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Internet Engineering Task Force                               M. Lichvar
Internet-Draft                                                   Red Hat
Updates: RFC5905 (if approved)                              Jun 09, 2020
Intended status: Standards Track
Expires: December 11, 2020

                          Alternative NTP port


   This document specifies an alternative port for the Network Time
   Protocol (NTP) which is restricted in the supported modes and
   extensions to not allow traffic amplification in order to make NTP
   safe for the Internet.

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 December 11, 2020.

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   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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1.  Introduction

   There are several modes specified for NTP.  NTP packets in versions
   2, 3, and 4 have a 3-bit field for the mode.  Modes 1, 2, 3, 4, and 5
   are used for synchronization of clocks.  They are specified in RFC
   5905 [RFC5905].  Modes 6 and 7 are used for other purposes, like
   monitoring and remote management of NTP servers and clients.  The
   mode 6 is specified in Control Messages Protocol for Use with Network
   Time Protocol Version 4 [I-D.ietf-ntp-mode-6-cmds].

   The first group of modes typically does not allow any traffic
   amplification, i.e. the response is not larger than the request.  An
   exception is Autokey specified in RFC 5906 [RFC5906].  Autokey is
   rarely supported on public NTP servers.

   However, the modes 6 and 7 allow significant traffic amplification,
   which has been exploited in large-scale denial-of-service (DoS)
   attacks over the Internet.

   Over time, network operators have been observed to implement the
   following mitigations:

   1.  Blocked UDP packets with destination or source port 123

   2.  Blocked UDP packets with destination or source port 123 and
       specific length (e.g. longer than 48 octets)

   3.  Blocked UDP packets with destination or source port 123 and NTP
       mode 6 or 7

   4.  Limited rate of UDP packets with destination or source port 123

   From those, only the 3rd approach does not have an impact on
   synchronization of clocks with NTP.

   The number of public servers in the pool.ntp.org project has dropped
   in large part due to the mitigations (citation?).

   Longer NTP packets (using extension fields) are needed by NTS

   This document specifies an alternative port for NTP which is
   restricted to the safe modes in order to enable synchronization of
   clocks in networks where the port 123 is blocked or rate limited.

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1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Alternative port

   The port TBD is an alternative port for NTP.  The protocol and the
   format of NTP packets sent from and to this port is unchanged.  Both
   NTP requests and responses MAY be sent from the port.  An NTP packet
   MUST NOT be sent from the alternative port if it is a response which
   has a longer UDP payload than the request, or the number of NTP
   packets in a single response is larger than one.

   In NTP versions 2, 3, and 4, only modes 1 (active), 2 (passive), 3
   (client), 4 (server), and 5 (broadcast) are generally usable on this

   An NTP server SHOULD receive requests in the client mode on both the
   original NTP port (123) and the alternative port (TBD).  If it
   responds, it MUST send the response from the port which received the
   request.  If the server supports any extension fields in NTP packets,
   it MUST verify that each response is not larger than the request,
   even if the number of extension fields is constant and they have a
   constant length.

   When an NTP client is started, it SHOULD send the first request to
   the alternative port.  The client SHOULD be switching between the two
   ports until a valid response is received.  The client MAY send a
   limited number of requests to both ports at the same time in order to
   speed up the discovery of the responding port.  When both ports are
   responding, the client SHOULD prefer the alternative port.

   An NTP server which supports NTS SHOULD include the NTPv4 Port
   Negotiation record in NTS-KE responses to specify the alternative
   port as the port to which the client should send NTP requests.

   In the symmetric modes (active and passive) NTP packets are
   considered to be requests and responses at the same time.  Therefore,
   the peers MUST send packets with an equal length in order to
   synchronize with each other.  The peers MAY use different polling
   intervals (packets sent at subsequent polls are considered to be
   separate requests and responses).

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3.  IANA Considerations

   IANA is requested to allocate a (system?)  UDP/TCP port.

4.  Acknowledgements

   The author would like to thank Daniel Franke and Ragnar Sundblad for
   their useful comments.

5.  References

5.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

5.2.  Informative References

              Haberman, B., "Control Messages Protocol for Use with
              Network Time Protocol Version 4", draft-ietf-ntp-mode-
              6-cmds-08 (work in progress), June 2020.

              Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
              Sundblad, "Network Time Security for the Network Time
              Protocol", draft-ietf-ntp-using-nts-for-ntp-28 (work in
              progress), March 2020.

   [RFC5906]  Haberman, B., Ed. and D. Mills, "Network Time Protocol
              Version 4: Autokey Specification", RFC 5906,
              DOI 10.17487/RFC5906, June 2010,

Author's Address

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   Miroslav Lichvar
   Red Hat
   Purkynova 115
   Brno  612 00
   Czech Republic

   Email: mlichvar@redhat.com

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