NTPv5 use cases and requirements
draft-ietf-ntp-ntpv5-requirements-00

Network Time Protocol                                       J. Gruessing
Internet-Draft                               Nederlandse Publieke Omroep
Intended status: Informational                              25 July 2022
Expires: 26 January 2023

                    NTPv5 use cases and requirements
                  draft-ietf-ntp-ntpv5-requirements-00

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

Status of This Memo

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Copyright Notice

   Copyright (c) 2022 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  . . . . . . . . . . . . . . . . .   3
   2.  Use cases and existing deployments of NTP . . . . . . . . . .   3
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Resource management . . . . . . . . . . . . . . . . . . .   3
     3.2.  Algorithms  . . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Timescales  . . . . . . . . . . . . . . . . . . . . . . .   4
     3.4.  Leap seconds  . . . . . . . . . . . . . . . . . . . . . .   5
     3.5.  Backwards compatibility with NTS and NTPv4  . . . . . . .   5
       3.5.1.  Dependent Specifications  . . . . . . . . . . . . . .   5
     3.6.  Extensibility . . . . . . . . . . . . . . . . . . . . . .   5
     3.7.  Security  . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Non-requirements  . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Server malfeasence detection  . . . . . . . . . . . . . .   6
   5.  Threat model  . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Delay-based attacks . . . . . . . . . . . . . . . . . . .   6
     5.2.  Payload manipulation  . . . . . . . . . . . . . . . . . .   7
     5.3.  Denial of Service and Amplification . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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.

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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.

2.  Use cases and existing deployments of NTP

   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 where reduced accuracy may be tolerable.
   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 local networks and over public internet connections
   where packet loss, delay, and filtering may occur.  It should be able
   to provide enough information for both basic time information and
   synchronisation.

3.1.  Resource management

   Historically there have been many documented instances of NTP servers
   receiving large amounts of unauthorised traffic [ntp-misuse] and the
   design of NTPv5 must ensure the risk of these can be minimised.

   Servers SHOULD have a new identifier that peers use as reference,
   this SHOULD NOT be a FQDN, an IP address, or an identifier tied to a
   public certificate.  Servers SHOULD be able to migrate and change
   their identifiers as stratum topologies or network configuration
   changes occur.

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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 attempting to maintain connectivity to a dead host and give
   network 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.

3.2.  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.

   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.

3.3.  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 capabilties, and
   have a rollover date sufficiently far into the future that the
   protocol's complete obsolescence is 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.

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3.4.  Leap seconds

   Tranmission 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 SHOULD be capable of transmitting upcoming leap seconds
   greater than 1 calendar day in advance.

   Leap second smearing SHOULD NOT be applied to timestamps transmitted
   by the server, however this should not prevent implementers from
   applying leap second smearing between the client and any clock it is
   training.

3.5.  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.

   The model for backward compatibility is: servers that support
   multiple versions of NTP must 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.

   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.

3.5.1.  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.  Any changes to the data formats should
   consider the potential implementation complexity that may be
   incurred.

3.6.  Extensibility

   The protocol MUST have the capability to be extended; 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.

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3.7.  Security

   Data authentication and optional data confidentiality MUST be
   integrated into the protocol, and downgrade attacks by an in-path
   attacker must be mitigated.

   Cryptographic agility 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 hardware capable of performing
   timestamping of packets SHOULD be able to add information to packets
   in flight without requiring modification or removal of authentication
   or confidentiality on the packet.

   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.

4.  Non-requirements

   This section covers topics that are explicitly out of scope.

4.1.  Server malfeasence 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.  Threat model

   The assumptions that apply to all of the threats and risks within
   this section are based on observations of the use cases defined
   earlier in this document, and focus on external threats outside of
   the trust boundaries which may be in place within a network.
   Internal threats and risks such as a trusted operator are out of
   scope.

5.1.  Delay-based attacks

   The risk that an on-path attacker can 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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5.2.  Payload manipulation

   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.  Message
   authentication with regular key rotation should mitigate both of
   these cases; however consideration should also be made for hardware-
   based timestamping.

5.3.  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
   many 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 and encouraging
   network operators to implement BCP 38 [RFC2827].  The NTPv5 protocol
   specification should reduce the amplification factor in request/
   response payload sizes [drdos-amplification] through the use of
   padding and consideration of payload data.

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

   [I-D.ietf-ntp-roughtime]
              Malhotra, A., Langley, A., Ladd, W., and M. Dansarie,
              "Roughtime", Work in Progress, Internet-Draft, draft-ietf-
              ntp-roughtime-06, 7 June 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-ntp-
              roughtime-06>.

   [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>.

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   [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>.

   [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>.

   [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>.

Appendix A.  Acknowledgements

   The author would like to thank Doug Arnold, Hal Murray, and Paul Gear
   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

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   Email: james.ietf@gmail.com

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