11. References ................................................. 22 11.1. Normative References .................................. 22 11.2. Informative References ................................ 22 12. Contributing Authors ....................................... 24 1. Introduction As time synchronization protocols are becoming increasingly common and widely deployed, concern about the resulting exposure to various security threats is increasing. If a time synchronization protocol is compromised, the applications it serves are prone to a range of possible attacks including Denial-of-Service or incorrect behavior. This document focuses on the security aspects of the Precision Time Protocol (PTP) [IEEE1588] and the Network Time Protocol [NTPv4]. The Network Time Protocol was defined with an inherent security protocol, defined in [NTPv4] and in [AutoKey]. The IEEE 1588 includes an experimental security protocol, defined in Annex K of the standard, but this Annex was never formalized into a fully defined security protocol. Many of the existing packet timing deployments do not use any security mechanisms. The absence of a standard security solution for Tal Mizrahi Expires March 14, 2013 [Page 3] Internet-Draft TICTOC Security Requirements September 2012 PTP undoubtedly contributed to the wide deployment of unsecured time synchronization solutions. However, in some cases security mechanisms may not be strictly necessary, e.g., due to other security practices in place, or due to the architecture of the network. A time synchronization security solution, much like any security solution, is comprised of various building blocks, and must be carefully tailored for the specific system it is deployed in. Based on a system-specific threat assessment, the benefits of a security solution must be weighed against the potential risks, and based on this tradeoff an optimal security solution can be selected. This document attempts to add clarity to the time synchronization protocol security requirements discussion by addressing a series of questions: (1) What are the threats that need to be addressed for the time synchronization protocol, and thus what security services need to be provided? (e.g. a malicious NTP server or PTP master) (2) What external security practices impact the security and performance of time keeping, and what can be done to mitigate these impacts? (e.g. an IPSec tunnel in the synchronization traffic path) (3) What are the security impacts of time synchronization protocol practices? (e.g. on-the-fly modification of timestamps) (4) What are the dependencies between other security services and time synchronization? (e.g. which comes first - the certificate or the timestamp?) In light of the questions above, this document defines a set of requirements for security solutions for time synchronization protocols, focusing on PTP and NTP. 2. Conventions Used in this Document 2.1. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [KEYWORDS]. This document describes security requirements, and thus requirements are phrased in the document in the form "the security mechanism MUST/SHOULD/...". Note, that the phrasing does not imply that this document defines a specific security mechanism, but defines the requirements that every security mechanism should comply to. Tal Mizrahi Expires March 14, 2013 [Page 4] Internet-Draft TICTOC Security Requirements September 2012 This document refers to both PTP and NTP. For the sake of consistency, throughout the document the term "master" applies to both a PTP master and an NTP server. Similarly, the term "slave" applies to both PTP slaves and NTP clients. The general term "clock" refers to masters, slaves and PTP Transparent Clocks (TC). The term "protocol packets" is refers generically to PTP and NTP messages. 2.2. Terms & Abbreviations BC Boundary Clock MITM Man In The Middle NTP Network Time Protocol OC Ordinary Clock PTP Precision Time Protocol Secured clock A clock that supports a security mechanism that complies to the requirements in this document TC Transparent Clock Unsecured clock A clock that does not support a security mechanism according to the requirments in this document 3. Security Threats This section discusses the possible attacker types, and analyzes various attacks against time synchronization protocols. The literature is rich with security threats of time synchronization protocols, e.g., [Traps], [AutoKey], [TM], [SecPTP], and [SecSen]. The threat analysis in this document is mostly based on [TM]. 3.1. Threat Model A time synchronization protocol can be attacked by various types of attackers. The analysis in this documents classifies attackers according to 2 criteria, as described in 3.1.1. and 3.1.2. Tal Mizrahi Expires March 14, 2013 [Page 5] Internet-Draft TICTOC Security Requirements September 2012 3.1.1. Internal vs. External Attackers In the context of internal and external attackers, the underlying assumption is that the time synchronization protocol is secured either by an encryption or an authentication mechanism. Internal attackers either have access to a trusted segment of the network, or possess the encryption or authentication keys. External attackers, on the other hand, do not have the keys, and are exposed only to the encrypted or authenticated traffic. Thus, an internal attacker can maliciously tamper with legitimate traffic in the network, as well as generate its own traffic and make it appear legitimate to its attacked nodes. Obviously, in the absence of a security mechanism there is no distinction between internal and external attackers, since all attackers are internal in practice. 3.1.2. Man in the Middle (MITM) vs. Packet Injector MITM attackers are located in a position that allows interception and modification of in-flight protocol packets. A traffic injector is not located in an MITM position, but can attack by generatating protocol packets. An injector can also potentially eavesdrop to protocol packets sent as multicast, record them and replay them later. 3.2. Threat Analysis 3.2.1. Packet Interception and Manipulation A packet interception and manipulation attack results when a Man-In- The-Middle (MITM) attacker intercepts timing protocol packets, alters them and relays them to their destination, allowing the attacker to maliciously tamper with the protocol. This can result in a situation where the time protocol is apparently operational but providing intentionally inaccurate information. 3.2.2. Spoofing In spoofing, an attacker masquerades as a legitimate node in the network by generating and transmitting protocol packets. For example, an attacker can impersonate the master, allowing malicious distribution of false timing information. As with packet interception and manipulation, this can result in a situation where the time Tal Mizrahi Expires March 14, 2013 [Page 6] Internet-Draft TICTOC Security Requirements September 2012 protocol is apparently operational but providing intentionally inaccurate information. 3.2.3. Replay Attack In a replay attack, an attacker records protocol packets and replays them at a later time without any modification. This can also result in a situation where the time protocol is apparently operational but providing intentionally inaccurate information. 3.2.4. Rogue Master Attack In a rogue master attack, an attacker causes other nodes in the network to believe it is a legitimate master. As opposed to the spoofing attack, in the Rouge Master attack the attacker does not fake its identity, but rather manipulates the master election process. For example, in PTP, an attacker can manipulate the Best Master Clock Algorithm (BMCA), and cause other nodes in the network to believe it is the most eligible candidate to be a grandmaster. 3.2.5. Packet Interception and Removal A packet interception and removal attack results when a Man-In-The- Middle attacker intercepts and drops protocol packets, preventing the destination node from receiving the timing information. 3.2.6. Packet Delay Manipulation In a packet delay manipulation scenario, a Man-In-The-Middle attacker intercepts protocol packets, and relays them to their destination after adding a maliciously computed delay. Note that the attackee still receives one copy of each packet, contrary to the replay attack, where a packet is received by the attackee more than once. 3.2.7. Cryptographic Performance Attacks In cryptographic performance attacks, an attacker transmits fake protocol packet, causing high utilization of the cryptographic engine at the receiver, which attempts to verify the integrity of these fake packets. Tal Mizrahi Expires March 14, 2013 [Page 7] Internet-Draft TICTOC Security Requirements September 2012 3.2.8. L2/L3 DoS Attacks There are many possible Layer 2 and Layer 3 Denial of Service attacks. As the target's availability is compromised, the timing protocol is affected accordingly. 3.2.9. Master Time Source Spoofing (e.g. GPS fraud) In time source spoofing, an attacker spoofs the accurate time source of the master. For example, if the master uses a GPS based clock as its reference source, an attacker can spoof the GPS satellites, causing the master to use a false reference time. 3.3. Threat Analysis Summary The two key factors to a threat analysis are the severity and the likelihood of each of the analyzed attacks. Table 1 summarizes the security attacks presented in 3.2. For each attack, the table specifies its impact, and its applicability to each of the attacker types presented in 3.1. The Impact column provides an intuition to the severity of each attack, and the relevant Attacker Type columns provide an intuition about the how difficult each attack is to implement, and hence about the likelihood of each attack. The impact column in Table 1 can have one of 3 values: o DoS - the attack causes a denial of service to the attacked node, the impact of which is not restricted to the time synchronization protocol. o False time - slaves align to a false time or frequency value due to the attack. Note that if the time synchronization service aligns to a false time, it may cause denial of service to other applications that rely on accurate time. However, for the purpose of the analysis in this section we distinguish this implication from "DoS", which refers to a DoS attack that is not necessarily aimed at the time synchronization protocol. o Accuracy degradation - the attack yields a degradation in the slave accuracy, but does not completely compromise the slaves' time and frequency. The Attacket Type columns refer to the 4 possible combinations of the attacker types defined in 3.1. Tal Mizrahi Expires March 14, 2013 [Page 8] Internet-Draft TICTOC Security Requirements September 2012 +-----------------------------+-------------------++-------------------+ | Attack | Impact || Attacker Type | | +-----+--------+----++---------+---------+ | |False|Accuracy| ||Internal | Extenal | | |Time |Degrad. |DoS ||MITM|Inj.|MITM|Inj.| +-----------------------------+-----+--------+----++----+----+----+----+ |Interception and manipulation| + | | || + | | | | +-----------------------------+-----+--------+----++----+----+----+----+ |Spoofing | + | | || + | + | | | +-----------------------------+-----+--------+----++----+----+----+----+ |Replay attack | + | | || + | + | | | +-----------------------------+-----+--------+----++----+----+----+----+ |Rogue master attack | + | | || + | + | | | +-----------------------------+-----+--------+----++----+----+----+----+ |Interception and Removal | | + | || + | | + | | +-----------------------------+-----+--------+----++----+----+----+----+ |Packet delay manipulation | + | | || + | | + | | +-----------------------------+-----+--------+----++----+----+----+----+ |Crypt. performance attacks | | | + || + | + | + | + | +-----------------------------+-----+--------+----++----+----+----+----+ |DoS attacks | | | + || + | + | + | + | +-----------------------------+-----+--------+----++----+----+----+----+ |Master Time source spoofing | + | | || + | + | + | + | |(e.g. GPS spoofing) | | | || | | | | +-----------------------------+-----+--------+----++----+----+----+----+ Table 1 Threat Analysis - Summary 4. Security Requirements This section defines a set of requirements from the security mechanisms used for PTP and NTP. These requirements are phrased in the form "the security mechanism MUST/SHOULD/MAY...". However, this document does not specify how these requirements can be met; While these requirments can be satisfied by extending the time protocols, at least a subset of the requirements can be met by applying common security practices to the network or by using existing security protocols, such as IPsec or MACsec. Thus, security solutions that address these requirements are outside the scope of this document. 4.1. Clock Identity Authentication Requirement Tal Mizrahi Expires March 14, 2013 [Page 9] Internet-Draft TICTOC Security Requirements September 2012 The security mechanism MUST provide a means for each clock to authenticate the sender of a protocol packet. Discussion In the context of this document, authentication refers to: o Identification: verifying the identity of the peer clock. o Authorization: verifying that the peer clock is permitted to play the role that it plays in the protocol. For example, some nodes may be permitted to be masters, while other nodes are only permitted to be slaves or TCs. The following subsections describe 4 distinct cases of clock authentication. 4.1.1. Authentication of Masters Requirement The security mechanism MUST support an authentication mechanism, allowing slave clocks to authenticate the identity of master clocks. 4.1.2. Recursive Authentication of Masters (Chain of Trust) Requirement The security mechanism MUST support recursive authentication of the master, to be used in cases where end-to-end authentication is not possible. Discussion Clocks authenticate masters in order to ensure the authenticity of the time source. In some cases a slave is connected to an intermediate master, that is not the primary time source. For example, in PTP a slave can be connected to a Boundary Clock (BC), which in turn is connected to a grandmaster. A similar example in NTP is when a client is connected to a stratum 2 server, which is connected to a stratum 1 server. In both the PTP and the NTP cases, the slave authenticates the intermediate master, and the intermediate master authenticates the primary master. This inductive authentication process is referred to in [AutoKey] as proventication. Tal Mizrahi Expires March 14, 2013 [Page 10] Internet-Draft TICTOC Security Requirements September 2012 4.1.3. Authentication of Slaves Requirement The security mechanism SHOULD provide a means for a master to authenticate its slaves. Discussion Slaves are authenticated by masters in order to verify that the slave is authorized to receive timing services from the master. Authentication of slaves prevents unauthorized clocks from receiving time services, and also reduces unnecessary load on the master clock, by preventing the master from serving unauthorized clocks. It could be argued that the authentication of slaves could put a higher load on the master then serving the unauthorized clock, and hence this requirement is a SHOULD. 4.1.4. PTP: Authentication of Transparent Clocks Requirement The security mechanism for PTP SHOULD provide a means for a master to authenticate the identity of the P2P TCs directly connected to it. Discussion P2P TCs that are one hop from the master use the PDelay_Req and PDelay_Resp handshake to compute the link delay between the master and TC. These TCs are authenticated by the master. Authentication of TCs, much like authentication of slaves, reduces unnecessary load on the master clock and peer TCs, by preventing the master from serving unauthorized clocks. 4.1.5. PTP: Authentication of Announce Messages Requirement The security mechanism for PTP MUST support authentication of Announce messages. Discussion Master election is performed in PTP using the Best Master Clock Algorithm (BMCA). Each Ordinary Clock (OC) announces its clock Tal Mizrahi Expires March 14, 2013 [Page 11] Internet-Draft TICTOC Security Requirements September 2012 attributes using Announce messages, and the best master is elected based on the information gathered from all the candidates. Announce messages must be authenticated in order to prevent malicious master attacks. Note, that this subsection specifies a requirement that is not necessarily included in 4.1.1. or in 4.1.3. , since the BMCA is initiated before clocks have been defined as masters or slaves. 4.2. Data integrity Requirement The security mechanism MUST protect the integrity of protocol packets. Discussion While subsection 4.1. refers to ensuring WHO sent the protocol packet, this subsection refers to ensuring that the packet arrived intact. The integrity protection mechanism ensures the authenticity and completeness of data from the data originator. 4.2.1. PTP: Hop-by-hop vs. End-to-end Integrity Protection Requirement A security mechanism for PTP MUST support hop-by-hop integrity protection. Requirement A security mechanism for PTP SHOULD support end-to-end integrity protection. Discussion Specifically in PTP, when protocol packets are subject to modification by TCs, the integrity protection can be enforced in one of two approaches, end-to-end or hop-by-hop. 4.2.1.1. Hop by Hop Integrity Protection Each hop that needs to modify a protocol packet: o Verifies its integrity. Tal Mizrahi Expires March 14, 2013 [Page 12] Internet-Draft TICTOC Security Requirements September 2012 o Modifies the packet, i.e., modifies the correctionField. o Re-generates the integrity protection, e.g., re-computes a Message Authentication Code. In the hop-by-hop approach, the integrity of protocol packets is protected by induction on the path from the originator to the receiver. This approach is simple, but allows malicious TCs to modify protocol packets. 4.2.1.2. End to End Integrity Protection In this approach, the integrity protection is maintained on the path from the originator of a protocol packet to the receiver. This allows the receiver to validate the protocol packet without the ability of intermediate TCs to manipulate the packet. Since TCs need to modify the correctionField, a separate integrity protection mechanism is used specifically for the correctionField. The end-to-end approach limits the TC's impact to the correctionField alone, while the rest of the protocol packet is protected on an end- to-end basis. It should be noted that this approach is more difficult to implement than the hop-by-hop approach, as it requires separate layers of protection for the correctionField and for the rest of the packet, using different cryptographic mechanisms and keys. 4.3. Availability Requirement The security mechanism MUST protect the time synchronization protocol from DoS attacks by external attackers. Discussion The protocol availability can be compromised by several different attacks. An attacker can inject protocol messages to implement the spoofing attack (Section 3.2.2. ) or the rogue master attack (Section 3.2.4. ), causing denial of service to the attackee. An authentication mechanism (Section 4.1. ) limits these attacks strictly to internal attackers, and thus prevents external attackers from performing them. Tal Mizrahi Expires March 14, 2013 [Page 13] Internet-Draft TICTOC Security Requirements September 2012 Note that a security mechanism applied at the time synchronization layer cannot, by itself, prevent DoS attacks described in Section 3.2.8. DoS attacks at lower layers of the protocol stack (Section 3.2.8. ) can still be implemented by external attackers even in the presence of an authentication mechanism. 4.4. Replay Protection Requirement Protocol messages MUST be resistant to replay attacks. 4.5. Cryptographic Keys & Security Associations 4.5.1. Security Association Requirement The security protocol SHOULD support an association protocol where: o Two or more clocks authenticate each other. o The clocks generate and agree on a cryptographic session key. Discussion The security requirements in 4.1. and 4.2. require usage of cryptographich mechanisms, deploying cryptographic keys. A security association is an essential building block in these mechanisms. 4.5.2. Unicast and Multicast Requirement The security mechanism SHOULD support security association protocols for unicast and for multicast associations. Discussion A unicast protocol requires an association protocol between two clocks, whereas a multicast protocol requires an association protocol among two or more clocks, where one of the clocks is a master. 4.5.3. Key Freshness Requirement Tal Mizrahi Expires March 14, 2013 [Page 14] Internet-Draft TICTOC Security Requirements September 2012 The cryptographic keys MUST be refreshed periodically. Requirement The association protocol MUST be invoked periodically, where each instance of the association protocol MUST produce a different session key. 4.6. Performance Requirement The security mechanism MUST be designed in such a way that it does not degrade the quality of the time transfer. Requirement The mechanism SHOULD be relatively lightweight, as client restrictions often dictate a low processing and memory footprint, and because the server may have extensive fan-out. Requirement The mechanism also SHOULD not require excessive storage of client state in the master, nor significantly increase bandwidth consumption. Discussion Note that the performance requirements refer to a time- synchronization-specific security mechanism. In systems where a security protocol is used for other types of traffic as well, this document does not place any performance requirements on the security protocol performance. For example, if IPsec encryption is used for securing all information between the master and slave node, including information that is not part of the time protocol, the requirements in this subsection are not necessarily applicable. 4.7. Confidentiality Requirement The security mechanism MAY provide confidentiality protection of the protocol packets. Discussion Tal Mizrahi Expires March 14, 2013 [Page 15] Internet-Draft TICTOC Security Requirements September 2012 In the context of time synchronization, confidentiality is typically of low importance, since timing information is typically not considered secret information. Confidentiality can play an important role when service providers charge payment for time synchronization services, but these cases are rather esoteric. Confidentiality can also prevent an MITM attacker from identifying protocol packets. Thus, confidentiality can assist in protecting the timing protocol against packet delay attacks, where the attacker selectively adds delay to time protocol packets. Note, that time protocols have predictable behavior such as packet transmission rates and packet lengths, and thus packet encryption does not prevent delay attacks, but rather makes these attacks more difficult to implement. 4.8. Protection against packet delay attacks Requirement The security mechanism MAY include a means to detect packet delay attacks. Requirement The security mechanism MAY include a redundancy mechanism that allows a node that detects a delay attack to switch over to a secondary master. Discussion While this document does not define specific security solutions, we note that common practices for protection against delay attacks use redundant masters (e.g. [NTPv4]), or redundant paths between the master and slave (e.g. [DelayAtt]). If one of the time sources indicates a time value that is significantly different than the other sources, it is assumed to be erroneous or under attack, and is therefore ignored. This requirement is a "may" requirement since both master redundancy and path redundancy are not necessarily possible in all network topologies. 4.9. Combining Secured with Unsecured Nodes Integrating a security mechanism into a time synchronized system is a complex process, and in some cases may require a gradual process, Tal Mizrahi Expires March 14, 2013 [Page 16] Internet-Draft TICTOC Security Requirements September 2012 where new equipment supports the security mechanism, and is required to interoperate with legacy equipment without the security features. 4.9.1. Secure Mode Requirement The security mechanism MUST support a secure mode, where only secured clocks are permitted to take part in the synchronization protocol. A protocol packet received from an unsecured clock MUST be discarded. Discussion While the requirement in this subsection is a bit similar to the one in 4.1. , it explicitly defines the secure mode, as opposed to the hybrid mode presented in the next subsection. 4.9.2. Hybrid Mode Requirement The security protocol MAY support a hybrid mode, where both secured and unsecured clocks are permitted to take part in the protocol. Discussion The hybrid mode allows both secured and unsecured clocks to take part in the synchronization protocol. NTP, for example, allows a mixture of secured and unsecured nodes. Requirement A master in the hybrid mode SHOULD be a secured clock. A secured slave in the hybrid mode SHOULD discard all protocol packets received from unsecured clocks. Discussion This requirement ensures that the existence of unsecured clocks does not compromise the security provided to secured clocks. Hence, secured slaves only "trust" protocol packets received from a secured clock. An unsecured clock can receive protocol packets from either secured clocks, or unsecured clocks. Note that the security scheme in [NTPv4] with [AutoKey] does not satisfy this requirement, since nodes prefer the server with the best Tal Mizrahi Expires March 14, 2013 [Page 17] Internet-Draft TICTOC Security Requirements September 2012 clock, and not necessarily the server that supports authentication. For example, a stratum 2 server is connected to two stratum 1 servers, Server A, supporting authentication, and server B, without authentication. If server B has a more accurate clock than A, the stratum 2 server chooses server B, in spite of the fact it does not support authentication. 5. Summary of Requirements +-----------+--------------------------------------+---------------+ | Section | Requirement | Type | +-----------+--------------------------------------+---------------+ | 4.1. | Authentication of sender. | MUST | | +--------------------------------------+---------------+ | | Authentication of master. | MUST | | +--------------------------------------+---------------+ | | Recursive authentication. | MUST | | +--------------------------------------+---------------+ | | Authentication of slaves. | SHOULD | | +--------------------------------------+---------------+ | | PTP: Authentication of TCs. | SHOULD | | +--------------------------------------+---------------+ | | PTP: Authentication of Announce | SHOULD | | | messages. | | +-----------+--------------------------------------+---------------+ | 4.2. | Integrity protection. | MUST | | +--------------------------------------+---------------+ | | PTP: hop-by-hop integrity protection.| MUST | | +--------------------------------------+---------------+ | | PTP: end-to-end integrity protection.| SHOULD | +-----------+--------------------------------------+---------------+ | 4.3. | Protection against DoS attacks. | MUST | +-----------+--------------------------------------+---------------+ | 4.4. | Replay protection. | MUST | +-----------+--------------------------------------+---------------+ | 4.5. | Security association. | SHOULD | | +--------------------------------------+---------------+ | | Unicast and multicast associations. | SHOULD | | +--------------------------------------+---------------+ | | Key freshness. | MUST | +-----------+--------------------------------------+---------------+ | 4.6. | Performance: no degradation in | MUST | Tal Mizrahi Expires March 14, 2013 [Page 18] Internet-Draft TICTOC Security Requirements September 2012 | | quality of time transfer. | | | +--------------------------------------+---------------+ | | Performance: lightweight. | SHOULD | | +--------------------------------------+---------------+ | | Performance: storage, bandwidth. | MUST | +-----------+--------------------------------------+---------------+ | 4.7. | Confidentiality protection. | MAY | +-----------+--------------------------------------+---------------+ | 4.8. | Protection against delay attacks. | MAY | +-----------+--------------------------------------+---------------+ | 4.9. | Secure mode. | MUST | | +--------------------------------------+---------------+ | | Hybrid mode. | MAY | +-----------+--------------------------------------+---------------+ Table 2 Summary of Security Requirements 6. Additional security implications This section discusses additional implications of the interaction between time synchronization protocols and security mechanisms. This section refers to time synchronization security mechanisms, as well as to "external" security mechanisms, i.e., security mechanisms that are not strictly related to the time synchronization protocol. 6.1. Security and on-the-fly Timestamping Time synchronization protocols often require protocol packets to be modified during transmission and reception. Both NTP and PTP in one- step mode require clocks to modify protocol packets with the time of transmission or reception. In the presence of a security mechanism, whether encryption or integrity protection: o During transmission the security protocol must be applied after integrating the timestamp into the packet. o During reception, the encryption or integrity check must be performed before modifying the packet with the time of reception. To allow high accuracy, timestamping is typically performed as close to the transmission or reception time as possible. However, since the security engine must be placed between the timestamping function and the physical interface, in some cases it may introduce non- Tal Mizrahi Expires March 14, 2013 [Page 19] Internet-Draft TICTOC Security Requirements September 2012 deterministic latency that causes accuracy degradation. These performance aspects have been analyzed in the literature, e.g., in [1588IPsec] and [Tunnel]. 6.2. Security and Two-Step Timestamping PTP supports a two-step mode of operation, where the time of transmission and the time of reception of protocol packets are measured without modifying the packets. As opposed to one-step mode, two step timestamping can be performed at the physical interface even in the presence of a security mechanism. Note that if an encryption mechanism such as IPsec is used, it presents a challenge to the timestamping mechanism, since time protocol packets are encrypted when traversing the physical interface, and are thus impossible to identify. A possible solution to this problem [IPsecSync] is to include an indication in the encryption header that identifies time synchronization packets. 6.3. Intermediate Clocks A time synchronization protocol allows slaves to receive time information from an accurate time source. Time information is sent over a path that often traverses one or more intermediate clocks. o In NTP, time information originated from a stratum 1 server can be distributed to stratum 2 servers, and in turn distributed from the stratum 2 servers to NTP clients. In this case, the stratum 2 servers are a layer of intermediate clocks. o In PTP, BCs and TCs are intermediate nodes used to improve the accuracy of time information conveyed between the grandmaster and the slaves. A common rule of thumb in network security is that end-to-end security is the best policy, as it secures the entire path between the data originator and its receiver. The usage of intermediate nodes implies that if a security mechanism is deployed in the network, all intermediate nodes must be exposed to the security key since they must be able to send time information to the slaves, or to modify time information sent through them. This inhehrent property of using intermediate clocks increases the system's exposure to internal threats, as there is a large number of nodes that are exposed to the security keys. Tal Mizrahi Expires March 14, 2013 [Page 20] Internet-Draft TICTOC Security Requirements September 2012 6.4. The Effect of External Security Protocols on Time Synchronization Time synchronization protocols are often deployed in systems that use security mechanisms and protocols. A typical example is the 3GPP Femtocell network [3GPP], where IPsec is used for securing traffic between a Femtocell and the Femto Gateway. In some cases, all traffic between these two nodes may be secured by IPsec, including the time synchronization protocol traffic. This use-case is thoroughly discussed in [IPsecSync]. Another typical example is the usage of MACsec encryption in L2 networks that deploy time synchronization [AvbAssum]. The usage of external security mechanisms may affect time synchronization protocols as follows: o Timestamping accuracy can be affected, as described in 6.1. o If traffic is secured between two nodes in the network, no intermediate clocks can be used between these two nodes. In the [3GPP] example, if traffic between the Femtocell and the Femto Gateway is encrypted, then time protocol packets are sent over the underlying network without modification, and thus cannot enjoy the improved accuracy provided by intermediate clock nodes. 6.5. External Security Services Requiring Time Synchronization Certificate validation requires the sender and receiver to be roughly time synchronized. Thus, synchronization is required for establishing security protocols such as IKEv2 and TLS. An even stronger interdependence between a time synchronization protocol and a security mechanism is defined in [AutoKey], which defines mutual dependence between the acquired time information, and the authentication protocol that secures it. 7. Issues for Further Discussion o The key distribution is outside the scope of this document. Although this is a cardinal element in any security system, it is not a security requirement, and is thus not described here. 8. Security Considerations The security considerations of network timing protocols are presented throughout this document. Tal Mizrahi Expires March 14, 2013 [Page 21] Internet-Draft TICTOC Security Requirements September 2012 9. IANA Considerations There are no new IANA considerations implied by this document. 10. Acknowledgments The authors gratefully acknowledge Stefano Ruffini, Dieter Sibold and Dan Grossman for their thorough review and helpful comments. The authors would also like to thank members of the TICTOC WG for providing feedback on the TICTOC mailing list. This document was prepared using 2-Word-v2.0.template.dot. 11. References 11.1. Normative References [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [NTPv4] Mills, D., Martin, J., Burbank, J., Kasch, W., "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, June 2010. [AutoKey] Haberman, B., Mills, D., "Network Time Protocol Version 4: Autokey Specification", RFC 5906, June 2010. [IEEE1588] IEEE TC 9 Test and Measurement Society 2000, "1588 IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems Version 2", IEEE Standard, 2008. 11.2. Informative References [Traps] Treytl, A., Gaderer, G., Hirschler, B., Cohen, R., "Traps and pitfalls in secure clock synchronization" in Proceedings of 2007 International Symposium for Precision Clock Synchronization for Measurement, Control and Communication, ISPCS 2007, pp. 18-24, 2007. [TM] T. Mizrahi, "Time synchronization security using IPsec and MACsec", ISPCS 2011, pp. 38-43, 2011. Tal Mizrahi Expires March 14, 2013 [Page 22] Internet-Draft TICTOC Security Requirements September 2012 [SecPTP] J. Tsang, K. Beznosov, "A security analysis of the precise time protocol (short paper)," 8th International Conference on Information and Communication Security (ICICS 2006), pp. 50-59, 2006. [SecSen] S. Ganeriwal, C. Popper, S. Capkun, M. B. Srivastava, "Secure Time Synchronization in Sensor Networks", ACM Trans. Info. and Sys. Sec., Volume 11, Issue 4, July 2008. [AvbAssum] D. Pannell, "Audio Video Bridging Gen 2 Assumptions", IEEE 802.1 AVB Plenary, work in progress, May 2012. [IPsecSync] Y. Xu, "IPsec security for packet based synchronization", IETF, draft-xu-tictoc-ipsec- security-for-synchronization (work in progress), 2011. [3GPP] 3GPP, "Security of Home Node B (HNB) / Home evolved Node B (HeNB)", 3GPP TS 33.320 10.4.0 (work in progress), 2011. [1588IPsec] A. Treytl, B. Hirschler, "Securing IEEE 1588 by IPsec tunnels - An analysis", in Proceedings of 2010 International Symposium for Precision Clock Synchronization for Measurement, Control and Communication, ISPCS 2010, pp. 83-90, 2010. [Tunnel] A. Treytl, B. Hirschler, and T. Sauter, "Secure tunneling of high precision clock synchronisation protocols and other timestamped data", in Proceedings of the 8th IEEE International Workshop on Factory Communication Systems (WFCS), vol. ISBN 978-1-4244- 5461-7, pp. 303-313, 2010. [DelayAtt] T. Mizrahi, "A Game Theoretic Analysis of Delay Attacks against Time Synchronization Protocols", accepted, to appear in Proceedings of the International IEEE Symposium on Precision Clock Synchronization for Measurement, Control and Communication, ISPCS, 2012. Tal Mizrahi Expires March 14, 2013 [Page 23] Internet-Draft TICTOC Security Requirements September 2012 12. Contributing Authors Karen O'Donoghue ISOC Email: odonoghue@isoc.org Authors' Addresses Tal Mizrahi Marvell 6 Hamada St. Yokneam, 20692 Israel Email: talmi@marvell.com Tal Mizrahi Expires March 14, 2013 [Page 24]