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)




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.

このドキュメントはInternet Standards Trackの仕様ではありません。試験、実験、評価のために公開されています。

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.

このドキュメントでは、インターネットコミュニティの実験プロトコルを定義します。このドキュメントは、IETF(Internet Engineering Task Force)の製品です。これは、IETFコミュニティのコンセンサスを表しています。公開レビューを受け、インターネットエンジニアリングステアリンググループ(IESG)による公開が承認されました。 IESGによって承認されたすべてのドキュメントが、あらゆるレベルのインターネット標準の候補になるわけではありません。 RFC 5741のセクション2をご覧ください。

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at


Copyright Notice


Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

Copyright(c)2016 IETF Trustおよびドキュメントの作成者として識別された人物。全著作権所有。

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.

この文書は、BCP 78およびIETF文書に関するIETFトラストの法的規定(の対象であり、この文書の発行日に有効です。これらのドキュメントは、このドキュメントに関するあなたの権利と制限を説明しているため、注意深く確認してください。このドキュメントから抽出されたコードコンポーネントには、Trust Legal Provisionsのセクション4.eに記載されているSimplified BSD Licenseのテキストが含まれている必要があり、Simplified BSD Licenseに記載されているように保証なしで提供されます。

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
1. Introduction
1. はじめに

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
1.1. 中間エンティティ

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.

図1に示すように、正確なタイムスタンプを容易にするために、実装ではハードウェアベースのタイムスタンプエンジンを使用できます。このような場合、NTPパケットはソフトウェア層によって送受信されますが、タイムスタンプエンジンは、正確な送信時間をパケットの<Transmit Timestamp>フィールドに入力します。

                      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".

パケットネットワーク上のクロック同期の精度は、基盤となるネットワークの遅延ジッターに非常に敏感です。これはクロックの精度に劇的な影響を与えます。この課題に対処するために、プレシジョンタイムプロトコル(PTP)[IEEE1588]は、透過クロック(TC)を定義しています。遅延を追加することにより、PTPパケットの「補正フィールド」を更新することにより、エンドツーエンドのクロック精度を向上させます。現在のTCが原因です。 NTPでは、同等のエンティティは現在定義されていませんが、NTPの将来のバージョンでは、「修正フィールド」を使用して、途中のNTPパケットを変更する中間ノードを定義する可能性があります。

1.2. Updating the UDP Checksum
1.2. UDPチェックサムの更新

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

UDPペイロードが中間エンティティによって変更された場合、UDPチェックサムフィールドは、その正確性を維持するために更新する必要があります。 UDP over IPv4 [UDP]を使用する場合、更新できない中間エンティティ

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.

UDPチェックサムの値は、チェックサムフィールドにゼロの値を割り当てる以外に選択肢がありません。これにより、受信者はチェックサムフィールドを無視し、破損したパケットを受け入れる可能性があります。 [IPv6]で定義されているUDP over IPv6は、特定の場合[ZeroChecksum]を除いて、ゼロチェックサムを許可しません。 [ZeroChecksum]で説明したように、ゼロチェックサムの使用は一般に推奨されておらず、可能な限り回避する必要があります。

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.

パケットデータがシリアル順に処理される場合、タイムスタンプを更新してから更新するのではなく、最初にタイムスタンプフィールドを更新してからチェックサム補完を更新する方が簡単であるため、チェックサム補完の使用により、実装が簡素化される場合があります。 UDPヘッダーにあるUDPチェックサム。 UDPチェックサムを更新するハードウェアタイムスタンプを実装することは不可能ではありませんが、チェックサム補完を使用すると、実装を大幅に簡略化できることに注意してください。

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].

[RFC7820]は、一方向アクティブ測定プロトコル(OWAMP)および双方向アクティブ測定プロトコル(TWAMP)のチェックサム補完メカニズムを定義しています。 [IEEE1588]の付録Eにも同様のメカニズムが示されています。

2. Conventions Used in This Document
2. このドキュメントで使用される規則
2.1. Terminology
2.1. 用語

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].

このドキュメントのキーワード「MUST」、「MUST NOT」、「REQUIRED」、「SHALL」、「SHALL NOT」、「SHOULD」、「SHOULD NOT」、「RECOMMENDED」、「MAY」、および「OPTIONAL」は、 [キーワード]で説明されているように解釈されます。

2.2. Abbreviations
2.2. 略語

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. NTPでのUDPチェックサム補完の使用
3.1. Overview
3.1. 概観

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.


3.2. Checksum Complement in NTP Packets
3.2. NTPパケットのチェックサム補完

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
3.2.1. チェックサム補完の使用

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


Field Type


A dedicated Field Type value is used to identify the Checksum Complement extension. See Section 5.




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.

この長さは、パケットにMACが含まれているかどうかに関係なく、パケットを受信するホストがそれを正しく解析することを保証します。 [RFC7822]は、MACがない場合の拡張フィールドの長さに関する詳細を提供します。



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.

拡張フィールドには、22オクテットのMBZ(ゼロでなければならない)フィールドが含まれています。このフィールドは0に設定する必要があり、受信者は無視する必要があります。 MBZフィールドは、拡張フィールドを28オクテットにパディングするために使用されます。

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
3.2.2. チェックサム補完を使用したNTPの送信

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


3.2.3. Updates of NTP with Checksum Complement
3.2.3. チェックサム補完によるNTPの更新

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
3.2.4. チェックサム補完を使用したNTPの受信

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
3.3. 既存の実装との相互運用性

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.

このドキュメントで定義されている動作は、[RFC7822]で定義されている要件を超えて、NTPパケットの受信に新しい要件を課しません。 [RFC7822]で定義されているように、不明な拡張フィールドを持つNTPメッセージを受信するホストは拡張フィールドを無視し、ポリシーで必要な場合はパケットをドロップする必要があることに注意してください。したがって、チェックサム補完をサポートするトランスミッタと中間エンティティは、これらのレシーバが不明な拡張フィールドを無視する限り、チェックサム補完に対応していないレシーバと透過的に相互運用できます。未知の拡張フィールドを持つパケットを破棄する既存の実装は、チェックサム補完を使用する送信機と相互運用できないことに注意してください。

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
3.4. チェックサム補完と認証

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.


4. Security Considerations
4. セキュリティに関する考慮事項

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.

この拡張機能の目的は、図1に示すように、正確なタイムスタンプエンジンの実装を容易にすることです。拡張機能は、NTPクライアントまたはサーバーの内部で使用することを目的としています。この拡張機能は、クライアントとサーバーの間に存在するスイッチとルーターで使用することを目的としていません。 PTP [IEEE1588]とは対照的に、NTPはNTPメッセージの内容を変更するために中間スイッチまたはルーターを必要としないため、このような変更は悪意のある中間者(MITM)攻撃と見なされます。

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
5. IANAに関する考慮事項

IANA has allocated a new value in the "NTP Extension Field Types" registry:


0x2005 Checksum Complement


6. References
6. 参考文献
6.1. Normative References
6.1. 引用文献

[Checksum] Rijsinghani, A., Ed., "Computation of the Internet Checksum via Incremental Update", RFC 1624, DOI 10.17487/RFC1624, May 1994, <>.

[チェックサム] Rijsinghani、A。、編、「インクリメンタルアップデートによるインターネットチェックサムの計算」、RFC 1624、DOI 10.17487 / RFC1624、1994年5月、<> 。

[IPv6] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998, <>.

[IPv6] Deering、S。およびR. Hinden、「インターネットプロトコル、バージョン6(IPv6)仕様」、RFC 2460、DOI 10.17487 / RFC2460、1998年12月、< rfc2460>。

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

[キーワード] Bradner、S。、「RFCで使用して要件レベルを示すためのキーワード」、BCP 14、RFC 2119、DOI 10.17487 / RFC2119、1997年3月、< 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, <>.

[NTPv4] Mills、D.、Martin、J.、Ed。、Burbank、J。、およびW. Kasch、「Network Time Protocol Version 4:Protocol and Algorithms Specification」、RFC 5905、DOI 10.17487 / RFC5905、2010年6月、 <>。

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

[RFC7822] Mizrahi、T。およびD. Mayer、「Network Time Protocol Version 4(NTPv4)Extension Fields」、RFC 7822、DOI 10.17487 / RFC7822、March 2016、< / rfc7822>。

[UDP] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC768, August 1980, <>.

[UDP] Postel、J。、「ユーザーデータグラムプロトコル」、STD 6、RFC 768、DOI 10.17487 / RFC768、1980年8月、<>。

6.2. Informative References
6.2. 参考引用

[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.

[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, <>.

[RFC7820]ミズラヒ、T。、「一方向アクティブ測定プロトコル(OWAMP)および双方向アクティブ測定プロトコル(TWAMP)におけるUDPチェックサム補完」、RFC 7820、DOI 10.17487 / RFC7820、2016年3月、<http:/ />。

[SecTime] Mizrahi, T., "Security Requirements of Time Protocols in Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384, October 2014, <>.

[SecTime] Mizrahi、T。、「パケット交換ネットワークにおけるタイムプロトコルのセキュリティ要件」、RFC 7384、DOI 10.17487 / RFC7384、2014年10月、<>。

[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, <>.

[ZeroChecksum] Fairhurst、G。およびM. Westerlund、「ゼロチェックサムを使用したIPv6 UDPデータグラムの使用に関する適用性声明」、RFC 6936、DOI 10.17487 / RFC6936、2013年4月、< / info / rfc6936>。

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:

クライアントのソフトウェア層(図1を参照)は、Origin Timestamp TとUDPチェックサム値Uを持つNTPパケットを生成します。Uの値は、UDPヘッダー、UDPペイロード、および疑似ヘッダーのチェックサムです。したがって、Uは次と等しくなります。

                         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.

「Const」は、Origin Timestamp Tを除く、チェックサムの対象となるすべてのフィールドのチェックサムです。

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:

クライアントのタイムスタンプエンジンは、Origin Timestampフィールドを正確な時刻に更新し、その値をTからT 'に変更します。エンジンはまた、チェックサム補完フィールドをゼロから新しい値Cに更新します。

                   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.




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

著者は、レビューと役立つコメントを提供してくれたDanny Mayer、Miroslav Lichvar、Paul Kyzivat、Suresh Krishnan、およびBrian Habermanに感謝します。

Author's Address


Tal Mizrahi Marvell 6 Hamada St. Yokneam, 20692 Israel

Tal Mizrahi Marvell 6 Hamada St. Yokneam、20692 Israel