draft-ietf-ntp-using-nts-for-ntp-23.txt   draft-ietf-ntp-using-nts-for-ntp-24.txt 
NTP Working Group D. Franke NTP Working Group D. Franke
Internet-Draft Akamai Internet-Draft Akamai
Intended status: Standards Track D. Sibold Intended status: Standards Track D. Sibold
Expires: September 4, 2020 K. Teichel Expires: September 19, 2020 K. Teichel
PTB PTB
M. Dansarie M. Dansarie
R. Sundblad R. Sundblad
Netnod Netnod
March 3, 2020 March 18, 2020
Network Time Security for the Network Time Protocol Network Time Security for the Network Time Protocol
draft-ietf-ntp-using-nts-for-ntp-23 draft-ietf-ntp-using-nts-for-ntp-24
Abstract Abstract
This memo specifies Network Time Security (NTS), a mechanism for This memo specifies Network Time Security (NTS), a mechanism for
using Transport Layer Security (TLS) and Authenticated Encryption using Transport Layer Security (TLS) and Authenticated Encryption
with Associated Data (AEAD) to provide cryptographic security for the with Associated Data (AEAD) to provide cryptographic security for the
client-server mode of the Network Time Protocol (NTP). client-server mode of the Network Time Protocol (NTP).
NTS is structured as a suite of two loosely coupled sub-protocols. NTS is structured as a suite of two loosely coupled sub-protocols.
The first (NTS-KE) handles initial authentication and key The first (NTS-KE) handles initial authentication and key
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 4, 2020. This Internet-Draft will expire on September 19, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Protocol Overview . . . . . . . . . . . . . . . . . . . . 5 1.2. Protocol Overview . . . . . . . . . . . . . . . . . . . . 5
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 7 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 7
3. TLS profile for Network Time Security . . . . . . . . . . . . 7 3. TLS profile for Network Time Security . . . . . . . . . . . . 7
4. The NTS Key Establishment Protocol . . . . . . . . . . . . . 8 4. The NTS Key Establishment Protocol . . . . . . . . . . . . . 8
4.1. NTS-KE Record Types . . . . . . . . . . . . . . . . . . . 10 4.1. NTS-KE Record Types . . . . . . . . . . . . . . . . . . . 10
4.1.1. End of Message . . . . . . . . . . . . . . . . . . . 10 4.1.1. End of Message . . . . . . . . . . . . . . . . . . . 11
4.1.2. NTS Next Protocol Negotiation . . . . . . . . . . . . 11 4.1.2. NTS Next Protocol Negotiation . . . . . . . . . . . . 11
4.1.3. Error . . . . . . . . . . . . . . . . . . . . . . . . 11 4.1.3. Error . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1.4. Warning . . . . . . . . . . . . . . . . . . . . . . . 12 4.1.4. Warning . . . . . . . . . . . . . . . . . . . . . . . 12
4.1.5. AEAD Algorithm Negotiation . . . . . . . . . . . . . 12 4.1.5. AEAD Algorithm Negotiation . . . . . . . . . . . . . 12
4.1.6. New Cookie for NTPv4 . . . . . . . . . . . . . . . . 13 4.1.6. New Cookie for NTPv4 . . . . . . . . . . . . . . . . 13
4.1.7. NTPv4 Server Negotiation . . . . . . . . . . . . . . 13 4.1.7. NTPv4 Server Negotiation . . . . . . . . . . . . . . 13
4.1.8. NTPv4 Port Negotiation . . . . . . . . . . . . . . . 13 4.1.8. NTPv4 Port Negotiation . . . . . . . . . . . . . . . 14
4.2. Key Extraction (generally) . . . . . . . . . . . . . . . 14 4.2. Key Extraction (generally) . . . . . . . . . . . . . . . 14
5. NTS Extension Fields for NTPv4 . . . . . . . . . . . . . . . 14 5. NTS Extension Fields for NTPv4 . . . . . . . . . . . . . . . 15
5.1. Key Extraction (for NTPv4) . . . . . . . . . . . . . . . 14 5.1. Key Extraction (for NTPv4) . . . . . . . . . . . . . . . 15
5.2. Packet Structure Overview . . . . . . . . . . . . . . . . 15 5.2. Packet Structure Overview . . . . . . . . . . . . . . . . 15
5.3. The Unique Identifier Extension Field . . . . . . . . . . 15 5.3. The Unique Identifier Extension Field . . . . . . . . . . 16
5.4. The NTS Cookie Extension Field . . . . . . . . . . . . . 16 5.4. The NTS Cookie Extension Field . . . . . . . . . . . . . 16
5.5. The NTS Cookie Placeholder Extension Field . . . . . . . 16 5.5. The NTS Cookie Placeholder Extension Field . . . . . . . 17
5.6. The NTS Authenticator and Encrypted Extension Fields 5.6. The NTS Authenticator and Encrypted Extension Fields
Extension Field . . . . . . . . . . . . . . . . . . . . . 17 Extension Field . . . . . . . . . . . . . . . . . . . . . 17
5.7. Protocol Details . . . . . . . . . . . . . . . . . . . . 19 5.7. Protocol Details . . . . . . . . . . . . . . . . . . . . 19
6. Suggested Format for NTS Cookies . . . . . . . . . . . . . . 24 6. Suggested Format for NTS Cookies . . . . . . . . . . . . . . 23
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
7.1. Service Name and Transport Protocol Port Number Registry 25 7.1. Service Name and Transport Protocol Port Number Registry 25
7.2. TLS Application-Layer Protocol Negotiation (ALPN) 7.2. TLS Application-Layer Protocol Negotiation (ALPN)
Protocol IDs Registry . . . . . . . . . . . . . . . . . . 25 Protocol IDs Registry . . . . . . . . . . . . . . . . . . 25
7.3. TLS Exporter Labels Registry . . . . . . . . . . . . . . 26 7.3. TLS Exporter Labels Registry . . . . . . . . . . . . . . 26
7.4. NTP Kiss-o'-Death Codes Registry . . . . . . . . . . . . 26 7.4. NTP Kiss-o'-Death Codes Registry . . . . . . . . . . . . 26
7.5. NTP Extension Field Types Registry . . . . . . . . . . . 26 7.5. NTP Extension Field Types Registry . . . . . . . . . . . 26
7.6. Network Time Security Key Establishment Record Types 7.6. Network Time Security Key Establishment Record Types
Registry . . . . . . . . . . . . . . . . . . . . . . . . 27 Registry . . . . . . . . . . . . . . . . . . . . . . . . 27
7.7. Network Time Security Next Protocols Registry . . . . . . 28 7.7. Network Time Security Next Protocols Registry . . . . . . 28
7.8. Network Time Security Error and Warning Codes Registries 29 7.8. Network Time Security Error and Warning Codes Registries 29
8. Implementation Status - RFC EDITOR: REMOVE BEFORE PUBLICATION 30 8. Implementation Status - RFC EDITOR: REMOVE BEFORE PUBLICATION 30
8.1. Implementation 1 . . . . . . . . . . . . . . . . . . . . 31 8.1. Implementation 1 . . . . . . . . . . . . . . . . . . . . 30
8.1.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 31 8.1.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 30
8.1.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 31 8.1.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 31
8.1.3. Contact Information . . . . . . . . . . . . . . . . . 31 8.1.3. Contact Information . . . . . . . . . . . . . . . . . 31
8.1.4. Last Update . . . . . . . . . . . . . . . . . . . . . 31 8.1.4. Last Update . . . . . . . . . . . . . . . . . . . . . 31
8.2. Implementation 2 . . . . . . . . . . . . . . . . . . . . 31 8.2. Implementation 2 . . . . . . . . . . . . . . . . . . . . 31
8.2.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 31 8.2.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 31
8.2.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 32 8.2.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 31
8.2.3. Contact Information . . . . . . . . . . . . . . . . . 32 8.2.3. Contact Information . . . . . . . . . . . . . . . . . 31
8.2.4. Last Update . . . . . . . . . . . . . . . . . . . . . 32 8.2.4. Last Update . . . . . . . . . . . . . . . . . . . . . 31
8.3. Implementation 3 . . . . . . . . . . . . . . . . . . . . 32 8.3. Implementation 3 . . . . . . . . . . . . . . . . . . . . 32
8.3.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 32 8.3.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 32
8.3.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 32 8.3.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 32
8.3.3. Contact Information . . . . . . . . . . . . . . . . . 32 8.3.3. Contact Information . . . . . . . . . . . . . . . . . 32
8.3.4. Last Update . . . . . . . . . . . . . . . . . . . . . 32 8.3.4. Last Update . . . . . . . . . . . . . . . . . . . . . 32
8.4. Implementation 4 . . . . . . . . . . . . . . . . . . . . 33 8.4. Implementation 4 . . . . . . . . . . . . . . . . . . . . 32
8.4.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 33 8.4.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 32
8.4.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 33 8.4.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 33
8.4.3. Contact Information . . . . . . . . . . . . . . . . . 33 8.4.3. Contact Information . . . . . . . . . . . . . . . . . 33
8.4.4. Last Update . . . . . . . . . . . . . . . . . . . . . 33 8.4.4. Last Update . . . . . . . . . . . . . . . . . . . . . 33
8.5. Implementation 5 . . . . . . . . . . . . . . . . . . . . 33 8.5. Implementation 5 . . . . . . . . . . . . . . . . . . . . 33
8.5.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 33 8.5.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 33
8.5.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 34 8.5.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 33
8.5.3. Contact Information . . . . . . . . . . . . . . . . . 34 8.5.3. Contact Information . . . . . . . . . . . . . . . . . 33
8.5.4. Last Update . . . . . . . . . . . . . . . . . . . . . 34 8.5.4. Last Update . . . . . . . . . . . . . . . . . . . . . 33
8.6. Implementation 6 . . . . . . . . . . . . . . . . . . . . 34 8.6. Implementation 6 . . . . . . . . . . . . . . . . . . . . 33
8.6.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 34 8.6.1. Coverage . . . . . . . . . . . . . . . . . . . . . . 34
8.6.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 34 8.6.2. Licensing . . . . . . . . . . . . . . . . . . . . . . 34
8.6.3. Contact Information . . . . . . . . . . . . . . . . . 34 8.6.3. Contact Information . . . . . . . . . . . . . . . . . 34
8.6.4. Last Update . . . . . . . . . . . . . . . . . . . . . 34 8.6.4. Last Update . . . . . . . . . . . . . . . . . . . . . 34
8.7. Interoperability . . . . . . . . . . . . . . . . . . . . 34 8.7. Interoperability . . . . . . . . . . . . . . . . . . . . 34
9. Security Considerations . . . . . . . . . . . . . . . . . . . 35 9. Security Considerations . . . . . . . . . . . . . . . . . . . 34
9.1. Protected Modes . . . . . . . . . . . . . . . . . . . . . 35 9.1. Protected Modes . . . . . . . . . . . . . . . . . . . . . 34
9.2. Sensitivity to DDoS Attacks . . . . . . . . . . . . . . . 35 9.2. Sensitivity to DDoS Attacks . . . . . . . . . . . . . . . 35
9.3. Avoiding DDoS Amplification . . . . . . . . . . . . . . . 36 9.3. Avoiding DDoS Amplification . . . . . . . . . . . . . . . 35
9.4. Initial Verification of Server Certificates . . . . . . . 36 9.4. Initial Verification of Server Certificates . . . . . . . 36
9.5. Delay Attacks . . . . . . . . . . . . . . . . . . . . . . 37 9.5. Delay Attacks . . . . . . . . . . . . . . . . . . . . . . 37
9.6. Random Number Generation . . . . . . . . . . . . . . . . 38 9.6. NTS Stripping . . . . . . . . . . . . . . . . . . . . . . 37
9.7. NTS Stripping . . . . . . . . . . . . . . . . . . . . . . 38
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 38 10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 38
10.1. Unlinkability . . . . . . . . . . . . . . . . . . . . . 38 10.1. Unlinkability . . . . . . . . . . . . . . . . . . . . . 38
10.2. Confidentiality . . . . . . . . . . . . . . . . . . . . 39 10.2. Confidentiality . . . . . . . . . . . . . . . . . . . . 39
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 40 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 39
12.1. Normative References . . . . . . . . . . . . . . . . . . 40 12.1. Normative References . . . . . . . . . . . . . . . . . . 39
12.2. Informative References . . . . . . . . . . . . . . . . . 42 12.2. Informative References . . . . . . . . . . . . . . . . . 41
Appendix A. Terms and Abbreviations . . . . . . . . . . . . . . 43 Appendix A. Terms and Abbreviations . . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43
1. Introduction 1. Introduction
This memo specifies Network Time Security (NTS), a cryptographic This memo specifies Network Time Security (NTS), a cryptographic
security mechanism for network time synchronization. A complete security mechanism for network time synchronization. A complete
specification is provided for application of NTS to the client-server specification is provided for application of NTS to the client-server
mode of the Network Time Protocol (NTP) [RFC5905]. mode of the Network Time Protocol (NTP) [RFC5905].
1.1. Objectives 1.1. Objectives
The objectives of NTS are as follows: The objectives of NTS are as follows:
o Identity: Through the use of the X.509 public key infrastructure, o Identity: Through the use of a X.509 public key infrastructure,
implementations may cryptographically establish the identity of implementations may cryptographically establish the identity of
the parties they are communicating with. the parties they are communicating with.
o Authentication: Implementations may cryptographically verify that o Authentication: Implementations may cryptographically verify that
any time synchronization packets are authentic, i.e., that they any time synchronization packets are authentic, i.e., that they
were produced by an identified party and have not been modified in were produced by an identified party and have not been modified in
transit. transit.
o Confidentiality: Although basic time synchronization data is o Confidentiality: Although basic time synchronization data is
considered non-confidential and sent in the clear, NTS includes considered non-confidential and sent in the clear, NTS includes
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networks came from the same client. networks came from the same client.
o Non-amplification: Implementations (especially server o Non-amplification: Implementations (especially server
implementations) may avoid acting as distributed denial-of-service implementations) may avoid acting as distributed denial-of-service
(DDoS) amplifiers by never responding to a request with a packet (DDoS) amplifiers by never responding to a request with a packet
larger than the request packet. larger than the request packet.
o Scalability: Server implementations may serve large numbers of o Scalability: Server implementations may serve large numbers of
clients without having to retain any client-specific state. clients without having to retain any client-specific state.
o Performance: NTS must not significantly degrade the quality of the
time transfer. The encryption and authentication used when
actually transferring time should be lightweight (see RFC 7384,
Section 5.7 [RFC7384]).
1.2. Protocol Overview 1.2. Protocol Overview
The Network Time Protocol includes many different operating modes to The Network Time Protocol includes many different operating modes to
support various network topologies (see RFC 5905, Section 3 support various network topologies (see RFC 5905, Section 3
[RFC5905]). In addition to its best-known and most-widely-used [RFC5905]). In addition to its best-known and most-widely-used
client-server mode, it also includes modes for synchronization client-server mode, it also includes modes for synchronization
between symmetric peers, a control mode for server monitoring and between symmetric peers, a control mode for server monitoring and
administration, and a broadcast mode. These various modes have administration, and a broadcast mode. These various modes have
differing and partly contradictory requirements for security and differing and partly contradictory requirements for security and
performance. Symmetric and control modes demand mutual performance. Symmetric and control modes demand mutual
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establishing key material for use with the NTS Extension Fields establishing key material for use with the NTS Extension Fields
for NTPv4. It uses TLS to exchange keys, provide the client with for NTPv4. It uses TLS to exchange keys, provide the client with
an initial supply of cookies, and negotiate some additional an initial supply of cookies, and negotiate some additional
protocol options. After this exchange, the TLS channel is closed protocol options. After this exchange, the TLS channel is closed
with no per-client state remaining on the server side. with no per-client state remaining on the server side.
The typical protocol flow is as follows: The client connects to an The typical protocol flow is as follows: The client connects to an
NTS-KE server on the NTS TCP port and the two parties perform a TLS NTS-KE server on the NTS TCP port and the two parties perform a TLS
handshake. Via the TLS channel, the parties negotiate some handshake. Via the TLS channel, the parties negotiate some
additional protocol parameters and the server sends the client a additional protocol parameters and the server sends the client a
supply of cookies along with an IP address to the NTP server for supply of cookies along with an address of an NTP server for which
which the cookies are valid. The parties use TLS key export the cookies are valid. The parties use TLS key export [RFC5705] to
[RFC5705] to extract key material which will be used in the next extract key material which will be used in the next phase of the
phase of the protocol. This negotiation takes only a single round protocol. This negotiation takes only a single round trip, after
trip, after which the server closes the connection and discards all which the server closes the connection and discards all associated
associated state. At this point the NTS-KE phase of the protocol is state. At this point the NTS-KE phase of the protocol is complete.
complete. Ideally, the client never needs to connect to the NTS-KE Ideally, the client never needs to connect to the NTS-KE server
server again. again.
Time synchronization proceeds with one of the indicated NTP servers Time synchronization proceeds with the indicated NTP server over the
over the NTP UDP port. The client sends the server an NTP client NTP UDP port. The client sends the server an NTP client packet which
packet which includes several extension fields. Included among these includes several extension fields. Included among these fields are a
fields are a cookie (previously provided by the key exchange server) cookie (previously provided by the key exchange server) and an
and an authentication tag, computed using key material extracted from authentication tag, computed using key material extracted from the
the NTS-KE handshake. The NTP server uses the cookie to recover this NTS-KE handshake. The NTP server uses the cookie to recover this key
key material and send back an authenticated response. The response material and send back an authenticated response. The response
includes a fresh, encrypted cookie which the client then sends back includes a fresh, encrypted cookie which the client then sends back
in the clear in a subsequent request. (This constant refreshing of in the clear in a subsequent request. (This constant refreshing of
cookies is necessary in order to achieve NTS's unlinkability goal.) cookies is necessary in order to achieve NTS's unlinkability goal.)
Figure 1 provides an overview of the high-level interaction between Figure 1 provides an overview of the high-level interaction between
the client, the NTS-KE server, and the NTP server. Note that the the client, the NTS-KE server, and the NTP server. Note that the
cookies' data format and the exchange of secrets between NTS-KE and cookies' data format and the exchange of secrets between NTS-KE and
NTP servers are not part of this specification and are implementation NTP servers are not part of this specification and are implementation
dependent. However, a suggested format for NTS cookies is provided dependent. However, a suggested format for NTS cookies is provided
in Section 6. in Section 6.
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[[TBD1]]. The two endpoints carry out a TLS handshake in conformance [[TBD1]]. The two endpoints carry out a TLS handshake in conformance
with Section 3, with the client offering (via an ALPN [RFC7301] with Section 3, with the client offering (via an ALPN [RFC7301]
extension), and the server accepting, an application-layer protocol extension), and the server accepting, an application-layer protocol
of "ntske/1". Immediately following a successful handshake, the of "ntske/1". Immediately following a successful handshake, the
client SHALL send a single request as Application Data encapsulated client SHALL send a single request as Application Data encapsulated
in the TLS-protected channel. Then, the server SHALL send a single in the TLS-protected channel. Then, the server SHALL send a single
response followed by a TLS "Close notify" alert and then discard the response followed by a TLS "Close notify" alert and then discard the
channel state. channel state.
The client's request and the server's response each SHALL consist of The client's request and the server's response each SHALL consist of
a sequence of records formatted according to Figure 2. Requests and a sequence of records formatted according to Figure 2. The request
non-error responses each SHALL include exactly one NTS Next Protocol and a non-error response each SHALL include exactly one NTS Next
Negotiation record. The sequence SHALL be terminated by a "End of Protocol Negotiation record. The sequence SHALL be terminated by a
Message" record. The requirement that all NTS-KE messages be "End of Message" record. The requirement that all NTS-KE messages be
terminated by an End of Message record makes them self-delimiting. terminated by an End of Message record makes them self-delimiting.
Clients and servers MAY enforce length limits on requests and Clients and servers MAY enforce length limits on requests and
responses, however, servers MUST accept requests of at least 1024 responses, however, servers MUST accept requests of at least 1024
octets and clients SHOULD accept responses of at least 65536 octets. octets and clients SHOULD accept responses of at least 65536 octets.
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| Record Type | Body Length | |C| Record Type | Body Length |
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| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: NTS-KE Record Format Figure 2: NTS-KE Record Format
The fields of an NTS-KE record are defined as follows: The fields of an NTS-KE record are defined as follows:
C (Critical Bit): Determines the disposition of unrecognized C (Critical Bit): Determines the disposition of unrecognized
Record Types. Implementations which receive a record with an Record Types. Implementations which receive a record with an
unrecognized Record Type MUST ignore the record if the Critical unrecognized Record Type MUST ignore the record if the Critical
Bit is 0 and MUST treat it as an error if the Critical Bit is 1. Bit is 0 and MUST treat it as an error if the Critical Bit is 1
(see Section 4.1.3).
Record Type Number: A 15-bit integer in network byte order. The Record Type Number: A 15-bit integer in network byte order. The
semantics of record types 0-7 are specified in this memo. semantics of record types 0-7 are specified in this memo.
Additional type numbers SHALL be tracked through the IANA Network Additional type numbers SHALL be tracked through the IANA Network
Time Security Key Establishment Record Types registry. Time Security Key Establishment Record Types registry.
Body Length: The length of the Record Body field, in octets, as a Body Length: The length of the Record Body field, in octets, as a
16-bit integer in network byte order. Record bodies MAY have any 16-bit integer in network byte order. Record bodies MAY have any
representable length and need not be aligned to a word boundary. representable length and need not be aligned to a word boundary.
Record Body: The syntax and semantics of this field SHALL be Record Body: The syntax and semantics of this field SHALL be
determined by the Record Type. determined by the Record Type.
For clarity regarding bit-endianness: the Critical Bit is the most- For clarity regarding bit-endianness: the Critical Bit is the most-
significant bit of the first octet. In C, given a network buffer significant bit of the first octet. In the C programming language,
`unsigned char b[]` containing an NTS-KE record, the critical bit is given a network buffer `unsigned char b[]` containing an NTS-KE
`b[0] >> 7` while the record type is `((b[0] & 0x7f) << 8) + b[1]`. record, the critical bit is `b[0] >> 7` while the record type is
`((b[0] & 0x7f) << 8) + b[1]`.
Note that, although the Type-Length-Body format of an NTS-KE record Note that, although the Type-Length-Body format of an NTS-KE record
is similar to that of an NTP extension field, the semantics of the is similar to that of an NTP extension field, the semantics of the
length field differ. While the length subfield of an NTP extension length field differ. While the length subfield of an NTP extension
field gives the length of the entire extension field including the field gives the length of the entire extension field including the
type and length subfields, the length field of an NTS-KE record gives type and length subfields, the length field of an NTS-KE record gives
just the length of the body. just the length of the body.
Figure 3 provides a schematic overview of the key exchange. It Figure 3 provides a schematic overview of the key exchange. It
displays the protocol steps to be performed by the NTS client and displays the protocol steps to be performed by the NTS client and
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The NTS Next Protocol Negotiation record has a Record Type number of The NTS Next Protocol Negotiation record has a Record Type number of
1. It MUST occur exactly once in every NTS-KE request and response. 1. It MUST occur exactly once in every NTS-KE request and response.
Its body consists of a sequence of 16-bit unsigned integers in Its body consists of a sequence of 16-bit unsigned integers in
network byte order. Each integer represents a Protocol ID from the network byte order. Each integer represents a Protocol ID from the
IANA Network Time Security Next Protocols registry. The Critical Bit IANA Network Time Security Next Protocols registry. The Critical Bit
MUST be set. MUST be set.
The Protocol IDs listed in the client's NTS Next Protocol Negotiation The Protocol IDs listed in the client's NTS Next Protocol Negotiation
record denote those protocols which the client wishes to speak using record denote those protocols which the client wishes to speak using
the key material established through this NTS-KE session. The the key material established through this NTS-KE session. Protocol
Protocol IDs listed in the server's response MUST comprise a subset IDs listed in the NTS-KE server's response MUST comprise a subset of
of those listed in the request and denote those protocols which the those listed in the request and denote those protocols which the NTP
server is willing and able to speak using the key material server is willing and able to speak using the key material
established through this NTS-KE session. The client MAY proceed with established through this NTS-KE session. The client MAY proceed with
one or more of them. The request MUST list at least one protocol, one or more of them. The request MUST list at least one protocol,
but the response MAY be empty. but the response MAY be empty.
4.1.3. Error 4.1.3. Error
The Error record has a Record Type number of 2. Its body is exactly The Error record has a Record Type number of 2. Its body is exactly
two octets long, consisting of an unsigned 16-bit integer in network two octets long, consisting of an unsigned 16-bit integer in network
byte order, denoting an error code. The Critical Bit MUST be set. byte order, denoting an error code. The Critical Bit MUST be set.
Clients MUST NOT include Error records in their request. If clients Clients MUST NOT include Error records in their request. If clients
receive a server response which includes an Error record, they MUST receive a server response which includes an Error record, they MUST
discard any negotiated key material and MUST NOT proceed to the Next discard any key material negotiated during the initial TLS exchange
Protocol. and MUST NOT proceed to the Next Protocol.
The following error codes are defined: The following error codes are defined:
Error code 0 means "Unrecognized Critical Record". The server Error code 0 means "Unrecognized Critical Record". The server
MUST respond with this error code if the request included a record MUST respond with this error code if the request included a record
which the server did not understand and which had its Critical Bit which the server did not understand and which had its Critical Bit
set. The client SHOULD NOT retry its request without set. The client SHOULD NOT retry its request without
modification. modification.
Error code 1 means "Bad Request". The server MUST respond with Error code 1 means "Bad Request". The server MUST respond with
this error if, upon the expiration of an implementation-defined this error if the request is not complete and syntactically well-
timeout, it has not yet received a complete and syntactically formed, or, upon the expiration of an implementation-defined
well-formed request from the client. timeout, it has not yet received such a request.
Error code 2 means "Internal Server Error". The server MUST Error code 2 means "Internal Server Error". The server MUST
respond with this error if it is unable to respond properly due to respond with this error if it is unable to respond properly due to
an internal condition. an internal condition.
4.1.4. Warning 4.1.4. Warning
The Warning record has a Record Type number of 3. Its body is The Warning record has a Record Type number of 3. Its body is
exactly two octets long, consisting of an unsigned 16-bit integer in exactly two octets long, consisting of an unsigned 16-bit integer in
network byte order, denoting a warning code. The Critical Bit MUST network byte order, denoting a warning code. The Critical Bit MUST
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Clients MUST NOT send records of this type. Servers MUST send at Clients MUST NOT send records of this type. Servers MUST send at
least one record of this type, and SHOULD send eight of them, if the least one record of this type, and SHOULD send eight of them, if the
Next Protocol Negotiation response record contains Protocol ID 0 Next Protocol Negotiation response record contains Protocol ID 0
(NTPv4) and the AEAD Algorithm Negotiation response record is not (NTPv4) and the AEAD Algorithm Negotiation response record is not
empty. The Critical Bit SHOULD NOT be set. empty. The Critical Bit SHOULD NOT be set.
4.1.7. NTPv4 Server Negotiation 4.1.7. NTPv4 Server Negotiation
The NTPv4 Server Negotiation record has a Record Type number of 6. The NTPv4 Server Negotiation record has a Record Type number of 6.
Its body consists of an ASCII-encoded [ANSI.X3-4.1986] string. The Its body consists of an ASCII-encoded [RFC0020] string. The contents
contents of the string SHALL be either an IPv4 address, an IPv6 of the string SHALL be either an IPv4 address, an IPv6 address, or a
address, or a fully qualified domain name (FQDN). IPv4 addresses fully qualified domain name (FQDN). IPv4 addresses MUST be in dotted
MUST be in dotted decimal notation. IPv6 addresses MUST conform to decimal notation. IPv6 addresses MUST conform to the "Text
the "Text Representation of Addresses" as specified in RFC 4291 Representation of Addresses" as specified in RFC 4291 [RFC4291] and
[RFC4291] and MUST NOT include zone identifiers [RFC6874]. If a MUST NOT include zone identifiers [RFC6874]. If a label contains at
label contains at least one non-ASCII character, an A-LABEL MUST be least one non-ASCII character, it is an internationalized domain name
used as defined in section 2.3.2.1 of RFC 5890 [RFC5890]. and an A-LABEL MUST be used as defined in section 2.3.2.1 of RFC 5890
[RFC5890]. If the record contains a domain name, the recipient MUST
treat it as a FQDN, e.g. by making sure it ends with a dot.
When NTPv4 is negotiated as a Next Protocol and this record is sent When NTPv4 is negotiated as a Next Protocol and this record is sent
by the server, the body specifies the hostname or IP address of the by the server, the body specifies the hostname or IP address of the
NTPv4 server with which the client should associate and which will NTPv4 server with which the client should associate and which will
accept the supplied cookies. If no record of this type is sent, the accept the supplied cookies. If no record of this type is sent, the
client SHALL interpret this as a directive to associate with an NTPv4 client SHALL interpret this as a directive to associate with an NTPv4
server at the same IP address as the NTS-KE server. Servers MUST NOT server at the same IP address as the NTS-KE server. Servers MUST NOT
send more than one record of this type. send more than one record of this type.
When this record is sent by the client, it indicates that the client When this record is sent by the client, it indicates that the client
wishes to associate with the specified NTP server. The NTS-KE server wishes to associate with the specified NTP server. The NTS-KE server
MAY incorporate this request when deciding what NTPv4 Server MAY incorporate this request when deciding what NTPv4 Server
Negotiation records to respond with, but honoring the client's Negotiation records to respond with, but honoring the client's
preference is OPTIONAL. The client MUST NOT send more than one preference is OPTIONAL. The client MUST NOT send more than one
record of this type. record of this type.
If the client has sent a record of this type, the NTS-KE server
SHOULD reply with the same record if it is valid and the server is
able to supply cookies for it. If the client has not sent any record
of this type, the NTS-KE server SHOULD respond with either an NTP
server address in the same family as the NTS-KE session or a FQDN
that can be resolved to an address in that family, if such
alternatives are available.
Servers MAY set the Critical Bit on records of this type; clients Servers MAY set the Critical Bit on records of this type; clients
SHOULD NOT. SHOULD NOT.
4.1.8. NTPv4 Port Negotiation 4.1.8. NTPv4 Port Negotiation
The NTPv4 Port Negotiation record has a Record Type number of 7. Its The NTPv4 Port Negotiation record has a Record Type number of 7. Its
body consists of a 16-bit unsigned integer in network byte order, body consists of a 16-bit unsigned integer in network byte order,
denoting a UDP port number. denoting a UDP port number.
When NTPv4 is negotiated as a Next Protocol and this record is sent When NTPv4 is negotiated as a Next Protocol and this record is sent
skipping to change at page 15, line 42 skipping to change at page 16, line 11
authentication tag and possible ciphertext). The corresponding authentication tag and possible ciphertext). The corresponding
plaintext, if non-empty, consists of some extension fields which plaintext, if non-empty, consists of some extension fields which
benefit from both encryption and authentication. benefit from both encryption and authentication.
Possibly, some additional extension fields which are neither Possibly, some additional extension fields which are neither
encrypted nor authenticated. In general, these are discarded by encrypted nor authenticated. In general, these are discarded by
the receiver. the receiver.
Always included among the authenticated or authenticated-and- Always included among the authenticated or authenticated-and-
encrypted extension fields are a cookie extension field and a unique encrypted extension fields are a cookie extension field and a unique
identifier extension field. The purpose of the cookie extension identifier extension field, as described in Section 5.7. The purpose
field is to enable the server to offload storage of session state of the cookie extension field is to enable the server to offload
onto the client. The purpose of the unique identifier extension storage of session state onto the client. The purpose of the unique
field is to protect the client from replay attacks. identifier extension field is to protect the client from replay
attacks.
5.3. The Unique Identifier Extension Field 5.3. The Unique Identifier Extension Field
The Unique Identifier extension field provides the client with a The Unique Identifier extension field provides the client with a
cryptographically strong means of detecting replayed packets. It has cryptographically strong means of detecting replayed packets. It has
a Field Type of [[TBD2]]. When the extension field is included in a a Field Type of [[TBD2]]. When the extension field is included in a
client packet (mode 3), its body SHALL consist of a string of octets client packet (mode 3), its body SHALL consist of a string of octets
generated uniformly at random. The string MUST be at least 32 octets generated uniformly at random. The string MUST be at least 32 octets
long. When the extension field is included in a server packet (mode long. When the extension field is included in a server packet (mode
4), its body SHALL contain the same octet string as was provided in 4), its body SHALL contain the same octet string as was provided in
skipping to change at page 16, line 49 skipping to change at page 17, line 20
send additional cookies in its response. This extension field MUST send additional cookies in its response. This extension field MUST
NOT be included in NTP packets whose mode is other than 3. NOT be included in NTP packets whose mode is other than 3.
Whenever an NTS Cookie Placeholder extension field is present, it Whenever an NTS Cookie Placeholder extension field is present, it
MUST be accompanied by an NTS Cookie extension field. The body MUST be accompanied by an NTS Cookie extension field. The body
length of the NTS Cookie Placeholder extension field MUST be the same length of the NTS Cookie Placeholder extension field MUST be the same
as the body length of the NTS Cookie extension field. This length as the body length of the NTS Cookie extension field. This length
requirement serves to ensure that the response will not be larger requirement serves to ensure that the response will not be larger
than the request, in order to improve timekeeping precision and than the request, in order to improve timekeeping precision and
prevent DDoS amplification. The contents of the NTS Cookie prevent DDoS amplification. The contents of the NTS Cookie
Placeholder extension field's body are undefined and, aside from Placeholder extension field's body SHOULD be all zeros and, aside
checking its length, MUST be ignored by the server. from checking its length, MUST be ignored by the server.
5.6. The NTS Authenticator and Encrypted Extension Fields Extension 5.6. The NTS Authenticator and Encrypted Extension Fields Extension
Field Field
The NTS Authenticator and Encrypted Extension Fields extension field The NTS Authenticator and Encrypted Extension Fields extension field
is the central cryptographic element of an NTS-protected NTP packet. is the central cryptographic element of an NTS-protected NTP packet.
Its Field Type is [[TBD5]]. It SHALL be formatted according to Its Field Type is [[TBD5]]. It SHALL be formatted according to
Figure 4 and include the following fields: Figure 4 and include the following fields:
Nonce Length: Two octets in network byte order, giving the length Nonce Length: Two octets in network byte order, giving the length
of the Nonce field, excluding any padding, interpreted as an of the Nonce field, excluding any padding, interpreted as an
unsigned integer. unsigned integer.
Ciphertext Length: Two octets in network byte order, giving the Ciphertext Length: Two octets in network byte order, giving the
length of the Ciphertext field, excluding any padding, interpreted length of the Ciphertext field, excluding any padding, interpreted
as an unsigned integer. as an unsigned integer.
Nonce: A nonce as required by the negotiated AEAD Algorithm. The Nonce: A nonce as required by the negotiated AEAD Algorithm. The
field is zero-padded to a word (four octets) boundary. end of the field is zero-padded to a word (four octets) boundary.
Ciphertext: The output of the negotiated AEAD Algorithm. The Ciphertext: The output of the negotiated AEAD Algorithm. The
structure of this field is determined by the negotiated algorithm, structure of this field is determined by the negotiated algorithm,
but it typically contains an authentication tag in addition to the but it typically contains an authentication tag in addition to the
actual ciphertext. The field is zero-padded to a word (four actual ciphertext. The end of the field is zero-padded to a word
octets) boundary. (four octets) boundary.
Additional Padding: Clients which use a nonce length shorter than Additional Padding: Clients which use a nonce length shorter than
the maximum allowed by the negotiated AEAD algorithm may be the maximum allowed by the negotiated AEAD algorithm may be
required to include additional zero-padding. The necessary length required to include additional zero-padding. The necessary length
of this field is specified below. of this field is specified below.
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nonce Length | Ciphertext Length | | Nonce Length | Ciphertext Length |
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The NTS Authenticator and Encrypted Extension Fields extension field The NTS Authenticator and Encrypted Extension Fields extension field
MUST NOT be included in NTP packets whose mode is other than 3 MUST NOT be included in NTP packets whose mode is other than 3
(client) or 4 (server). (client) or 4 (server).
5.7. Protocol Details 5.7. Protocol Details
A client sending an NTS-protected request SHALL include the following A client sending an NTS-protected request SHALL include the following
extension fields as displayed in Figure 5: extension fields as displayed in Figure 5:
Exactly one Unique Identifier extension field which MUST be Exactly one Unique Identifier extension field which MUST be
authenticated, MUST NOT be encrypted, and whose contents MUST NOT authenticated, MUST NOT be encrypted, and whose contents MUST be
duplicate those of any previous request. the output of a cryptographically secure random number generator.
[RFC4086]
Exactly one NTS Cookie extension field which MUST be authenticated Exactly one NTS Cookie extension field which MUST be authenticated
and MUST NOT be encrypted. The cookie MUST be one which has been and MUST NOT be encrypted. The cookie MUST be one which has been
previously provided to the client; either from the key exchange previously provided to the client, either from the key exchange
server during the NTS-KE handshake or from the NTP server in server during the NTS-KE handshake or from the NTP server in
response to a previous NTS-protected NTP request. response to a previous NTS-protected NTP request.
Exactly one NTS Authenticator and Encrypted Extension Fields Exactly one NTS Authenticator and Encrypted Extension Fields
extension field, generated using an AEAD Algorithm and C2S key extension field, generated using an AEAD Algorithm and C2S key
established through NTS-KE. established through NTS-KE.
To protect the client's privacy, the client SHOULD avoid reusing a To protect the client's privacy, the client SHOULD avoid reusing a
cookie. If the client does not have any cookies that it has not cookie. If the client does not have any cookies that it has not
already sent, it SHOULD initiate a re-run the NTS-KE protocol. The already sent, it SHOULD initiate a re-run of the NTS-KE protocol.
client MAY reuse cookies in order to prioritize resilience over The client MAY reuse cookies in order to prioritize resilience over
unlinkability. Which of the two that should be prioritized in any unlinkability. Which of the two that should be prioritized in any
particular case is dependent on the application and the user's particular case is dependent on the application and the user's
preference. Section 10.1 describes the privacy considerations of preference. Section 10.1 describes the privacy considerations of
this in further detail. this in further detail.
The client MAY include one or more NTS Cookie Placeholder extension The client MAY include one or more NTS Cookie Placeholder extension
fields which MUST be authenticated and MAY be encrypted. The number fields which MUST be authenticated and MAY be encrypted. The number
of NTS Cookie Placeholder extension fields that the client includes of NTS Cookie Placeholder extension fields that the client includes
SHOULD be such that if the client includes N placeholders and the SHOULD be such that if the client includes N placeholders and the
server sends back N+1 cookies, the number of unused cookies stored by server sends back N+1 cookies, the number of unused cookies stored by
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fields will equal the number of dropped packets since the last fields will equal the number of dropped packets since the last
successful volley. successful volley.
In rare circumstances, it may be necessary to include fewer NTS In rare circumstances, it may be necessary to include fewer NTS
Cookie Placeholder extensions than recommended above in order to Cookie Placeholder extensions than recommended above in order to
prevent datagram fragmentation. When cookies adhere the format prevent datagram fragmentation. When cookies adhere the format
recommended in Section 6 and the AEAD in use is the mandatory-to- recommended in Section 6 and the AEAD in use is the mandatory-to-
implement AEAD_AES_SIV_CMAC_256, senders can include a cookie and implement AEAD_AES_SIV_CMAC_256, senders can include a cookie and
seven placeholders and still have packet size fall comfortably below seven placeholders and still have packet size fall comfortably below
1280 octets if no non-NTS-related extensions are used; 1280 octets is 1280 octets if no non-NTS-related extensions are used; 1280 octets is
the minimum prescribed MTU for IPv6 and is in practice also safe for the minimum prescribed MTU for IPv6 and is generally safe for
avoiding IPv4 fragmentation. Nonetheless, senders SHOULD include avoiding IPv4 fragmentation. Nonetheless, senders SHOULD include
fewer cookies and placeholders than otherwise indicated if doing so fewer cookies and placeholders than otherwise indicated if doing so
is necessary to prevent fragmentation. is necessary to prevent fragmentation.
+---------------------------------------+ +---------------------------------------+
| - Verify time request message | | - Verify time request message |
| - Generate time response message | | - Generate time response message |
| - Included NTPv4 extension fields | | - Included NTPv4 extension fields |
| o Unique Identifier EF | | o Unique Identifier EF |
| o NTS Authentication and | | o NTS Authentication and |
skipping to change at page 21, line 42 skipping to change at page 21, line 42
| o Unique Identifier EF | |- Extract cookie(s) | | o Unique Identifier EF | |- Extract cookie(s) |
| o NTS Cookie EF | |- Time synchronization | | o NTS Cookie EF | |- Time synchronization |
| o <NTS Cookie Placeholder EF> | | processing | | o <NTS Cookie Placeholder EF> | | processing |
| | +------------------------+ | | +------------------------+
|- Generate AEAD tag of NTP message| |- Generate AEAD tag of NTP message|
|- Add NTS Authentication and | |- Add NTS Authentication and |
| Encrypted Extension Fields EF | | Encrypted Extension Fields EF |
|- Transmit time request packet | |- Transmit time request packet |
+----------------------------------+ +----------------------------------+
Figure 5: NTS Time Synchronization Messages Figure 5: NTS-protected NTP Time Synchronization Messages
The client MAY include additional (non-NTS-related) extension fields The client MAY include additional (non-NTS-related) extension fields
which MAY appear prior to the NTS Authenticator and Encrypted which MAY appear prior to the NTS Authenticator and Encrypted
Extension Fields extension fields (therefore authenticated but not Extension Fields extension fields (therefore authenticated but not
encrypted), within it (therefore encrypted and authenticated), or encrypted), within it (therefore encrypted and authenticated), or
after it (therefore neither encrypted nor authenticated). In after it (therefore neither encrypted nor authenticated). The server
general, however, the server MUST discard any unauthenticated MUST discard any unauthenticated extension fields. Future
extension fields and process the packet as though they were not specifications of extension fields MAY provide exceptions to this
present. Servers MAY implement exceptions to this requirement for rule.
particular extension fields if their specification explicitly
provides for such.
Upon receiving an NTS-protected request, the server SHALL (through Upon receiving an NTS-protected request, the server SHALL (through
some implementation-defined mechanism) use the cookie to recover the some implementation-defined mechanism) use the cookie to recover the
AEAD Algorithm, C2S key, and S2C key associated with the request, and AEAD Algorithm, C2S key, and S2C key associated with the request, and
then use the C2S key to authenticate the packet and decrypt the then use the C2S key to authenticate the packet and decrypt the
ciphertext. If the cookie is valid and authentication and decryption ciphertext. If the cookie is valid and authentication and decryption
succeed, the server SHALL include the following extension fields in succeed, the server SHALL include the following extension fields in
its response: its response:
Exactly one Unique Identifier extension field which MUST be Exactly one Unique Identifier extension field which MUST be
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within the NTS Authenticator and Encrypted Extensions extension within the NTS Authenticator and Encrypted Extensions extension
field. While the body of an NTS Cookie extension field will field. While the body of an NTS Cookie extension field will
generally consist of some sort of AEAD output (regardless of whether generally consist of some sort of AEAD output (regardless of whether
the recommendations of Section 6 are precisely followed), this is not the recommendations of Section 6 are precisely followed), this is not
sufficient to make the extension field "encrypted". sufficient to make the extension field "encrypted".
The server MAY include additional (non-NTS-related) extension fields The server MAY include additional (non-NTS-related) extension fields
which MAY appear prior to the NTS Authenticator and Encrypted which MAY appear prior to the NTS Authenticator and Encrypted
Extension Fields extension field (therefore authenticated but not Extension Fields extension field (therefore authenticated but not
encrypted), within it (therefore encrypted and authenticated), or encrypted), within it (therefore encrypted and authenticated), or
after it (therefore neither encrypted nor authenticated). In after it (therefore neither encrypted nor authenticated). The client
general, however, the client MUST discard any unauthenticated MUST discard any unauthenticated extension fields. Future
extension fields and process the packet as though they were not specifications of extension fields MAY provide exceptions to this
present. Clients MAY implement exceptions to this requirement for rule.
particular extension fields if their specification explicitly
provides for such.
Upon receiving an NTS-protected response, the client MUST verify that Upon receiving an NTS-protected response, the client MUST verify that
the Unique Identifier matches that of an outstanding request, and the Unique Identifier matches that of an outstanding request, and
that the packet is authentic under the S2C key associated with that that the packet is authentic under the S2C key associated with that
request. If either of these checks fails, the packet MUST be request. If either of these checks fails, the packet MUST be
discarded without further processing. discarded without further processing.
If the server is unable to validate the cookie or authenticate the If the server is unable to validate the cookie or authenticate the
request, it SHOULD respond with a Kiss-o'-Death (KoD) packet (see RFC request, it SHOULD respond with a Kiss-o'-Death (KoD) packet (see RFC
5905, Section 7.4 [RFC5905]) with kiss code "NTSN", meaning "NTS 5905, Section 7.4 [RFC5905]) with kiss code "NTSN", meaning "NTS NAK"
negative-acknowledgment (NAK)". It MUST NOT include any NTS Cookie (NTS negative-acknowledgment). It MUST NOT include any NTS Cookie or
or NTS Authenticator and Encrypted Extension Fields extension fields. NTS Authenticator and Encrypted Extension Fields extension fields.
If the NTP server has previously responded with authentic NTS- If the NTP server has previously responded with authentic NTS-
protected NTP packets (i.e., packets containing the NTS Authenticator protected NTP packets, the client MUST verify that any KoD packets
and Encrypted Extension Fields extension field), the client MUST received from the server contain the Unique Identifier extension
verify that any KoD packets received from the server contain the field and that the Unique Identifier matches that of an outstanding
Unique Identifier extension field and that the Unique Identifier request. If this check fails, the packet MUST be discarded without
matches that of an outstanding request. If this check fails, the further processing. If this check passes, the client MUST comply
packet MUST be discarded without further processing. If this check with RFC 5905, Section 7.4 [RFC5905] where required. A client MAY
passes, the client MUST comply with RFC 5905, Section 7.4 [RFC5905] automatically re-run the NTS-KE protocol upon forced disassociation
where required. A client MAY automatically re-run the NTS-KE from an NTP server. In that case, it MUST be able to detect and stop
protocol upon forced disassociation from an NTP server. In that looping between the NTS-KE and NTP servers by rate limiting the
case, it MUST be able to detect and stop looping between the NTS-KE retries using e.g. exponential retry intervals.
and NTP servers by rate limiting the retries using e.g. exponential
retry intervals.
Upon reception of the NTS NAK kiss code, the client SHOULD wait until Upon reception of the NTS NAK kiss code, the client SHOULD wait until
the next poll for a valid NTS-protected response and if none is the next poll for a valid NTS-protected response and if none is
received, initiate a fresh NTS-KE handshake to try to renegotiate new received, initiate a fresh NTS-KE handshake to try to renegotiate new
cookies, AEAD keys, and parameters. If the NTS-KE handshake cookies, AEAD keys, and parameters. If the NTS-KE handshake
succeeds, the client MUST discard all old cookies and parameters and succeeds, the client MUST discard all old cookies and parameters and
use the new ones instead. As long as the NTS-KE handshake has not use the new ones instead. As long as the NTS-KE handshake has not
succeeded, the client SHOULD continue polling the NTP server using succeeded, the client SHOULD continue polling the NTP server using
the cookies and parameters it has. the cookies and parameters it has.
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Servers should select an AEAD algorithm which they will use to Servers should select an AEAD algorithm which they will use to
encrypt and authenticate cookies. The chosen algorithm should be one encrypt and authenticate cookies. The chosen algorithm should be one
such as AEAD_AES_SIV_CMAC_256 [RFC5297] which resists accidental such as AEAD_AES_SIV_CMAC_256 [RFC5297] which resists accidental
nonce reuse. It need not be the same as the one that was negotiated nonce reuse. It need not be the same as the one that was negotiated
with the client. Servers should randomly generate and store a master with the client. Servers should randomly generate and store a master
AEAD key `K`. Servers should additionally choose a non-secret, unique AEAD key `K`. Servers should additionally choose a non-secret, unique
value `I` as key-identifier for `K`. value `I` as key-identifier for `K`.
Servers should periodically (e.g., once daily) generate a new pair Servers should periodically (e.g., once daily) generate a new pair
(I,K) and immediately switch to using these values for all newly- (I,K) and immediately switch to using these values for all newly-
generated cookies. Immediately following each such key rotation, generated cookies. Following each such key rotation, servers should
servers should securely erase any keys generated two or more rotation securely erase any prevoiusly generated keys that should now be
periods prior. Servers should continue to accept any cookie expired, e.g. two or more rotation periods prior. Servers should
generated using keys that they have not yet erased, even if those continue to accept any cookie generated using keys that they have not
keys are no longer current. Erasing old keys provides for forward yet erased, even if those keys are no longer current. Erasing old
secrecy, limiting the scope of what old information can be stolen if keys provides for forward secrecy, limiting the scope of what old
a master key is somehow compromised. Holding on to a limited number information can be stolen if a master key is somehow compromised.
of old keys allows clients to seamlessly transition from one Holding on to a limited number of old keys allows clients to
generation to the next without having to perform a new NTS-KE seamlessly transition from one generation to the next without having
handshake. to perform a new NTS-KE handshake.
The need to keep keys synchronized between NTS-KE and NTP servers as The need to keep keys synchronized between NTS-KE and NTP servers as
well as across load-balanced clusters can make automatic key rotation well as across load-balanced clusters can make automatic key rotation
challenging. However, the task can be accomplished without the need challenging. However, the task can be accomplished without the need
for central key-management infrastructure by using a ratchet, i.e., for central key-management infrastructure by using a ratchet, i.e.,
making each new key a deterministic, cryptographically pseudo-random making each new key a deterministic, cryptographically pseudo-random
function of its predecessor. A recommended concrete implementation function of its predecessor. A recommended concrete implementation
of this approach is to use HKDF [RFC5869] to derive new keys, using of this approach is to use HKDF [RFC5869] to derive new keys, using
the key's predecessor as Input Keying Material and its key identifier the key's predecessor as Input Keying Material and its key identifier
as a salt. as a salt.
skipping to change at page 26, line 20 skipping to change at page 26, line 13
Reference: [[this memo]], Section 4 Reference: [[this memo]], Section 4
7.3. TLS Exporter Labels Registry 7.3. TLS Exporter Labels Registry
IANA is requested to allocate the following entry in the TLS Exporter IANA is requested to allocate the following entry in the TLS Exporter
Labels Registry [RFC5705]: Labels Registry [RFC5705]:
+--------------------+---------+-------------+---------------+------+ +--------------------+---------+-------------+---------------+------+
| Value | DTLS-OK | Recommended | Reference | Note | | Value | DTLS-OK | Recommended | Reference | Note |
+--------------------+---------+-------------+---------------+------+ +--------------------+---------+-------------+---------------+------+
| EXPORTER-network- | Y | Y | [[this | | | EXPORTER-network- | Y | N | [[this | |
| time-security/1 | | | memo]], | | | time-security/1 | | | memo]], | |
| | | | Section 4.2 | | | | | | Section 4.2 | |
+--------------------+---------+-------------+---------------+------+ +--------------------+---------+-------------+---------------+------+
7.4. NTP Kiss-o'-Death Codes Registry 7.4. NTP Kiss-o'-Death Codes Registry
IANA is requested to allocate the following entry in the registry of IANA is requested to allocate the following entry in the registry of
NTP Kiss-o'-Death Codes [RFC5905]: NTP Kiss-o'-Death Codes [RFC5905]:
+------+---------------------------------------+--------------------+ +------+---------------------------------------+--------------------+
skipping to change at page 28, line 6 skipping to change at page 27, line 38
The policy for allocation of new entries in this registry SHALL vary The policy for allocation of new entries in this registry SHALL vary
by the Record Type Number, as follows: by the Record Type Number, as follows:
0-1023: IETF Review 0-1023: IETF Review
1024-16383: Specification Required 1024-16383: Specification Required
16384-32767: Private and Experimental Use 16384-32767: Private and Experimental Use
Applications for new entries SHALL specify the contents of the Applications for new entries SHALL specify the contents of the
Description, Set Critical Bit, and Reference fields as well as which Description and Reference fields as well as which of the above ranges
of the above ranges the Record Type Number should be allocated from. the Record Type Number should be allocated from. Applicants MAY
Applicants MAY request a specific Record Type Number and such request a specific Record Type Number and such requests MAY be
requests MAY be granted at the registrar's discretion. granted at the registrar's discretion.
The initial contents of this registry SHALL be as follows: The initial contents of this registry SHALL be as follows:
+-------------+-------------------------+---------------------------+ +-------------+-------------------------+---------------------------+
| Record Type | Description | Reference | | Record Type | Description | Reference |
| Number | | | | Number | | |
+-------------+-------------------------+---------------------------+ +-------------+-------------------------+---------------------------+
| 0 | End of Message | [[this memo]], | | 0 | End of Message | [[this memo]], |
| | | Section 4.1.1 | | | | Section 4.1.1 |
| 1 | NTS Next Protocol | [[this memo]], | | 1 | NTS Next Protocol | [[this memo]], |
skipping to change at page 35, line 41 skipping to change at page 35, line 25
The main protection against these attacks in NTS lies in that the use The main protection against these attacks in NTS lies in that the use
of asymmetric cryptosystems is only necessary in the initial NTS-KE of asymmetric cryptosystems is only necessary in the initial NTS-KE
phase of the protocol. Since the protocol design enables separation phase of the protocol. Since the protocol design enables separation
of the NTS-KE and NTP servers, a successful DDoS attack on an NTS-KE of the NTS-KE and NTP servers, a successful DDoS attack on an NTS-KE
server separated from the NTP service it supports will not affect NTP server separated from the NTP service it supports will not affect NTP
users that have already performed initial authentication, AEAD key users that have already performed initial authentication, AEAD key
extraction, and cookie exchange. extraction, and cookie exchange.
NTS users should also consider that they are not fully protected NTS users should also consider that they are not fully protected
against DDoS attacks by on-path adversaries. In addition to dropping against DoS attacks by on-path adversaries. In addition to dropping
packets and attacks such as those described in Section 9.5, an on- packets and attacks such as those described in Section 9.5, an on-
path attacker can send spoofed kiss-o'-death replies, which are not path attacker can send spoofed kiss-o'-death replies, which are not
authenticated, in response to NTP requests. This could result in authenticated, in response to NTP requests. This could result in
significantly increased load on the NTS-KE server. Implementers have significantly increased load on the NTS-KE server. Implementers have
to weigh the user's need for unlinkability against the added to weigh the user's need for unlinkability against the added
resilience that comes with cookie reuse in cases of NTS-KE server resilience that comes with cookie reuse in cases of NTS-KE server
unavailability. unavailability.
9.3. Avoiding DDoS Amplification 9.3. Avoiding DDoS Amplification
skipping to change at page 38, line 13 skipping to change at page 37, line 43
client will tolerate before concluding that the server is unsuitable client will tolerate before concluding that the server is unsuitable
for synchronization. The standard value for MAXDIST is one second, for synchronization. The standard value for MAXDIST is one second,
although some implementations use larger values. Whatever value a although some implementations use larger values. Whatever value a
client chooses, the maximum error which can be introduced by a delay client chooses, the maximum error which can be introduced by a delay
attack is MAXDIST/2. attack is MAXDIST/2.
Usage of multiple time sources, or multiple network paths to a given Usage of multiple time sources, or multiple network paths to a given
time source [Shpiner], may also serve to mitigate delay attacks if time source [Shpiner], may also serve to mitigate delay attacks if
the adversary is in control of only some of the paths. the adversary is in control of only some of the paths.
9.6. Random Number Generation 9.6. NTS Stripping
At various points in NTS, the generation of cryptographically secure
random numbers is required. Whenever this draft specifies the use of
random numbers, cryptographically secure random number generation
MUST be used. RFC 4086 [RFC4086] contains guidelines concerning this
topic.
9.7. NTS Stripping
Implementers must be aware of the possibility of "NTS stripping" Implementers must be aware of the possibility of "NTS stripping"
attacks, where an attacker tricks clients into reverting to plain attacks, where an attacker tricks clients into reverting to plain
NTP. Naive client implementations might, for example, revert NTP. Naive client implementations might, for example, revert
automatically to plain NTP if the NTS-KE handshake fails. A man-in- automatically to plain NTP if the NTS-KE handshake fails. A man-in-
the-middle attacker can easily cause this to happen. Even clients the-middle attacker can easily cause this to happen. Even clients
that already hold valid cookies can be vulnerable, since an attacker that already hold valid cookies can be vulnerable, since an attacker
can force a client to repeat the NTS-KE handshake by sending faked can force a client to repeat the NTS-KE handshake by sending faked
NTP mode 4 replies with the NTS NAK kiss code. Forcing a client to NTP mode 4 replies with the NTS NAK kiss code. Forcing a client to
repeat the NTS-KE handshake can also be the first step in more repeat the NTS-KE handshake can also be the first step in more
skipping to change at page 39, line 16 skipping to change at page 38, line 37
data that could be used to link a device's network address. NTS does data that could be used to link a device's network address. NTS does
not rectify legacy linkability issues that are already present in not rectify legacy linkability issues that are already present in
NTP. Thus, a client that requires unlinkability must also minimize NTP. Thus, a client that requires unlinkability must also minimize
information transmitted in a client query (mode 3) packet as information transmitted in a client query (mode 3) packet as
described in the draft [I-D.ietf-ntp-data-minimization]. described in the draft [I-D.ietf-ntp-data-minimization].
The unlinkability objective only holds for time synchronization The unlinkability objective only holds for time synchronization
traffic, as opposed to key exchange traffic. This implies that it traffic, as opposed to key exchange traffic. This implies that it
cannot be guaranteed for devices that function not only as time cannot be guaranteed for devices that function not only as time
clients, but also as time servers (because the latter can be clients, but also as time servers (because the latter can be
externally triggered to send authentication data). externally triggered to send linkable data, such as the TLS
certificate).
It should also be noted that it could be possible to link devices It should also be noted that it could be possible to link devices
that operate as time servers from their time synchronization traffic, that operate as time servers from their time synchronization traffic,
using information exposed in (mode 4) server response packets (e.g. using information exposed in (mode 4) server response packets (e.g.
reference ID, reference time, stratum, poll). Also, devices that reference ID, reference time, stratum, poll). Also, devices that
respond to NTP control queries could be linked using the information respond to NTP control queries could be linked using the information
revealed by control queries. revealed by control queries.
Note that the unlinkability objective does not prevent a client Note that the unlinkability objective does not prevent a client
device to be tracked by its time servers. device to be tracked by its time servers.
skipping to change at page 39, line 48 skipping to change at page 39, line 25
the client making the request. the client making the request.
Future extension fields could hypothetically contain sensitive Future extension fields could hypothetically contain sensitive
information, in which case NTS provides a mechanism for encrypting information, in which case NTS provides a mechanism for encrypting
them. them.
11. Acknowledgements 11. Acknowledgements
The authors would like to thank Richard Barnes, Steven Bellovin, The authors would like to thank Richard Barnes, Steven Bellovin,
Patrik Faeltstroem (Faltstrom), Scott Fluhrer, Sharon Goldberg, Russ Patrik Faeltstroem (Faltstrom), Scott Fluhrer, Sharon Goldberg, Russ
Housley, Martin Langer, Miroslav Lichvar, Aanchal Malhotra, Dave Housley, Benjamin Kaduk, Suresh Krishnan, Martin Langer, Barry Leiba,
Mills, Danny Mayer, Karen O'Donoghue, Eric K. Rescorla, Stephen Miroslav Lichvar, Aanchal Malhotra, Dave Mills, Sandra Murphy, Danny
Roettger, Kurt Roeckx, Kyle Rose, Rich Salz, Brian Sniffen, Susan Mayer, Karen O'Donoghue, Eric K. Rescorla, Stephen Roettger, Kurt
Roeckx, Dan Romascanu, Kyle Rose, Rich Salz, Brian Sniffen, Susan
Sons, Douglas Stebila, Harlan Stenn, Joachim Stroembergsson Sons, Douglas Stebila, Harlan Stenn, Joachim Stroembergsson
(Strombergsson), Martin Thomson, Richard Welty, and Christer Weinigel (Strombergsson), Martin Thomson, Eric (Eric) Vyncke, Richard Welty,
for contributions to this document and comments on the design of NTS. Christer Weinigel, and Magnus Westerlund for contributions to this
document and comments on the design of NTS.
12. References 12. References
12.1. Normative References 12.1. Normative References
[ANSI.X3-4.1986]
American National Standards Institute, "Coded Character
Set - 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[IANA-AEAD] [IANA-AEAD]
IANA, "Authenticated Encryption with Associated Data IANA, "Authenticated Encryption with Associated Data
(AEAD) Parameters", (AEAD) Parameters",
<https://www.iana.org/assignments/aead-parameters/>. <https://www.iana.org/assignments/aead-parameters/>.
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>. 2006, <https://www.rfc-editor.org/info/rfc4291>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<https://www.rfc-editor.org/info/rfc5116>. <https://www.rfc-editor.org/info/rfc5116>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5297] Harkins, D., "Synthetic Initialization Vector (SIV) [RFC5297] Harkins, D., "Synthetic Initialization Vector (SIV)
Authenticated Encryption Using the Advanced Encryption Authenticated Encryption Using the Advanced Encryption
Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October Standard (AES)", RFC 5297, DOI 10.17487/RFC5297, October
2008, <https://www.rfc-editor.org/info/rfc5297>. 2008, <https://www.rfc-editor.org/info/rfc5297>.
[RFC5705] Rescorla, E., "Keying Material Exporters for Transport [RFC5705] Rescorla, E., "Keying Material Exporters for Transport
Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705, Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
March 2010, <https://www.rfc-editor.org/info/rfc5705>. March 2010, <https://www.rfc-editor.org/info/rfc5705>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.
[RFC5890] Klensin, J., "Internationalized Domain Names for [RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework", Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010, RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/info/rfc5890>. <https://www.rfc-editor.org/info/rfc5890>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms "Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>. <https://www.rfc-editor.org/info/rfc5905>.
skipping to change at page 41, line 29 skipping to change at page 41, line 22
[RFC6874] Carpenter, B., Cheshire, S., and R. Hinden, "Representing [RFC6874] Carpenter, B., Cheshire, S., and R. Hinden, "Representing
IPv6 Zone Identifiers in Address Literals and Uniform IPv6 Zone Identifiers in Address Literals and Uniform
Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874, Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874,
February 2013, <https://www.rfc-editor.org/info/rfc6874>. February 2013, <https://www.rfc-editor.org/info/rfc6874>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan, [RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol "Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301, Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>. July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
<https://www.rfc-editor.org/info/rfc7507>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer "Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>. 2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC7822] Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4 [RFC7822] Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
(NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822, (NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
March 2016, <https://www.rfc-editor.org/info/rfc7822>. March 2016, <https://www.rfc-editor.org/info/rfc7822>.
skipping to change at page 42, line 15 skipping to change at page 41, line 50
12.2. Informative References 12.2. Informative References
[I-D.ietf-ntp-data-minimization] [I-D.ietf-ntp-data-minimization]
Franke, D. and A. Malhotra, "NTP Client Data Franke, D. and A. Malhotra, "NTP Client Data
Minimization", draft-ietf-ntp-data-minimization-04 (work Minimization", draft-ietf-ntp-data-minimization-04 (work
in progress), March 2019. in progress), March 2019.
[Mizrahi] Mizrahi, T., "A game theoretic analysis of delay attacks [Mizrahi] Mizrahi, T., "A game theoretic analysis of delay attacks
against time synchronization protocols", in Proceedings against time synchronization protocols", in Proceedings
of Precision Clock Synchronization for Measurement Control of Precision Clock Synchronization for Measurement Control
and Communication, ISPCS 2012, pp. 1-6, September 2012. and Communication, ISPCS 2012, pp. 1-6
DOI 10.1109/ISPCS.2012.6336612, September 2012.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086, "Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005, DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>. <https://www.rfc-editor.org/info/rfc4086>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without "Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077, Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>. January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.
[RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in [RFC7384] Mizrahi, T., "Security Requirements of Time Protocols in
Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384, Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
October 2014, <https://www.rfc-editor.org/info/rfc7384>. October 2014, <https://www.rfc-editor.org/info/rfc7384>.
[RFC8573] Malhotra, A. and S. Goldberg, "Message Authentication Code [RFC8573] Malhotra, A. and S. Goldberg, "Message Authentication Code
for the Network Time Protocol", RFC 8573, for the Network Time Protocol", RFC 8573,
DOI 10.17487/RFC8573, June 2019, DOI 10.17487/RFC8573, June 2019,
<https://www.rfc-editor.org/info/rfc8573>. <https://www.rfc-editor.org/info/rfc8573>.
[Shpiner] Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time [Shpiner] Shpiner, A., Revah, Y., and T. Mizrahi, "Multi-path Time
Protocols", in Proceedings of IEEE International Symposium Protocols", in Proceedings of IEEE International Symposium
on Precision Clock Synchronization for Measurement, on Precision Clock Synchronization for Measurement,
Control and Communication (ISPCS), September 2013. Control and Communication (ISPCS)
DOI 10.1109/ISPCS.2013.6644754, September 2013.
Appendix A. Terms and Abbreviations Appendix A. Terms and Abbreviations
AEAD Authenticated Encryption with Associated Data [RFC5116] AEAD Authenticated Encryption with Associated Data [RFC5116]
ALPN Application-Layer Protocol Negotiation [RFC7301] ALPN Application-Layer Protocol Negotiation [RFC7301]
C2S Client-to-server C2S Client-to-server
DoS Denial-of-Service
DDoS Distributed Denial-of-Service DDoS Distributed Denial-of-Service
EF Extension Field [RFC5905] EF Extension Field [RFC5905]
HKDF Hashed Message Authentication Code-based Key Derivation HKDF Hashed Message Authentication Code-based Key Derivation
Function [RFC5869] Function [RFC5869]
KoD Kiss-o'-Death [RFC5905] KoD Kiss-o'-Death [RFC5905]
NTP Network Time Protocol [RFC5905] NTP Network Time Protocol [RFC5905]
NTS Network Time Security NTS Network Time Security
NTS NAK NTS negative-acknowledgment
NTS-KE Network Time Security Key Exchange NTS-KE Network Time Security Key Exchange
PRF Pseudorandom Function PRF Pseudorandom Function
S2C Server-to-client S2C Server-to-client
SCSV Signaling Cipher Suite Value [RFC7507]
TLS Transport Layer Security [RFC8446] TLS Transport Layer Security [RFC8446]
Authors' Addresses Authors' Addresses
Daniel Fox Franke Daniel Fox Franke
Akamai Technologies Akamai Technologies
150 Broadway 145 Broadway
Cambridge, MA 02142 Cambridge, MA 02142
United States United States
Email: dafranke@akamai.com Email: dafranke@akamai.com
URI: https://www.dfranke.us
Dieter Sibold Dieter Sibold
Physikalisch-Technische Physikalisch-Technische
Bundesanstalt Bundesanstalt
Bundesallee 100 Bundesallee 100
Braunschweig D-38116 Braunschweig D-38116
Germany Germany
Phone: +49-(0)531-592-8420 Phone: +49-(0)531-592-8420
Fax: +49-531-592-698420 Fax: +49-531-592-698420
Email: dieter.sibold@ptb.de Email: dieter.sibold@ptb.de
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