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Network Working Group                                         K. Kinnear
INTERNET DRAFT                             American Internet Corporation
                                                                 R. Cole
                                                                AT&T MNS
                                                                R. Droms
                                                     Bucknell University
                                                               July 1997
                                                    Expires January 1998


                   An Inter-server Protocol for DHCP
                  <draft-ietf-dhc-interserver-02.txt>


Status of this Memo

   This document is an Internet-Draft. Internet-Drafts are working docu-
   ments of the Internet Engineering Task Force (IETF), its areas, and
   its working groups. Note that other groups may also distribute work-
   ing 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 mate-
   rial 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), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).

Abstract

   The DHCP protocol is designed to allow for multiple DHCP servers, so
   that reliability of DHCP service can be improved through the use of
   redundant servers.  To provide redundant service, all of the DHCP
   servers must be configured with the same information about assigned
   IP addresses and parameters; i.e., all of the servers must be config-
   ured with the same bindings.  Because DHCP servers may dynamically
   assign new addresses or configuration parameters, or extend the lease
   on an existing address assignment, the bindings on some servers may
   become out of date.  The DHCP inter-server protocol provides an auto-
   matic mechanism for synchronization of the bindings stored on a set
   of cooperating DHCP servers.

   This draft is a direct extension of draft-ietf-dhc-
   interserver-00.txt, and represents the merging of ideas from both



Kinnear, Cole & Droms                                           [Page 1]


DRAFT                                                          July 1997


   draft-ietf-dhc-interserver-alt-00.txt and draft-ietf-dhc-
   interserver-01.txt.  The basic protocol semantics from draft-ietf-
   dhc-interserver-alt-00.txt were used with the underlying message map-
   ping to SCSP from draft-ietf-dhc-interserver-01.txt.  Considerable
   additional work has been included in this current draft in the area
   of protocol correctness, detailed work on mapping the protocol to
   SCSP, and organization of the draft itself.


1.  Introduction

   DHCP servers manage the assignment of IP address and configuration
   parameters to IP hosts.  The DHCP protocol specification [1] refers
   to the collection of configuration information assigned to a client
   as a "binding".  The DHCP protocol is designed to allow for multiple
   DHCP servers, so that reliability of DHCP service can be improved
   through the use of redundant servers.  To provide redundant service,
   all of the DHCP servers must be configured with the same information
   about assigned IP addresses and parameters; i.e., all of the servers
   must be configured with the same bindings.  Because DHCP servers may
   dynamically assign new addresses or configuration parameters, or
   extend the lease on an existing address assignment, the bindings on
   some servers may become out of date.

   The DHCP inter-server protocol provides an automatic mechanism for
   synchronization of the bindings stored on a set of cooperating DHCP
   servers.

   The remainder of this document is organized in the following sec-
   tions:

     2.  Goals and Requirements

         Defines the requirements and goals for the protocol.  Discusses
         limitations of the protocol.  Also contains a definition of
         several classes of failures as well as a list of specific fail-
         ures (which provide a useful common ground for discussion).

     3.  Overview

         Discusses in a general way the content of the information com-
         municated between servers implementing this protocol as well as
         the way that information is communicated.

         Introduces the three aspects of the protocol: client binding
         management, address management, and group management.





Kinnear, Cole & Droms                                           [Page 2]


DRAFT                                                          July 1997


         Defines some key concepts surrounding the allowable "states" of
         an IP address, including extensions critical to the operation
         of this protocol.

         Gives a brief sketch of the actions required by this protocol
         for each DHCP client request received by the server.

     4.  Client Binding Management

         Discusses the fundamental messages used by this portion of the
         protocol, and the ways in which these messages are combined to
         form higher level operations.  Required responses to incoming
         client binding management requests are explained in this sec-
         tion.  The required responses to incoming DHCP client requests
         are explained in Section 6 below.

     5.  Address Management

         The fundamental messages used by the address management portion
         of the protocol are explained, as well as how they are combined
         into higher level operations.  The required responses to incom-
         ing address management requests are explained in this section,
         while the required responses to incoming DHCP client requests
         are explained in Section 6 below.

     6.  Actions in Response to DHCP Client Messages and Events

         The required responses to incoming DHCP client messages and
         events are discussed in this section.

     7.  Group Management

         The fundamental messages and their combination into higher
         level operations for the group management portion of the proto-
         col are explained.  The actions to take when receiving any of
         these messages as well as how to utilize them to join or leave
         a server group are explained.

     8.  SCSP Message Mapping

         The messages described in sections 4, 5, and 7 are mapped into
         underlying SCSP messages in this section.  This includes
         detailed information on the format of each SCSP message.

     9.  IP Address State Transition

         This protocol expands the possible states for an IP address.
         The new states are described in Section 3.3.  This section



Kinnear, Cole & Droms                                           [Page 3]


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         describes all of the transitions between states in detail.

     10. Security

         The security implications of this draft are discussed in this
         section.

     11. Open Questions

         Poses open questions about the protocol.  Some questions from
         draft-ietf-dhc-interserver-00.txt are included verbatim with
         answers and questions (and some answers) new to this draft are
         included as well.

     12. Acknowledgments

     13. References

     14. Author's Information

     A.  Appendix A: An Overview of SCSP


1.1.  The Language of Requirements

   Throughout this document, the words that are used to define the sig-
   nificance of particular requirements are capitalized.  These words
   are:

     o "MUST"

       This word or the adjective "REQUIRED" means that the item is an
       absolute requirement of this specification.

     o "MUST NOT"

       This phrase means that the item is an absolute prohibition of
       this specification.

     o "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 should be understood and the case
       carefully weighed before choosing a different course.

     o "SHOULD NOT"




Kinnear, Cole & Droms                                           [Page 4]


DRAFT                                                          July 1997


       This phrase means that there may exist valid reasons in particu-
       lar circumstances when the listed behavior is acceptable or even
       useful, but the full implications should be understood and the
       case carefully weighed before implementing any behavior described
       with this label.

     o "MAY"

       This word or the adjective "OPTIONAL" means that this item is
       truly optional.  One vendor may choose to include the item
       because a particular marketplace requires it or because it
       enhances the product, for example; another vendor may omit the
       same item.


1.2.  Terminology

   This document uses the following terms:

     o "DHCP client"

       A DHCP client is an Internet host using DHCP to obtain configura-
       tion parameters such as a network address.

     o "client"

       Whenever the term client is used in this draft, it refers to a
       DHCP client (and not a server communicating with another server
       using this protocol).

     o "DHCP server"

       A DHCP server is an Internet host that returns configuration
       parameters to DHCP clients.

     o "binding"

       A binding is a collection of configuration parameters, including
       at least an IP address, associated with or "bound to" a DHCP
       client.  Bindings are managed by DHCP servers.

     o "active server"

       An active server is one which is capable of offering IP addresses
       to clients.

     o "stable storage"




Kinnear, Cole & Droms                                           [Page 5]


DRAFT                                                          July 1997


       Every DHCP server is assumed to have some form of what is called
       "stable storage".  Stable storage is used to hold information
       concerning IP address bindings (among other things) so that this
       information is not lost in the event of a server failure which
       requires restart of the server.


2.  Goals and Requirements

   There are several levels of goals for this protocol.  There are a set
   of requirements with which it must comply, and then there are a set
   of goals for the protocol and the way that it is used that are listed
   in priority order.


2.1.  Requirements on this Protocol

   The following list of requirements must be (and are) achieved by this
   protocol.

     1. Implementations of this protocol work with existing DHCP client
        implementations based on the DHCP protocol [1].  It must work
        with today's clients!

     2. Implementation works with existing BOOTP relay implementations.

     3. Can be specified with sufficient clarity that unique implementa-
        tions will work well together the first time (e.g. DHCP today
        largely meets this requirement).

     4. Work well with minimum of two and a maximum of 16 servers.

2.2.  Goals of this Protocol

   The following are the goals of this protocol.  These goals are listed
   in priority order.  The protocol meets all of these goals.

     1. Avoid binding an IP address to a client while that binding is
        currently valid for another client.  In other words, don't allo-
        cate the same IP address to two clients.

     2. Ensure that an existing client can keep its existing IP address
        binding if it can communicate with any DHCP server using this
        protocol -- not just the server that originally offered it the
        binding.

        DISCUSSION:




Kinnear, Cole & Droms                                           [Page 6]


DRAFT                                                          July 1997


           There is a subtle but very important point here.  For exam-
           ple, assume that there are five servers using this protocol.
           Everything is running fine, and then the network becomes par-
           titioned, and three servers can communicate among themselves,
           and the other two can communicate among themselves -- but the
           set of three cannot communicate with the set of two.  Each
           set, however, can communicate with some clients.

           In this situation, every client that can communicate with a
           DHCP server in either set should be able to continue to use
           its existing binding, even if the server that originally cre-
           ated the binding is not included in the set of servers with
           which it can communicate.

     3. Do not add any requirement for communication with another server
        to the processing between a DHCPDISCOVER and a DHCPOFFER or
        between a DHCPREQUEST and a DHCPACK.

        DISCUSSION:

           This is another subtle point.  The implications of this goal
           are that "lazy" update of IP address binding information is
           required.  In other words, because of this goal, the protocol
           cannot require one server to update another server with
           information concerning a new IP address binding prior to
           sending the DHCPACK to the DHCP client.

        As a result of this goal, a server may fail immediately after
        sending the DHCPACK to the client but prior to successfully
        sending a record of that information to any other server.
        Should this happen, the DHCP client is the only operational
        machine with a record of this binding -- and the protocol must
        be (and has been) designed to properly deal with this situation.

     3. Ensure that a new client can get an IP address from some server.

     4. If a server goes down, and an external agent determines that it
        is actually down as opposed to running but simply unable to com-
        municate with other servers, then the addresses that it cur-
        rently owns but are not yet bound may be recovered for use by
        other servers.

     5. Ensure that in the face of partition, where servers continue to
        run but cannot communicate with each other, the above goals and
        requirements are met.  In addition, when the partition condition
        is removed, allow graceful automatic re-integration without
        requiring human intervention.




Kinnear, Cole & Droms                                           [Page 7]


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2.3.  Limitations of this Protocol

   The following are explicit limitations of this protocol.  This is not
   to say that they are not useful capabilities to have (that's why they
   are explicitly listed, so that it will be clear that this protocol
   does not supply them).

     1. Determination of permanent server failure.

        The protocol provides a way to propagate information about the
        permanent failure of a server, but no way to detect a permanent
        failure.  Transient failures are detected, but there is no mech-
        anism in this protocol to determine when a transient failure is
        really a permanent failure.  Some external agent must make this
        determination -- and must ensure that the server declared perma-
        nently failed is not simply partitioned from the other servers
        and unable to communicate with them.  The server which has been
        declared permanently failed by the external agent MUST be
        informed of that declaration prior to restart.

        DISCUSSION:

           The existing configuration messages allow one server to
           declare another server as permanently failed and remove it
           from the group.  That is not the issue.  What makes fully
           automatic determination of permanent server failure impracti-
           cal is distinguishing between permanent server failure (which
           is easily defined as  transient server failure that has gone
           on too long) and partition of the group of servers.

           Once communication fails with a server, the other servers
           cannot know if it is still operating or not, and removing an
           operating server from the group is an activity fraught with
           peril.

           This protocol is designed so that a server which is parti-
           tioned from the group will re-integrate cleanly when it can
           communicate again with the rest of the group.

           Group membership protocols typically handle a partition situ-
           ation (when they bother to handle it at all) by having the
           partitioned server determine that it has been partitioned and
           shut itself down.  It detects a partition condition in one of
           two ways:  either it can't communicate with the "master", or
           it can't communicate with the "majority" of the group.  In
           either case, it shuts down.

           We believe that this is not an appropriate response for a



Kinnear, Cole & Droms                                           [Page 8]


DRAFT                                                          July 1997


           DHCP server.  If my DHCP client can talk to a DHCP server, I
           want my client to continue to operate -- I'm not interested
           in having the only DHCP server to which I can talk shut
           itself down!

     2. Some addresses are temporarily unavailable during transient
        server failure.

        The full range of existing IP addresses that are potentially
        available for allocation is reduced during the period of a tran-
        sient server failure.  The size of the pool of addresses that
        are available for allocation but not yet allocated SHOULD be
        configurable for each server.  If the server is subsequently
        declared to have undergone a permanent failure, these addresses
        will be made available again.

        Note that it is only the addresses not yet allocated but avail-
        able for allocation that are unusable during the period of a
        transient server failure.  IP addresses that have been allocated
        to clients may continue to be used by those clients even during
        server failure.  Indeed -- to allow existing clients to be able
        to renew their existing IP addresses even if the server who
        granted them the lease has failed is a primary reason why this
        protocol exists.


2.4.  Failures

   This section makes explicit both classes of failures as well as a
   list of specific failure scenarios in order to facilitate discussion
   of the capabilities of this protocol.

     o "transient server failure"

       A transient server failure is one where a server is unable to
       respond to requests, but later becomes operational and able to
       respond to requests.  Its local stable storage (i.e., whatever
       mechanism it uses to preserve its binding information) is accu-
       rate as of the time that transient server failure began.

     o "permanent server failure"

       A permanent server failure is one where a server is unable to
       respond to requests -- probably for an extended period. While the
       protocol defined in this document supports declaration of a per-
       manent server failure, the decision that a transient server fail-
       ure is in reality a permanent server failure is beyond the scope
       of this protocol.



Kinnear, Cole & Droms                                           [Page 9]


DRAFT                                                          July 1997


       This determination will be likely be performed by some adminis-
       trative entity, although in the future a group membership proto-
       col could be integrated with the protocol defined in this docu-
       ment to make such determinations automatically.

     o "partition"

       A network partition is caused by a failure of the underlying com-
       munications substrate, such that two systems that could previ-
       ously communicate cannot now do so.  This may mimic transient
       server failure, but is not the same because in this case the
       server that appears to have failed may still be operational and
       interacting with clients.

       There is a form of partition known as "partial partition", where
       the transitivity of communication usually expected is not
       achieved.  Imagine a set of servers organized (for the purposes
       of exposition only) as a ring where each server can communicate
       with its neighbors, but nobody else -- and when the number of
       servers is greater than three, a partial partition situation
       exists.

       This term may also be used as a noun, as in "each partition may
       communicate with ...", and in this case it refers to the group of
       servers which can communicate normally (as distinguished from
       those with which that group cannot communicate).

     o "communication failure"

       Communications failure describes the condition where the communi-
       cation channel between two servers becomes impossible.  "Partial
       communication failure" describes the case where the normally
       bidirectional communications channel becomes unidirectional,
       where one server can send to but not receive from another server.

   Some examples of the above failures are given below:

     1. A single server crashes and reboots. [transient failure]

     2. A single server crashes and stays down for a period of hours and
        then reboots (either automatically or through some external
        agent).  [transient failure]

     3. A single server fails and never returns.  No permanent failure
        is declared for this server.  [transient failure]

     4. A single server fails. A permanent failure is declared for this
        server. [permanent failure]



Kinnear, Cole & Droms                                          [Page 10]


DRAFT                                                          July 1997


     5. A group of two servers are partitioned so that they cannot com-
        municate, but each can communicate to some clients.  [partition]

     6. A group of five servers are partitioned so that three can commu-
        nicate together and the remaining two can also communicate, but
        the two partitions cannot communicate.  Each partition can com-
        municate with a subset of the clients, and these subsets are
        disjoint.  [partition]

     7. A group of five servers are partitioned so that three can commu-
        nicate together and the remaining two can also communicate, but
        the two partitions cannot communicate.  Each server continues to
        be able to communicate with all of the clients.  [partition]

        DISCUSSION:

           This situation is unlikely to occur, but the protocol should
           be able to handle it.

     8. Server A can send packets to server B, but cannot receive pack-
        ets from server B. [partial communications failure]

     9. There are four servers, A, B, C, and D.  A cannot communicate
        with C, B cannot communicate with D. [partial partition]

   DISCUSSION:

      This section on failures may well not belong in the final docu-
      ment.  For the purposes of review of the rest of the protocol,
      however, defining a common language to describe failures and giv-
      ing specific examples of failures as an aid to discussion seemed
      useful.


3.  Overview

   At the most basic level, the DHCP protocol specifies the behavior of
   DHCP servers which communicate with DHCP clients in order to allocate
   IP address to the clients as well as provide a variety of configura-
   tion parameters information to them.  It is the allocation of IP
   addresses to clients by the server that creates a requirement to
   update what is known as "stable storage"  -- typically held on disk.
   This information is used to "remember" the IP address bindings that
   have been made by the DHCP server in order to avoid allocating the
   same IP address to two clients.

   The key motivation for an inter-server protocol is the desire to
   allow a client to continue to use its IP address (i.e., be able to



Kinnear, Cole & Droms                                          [Page 11]


DRAFT                                                          July 1997


   renew its lease on an IP address) even if the server who initially
   offered it the lease on its IP address is unavailable for some rea-
   son.  In addition, no IP address should ever be bound to two clients
   simultaneously.

   Providing multiple DHCP servers to which each client can communicate
   is the first step in creating this reliable DHCP capability.

   In addition, these DHCP servers must communicate among themselves in
   order to provide this reliable DHCP capability.


3.1.  Information Communicated by the Protocol

   There are three types of information which must be communicated
   between servers implementing the server server protocol.


     o Client Binding Information

       This entire interserver protocol exists in order to allow servers
       to share information about client bindings of IP addresses.
       Servers must be able to update other servers about client bind-
       ings that they have created, and must be able to receive similar
       updates from other servers about client bindings that the other
       servers have made or changed.

     o Address Management Information

       In order to implement an effective strategy for client binding
       information updates, this protocol defines some additional states
       for an IP address beyond those defined or implied by RFC 2131 [1]
       that are not directly connected with client binding information.
       The servers need to communicate among themselves concerning these
       states, and this communication is enabled by the address manage-
       ment information portion of the protocol.

     o Group Management Information

       While it is possible to conceive of a group of servers statically
       configured to be part of a server group, the operational charac-
       teristics of such an approach are far from pleasant.  The group
       management portion of this protocol allows a server to determine
       the groups to which another server belongs; determine for each
       group the current membership in the group; determine for each
       group the subnets and IP addresses managed by that group; and
       join or leave a server group.




Kinnear, Cole & Droms                                          [Page 12]


DRAFT                                                          July 1997


3.2.  Server Groups

   Fundamental to this protocol is the "group" of servers which are com-
   municating and with which the clients can communicate in order to
   provide a reliable DHCP service.

   Each server group (SG) to which a server belongs is associated with a
   particular set of address pools.  These address pools are those which
   exist on a single network segment (sometimes called a single "wire").

   An active server can be (and typically would be) a member of several
   groups simultaneously.  This protocol allows a server to join an
   existing SG.  Which SGs a server would join is a configuration issue
   for a particular server, and outside of the realm of this protocol --
   although considerable support is provided in order to make this a
   solvable problem.

   The membership of a particular SG will change over time, and in order
   to ensure that each server is made aware of any changes in group mem-
   bership in a timely way, every protocol message which is sent in the
   inter-server protocol includes a group generation number (with a few
   exceptions).

   Whenever a message is received, the group management layer of the
   software MUST verify that the group generation number matches the
   current group generation number for that SG stored in the server.  If
   there is a mismatch, the group management layer will discard the mes-
   sage.  It will then attempt to update its knowledge of the current
   group (and incidentally bring its generation number up to date in the
   process).

   In this way, any changes in group membership become spread throughout
   the group as fast as possible -- and no messages that are out of syn-
   chronization with the latest concept of group membership can be
   received.

   A server attempts to become a member of a particular group by using
   the configuration messages described in Section 7 below.  In addi-
   tion, a server can remove another server from the group using these
   messages -- but in this case an external agent must ensure that the
   server being removed is truly inactive and not just partitioned.


3.3.  Messages and Operations Defined by the Protocol

   The protocol requires that servers who implement it can communicate,
   each with the other, in a point-to-point manner (when all are operat-
   ing correctly).  It allows for the possibility that they can fail



Kinnear, Cole & Droms                                          [Page 13]


DRAFT                                                          July 1997


   entirely (i.e., crash) or be unable to communicate with each other
   for a variety of reasons.

   Each server will periodically need to communicate with other servers
   in the group.  There are several recurring styles of communication
   that, if defined, will assist in explaining the major concepts of
   this protocol.  These major styles of group communication are as fol-
   lows:

   There are "messages", which for the purpose of this specification
   consist of a communication between two servers.  Messages are gath-
   ered into higher level generic "operations", which describe the form
   of the operation, and are made up of messages communicated between
   more than one server.  These generic operations are then instantiated
   into specific operations as part of the various portions of the pro-
   tocol.

3.3.1.  Generic Protocol Messages

   Messages are used to communicate between a pair of servers.

     o QUERY

       A QUERY operation is performed when one server wishes to obtain
       knowledge about the server cache of another server.

     o UPDATE

       An UPDATE operation is performed when one server wishes to update
       the information in the cache of another server.

3.3.2.  Generic Protocol Operations

   These generic protocol operations are used when a server must commu-
   nicate with more than one other server.

     o POLL

       A POLL operation is used when one server must contact every other
       server in the group using a QUERY message in order to request
       that they respond with some information (typically concerning an
       IP address).  Usually, if the server executing the POLL cannot
       contact all of the other servers using the QUERY message, it will
       use whatever information it could glean from those it could con-
       tact.

     o COMPLETE POLL




Kinnear, Cole & Droms                                          [Page 14]


DRAFT                                                          July 1997


       A COMPLETE POLL is like a POLL in that one server attempts to
       contact every other server using a QUERY message -- but in a COM-
       PLETE POLL it must successfully complete a QUERY with each of
       them or the operation itself fails to complete.

     o PUSH

       A PUSH operation is used when one server wants to update all of
       the other servers using an UPDATE message.  In a way similar to
       the POLL operation, a PUSH operation will succeed if the server
       employing it has managed to contact at least one other server in
       the group with a successful UPDATE.

     o COMPLETE PUSH

       A COMPLETE PUSH is analogous to a COMPLETE POLL -- the COMPLETE
       PUSH operation requires the server to attempt to UPDATE every
       other server in the group.  If every server responds successfully
       to the UPDATE, the COMPLETE PUSH succeeds, otherwise the COMPLETE
       PUSH fails.

   Note that both PUSH and POLL involve operations to all of the servers
   in the group.


3.3.3.  Specific Protocol Operations

   These above generic forms of inter-server communication are utilized
   in the following ways in the Client Binding and Address Management.

   Client Binding Management:

     o CLIENT BINDING POLL (operation)

       This operation involves one server asking every other server
       using a QUERY for client binding information concerning a partic-
       ular IP address.  If all of the other servers are not opera-
       tional, the requesting server will use any information it
       receives.

     o CLIENT BINDING COMPLETE PUSH (operation)

       This operation involves one server informing all of the other
       servers using an UPDATE about updated client binding information.
       While there is utility in reaching even one other server (in some
       cases) the operation is not deemed to have succeeded unless all
       of the other servers were successfully updated with the new
       information.



Kinnear, Cole & Droms                                          [Page 15]


DRAFT                                                          July 1997


   Address Management:

     o UNBINDABLE COMPLETE POLL (operation)

       In this operation, all of the other servers are contacted using a
       QUERY concerning one (or more) IP addresses, and they all report
       on whether that IP address(es) is UNBINDABLE or not.  This opera-
       tion fails if any server fails to respond to the QUERY or if any
       server responds to the QUERY with a negative answer (i.e., the IP
       address is not currently UNBINDABLE).  It succeeds only when all
       of the servers in the server group answer that the address is
       UNBINDABLE.

     o TRANSFER (message)

       This message is used to transfer BINDABLE IP addresses from one
       server to another (used when the SG is partitioned and the normal
       UNBINDABLE COMPLETE POLL cannot be used to make an IP address
       BINDABLE, but also when all of the UNBINDABLE IP addresses have
       already been made BINDABLE by some server).

       The information is sent from the initiating to the responding
       server as a QUERY and includes the subnet specification and the
       number of BINDABLE IP addresses the initiating server has avail-
       able for that address pool, and the number of BINDABLE IP
       addresses it is requesting.

       The responding server is free to give the initiating server all,
       some, or none of the number of IP addresses the initiating server
       has requested.


3.4.  IP Address State

   The concept of the state of an IP address is largely implicit in the
   DHCP RFC [1].  However, in order to manage pools of IP addresses with
   multiple servers, the states and transitions between them must be
   made quite explicit.


3.4.1.  IP Address State: Basic DHCP Protocol

   When an IP address is always controlled by a single DHCP server
   (implicit in the definition of DHCP in the current DHCP draft [1])
   the IP address is either in the BINDABLE state or the BOUND state.
   The following state diagram represents the states that an IP address
   may occupy based on the current DHCP draft.  (Note that these terms
   do not appear in [1], but are terms that describe concepts that are



Kinnear, Cole & Droms                                          [Page 16]


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   implicit in the RFC.)



                        +-----------------+
                        |                 |
                        |     BINDABLE    |<--+
                        |                 |   |
                        +-----------------+   |
                                 |            |
                                 V            |
                        +-----------------+   |
                        |                 |   |
                        |      BOUND      |---+
                        |                 |
                        +-----------------+

      Figure 3.4.1-1: Basic DHCP IP address state transition diagram



   When an IP address transitions from BINDABLE to BOUND, that transi-
   tion must be recorded in the server's stable storage prior to the
   transition being "published" to any observer outside of the server.


3.4.2.  IP Address State: Extensions to Support the Interserver Protocol

   The situation is more complex when multiple servers are managing the
   same set of IP addresses as required by this protocol.  Three new
   states are defined for an IP address: UNBINDABLE, POLLING, PUSHED and
   EXPIRED.

   This is the state diagram for IP address state required by this pro-
   tocol:
















Kinnear, Cole & Droms                                          [Page 17]


DRAFT                                                          July 1997


                        +-----------------+
                        |                 |
                        |    UNBINDABLE   |<--------+
                        |                 |         |
                        +-----------------+         |
                                 |                  |
                                 V                  |
                        +-----------------+         |
                        |                 |         |
                        |     POLLING     |-------->|
                        |                 |         |
                        +-----------------+         |
                                 |                  |
                                 V                  |
                        +-----------------+         |
                        |                 |         |
                        |    BINDABLE     |-------->|
                        |                 |         |
                        +-----------------+         |
                                 |                  |
                   -----------------------------    |
                                 V                  |
                        +-----------------+         |
                        |                 |         |
                    +-->|     BOUND       |-------->|
                    |   |                 |         |
                    |   +-----------------+         |
                    |      |     |                  |
                    |      |     V                  |
                    |      |  +-----------------+   |
                    |      |  |                 |   |
                    |      |  |     PUSHED      |-->|
                    |      |  |                 |   |
                    |      |  +-----------------+   |
                    |      |     |                  |
                    |      V     V                  |
                    |   +-----------------+         |
                    |   |                 |         |
                    +<--|    EXPIRED      |-------->+
                        |                 |
                        +-----------------+

     Figure 3.4.2-1:  Extended DHCP IP address state transition diagram
                      required for the Inter-server protocol.







Kinnear, Cole & Droms                                          [Page 18]


DRAFT                                                          July 1997


   For every server which cooperates using this protocol, an IP address
   is in one of the following six states:

     o UNBINDABLE

       This state represents the default state for every IP address.
       Explicit action must be taken to move an IP address from this
       state into the BINDABLE state.  An UNBINDABLE COMPLETE POLL must
       be performed and must complete successfully.

       Any IP address that has previously been BOUND must retain infor-
       mation concerning the server that PUSHED the binding information,
       the client to which it was bound, and the lease time for the
       binding.  This information is used when a server is removed from
       the server group.

     o POLLING

       While an UNBINDABLE COMPLETE POLL operation is being performed,
       an IP address is in the POLLING state.  This ensures that if two
       servers are simultaneously performing an UNBINDABLE COMPLETE POLL
       operation that involves the same address that neither of them
       will succeed in making that address BINDABLE.

     o BINDABLE

       In this state, the IP address is available to be offered to a
       DHCP client, and if the client accepts the offer, it may be bound
       to that client.

       An IP address is only BINDABLE by a single server at a time.  A
       server must know for precisely which IP addresses it has on its
       list of BINDABLE addresses.  A server does not know about any
       other server's list of BINDABLE addresses. (Although performance
       optimizations are possible where a server may develop hints about
       this information, they are not required).

       An IP address can move from the BINDABLE state into the BOUND
       state through the normal activity of the DHCP protocol where a
       server interacts with a client.  When this happens, the Client
       Binding Management portion of the protocol is used to inform
       other servers of the change.

       A server can also transfer ownership of a BINDABLE IP address to
       another server upon request from that other server (and without
       any interaction beyond that with the other server).





Kinnear, Cole & Droms                                          [Page 19]


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     o BOUND

       An address that is BOUND is associated with a particular DHCP
       client, and usually is in use by that client (although it may
       have abandoned the lease on that IP address).  It may be termed
       BOUND to that client.  In the BOUND state the information about
       the client binding has not been propagated to all of the other
       servers in the server group.

     o PUSHED

       An address that is PUSHED is associated with a client in the same
       was as a BOUND address.  However, an address in the PUSHED state
       indicates that all of the other servers in the server group have
       been informed of the existence of the binding to this client.

       When a DHCP client releases a lease on an IP address it moves
       from either the BOUND or PUSHED state into the UNBINDABLE state,
       but no explicit PUSH operation is required.

       When the lease time and any grace period implemented by a server
       both expire, then an IP address moves into the EXPIRED state.

       Note that only a server that actually completes a CLIENT BINDING
       COMPLETE PUSH will place its IP address into the PUSHED state.
       The servers who receive the CLIENT BINDING COMPLETE PUSH will
       place their IP addresses into the BOUND state.

       DISCUSSION:

            Many DHCP servers implement something called a "grace
            period", which is a period after the the lease on a binding
            expires that an IP address will not be offered to another
            DHCP client.  A lease which is in this "grace period" is
            still BOUND or PUSHED as far as the inter-server protocol is
            concerned.

     o EXPIRED

       An IP address is EXPIRED when it was BOUND and the term of the
       lease (and any implemented grace period) has run out.  It may be
       termed EXPIRED to that client.

       An EXPIRED IP address will transition to the UNBINDABLE state
       when the server who shows it as EXPIRED receives an UNBINDABLE
       COMPLETE POLL.  It will respond to the UNBINDABLE COMPLETE POLL
       after making the IP address UNBINDABLE.




Kinnear, Cole & Droms                                          [Page 20]


DRAFT                                                          July 1997


       It may be moved back into the BOUND state by an REQUEST/INIT-
       REBOOT request from the previously bound client.

   Note that an IP address can never go from BOUND to one client to
   BOUND to another client without first passing through the UNBINDABLE
   state.  The line across the middle of the state transition diagram
   helps to illustrate this.

   Further, note that the transition from POLLING to BINDABLE requires
   the successful completion of an UNBINDABLE COMPLETE POLL.


3.5.  Overview of Server Operation

   This section will give a brief sketch of the of the core elements of
   the Client Binding Management and Address Management parts of the
   protocol (from the perspective of an already configured group of
   servers).  Many of the possible cases are not described here, and
   this section is not to be considered definitive. The definitive
   description of this information is contained in Section 6 and in the
   case of conflicts with information found there, the information in
   Section 6 will govern.


3.5.1.  DISCOVER

   Prior to the receipt of a DISCOVER message, each server should have
   built up a list of BINDABLE IP addresses -- for two reasons.  First,
   because an UNBINDABLE COMPLETE POLL is required to move an IP address
   into the BINDABLE state, and an UNBINDABLE COMPLETE POLL may not be
   possible due to server failure at any given instant.  Second, because
   even if an UNBINDABLE COMPLETE POLL was possible it would generally
   take too long to do between a DISCOVER and an OFFER message.

   A server should offer a BINDABLE address to a client upon receipt of
   a DISCOVER message.

   There are no inter-server protocol activities required when a DIS-
   COVER is processed and an OFFER is returned to the client (assuming
   of course that a BINDABLE address was available to be offered).


3.5.2.  REQUEST/SELECTING

   When a client accepts an offer by sending a SELECTING message, then
   the server updates its stable storage with the binding information
   and ACKs the client.  It must then perform a CLIENT BINDING COMPLETE
   PUSH operation to push the binding information to all of the other



Kinnear, Cole & Droms                                          [Page 21]


DRAFT                                                          July 1997


   servers (to which it can communicate at that time).  There are some
   limitations on the lease time that can be offered to the client until
   at least one successful CLIENT BINDING COMPLETE PUSH has succeeded
   for the offering server.  See Section 4.4.1 for additional details.


3.5.3.  REQUEST/INIT-REBOOT

   In the usual case where the server who created the binding for the
   requesting client managed to PUSH that information to the other
   servers using a CLIENT BINDING COMPLETE PUSH, the receiving server
   will have the binding information for this client.  If this informa-
   tion can be verified, then ACK the client -- else NAK it.

   If the IP address was in the EXPIRED state, then move the IP address
   to the PUSHED state.


3.5.4.  REQUEST/RENEWING

   Upon receipt of a RENEWAL message (which is unicast from the client
   to the server), it is expected that the server will have accurate
   information concerning the binding of the client.  If it does not,
   process the message like a REBINDING, below.  Given that the server
   has information sufficient to extend the lease, it should update its
   stable storage with the lease extension, and then ACK the client with
   the extended time.  Then it must perform a CLIENT BINDING COMPLETE
   PUSH operation to the other servers with the updated binding informa-
   tion.


3.5.5.  REQUEST/REBINDING

   Upon receipt of a REBINDING message (which is broadcast from the
   client), the server will check to see if it has any information about
   the binding for this client.  There are several possible cases:

     1. Current information shows that this client owns the IP address.

        Extend the lease, update stable storage, ACK the client, and
        perform a CLIENT BINDING COMPLETE PUSH with the information to
        the other servers.

     2. Current information shows that some other client is BOUND to
        this IP address.

        This is a problem. Make the IP address UNAVAILABLE (see Section
        12 for details).



Kinnear, Cole & Droms                                          [Page 22]


DRAFT                                                          July 1997


     3. Current information says this IP address is UNBINDABLE.

        In this case, a server has probably created a binding and then
        failed to propagate the information to this server.   Perform a
        POLL operation to see if any communicating server has any better
        information.

        If information is returned, then move to the appropriate case in
        this list.

        If no information is returned, then extend the lease on the IP
        address, update stable storage, ACK the client, and PUSH the
        information to the other servers.


3.5.6.  RELEASE

   When a release is received, if the client matches the binding infor-
   mation in the server, then update stable storage with the release,
   set the IP address UNBINDABLE, and perform a CLIENT BINDING COMPLETE
   PUSH to inform other servers.

   If the CLIENT BINDING COMPLETE PUSH operation fails due to inability
   of an UPDATE message to succeed to another server, do nothing.


3.5.7.  Expiration

   When a lease on an IP address expires, move the lease to the EXPIRED
   state and update stable storage with this information.  From now on,
   if some server performs an UNBINDABLE COMPLETE POLL operation to
   gather information about this IP address, make the IP address UNBIND-
   ABLE, update stable storage, and respond with the state of the IP
   address as UNBINDABLE.


3.6.  When a server is down or partitioned and can't be contacted

   When a server is down or partitioned (i.e., can't be reached), then
   some aspects of the normal DHCP client processing are different.
   This section summarizes those differences:

     o Client lease times for new clients will never be greater than
       MAXIMUM_UNPUSHED_LEASE_TIME, since a CLIENT BINDING COMPLETE PUSH
       cannot succeed.

     o No UNBINDABLE COMPLETE PUSH will succeed, and thus no server will
       be able to transition an address from the UNBINDABLE state into



Kinnear, Cole & Droms                                          [Page 23]


DRAFT                                                          July 1997


       the BINDABLE state.  If a server runs low on addresses, it will
       have to use TRANSFER messages to acquire new addresses from other
       servers.

4.  Client Binding Management

   Client binding management is the aspect of the protocol which is con-
   cerned with communicating information about client bindings from one
   server to another.  It is the core of the inter-server protocol.

   The following messages and operations are used explicitly by a server
   participating in the interserver protocol when DHCP client requests
   and events require it, and are used implicitly by the SCSP cache
   alignment procedure whenever a server (re)establishes communication
   with another server.


4.1.  Client Binding Messages


     o CLIENT BINDING UPDATE

       Update a single server with client binding information.  This
       operation will not complete successfully unless and until that
       server is updated with the information being sent.

     o CLIENT BINDING QUERY

       Query a single server for its client binding information.

4.2.  Client Binding Operations

   The operations defined in for client binding management are:

     o CLIENT BINDING COMPLETE PUSH

       This operation involves one server using the UPDATE message to
       inform all of the other servers about updated client binding
       information.  While there is utility in reaching even one other
       server (in some cases) the operation is not deemed to have suc-
       ceeded unless all of the other servers were successfully updated
       with the new information.

     o CLIENT BINDING POLL

       This operation involves one server using the QUERY message to
       inquire of every other server about client binding information
       concerning a particular IP address.  If all of the other servers



Kinnear, Cole & Droms                                          [Page 24]


DRAFT                                                          July 1997


       are not operational, the requesting server will use any informa-
       tion it receives.

4.3.  Client Binding Information

   When binding data is sent as part of message concerned with client
   binding management it contains the following information:

     o IP Address

     o Expiration [expressed as a delta seconds from the current time]

     o Client ID

     o MAC Address [including the hardware type]

     o Last Transaction [selected from the list below]

     o Last Transaction Time [expressed as a delta seconds from the cur-
       rent time]

     o Last Transaction Server [an IP address]

   Each server must maintain as part of the binding information the
   "last transaction time", the "last transaction", and the "last trans-
   action server" associated with that binding.

   The last transaction time is the time at which the binding changed in
   response to a request (the last transaction) from the client.  The
   last transaction time is returned in an address information message
   as a number of seconds from "now".

   The possible last transactions are listed below.  This list is
   ordered by the precedence of the transactions and is used to help
   determine if a response to an address information message contains
   more recent information than that currently held by a server.

   The last transaction is one of the following:

     o DHCPREQUEST/SELECTING

     o DHCPREQUEST/REBINDING

     o DHCPREQUEST/INIT-REBOOT

     o DHCPREQUEST/RENEWING





Kinnear, Cole & Droms                                          [Page 25]


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     o DHCPRELEASE

     o EXPIRATION

   The IP address state information is transmitted as well, and it con-
   sists of one of the following states:


     o UNBINDABLE

     o POLLING

     o BINDABLE

     o BOUND

     o PUSHED

     o EXPIRED

4.4.  Initiating Client Binding Operations and Messages


4.4.1.  CLIENT BINDING COMPLETE PUSH

   The CLIENT BINDING COMPLETE PUSH operation is initiated whenever the
   state of a server's client binding cache is changed, typically by the
   receipt of a DHCP client request or expiration of a lease.

   The lease time that is offered to a DHCP client must not be greater
   than the MAXIMUM-UNPUSHED-LEASE-TIME for that SG until at least one
   CLIENT BINDING COMPLETE PUSH has succeeded for that client binding.
   Thus, as long as the state of the IP address is BOUND, then the
   client should be offered the MAXIMUM-UNPUSHED-LEASE-TIME.

   The lease time that is sent to the other servers in the CLIENT BIND-
   ING COMPLETE PUSH is the lease time that the server would like to
   give to the DHCP client, and once a CLIENT BINDING COMPLETE PUSH has
   succeeded with that lease time in it (and the IP address state is set
   to PUSHED), then the server is free to actually extend the client's
   lease on the IP address with that lease time.

   The servers which receive the CLIENT BINDING COMPLETE PUSH will place
   their IP addresses into the BOUND state, not the PUSHED state.







Kinnear, Cole & Droms                                          [Page 26]


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4.4.2.  CLIENT BINDING POLL

   The CLIENT BINDING POLL is used when the server has received a DHCP
   client request but believes that it has insufficient or out-of-date
   information concerning this client's binding.  Thus, the CLIENT BIND-
   ING POLL is an attempt to gather more recent and up-to-date informa-
   tion from the other servers in the SG.

     DISCUSSION:

          Is this really necessary?  Given that SCSP will "align" the
          caches of the servers at every reconnect, then what is the
          value of asking "again"?


4.4.3.  CLIENT BINDING UPDATE

   The CLIENT BINDING UPDATE is initiated in three ways.

   It is initiated at the client binding management level as the under-
   lying operation in a CLIENT BINDING COMPLETE PUSH.  It is initiated
   at the client binding management level when a server realizes that
   the server who returned information as a result of a CLIENT BINDING
   QUERY returned information which was less up-to-date than that avail-
   able to the current server.  It is initiated at the SCSP level as
   part of the cache state alignment process.


4.5.  Responding to Client Binding Messages

   When a server receives the following client binding messages, it
   should respond as detailed below.  Note that operations consist of
   multiple messages at the initiator, but that when processing incoming
   requests, only individual messages are evident.


4.5.1.  CLIENT BINDING QUERY

   The proper response to a CLIENT BINDING QUERY is to respond with the
   current information in the client binding cache.


4.5.2.  CLIENT BINDING UPDATE

   The proper response to a CLIENT BINDING UPDATE is to determine if the
   information received is more current than that available in the
   server's cache.  If it is not, then respond negatively to this
   request.  If it is, then update the client binding cache, ensure that



Kinnear, Cole & Droms                                          [Page 27]


DRAFT                                                          July 1997


   the changes have been written to stable storage, and respond success-
   fully.  Note that no CLIENT BINDING UPDATE should generate additional
   client binding message activity (i.e., the CLIENT BINDING UPDATE
   should not generate a CLIENT BINDING COMPLETE PUSH).

   When a CLIENT BINDING UPDATE is received, the IP address should be
   placed into the BOUND state, not the PUSHED state.  Only the actual
   server performing the CLIENT BINDING COMPLETE PUSH will place its IP
   address into the PUSHED state.


5.  Address Managment

   Address management is the aspect of the protocol concerned with man-
   aging the state of IP addresses that are not currently bound to any
   client.  It is a necessary part of the protocol in order to support
   certain goals in the client binding management part of the protocol,
   principally that of allowing a server to continue to operate even
   though it was partitioned from other servers in the server group.


5.1.  Address Management Operations


     o UNBINDABLE COMPLETE POLL

       In this operation, all of the other servers are contacted using a
       QUERY operation concerning one (or more) IP addresses, and they
       all report on whether that IP address(es) is UNBINDABLE or not.
       If they are UNBINDABLE, then the current information on that IP
       address is also reported (as in a CLIENT BINDING POLL). In con-
       trast to a CLIENT BINDING POLL, this operation fails if any
       server cannot be contacted or if any server answers the QUERY
       with a negative answer (i.e., the IP address is not currently
       UNBINDABLE).  It succeeds when all of the servers answer that the
       address is UNBINDABLE.

       There is a subtle interaction required with the group management
       layer of the protocol.   A successful UNBINDABLE COMPLETE POLL
       must be inhibited in certain cases where a server has been
       removed from a server group.

       The case is question is that where a server is removed from a
       server group by a different server.  Immediately after this hap-
       pens, all UNBINDABLE COMPLETE POLLS must fail for a period equal
       to the MAXIMUM-UNPUSHED-LEASE-TIME.  After that time passes, then
       UNBINDABLE COMPLETE POLLS may operate as they normally do.




Kinnear, Cole & Droms                                          [Page 28]


DRAFT                                                          July 1997


       DISCUSSION:

          This covers the situation where a server gives a lease to a
          while both the client and server are partitioned.  Then, the
          server goes away completely.  The client stays up, but remains
          partitioned.  Then, the dead server is removed by another
          server from the server group.  At this point, UNBINDABLE COM-
          PLETE POLL operations could (except for the above restriction)
          begin to complete successfully.  However, the client that was
          given a lease while partitioned along with the server that
          died certainly has an address, and when the partition is
          removed (just after the UNBINDABLE COMPLETE POLL operation
          which declared its IP address now BINDABLE for some server),
          there would be a very dangerous situation developing.

       The solution is to only offer leases to clients of the MAXIMUM-
       UNPUSHED-LEASE-TIME until the information concerning their client
       binding reaches all of the other servers in the group.  Once that
       happens, then they can be offered the normal lease time.

       Thus, whenever any server is removed from the group (where it
       doesn't remove itself), then there is a possibility that it may
       have offered leases to clients about which no other server would
       have any record.  In this case, the remaining servers must wait
       the MAXIMUM-UNPUSHED-LEASE-TIME before being able to complete an
       UNBINDABLE COMPLETE POLL and reuse the BINDABLE addresses that
       the removed server was using.


5.2.  Address Management Messages

   The following messages are part of the address management portion of
   the protocol.

     o TRANSFER

       This message is used to transfer BINDABLE IP addresses from one
       server to another (especially when the SG is partitioned and the
       normal UNBINDABLE COMPLETE POLL cannot be used to make an IP
       address BINDABLE, but also when all of the UNBINDABLE IP
       addresses have already been made BINDABLE by some server).

       The information sent from the initiating to the responding server
       includes the subnet specification and the number of BINDABLE IP
       addresses the initiating server has available for that address
       pool, and the number of BINDABLE IP addresses it is requesting.





Kinnear, Cole & Droms                                          [Page 29]


DRAFT                                                          July 1997


       The responding server is free to give the initiating server all,
       some, or none of the number of IP addresses the initiating server
       has requested.

     o UNBINDABLE QUERY

       The UNBINDABLE QUERY operation is the primitive query from which
       the UNBINDABLE COMPLETE POLL is constructed.  It is identical to
       the CLIENT BINDING QUERY defined above in terms of the data
       returned, although the actions taken when it is received are
       slightly different.

5.3.  Initiating Address Management Operations and Messages


     o UNBINDABLE COMPLETE POLL (operation)

       This operation is initiated when the server detects that it needs
       to generate more BINDABLE IP addresses.  It will initiate this
       operation whenever the number of BINDABLE IP addresses drops
       below a configurable threshold.

       Prior to initiating this operation, the server must change the
       state for each IP address that will be part of the UNBINDABLE
       COMPLETE POLL from UNBINDABLE to POLLING, and commit this state
       change to stable storage.

       DISCUSSION:

          Is the commit to stable storage really necessary? Given that
          we will abandon the POLL if we reboot (presumably), what is
          the value of remembering that we were doing it?

       For every IP address for which the UNBINDABLE COMPLETE POLL oper-
       ation fails (i.e., some server responds in such a way that indi-
       cates that the IP address is not UNBINDABLE, or some server fails
       to respond at all), the IP address' state should be reset to
       UNBINDABLE.

     o TRANSFER (message)

       The TRANSFER message, which attempts to transfer some IP
       addresses from some other server to the initiating server, is
       initiated whenever the number of BINDABLE IP addresses in an
       address pool falls below a configurable threshold.






Kinnear, Cole & Droms                                          [Page 30]


DRAFT                                                          July 1997


5.4.  Responding to Address Management Messages


     o TRANSFER

       When receiving a TRANSFER message, the responding server inspects
       its list of BINDABLE addresses for the address pool to which the
       TRANSFER operation refers.  It will attempt to offer the initiat-
       ing server as many addresses as it requested, with the limitation
       that it will never give away more than half of its pool of BIND-
       ABLE addresses in any one request.

     o UNBINDABLE QUERY

       The responding server will respond to this query just like it
       responds to a CLIENT BINDING QUERY as far as the information com-
       municated to the initiating server is concerned.

       In addition, if the IP address mentioned in this query was in the
       EXPIRED state, prior to responding to this message, the respond-
       ing server will move that IP address to the UNBINDABLE state,
       commit this change to stable storage, and then respond with
       information that indicates the IP address in question was UNBIND-
       ABLE.

       Note that an UNBINDABLE QUERY will not be generated to any server
       if at least one server in the SG is currently not able to be con-
       tacted, as known by the SCSP "Hello" subprotocol.  This will pre-
       vent unnecessary transitions from the EXPIRED to the UNBINDABLE
       state when an UNBINDABLE COMPLETE POLL would not be able to com-
       plete in any case.


6.  Actions in Response to DHCP Client Messages and Events

   This section defines the actions that should be taken in the client
   binding and address management portions of the protocol when incoming
   DHCP requests (messages) are received.

   DISCUSSION:

      There is considerable commonality in the sections that describe
      the various DHCP client messages below.  Once the details have
      stabilized, it should be possible to compress the explanations.







Kinnear, Cole & Droms                                          [Page 31]


DRAFT                                                          July 1997


6.1.  DISCOVER

   Prior to the receipt of a DISCOVER message, each server should have
   built of a list of BINDABLE IP addresses -- for two reasons.  First,
   because a CLIENT BINDING COMPLETE POLL is required to get a BINDABLE
   IP address, and a CLIENT BINDING COMPLETE POLL may not be possible
   due to server failure at any given instant.  Second, because even if
   a CLIENT BINDING COMPLETE POLL were possible, it would be unwise to
   require such an operation between a receipt of a DISCOVER message and
   the response of an OFFER to a client.

   There are several cases involved in processing a DISCOVER request,
   depending on the state of the requested IP address in the DISCOVER
   request:

     o No specific IP address requested.

       Offer a BINDABLE address to the client.  Record that this address
       was offered in the cache memory of the server, but there is no
       need to update the stable storage of the server with any informa-
       tion.  The IP address continues to be BINDABLE as far as the
       inter-server protocol is concerned.

     o Requested IP address is UNBINDABLE.

       If the IP address is UNBINDABLE, then perform a UNBINDABLE COM-
       PLETE POLL operation in an attempt to make the IP address BIND-
       ABLE.  If the operation is successful, then respond as though the
       IP address were BINDABLE, below.  If the results of the attempt
       to make the IP address BINDABLE resulted in a discovery that the
       IP address is now BOUND or PUSHED, then respond as for BOUND our
       PUSHED, below.  Otherwise (i.e., the IP address is BINDABLE for
       some other server, or no an UNBINDABLE COMPLETE POLL was not pos-
       sible) then respond as above for "No specific IP address
       requested".

     o Requested IP address is BINDABLE.

       Offer the IP address to the client.  IP address remains BINDABLE.

     o Requested IP address is BOUND or EXPIRED.

       If the IP address is BOUND or EXPIRED to the requesting client,
       then set it to BOUND and offer it to the client -- with a lease
       time of MAXIMUM-UNPUSHED-LEASE-TIME.  Otherwise (i.e., the IP
       address is BOUND or EXPIRED to some other client), respond as in
       "No specific IP address requested", above.




Kinnear, Cole & Droms                                          [Page 32]


DRAFT                                                          July 1997


     o Requested IP address is PUSHED.

       If the IP address is PUSHED to the requesting client, then offer
       it to the client -- with a normal lease time.  Otherwise (i.e.,
       the IP address is PUSHED to some other client), respond as in "No
       specific IP address requested", above.


6.2.  REQUEST/SELECTING

   The client uses a REQUEST/SELECTING to accept the offer of a lease
   made by a server.  When a server receives such a message, and where
   the server-id option reflects the IP address of that server, then if
   the IP address is in the following states the server should respond
   in the following way:

     o UNBINDABLE

       If the IP address is UNBINDABLE, then perform a UNBINDABLE COM-
       PLETE POLL operation in an attempt to make the IP address BIND-
       ABLE.  If that operation is successful, then respond as though
       the IP address were BINDABLE, below.  If the results of the
       attempt to make the IP address BINDABLE resulted in a discovery
       that the IP address is now BOUND, then respond as for BOUND,
       below.  Otherwise (i.e., the IP address is BINDABLE for some
       other server, or no a complete POLL was not possible) NAK the
       REQUEST.

     o BINDABLE

       If the IP address is BINDABLE and has been offered to the
       requester, then bind the IP address to the client, set the IP
       address BOUND, and update stable storage.  Then, ACK the client,
       and finally perform a PUSH operation of the binding information
       to the other servers.

     o BOUND or EXPIRED

       If the IP address is BOUND or EXPIRED to the requesting client,
       then set the state to BOUND, update the expiration time using the
       normal lease time, update stable storage, ACK the client with the
       MAXIMUM-UNPUSHED-LEASE-TIME, and perform a CLIENT BINDING COM-
       PLETE PUSH with the normal lease time.

       If the IP address is BOUND or EXPIRED to a different client, then
       NAK this REQUEST.





Kinnear, Cole & Droms                                          [Page 33]


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     o PUSHED

       If the IP address is PUSHED to the requesting client, set the IP
       address to be PUSHED, update the expiration time, update stable
       storage, and ACK the client.  Finally, perform a CLIENT BINDING
       COMPLETE PUSH operation of the updated binding information to the
       other servers.

       Use the normal lease time in all of the above operations.

       If the IP address is PUSHED to some other client, then NAK the
       request.


6.3.  REQUEST/INIT-REBOOT

   The client uses a REQUEST/INIT-REBOOT to query the server (as part of
   the client boot process) to determine if a "remembered" binding is
   still valid.  If the requested IP address will be in one of the fol-
   lowing states:

     o UNBINDABLE

       If the IP address is UNBINDABLE, then perform a UNBINDABLE COM-
       PLETE POLL operation in an attempt to make the IP address BIND-
       ABLE.  If the operation is successful, then respond as though the
       IP address were BINDABLE, below.  If the results of the attempt
       to make the IP address BINDABLE resulted in a discovery that the
       IP address is now BOUND, then respond as for BOUND, below.  Oth-
       erwise (i.e., the IP address is BINDABLE for some other server,
       or a complete POLL was not possible) NAK the REQUEST.

       DISCUSSION:

          This means that if a server creates a binding for a client and
          fails to PUSH the information to any other server prior to
          undergoing a server failure, and if the client is powered off
          prior to the time when it will issue a REBINDING message, it
          will not get back the same lease when it is powered back on.
          The reasoning for this (and the difference from the REBINDING
          case below) is that in this case the server has no way to
          determine if the requested address in the INIT-REBOOT request
          is current or perhaps very old indeed.  In the REBINDING case
          the client is currently using the address, so the client at
          least believes that it is current and not in use by some other
          client.  In this case, however, no such assumption is possi-
          ble.




Kinnear, Cole & Droms                                          [Page 34]


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       In the case where a server which creates a binding fails prior to
       PUSHing the information about a lease to some other server, and
       the client which receives that binding makes a REBINDING request
       prior to either failing or being shutdown, it will get back the
       existing binding upon restart and INIT-REBOOT -- since the
       REBINDING will have caused a recovery of the binding information
       and that will have been distributed through a CLIENT BINDING COM-
       PLETE PUSH.

     o BINDABLE

       If the IP address is BINDABLE, then bind the IP address to the
       client, set the IP address BOUND, and update stable storage.
       Then, ACK the client, and finally perform a PUSH operation of the
       binding information to the other servers.

     o BOUND or EXPIRED

       If the IP address is BOUND or EXPIRED to the requesting client,
       then set the state to BOUND, update the expiration time using the
       normal lease time, update stable storage, ACK the client with the
       MAXIMUM-UNPUSHED-LEASE-TIME, and perform a CLIENT BINDING COM-
       PLETE PUSH with the normal lease time.

       If the IP address is BOUND or EXPIRED to a different client, then
       NAK this REQUEST.

     o PUSHED

       If the IP address is PUSHED to the requesting client then set the
       IP address PUSHED, update the expiration time, update stable
       storage, and ACK the client.  Finally, perform a CLIENT BINDING
       COMPLETE PUSH operation of the updated binding information to the
       other servers.  Use the normal lease time for all of the above
       operations.

       If the IP address is PUSHED to some other client, then NAK the
       request.


6.4.  REQUEST/RENEWING

   Upon receipt of a RENEWAL message (which is unicast from the client
   to the server), it is expected that the server will have accurate
   information concerning the binding of the client since this is the
   server that the client believes most recently sent an ACK to the
   client concerning this IP address binding.




Kinnear, Cole & Droms                                          [Page 35]


DRAFT                                                          July 1997


   Perform the following actions if the IP address being renewed (i.e.,
   the IP address in ciaddr) is in one of these states:

     o UNBINDABLE

       If the IP address is UNBINDABLE, then perform an UNBINDABLE COM-
       PLETE POLL operation in an attempt to make the IP address BIND-
       ABLE.  If the operation is successful, then respond as though the
       IP address were BINDABLE, below.  If the results of the attempt
       to make the IP address BINDABLE resulted in a discovery that the
       IP address is now BOUND, then respond as for BOUND, below.

       If the IP address is determined to be BINDABLE for some other
       server, then NAK the request, and set the IP address to be
       UNAVAILABLE since this likely represents a duplicate allocation
       of an IP address (see Section 11, Open Questions, for details).

       Otherwise NAK the request.

     o BINDABLE

       If the IP address is BINDABLE, then bind the IP address to the
       client, set the IP address BOUND, and update stable storage.
       Then, ACK the client, and finally perform a PUSH operation of the
       binding information to the other servers.

     o BOUND or EXPIRED

       If the IP address is BOUND or EXPIRED to the requesting client,
       then set the state to BOUND, update the expiration time using the
       normal lease time, update stable storage, ACK the client with the
       MAXIMUM-UNPUSHED-LEASE-TIME, and perform a CLIENT BINDING COM-
       PLETE PUSH with the normal lease time.

       If the IP address is BOUND or EXPIRED to a different client, then
       NAK this REQUEST.

     o PUSHED

       If the IP address is PUSHED to the requesting client then set the
       IP address PUSHED, update the expiration time, update stable
       storage, and ACK the client.  Finally, perform a CLIENT BINDING
       COMPLETE PUSH operation of the updated binding information to the
       other servers.  Use the normal lease time for all of the above
       operations.

       If the IP address is PUSHED to some other client, then NAK the
       request and set the IP address to UNAVAILABLE.  (see Section 11,



Kinnear, Cole & Droms                                          [Page 36]


DRAFT                                                          July 1997


       Open Questions, for details).


6.5.  REQUEST/REBINDING

   Upon receipt of a REBINDING message (which is broadcast from the
   client), the server will check to the state of the address requested
   for rebinding (i.e., the ciaddr).  There are several cases possible:

     o UNBINDABLE

       If the IP address is UNBINDABLE, then perform an UNBINDABLE COM-
       PLETE POLL operation in an attempt to make the IP address BIND-
       ABLE.  If the operation is successful, then respond as though the
       IP address were BINDABLE, below.  If the results of the attempt
       to make the IP address BINDABLE resulted in a discovery that the
       IP address is now BOUND, then respond as for BOUND, below.

       If the IP address is determined to be BINDABLE for some other
       server, then NAK the request.  Set the IP address to be UNAVAIL-
       ABLE since this likely represents a duplicate allocation of an IP
       address (see Section 11, Open Questions, for details).

       If no information is returned from any server that this IP
       address is anything but UNBINDABLE, then consider the address
       BOUND to this client, and proceed as in BOUND below.

       DISCUSSION:

          This is one of the key points of the inter-server protocol.
          In this case, a server has created a binding and then failed
          prior to telling any other server about that binding.  Eventu-
          ally, the client to whom that binding was made will attempt a
          REQUEST/REBINDING and contact a different server.  That dif-
          ferent server will be able to determine nothing about that IP
          address.  As far as can be determined, it is not BOUND to any
          client, and it is not BINDABLE for any other server.  In this
          restricted case, the server will renew the lease for the
          client and move the IP address into the BOUND state -- and
          PUSH this information to the rest of the servers.

          How can this be safe?  Well, remember that the client is
          presently using the IP address to make this request.  In this
          limited case where a server crashes before PUSHing information
          about a BOUND IP address to any other server, the client to
          whom the IP address is BOUND is the only running machine with
          any record of that binding.  In this case, the DHCP servers
          will accept that client's information about the binding as



Kinnear, Cole & Droms                                          [Page 37]


DRAFT                                                          July 1997


          correct.

     o BINDABLE

       If the IP address is BINDABLE, then bind the IP address to the
       client, set the IP address BOUND, and update stable storage.
       Then, ACK the client, and finally perform a PUSH operation of the
       binding information to the other servers.

     o BOUND or EXPIRED

       If the IP address is BOUND or EXPIRED to the requesting client,
       then set the state to BOUND, update the expiration time using the
       normal lease time, update stable storage, ACK the client with the
       MAXIMUM-UNPUSHED-LEASE-TIME, and perform a CLIENT BINDING COM-
       PLETE PUSH with the normal lease time.

       If the IP address is BOUND or EXPIRED to a different client, then
       NAK this REQUEST.

     o PUSHED

       If the IP address is PUSHED to the requesting client then set the
       IP address PUSHED, update the expiration time, update stable
       storage, and ACK the client.  Finally, perform a CLIENT BINDING
       COMPLETE PUSH operation of the updated binding information to the
       other servers.  Use the normal lease time for all of the above
       operations.

       If the IP address is PUSHED to some other client, then NAK the
       request and set the IP address to UNAVAILABLE.  (see Section 11,
       Open Questions, for details).

6.6.  RELEASE

   When a RELEASE is received, an IP address will be in one of the fol-
   lowing states:

     o UNBINDABLE

       If the IP address is UNBINDABLE, then perform a CLIENT BINDING
       POLL operation in an attempt to determine if this IP address is
       BOUND to any client.

       If the results of the POLL operation indicate that the IP address
       is now BOUND, then respond as for BOUND, below.





Kinnear, Cole & Droms                                          [Page 38]


DRAFT                                                          July 1997


       If the IP address is determined to be BINDABLE for some other
       server, then NAK the request.  Set the IP address to be UNAVAIL-
       ABLE since this likely represents a duplicate allocation of an IP
       address (see Section 11, Open Questions, for details).

       Otherwise, ignore the RELEASE.

     o BINDABLE

       If the IP address is BINDABLE, ignore the RELEASE.

     o BOUND, PUSHED, or EXPIRED

       If the IP address is BOUND, PUSHED, or EXPIRED to the requesting
       client set the IP address to be UNBINDABLE, update stable stor-
       age, and perform a CLIENT BINDING COMPLETE PUSH to update the
       other servers with this information.


6.7.  Lease Period Expiration

   When the lease period on a BOUND or PUSHED IP address expires, set
   the IP address to be EXPIRED and update stable storage.


7.  Group Management

   The group management part of the protocol is concerned with configur-
   ing a server into or out of a server group (SG).  It allows discovery
   of information concerning the configuration of an existing server
   group as well as the address pools that are managed by a server
   group.  While it is possible to conceive of a statically defined
   server group, the operational characteristics (both for group startup
   as well as removal of a server from a group) are quite painful.

   Group management messages are used add a server to a group as well as
   to remove a server from a group.  A server must add itself to a group
   -- it cannot be added by another server.  A server may be removed by
   any server in the group, including itself.

   In addition to changing the group membership, group management mes-
   sages are used to keep the various servers up to date with respect to
   the current membership of the group.

   Once a server successfully become part of a group using the group
   management messages, it the goes into the SCSP protocol.  This proto-
   col determines which servers in the SG are currently in communication
   with this server, and starts an automatch "cache alignment" process



Kinnear, Cole & Droms                                          [Page 39]


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   with each connected server.


7.1.  Group Management Operations


     o SG CHANGE

       The SG CHANGE operation is a two-stage operation made up of a
       propose and then a commit phase.  It uses the SG PROPOSE CHANGE
       and SG COMMIT CHANGE messages as part of this operation.  It is
       used to change the membership of the group, either to add a
       server or to remove a server.

7.2.  Group Management Messages

     o SG DISCOVERY QUERY

       The first stage of becoming a server participating in the inter-
       server protocol is to determine the existing SG ID for each SG
       for which participation in the inter-server protocol is desired.

       Assuming that a server has been provided or can discover the IP
       address of a server maybe in a group to which it wants to join, a
       server who wants to become a member of a group will send a SG
       DISCOVERY QUERY message to that server.

       The reply to the SG DISCOVERY QUERY message is a message which
       contains the list of SG identifiers for all of the groups to
       which the replying server belongs.  These SG ids can then be used
       in SG CONFIGURATION messages to determine more information about
       each SG.

       This operation is performed only upon one server at a time, since
       at this point there is no notion of a "current" server group.

     o SG CONFIGURATION QUERY

       The SG CONFIGURATION QUERY operation has several suboperations,
       corresponding to the following types of configuration informa-
       tion:  subnets, IP addresses, client configuration information,
       and vendor specific information.

       Each SG CONFIGURATION QUERY operation is read-only to the receiv-
       ing server.  The particular SG CONFIGURATION QUERY suboperations
       are:





Kinnear, Cole & Droms                                          [Page 40]


DRAFT                                                          July 1997


       o Subnets

         The specific subnets managed by this SG are returned in this as
         part of this operation.

       o IP Addresses

         The IP addresses which are managed by this SG within this sub-
         net are return as the result of this operation.

       o Client Configuration Information

         The client configuration information associated with this sub-
         net is returned as the result of this operation.

       o Vendor Specific Information

         Provision is made for vendor specific configuration information
         to be returned in the SG CONFIGURATION message.  Its format is
         TBD, but should be regular even though vendor specific.

     o SG PROPOSE CHANGE UPDATE

       The SG PROPOSE CHANGE UPDATE message is sent to all of the
       servers in a SG to propose a new membership in the server group.
       The information sent with this message is an updated list of the
       servers in the group.  The servers to add to the group and
       servers to remove from the group are both listed in the same mes-
       sage.

     o SG COMMIT CHANGE UPDATE

       The SG COMMIT CHANGE UPDATE message is sent to all of the servers
       in the SG to commit a change the was proposed in a SG PROPOSE
       CHANGE operation.


7.3.  Initiating Group Management Operations and Messages


7.3.1.  SG CHANGE (operation)

   The SG CHANGE operation consists of the the following steps:

     o Determine the group membership using an SG CONFIGURATION message.

       Find out to whom to send all of the SG CHANGE messages.




Kinnear, Cole & Droms                                          [Page 41]


DRAFT                                                          July 1997


     o Send a SG PROPOSE CHANGE message to every member of the SG.

       This message has the current group specifier in the message,
       along with the new group membership.  As the joining server
       cycles through the existing members of the group, it will be
       rationalizing the group specifiers among the group and the entire
       group's picture of the membership of the group.  If it encounters
       a server whose view of the group membership lags behind that of
       the server from which the joining server received its idea of
       group membership, then it will bring that server up to date.

       If, on the other hand, it encounters a server that has a more up
       to date version of the group membership than the one from which
       it is operating, it will have to update its idea of the group
       membership and then start the proposal sequence over.  All of the
       servers with which it has created proposals will be forced to
       update their view of group membership as part of this process.

       At the end of this process of proposal generation, all of the
       servers in the group share a common picture of both the group
       membership as well as the current proposal.

     o Reverify the group membership from at lease one server using an
       SG CONFIGURATION message.

       This is to ensure that all of the members of the group have actu-
       ally been sent a SG PROPOSE CHANGE message.

     o Check the proposal timer.

       The initiating server must have started a timer when it sent out
       the first SG PROPOSE CHANGE message, and if that timer has less
       than time/2 time left on it, the joining server SHOULD start the
       process over.

     o Send a SG COMMIT CHANGE message to every member of the SG.

       As soon as this completes successfully with one server, the
       server has changed the membership of the group, but the initiat-
       ing server MUST continue to try to update the other servers as
       long as they remain in the server group.


7.3.2.  SG DISCOVERY QUERY (message)

   This is sent when a server wishes to know the groups to which another
   server is a member.  It is used primarily when starting up a server
   in the initial discovery of the server group configuration.



Kinnear, Cole & Droms                                          [Page 42]


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7.3.3.  SG CONFIGURATION QUERY (message)

   This message is sent to determine the details of the configuration of
   the server group.  A server would typically initiate these messages
   as part of the process of confirming that it wished to be part of a
   particular server group.

   The SG CONFIGURATION QUERY operation has several suboperations, cor-
   responding to the following types of configuration information:

     o Subnets

       The specific subnets managed by this SG are returned in this as
       part of this operation.

     o IP Addresses

       The IP addresses which are managed by this SG within this subnet
       are return as the result of this operation.

     o Client Configuration Information

       The client configuration information associated with this subnet
       is returned as the result of this operation.

     o Vendor Specific Information

       Provision is made for vendor specific configuration information
       to be returned in the SG CONFIGURATION QUERY message.  Its format
       is TBD, but should be regular even though vendor specific.


7.4.  Responding to Group Management Messages


7.4.1.  SG PROPOSE CHANGE UPDATE


   Upon receipt of a SG PROPOSE CHANGE UPDATE message, if no existing
   proposal exists that has not timed out, a server will create a single
   "proposed" group specifier from the current group specifier by incre-
   menting the group sequence number by 1.   The creation of this pro-
   posed group specifier will inhibit the creation of another proposed
   group specifier for a 30 seconds.

   If an existing proposal exists that has not timed out, the responding
   will respond negatively to the SG PROPOSE CHANGE UPDATE message.




Kinnear, Cole & Droms                                          [Page 43]


DRAFT                                                          July 1997


   DISCUSSION:

      Clearly a deadlock situation can occur where two servers are try-
      ing to join a group at the same time, and each is working from
      "opposite ends" of the group.  In this case, where the joining
      server gets a failure from a SG PROPOSE CHANGE UPDATE message due
      to the existence of a valid proposal that has not timed out, then
      the joining server should backoff an amount of time that is based
      in part on its IP address before trying again.  The exact algo-
      rithm is TBD.

   This proposed group specifier will not be used in any messages until
   it moves to the accepted stage and become the current group specifier
   (see below for how it does that).

   If a second SG PROPOSE CHANGE UPDATE request is received from a
   server, that message will supersede the existing proposal and the
   timer will be reset.

   DISCUSSION

      Is there some possible attack here?  Should we limit one servers
      proposals from tying up the "proposal" for more than 3 minutes at
      a time, for instance?


7.4.2.  SG COMMIT CHANGE UPDATE

   Upon receipt of a SG COMMIT CHANGE UPDATE message, the current pro-
   posal is compared with the data in the SG COMMIT CHANGE UPDATE mes-
   sage, and if it compares successfully, the proposed new group becomes
   the current group and the group specifier is changed.

   Once a SG COMMIT CHANGE UPDATE message is received, the receiving
   server MUST examine all of its IP addresses.  For every IP address
   for which the "last transaction server" is a server which was previ-
   ously in the group and is now not in the group, the following action
   should be taken:

   If the IP address is shown as ever having been BOUND to a client, and
   if that client does not now have a different IP address, then the IP
   address should be set to BOUND to that client, the lease time should
   be restarted for the previously recorded lease time.

   DISCUSSION:

      This is a key aspect of the protocol in terms of safely removing
      possibly partitioned servers from the group.  The specific case



Kinnear, Cole & Droms                                          [Page 44]


DRAFT                                                          July 1997


      that this protects against is as follows.

      If a connected server creates a client binding, and successfully
      performs a CLIENT BINDING COMPLETE PUSH operation, and then renews
      its client's lease for the full lease time -- and then becomes
      partitioned, there can be problems if that server is ultimately
      removed from the group much later.  If the server is partitioned
      for longer than the client's lease time, and if all of the other
      servers move this IP address to EXPIRED, and if then some server
      tries (unsuccessfully) to perform an UNBINDABLE COMPLETE POLL --
      which will move the EXPIRED addresses to UNBINDABLE.  Now, the
      partitioned server has updated the client several times, and the
      other servers by this time all believe that the IP address is
      UNBINDABLE.  If the partitioned server then fails and is removed
      from the SG -- the other servers could (in the absence of the
      above algorithm) believe that they only need wait the MAXIMUM-
      UNPUSHED-LEASE-TIME before then can make those UNBINDABLE
      addresses BINDABLE.  But in this case that would cause a failure.
      Thus, when a server is removed from a SG, each remaining server
      must look around for any IP addresses that it previously PUSHED,
      and set them up with their previous maximum lease time in order to
      catch this case.

7.4.3.  SG DISCOVERY QUERY

   The server groups to which the current server belongs are returned as
   the response to an SG DISCOVERY QUERY message.


7.4.4.  SG CONFIGURATION QUERY

   The SG CONFIGURATION QUERY operation has several suboperations, cor-
   responding to the following types of configuration information:

     o Subnets

       The specific subnets managed by this SG are returned in this as
       part of this operation.

     o IP Addresses

       The IP addresses which are managed by this SG within this subnet
       are return as the result of this operation.

     o Client Configuration Information

       The client configuration information associated with this subnet
       is returned as the result of this operation.



Kinnear, Cole & Droms                                          [Page 45]


DRAFT                                                          July 1997


     o Vendor Specific Information

       Provision is made for vendor specific configuration information
       to be returned in the SG CONFIGURATION QUERY message.  Its format
       is TBD, but should be regular even though vendor specific.


8.  SCSP Message Mapping

   This section develops the SCSP capabilities supporting the DHCP
   interserver protocol.  The Server Cache Synchronization Protocol
   (SCSP) is found in [1].  The organization of this section is 1) we
   present a brief overview of SCSP (and refer to appendices for a more
   detailed discussion), 2) we discuss the mapping of the DHCP inter-
   server protocol onto SCSP and how the various DCHP interserver mes-
   sages are mapped into SCSP messages, 3) we identify the modifications
   to the SCSP protocol as identified in [1] necessary for the mapping
   of the DHCP interserver protocol onto SCSP, 4) we present the spe-
   cific formats of the DHCP protocol specific SCSP records and 5) we
   present a list of the open issues with respect to the mapping onto
   SCSP.

8.1.  SCSP Overview

   The Server Cache Synchronization Protocol (SCSP) is a protocol which
   provides the generic functions necessary to provide loose synchro-
   nization between a set of distributed databases.  The protocol, which
   is presented in [2], was developed to specifically address to issues
   associated with synchronizing the caches of redundant servers which
   provide the server functionality of a specific client-server proto-
   col.  SCSP was built based upon the extensive experience in develop-
   ing and running link state routing protocols such as OSPF [3].
   Client server protocols for which a redundant server capability is
   being developed using SCSP are NHRP [4] and ATM ARP [5].  Here we
   present the use of SCSP to synchronize servers supporting the DHCPv4
   client-server protocol.

   The SCSP protocol consist of three separate sub-protocols, i.e.,

     o The "Hello" protocol:  this protocol defines and maintains the
       status of the inter-server connection,

     o The "Cache Alignment" protocol: this protocol defines the cache
       synchronization capability for new servers and servers that, for
       whatever reason, have lost synchronization, and

     o The "Client State Update" protocol:  this protocol provides the
       ongoing server cache synchronization through asynchronous client



Kinnear, Cole & Droms                                          [Page 46]


DRAFT                                                          July 1997


       state updates.

   These sub-protocols define the semantics and high-level syntax of
   generic message sets and their exchanges in support of the capabili-
   ties provided.  The SCSP associates replica databases into Server
   Groups (SG).  The SCSP supports both point-to-point and point-to-
   multipoint connections between the local servers (LS) and the
   directly connected servers DCS(es).   We discuss each of these sub-
   protocols in more detail in the appendices below.

   SCSP defines five message types in the operation of the above subpro-
   tocols:

     o Hello

     o Cache Alignment (CA)

     o Cache State Update (CSU) Solicit (CSU_Sol)

     o CSU Request (CSU_Req)

     o CSU Reply (CSU_Rep).

   The Hello and the CA messages are used within the Hello and the Cache
   Alignment subprotocol respectively.  The CSU_Sol, CSU_Req and CSU_Rep
   messages are used to distribute cache records between the distributed
   servers of a server group.  Full records are called Client State
   Advertisement (CSA) records.  Summary records, which are essentially
   pointers to the full records, are called Client State Advertisement
   Summary (CSAS) records.

   For a server to request a particular record, it can send a CSU_Sol
   message containing the CSAS to indicate the full record of interest.
   A server which receives a CSU_Sol is required to respond with a
   CSU_Req message containing the full CSA record associated with the
   CSAS of the CSU_Sol.  The soliciting server follows the receipt of
   the CSU_Req with a CSU_Rep to acknowledge receipt.  A server which
   wishes to communicate a full record to the rest of the SG would
   transmit a CSU_Req message containing the full CSA record.  This is
   acknowledged with a CSU_Rep message.

   DISCUSSION

      In some cases the CSU_Sol, CSU_Req, CSU_Rep sequence is overkill
      when one wants to perform a simple query operation.  See the dis-
      cussion at the end of Section 8.3 for more details.

   For now we accept that these capabilities are generically provided



Kinnear, Cole & Droms                                          [Page 47]


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   discuss the DHCPv4 interserver protocol specific overlay on SCSP.

8.2.  Mapping DHCP interserver onto SCSP

   This section presents the relationship of SCSP to the DHCP inter-
   server protocol, the assumptions made in developing this relationship
   and the specific mappings of DHCP interserver messages into SCSP.

   The assumptions made in defining the DHCP client/server protocol map-
   ping onto SCSP are the following:

     o On the Issue of Protocol Encapsulation:

       The assumption is that the SCSP messages, and in fact all inter-
       server messages, are to be defined over UDP.  Currently the SCSP
       messages within [2] are LLC/SNAP encapsulated.

     o On the Interserver over SCSP Layering Model:

       The interserver group management protocol will initialize a
       server into the group upon initial join, re-booting or re-
       connecting.  Once this is complete the interserver group manage-
       ment protocol will initialize the SCSP protocol to handle the
       ongoing operation of the interserver cache alignment and address
       management functions.

     o On the DHCP Interserver Sub-Protocols:

       The current thinking goes as follows.  The draft specification
       defines three DHCP interserver sub-protocols, i.e., the 'Client
       Binding Management' protocol (see Section 4), the 'Address Man-
       agement' protocol (see Section 5), and the 'Group Management'
       protocol (see Section 7).  The 'Client Binding Management' sub-
       protocol addresses the core of the interserver protocol in that
       it distributes and maintains the client binding records over the
       distributed SG.  This sub-protocol is to be mapped onto SCSP and
       is assigned a unique SCSP 'Protocol ID' value, e.g., the SCSP
       ProtID = 4 assigned to DCHP.  For this draft we assume that the
       Group Management sub-protocol is run on a separate UDP port from
       the SCSP UDP port.  The Group Mgmt sub-protocols will be assigned
       a unique UDP port number = tbd.  We had no compelling reason to
       carry the Address Management subprotocol on SCSP as for the
       Client Binding protocol, however for this draft we mantain both
       these sub-protocols within SCSP.  If at a later date it is deemed
       useful to separate these two protocol 1) we can define separate
       SCSP protocol types for the Cache Management and the Address Man-
       agement protocols, yet support them with a common Hello protocol
       link via the Hello protocol Family type field or 2)we can move



Kinnear, Cole & Droms                                          [Page 48]


DRAFT                                                          July 1997


       the address management sub-protocol out from SCSP as in the case
       of the Group management sub-protocol.

       The mappings between the interserver messages and the SCSP mes-
       sages will cover the interserver messages handling client binding
       and address management, but not the group management protocol
       functions of the interserver protocol.  The  group management
       messages are to be defined outside of SCSP, however these mes-
       sages will follow the syntax of the SCSP message sets to simplify
       the parsing of the total message sets required within the DHCP
       interserver protocol.

       The client binding management operations are CLIENT BINDING COM-
       PLETE PUSH and CLIENT BINDING POLL.  CLIENT BINDING COMPLETE PUSH
       is required to distribute binding information and to increase the
       initial lease period to the desirable lease period.  The CLIENT
       BINDING POLL is required to solicit information on client bind-
       ings in the event that the specific server has no record of the
       client requested binding.  The Interserver messages supporting
       these operations are the CLIENT BINDING UPDATE and the CLIENT
       BINDING QUERY messages, respectively.  The SCSP records for these
       operations are 'Binding' records for the update and query mes-
       sages.

       The Address Management operations are UNBINDABLE COMPLETE POLL
       and TRANSFER.  The UNBINDABLE COMPLETE POLL initializes an
       address as bindable by the LS.  The TRANSFER allows for the
       transfer of a block of bindable addresses between servers.  The
       Interserver messages supporting these operations are the UNBIND-
       ABLE QUERY and the TRANSFER messages.  The SCSP records for these
       operations are 'Address' records for the UNBINDABLE QUERY and
       'Bindable Block Address' records for the TRANSFER messages.

       The Group Management messages are SG DISCOVERY Query, SG CONFIGU-
       RATION QUERY, SG PROPOSE CHANGE UPDATE and SG COMMIT CHANGE
       UPDATE.  The SCSP records associated with these operations are
       'SG Specifier' records for the SG DISCOVERY QUERY, 'SG Subnets'
       records for the SG CONFIGURATION QUERY, 'SG Members' records for
       the SG DISCOVERY Query, and 'SG Proposed Members' records for the
       SG PROPOSE CHANGE UPDATE and SG COMMIT CHANGE UPDATE messages.

     o On DHCP Interserver Authentication:

       The interserver protocol will rely on the authentication exten-
       sions within SCSP for the SCSP message authentication between
       servers within a server group.  The authentication of the inter-
       server group management protocol messages are tbd.




Kinnear, Cole & Droms                                          [Page 49]


DRAFT                                                          July 1997


     o On the Notion of Server Ownership of Binding Records:

       It will be assumed that once the initial client binding record is
       generated by a particular server, that record will indicate that
       server as the originating server in the SCSP 'Originating Server
       ID' field.  Any further changes to that binding, whether by the
       originating server or by another server, e.g., the originating
       server is down and the client is Rebinding and getting a lease
       extension from another server, that server does change the Origi-
       nating Server ID in the SCSP record field to indicate itself as
       the last transaction server.

     o On a More Efficient Cache Alignment Process:

       The cache alignment process can be made more efficient if the
       servers time stamp their cache records.  In the event that the
       connections between servers fails, the servers determine and
       record the failure time.  Upon reconnecting and cache alignment,
       the SCSP CRL list can be limited to those records that are more
       'recent' than the failure and therefore greatly reduce the time
       and the bandwidth required.  The details are presented below.

       Also, it is not necessary to perform a cache alignment of the
       address records for the proper operation of the Interserver pro-
       tocol.  Therefore, we assume that the SCSP cache alignment pro-
       cess will not include these address records when building the
       SCSP CRL.

     o On the More Recent Record Determination:

       SCSP relies on the ability of identifying the more recent-ness of
       records when aligning and updating the cache based upon the CSA
       Sequence Number.  For binding records this implies that in situa-
       tions where it is clear that a single server is updating the
       binding, e.g., extending the lease, then it should increment the
       CSA Sequence number by one.  However there are situations in DHCP
       where multiple servers can simultaneously update the client bind-
       ing and it is not clear which of these updated bindings is
       accepted by the client, e.g., the client is in the rebinding
       state and the originating server is down and the other servers
       received the client broadcast request and the client gets multi-
       ple DHCPACKs extending the lease.  In these situations the
       servers are required to increment the CSA sequence numbers by one
       and indicate that they are the last transaction server.  Then,
       when a server caches the record, if it already has a cache record
       for that binding (as indicated by the Cache Key) it should
       replace the existing record only if the new record indicates a
       lease period which is greater than the existing record.



Kinnear, Cole & Droms                                          [Page 50]


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     o On Maximally Defined Binding Records (or the B.Hibbs' Question):

       B.Hibbs' posed the question regarding the nature of the configu-
       ration synchronization of the servers within the same SG; Does
       the DHCP Interserver protocol require synchronization of all con-
       figuration parameters or a subset? We are assuming that there is
       a minimal set of configuration and client binding information to
       be synchronized across the members of the SG to ensure the cor-
       rect operation of the DHCP Client/Server protocol.  This informa-
       tion must be carried in the interserver messages to synchronize
       the members in the SG with respect to this information.  Further,
       there may be other client binding information that the members
       want to communicate; we currently have this information encoded
       as optional in this draft.

       The parameters encoded into the 'Client Binding' records are
       those which are minimally required for the correct operation of
       the DHCP Client/Server protocol.  The interserver protocol should
       allow for situations where the configuration of the servers of
       the same server group are not strictly aligned; their configura-
       tions are only required to be aligned in the specification of the
       subnets and masks that are covered with a SG and the list of
       assignable addresses within each of the subnets.  However,
       because clients DHCPDISCOVER messages can contain client specific
       requests for parameters, it may be desirable to embed a fuller
       set of parameters (committed to the client in the DHCPOFFER mes-
       sage) within the CSA record.  This fuller set of parameters may
       be included in the initial CLIENT BINDING COMPLETE PUSH (encoded
       in the optional fields location in the record).  The server in
       receipt of a CLIENT BINDING COMPLETE PUSH may chose not to cache
       or forward these optional parameters.

     o On Knowledge Obtained Through the SCSP Hello protocol:

       The SCSP Hello protocol maintains current status of the inter-
       server connectivity through a polling mechanism.  This status
       information can be used to influence the actions of the LS, e.g.,
       in the event that the LS has lost connectivity from a DCS, then
       it should not perform a COMPLETE POLL operation.

     o On the SG Connectivity:

       It is likely that the servers of the SG are required to be fully
       interconnected, i.e., a LS is a DCS to all other servers of the
       SG.  It was first thought that this would aid in determining the
       status of the SG, i.e., whether the SG was 'up' (fully function-
       ing) or 'down' (not fully functioning).  However on further
       inspection this is not true, i.e., the loss of connectivity



Kinnear, Cole & Droms                                          [Page 51]


DRAFT                                                          July 1997


       between a pair of servers in a fully connected SG does not imply
       that the other servers are not still connected to the other
       servers.  Full mesh connectivity may still be required for the
       correct operation of the Address Management protocol.  This is
       currently under study.

   When a new server wishes to join a server group, it must initialize
   itself to the other members of the server group through the above
   defined interserver Group Management Protocol.  Once this has
   occurred, the local server must initiate SCSP which then will align
   its client binding cache to that of the server group.  It should then
   acquire Bindable addresses and fully participate in the on-going
   client binding update functions of the server group.

   This process is outlined in the below state diagram for the DHCP
   interserver protocol.  The Group Management protocol handles the new
   server joining the group.  Once this has occurred, the new server and
   all the other servers of the server group initiate the SCSP Hello
   Protocol on a pairwise basis.  Per the discussion in the SCSP speci-
   fication, once bi-directional connectivity is re-verified and now
   monitored within the SCSP Hello protocol, the servers enter into the
   cache alignment and then the ongoing cache and address management
   functions.  In the event that the servers transition to the 'DOWN'
   state, polling will continue until connectivity is re-established.

   The Group Management Protocol does not allow additions to the member-
   ship in the event that the SG is down.  However it does allow for the
   removal of a server from the SG while another server is re-booting or
   disconnected.  Therefore a re-booting or re-connecting server cannot
   be assured that the SG generation has remained constant during the
   'DOWN' period.  Therefore, in the event that the generation number of
   the SG has changed as indicated through the generation number con-
   tained within the interserver messages, the server needs to update
   its notion of the server group through the procedures identified in
   the group management protocol prior to aligning its cache.
















Kinnear, Cole & Droms                                          [Page 52]


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                           +------------+
                           |   Group    |
                           | Management |
                           |  Protocol  |
                           +------------+
                                 |
                                 |
                                 V
                          +------------+
                          |    SCSP    |
                          |   Hello    |
                          +------------+
                           /     ^   \
                          /      |    \
                         V       |     V
              +--------------+   |    +---------------+
              |'Binding Mgmt'|   |    |Null'Addr Mgmt'|
              |    Cache     |---+----|     Cache     |
              |  Alignment   |   |    |   Alignment   |
              +--------------+   |    +---------------+
                     |           |           |
                     |           |           |
                     V           |           V
              +--------------+   |    +------------+
              |'Binding Mgmt'|   |    | 'Addr Mgmt'|
              | Cache Update |---+----|Cache Update|
              +--------------+        +------------+


              Figure 8.2-1  Interserver State Flow Diagram

   For operational efficiency, the servers should implement a scheme to
   limit the number of cache records to exchange during the cache align-
   ment process.  For example, a SG could easily be managing 10,000
   client records and the bandwidth requirements to pass even the sum-
   mary records required to build the CRL table can be quite large.
   Therefore, for the 'Cache Management' sub-protocol, the servers
   should record the times at which the cache entries were received or
   created or modified.  When the CAFSM transitions for a particular DCS
   to the down state, t(down) should be recorded.  Then when the CAFSM
   enters the cache alignment state, the CRL list is to be built up
   based upon only those records with time stamps more recent then
   t(down) - F, where F is a factor to be set to a multiple of the Hel-
   loInterval x DeadFactor.  We recommend that the multiple be 10.  In
   the event that the LS crashed (causing the transition to the down
   state), then t(down) should be set to the last record time stamp when
   the LS reboots.  In the event that the server has just joined the SG,
   the CRL should be built up from all of the current cache records.



Kinnear, Cole & Droms                                          [Page 53]


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   The interserver messages associated with the Client Binding Manage-
   ment are:  CLIENT BINDING QUERY for the CLIENT BINDING POLL opera-
   tion, and CLIENT BINDING UPDATE for the CLIENT BINDING COMPLETE PUSH
   operation.  These are discussed in detail in the following list
   items:

     o The CLIENT BINDING QUERY message queries another server regarding
       the status of a particular binding.  Within the SCSP protocol,
       this exchange is accomplished by the LS sending a Client State
       Update_Solicit (CSUS) message with the Client State Advertisement
       Summary (CSAS) 'Address record' of the IP address in question.
       The DCS responds with the CSU_Request message with the Client
       State Update (CSU) record associated with the CSAS.  The LS then
       replies with a CSU_Reply with the 'A-bit' set.

     o The CLIENT BINDING UPDATE message updates another server with a
       new, or changed, client binding.  Within the SCSP protocol, this
       exchange is accomplished with the CSU_Request message carrying
       the specific CSA 'Binding record' of the client binding in ques-
       tion.  The DCS responds with the CSA-Reply with the 'A-bit' set.

   The interserver messages associated with the Address Management are:
   UNBINDABLE QUERY for the UNBINDABLE COMPLETE POLL operation, and
   TRANSFER messages for the TRANSFER operation.  These are discussed in
   detail in the following list items:

     o The UNBINDABLE QUERY message queries another server of the SG
       regarding the status of a particular address with the intent of
       making that address bindable to the LS.  Within the SCSP proto-
       col, this exchange is accomplished by the LS sending a
       CSU_Solicit with the CSAS 'Address' record of the IP address in
       question to all other servers of the SG.  The DCSes respond with
       the CSU_Request message with the CSA 'Address' record indicating
       the status of the address within the DCS.  The LS then replies
       with the CSU_Reply message to the DCS with the 'A-bit' set.

     o The 'TRANSFER' operation is initiated by the LS to request a
       transfer of bindable addresses from the DCS to the LS.  Within
       the SCSP protocol, this exchange is accomplished by a two step
       process.  First, the LS sends a CSU_Request message with the CSA
       'Subnet Bindable Addresses' record to the DCS, which then
       responds with a CSU_Reply.  The CSA 'Subnet Bindable Addresses'
       record indicates the subnet in question, the number of BINDABLE
       addresses owned by the LS and the number of additional BINDABLE
       addresses the LS is requesting.  Second, this is immediately fol-
       lowed by the DCS sending a CSU_Request message with a CSA 'Subnet
       Bindable Address' record for the given subnet in question.  The
       DCS' CSA 'Subnet Bindable Addresses' record indicates the subnet



Kinnear, Cole & Droms                                          [Page 54]


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       in question and the number and address of the IP addresses that
       the DCS is transferring to the LS based upon it's previous
       request.  This is based upon the DCS' current understanding of
       the supply of bindable addresses within the LS and its local
       knowledge of its own set of bindable addresses for this subnet.
       This CSU_Request will generate a CSU_Reply from the originating
       LS.  When sending the CSU_Request message, the DCS sets the
       addresses it is transferring to the LS as UNBINDABLE.  The LS
       then moves these addresses to its list of BINDABLE addresses and
       sends a CSU_Reply to the DCS with the 'A-bit' set.

   The interserver messages associated with the Group Management opera-
   tions are:  SG DISCOVERY QUERY, SG CONFIGURATION QUERY, SG PROPOSE
   CHANGE UPDATE, and SG COMMIT CHANGE UPDATE messages.  These are dis-
   cussed in detail in the following list items:

     o The SG DISCOVERY QUERY message queries the DCS for its list of
       current SG in which it is participating.  Within the SCSP proto-
       col, this exchange is accomplished by the LS sending a
       CSU_Solicit with the CSAS 'Server Groups' record and the DCS
       replys with the CSU_Request message containing the CSA 'Server
       Groups' record.  This record contains the list SG specifiers,
       i.e., SG ID and SG Generation Number (GN) pairs.  The LS replies
       with a CSU_Reply.

     o The SG CONFIGURATION QUERY message queries the DCS for its con-
       figuration information.  This information is passed within the
       'SG Subnets Configuration' record.  The LS initiates this query
       by sending a CSU_Solicit containing the CSAS 'SG Subnets Configu-
       ration' summary record.  The responds with a CSU_Request contain-
       ing the CSA 'SG Subnets Configuration' record.  The LS replies
       with the CSU_Reply message.

     o The SG PROPOSE CHANGE UPDATE message proposes the new member to
       the rest of the SG.  This is accomplished with a SCSP CSU_Req
       message carrying the 'SG Proposed Members' record.  The SG COMMIT
       CHANGE UPDATE message consummates the new server joining the SG.
       Once the joining member has received positive CSU_Reply from all
       of the current members of the SG as part of the proposal phase,
       it then moves to the join commit phase.  The new server now
       issues an SCSP CSU_Req message with the 'SG Members' record car-
       rying the newly joined member to the list of servers of the SG.

     o The SG PROPOSE CHANGE UPDATE message may also be used to propose
       the removal of an existing server from the membership of the SG.
       This is accomplished with a SCSP CSU_Req message carrying the 'SG
       Proposed Members' record containing all of the existing members
       of the SG minus the server ID to be removed.  The SG COMMIT



Kinnear, Cole & Droms                                          [Page 55]


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       CHANGE UPDATE message consummates the existing server leaving the
       SG.  Once the removing member, i.e., the member who is actively
       removing the existing member from the group, has received posi-
       tive CSU_Reply from all of the current members of the SG (except
       for the member being removed) as part of the proposal phase, it
       then moves to the remove commit phase.  The removing server now
       issues an SCSP CSU_Req message with the 'SG Members' record car-
       rying the new membership minus the removed server.

8.3.  Necessary Modifications to SCSP

   The SCSP modifications required to support the DHCP interserver pro-
   tocol are as follows:

     o The operation of the SCSP protocol in this application is initi-
       ated upon the successful completion of the interserver 'Group
       Management Protocol'.

     o The SCSP messages, and in fact all of the DHCP interserver mes-
       sages are carried in UDP packets.  Therefore a UDP port number
       needs to be defined for SCSP.

       DISCUSSION:

          Currently SCSP is defined only for NMBA networks.  This mani-
          fests itself in two ways; a) the operation of the SCSP proto-
          col is initiated upon the establishment of NBMA connectivity,
          i.e., a virtual circuit being established, and b) the SCSP
          messages are encapsulated into link level frames using the
          LLC/SNAP encapsulation method.

          Instead of relying upon the establishment of a virtual circuit
          connection, the interserver protocol will initiate the SCSP
          protocol based upon the results of the 'Group Management Pro-
          tocol'.  This divorces the operation of the interserver proto-
          col from the specifics of the link layer.  Also, by carrying
          the messages within UDP, the protocol achieves independence in
          the deployment and proximity of the servers which are members
          of the same server group, i.e., servers are not required to
          have an interface on a common subnet.

          Because SCSP provides a generic capability to synchronize
          caches in distributed servers, it is best to define a separate
          UDP port number for the 'generic' SCSP protocol and a separate
          UDP port for the DHCP interserver Group Management protocol.
          These UPD port numbers are tbd.





Kinnear, Cole & Droms                                          [Page 56]


DRAFT                                                          July 1997


     o A SG Generation Number SCSP extension field needs to be defined.

       DISCUSSION:

          We have defined the notion of a Server Group Generation Number
          to distinguish between the various instantiations of a partic-
          ular SG.  The membership of a particular SG will change over
          time.  Because it is necessary for the correct operation of
          the DHCP interserver protocol for each server to know the cur-
          rent membership, it was deemed necessary to define a Genera-
          tion Number which is incremented each time a new server joins
          the SG or an existing server is removed from the SG.  This GN
          is to be carried in every interserver message.  No obvious
          place existed with the SCSP message formats to carry such
          information.  Therefore, we have chosen to define a new SCSP
          extension type and will carry the GN in this method.

     o Some modification to the Authentication extension in the SCSP
       protocol may be required.

       DISCUSSION:

          Currently SCSP states that the authentication extension covers
          the SCSP message other than the extensions.  However we have
          chosen to carry a new extension within the SCSP messages; the
          Generation Number.  Ideally we would prefer that this exten-
          sion be protected by the authentication extension.  Because it
          is not, we will also include the Generation Number in the SG
          Specifier record.  Through this record a server may reverify
          the current Generation Number through a protected channel.

     o The three step Solicit_Request_Reply seems excessive when one
       server wishes to simply query another server.  Perhaps this could
       be simplified (when desirable) by adding a bit to the CSU_Solicit
       message indicating whether the soliciting server wishes the DCS
       to expect or not to expect a CSU-Rep from the soliciting server.

       DISCUSSION:

          Currently SCSP states that the three step process of CSU_Sol
          followed by a CSU_Req which is then followed by a CSU_Rep.  In
          certain situations this may be a desirable sequence.  However,
          in other situations it may not be necessary.  When the CSU_Sol
          is sent a CSUSReXmtInterval timer is set which tracks the sta-
          tus of the receipt of the requested CSU_Req records.  For sim-
          ply queries, this re-transmit timer may be sufficient.  There-
          fore, it seems reasonable that DCS should expect a CSU_Rep
          from the LS which sent the CSU_Sol message.



Kinnear, Cole & Droms                                          [Page 57]


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8.4.  DHCP Specific CSA and CSAS Records

   This section presents the CSA and the CSAS records specific to the
   DHCP inter-server protocol.  The mappings of the interserver protocol
   onto SCSP messages discussed in the previous section relys upon the
   definition of a number of record types.  These record types will be
   distinguished within the CSAS defined 'Cache Key', which for the pur-
   pose of running the DHCP interserver protocol will consist of a
   TYPE/Key pair.  The following CSAS and CSA record types are required
   to run the interserver protocol:

   For Client Binding Management:

     o Binding Record - contains the complete client binding informa-
       tion.

   For Address Management:

     o Address Record - contains the status of a specific IP address,
       e.g., unbindable, bindable, bound, expired, etc.

     o Subnet Bindable Record - contains information regarding the sub-
       net addresses, e.g., number of bindable addresses.

   For Group Management:

     o SG Specifier Record - contains the current Server Group speci-
       fiers, i.e., the SG ID (which is fixed for the duration of the
       life of the SG) and the SG Generation Number which is incremented
       for each new server add or old server delete.

     o SG Members Record - contains the current list of member servers
       of the SG.

     o SG Subnets Configuration Record - contains a list of all subnets,
       i.e., subnet address and mask, for all of the subnets served by
       the SG as well as the assignable addresses per subnet, and poten-
       tially other configuration parameters necessary for the proper
       operation of the DHCP interserver protocol.

     o SG Proposed Members Record - contains a list of the proposed mem-
       ber servers of the SG used in the group join proposal process.
       This record has a finite duration associated with it and times
       out if the proposed join fails.







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8.4.1.  The SCSP CSAS Records for the Interserver Protocol

   The CSAS record is completely specified in [2].  The format of the
   CSAS record is:


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Hop Count             |       Record Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Cache Key Len | Orig ID Len   |N|      unused                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      CSA Sequence Number                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Cache Key    (variable)                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Originator ID  (variable)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                Figure 8.4.1-1  SCSP CSAS Record Format
   where:

     o Hop Count - this represents the number of hops that the record
       may take before being dropped.

     o Record Length - this is the length in bytes of the CSAS record if
       stand-alone, otherwise it is the length in bytes of the CSAS
       record and the protocol specific part of the cache entry com-
       bined, i.e., the length of the CSA record.

     o Cache Key Length - this is the length of the Cache Key field in
       bytes.

     o Originator ID Length - this is the length of the Originator ID
       field in bytes.

     o N bit -  this bit, when set, signifies a Null record.  This may
       be the case when the LS receives a solicitation for a record that
       has been released by the DHCP client.

     o CSA Sequence Number - this field contains the sequence number
       that identifies the 'newness' of a CSA record instance being sum-
       marized.  This number is assigned by the originator of the CSA
       record, i.e., the last transaction server.





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     o Cache Key - is an opaque string used by the receiving server to
       identify the cache entry referred to by the record.  For the pur-
       poses of running the DHCP interserver protocol, the Cache Key
       will be encoded as a Type/Key pair, where the type is an 8 bit
       field and the length of the Key is derived from the Cache Key
       Length field in the header.  The Type indicates the type of
       record and equivalently the Interserver message type, e.g.,
       Unbindable Address Query, SG Configuration Query, etc.  The 8 bit
       type encodings are defined in the table below.

     o Originator ID - this field contains an ID which is administra-
       tively assigned to the server which is the originator of the CSA
       record.  For the DHCP interserver mapping, the the Originating
       Server ID is chosen to be the IP address of the server.  In the
       event that the server has multiple IP addresses assigned to it,
       then the Originating Server ID is set to the IP address with the
       highest value.

   The CSAS record is specified by SCSP except for the specifics of the
   Cache Key and the Originator ID.

   For the purpose of the DHCP interserver specification, the Originat-
   ing Server ID is chosen to be the IP address of the server.  In the
   event that the server has multiple IP addresses assigned to it, then
   the Originating Server ID is set to the IP address with the highest
   value.

   The Cache Key used is dependent upon the specific CSAS record in
   question.  The table below identifies the specific Cache Keys for the
   various CSAS records within the DHCP interserver protocol.  These are
   composed of a type and key field, both of which are identified in the
   table.



















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        Table 8.4.1-1  Cache Keys for the various CSAS and CSA records

         Record Type       | Encoding      |   Key
   --------------------------------------------------
                           |               |
   Client Binding          |   0x00        |  Client ID
                           |               |  or hwaddr
   Address                 |   0x10        |  IP addr
                           |               |
   Subnet Bindable Addrs   |   0x11        |  Subnet/Mask *
                           |               |
   SG Specifiers           |   0x20        |  IP addr
                           |               |
   SG Subnet Configs       |   0x21        |  SG ID
                           |               |
   SG Members              |   0x22        |  SG ID/SG GN **
                           |               |
   SG Proposed Members     |   0x23        |  SG ID/SG GN **


      * The subnet address and the subnet mask will be encoded as 32 bit
      strings with the subnet address followed by the subnet mask.

      ** The SG ID and SG GN are encoded as 16 bit strings with the SG
      ID first, immediately followed by the SG GN.

8.4.2.  The SCSP CSA Records for the Interserver Protocol

   There are several types of DHCP specific CSA records defined corre-
   sponding to each of the CSAS record types discussed above and found
   in Table 8.4.1-1.

   For many of these records, DHCP options appear in the records in the
   same format as specified in [7].

   The records are:

     o The Client Binding record carries the complete client binding
       information.  The Key for this record is the chaddr or the
       'client ID' from the optional DHCP extension.  This is utilized
       in the Cache Mgmt sub-protocol in handling the COMPLETE PUSH,
       POLL and SCSP cache alignment operations.

     o The Address record carries the information required to achieve
       the desired response from the CSU_Solicit message.  The Key is
       the IP address.  This is utilized in the Address Mgmt sub-
       protocol in handling the UNBINDABLE COMPLETE POLL operation.




Kinnear, Cole & Droms                                          [Page 61]


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     o The Subnet Bindable Address record carries the information
       required to determine the status of the available IP addresses
       which are bindable to the DCS and which it is will to transfer to
       the LS.  The Key for this record is the subnet address and mask
       of the subnet in question.  This is utilized in the Address Mgmt
       sub-protocol by the TRANSFER operation.

     o The SG Specifier record contains the total list of SG specifiers,
       i.e., SG ID and SG GN pairs, of which the server in question is
       currently a member.  This is utilized in the Group Mgmt sub-
       protocol by the DISCOVERY operation.  The Key for this record is
       the Server ID, i.e., the IP address of the server.

     o The SG Members record contains a list of the Server IDs which
       comprise the SG in question.  This is utilized in the Group Mgmt
       sub-protocol by the DISCOVER MEMBERS operation.  The Key for this
       record is the SG Specifier, i.e., the SGID and SG GN pair.

     o The SG Proposed Members record contains a list of the SG members,
       including the newly proposed member, of the server group.  This
       is utilized in the Group Mgmt sub-protocol by the PROPOSE JOIN
       operation.  The Key for this record is the SG Specifier, i.e.,
       the SGID and SG GN pair where the SG GN is one greater than the
       current GN of the SG.

8.4.2.1.  Binding Records

   The approach taken in defining the Client Binding record is as fol-
   lows.  It is possible, while still maintaining the correct operation
   of the DHCP client/server protocol, to have the different server con-
   figurations within the same server group with respect to certain
   parameters.  For these parameters we do not require synchronization
   of the server configurations and we make the passing of these parame-
   ters as optional.  However there are some configuration parameters
   and binding information which is critical to the correct operation of
   the protocol.  For these client parameters we require that they be
   included in the Client Binding records.  The minimal, required set of
   parameters to be sent in the Client Binding are the IP address
   (ciaddr), the lease period, the last transaction type, the client
   hardware address, the Client-Identifier and the Renewel (T1) and
   Rebinding (T2) Time values (if present in the DHCP options extensions
   of the DHCPACK).

   The format of the CSA Binding record for the DCHP inter-server proto-
   col is:






Kinnear, Cole & Droms                                          [Page 62]


DRAFT                                                          July 1997


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      CSAS Record  (variable)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  LTT  |resrv'd|   HTYPE       |    HLEN       |    resrv'd    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       CHADDR  (HLEN in octets)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       CIADDR  (4 octet)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Last Transaction Time (4 octet)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     IP Address Lease Time (encoded as tag=51) (6 octet)       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Optional ClientID (encoded as tag=61) (variable)          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Optional Renewal Time (encoded as tag=58) (6 octet)       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Optional Rebinding Time (encoded as tag=59) (6 octet)     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Other desirable DCHP extensions   (variable)              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   End Option (encoded as in BOOTP options, tag=255) (1 octet) |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     Figure 8.4.2.1-1  DHCP inter-server CSA Binding record format

   where:

     o CSAS Record - represents the full CSAS record as identified in
       Section 8.4.1.

     o LLT - indicates the Last Transaction Type.  The allowed LTTs are:
       DHCPREQUEST/SELECTING (0x0), DHCPREQUEST/REBINDING (0x3), DHCPRE-
       QUEST/RENEWING(0x2), DHCPREQUEST/INIT-REBOOT (0x1), DHCPRELEASE
       (0x4), and EXPIRATION (0x5).

     o HTYPE - hardware address type (defined in [1])

     o HLEN - hardware address length

     o CHADDR - client hardware address

     o CIADDR - client IP address (if assigned).  If not assigned, this
       field is all 0s.




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     o Last Transaction Time - the time from now in seconds of the last
       transaction time associated with the LTT as indicated in the mes-
       sage.

     o IP Address Lease Time - the IP Address Lease Time encoded as in
       the DHCP options and BOOTP vendor extensions defined in [7].
       This represents the time from now that the client lease is to
       expire.

     o (Optional) Client ID - this field is the optional Client ID
       encoded as in the DHCP options and BOOTP vendor extensions
       defined in RFC 2132 [7].  If present, the Client ID is the
       'search string'.

     o (Optional) Renewal Time - this field is the optional Client
       Renewal Time (T1) as encoded in the DHCP options and BOOTP vendor
       extensions defined in RFC 2132 [7].

     o (Optional) Rebinding Time - this field is the optional Client
       Rebinding Time (T2) as encoded in the DHCP options and BOOTP ven-
       dor extensions defined in RFC 2132 [7].

     o Remaining Options - any remaining options carried in the original
       DHCPOFFER message to the client encoded as in the DHCP options
       and BOOTP vendor extensions defined in [7]

     o End option - determines the end of the CSAS record

       DISCUSSION:

          As discussed in the previous section on the CSAS record for-
          mat, the format shown above is intended to be the Binding type
          CSA record.  The binding record is used in the PUSH and COM-
          PLETE PUSH operations to transfer to the DCSes the newly cre-
          ated or changed binding and in the cache alignment procedures.
          The structure of the Client Binding is defined, for the pro-
          pose of the DHCP interserver protocol into a mandatory part
          and an optional part.  The mandatory part is everything upto
          and including the (Optional) Rebinding Time.  The optional
          part is everything following the (Optional) Rebinding Time.
          The PUSHing server may include any additional parameters which
          were part of the DHCPACK message to the client within the
          Client Binding Record and encode this as defined in the the
          DHCP options and BOOTP vendor extensions defined in RFC 2132
          [7].  The server which is the recipient of the PUSH may chose
          to save and forward these optional parameters in the record or
          may chose not to save and forward these optional parameters.




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8.4.2.2.  Address Records

   The format of the CSA Address record for the DCHP inter-server proto-
   col is:


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      CSAS Record  (variable)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  ST   |                     reserved                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     Figure 8.4.2.2-1  DHCP inter-server CSA Address record format

   where:

     o CSAS Record - represents the full CSAS record as identified in
       Section 8.4.1.

     o ST - represents the state of the (client) record, e.g., unbind-
       able, bindable, bound, expired, polling, static

       DISCUSSION:

          The Address record is used within the UNBINDABLE COMPLETE POLL
          operation to move an unbindable address to a bindable address.
          The POLLed server returns the Address record indicating the
          current status of the address within the server.  If all of
          the servers indicate that the address is unbindable, then and
          only then will the LS move the address to its Bindable pool.

          The ST field indicates the servers view of the state of the
          address.  The states (defined in Section 3.4.2) are: UNBIND-
          ABLE, POLLING, BINDABLE, BOUND, PUSHED, and EXPIRED.

   The IP address states are encoded in the following manner:












Kinnear, Cole & Droms                                          [Page 65]


DRAFT                                                          July 1997


          Table 8.4.2.2-1  IP Address State Encodings

           IP Address State  | Encoding
     --------------------------------------------------
                             |
     UNBINDABLE              |   0x01
     POLLING                 |   0x02
     BINDABLE                |   0x03
     BOUND                   |   0x04
     PUSHED                  |   0x05
     EXPIRED                 |   0x06



8.4.2.3.  Subnet Bindable Addresses Record

   The CSA Subnet Bindable Addresses record indicates the set of
   addresses that a server is willing to TRANSFER to a requesting
   server.  This record is used in the TRANSFER operation.

   The format of the CSA Subnet Bindable Addresses record for the DCHP
   inter-server protocol is:


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      CSAS Record  (variable)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | No. Addresses |No. Addr.Ranges|R|  reserved   |No.Ownd|No.Reqd|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               List of IP Addresses                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Figure 8.4.2.3-1  DHCP inter-server CSA Subnet Bindable Addresses record
                                 format

   where:

     o CSAS Record - represents the full CSAS record as identified in
       Section 8.4.1.

     o No. Address - indicates the number of IP addresses contained
       within the subnet record.  These are the addresses that the DCS
       is transferring to the LS as part of the TRANSFER operation.
       This is set to 0 when the R-bit is set to 1 (see R-bit below).




Kinnear, Cole & Droms                                          [Page 66]


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     o No. Addr. Ranges - indicates the number of IP address ranges of
       the form 135.16.114.5 to 135.16.114.235.  These will immediately
       follow the listing of the individual addresses.  This is set to 0
       when the R-bit is set to 1 (see R-bit below).

     o R - represents the request bit.  When this bit is set to 1, it
       indicates that the LS is requesting BINDABLE addresses from the
       DCS as part of the TRANSFER operation.  When it is set to 0, it
       indicates that the DCS is transferring these addresses to the LS.

     o No. Ownd - indicates the current number of BINDABLE addresses
       owned by the LS when the R-bit is set to 1.

     o No.Reqd - indicates the number of additional BINDABLE addresses
       requested by the LS when the R-bit is set to 1.

     o List of IP Addresses - this is a consecutive list of IP address
       and address ranges.

       DISCUSSION:

          The Subnet record is used in the TRANSFER operation to indi-
          cate 1) the list of bindable IP addresses that the DCS is
          willing to transfer to the LS when the R bit is 0, and 2) the
          IP addresses that the LS is requesting when the R bit is 1.

          Further, it may be useful to develop similar records for Sub-
          net UNBINDABLE, BOUND, PUSHED, and EXPIRED address.  They can
          have an identical record format and be distinguished through
          the 8 bit type field encoded into the SCSP Cache Key.  The
          utility of these record types is TBD.

8.4.2.4.  SG Specifier Record

   The CSA SG Specifier Record indicates the total list of DHCP Inter-
   server protocol Server Groups that the DCS is currently a member.
   This is used in the Group Management subprotocol during the initial
   contact of a prospective new member to the Server Group.

   The format of the CSA SG Specifier Record for the DCHP inter-server
   protocol is:










Kinnear, Cole & Droms                                          [Page 67]


DRAFT                                                          July 1997


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      CSAS Record  (variable)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |No. Specifiers |             reserved                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               List of Specifier Pairs                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


  Figure 8.4.2.4-1  DHCP inter-server CSA SG Specifiers record format

   where:

     o CSAS Record - represents the full CSAS record as identified in
       Section 8.4.1.

     o No. Specifiers - is a count of the number of specifier pairs con-
       tained within this CSA record.

     o List of Specifier Pairs - represents a consecutive listing of the
       specifier pairs of which the DCS is current a mamber.  The encod-
       ing of the specifier pairs is SG ID first, which is a 16 bit
       string, followed by the SG Generation Number, which is also a
       16-bit string.

       DISCUSSION:

          This record is initially requested by a server which is inter-
          ested in joining a DHCP Interserver Server Group and has been
          configured with the IP address of a server to first contact.
          The first contacted server then replies with the SG Specifier
          record.  This record can also be solicited when a server,
          which an existing member of a group becomes uncertain regard-
          ing the current Generation Number of the group.

          The SG Generation Number, obtained from this record, is car-
          ried in every DHCP Interserver protocol message, encoded as an
          extension to the SCSP message extension fields.  The extension
          encoding is TBD.

8.4.2.5.  SG Subnets Configuration Record

   The CSA SG Subnet Configuration Record carries SG configuration
   information necessary to ensure the correct protocol operation of the
   group.  The encoding of this record is essentially the subnet address
   and mask followed by the pool of addresses which are dynamically



Kinnear, Cole & Droms                                          [Page 68]


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   managed by the Server Group for this subnet.  The encoding of the
   address pool with be consistent with the address pool encoding of the
   Subnet Bindable Addresses Record discussed in Section 8.4.2.3 above.
   Other configuration parameters may be including if deemed important
   to the correct operation of the DHCP interserver protocol.

   Section 7.2 specifies that additional information (specifically
   client configuration information and vendor specific configuration
   information) will be also be available.  The precise details of how
   this information is encoded is TBD.

   The format of the CSA SG Subnets Configuration Record for the DCHP
   inter-server protocol is:


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      CSAS Record  (variable)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | No. Subnets |               reserved                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Subnet Address                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Subnet Mask                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Address Pool of first subnet (variable)    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Subnet Address                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                              ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Address Pool of last subnet  (variable)    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



Figure 8.4.2.5-1  DHCP inter-server CSA SG Subnets Configuration record
format

   where:

     o CSAS Record - represents the full CSAS record as identified in
       Section 8.4.1.

     o No. Subnets - indicates the number of subnet configurations con-
       tained in this record.





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     o Subnet Address - this is the subnet address of the subnet for
       which the following address pool is related.

     o Subnet Mask - this is the mask of the subnet in question.

     o Address pool of subnet - this is a listing of the address pool
       for which this SG can allocate from for this particular subnet.
       The encoding will follow the address pool encoding for the Subnet
       Bindable Addresses record.  Therefore, the address pool should
       contain two count fields, the first indicating the number of
       individually listed addresses, followed by another field indicat-
       ing the number of address ranges.  These are then followed by the
       list of individual IP addresses and then the list of address
       ranges.

       DISCUSSION:

          The total list of configuration items to be incorporated into
          this record needs to be further fleshed out.  Currently this
          record is planned to contain a list of the subnets and the
          address pools associated with each from which this SG can
          allocate.  If other configuration parameters are deemed neces-
          sary for the proper operation of the DHCP Interserver proto-
          col, then these need to be incorporated into this record.

8.4.2.6.  SG Members Record

   The CSA SG Members Record indicates the list of the current SG mem-
   bers, in the opinion of the sending server, including itself.

   The format of the CSA SG Members Record for the DCHP inter-server
   protocol is:


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      CSAS Record  (variable)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | No. Server IDs|P|             reserved                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     List of Server IDs                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


    Figure 8.4.2.6-1  DHCP inter-server CSA SG Members record format

   where:



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     o CSAS Record - represents the full CSAS record as identified in
       Section 8.4.1.

     o No. Server IDs - this is the number of Server IDs contained
       within this record.

     o P bit - the Proposal bit is used to indicate that this record is
       a current group members record (here set to 0) or a proposed
       group members record (discussed in the next section).

     o List of the Server IDs - this is a consecutive list of Server IDs
       which comprise this server's view of the current SG membership.
       The Server IDs are IP addresses associated with one of the
       server's interfaces.

8.4.2.7.  SG Proposed Members Record

   The CSA SG Proposed Members Record indicates the list of the current
   SG members, in the opinion of the sending server, and adding itself.
   This is a temporary record (with a lifetime associated with the
   period during which a Group Management SG CHANGE operation has to
   complete).  Once the SG COMMIT CHANGE UPDATE is received, this record
   replaces the old SG Members record as the new member record contain-
   ing the newly joined server.

   The format of the CSA SG Proposed Members Record for the DCHP inter-
   server protocol is:


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      CSAS Record  (variable)                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | No. Server IDs|P|           reserved                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               List of Server IDs                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Figure 8.4.2.7-1  DHCP inter-server CSA SG Proposed Members format

   where:

     o CSAS Record - represents the full CSAS record as identified in
       Section 8.4.1.





Kinnear, Cole & Droms                                          [Page 71]


DRAFT                                                          July 1997


     o No. Server IDs - this is the number of Server IDs contained
       within this record.

     o P bit - the Proposal bit is used to indicate that this record is
       a proposed group members record (here set to 1) or a current
       group members record (discussed in the previous section).

     o List of the Server IDs - this is a consecutive list of Server IDs
       which comprise the sending server's view of the proposed SG mem-
       bership.  The Server IDs are IP addresses associated with one of
       the server's interfaces.

       DISCUSSION:

          This record contains the proposed group membership from the
          view of the proposing server.  This record conceptually has a
          temporary lifetime associated with the period for which a
          group join proposal can live.  If a server receives a SG COM-
          MIT CHANGE UPDATE message, then this record becomes the new SG
          Members record.  If a SG COMMIT CHANGE UPDATE message is not
          received within the appropriate period, then this record
          expires.  If the server receives a second SG PROPOSE CHANGE
          UPDATE message while another Proposed Members record is
          active, it should NAK this second Proposed Members record.
          Only one group join can be in process at any given time.

8.5.  Open Questions with the Mapping onto SCSP

   The following questions are identified as outstanding issues to be
   resolved for the CSAS and CSA record definitions to be considered
   complete:

     o SCSP is currently LLC/SNAP encapsulated.  We are proposing that a
       UDP port be defined to carry SCSP messages for DHCP.  In fact we
       are proposing that the entire DHCP interserver protocol be run
       over UDP.

     o SCSP has currently reserved its Protocol ID = 4 for DHCP. This
       draft discusses DHCPv4 Interserver protocol and therefore the
       SCSP Protocol ID reservation should reflect that fact.  If a
       DHCPv6 extension to this draft were developed it would require a
       separate SCSP Protocol ID.

     o SCSP dropped support for message fragmentation.  We need to look
       into the size required for the various records defined in this
       draft and, if necessary, consider how to handle records larger
       than can fit into a single UDP packet.




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DRAFT                                                          July 1997


     o Need to give further thought to the partitioning of the DHCP
       interserver protocol into three separate but related subproto-
       cols; the Group Management, the Binding Management and the
       Address Management subprotocols.  Currently this draft has these
       as separate subprotocols, with the Group Management subprotocol
       run separate from the SCSP protocol and in fact on a different
       UDP port as the SCSP protocol.  The Group Management does however
       share common message semantics and syntax with the SCSP messages
       in order to simplify parsing the various messages associated with
       the DHCP interserver protocol.  The Binding Management and the
       Address Management subprotocols are run on top of SCSP with a
       single Protocol ID.

     o We need to explicitly discuss the method used to authenticate the
       DHCP Interserver protocol messages.  Current thinking is to use
       the SCSP authentication extensions.  This should be investigated
       and should be consistent with the 'Security Architecture for
       DHCP' draft [8].

9.  IP Address State Transitions

   The possible states of an IP address were defined in Section 3.2.2,
   and the state transition diagram appears there.  The state transi-
   tions though which an IP address can move were discussed implicitly
   in Section 6 in the context of the receipt of DHCP messages from DHCP
   clients.  However, an explicit examination of the processing required
   of a server by this protocol on each of the state transitions will
   serve to highlight some important aspects of this protocol.

   The IP address state transitions are handled in the following way:

     o UNBINDABLE -> POLLING

       When a server attempts to make a particular IP address BINDABLE,
       it first moves that IP address into the POLLING state.  Once in
       this state, if queried about whether that IP address is UNBIND-
       ABLE, the server will reply negatively.

     o UNBINDABLE -> BOUND

       When a server is removed from a server group, all of the IP
       addresses must be scanned to see if any of them show that server
       as the server who performed the last transaction (as set by that
       server successfully completing a CLIENT BINDING COMPLETE PUSH).
       For all of those IP addresses, if there is a client recorded in
       the IP address, and if that client does not have a currently dif-
       ferent binding, then that IP address must be set to BOUND and the
       lease time must be reset to the value sent in the latest CLIENT



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       BINDING COMPLETE PUSH.

       The only states from which this transition will be made are
       UNBINDABLE and EXPIRED.

     o POLLING -> BINDABLE

       A fundamental point and guarantee of this state transition dia-
       gram is that for an IP address to move from the UNBINDABLE state
       (where it is not owned by any server) through the POLLING state
       and on to the BINDABLE state (where it is owned by a single
       server) requires the server seeking to own the IP address to con-
       tact all of the other servers in the group.  It requires an
       UNBINDABLE COMPLETE POLL to complete successfully.

       The server attempting to move an IP address from the UNBINDABLE
       through the POLLING and on to the BINDABLE state must ask every
       other server in the group if it believes that the IP address is
       currently UNBINDABLE using an UNBINDABLE COMPLETE POLL.  If any
       server says that the IP address is either BINDABLE (i.e., it cur-
       rently owns the IP address) or BOUND (i.e., a client currently
       owns the IP address), then the server attempting to move the IP
       address from the UNBINDABLE to BINDABLE state MUST abandon the
       attempt.  If any server fails to respond at all, the server MUST
       abandon the attempt as well.

       DISCUSSION:

          In addition (and this is important!) if the server attempting
          to move the IP address from the UNBINDABLE state through the
          POLLING state and on to the BINDABLE state fails to hear from
          some other server, then the attempt cannot complete.  This
          means that if a server cannot communicate with every other
          server (due to communications failure, transient server fail-
          ure, or network partition) then this state transition cannot
          be made.

       Thus, all addresses in the UNBINDABLE state will stay in that
       state while any server in the group is out of communication with
       the group for any reason at all.

       Of course, the detailed description of the protocol suggests that
       a server build up a supply of BINDABLE IP addresses so that in
       the event of server failure it has BINDABLE addresses that are
       available to offer to new DHCP clients.

     o BINDABLE -> BOUND




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       Once an IP address is BINDABLE it may be BOUND to a client
       through the normal actions of the DHCP protocol.  Once a server
       has received a DHCPREQUEST/SELECTING message from a client it can
       move the IP address into the BOUND state, update its stable stor-
       age, and reply with a DHCPACK message to the client.

       After the DHCPACK has been sent, the DHCP server MUST also
       attempt to update all servers in the group with information indi-
       cating that the IP address is now BOUND to a particular client.
       It must perform a CLIENT BINDING COMPLETE PUSH operation with
       this information.

       An IP address that is BOUND will always result in a lease time
       that is no greater than the MAXIMUM-UNPUSHED-LEASE-TIME when
       given to a client, although the normal lease time is used in all
       interactions with other servers.

       DISCUSSION:

          In an ideal world, the server who created the binding would
          always succeed in updating all other servers in the group with
          the binding information.  Then, in the event that the binding
          server failed at some later time, another server to whom the
          client could broadcast would receive a DHCPREQUEST/REBINDING
          request and could reply with updated binding information.

          However, there is obviously a window where a server can crash
          after sending a DHCPACK and prior to updating even one addi-
          tional server.  This protocol has been designed so that not
          only is the process of updating all of the servers in the
          group with information concerning a new binding "lazy" (i.e.,
          performed after the actual binding is made), but also unneces-
          sary for correct operation.  The protocol only requires that a
          server try to update the other servers -- not that it succeed
          at updating even one server.

          The protocol accomplishes this by allowing a server to respond
          to a DHCPREQUEST/REBINDING message from a client without any
          information having been propagated from the server who created
          the binding.  Thus, a server who receives a rebinding request
          for an IP address about which it has no information must check
          with all available servers in the group, but in the absence of
          information to the contrary arriving within a relatively short
          timeout period, the server should respond to the rebinding
          request with an extension of the existing lease on the IP
          address.





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     o BINDABLE -> UNBINDABLE

       A server can relinquish an IP address in the BINDABLE state that
       it owns simply by responding to requests for information about
       the IP address as if it were UNBINDABLE.  No explicit action need
       be taken other than to respond correctly to POLL operations from
       other servers.

     o BOUND -> PUSHED

       Once an IP address that is BOUND to a client has a CLIENT BINDING
       COMPLETE PUSH succeed (and that means succeed to all of the
       servers), then it moves from the BOUND to the PUSHED state.  At
       this point, the normal lease time may be returned to the client
       on the next renewal or discover or rebinding.

       Note that only the server which executes the CLIENT BINDING COM-
       PLETE PUSH will set its IP address into the PUSHED state.  The
       state that it PUSHes to the other servers is BOUND.

     o BOUND -> UNBINDABLE

       In order for an IP address to move from the BOUND to the UNBIND-
       ABLE state, the client that owns the IP address (i.e., to which
       it is BOUND) must send a DHCPRELEASE message.  In this case, the
       receiving server (which may or may not be the server who created
       original binding) will update its stable storage with information
       that the IP address is not currently BOUND by any client.  It
       should then transmit this information to all other servers to
       which it can communicate at that time by performing a CLIENT
       BINDING COMPLETE PUSH operation.

       In the event that the server fails to update any other server
       with the new information about the IP address prior to undergoing
       some failure, then the worst that will happen is that the other
       servers will believe that an IP address is in the BOUND state
       when it need not be.  Ultimately the lease on the IP address will
       expire.

     o BOUND -> EXPIRED

       Any server which has information concerning a BOUND IP address
       may determine that the lease on the IP address has expired, and
       after an appropriate grace period has elapsed, that the IP
       address should be moved to the EXPIRED state.  A record of the
       client to which the IP address was BOUND must be kept.





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     o PUSHED -> UNBINDABLE

       In order for an IP address to move from the PUSHED to the UNBIND-
       ABLE state, the client that owns the IP address (i.e., to which
       it is BOUND) must send a DHCPRELEASE message.  In this case, the
       receiving server (which may or may not be the server who created
       original binding) will update its stable storage with information
       that the IP address is not currently BOUND by any client.  It
       should then transmit this information to all other servers to
       which it can communicate at that time by performing a CLIENT
       BINDING COMPLETE PUSH operation.

       In the event that the server fails to update any other server
       with the new information about the IP address prior to undergoing
       some failure, then the worst that will happen is that the other
       servers will believe that an IP address is in the PUSHED state
       when it need not be.  Ultimately the lease on the IP address will
       expire.

     o PUSHED -> EXPIRED

       Any server which has information concerning a PUSHED IP address
       may determine that the lease on the IP address has expired, and
       after an appropriate grace period has elapsed, that the IP
       address should be moved to the EXPIRED state.  A record of the
       client to which the IP address was PUSHED must be kept.

     o EXPIRED -> UNBINDABLE

       If any server asks for information concerning this IP address,
       then the receiving server should set the IP address to be UNBIND-
       ABLE, update its stable storage, and respond to the requesting
       server.

     o EXPIRED -> BOUND

       If a server receives a message from a client and the IP address
       is EXPIRED, but was last BOUND or PUSHED to that client, then the
       IP address can be moved back into the BOUND state.  This is pos-
       sible because no other server can have attempted to make this IP
       address BINDABLE.  If it had, the IP address would not be in the
       EXPIRED state anymore, but in the UNBINDABLE state (see the
       EXPIRED -> UNBINDABLE transition above).

       Another reason this transition can occur is as follows.  When a
       server is removed from a server group, all of the IP addresses
       must be scanned to see if any of them show that server as the
       server who performed the last transaction (as set by that server



Kinnear, Cole & Droms                                          [Page 77]


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       successfully completing a CLIENT BINDING COMPLETE PUSH).  For all
       of those IP addresses, if there is a client recorded in the IP
       address, and if that client does not have a currently different
       binding, then that IP address must be set to BOUND and the lease
       time must be reset to the value sent in the latest CLIENT BINDING
       COMPLETE PUSH.

       The only states from which this transition will be made are
       UNBINDABLE and EXPIRED.


10.  Security Considerations

   Minimal security would be provided by configuring every server in a
   group with the IP addresses of the allowable servers that could ever
   join that group.

   Some additional security is created by using the SCSP security mecha-
   nism, although there are limitations to that for other than the
   client binding management part of the protocol.

   Other, more powerful security approaches are and must be addressed
   prior to further progress on this protocol.


11.  Open Questions

   The following open questions set off by the "*" character remain from
   Ralph Droms' original draft:  draft-ietf-dhc-interserver-00.txt.
   Comments have been added in square brackets [].  Additional open
   questions new to this draft are listed with the "o" character.


     * Each server must know all other servers.

       Requiring each server to know about every other server imposes
       additional administrative overhead in the configuration of DHCP
       servers.  However, this configuration overhead is probably mini-
       mal relative to any other configuration required for DHCP
       servers.

       [The group management messages in Section 7 provide a step
       towards an answer here.  A server needs to know only one other
       server.]

     * Each server must contact all other servers before reassigning an
       address.




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       [This is fundamental if we wish to use the "lazy synchronization"
       mode -- you can't get one without the other.]

       There is a potential issue here in which no new DHCP clients can
       be configured if any of the DHCP servers cannot be contacted.
       Servers can mitigate this problem by maintaining a list of pre-
       checked addresses that can be allocated without contacting all
       other servers at the time of address allocation.

       The protocol may need additional definition of specific actions
       on the part of DHCP servers in response to situations in which a
       server cannot contact all other servers.  [Added a lot of these
       in this draft.]

     * Servers cooperating to achieve "fair" distribution of available
       addresses.

       The protocol may need additional mechanisms or definition of
       default behavior through which servers cooperate among themselves
       to ensure that each has a sufficient pool of prechecked-addresses
       on each network.

       [Not yet addressed, and needs work. Initial thinking is that all
       addresses should be allocated to some server, so that if the
       event of a SG where one member can't be contacted, the maximum
       addresses are available for TRANSFER operations as necessary.]

     * User intervention in case of database incoherency.

       Fixing the collective database on the DHCP servers in case of a
       problem could be a *real* nightmare.

     * Potential deadlock in checking address - suppose two servers
       check the same address for reassignment simultaneously?

       [Solved with the introduction of the POLLING state.]

     * Potential configuration for new server?

       One ancillary use of the inter-server protocol might be in con-
       figuring new DHCP servers.  Suppose the inter-server protocol
       were extended to allow download of a server's configuration file
       and to allow addition of a new server to the list of DHCP
       servers.  A new server might be configured by simply giving it
       the address of an existing server.  The new server could then
       download a list of all other known servers, the pool of candidate
       addresses, any special configuration information (e.g., vendor
       class information) and the existing bindings.  The new server



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       could also announce itself to all of the other existing servers.

       [Much of this is in the current draft, principally in the group
       management configuration messages.  At this stage, a server can
       figure out which groups correspond with which subnets, which
       addresses that group manages on that subnet, and some additional
       configuration information.  This is considerable distance towards
       both ensuring that all servers in the SG have compatible configu-
       rations, as well as towards one server downloading configuration
       data from another server.

       Downloading configuration files would not be a great idea for
       servers which don't use configuration files.]

     * DHCP server maintenance

       There is likely an opportunity for the development of a server
       management tool that would download the database information from
       all servers and check for conflicts/inconsistencies such as
       assignment of an IP address to multiple clients, bindings that
       are not replicated across all servers, bindings that have incon-
       sistent lease expiration times, etc.

     o Group-id selection.

       The group-id's for various groups need to be sufficiently unique
       that no server will ever be a member of two groups with the same
       group-id.  No mechanism is provided yet in this protocol to gen-
       erate group-id's which conform to this requirement.

       Possibly a group-id can be synthesized in some manner to ensure
       that they conform to this requirement.

     o The original draft discussed the requirement for each server to
       have a synchronized clock using available time synchronization
       protocols.  That requirement has been removed in this draft, and
       in its place all times are sent in "seconds from now" as a signed
       32 bit number.  There is clearly a bit of additional complexity
       required to do this, but we have been so impressed at how well
       DHCP works with "relative" instead of "absolute" time that we
       felt the complexity of using relative time worth it (since using
       synchronized time is not without its own complexities).

     o UNAVAILABLE IP addresses

       There are several cases where a server can determine that some
       sort of serious error has occurred, and apparently an IP address
       is in an inconsistent state.  In these cases, the server should



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       make the IP address UNAVAILABLE -- i.e., no other server should
       be able to operate on it.  Just what is necessary to make this
       happen?  Could it be a passive response to address information
       messages, or must it involve a complete push to all of the other
       servers, and a new IP address state?


12.  Acknowledgments

   Many of the ideas in this proposal are due to Jeff Mogul, Greg Min-
   shall, Rob Stevens, Walt Wimer, Ted Lemon and the DHC working group.
   Thanks to all who have contributed their ideas and participated in
   the discussion of the inter-server protocol.

   At American Internet, Brad Parker and Mark Stapp have been key con-
   tributors to the design discussions that have resulted in our contri-
   butions to the this draft.  They have each invested many hours of
   work in this protocol.


13.  References


     [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
         March 1997.

     [2] Luciani, J., Armitage, G., Halpern, J., "Server Cache Synchro-
         nization Protocol (SCSP)",  draft-ietf-ion-scsp-01.txt.

     [3] Moy, J.  "OSPF Version 2", IETF RFC1247, July 1991.

     [4] Luciani, J.,  "A Distributed NHRP Service Using SCSP", draft-
         ietf-ion-scsp-nhrp-00.txt.

     [5] Luciani, J., Fox, B., "A Distributed ATMARP Service Using
         SCSP", draft-ietf-ion-scsp-atmarp-00.txt.

     [6] Reynolds, J., Postel, J., "Assigned Numbers", Internet STD 2,
         Internet RFC 1340,  USC/Information Sciences Institute, July
         1992.

     [7] Alexander, S.,  Droms, R., "DHCP Options and BOOTP Vendor
         Extensions", Internet RFC 2132, March 1997.

     [8] Gudmundsson, Olafur, "Security Architecture for DHCP", draft-
         ietf-dhc-security-arch-00.txt.





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14.  Author's information

      Kim Kinnear
      American Internet Corporation
      4 Preston Ct.
      Bedford, MA  01730-2334

      Phone: (617) 276-4587
      EMail: kinnear@american.com


      Robert G. Cole
      AT&T Laboratories
      Managed Network Solutions Division
      Rm. 3L-533
      101 Crawfords Corner Road
      Holmdel, NJ  07733

      Phone: (908) 949-1950
      EMail: rgc@qsun.att.com


      Ralph Droms
      Computer Science Department
      323 Dana Engineering
      Bucknell University
      Lewisburg, PA 17837

      Phone: (717) 524-1145
      EMail: droms@bucknell.edu





















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Appendix A:  An Overview of SCSP

   This appendix presents an overview of the SCSP protocol and supple-
   ments Section 8.2 in the main text of this specification.  For a com-
   plete discussion of the SCSP protocol see [2].

   This appendix is divided into three following sections on the SCSP
   Hello, Cache Alignment and Cache Update subprotocols respectively.
   The last section of this appendix presents a summary of the SCSP mes-
   sage sets.

A.1 The SCSP "Hello" Sub-protocol Overview

   The function of the SCSP "Hello" protocol is to monitor the status of
   the LS to DCS connection.  The LS must be configured with the
   addresses of its DCSs.  The protocol contains a 'Family ID' which
   allows for the multiplexing of multiple protocol specific SCSP imple-
   mentations to rely on a single Hello mechanism between each server
   pair.  For each DCS (whether the low level connection is point-to-
   point or point-to-multipoint), the LS maintains an Hello Finite State
   Machine (HFSM).  The HFSM  is shown in the figure below.


                          +---------------+
                          |               |
                 +------->|     DOWN      |<-------+
                 |        |               |        |
                 |        +---------------+        |
                 |            |       ^            |
                 |            |       |            |
                 |            |       |            |
                 |            |       |            |
                 |            V       |            |
                 |        +---------------+        |
                 |        |               |        |
                 |        |    WAITING    |        |
                 |     +--|               |--+     |
                 |     |  +---------------+  |     |
                 |     |    ^           ^    |     |
                 |     |    |           |    |     |
                 |     V    |           |    V     |
               +---------------+     +---------------+
               |  BIDIRECTION  |---->|  UNIDIRECTION |
               |               |     |               |
               |  CONNECTION   |<----|  CONNECTION   |
               +---------------+     +---------------+

              Figure A.1-1  The Hello Finite State Machine



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   Key:


     1: Link layer connection is established

     2: Transition based upon the receipt of a Hello message (and
        whether the LS ID is found in the  Rec ID portion of the message

     3: Hello Interval * Dead Factor exceeded

     4: Loss of link layer connectivity

   The LS to DCS connections are initialized into the down state.  The
   numbers in the figure refer to the actions discussed in the Key that
   cause a transition in the HFSM (Note:  These numbers didn't appear in
   the original figure in [2], and are TBD).  The Hello protocol employs
   poll messages to monitor the status of the LS to DCS connections.

   The Hello messages contain the ID s of the DCS s that the LS has
   received a Hello message from.  The LS' HFSM uses these ID s to
   determine the status of the HFSM for each of the DCS s.  Multiple DCS
   ID s are present in order to support point-to-multipoint connections.
   The messages also contain two fields; the Polling Interval and the
   Dead Factor.  The product of the Polling Interval and the Dead Factor
   determines the length of time that the HFSM will hold open a connec-
   tion without receiving a Hello from a peer DCS and transitioning the
   HFSM for that DCS to the Wait state.


A.2  The SCSP "Cache Alignment" Sub-protocol

   The Cache Alignment protocol supports the initial server cache syn-
   chronization process of an LS with its DCSs.  This process may occur
   at initial boot time of the server, at reconnect time of the server
   to the network, or other possible initialization or failure recovery
   scenarios.  Like the Hello protocol, the Cache Alignment (CA) proto-
   col maintains a Cache Alignment Finite State Machine (CAFSM) for each
   of its DCSs to monitor the status of its cache alignment.  The figure
   below shows the CAFSM and indicates some of the triggers that would
   cause the state transitions to occur.











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                      +------------+
                      |            |
                 +--->|    DOWN    |
                 |    |            |
                 |    +------------+
                 |          |
                 |          |
                 |          V
                 |    +------------+
                 |    |Master/Slave|
                 |----|            |<---+
                 |    |Negotiation |    |
                 |    +------------+    |
                 |          |           |
                 |          |           |
                 |          V           |
                 |    +------------+    |
                 |    |   Cache    |    |
                 |----|            |----|
                 |    | Summarize  |    |
                 |    +------------+    |
                 |          |           |
                 |          |           |
                 |          V           |
                 |    +------------+    |
                 |    |   Update   |    |
                 |----|            |----|
                 |    |   Cache    |    |
                 |    +------------+    |
                 |          |           |
                 |          |           |
                 |          V           |
                 |    +------------+    |
                 |    |            |    |
                 +----|  Aligned   |----+
                      |            |
                      +------------+


           Figure A.2-1  Cache Alignment Finite State Machine


Key:


     1: When HFSM reaches Bi-directional state





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     2: HFSM transitions out of Bi-directional state

     3: Master/Slave relationship is established

     4: Once both LS and DCS exchange CA messages, both with O-bit set
        to 0, then CRL is complete

     5: E.g., Errored sequence number

     6: Full cache update achieved

   (Note: The key numbers don't appear in the figure in [2],a and are
   TBD.)

   Each of the CAFSMs is coupled with the respective HFSMs in the LS.
   The CAFSM is initialized in the Down state.  It transitions to the
   Master/Slave Negotiation state when the corresponding HFSM transi-
   tions to the Bi-Directional  state.  The CAFSM transitions back to
   the Down state in the event that the corresponding HFSM transitions
   out of the Bi-Directional state.

   In the Master/Slave state the LS-DCS pair negotiate who is to be the
   master of the connection during the cache alignment process.  In the
   Cache Summary state the LS/DCS pair exchange Client State Advertise-
   ment Summary (CSAS) records within the CA messages.  The servers use
   these message exchanges to build a Client State Advertisement Request
   List (CRL).  The CRL indicates the portions of the respective server
   caches that are out of alignment.  The cache mis-alignment (as indi-
   cated in the local CRL) is resolved in the Update Cache state where
   the servers exchange full client state information in CSA records
   within the CSU messages, only where mis-alignment occurs.  Once the
   CRL is resolved, the LS/DCS caches are aligned and the CAFSM transi-
   tions to the Aligned state.

   The protocol further defines the high-level syntax of a generic CA
   message as discussed in a later section of this appendix.

A.3  The SCSP "Client State Update" Sub-protocol Overview

   The purpose of the Client State Update (CSU) protocol is to provide a
   capability to constantly update the server caches  through asyn-
   chronous CSU message exchanges.  These updates are necessary because
   the status of the clients are in constant flux.  Unlike the other two
   sub-protocols, the Client State Update protocol does not maintain a
   separate finite state machine.  Instead, the activity of this proto-
   col is tied to the CAFSM.

   Each CSU can contain zero or more Client State Advertisement records.



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   The LS may send and receive CSUs when the corresponding CAFSM is in
   either the Aligned or the Cache Update states.  The CSU protocol
   defines both CSU requests and reply messages.  As consistent through-
   out the definition of the SCSP, the CSU protocol supports both point-
   to-point and point-to-multipoint connections.

A.4  The SCSP Message Set Overview

   The structure of the SCSP messages is a)a fixed length, generic
   header, b) a SCSP message specific part header of variable length, c)
   an fixed length, message field and d) zero, one or more SCSP message
   specific records.  This is shown in the following figure.


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Version       |   type        |       Packet Size             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     IP Checksum               |    Start of Extensions        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          SCSP Message Specific part (variable)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Protocol ID             |           SG  ID              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       unused                  |          Flags                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Sender ID Len | Recvr  ID Len |       No. of Records          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Sender ID   (variable)                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Receiver ID   (variable)                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         SCSP Message Specific Records   (variable)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                   Figure A.4-1  SCSP Message Format

   where

     o Version - is the version of the SCSP protocol defined in [2]

     o type - represents the SCSP message type, i.e., CA, Hello,
       CSU_Req, CSU_Reply, and CSU_Solicit

     o Packet Size -




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   The SCSP messages have identical syntax except for the 1) the SCSP
   message specific part header and 2) the SCSP message specific part
   record.  The following table summarizes the content of these specific
   parts:



                    Table A.4-1  SCSP Message Specific Parts

               |  Hello    |   CA      |   CSUS    | CSU_Req   |  CSU_Reply
   ------------------------------------------------------------------------
               |           |           |           |           |
   SCSP mesg   | hello int,|CSA Seq.No.|  null     |  null     |   null
   spec header | dead fac.,|           |           |           |
               | Family ID |           |           |           |
   ------------------------------------------------------------------------
               |           |           |           |           |
   SCSP mesg   |Additional |CSAS Rec.  | CSAS Rec. | CSA  Rec. | CSAS Rec.
   spec record | Recvr ID  |           |           |           |
               | records   |           |           |           |



   The detailed formats of the various SCSP messages are given in [2].
   However, two SCSP message specific records are of particular interest
   to the development of the DHCP interserver specification.  These are:
   1) the CSAS record and 2) the CSA record.  The CSAS record is defined
   within the SCSP specification as:


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Hop Count             |       Record Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Cache Key Len | Orig ID Len   |N|      unused                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      CSA Sequence Number                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Cache Key    (variable)                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Originator ID  (variable)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                 Figure A.4-2  SCSP CSAS Record Format

   See Section 8.4.1 for details.



Kinnear, Cole & Droms                                          [Page 88]


DRAFT                                                          July 1997


   The CSA record is defined within the SCSP specification as:


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        CSAS Record                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Client/Server Protocol Specific Part Cache Entry     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                  Figure A.4-3  SCSP CSA Record Format

   The CSA records for the DHCP interserver mapping to SCSP are defined
   in Section 8.4.2.


   [end of document <draft-ietf-dhc-interserver-02.txt>]
































Kinnear, Cole & Droms                                          [Page 89]


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