| Rfc | 7384 |
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| Title | Security Requirements of Time Protocols in Packet Switched Networks |
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| Author | T. Mizrahi |
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| Date | October 2014 |
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| Format: | TXT, HTML |
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| Status: | INFORMATIONAL |
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Internet Engineering Task Force (IETF) T. Mizrahi
Request for Comments: 7384 Marvell
Category: Informational October 2014
ISSN: 2070-1721
Security Requirements of Time Protocols
in Packet Switched Networks
Abstract
As time and frequency distribution 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 protocols, focusing on the
Precision Time Protocol (PTP) and the Network Time Protocol (NTP).
This document also discusses the security impacts of time protocol
practices, the performance implications of external security
practices on time protocols, and the dependencies between other
security services and time synchronization.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7384.
Copyright Notice
Copyright (c) 2014 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 ....................................................4
2. Terminology .....................................................5
2.1. Requirements Language ......................................5
2.2. Abbreviations ..............................................6
2.3. Common Terminology for PTP and NTP .........................6
2.4. Terms Used in This Document ................................6
3. Security Threats ................................................7
3.1. Threat Model ...............................................8
3.1.1. Internal vs. External Attackers .....................8
3.1.2. Man in the Middle (MITM) vs. Packet Injector ........8
3.2. Threat Analysis ............................................9
3.2.1. Packet Manipulation .................................9
3.2.2. Spoofing ............................................9
3.2.3. Replay Attack .......................................9
3.2.4. Rogue Master Attack .................................9
3.2.5. Packet Interception and Removal ....................10
3.2.6. Packet Delay Manipulation ..........................10
3.2.7. L2/L3 DoS Attacks ..................................10
3.2.8. Cryptographic Performance Attacks ..................10
3.2.9. DoS Attacks against the Time Protocol ..............11
3.2.10. Grandmaster Time Source Attack (e.g., GPS Fraud) ..11
3.2.11. Exploiting Vulnerabilities in the Time Protocol ...11
3.2.12. Network Reconnaissance ............................11
3.3. Threat Analysis Summary ...................................12
4. Requirement Levels .............................................13
5. Security Requirements ..........................................14
5.1. Clock Identity Authentication and Authorization ...........14
5.1.1. Authentication and Authorization of Masters ........15
5.1.2. Recursive Authentication and Authorization
of Masters (Chain of Trust) ........................16
5.1.3. Authentication and Authorization of Slaves .........17
5.1.4. PTP: Authentication and Authorization of
P2P TCs by the Master ..............................18
5.1.5. PTP: Authentication and Authorization of
Control Messages ...................................18
5.2. Protocol Packet Integrity .................................19
5.2.1. PTP: Hop-by-Hop vs. End-to-End Integrity
Protection .........................................20
5.2.1.1. Hop-by-Hop Integrity Protection ...........20
5.2.1.2. End-to-End Integrity Protection ...........21
5.3. Spoofing Prevention .......................................21
5.4. Availability ..............................................22
5.5. Replay Protection .........................................23
5.6. Cryptographic Keys and Security Associations ..............23
5.6.1. Key Freshness ......................................23
5.6.2. Security Association ...............................24
5.6.3. Unicast and Multicast Associations .................24
5.7. Performance ...............................................25
5.8. Confidentiality ...........................................26
5.9. Protection against Packet Delay and Interception Attacks ..27
5.10. Combining Secured with Unsecured Nodes ...................27
5.10.1. Secure Mode .......................................28
5.10.2. Hybrid Mode .......................................28
6. Summary of Requirements ........................................29
7. Additional Security Implications ...............................31
7.1. Security and On-the-Fly Timestamping ......................31
7.2. PTP: Security and Two-Step Timestamping ...................31
7.3. Intermediate Clocks .......................................32
7.4. External Security Protocols and Time Protocols ............32
7.5. External Security Services Requiring Time .................33
7.5.1. Timestamped Certificates ...........................33
7.5.2. Time Changes and Replay Attacks ....................33
8. Issues for Further Discussion ..................................34
9. Security Considerations ........................................34
10. References ....................................................34
10.1. Normative References .....................................34
10.2. Informative References ...................................34
Acknowledgments ...................................................36
Contributors ......................................................36
Author's Address ..................................................36
1. Introduction
As time protocols are becoming increasingly common and widely
deployed, concern about the resulting exposure to various security
threats is increasing. If a time protocol is compromised, the
applications it serves are prone to a range of possible attacks
including Denial of Service (DoS) or incorrect behavior.
This document discusses the security aspects of time distribution
protocols in packet networks and focuses on the two most common
protocols: the Network Time Protocol [NTPv4] and the Precision Time
Protocol (PTP) [IEEE1588]. Note that although PTP was not defined by
the IETF, it is one of the two most common time protocols; hence, it
is included in the discussion.
The Network Time Protocol was defined with an inherent security
protocol; [NTPv4] defines a security protocol that is based on a
symmetric key authentication scheme, and [AutoKey] presents an
alternative security protocol, based on a public key authentication
scheme. [IEEE1588] includes an experimental security protocol,
defined in Annex K of the standard, but this Annex was never
formalized into a fully defined security protocol.
While NTP includes an inherent security protocol, the absence of a
standard security solution for 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 in which it is deployed. Based on a system-specific threat
assessment, the benefits of a security solution must be weighed
against the potential risks, and based on this trade-off an optimal
security solution can be selected.
The target audience of this document includes:
o Timing and networking equipment vendors - can benefit from this
document by deriving the security features that should be
supported in the time/networking equipment.
o Standards development organizations - can use the requirements
defined in this document when specifying security mechanisms for a
time protocol.
o Network operators - can use this document as a reference when
designing a network and its security architecture. As stated
above, the requirements in this document may be deployed
selectively based on a careful per-system threat analysis.
This document attempts to add clarity to the time protocol security
requirements discussion by addressing a series of questions:
(1) What are the threats that need to be addressed for the time
protocol and 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 time protocol traffic
path)?
(3) What are the security impacts of time protocol practices (e.g.,
on-the-fly modification of timestamps)?
(4) What are the dependencies between other security services and
time protocols? (For example, 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 protocols, focusing on
PTP and NTP.
2. Terminology
2.1. Requirements Language
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; 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 that it defines
the requirements with which every security mechanism should comply.
2.2. Abbreviations
BC Boundary Clock [IEEE1588]
BMCA Best Master Clock Algorithm [IEEE1588]
DoS Denial of Service
MITM Man in the Middle
NTP Network Time Protocol [NTPv4]
OC Ordinary Clock [IEEE1588]
P2P TC Peer-to-Peer Transparent Clock [IEEE1588]
PTP Precision Time Protocol [IEEE1588]
TC Transparent Clock [IEEE1588]
2.3. Common Terminology for PTP and NTP
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 term "protocol
packets" refers generically to PTP and NTP messages.
2.4. Terms Used in This Document
o Clock - A node participating in the protocol (either PTP or NTP).
A clock can be a master, a slave, or an intermediate clock (see
corresponding definitions below).
o Control packets - Packets used by the protocol to exchange
information between clocks that is not strictly related to the
time. NTP uses NTP Control Messages. PTP uses Announce,
Signaling, and Management messages.
o End-to-end security - A security approach where secured packets
sent from a source to a destination are not modified by
intermediate nodes, allowing the destination to authenticate the
source of the packets and to verify their integrity. In the
context of confidentiality, end-to-end encryption guarantees that
intermediate nodes cannot eavesdrop to en route packets. However,
as discussed in Section 5, confidentiality is not a strict
requirement in this document.
o Grandmaster - A master that receives time information from a
locally attached clock device and not through the network. A
grandmaster distributes its time to other clocks in the network.
o Hop-by-hop security - A security approach where secured packets
sent from a source to a destination may be modified by
intermediate nodes. In this approach intermediate nodes share the
encryption key with the source and destination, allowing them to
re-encrypt or re-authenticate modified packets before relaying
them to the destination.
o Intermediate clock - A clock that receives timing information from
a master and sends timing information to other clocks. In NTP,
this term refers to an NTP server that is not a Stratum 1 server.
In PTP, this term refers to a BC or a TC.
o Master - A clock that generates timing information to other clocks
in the network. In NTP, 'master' refers to an NTP server. In
PTP, 'master' refers to a master OC (aka grandmaster) or to a port
of a BC that is in the master state.
o Protocol packets - Packets used by the time protocol. The
terminology used in this document distinguishes between time
packets and control packets.
o Secured clock - A clock that supports a security mechanism that
complies to the requirements in this document.
o Slave - A clock that receives timing information from a master.
In NTP, 'slave' refers to an NTP client. In PTP, 'slave' refers
to a slave OC or to a port of a BC that is in the slave state.
o Time packets - Protocol packets carrying time information.
o Unsecured clock - A clock that does not support a security
mechanism according to the requirements in this document.
3. Security Threats
This section discusses the possible attacker types and analyzes
various attacks against time protocols.
The literature is rich with security threats of time protocols, e.g.,
[Traps], [AutoKey], [TimeSec], [SecPTP], and [SecSen]. The threat
analysis in this document is mostly based on [TimeSec].
3.1. Threat Model
A time protocol can be attacked by various types of attackers.
The analysis in this document classifies attackers according to two
criteria, as described in Sections 3.1.1 and 3.1.2.
3.1.1. Internal vs. External Attackers
In the context of internal and external attackers, the underlying
assumption is that the time protocol is secured by either an
encryption mechanism, an authentication mechanism, or both.
Internal attackers either have access to a trusted segment of the
network or possess the encryption or authentication keys. An
internal attack can also be performed by exploiting vulnerabilities
in devices; for example, by installing malware or obtaining
credentials to reconfigure the device. 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.
Note that internal attacks are a special case of Byzantine failures,
where a node in the system may fail in arbitrary ways; by crashing,
by omitting messages, or by malicious behavior. This document
focuses on nodes that demonstrate malicious behavior.
External attackers, on the other hand, do not have the keys and have
access only to the encrypted or authenticated traffic.
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. It is assumed that an
MITM attacker has physical access to a segment of the network or has
gained control of one of the nodes in the network.
A traffic injector is not located in an MITM position, but can attack
by generating protocol packets. An injector can reside either within
the attacked network or on an external network that is connected to
the attacked network. An injector can also potentially eavesdrop on
protocol packets sent as multicast, record them, and replay them
later.
3.2. Threat Analysis
3.2.1. Packet Manipulation
A packet manipulation attack results when an MITM attacker receives
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 injector masquerades as a legitimate node in the
network by generating and transmitting protocol packets or control
packets. Two typical examples of spoofing attacks:
o An attacker can impersonate the master, allowing malicious
distribution of false timing information.
o An attacker can impersonate a legitimate clock, a slave, or an
intermediate clock, by sending malicious messages to the master,
causing the master to respond to the legitimate clock with
protocol packets that are based on the spoofed messages.
Consequently, the delay computations of the legitimate clock are
based on false information.
As with packet manipulation, this attack can result in a situation
where the time 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 rogue master attack the attacker does not
fake its identity, but rather manipulates the master election process
using malicious control packets. 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.
In PTP, a possible variant of this attack is the rogue TC/BC attack.
Similar to the rogue master attack, an attacker can cause victims to
believe it is a legitimate TC or BC, allowing the attacker to
manipulate the time information forwarded to the victims.
3.2.5. Packet Interception and Removal
A packet interception and removal attack results when an MITM
attacker intercepts and drops protocol packets, preventing the
destination node from receiving some or all of the protocol packets.
3.2.6. Packet Delay Manipulation
In a packet delay manipulation scenario, an MITM attacker receives
protocol packets and relays them to their destination after adding a
maliciously computed delay. The attacker can use various delay
attack strategies; the added delay can be constant, jittered, or
slowly wandering. Each of these strategies has a different impact,
but they all effectively manipulate the attacked clock.
Note that the victim still receives one copy of each packet, contrary
to the replay attack, where some or all of the packets may be
received by the victim more than once.
3.2.7. L2/L3 DoS Attacks
There are many possible Layer 2 and Layer 3 DoS attacks, e.g., IP
spoofing, ARP spoofing [Hack], MAC flooding [Anatomy], and many
others. As the target's availability is compromised, the timing
protocol is affected accordingly.
3.2.8. Cryptographic Performance Attacks
In cryptographic performance attacks, an attacker transmits fake
protocol packets, causing high utilization of the cryptographic
engine at the receiver, which attempts to verify the integrity of
these fake packets.
This DoS attack is applicable to all encryption and authentication
protocols. However, when the time protocol uses a dedicated security
mechanism implemented in a dedicated cryptographic engine, this
attack can be applied to cause DoS specifically to the time protocol.
3.2.9. DoS Attacks against the Time Protocol
An attacker can attack a clock by sending an excessive number of time
protocol packets, thus degrading the victim's performance. This
attack can be implemented, for example, using the attacks described
in Sections 3.2.2 and 3.2.4.
3.2.10. Grandmaster Time Source Attack (e.g., GPS Fraud)
Grandmasters receive their time from an external accurate time
source, such as an atomic clock or a GPS clock, and then distribute
this time to the slaves using the time protocol.
Time source attacks are aimed at the accurate time source of the
grandmaster. For example, if the grandmaster uses a GPS-based clock
as its reference source, an attacker can jam the reception of the GPS
signal, or transmit a signal similar to one from a GPS satellite,
causing the grandmaster to use a false reference time.
Note that this attack is outside the scope of the time protocol.
While various security measures can be taken to mitigate this attack,
these measures are outside the scope of the security requirements
defined in this document.
3.2.11. Exploiting Vulnerabilities in the Time Protocol
Time protocols can be attacked by exploiting vulnerabilities in the
protocol, implementation bugs, or misconfigurations (e.g.,
[NTPDDoS]). It should be noted that such attacks cannot typically be
mitigated by security mechanisms. However, when a new vulnerability
is discovered, operators should react as soon as possible, and take
the necessary measures to address it.
3.2.12. Network Reconnaissance
An attacker can exploit the time protocol to collect information such
as addresses and locations of nodes that take part in the protocol.
Reconnaissance can be applied by either passively eavesdropping on
protocol packets or sending malicious packets and gathering
information from the responses. By eavesdropping on a time protocol,
an attacker can learn the network latencies, which provide
information about the network topology and node locations.
Moreover, properties such as the frequency of the protocol packets,
or the exact times at which they are sent, can allow fingerprinting
of specific nodes; thus, protocol packets from a node can be
identified even if network addresses are hidden or encrypted.
3.3. Threat Analysis Summary
The two key factors to a threat analysis are the impact and the
likelihood of each of the analyzed attacks.
Table 1 summarizes the security attacks presented in Section 3.2.
For each attack, the table specifies its impact, and its
applicability to each of the attacker types presented in Section 3.1.
Table 1 clearly shows the distinction between external and internal
attackers, and motivates the usage of authentication and integrity
protection, significantly reducing the impact of external attackers.
The Impact column provides an intuitive measure of the severity of
each attack, and the relevant Attacker Type column provides an
intuition about 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 three values:
o DoS - the attack causes denial of service to the attacked node,
the impact of which is not restricted to the time protocol.
o Accuracy degradation - the attack yields a degradation in the
slave accuracy, but does not completely compromise the slaves'
time and frequency.
o False time - slaves align to a false time or frequency value due
to the attack. Note that if the time protocol aligns to a false
time, it may cause DoS 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 protocol. All
attacks that have a '+' for 'False Time' implicitly have a '+' for
'Accuracy Degradation'. Note that 'False Time' necessarily
implies 'Accuracy Degradation'. However, two different terms are
used, indicating two levels of severity.
The Attacker Type column refers to the four possible combinations of
the attacker types defined in Section 3.1.
+-----------------------------+-------------------++-------------------+
| Attack | Impact || Attacker Type |
| +-----+--------+----++---------+---------+
| |False|Accuracy| ||Internal |External |
| |Time |Degrad. |DoS ||MITM|Inj.|MITM|Inj.|
+-----------------------------+-----+--------+----++----+----+----+----+
|Manipulation | + | | || + | | | |
+-----------------------------+-----+--------+----++----+----+----+----+
|Spoofing | + | | || + | + | | |
+-----------------------------+-----+--------+----++----+----+----+----+
|Replay attack | + | | || + | + | | |
+-----------------------------+-----+--------+----++----+----+----+----+
|Rogue master attack | + | | || + | + | | |
+-----------------------------+-----+--------+----++----+----+----+----+
|Interception and removal | | + | + || + | | + | |
+-----------------------------+-----+--------+----++----+----+----+----+
|Packet delay manipulation | + | | || + | | + | |
+-----------------------------+-----+--------+----++----+----+----+----+
|L2/L3 DoS attacks | | | + || + | + | + | + |
+-----------------------------+-----+--------+----++----+----+----+----+
|Crypt. performance attacks | | | + || + | + | + | + |
+-----------------------------+-----+--------+----++----+----+----+----+
|Time protocol DoS attacks | | | + || + | + | | |
+-----------------------------+-----+--------+----++----+----+----+----+
|Master time source attack | + | | || + | + | + | + |
|(e.g., GPS spoofing) | | | || | | | |
+-----------------------------+-----+--------+----++----+----+----+----+
Table 1: Threat Analysis - Summary
The threats discussed in this section provide the background for the
security requirements presented in Section 5.
4. Requirement Levels
The security requirements are presented in Section 5. Each
requirement is defined with a requirement level, in accordance with
the requirement levels defined in Section 2.1.
The requirement levels in this document are affected by the following
factors:
o Impact:
The possible impact of not implementing the requirement, as
illustrated in the Impact column of Table 1. For example, a
requirement that addresses a threat that can be implemented by an
external injector is typically a 'MUST', since the threat can be
implemented by all the attacker types analyzed in Section 3.1.
o Difficulty of the corresponding attack:
The level of difficulty of the possible attacks that become
possible by not implementing the requirement. The level of
difficulty is reflected in the Attacker Type column of Table 1.
For example, a requirement that addresses a threat that only
compromises the availability of the protocol is typically no more
than a 'SHOULD'.
o Practical considerations:
Various practical factors that may affect the requirement. For
example, if a requirement is very difficult to implement, or is
applicable to very specific scenarios, these factors may reduce
the requirement level.
Section 5 lists the requirements. For each requirement, there is a
short explanation detailing the reason for its requirement level.
5. Security Requirements
This section defines a set of security requirements. 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 requirements can be
satisfied by defining explicit security mechanisms for 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.
5.1. Clock Identity Authentication and Authorization
Requirement
The security mechanism MUST support authentication.
Requirement
The security mechanism MUST support authorization.
Requirement Level
The requirements in this subsection address the spoofing attack
(Section 3.2.2) and the rogue master attack (Section 3.2.4).
The requirement level of these requirements is 'MUST' since, in
the absence of these requirements, the protocol is exposed to
attacks that are easy to implement and have a high impact.
Discussion
Authentication refers to verifying the identity of the peer clock.
Authorization, on the other hand, refers to 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.
Authentication is typically implemented by means of a
cryptographic signature, allowing the verification of the identity
of the sender. Authorization requires clocks to maintain a list
of authorized clocks, or a "black list" of clocks that should be
denied service or revoked.
It is noted that while the security mechanism is required to
provide an authorization mechanism, the deployment of such a
mechanism depends on the nature of the network. For example, a
network that deploys PTP may consist of a set of identical OCs,
where all clocks are equally permitted to be a master. In such a
network, an authorization mechanism may not be necessary.
The following subsections describe five distinct cases of clock
authentication.
5.1.1. Authentication and Authorization of Masters
Requirement
The security mechanism MUST support an authentication mechanism,
allowing slaves to authenticate the identity of masters.
Requirement
The authentication mechanism MUST allow slaves to verify that the
authenticated master is authorized to be a master.
Requirement Level
The requirements in this subsection address the spoofing attack
(Section 3.2.2) and the rogue master attack (Section 3.2.4).
The requirement level of these requirements is 'MUST' since, in
the absence of these requirements, the protocol is exposed to
attacks that are easy to implement and have a high impact.
Discussion
Clocks authenticate masters in order to ensure the authenticity of
the time source. It is important for a slave to verify the
identity of the master, as well as to verify that the master is
indeed authorized to be a master.
5.1.2. Recursive Authentication and Authorization of Masters (Chain of
Trust)
Requirement
The security mechanism MUST support recursive authentication and
authorization of the master, to be used in cases where time
information is conveyed through intermediate clocks.
Requirement Level
The requirement in this subsection addresses the spoofing attack
(Section 3.2.2) and the rogue master attack (Section 3.2.4).
The requirement level of this requirement is 'MUST' since, in the
absence of this requirement, the protocol is exposed to attacks
that are easy to implement and have a high impact.
Discussion
In some cases, a slave is connected to an intermediate clock that
is not the primary time source. For example, in PTP, a slave can
be connected to a Boundary Clock (BC) or a Transparent Clock (TC),
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 clock, and the
intermediate clock authenticates the grandmaster. This recursive
authentication process is referred to in [AutoKey] as
proventication.
Specifically in PTP, this requirement implies that if a slave
receives time information through a TC, it must authenticate the
TC to which it is attached, as well as authenticate the master
from which it receives the time information, as per Section 5.1.1.
Similarly, if a TC receives time information through an attached
TC, it must authenticate the attached TC.
5.1.3. Authentication and Authorization of Slaves
Requirement
The security mechanism MAY provide a means for a master to
authenticate its slaves.
Requirement
The security mechanism MAY provide a means for a master to verify
that the sender of a protocol packet is authorized to send a
packet of this type.
Requirement Level
The requirement in this subsection prevents DoS attacks against
the master (Section 3.2.9).
The requirement level of this requirement is 'MAY' since:
o Its impact is low, i.e., in the absence of this requirement the
protocol is only exposed to DoS.
o Practical considerations: requiring an NTP server to
authenticate its clients may significantly impose on the
server's performance.
Note that while the requirement level of this requirement is
'MAY', the requirement in Section 5.1.1 is 'MUST'; the security
mechanism must provide a means for authentication and
authorization, with an emphasis on the master. Authentication and
authorization of slaves are specified in this subsection as 'MAY'.
Discussion
Slaves and intermediate clocks are authenticated by masters in
order to verify that they are authorized to receive timing
services from the master.
Authentication of slaves prevents unauthorized clocks from
receiving time services. Preventing the master from serving
unauthorized clocks can help in mitigating DoS attacks against the
master. Note that the authentication of slaves might put a higher
load on the master than serving the unauthorized clock; hence,
this requirement is 'MAY'.
5.1.4. PTP: Authentication and Authorization of P2P TCs by the Master
Requirement
The security mechanism for PTP MAY provide a means for a master to
authenticate the identity of the P2P TCs directly connected to it.
Requirement
The security mechanism for PTP MAY provide a means for a master to
verify that P2P TCs directly connected to it are authorized to be
TCs.
Requirement Level
The requirement in this subsection prevents DoS attacks against
the master (Section 3.2.9).
The requirement level of this requirement is 'MAY' for the same
reasons specified in Section 5.1.3.
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 and peer TCs, by preventing the
master from serving unauthorized clocks.
5.1.5. PTP: Authentication and Authorization of Control Messages
Requirement
The security mechanism for PTP MUST support authentication of
Announce messages. The authentication mechanism MUST also verify
that the sender is authorized to be a master.
Requirement
The security mechanism for PTP MUST support authentication and
authorization of Management messages.
Requirement
The security mechanism MAY support authentication and
authorization of Signaling messages.
Requirement Level
The requirements in this subsection address the spoofing attack
(Section 3.2.2) and the rogue master attack (Section 3.2.4).
The requirement level of the first two requirements is 'MUST'
since, in the absence of these requirements, the protocol is
exposed to attacks that are easy to implement and have a high
impact.
The requirement level of the third requirement is 'MAY' since its
impact greatly depends on the application for which the Signaling
messages are used.
Discussion
Master election is performed in PTP using the Best Master Clock
Algorithm (BMCA). Each Ordinary Clock (OC) announces its clock
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 rogue
master attacks (Section 3.2.4). Note that this subsection
specifies a requirement that is not necessarily included in
Sections 5.1.1 or 5.1.3, since the BMCA is initiated before clocks
have been defined as masters or slaves.
Management messages are used to monitor or configure PTP clocks.
Malicious usage of Management messages enables various attacks,
such as the rogue master attack or DoS attack.
Signaling messages are used by PTP clocks to exchange information
that is not strictly related to time information or to master
selection, such as unicast negotiation. Authentication and
authorization of Signaling messages may be required in some
systems, depending on the application for which these messages are
used.
5.2. Protocol Packet Integrity
Requirement
The security mechanism MUST protect the integrity of protocol
packets.
Requirement Level
The requirement in this subsection addresses the packet
manipulation attack (Section 3.2.1).
The requirement level of this requirement is 'MUST' since, in the
absence of this requirement, the protocol is exposed to attacks
that are easy to implement and have high impact.
Discussion
While Section 5.1 refers to ensuring the identity an authorization
of the source of a 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.
Integrity protection is typically implemented by means of an
Integrity Check Value (ICV) that is included in protocol packets
and is verified by the receiver.
5.2.1. PTP: Hop-by-Hop vs. End-to-End Integrity Protection
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.
5.2.1.1. Hop-by-Hop Integrity Protection
Each hop that needs to modify a protocol packet:
o Verifies its integrity.
o Modifies the packet, i.e., modifies the correctionField. Note:
TCs improve the end-to-end accuracy by updating a correctionField
(Clause 6.5 in [IEEE1588]) in the PTP packet by adding the latency
caused by the current TC.
o Re-generates the integrity protection, e.g., re-computes a Message
Authentication Code (MAC).
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 rogue TCs to modify protocol
packets.
5.2.1.2. 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 directly 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
the correctionField to be protected separately from the other fields
of the packet, possibly using different cryptographic mechanisms and
keys.
5.3. Spoofing Prevention
Requirement
The security mechanism MUST provide a means to prevent master
spoofing.
Requirement
The security mechanism MUST provide a means to prevent slave
spoofing.
Requirement
PTP: The security mechanism MUST provide a means to prevent P2P TC
spoofing.
Requirement Level
The requirements in this subsection address spoofing attacks. As
described in Section 3.2.2, when these requirements are not met,
the attack may have a high impact, causing slaves to rely on false
time information. Thus, the requirement level is 'MUST'.
Discussion
Spoofing attacks may take various forms, and they can potentially
cause significant impact. In a master spoofing attack, the
attacker causes slaves to receive false information about the
current time by masquerading as the master.
By spoofing a slave or an intermediate node (the second example of
Section 3.2.2), an attacker can tamper with the slaves' delay
computations. These attacks can be mitigated by an authentication
mechanism (Sections 5.1.3 and 5.1.4) or by other means, for
example, a PTP Delay_Req can include a MAC that is included in the
corresponding Delay_Resp message, allowing the slave to verify
that the Delay_Resp was not sent in response to a spoofed message.
5.4. Availability
Requirement
The security mechanism SHOULD include measures to mitigate DoS
attacks against the time protocol.
Requirement Level
The requirement in this subsection prevents DoS attacks against
the protocol (Section 3.2.9).
The requirement level of this requirement is 'SHOULD' due to its
low impact, i.e., in the absence of this requirement the protocol
is only exposed to DoS.
Discussion
The protocol availability can be compromised by several different
attacks. An attacker can inject protocol packets to implement the
spoofing attack (Section 3.2.2) or the rogue master attack
(Section 3.2.4), causing DoS to the victim (Section 3.2.9).
An authentication mechanism (Section 5.1) limits these attacks
strictly to internal attackers; thus, it prevents external
attackers from performing them. Hence, the requirements of
Section 5.1 can be used to mitigate this attack. Note that
Section 5.1 addresses a wider range of threats, whereas the
current section is focused on availability.
The DoS attacks described in Section 3.2.7 are performed at lower
layers than the time protocol layer, and they are thus outside the
scope of the security requirements defined in this document.
5.5. Replay Protection
Requirement
The security mechanism MUST include a replay prevention mechanism.
Requirement Level
The requirement in this subsection prevents replay attacks
(Section 3.2.3).
The requirement level of this requirement is 'MUST' since, in the
absence of this requirement, the protocol is exposed to attacks
that are easy to implement and have a high impact.
Discussion
The replay attack (Section 3.2.3) can compromise both the
integrity and availability of the protocol. Common encryption and
authentication mechanisms include replay prevention mechanisms
that typically use a monotonously increasing packet sequence
number.
5.6. Cryptographic Keys and Security Associations
5.6.1. Key Freshness
Requirement
The security mechanism MUST provide a means to refresh the
cryptographic keys.
The cryptographic keys MUST be refreshed frequently.
Requirement Level
The requirement level of this requirement is 'MUST' since key
freshness is an essential property for cryptographic algorithms,
as discussed below.
Discussion
Key freshness guarantees that both sides share a common updated
secret key. It also helps in preventing replay attacks. Thus, it
is important for keys to be refreshed frequently. Note that the
term 'frequently' is used without a quantitative requirement, as
the precise frequency requirement should be considered on a per-
system basis, based on the threats and system requirements.
5.6.2. Security Association
Requirement
The security protocol SHOULD support a security association
protocol where:
o Two or more clocks authenticate each other.
o The clocks generate and agree on a cryptographic session
key.
Requirement
Each instance of the association protocol SHOULD produce a
different session key.
Requirement Level
The requirement level of this requirement is 'SHOULD' since it may
be expensive in terms of performance, especially in low-cost
clocks.
Discussion
The security requirements in Sections 5.1 and 5.2 require usage of
cryptographic mechanisms, deploying cryptographic keys. A
security association (e.g., [IPsec]) is an important building
block in these mechanisms.
It should be noted that in some cases, different security
association mechanisms may be used at different levels of clock
hierarchies. For example, the association between a Stratum 2
clock and a Stratum 3 clock in NTP may have different
characteristics than an association between two clocks at the same
stratum level. On a related note, in some cases, a hybrid
solution may be used, where a subset of the network is not secured
at all (see Section 5.10.2).
5.6.3. Unicast and Multicast Associations
Requirement
The security mechanism SHOULD support security association
protocols for unicast and for multicast associations.
Requirement Level
The requirement level of this requirement is 'SHOULD' since it may
be expensive in terms of performance, especially for low-cost
clocks.
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.
5.7. Performance
Requirement
The security mechanism MUST be designed in such a way that it does
not significantly degrade the quality of the time transfer.
Requirement
The mechanism SHOULD minimize computational load.
Requirement
The mechanism SHOULD minimize storage requirements of client state
in the master.
Requirement
The mechanism SHOULD minimize the bandwidth overhead required by
the security protocol.
Requirement Level
While the quality of the time transfer is clearly a 'MUST', the
other three performance requirements are 'SHOULD', since some
systems may be more sensitive to resource consumption than others;
hence, these requirements should be considered on a per-system
basis.
Discussion
Performance efficiency is important since client restrictions
often dictate a low processing and memory footprint and because
the server may have extensive fan-out.
Note that the performance requirements refer to a time-protocol-
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.
5.8. Confidentiality
Requirement
The security mechanism MAY provide confidentiality protection of
the protocol packets.
Requirement Level
The requirement level of this requirement is 'MAY' since the
absence of this requirement does not expose the protocol to severe
threats, as discussed below.
Discussion
In the context of time protocols, confidentiality is typically of
low importance, since timing information is usually not considered
secret information.
Confidentiality can play an important role when service providers
charge their customers for time synchronization services; thus, an
encryption mechanism can prevent eavesdroppers from obtaining the
service without payment. Note that these cases are, for now,
rather esoteric.
Confidentiality can also prevent an MITM attacker from identifying
protocol packets. Thus, confidentiality can assist in protecting
the timing protocol against MITM attacks such as packet delay
(Section 3.2.6), manipulation and interception, and removal
attacks. Note that time protocols have predictable behavior even
after encryption, such as packet transmission rates and packet
lengths. Additional measures can be taken to mitigate encrypted
traffic analysis by random padding of encrypted packets and by
adding random dummy packets. Nevertheless, encryption does not
prevent such MITM attacks, but rather makes these attacks more
difficult to implement.
5.9. Protection against Packet Delay and Interception Attacks
Requirement
The security mechanism MUST include means to protect the protocol
from MITM attacks that degrade the clock accuracy.
Requirement Level
The requirements in this subsection address MITM attacks such as
the packet delay attack (Section 3.2.6) and packet interception
attacks (Sections 3.2.5 and 3.2.1).
The requirement level of this requirement is 'MUST'. In the
absence of this requirement, the protocol is exposed to attacks
that are easy to implement and have a high impact. Note that in
the absence of this requirement, the impact is similar to packet
manipulation attacks (Section 3.2.1); thus, this requirement has
the same requirement level as integrity protection (Section 5.2).
It is noted that the implementation of this requirement depends on
the topology and properties of the system.
Discussion
While this document does not define specific security solutions,
we note that common practices for protection against MITM 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.
Thus, MITM attack prevention derives a requirement from the
security mechanism and a requirement from the network topology.
While the security mechanism should support the ability to detect
delay attacks, it is noted that in some networks it is not
possible to provide the redundancy needed for such a detection
mechanism.
5.10. Combining Secured with Unsecured Nodes
Integrating a security mechanism into a time-synchronized system is a
complex and expensive process, and hence in some cases may require
incremental deployment, where new equipment supports the security
mechanism, and is required to interoperate with legacy equipment
without the security features.
5.10.1. Secure Mode
Requirement
The security mechanism MUST support a secure mode, where only
secured clocks are permitted to take part in the time protocol.
In this mode every protocol packet received from an unsecured
clock MUST be discarded.
Requirement Level
The requirement level of this requirement is 'MUST' since the full
capacity of the security requirements defined in this document can
only be achieved in secure mode.
Discussion
While the requirement in this subsection is similar to the one in
Section 5.1, it refers to the secure mode, as opposed to the
hybrid mode presented in the next subsection.
5.10.2. Hybrid Mode
Requirement
The security protocol SHOULD support a hybrid mode, where both
secured and unsecured clocks are permitted to take part in the
protocol.
Requirement Level
The requirement level of this requirement is 'SHOULD'; on one
hand, hybrid mode enables a gradual transition from unsecured to
secured mode, which is especially important in large-scaled
deployments. On the other hand, hybrid mode is not required in
all systems; this document recommends deployment of the 'secure
mode' described in Section 5.10.1, where possible.
Discussion
The hybrid mode allows both secured and unsecured clocks to take
part in the time 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.
Requirement Level
The requirement level of this requirement is 'SHOULD' since it may
not be applicable to all deployments. For example, a hybrid
network may require the usage of unsecured masters or TCs.
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 slave can receive protocol packets from either
unsecured clocks or secured clocks. Note that the latter does not
apply when encryption is used. When integrity protection is used,
the unsecured slave can receive secured packets ignoring the
integrity protection.
Note that the security scheme in [NTPv4] with [AutoKey] does not
satisfy this requirement, since nodes prefer the server with the
most accurate clock, which is 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.
6. Summary of Requirements
+-----------+---------------------------------------------+--------+
| Section | Requirement | Type |
+-----------+---------------------------------------------+--------+
| 5.1 | Authentication & authorization of sender | MUST |
| +---------------------------------------------+--------+
| | Authentication & authorization of master | MUST |
| +---------------------------------------------+--------+
| | Recursive authentication & authorization | MUST |
| +---------------------------------------------+--------+
| | Authentication & authorization of slaves | MAY |
| +---------------------------------------------+--------+
| | PTP: Authentication & authorization of | MAY |
| | P2P TCs by master | |
+-----------+---------------------------------------------+--------+
+-----------+---------------------------------------------+--------+
|5.1 (cont) | PTP: Authentication & authorization of | MUST |
| | Announce messages | |
| +---------------------------------------------+--------+
| | PTP: Authentication & authorization of | MUST |
| | Management messages | |
| +---------------------------------------------+--------+
| | PTP: Authentication & authorization of | MAY |
| | Signaling messages | |
+-----------+---------------------------------------------+--------+
| 5.2 | Integrity protection | MUST |
+-----------+---------------------------------------------+--------+
| 5.3 | Spoofing prevention | MUST |
+-----------+---------------------------------------------+--------+
| 5.4 | Protection from DoS attacks against the | SHOULD |
| | time protocol | |
+-----------+---------------------------------------------+--------+
| 5.5 | Replay protection | MUST |
+-----------+---------------------------------------------+--------+
| 5.6 | Key freshness | MUST |
| +---------------------------------------------+--------+
| | Security association | SHOULD |
| +---------------------------------------------+--------+
| | Unicast and multicast associations | SHOULD |
+-----------+---------------------------------------------+--------+
| 5.7 | Performance: no degradation in quality of | MUST |
| | time transfer | |
| +---------------------------------------------+--------+
| | Performance: computation load | SHOULD |
| +---------------------------------------------+--------+
| | Performance: storage | SHOULD |
| +---------------------------------------------+--------+
| | Performance: bandwidth | SHOULD |
+-----------+---------------------------------------------+--------+
| 5.8 | Confidentiality protection | MAY |
+-----------+---------------------------------------------+--------+
| 5.9 | Protection against delay and interception | MUST |
| | attacks | |
+-----------+---------------------------------------------+--------+
| 5.10 | Secure mode | MUST |
| +---------------------------------------------+--------+
| | Hybrid mode | SHOULD |
+-----------+---------------------------------------------+--------+
Table 2: Summary of Security Requirements
7. Additional Security Implications
This section discusses additional implications of the interaction
between time protocols and security mechanisms.
This section refers to time protocol security mechanisms, as well as
to "external" security mechanisms, i.e., security mechanisms that are
not strictly related to the time protocol.
7.1. Security and On-the-Fly Timestamping
Time protocols often require that protocol packets be modified during
transmission. Both NTP and PTP in one-step mode require clocks to
modify protocol packets based on the time of transmission and/or
reception.
In the presence of a security mechanism, whether encryption or
integrity protection:
o During transmission the encryption and/or integrity protection
MUST be applied after integrating the timestamp into the packet.
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, it may introduce non-deterministic
latency that causes accuracy degradation. These performance aspects
have been analyzed in literature, e.g., [1588IPsec] and [Tunnel].
7.2. PTP: Security and Two-Step Timestamping
PTP supports a two-step mode of operation, where the time of
transmission of protocol packets is communicated without modifying
the packets. As opposed to one-step mode, two-step timestamping can
be performed without the requirement to encrypt after timestamping.
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 protocol packets.
7.3. Intermediate Clocks
A time 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. These intermediate
clocks are referred to as "secondary servers" in [NTPv4].
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, a hop-by-hop security scheme must be used, since
intermediate nodes must be able to send time information to the
slaves, or to modify time information sent through them.
This inherent property of using intermediate clocks increases the
system's exposure to internal threats, as a large number of nodes
possess the security keys.
Thus, there is a trade-off between the achievable clock accuracy of a
system, and the robustness of its security solution. On one hand,
high clock accuracy calls for hop-by-hop involvement in the protocol,
also known as on-path support. On the other hand, a robust security
solution calls for end-to-end data protection.
7.4. External Security Protocols and Time Protocols
Time 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 protocol traffic. This use-case
is thoroughly discussed in [IPsecSync].
Another typical example is the usage of MACsec encryption ([MACsec])
in L2 networks that deploy time synchronization [AvbAssum].
The usage of external security mechanisms may affect time protocols
as follows:
o Timestamping accuracy can be affected, as described in Section
7.1.
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 necessarily
transported over the underlying network without modification and,
thus, cannot enjoy the improved accuracy provided by intermediate
clock nodes.
7.5. External Security Services Requiring Time
Cryptographic protocols often use time as an important factor in the
cryptographic algorithm. If a time protocol is compromised, it may
consequently expose the security protocols that rely on it to various
attacks. Two examples are presented in this section.
7.5.1. Timestamped Certificates
Certificate validation requires the sender and receiver to be roughly
time synchronized. Thus, synchronization is required for
establishing security protocols such as Internet Key Exchange
Protocol version 2 (IKEv2) and Transport Layer Security (TLS). Other
authentication and key exchange mechanisms, such as Kerberos, also
require the parties involved to be synchronized [Kerb].
An even stronger interdependence between a time 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. This bootstrapping behavior
results from the fact that trusting the received time information
requires a valid certificate, and validating a certificate requires
knowledge of the time.
7.5.2. Time Changes and Replay Attacks
A successful attack on a time protocol may cause the attacked clocks
to go back in time. The erroneous time may expose cryptographic
algorithms that rely on time, as a node may use a key that was
already used in the past and has expired.
8. Issues for Further Discussion
The Key distribution is outside the scope of this document. Although
this is an essential element of any security system, it is outside
the scope of this document.
9. Security Considerations
The security considerations of network timing protocols are presented
throughout this document.
10. References
10.1. Normative References
[IEEE1588] IEEE, "1588-2008 - IEEE Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems", IEEE Standard 1588-2008, July 2008.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[NTPv4] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and
Algorithms Specification", RFC 5905, June 2010,
<http://www.rfc-editor.org/info/rfc5905>.
10.2. Informative References
[1588IPsec] Treytl, A. and 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, September 2010.
[3GPP] 3GPP, "Security of Home Node B (HNB) / Home evolved
Node B (HeNB)", 3GPP TS 33.320 11.6.0, November 2012.
[Anatomy] Nachreiner, C., "Anatomy of an ARP Poisoning Attack",
2003.
[AutoKey] Haberman, B., Ed., and D. Mills, "Network Time Protocol
Version 4: Autokey Specification", RFC 5906, June 2010,
<http://www.rfc-editor.org/info/rfc5906>.
[AvbAssum] Pannell, D., "Audio Video Bridging Gen 2 Assumptions",
IEEE 802.1 AVB Plenary, Work in Progress, May 2012.
[DelayAtt] Mizrahi, T., "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,
September 2012.
[Hack] McClure, S., Scambray, J., and G. Kurtz, "Hacking
Exposed: Network Security Secrets and Solutions",
McGraw-Hill, 2009.
[IPsec] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005,
<http://www.rfc-editor.org/info/rfc4301>.
[IPsecSync] Xu, Y., "IPsec security for packet based
synchronization", Work in Progress, draft-xu-tictoc-
ipsec-security-for-synchronization-02, September 2011.
[Kerb] Sakane, S., Kamada, K., Thomas, M., and J. Vilhuber,
"Kerberized Internet Negotiation of Keys (KINK)",
RFC 4430, March 2006,
<http://www.rfc-editor.org/info/rfc4430>.
[MACsec] IEEE, "IEEE Standard for Local and metropolitan area
networks - Media Access Control (MAC) Security", IEEE
Standard 802.1AE, August 2006.
[NTPDDoS] "Attackers use NTP reflection in huge DDoS attack",
TICTOC mail archive, 2014.
[SecPTP] Tsang, J. and K. Beznosov, "A Security Analysis of the
Precise Time Protocol (Short Paper)," 8th International
Conference on Information and Communication Security
(ICICS) Lecture Notes in Computer Science Volume 4307,
pp. 50-59, 2006.
[SecSen] Ganeriwal, S., Popper, C., Capkun, S., and M. B.
Srivastava, "Secure Time Synchronization in Sensor
Networks", ACM Trans. Inf. Syst. Secur., Volume 11,
Issue 4, Article 23, July 2008.
[TimeSec] Mizrahi, T., "Time synchronization security using IPsec
and MACsec", ISPCS 2011, pp. 38-43, September 2011.
[Traps] Treytl, A., Gaderer, G., Hirschler, B., and R. Cohen,
"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,
October 2007.
[Tunnel] Treytl, A., Hirschler, B., and T. Sauter, "Secure
tunneling of high-precision clock synchronisation
protocols and other time-stamped data", in Proceedings
of the 8th IEEE International Workshop on Factory
Communication Systems (WFCS), pp. 303-313, May 2010.
Acknowledgments
The author gratefully acknowledges Stefano Ruffini, Doug Arnold,
Kevin Gross, Dieter Sibold, Dan Grossman, Laurent Montini, Russell
Smiley, Shawn Emery, Dan Romascanu, Stephen Farrell, Kathleen
Moriarty, and Joel Jaeggli for their thorough review and helpful
comments. The author would also like to thank members of the TICTOC
WG for providing feedback on the TICTOC mailing list.
Contributors
Karen O'Donoghue
ISOC
EMail: odonoghue@isoc.org
Author's Address
Tal Mizrahi
Marvell
6 Hamada St.
Yokneam, 20692 Israel
EMail: talmi@marvell.com