Internet Engineering Task Force                               M. Lichvar
Internet-Draft                                                   Red Hat
Intended status: Standards Track                            Jun 24, 2021
Expires: December 26, 2021

                              NTP Over PTP


   This document specifies a transport for the Network Time Protocol
   (NTP) client-server mode using the Precision Time Protocol (PTP) to
   enable hardware timestamping on hardware that can timestamp PTP
   messages but not NTP messages.

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1.  Introduction

   The Precision Time Protocol (PTP) [IEEE1588] was designed for highly
   accurate synchronization of clocks in a network.  It relies on
   hardware timestamping supported in network devices (e.g.  interface
   controllers, switches, and routers) to eliminate the impact of
   processing and queueing delays on PTP measurements.

   PTP was originally designed for multicast communication.  Later was
   added a unicast mode, which can be used in larger networks with
   partial on-path PTP support (e.g. telecom profiles G.8265.1 and

   The Network Time Protocol [RFC5905] does not rely on hardware
   timestamping support, but implementations can use it if it is
   available to avoid the impact of processing and queueing delays,
   similarly to PTP.  The client-server mode of NTP is functionally
   similar to the PTP unicast mode.

   An issue for NTP is hardware that can specifically timestamp only PTP
   packets.  This limitation comes from their design, which does not
   allow the timestamps to be captured or retrieved at the same rate as
   packets can be received or transmitted.  A filter needs to be
   implemented in the hardware to inspect each packet and timestamp only
   those that actually need it.  The filter can be usually configured
   for the PTP transport (e.g.  UDPv4, UDPv6, 802.3) and sometimes even
   the message type (e.g.  sync message or delay request) to further
   reduce the rate of timestamps on the server or client side.  This
   limitation prevents hardware timestamping of NTP messages.  It also
   prevents timestamping of PTP messages if they are secured at the
   transport layer or below (e.g.  IPSec or MACSec).

   This document specifies a new transport for NTP to enable the PTP-
   specific timestamping support.  It adds a new extension field (TLV)
   for PTP to contain NTP messages.

   NTP over PTP does not disrupt normal operation of PTP.  A network and
   even a single host can support both at the same time.

   The specification does not take advantage of the PTP correctionField
   modified by PTP transparent clocks as their support for the unicast
   mode seems to be rare or nonexistent.

   The client/server mode of NTP, even if using the PTP transport, has
   several advantages when compared to the PTP unicast mode:

   o  It is more secure.  It can use existing security mechanisms
      specified for NTP like Network Time Security [RFC8915], not losing

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      any of its features.  The PTP unicast mode allows an almost-
      infinite traffic amplification, which can be exploited for denial-
      of-service attacks and can only be limited by security mechanisms
      using client authentication.

   o  It needs fewer messages and less network bandwith to get the same
      number of timestamps.

   o  It is better suited for synchronization in networks without full
      on-path support.  It does not assume the network delay is constant
      and the number of measurements in opposite directions is symmetric
      (in PTP sync messages and delay requests have independent timing).

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.  PTP transport for NTP

   A new TLV is defined for PTP to contain NTP messages in the client
   and server mode.  Using other NTP modes in the TLV is not specified.
   Any transport specified for PTP that supports unicast messaging can
   be used for NTP over PTP, e.g.  UDP on IPv4 and IPv6.

   The type value of the NTP TLV is TBD.  The TLV contains the whole NTP
   message as would normally be the UDP payload, without any
   modifications.  The TLV does not propagate through boundary clocks.

   If the UDP transport is used for PTP, the UDP source and destination
   port numbers MUST be the PTP event port (319).  Client port
   randomization would break the timestamping.

   The NTP TLV MUST be included in a delay request message.  The
   originTimestamp field and all fields of the header SHOULD be zero,

   o  messageType is 1 (delay request)

   o  versionPTP is 2

   o  messageLength is the length of the PTP message including the NTP

   o  domainNumber is TBD

   o  flagField has the unicastFlag bit set

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   An NTP client using the PTP transport sends NTP requests in PTP
   messages to the server at the same rate as it would normally send
   them over UDP.

   A server which supports the NTP TLV MUST check for the domainNumber
   of TBD and respond to an NTP request with a single PTP message
   containing the NTP response using the same PTP message format.  It
   MUST NOT send a delay response message.

   A server which does not support the NTP TLV will not recognize the
   domain number and ignore the message.  If it responded to messages in
   the domain (e.g. due to misconfiguration), it would send a delay
   response (to port 320 if using the UDP transport), which would be
   ignored by the client.

   Any authenticator fields included in the NTP messages MUST be
   calculated only over the NTP message following the header of the NTP

   Timestamps SHOULD NOT be adjusted for the beginning of the NTP data
   in the PTP message.  They SHOULD still correspond to the ending of
   the transmission and beginning of the reception (e.g. start of
   delimiter in the Ethernet frame).

   Any modifications of the correctionField made by potential one-step
   end-to-end transparent clocks in the network SHOULD be ignored by the
   server and client.

3.  Security Considerations

   The PTP transport prevents NTP clients from randomizing their source
   port.  It has no other impact on security of NTP.

4.  References

4.1.  Normative References

              Institute of Electrical and Electronics Engineers, "IEEE
              std. 1588-2019, "IEEE Standard for a Precision Clock
              Synchronization for Networked Measurement and Control
              Systems."", 11 2019, <>.

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

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   [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,

4.2.  Informative References

   [RFC8915]  Franke, D., Sibold, D., Teichel, K., Dansarie, M., and R.
              Sundblad, "Network Time Security for the Network Time
              Protocol", RFC 8915, DOI 10.17487/RFC8915, September 2020,

Author's Address

   Miroslav Lichvar
   Red Hat
   Purkynova 115
   Brno  612 00
   Czech Republic


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