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rfc:rfc5357

Network Working Group K. Hedayat Request for Comments: 5357 Brix Networks Category: Standards Track R. Krzanowski

                                                               Verizon
                                                             A. Morton
                                                             AT&T Labs
                                                                K. Yum
                                                      Juniper Networks
                                                            J. Babiarz
                                                       Nortel Networks
                                                          October 2008
           A Two-Way Active Measurement Protocol (TWAMP)

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 RFC
 4656, 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.

Hedayat, et al. Standards Track [Page 1] RFC 5357 Two-Way Active Measurement Protocol October 2008

Table of Contents

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

1. Introduction

 The Internet Engineering Task Force (IETF) has completed a Proposed
 Standard for the round-trip delay [RFC2681] metric.  The IETF has
 also completed a protocol for the control and collection of one-way
 measurements, the One-way Active Measurement Protocol (OWAMP)
 [RFC4656].  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

Hedayat, et al. Standards Track [Page 2] RFC 5357 Two-Way Active Measurement Protocol October 2008

 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 Section
 2.6 of [RFC2681].  This memo specifies the Two-Way Active Measurement
 Protocol, or TWAMP.  TWAMP uses the methodology and architecture of
 OWAMP [RFC4656] 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 RFC 2119 [RFC2119].

1.1. Relationship of Test and Control Protocols

 Similar to OWAMP [RFC4656], TWAMP consists of two inter-related
 protocols: TWAMP-Control and TWAMP-Test.  The relationship of these
 protocols is as defined in Section 1.1 of OWAMP [RFC4656].  TWAMP-
 Control is used to initiate, start, and stop test sessions, whereas
 TWAMP-Test is used to exchange test packets between two TWAMP
 entities.

1.2. Logical Model

 The role and definition of the logical entities are as defined in
 Section 1.2 of OWAMP [RFC4656] with the following exceptions:
  1. 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.
  1. 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.

Hedayat, et al. Standards Track [Page 3] RFC 5357 Two-Way Active Measurement Protocol October 2008

  1. 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 |<-TWAMP-Test-->| Session-Reflector |
       +----------------+               +-------------------+
         ^                                     ^
         |                                     |
         |                                     |
         |                                     |
         |  +----------------+<----------------+
         |  |     Server     |
         |  +----------------+
         |    ^
         |    |
         | TWAMP-Control
         |    |
         v    v
       +----------------+
       | Control-Client |
       +----------------+
 As in OWAMP [RFC4656], 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  |<--TWAMP-Control-->|      Server       |
        |                 |                   |                   |
        | Session-Sender  |<--TWAMP-Test----->| Session-Reflector |
        +-----------------+                   +-------------------+

1.3. Pronunciation Guide

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

Hedayat, et al. Standards Track [Page 4] RFC 5357 Two-Way Active Measurement Protocol October 2008

2. Protocol Overview

 The Two-Way Active Measurement Protocol is an open protocol for
 measurement of two-way metrics.  It is based on OWAMP [RFC4656] and
 adheres to OWAMP's overall architecture and design.  The TWAMP-
 Control and TWAMP-Test protocols accomplish their testing tasks as
 outlined below:
  1. 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.
  1. 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.
  1. 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.
  1. The Control-Client initiates all requested testing with a Start-

Sessions message, and the Server acknowledges.

  1. The Session-Sender and the Session-Reflector exchange test packets

according to the TWAMP-Test protocol for each active session.

  1. 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.

Hedayat, et al. Standards Track [Page 5] RFC 5357 Two-Way Active Measurement Protocol October 2008

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

3. TWAMP-Control

 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 Section 3 of OWAMP
 [RFC4656] with the exceptions outlined in the following sections.
 One such exception is the Fetch-Session command, which is not used in
 TWAMP.

3.1. Connection Setup

 Connection establishment of TWAMP follows the same procedure defined
 in Section 3.1 of OWAMP [RFC4656].  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.

Hedayat, et al. Standards Track [Page 6] RFC 5357 Two-Way Active Measurement Protocol October 2008

 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 Appendix B of [RFC5198] (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.

3.2. Integrity Protection

 Integrity protection of TWAMP follows the same procedure defined in
 Section 3.2 of OWAMP [RFC4656].  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.

3.3. Values of the Accept Field

 Accept values used in TWAMP are the same as the values defined in
 Section 3.3 of OWAMP [RFC4656].

3.4. TWAMP-Control Commands

 TWAMP-Control commands conform to the rules defined in Section 3.4 of
 OWAMP [RFC4656].
 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.

Hedayat, et al. Standards Track [Page 7] RFC 5357 Two-Way Active Measurement Protocol October 2008

3.5. Creating Test Sessions

 Test session creation follows the same procedure as defined in
 Section 3.5 of OWAMP [RFC4656].  The Request-TW-Session command is
 based on the OWAMP Request-Session command, and uses the message
 format as described in Section 3.5 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

Hedayat, et al. Standards Track [Page 8] RFC 5357 Two-Way Active Measurement Protocol October 2008

 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 [RFC4656].  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 [RFC4656].
 The Timeout is interpreted differently from the definition in OWAMP
 [RFC4656].  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 [RFC4656].  The only
 capability of this field is to set the Differentiated Services Code
 Point (DSCP) as defined in [RFC2474].  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 [RFC4656].  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.

Hedayat, et al. Standards Track [Page 9] RFC 5357 Two-Way Active Measurement Protocol October 2008

 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.

3.6. Send Schedules

 The send schedule for test packets defined in Section 3.6 of OWAMP
 [RFC4656] 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.

3.7. Starting Test Sessions

 The procedure and guidelines for starting test sessions is the same
 as defined in Section 3.7 of OWAMP [RFC4656].

3.8. Stop-Sessions

 The procedure and guidelines for stopping test sessions is similar to
 that defined in Section 3.8 of OWAMP [RFC4656].  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.

Hedayat, et al. Standards Track [Page 10] RFC 5357 Two-Way Active Measurement Protocol October 2008

 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 Section 3.3 of [RFC4656],
 "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).

Hedayat, et al. Standards Track [Page 11] RFC 5357 Two-Way Active Measurement Protocol October 2008

3.9. Fetch-Session

 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.

4. TWAMP-Test

 The TWAMP-Test protocol is similar to the OWAMP-test protocol
 [RFC4656] 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
 [RFC4656], there are three modes: unauthenticated, authenticated, and
 encrypted.

4.1. Sender Behavior

 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 [RFC4656].  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.

4.1.1. Packet Timings

 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).

4.1.2. Packet Format and Content

 The Session-Sender packet format and content follow the same
 procedure and guidelines as defined in Section 4.1.2 of OWAMP
 [RFC4656] (with the exception of the reference to the send schedule).

Hedayat, et al. Standards Track [Page 12] RFC 5357 Two-Way Active Measurement Protocol October 2008

 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.

4.2. Reflector Behavior

 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:
  1. 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).
  1. 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.
  1. Copy the packet sequence number into the corresponding reflected

packet to the Session-Sender.

  1. 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.
  1. In authenticated and encrypted modes, the HMAC MUST be calculated

first, then the appropriate portion of the packet body is

    encrypted.
  1. 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
    Section 4.2.1.  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.

Hedayat, et al. Standards Track [Page 13] RFC 5357 Two-Way Active Measurement Protocol October 2008

  1. 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.

4.2.1. TWAMP-Test Packet Format and Content

 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.

Hedayat, et al. Standards Track [Page 14] RFC 5357 Two-Way Active Measurement Protocol October 2008

 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                        .
 .                                                               .
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Hedayat, et al. Standards Track [Page 15] RFC 5357 Two-Way Active Measurement Protocol October 2008

 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)                       |
 |                                                               |
 |                                                               |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|

Hedayat, et al. Standards Track [Page 16] RFC 5357 Two-Way Active Measurement Protocol October 2008

 |                                                               |
 .                                                               .
 .                         Packet Padding                        .
 .                                                               .
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Note that all timestamps have the same format as OWAMP [RFC4656] 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, Section 4.1.2 in
 [RFC4656].
 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.

Hedayat, et al. Standards Track [Page 17] RFC 5357 Two-Way Active Measurement Protocol October 2008

 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 section 4.1.2 of [RFC4656].
 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.

Hedayat, et al. Standards Track [Page 18] RFC 5357 Two-Way Active Measurement Protocol October 2008

 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.

Hedayat, et al. Standards Track [Page 19] RFC 5357 Two-Way Active Measurement Protocol October 2008

5. Implementers' Guide

 This section serves as guidance to implementers of TWAMP.  The
 example architecture presented here is not a requirement.  Similar to
 OWAMP [RFC4656], 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  |<--TWAMP-Control-->| Server            |
        |                 |                   |                   |
        | Session-Sender  |<--TWAMP-Test----->| 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 Section 4.2.
 Appendix I provides an example for purely informational purposes.  It
 suggests an incremental path to adopting TWAMP, by implementing the
 TWAMP-Test protocol first.

6. Security Considerations

 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 [RFC4656] applies to TWAMP.
 The Server-Greeting message (defined in OWAMP, Section 3.1 of
 [RFC4656]) 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

Hedayat, et al. Standards Track [Page 20] RFC 5357 Two-Way Active Measurement Protocol October 2008

 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.

7. Acknowledgements

 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.

8. IANA Considerations

 IANA has allocated a well-known TCP port number (861) for the OWAMP-
 Control part of the OWAMP [RFC4656] protocol.
 ...
 owamp-control   861/tcp    OWAMP-Control
 owamp-control   861/udp    OWAMP-Control
 #                          [RFC4656]
 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
 #                          [RFC5357]
 #               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 [RFC4656] but is
 called out for each command.  This field is a recognized extension
 mechanism for TWAMP.

Hedayat, et al. Standards Track [Page 21] RFC 5357 Two-Way Active Measurement Protocol October 2008

8.1. Registry Specification

 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 Section 3.5 of [RFC4656], and modified by this
 document.  Thus, this registry may contain sixteen possible values.

8.2. Registry Management

 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 [RFC5226] -- 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.

8.3. Experimental Numbers

 [RFC3692] 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.

8.4. Initial Registry Contents

 TWAMP-Control Command Number 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      this document, Section 3.5
 6      Experimentation         undefined, see Section 8.3.

9. Internationalization Considerations

 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, RFC5198].

Hedayat, et al. Standards Track [Page 22] RFC 5357 Two-Way Active Measurement Protocol October 2008

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      |<----------------->|                   |
        | Control-Client  |                   | Session-Reflector |
        | Session-Sender  |<--TWAMP-Test----->|                   |
        +-----------------+                   +-------------------+
 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 section 4.2 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 Section 4.2.1. 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.

Hedayat, et al. Standards Track [Page 23] RFC 5357 Two-Way Active Measurement Protocol October 2008

 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

 [RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
           Zekauskas, "A One-way Active Measurement Protocol (OWAMP)",
           RFC 4656, September 2006.
 [RFC2681] Almes, G., Kalidindi, S., and M. Zekauskas, "A Round-trip
           Delay Metric for IPPM", RFC 2681, September 1999.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
           "Definition of the Differentiated Services Field (DS Field)
           in the IPv4 and IPv6 Headers", RFC 2474, December 1998.
 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
           IANA Considerations Section in RFCs", BCP 26, RFC 5226, May
           2008.
 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
           STD 63, RFC 3629, November 2003.
 [RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
           Interchange", RFC 5198, March 2008.

Informative References

 [RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
           Considered Useful", BCP 82, RFC 3692, January 2004.

Hedayat, et al. Standards Track [Page 24] RFC 5357 Two-Way Active Measurement Protocol October 2008

Authors' Addresses

 Kaynam Hedayat
 Brix Networks
 285 Mill Road
 Chelmsford, MA  01824
 USA
 EMail: khedayat@brixnet.com
 URI:   http://www.brixnet.com/
 Roman M. Krzanowski, Ph.D.
 Verizon
 500 Westchester Ave.
 White Plains, NY
 USA
 EMail: roman.krzanowski@verizon.com
 URI:   http://www.verizon.com/
 Al Morton
 AT&T Labs
 Room D3 - 3C06
 200 Laurel Ave. South
 Middletown, NJ 07748
 USA
 Phone  +1 732 420 1571
 EMail: acmorton@att.com
 URI:   http://home.comcast.net/~acmacm/
 Kiho Yum
 Juniper Networks
 1194 Mathilda Ave.
 Sunnyvale, CA
 USA
 EMail: kyum@juniper.net
 URI:   http://www.juniper.com/
 Jozef Z. Babiarz
 Nortel Networks
 3500 Carling Avenue
 Ottawa, Ont  K2H 8E9
 Canada
 Email: babiarz@nortel.com
 URI:   http://www.nortel.com/

Hedayat, et al. Standards Track [Page 25] RFC 5357 Two-Way Active Measurement Protocol October 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, 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
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 Copies of IPR disclosures made to the IETF Secretariat and any
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 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]

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