GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc7821

Internet Engineering Task Force (IETF) T. Mizrahi Request for Comments: 7821 Marvell Category: Experimental March 2016 ISSN: 2070-1721

     UDP Checksum Complement in the Network Time Protocol (NTP)

Abstract

 The Network Time Protocol (NTP) allows clients to synchronize to a
 time server using timestamped protocol messages.  To facilitate
 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
 implementations.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  This document is a product of the Internet Engineering
 Task Force (IETF).  It represents the consensus of the IETF
 community.  It has received public review and has been approved for
 publication by the Internet Engineering Steering Group (IESG).  Not
 all documents approved by the IESG are a candidate for any level of
 Internet Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7821.

Mizrahi Experimental [Page 1] RFC 7821 NTP Checksum Complement March 2016

Copyright Notice

 Copyright (c) 2016 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
 (http://trustee.ietf.org/license-info) 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.

Table of Contents

 1. Introduction ....................................................3
    1.1. Intermediate Entities ......................................3
    1.2. Updating the UDP Checksum ..................................4
 2. Conventions Used in This Document ...............................5
    2.1. Terminology ................................................5
    2.2. Abbreviations ..............................................6
 3. Using the UDP Checksum Complement in NTP ........................6
    3.1. Overview ...................................................6
    3.2. Checksum Complement in NTP Packets .........................7
         3.2.1. Using the Checksum Complement .......................7
         3.2.2. Transmission of NTP with Checksum Complement ........8
         3.2.3. Updates of NTP with Checksum Complement .............8
         3.2.4. Reception of NTP with Checksum Complement ...........8
    3.3. Interoperability with Existing Implementations .............9
    3.4. The Checksum Complement and Authentication .................9
 4. Security Considerations ........................................10
 5. IANA Considerations ............................................10
 6. References .....................................................11
    6.1. Normative References ......................................11
    6.2. Informative References ....................................11
 Appendix A. Checksum Complement Usage Example .....................13
 Acknowledgments ...................................................14
 Author's Address ..................................................14

Mizrahi Experimental [Page 2] RFC 7821 NTP Checksum Complement March 2016

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.

1.1. Intermediate Entities

 In this document, we use the term "intermediate entity" to refer to
 an entity that resides on the path between the sender and the
 receiver of an NTP packet and that modifies this NTP packet en route.
 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.

Mizrahi Experimental [Page 3] RFC 7821 NTP Checksum Complement March 2016

                    NTP client/server
                  +-------------------+
                  |                   |
                  |   +-----------+   |
   Software       |   |    NTP    |   |
                  |   | protocol  |   |
                  |   +-----+-----+   |
                  |         |         |     +-----------------------+
                  |   +-----+-----+   |    / Intermediate entity    |
                  |   | Accurate  |   |   /  in charge of:          |
   ASIC/FPGA      |   | Timestamp |   |  /__ - Timestamping         |
                  |   |  engine   |   |     |- Updating checksum or |
                  |   +-----------+   |     |  Checksum Complement  |
                  |         |         |     +-----------------------+
                  +---------+---------+
                            |
                            |NTP packets
                            |
                        ___ v _
                       /   \_/ \__
                      /           \_
                     /     IP      /
                     \_  Network  /
                      /           \
                      \__/\_   ___/
                            \_/
   ASIC: Application-Specific Integrated Circuit
   FPGA: Field-Programmable Gate Array
                Figure 1: Accurate Timestamping in NTP
 The accuracy of clock synchronization over packet networks is highly
 sensitive to delay jitters in the underlying network; this
 dramatically affects clock accuracy.  To address this challenge, the
 Precision Time Protocol (PTP) [IEEE1588] defines Transparent Clocks
 (TCs) -- switches and routers that improve end-to-end clock 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".

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

Mizrahi Experimental [Page 4] RFC 7821 NTP Checksum Complement March 2016

 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].
 This document defines the Checksum Complement for [NTPv4].  The
 Checksum Complement is a 2-octet field that resides at the end of the
 UDP payload.  It allows intermediate entities to update NTP packets
 and maintain the correctness of the UDP Checksum by modifying the
 last 2 octets of the packet, instead of updating the UDP Checksum
 field.  This is performed by adding an NTP extension field at the end
 of the packet, in which the last 2 octets are used as a Checksum
 Complement.
 The usage of the Checksum Complement can in some cases simplify the
 implementation, because if the packet data is processed in 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.  Note
 that while it is not impossible to implement a hardware timestamper
 that updates the UDP Checksum, using the Checksum Complement instead
 can significantly simplify the implementation.
 Note that the software layer and the intermediate entity (see
 Figure 1) are two modules in a single NTP clock.  It is assumed that
 these two modules are in agreement regarding whether transmitted NTP
 packets include the Checksum Complement or not.
 [RFC7820] defines the Checksum Complement mechanism for the One-Way
 Active Measurement Protocol (OWAMP) and the Two-Way Active
 Measurement Protocol (TWAMP).  A similar mechanism is presented in
 Annex E of [IEEE1588].

2. Conventions Used in This Document

2.1. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [KEYWORDS].

Mizrahi Experimental [Page 5] RFC 7821 NTP Checksum Complement March 2016

2.2. Abbreviations

 MAC      Message Authentication Code
 NTP      Network Time Protocol
 PTP      Precision Time Protocol
 UDP      User Datagram Protocol

3. Using the UDP Checksum Complement in NTP

3.1. Overview

 The UDP Checksum Complement is a 2-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.
                       +--------------------------------+
                       |        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 Packets
 The Checksum Complement is used to compensate for changes performed
 in the NTP packet by intermediate entities, as described in the
 Introduction (Section 1).  An example of the usage of the Checksum
 Complement is provided in Appendix A.

Mizrahi Experimental [Page 6] RFC 7821 NTP Checksum Complement March 2016

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 3.
 If the NTP packet includes more than one extension field, the
 Checksum Complement extension is always the last extension field.
 Thus, the Checksum Complement is the last 2 octets in the UDP payload
 and is located at (UDP Length - 2 octets) after the beginning of the
 UDP header.  Note that the Checksum Complement is not used in
 authenticated NTP packets, as further discussed in Section 3.4.

3.2.1. Using the Checksum Complement

 As described in Section 1, an intermediate entity that updates the
 timestamp in the NTP packet can use the Checksum Complement in order
 to maintain the correctness of the UDP Checksum field.  Specifically,
 if the value of the timestamp is updated, this update yields a change
 in the UDP Checksum value; thus, the intermediate entity assigns a
 new value in the Checksum Complement that cancels this change,
 leaving the current value of the UDP Checksum correct.  An example of
 the usage of the Checksum Complement is provided in Appendix A.
   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       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  |                              MBZ                              |
  |                                                               |
  |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                               |      Checksum Complement      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Figure 3: NTP Checksum Complement Extension Field

Mizrahi Experimental [Page 7] RFC 7821 NTP Checksum Complement March 2016

 Field Type
    A dedicated Field Type value is used to identify the Checksum
    Complement extension.  See Section 5.
 Length
    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.
    [RFC7822] provides further details about the length of an
    extension field in the absence of a MAC.
 MBZ
    The extension field includes a 22-octet MBZ (MUST be zero) field.
    This field MUST be set to 0 and MUST be ignored by the recipient.
    The MBZ field is used for padding the extension field to
    28 octets.
 Checksum Complement
    The Checksum Complement extension includes the Checksum Complement
    field, residing in the last 2 octets of the extension.

3.2.2. Transmission of NTP with Checksum Complement

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

3.2.3. Updates of NTP with Checksum Complement

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

3.2.4. 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.

Mizrahi Experimental [Page 8] RFC 7821 NTP Checksum Complement March 2016

3.3. Interoperability with Existing Implementations

 The behavior defined in this document does not impose new
 requirements on the reception of NTP packets beyond the requirements
 defined in [RFC7822].  Note that, as defined in [RFC7822], a host
 that receives an NTP message with an unknown extension field SHOULD
 ignore the extension field and MAY drop the packet if policy requires
 it.  Thus, transmitters and intermediate entities that support the
 Checksum Complement can transparently interoperate with receivers
 that are not Checksum Complement compliant, as long as these
 receivers ignore unknown extension fields.  It is noted that existing
 implementations that discard packets with unknown extension fields
 cannot interoperate with transmitters that use the Checksum
 Complement.
 It should be noted that when hardware-based timestamping is used, it
 will likely be used at both ends, and thus both hosts that take part
 in the protocol will support the functionality described in this
 memo.  If only one of the hosts uses hardware-based timestamping,
 then the Checksum Complement can only be used if it is known that the
 peer host can accept the Checksum Complement.

3.4. The Checksum Complement and Authentication

 A Checksum Complement MUST NOT be used when authentication is
 enabled.  The Checksum Complement is useful 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 recompute the
 Message Authentication Code (MAC) accordingly.  In this case, it is
 not possible to update the Checksum Complement; updating the Checksum
 Complement would result in having to recalculate the MAC, and there
 would be a cyclic dependency between the MAC and the Checksum
 Complement.  Hence, when updating the MAC, it is necessary to update
 the UDP Checksum field, making the Checksum Complement field
 unnecessary in the presence of authentication.

Mizrahi Experimental [Page 9] RFC 7821 NTP Checksum Complement March 2016

4. Security Considerations

 This document describes how a Checksum Complement extension can be
 used for maintaining the correctness of the UDP Checksum.  The
 security considerations of time protocols in general are discussed in
 [SecTime], and the security considerations of NTP are discussed in
 [NTPv4].
 The purpose of this extension is to ease the implementation of
 accurate timestamping engines, as illustrated in Figure 1.  The
 extension is intended to be used internally in an NTP client or
 server.  This extension is not intended to be used by switches and
 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 man-in-the-middle (MITM) attack.
 It is important to emphasize that the scheme described in this
 document does not increase the protocol's vulnerability to MITM
 attacks; a MITM attacker 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 discussed in Section 3.4, if a
 cryptographic security mechanism is used, then the Checksum
 Complement does not simplify the implementation compared to using the
 conventional Checksum, and therefore the Checksum Complement is not
 used.

5. IANA Considerations

 IANA has allocated a new value in the "NTP Extension Field Types"
 registry:
    0x2005 Checksum Complement

Mizrahi Experimental [Page 10] RFC 7821 NTP Checksum Complement March 2016

6. References

6.1. Normative References

 [Checksum]  Rijsinghani, A., Ed., "Computation of the Internet
             Checksum via Incremental Update", RFC 1624,
             DOI 10.17487/RFC1624, May 1994,
             <http://www.rfc-editor.org/info/rfc1624>.
 [IPv6]      Deering, S. and R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
             December 1998, <http://www.rfc-editor.org/info/rfc2460>.
 [KEYWORDS]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119,
             DOI 10.17487/RFC2119, March 1997,
             <http://www.rfc-editor.org/info/rfc2119>.
 [NTPv4]     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, <http://www.rfc-editor.org/info/rfc5905>.
 [RFC7822]   Mizrahi, T. and D. Mayer, "Network Time Protocol
             Version 4 (NTPv4) Extension Fields", RFC 7822,
             DOI 10.17487/RFC7822, March 2016,
             <http://www.rfc-editor.org/info/rfc7822>.
 [UDP]       Postel, J., "User Datagram Protocol", STD 6, RFC 768,
             DOI 10.17487/RFC768, August 1980,
             <http://www.rfc-editor.org/info/rfc768>.

6.2. Informative References

 [IEEE1588]  IEEE, "IEEE Standard for a Precision Clock
             Synchronization Protocol for Networked Measurement and
             Control Systems", IEEE Std 1588-2008,
             DOI 10.1109/IEEESTD.2008.4579760, July 2008.
 [RFC7820]   Mizrahi, T., "UDP Checksum Complement in the One-Way
             Active Measurement Protocol (OWAMP) and Two-Way Active
             Measurement Protocol (TWAMP)", RFC 7820,
             DOI 10.17487/RFC7820, March 2016,
             <http://www.rfc-editor.org/info/rfc7820>.

Mizrahi Experimental [Page 11] RFC 7821 NTP Checksum Complement March 2016

 [SecTime]   Mizrahi, T., "Security Requirements of Time Protocols in
             Packet Switched Networks", RFC 7384,
             DOI 10.17487/RFC7384, October 2014,
             <http://www.rfc-editor.org/info/rfc7384>.
 [ZeroChecksum]
             Fairhurst, G. and M. Westerlund, "Applicability Statement
             for the Use of IPv6 UDP Datagrams with Zero Checksums",
             RFC 6936, DOI 10.17487/RFC6936, April 2013,
             <http://www.rfc-editor.org/info/rfc6936>.

Mizrahi Experimental [Page 12] RFC 7821 NTP Checksum Complement March 2016

Appendix A. Checksum Complement Usage Example

 Consider an NTP packet sent by an NTP client to an NTP server.
 The client's software layer (see Figure 1) generates an NTP packet
 with an Origin Timestamp T and a UDP Checksum value U.  The value of
 U is the checksum of the UDP header, UDP payload, and pseudo-header.
 Thus, U is equal to:
                       U = Const + checksum(T)                     (1)
 Where "Const" is the checksum of all the fields that are covered by
 the checksum, except the Origin Timestamp T.
 Recall that the client's software emits the NTP packet with a
 Checksum Complement extension field, which resides at the end of the
 PTP packet.  It is assumed that the client initially assigns zero to
 the value of the Checksum Complement.
 The client's timestamping engine updates the Origin Timestamp field
 to the accurate time, changing its value from T to T'.  The engine
 also updates the Checksum Complement field from zero to a new value
 C, such that:
                 checksum(C) = checksum(T) - checksum(T')          (2)
 When the NTP packet is transmitted by the client's timestamping
 engine, the value of the checksum remains U as before:
    U = Const + checksum(T) = Const + checksum(T) + checksum(T') -
        checksum(T') = Const + checksum(T') + checksum(C)          (3)
 Thus, after the timestamping engine has updated the timestamp,
 U remains the correct checksum of the packet.
 When the NTP packet reaches the NTP server, the server performs a
 conventional UDP Checksum computation, and the computed value is U.
 Since the Checksum Complement is part of the extension field, its
 value (C) is transparently included in the computation, as per
 Equation (3), without requiring special treatment by the server.

Mizrahi Experimental [Page 13] RFC 7821 NTP Checksum Complement March 2016

Acknowledgments

 The author gratefully thanks Danny Mayer, Miroslav Lichvar, Paul
 Kyzivat, Suresh Krishnan, and Brian Haberman for their review and
 helpful comments.

Author's Address

 Tal Mizrahi
 Marvell
 6 Hamada St.
 Yokneam, 20692
 Israel
 Email: talmi@marvell.com

Mizrahi Experimental [Page 14]

/data/webs/external/dokuwiki/data/pages/rfc/rfc7821.txt · Last modified: 2016/03/31 00:41 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki