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Versions: 00 01                                                         
INTERNET DRAFT                                             Pat R. Calhoun
Category: Standards Track                          Sun Microsystems, Inc.
Title: draft-calhoun-diameter-reliable-00.txt             Allan C. Rubens
Date: November 1998                                  Ascend Communications

                     Reliable Transport Extensions

Status of this Memo

   This document is an Internet-Draft.  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.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or
   ftp.isi.edu (US West Coast).


   Many services that require DIAMETER need retransmission and timeout
   faster than TCP can provide.

   An example would be in a NAS environment where DIAMETER is used for
   the authentication and authorization of users. The amount of time
   that it takes for TCP to determine that a connection to a server is
   broken is longer than the disonnect timeout of the PPP clients on
   whose behalf the server is being contacted.

   RADIUS has been able to handle this situation by operating over UDP.
   However, RADIUS fails to define a standard retransmission and timeout
   scheme, which has resulted in many different methods across

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   This DIAMETER specification defines the extensions necessary for the
   base protocol to operate over a non-reliable transport (e.g. UDP).

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Table of Contents

      1.0    Introduction
      1.1    Definitions
      2.0    Protocol Overview
      2.1    Flow Control
      2.2    Suggested implementation
      2.3    Peer failure recovery
      3.0    Extended Header Format
      4.0    DIAMETER AVPs
      4.1    Receive-Window
      5.0    References
      6.0    Acknowledgements
      7.0    Author's Address
      Appendix A: Acknowledgment Timeouts
      A.1    Calculating Adaptive Acknowledgment Timeout
      A.2    Flow Control: Adjusting for Timeout
      Appendix B: Examples of sequence numbering
      B.1    Lock-step tunnel establishment
      B.2    Multiple packets acknowledged
      B.3    Lost packet with retransmission

1.0  Introduction

   The extensions defined in this specification are mandatory for all
   DIAMETER extensions operating over a non-reliable transport (e.g.

1.1  Definitions

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.

   MUST      This word, or the adjective "required", means that the
             definition is an absolute requirement of the

   MUST NOT  This phrase means that the definition is an absolute
             prohibition of the specification.

   SHOULD    This word, or the adjective "recommended", means that
             there may exist valid reasons in particular circumstances
             to ignore this item, but the full implications must be
             understood and carefully weighed before choosing a
             different course.

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   MAY       This word, or the adjective "optional", means that this
             item is one of an allowed set of alternatives.  An
             implementation which does not include this option MUST
             be prepared to interoperate with another implementation
             which does include the option.

2.0  Protocol Overview

   This section provides a detailed overview of how reliable transport
   can be optionally provided by DIAMETER.  No negotiation mechanism for
   determining if this optional capability is required by either peer of
   a DIAMETER session is defined herein.  The mechanism for deciding
   this is beyond the scope of this document.

2.1  Flow Control

   There are two different types of DIAMETER messages; A DIAMETER
   message that only contains the header and no Attribute-Value Pairs
   (AVPs) is known as a zero length body message (ZLB). ZLB messages are
   used for explicitly acknowledging packets to the peer. Non-ZLB
   DIAMETER messages are messages that contain AVPs and can be of any
   type defined in [10].

   Two optional fields in the DIAMETER header that are important to the
   operation of DIAMETER when it is not being run over TCP are Nr (Next
   Received) Ns (Next Send). A single sequence number state is
   maintained for all DIAMETER messages to a given peer. The sequence
   number starts at 0. Each subsequent non-ZLB packet is sent with the
   next increment of the sequence number.

   The sequence number is thus a free running counter represented modulo
   65536. For purposes of detecting duplication, a received sequence
   value is considered less than or equal to the last received value if
   its value lies in the range of the last value and its 32767 successor
   values. For example, if the last received sequence number was 15,
   then received packets with Ns values in the range ( 32783, ... 65535,
   0, ... 15 ) would be considered duplicates and would be silently
   discarded.  A packet with sequence number 16 would be treated as the
   next in-sequence packet and packets with other sequences numbers are

   It is an implementation decision as to whether DIAMETER Messages
   received out-of-order are queued for later processing or silently
   discarded. The former is recommended when possible.

   In this document, the sequence number state for each peer is

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   represented for clarity of discussion by distinct pairs of state
   variables, Sr and Ss. Sr represents the value of the next in-sequence
   message expected to be received for a given session by a peer. Ss
   represents the sequence number to be placed in the Ns field of the
   next message sent to a given peer. Each state is initialized such
   that the first message sent and the first message expected to be
   received to/from each peer has an Ns value of 0. This corresponds to
   initializing Ss and Sr to 0 for each peer.

   As messages are sent to a given peer, Nr is set in these messages to
   reflect one more than the Ns value of the highest (modulo 2^16) in-
   order message received from that peer; if sent before any packet is
   received Nr will be 0, indicating that the peer expects the next new
   Ns value to be 0.

   When a non-ZLB message is received with an Ns value that matches the
   peer's current Sr value, Sr is incremented by 1 (modulo 2^16). It is
   important to note that Sr is not modified if a message is received
   with a value of Ns greater than the current Sr value. Retransmission
   of lost packets will eventually provide the receiving peer with its
   next expected message.

   Every time a peer sends a non-ZLB message it increments its Ss value
   for that peer by 1 (modulo 2^16). This increment takes place after
   the current Ss value is copied to Ns in the message to be sent.  New
   outgoing messages normally include the current value of Sr for the
   corresponding peer in their Nr field.  A peer may not wish to send
   the latest Sr value back to its peer due to congestion (i.e., its
   receive buffer for the session is full).  In this case it is
   permissible for the peer to send back an Nr value containing the Ns
   value of the first message in the window.  It is preferable to return
   an acknowledgment with this old Nr value rather than to withhold
   acknowledgments entirely when the receive window is full.

   Retransmitted messages should also include the current value of Sr in
   their Nr field, but some implementations may choose not to update Nr
   to avoid having to perform another hash in the Integrity-Check-Vector
   AVP. Note that the hash would only have to be recomputed if the Nr
   value had changed. This restriction does not apply to end-to-end
   integrity since the Ns and Nr fields are mutable. When retransmitting
   a message the identifier in the protocol header MUST NOT be changed.

   When transmitting packets, a DIAMETER peer must obey the receive
   window size offerred by its peer.  The default window size is 7.  A
   DIAMETER peer MUST NOT send new packets when its peer's window is
   closed (the number of packets unacknowledged is equal to the
   advertised, or assumed, window size). Previously transmitted packets
   may be retransmitted while the peer's window is closed.  A peer

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   communicating via UDP can specify the window size it is providing to
   its peer by specifying this value in the Device-Reboot-Ind message.

   A ZLB message is used to communicate Nr and Ns fields. The Nr and the
   Ns fields are filled in as above, but the sequence number state, Ss,
   is not modified. Thus a ZLB message sent after a non-ZLB message will
   contain the new Ss value while a non-ZLB message sent after a ZLB
   message will contain the same value of Ns as the ZLB message did.

   Upon receipt of an in-order non-ZLB message, the receiving peer must
   increment its Sr value and may acknowledge the message by sending
   back the updated value of Sr in the Nr field of the next outgoing
   message. This updated Sr value can be piggybacked in the Nr field of
   any outgoing messages that the peer may happen to send back.

   If a peer does not have a message queued to transmit at the time a
   non-ZLB message is received then it should delay a short time before
   sending a ZLB message containing the latest values of Sr and Ss, as
   described above.  This short delay is to allow for the possible
   arrival of a message to be transmitted back to its peer, thus
   avoiding the need to issue a ZLB.  The suggested value for this time
   delay is 1/4 the receiving peer's value of Round-Trip-Time (RTT - see
   Appendix A), if it computes RTT, or a maximum of 1/2 of its fixed
   acknowledgment timeout interval otherwise. This timeout should
   provide a reasonable opportunity for the receiving peer to obtain a
   payload message destined for its peer, upon which the ACK of the
   received message can be piggybacked. Note that if a peer's window is
   full, it MAY advertise an older Nr value if it is not ready to accept
   new messages.

   This delay value should be treated as a suggested maximum; an
   implementation could make this delay quite small without adversely
   affecting the protocol. The default time delay is 2 seconds. To
   provide for better throughput, the receiving peer should skip this
   delay entirely and send a ZLB message immediately in the case where
   its receive window is filled and it has no queued data to send for
   this connection or it can't send queued data because the transmit
   window is closed.

   See Appendix B for some examples of how sequence numbers progress.

2.2 Suggested implementation

   A suggested implementation of this delay is as follows: Upon
   receiving a non-ZLB message, the receiver starts a timer that will
   expire in the recommended time interval. A variable, Lr (Last Nr
   value sent), is used by the transmitter to store the last value sent

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   in the Nr field of a transmitted payload message for this connection.
   Upon expiration of this timer, Sr is compared to Lr and, if they are
   not equal, a ZLB ACK is issued. If they are equal, then no ACK's are
   outstanding and no action needs to be taken.

   This timer should not be reinitialized if a new message is received
   while it is active since such messages will be acknowledged when the
   timer expires. This ensures that periodic ACK's are issued with a
   maximum period equal to the recommended delay time interval. This
   interval should be short enough to not cause false acknowledgement
   timeouts at the transmitter when payload messages are being sent in
   one direction only. Since such ACK's are being sent on what would
   otherwise be an idle data path, their affect on performance should be
   small, of not negligible.

   In order for a DIAMETER implementation to be able to retransmit
   messages, it MUST queue transmitted messages until the messages are
   acknowledged.  It must also maintain a retransmission timer that
   determines when to assume that either a sent message did not arrive
   at the peer or the acknowledgment sent by the peer was lost.  See
   Appendix A for a recommended retransmit timer implementation. There
   are two recommended methods for implementing the retransmission
   procedure. One method is for the sender to resend the entire window
   of unacknowledged messages when the retransmit timeout expires.  This
   is the simplest method, but is inefficient when a receiver is not
   rotating the window due to congestion. The alternative method is to
   only resend the first message in the window (the first unacknowledged
   message) until an acknowledgment is received.  This acknowledgment
   will indicate to the receiver the next, if any, message in the
   current window that needs to be retransmitted.  A particular
   implementation may use either or both methods if desired.

   When a DIAMETER node has retransmitted a message to a given peer the
   maximum number of times (the recommended value is 3), it may send the
   request to an alternate DIAMETER server. This procedure may continue
   until either all of the servers have been tried, or the node
   selectively issues a failure to the requestor.

2.3 Peer failure recovery

   A DIAMETER message with the Command-Code AVP set to Device-Reboot-Ind
   and the Ns and Nr values set to zero (0) indicates that the peer has
   rebooted.  This message MUST be recognized and supported by a
   DIAMETER implementation. When this event occurs, the Ss and Sr values
   must be reset and the retransmission queue MUST be cleared. Since the
   protocol requires that all new messages include a random identifier
   in the protocol header, a Device-Reboot-Ind that is received with the

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   same identifier as the last processed Device-Reboot-Ind is considered
   a retransmission and SHOULD NOT change the peer's state to inactive.

   Messages other than the Device-Reboot-Ind MUST NOT be sent to the
   peer until both the acknowledgement for the transmitted Device-
   Reboot-Ind AND the peer's Device-Reboot-Ind have been received. When
   both of these have been received, the peer is considered to be in the
   active state.

3.0  Extended Header Format

   The DIAMETER Base Protocol [12] assumes that the underlying transport
   is reliable (e.g. TCP). This section defines the optional fields in
   the DIAMETER header that allow DIAMETER to provide reliability.

   See [12] for a full description of the header fields not introduced
   in this document.

   A summary of the DIAMETER data format is shown below. The fields are
   transmitted from left to right.

      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
      |  RADIUS PCC   |Flags|A|W| Ver |         Packet Length         |
      |                          Identifier                           |
      |         Next Send (Ns)        |       Next Received (Nr)      |
      |  AVPs ...

   PKT Flags

      The Packet Flags field is five bits, and is used in order to
      identify any options. This field MUST be initialized to zero. The
      following flags may be set:

         The 'W' bit (Window-Present) is set when the Next Send (Ns) and
         Next Received (Nr) fields are present in the header. This
         SHOULD be set unless the underlying layer provides reliability
         (i.e. TCP).

         The 'A' bit is set to indicate that the packet is an
         acknowledgement only and does not contain a Command-Code AVP
         following the header. Note that the Security AVPs MUST still be

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         present within an acknowledgment message.

   Next Send

      This field is present when the Window-Present bit is set in the
      header flags. The Next Send (Ns) is copied from the send sequence
      number state variable, Ss, at the time the message is transmitted.
      Ss is incremented after copying if the message is not a ZLB ACK.

   Next Received

      This field is present when the Window-Present bit is set in the
      header flags. Nr is copied from the receive sequence number state
      variable, Sr, and indicates the sequence number, Ns, +1 of the
      highest (modulo 2^16) in-sequence message received. See section
      2.0 for more information.


   This section defines a mandatory AVP which MUST be supported by all
   DIAMETER implementations supporting this extension.

   The following AVP is defined in this document:

      Attribute Name       Attribute Code
      Receive-Window            277

4.1  Receive-Window


      This AVP is used by a peer to inform its peer of its local receive
      window size. The size indicated is the number of packets that it
      is willing to accept before the window is full.

      A sending peer MUST stop sending new DIAMETER messages when this
      many messages are outstanding (sent but not yet acknowledged).

      If a peer does not issue this attribute, a receive window size of
      7 is assumed by its peer.

      This attribute is only valid in the Device-Reboot-Ind message.

   AVP Format

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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
      |                           AVP Code                            |
      |          AVP Length           |     Reserved      |P|T|V|E|H|M|
      |                           Integer32                           |

      AVP Code

         277     Receive-Window

      AVP Length

         The length of this attribute MUST be 12.

      AVP Flags

         The 'M' bit MUST be set. The 'H' and 'E' MAY be set depending
         upon the security model used. The 'V', 'T' and the 'P' bits
         MUST NOT be set.


         This field contains the receive window size.

5.0  References

    [1] Reynolds, Postel, "Assigned Numbers", RFC 1700,
        October 1994.
    [2] Postel, "User Datagram Protocol", RFC 768, August 1980.
    [3] Calhoun, Zorn, Pan, "DIAMETER Framework", Internet-
        Draft, draft-calhoun-diameter-framework-00.txt, May 1998
    [4] Calhoun, Rubens, "DIAMETER Base Protocol", Internet-Draft,
        draft-calhoun-diameter-05.txt, May 1998.
    [5] K. Hamzeh, T. Kolar, M. Littlewood, G. Singh Pall, J. Taarud,
        A. J. Valencia, W. Verthein, W.M. Townsley, B. Palter,
        A. Rubens "Layer Two Tunneling Protocol (L2TP)",
        Internet-Draft, May 1998

6.0  Acknowledgements

   The Authors would like to acknowledge the following people for their
   contribution in the development of the DIAMETER protocol:

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   Bernard Aboba, Jari Arkko, William Bulley, Daniel C. Fox, Lol Grant,
   Nancy Greene, Peter Heitman, Ryan Moats, Victor Muslin, Kenneth
   Peirce, Sumit Vakil, John R. Vollbrecht, Jeff Weisberg and Glen Zorn

   The authors would also like to thank the authors of the L2TP spec
   since most of the windowing text in this draft was shamefully copied
   from that spec.

7.0  Author's Address

   Questions about this memo can be directed to:

      Pat R. Calhoun
      Technology Development
      Sun Microsystems, Inc.
      15 Network Circle
      Menlo Park, California, 94025

       Phone:  1-650-786-7733
         Fax:  1-650-786-6445
      E-mail:  pcalhoun@eng.sun.com

      Allan C. Rubens
      Ascend Communications
      1678 Broadway
      Ann Arbor, MI 48105-1812

       Phone:  1-734-761-6025
      E-Mail:  acr@del.com

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Appendix A: Acknowledgment Timeouts

   DIAMETER uses sliding windows and timeouts to provide flow-control
   across the underlying medium and to perform efficient data buffering
   to keep two DIAMETER peers' receive window full without causing
   receive buffer overflow. DIAMETER requires that a timeout be used to
   recover from dropped packets.

   When the timeout for a peer expires, the previously transmitted
   message with Ns value equal to the highest in-sequence value of Nr
   received from the peer is retransmitted. The receiving peer does not
   advance its value for the receive sequence number state, Sr, until it
   receives a message with Ns equal to its current value of Sr.

   This rule assures that all subsequent acknowledgements to this peer
   will contain an Nr value equal to the Ns value of the first missing
   message until a message with the missing Ns value is received.

   The exact implementation of the acknowledgment timeout is vendor-
   specific.  It is suggested that an adaptive timeout be implemented
   with backoff for flow control.  The timeout mechanism proposed here
   has the following properties:

      Independent timeouts for each peer.  A device will have to
      maintain and calculate timeouts for every active peer.

      An administrator-adjustable maximum timeout, MaxTimeOut, unique to
      each device.

      An adaptive timeout mechanism that compensates for changing
      throughput.  To reduce packet processing overhead, vendors may
      choose not to recompute the adaptive timeout for every received
      acknowledgment.  The result of this overhead reduction is that the
      timeout will not respond as quickly to rapid network changes.

      Timer backoff on timeout to reduce congestion.  The backed-off
      timer value is limited by the configurable maximum timeout value.
      Timer backoff is done every time an acknowledgment timeout occurs.

   In general, this mechanism has the desirable behavior of quickly
   backing off upon a timeout and of slowly decreasing the timeout value
   as packets are delivered without errors.

A.1  Calculating Adaptive Acknowledgment Timeout

   We must decide how much time to allow for acknowledgments to return.
   If the timeout is set too high, we may wait an unnecessarily long

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   time for dropped packets.  If the timeout is too short, we may time
   out just before the acknowledgment arrives.  The acknowledgment
   timeout should also be reasonable and responsive to changing network

   The suggested adaptive algorithm detailed below is based on the TCP
   1989 implementation and is explained in Richard Steven's book TCP/IP
   Illustrated, Volume 1 (page 300).  'n' means this iteration of the
   calculation, and 'n-1' refers to values from the last calculation.

      DIFF[n] = SAMPLE[n] - RTT[n-1]
      DEV[n] = DEV[n-1] + (beta * (|DIFF[n]| - DEV[n-1]))
      RTT[n] = RTT[n-1] + (alpha * DIFF[n])
      ATO[n] = MIN (RTT[n] + (chi * DEV[n]), MaxTimeOut)

      DIFF represents the error between the last estimated round-trip
      time and the measured time.  DIFF is calculated on each iteration.

      DEV is the estimated mean deviation.  This approximates the
      standard deviation.  DEV is calculated on each iteration and
      stored for use in the next iteration.  Initially, it is set to 0.

      RTT is the estimated round-trip time of an average packet.  RTT is
      calculated on each iteration and stored for use in the next
      iteration.  Initially, it is set to PPD.

      ATO is the adaptive timeout for the next transmitted packet.  ATO
      is calculated on each iteration.  Its value is limited, by the MIN
      function, to be a maximum of the configured MaxTimeOut value.

      Alpha is the gain for the round trip estimate error and is
      typically 1/8 (0.125).

      Beta is the gain for the deviation and is typically 1/4 (0.250).

      Chi is the gain for the timeout and is typically set to 4.

   To eliminate division operations for fractional gain elements, the
   entire set of equations can be scaled.  With the suggested gain
   constants, they should be scaled by 8 to eliminate all division.  To
   simplify calculations, all gain values are kept to powers of two so
   that shift operations can be used in place of multiplication or
   division.  The above calculations are carried out each time an
   acknowledgment is received for a packet that was not retransmitted
   (no timeout occured).

A.2  Flow Control: Adjusting for Timeout

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   This section describes how the calculation of ATO is modified in the
   case where a timeout does occur.  When a timeout occurs, the timeout
   value should be adjusted rapidly upward. To compensate for shifting
   internetwork time delays, a strategy must be employed to increase the
   timeout when it expires.  A simple formula called Karn's Algorithm is
   used in TCP implementations and may be used in implementing the
   backoff timers for the DIAMETER peers.  Notice that in addition to
   increasing the timeout, we also shrink the size of the window as
   described in the next section.

   Karn's timer backoff algorithm, as used in TCP, is:

      NewTIMEOUT = delta * TIMEOUT

      Adapted to our timeout calculations, for an interval in which a
      timeout occurs, the new timeout interval ATO is calculated as:

      RTT[n] = delta * RTT[n-1]
      DEV[n] = DEV[n-1]
      ATO[n] = MIN (RTT[n] + (chi * DEV[n]), MaxTimeOut)

   In this modified calculation of ATO, only the two values that
   contribute to ATO and that are stored for the next iteration are
   calculated.  RTT is scaled by delta, and DEV is unmodified.  DIFF is
   not carried forward and is not used in this scenario.  A value of 2
   for Delta, the timeout gain factor for RTT, is suggested.

Appendix B: Examples of sequence numbering

   This appendix uses several common scenarios to illustrate how
   sequence number state progresses and is interpreted.

B.1  Lock-step session establishment

   In this example, a DIAMETER host establishes communication with a
   peer, with the exchange involving each side alternating in the
   sending of messages.  This example is contrived, in that the final
   acknowledgement typically would be included in the Device-Watchdog-
   Ind message.

        DIAMETER Host A                             DIAMETER Host B
             ->    Device-Reboot-Ind
                   Nr: 0, Ns: 0

                                                 (ZLB)   <-
                                          Nr: 1, Ns: 0

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             ->    Device-Watchdog-Ind
                   Nr: 0, Ns: 1


                                                 (ZLB)    <-
                                          Nr: 2, Ns: 0

B.2  Multiple packets acknowledged

   This example shows a flow of packets from DIAMETER Host B to Host A,
   with Host A having no traffic of its own. Host A is waiting 1/4 of
   its timeout interval, and then acknowledging all packets seen since
   the last interval.

        DIAMETER Host A                             DIAMETER Host B
              (previous packet flow precedes this)

              ->    (ZLB)
                    Nr: 7000, Ns: 1000
                                             (non-ZLB)    <-
                                    Nr: 1000, Ns: 7000
                                             (non-ZLB)    <-
                                    Nr: 1000, Ns: 7001
                                             (non-ZLB)    <-
                                    Nr: 1000, Ns: 7002

              (Host A's timer indicates it should acknowledge pending

              ->    (ZLB)
                    Nr: 7003, Ns: 1000

B.3  Lost packet with retransmission

   Host A attempts to communicate with Host B. The Device-Reboot-Ind
   sent from B to A is lost and must be retransmitted by Host B.

        DIAMETER Host A                             DIAMETER Host B
             ->    Device-Reboot-Ind
                   Nr: 0, Ns: 0

                       (packet lost) Device-Reboot-Ind    <-
                                          Nr: 1, Ns: 0

             (pause; Host A's timer started first, so fires first)

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             ->    Device-Reboot-Ind
                   Nr: 0, Ns: 0

             (Host B realizes it has already seen this packet)
             (Host B might use this as a cue to retransmit, as in this

                                     Device-Reboot-Ind    <-
                                          Nr: 1, Ns: 0
             ->    Device-Watchdog-Ind
                   Nr: 1, Ns: 1


                                                 (ZLB)    <-
                                          Nr: 2, Ns: 1

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