NTPv5 Use Cases and Requirements
draft-ietf-ntp-ntpv5-requirements-04

Network Time Protocol                                       J. Gruessing
Internet-Draft                               Nederlandse Publieke Omroep
Intended status: Informational                           25 January 2024
Expires: 28 July 2024

                    NTPv5 Use Cases and Requirements
                  draft-ietf-ntp-ntpv5-requirements-04

Abstract

   This document describes the use cases, requirements, and
   considerations that should be factored in the design of a successor
   protocol to supersede version 4 of the NTP protocol presently
   referred to as NTP version 5 ("NTPv5").  It aims to define what
   capabilities and requirements such a protocol possesses, informing
   the design of the protocol in addition to capturing any working group
   consensus made in development.

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/draft-gruessing-ntp-
   ntpv5-requirements (https://github.com/fiestajetsam/draft-gruessing-
   ntp-ntpv5-requirements).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 28 July 2024.

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Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Use Cases and Existing Deployments of NTP . . . . . . . . . .   3
   3.  Threat Analysis and Modeling  . . . . . . . . . . . . . . . .   4
     3.1.  Denial of Service and Amplification . . . . . . . . . . .   4
     3.2.  Accuracy Degradation  . . . . . . . . . . . . . . . . . .   4
     3.3.  False Time  . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Resource Management . . . . . . . . . . . . . . . . . . .   5
     4.2.  Data Minimisation . . . . . . . . . . . . . . . . . . . .   6
     4.3.  Algorithms  . . . . . . . . . . . . . . . . . . . . . . .   6
     4.4.  Timescales  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.5.  Leap seconds  . . . . . . . . . . . . . . . . . . . . . .   7
     4.6.  Backwards Compatibility with NTS and NTPv4  . . . . . . .   7
       4.6.1.  Dependent Specifications  . . . . . . . . . . . . . .   8
     4.7.  Extensibility . . . . . . . . . . . . . . . . . . . . . .   8
     4.8.  Security  . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Non-requirements  . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Server Malfeasance Detection  . . . . . . . . . . . . . .   9
     5.2.  Additional Time Information and Metadata  . . . . . . . .   9
     5.3.  Remote Monitoring Support . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

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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 has also fallen victim to
   vulnerabilities that have been used for distributed denial of service
   (DDoS) amplification attacks.  In order to advance the protocol and
   address these known issues alongside add capabilities for future
   usage this document defines the current known and applicable use
   cases in existing NTPv4 deployments and defines requirements for the
   future.

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.

   Use of time specific terminology used in this document may further be
   specified in [RFC7384] or NTP specific terminology and concepts
   within [RFC5905].

2.  Use Cases and Existing Deployments of NTP

   As a protocol, NTP is used synchronise large amounts of computers via
   both private networks and the open internet, and there are several
   common scenarios for existing 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 are used to synchronise devices such as customer-premises
   equipment.  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 have also deployed and
   offer NTP services both for internal use and for customers,
   particularly where the network is unable to offer or does not require
   the capabilities other protocols can provide, and where there may
   already be familiarity with NTP.  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, particularly through the use of modifications
   such as the use of bespoke algorithms.

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3.  Threat Analysis and Modeling

   A considerable motivation towards a new version of the protocol is
   the inclusion of security primitives such as authentication and
   encryption to bring the protocol in-line with current best practices
   for protocol design.

   There are numerous potential threats to a deployment or network
   handling traffic time synchronisation protocols that [RFC7384]
   section 3 describes, which can be summarised into three basic groups:
   Denial of Service (DoS), degradation of accuracy, and false time, all
   of which in various forms apply to NTP.  However, not all threats
   apply specifically to NTP directly, most notable attacks on time
   sources (section 3.2.10) and L2/L3 DoS Attacks (section 3.2.7) as
   both are outside the scope of the protocol, and the protocol itself
   cannot provide much in the way of mitigations.

3.1.  Denial of Service and Amplification

   NTPv4 has previously suffered from DDoS amplification attacks using a
   combination of IP address spoofing and private mode commands used in
   some NTP implementations, leading to an attacker being able to direct
   very large volumes of traffic to a victim IP address.  Current
   mitigations are disabling private mode commands susceptible to
   attacks and encouraging network operators to implement BCP 38
   [RFC2827] as well as source address validation where possible.

   The NTPv5 protocol specification should be designed with current best
   practices for UDP based protocols in mind [RFC8085].  It should
   reduce the potential amplification factors in request/response
   payload sizes [drdos-amplification] through the use of padding of
   payload data, in addition to restricting command and diagnostic modes
   which could be exploited.

3.2.  Accuracy Degradation

   The risk that an on-path attacker can systemically delay packets
   between a client and server exists in all time protocols operating on
   insecure networks and its mitigations within the protocol are limited
   for a clock which is not yet synchronised.  Increased path diversity
   and protocol support for synchronisation across multiple
   heterogeneous sources are likely the most effective mitigations.

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3.3.  False Time

   Conversely, on-path attackers who can manipulate timestamps could
   also speed up a client's clock resulting in drift-related
   malfunctions and errors such as premature expiration of certificates
   on affected hosts.  An attacker may also manipulate other data in
   flight to disrupt service and cause de-synchronisation.  Additionally
   attacks via replaying previously transmitted packets can also delay
   or confuse receiving clocks, impacting ongoing synchronisation.

   Message authentication with regular key rotation should mitigate all
   of these cases; however deployments should consider finding an
   appropriate compromise between the frequency of rotation to balance
   the window of attack and the rate of re-keying.

4.  Requirements

   At a high level, NTPv5 should be a protocol that is capable of
   operating in local networks and over public internet connections
   where packet loss, delay, and filtering may occur.  It should provide
   both basic time information and synchronisation.

4.1.  Resource Management

   Historically there have been many documented instances of NTP servers
   receiving ongoing large volumes of unauthorised traffic [ntp-misuse]
   and the design of NTPv5 must ensure the risk of these can be
   minimised through the use of signalling unwanted traffic (e.g Kiss of
   Death) or easily identifiable packet formats which make rate-
   limiting, filtering, or blocking by firewalls possible.

   The protocol's loop avoidance mechanisms SHOULD be able to use
   identifiers that change over time.  Identifiers MUST NOT relate to
   network topology, in particular such mechanism should not rely on any
   FQDN, IP address or identifier tied to a public certificate used or
   owned by the server.  Servers SHOULD be able to migrate and change
   any identifier used as stratum topologies or network configuration
   changes occur.

   An additional identifier mechanism MAY be considered for the purposes
   of client allow/deny lists, logging and monitoring.  Such a mechanism
   when included, SHOULD be independent of any loop avoidance mechanism,
   and authenticity requirements SHOULD be considered.

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   The protocol MUST have the capability for servers to notify clients
   that the service is unavailable and clients MUST have clearly defined
   behaviours for honouring this signalling.  In addition servers SHOULD
   be able to communicate to clients that they should reduce their query
   rate when the server is under high load or has reduced capacity.

   Clients SHOULD periodically re-establish connections with servers to
   prevent maintaining prolonged connectivity to unavailable hosts and
   give operators the ability to move traffic away from hosts in a
   timely manner.

   The protocol SHOULD have provisions for deployments where Network
   Address Translation occurs and define behaviours when NAT rebinding
   occurs.  This should also not compromise any DDoS mitigation(s) that
   the protocol may define.

   Client and server protocol modes MUST be supported.  Other modes such
   as symmetric and broadcast MAY be supported by the protocol but
   SHOULD NOT be required by implementers to implement.  Considerations
   should be made in these modes to avoid implementation vulnerabilities
   and to protect deployments from attacks.

4.2.  Data Minimisation

   To minimise ongoing use of deprecated fields and exposing identifying
   information of implementations and deployments, payload formats
   SHOULD use the least amount of fields and information where possible,
   realising that data minimisation and resource management can be at
   odds with one another.  The use of extensions should be preferred
   when transmitting optional data.

4.3.  Algorithms

   The use of algorithms describing functions such as clock filtering,
   selection, and clustering SHOULD have agility, allowing for
   implementations to develop and deploy new algorithms independently.
   Signalling of algorithm use or preference SHOULD NOT be transmitted
   by servers, however essential properties of the algorithm (e.g.
   precision) SHOULD be obvious.

   The working group should consider creating a separate informational
   document to describe an algorithm to assist with implementation, and
   consider adopting future documents which describe new algorithms as
   they are developed.  Specifying client algorithms separately from the
   protocol will allow NTPv5 to meet the needs of applications with a
   variety of network properties and performance requirements.

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4.4.  Timescales

   The protocol should adopt a linear, monotonic timescale as the basis
   for communicating time.  The format should provide sufficient scale,
   precision, and resolution to meet or exceed NTPv4's capabilities, and
   have a roll-over date sufficiently far into the future that the
   protocol's complete obsolescence is likely to occur first.  Ideally
   it should be similar or identical to the existing epoch and data
   model that NTPv4 defines to allow for implementations to better
   support both versions of the protocol, simplifying implementation.

   The timescale, in addition to any other time-sensitive information,
   MUST be sufficient to calculate representations of both UTC and TAI
   [TF.460-6], noting that UTC itself as the current timescale used in
   NTPv4 is neither linear nor monotonic unlike TAI.  Through extensions
   the protocol SHOULD support additional timescale representations
   outside of the main specification, and all transmissions of time data
   MUST indicate the timescale in use.

4.5.  Leap seconds

   Transmission of UTC leap second information MUST be included in the
   protocol in order for clients to generate a UTC representation, but
   must be transmitted as separate information to the timescale.  The
   specification MUST require that servers transmit upcoming leap
   seconds greater than 24 hours in linear timescale in advance if that
   information is known by the server.  If the server learns of a leap
   second less than 24 hours before an upcoming leap second event, it
   MUST start transmitting the information immediately.

   Smearing [google-smear] of leap seconds SHOULD be supported in the
   protocol, and the protocol MUST support servers transmitting
   information if they are configured to smear leap seconds and if they
   are actively doing so.  Behaviours for both client and server in
   handling leap seconds MUST be part of the specification; in
   particular how clients handle multiple servers where some may use
   leap seconds and others smearing, that servers should not apply both
   leap seconds and smearing, as well as details around smearing
   timescales.  Supported smearing algorithms MUST be defined or
   referenced.

4.6.  Backwards Compatibility with NTS and NTPv4

   The desire for compatibility with older protocols should not prevent
   addressing deployment issues or cause ossification of the protocol
   caused by middleboxes [RFC9065].

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   Servers that support multiple versions of NTP MUST send a response in
   the same version as the request as the model of backwards
   compatibility.  This does not preclude servers from acting as a
   client in one version of NTP and a server in another.

   Protocol ossification MUST be addressed to prevent existing NTPv4
   deployments which respond incorrectly 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.

4.6.1.  Dependent Specifications

   Many other documents make use of NTP's data formats ([RFC5905]
   Section 6) for representing time, notably for media and packet
   timestamp measurements, such as SDP [RFC4566] and STAMP [RFC8762].
   Any changes to the data formats should consider the potential
   implementation complexity that may be incurred.

4.7.  Extensibility

   The protocol MUST have the capability to be extended; implementations
   MUST ignore unknown extensions.  Unknown extensions received from a
   lower stratum server SHALL NOT be re-transmitted towards higher
   stratum servers.

4.8.  Security

   Data authentication and integrity MUST be supported by the protocol,
   with optional support for data confidentiality.  Downgrade attacks by
   an in-path attacker must be mitigated.  The protocol MUST define at
   least one common mechanism to ensure interoperability, but should
   also include support for different mechanisms to support different
   deployment use cases.  Extensions and additional modes SHOULD also
   incorporate authentication and integrity on data which could be
   manipulated by an attacker, on-path or off-path.

   Upgrading cryptographic algorithms must be supported, allowing for
   more secure cryptographic primitives to be incorporated as they are
   developed and as attacks and vulnerabilities with incumbent
   primitives are discovered.

   Intermediate devices such as networking equipment capable of
   modifying NTP packets, for example to adjust timestamps MUST be able
   to do so without compromising authentication or confidentiality.
   Extension fields with separate authentication may be used to
   facilitate this.

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   Consideration must be given to 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.

5.  Non-requirements

   This section covers topics that are explicitly out of scope.

5.1.  Server Malfeasance Detection

   Detection and reporting of server malfeasance should remain out of
   scope as [I-D.ietf-ntp-roughtime] already provides this capability as
   a core functionality of the protocol.

5.2.  Additional Time Information and Metadata

   Previous versions of NTP do not transmit additional time information
   such as time zone data or historical leap seconds, and NTPv5 should
   not explicitly add support for it by default as existing protocols
   (e.g.  TZDIST [RFC7808]) already provide mechanisms to do so.  This
   does not prevent however, further extensions enabling this.

5.3.  Remote Monitoring Support

   Largely due to previous DDoS amplification attacks, mode 6 messages
   which have historically provided the ability for monitoring of
   servers SHOULD NOT be supported in the core of the protocol.
   However, it may be provided as a separate extension specification.
   It is likely that even with a new version of the protocol middleboxes
   may continue to block this mode in default configurations into the
   future.

6.  IANA Considerations

   This document makes no requests of IANA.

7.  Security Considerations

   As this document is intended to create discussion and consensus, it
   introduces no security considerations of its own.

8.  References

8.1.  Normative References

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   [I-D.ietf-ntp-roughtime]
              Malhotra, A., Langley, A., Ladd, W., and M. Dansarie,
              "Roughtime", Work in Progress, Internet-Draft, draft-ietf-
              ntp-roughtime-08, 18 October 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-ntp-
              roughtime-08>.

   [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/rfc/rfc2119>.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <https://www.rfc-editor.org/rfc/rfc2827>.

   [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/rfc/rfc5905>.

   [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
              October 2014, <https://www.rfc-editor.org/rfc/rfc7384>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/rfc/rfc8085>.

   [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/rfc/rfc8174>.

8.2.  Informative References

   [drdos-amplification]
              "Amplification and DRDoS Attack Defense -- A Survey and
              New Perspectives", n.d.,
              <https://arxiv.org/abs/1505.07892>.

   [google-smear]
              "Google Leap Smear", n.d.,
              <https://developers.google.com/time/smear>.

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   [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>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/rfc/rfc4566>.

   [RFC7808]  Douglass, M. and C. Daboo, "Time Zone Data Distribution
              Service", RFC 7808, DOI 10.17487/RFC7808, March 2016,
              <https://www.rfc-editor.org/rfc/rfc7808>.

   [RFC8762]  Mirsky, G., Jun, G., Nydell, H., and R. Foote, "Simple
              Two-Way Active Measurement Protocol", RFC 8762,
              DOI 10.17487/RFC8762, March 2020,
              <https://www.rfc-editor.org/rfc/rfc8762>.

   [RFC9065]  Fairhurst, G. and C. Perkins, "Considerations around
              Transport Header Confidentiality, Network Operations, and
              the Evolution of Internet Transport Protocols", RFC 9065,
              DOI 10.17487/RFC9065, July 2021,
              <https://www.rfc-editor.org/rfc/rfc9065>.

   [TF.460-6] "Standard-frequency and time-signal emissions", n.d.,
              <https://www.itu.int/rec/R-REC-TF.460-6-200202-I/en>.

Appendix A.  Acknowledgements

   The author would like to thank Doug Arnold, Hal Murray, Paul Gear,
   and David Venhoek 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
   Nederlandse Publieke Omroep
   Netherlands
   Email: james.ietf@gmail.com

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