Network Time Protocol J. Gruessing
Internet-Draft May 22, 2021
Intended status: Informational
Expires: November 23, 2021
NTPv5 use cases and requirements
draft-gruessing-ntp-ntpv5-requirements-02
Abstract
This document describes the use cases, requirements, and
considerations that should be factored in the design of a successor
protocol to supercede version 4 of the NTP protocol [RFC5905]
presently referred to as NTP version 5 ("NTPv5"). This document is
non-exhaustive and does not in its current version represent working
group consensus.
Note to Readers
_RFC Editor: please remove this section before publication_
Source code and issues for this draft can be found at
https://github.com/fiestajetsam/I-D/tree/main/draft-gruessing-ntp-
ntpv5-requirements [1].
Status of This Memo
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 2
2. Use cases and existing deployments of NTP . . . . . . . . . . 3
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Resource mamangement . . . . . . . . . . . . . . . . . . 3
3.2. Algorithms . . . . . . . . . . . . . . . . . . . . . . . 4
3.3. Timescales . . . . . . . . . . . . . . . . . . . . . . . 4
3.4. Leap seconds . . . . . . . . . . . . . . . . . . . . . . 4
3.5. Backwards compatibility to NTS and NTPv4 . . . . . . . . 4
3.6. Extensibility . . . . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . 6
6.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
NTP version 4 [RFC5905] has seen active use for over a decade, and
within this time period the protocol has not only been extended to
support new requirements but also fallen victim to vulnerabilities
that have made it used for distributed denial of service (DDoS)
amplification attacks.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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2. Use cases and existing deployments of NTP
There are several common scenarios for exxisting NTPv4 deployments;
publicly accessible NTP services such as the NTP Pool [ntppool] are
used to offer clock synchronisation for end users and embedded
devices, ISP provided servers to synchronise devices such as
customer-premesis equipment where reduced accuracy may be tollerable.
Depending on the network and path these deployments may be affected
by variable latency as well as throttling or blocking by providers.
Data centres and cloud computing providers also have deployed and
offer NTP services both for internal use and for customers,
particularly where the network is unable to offer or does not require
PTP [IEEE-1588-2008]. As these deployments are less likely to be
constrained by network latency or power the potential for higher
levels of accuracy and precision within the bounds of the protocol
are possible.
3. Requirements
At a high level, NTPv5 should be a protocol that is capable of
operating in both local networks and also over public internet
connections where packet loss, delay, and even filtering may occur.
3.1. Resource mamangement
Historically there have been many documented instances of NTP servers
taking a large increase in unauthorised traffic [ntp-misuse] and the
design of NTPv5 must ensure the risk of these can be minimised to the
fullest extent.
Servers SHOULD have a new identifier that peers use as reference,
this SHOULD NOT be a FQDN, an IP address or identifier tied to a
public certificate. Servers SHOULD be able to migrate and change
their identifiers as stratum topologies or network configuration
changes occur.
The specification MUST have support for servers to notify clients
that the service is unavailable, and clients MUST have clearly
defined behaviours honouring this signalling. In addition to this
servers SHOULD be able to communicate to clients that they should
reduce their query interval rate when the server is under high
bandwidth or has reduced capacity.
Clients SHOULD re-establish connections with servers at an interval
to prevent attempting to maintain connectivity to a dead host and
give network operators the ability to move traffic away from hosts in
a timely manner.
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3.2. Algorithms
Algorithms describing functions such as clock filtering, selection
and clustering SHOULD be omitted from the protocol specification; the
specification should instead only provide what is necessary to
describe protocol semantics and normative behaviours.
The working group should consider creating a separate informational
document to describe an algorithm to assist with implementation, and
to consider adopting future documents which describe new algorithms
as they are developed. Specifying client algorithms separately from
the protocol allows will allow NTPv5 to meet the needs of
applications with a variety of network properties and performance
requirements. It also allows for innovation in implementations
without sacrificing basic interoperability.
3.3. Timescales
The protocol SHOULD adopt a linear, monotonic timescale as the basis
for communicating time. The format should meet sufficient scale and
precision with resolution either meeting or exceeding NTPv4, and have
a rollover date sufficiently far enough into the future that the
protocol's complete obsolescence is most likely to occur first. The
timescale in addition to any other time sensitive information must be
sufficient to calculate representations of both UTC and TAI. Through
extensions the protocol SHOULD support additional timescale
representations outside of the main specification, and all
transmissions of time data SHALL indicate the timescale in use.
3.4. Leap seconds
Support for UTC leap second information MUST be included in the
protocol specification in order for clients to generate a UTC
representation but must be transmitted as separate information to the
timescale. The specification SHOULD also be capable of transmitting
upcoming leap seconds greater than 1 calendar day in advance.
Leap second smearing SHOULD NOT be part of the wire specification,
however this should not prevent implementors from applying leap
second smearing between the client and any clock it is training but
MUST NOT be applied to downstream clients.
3.5. Backwards compatibility to NTS and NTPv4
The support for compatibility with other protocols should not prevent
addressing issues that have previous caused issues in deployments or
cause ossification of the protocol.
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Protocol ossification MUST be addressed to prevent existing NTPv4
deployments which incorrectly respond to clients posing as NTPv5 from
causing issues. Forward prevention of ossification (for a potential
NTPv6 protocol in the future) should also be taken into
consideration.
The model for backward compatibility is servers that support mutliple
versions NTP and send a response in the same version as the request.
This does not preclude servers from acting as a client in one version
of NTP and a server in another.
3.6. Extensibility
The protocol MUST have the capability to be extended, and that
implementations MUST ignore unknown extensions. Unknown extensions
received by a server from a lower stratum server SHALL not be added
to response messages sent by the server receiving these extensions.
4. IANA Considerations
Considerations should be made about the future of the existing IANA
registry for NTPv4 parameters. If NTPv5 becomes incompatible with
these parameters a new registry SHOULD be created.
5. Security Considerations
Encryption and authentication MUST be provided by the protocol
specification as a default and MUST be resistent to downgrade
attacks. The encryption used must have agility, allowing for the
protocol to update as more secure cryptography becomes known and
vulnerabilities are discovered.
The specification MAY consider leaving room for middleboxes which may
deliberately modify packets in flight for legitimate purposes.
Thought must be given around how this will be incorporated into any
applicable trust model. Downgrading attacks that could lead to an
adversary disabling or removing encryption or authentication MUST NOT
be possible in the design of the protocol.
Detection and reporting of server malfeasence SHOULD remain out of
scope of this specification as [I-D.ietf-ntp-roughtime] already
provides this capability as a core functionality of the protocol.
6. References
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6.1. Normative References
[I-D.ietf-ntp-roughtime]
Malhotra, A., Langley, A., Ladd, W., and M. Dansarie,
"Roughtime", draft-ietf-ntp-roughtime-04 (work in
progress), February 2021.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC7808] Douglass, M. and C. Daboo, "Time Zone Data Distribution
Service", RFC 7808, DOI 10.17487/RFC7808, March 2016,
<https://www.rfc-editor.org/info/rfc7808>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
6.2. Informative References
[IEEE-1588-2008]
"IEEE Standard for a Precision Clock Synchronization
Protocol for Networked Measurement and Control Systems",
n.d..
[ntp-misuse]
"NTP server misuse and abuse", n.d.,
<https://en.wikipedia.org/wiki/
NTP_server_misuse_and_abuse>.
[ntppool] "pool.ntp.org: the internet cluster of ntp servers", n.d.,
<https://www.ntppool.org>.
6.3. URIs
[1] https://github.com/fiestajetsam/I-D/tree/main/draft-gruessing-
ntp-ntpv5-requirements
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Appendix A. Acknowledgements
The author would like to thank Doug Arnold and Hal Murray for
contributions to this document, and would like to acknowledge Daniel
Franke, Watson Ladd, Miroslav Lichvar for their existing documents
and ideas. The author would also like to thank Angelo Moriondo,
Franz Karl Achard, and Malcom McLean for providing the author with
motivation.
Author's Address
James Gruessing
Email: james.ietf@gmail.com
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