NTP Working Group                                             T. Mizrahi
Internet Draft                                                   Marvell
Intended status: Experimental
Expires: January 2016                                      July 21, 2015

        UDP Checksum Complement in the Network Time Protocol (NTP)


   The Network Time Protocol (NTP) allows clients to synchronize to a
   time server using timestamped protocol messages. To faciliate
   accurate timestamping, some implementations use hardware-based
   timestamping engines that integrate the accurate transmission time
   into every outgoing NTP packet during transmission. Since these
   packets are transported over UDP, the UDP checksum field is then
   updated to reflect this modification. This document proposes an
   extension field that includes a 2-octet Checksum Complement, allowing
   timestamping engines to reflect the checksum modification in the last
   2 octets of the packet rather than in the UDP checksum field. The
   behavior defined in this document is interoperable with existing NTP

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Copyright Notice

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Table of Contents

   1. Introduction...................................................2
      1.1. Intermediate Entities.....................................3
      1.2. Updating the UDP Checksum.................................5
   2. Conventions used in this document..............................6
      2.1. Terminology...............................................6
      2.2. Abbreviations.............................................6
   3. Using UDP Checksum Complements in NTP..........................6
      3.1. Overview..................................................6
      3.2. Checksum Complement in NTP Packets........................7
         3.2.1. Transmission of NTP with Checksum Complement.........8
         3.2.2. Intermediate Updates of NTP with Checksum Complement.9
         3.2.3. Reception of NTP with Checksum Complement............9
      3.3. Interoperability with Existing Implementations............9
      3.4. Using the Checksum Complement with or without Authentication
   4. Security Considerations........................................9
   5. IANA Considerations...........................................10
   6. Acknowledgments...............................................10
   7. References....................................................10
      7.1. Normative References.....................................10
      7.2. Informative References...................................11

1. Introduction

   The Network Time Protocol [NTPv4] allows clients to synchronize their
   clocks to a time server by exchanging NTP packets. The increasing
   demand for highly accurate clock synchronization motivates
   implementations that provide accurate timestamping.

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1.1. Intermediate Entities

   In this document we use the term 'intermediate entity', referring to
   an entity that resides on the path between the sender and the
   receiver of an NTP packet, that modifies this NTP packet en-route.
   Two examples of intermediate entities are presented below.

   In order to facilitate accurate timestamping, an implementation can
   use a hardware based timestamping engine, as shown in Figure 1. In
   such cases, NTP packets are sent and received by a software layer,
   whereas a timestamping engine modifies every outgoing NTP packet by
   incorporating its accurate transmission time into the <Transmit
   Timestamp> field in the packet.

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                          NTP client/server
                        |                   |
                        |   +-----------+   |
       Software         |   |    NTP    |   |
                        |   | protocol  |   |
                        |   +-----+-----+   |
                        |         |         |
                        |   +-----+-----+   |
                        |   | Accurate  |   |
       ASIC/FPGA        |   | Timestamp |   |
                        |   |  engine   |   |
                        |   +-----------+   |
                        |         |         |
                                  |NTP packets
                              ___ v _
                             /   \_/ \__
                            /           \_
                           /     IP      /
                           \_  Network  /
                            /           \
                            \__/\_   ___/

                   Figure 1 Accurate Timestamping in NTP

   The accuracy of clock synchronization over packet networks is highly
   sensitive to delay jitters in the underlying network, which
   dramatically affects the clock accuracy. To address this challenge,
   the Precision Time Protocol (PTP) [IEEE1588] defines Transparent
   Clocks (TCs), intermediate switches and routers that improve the end-
   to-end accuracy by updating a "Correction Field" in the PTP packet by
   adding the latency caused by the current TC. In NTP no equivalent
   entity is currently defined, but future versions of NTP may define an
   intermediate node that modifies en-route NTP packets using a
   "Correction Field".

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1.2. Updating the UDP Checksum

   When the UDP payload is modified by an intermediate entity, the UDP
   Checksum field needs to be updated to maintain its correctness. When
   using UDP over IPv4 ([UDP]), an intermediate entity that cannot
   update the value of the UDP checksum has no choice except to assign a
   value of zero to the checksum field, causing the receiver to ignore
   the checksum field and potentially accept corrupted packets. UDP over
   IPv6, as defined in [IPv6], does not allow a zero checksum, except in
   specific cases [ZeroChecksum]. As discussed in [ZeroChecksum], the
   use of a zero checksum is generally not recommended, and should be
   avoided to the extent possible.

   Since an intermediate entity only modifies a specific field in the
   packet, i.e. the timestamp field, the UDP checksum update can be
   performed incrementally, using the concepts presented in [Checksum].

   A similar problem is addressed in Annex E of [IEEE1588]. When the
   Precision Time Protocol (PTP) is transported over IPv6, two octets
   are appended to the end of the PTP payload for UDP checksum updates.
   The value of these two octets can be updated by an intermediate
   entity, causing the value of the UDP checksum field to remain

   This document defines a similar concept for [NTP], allowing
   intermediate entities to update NTP packets and maintain the
   correctness of the UDP checksum by modifying the last 2 octets of the
   packet. This is performed by adding an NTP extension field at the end
   of the packet, in which the last two bytes are used as a checksum

   The term Checksum Complement is used throughout this document and
   refers to the 2 octets at the end of the UDP payload, used for
   updating the UDP checksum by intermediate entities.

   The usage of the Checksum Complement can in some cases simplify the
   implementation, since if the packet data is processed in a serial
   order, it is simpler to first update the timestamp field, and then
   update the Checksum Complement rather than to update the timestamp
   and then update the UDP checksum, residing at the UDP header.

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2. Conventions used in this document

2.1. Terminology

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

2.2. Abbreviations

   MAC      Message Authentication Code

   NTP      Network Time Protocol

   PTP      Precision Time Protocol

   UDP      User Datagram Protocol

3. Using UDP Checksum Complements in NTP

3.1. Overview

   The UDP Checksum Complement is a two-octet field that is appended at
   the end of the UDP payload using an NTP extension field. Figure 2
   illustrates the packet format of an NTP packet with a Checksum
   Complement extension. The figure illustrates an unauthenticated NTP
   packet. Section 3.4.  provides further details about using the
   Checksum Complement in authenticated packets.

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                    |      IPv4 / IPv6 Header        |
                    |           UDP Header           |
            ^       |                                |
            |       |           NTP packet           |
            |       |                                |
            |       +--------------------------------+
           UDP      | Optional NTP Extension Fields  |
         Payload    +--------------------------------+
            |       |    UDP Checksum Complement     |
            |       |   Extension Field (28 octets)  |
            v       +--------------------------------+

        Figure 2 Checksum Complement in NTP Unauthenticated Packets

3.2. Checksum Complement in NTP Packets

   NTP is transported over UDP, either over IPv4 or over IPv6. This
   document applies to both NTP over IPv4, and NTP over IPv6.

   NTP packets may include one or more extension fields, as defined in
   [NTPv4]. The Checksum Complement in NTP packets resides in a
   dedicated NTP extension field, as shown in Figure 2.

   In unauthenticated mode, if the NTP packet includes more than one
   extension field, the Checksum Complement extension is always the last
   extension field. Thus, when NTP authentication is disabled, the
   Checksum Complement is the last 2 octets in the UDP payload, and thus
   the Checksum Complement is located at (UDP Length - 2 octets) after
   the beginning of the UDP header.

   When NTP authentication is enabled, the Checksum Complement is the
   last 2 octets before the Message Authentication Code (MAC).

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    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 octets       |
   |                                                               |
   |                           Padding                             |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |      Checksum Complement      |
             Figure 3 NTP Checksum Complement Extension Field

   Field Type

     A dedicated Field Type value is used to identify the Checksum
     Complement extension. See Section 6 for further details.


     The Checksum Complement extension field length is 28 octets.

     This length guarantees that the host that receives the packet
     parses it correctly, whether the packet includes a MAC or not.
     [NTP-Ext] provides further details about the length of an
     extension field in the absence of a MAC.


     The extension field includes 22 octets of padding. This field
     SHOULD be set to 0, and SHOULD be ignored by the recipient.

   Checksum Complement

     Includes the UDP Checksum Complement field.

3.2.1. Transmission of NTP with Checksum Complement

   The transmitter of an NTP packet MAY include a Checksum Complement
   extension field.

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3.2.2. Intermediate Updates of NTP with Checksum Complement

   An intermediate node that receives and alters an NTP packet
   containing a Checksum Complement extension MAY use the Checksum
   Complement to maintain a correct UDP checksum value.

3.2.3. Reception of NTP with Checksum Complement

   This document does not impose new requirements on the receiving end
   of an NTP packet.

   The UDP layer at the receiving end verifies the UDP Checksum of
   received NTP packets, and the NTP layer SHOULD ignore the Checksum
   Complement extension field.

3.3. Interoperability with Existing Implementations

   The behavior defined in this document does not impose new
   requirements on the reception of NTP packets. Thus, transmitters and
   intermediate nodes that support the Checksum Complement can
   transparently interoperate with existing implementations.

3.4. Using the Checksum Complement with or without Authentication

   A Checksum Complement SHOULD NOT be used when authentication is
   enabled. The Checksum Complement is effective in unauthenticated
   mode, allowing the intermediate entity to perform serial processing
   of the packet without storing-and-forwarding it.

   On the other hand, when message authentication is used, an
   intermediate entity that alters NTP packets must also re-compute the
   Message Authentication Code (MAC) accordingly. The MAC update
   typically requires the intermediate entity to store the packet, re-
   compute its MAC, and then forward it. Thus, from an implementer's
   perspective, the Checksum Complement has very little value in
   authenticated mode, as it does not necessarily simplify the

4. Security Considerations

   This document describes how a Checksum Complement extension can be
   used for maintaining the correctness of the UDP checksum.

   The purpose of this extension is to ease the implementation of
   accurate timestamping engines, as described in Figure 1. The
   extension is intended to be used internally in an NTP client or
   server, and not intended to be used by intermediate switches and

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   routers that reside between the client and the server. As opposed to
   PTP [IEEE1588], NTP does not require intermediate switches or routers
   to modify the content of NTP messages, and thus any such modification
   should be considered as a malicious MITM attack.

   It is important to emphasize that the scheme described in this
   document does not increase the protocol's vulnerabitliy to MITM
   attacks; a MITM who maliciously modifies a packet and its Checksum
   Complement is logically equivalent to a MITM attacker who modifies a
   packet and its UDP Checksum field.

   The concept described in this document is intended to be used only in
   unauthenticated mode. As described in Section 3.4. , in authenticated
   mode using the Checksum Complement does not simplify the
   implementation compared to using the conventional Checksum, and
   therefore the Checksum Complement should not be used.

5. IANA Considerations

   IANA is requested to allocate an NTP extension Field Type value for
   the Checksum Complement extension.

6. Acknowledgments

   This document was prepared using 2-Word-v2.0.template.dot.

7. References

7.1. Normative References

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

   [IPv6]        Deering, S., Hinden, R., "Internet Protocol, Version 6
                 (IPv6) Specification", RFC 2460, December 1998.

   [Checksum]    Rijsinghani, A., "Computation of the Internet Checksum
                 via Incremental Update", RFC 1624, May 1994.

   [UDP]         Postel, J., "User Datagram Protocol", RFC 768, August

   [NTPv4]       Mills, D., Martin, J., Burbank, J., Kasch, W.,
                 "Network Time Protocol Version 4: Protocol and
                 Algorithms Specification", RFC 5905, June 2010.

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7.2. Informative References

   [IEEE1588]    IEEE TC 9 Instrumentation and Measurement Society
                 2000, "1588 IEEE Standard for a Precision Clock
                 Synchronization Protocol for Networked Measurement and
                 Control Systems Version 2", IEEE Standard, 2008.

   [NTP-Ext]     Mizrahi, T., Mayer, D., "The Network Time Protocol
                 Version 4 (NTPv4) Extension Fields", draft-ietf-ntp-
                 extension-field (work in progress), June 2015.

   [ZeroChecksum] Fairhurst, G., Westerlund, M., "Applicability
                 Statement for the Use of IPv6 UDP Datagrams with Zero
                 Checksums", RFC 6936, April 2013.

Authors' Addresses

   Tal Mizrahi
   6 Hamada St.
   Yokneam, 20692 Israel

   Email: talmi@marvell.com

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