Internet Engineering Task Force                               M. Lichvar
Internet-Draft                                                   Red Hat
Intended status: Experimental                           October 30, 2017
Expires: May 3, 2018

                          NTP Correction Field


   This document specifies an extension field for the Network Time
   Protocol (NTP) which allows network devices such as switches and
   routers to modify NTP packets with corrections to improve accuracy of
   the synchronization in the network.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 3, 2018.

Copyright Notice

   Copyright (c) 2017 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
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Lichvar                    Expires May 3, 2018                  [Page 1]

Internet-Draft            NTP Correction Field              October 2017

1.  Introduction

   Processing and queueing delays in network switches and routers may be
   a significant source of jitter and asymmetry in network delay, which
   has a negative impact on accuracy and stability of synchronization of
   clocks in local networks using NTP [RFC5905].

   If all network devices on the paths between NTP clients and servers
   implemented NTP and supported an operation as a server and client,
   the impact of the delays could be avoided by configuring NTP to make
   measurements only between devices and hosts that are directly
   connected to one another.  In the Precision Time Protocol (PTP)
   [IEEE1588], which is a different protocol for synchronization of
   clocks in networks, such devices would be called Boundary Clocks

   A different approach supported by PTP to improve the accuracy uses
   Transparent Clocks (TC).  Instead of fully implementing PTP in order
   to support an operation as a BC, the devices only modify a correction
   field in forwarded PTP packets with the time that the packets had to
   wait for transmission.  The final value of the correction is included
   in the calculation of the delay and offset, which may significantly
   improve the accuracy and stability of the synchronization.

   This document describes an NTP extension field which allows the
   devices to make similar correction in forwarded NTP packets.

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.  Format of Correction Field

   The Correction Field is an NTP extension field following RFC 7822
   [RFC7822].  The format of the extension field is shown in Figure 1.

Lichvar                    Expires May 3, 2018                  [Page 2]

Internet-Draft            NTP Correction Field              October 2017

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |          Field Type           |          Length (28)          |
   |      Receive Correction       |      Transmit Correction      |
   |                                                               |
   |                       Origin Correction                       |
   |                                                               |
   |                                                               |
   |                        Delay Correction                       |
   |                                                               |
   |   Origin ID   |    Path ID    |     Checksum complement       |

                   Figure 1: Format of Correction Field

   The extension field has the following fields:

   Field Type
           The type which identifies the Correction extension field.

           The length of the extension field, which is 28 octets.

   Receive Correction
           A 16-bit extension of the receive timestamp in the NTP

   Transmit Correction
           A 16-bit extension of the transmit timestamp in the NTP

   Origin Correction
           A field which contains a copy of the final delay correction
           from the previous packet in the NTP exchange.

   Delay Correction
           A fixed point number with 16 unsigned integer bits and 48
           fractional bits, which represents the current correction of
           the network delay that has accumulated for this packet on the
           path from the source to the destination.

   Origin ID

Lichvar                    Expires May 3, 2018                  [Page 3]

Internet-Draft            NTP Correction Field              October 2017

           A field which contains a copy of the final path ID from the
           previous packet in the NTP exchange.

   Path ID
           An 8-bit identification number of the path where the delay
           correction was updated.

   Checksum Complement
           A field which can be modified to not change the UDP checksum
           after updating the other fields.  The same field is described
           in RFC 7821 [RFC7821].

3.  Network devices

   A network device which is forwarding a packet and supports the
   Correction Field MUST NOT modify the packet unless all of the
   following applies:

   1.  The packet is an IPv4 or IPv6 UDP packet.

   2.  The source port or destination port is 123.

   3.  The NTP version is 4.

   4.  The NTP mode is 1, 2, 3, 4, or 5.

   5.  The format of the packet is valid per RFC 7822.

   6.  The packet contains an extension field which has a type of TBD
       and length of 28 octets.

   The device SHOULD add to the delay correction field of the extension
   field the time difference between the following points:

   1.  The end of the reception (trailer timestamp).

   2.  The beginning of the transmission (preamble timestamp).

   If the device updates the delay correction, it SHOULD also add the
   identification numbers of the incoming and outgoing port to the path

   The device MAY update the checksum complement field in order to keep
   the UDP checksum valid.

   TBD: Requirements on frequency accuracy of the clock.

Lichvar                    Expires May 3, 2018                  [Page 4]

Internet-Draft            NTP Correction Field              October 2017

4.  NTP hosts

   When an NTP client sends a request to a server and the association is
   configured to use the Correction Field, it SHOULD add the extension
   field to the packet.  All fields of the extension field except type
   and length SHOULD be set to zero.

   When the server receives a packet which includes the extension field,
   the response SHOULD also include the extension field.

   If the server's clock has a better precision than resolution of the
   64-bit NTP timestamp format, the server SHOULD save the additional
   bits in the receive and transmit correction fields and set the
   precision field to the corresponding number, which is smaller than
   -32.  Otherwise, the receive and transmit correction fields SHOULD be

   The origin correction and origin path ID SHOULD be set to the delay
   correction and path ID from the request.  The other fields of the
   Correction Field SHOULD be zero.

   When the client receives a response which contains the extension
   field, it SHOULD check the value of both the origin and delay
   correction fields.  If a correction is larger than a specified
   maximum (e.g. 1 second), the extension field SHOULD be ignored.

   The client MAY log a warning if the origin ID and path ID are not
   equal, which indicates the network path between the server and client
   is not symmetric.

   If the client's clock has a better precision than resolution of the
   64-bit NTP format and the precision field in the response contains a
   number smaller than -32, the client SHOULD extend the receive and
   transmit timestamps with the additional bits in the receive and
   transmit correction fields.

   When the client computes the offset and delay using the formulas from
   RFC 5905, the origin correction is substracted from the receive
   timestamp and the delay correction is added to the transmit

   TBD: Other modes.

5.  Acknowledgements

   The Correction Field extension is based on the PTP correction field
   specified in IEEE 1588-2008.

Lichvar                    Expires May 3, 2018                  [Page 5]

Internet-Draft            NTP Correction Field              October 2017

6.  IANA Considerations

   IANA is requested to allocate an Extension Field Type for the
   Correction Field.

7.  Security Considerations

   Packets using the Correction Field cannot be authenticated by legacy
   MACs, because network devices which are supposed to update the field
   do not have access to the key.

   Packets can be partially protected if the Correction Field is placed
   in the packet after an authentication extension field, e.g. the NTS
   Authenticator and Encrypted Extensions extension field

   A man-in-the-middle attacker can delay packets in the network in
   order to increase the measured delay and shift the measured offset by
   up to half of the extra delay.  If the packets contain the Correction
   Field, the attacker can reduce the delay calculated by the client or
   peer and shift the offset even more.  The maximum correction should
   be limited (e.g. to 1 second) to prevent the attacker from injecting
   a larger offset to the measurements.

8.  References

8.1.  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,

   [RFC7822]  Mizrahi, T. and D. Mayer, "Network Time Protocol Version 4
              (NTPv4) Extension Fields", RFC 7822, DOI 10.17487/RFC7822,
              March 2016, <>.

8.2.  Informative References

              Franke, D., Sibold, D., and K. Teichel, "Network Time
              Security for the Network Time Protocol", draft-ietf-ntp-
              using-nts-for-ntp-09 (work in progress), June 2017.

Lichvar                    Expires May 3, 2018                  [Page 6]

Internet-Draft            NTP Correction Field              October 2017

              IEEE std. 1588-2008, "IEEE Standard for a Precision Clock
              Synchronization Protocol for Networked Measurement and
              Control Systems", 2008.

   [RFC7821]  Mizrahi, T., "UDP Checksum Complement in the Network Time
              Protocol (NTP)", RFC 7821, DOI 10.17487/RFC7821, March
              2016, <>.

Author's Address

   Miroslav Lichvar
   Red Hat
   Purkynova 115
   Brno  612 00
   Czech Republic


Lichvar                    Expires May 3, 2018                  [Page 7]