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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.







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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


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      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


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   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.


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   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.






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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



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   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.







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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.


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+-----------------------------+-------------------++-------------------+
| 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


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   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.



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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


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   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.



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   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.




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   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



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   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



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   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,


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   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


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   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          |


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   |           | 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-


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   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.




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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.


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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.





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   [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.









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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































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