<pre>Network Working Group                                         K. Hedayat
Request for Comments: 5357                                 Brix Networks
Category: Standards Track                                  R. Krzanowski
                                                                 Verizon
                                                               A. Morton
                                                               AT&amp;T Labs
                                                                  K. Yum
                                                        Juniper Networks
                                                              J. Babiarz
                                                         Nortel Networks
                                                            October 2008


             <span class="h1">A Two-Way Active Measurement Protocol (TWAMP)</span>

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   The One-way Active Measurement Protocol (OWAMP), specified in <a href="./rfc4656">RFC</a>
   <a href="./rfc4656">4656</a>, provides a common protocol for measuring one-way metrics
   between network devices.  OWAMP can be used bi-directionally to
   measure one-way metrics in both directions between two network
   elements.  However, it does not accommodate round-trip or two-way
   measurements.  This memo specifies a Two-Way Active Measurement
   Protocol (TWAMP), based on the OWAMP, that adds two-way or round-trip
   measurement capabilities.  The TWAMP measurement architecture is
   usually comprised of two hosts with specific roles, and this allows
   for some protocol simplifications, making it an attractive
   alternative in some circumstances.















<span class="grey">Hedayat, et al.             Standards Track                     [Page 1]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-2" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-2">2</a>
      <a href="#section-1.1">1.1</a>. Relationship of Test and Control Protocols .................<a href="#page-3">3</a>
      <a href="#section-1.2">1.2</a>. Logical Model ..............................................<a href="#page-3">3</a>
      <a href="#section-1.3">1.3</a>. Pronunciation Guide ........................................<a href="#page-4">4</a>
   <a href="#section-2">2</a>. Protocol Overview ...............................................<a href="#page-5">5</a>
   <a href="#section-3">3</a>. TWAMP-Control ...................................................<a href="#page-6">6</a>
      <a href="#section-3.1">3.1</a>. Connection Setup ...........................................<a href="#page-6">6</a>
      <a href="#section-3.2">3.2</a>. Integrity Protection .......................................<a href="#page-7">7</a>
      <a href="#section-3.3">3.3</a>. Values of the Accept Field .................................<a href="#page-7">7</a>
      <a href="#section-3.4">3.4</a>. TWAMP-Control Commands .....................................<a href="#page-7">7</a>
      <a href="#section-3.5">3.5</a>. Creating Test Sessions .....................................<a href="#page-8">8</a>
      <a href="#section-3.6">3.6</a>. Send Schedules ............................................<a href="#page-10">10</a>
      <a href="#section-3.7">3.7</a>. Starting Test Sessions ....................................<a href="#page-10">10</a>
      <a href="#section-3.8">3.8</a>. Stop-Sessions .............................................<a href="#page-10">10</a>
      <a href="#section-3.9">3.9</a>. Fetch-Session .............................................<a href="#page-12">12</a>
   <a href="#section-4">4</a>. TWAMP-Test .....................................................<a href="#page-12">12</a>
      <a href="#section-4.1">4.1</a>. Sender Behavior ...........................................<a href="#page-12">12</a>
           <a href="#section-4.1.1">4.1.1</a>. Packet Timings .....................................<a href="#page-12">12</a>
           <a href="#section-4.1.2">4.1.2</a>. Packet Format and Content ..........................<a href="#page-12">12</a>
      <a href="#section-4.2">4.2</a>. Reflector Behavior ........................................<a href="#page-13">13</a>
           <a href="#section-4.2.1">4.2.1</a>. TWAMP-Test Packet Format and Content ...............<a href="#page-14">14</a>
   <a href="#section-5">5</a>. Implementers' Guide ............................................<a href="#page-20">20</a>
   <a href="#section-6">6</a>. Security Considerations ........................................<a href="#page-20">20</a>
   <a href="#section-7">7</a>. Acknowledgements ...............................................<a href="#page-21">21</a>
   <a href="#section-8">8</a>. IANA Considerations ............................................<a href="#page-21">21</a>
      <a href="#section-8.1">8.1</a>. Registry Specification ....................................<a href="#page-22">22</a>
      <a href="#section-8.2">8.2</a>. Registry Management .......................................<a href="#page-22">22</a>
      <a href="#section-8.3">8.3</a>. Experimental Numbers ......................................<a href="#page-22">22</a>
      <a href="#section-8.4">8.4</a>. Initial Registry Contents .................................<a href="#page-22">22</a>
   <a href="#section-9">9</a>. Internationalization Considerations ............................<a href="#page-22">22</a>
   <a href="#appendix-I">Appendix I</a> - TWAMP Light (Informative) ............................<a href="#page-23">23</a>
   Normative References ..............................................<a href="#page-24">24</a>
   Informative References ............................................<a href="#page-24">24</a>

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   The Internet Engineering Task Force (IETF) has completed a Proposed
   Standard for the round-trip delay [<a href="./rfc2681" title="&quot;A Round-trip Delay Metric for IPPM&quot;">RFC2681</a>] metric.  The IETF has
   also completed a protocol for the control and collection of one-way
   measurements, the One-way Active Measurement Protocol (OWAMP)
   [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].  However, OWAMP does not accommodate round-trip or two-way
   measurements.

   Two-way measurements are common in IP networks, primarily because
   synchronization between local and remote clocks is unnecessary for
   round-trip delay, and measurement support at the remote end may be



<span class="grey">Hedayat, et al.             Standards Track                     [Page 2]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-3" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   limited to a simple echo function.  However, the most common facility
   for round-trip measurements is the ICMP Echo Request/Reply (used by
   the ping tool), and issues with this method are documented in <a href="./rfc2681#section-2.6">Section</a>
   <a href="./rfc2681#section-2.6">2.6 of [RFC2681]</a>.  This memo specifies the Two-Way Active Measurement
   Protocol, or TWAMP.  TWAMP uses the methodology and architecture of
   OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>] to define an open protocol for measurement of two-way
   or round-trip metrics (henceforth in this document the term two-way
   also signifies round-trip), in addition to the one-way metrics of
   OWAMP.  TWAMP employs time stamps applied at the echo destination
   (reflector) to enable greater accuracy (processing delays can be
   accounted for).  The TWAMP measurement architecture is usually
   comprised of only two hosts with specific roles, and this allows for
   some protocol simplifications, making it an attractive alternative to
   OWAMP in some circumstances.

   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 <a href="./rfc2119">RFC 2119</a> [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Relationship of Test and Control Protocols</span>

   Similar to OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>], TWAMP consists of two inter-related
   protocols: TWAMP-Control and TWAMP-Test.  The relationship of these
   protocols is as defined in <a href="#section-1.1">Section 1.1</a> of OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].  TWAMP-
   Control is used to initiate, start, and stop test sessions, whereas
   TWAMP-Test is used to exchange test packets between two TWAMP
   entities.

<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  Logical Model</span>

   The role and definition of the logical entities are as defined in
   <a href="#section-1.2">Section 1.2</a> of OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>] with the following exceptions:

   -  The Session-Receiver is called the Session-Reflector in the TWAMP
      architecture.  The Session-Reflector has the capability to create
      and send a measurement packet when it receives a measurement
      packet.  Unlike the Session-Receiver, the Session-Reflector does
      not collect any packet information.

   -  The Server is an end system that manages one or more TWAMP
      sessions, and is capable of configuring per-session state in the
      endpoints.  However, a Server associated with a Session-Reflector
      would not have the capability to return the results of a test
      session, and this is a difference from OWAMP.







<span class="grey">Hedayat, et al.             Standards Track                     [Page 3]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-4" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   -  The Fetch-Client entity does not exist in the TWAMP architecture,
      as the Session-Reflector does not collect any packet information
      to be fetched.  Consequently, there is no need for the Fetch-
      Client.

   An example of possible relationship scenarios between these roles is
   presented below.  In this example, different logical roles are played
   on different hosts.  Unlabeled links in the figure are unspecified by
   this document and may be proprietary protocols.

         +----------------+               +-------------------+
         | Session-Sender |&lt;-TWAMP-Test--&gt;| Session-Reflector |
         +----------------+               +-------------------+
           ^                                     ^
           |                                     |
           |                                     |
           |                                     |
           |  +----------------+&lt;----------------+
           |  |     Server     |
           |  +----------------+
           |    ^
           |    |
           | TWAMP-Control
           |    |
           v    v
         +----------------+
         | Control-Client |
         +----------------+

   As in OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>], different logical roles can be played by the
   same host.  For example, in the figure above, there could actually be
   two hosts: one playing the roles of Control-Client and Session-
   Sender, and the other playing the roles of Server and Session-
   Reflector.  This example is shown below.

          +-----------------+                   +-------------------+
          | Control-Client  |&lt;--TWAMP-Control--&gt;|      Server       |
          |                 |                   |                   |
          | Session-Sender  |&lt;--TWAMP-Test-----&gt;| Session-Reflector |
          +-----------------+                   +-------------------+

<span class="h3"><a class="selflink" id="section-1.3" href="#section-1.3">1.3</a>.  Pronunciation Guide</span>

   The acronym OWAMP is usually pronounced in two syllables, Oh-wamp.

   The acronym TWAMP is also pronounced in two syllables, Tee-wamp.





<span class="grey">Hedayat, et al.             Standards Track                     [Page 4]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-5" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Protocol Overview</span>

   The Two-Way Active Measurement Protocol is an open protocol for
   measurement of two-way metrics.  It is based on OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>] and
   adheres to OWAMP's overall architecture and design.  The TWAMP-
   Control and TWAMP-Test protocols accomplish their testing tasks as
   outlined below:

   -  The Control-Client initiates a TCP connection on TWAMP's well-
      known port, and the Server (its role now established) responds
      with its Greeting message, indicating the security/integrity
      mode(s) it is willing to support.

   -  The Control-Client responds with the chosen mode of communication
      and information supporting integrity protection and encryption, if
      the mode requires them.  The Server responds to accept the mode
      and give its start time.  This completes the control-connection
      setup.

   -  The Control-Client requests (and describes) a test session with a
      unique TWAMP-Control message.  The Server responds with its
      acceptance and supporting information.  More than one test session
      may be requested with additional messages.

   -  The Control-Client initiates all requested testing with a Start-
      Sessions message, and the Server acknowledges.

   -  The Session-Sender and the Session-Reflector exchange test packets
      according to the TWAMP-Test protocol for each active session.

   -  When appropriate, the Control-Client sends a message to stop all
      test sessions.

   There are two recognized extension mechanisms in the TWAMP Protocol.

   1) The Modes field is used to establish the communication options
      during TWAMP-Control Connection Setup.

   2) The TWAMP-Control Command Number is another intended extension
      mechanism, allowing additional commands to be defined in the
      future.

   The TWAMP-Control protocol resolves different capability levels
   between the Control-Client and Server.

   All multi-octet quantities defined in this document are represented
   as unsigned integers in network byte order, unless specified
   otherwise.



<span class="grey">Hedayat, et al.             Standards Track                     [Page 5]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-6" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   Throughout this memo, the bits marked MBZ (Must Be Zero) MUST be set
   to zero by senders and MUST be ignored by receivers.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  TWAMP-Control</span>

   TWAMP-Control is a derivative of the OWAMP-Control for two-way
   measurements.  All TWAMP-Control messages are similar in format and
   follow similar guidelines to those defined in <a href="#section-3">Section 3</a> of OWAMP
   [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>] with the exceptions outlined in the following sections.
   One such exception is the Fetch-Session command, which is not used in
   TWAMP.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Connection Setup</span>

   Connection establishment of TWAMP follows the same procedure defined
   in <a href="#section-3.1">Section 3.1</a> of OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].  The Modes field is a recognized
   extension mechanism in TWAMP, and the current mode values are
   identical to those used in OWAMP.  The only exception is the well-
   known port number for TWAMP-Control.  A Client opens a TCP connection
   to the Server on well-known port 862.  The host that initiates the
   TCP connection takes the roles of Control-Client and (in the two-host
   implementation) the Session-Sender.  The host that acknowledges the
   TCP connection accepts the roles of Server and (in the two-host
   implementation) the Session-Reflector.

   The Control-Client MAY set a desired code point in the Diffserv Code
   Point (DSCP) field in the IP header for ALL packets of a specific
   control connection.  The Server SHOULD use the DSCP of the Control-
   Client's TCP SYN in ALL subsequent packets on that connection
   (avoiding any ambiguity in case of re-marking).

   The possibility exists for Control-Client failure after TWAMP-
   Control connection establishment, or the path between the Control-
   Client and Server may fail while a connection is in progress.  The
   Server MAY discontinue any established control connection when no
   packet associated with that connection has been received within
   SERVWAIT seconds.  The Server SHALL suspend monitoring control
   connection activity after receiving a Start-Sessions command, and
   SHALL resume after receiving a Stop-Sessions command (IF the SERVWAIT
   option is supported).  Note that the REFWAIT timeout (described
   below) covers failures during test sessions, and if REFWAIT expires
   on ALL test sessions initiated by a TWAMP-Control connection, then
   the SERVWAIT monitoring SHALL resume (as though a Stop-Sessions
   command had been received).  An implementation that supports the
   SERVWAIT timeout SHOULD also implement the REFWAIT timeout.  The
   default value of SERVWAIT SHALL be 900 seconds, and this waiting time
   MAY be configurable.  This timeout allows the Server to free up
   resources in case of failure.



<span class="grey">Hedayat, et al.             Standards Track                     [Page 6]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-7" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   Both the Server and the Client use the same mappings from KeyIDs to
   shared secrets.  The Server, being prepared to conduct sessions with
   more than one Client, uses KeyIDs to choose the appropriate secret
   key; a Client would typically have different secret keys for
   different Servers.  The shared secret is a passphrase.  To maximize
   passphrase interoperability, the passphrase character set MUST be
   encoded using <a href="./rfc5198#appendix-B">Appendix&nbsp;B of [RFC5198]</a> (the ASCII Network Virtual
   Terminal Definition).  It MUST not contain newlines (any combination
   of Carriage-Return (CR) and/or Line-Feed (LF) characters), and
   control characters SHOULD be avoided.

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Integrity Protection</span>

   Integrity protection of TWAMP follows the same procedure defined in
   <a href="#section-3.2">Section 3.2</a> of OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].  As in OWAMP, each HMAC (Hashed
   Message Authentication Code) sent covers everything sent in a given
   direction between the previous HMAC (but not including it) and the
   start of the new HMAC.  This way, once encryption is set up, each bit
   of the TWAMP-Control connection is authenticated by an HMAC exactly
   once.

   Note that the Server-Start message (sent by a Server during the
   initial control-connection exchanges) does not terminate with an HMAC
   field.  Therefore, the HMAC in the first Accept-Session message also
   covers the Server-Start message and includes the Start-Time field in
   the HMAC calculation.

   Also, in authenticated and encrypted modes, the HMAC in TWAMP-Control
   packets is encrypted.

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  Values of the Accept Field</span>

   Accept values used in TWAMP are the same as the values defined in
   <a href="#section-3.3">Section 3.3</a> of OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].

<span class="h3"><a class="selflink" id="section-3.4" href="#section-3.4">3.4</a>.  TWAMP-Control Commands</span>

   TWAMP-Control commands conform to the rules defined in <a href="#section-3.4">Section 3.4</a> of
   OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].

   The following commands are available for the Control-Client:
   Request-TW-Session, Start-Sessions, and Stop-Sessions.  The Server
   can send specific messages in response to the commands it receives
   (as described in the sections that follow).

   Note that the OWAMP Request-Session command is replaced by the TWAMP
   Request-TW-Session command, and the Fetch-Session command does not
   appear in TWAMP.



<span class="grey">Hedayat, et al.             Standards Track                     [Page 7]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-8" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


<span class="h3"><a class="selflink" id="section-3.5" href="#section-3.5">3.5</a>.  Creating Test Sessions</span>

   Test session creation follows the same procedure as defined in
   <a href="#section-3.5">Section 3.5</a> of OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].  The Request-TW-Session command is
   based on the OWAMP Request-Session command, and uses the message
   format as described in <a href="#section-3.5">Section 3.5</a> of OWAMP, but without the Schedule
   Slot Descriptions field(s) and uses only one HMAC.  The description
   of the Request-TW-Session format follows.

   In TWAMP, the first octet is referred to as the Command Number, and
   the Command Number is a recognized extension mechanism.  Readers are
   encouraged to consult the TWAMP-Control Command Number registry to
   determine if there have been additional values assigned.

   The Command Number value of 5 indicates a Request-TW-Session command,
   and the Server MUST interpret this command as a request for a two-way
   test session using the TWAMP-Test protocol.

   If a TWAMP Server receives an unexpected Command Number, it MUST
   respond with the Accept field set to 3 (meaning "Some aspect of
   request is not supported") in the Accept-Session message.  Command
   Numbers that are Forbidden (and possibly numbers that are Reserved)
   are unexpected.

   In OWAMP, the Conf-Sender field is set to 1 when the Request-Session
   message describes a task where the Server will configure a one-way
   test packet sender.  Likewise, the Conf-Receiver field is set to 1
   when the message describes the configuration for a Session-Receiver.
   In TWAMP, both endpoints send and receive test packets, with the
   Session-Sender first sending and then receiving test packets,
   complimented by the Session-Reflector first receiving and then
   sending.

   Both the Conf-Sender field and Conf-Receiver field MUST be set to 0
   since the Session-Reflector will both receive and send packets, and
   the roles are established according to which host initiates the TCP
   connection for control.  The Server MUST interpret any non-zero value
   as an improperly formatted command, and MUST respond with the Accept
   field set to 3 (meaning "Some aspect of request is not supported") in
   the Accept-Session message.

   The Session-Reflector in TWAMP does not process incoming test packets
   for performance metrics and consequently does not need to know the
   number of incoming packets and their timing schedule.  Consequently
   the Number of Scheduled Slots and Number of Packets MUST be set to 0.

   The Sender Port is the UDP port from which TWAMP-Test packets will be
   sent and the port to which TWAMP-Test packets will be sent by the



<span class="grey">Hedayat, et al.             Standards Track                     [Page 8]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-9" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   Session-Reflector (the Session-Sender will use the same UDP port to
   send and receive packets).  The Receiver Port is the desired UDP port
   to which TWAMP-Test packets will be sent by the Session-Sender (the
   port where the Session-Reflector is asked to receive test packets).
   The Receiver Port is also the UDP port from which TWAMP-Test packets
   will be sent by the Session-Reflector (the Session-Reflector will use
   the same UDP port to send and receive packets).

   The Sender Address and Receiver Address fields contain, respectively,
   the sender and receiver addresses of the endpoints of the Internet
   path over which a TWAMP-Test session is requested.  They MAY be set
   to 0, in which case the IP addresses used for the Control-Client to
   Server TWAMP-Control message exchange MUST be used in the test
   packets.

   The Session Identifier (SID) is as defined in OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].  Since
   the SID is always generated by the receiving side, the Server
   determines the SID, and the SID in the Request-TW-Session message
   MUST be set to 0.

   The Start Time is as defined in OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].

   The Timeout is interpreted differently from the definition in OWAMP
   [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].  In TWAMP, Timeout is the interval that the Session-
   Reflector MUST wait after receiving a Stop-Sessions message.  In case
   there are test packets still in transit, the Session-Reflector MUST
   reflect them if they arrive within the Timeout interval following the
   reception of the Stop-Sessions message.  The Session-Reflector MUST
   NOT reflect packets that are received beyond the timeout.

   Type-P descriptor is as defined in OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].  The only
   capability of this field is to set the Differentiated Services Code
   Point (DSCP) as defined in [<a href="./rfc2474" title="&quot;Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers&quot;">RFC2474</a>].  The same value of DSCP MUST be
   used in test packets reflected by the Session-Reflector.

   Since there are no Schedule Slot Descriptions, the Request-TW-Session
   message is completed by MBZ (Must Be Zero) and HMAC fields.  This
   completes one logical message, referred to as the Request-TW-Session
   command.

   The Session-Reflector MUST respond to each Request-TW-Session command
   with an Accept-Session message as defined in OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].  When
   the Accept field = 0, the Port field confirms (repeats) the port to
   which TWAMP-Test packets are sent by the Session-Sender toward the
   Session-Reflector.  In other words, the Port field indicates the port
   number where the Session-Reflector expects to receive packets from
   the Session-Sender.




<span class="grey">Hedayat, et al.             Standards Track                     [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   When the requested Receiver Port is not available (e.g., port in
   use), the Server at the Session-Reflector MAY suggest an alternate
   and available port for this session in the Port field.  The Session-
   Sender either accepts the alternate port, or composes a new Session-
   Request message with suitable parameters.  Otherwise, the Server at
   the Control-Client uses the Accept field to convey other forms of
   session rejection or failure and MUST NOT suggest an alternate port;
   in this case, the Port field MUST be set to zero.

<span class="h3"><a class="selflink" id="section-3.6" href="#section-3.6">3.6</a>.  Send Schedules</span>

   The send schedule for test packets defined in <a href="#section-3.6">Section 3.6</a> of OWAMP
   [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>] is not used in TWAMP.  The Control-Client and Session-
   Sender MAY autonomously decide the send schedule.  The Session-
   Reflector SHOULD return each test packet to the Session-Sender as
   quickly as possible.

<span class="h3"><a class="selflink" id="section-3.7" href="#section-3.7">3.7</a>.  Starting Test Sessions</span>

   The procedure and guidelines for starting test sessions is the same
   as defined in <a href="#section-3.7">Section 3.7</a> of OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].

<span class="h3"><a class="selflink" id="section-3.8" href="#section-3.8">3.8</a>.  Stop-Sessions</span>

   The procedure and guidelines for stopping test sessions is similar to
   that defined in <a href="#section-3.8">Section 3.8</a> of OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].  The Stop-Sessions
   command can only be issued by the Control-Client.  The message MUST
   NOT contain any session description records or skip ranges.  The
   message is terminated with a single block HMAC to complete the Stop-
   Sessions command.  Since the TWAMP Stop-Sessions command does not
   convey SIDs, it applies to all sessions previously requested and
   started with a Start-Sessions command.



















<span class="grey">Hedayat, et al.             Standards Track                    [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   Thus, the TWAMP Stop-Sessions command is constructed as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      3        |    Accept     |              MBZ              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Number of Sessions                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        MBZ (8 octets)                         |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                       HMAC (16 octets)                        |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Above, the Command Number in the first octet (3) indicates that this
   is the Stop-Sessions command.

   Non-zero Accept values indicate a failure of some sort.  Zero values
   indicate normal (but possibly premature) completion.  The full list
   of available Accept values is described in <a href="./rfc4656#section-3.3">Section&nbsp;3.3 of [RFC4656]</a>,
   "Values of the Accept Field".

   If Accept has a non-zero value, results of all TWAMP-Test sessions
   spawned by this TWAMP-Control session SHOULD be considered invalid.
   If the Accept-Session message was not transmitted at all (for
   whatever reason, including failure of the TCP connection used for
   TWAMP-Control), the results of all TWAMP-Test sessions spawned by
   this TWAMP-Control session MAY be considered invalid.

   Number of Sessions indicates the number of sessions that the
   Control-Client intends to stop.

   Number of Sessions MUST contain the number of send sessions started
   by the Control-Client that have not been previously terminated by a
   Stop-Sessions command (i.e., the Control-Client MUST account for each
   accepted Request-Session).  If the Stop-Sessions message does not
   account for exactly the number of sessions in progress, then it is to
   be considered invalid, the TWAMP-Control connection SHOULD be closed,
   and any results obtained considered invalid.

   Upon receipt of a TWAMP-Control Stop-Sessions command, the Session-
   Reflector MUST discard any TWAMP-Test packets that arrive at the
   current time plus the Timeout (in the Request-TW-Session command).




<span class="grey">Hedayat, et al.             Standards Track                    [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


<span class="h3"><a class="selflink" id="section-3.9" href="#section-3.9">3.9</a>.  Fetch-Session</span>

   One purpose of TWAMP is measurement of two-way metrics.  Two-way
   measurement methods do not require packet-level data to be collected
   by the Session-Reflector (such as sequence number, timestamp, and
   Time to Live (TTL)) because this data is communicated in the
   "reflected" test packets.  As such, the protocol does not require the
   retrieval of packet-level data from the Server and the OWAMP Fetch-
   Session command is not used in TWAMP.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  TWAMP-Test</span>

   The TWAMP-Test protocol is similar to the OWAMP-test protocol
   [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>] with the exception that the Session-Reflector transmits
   test packets to the Session-Sender in response to each test packet it
   receives.  TWAMP defines two different test packet formats, one for
   packets transmitted by the Session-Sender and one for packets
   transmitted by the Session-Reflector.  As with OWAMP-test protocol
   [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>], there are three modes: unauthenticated, authenticated, and
   encrypted.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Sender Behavior</span>

   The sender behavior is determined by the configuration of the
   Session-Sender and is not defined in this standard.  Further, the
   Session-Reflector does not need to know the Session-Sender behavior
   to the degree of detail as needed in OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>].  Additionally,
   the Session-Sender collects and records the necessary information
   provided from the packets transmitted by the Session-Reflector for
   measuring two-way metrics.  The information recording based on the
   packet(s) received by the Session-Sender is implementation dependent.

<span class="h4"><a class="selflink" id="section-4.1.1" href="#section-4.1.1">4.1.1</a>.  Packet Timings</span>

   Since the send schedule is not communicated to the Session-Reflector,
   there is no need for a standardized computation of packet timing.

   Regardless of any scheduling delays, each packet that is actually
   sent MUST have the best possible approximation of its real time of
   departure as its timestamp (in the packet).

<span class="h4"><a class="selflink" id="section-4.1.2" href="#section-4.1.2">4.1.2</a>.  Packet Format and Content</span>

   The Session-Sender packet format and content follow the same
   procedure and guidelines as defined in <a href="#section-4.1.2">Section 4.1.2</a> of OWAMP
   [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>] (with the exception of the reference to the send schedule).





<span class="grey">Hedayat, et al.             Standards Track                    [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   Note that the Reflector test packet formats are larger than the
   Sender's formats.  The Session-Sender MAY append sufficient Packet
   Padding to allow the same IP packet payload lengths to be used in
   each direction of transmission (this is usually desirable).  To
   compensate for the Reflector's larger test packet format, the Sender
   appends at least 27 octets of padding in unauthenticated mode, and at
   least 56 octets in authenticated and encrypted modes.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Reflector Behavior</span>

   TWAMP requires the Session-Reflector to transmit a packet to the
   Session-Sender in response to each packet it receives.

   As packets are received, the Session-Reflector will do the following:

   -  Timestamp the received packet.  Each packet that is actually
      received MUST have the best possible approximation of its real
      time of arrival entered as its Received Timestamp (in the packet).

   -  In authenticated or encrypted mode, decrypt the appropriate
      sections of the packet body (first block (16 octets) for
      authenticated, 96 octets for encrypted), and then check integrity
      of sections covered by the HMAC.

   -  Copy the packet sequence number into the corresponding reflected
      packet to the Session-Sender.

   -  Extract the Sender TTL value from the TTL/Hop Limit value of
      received packets.  Session-Reflector implementations SHOULD fetch
      the TTL/Hop Limit value from the IP header of the packet,
      replacing the value of 255 set by the Session-Sender.  If an
      implementation does not fetch the actual TTL value (the only good
      reason not to do so is an inability to access the TTL field of
      arriving packets), it MUST set the Sender TTL value as 255.

   -  In authenticated and encrypted modes, the HMAC MUST be calculated
      first, then the appropriate portion of the packet body is
      encrypted.

   -  Transmit a test packet to the Session-Sender in response to every
      received packet.  The response MUST be generated as immediately as
      possible.  The format and content of the test packet is defined in
      <a href="#section-4.2.1">Section 4.2.1</a>.  Prior to the transmission of the test packet, the
      Session-Reflector MUST enter the best possible approximation of
      its actual sending time as its Timestamp (in the packet).  This
      permits the determination of the elapsed time between the
      reception of the packet and its transmission.




<span class="grey">Hedayat, et al.             Standards Track                    [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   -  Packets not received within the Timeout (following the Stop-
      Sessions command) MUST be ignored by the Reflector.  The Session-
      Reflector MUST NOT generate a test packet to the Session-Sender
      for packets that are ignored.

   The possibility exists for Session-Sender failure during a session,
   or the path between the Session-Sender and Session-Reflector may fail
   while a test session is in progress.  The Session-Reflector MAY
   discontinue any session that has been started when no packet
   associated with that session has been received for REFWAIT seconds.
   The default value of REFWAIT SHALL be 900 seconds, and this waiting
   time MAY be configurable.  This timeout allows a Session-Reflector to
   free up resources in case of failure.

<span class="h4"><a class="selflink" id="section-4.2.1" href="#section-4.2.1">4.2.1</a>.  TWAMP-Test Packet Format and Content</span>

   The Session-Reflector MUST transmit a packet to the Session-Sender in
   response to each packet received.  The Session-Reflector SHOULD
   transmit the packets as immediately as possible.  The Session-
   Reflector SHOULD set the TTL in IPv4 (or Hop Limit in IPv6) in the
   UDP packet to 255.

   The test packet will have the necessary information for calculating
   two-way metrics by the Session-Sender.  The format of the test packet
   depends on the mode being used.  The two formats are presented below.


























<span class="grey">Hedayat, et al.             Standards Track                    [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   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        |           MBZ                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Receive Timestamp                    |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sender Sequence Number                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Sender Timestamp                         |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Sender Error Estimate    |           MBZ                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Sender TTL   |                                               |
   +-+-+-+-+-+-+-+-+                                               +
   |                                                               |
   .                                                               .
   .                         Packet Padding                        .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





















<span class="grey">Hedayat, et al.             Standards Track                    [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   For authenticated and encrypted modes:

   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                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        MBZ (12 octets)                        |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Timestamp                            |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Error Estimate        |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                        MBZ (6 octets)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Receive Timestamp                      |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        MBZ (8 octets)                         |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sender Sequence Number                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        MBZ (12 octets)                        |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Sender Timestamp                         |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Sender Error Estimate    |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |                        MBZ (6 octets)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Sender TTL   |                                               |
   +-+-+-+-+-+-+-+-+                                               +
   |                                                               |
   |                                                               |
   |                        MBZ (15 octets)                        |
   +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
   |                        HMAC (16 octets)                       |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|



<span class="grey">Hedayat, et al.             Standards Track                    [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   |                                                               |
   .                                                               .
   .                         Packet Padding                        .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Note that all timestamps have the same format as OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>] as
   follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Integer part of seconds                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Fractional part of seconds                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Sequence Number is the sequence number of the test packet according
   to its transmit order.  It starts with zero and is incremented by one
   for each subsequent packet.  The Sequence Number generated by the
   Session-Reflector is independent from the sequence number of the
   arriving packets.

   Timestamp and Error Estimate are the Session-Reflector's transmit
   timestamp and error estimate for the reflected test packet,
   respectively.  The format of all timestamp and error estimate fields
   follow the definition and formats defined by OWAMP, <a href="./rfc4656#section-4.1.2">Section&nbsp;4.1.2 in
   [RFC4656]</a>.

   Sender Timestamp and Sender Error Estimate are exact copies of the
   timestamp and error estimate from the Session-Sender test packet that
   corresponds to this test packet.

   Sender TTL is 255 when transmitted by the Session-Sender.  Sender TTL
   is set to the Time To Live (or Hop Count) value of the received
   packet from the IP packet header when transmitted by the Session-
   Reflector.

   Receive Timestamp is the time the test packet was received by the
   reflector.  The difference between Timestamp and Receive Timestamp is
   the amount of time the packet was in transition in the Session-
   Reflector.  The Error Estimate associated with the Timestamp field
   also applies to the Receive Timestamp.

   Sender Sequence Number is a copy of the Sequence Number of the packet
   transmitted by the Session-Sender that caused the Session-Reflector
   to generate and send this test packet.



<span class="grey">Hedayat, et al.             Standards Track                    [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   The HMAC field in TWAMP-Test packets covers the same fields as the
   Advanced Encryption Standard (AES) encryption.  Thus, in
   authenticated mode, HMAC covers the first block (16 octets); in
   encrypted mode, HMAC covers the first six blocks (96 octets).  In
   TWAMP-Test, the HMAC field MUST NOT be encrypted.

   Packet Padding in TWAMP-Test SHOULD be pseudo-random (it MUST be
   generated independently of any other pseudo-random numbers mentioned
   in this document).  However, implementations MUST provide a
   configuration parameter, an option, or a different means of making
   Packet Padding consist of all zeros.  Packet Padding MUST NOT be
   covered by the HMAC and MUST NOT be encrypted.

   The minimum data segment length of TWAMP-Test packets in
   unauthenticated mode is 41 octets, and 104 octets in both
   authenticated mode and encrypted modes.

   Note that the Session-Reflector Test packet formats are larger than
   the Sender's formats.  The Session-Reflector SHOULD reduce the length
   of the Sender's Packet Padding to achieve equal IP packet payload
   lengths in each direction of transmission, when sufficient padding is
   present.  The Session-Reflector MAY re-use the Sender's Packet
   Padding (since the requirements for padding generation are the same
   for each), and in this case the Session-Reflector SHOULD truncate the
   padding such that the highest-number octets are discarded.

   In unauthenticated mode, encryption or authentication MUST NOT be
   applied.

   The TWAMP-Test packet layout is identical in authenticated and
   encrypted modes.  The encryption operation for a Session-Sender
   packet follows the same rules of Session-Sender packets as defined in
   OWAMP <a href="./rfc4656#section-4.1.2">section&nbsp;4.1.2 of [RFC4656]</a>.

   The main difference between authenticated mode and encrypted mode is
   the portion of the test packets that are covered by HMAC and
   encrypted.  Authenticated mode permits the timestamp to be fetched
   after a portion of the packet is encrypted, but in encrypted mode all
   the sequence numbers and timestamps are fetched before encryption to
   provide maximum data-integrity protection.

   In authenticated mode, only the sequence number in the first block is
   encrypted, and the subsequent timestamps and sequence numbers are
   sent in clear text.  Sending the timestamp in clear text allows one
   to reduce the time between when a timestamp is obtained by a
   Session-Reflector and when that packet is sent out.  This potentially
   improves the timestamp accuracy, because the sequence number can be
   encrypted before the timestamp is fetched.



<span class="grey">Hedayat, et al.             Standards Track                    [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   In encrypted mode, the reflector MUST fetch the timestamps, generate
   the HMAC, and encrypt the packet, then send it.

   Obtaining the keys and encryption methods follows the same procedure
   as OWAMP as described below.  Each TWAMP-Test session has two keys,
   an AES Session-key and an HMAC Session-key, and the keys are derived
   from the TWAMP-Control keys and the SID.

   The TWAMP-Test AES Session-key is obtained as follows: the TWAMP-
   Control AES Session-key (the same AES Session-key as used for the
   corresponding TWAMP-Control session) is encrypted with the 16-octet
   session identifier (SID) as the key, using a single-block AES-ECB
   encryption.  The result is the TWAMP-Test AES Session-key to be used
   in encrypting (and decrypting) the packets of the particular TWAMP-
   Test session.  Note that the TWAMP-Test AES Session-key, TWAMP-
   Control AES Session-key, and the SID are all comprised of 16 octets.

   The TWAMP-Test HMAC Session-key is obtained as follows: the TWAMP-
   Control HMAC Session-key (the same HMAC Session-key as used for the
   corresponding TWAMP-Control session) is encrypted using AES-CBC
   (Cipher Block Chaining) with the 16-octet session identifier (SID) as
   the key.  This is a two-block CBC encryption that is always performed
   with IV=0.  Note that the TWAMP-Test HMAC Session-key and TWAMP-
   Control HMAC Session-key are comprised of 32 octets, while the SID is
   16 octets.

   In authenticated mode, the first block (16 octets) of each TWAMP-Test
   packet is encrypted using the AES Electronic Codebook (ECB) mode.
   This mode does not involve any chaining, and lost, duplicated, or
   reordered packets do not cause problems with deciphering any packet
   in a TWAMP-Test session.

   In encrypted mode, the first six blocks (96 octets) are encrypted
   using AES-CBC mode.  The AES Session-key to use is obtained in the
   same way as the key for authenticated mode.  Each TWAMP-Test packet
   is encrypted as a separate stream, with just one chaining operation;
   chaining does not span multiple packets so that lost, duplicated, or
   reordered packets do not cause problems.  The initialization vector
   for the CBC encryption is a value with all bits equal to zero.

   Implementation Note: Naturally, the key schedule for each TWAMP-Test
   session MUST be set up at most once per session, not once per packet.









<span class="grey">Hedayat, et al.             Standards Track                    [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Implementers' Guide</span>

   This section serves as guidance to implementers of TWAMP.  The
   example architecture presented here is not a requirement.  Similar to
   OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>], TWAMP is designed with enough flexibility to allow
   different architectures that suit multiple system requirements.

   In this example, the roles of Control-Client and Session-Sender are
   implemented in one host referred to as the controller, and the roles
   of Server and Session-Reflector are implemented in another host
   referred to as the responder.

              controller                              responder
          +-----------------+                   +-------------------+
          | Control-Client  |&lt;--TWAMP-Control--&gt;| Server            |
          |                 |                   |                   |
          | Session-Sender  |&lt;--TWAMP-Test-----&gt;| Session-Reflector |
          +-----------------+                   +-------------------+

   This example provides an architecture that supports the full TWAMP
   standard.  The controller establishes the test session with the
   responder through the TWAMP-Control protocol.  After the session is
   established, the controller transmits test packets to the responder.
   The responder follows the Session-Reflector behavior of TWAMP as
   described in <a href="#section-4.2">Section 4.2</a>.

   <a href="#appendix-I">Appendix I</a> provides an example for purely informational purposes.  It
   suggests an incremental path to adopting TWAMP, by implementing the
   TWAMP-Test protocol first.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Security Considerations</span>

   Fundamentally, TWAMP and OWAMP use the same protocol for
   establishment of Control and Test procedures.  The main difference
   between TWAMP and OWAMP is the Session-Reflector behavior in TWAMP
   vs. the Session-Receiver behavior in OWAMP.  This difference in
   behavior does not introduce any known security vulnerabilities that
   are not already addressed by the security features of OWAMP.  The
   entire security considerations of OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>] applies to TWAMP.

   The Server-Greeting message (defined in OWAMP, <a href="./rfc4656#section-3.1">Section&nbsp;3.1 of
   [RFC4656]</a>) includes a Count field to specify the iteration counter
   used in PKCS #5 to generate keys from shared secrets.  OWAMP
   recommends a lower limit of 1024 iterations, but no upper limit.  The
   Count field provides an opportunity for a denial-of-service (DOS)
   attack because it is 32 bits long.  If an attacking system set the
   maximum value in Count (2**32), then the system under attack would
   stall for a significant period of time while it attempts to generate



<span class="grey">Hedayat, et al.             Standards Track                    [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   keys.  Therefore, TWAMP-compliant systems SHOULD have a configuration
   control to limit the maximum Count value.  The default maximum Count
   value SHOULD be 32768.  As suggested in OWAMP, this value MAY be
   increased when greater computing power becomes common.  If a
   Control-Client receives a Server-Greeting message with Count greater
   that its maximum configured value, it SHOULD close the control
   connection.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Acknowledgements</span>

   We would like to thank Nagarjuna Venna, Sharee McNab, Nick Kinraid,
   Stanislav Shalunov, Matt Zekauskas, Walt Steverson, Jeff Boote,
   Murtaza Chiba, and Kevin Earnst for their comments, suggestions,
   reviews, helpful discussion, and proof-reading.  Lars Eggert, Sam
   Hartman, and Tim Polk contributed very useful AD-level reviews, and
   the authors thank them for their contributions to the memo.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  IANA Considerations</span>

   IANA has allocated a well-known TCP port number (861) for the OWAMP-
   Control part of the OWAMP [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>] protocol.

   ...
   owamp-control   861/tcp    OWAMP-Control
   owamp-control   861/udp    OWAMP-Control
   #                          [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>]

   IANA has also allocated a well-known TCP/UDP port number for the
   TWAMP-Control protocol.

   ...
   twamp-control   862/tcp    Two-way Active Measurement Protocol
                              (TWAMP) Control
   twamp-control   862/udp    Two-way Active Measurement Protocol
                              (TWAMP) Control
   #                          [<a href="./rfc5357">RFC5357</a>]
   #               863-872    Unassigned

   Since TWAMP adds an additional Control command beyond the OWAMP-
   Control specification and describes behavior when this control
   command is used, IANA has created a registry for the TWAMP Command
   Number field.  The field is not explicitly named in [<a href="./rfc4656" title="&quot;A One-way Active Measurement Protocol (OWAMP)&quot;">RFC4656</a>] but is
   called out for each command.  This field is a recognized extension
   mechanism for TWAMP.







<span class="grey">Hedayat, et al.             Standards Track                    [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  Registry Specification</span>

   IANA has created a TWAMP-Control Command Number registry.  TWAMP-
   Control commands are specified by the first octet in OWAMP-Control
   messages as shown in <a href="./rfc4656#section-3.5">Section&nbsp;3.5 of [RFC4656]</a>, and modified by this
   document.  Thus, this registry may contain sixteen possible values.

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Registry Management</span>

   Because the registry may only contain sixteen values, and because
   OWAMP and TWAMP are IETF protocols, this registry must only be
   updated by "IETF Consensus" as specified in [<a href="./rfc5226" title="">RFC5226</a>] -- an RFC
   documenting the use that is approved by the IESG.  We expect that new
   values will be assigned as monotonically increasing integers in the
   range [0-15], unless there is a good reason to do otherwise.

<span class="h3"><a class="selflink" id="section-8.3" href="#section-8.3">8.3</a>.  Experimental Numbers</span>

   [<a id="ref-RFC3692">RFC3692</a>] recommends allocating an appropriate number of values for
   experimentation and testing.  It is not clear to the authors exactly
   how many numbers might be useful in this space, or if it would be
   useful that they were easily distinguishable or at the "high end" of
   the number range.  Two might be useful, say one for session control,
   and one for session fetch.  On the other hand, a single number would
   allow for unlimited extension, because the format of the rest of the
   message could be tailored, with allocation of other numbers done once
   usefulness has been proven.  Thus, this document allocates one number
   (6) as designated for experimentation and testing.

<span class="h3"><a class="selflink" id="section-8.4" href="#section-8.4">8.4</a>.  Initial Registry Contents</span>

   TWAMP-Control Command Number Registry

   Value  Description             Semantics Definition
   0      Reserved
   1      Forbidden
   2      Start-Sessions          <a href="./rfc4656#section-3.7">RFC 4656, Section&nbsp;3.7</a>
   3      Stop-Sessions           <a href="./rfc4656#section-3.8">RFC 4656, Section&nbsp;3.8</a>
   4      Reserved
   5      Request-TW-Session      this document, <a href="#section-3.5">Section 3.5</a>
   6      Experimentation         undefined, see <a href="#section-8.3">Section 8.3</a>.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Internationalization Considerations</span>

   The protocol does not carry any information in a natural language,
   with the possible exception of the KeyID in TWAMP-Control, which is
   encoded in UTF-8 [RFC3629, <a href="./rfc5198">RFC5198</a>].




<span class="grey">Hedayat, et al.             Standards Track                    [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


Appendix I - TWAMP Light (Informative)

   In this example, the roles of Control-Client, Server, and Session-
   Sender are implemented in one host referred to as the controller, and
   the role of Session-Reflector is implemented in another host referred
   to as the responder.

              controller                              responder
          +-----------------+                   +-------------------+
          |     Server      |&lt;-----------------&gt;|                   |
          | Control-Client  |                   | Session-Reflector |
          | Session-Sender  |&lt;--TWAMP-Test-----&gt;|                   |
          +-----------------+                   +-------------------+

   This example provides a simple architecture for responders where
   their role will be to simply act as light test points in the network.
   The controller establishes the test session with the Server through
   non-standard means.  After the session is established, the controller
   transmits test packets to the responder.  The responder follows the
   Session-Reflector behavior of TWAMP as described in <a href="#section-4.2">section 4.2</a> with
   the following exceptions.

   In the case of TWAMP Light, the Session-Reflector does not
   necessarily have knowledge of the session state.  IF the Session-
   Reflector does not have knowledge of the session state, THEN the
   Session-Reflector MUST copy the Sequence Number of the received
   packet to the Sequence Number field of the reflected packet.  The
   controller receives the reflected test packets and collects two-way
   metrics.  This architecture allows for collection of two-way metrics.

   This example eliminates the need for the TWAMP-Control protocol, and
   assumes that the Session-Reflector is configured and communicates its
   configuration with the Server through non-standard means.  The
   Session-Reflector simply reflects the incoming packets back to the
   controller while copying the necessary information and generating
   sequence number and timestamp values per <a href="#section-4.2.1">Section 4.2.1</a>. TWAMP Light
   introduces some additional security considerations.  The non-standard
   means to control the responder and establish test sessions SHOULD
   offer the features listed below.

   The non-standard responder control protocol SHOULD have an
   authenticated mode of operation.  The responder SHOULD be
   configurable to accept only authenticated control sessions.

   The non-standard responder control protocol SHOULD have a means to
   activate the authenticated and encrypted modes of the TWAMP-Test
   protocol.




<span class="grey">Hedayat, et al.             Standards Track                    [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


   When the TWAMP Light test sessions operate in authenticated or
   encrypted mode, the Session-Reflector MUST have some mechanism for
   generating keys (because the TWAMP-Control protocol normally plays a
   role in this process, but is not present here).  The specification of
   the key generation mechanism is beyond the scope of this memo.

Normative References

   [<a id="ref-RFC4656">RFC4656</a>] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
             Zekauskas, "A One-way Active Measurement Protocol (OWAMP)",
             <a href="./rfc4656">RFC 4656</a>, September 2006.

   [<a id="ref-RFC2681">RFC2681</a>] Almes, G., Kalidindi, S., and M. Zekauskas, "A Round-trip
             Delay Metric for IPPM", <a href="./rfc2681">RFC 2681</a>, September 1999.

   [<a id="ref-RFC2119">RFC2119</a>] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-RFC2474">RFC2474</a>] Nichols, K., Blake, S., Baker, F., and D. Black,
             "Definition of the Differentiated Services Field (DS Field)
             in the IPv4 and IPv6 Headers", <a href="./rfc2474">RFC 2474</a>, December 1998.

   [<a id="ref-RFC5226">RFC5226</a>] Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>, <a href="./rfc5226">RFC 5226</a>, May
             2008.

   [<a id="ref-RFC3629">RFC3629</a>] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
             STD 63, <a href="./rfc3629">RFC 3629</a>, November 2003.

   [<a id="ref-RFC5198">RFC5198</a>] Klensin, J. and M. Padlipsky, "Unicode Format for Network
             Interchange", <a href="./rfc5198">RFC 5198</a>, March 2008.

Informative References

   [<a id="ref-RFC3692">RFC3692</a>] Narten, T., "Assigning Experimental and Testing Numbers
             Considered Useful", <a href="https://www.rfc-editor.org/bcp/bcp82">BCP 82</a>, <a href="./rfc3692">RFC 3692</a>, January 2004.















<span class="grey">Hedayat, et al.             Standards Track                    [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


Authors' Addresses

   Kaynam Hedayat
   Brix Networks
   285 Mill Road
   Chelmsford, MA  01824
   USA
   EMail: khedayat@brixnet.com
   URI:   <a href="http://www.brixnet.com/">http://www.brixnet.com/</a>

   Roman M. Krzanowski, Ph.D.
   Verizon
   500 Westchester Ave.
   White Plains, NY
   USA
   EMail: roman.krzanowski@verizon.com
   URI:   <a href="http://www.verizon.com/">http://www.verizon.com/</a>

   Al Morton
   AT&amp;T Labs
   Room D3 - 3C06
   200 Laurel Ave. South
   Middletown, NJ 07748
   USA
   Phone  +1 732 420 1571
   EMail: acmorton@att.com
   URI:   <a href="http://home.comcast.net/~acmacm/">http://home.comcast.net/~acmacm/</a>

   Kiho Yum
   Juniper Networks
   1194 Mathilda Ave.
   Sunnyvale, CA
   USA
   EMail: kyum@juniper.net
   URI:   <a href="http://www.juniper.com/">http://www.juniper.com/</a>

   Jozef Z. Babiarz
   Nortel Networks
   3500 Carling Avenue
   Ottawa, Ont  K2H 8E9
   Canada
   Email: babiarz@nortel.com
   URI:   <a href="http://www.nortel.com/">http://www.nortel.com/</a>








<span class="grey">Hedayat, et al.             Standards Track                    [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc5357">RFC 5357</a>          Two-Way Active Measurement Protocol       October 2008</span>


Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a>, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and <a href="https://www.rfc-editor.org/bcp/bcp79">BCP 79</a>.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   <a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.












Hedayat, et al.             Standards Track                    [Page 26]
</pre>