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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 4656

Network Working Group                                 Stanislav Shalunov
Internet Draft                                       Benjamin Teitelbaum
Expiration Date: April 2005                                 Anatoly Karp
                                                           Jeff W. Boote
                                                    Matthew J. Zekauskas
                                                               Internet2
                                                            October 2004

             A One-way Active Measurement Protocol (OWAMP)
                     <draft-ietf-ippm-owdp-11.txt>


Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

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   http://www.ietf.org/shadow.html.

Copyright Notice

   Copyright (C) The Internet Society 2004.  All Rights Reserved.

Abstract

   With growing availability of good time sources to network nodes, it
   becomes increasingly possible to measure one-way IP performance
   metrics with high precision.  To do so in an interoperable manner, a
   common protocol for such measurements is required.  The One-Way
   Active Measurement Protocol (OWAMP) can measure one-way delay, as
   well as other unidirectional characteristics, such as one-way loss.




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Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . .   3
      1.1. Relationship of Test and Control Protocols . . . . . .   4
      1.2. Logical Model  . . . . . . . . . . . . . . . . . . . .   5
   2. Protocol Overview . . . . . . . . . . . . . . . . . . . . .   6
   3. OWAMP-Control . . . . . . . . . . . . . . . . . . . . . . .   7
      3.1. Connection Setup . . . . . . . . . . . . . . . . . . .   7
      3.2. Values of the Accept Field . . . . . . . . . . . . . .  10
      3.3. OWAMP-Control Commands . . . . . . . . . . . . . . . .  11
      3.4. Creating Test Sessions . . . . . . . . . . . . . . . .  11
      3.5. Send Schedules . . . . . . . . . . . . . . . . . . . .  16
      3.6. Starting Test Sessions . . . . . . . . . . . . . . . .  17
      3.7. Stop-Sessions  . . . . . . . . . . . . . . . . . . . .  19
      3.8. Fetch-Session  . . . . . . . . . . . . . . . . . . . .  22
   4. OWAMP-Test  . . . . . . . . . . . . . . . . . . . . . . . .  26
      4.1. Sender Behavior  . . . . . . . . . . . . . . . . . . .  26
         4.1.1. Packet Timings  . . . . . . . . . . . . . . . . .  26
         4.1.2. Packet Format and Content . . . . . . . . . . . .  27
      4.2. Receiver Behavior  . . . . . . . . . . . . . . . . . .  30
   5. Computing Exponentially Distributed Pseudo-Random Numbers .  32
      5.1. High-Level Description of the Algorithm  . . . . . . .  32
      5.2. Data Types, Representation, and Arithmetic . . . . . .  33
      5.3. Uniform Random Quantities  . . . . . . . . . . . . . .  34
   6. Security Considerations . . . . . . . . . . . . . . . . . .  35
      6.1. Introduction . . . . . . . . . . . . . . . . . . . . .  35
      6.2. Preventing Third-Party Denial of Service . . . . . . .  35
      6.3. Covert Information Channels  . . . . . . . . . . . . .  35
      6.4. Requirement to Include AES in Implementations  . . . .  35
      6.5. Resource Use Limitations . . . . . . . . . . . . . . .  36
      6.6. Use of Cryptographic Primitives in OWAMP . . . . . . .  37
      6.7. Required Properties of MD5 . . . . . . . . . . . . . .  38
      6.8. The Use of AES-CBC-MAC . . . . . . . . . . . . . . . .  39
   7. IANA Considerations . . . . . . . . . . . . . . . . . . . .  40
   8. Internationalization Considerations . . . . . . . . . . . .  40
   9. Appendix A: Sample C Code for Exponential Deviates  . . . .  41
   10. Appendix B: Test Vectors for Exponential Deviates  . . . .  46
   11. Normative References . . . . . . . . . . . . . . . . . . .  46
   12. Informative References . . . . . . . . . . . . . . . . . .  47
   13. Authors' Addresses . . . . . . . . . . . . . . . . . . . .  48











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1. Introduction

   The IETF IP Performance Metrics (IPPM) working group has proposed
   draft standard metrics for one-way packet delay [RFC2679] and loss
   [RFC2680] across Internet paths.  Although there are now several
   measurement platforms that implement collection of these metrics
   [SURVEYOR], [RIPE], there is not currently a standard that would
   permit initiation of test streams or exchange of packets to collect
   singleton metrics in an interoperable manner.

   With the increasingly wide availability of affordable global
   positioning systems (GPS) and CDMA-based time sources, hosts
   increasingly have available to them very accurate time
   sources--either directly or through their proximity to Network Time
   Protocol (NTP) primary (stratum 1) time servers.  By standardizing a
   technique for collecting IPPM one-way active measurements, we hope to
   create an environment where IPPM metrics may be collected across a
   far broader mesh of Internet paths than is currently possible.  One
   particularly compelling vision is of widespread deployment of open
   OWAMP servers that would make measurement of one-way delay as
   commonplace as measurement of round-trip time using an ICMP-based
   tool like ping.

   Additional design goals of OWAMP include being hard to detect and
   manipulate, security, logical separation of control and test
   functionality, and support for small test packets.

   OWAMP test traffic is hard to detect because it is simply a stream of
   UDP packets from and to negotiated port numbers, with potentially
   nothing static in the packets (size is negotiated, as well).  OWAMP
   also supports an encrypted mode that further obscures the traffic, at
   the same time making it impossible to alter timestamps undetectably.

   Security features include optional authentication and/or encryption
   of control and test messages.  These features may be useful to
   prevent unauthorized access to results or man-in-the-middle attackers
   who attempt to provide special treatment to OWAMP test streams or who
   attempt to modify sender-generated timestamps to falsify test
   results.

   The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
   in this document are to be interpreted as described in [RFC2119].









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1.1. Relationship of Test and Control Protocols

   OWAMP actually consists of two inter-related protocols: OWAMP-Control
   and OWAMP-Test.  OWAMP-Control is used to initiate, start, and stop
   test sessions and fetch their results, while OWAMP-Test is used to
   exchange test packets between two measurement nodes.

   Although OWAMP-Test may be used in conjunction with a control
   protocol other than OWAMP-Control, the authors have deliberately
   chosen to include both protocols in the same draft to encourage the
   implementation and deployment of OWAMP-Control as a common
   denominator control protocol for one-way active measurements.  Having
   a complete and open one-way active measurement solution that is
   simple to implement and deploy is crucial to assuring a future in
   which inter-domain one-way active measurement could become as
   commonplace as ping.  We neither anticipate nor recommend that
   OWAMP-Control form the foundation of a general-purpose extensible
   measurement and monitoring control protocol.

   OWAMP-Control is designed to support the negotiation of one-way
   active measurement sessions and results retrieval in a
   straightforward manner. At session initiation, there is a negotiation
   of sender and receiver addresses and port numbers, session start
   time, session length, test packet size, the mean Poisson sampling
   interval for the test stream, and some attributes of the very general
   RFC 2330 notion of packet type, including packet size and per-hop
   behavior (PHB) [RFC2474], which could be used to support the
   measurement of one-way network characteristics across differentiated
   services networks.  Additionally, OWAMP-Control supports per-session
   encryption and authentication for both test and control traffic,
   measurement servers that can act as proxies for test stream
   endpoints, and the exchange of a seed value for the pseudo-random
   Poisson process that describes the test stream generated by the
   sender.

   We believe that OWAMP-Control can effectively support one-way active
   measurement in a variety of environments, from publicly accessible
   measurement beacons running on arbitrary hosts to network monitoring
   deployments within private corporate networks.  If integration with
   Simple Network Management Protocol (SNMP) or proprietary network
   management protocols is required, gateways may be created.










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1.2. Logical Model

   Several roles are logically separated to allow for broad flexibility
   in use.  Specifically, we define:

   Session-Sender      the sending endpoint of an OWAMP-Test session;

   Session-Receiver    the receiving endpoint of an OWAMP-Test session;

   Server              an end system that manages one or more OWAMP-Test
                       sessions, is capable of configuring per-session
                       state in session endpoints, and is capable of
                       returning the results of a test session;

   Control-Client      an end system that initiates requests for
                       OWAMP-Test sessions, triggers the start of a set
                       of sessions, and may trigger their termination; and

   Fetch-Client        an end system that initiates requests to fetch
                       the results of completed OWAMP-Test sessions.

   One possible scenario of relationships between these roles is shown
   below.

       +----------------+               +------------------+
       | Session-Sender |--OWAMP-Test-->| Session-Receiver |
       +----------------+               +------------------+
         ^                                     ^
         |                                     |
         |                                     |
         |                                     |
         |  +----------------+<----------------+
         |  |     Server     |<-------+
         |  +----------------+        |
         |    ^                       |
         |    |                       |
         | OWAMP-Control         OWAMP-Control
         |    |                       |
         v    v                       v
       +----------------+     +-----------------+
       | Control-Client |     |   Fetch-Client  |
       +----------------+     +-----------------+

   (Unlabeled links in the figure are unspecified by this draft and may
   be proprietary protocols.)

   Different logical roles can be played by the same host.  For example,
   in the figure above, there could actually be only two hosts: one



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   playing the roles of Control-Client, Fetch-Client, and
   Session-Sender, and the other playing the roles of Server and
   Session-Receiver. This is shown below.

       +-----------------+                   +------------------+
       | Control-Client  |<--OWAMP-Control-->| Server           |
       | Fetch-Client    |                   |                  |
       | Session-Sender  |---OWAMP-Test----->| Session-Receiver |
       +-----------------+                   +------------------+

   Finally, because many Internet paths include segments that transport
   IP over ATM, delay and loss measurements can include the effects of
   ATM segmentation and reassembly (SAR).  Consequently, OWAMP has been
   designed to allow for small test packets that would fit inside the
   payload of a single ATM cell (this is only achieved in
   unauthenticated and encrypted modes).


2. Protocol Overview

   As described above, OWAMP consists of two inter-related protocols:
   OWAMP-Control and OWAMP-Test.  The former is layered over TCP and is
   used to initiate and control measurement sessions and to fetch their
   results.  The latter protocol is layered over UDP and is used to send
   singleton measurement packets along the Internet path under test.

   The initiator of the measurement session establishes a TCP connection
   to a well-known port on the target point and this connection remains
   open for the duration of the OWAMP-Test sessions.  IANA will be
   requested to allocate a well-known port number for OWAMP-Control
   sessions.  An OWAMP server SHOULD listen to this well-known port.

   OWAMP-Control messages are transmitted only before OWAMP-Test
   sessions are actually started and after they complete (with the
   possible exception of an early Stop-Sessions message).

   The OWAMP-Control and OWAMP-Test protocols support three modes of
   operation: unauthenticated, authenticated, and encrypted.  The
   authenticated or encrypted modes require endpoints to possess a
   shared secret.

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







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3. OWAMP-Control

   Each type of OWAMP-Control message has a fixed length.  The recipient
   will know the full length of a message after examining the first 16
   octets of it.  No message is shorter than 16 octets.

   If the full message is not received within 30 minutes after it is
   expected, connection SHOULD be dropped.


3.1. Connection Setup

   Before either a Control-Client or a Fetch-Client can issue commands
   of a Server, it has to establish a connection to the server.

   First, a client opens a TCP connection to the server on a well-known
   port.  The server responds with a server greeting:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                      Unused (12 octets)                       |
     |                                                               |
     |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Modes                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                     Challenge (16 octets)                     |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The following Mode values are meaningful: 1 for unauthenticated, 2
   for authenticated, and 4 for encrypted.  The value of the Modes field
   sent by the server is the bit-wise OR of the mode values that it is
   willing to support during this session.  Thus, the last three bits of
   the Modes 32-bit value are used.  The first 29 bits MUST be zero.  A
   client MUST ignore the values in the first 29 bits of the Modes
   value.  (This way, the bits are available for future protocol
   extensions.  This is the only intended extension mechanism.)

   Challenge is a random sequence of octets generated by the server; it
   is used subsequently by the client to prove possession of a shared
   secret in a manner prescribed below.

   If Modes value is zero, the server does not wish to communicate with
   the client and MAY close the connection immediately.  The client



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   SHOULD close the connection if it receives a greeting with Modes
   equal to zero.  The client MAY close the connection if the client's
   desired mode is unavailable.

   Otherwise, the client MUST respond with the following message:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                             Mode                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                                                               .
     .                     Username (16 octets)                      .
     .                                                               .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                                                               .
     .                       Token (32 octets)                       .
     .                                                               .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                                                               .
     .                     Client-IV (16 octets)                     .
     .                                                               .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Here Mode is the mode that the client chooses to use during this
   OWAMP-Control session.  It will also be used for all OWAMP-Test
   sessions started under control of this OWAMP-Control session.  In
   Mode, one or zero bits MUST be set within last three bits.  The first
   29 bits of Mode MUST be zero.  A server MUST ignore the values of the
   first 29 bits.

   In unauthenticated mode, Username, Token, and Client-IV are unused.

   Otherwise, Username is a 16-octet indicator that tells the server
   which shared secret the client wishes to use to authenticate or
   encrypt, while Token is the concatenation of a 16-octet challenge and
   a 16-octet Session-key, encrypted using the AES (Advanced Encryption
   Standard) [AES] in Cipher Block Chaining (CBC). Encryption MUST be
   performed using an Initialization Vector (IV) of zero and a key value
   that is the shared secret associated with Username.  (Both the server
   and the client use the same mappings from user names to secret keys.
   The server, being prepared to conduct sessions with more than one



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   client, uses user names to choose the appropriate secret key; a
   client would typically have different secret keys for different
   servers.  The situation is analogous to that of passwords, except
   that secret keys, rather than being the typical low-entropy
   passwords, are suitable for use as AES keys.)  The shared secret will
   typically be provided as a passphrase; in this case, the MD5 sum
   [RFC1321] of the passphrase (without possible newline character(s) at
   the end of the passphrase) SHOULD be used as a key for encryption by
   the client and decryption by the server (the passphrase also SHOULD
   NOT contain newlines in the middle).

   Session-key and Client-IV are generated randomly by the client.
   Session-key MUST be generated with sufficient entropy not to reduce
   the security of the underlying cipher.  Client-IV merely needs to be
   unique (i.e., it MUST never be repeated for different sessions using
   the same secret key; a simple way to achieve that without the use of
   cumbersome state is to generate the Client-IV strings using a
   cryptographically secure pseudo-random number source: if this is
   done, the first repetition is unlikely to occur before 2^64 sessions
   with the same secret key are conducted).

   The server MUST respond with the following message:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                      Unused, MBZ (15 octets)                  |
     |                                                               |
     |                                               +-+-+-+-+-+-+-+-+
     |                                               |   Accept      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                     Server-IV (16 octets)                     |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Uptime (Timestamp)                       |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         IZP (8 octets)                        |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Unused 15-octet part MUST be zero.  The client MUST ignore its
   value.  MBZ (MUST be zero) fields here and hereafter have the same
   semantics: the party that sends the message MUST set the field to a
   string of zero bits; the party that interprets the message MUST



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   ignore the value.  (This way the field could be used for future
   extensions.)

   Server-IV is generated randomly by the server.  In unauthenticated
   mode, Server-IV is unused.

   The Accept field indicates the server's willingness to continue
   communication.  A zero value in the Accept field means that the
   server accepts the authentication and is willing to conduct further
   transactions.  Non-zero values indicate that the server does not
   accept the authentication or, for some other reason, is not willing
   to conduct further transactions in this OWAMP-Control session.  The
   full list of available Accept values is described in the ``Values of
   the Accept Field'' section.

   If a negative (non-zero) response is sent, the server MAY and the
   client SHOULD close the connection after this message.

   Uptime is a timestamp representing the time when the current
   instantiation of the server started operating.  (For example, in a
   multi-user general purpose operating system (OS), it could be the
   time when the server process was started.)  If Accept is non-zero,
   Uptime SHOULD be set to a string of zeros. In authenticated and
   encrypted modes, Uptime is encrypted as described in the next
   section, unless Accept is non-zero. (Authenticated and encrypted mode
   cannot be entered unless the control connection can be initialized.)

   Timestamp format is described in ``Sender Behavior'' section below.
   The same instantiation of the server SHOULD report the same exact
   Uptime value to each client in each session.

   Integrity Zero Padding (IZP) is treated the same way as IZP in the
   next section and beyond.

   The previous transactions constitute connection setup.


3.2. Values of the Accept Field

   Accept values are used throughout the OWAMP-Control protocol to
   communicate the server response to client requests.  The full set of
   valid Accept field values are:

     0    OK.

     1    Failure, reason unspecified (catch-all).

     2    Internal error.



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     3    Some aspect of request is not supported.

     4    Cannot perform request due to permanent resource limitations.

     5    Cannot perform request due to temporary resource limitations.

   All other values are reserved.  The sender of the message MAY use the
   value of 1 for all non-zero Accept values.  A message sender SHOULD
   use the correct Accept value if it is going to use other values.  The
   message receiver MUST interpret all values of Accept other than these
   reserved values as 1.  This way, other values are available for
   future extensions.


3.3. OWAMP-Control Commands

   In authenticated or encrypted mode (which are identical as far as
   OWAMP-Control is concerned, and only differ in OWAMP-Test) all
   further communications are encrypted with the Session-key, using CBC
   mode.  The client encrypts its stream using Client-IV.  The server
   encrypts its stream using Server-IV.

   The following commands are available for the client: Request-Session,
   Start-Sessions, Stop-Sessions, and Fetch-Session.  The command
   Stop-Sessions is available to both the client and the server.  (The
   server can also send other messages in response to commands it
   receives.)

   After Start-Sessions is sent/received by the client/server, and
   before it both sends and receives Stop-Sessions (order unspecified),
   it is said to be conducting active measurements.

   While conducting active measurements, the only command available is
   Stop-Sessions.

   These commands are described in detail below.


3.4. Creating Test Sessions

   Individual one-way active measurement sessions are established using
   a simple request/response protocol. An OWAMP client MAY issue zero or
   more Request-Session messages to an OWAMP server, which MUST respond
   to each with an Accept-Session message.  An Accept-Session message
   MAY refuse a request.

   The format of Request-Session message is as follows:




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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      1        |  MBZ  | IPVN  |  Conf-Sender  | Conf-Receiver |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Number of Schedule Slots                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Number of Packets                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Sender Port          |         Receiver Port         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Sender Address                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |              Sender Address (cont.) or MBZ                    |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Receiver Address                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |              Receiver Address (cont.) or MBZ                  |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                        SID (16 octets)                        |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Padding Length                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Start Time                          |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Timeout                            |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Type-P Descriptor                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              MBZ                              |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                        IZP (16 octets)                        |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This is immediately followed by one or more schedule slot



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   descriptions (the number of schedule slots is specified in the
   `Number of Schedule Slots' field above):

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Slot Type  |                                               |
     +-+-+-+-+-+-+-+-+              MBZ                              |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Slot Parameter (Timestamp)                    |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   These are immediately followed by IZP:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                        IZP (16 octets)                        |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   All these messages comprise one logical message: the Request-Session
   command.

   Above, the first octet (1) indicates that this is Request-Session
   command.

   IPVN is the IP version numbers for Sender and Receiver.  When the IP
   version number is 4, 12 octets follow the 4-octet IPv4 address stored
   in Sender Address and Receiver Address.  These octets MUST be set to
   zero by the client and MUST be ignored by the server.  Currently
   meaningful IPVN values are 4 and 6.

   Conf-Sender and Conf-Receiver MUST be set to 0 or 1 by the client.
   The server MUST interpret any non-zero value as 1.  If the value is
   1, the server is being asked to configure the corresponding agent
   (sender or receiver).  In this case, the corresponding Port value
   SHOULD be disregarded by the server.  At least one of Conf-Sender and
   Conf-Receiver MUST be 1.  (Both can be set, in which case the server
   is being asked to perform a session between two hosts it can
   configure.)

   Number of Schedule Slots, as mentioned before, specifies the number
   of slot records that go between the two blocks of IZP.  It is used by



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   the sender to determine when to send test packets (see next section).

   Number of Packets is the number of active measurement packets to be
   sent during this OWAMP-Test session (note that both server and client
   can abort the session early).

   If Conf-Sender is not set, Sender Port is the UDP port from which
   OWAMP-Test packets will be sent.  If Conf-Receiver is not set,
   Receiver Port is the UDP port OWAMP-Test to which packets are
   requested to be sent.

   The Sender Address and Receiver Address fields contain, respectively,
   the sender and receiver addresses of the end points of the Internet
   path over which an OWAMP test session is requested.

   SID is the session identifier.  It can be used in later sessions as
   an argument for the Fetch-Session command.  It is meaningful only if
   Conf-Receiver is 0.  This way, the SID is always generated by the
   receiving side.  See the end of the section for information on how
   the SID is generated.

   Padding length is the number of octets to be appended to the normal
   OWAMP-Test packet (see more on padding in discussion of OWAMP-Test).

   Start Time is the time when the session is to be started (but not
   before Start-Sessions command is issued).  This timestamp is in the
   same format as OWAMP-Test timestamps.

   Timeout (or a loss threshold) is an interval of time (expressed as a
   timestamp).  A packet belonging to the test session that is being set
   up by the current Request-Session command will be considered lost if
   it is not received during Timeout seconds after it is sent.

   Type-P Descriptor covers only a subset of (very large) Type-P space.
   If the first two bits of the Type-P Descriptor are 00, then
   subsequent six bits specify the requested Differentiated Services
   Codepoint (DSCP) value of sent OWAMP-Test packets, as defined in
   RFC 2474.  If the first two bits of Type-P descriptor are 01, then
   the subsequent 16 bits specify the requested PHB Identification Code
   (PHB ID), as defined in RFC 2836.

   Therefore, the value of all zeros specifies the default best-effort
   service.

   If Conf-Sender is set, the Type-P Descriptor is to be used to
   configure the sender to send packets according to its value.  If
   Conf-Sender is not set, the Type-P Descriptor is a declaration of how
   the sender will be configured.



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   If Conf-Sender is set and the server does not recognize the Type-P
   Descriptor, or it cannot or does not wish to set the corresponding
   attributes on OWAMP-Test packets, it SHOULD reject the session
   request.  If Conf-Sender is not set, the server SHOULD accept or
   reject the session paying no attention to the value of the Type-P
   Descriptor.

   IZP MUST be all zeros in this and all messages that use IZP.  The
   recipient of a message where IZP is not zero MUST reject the message,
   as it is an indication of tampering with the content of the message
   by an intermediary (or brokenness).  If the message is part of
   OWAMP-Control, the session MUST be terminated and results
   invalidated.  If the message is part of OWAMP-Test, it MUST be
   silently ignored.  This will ensure data integrity.  In
   unauthenticated mode, IZP is nothing more than a simple check.  In
   authenticated and encrypted modes, however, it ensures, in
   conjunction with properties of CBC chaining mode, that everything
   received before was not tampered with.  For this reason, it is
   important to check the IZP field as soon as possible, so that bad
   data doesn't get propagated.

   To each Request-Session message, an OWAMP server MUST respond with an
   Accept-Session message:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Accept     |  Unused       |            Port               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |                                                               |
     |                        SID (16 octets)                        |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                        IZP (12 octets)                        |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In this message, zero in the Accept field means that the server is
   willing to conduct the session.  A non-zero value indicates rejection
   of the request.  The full list of available Accept values is
   described in the ``Values of the Accept Field'' section.

   If the server rejects a Request-Session message, it SHOULD not close
   the TCP connection.  The client MAY close it if it receives negative
   response to the Request-Session message.




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   The meaning of Port in the response depends on the values of
   Conf-Sender and Conf-Receiver in the query that solicited the
   response.  If both were set, the Port field is unused.  If only
   Conf-Sender was set, Port is the port from which to expect OWAMP-Test
   packets.  If only Conf-Receiver was set, Port is the port to which
   OWAMP-Test packets are sent.

   If only Conf-Sender was set, the SID field in the response is unused.
   Otherwise, SID is a unique server-generated session identifier.  It
   can be used later as handle to fetch the results of a session.

   SIDs SHOULD be constructed by concatenation of the 4-octet IPv4 IP
   number belonging to the generating machine, an 8-octet timestamp, and
   a 4-octet random value.  To reduce the probability of collisions, if
   the generating machine has any IPv4 addresses (with the exception of
   loopback), one of them SHOULD be used for SID generation, even if all
   communication is IPv6-based.  If it has no IPv4 addresses at all, the
   last four octets of an IPv6 address MAY be used instead.  Note that
   SID is always chosen by the receiver.  If truly random values are not
   available, it is important that the SID be made unpredictable, as
   knowledge of the SID might be used for access control.


3.5. Send Schedules

   The sender and the receiver both need to know the same send schedule.
   This way, when packets are lost, the receiver knows when they were
   supposed to be sent.  It is desirable to compress common schedules
   and still to be able to use an arbitrary one for the test sessions.
   In many cases, the schedule will consist of repeated sequences of
   packets: this way, the sequence performs some test, and the test is
   repeated a number of times to gather statistics.

   To implement this, we have a schedule with a given number of slots.
   Each slot has a type and a parameter.  Two types are supported:
   exponentially distributed pseudo-random quantity (denoted by a code
   of 0) and a fixed quantity (denoted by a code of 1).  The parameter
   is expressed as a timestamp and specifies a time interval.  For a
   type 0 slot (exponentially distributed pseudo-random quantity) this
   interval is the mean value (or 1/lambda if the distribution density
   function is expressed as lambda*exp(-lambda*x) for positive values of
   x).  For a type 1 (fixed quantity) slot, the parameter is the delay
   itself.  The sender starts with the beginning of the schedule, and
   executes the instructions in the slots: for a slot of type 0, wait an
   exponentially distributed time with a mean of the specified parameter
   and then send a test packet (and proceed to the next slot); for a
   slot of type 1, wait the specified time and send a test packet (and
   proceed to the next slot).  The schedule is circular: when there are



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   no more slots, the sender returns to the first slot.

   The sender and the receiver need to be able to reproducibly execute
   the entire schedule (so, if a packet is lost, the receiver can still
   attach a send timestamp to it). Slots of type 1 are trivial to
   reproducibly execute.  To reproducibly execute slots of type 0, we
   need to be able to generate pseudo-random exponentially distributed
   quantities in a reproducible manner.  The way this is accomplished is
   discussed later.

   Using this mechanism one can easily specify common testing scenarios.
   Some examples include:


   +  Poisson stream: a single slot of type 0;

   +  Periodic stream: a single slot of type 1;

   +  Poisson stream of back-to-back packet pairs: two slots -- type 0
      with a non-zero parameter and type 1 with a zero parameter.


   Further, a completely arbitrary schedule can be specified (albeit
   inefficiently) by making the number of test packets equal to the
   number of schedule slots.  In this case, the complete schedule is
   transmitted in advance of an OWAMP-Test session.


3.6. Starting Test Sessions

   Having requested one or more test sessions and received affirmative
   Accept-Session responses, an OWAMP client MAY start the execution of
   the requested test sessions by sending a Start-Sessions message to
   the server.

   The format of this message is as follows:















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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      2        |                                               |
     +-+-+-+-+-+-+-+-+                                               |
     |                      Unused (15 octets)                       |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                        IZP (16 octets)                        |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The server MUST respond with an Start-Ack message (which SHOULD be
   sent as quickly as possible). Start-Ack messages have the following
   format:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Accept    |                                               |
     +-+-+-+-+-+-+-+-+                                               |
     |                      Unused (15 octets)                       |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                        IZP (16 octets)                        |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If Accept is non-zero, the Start-Sessions request was rejected; zero
   means that the command was accepted.  The full list of available
   Accept values is described in the ``Values of the Accept Field''
   section.  The server MAY, and the client SHOULD, close the connection
   in the case of a rejection.

   The server SHOULD start all OWAMP-Test streams immediately after it
   sends the response or immediately after their specified start times,
   whichever is later.  If the client represents a Sender, the client
   SHOULD start its OWAMP-Test streams immediately after it sees the
   Start-Ack response from the Server (if the Start-Sessions command was
   accepted) or immediately after their specified start times, whichever
   is later.  See more on OWAMP-Test sender behavior in a separate
   section below.



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3.7. Stop-Sessions

   The Stop-Sessions message may be issued by either the Control-Client
   or the Server.  The format of this 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      3        |    Accept     |            Unused             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Number of Sessions                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Unused (8 octets)                        |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This is immediately followed by zero or more session description
   records (the number of session description records is specified in
   the ``Number of Sessions'' field above).  The session description
   record is used to indicate which packets were actually sent by the
   sender process (rather than skipped).  The header of the session
   description record 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |                                                               |
     |                        SID (16 octets)                        |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Next Seqno                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Number of Skip Ranges                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This is immediately followed by zero or more Skip Range descriptions
   as specified by the ``Number of Skip Ranges'' field above.  Skip
   Ranges are simply two sequence numbers that, together, indicate a
   range of packets that were not sent:











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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |                      First Seqno Skipped                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Last Seqno Skipped                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The last (possibly full, possibly incomplete) block (16 octets) of
   data MUST be padded with zeros, if necessary. This ensures that the
   next session description record starts on a block boundary.

   Finally, a single block (16 octets) of IZP is concatenated on the end
   to complete the Stop-Sessions message.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                        IZP (16 octets)                        |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   All these records comprise one logical message: the Stop-Sessions
   command.

   Above, 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 the ``Values of the Accept
   Field'' section.

   If Accept had a non-zero value (from either party), results of all
   OWAMP-Test sessions spawned by this OWAMP-Control session SHOULD be
   considered invalid, even if a Fetch-Session with SID from this
   session works for a different OWAMP-Control session.  If Accept was
   not transmitted at all (for whatever reason, including the TCP
   connection used for OWAMP-Control breaking), the results of all
   OWAMP-Test sessions spawned by this OWAMP-control session MAY be
   considered invalid.

   Number of Sessions indicates the number of session description
   records that immediately follow the Stop-Sessions header.

   Number of Sessions MUST contain the number of send sessions started
   by the local side of the control connection that have not been
   previously terminated by a Stop-Sessions command (i.e., the



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   Control-Client MUST account for each accepted Request-Session where
   Conf-Receiver was set; the Control-Server MUST account for each
   accepted Request-Session where Conf-Sender was set).  If the
   Stop-Sessions message does not account for exactly the send sessions
   controlled by that side, then it is to be considered invalid and the
   connection SHOULD be closed and any results obtained considered
   invalid.

   Each session description record represents one OWAMP-Test session.

   SID is the session identifier (SID) used to indicate which send
   session is being described.

   Next Seqno indicates the next sequence number that would have been
   sent from this send session. For completed sessions, this will equal
   NumPackets from the Request-Session.

   Number of Skip Ranges indicates the number of holes that actually
   occurred in the sending process. This is a range of packets that were
   never actually sent by the sending process. For example, if a send
   session is started too late for the first 10 packets to be sent and
   this is the only hole in the schedule, then ``Number of Skip Ranges''
   would be 1. The single Skip Range description will have First Seqno
   Skipped equal to 0 and Last Seqno Skipped equal to 9.  This is
   described further in the ``Sender Behavior'' section.

   If the OWAMP-Control connection breaks when the Stop-Sessions command
   is sent, the receiver MAY not completely invalidate the session
   results.  It MUST discard all record of packets that follow (in other
   words, have greater sequence number than) the last packet that was
   actually received before before any lost packet records.  This will
   help differentiate between packet losses that occurred in the network
   and packets the sending process may have never sent.

   If a receiver of an OWAMP-Test session learns, through an OWAMP-
   Control Stop-Sessions message, that the OWAMP-Test sender's last
   sequence number is lower than any sequence number actually received,
   the results of the complete OWAMP-Test session MUST be invalidated.

   A receiver of an OWAMP-Test session, upon receipt of an OWAMP-Control
   Stop-Sessions command, MUST discard any packet records -- including
   lost packet records -- with a (computed) send time that falls between
   the current time minus Timeout and the current time. This ensures
   statistical consistency for the measurement of loss and duplicates in
   the event that the Timeout is greater than the time it takes for the
   Stop-Sessions command to take place.

   To effect complete sessions, each side of the control connection



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   SHOULD wait until all sessions are complete before sending the
   Stop-Sessions message. The completed time of each sessions is
   determined as Timeout after the scheduled time for the last sequence
   number.  Endpoints MAY add a small increment to the computed
   completed time for send endpoints to ensure the Stop-Sessions message
   reaches the receiver endpoint after Timeout.

   To effect a premature stop of sessions, the party that initiates this
   command MUST stop its OWAMP-Test send streams to send the Session
   Packets Sent values before sending this command. That party SHOULD
   wait until receiving the response Stop-Sessions message before
   stopping the receiver streams so that it can use the values from the
   received Stop-Sessions message to validate the data.


3.8. Fetch-Session

   The format of this client 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      4        |                                               |
     +-+-+-+-+-+-+-+-+                                               |
     |                      Unused (7 octets)                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Begin Seq                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          End Seq                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                        SID (16 octets)                        |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                        IZP (16 octets)                        |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Begin Seq is the sequence number of the first requested packet.  End
   Seq is the sequence number of the last requested packet.  If Begin
   Seq is all zeros and End Seq is all ones, complete session is said to
   be requested.

   If a complete session is requested and the session is still in
   progress, or has terminated in any way other than normal, the request



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   to fetch session results MUST be denied.  If an incomplete session is
   requested, all packets received so far that fall into the requested
   range SHOULD be returned.  Note that, since no commands can be issued
   between Start-Sessions and Stop-Sessions, incomplete requests can
   only happen on a different OWAMP-Control connection (from the same or
   different host as Control-Client).

   The server MUST respond with a Fetch-Ack message.  The format of this
   server response 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Accept    | Complete      |       Unused (2 octets)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Next Seqno                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Number of Skip Ranges                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Number of Records                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                        IZP (16 octets)                        |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Again, non-zero in the Accept field means a rejection of command.
   The server MUST specify zero for all remaining fields if Accept is
   non-zero. The client MUST ignore all remaining fields (except for the
   IZP) if Accept is non-zero.  The full list of available Accept values
   is described in the ``Values of the Accept Field'' section.

   Complete is non-zero if the OWAMP-Test session has terminated.

   Next Seqno indicates the next sequence number that would have been
   sent from this send session. For completed sessions, this will equal
   NumPackets from the Request-Session.

   Number of Skip Ranges indicates the number of holes that actually
   occurred in the sending process.

   Number of Records is the number of packet records that fall within
   the requested range.  This number might be less than the Number of
   Packets in the reproduction of the Request-Session command because of
   a session that ended prematurely or it might be greater because of
   duplicates.




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   If Accept was non-zero, this concludes the response to the Fetch-
   Session message. If Accept was 0, the server then MUST immediately
   send the OWAMP-Test session data in question.

   The OWAMP-Test session data consists of the following (concatenated):

   +  A reproduction of the Request-Session command that was used to
      start the session; it is modified so that actual sender and
      receiver port numbers that were used by the OWAMP-Test session
      always appear in the reproduction.

   +  16 octets of IZP.

   +  Zero or more (as specified) Skip Range descriptions.  The last
      (possibly full, possibly incomplete) block (16 octets) of Skip
      Range descriptions is padded with zeros if necessary.  (These
      zeros are simple padding and should be distinguished from the 16
      octets of IZP that follow.)

   +  16 octets of IZP.

   +  Zero or more (as specified) packet records.  The last (possibly
      full, possibly incomplete) block (16 octets) of data is padded
      with zeros if necessary.  (These zeros are simple padding and
      should be distinguished from the 16 octets of IZP that follow.)

   +  16 octets of IZP.


   Skip Range descriptions are simply two sequence numbers that,
   together, indicate a range of packets that were not sent:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |                      First Seqno Skipped                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Last Seqno Skipped                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Skip Range descriptions should be sent out in order, as sorted by
   First Seqno. If any Skip Ranges overlap, or are out of order, the
   session data is to be considered invalid and the connection SHOULD be
   closed and any results obtained considered invalid.

   Each packet record is 25 octets, and includes 4 octets of sequence
   number, 8 octets of send timestamp, 2 octets of send timestamp error
   estimate, 8 octets of receive timestamp, 2 octets of receive



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   timestamp error estimate, and 1 octet of Time To Live (TTL), or Hop
   Limit in IPv6:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     00|                          Seq Number                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     04|      Send Error Estimate      |    Receive Error Estimate     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     08|                         Send Timestamp                        |
     12|                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     16|                       Receive Timestamp                       |
     20|                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     24|    TTL        |
       +-+-+-+-+-+-+-+-+

   Packet records are sent out in the same order the actual packets were
   received.  Therefore, the data is in arrival order.

   Note that lost packets (if any losses were detected during the
   OWAMP-Test session) MUST appear in the sequence of packets.  They can
   appear either at the point when the loss was detected or at any later
   point.  Lost packet records are distinguished as follows:


   +  A send timestamp filled with the presumed send time (as computed
      by the send schedule).

   +  A send error estimate filled with Multiplier=1, Scale=64, and S=0
      (see the OWAMP-Test description for definition of these quantities
      and explanation of timestamp format and error estimate format).

   +  A normal receive error estimate as determined by the error of the
      clock being used to declare the packet lost.  (It is declared lost
      if it is not received by the Timeout after the presumed send time,
      as determined by the receiver's clock.)

   +  A receive timestamp consisting of all zero bits.

   +  A TTL value of 255.








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4. OWAMP-Test

   This section describes OWAMP-Test protocol.  It runs over UDP using
   sender and receiver IP and port numbers negotiated during the
   Request-Session exchange.

   As with OWAMP-Control, OWAMP-Test has three modes: unauthenticated,
   authenticated, and encrypted.  All OWAMP-Test sessions that are
   spawned by an OWAMP-Control session inherit its mode.

   OWAMP-Control client, OWAMP-Control server, OWAMP-Test sender, and
   OWAMP-Test receiver can potentially all be different machines.  (In a
   typical case, we expect that there will be only two machines.)


4.1. Sender Behavior


4.1.1. Packet Timings

   Send schedules based on slots, described previously, in conjunction
   with scheduled session start time, enable the sender and the receiver
   to compute the same exact packet sending schedule independently of
   each other.  These sending schedules are independent for different
   OWAMP-Test sessions, even if they are governed by the same
   OWAMP-Control session.

   Consider any OWAMP-Test session.  Once Start-Sessions exchange is
   complete, the sender is ready to start sending packets.  Under normal
   OWAMP use circumstances, the time to send the first packet is in the
   near future (perhaps a fraction of a second away).  The sender SHOULD
   send packets as close as possible to their scheduled time, with the
   following exception: if the scheduled time to send is in the past,
   and separated from the present by more than Timeout time, the sender
   MUST NOT send the packet.  (Indeed, such a packet would be considered
   lost by the receiver anyway.)  The sender MUST keep track of which
   packets it does not send.  It will use this to tell the receiver what
   packets were not sent by setting Skip Ranges in the Stop-Sessions
   message from the sender to the receiver upon completion of the test.
   The Skip Ranges are also sent to a Fetch-Client as part of the
   session data results.  These holes in the sending schedule can happen
   if a time in the past was specified in the Request-Session command,
   or if the Start-Sessions exchange took unexpectedly long, or if the
   sender could not start serving the OWAMP-Test session on time due to
   internal scheduling problems of the OS.  Packets in the past, but
   separated from the present by less than Timeout value, SHOULD be sent
   as quickly as possible.  With normal test rates and timeout values,
   the number of packets in such a burst is limited.  Nevertheless,



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   hosts SHOULD NOT intentionally schedule sessions so that such bursts
   of packets occur.

   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 sender sends the receiver a stream of packets with the schedule
   specified in the Request-Session command.  The sender SHOULD set the
   TTL in IPv4 (or Hop Limit in IPv6) in the UDP packet to 255.  The
   format of the body of a UDP packet in the stream depends on the mode
   being used.

   For unauthenticated mode:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Sequence Number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Timestamp                            |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Error Estimate         |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                                                               |
     .                                                               .
     .                         Packet Padding                        .
     .                                                               .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   For authenticated and encrypted modes:















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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Sequence Number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                        IZP (12 octets)                        |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Timestamp                            |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Error Estimate         |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
     |                         IZP (6 octets)                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                                                               .
     .                        Packet Padding                         .
     .                                                               .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format of the timestamp is the same as in [RFC 1305] and is as
   follows: first 32 bits represent the unsigned integer number of
   seconds elapsed since 0h on 1 January 1900; next 32 bits represent
   the fractional part of a second that has elapsed since then.

   So, Timestamp is represented 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                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Error Estimate specifies the estimate of the error and
   synchronization.  It has the following format:

         0                   1
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |S|Z|   Scale   |   Multiplier  |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The first bit S SHOULD be set if the party generating the timestamp
   has a clock that is synchronized to UTC using an external source



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   (e.g., the bit should be set if GPS hardware is used and it indicates
   that it has acquired current position and time or if NTP is used and
   it indicates that it has synchronized to an external source, which
   includes stratum 0 source, etc.); if there is no notion of external
   synchronization for the time source, the bit SHOULD NOT be set.  The
   next bit has the same semantics as MBZ fields elsewhere: it MUST be
   set to zero by the sender and ignored by everyone else.  The next six
   bits, Scale, form an unsigned integer; Multiplier is an unsigned
   integer as well.  They are interpreted as follows: the error estimate
   is equal to Multiplier*2^(-32)*2^Scale (in seconds).  [Notation
   clarification: 2^Scale is two to the power of Scale.]  Multiplier
   MUST NOT be set to zero.  If Multiplier is zero, the packet SHOULD be
   considered corrupt and discarded.

   Sequence numbers start with zero and are incremented by one for each
   subsequent packet.

   The minimum data segment length is, therefore, 14 octets in
   unauthenticated mode, and 32 octets in both authenticated mode and
   encrypted modes.

   The OWAMP-Test packet layout is the same in authenticated and
   encrypted modes.  The encryption operations are, however, different.
   The difference is that in encrypted mode both the sequence number and
   the timestamp are encrypted to provide maximum data integrity
   protection while in authenticated mode the sequence number is
   encrypted and the timestamp is sent in clear text.  Sending the
   timestamp in clear text in authenticated mode allows one to reduce
   the time between when a timestamp is obtained by a sender and when
   the packet is shipped out.  In encrypted mode, the sender has to
   fetch the timestamp, encrypt it, and send it; in authenticated mode,
   the middle step is removed, improving accuracy (the sequence number
   can be encrypted before the timestamp is fetched).

   In authenticated mode, the first block (16 octets) of each packet is
   encrypted using AES Electronic Cookbook (ECB) mode.  The key to use
   is the same key as is used for the corresponding OWAMP-Control
   session (where it is used in a different chaining mode).  ECB mode
   does not involve any actual chaining; this way, lost, duplicated, or
   reordered packets do not cause problems with deciphering any packet
   in an OWAMP-Test session.

   In encrypted mode, the first two blocks (32 octets) are encrypted
   using AES CBC mode.  The key to use is the same key as is used for
   the corresponding OWAMP-Control session.  Each OWAMP-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.



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   In unauthenticated mode, no encryption is applied.

   Packet Padding in OWAMP-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.

   The time elapsed between packets is computed according to the slot
   schedule as mentioned in Request-Session command description.  At
   that point, we skipped over the issue of computing exponentially
   distributed pseudo-random numbers in a reproducible fashion.  It is
   discussed later in a separate section.


4.2. Receiver Behavior

   The receiver knows when the sender will send packets.  The following
   parameter is defined: Timeout (from Request-Session).  Packets that
   are delayed by more than Timeout are considered lost (or `as good as
   lost').  Note that there is never an actual assurance of loss by the
   network: a `lost' packet might still be delivered at any time.  The
   original specification for IPv4 required that packets be delivered
   within TTL seconds or never (with TTL having a maximum value of 255).
   To the best of the authors' knowledge, this requirement was never
   actually implemented (and, of course, only a complete and universal
   implementation would ensure that packets do not travel for longer
   than TTL seconds). In fact, in IPv6, the name of this field has
   actually been changed to Hop Limit.  Further, IPv4 specification
   makes no claims about the time it takes the packet to traverse the
   last link of the path.

   The choice of a reasonable value of Timeout is a problem faced by a
   user of OWAMP protocol, not by an implementor.  A value such as two
   minutes is very safe.  Note that certain applications (such as
   interactive `one-way ping') might wish to obtain the data faster than
   that.

   As packets are received,

   +  Timestamp the received packet.

   +  In authenticated or encrypted mode, decrypt the first block (16
      octets) of the packet body.

   +  Store the packet sequence number, send time, receive time, and the
      TTL for IPv4 (or Hop Limit for IPv6) from the packet IP header for
      the results to be transferred.



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   +  Packets not received within the Timeout are considered lost.  They
      are recorded with their true sequence number, presumed send time,
      receive time consisting of a string of zero bits, and TTL (or Hop
      Limit) of 255.


   Implementations SHOULD fetch the TTL/Hop Limit value from the IP
   header of the packet.  If an implementation does not fetch the actual
   TTL value (the only good reason to not do so is inability to access
   the TTL field of arriving packets), it MUST record the TTL value as
   255.

   Packets that are actually received are recorded in the order of
   arrival.  Lost packet records serve as indications of the send times
   of lost packets.  They SHOULD be placed either at the point where the
   receiver learns about the loss or at any later point; in particular,
   one MAY place all the records that correspond to lost packets at the
   very end.

   Packets that have send time in the future MUST be recorded normally,
   without changing their send timestamp, unless they have to be
   discarded.  (Send timestamps in the future would normally indicate
   clocks that differ by more than the delay.  Some data -- such as
   jitter -- can be extracted even without knowledge of time difference.
   For other kinds of data, the adjustment is best handled by the data
   consumer on the basis of the complete information in a measurement
   session, as well as, possibly, external data.)

   Packets with a sequence number that was already observed (duplicate
   packets) MUST be recorded normally.  (Duplicate packets are sometimes
   introduced by IP networks.  The protocol has to be able to measure
   duplication.)

   If any of the following is true, the packet MUST be discarded:

   +  Send timestamp is more than Timeout in the past or in the future.

   +  Send timestamp differs by more than Timeout from the time when the
      packet should have been sent according to its sequence number.

   +  In authenticated or encrypted mode, any of the bits of zero
      padding inside the first 16 octets of packet body is non-zero.









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5. Computing Exponentially Distributed Pseudo-Random Numbers

   Here we describe the way exponential random quantities used in the
   protocol are generated.  While there is a fair number of algorithms
   for generating exponential random variables, most of them rely on
   having logarithmic function as a primitive, resulting in potentially
   different values, depending on the particular implementation of the
   math library.      We use algorithm 3.4.1.S in [KNUTH], which is free
   of the above-mentioned problem, and guarantees the same output on any
   implementation.  The algorithm belongs to the ziggurat family
   developed in the 1970s by G. Marsaglia, M. Sibuya and J. H. Ahrens
   [ZIGG]. It replaces the use of logarithmic function by clever bit
   manipulation, still producing the exponential variates on output.


5.1. High-Level Description of the Algorithm

   For ease of exposition, the algorithm is first described with all
   arithmetic operations being interpreted in their natural sense.
   Later, exact details on data types, arithmetic, and generation of the
   uniform random variates used by the algorithm are given. It is an
   almost verbatim quotation from [KNUTH], p.133.

   Algorithm S: Given a real positive number 'mu', produce an
   exponential random variate with mean 'mu'.

   First, the constants

   Q[k] = (ln2)/(1!) + (ln2)^2/(2!) + ... + (ln2)^k/(k!),  1 <= k <= 11

   are computed in advance. The exact values which MUST be used by all
   implementations are given in the reference code (see Appendix A).
   This is necessary to insure that exactly the same pseudo-random
   sequences are produced by all implementations.

   S1. [Get U and shift.] Generate a 32-bit uniform random binary
   fraction

             U = (.b0 b1 b2 ... b31)    [note the binary point]

   Locate the first zero bit b_j, and shift off the leading (j+1) bits,
   setting U <- (.b_{j+1} ... b31)

   Note: In the rare case that the zero has not been found, it is
   prescribed that the algorithm return (mu*32*ln2).

   S2. [Immediate acceptance?] If U < ln2, set X <- mu*(j*ln2 + U) and
   terminate the algorithm. (Note that Q[1] = ln2.)



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   S3. [Minimize.] Find the least k >= 2 such that U < Q[k]. Generate k
   new uniform random binary fractions U1,...,Uk and set V <-
   min(U1,...,Uk).

   S4. [Deliver the answer.] Set X <- mu*(j + V)*ln2.


5.2. Data Types, Representation, and Arithmetic

   The high-level algorithm operates on real numbers -- typically
   represented as floating point numbers. This specification prescribes
   that unsigned 64-bit integers be used instead.

   u_int64_t integers are interpreted as real numbers by placing the
   decimal point after the first 32 bits. In other words, conceptually,
   the interpretation is given by the map:

          u_int64_t u;

          u  |--> (double)u / (2**32)

   The algorithm produces a sequence of such u_int64_t integers that,
   for any given value of SID, is guaranteed to be the same on any
   implementation.

   We specify that the u_int64_t representations of the first 11 values
   of the Q array in the high-level algorithm be as follows:

   #1      0xB17217F8,
   #2      0xEEF193F7,
   #3      0xFD271862,
   #4      0xFF9D6DD0,
   #5      0xFFF4CFD0,
   #6      0xFFFEE819,
   #7      0xFFFFE7FF,
   #8      0xFFFFFE2B,
   #9      0xFFFFFFE0,
   #10     0xFFFFFFFE,
   #11     0xFFFFFFFF

   For example, Q[1] = ln2 is indeed approximated by 0xB17217F8/(2**32)
   = 0.693147180601954; for j > 11, Q[j] is 0xFFFFFFFF.

   Small integer j in the high-level algorithm is represented as
   u_int64_t value j * (2**32).

   Operation of addition is done as usual on u_int64_t numbers; however,
   the operation of multiplication in the high-level algorithm should be



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   replaced by

      (u, v) |---> (u * v) >> 32.

   Implementations MUST compute the product (u * v) exactly.  For
   example, a fragment of unsigned 128-bit arithmetic can be implemented
   for this purpose (see sample implementation below).


5.3. Uniform Random Quantities

   The procedure for obtaining a sequence of 32-bit random numbers (such
   as U in algorithm S) relies on using AES encryption in counter mode.
   To describe the exact working of the algorithm, we introduce two
   primitives from Rijndael. Their prototypes and specification are
   given below, and they are assumed to be provided by the supporting
   Rijndael implementation, such as [RIJN].


   +  A function that initializes a Rijndael key with bytes from seed
      (the SID will be used as the seed):

      void KeyInit(unsigned char seed[16]);

   +  A function that encrypts the 16-octet block inblock with the
      specified key, returning a 16-octet encrypted block. Here
      keyInstance is an opaque type used to represent Rijndael keys:

      void BlockEncrypt(keyInstance key, unsigned char inblock[16]);


   Algorithm Unif: given a 16-octet quantity seed, produce a sequence of
   unsigned 32-bit pseudo-random uniformly distributed integers. In
   OWAMP, the SID (session ID) from Control protocol plays the role of
   seed.

   U1. [Initialize Rijndael key] key <- KeyInit(seed) [Initialize an
   unsigned 16-octet (network byte order) counter] c <- 0     U2. [Need
   more random bytes?]  Set i <- c mod 4.  If (i == 0) set s <-
   BlockEncrypt(key, c)

   U3. [Increment the counter as unsigned 16-octet quantity] c <- c + 1

   U4. [Do output] Output the i_th quartet of octets from s starting
   from high-order octets, converted to native byte order and
   represented as OWPNum64 value (as in 3.b).

   U5. [Loop] Go to step U2.



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6. Security Considerations


6.1. Introduction

   The goal of authenticated mode to let one passphrase-protect service
   provided by a particular OWAMP-Control server.  One can imagine a
   variety of circumstances where this could be useful.  Authenticated
   mode is designed to prohibit theft of service.

   An additional design objective of the authenticated mode was to make
   it impossible for an attacker who cannot read traffic between OWAMP-
   Test sender and receiver to tamper with test results in a fashion
   that affects the measurements, but not other traffic.

   The goal of encrypted mode is quite different: to make it hard for a
   party in the middle of the network to make results look `better' than
   they should be.  This is especially true if one of client and server
   does not coincide with either sender or receiver.

   Encryption of OWAMP-Control using AES CBC mode with blocks of zeros
   after each message aims to achieve two goals: (i) to provide secrecy
   of exchange; (ii) to provide authentication of each message.


6.2. Preventing Third-Party Denial of Service

   OWAMP-Test sessions directed at an unsuspecting party could be used
   for denial of service (DoS) attacks.  In unauthenticated mode,
   servers SHOULD limit receivers to hosts they control or to the OWAMP-
   Control client.


6.3. Covert Information Channels

   OWAMP-Test sessions could be used as covert channels of information.
   Environments that are worried about covert channels should take this
   into consideration.


6.4. Requirement to Include AES in Implementations

   Notice that AES, in counter mode, is used for pseudo-random number
   generation, so implementation of AES MUST be included, even in a
   server that only supports unauthenticated mode.






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6.5. Resource Use Limitations

   An OWAMP server can consume resources of various kinds.  The two most
   important kinds of resources are network capacity and memory (primary
   or secondary) for storing test results.

   Any implementation of OWAMP server MUST include technical mechanisms
   to limit the use of network capacity and memory.  Mechanisms for
   managing the resources consumed by unauthenticated users and users
   authenticated with a username and passphrase SHOULD be separate.  The
   default configuration of an implementation MUST enable these
   mechanisms and set the resource use limits to conservatively low
   values.

   One way to design the resource limitation mechanisms is as follows:
   assign each session to a user class.  User classes are partially
   ordered with ``includes'' relation, with one class (``all users'')
   that is always present and that includes any other class.  The
   assignment of a session to a user class can be based on the presence
   of authentication of the session, the user name, IP address range,
   time of day, and, perhaps, other factors.  Each user class would have
   a limit for usage of network capacity (specified in units of
   bit/second) and memory for storing test results (specified in units
   of octets).  Along with the limits for resource use, current use
   would be tracked by the server.  When a session is requested by a
   user in a specific user class, the resources needed for this session
   are computed: the average network capacity use (based on the sending
   schedule) and the maximum memory use (based on the number of packets
   and number of octets each packet would need to be stored internally
   -- note that outgoing sessions would not require any memory use).
   These resource use numbers are added to the current resource use
   numbers for the given user class; if such addition would take the
   resource use outside of the limits for the given user class, the
   session is rejected.  When resources are reclaimed, corresponding
   measures are subtracted from the current use.  Network capacity is
   reclaimed as soon as the session ends.  Memory is reclaimed when the
   data is deleted.  For unauthenticated sessions, memory consumed by an
   OWAMP-Test session SHOULD be reclaimed after the OWAMP-Control
   connection that initiated the session is closed (gracefully or
   otherwise).  For authenticated sessions, the administrator who
   configures the service should be able to decide the exact policy, but
   useful policy mechanisms that MAY be implemented are the ability to
   automatically reclaim memory when the data is retrieved and the
   ability to reclaim memory after a certain configurable (based on user
   class) period of time passes after the OWAMP-Test session terminates.






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6.6. Use of Cryptographic Primitives in OWAMP

   At an early stage in designing the protocol, we considered using
   Transport Layer Security (TLS) and IPsec as cryptographic security
   mechanisms for OWAMP.  The disadvantages of those are as follows (not
   an exhaustive list):

   Regarding TLS:

   +  While TLS could be used to secure TCP-based OWAMP-Control, but
      difficult to use to secure UDP-based OWAMP-Test: OWAMP-Test
      packets, if lost, are not resent, so packets have to be
      (optionally) encrypted and authenticated while retaining
      individual usability.  Stream-based TLS is not conducive of this.

   +  Dealing with streams, does not authenticate individual messages
      (even in OWAMP-Control).  The easiest way out would be to add some
      known-format padding to each message and verify that the format of
      the padding is intact before using the message.  The solution
      would thus lose some of its appeal (``just use TLS''); it would
      also be much more difficult to evaluate the security of this
      scheme with the various modes and options of TLS -- it would
      almost certainly not be secure with all.  The capacity of an
      attacker to replace parts of messages (namely, the end) with
      random garbage could have serious security implications and would
      need to be analyzed carefully: suppose, for example, that a
      parameter that is used in some form to control the rate were
      replaced by random garbage -- chances are the result (an unsigned
      integer) would be quite large.

   +  Dependent on the mode of use, one can end up with a requirement
      for certificates for all users and a PKI.  Even if one is to
      accept that PKI is desirable, there just isn't a usable one today.

   +  TLS requires a fairly large implementation.  OpenSSL, for example,
      is larger than our implementation of OWAMP as a whole.  This can
      matter for embedded implementations.


   Regarding IPsec:

   +  What we now call authenticated mode would not be possible (in
      IPsec you can't authenticate part of a packet).

   +  The deployment paths of IPsec and OWAMP could be separate if OWAMP
      does not depend on IPsec.  After nine years of IPsec, only 0.05%
      of traffic on an advanced backbone network such as Abilene uses
      IPsec (for comparison purposes with encryption above layer 4, SSH



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      use is at 2-4% and HTTPS use is at 0.2-0.6%).  It is desirable to
      be able to deploy OWAMP on as large of a number of different
      platforms as possible.

   +  The deployment problems of a protocol dependent on IPsec would be
      especially acute in the case of lightweight embedded devices.
      Ethernet switches, DSL ``modems,'' and other such devices mostly
      do not support IPsec.

   +  The API for manipulation IPsec from an application is currently
      poorly understood.  Writing a program that needs to encrypt some
      packets, authenticate some packets, and leave some open -- for the
      same destination -- would become more of an exercise in IPsec
      rather than IP measurement.


   For the enumerated reasons, we decided to use a simple cryptographic
   protocol (based on a block cipher in CBC mode) that is different from
   TLS and IPsec.


6.7. Required Properties of MD5

   The protocol makes use of the MD5 hash function to convert a
   user-supplied passphrase into a key that will be used to encrypt a
   short piece of random data (the session key).

   In this document we use cryptographic terminology of [MENEZES].

   It has long been suspected, and has been conclusively shown recently
   that MD5 is not a collision-resistant hash function.  Since collision
   resistance was one of design goals of MD5, this casts strong
   suspicion on the other design goals of MD5, namely preimage
   resistance and 2nd preimage resistance.

   OWAMP does not rely on any of these properties.

   The properties of MD5 that are necessary are as follows: (1) it is a
   function that maps arbitrary length inputs into 128-bit outputs
   [fixed-length hash function], (2) a change in any bit of the input
   usually results in a change of a few bits of output [weakened
   avalanche property], (3) many 128-bit strings have preimages [almost
   surjective], and (4) the visible special structure of
   natural-language text possibly present in the passphrase is concealed
   after application of the function.  These are very weak requirements
   that many functions satisfy.  Something resembling CRC-128 would work
   just as well.




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   We chose MD5 here because it has the required properties and is
   widely implemented, understood, and documented.  Alternatives would
   include (1) a non-cryptographic primitive, such as CRC-128, (2) SHA-1
   truncated to 128 bits, or (3) a hash function based on AES (using
   Matyas-Meyer-Oseas, Davies-Meyer, or Miyaguchi-Preneel constructions;
   we would probably gravitate towards the last one if a block-cipher-
   based cryptographically secure hash function were required).  Note
   that option 1 would not have any cryptographically relevant
   properties.  We chose not to use it because of lack of
   well-documented 128-bit checksums; this specification would incur an
   unnecessary burden precisely defining one, providing test vectors,
   etc., with no advantage over MD5.  Option 2, SHA-1, belongs to the
   MD4 family that appears to be under suspicion in light of recent
   developments.  To avoid creating an impression that any potential
   future changes in the status of SHA-1 can affect the status of OWAMP
   we chose not to use it.  Option 3 would result in a hash function
   that, with the current state of knowledge, would probably be one of
   the most cryptographically sound.  Our requirements 1-4 from the
   preceding paragraph, however, do not call for a cryptographically
   sound hash function.  Just as with CRC-128, this specification would
   need to define the hash function and provide test vectors (and
   perhaps sample code); we see no advantage in this approach versus
   using MD5.  (Note that the performance advantages of MD5 are
   irrelevant for this application, as the hash is computed on a
   relatively short human-supplied string only once per OWAMP-Control
   session, so if the Miyaguchi-Preneel construction were documented in
   an RFC, we might just as well have used that.)


6.8. The Use of AES-CBC-MAC

   OWAMP relies on AES-CBC-MAC for message authentication.  Random IV
   choice is important for prevention of a codebook attack on the first
   block; it is unimportant for the purposes of CBC-MAC authentication
   (it should also be noted that, with its 128-bit block size, AES is
   more resistant to codebook attacks than ciphers with shorter blocks;
   we use random IV anyway).

   IZP, when decrypted, MUST be zero.  It is crucial to check for this
   before using the message, otherwise existential forgery becomes
   possible.  The complete message for which IZP is decrypted to non-
   zero MUST be discarded (for both short messages consisting of a few
   blocks and potentially long messages, such as a response to the
   Fetch-Session command).

   Since OWAMP messages can have different numbers of blocks, the
   existential forgery attack described in example 9.62 of [MENEZES]
   becomes a concern.  To prevent it (and to simplify implementation),



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   the length of any message becomes known after decrypting the first
   block of it.

   A special case is the first (fixed-length) message sent by the
   client.  There, the token is a concatenation of the 128-bit challenge
   (transmitted by the server in the clear) and a 128-bit session key
   (generated randomly by the client, encrypted with AES-CBC with IV=0.
   Since IV=0, the challenge (a single cipher block) is simply encrypted
   with the secret key.  Therefore, we rely on resistance of AES to
   chosen plaintext attacks (as the challenge could be substituted by an
   attacker).  It should be noted that the number of blocks of chosen
   plaintext an attacker can have encrypted with the secret key is
   limited by the number of sessions the client wants to initiate.  An
   attacker who knows the encryption of a server's challenge can produce
   an existential forgery of the session key and thus disrupt the
   session; however, any attacker can disrupt a session by corrupting
   the protocol messages in an arbitrary fashion, therefore no new
   threat is created here; nevertheless, we require that the server
   never issues the same challenge twice (if challenges are generated
   randomly, a repetition would occur, on average, after 2^64 sessions;
   we deem this satisfactory as this is enough even for an implausibly
   busy server that participates in 1,000,000 sessions per second to go
   without repetitions for more than 500 centuries).  With respect to
   the second part of the token, an attacker can produce an existential
   forgery of the session key by modifying the second half of the
   client's token while leaving the first part intact.  This forgery,
   however, would be immediately discovered by the client when the IZP
   on the server's next message (acceptance or rejection of the
   connection) does not verify.


7. IANA Considerations

   IANA is requested to allocate a well-known TCP port number for the
   OWAMP-Control part of the OWAMP protocol.


8. Internationalization Considerations

   The protocol does not carry any information in a natural language.











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9. Appendix A: Sample C Code for Exponential Deviates

   The values in array Q[] are the exact values that MUST be used by all
   implementations.  The rest of this appendix only serves for
   illustrative purposes.


/*
** Example usage: generate a stream of exponential (mean 1)
** random quantities (ignoring error checking during initialization).
** If a variate with some mean mu other than 1 is desired, the output
** of this algorithm can be multiplied by mu according to the rules
** of arithmetic we described.

** Assume that a 16-octet 'seed' has been initialized
** (as the shared secret in OWAMP, for example)
** unsigned char seed[16];

** OWPrand_context next;

** (initialize state)
** OWPrand_context_init(&next, seed);

** (generate a sequence of exponential variates)
** while (1) {
**    u_int64_t num = OWPexp_rand64(&next);
      <do something with num here>
                 ...
** }
*/

#include <stdlib.h>

typedef u_int64_t u_int64_t;

/* (K - 1) is the first k such that Q[k] > 1 - 1/(2^32). */
#define K 12

#define BIT31   0x80000000UL    /* See if first bit in the lower
                                   32 bits is zero. */
#define MASK32(n)       ((n) & 0xFFFFFFFFUL)

#define EXP2POW32       0x100000000ULL

typedef struct OWPrand_context {
        unsigned char counter[16]; /* Counter (network byte order). */
        keyInstance key;           /* Key to encrypt the counter. */
        unsigned char out[16];     /* The encrypted block. */



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} OWPrand_context;

/*
** The array has been computed according to the formula:
**
**       Q[k] = (ln2)/(1!) + (ln2)^2/(2!) + ... + (ln2)^k/(k!)
**
** as described in algorithm S. (The values below have been
** multiplied by 2^32 and rounded to the nearest integer.)
** These exact values MUST be used so that different implementation
** produce the same sequences.
*/
static u_int64_t Q[K] = {
        0,          /* Placeholder - so array indices start from 1. */
        0xB17217F8,
        0xEEF193F7,
        0xFD271862,
        0xFF9D6DD0,
        0xFFF4CFD0,
        0xFFFEE819,
        0xFFFFE7FF,
        0xFFFFFE2B,
        0xFFFFFFE0,
        0xFFFFFFFE,
        0xFFFFFFFF
};

/* this element represents ln2 */
#define LN2 Q[1]

/*
** Convert an unsigned 32-bit integer into a u_int64_t number.
*/
u_int64_t
OWPulong2num64(u_int32_t a)
{
        return ((u_int64_t)1 << 32) * a;
}

/*
** Arithmetic functions on u_int64_t numbers.
*/

/*
** Addition.
*/
u_int64_t
OWPnum64_add(u_int64_t x, u_int64_t y)



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{
        return x + y;
}

/*
** Multiplication.  Allows overflow.  Straightforward implementation
** of Algorithm 4.3.1.M (p.268) from [KNUTH].
*/
u_int64_t
OWPnum64_mul(u_int64_t x, u_int64_t y)
{
        unsigned long w[4];
        u_int64_t xdec[2];
        u_int64_t ydec[2];

        int i, j;
        u_int64_t k, t, ret;

        xdec[0] = MASK32(x);
        xdec[1] = MASK32(x>>32);
        ydec[0] = MASK32(y);
        ydec[1] = MASK32(y>>32);

        for (j = 0; j < 4; j++)
                w[j] = 0;

        for (j = 0; j < 2; j++) {
                k = 0;
                for (i = 0; ; ) {
                        t = k + (xdec[i]*ydec[j]) + w[i + j];
                        w[i + j] = t%EXP2POW32;
                        k = t/EXP2POW32;
                        if (++i < 2)
                                continue;
                        else {
                                w[j + 2] = k;
                                break;
                        }
                }
        }

        ret = w[2];
        ret <<= 32;
        return w[1] + ret;
}


/*



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** Seed the random number generator using a 16-byte quantity 'seed'
** (== the session ID in OWAMP). This function implements step U1
** of algorithm Unif.
*/

void
OWPrand_context_init(OWPrand_context *next, unsigned char *seed)
{
        int i;

        /* Initialize the key */
        rijndaelKeyInit(next->key, seed);

        /* Initialize the counter with zeros */
        memset(next->out, 0, 16);
        for (i = 0; i < 16; i++)
                next->counter[i] = 0UL;
}


/*
** Random number generating functions.
*/

/*
** Generate and return a 32-bit uniform random string (saved in the less
** significant half of the u_int64_t).  This function implements steps
** U2-U4 of the algorithm Unif.
*/
u_int64_t
OWPunif_rand64(OWPrand_context *next)
{
        int j;
        u_int8_t  *buf;
        u_int64_t  ret = 0;

        /* step U2 */
        u_int8_t i = next->counter[15] & (u_int8_t)3;
        if (!i)
                rijndaelEncrypt(next->key, next->counter, next->out);

        /* Step U3.  Increment next.counter as a 16-octet single
           quantity in network byte order for AES counter mode. */
        for (j = 15; j >= 0; j--)
                if (++next->counter[j])
                        break;

        /* Step U4.  Do output.  The last 4 bytes of ret now contain the



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           random integer in network byte order */
        buf = &next->out[4*i];
        for (j=0; j<4; j++) {
                ret <<= 8;
                ret += *buf++;
        }
        return ret;
}

/*
** Generate an exponential deviate with mean 1.
*/
u_int64_t
OWPexp_rand64(OWPrand_context *next)
{
        unsigned long i, k;
        u_int32_t j = 0;
        u_int64_t U, V, J, tmp;

        /* Step S1. Get U and shift */
        U = OWPunif_rand64(next);

        while ((U & BIT31) && (j < 32)) { /* Shift until first 0. */
                U <<= 1;
                j++;
        }
        /* Remove the 0 itself. */
        U <<= 1;

        U = MASK32(U);  /* Keep only the fractional part. */
        J = OWPulong2num64(j);

        /* Step S2.  Immediate acceptance? */
        if (U < LN2)       /* return  (j*ln2 + U) */
                return OWPnum64_add(OWPnum64_mul(J, LN2), U);

        /* Step S3.  Minimize. */
        for (k = 2; k < K; k++)
                if (U < Q[k])
                        break;
        V = OWPunif_rand64(next);
        for (i = 2; i <= k; i++) {
                tmp = OWPunif_rand64(next);
                if (tmp < V)
                        V = tmp;
        }

        /* Step S4.  Return (j+V)*ln2 */



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        return OWPnum64_mul(OWPnum64_add(J, V), LN2);
}


10. Appendix B: Test Vectors for Exponential Deviates

   It is important that the test schedules generated by different
   implementations from identical inputs be identical.  The non-trivial
   part is the generation of pseudo-random exponentially distributed
   deviates.  To aid implementors in verifying interoperability, several
   test vectors are provided.  For each of the four given 128-bit values
   of SID represented as hexadecimal numbers, 1,000,000 exponentially
   distributed 64-bit deviates are generated as described above.  As
   they are generated, they are all added to each other.  The sum of all
   1,000,000 deviates is given as a hexadecimal number for each SID.  An
   implementation MUST produce exactly these hexadecimal numbers.  To
   aid in the verification of the conversion of these numbers to values
   of delay in seconds, approximate values are given (assuming
   lambda=1).  An implementation SHOULD produce delay values in seconds
   that are close to the ones given below.

       SID = 0x2872979303ab47eeac028dab3829dab2
       SUM[1000000] = 0x000f4479bd317381 (1000569.739036 seconds)

       SID = 0x0102030405060708090a0b0c0d0e0f00
       SUM[1000000] = 0x000f433686466a62 (1000246.524512 seconds)

       SID = 0xdeadbeefdeadbeefdeadbeefdeadbeef
       SUM[1000000] = 0x000f416c8884d2d3 (999788.533277 seconds)

       SID = 0xfeed0feed1feed2feed3feed4feed5ab
       SUM[1000000] = 0x000f3f0b4b416ec8 (999179.293967 seconds)


11. Normative References


   [AES] Advanced Encryption Standard (AES),
        http://csrc.nist.gov/encryption/aes/

   [RFC1305] D. Mills, `Network Time Protocol (Version 3) Specification,
        Implementation and Analysis', RFC 1305, March 1992.

   [RFC1321] R. Rivest, `The MD5 Message-Digest Algorithm', RFC 1321,
        April 1992.

   [RFC2026] S. Bradner, `The Internet Standards Process -- Revision 3',
        RFC 2026, October 1996.



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

   [RFC2330] V. Paxon, G. Almes, J. Mahdavi, M. Mathis, `Framework for
        IP Performance Metrics' RFC 2330, May 1998.

   [RFC2474] K. Nichols, S. Blake, F. Baker, D. Black, `Definition of
        the Differentiated Services Field (DS Field) in the IPv4 and
        IPv6 Headers', RFC 2474, December 1998.

   [RFC2679] G. Almes, S. Kalidindi, and M. Zekauskas, `A One-way Delay
        Metric for IPPM', RFC 2679, September 1999.

   [RFC2680] G. Almes, S. Kalidindi, and M. Zekauskas, `A One-way Packet
        Loss Metric for IPPM', RFC 2680, September 1999.

   [RFC2836] S. Brim, B. Carpenter, F. Le Faucheur, `Per Hop Behavior
        Identification Codes', RFC 2836, May 2000.


12. Informative References


   [ZIGG] G. Marsaglia, M. Sibuya, and J. H. Ahrens, Communications of
        ACM, 15 (1972), 876-877.

   [MENEZES] A. J. Menezes, P. C. van Oorschot, and S. A. Vanstone,
        Handbook of Applied Cryptography, CRC Press, revised reprint
        with updates, 1997.

   [KNUTH] D. Knuth, The Art of Computer Programming, vol.2, 3rd
        edition, 1998.

   [RIJN] Reference ANSI C Implementation of Rijndael
        http://www.esat.kuleuven.ac.be/~rijmen/rijndael/rijndaelref.zip

   [RIPE] RIPE NCC Test-Traffic Measurements home,
        http://www.ripe.net/test-traffic/.

   [RIPE-NLUUG] H. Uijterwaal and O. Kolkman, `Internet Delay
        Measurements Using Test-Traffic', Spring 1998 Dutch Unix User
        Group Meeting,
        http://www.ripe.net/test-traffic/Talks/9805_nluug.ps.gz.

   [SURVEYOR] Surveyor Home Page, http://www.advanced.org/surveyor/.

   [SURVEYOR-INET] S. Kalidindi and M. Zekauskas, `Surveyor: An
        Infrastructure for Network Performance Measurements',



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        Proceedings of INET'99, June 1999.
        http://www.isoc.org/inet99/proceedings/4h/4h_2.htm


13. Authors' Addresses

   Stanislav Shalunov
   Internet2
   3025 Boardwalk Dr, Suite 200
   Ann Arbor, MI 48108
   Telephone: +1-734-995-7060
   Email: shalunov@internet2.edu
   SIP: shalunov@internet2.edu

   Benjamin Teitelbaum
   Internet2
   3025 Boardwalk Dr, Suite 200
   Ann Arbor, MI 48108
   Email: ben@internet2.edu
   SIP: ben@internet2.edu

   Anatoly Karp
   4710 Regent St, Apt 81B
   Madison, WI 53705
   Telephone: +1-608-347-6255
   Email: ankarp@charter.net

   Jeff W. Boote
   Internet2
   3025 Boardwalk Dr, Suite 200
   Ann Arbor, MI 48108
   Email: boote@internet2.edu
   SIP: boote@internet2.edu

   Matthew J. Zekauskas
   Internet2
   3025 Boardwalk Dr, Suite 200
   Ann Arbor, MI 48108
   Email: matt@internet2.edu

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



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   on the procedures with respect to rights in RFC documents can be found
   in BCP 78 and BCP 79.

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

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

Disclaimer of Validity

   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 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.

Copyright Statement

   Copyright (C) The Internet Society (2004).  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.

Acknowledgments

   We would like to thank Guy Almes, Hamid Asgari, Steven Van den
   Berghe, Eric Boyd, Robert Cole, Joan Cucchiara, Stephen Donnelly,
   Kaynam Hedayat, Petri Helenius, Kitamura Yasuichi, Daniel H. T. R.
   Lawson, Will E. Leland, Bruce A. Mah, Allison Mankin, Al Morton,
   Attila Pasztor, Randy Presuhn, Matthew Roughan, Andy Scherrer, Henk
   Uijterwaal, and Sam Weiler for their comments, suggestions, reviews,
   helpful discussion and proof-reading.

Expiration date: April 2005








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