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rfc:rfc7820

Internet Engineering Task Force (IETF) T. Mizrahi Request for Comments: 7820 Marvell Category: Experimental March 2016 ISSN: 2070-1721

                     UDP Checksum Complement in
        the One-Way Active Measurement Protocol (OWAMP) and
            Two-Way Active Measurement Protocol (TWAMP)

Abstract

 The One-Way Active Measurement Protocol (OWAMP) and the Two-Way
 Active Measurement Protocol (TWAMP) are used for performance
 monitoring in IP networks.  Delay measurement is performed in these
 protocols by using timestamped test packets.  Some implementations
 use hardware-based timestamping engines that integrate the accurate
 transmission time into every outgoing OWAMP/TWAMP test packet during
 transmission.  Since these packets are transported over UDP, the UDP
 Checksum field is then updated to reflect this modification.  This
 document proposes to use the last 2 octets of every test packet as a
 Checksum Complement, allowing timestamping engines to reflect the
 checksum modification in the last 2 octets rather than in the UDP
 Checksum field.  The behavior defined in this document is completely
 interoperable with existing OWAMP/TWAMP 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/rfc7820.

Mizrahi Experimental [Page 1] RFC 7820 OWAMP and TWAMP 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
 2. Conventions Used in This Document ...............................5
    2.1. Terminology ................................................5
    2.2. Abbreviations ..............................................5
 3. Using the UDP Checksum Complement in OWAMP and TWAMP ............6
    3.1. Overview ...................................................6
    3.2. OWAMP/TWAMP Test Packets with Checksum Complement ..........6
         3.2.1. Transmission of OWAMP/TWAMP with Checksum
                Complement .........................................10
         3.2.2. Intermediate Updates of OWAMP/TWAMP with
                Checksum Complement ................................10
         3.2.3. Reception of OWAMP/TWAMP with Checksum Complement ..10
    3.3. Interoperability with Existing Implementations ............10
    3.4. Using the Checksum Complement with or without
         Authentication ............................................11
         3.4.1. Checksum Complement in Authenticated Mode ..........11
         3.4.2. Checksum Complement in Encrypted Mode ..............11
 4. Security Considerations ........................................12
 5. References .....................................................12
    5.1. Normative References ......................................12
    5.2. Informative References ....................................13
 Appendix A. Checksum Complement Usage Example .....................14
 Acknowledgments ...................................................15
 Author's Address ..................................................15

Mizrahi Experimental [Page 2] RFC 7820 OWAMP and TWAMP Checksum Complement March 2016

1. Introduction

 The One-Way Active Measurement Protocol [OWAMP] and the Two-Way
 Active Measurement Protocol [TWAMP] are used for performance
 monitoring in IP networks.
 Delay and delay variation are two of the metrics that OWAMP/TWAMP can
 measure.  Measurement is performed using timestamped test packets.
 In some use cases, such as carrier networks, these two metrics are an
 essential aspect of the Service Level Agreement (SLA) and therefore
 must be measured with a high degree of accuracy.  If packets are
 timestamped in hardware as they exit the host, then greater accuracy
 is possible in comparison to higher-layer timestamps (as explained
 further below).
 The accuracy of delay measurements relies on the timestamping method
 and its implementation.  In order to facilitate accurate
 timestamping, an implementation can use a hardware-based timestamping
 engine, as shown in Figure 1.  In such cases, the OWAMP/TWAMP packets
 are sent and received by a software layer, whereas the timestamping
 engine modifies every outgoing test packet by incorporating its
 accurate transmission time into the Timestamp field in the packet.

Mizrahi Experimental [Page 3] RFC 7820 OWAMP and TWAMP Checksum Complement March 2016

                OWAMP/TWAMP-enabled Node
                  +-------------------+
                  |                   |
                  |   +-----------+   |
   Software       |   |OWAMP/TWAMP|   |
                  |   | protocol  |   |
                  |   +-----+-----+   |
                  |         |         |     +-----------------------+
                  |   +-----+-----+   |    / Intermediate entity    |
                  |   | Accurate  |   |   /  in charge of:          |
   ASIC/FPGA      |   | Timestamp |   |  /__ - Timestamping         |
                  |   |  engine   |   |     |- Updating checksum or |
                  |   +-----------+   |     |  Checksum Complement  |
                  |         |         |     +-----------------------+
                  +---------+---------+
                            |
                            |test packets
                            |
                        ___ v _
                       /   \_/ \__
                      /           \_
                     /     IP      /
                     \_  Network  /
                      /           \
                      \__/\_   ___/
                            \_/
   ASIC: Application-Specific Integrated Circuit
   FPGA: Field-Programmable Gate Array
            Figure 1: Accurate Timestamping in OWAMP/TWAMP
 OWAMP/TWAMP test packets are transported over UDP.  When the UDP
 payload is changed by an intermediate entity such as the timestamping
 engine, the UDP Checksum field must be updated to reflect the new
 payload.  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].

Mizrahi Experimental [Page 4] RFC 7820 OWAMP and TWAMP Checksum Complement March 2016

 A similar problem is addressed in Annex E of [IEEE1588].  When the
 Precision Time Protocol (PTP) is transported over IPv6, 2 octets are
 appended to the end of the PTP payload for UDP Checksum updates.  The
 value of these 2 octets can be updated by an intermediate entity,
 causing the value of the UDP Checksum field to remain correct.
 This document defines a similar concept for [OWAMP] and [TWAMP],
 allowing intermediate entities to update OWAMP/TWAMP test packets and
 maintain the correctness of the UDP Checksum by modifying the last
 2 octets of the packet.
 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, 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.
 The Checksum Complement mechanism is also defined for the Network
 Time Protocol in [RFC7821].

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].

2.2. Abbreviations

 HMAC     Hashed Message Authentication Code
 OWAMP    One-Way Active Measurement Protocol
 PTP      Precision Time Protocol
 TWAMP    Two-Way Active Measurement Protocol
 UDP      User Datagram Protocol

Mizrahi Experimental [Page 5] RFC 7820 OWAMP and TWAMP Checksum Complement March 2016

3. Using the UDP Checksum Complement in OWAMP and TWAMP

3.1. Overview

 The UDP Checksum Complement is a 2-octet field that is piggybacked at
 the end of the test packet.  It resides in the last 2 octets of the
 UDP payload.
                 +----------------------------------+
                 |         IPv4/IPv6 Header         |
                 +----------------------------------+
                 |            UDP Header            |
                 +----------------------------------+
          ^      |                                  |
          |      |           OWAMP/TWAMP            |
         UDP     |             packet               |
        Payload  +----------------------------------+
          |      |UDP Checksum Complement (2 octets)|
          v      +----------------------------------+
       Figure 2: Checksum Complement in OWAMP/TWAMP Test Packets
 The Checksum Complement is used to compensate for changes performed
 in the 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. OWAMP/TWAMP Test Packets with Checksum Complement

 The One-Way Active Measurement Protocol [OWAMP] and the Two-Way
 Active Measurement Protocol [TWAMP] both make use of timestamped test
 packets.  A Checksum Complement MAY be used in the following cases:
 o  In OWAMP test packets sent by the sender to the receiver.
 o  In TWAMP test packets sent by the sender to the reflector.
 o  In TWAMP test packets sent by the reflector to the sender.
 OWAMP/TWAMP test packets are transported over UDP, either over IPv4
 or over IPv6.  This document applies to both OWAMP and TWAMP over
 IPv4 and over IPv6.

Mizrahi Experimental [Page 6] RFC 7820 OWAMP and TWAMP Checksum Complement March 2016

 OWAMP/TWAMP test packets contain a Packet Padding field.  This
 document proposes to use the last 2 octets of the Packet Padding
 field as the Checksum Complement.  In this case, the Checksum
 Complement is always the last 2 octets of the UDP payload, and thus
 the field is located at (UDP Length - 2 octets) after the beginning
 of the UDP header.
 Figure 3 illustrates the OWAMP test packet format, including the UDP
 Checksum Complement.
   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        Sequence Number                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          Timestamp                            |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |        Error Estimate         |                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
  |                                                               |
  .                         Packet Padding                        .
  .                                                               .
  |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                               |      Checksum Complement      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          Figure 3: Checksum Complement in OWAMP Test Packets

Mizrahi Experimental [Page 7] RFC 7820 OWAMP and TWAMP Checksum Complement March 2016

 Figure 4 illustrates the TWAMP test packet format, including the UDP
 Checksum Complement.  ("TTL" means "Time to Live", and "MBZ" refers
 to the "MUST be zero" field [IPPMIPsec].)
   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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        Sequence Number                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          Timestamp                            |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |         Error Estimate        |           MBZ                 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Receive Timestamp                         |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                   Sender Sequence Number                      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                      Sender Timestamp                         |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |      Sender Error Estimate    |           MBZ                 |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Sender TTL   |                                               |
  +-+-+-+-+-+-+-+-+                                               +
  |                                                               |
  .                                                               .
  .                         Packet Padding                        .
  .                                                               .
  |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                               |     Checksum Complement       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          Figure 4: Checksum Complement in TWAMP Test Packets
 The length of the Packet Padding field in test packets is announced
 during the session initiation through the <Padding Length> field in
 the Request-Session message [OWAMP] or in the Request-TW-Session
 message [TWAMP].

Mizrahi Experimental [Page 8] RFC 7820 OWAMP and TWAMP Checksum Complement March 2016

 When a Checksum Complement is included, the padding length MUST be
 sufficiently long to include the Checksum Complement:
 o  In OWAMP, the padding length is at least 2 octets, allowing the
    sender to incorporate the Checksum Complement in the last 2 octets
    of the padding.
 o  In TWAMP, the padding length is at least 29 octets in
    unauthenticated mode and at least 58 octets in authenticated mode.
    The additional padding is required, since the header of reflector
    test packets is longer than the header of sender test packets.
    The difference between the sender packet and the reflector packet
    is 27 octets in unauthenticated mode and 56 octets in
    authenticated mode.  Thus, the padding in reflector test packets
    is shorter than the padding in sender packets.  Using at least
    29 octets of padding (58 in authenticated mode) in sender test
    packets allows both the sender and the reflector to use a 2-octet
    Checksum Complement.  Note: If the minimal length requirement is
    not met, the reflector cannot use a Checksum Complement in the
    reflected test packets, but the sender can use a Checksum
    Complement in the test packets it transmits.
 o  Two optional TWAMP features are defined in [TWAMP-Reflect]:
    octet reflection and symmetrical size.  When at least one of these
    features is enabled, the Request-TW-Session message includes the
    <Padding Length> field, as well as a <Length of padding to
    reflect> field.  In this case, both fields must be sufficiently
    long to allow at least 2 octets of padding in both sender test
    packets and reflector test packets.  Specifically, when octet
    reflection is enabled, the two Length fields must be defined such
    that the padding expands at least 2 octets beyond the end of the
    reflected octets.
 As described in Section 1, the extensions described in this document
 are implemented by two logical layers -- a protocol layer and a
 timestamping layer.  It is assumed that the two layers are
 synchronized regarding whether the usage of the Checksum Complement
 is enabled or not; since both logical layers reside in the same
 network device, it is assumed that there is no need for a protocol
 that synchronizes this information between the two layers.  When
 Checksum Complement usage is enabled, the protocol layer must take
 care to verify that test packets include the necessary padding,
 thereby avoiding the need for the timestamping layer to verify that
 en-route test packets include the necessary padding.

Mizrahi Experimental [Page 9] RFC 7820 OWAMP and TWAMP Checksum Complement March 2016

3.2.1. Transmission of OWAMP/TWAMP with Checksum Complement

 The transmitter of an OWAMP/TWAMP test packet MAY include a Checksum
 Complement field, incorporated in the last 2 octets of the padding.
 A transmitter that includes a Checksum Complement in its outgoing
 test packets MUST include a Packet Padding field in these packets,
 the length of which MUST be sufficient to include the Checksum
 Complement.  The length of the Packet Padding field is negotiated
 during session initiation, as described in Section 3.2.

3.2.2. Intermediate Updates of OWAMP/TWAMP with Checksum Complement

 An intermediate entity that receives and alters an OWAMP/TWAMP
 test packet can alter either the UDP Checksum field or the Checksum
 Complement field in order to maintain the correctness of the
 UDP Checksum value.

3.2.3. Reception of OWAMP/TWAMP with Checksum Complement

 This document does not impose new requirements on the receiving end
 of an OWAMP/TWAMP test packet.
 The UDP layer at the receiving end verifies the UDP Checksum of
 received test packets, and the OWAMP/TWAMP layer should treat the
 Checksum Complement as part of the padding.

3.3. Interoperability with Existing Implementations

 The behavior defined in this document does not impose new
 requirements on the reception behavior of OWAMP/TWAMP test packets.
 The protocol stack of the receiving host performs the conventional
 UDP Checksum verification; thus, from the perspective of the
 receiving host, the existence of the Checksum Complement is
 transparent.  Therefore, the functionality described in this document
 allows interoperability with existing implementations that comply
 with [OWAMP] or [TWAMP].

Mizrahi Experimental [Page 10] RFC 7820 OWAMP and TWAMP Checksum Complement March 2016

3.4. Using the Checksum Complement with or without Authentication

 Both OWAMP and TWAMP may use authentication or encryption, as defined
 in [OWAMP] and [TWAMP].

3.4.1. Checksum Complement in Authenticated Mode

 OWAMP and TWAMP test packets can be authenticated using an HMAC
 (Hashed Message Authentication Code).  The HMAC covers some of the
 fields in the test packet header.  The HMAC does not cover the
 Timestamp field and the Packet Padding field.
 A Checksum Complement MAY be used when authentication is enabled.  In
 this case, an intermediate entity can timestamp test packets and
 update their Checksum Complement field without modifying the HMAC.

3.4.2. Checksum Complement in Encrypted Mode

 When OWAMP and TWAMP are used in encrypted mode, the Timestamp field
 is encrypted.
 A Checksum Complement SHOULD NOT be used in encrypted mode.  The
 Checksum Complement is effective in both unauthenticated mode and
 authenticated mode, allowing the intermediate entity to perform
 serial processing of the packet without storing and forwarding it.
 On the other hand, in encrypted mode, an intermediate entity that
 timestamps a test packet must also re-encrypt the packet accordingly.
 Re-encryption typically requires the intermediate entity to store the
 packet, re-encrypt it, and then forward it.  Thus, from an
 implementer's perspective, the Checksum Complement has very little
 value in encrypted mode, as it does not necessarily simplify the
 implementation.
 Note: While [OWAMP] and [TWAMP] include an inherent security
 mechanism, these protocols can be secured by other measures, e.g.,
 [IPPMIPsec].  For reasons similar to those described above, a
 Checksum Complement SHOULD NOT be used in this case.

Mizrahi Experimental [Page 11] RFC 7820 OWAMP and TWAMP 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 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 OWAMP/TWAMP-enabled
 node, and not intended to be used by intermediate switches and
 routers that reside between the sender and the receiver/reflector.
 Any modification of a test packet by intermediate switches or routers
 should be considered 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 or authenticated mode.  As described in
 Section 3.4.2, using the Checksum Complement in encrypted mode does
 not simplify the implementation compared to using the conventional
 checksum, and therefore the Checksum Complement should not be used.

5. References

5.1. 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>.
 [OWAMP]     Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
             Zekauskas, "A One-way Active Measurement Protocol
             (OWAMP)", RFC 4656, DOI 10.17487/RFC4656, September 2006,
             <http://www.rfc-editor.org/info/rfc4656>.

Mizrahi Experimental [Page 12] RFC 7820 OWAMP and TWAMP Checksum Complement March 2016

 [TWAMP]     Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
             Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
             RFC 5357, DOI 10.17487/RFC5357, October 2008,
             <http://www.rfc-editor.org/info/rfc5357>.
 [TWAMP-Reflect]
             Morton, A. and L. Ciavattone, "Two-Way Active Measurement
             Protocol (TWAMP) Reflect Octets and Symmetrical Size
             Features", RFC 6038, DOI 10.17487/RFC6038, October 2010,
             <http://www.rfc-editor.org/info/rfc6038>.
 [UDP]       Postel, J., "User Datagram Protocol", STD 6, RFC 768,
             DOI 10.17487/RFC768, August 1980,
             <http://www.rfc-editor.org/info/rfc768>.

5.2. 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.
 [IPPMIPsec] Pentikousis, K., Ed., Zhang, E., and Y. Cui,
             "IKEv2-Derived Shared Secret Key for the One-Way Active
             Measurement Protocol (OWAMP) and Two-Way Active
             Measurement Protocol (TWAMP)", RFC 7717,
             DOI 10.17487/RFC7717, December 2015,
             <http://www.rfc-editor.org/info/rfc7717>.
 [RFC7821]   Mizrahi, T., "UDP Checksum Complement in the Network Time
             Protocol (NTP)", RFC 7821, DOI 10.17487/RFC7821,
             March 2016, <http://www.rfc-editor.org/info/rfc7821>.
 [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 13] RFC 7820 OWAMP and TWAMP Checksum Complement March 2016

Appendix A. Checksum Complement Usage Example

 Consider a session between an OWAMP sender and an OWAMP receiver, in
 which the sender transmits test packets to the receiver.
 The sender's software layer generates an OWAMP test packet with a
 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 timestamp T.
 Recall that the sender's software emits the test packet with a
 Checksum Complement field, which is simply the last 2 octets of the
 padding.  In this example, it is assumed that the sender initially
 assigns zero to these 2 octets.
 The sender's timestamping engine updates the Timestamp field to the
 accurate time, changing its value from T to T'.  The sender also
 updates the Checksum Complement field from zero to a new value C,
 such that:
                checksum(C) = checksum(T) - checksum(T')           (2)
 When the test packet is transmitted by the sender'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 test packet reaches the receiver, the receiver performs a
 conventional UDP Checksum computation, and the computed value is U.
 Since the Checksum Complement is part of the padding, the value of
 checksum(C) is transparently included in the computation, as per
 Equation (3), without requiring special treatment by the receiver.

Mizrahi Experimental [Page 14] RFC 7820 OWAMP and TWAMP Checksum Complement March 2016

Acknowledgments

 The author gratefully acknowledges Al Morton, Greg Mirsky, Steve
 Baillargeon, Brian Haberman, and Spencer Dawkins for their helpful
 comments.

Author's Address

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

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