Internet Draft
INTERNET DRAFT                                             Pat R. Calhoun
Category: Standards Track                          Sun Laboratories, Inc.
Title: draft-calhoun-diameter-08.txt                      Allan C. Rubens
Date: August 1999                                   Ascend Communications



                         DIAMETER Base Protocol



Status of this Memo

   This document is an individual contribution for consideration by the
   AAA Working Group of the Internet Engineering Task Force.  Comments
   should be submitted to the diameter@ipass.com mailing list.

   Distribution of this memo is unlimited.

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  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

   The list of Internet-Draft Shadow Directories can be accessed at:

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


Abstract

   The DIAMETER base protocol is intended to provide a framework for any
   services which require AAA/Policy support. The protocol is intended
   to be flexible enough to allow services to add building blocks (or
   extensions) to DIAMETER in order to meet their requirements.

   This draft specifies the message format and transport to be used by
   all DIAMETER extensions and MUST be supported by all DIAMETER
   implementations, regardless of the specific underlying service.











Table of Contents

      1.0  Introduction
      1.1  Copyright Statement
      1.2  Requirements language
      1.3  Terminology
      1.4  Changes in Revision 8
      2.0  Protocol Overview
            2.1  Header Format
                  2.1.1  ZLB Message Format
            2.2  AVP Format
            2.3  Error Reporting
      3.0  Reliable Transport
            3.1  Flow Control
            3.2  Suggested implementation
            3.3  Peer failure recovery
      4.0  DIAMETER AVPs
            4.1  DIAMETER-Command AVP
                  4.1.1  Message-Reject-Ind
                  4.1.2  Device-Reboot-Ind
                  4.1.3  Device-Watchdog-Ind
            4.2  Host-IP-Address
            4.3  Host-Name
            4.4  State
            4.5  Class
            4.6  Session-Timeout
            4.7  Extension-Id
            4.8  Integrity-Check-Vector
            4.9  Nonce
            4.10 Timestamp
            4.11 Session-Id
            4.12 Vendor-Name
            4.13 Firmware-Revision
            4.14 Result-Code
            4.15 Error-Code
            4.16 Unrecognized-Command-Code
            4.17 Reboot-Type
            4.18 Reboot-Time
            4.19 Failed-AVP-Code
            4.20 User-Name
            4.21 Receive-Window
            4.22 Proxy-State
      5.0  Protocol Definition
            5.1  DIAMETER Bootstrap Message
                  5.1.1  State Machine
            5.2  Keepalive Exchange
            5.3  Unrecognized Command Support
            5.4  The art of AVP Tagging
            5.5  Using the Integrity-Check-Vector
            5.6  DIAMETER Proxying
            5.7  AVP Encryption with Shared Secrets
      6.0  IANA Considerations
            6.1  AVP Attributes
            6.2  Command Code AVP Values
            6.3  Extension Identifier Values
            6.4  Result Code AVP Values
            6.5  Integrity Check Vector Transform Values
            6.7  AVP Header Bits
            6.6  Reboot Type Values
      7.0  References
      8.0  Acknowledgements
      9.0  Author's Address
      10.0 Full Copyright Statement
      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

   Since the RADIUS protocol is being used today for much more than
   simple authentication and accounting of dial-up users (i.e.
   authentication of WWW clients, etc), a more extensible protocol was
   necessary which could support new services being deployed in the
   internet and corporate networks.

   RADIUS in itself is not an extensible protocol largely due to its
   very limited command and attribute address space. In addition, the
   RADIUS protocol assumes that there cannot be any unsolicited messages
   from a server to a client. In order to support new services it is
   imperative that a server be able to send unsolicited messages to
   clients on a network, and this is a requirement for any DIAMETER
   implementation.

   This document describes the base DIAMETER protocol, which is used as
   the transport for all DIAMETER extensions. This document in itself is
   not complete and MUST be used with an accompanying applicability
   extension document.

   An example of such a document would be [7] that defines extensions to
   the base protocol to support user authentication and [XXX] which
   defines extensions to support accounting.


1.1  Copyright Statement

   Copyright   (C) The Internet Society 1999.  All Rights Reserved.


1.2  Requirements language

   In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
   "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
   described in [13].


1.3  Terminology

   AVP

      The DIAMETER protocol consists of a header followed by objects.
      Each object is encapsulated in a header known as an Attribute-
      Value-Pair (AVP).

   DIAMETER Device

      A Diameter device is a client or server system that supports the
      Diameter Base protocol and 0 or more extensions.

   ICV

      An Integrity Check Vector (ICV) is a hash of the packet with a
      shared secret.

   Session

      The DIAMETER protocol is session based. When an authentication
      request is initially transmitted, it includes a session identifier
      that is used for the duration of the session. The Session-
      Identifier AVP contains the identifier and must be globally
      unique. Section 4.11 describes the semantics of a session
      identifier.


1.4  Changes in Revision 8

   The following changes were made to version 8 of this specification:

      - Added text to clear up the Identifier field description.

      - Clarified the text for all AVPs of type "time".

      - Added a warning about all "time" AVPs in regards to the end of
      life of a 32 bit time value.

      - Added a placeholder in the Session-Id AVP description about the
      protocol's interactions with NAT.

      - Renamed the Initialization-Vector AVP to the Nonce AVP. This
      makes sense since the IV was also used for authentication
      purposes, and an IV is normally used for encryption purposes.

      - Added a placeholder to the Nonce AVP section regarding the fact
      that some crypto transforms have known attacks if there is no
      random value in the plaintext early within a message.

      - Clarification of the "Tag" and the "Mandatory" bits in the AVP
      header.

      - Added text specifying that the Session-Id AVP can only appear
      once in a message.

      - Clarified the conditions that cause a "Bad Packet" situation.

      - Removed all support for TCP.

      - Removed all references to the 'P' and 'E' bits, given that these
      bit are defined in the proxy draft, and should not be specified in
      the base protocol.

      - The removal of the 'E' bit caused a shift in the bits, changing
      the AVP header.

      - A statement was added in the AVP Header definition that new AVP
      flags may be added in the future and that an unrecognized flag
      SHOULD be considered an error.

      - Most AVPs flag field requirements have changed.

      - Added descriptions for the 'A' bit, the Ns and Nr in the
      DIAMETER header in section 2.1.

      - Added section 2.1.1, which describes DIAMETER Acknowledgements.

      - Added Command-Specific bits in the AVP Header in section 2.2.
      This will eliminate the overlap problem found between the proxy
      draft and the authent draft.

      - Added section 3.0 (Reliable Transport) (from the Reliable
      Transport document).

      - Fixed up text in section 3.1 about updating the time in the
      Timestamp AVP in retransmissions.

      - Section 4.1 does not allow the Command Code AVP to be encrypted.

      - Cleaned up some language in section 4.2, describing when a
      Device-Reboot-Ind should be used.

      - The Integrity-Check-Vector description now clearly states that
      any AVPs found after it must be ignored.

      - Added section 4.21, which is the Receive-Window AVP (from the
      Reliable Transport document).

      - Added section 4.22, which is the Proxy-State AVP. This AVP used
      to be defined in the proxy extension, but has been deemed more
      appropriate in the base protocol.

      - The Timestamp AVP (section 4.10) was incorrect since it stated
      that NTP time started on January 1st, 1970 instead of 1900.

      - Added section 5.1.1 that describes the DIAMETER state machine.

      - Fixed up a problem in the definition of Hop-by-Hop encryption
      (section 4.6) since the original text defined using the two octet
      Command Code instead of four octets.

      - Added section 5.6, which provides a detailed description of how
      DIAMETER server should proxy messages.

      - Added IANA Considerations

      - Removed all references to the DIAMETER Reliable Transport
      document.

      - Added appendix A and B (from the Reliable Transport document).


2.0  Protocol Overview


2.1  Header Format

   The base DIAMETER protocol is run over UDP port 1812. Due to the fact
   that both the client and server can receive unsolicited messages, it
   is highly recommended that the source and destination field for all
   DIAMETER messages be 1812.

   When a request is received, the source and destination ports in the
   reply are reversed.


   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 ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-

   RADIUS PCC (Packet Compatibility Code)

      The RADIUS PCC  field is a one octet field which is used for
      backward compatibility with RADIUS. In order to easily distinguish
      DIAMETER packets from RADIUS a special value has been reserved and
      allows an implementation to support both protocols concurrently
      using the first octet in the header. The RADIUS PCC field MUST be
      set as follows:

          254       DIAMETER packet

   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 flag 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. Should
         DIAMETER be implemented over a reliable transport, the 'W'
         should not be set.

         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
         present within an acknowledgment message.

   Version

      The Version field is three bits, and indicates the version number
      which is associated with the packet received. This field MUST be
      set to 1 to indicate DIAMETER Version 1.

   Packet Length

      The Packet Length field is two octets.  It indicates the length of
      the packet including the header fields. For messages received via
      UDP, octets outside the range of the Length field should be
      treated as padding and should be ignored upon receipt.

   Identifier

      The Identifier field is four octets, and aids in matching requests
      and replies. The sender MUST ensure that the identifier in a
      message is locally unique (to the sender) at any given time, and
      MAY attempt to ensure that the number is unique across reboots.
      The identifier is normally a monotonically increasing number,
      whose start value was randomly generated. The random algorithm
      used should ensure low probability of duplication. Given the size
      of the Identifier field it is unlikely that 2^32 requests could be
      outstanding at any given time.

   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.


   AVPs

      AVPs is a method of encapsulating information relevant to the
      DIAMETER message. See section 2.2 for more information on AVPs.

2.1.1  ZLB Message Format

   Zero Length Body messages are used to explicitly acknowledge one or
   more DIAMETER message, and contain no additional Authentication,
   Authorization or Accounting related AVPs. ZLB messages must contain
   authentication AVPs, otherwise attacks could be mounted against
   DIAMETER nodes. Consider the following example.

          +------+       ----->       +------+
          |      |        Ns=10       |      |
          | DIA1 +--------------------+ DIA3 |
          |      |        Ns=40       |      |
          +------+       <-----       +-+----+
                                       /
                                     /
                      +------+     / Nr = 41
                      |Malici|   /
                      | ous  +-/
                      | Node |
                      +------+

   In the above figure, DIA3 sends a stream of messages to DIA, with
   sequence number 40 being the last message sent. A malicious user
   could send an acknowledgement for Ns 40 to DIA3, effectively opening
   up the window. Furthermore, if any of the messages from DIA3 were
   lost in transit to DIA1, DIA3 would not attempt to retransmit them
   since it received an acknowledgement. Therefore, it is necessary that
   all acknowledgement messages also include the same authentication
   related AVPs are normal DIAMETER messages.

   The format of a ZLB message will be as follows:

       ::= 
                        
                        
                       { ||
                        2].

      AVP numbers 256 and above are used for DIAMETER. Each service MUST
      allocate AVP numbers through the IANA (see section 6.0).

      If the Vendor-Specific-AVP flag is set, the AVP Code is allocated
      from the vendor's private address space.

   AVP Length

      The AVP Length field is two octets, and indicates the length of
      this Attribute including the AVP Code, AVP Length, AVP Flags,
      Reserved, the Tag and Vendor ID fields if present and the AVP
      data. If a packet is received with an Invalid attribute length,
      the packet SHOULD be rejected.

   Command Flags

      The Command Flag field is a bit-field that can be used by
      individual command codes. Any Command Code that makes use of these
      bits MUST define their value, and how they are used. Note that
      only AVPs with the AVP Code set to Command-Code may use these
      bits, otherwise the bits MUST be set to zero (0).

   Reserved

      The Reserved field MUST be set to zero (0).

   AVP Flags

      The AVP Flags field informs the DIAMETER host how each attribute
      must be handled. Note that subsequent DIAMETER extensions MAY
      define bits to be used within the AVP Header, and an unrecognized
      bit should be considered an error.

      The 'M' Bit, known as the Mandatory bit, indicates whether support
      of the AVP is required. If an AVP is received with the 'M' bit
      enabled and the receiver does not support the AVP, the message
      MUST be rejected.

      AVPs without the 'M' bit enabled are informational only and a
      receiver that receives a message with such an AVP that is not
      supported MAY simply ignore the AVP.

      When the 'H' bit is enabled it indicates that the AVP data is
      encrypted using hop-by-hop encryption. See section 4.5 for more
      information.

      The 'V' bit, known as the Vendor-Specific bit, indicates whether
      the optional Vendor ID field is present in the AVP header. When
      set the AVP Code belongs to the specific vendor code address
      space.

      The 'T' bit, known as the Tag bit, is used to group sets of AVPs
      together. Grouping of AVPs is necessary when more than one AVP is
      needed to express a condition. If this bit is set, the optional
      Tag field will be present.


   Vendor ID

      The optional four octet Vendor ID field contains the IANA assigned
      "SMI Network Management Private Enterprise Codes" value, encoded
      in network byte order. Any vendor wishing to implement DIAMETER
      extensions can use their own Vendor ID along with private
      Attribute values, guaranteeing that they will not collide with any
      other vendor's extensions, nor with future IETF extensions.

      The value zero, reserved in this protocol, corresponds to IETF
      adopted Attribute values, defined within this document; zero MUST
      NOT be used in an AVP.

   Tag

      The Tag field is four octet in length and is intended to provide a
      means of grouping attributes in the same packet which refer to the
      same tunnel. If the Tag field is unused, the 'T' bit MUST NOT be
      set..

   Data

      The Data field is zero or more octets and contains information
      specific to the Attribute. The format and length of the Data field
      is determined by the AVP Code and AVP Length fields.

      The format of the value field MAY be one of six data types. It is
      possible for an attribute to have a structure and this MUST be
      defined along with the attribute.

      Data

         0-65400 octets of arbitrary data.

      String

         0-65400 octets of string data using the UTF-8 character set.

      Address

         32 bit or 128 bit value, most significant octet first. The
         length of the attribute is determined by the length.

      Integer32

         32 bit value, most significant octet first.

      Integer64

         64 bit value, most significant octet first.

      Time

         32 bit unsigned value, most significant octet first -- seconds
         since 00:00:00 GMT, January 1, 1900. Note that this field has
         the problem that it will expire sometime in 2038, as will the
         current NTP time format. More investigation is needed here to
         determine whether there exists a 64 bit time format.


2.3  Error Reporting

   There are five different types of errors within DIAMETER. The first
   being where a DIAMETER message is poorly formatted and
   unrecognizable, indicated below by "Bad Packet". This error condition
   applies if a message is received with an unexpected AVP (e.g. more
   than one Session-Id or DIAMETER-Command AVP).

   The second case involves receiving a DIAMETER-Command AVP that is not
   supported, which is shown below by "Unknown Command". The third case
   is where an AVP is received, marked mandatory and is unknown by the
   receiver, which is labeled below as "Unknown AVP".

   This fourth case involves receiving a message with a known AVP, yet
   the value is either unknown or illegal, which is shown below as "Bad
   Value".  The last case occurs when an error occurs while processing a
   specific extension command, which is not related to the packet format
   and is labeled "Extension Error" below.

   Error Type           Ignore Message          Send         Extension
                                         Message-Reject-Ind  Response /w
                                                             Result-Code
   Bad Packet                 X
   Unknown Command                               X
   Unknown AVP                                   X
   Bad Value                                     X
   Extension Error                                               X

   "Ignore Message" indicates that the message is simply dropped. The
   "Message-Reject-Ind" indicates that a Message-Reject-Ind message MUST
   be sent to the peer as described in the appropriate section. The
   "Extension Error w/ Result-Code" indicates that the appropriate
   Response to the message MUST be sent with the Result-Code or Error-
   Code AVP set to a value that enables the peer to understand the
   nature of the problem.


3.0  Reliable Transport

   This section provides a detailed overview of how DIAMETER is reliably
   transported over UDP.


3.1  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, and contain no
   additional data.

   Two fields in the DIAMETER header that are important for DIAMETER to
   be operated reliable over UDP are the Nr (Next Received) and 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
   out-of-order.

   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
   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.
   If the Nr value changes, and the ICV must be re-computed, it is
   strongly recommended that the time in the Timestamp AVP be updated as
   well.

   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
   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.


3.2 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
   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.


3.3 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
   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.


4.0  DIAMETER AVPs

   This section will define the mandatory AVPs that MUST be supported by
   all DIAMETER implementations. Note the first 256 AVP numbers are
   reserved for RADIUS compatibility.

   The following AVPs are defined in this document:

      Attribute Name       Attribute Code
      -----------------------------------
      DIAMETER-Command          256
      Host-IP-Address             4
      Host-Name                  32
      State                      24
      Class                      25
      Session-Timeout            27
      Extension-Id              258
      Integrity-Check-Vector    259
      Nonce                     261
      Timestamp                 262
      Session-Id                263
      Vendor-Name               266
      Firmware-Revision         267
      Result-Code               268
      Error-Code                269
      Unrecognized-Command-Code 270
      Reboot-Type               271
      Reboot-Time               272
      Failed-AVP-Code           279
      Receive-Window        277
      Proxy-State                33


4.1  DIAMETER-Command AVP

   Description

      The DIAMETER-Command AVP MUST be the first AVP following the
      DIAMETER header. This AVP is used in order to communicate the
      command associated with the message. A DIAMETER message can have
      at most one DIAMETER-Command AVP. Unless noted otherwise, all
      command codes defined in this document will use 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           | Cmd Flags | Reserved  |T|V|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Command Code                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      AVP Code

         256     DIAMETER-Command

      AVP Length

         The length of this attribute MUST be at least 12. The exact
         length of the AVP is determined by the actual Command and is
         defined with each command.

      Command Flags

         All Command Codes defined in this spec MUST set all bits in
         this field to zero (0).

      AVP Flags

         The 'M' bit MUST be set. The 'V' MAY be set if the Command Code
         is vendor specific. The 'H', 'T' bits MUST NOT be set.


      Command Code

         The Command Code field contains the command number. The
         following commands are defined and MUST be supported by all
         DIAMETER implementations in order to conform to the base
         protocol specification:

            Command Name          Command Code
            -----------------------------------
            Message-Reject-Ind        256
            Device-Reboot-Ind         257
            Device-Watchdog-Ind       258


4.1.1  Message-Reject-Ind (MRI)

   Description

      The Message-Reject-Ind command provides a generic means of
      completing transactions by indicating errors in the messages which
      initiated them. The Message-Reject-Ind command is a possible
      response to any DIAMETER command, but some DIAMETER commands MAY
      expect more specialized error messages, depending on the error
      type.

      The Message-Reject-Ind message MUST contain the same
      identification in the header and include the Session-Id if it was
      present in the original message that it is responding to, even if
      the identification is erroneous. The receiver of a Message-Reject-
      Ind SHOULD examine the Result-Code AVP provided before processing
      the identification, in order to handle the letter appropriately.

   Message Format

      The structure of the Message-Reject message is defined as follows:

       ::= 
                                       
                                       
                                       []
                                       []
                                       
                                       [ ]
                                       { ||
                                        }
                                       
                                       
                                       { ||
                                         ::= 
                              
                              
                              []
                              
                              []
                              
                              
                              
                              []
                              []
                              
                              
                              { ||
                                ::= 
                                
                                
                                []
                                
                                
                                { ||
                                 

      It is suggested that the monotonically increasing 32 bit value NOT
      start at zero upon reboot, but rather start at a random value.
      This will minimize the possibility of overlapping Session-Ids
      after a reboot. The optional value is implementation specific but
      may include a modem's device Id, a random value, etc.

      The session Id is created by the DIAMETER device initiating the
      session, which in most cases is done by the client. Note that a
      Session-Id can be used by more than one extension.

      NOTE: The fact that the Sender's IP Address is used in the
      construction of the Session-Id means that the introduction of
      Network Address Translation can cause two hosts to represent the
      same Session Identifier.  This area needs to be investigated
      further to be able to support DIAMETER hosts on a private network.

   AVP 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |        Reserved       |T|V|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Data ...
      +-+-+-+-+-+-+-+-+

      AVP Code

         263     Session-Id

      AVP Length

         The length of this attribute MUST be at least 9.

      AVP Flags

         The 'M' bit MUST be set. The 'T' and 'H' bits MAY be set. The
         'V' bit MUST NOT be set.

      Data

         The Data field contains the session identifier assigned to the
         session.


4.12  Vendor-Name

   Description

      The Vendor-Name attribute is used in order to inform a DIAMETER
      peer of the Vendor Name of the DIAMETER device. This MAY be used
      in order to know which vendor specific attributes may be sent to
      the peer.

      It is also envisioned that the combination of the Vendor-Name and
      the Firmware-Revision AVPs can provide very useful debugging
      information.

   AVP 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |        Reserved       |T|V|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   String ...
      +-+-+-+-+-+-+-+-+

      AVP Code

         266     Vendor-Name

      AVP Length

         The length of this attribute MUST be at least 9.

      AVP Flags

         The 'H' bits MAY be set. The 'T', 'V' and 'M' bits MUST NOT be
         set.

      String

         The String field contains the vendor name.


4.13  Firmware-Revision

   Description

      The Firmware-Revision AVP is used to inform a DIAMETER peer of the
      firmware revision of the issuing device.

      For devices which do not have a firmware revision (general purpose
      computers running DIAMETER software modules, for instance), the
      revision of the DIAMETER software module may be reported instead.

   AVP 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |        Reserved       |T|V|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Integer32                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      AVP Code

         267     Firmware-Revision

      AVP Length

         The length of this attribute MUST be at least 12.

      AVP Flags

         The 'H' bits MAY be set. The 'T', 'V' and 'M' bits MUST NOT be
         set.

      Integer32

         The Integer32 field contains the firmware revision number of
         the issuing device.


4.14  Result-Code

   Description

      The Result-Code AVP is used in order to indicate whether a
      particular command was completed successfully or whether an error
      occurred. The Result-Code AVP MUST be present in all DIAMETER
      messages of type -Request or -Answer.

   AVP 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |        Reserved       |T|V|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Integer32                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      AVP Code

         268     Result-Code

      AVP Length

         The length of this attribute MUST be 12.

      AVP Flags

         The 'M' bit MUST be set. The 'H' bits MAY be set. The 'T' and
         'V' bits MUST NOT be set.

      Integer32

         The Integer32 field contains the result code associated with
         the DIAMETER command. The following codes have been defined:

            DIAMETER_SUCCESS                0
               The Request was successfully completed.

            DIAMETER_FAILURE                1
               The Request was not successfully completed for an
               unspecified reason.  A DIAMETER Message-Reject message
               returning this result SHOULD whenever possible also
               contain one or more Failed-AVP-Code AVPs indicating the
               attributes which caused the failure.

            DIAMETER_POOR_REQUEST           2
               The Request was poorly constructed.  A DIAMETER Message-
               Reject message returning this result SHOULD whenever
               possible also contain one or more Failed-AVP-Code AVPs
               indicating the attributes which caused the failure.

            DIAMETER_INVALID_MAC            3
               The Request did not contain a valid Integrity-Check-
               Vector or Digital-Signature [11].

            DIAMETER_UNKNOWN_SESSION_ID     4
               The Request contained an unknown Session-Id.

            DIAMETER_SEE_ERROR_CODE         5
               The Request failed. The message MUST also contain an
               Error-Code AVP which provides command-specific
               information on the failure.  A DIAMETER Message-Reject-
               Ind message returning this result SHOULD whenever
               possible also contain one or more Failed-AVP-Code AVPs
               indicating the attributes which caused the failure.

            DIAMETER_COMMAND_UNSUPPORTED    6

               The  Request contained a command code which the DIAMETER
               implementation does not recognize or does not support.
               The Message-Reject-Ind message MUST also contain an
               Unrecognized-Command-Code AVP which contains the Command
               Code value which was rejected.

            DIAMETER_ATTRIBUTE_UNSUPPORTED  8

               The Request contained an AVP with an AVP Code which the
               DIAMETER implementation does not recognize or does not
               support. An DIAMETER Message-Reject-Ind message returning
               this result MUST also contain one or more Failed-AVP-Code
               AVPs indicating the AVP Codes which caused the failure.


4.15  Error-Code

   Description

      The Error-Code AVP contains the message specific error code, if
      any.  This AVP only needs to be present if the Result-Code AVP is
      present with the DIAMETER_SEE_ERROR_CODE.

      Error-Code values and corresponding semantics are specific to the
      command to which the Error-Code is a response, and MUST therefore
      be documented as part of the description of that command.

   AVP 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |        Reserved       |T|V|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Integer32                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      AVP Code

         269     Error-Code

      AVP Length

         The length of this attribute MUST be 12.

      AVP Flags

         The 'M' bit MUST be set. The 'H' bits MAY be set. The 'T' and
         'V' bits MUST NOT be set.

      Integer32

         The Integer32 field contains the error code.


4.16  Unrecognized-Command-Code

   Description

      The Unrecognized-Command-Code AVP contains the offending Command
      Code that resulted in sending the Message-Reject-Ind message.

   AVP 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |        Reserved       |T|V|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Integer32                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      AVP Code

         270     Unrecognized-Command-Code

      AVP Length

         The length of this attribute MUST be 12.

      AVP Flags

         The 'M' bit MUST be set. The 'H' bits MAY be set. The 'T' and
         'V' bits MUST NOT be set.

      Integer32

         The Integer32 field contains the unrecognized command code that
         resulted in sending an Message-Reject-Ind message.


4.17  Reboot-Type

   Description

      The Reboot-Type AVP MUST be present in the Device-Reboot-
      Indication message and contains an indication of the type of
      reboot.

   AVP 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |        Reserved       |T|V|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Integer32                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      AVP Code

         271     Reboot-Type

      AVP Length

         The length of this attribute MUST be 12.

      AVP Flags

         The 'M' bit MUST be set. The 'H' bits MAY be set. The 'T' and
         'V' bits MUST NOT be set.

      Integer32

         The Integer32 field contains the reboot type associated with
         the DRI command. The following value are currently defined:

            REBOOT_IMMINENT                 1
               When the Reboot-Type AVP is set to this value it is an
               indication that the DIAMETER peer is about to reboot and
               should not be sent any additional DIAMETER messages
               besides the acknowledgement.

            REBOOTED                        2
               When the Reboot-Type AVP is set to this value it is an
               indication that the DIAMETER peer has recently rebooted
               and is ready to accept new DIAMETER messages.

            CLEAN_REBOOT                    3
               When the Reboot-Type AVP is set to this value the server
               is in the process of shutting down and MAY be available
               at some time in the future.


4.18  Reboot-Time

   Description

      The Reboot-Time AVP MAY be present in the DRI and indicates the
      number of seconds before the issuer expects to be ready to receive
      new DIAMETER messages. This AVP MUST only be present when the
      Reboot-Type AVP is set to REBOOT_IMMINENT. The value indicated by
      this AVP should be used as an estimate and is not a hard rule.

   AVP 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |        Reserved       |T|V|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           Integer32                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      AVP Code

         272     Reboot-Time

      AVP Length

         The length of this attribute MUST be 12.

      AVP Flags

         The 'M' bit MUST be set. The 'H' bits MAY be set. The 'T' and
         'V' bits MUST NOT be set.

      Integer32

         The Integer32 field contains the expected amount of seconds
         before the issuer of the DRI expects to be receive to receive
         new DIAMETER messages.


4.19  Failed-AVP-Code

   Description

      The Failed-AVP-Code AVP provides debugging information in cases
      where a request is rejected or not fully processed due to
      erroneous information in a specific AVP. The documentation of the
      Result-Code AVP and of the Message-Reject-Ind command provide
      information on the use of the Failed-AVP-Code AVP. This AVP has
      the following format:

   AVP 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |        Reserved       |T|V|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Data...
      +-+-+-+-+-+-+-+-+

      AVP Code

         279      Failed-AVP-Code

      AVP Length

         The length of this attribute MUST be 12.

      AVP Flags

         The 'M' bit MUST be set. The 'H' bits MAY be set. The 'T' and
         'V' bits MUST NOT be set.

      Data

         The Data field contains the complete AVP that could not be
         processed successfully. Possible reasons for this are an
         improperly-constructed AVP, an unsupported or unrecognized AVP
         Code, or an invalid value.


4.20  User-Name

   Description

      This attribute contains the User-Name in a format consistent with
      the NAI specification [8].

      A summary of the User-Name AVP format is shown below. The fields
      are transmitted from left to right.

   AVP 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |        Reserved       |T|V|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   String ...
      +-+-+-+-+-+-+-+-+

      Type

         1 for User-Name.

      AVP Length

         The length of this AVP MUST be at least 9.

      AVP Flags

         The 'M' bit MUST be set. The 'H' bits MAY be set. The 'T' and
         'V' bits MUST NOT be set.

      String

         The String field is one or more octets. All DIAMETER systems
         SHOULD support User-Name lengths of at least 63 octets. The
         format of the User-Name SHOULD follow the format defined in
         [8].


4.21  Receive-Window

   Description

      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

      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       |T|V|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' bits MAY be set. The 'T' and
         'V' bits MUST NOT be set.

      Integer32

         This field contains the receive window size.


4.22  Proxy-State

   Description

      The Proxy-State AVP is used by proxy servers when forwarding
      requests and contains opaque data that is used by the proxy to
      further process the response. Such data may include AVPs that are
      to be added to the response, information about the downstream
      peer, etc.

      A DIAMETER node that receives such an AVP in a request MUST return
      the identical AVP in the response. Furthermore, only one such AVP
      may be present in a message at any given time, so implmentations
      MUST ensure that they remove any Proxy-State AVPs before adding
      their own.

      If the Proxy-State AVP was removed from a request, the same AVP
      must be inserted in the corresponding response before forwarding
      the message to the downstream peer.

      The Proxy-State AVP's Address field is intended to be used by
      DIAMETER hosts in order to assist in determining if the AVP was
      locally generated.

   AVP Format

      A summary of the Proxy-State AVP 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           AVP Code                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          AVP Length           |       Reserved        |T|V|H|M|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            Address                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Data ...
      +-+-+-+-+-+-+-+-+

      AVP Code

         33 for Proxy-State.

      AVP Length

         The length of this attribute MUST be at least 13.

      AVP Flags

         The 'M' bit MUST be set. The 'V', 'H' and 'T' bits MUST NOT be
         set.

      Address

         The Address field contains the IP Address of the system that
         created the Proxy-State AVP. This field is intended to assist
         hosts in determining if a Proxy-State AVP in a message was
         locally created.

      Data

         The Data field is one or more octets. The actual format of the
         information is site or application specific, and a robust
         implementation SHOULD support the field as undistinguished
         octets.


5.0  Protocol Definition

   This section will describe how the base protocol works (or is at
   least an attempt to).


5.1  DIAMETER Bootstrap Message

   DIAMETER provides a message that is used to indicate either an
   imminent reboot, or that a reboot has occurred. The DRI message MUST
   be sent to all known DIAMETER peers both previous to a reboot when
   possible as well as following a reboot.

   The Reboot-Type AVP is used to indicate the type of reboot associated
   with the DRI. When set to REBOOT_IMMINENT, all peers should be warned
   that any new DIAMETER requests sent to the issuer will probably not
   be received or processed. If a request MUST be sent it would be
   preferable to issue the request to an alternate peer if available.

   The message includes an optional Reboot-Time AVP that specifies an
   estimate of how long before the issuer is available to receive new
   DIAMETER messages.

   Upon reboot, the host MUST issue a DRI message with the Reboot-Type
   AVP set to REBOOTED. This is an indication that new DIAMETER messages
   may be sent to the transmitter of the DRI.

   Note that the Reboot-Time AVP is not required, and when present
   provides an estimate and should not be used as a hard value. In the
   case of a software implementation (server) running on a general
   purpose operating system, the Reboot-Time AVP will probably not be
   present since it is possible that the DIAMETER server has been
   stopped and it is not possible to know how long before (and if) it
   will be restarted.

   Upon receipt of this message the peer's Ss and Sr variables must be
   reset.  It is possible for this message to be received outside the
   window (Ns and Nr set to zero) when it follows a reboot.

   The DIAMETER Reboot-Ind message does not require a reply. The message
   is acknowledged using DIAMETER's reliable transport.


5.1.1  State Machine

   A DIAMETER node initially considers all known peers to be in the
   closed state, and should not process any DIAMETER message with the
   exception of acknowledgements and the DRI. Once the DIAMETER peer is
   set to the open state, any DIAMETER message may be accepted and
   processed. The following is a suggested state machine.

      State           Event             Action               New State
      -----           -----             ------               ---------
      closed         Local Open        send DRI             wait-ack1
                      Request

      closed          receive DRI       send ACK             wait-ack2
                                        send DRI

      closed          receive invalid   cleanup              closed
                      DRI

      wait-ack1       receive ACK       accept Incoming      wait-ack1
                                        Messages

      wait-ack1       receive DRI       send ACK             open
                                        Accept Incoming
                                        Messages

      wait-ack2       received ACK      Accept Incoming      open
                                        Messages

      open            receive DRI       cleanup              closed

      open            receive DWI       send ACK             open

      open            receive other     send ACK             open
                      messages


5.2  Keepalive Exchange

   DIAMETER uses the Device-Watchdog-Ind message as a keepalive
   mechanism.  DIAMETER entities that need to ensure that connectivity
   with a peer is not lost may use this mechanism.

   A DIAMETER Client can use this mechanism to ensure that failover to
   an alternate server occurs even without any AAA traffic. DIAMETER
   Servers use this mechanism to identify when a particular client is no
   longer reachable. Redundant DIAMETER Servers can use this mechanism
   to identify when the primary server is no longer available. Proxy
   Servers can equally use this method to identify when a particular
   domain's server is no longer reachable.

   The DIAMETER Device-Watchdog-Ind message does not require a reply.
   The message is acknowledged using DIAMETER's reliable transport.


5.3  Unrecognized Command Support

   The DIAMETER protocol provides a message that is used to inform a
   peer that a DIAMETER message was received with an unrecognized
   command. The following provides a DIAMETER message that is sent to a
   peer:

    ::= 
                         
                         
                         []
                         
                         
                         { ||
                           ::= 
                            
                            
                            
                            []
                            
                            
                            { ||
                              ::= 
                          
                          []
                          
                          
                          

   Any AVPs in a message that is not succeeded by the Integrity-Check-
   Vector AVP MUST be ignored.


5.6  DIAMETER Proxying

   This section will describe how DIAMETER server implementations can
   proxy requests to upstream servers. Consider the following diagram,
   which depicts DIA1 sending a request to DIA2. Typically, the request
   will contain the User-Name AVP (section 4.20), which conforms to the
   format defined in the NAI specification [8]. Server DIA2 normally
   will extract that domain name portion of the NAI to determine if the
   request can be locally processed, or if the request must be proxied
   to an upstream server (in this case DIA3).

                   (Request)                  (Request)
           (User-Name = joe@abc.com)       (Proxy-State=1)
      +------+      ------>      +------+      ------>      +------+
      |      |                   |      |                   |      |
      | DIA1 +-------------------+ DIA2 +-------------------+ DIA3 |
      |      |                   |      |                   |      |
      +------+      <------      +------+      <------      +------+
                   (Response)                 (Response)
                                           (Proxy-State=1)
      mno.net                    xyz.com                    abc.com

   Prior to forwarding the request, DIA2 must establish some state
   information in order to be able to forward the corresponding response
   from DIA3 to DIA1.  There are two methods of doing so:

      1. DIA2 can maintain state information locally, and using the
      session-Id and possible the Identifier in the header, can match
      the request with the response. The state information would contain
      sufficient information for it to know where the response should be
      forwarded. Additionally, it may be necessary for DIA2 to attach
      AVPs to the response that were created when the request was
      received. These AVPs could be kept in the state table.

      2. DIA2 can attach a Proxy-State AVP (section 4.22), which may
      contain any information, including information regarding the
      source of the request, additional AVPs that must be attached to
      the response, etc. Upon receipt of the response, DIA2 must find
      the Proxy-State AVP, determine if the AVP was created locally, and
      if so use the information included within the AVP. If AVPs were
      found within the Proxy-State AVP, they could easily be attached to
      the response. Finally, the Proxy-State AVP is removed from the
      response before being forwarded to DIA1.

      Althought both methods work, the latter is much simpler as it
      reduces the amount of state information each proxy must maintain
      on a per request basis.

      When DIA3 receives a request that includes the Proxy-State AVP, it
      MUST include the same AVP in the corresponding response.
      Furthermore, should DIA3 have to proxy the request to another
      upstream server, it would have to replace the existing Proxy-State
      AVP with its own. It must, however, be able to replace the Proxy-
      State AVP in the corresponding response back to the one it had
      received in the request. One suggested implementation is to
      include the Proxy-State AVPs in a newly created Proxy-State AVP,
      allowing a server to easily replace the Proxy-State AVPs in the
      responses.


5.7  AVP Encryption with Shared Secrets

   This method of encrypting AVP data is the simplest to use and MUST be
   supported by all DIAMETER implementations. However, local policy MAY
   determine that the use of this mechanism is not permitted.

   The 'H' bit MUST only be set if a shared secret exists between both
   DIAMETER peers. If the 'H' bit is set in any DIAMETER AVP, the Nonce
   AVP MUST be present prior to the first encrypted AVP.

   The length of the AVP value to be encrypted is first encoded in the
   following Subformat, which is included in the AVP's data field.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Length of ClearText Data    |       ClearText Data ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Padding ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length

      The Length field contains the length of the decrypted data.

   ClearText Data

      Data of AVP that is to be obscured.

   Padding

      Random additional octets used to obscure length of the ClearText
      Data.

   The resulting subformat MAY be padded to a multiple of 16 octets in
   length. For example, if the ClearText Data to be obscured is a string
   containing 6 characters (e.g. password 'foobar'), then 8 + n * 16
   octets of padding would be applied. Padding does NOT alter the value
   placed in the Length of the ClearText Data, only the length of the
   AVP itself.

   Next, An MD5 hash is performed on the concatenation of:

      - the four octet Command Code of the AVP
      - the shared authentication secret
      - an arbitrary length random vector

   The value of the random vector used in this hash is passed in the
   Data field of a Nonce AVP.  This Nonce AVP must appear in the message
   before any hidden AVPs. The same Nonce may be used for more than one
   hidden AVP in the same message.  If a different Nonce is used for the
   hiding of subsequent AVPs then a new Nonce AVP must be placed before
   the first AVP to which it applies.

   The MD5 hash value is then XORed with the first 16 octet or less
   segment of the AVP Subformat and placed in the Data field of the AVP.
   If the AVP Subformat is less than 16 octets, the Subformat is
   transformed as if the Value field had been padded to 16 octets before
   the XOR, but only the actual octets present in the Subformat are
   modified, and the length of the AVP is not altered.

   If the Subformat is longer than 16 octets, a second one-way MD5 hash
   is calculated over a stream of octets consisting of the shared secret
   followed by the result of the first XOR.  That hash is XORed with the
   second 16 octet or less segment of the Subformat and placed in the
   corresponding octets of the Data field of the AVP.

   If necessary, this operation is repeated, with each XOR result being
   used along with the shared secret to generate the next hash to XOR
   the next segment of the value with.  This technique results in the
   content of the AVP being obscured, although the length of the AVP is
   still known.

   On receipt, the Nonce is taken from the last Nonce AVP encountered in
   the message prior to the AVP to be decrypted. The above process is
   then reversed to yield the original value.  For more details on this
   hiding method, consult RFC2138 [1].

   Please note that in the case where the DIAMETER message needs to be
   processed by an intermediate non-trusted DIAMETER server (also known
   as a proxy server, depicted as DIA2 in the figure below) the AVP
   needs to be decrypted using Shared-Secret-1 and re-encrypted by DIA2
   using Shared-Secret-2.

               (Shared-Secret-1)          (Shared-Secret-2)
      +------+       ----->      +------+      ------>      +------+
      |      |                   |      |                   |      |
      | DIA1 +-------------------+ DIA2 +-------------------+ DIA3 |
      |      |                   |      |                   |      |
      +------+                   +------+                   +------+

   Unfortunately in this case the non-trusted server DIA2 has access to
   sensitive information (such as a password).


6.0  IANA Considerations

   This document defines a number of "magic" numbers to be maintained by
   the IANA.  This section explains the criteria to be used by the IANA
   to assign additional numbers in each of these lists. The following
   subsections describe the assignment policy for the namespaces defined
   elsewhere in this document.


6.1  AVP Attributes

   As defined in Section 4.0, AVPs contain vendor ID, Attribute and
   Value fields. For vendor ID value of 0, IANA will maintain a registry
   of assigned Attributes and in some case also values. Attribute 0-254
   are assigned from the RADIUS protocol [1], whose attributes are also
   maintained through IANA. Attributes 256-280 are assigned within this
   document in section 4.0. The remaining values are available for
   assignment through IETF Consensus [12].


6.2  Command Code AVP Values

   As defined in Section 4.1, the Command Code AVPs (AVP Code 256) have
   an associated value maintained by IANA. Values 0-255 are reserved for
   backward RADIUS compatibility, and values 256-258 are defined in this
   specification. The remaining values are available for assignment via
   IETF Consensus [12].


6.3  Extension Identifier Values

   as defined in Section 4.7, the Extension Identifier is used to
   identify a specific DIAMETER Extension. All values, other than zero
   (0) are available for assignment via IETF Consensus [12].


6.4  Result Code AVP Values

   As defined in Section 4.14, the Result Code AVP (AVP Code 268)
   defines the values 0-8. All remaining values are available for
   assignment via IETF Consensus [12].


6.5  Integrity Check Vector Transform Values

   Section 4.8 defines the Integrity-Check-Vector AVP (AVP Code 259)
   which contains a field called the Transform. This document reserves
   the value 1. All remaining values are available for assignment via
   IETF Consensus [12].


6.6  Reboot Type Values

   Section 4.17 defines the Reboot-Type AVP (AVP Code 271), which is
   used to inform the peer of the cause for the reboot. This document
   defines the values 1-3. All remaining values are available for
   assignment via IETF Consensus [12].


6.7  AVP Header Bits

   There are six remaining reserved bits in the AVP header. Additional
   bits should only be assigned via a Standards Action [12].


7.0  References

    [1]  Rigney, et alia, "RADIUS", RFC-2138, April 1997
    [2]  Reynolds, Postel, "Assigned Numbers", RFC 1700,
         October 1994.
    [3]  Postel, "User Datagram Protocol", RFC 768, August 1980.
    [4]  Rivest, Dusse, "The MD5 Message-Digest Algorithm",
         RFC 1321, April 1992.
    [5]  Kaufman, Perlman, Speciner, "Network Security: Private
         Communications in a Public World", Prentice Hall,
         March 1995, ISBN 0-13-061466-1.
    [6]  Krawczyk, Bellare, Canetti, "HMAC: Keyed-Hashing for Message
         Authentication", RFC 2104, January 1997.
    [7]  Calhoun, Bulley, "DIAMETER User Authentication Extensions",
         draft-calhoun-diameter-authen-06.txt, Work in Progress,
         August 1999.
    [8]  Aboba, Beadles "The Network Access Identifier." RFC 2486.
         January 1999.
    [9]  Calhoun, Zorn, Pan, "DIAMETER Framework",
         draft-calhoun-diameter-framework-02.txt, Work in Progress,
         December 1998.
    [10] Zorn, Leifer, Rubens, Shriver, "RADIUS attributes for
         Tunnel Protocol Support",
         draft-ietf-radius-tunnel-auth-05.txt, Work in Progress,
         April 1998.
    [11] Calhoun, Bulley, "DIAMETER Proxy Extension",
         draft-calhoun-diameter-proxy-02.txt, Work in Progress,
         August 1999.
    [12] Narten, Alvestrand,"Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434,
         October 1998
    [13] S. Bradner, "Key words for use in RFCs to Indicate
         Requirement Levels", BCP 14, RFC 2119, March 1997.

8.0  Acknowledgements

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

   Bernard Aboba, Jari Arkko, William Bulley, Daniel C. Fox, Lol Grant,
   Ignacio Goyret, Nancy Greene, Erik Guttman, Peter Heitman, Paul
   Krumviede, Fergal Ladley, Ryan Moats, Victor Muslin, Kenneth Peirce,
   Nenad Trifunovic, Sumit Vakil, John R. Vollbrecht, Jeff Weisberg and
   Glen Zorn


9.0  Author's Address

   Questions about this memo can be directed to:

      Pat R. Calhoun
      Network and Security Research Center, Sun Labs
      Sun Microsystems, Inc.
      15 Network Circle
      Menlo Park, California, 94025
      USA

       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
      USA

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


10.0  Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied  and  furnished
   to others,  and  derivative works that comment on or otherwise
   explain it or assist in its implmentation may be prepared, copied,
   published  and distributed,  in  whole  or  in part, without
   restriction of any kind, provided that the  above  copyright  notice
   and  this  paragraph  are included on all such copies and derivative
   works.  However, this docu- ment itself may not be modified in any
   way, such as  by  removing  the copyright notice or references to the
   Internet Society or other Inter- net organizations, except as needed
   for  the  purpose  of  developing Internet standards in which case
   the procedures for copyrights defined in the Internet Standards
   process must be followed, or as required  to translate it into
   languages other than   English.  The limited permis- sions granted
   above are perpetual and  will  not  be  revoked  by  the Internet
   Society or its successors or assigns.  This document and the
   information contained herein is provided on an "AS IS" basis  and
   THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE
   DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT
   LIMITED TO ANY  WAR- RANTY  THAT  THE  USE  OF THE INFORMATION HEREIN
   WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS  FOR  A PARTICULAR PURPOSE."


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
   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
   conditions.

   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

   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

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

             (delay)

                                                 (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
        traffic)

              ->    (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)

             ->    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
        example)

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

             (delay)

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