    Internet Engineering Task Force (IETF)                         A. Morton
    Request for Comments: 6038                                 L. Ciavattone
    Updates: 5357                                                  AT&T Labs
    Category: Standards Track                                   October 2010
    ISSN: 2070-1721

         Two-Way Active Measurement Protocol (TWAMP) Reflect Octets and
                           Symmetrical Size Features

    Abstract

       This memo describes two closely related features for the core
       specification of the Two-Way Active Measurement Protocol (TWAMP): an
       optional capability where the responding host returns some of the
       command octets or padding octets to the sender, and an optional
       sender packet format that ensures equal test packet sizes are used in
       both directions.

    Status of This Memo

       This is an Internet Standards Track document.

       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).  Further information on
       Internet Standards is available in 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/rfc6038.

    Copyright Notice

       Copyright (c) 2010 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.

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       This document may contain material from IETF Documents or IETF
       Contributions published or made publicly available before November
       10, 2008.  The person(s) controlling the copyright in some of this
       material may not have granted the IETF Trust the right to allow
       modifications of such material outside the IETF Standards Process.
       Without obtaining an adequate license from the person(s) controlling
       the copyright in such materials, this document may not be modified
       outside the IETF Standards Process, and derivative works of it may
       not be created outside the IETF Standards Process, except to format
       it for publication as an RFC or to translate it into languages other
       than English.

    Table of Contents

       1. Introduction ....................................................3
       2. Requirements Language ...........................................3
       3. Purpose and Scope ...............................................4
       4. TWAMP Control Extensions ........................................4
          4.1. Connection Setup with New Features .........................5
          4.2. Reflect Octets: Request-TW-Session Packet Format ...........6
          4.3. Reflect Octets: Accept Session Packet Format ...............7
          4.4. Additional Considerations ..................................9
       5. Extended TWAMP Test .............................................9
          5.1. Sender Behavior ............................................9
               5.1.1. Packet Timings ......................................9
               5.1.2. Reflect Octets: Packet Formats and Contents .........9
               5.1.3. Reflect Octets: Interaction with Padding
                      Truncation .........................................11
               5.1.4. Symmetrical Size: Session-Sender Packet Format .....13
               5.1.5. Symmetrical Size AND Reflect Octets:
                      Session-Sender Packet ..............................14
          5.2. Reflector Behavior ........................................14
               5.2.1. Reflect Octets: Session-Reflector Packet
                      Format and Contents ................................15
               5.2.2. Symmetrical Size: Session-Reflector Packet Format ..16
               5.2.3. Symmetrical Size AND Reflect Octets:
                      Session-Sender Packet Format .......................16
       6. Security Considerations ........................................17
       7. IANA Considerations ............................................17
          7.1. Registry Specification ....................................17
          7.2. Registry Contents .........................................17
       8. Acknowledgements ...............................................17
       9. Normative References ...........................................18

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    1.  Introduction

       The Two-Way Active Measurement Protocol (TWAMP) [RFC5357] is an
       extension of the One-way Active Measurement Protocol (OWAMP)
       [RFC4656].  The TWAMP specification gathered wide review as it
       approached completion, and the by-products were several
       recommendations for new features in TWAMP.

       This memo describes two closely related features for TWAMP.

       One is the OPTIONAL capability for the responder host to return a
       limited number of unassigned (padding) octets to the Control-Client
       or Session-Sender entities in both the TWAMP-Control and TWAMP-Test
       protocols.  With this capability, the Control-Client or Session-
       Sender can embed octets of information it deems useful and have the
       assurance that the corresponding reply/test packet will contain that
       information when it is reflected and returned (by the Server or
       Session-Reflector).

       The memo also adds an OPTIONAL capability to ensure that reflected
       test packets are the same size in both directions of transmission.
       This is accomplished by specifying a new TWAMP-Test Session-Sender
       packet format.  Although TWAMP [RFC5357] recommends padding
       truncation to achieve symmetrical sizes (to compensate for the
       Session-Reflector's larger test packet header), it's not guaranteed
       that the Session-Reflector will truncate the padding, and it's not
       possible if there's insufficient padding.

       This memo is an update to the TWAMP core protocol specified in
       [RFC5357].  Measurement systems are not required to implement the
       features described in this memo to claim compliance with [RFC5357].

       Throughout this memo, the bits marked MBZ (Must Be Zero) MUST be set
       to zero by senders and MUST be ignored by receivers.  Also, the HMAC
       (Hashed Message Authentication Code) MUST be calculated as defined in
       Section 3.2 of [RFC4656].

    2.  Requirements Language

       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 RFC 2119 [RFC2119].

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    3.  Purpose and Scope

       The purpose of this memo is to define two OPTIONAL closely related
       features for TWAMP [RFC5357].  The features enhance the TWAMP host's
       capabilities to perform a simple operation on control and test
       packets: the reflection of octets or padding, and the capability to
       ensure symmetrical size TWAMP-Test packets.  Motivations include
       permitting the controller host to tag packets with an index for
       simplified identification, and/or assert that the same size test
       packets will be used in each direction.

       The scope of the memo is limited to specifications of the following
       features:

       o  Reflect Octets: the capability of the Server/Session-Reflector to
          reflect specific octets back to the Client/Session-Sender, and a
          similar service provided by the Client and Session-Sender.

       o  Symmetrical Size: the capability to ensure that TWAMP-Test
          protocol uses the same packet size in both directions through
          support of a new TWAMP-Test Session-Sender test packet format in
          both the Session-Sender and the Session-Reflector.  Only the
          Session-Sender test packet format is new.

       This memo extends the modes of operation through assignment of two
       new values in the Modes field (see Section 3.1 of [RFC4656] for the
       format of the Server Greeting message), while retaining backward
       compatibility with the core TWAMP [RFC5357] implementations.  The two
       new values correspond to the two features defined in this memo.

       When the Server and Control-Client have agreed to use the Reflect
       Octets mode during control connection setup, then the Control-Client,
       the Server, the Session-Sender, and the Session-Reflector MUST all
       conform to the requirements of that mode, as identified below.

       When the Server and Control-Client have agreed to use the Symmetrical
       Size mode during control connection setup, then the Control-Client,
       the Server, the Session-Sender, and the Session-Reflector MUST all
       conform to the requirements of that mode, as identified below.

    4.  TWAMP Control Extensions

       TWAMP-Control protocol [RFC5357] uses the Modes field to identify and
       select specific communication capabilities, and this field is a
       recognized extension mechanism.  The following sections describe two
       such extensions.

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    4.1.  Connection Setup with New Features

       TWAMP connection establishment follows the procedure defined in
       Section 3.1 of [RFC4656] and Section 3.1 of [RFC5357].  The new
       features require two new bit positions (and values) to identify the
       ability of the Server/Session-Reflector to reflect specific octets
       back to the Control-Client/Session-Sender, and to support the new
       Session-Sender packet format in the TWAMP-Test protocol.  See the
       IANA section for details on the assigned values and bit positions.

       The Server sets one or both of the new bit positions in the Modes
       field of the Server Greeting message to indicate its capabilities and
       willingness to operate in either of these modes (or both) if desired.

       If the Control-Client intends to operate all test sessions invoked
       with this control connection using one or both of the new modes, it
       MUST set the Mode field bit corresponding to each function in the
       Setup Response message.  With this and other extensions, the Control-
       Client MAY set multiple Mode field bits in the Setup Response
       message.

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    4.2.  Reflect Octets: Request-TW-Session Packet Format

       The bits designated for the Reflect Octets feature in the Request-TW-
       Session command are as shown in the packet format below.

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      5        |  MBZ  | IPVN  |  Conf-Sender  | Conf-Receiver |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Number of Schedule Slots                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .           ... Many fields (66 octets) not shown ...           .
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                    Padding Length  (4 octets)                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Start Time  (8 octets)                   |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Timeout  (8 octets)                     |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Type-P Descriptor                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Octets to be reflected    |  Length of padding to reflect |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       MBZ (4 octets)                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                       HMAC (16 octets)                        |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       It is important to note that the Padding Length field continues to
       specify the number of padding octets that the Session-Sender will
       append to ALL TWAMP-Test packets associated with this test session.
       See below for considerations on the minimum length of the padding
       octets following the definitions of the two new fields that follow
       the Type-P Descriptor.

       Note that the number of padding octets appended to the Session-
       Reflector's test packet depends on support for the truncation process
       that TWAMP recommends, see Section 4.2.1 of [RFC5357].

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       The Octets to be reflected field SHALL be 2 octets long, as shown,
       and contain the octets that the Server MUST reflect in the Accept
       Session message as specified below.

       The Length of padding to reflect field SHALL be 2 octets long and
       contain an unsigned binary value in units of octets.  This field
       communicates the length of the padding in the TWAMP-Test Packet that
       the Session-Sender expects to be reflected and the length of octets
       that the Session-Reflector SHALL return in its TWAMP-Test packet
       format (see Section 5.2).  By including this length field in the
       Request-TW-Session message, a Server is able to determine if it can
       comply with a specific request to reflect padding in the TWAMP-Test
       packets and to arrange for the Session-Reflector processing in
       advance.

       The Padding Length SHOULD be >= 27 octets when specifying a test
       session using the Unauthenticated TWAMP-Test mode to allow for the
       truncation process that TWAMP recommends, see Section 4.2.1 of
       [RFC5357].

       The Padding Length SHOULD be >= 56 octets when specifying a test
       session using the Authenticated or Encrypted TWAMP-Test modes to
       allow for the truncation process that TWAMP recommends, see Section
       4.2.1 of [RFC5357].

       The Padding Length SHALL be > the Length of padding to reflect when
       specifying a test session using the OPTIONAL Reflect Octets mode.

       In Unauthenticated TWAMP-Test mode, the Padding Length SHALL be >= 27
       + Length of padding to reflect octets when specifying a test session
       using both the OPTIONAL Reflect Octets mode and the truncation
       process that TWAMP recommends, see Section 4.2.1 of [RFC5357].

       In Authenticated or Encrypted TWAMP-Test modes, the Padding Length
       SHALL be >= 56 + Length of padding to reflect octets when specifying
       a test session using both the OPTIONAL Reflect Octets mode and the
       truncation process that TWAMP recommends, see Section 4.2.1 of
       [RFC5357].

    4.3.  Reflect Octets: Accept Session Packet Format

       The bits designated for the Reflect Padding feature in the Accept
       Session command are as shown in the packet format below.

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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Accept     |      MBZ      |            Port               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
       |                                                               |
       |                        SID (16 octets)                        |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Reflected octets        |         Server octets         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       MBZ (8 octets)                          |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                       HMAC (16 octets)                        |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       The Reflected octets field SHALL contain the octets from the Request-
       TW-Session Octets to be reflected field and be 2 octets long, as
       shown.

       The Server octets field SHALL contain information that the Server
       intends to be returned in the TWAMP-Test Packet Padding (to be
       reflected) field, OR SHALL be zero, and be 2 octets long, as shown.
       Although the Server determines the SID, this field is very long (16
       octets) and does not normally appear in TWAMP-Test packets.  The
       following items MUST be part of compliant implementations using the
       Reflect Octets feature:

       o  When a Server does not require octets to be returned in TWAMP-Test
          packets, it MUST send all zeros in the Server octets field.

       o  When a Server intends octets to be returned in TWAMP-Test packets,
          it MUST send a non-zero value in the Server octets field, and the
          TWAMP-Test Sender MUST include those octets at the beginning of
          the Packet Padding (to be reflected) field.

       o  Section 4.1.2 defines how Server octets MUST be included in the
          TWAMP-Test packet padding when this service is desired by the
          Server (indicated in the second of two figures in this section).

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       When performing the truncation process that TWAMP recommends, see
       Section 4.2.1 of [RFC5357], if calculations on the Padding lengths
       reveal that there are insufficient octets supplied to produce equal-
       length Session-Sender and Session-Reflector test packets, then the
       Accept field MUST be set to 3 to indicate that some aspect of the
       request is not supported.

    4.4.  Additional Considerations

       The value of the Modes field sent by the Server in the Server
       Greeting message is the bit-wise OR of the mode values that it is
       willing to support during this session.

       With the publication of this memo as an RFC, the last 7 bit positions
       of the Modes 32-bit field are used.  A Control-Client conforming to
       this extension of [RFC5357] MAY ignore the values in the higher bits
       of the Modes field, or it MAY support other features that are
       communicated in those bit positions.  The other bits are available
       for future protocol extensions.

    5.  Extended TWAMP Test

       The TWAMP-Test protocol is similar to the OWAMP [RFC4656] test
       protocol with the exception that the Session-Reflector transmits test
       packets to the Session-Sender in response to each test packet it
       receives.  TWAMP, see Section 4 of [RFC5357], defines two additional
       test packet formats for packets transmitted by the Session-Reflector.
       The appropriate format depends on the security mode chosen.  The new
       modes specified here utilize some of the padding octets within each
       test packet format or require truncation of those octets depending on
       the security mode in use.

    5.1.  Sender Behavior

       This section describes extensions to the behavior of the TWAMP
       Session-Sender.

    5.1.1.  Packet Timings

       The Send Schedule is not utilized in TWAMP, and this is unchanged in
       this memo.

    5.1.2.  Reflect Octets: Packet Formats and Contents

       The Session-Sender packet format and content follow the same
       procedure and guidelines as defined in Section 4.1.2 of [RFC4656] (as
       indicated in Section 4.1.2 of [RFC5357]).

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       The Reflect Octets mode re-designates the original TWAMP-Test Packet
       Padding field (see Section 4.1.2 of [RFC4656]), as shown below for
       Unauthenticated mode:

        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 (to be reflected)             |
       .               (length in octets specified in command)         .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                  Additional Packet Padding                    .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       The Packet Padding (to be reflected) field SHALL correspond to the
       length of octets specified in the Request-TW-Session Length of
       padding to reflect field to this test session.  These are the octets
       that the Session-Sender expects will be returned by the Session-
       Reflector.

       The length of the Additional Packet Padding field is the difference
       between two fields in the Request-TW-Session command, as follows:

       Additional Packet Padding, in octets =

       Padding Length - Length of padding to reflect

       When a Server intends octets to be returned in TWAMP-Test packets, it
       MUST send a non-zero value in the Server octets field, and the TWAMP-
       Test Session-Sender MUST include those octets in the first 2 octets
       of the Packet Padding (to be reflected) field as shown below:

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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
                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                       |         Server octets         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |              Remaining Packet Padding (to be reflected)       |
       .             (total length in octets specified in command)     .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       The Server octets field contains the same information that the Server
       returned to the Control-Client in the Accept-Session message
       corresponding to this specific test session (see Section 4.3).  At
       the Session-Reflector, these octets MUST be reflected the same as the
       rest of the Packet Padding (to be reflected) field.

       Note that it is permissible for the Session-Sender to insert the same
       octets used in the Octets to be reflected field of the Request-TW-
       Session command elsewhere in the Packet Padding (to be reflected)
       field.

    5.1.3.  Reflect Octets: Interaction with Padding Truncation

       When the Reflect Octets mode is selected, and performing the
       truncation process that TWAMP recommends, see Section 4.2.1 of
       [RFC5357], the Session-Sender MUST anticipate a minimum padding
       required to achieve equal-size test packets in both directions.  The
       amount of padding needed to achieve symmetrical packet size depends
       on both the security mode (Unauthenticated/Authenticated/Encrypted)
       and whether the Reflect Octets mode is selected simultaneously.

       When using the truncation process in TWAMP alone, see Section 4.2.1
       of [RFC5357], the Session-Sender MUST append sufficient Packet
       Padding octets to allow the same IP packet payload lengths to be used
       in each direction of transmission (this is usually desirable).  To
       compensate for the Session-Reflector's larger test packet format, the
       Session-Sender MUST append at least 27 octets of padding in
       Unauthenticated mode, and at least 56 octets in Authenticated and
       Encrypted modes.  The sizes of TWAMP-Test protocol packets and the
       resulting truncated padding to achieve equal packet sizes in both
       directions are shown in the table below:

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        +-------------------+----------------------+---------------------+
        | Octets in:        | Unauthenticated Mode | Auth/Encrypted Mode |
        +-------------------+----------------------+---------------------+
        | Reflector Header  | 41                   | 104                 |
        | Sender Header     | 14                   | 48                  |
        | Truncated Padding | 27                   | 56                  |
        +-------------------+----------------------+---------------------+

                           TWAMP-Test Padding Truncation

       When using the Reflect Octets mode simultaneously with the truncation
       process that TWAMP recommends in Section 4.2.1 of [RFC5357], the
       Session-Sender MUST append at least 27 octets of padding plus the
       Length of the padding to reflect octets when operating in
       Unauthenticated mode.  The Session-Sender MUST append at least 56
       octets of padding plus the Length of the padding to reflect octets
       when operating in Authenticated and Encrypted modes.

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    5.1.4.  Symmetrical Size: Session-Sender Packet Format

       When the Symmetrical Size mode is selected, the Session-Sender SHALL
       use the following TWAMP-Test Packet Format in Unauthenticated mode:

       Unauthenticated Mode

        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 (27 octets)                       |
       |                                                               |
       |                                                               |
       |                                                               |
       +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               |                                               |
       +-+-+-+-+-+-+-+-+                                               +
       .                                                               .
       .                        Packet Padding                         .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       This feature REQUIRES only a new Session-Sender test packet format,
       the Session-Reflector test packet format is unchanged.

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    5.1.5.  Symmetrical Size AND Reflect Octets: Session-Sender Packet
            Format

       When both the Symmetrical Size mode and the Reflect Octets mode are
       selected, the Session-Sender SHALL use the following TWAMP-Test
       Packet Format in Unauthenticated mode:

       Unauthenticated Mode

        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 (27 octets)                       |
       |                                                               |
       |                                                               |
       |                                                               |
       +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               |                                               |
       +-+-+-+-+-+-+-+-+                                               +
       |                  Packet Padding (to be reflected)             |
       .               (length in octets specified in command)         .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                  Additional Packet Padding                    .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       In this combined mode, the Packet Padding to be reflected follows the
       27 MBZ octets.  In Authenticated or Encrypted modes, the Packet
       Padding to be reflected follows the 56 MBZ octets.

    5.2.  Reflector Behavior

       The TWAMP Reflector follows the procedures and guidelines in Section
       4.2 of [RFC5357], with the following additional functions:

       o  Reflect Octets mode: Designated octets in the Packet Padding (to
          be reflected) field of the Session-Sender's test packet MUST be
          included in the Session-Reflector's test packet.

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       o  Symmetrical Size mode: The Session-Reflector MUST operate using
          the Session_Reflector Packet Format defined in Section 5.1.4,
          where the Padding Octets are separated from the information
          fields.

    5.2.1.  Reflect Octets: Session-Reflector Packet Format and Contents

       The Reflect Padding feature re-designates the Packet Padding field,
       as shown below.  When the Reflect Octets mode is selected, the
       Session-Sender SHALL use the following TWAMP-Test Packet Format in
       Unauthenticated mode:

        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 (from Session-Sender)  |
        +-+-+-+-+-+-+-+-+                                               +
        .                                                               .
        +                                               +-+-+-+-+-+-+-+-+
        |          Packet Padding (from Session-Sender) |               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
        |                                                               |
        |                                                               |
        .                  Additional Packet Padding                    .
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       The Packet Padding (from Session-Sender) field MUST be the same
       octets as the Packet Padding (to be reflected) field in the Session-
       Sender's test packet, and therefore MUST conform to the length
       specified in the Request-TW-Session message.

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    RFC 6038             Reflect Octets & Symmetric Size        October 2010

       When the Server has returned a non-zero value in the Server octets
       field of the Accept Session message (as described in Section 4.3),
       then the Session-Reflector SHALL reflect these octets the same as the
       rest of the Packet Padding (to be reflected) field.

       Section 4.2.1 of [RFC5357] recommends a padding truncation process
       for use in TWAMP.  When using that process in conjunction with the
       Reflect Octets mode, the Session-Reflector MUST reflect the
       designated octets from the Session-Sender's test packet in the Packet
       Padding (from Session-Sender) field, and MAY re-use additional Packet
       Padding from the Session-Sender.  The Session-Reflector MUST truncate
       the padding such that the highest number octets are discarded, and
       the test packet length equals the Session-Sender's packet length.
       When using the recommended truncation process, the Session-Reflector
       MUST truncate exactly 27 octets of padding in Unauthenticated mode,
       and exactly 56 octets in Authenticated and Encrypted modes.

       In any case, the Session-Reflector MAY re-use the Sender's Packet
       Padding (since the requirements for padding generation are the same
       for each).

    5.2.2.  Symmetrical Size: Session-Reflector Packet Format

       When Symmetrical Size mode is selected, the Session-Reflector packet
       formats for Unauthenticated and Authenticated/Encrypted modes are
       identical to the core TWAMP specification, see Section 4.2.1 of
       [RFC5357].  Thus, the Session-Reflector test packet format is
       unchanged.

       The Session-Reflector MUST construct its test packet using the
       information in the Session-Sender's test packet.  The length of the
       Session-Reflector's test packet SHALL equal the length of the
       Session-Sender's test packet.

    5.2.3.  Symmetrical Size AND Reflect Octets: Session-Sender Packet
            Format

       When both the Symmetrical Size mode and the Reflect Octets mode are
       selected, the Session-Reflector MUST operate using the Session-Sender
       Packet Format defined in Section 5.1.5, where the Padding Octets are
       separated from the information fields, and the Packet Padding (to be
       reflected) field precedes the Additional Padding.

       The Session-Reflector SHALL use the same TWAMP-Test Packet Format as
       specified in Section 5.2.1, above.

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    RFC 6038             Reflect Octets & Symmetric Size        October 2010

    6.  Security Considerations

       These extended modes of operation do not appear to permit any new
       attacks on hosts communicating with core TWAMP [RFC5357].

       The security considerations that apply to any active measurement of
       live networks are relevant here as well.  See [RFC4656] and
       [RFC5357].

    7.  IANA Considerations

       This memo adds two modes to the IANA registry for the TWAMP Modes
       field, and describes behavior when the new modes are used.  This
       field is a recognized extension mechanism for TWAMP.

    7.1.  Registry Specification

       IANA has created a TWAMP-Modes registry (as requested in [RFC5618]).
       TWAMP-Modes are specified in TWAMP Server Greeting messages and Setup
       Response messages, as described in Section 3.1 of [RFC5357],
       consistent with Section 3.1 of [RFC4656], and extended by this memo.
       Modes are indicated by setting bits in the 32-bit Modes field that
       correspond to values in the Modes registry.  For the TWAMP-Modes
       registry, we expect that new features will be assigned increasing
       registry values that correspond to single bit positions, unless there
       is a good reason to do otherwise (more complex encoding than single-
       bit positions may be used in the future to access the 2^32 value
       space).

    7.2.  Registry Contents

       The TWAMP-Modes registry has been augmented as follows:

       Value  Description             Semantics Definition
       --------------------------------------------------------
       32     Reflect Octets          this memo, Section 3.1
              Capability              new bit position (5)
       64     Symmetrical Size        this memo, Section 3.1
              Sender Test Packet Format new bit position (6)

    8.  Acknowledgements

       The authors thank Steve Baillargeon, Walt Steverson, and Stina Ross
       for helpful review and comments.

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    RFC 6038             Reflect Octets & Symmetric Size        October 2010

    9.  Normative References

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

       [RFC4656]  Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
                  Zekauskas, "A One-way Active Measurement Protocol
                  (OWAMP)", RFC 4656, September 2006.

       [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
                  Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
                  RFC 5357, October 2008.

       [RFC5618]  Morton, A. and K. Hedayat, "Mixed Security Mode for the
                  Two-Way Active Measurement Protocol (TWAMP)", RFC 5618,
                  August 2009.

    Authors' Addresses

       Al Morton
       AT&T Labs
       200 Laurel Avenue South
       Middletown, NJ  07748
       USA

       Phone: +1 732 420 1571
       Fax:   +1 732 368 1192
       EMail: acmorton@att.com
       URI:   http://home.comcast.net/~acmacm/

       Len Ciavattone
       AT&T Labs
       200 Laurel Avenue South
       Middletown, NJ  07748
       USA

       Phone: +1 732 420 1239
       EMail: lencia@att.com

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