





Internet Engineering Task Force (IETF)                    S. Baillargeon
Request for Comments: 6802                                     C. Flinta
Category: Informational                                      A. Johnsson
ISSN: 2070-1721                                                 Ericsson
                                                           November 2012


Ericsson Two-Way Active Measurement Protocol (TWAMP) Value-Added Octets

Abstract

   This memo describes an extension to the Two-Way Active Measurement
   Protocol (TWAMP).  Specifically, it extends the TWAMP-Test protocol,
   which identifies and manages packet trains, in order to measure
   capacity metrics like the available path capacity, tight section
   capacity, and UDP delivery rate in the forward and reverse path
   directions.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It 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/rfc6802.

Copyright Notice

   Copyright (c) 2012 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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Table of Contents

   1. Introduction ....................................................2
      1.1. Requirements Language ......................................3
   2. Purpose and Scope ...............................................3
   3. Capacity Measurement Principles .................................4
   4. TWAMP-Control Extensions ........................................5
      4.1. Additional Considerations ..................................6
   5. Extended TWAMP-Test .............................................6
      5.1. Sender Behavior ............................................6
           5.1.1. Packet Timings ......................................7
           5.1.2. Session-Sender Packet Format ........................7
      5.2. Reflector Behavior ........................................12
           5.2.1. Session-Reflector Packet Format ....................13
      5.3. Additional Considerations .................................13
   6. Experiments ....................................................14
   7. Security Considerations ........................................14
   8. Acknowledgements ...............................................14
   9. References .....................................................15
      9.1. Normative References ......................................15
      9.2. Informative References ....................................15

1.  Introduction

   The notion of embedding a number of meaningful fields in the padding
   octets has been established as a viable methodology for carrying
   additional information within the TWAMP-Test protocol running between
   a Session-Sender and a Session-Reflector [RFC5357] [RFC6038].

   This memo describes an optional extension to the Two-Way Active
   Measurement Protocol (TWAMP) [RFC5357].  It is called the Ericsson
   TWAMP Value-Added Octets feature.  This memo defines version 1.

   This feature enables the controller host to measure capacity metrics
   like the IP-type-P available path capacity (APC) [RFC5136], IP-layer
   tight section capacity (TSC) [Y1540], and UDP delivery rate on both
   forward and reverse paths using a single TWAMP test session.  The
   actual method to calculate the APC, TSC, or the UDP delivery rate
   from packet-level performance data is not discussed in this memo.

   The Valued-Added Octets feature consists of new behaviors for the
   Session-Sender and Session-Reflector and a set of value-added octets
   of information that are placed at the beginning of the Packet Padding
   [RFC5357] or immediately after the Server Octets in the Packet
   Padding (to be reflected) [RFC6038] by the Session-Sender and are
   reflected or returned by the Session-Reflector.  The length of the
   value-added octets in version 1 is 10 octets.  The Valued-Added




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   Octets feature does not change the basic roles and functions of the
   TWAMP hosts, which are still responsible to include timestamp(s) and
   sequence number(s) in the test packets.

1.1.  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].

2.  Purpose and Scope

   The purpose of this memo is to describe the Ericsson TWAMP Valued-
   Added Octets feature (version 1) for TWAMP [RFC5357].

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

   o  The definition of a structure for embedding a sequence of value-
      added fields at the beginning of the Packet Padding [RFC5357] or
      Packet Padding (to be reflected) [RFC6038] in the TWAMP-Test
      packets

   o  The definition of new Session-Sender and Session-Reflector
      behaviors

   The motivation for this feature is to enable the measurement of
   capacity metrics on both the forward and reverse paths using a single
   TWAMP test session.  Multiple TWAMP test sessions between a
   controller and a responder with different Diffserv Code Points
   (DSCPs) may also be used to evaluate the QoS impacts on the capacity
   metrics.

   This memo captures the prototype presented and demonstrated at IETF
   80.  It may be used as a reference for future work or may be used
   during benchmark analysis to compare the accuracy or performance of
   the available path capacity estimates under various condition or use
   cases.

   This memo does not extend the standard modes of operation through
   assignment of a new value in the Modes field (see Section 3.1 of
   [RFC4656] for the format of the Server Greeting message).  This memo
   does not define a vendor-specific or experimental mode since the
   Modes field as currently defined does not explicitly reserve a value
   or range of values for this purpose.






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   This memo assumes the TWAMP controller is capable to send test
   packets with value-added padding octets and the TWAMP responder is
   configured to interpret the value-added padding octets embedded in
   each TWAMP-Test packet.  Bootstrapping such behavior at the TWAMP
   responder is implementation specific.  By default, the feature MUST
   be disabled on the TWAMP host.  The Value-Added Octets feature MUST
   be deployed in an environment where both controller and responder are
   managed by the same administrative entity and such entity has
   established an agreement to operate the Value-Added Octets feature
   between the pair of hosts or between specific UDP endpoints between
   the pair of hosts.  See Sections 4 and 5.3 for additional
   considerations.

   The Value-Added Octets Version 1 feature is intended to work in
   conjunction with any TWAMP modes.  When the Server and Control-Client
   are configured or have agreed to use the Value-Added Octets Version 1
   feature, then the Control-Client, the Server, the Session-Sender, and
   the Session-Reflector must all conform to the requirements of that
   feature, as identified below.

3.  Capacity Measurement Principles

   Most capacity estimation methods for APC [RRBNC] [PDM] [ENHJMMB]
   [SBW] and for UDP delivery rate need to send and receive packets in
   groups, called "packet trains" or simply "trains".  Each train is
   sent at a specific transmission rate in a given direction.  These
   trains must be identified within each bidirectional test session
   stream.

   The first measurement principle is to send multiple trains within a
   test session stream from one IP node to another IP node in order to
   estimate the APC, TSC, or UDP delivery rate in the forward direction.
   Each train consists of a group of test packets that are separated
   from each other by a packet interval, as shown in the figure below.
   The packet interval is measured using either the first bit or the
   last bit of two consecutive packets.

         tt                      tt                      tt
   |<---------->|          |<---------->|          |<---------->|
   |            |          |            |          |            |
   +------------+          +------------+          +------------+
   |  Packet 1  |          |  Packet 2  |          |  Packet 3  |
   +------------+          +------------+          +------------+
   |                       |                       |
   |<--------------------->|<--------------------->|
       packet interval 1       packet interval 2





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   The test packet size and interval between consecutive packets for
   each train sent by the Session-Sender and reflected by the Session-
   Reflector MUST be calculated and determined by the controller or an
   application or entity communicating with the controller.  The packet
   size and interval MAY vary within a train and/or between trains.
   Determination of the packet size and interval is implementation
   specific.

   The transmission time tt to send one packet (i.e., determined by the
   interface speed and the IP packet size) is also shown in the figure
   above.  Observe that the packet interval MUST be larger than or equal
   to tt.

   At the Session-Reflector, each received test packet within a forward
   train is time stamped.  This provides a second set of packet interval
   values.  Methods for measuring the APC, TSC, and UDP delivery rate
   use the packet intervals obtained from both endpoints in the
   estimation process.  The method of measuring the UDP delivery rate
   may also require the rate of packet loss.  The estimation process
   itself, as well as any requirements on software or hardware, is
   implementation specific.

   The second measurement principle is referred to as "self-induced
   congestion".  According to this principle, in order to measure APC,
   TSC, and UDP delivery rates, some trains MUST cause momentary
   congestion on the network path.  In essence, this means that some
   trains MUST be sent at a higher rate than what is available on the
   network path.

   In order to fulfill the above measurement principles and to measure
   the APC, TSC, and UDP delivery rates in the reverse direction, the
   test packets at the Session-Reflector MUST be regrouped into trains
   and then transmitted back to the Session-Sender with a provided
   packet interval.

4.  TWAMP-Control Extensions

   TWAMP connection establishment follows the procedure defined in
   Section 3.1 of [RFC4656] and Section 3.1 of [RFC5357].  The TWAMP-
   Control protocol [RFC5357] uses the Modes field to identify and
   select specific communication capabilities.  According to the
   standard specifications, the Value-Added Octets feature requires one
   new bit position (and value) to identify the ability of the
   Server/Session-Reflector to read and act upon the new fields in the
   value-added octets.  Such bit position (and value) is not defined in
   this memo.  Bootstrapping the TWAMP Value-Added Octets Version 1
   feature is implementation specific.




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   Both the Reflect Octets mode and Symmetrical Size mode MAY be
   selected to ensure the reflection of the value-added padding octets
   by the Session-Reflector and symmetrical size TWAMP-Test packets in
   the forward and reverse directions of transmission.

4.1.  Additional Considerations

   In the TWAMP control architecture, the TWAMP reflector (server)
   signals the modes it wishes to operate and the TWAMP controller
   (control-client) selects the mode or modes supported by the
   responder.  This feature is designed to retain backward compatibility
   with the original TWAMP-Control and TWAMP-Test protocols.  As an
   alternative, the user may opt for TWAMP Light architecture, which
   does not require the TWAMP-Control protocol.

   The methods to determine if the Value-Added Octets feature is
   supported on a TWAMP reflector is implementation specific.  When the
   Value-Added Octets feature is not supported on a TWAMP reflector, the
   TWAMP controller MUST NOT select the Value-Added Octets feature and
   MUST NOT include any value-added octets in the test packets.  If the
   TWAMP controller inadvertently sends value-added octets in the test
   packets to a TWAMP responder that does not support such feature, the
   TWAMP responder shall treat the value-added octets as regular padding
   octets and return the test packets as quickly as possible to the
   Session-Sender as defined in [RFC5357].

5.  Extended TWAMP-Test

   The forward and reverse APC, TSC, and UDP delivery rate measurement
   characteristics depend on the size and packet intervals of the test
   packets.  This memo allows variable packet sizes and packet intervals
   between trains and even between packets in the same train.  The
   functionality is described below.

   The TWAMP-Test protocol carrying the value-added padding octets is
   identical to TWAMP [RFC5357] except for the definition of the first
   10 octets in the Packet Padding that the Session-Sender expects to be
   reflected.  The new octets define fields for Value-Added Octets
   Version, Flags, Last Sequence Number in Train, and Desired Reverse
   Packet Interval.  Each of these fields are described in detail below.

   The Session-Sender and Session-Reflector behaviors are also modified.

5.1.  Sender Behavior

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




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5.1.1.  Packet Timings

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

5.1.2.  Session-Sender Packet Format

   The Session-Sender packet format follows the same procedure and
   guidelines as defined in TWAMP [RFC5357] and TWAMP Reflect Octets and
   Symmetrical Size Features [RFC6038].

   This feature allows the Session-Sender to set the first few octets in
   the TWAMP-Test Packet Padding with information to communicate value-
   added padding version number, flag bits, sequence number of the last
   packet in a train, and desired reverse packet interval (or per-packet
   waiting time) for the reverse path direction of transmission.

   The Valued-Added Octets feature must be placed immediately after the
   TWAMP header or immediately after any new field that could be added
   to the TWAMP header or added to the beginning of the padding octets
   in the future.  Therefore, the placement of the first bit from the
   valued-added octets depends on the mode(s) being selected.

   A version number and a sequence of flag bits are defined at the very
   beginning of the value-added padding octets.  The version number
   identifies the version of the value-added padding octets and meaning
   of the flag bits and corresponding fields.  Each flag bit indicates
   if a specific field is used in the valued-added padding octets.  The
   version number and flag bits provide an effective method for
   extracting information at Session-Reflector and Session-Sender.  This
   document defines version 1 with two flag bits: L and I.

   The format of the test packet depends on the TWAMP modes.  The Value-
   Added Octets Version 1 feature is intended to work with any TWAMP
   modes.

   The Session-Sender SHALL use the following TWAMP-Test packet format
   when the Value-Added Octets Version 1 feature is selected in
   conjunction with the Unauthenticated mode:












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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Sequence Number                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Timestamp                            |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Error Estimate        |  Ver  |L|I|     Reserved      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Last Seqno In Train                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Desired Reverse Packet Interval                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   Additional Packet Padding                   |
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

































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   The Session-Sender SHALL use the following TWAMP-Test packet format
   when the Value-Added Octets Version 1 feature is selected in
   conjunction with the Unauthenticated mode, Symmetrical Size mode, and
   Reflect Octets 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)                       |
    |                                                               |
    |                                                               |
    |                                                               |
    |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               |  Ver  |L|I|      Reserved     |    Last...    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               Seqno in Train                  |   Desired...  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Reverse Packet Interval               | Additional... |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Packet Padding                          |
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



















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   The Session-Sender SHALL use the following TWAMP-Test packet format
   when the Value-Added Octets Version 1 feature is selected in
   conjunction with the Unauthenticated mode, Symmetrical Size mode, and
   Reflect Octets mode with a non-zero value in the Server Octets field:

     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)                       |
    |                                                               |
    |                                                               |
    |                                                               |
    |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |               |         Server Octets         |  Ver  |L|I|...|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Reserved    |               Last Seqno in...                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Train       |             Desired Reverse Packet...         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Interval    |         Additional Packet Padding             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In the mode using Reflect Octets illustrated above, the value-added
   padding octets are embedded in the Packet Padding (to be reflected).

   The Version (Ver) field MUST be encoded in the first 4 bits.  It
   identifies the version number of the value-added padding octets and
   meaning of the flag bits and the corresponding fields.  This memo
   defines version 1 with two flag bits: L and I.  When the Value-Added
   Octets Version 1 feature is selected, the Session-Sender MUST set the
   Ver field to 1.

   The 2 bits after the Version field are used for flags: L and I.





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   The Last Seqno in Train bit (L) is the first flag.  When the Value-
   Added Octets Version 1 feature is selected, the Session-Sender MAY
   set the Last Seqno in Train bit L to 1.

   The Desired Reverse Packet Interval bit (I) is the second flag.  When
   the Value-Added Octets Version 1 feature is selected, the Session-
   Sender MAY set the Desired Reverse Packet Interval bit I to 1.

   The Reserved field is reserved for future use.  All 10 bits of the
   Reserved field MUST be transmitted as zero by the Session-Sender.

   If the Last Seqno in Train bit is set to 1, then the Last Seqno in
   Train field MUST contain an unsigned 32-bit integer generated by the
   Session-Sender.  It MUST indicate the expected sequence number of the
   last packet in the train.  It SHOULD be used by the Session-Sender
   and Session-Reflector to identify the train to which a test packet
   belongs.  The packets belonging to a train are determined by
   observing the test packet Sequence Number in relation to the Last
   Seqno in Train. The Last Seqno in Train MUST be higher or equal to
   Sequence Number of the packet.  It must also be higher than the Last
   Seqno in Train for the previous train.  If the L bit is set to 0, the
   Session-Sender shall set all the bits in the Last Seqno in Train
   field to zero.

   If the Desired Reverse Packet Interval bit is set to 1, then the
   Desired Reverse Packet Interval field MUST contain an unsigned 32 bit
   integer generated by the Session-Sender.  It MUST indicate the
   desired packet interval (or the waiting time) that the Session-
   Reflector SHOULD use when transmitting the reflected test packets
   towards the Session-Sender.  The value 0 means the Session-Reflector
   SHOULD return the test packet to the Session-Sender as quickly as
   possible.  The format of this field MUST be a fractional part of a
   second as defined in the One-Way Active Measurement Protocol (OWAMP)
   [RFC4656].  If the I bit is set to 0, the Session-Sender shall set
   all the bits in the Desired Reverse Packet Interval field to zero.

   The values of the above fields are usually provided by a measurement
   method, tool, or algorithm.  This measurement algorithm is outside
   the scope of this specification.












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5.2.  Reflector Behavior

   The TWAMP Session-Reflector follows the procedures and guidelines in
   Section 4.2 of [RFC5357], with some changes and additional functions.

   When the Value-Added Octets Version 1 feature is selected, the
   behavior of the Session-Reflector SHALL be as follows:

   o  The Session-Reflector MUST read the Version field.  If Ver = 1,
      the Session-Reflector MUST read the L and I flag bits.

   o  If L=1 and I=1, the Session-Reflector MUST read and extract the
      information from the Last Seqno in Train field and the Desired
      Reverse Packet Interval field in the value-added padding octets.

      -  The Last Seqno in Train MUST be compared to Sequence Number in
         the same packet in order to determine when a complete train has
         been collected.  The Session-Reflector SHOULD buffer the
         packets belonging to the current train (or store the packet-
         level performance data).  After the last packet of the train
         has been received, the Session-Reflector SHOULD transmit the
         packets belonging to a reverse train with a waiting time
         (packet interval) for each packet indicated in the Desired
         Reverse Packet Interval field.  If the Desired Reverse Packet
         Interval field is set to zero, then the Session-Reflector
         SHOULD transmit the packet as quickly as possible.  The last
         packet within a train has Sender Sequence Number = Last Seqno
         in Train.

      -  The Last Seqno in Train of a packet MUST also be compared to
         the Last Seqno in Train of the previous packet in order to
         determine if a new train needs to be collected.  In case of
         packet loss, the Session-Reflector MUST transmit the incomplete
         train when it receives a packet with a Last Seqno in Train
         belonging to another train (e.g., next train) of the test
         session or after a timeout.  The timeout MAY be the REFWAIT
         timer specified in section 4.2 of [RFC5357].

      -  Packets arriving out-of-order within a train MUST be buffered
         at the Session-Reflector if the train is not yet transmitted to
         the Session-Sender.  If the train is already transmitted, the
         test packet SHOULD be returned to the Session-Sender as quickly
         as possible.  The Session-Reflector MUST NOT reorder the test
         packets if they happen to arrive out-of-sequence.

      -  Duplicate packets within a train MUST be buffered at the
         Session-Reflector if the train is not yet transmitted to the
         Session-Sender.  If the train is already transmitted, the



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         duplicate test packet SHOULD be returned to the Session-Sender
         as quickly as possible.  The Session-Reflector MUST NOT discard
         duplicate test packets.

   For any other combinations of the Version field and the L and I
   flags, the Session-Reflector SHOULD return the test packet to the
   Session-Sender as quickly as possible.

   The Session-Reflector MUST implement the changes described above when
   the Value-Added Octets Version 1 feature is selected.

5.2.1  Session-Reflector Packet Format

   The Session-Reflector packet format follows the same procedure and
   guidelines as defined in TWAMP [RFC5357] and TWAMP Reflect Octets and
   Symmetrical Size Features [RFC6038], with the following changes:

   o  The Session-Reflector MUST reuse (reflect) the value-added padding
      octets (10 octets) provided in the Sender's Packet Padding.

   o  The Session-Reflector MAY reuse the rest of the padding octets in
      the Sender's Packet Padding.

   The truncation process [RFC5357] is recommended when the Symmetrical
   mode is not used.  The Session-Reflector MUST truncate exactly 27
   octets of padding in Unauthenticated mode and exactly 56 octets in
   Authenticated and Encrypted modes.

5.3.  Additional Considerations

   The Session-Reflector supporting the Value-Added Octets feature
   should revert back to the standard Session-Reflector behavior if it
   cannot interpret the value-added padding octets in a given test
   packet.  Section 5.2 also describes such behavior.  For instance, the
   test packet is returned as quickly as possible to the Session-Sender
   when the Last Seqno in the Train is not what is expected.

   Capacity measurements introduce an additional consideration when the
   test sessions operate in TWAMP Light.  When the Session-Reflector
   does not have knowledge of the session state, the measurement system
   may be restricted to estimating or calculating the capacity metrics
   in the forward path direction of transmission only.  Capacity
   measurements in the reverse path direction is best handled with a
   Session-Reflector supporting knowledge of the session state and being
   capable of identifying the test packets belonging to a specific test
   session.  A method for creating a session state from the initial test
   packet may be implemented on the TWAMP Light Session-Reflector.  This
   is outside the scope of this specification.



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6.  Experiments

   This memo describes the protocol used in the current working
   prototype implementation of the Value-Added Octets feature in the
   Ericsson lab.  The prototype has been tested in real network
   environments.  The conclusion from these tests is that the Value-
   Added Octets feature is able to enable estimation of capacity metrics
   such as available path capacity in both the forward and reverse
   directions of the network path.

   During the experiments with the protocol described in this memo, we
   have identified a need for the controller and responder to use the
   same maximum train length.  The reflector must be able to buffer the
   whole train received from the controller.  In order to reduce the
   risk for buffer overrun, the maximum train length should be
   negotiated.  This can be resolved through configuration, introduction
   of a new field in the value-added octets, or a new maximum train
   length field in the Request-TW-Session message.

   The Sender Discriminator (SD) field, which was proposed in an early
   draft of this document, was removed because of complications with
   different Session-Reflector implementations.  A Session-Reflector may
   not be able to easily identify the SD field or associate it with a
   specific Session-Sender, which may skew the test results.

   The flags defined in the value-added octets now indicate the usage of
   fields and not the presence of fields.  This modification was needed
   to simplify the responder implementation in the working prototype.

7.  Security Considerations

   The value-added padding octets permit DoS attacks on the responder
   host communicating with core TWAMP [RFC5357].  For instance, a DoS
   condition could arise when the Last Seqno in Train is too large to
   handle, potentially causing undesirable processing delay or discard
   of the TWAMP-Test packets.  The responder host MUST provide a
   mechanism to protect or limit the use of its local memory, buffer
   space, or maximum transmission time for a train.

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

8.  Acknowledgements

   The authors thank Svante Ekelin for providing direction and comments
   on this document.




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9.  References

9.1.  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.

   [RFC5136]   Chimento, P. and J. Ishac, "Defining Network Capacity",
               RFC 5136, February 2008.

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

   [RFC6038]   Morton, A. and L. Ciavattone, "Two-Way Active Measurement
               Protocol (TWAMP) Reflect Octets and Symmetrical Size
               Features", RFC 6038, October 2010.

9.2.  Informative References

   [ENHJMMB]   Ekelin, S., Nilsson, M., Hartikainen, E., Johnsson, A.,
               Mangs, J., Melander, B., and M. Bjorkman, "Real-Time
               Measurement of End-to-End Available Bandwidth Using
               Kalman Filtering", Proceedings to the IEEE/IFIP Network
               Operations and Management Symposium, 2006.

   [PDM]       Dovrolis, C., Ramanathan, P., and D. Moore, "Packet-
               Dispersion Techniques and a Capacity-Estimation
               Methodology", IEEE/ACM Transactions on Networking,
               December 2004.

   [RRBNC]     Ribeiro, V., Riedi, R., Baraniuk, R., Navratil, J., and
               L. Cottrel, "pathChirp: Efficient Available Bandwidth
               Estimation for Network Paths", Passive and Active
               Monitoring Workshop, 2003.

   [SBW]       Sommers, J., Barford, P., and W. Willinger, "Laboratory-
               based Calibration of Available Bandwidth Estimation
               Tools", Microprocessors and Microsystems, 2007.

   [Y1540]     International Telecommunications Union, "Internet
               protocol data communication service - IP packet transfer
               and availability performance parameters", ITU-T
               Recommendation Y.1540, 2011.



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Authors' Addresses

   Steve Baillargeon
   Ericsson
   3500 Carling Avenue
   Ottawa, Ontario K2H 8E9
   Canada

   EMail: steve.baillargeon@ericsson.com


   Christofer Flinta
   Ericsson
   Farogatan 6
   Stockholm, 164 80
   Sweden

   EMail: christofer.flinta@ericsson.com


   Andreas Johnsson
   Ericsson
   Farogatan 6
   Stockholm, 164 80
   Sweden

   EMail: andreas.a.johnsson@ericsson.com
























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