





Internet Engineering Task Force (IETF)                         A. Morton
Request for Comments: 5938                                     AT&T Labs
Updates: 5357                                                   M. Chiba
Category: Standards Track                                  Cisco Systems
ISSN: 2070-1721                                              August 2010


                   Individual Session Control Feature
          for the Two-Way Active Measurement Protocol (TWAMP)

Abstract

   The IETF has completed its work on the core specification of TWAMP --
   the Two-Way Active Measurement Protocol.  This memo describes an
   OPTIONAL feature for TWAMP, that gives the controlling host the
   ability to start and stop one or more individual test sessions using
   Session Identifiers.  The base capability of the TWAMP protocol
   requires all test sessions that were previously requested and
   accepted to start and stop at the same time.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5938.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



Morton & Chiba               Standards Track                    [Page 1]

RFC 5938                  TWAMP Session Control              August 2010


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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
   2.  Purpose and Scope  . . . . . . . . . . . . . . . . . . . . . .  4
   3.  TWAMP Control Extensions . . . . . . . . . . . . . . . . . . .  4
     3.1.  Connection Setup with Individual Session Control . . . . .  5
     3.2.  Start-N-Sessions Command with Individual Session
           Control  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Start-N-Ack Command with Individual Session Control  . . .  7
     3.4.  Stop-N-Sessions Command with Individual Session Control  .  9
     3.5.  Stop-N-Ack Command with Individual Session Control . . . . 10
     3.6.  SERVWAIT Timeout Operation . . . . . . . . . . . . . . . . 12
     3.7.  Additional Considerations  . . . . . . . . . . . . . . . . 12
   4.  TWAMP Test with Individual Session Control . . . . . . . . . . 13
     4.1.  Sender Behavior  . . . . . . . . . . . . . . . . . . . . . 13
     4.2.  Reflector Behavior . . . . . . . . . . . . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
     6.1.  Registry Specification . . . . . . . . . . . . . . . . . . 14
     6.2.  Registry Management  . . . . . . . . . . . . . . . . . . . 14
     6.3.  Experimental Numbers . . . . . . . . . . . . . . . . . . . 15
     6.4.  Registry Contents  . . . . . . . . . . . . . . . . . . . . 15
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 16












Morton & Chiba               Standards Track                    [Page 2]

RFC 5938                  TWAMP Session Control              August 2010


1.  Introduction

   The IETF has completed its work on the core specification of TWAMP --
   the Two-Way Active Measurement Protocol [RFC5357].  TWAMP is an
   extension of the One-way Active Measurement Protocol, OWAMP
   [RFC4656].  The TWAMP specification gathered wide review as it
   approached completion, and the by-products were several
   recommendations for new features in TWAMP.  There are a growing
   number of TWAMP implementations at present, and widespread usage is
   expected.  There are even devices that are designed to test
   implementations for protocol compliance.

   This memo describes an OPTIONAL feature for TWAMP.  [RFC5357] TWAMP
   (and OWAMP) start all previously requested and accepted test sessions
   at once.  This feature allows the Control-Client to control
   individual test sessions on the basis of their Session Identifier
   (SID).  This feature permits a short-duration TWAMP test to start
   (and/or stop) during a longer test.  This feature permits a specific
   diagnostic test to begin if intermediate results indicate that the
   test is warranted, for example.

   This feature requires a Modes field bit position assignment and the
   use of two new TWAMP command numbers (for the augmented Start and
   Stop commands).  This feature also specifies the use of a new Stop-N-
   ACK Server response, to complete the symmetry of the session-stopping
   process in the same way as the Start-ACK (Start-N-ACK when used with
   this feature) response.

   The Individual Session Control feature gives the Control-Client new
   flexibility to manage any number of test sessions once they are
   established.  However, [RFC5357] test sessions are established in
   serial order and the total establishment time grows with the number
   of sessions and the round-trip time.  Therefore, implementers of this
   feature may also wish to implement the "Reflect Octets" feature,
   described in [REFLECT].  This feature allows a Control-Client to
   distinguish between parallel Request-TW-Session commands because a
   participating Server can return octets (e.g., the Control-Client's
   local index) in its reply to the request.  Thus, the Reflect Octets
   feature supports the efficient establishment of many simultaneous
   test sessions that the Individual Session Control feature can then
   manage (start/stop).

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






Morton & Chiba               Standards Track                    [Page 3]

RFC 5938                  TWAMP Session Control              August 2010


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

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 an additional OPTIONAL
   function and feature for TWAMP [RFC5357].

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

   1.  extension of the modes of operation through assignment of a new
       value in the Modes field to communicate feature capability and
       use,

   2.  the definitions of augmented start session and stop session
       commands (with corresponding acknowledgements), and

   3.  the definition of related procedures for TWAMP entities.

   The motivation for this feature is the ability to start and stop
   individual test sessions at will, using a single TWAMP-Control
   connection.

   When the Server and Control-Client have agreed to use the Individual
   Session Control mode during control connection setup, then the
   Control-Client, the Server, the Session-Sender, and the Session-
   Reflector MUST all conform to the requirements of that mode, as
   identified below.  The original TWAMP-Control Start and Stop commands
   MUST NOT be used.

3.  TWAMP Control Extensions

   The TWAMP-Control protocol is a derivative of the OWAMP-Control
   protocol, and provides two-way measurement capability.  TWAMP
   [RFC5357] uses the Modes field to identify and select specific
   communication capabilities, and this field is a recognized extension
   mechanism.  The following sections describe one such extension.





Morton & Chiba               Standards Track                    [Page 4]

RFC 5938                  TWAMP Session Control              August 2010


3.1.  Connection Setup with Individual Session Control

   TWAMP-Control connection establishment follows the procedure defined
   in Section 3.1 of [RFC4656] OWAMP.  The Individual Session Control
   mode requires one new bit position (and value) to identify the
   ability of the Server/Session-Reflector to start and stop specific
   sessions (according to their Session Identifier, or SID).  This new
   feature requires an additional TWAMP mode bit assignment as follows:

   Value  Description             Reference/Explanation
   0      Reserved
   1      Unauthenticated         RFC 4656, Section 3.1
   2      Authenticated           RFC 4656, Section 3.1
   4      Encrypted               RFC 4656, Section 3.1
   8      Unauth. TEST protocol,  RFC 5618, Section 3.1
          Encrypted CONTROL
   --------------------------------------------------------
   16     Individual Session      RFC 5938, bit position 4
          Control

   In the original OWAMP Modes field, setting bit positions 0, 1, or 2
   indicated the security mode of the Control protocol, and the Test
   protocol inherited the same mode (see Section 4 of [RFC4656]).  In
   the [RFC5618] memo, bit position (3) allows a different security mode
   in the Test protocol and uses the unauthenticated test packet format.

   If the Server sets the new bit position (bit position 4) in the
   Server Greeting message to indicate its capabilities, then the Server
   and Session-Reflector MUST comply with the requirements of this memo
   to control sessions on an individual basis if desired.

   If the Control-Client intends to control sessions on an individual
   basis (according to the requirements in this memo), it MUST set the
   mode bit (4, corresponding to the new mode) in the Setup Response
   message.  This means that:

   1.  The Control-Client and the Server MUST use the start and stop
       commands intended for individual session control and the
       corresponding acknowledgements, as defined in the sections that
       follow.

   2.  The Control-Client and the Server MUST NOT use the start and stop
       commands (2 and 3) and the acknowledgement defined in [RFC5357].

   The Control-Client MUST also set one mode bit to indicate the chosen
   security mode (currently bits 0, 1, 2, or 3), consistent with the
   modes offered by the Server.  The Control-Client MAY also set Modes




Morton & Chiba               Standards Track                    [Page 5]

RFC 5938                  TWAMP Session Control              August 2010


   field bit 4 with other features and bit positions (such as the
   reflect octets feature).

3.2.  Start-N-Sessions Command with Individual Session Control

   Having

   o  initiated Individual Session Control mode in the Setup Response,

   o  requested one or more test sessions, and

   o  received affirmative Accept-Session response(s),

   a TWAMP Client MAY start the execution of one or more test sessions
   by sending a Start-N-Sessions message to the Server (note that "N"
   indicates that this command is applicable to one or more sessions,
   and does not change with the number of sessions identified in the
   command).

   The format of the Start-N-Sessions message is 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |      7        |                                               |
   +-+-+-+-+-+-+-+-+                                               +
   |                        MBZ (11 octets)                        |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Number of Sessions                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |                                                               |
   |                    First SID (16 octets)                      |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |                                                               |
   .                 remaining SIDs (16 octets each)               .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |                                                               |
   |                       HMAC (16 octets)                        |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B

   Note: In figures, "B" indicates the boundary of a 16-octet word.



Morton & Chiba               Standards Track                    [Page 6]

RFC 5938                  TWAMP Session Control              August 2010


   The Command Number value of 7 indicates that this is a Start-N-
   Sessions command.  The Control-Client MUST compose this command, and
   the Server MUST interpret this command, according to the field
   descriptions below.

   The Number of Sessions field indicates the count of sessions that
   this Start command applies to, and MUST be one or greater.  The
   number of SID fields that follow MUST be equal to the value in the
   Number of Sessions field (otherwise, the command MUST NOT be affirmed
   with a zero Accept field in the Start-N-Ack response).

   All SID fields are constructed as defined in the last paragraph of
   Section 3.5 of OWAMP [RFC4656] (and referenced in TWAMP).  Note that
   the SID is assigned by the Server during the session request
   exchange.

   The message is terminated with a single block HMAC, as illustrated
   above.

   The Server MUST respond with one or more Start-N-Ack messages (which
   SHOULD be sent as quickly as possible).  Start-N-Ack messages SHALL
   have the format defined in the next session.

   When using Individual Session Control mode and its Start-N-Ack
   command as described in the next section, multiple Start-N-Sessions
   commands MAY be sent without waiting for acknowledgement, and the
   Start-N-sessions commands MAY arrive in any order.

3.3.  Start-N-Ack Command with Individual Session Control

   The Server responds to the Start-N-Sessions command (for one or more
   specific sessions referenced by their SIDs) with one or more Start-N-
   Ack commands with Accept fields corresponding to one or more of the
   SIDs.  This allows for the possibility that a Server cannot
   immediately start one or more of the sessions referenced in a
   particular Start-N-Sessions command, but can start one or more of the
   sessions.














Morton & Chiba               Standards Track                    [Page 7]

RFC 5938                  TWAMP Session Control              August 2010


   The format of the message is 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |      8        |     Accept    |            MBZ                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        MBZ (8 octets)                         |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Number of Sessions                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |                                                               |
   |                    First SID (16 octets)                      |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |                                                               |
   .                remaining SIDs (16 octets each)                .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |                                                               |
   |                       HMAC (16 octets)                        |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Command Number value of 8 indicates that this is a Start-N-Ack
   message.  The Server MUST compose this command, and the Control-
   Client MUST interpret this command, according to the field
   descriptions below.

   The Accept field values are defined in Section 3.3 of OWAMP
   [RFC4656].

   The Number of Sessions field indicates the count of sessions that
   this Start-N-Ack command applies to, and MUST be one or greater.  The
   number of SID fields that follow MUST be equal to the value in the
   Number of Sessions field.

   All SID fields are constructed as defined in the last paragraph of
   Section 3.5 of OWAMP [RFC4656] (and referenced in TWAMP).  Note that
   the SID is assigned by the Server during the session request
   exchange.

   The message is terminated with a single block HMAC, as illustrated
   above.



Morton & Chiba               Standards Track                    [Page 8]

RFC 5938                  TWAMP Session Control              August 2010


   Note that the SIDs for all Sessions with the same 'Accept' code can
   be acknowledged using the same Start-N-Ack message.

   For example, say that the Server receives a Start-N-Sessions command
   for SIDs 1, 2, 3, and 4.  The Server determines that the resources
   for SID=3 are temporarily unavailable.  The Server responds with two
   Start-N-Ack commands with fields as follows:

      Accept = 0   Number of Sessions = 3    SIDs 1, 2, 4

      Accept = 5   Number of Sessions = 1    SID 3

3.4.  Stop-N-Sessions Command with Individual Session Control

   The Stop-N-Sessions command can only be issued by the Control-Client.
   The command MUST contain at least one SID.

   The TWAMP Stop-N-Sessions command for use in Individual Session
   Control mode is formatted 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |      9        |                                               |
   +-+-+-+-+-+-+-+-+                                               +
   |                        MBZ (11 octets)                        |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Number of Sessions                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |                                                               |
   |                    First SID (16 octets)                      |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |                                                               |
   .                remaining SIDs (16 octets each)                .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |                                                               |
   |                       HMAC (16 octets)                        |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B






Morton & Chiba               Standards Track                    [Page 9]

RFC 5938                  TWAMP Session Control              August 2010


   The Command Number value of 9 indicates that this is a Stop-N-
   Sessions command.  The Control-Client MUST compose this command, and
   the Server MUST interpret this command, according to the field
   descriptions below.

   The Number of Sessions field indicates the count of sessions to which
   this Stop-N-Sessions command applies.  The SID is as defined in
   Section 3.5 of OWAMP [RFC4656] (and TWAMP), and the value MUST be one
   or greater.  The number of SID fields that follow MUST be equal to
   the value in the Number of Sessions field.

   The message is terminated with a single block HMAC, as illustrated
   above.

   The Server MUST respond with one or more Stop-N-Ack messages (which
   SHOULD be sent as quickly as possible).  Stop-N-Ack messages SHALL
   have the format defined in the next session.

3.5.  Stop-N-Ack Command with Individual Session Control

   In response to the Stop-N-Sessions command (for one or more specific
   sessions referenced by their SIDs), the Server MUST reply with one or
   more Stop-N-Ack commands with Accept fields corresponding to one or
   more of the SIDs.  This allows for the possibility that a Server
   cannot immediately stop one or more of the sessions referenced in a
   particular Stop-N-Sessions command, but can stop one or more of the
   sessions.
























Morton & Chiba               Standards Track                   [Page 10]

RFC 5938                  TWAMP Session Control              August 2010


   The format for the Stop-N-Ack command is 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |      10       |     Accept    |            MBZ                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        MBZ (8 octets)                         |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Number of Sessions                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |                                                               |
   |                    First SID (16 octets)                      |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |                                                               |
   .                 remaining SIDs (16 octets each)               .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+B
   |                                                               |
   |                       HMAC (16 octets)                        |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Command Number value of 10 indicates that this is a Stop-N-Ack
   message.  The Server MUST compose this command, and the Control-
   Client MUST interpret this command, according to the field
   descriptions below.

   The Accept Field values are defined in Section 3.3 of OWAMP
   [RFC4656].

   The Number of Sessions field indicates the count of sessions that
   this Stop-N-Ack command applies to, and MUST be one or greater.  The
   number of SID fields that follow MUST be equal to the value in the
   Number of Sessions field.

   All SID fields are constructed as defined in the last paragraph of
   Section 3.5 of OWAMP [RFC4656] (and referenced in TWAMP).  Note that
   the SID is assigned by the Server during the session request
   exchange.

   The message is terminated with a single block HMAC, as illustrated
   above.



Morton & Chiba               Standards Track                   [Page 11]

RFC 5938                  TWAMP Session Control              August 2010


   Note that the SIDs for all Sessions with the same 'Accept' code can
   be acknowledged using the same Stop-N-Ack message.

3.6.  SERVWAIT Timeout Operation

   Section 3.1 of [RFC5357] describes the operation of the optional
   SERVWAIT timer.  In normal TWAMP operation, the Server suspends
   monitoring the SERVWAIT timer while test sessions are in progress.
   When the Individual Session Control feature is utilized, this
   suspension is extended to cover the time when ANY test session is in
   progress.

   Thus, the Server SHALL suspend monitoring control connection activity
   after receiving any Start-N-Sessions command, and after receiving a
   Stop-N-Sessions command for all corresponding SIDs (and no test
   sessions are in progress), OR when REFWAIT expires on ALL test
   sessions initiated by a TWAMP-Control connection, then the SERVWAIT
   monitoring SHALL resume (as though a Stop-N-Sessions command had been
   received).  An implementation that supports the SERVWAIT timeout
   option SHOULD also implement the REFWAIT timeout option.

   The diagram below illustrates the operation of timers SERVWAIT and
   REFWAIT.

     SERVWAIT                   REFWAIT                  SERVWAIT
   +-+-+-+-+-+-+-+-+                                   +-+-+-+-+-+-+
   (no sessions
    in progress)
                    +-+-+-+-+-+-+-+-+-+-+-+
                            SID="1"

                                   +-+-+-+-+
                                    SID="2"

                                     +-+-+-+-+-+-+-+-+
                                           SID="3"

   >>>>>>>>>> Time >>>>>>>>>>>>>>>>>>> Time >>>>>>>>>>>>>>>> Time >>>>>

3.7.  Additional Considerations

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

   With the publication of this feature, bit positions 0 through 4 of
   the 32-bit Modes field are used.  A Control-Client MAY ignore bit
   positions greater than 2 in the Modes field, or it MAY support



Morton & Chiba               Standards Track                   [Page 12]

RFC 5938                  TWAMP Session Control              August 2010


   OPTIONAL features that are communicated in bit positions 3 and
   higher.  (The unassigned bits are available for future protocol
   extensions.)

   Other ways in which TWAMP extends OWAMP are described in [RFC5357].

4.  TWAMP Test with Individual Session Control

   The TWAMP test protocol is similar to the OWAMP [RFC4656] test
   protocol with the exception that the Session-Reflector transmits test
   packets to the Session-Sender in response to each test packet it
   receives.  TWAMP [RFC5357] 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 the OWAMP-Test
   protocol, there are three security modes: unauthenticated,
   authenticated, and encrypted.  The unauthenticated mode has one test
   packet format, while the authenticated and encrypted modes use
   another (common) format.

4.1.  Sender Behavior

   The individual session control feature requires that the sender MUST
   manage test sessions according to their SID.  Otherwise, the sender
   behavior is as described in Section 4.1 of [RFC5357].

4.2.  Reflector Behavior

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

   o  The Session-Reflector MUST manage all test sessions accepted
      according to their SID.

   o  Upon receipt of a TWAMP-Control Stop-N-Sessions command
      referencing a specific session/SID, the Session-Reflector MUST
      ignore TWAMP-Test packets (in the same session/SID) that arrive at
      the current time plus the Timeout (in the Request-TW-Session
      command and assuming subsequent acknowledgement).  The Session-
      Reflector MUST NOT generate a test packet to the Session-Sender
      for packets that are ignored.  (Note: The Request-TW-Session
      command includes sender address + port and receiver address +
      port, and this is usually sufficient to distinguish sessions.)

   o  If the REFWAIT timer is implemented, it SHOULD be enforced when
      any test session is in progress (started and not stopped).





Morton & Chiba               Standards Track                   [Page 13]

RFC 5938                  TWAMP Session Control              August 2010


5.  Security Considerations

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

6.  IANA Considerations

   As a result of this document, IANA has assigned one mode bit
   position/value for a mode in the IANA registry for the TWAMP Modes
   field, and this memo describes the behavior when the new mode is
   used.  This field is a recognized extension mechanism for TWAMP.

   As a result of this document, IANA has assigned four command numbers
   in the "TWAMP-Control Command Numbers" registry, and this memo
   describes the use of the new commands.  The command number field is a
   recognized extension mechanism for TWAMP.

6.1.  Registry Specification

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

   IANA has also created the "TWAMP-Control Command Numbers" registry.
   TWAMP-Control commands are specified by the first octet in TWAMP-
   Control messages as specified in Section 3.5 of [RFC5357], and
   augmented by this memo.  This registry may contain 256 possible
   values.

6.2.  Registry Management

   Because the "TWAMP-Control Command Numbers" registry can contain only
   256 values and "TWAMP-Modes" are based on only 32 bit positions with
   a maximum of 2^32 values, and because TWAMP is an IETF protocol,
   these registries must be updated only by "IETF Consensus" as
   specified in [RFC5226] (an RFC that documents registry use and is
   approved by the IESG).  Management of these registries is described
   in Section 8.2 of [RFC5357] and [RFC5618].




Morton & Chiba               Standards Track                   [Page 14]

RFC 5938                  TWAMP Session Control              August 2010


   The values 7, 8, 9, and 10 have been assigned in the "TWAMP-Control
   Command Numbers" Registry.  The value 16 corresponding to the next
   available bit position (4) (as described in Sections 3.1 and 3.7) has
   been assigned in the "TWAMP-Modes" registry.

6.3.  Experimental Numbers

   One experimental value has been assigned in the "TWAMP-Control
   Command Numbers" registry.

   No additional experimental values are assigned in the TWAMP-Modes
   registry.

6.4.  Registry Contents

   TWAMP-Control Command Numbers Registry

   Value  Description             Semantics Definition
    0      Reserved
    1      Forbidden
    2      Start-Sessions          RFC 4656, Section 3.7
    3      Stop-Sessions           RFC 4656, Section 3.8
    4      Reserved
    5      Request-TW-Session      RFC 5357, Section 3.5
    6      Experimentation         RFC 5357, Section 8.3
   ------------------------------------------------------------------
    7      Start-N-Sessions        RFC 5938, Section 3.2
    8      Start-N-Ack             RFC 5938, Section 3.3
    9      Stop-N-Sessions         RFC 5938, Section 3.4
   10      Stop-N-Ack              RFC 5938, Section 3.5


   TWAMP-Modes Registry

   Value  Description             Reference/Explanation
   0      Reserved
   1      Unauthenticated         RFC 4656, Section 3.1
   2      Authenticated           RFC 4656, Section 3.1
   4      Encrypted               RFC 4656, Section 3.1
   8      Unauth. TEST protocol,  RFC 5618, Section 3.1
          Encrypted CONTROL
   --------------------------------------------------------
   16     Individual Session       RFC 5938, Section 3.1
          Control                  bit position 4







Morton & Chiba               Standards Track                   [Page 15]

RFC 5938                  TWAMP Session Control              August 2010


7.  Acknowledgements

   The authors thank everyone who provided comments on this feature,
   especially Lars Eggert, Adrian Farrel, and Alexey Melnikov.

8.  References

8.1.  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.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

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

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

8.2.  Informative References

   [REFLECT]  Morton, A. and L. Ciavattone, "TWAMP Reflect Octets and
              Symmetrical Size Features", Work in Progress, June 2010.



















Morton & Chiba               Standards Track                   [Page 16]

RFC 5938                  TWAMP Session Control              August 2010


Authors' Addresses

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

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


   Murtaza Chiba
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   Phone: +1 800 553 NETS
   EMail: mchiba@cisco.com





























Morton & Chiba               Standards Track                   [Page 17]

