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NFSv4                                                     D. Noveck, Ed.
Internet-Draft                                                       EMC
Intended status: Informational                                 P. Shivam
Expires: September 22, 2013                                     C. Lever
                                                                B. Baker
                                                                  ORACLE
                                                          March 21, 2013


 NFSv4 migration: Implementation experience and spec issues to resolve
                  draft-ietf-nfsv4-migration-issues-03

Abstract

   The migration feature of NFSv4 provides for moving responsibility for
   a single filesystem from one server to another, without disruption to
   clients.  Recent implementation experience has shown problems in the
   existing specification for this feature.  This document discusses the
   issues which have arisen, explores the options available for curing
   the issues, and explains the choices made in updating the NFSv4.0 and
   NFSv4.1 specifications, to address migration.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on September 22, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  NFSv4.0 Implementation Experience . . . . . . . . . . . . . .   4
     3.1.  Implementation issues . . . . . . . . . . . . . . . . . .   4
       3.1.1.  Failure to free migrated state on client reboot . . .   4
       3.1.2.  Server reboots resulting in a confused lease
               situation . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.3.  Client complexity issues  . . . . . . . . . . . . . .   6
     3.2.  Sources of Protocol difficulties  . . . . . . . . . . . .   8
       3.2.1.  Issues with nfs_client_id4 generation and use . . . .   8
       3.2.2.  Issues with lease proliferation . . . . . . . . . . .   9
   4.  Issues to be resolved in NFSv4.0  . . . . . . . . . . . . . .  10
     4.1.  Possible changes to nfs_client_id4 client-string  . . . .  10
     4.2.  Possible changes to handle differing nfs_client_id4
           string values . . . . . . . . . . . . . . . . . . . . . .  11
     4.3.  Other issues within migration-state sections  . . . . . .  12
     4.4.  Issues within other sections  . . . . . . . . . . . . . .  12
   5.  Proposed resolution of NFSv4.0 protocol difficulties  . . . .  13
     5.1.  Proposed changes: nfs_client_id4 client-string  . . . . .  13
     5.2.  Proposed changes: merged (vs. synchronized) leases  . . .  13
     5.3.  Other proposed changes to migration-state sections  . . .  15
       5.3.1.  Proposed changes: Client ID migration . . . . . . . .  15
       5.3.2.  Proposed changes: Callback re-establishment . . . . .  16
       5.3.3.  Proposed changes: NFS4ERR_LEASE_MOVED rework  . . . .  16
     5.4.  Proposed changes to other sections  . . . . . . . . . . .  17
       5.4.1.  Proposed changes: callback update . . . . . . . . . .  17
       5.4.2.  Proposed changes: clientid4 handling  . . . . . . . .  17
       5.4.3.  Proposed changes: NFS4ERR_CLID_INUSE  . . . . . . . .  19
   6.  Results of proposed changes for NFSv4.0 . . . . . . . . . . .  19
     6.1.  Results: Failure to free migrated state on client reboot   20
     6.2.  Results: Server reboots resulting in confused lease
           situation . . . . . . . . . . . . . . . . . . . . . . . .  20
     6.3.  Results: Client complexity issues . . . . . . . . . . . .  22
     6.4.  Result summary  . . . . . . . . . . . . . . . . . . . . .  23
   7.  Issues for NFSv4.1  . . . . . . . . . . . . . . . . . . . . .  23
     7.1.  Addressing state merger in NFSv4.1  . . . . . . . . . . .  23
     7.2.  Addressing pNFS relationship with migration . . . . . . .  24
     7.3.  Addressing server owner changes in NFSv4.1  . . . . . . .  24
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26



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   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     11.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   This document is in the informational category, and while the facts
   it reports may have normative implications, any such normative
   significance reflects the readers' preferences.  For example, we may
   report that the reboot of a client with migrated state results in
   state not being promptly cleared and that this will prevent granting
   of conflicting lock requests at least for the lease time, which is a
   fact.  While it is to be expected that client and server implementers
   will judge this to be a situation that is best avoided, the judgment
   as to how pressing this issue should be considered is a judgment for
   the reader, and eventually the nfsv4 working group to make.

   We do explore possible ways in which such issues can be avoided, with
   minimal negative effects, given that the working group has decided to
   address these issues, but the choice of exactly how to address these
   is best given effect in one or more standards-track documents and/or
   errata.

   This document focuses on NFSv4.0, since that is where the majority of
   implementation experience has been.  Nevertheless, there is
   discussion of the implications of the NFSv4.0 experience for
   migration in NFSv4.1, as well as discussion of other issues with
   regard to the treatment of migration in NFSv4.1.

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   In the context of this informational document, these normative
   keywords will always occur in the context of a quotation, most often
   direct but sometimes indirect.  The context will make it clear
   whether the quotation is from:

   o  The current definitive definition of the NFSv4.0 protocol, whether
      that is the original NFSv4.0 specification [RFC3530], or its
      expected successor [RFC3530bis].

      As the identity of that document may change during the lifetime of
      this document, we will often refer to the current or pending



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      definition of NFSv4.0 and quote from portions of the documents
      that are identical among all existing drafts.  Given that RFC3530
      and all RFC3530bis drafts agree as to the issues under discussion,
      this should not cause undue difficulty.  Note that to simplify
      document maintenance, section names rather than section numbers
      are used when referring to sections in existing documents so that
      only minimal changes will be necessary as the identity of the
      document defining NFSv4.0 changes.

   o  The current definitive definition of the NFSv4.1 protocol
      [RFC5661].

   o  A proposed or possible text to serve as a replacement for the
      current definitive document text.  Sometimes, a number of possible
      alternative texts may be listed and benefits and detriments of
      each examined in turn.

3.  NFSv4.0 Implementation Experience

3.1.  Implementation issues

   Note that the examples below reflect current experience which arises
   from clients implementing the recommendation to use different
   nfs_client_id4 id strings for different server addresses, i.e.  using
   what is later referred to herein as the "non-uniform client-string
   approach."

   This is simply because that is the experience implementers have had.
   The reader should not assume that in all cases, this practice is the
   source of the difficulty.  It may be so in some cases but clearly it
   is not in all cases.

3.1.1.  Failure to free migrated state on client reboot

   The following sort of situation has proved troublesome:

   o  A client C establishes a clientid4 C1 with server ABC specifying
      an nfs_client_id4 with id string value "C-ABC" and boot verifier
      0x111.

   o  The client begins to access files in filesystem F on server ABC,
      resulting in generating stateids S1, S2, etc.  under the lease for
      clientid C1.  It may also access files on other filesystems on the
      same server.

   o  The filesystem is migrated from server ABC to server XYZ.  When
      transparent state migration is in effect, stateids S1 and S2 and
      clientid4 C1 are now available for use by client C at server XYZ.



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   o  Client C reboots and attempts to access data on server XYZ,
      whether in filesystem F or another.  It does a SETCLIENTID with an
      nfs_client_id4 with id string value "C-XYZ" and boot verifier
      0x112.  There is thus no occasion to free stateids S1 and S2 since
      they are associated with a different client name and so lease
      expiration is the only way that they can be gotten rid of.

   Note here that while it seems clear to us in this example that C-XYZ
   and C-ABC are from the same client, the server has no way to
   determine the structure of the "opaque" id string.  In the protocol,
   it really is treated as opaque.  Only the client knows which
   nfs_client_id4 values designate the same client on a different
   server.

3.1.2.  Server reboots resulting in a confused lease situation

   Further problems arise from scenarios like the following.

   o  Client C talks to server ABC using an nfs_client_id4 id string
      such as "C-ABC" and a boot verifier v1.  As a result, a lease with
      clientid4 c.i is established: {v1, "C-ABC", c.i}.

   o  fs_a1 migrates from server ABC to server XYZ along with its state.
      Now server XYZ also has a lease: {v1, "C-ABC", c.i}.

   o  Server ABC reboots.

   o  Client C talks to server ABC using an nfs_client_id4 id string
      such as "C-ABC" and a boot verifier v1.  As a result, a lease with
      clientid4 c.j is established: {v1, "C-ABC", c.j}.

   o  fs_a2 migrates from server ABC to server XYZ.  Now server XYZ also
      has a lease: {v1, "C-ABC", c.j}.

   o  Now server XYZ has two leases that match {v1, "C-ABC", *}, when
      the protocol clearly assumes there can be only one.

   Note that if the client used "C" (rather than "C-ABC") as the
   nfs_client_id4 id string, the exact same situation would arise.

   One of the first cases in which this sort of situation has resulted
   in difficulties is in connection with doing a SETCLIENTID for
   callback update.

   The SETCLIENTID for callback update only includes the nfs_client_id4,
   assuming there can only be one such with a given nfs_client_id4
   value.  If there were multiple, confirmed client records with
   identical nfs_client_id4 id string values, there would be no way to



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   map the callback update request to the correct client record.  Apart
   from the migration handling specified in [RFC3530] and [RFC3530bis],
   such a situation cannot arise.

   One possible accommodation for this particular issue that has been
   used is to add a RENEW operation along with SETCLIENTID (on a
   callback update) to disambiguate the client.

   When the client updates the callback info to the destination, the
   client would, by convention, send a compound like this:

   { RENEW clientid4, SETCLIENTID nfs_client_id4,verf,cb }

   The presence of the clientid4 in the compound would allow the server
   to differentiate among the various leases that it knows of, all with
   the same nfs_client_id4 value.

   While this would be a reasonable patch for an isolated protocol
   weakness, interoperable clients and servers would require that the
   protocol truly be updated to allow such a situation, specifically
   that of multiple clientid4's with the same nfs_client_id4 value.  The
   protocol is currently designed and implemented assuming this cannot
   happen.  We need to either prevent the situation from happening, or
   fully adapt to the possibilities which can arise.  See Section 4 for
   a discussion of such issues.

3.1.3.  Client complexity issues

   Consider the following situation:

   o  There are a set of clients C1 through Cn accessing servers S1
      through Sm.  Each server manages some significant number of
      filesystems with the filesystem count L being significantly
      greater than m.

   o  Each client Cx will access a subset of the servers and so will
      have up to m clientids, which we will call Cxy for server Sy.

   o  Now assume that for load-balancing or other operational reasons,
      numbers of filesystems are migrated among the servers.  As a
      result, each client-server pair will have up to m clientids and
      each client will have up to m**2 clientids.  If we add the
      possibility of server reboot, the only bound on a client's
      clientid count is L.

   Now, instead of a clientid4 identifying a client-server pair, we have
   many more entities for the client to deal with.  In addition, it
   isn't clear how new state is to be incorporated in this structure.



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   The limitations of the migrated state (inability to be freed on
   reboot) would argue against adding more such state but trying to
   avoid that would run into its own difficulties.  For example, a
   single lockowner string presented under two different clientids would
   appear as two different entities.

   Thus we have to choose between:

   o  indefinite prolongation of foreign clientids even after all
      transferred state is gone.

   o  having multiple requests for the same lockowner-string-named
      entity carried on in parallel by separate identically named
      lockowners under different clientid4's

   o  Adding serialization at the lock-owner string level, in addition
      to that at the lockowner level.

   In any case, we have gone (in adding migration as it was described)
   from a situation in which

   o  Each client has a single clientid4/lease for each server it talks
      to.

   o  Each client has a single nfs_client_id4 for each server it talks
      to.

   o  Every state id can be mapped to an associated lease based on the
      server it was obtained from.

   To one in which

   o  Each client may have multiple clientid4's for a single server.

   o  For each stateid, the client must separately record the clientid4
      that it is assigned to, or it must manage separate "state blobs"
      for each fsid and map those to clientid4's.

   o  Before doing an operation that can result in a stateid, the client
      must either find a "state blob" based on fsid or create a new one,
      possibly with a new clientid4.

   o  There may be multiple clientid4's all connected to the same server
      and using the same nfs_clientid4.

   This sort of additional client complexity is troublesome and needs to
   be eliminated.




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3.2.  Sources of Protocol difficulties

3.2.1.  Issues with nfs_client_id4 generation and use

   The current definitive definitions of the NFSv4.0 protocol, [RFC3530]
   and [RFC3530bis] both agree.  The section entitled "Client ID" says:

      The second field, id is a variable length string that uniquely
      defines the client.

   There are two possible interpretations of the phrase "uniquely
   defines" in the above:

   o  The relation between strings and clients is a function from such
      strings to clients so that each string designates a single client.

   o  The relation between strings and clients is a bijection between
      such strings and clients so that each string designates a single
      client and each client is named by a single string.

   The first interpretation would make these client-strings like phone
   numbers (a single person can have several) while the second would
   make them like social security numbers.

   Endless debate about the true meaning of "uniquely defines" in this
   context is quite possible but not very helpful.  The following points
   should be noted though:

   o  The second interpretation is more consistent with the way
      "uniquely defines" is used elsewhere in the spec.

   o  The spec as now written intends the first interpretation (or is
      internally inconsistent).  In fact, it recommends, although it
      doesn't "RECOMMEND" that a single client have at least as many
      client-strings as server addresses that it interacts with.  It
      says, in the third bullet point regarding construction of the
      string (which we shall henceforth refer to as client-string-BP3):

         The string should be different for each server network address
         that the client accesses, rather than common to all server
         network addresses.

   o  If internode interactions are limited to those between a client
      and its servers, there is no occasion for servers to be concerned
      with the question of whether two client-strings designate the same
      client, so that there is no occasion for the difference in
      interpretation to matter.




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   o  When transparent migration of client state occurs between two
      servers, it becomes important to determine when state on two
      different servers is for the same client or not, and this
      distinction becomes very important.

   Given the need for the server to be aware of client identity with
   regard to migrated state, either client-string construction rules
   will have to change or there will be a need to get around current
   issues, or perhaps a combination of these two will be required.
   Later sections will examine the options and propose a solution.

   One consideration that may indicate that this cannot remain exactly
   as it is today has to do with the fact that the current explanation
   for this behavior is not correct.  The current definitive definitions
   of the NFSv4.0 protocol, [RFC3530] and [RFC3530bis] both agree.  The
   section entitled "Client ID" says:

      The reason is that it may not be possible for the client to tell
      if the same server is listening on multiple network addresses.  If
      the client issues SETCLIENTID with the same id string to each
      network address of such a server, the server will think it is the
      same client, and each successive SETCLIENTID will cause the server
      to begin the process of removing the client's previous leased
      state.

   In point of fact, a "SETCLIENTID with the same id string" sent to
   multiple network addresses will be treated as all from the same
   client but will not "cause the server to begin the process of
   removing the client's previous leased state" unless the server
   believes it is a different instance of the same client, i.e.  if the
   id string is the same and there is a different boot verifier.  If the
   client does not reboot, the verifier should not change.  If it does
   reboot, the verifier will change, and the server should "begin the
   process of removing the client's previous leased state.

   The situation of multiple SETCLIENTID requests received by a server
   on multiple network addresses is exactly the same, from the protocol
   design point of view, as when multiple (i.e.  duplicate) SETCLIENTID
   requests are received by the server on a single network address.  The
   same protocol mechanisms that prevent erroneous state deletion in the
   latter case prevent it in the former case.  There is no reason for
   special handling of the multiple-network-appearance case, in this
   regard.

3.2.2.  Issues with lease proliferation

   It is often felt that this is a consequence of the client-string
   construction issues, and it is certainly the case that the two are



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   closely connected in that non-uniform client-strings make it
   impossible for the server to appropriately combine leases from the
   same client.

   However, even where the server could combine leases from the same
   client, it needs to be clear how and when it will do so, so that the
   client will be prepared.  These issues will have to be addressed at
   various places in the spec.

   This could be enough only if we are prepared to do away with the
   "should" recommending non-uniform client-strings and replace it with
   a "should not" or even a "SHOULD NOT".  Current client implementation
   patterns make this an unpalatable choice for use as a general
   solution, but it is reasonable to "RECOMMEND" this choice for a well-
   defined subset of clients.  One alternative would be to create a way
   for the server to infer from client behavior which leases are held by
   the same client and use this information to do appropriate lease
   mergers.  Prototyping and detailed specification work has shown that
   this could be done but the resulting complexity is such that a better
   choice is to "RECOMMEND" use of the uniform client-string approach
   for clients supporting the migration feature.

   Because of the discussion of client-string construction in [RFC3530]
   and [RFC3530bis], most existing clients implement the non-uniform
   client-string approach.  As a result, existing servers may not have
   been tested with clients implementing uniform client-strings.  As a
   consequence, care must be taken to preserve interoperability between
   UCS-capable clients and servers that don't tolerate uniform client
   strings for one reason or another.

4.  Issues to be resolved in NFSv4.0

4.1.  Possible changes to nfs_client_id4 client-string

   The fact that the reason given in client-string-BP3 is not valid
   makes the existing "should" insupportable.  We can't either

   o  Keep a reason we know is invalid.

   o  Keep saying "should" without giving a reason.

   What are often presented as reasons that motivate use of the non-
   uniform approach always turn out to be cases in which, if the uniform
   approach were used, the server will treat a client which accesses
   that server via two different IP addresses as part of a single
   client, as it in fact is.  This may be disconcerting to a client
   unaware that the two IP addresses connect to the same server.  This
   is not a reason to use the non-uniform approach but is better thought



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   of as an illustration of the fact that those using the uniform
   approach need to be aware of the possibility of server trunking and
   its effect on server behavior.

   If it is possible to reliably infer the existence of trunking of
   server IP addresses from observed server behavior, use of the uniform
   approach would be more desirable, although compatibility issues would
   have to be dealt with.

   It is always possible that a valid new reason will be found, but so
   far none has been proposed.  Given the history, the burden of proof
   should be on those asserting the validity of a proposed new reason.

   So we will assume for now that the "should" will have to go.  The
   question is what to replace it with.

   o  We can't say "MUST NOT", despite the problems this raises for
      migration since this is pretty late in the day for such a change.
      Many currently operating clients obey the existing "should".
      Similar considerations would apply for "SHOULD NOT" or "should
      not".

   o  Dropping client-string-BP3 entirely is a possibility but, given
      the context and history, it would just be a confusing version of
      "SHOULD NOT".

   o  Using "MAY" would clearly specify that both ways of doing this are
      valid choices for clients and that servers will have to deal with
      clients that make either choice.

   o  This might be modified by a "SHOULD" (or even a "MUST") for
      particular groups of clients.

   o  There will have to be some text explaining why a client might make
      either choice but, except for the particular cases referred to
      above, we will have to make sure that it is truly descriptive, and
      not slanted in either direction.

4.2.  Possible changes to handle differing nfs_client_id4 string values

   Given the difficulties caused by having different nfs_client_id4
   client-string values for the same client, we have two choices:

   o  Deprecate the existing treatment and basically say the client is
      on its own doing migration, if it follows it.






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   o  Introduce a way of having the client provide client identity
      information to the server, if it can be done compatibly while
      staying within the bounds of v4.0.

4.3.  Other issues within migration-state sections

   There are a number of issues where the existing text is unclear and/
   or wrong and needs to be fixed in some way.

   o  Lack of clarity in the discussion of moving clientids (as well as
      stateids) as part of moving state for migration.

   o  The discussion of synchronized leases is wrong in that there is no
      way to determine (in the current spec) when leases are for the
      same client and also wrong in suggesting a benefit from leases
      synchronized at the point of transfer.  What is needed is merger
      of leases, which is necessary to keep client complexity
      requirements from getting out of hand.

   o  Lack of clarity in the discussion of LEASE_MOVED handling,
      including failure to fully address situations in which transparent
      state migration did not occur.

4.4.  Issues within other sections

   There are a number of cases in which certain sections, not
   specifically related to migration, require additional clarification.
   This is generally because text that is clear in a context in which
   leases and clientids are created in one place and live there forever
   may need further refinement in the more dynamic environment that
   arises as part of migration.

   Some examples:

   o  Some people are under the impression that updating callback
      endpoint information for an existing client, as used during
      migration, may cause the destination server to free existing
      state.  There need to be additions to clarify the situation.

   o  The handling of the sets of clientid4's maintained by each server
      needs to be clarified.  In particular, the issue of how the client
      adapts to the presumably independent and uncoordinated clientid4
      sets needs to be clearly addressed

   o  Statements regarding handling of invalid clientid4's need to be
      clarified and/or refined in light of the possibilities that arise
      due to lease motion and merger.




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   o  Confusion and lack of clarity about NFS4ERR_CLID_INUSE.

5.  Proposed resolution of NFSv4.0 protocol difficulties

   This section lists the changes which we believe are necessary to
   resolve the difficulties mentioned above.  Such change, along with
   other clarifications found to be desirable during drafting and review
   are contained in [migr-v4.0-update].

5.1.  Proposed changes: nfs_client_id4 client-string

   We propose replacing client-string-BP3 with the following text and
   adding the following proposed to provide implementation guidance.

      The string MAY be different for each server network address that
      the client accesses, rather than common to all server network
      addresses.

   In addition, given the importance of the issue of client identity and
   the fact that both client string-approaches are to be considered
   valid, a greatly expanded treatment of client identity desirable.  It
   should have the following major elements.

   o  It should fully describe the consequences of making the string
      different for each network address (the non-uniform client-string
      approach) and of making it the same for all network addresses (the
      uniform client string approach).

   o  It should give helpful guidance about the factors that might
      affect client implementation choice between these approaches.

   o  It should describe the compatibility issues that might cause
      servers to be incompatible with the uniform approach and give
      guidance about dealing with these.

   o  It should describe how a client using the uniform approach might
      use server behavior to determine server address trunking patterns.

   o  It should present a clearer and more complete set of
      recommendations to guide client string construction.

5.2.  Proposed changes: merged (vs.  synchronized) leases

   The current definitive definitions of the NFSv4.0 protocol, [RFC3530]
   and [RFC3530bis] both agree.  The section entitled "Migration and
   State" says:





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      As part of the transfer of information between servers, leases
      would be transferred as well.  The leases being transferred to the
      new server will typically have a different expiration time from
      those for the same client, previously on the old server.  To
      maintain the property that all leases on a given server for a
      given client expire at the same time, the server should advance
      the expiration time to the later of the leases being transferred
      or the leases already present.  This allows the client to maintain
      lease renewal of both classes without special effort:

   There are a number of problems with this and any resolution of our
   difficulties must address them somehow.

   o  The current v4.0 spec recommends that the client make it
      essentially impossible to determine when two leases are from "the
      same client".

   o  It is not appropriate to speak of "maintain[ing] the property that
      all leases on a given server for a given client expire at the same
      time", since this is not a property that holds even in the absence
      of migration.  A server listening on multiple network addresses
      may have the same client appear as multiple clients with no way to
      recognize the client as the same.

   o  Even if the client identity issue could be resolved, advancing the
      lease time at the point of migration would not maintain the
      desired synchronization property.  The leases would be
      synchronized until one of them was renewed, after which they would
      be unsynchronized again.

   To avoid client complexity, we need to have no more than one lease
   between a single client and a single server.  This requires merger of
   leases since there is no real help from synchronizing them at a
   single instant.

   For the uniform approach, the destination server would simply merge
   leases as part of state transfer, since two leases with the same
   nfs_client_id4 values must be for the same client.

   We have made the following decisions as far as proposed normative
   statements regarding for state merger.  They reflect the facts that
   we want to support fully migration support in the simplest way
   possible and that we can't say MUST since we have older clients and
   servers to deal with.

   o  Clients SHOULD use the uniform client-string approach in order to
      get good migration support.




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   o  Servers SHOULD provide automatic lease merger during state
      migration so that clients using the uniform id approach get the
      support automatically.

   If the clients and the servers obey the SHOULD's, having more than a
   single lease for a given client-server pair will be a transient
   situation, cleaned up as part of adapting to use of migrated state.

   Since clients and servers will be a mixture of old and new and
   because nothing is a MUST we have to ensure that no combination will
   show worse behavior than is exhibited by current (i.e.  old) clients
   and servers.

5.3.  Other proposed changes to migration-state sections

5.3.1.  Proposed changes: Client ID migration

   The current definitive definitions of the NFSv4.0 protocol, [RFC3530]
   and [RFC3530bis] both agree.  The section entitled "Migration and
   State" says:

      In the case of migration, the servers involved in the migration of
      a filesystem SHOULD transfer all server state from the original to
      the new server.  This must be done in a way that is transparent to
      the client.  This state transfer will ease the client's transition
      when a filesystem migration occurs.  If the servers are successful
      in transferring all state, the client will continue to use
      stateids assigned by the original server.  Therefore the new
      server must recognize these stateids as valid.  This holds true
      for the client ID as well.  Since responsibility for an entire
      filesystem is transferred with a migration event, there is no
      possibility that conflicts will arise on the new server as a
      result of the transfer of locks.

   This poses some difficulties, mostly because the part about "client
   ID" is not clear:

   o  It isn't clear what part of the paragraph the "this" in the
      statement "this holds true ..." is meant to signify.

   o  The phrase "the client ID" is ambiguous, possibly indicating the
      clientid4 and possibly indicating the nfs_client_id4.

   o  If the text means to suggest that the same clientid4 must be used,
      the logic is not clear since the issue is not the same as for
      stateids of which there might be many.  Adapting to the change of
      a single clientid, as might happen as a part of lease migration,
      is relatively easy for the client.



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   We have decided that it is best to address this issue as follows:

   o  Make it clear that both clientid4 and nfs_client_id4 (including
      both id string and boot verifier) are to be transferred.

   o  Indicate that the initial transfer will result in the same
      clientid4 after transfer but this is not guaranteed since there
      may conflict with an existing clientid4 on the destination server
      and because lease merger can result in a change of the clientid4.

5.3.2.  Proposed changes: Callback re-establishment

   The current definitive definitions of the NFSv4.0 protocol, [RFC3530]
   and [RFC3530bis] both agree.  The section entitled "Migration and
   State" says:

      A client SHOULD re-establish new callback information with the new
      server as soon as possible, according to sequences described in
      sections "Operation 35: SETCLIENTID - Negotiate Client ID" and
      "Operation 36: SETCLIENTID_CONFIRM - Confirm Client ID".  This
      ensures that server operations are not blocked by the inability to
      recall delegations.

   The above will need to be fixed to reflect the possibility of merging
   of leases,

5.3.3.  Proposed changes: NFS4ERR_LEASE_MOVED rework

   The current definitive definitions of the NFSv4.0 protocol, [RFC3530]
   and [RFC3530bis] both agree.  The section entitled "Notification of
   Migrated Lease" says:

      Upon receiving the NFS4ERR_LEASE_MOVED error, a client that
      supports filesystem migration MUST probe all filesystems from that
      server on which it holds open state.  Once the client has
      successfully probed all those filesystems which are migrated, the
      server MUST resume normal handling of stateful requests from that
      client.

   There is a lack of clarity that is prompted by ambiguity about what
   exactly probing is and what the interlock between client and server
   must be.  This has led to some worry about the scalability of the
   probing process, and although the time required does scale linearly
   with the number of filesystems that the client may have state for
   with respect to a given server, the actual process can be done
   efficiently.





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   To address these issues we propose rewriting the above to be more
   clear and to give suggestions about how to do the required scanning
   efficiently.

5.4.  Proposed changes to other sections

5.4.1.  Proposed changes: callback update

   Some changes are necessary to reduce confusion about the process of
   callback information update and in particular to make it clear that
   no state is freed as a result:

   o  Make it clear that after migration there are confirmed entries for
      transferred clientid4/nfs_client_id4 pairs.

   o  Be explicit in the sections headed "otherwise," in the
      descriptions of SETCLIENTID and SETCLIENTID_CONFIRM, that these
      don't apply in the cases we are concerned about.

5.4.2.  Proposed changes: clientid4 handling

   To address both of the clientid4-related issues mentioned in
   Section 4.4, we propose replacing the last three paragraphs of the
   section entitled "Client ID" with the following:

      Once a SETCLIENTID and SETCLIENTID_CONFIRM sequence has
      successfully completed, the client uses the shorthand client
      identifier, of type clientid4, instead of the longer and less
      compact nfs_client_id4 structure.  This shorthand client
      identifier (a client ID) is assigned by the server and should be
      chosen so that it will not conflict with a client ID previously
      assigned by same server.  This applies across server restarts or
      reboots.

      Distinct servers MAY assign clientid4's independently, and will
      generally do so.  Therefore, a client has to be prepared to deal
      with multiple instances of the same clientid4 value received on
      distinct IP addresses, denoting separate entities.  When trunking
      of server IP addresses is not a consideration, a client should
      keep track of (IP-address, clientid4) pairs, so that each pair is
      distinct.  In the face of possible trunking of server IP
      addresses, the client will use the receipt of the same clientid4
      from multiple IP-addresses, as an indication that the two IP-
      addresses may be trunked and proceed to determine, from the
      observed server behavior whether the two addresses are in fact
      trunked.





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      When a clientid4 is presented to a server and that clientid4 is
      not recognized, the server will reject the request with the error
      NFS4ERR_STALE_CLIENTID.  This can occur for a number of reasons:

      *  A server reboot causing loss of the server's knowledge of the
         client

      *  Client error sending an incorrect clientid4 or a valid
         clientid4 to the wrong server.

      *  Loss of lease state due to lease expiration.

      *  Client or server error causing the server to believe that the
         client has rebooted (i.e.  receiving a SETCLIENTID with an
         nfs_client_id4 which has a matching id string and a non-
         matching boot verifier).

      *  Migration of all state under the associated lease causes its
         non-existence to be recognized on the source server.

      *  Merger of state under the associated lease with another lease
         under a different clientid causes the clientid4 serving as the
         source of the merge to cease being recognized on its server.

      In the event of a server reboot, or loss of lease state due to
      lease expiration, the client must obtain a new clientid4 by use of
      the SETCLIENTID operation and then proceed to any other necessary
      recovery for the server reboot case (See the section entitled
      "Server Failure and Recovery").  In cases of server or client
      error resulting in this error, use of SETCLIENTID to establish a
      new lease is desirable as well.

      In the last two cases, different recovery procedures are required.
      Note that in cases in which there is any uncertainty about which
      sort of handling is applicable, the distinguishing characteristic
      is that in reboot-like cases, the clientid4 and all associated
      stateids cease to exist while in migration-related cases, the
      clientid4 ceases to exist while the stateids are still valid.

      The client must also employ the SETCLIENTID operation when it
      receives a NFS4ERR_STALE_STATEID error using a stateid derived
      from its current clientid4, since this indicates a situation, such
      as server reboot which has invalidated the existing clientid4 and
      associated stateids (see the section entitled "lock-owner" for
      details).






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      See the detailed descriptions of SETCLIENTID and
      SETCLIENTID_CONFIRM for a complete specification of the
      operations.

5.4.3.  Proposed changes: NFS4ERR_CLID_INUSE

   It appears to be the intention that only a single principal be used
   for client establishment between any client-server pair.  However:

   o  There is no explicit statement to this effect.

   o  The error that indicates a principal conflict has a name which
      does not clarify this issue: NFS4ERR_CLID_INUSE.

   o  The definition of the error is also not very helpful: "The
      SETCLIENTID operation has found that a client id is already in use
      by another client".

   As a result, servers exist which reject a SETCLIENTID simply because
   there already exists a clientid for the same client, established
   using a different IP address.  Although this is generally understood
   to be erroneous, such servers still exist and the spec should make
   the correct behavior clear.

   Although the error name cannot be changed, the following changes
   should be made to avoid confusion:

   o  The definition of the error should be changed to read as follows:

         The SETCLIENTID operation has found that the specified
         nfs_client_id4 was previously presented with a different
         principal and that client instance currently holds an active
         lease.  A server MAY return this error if the same principal is
         used but a change in authentication flavor gives good reason to
         reject the new SETCLIENTID operation as not bona fide.

   o  In the description of SETCLIENTID, the phrase "then the server
      returns a NFS4ERR_CLID_INUSE error" should be expanded to read
      "then the server returns a NFS4ERR_CLID_INUSE error, since use of
      a single client with multiple principals is not allowed."

6.  Results of proposed changes for NFSv4.0

   The purpose of this section is to examine the troubling results
   reported in Section 3.1.  We will look at the scenarios as they would
   be handled within the proposal.





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   Because the choice of uniform vs.  non-uniform nfs_client_id4 id
   strings is a "SHOULD" in these cases, we will designate clients that
   follow this recommendation by SHOULD-UF-CID.

   We will also have to take account of any merger-related "SHOULD"
   clauses to better understand how they have addressed the issues seen.
   We abbreviate as follows:

   o  SHOULD-SVR-AM refers to the server obeying the SHOULD which
      RECOMMENDS that they merge leases with identical nfs_client_id4 id
      strings and boot verifiers.

6.1.  Results: Failure to free migrated state on client reboot

   Let's look at the troublesome situation cited in Section 3.1.1.  We
   have already seen what happens when SHOULD-UF-CID does not hold.  Now
   let's look at the situation in which SHOULD-UF-CID holds, whether
   SHOULD-SVR-AM is in effect or not.

   o  A client C establishes a clientid4 C1 with server ABC specifying
      an nfs_client_id4 with id string value "C" and boot verifier
      0x111.

   o  The client begins to access files in filesystem F on server ABC,
      resulting in generating stateids S1, S2, etc.  under the lease for
      clientid C1.  It may also access files on other filesystems on the
      same server.

   o  The filesystem is migrated from ABC to server XYZ.  When
      transparent state migration is in effect, stateids S1 and S2 and
      lease {0x111, "C", C1} are now available for use by client C at
      server XYZ.

   o  Client C reboots and attempts to access data on server XYZ,
      whether in filesystem F or another.  It does a SETCLIENTID with an
      nfs_client_id4 with id string value "C" and boot verifier 0x112.
      The state associated with lease {0x111, "C", C1} is deleted as
      part of creating {0x112, "C", C2}.  No problem.

   The correctness signature for this issue is

      SHOULD-UF-CID

   so if you have clients and servers that obey the SHOULD clauses, the
   problem is gone regardless of the choice on the MAY.

6.2.  Results: Server reboots resulting in confused lease situation




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   Now let's consider the scenario given in Section 3.1.2.  We have
   already seen what happens when SHOULD-UF-CID does not hold .  Now
   let's look at the situation in which SHOULD-UF-CID holds and SHOULD-
   SVR-AM holds as well.

   o  Client C talks to server ABC using an nfs_client_id4 id string
      such as "C-ABC" and boot verifier v1.  As a result a lease with
      clientid4 c.i established: {v1, "C-ABC", c.i}.

   o  Filesystem fs_a1 migrates from server ABC to server XYZ along with
      its state.  Now server XYZ also has a lease: {v1, "C-ABC", c.i}

   o  Server ABC reboots.

   o  Client C talks to server ABC using an nfs_client_id4 id string
      such as "C-ABC" and boot verifier v1.  As a result a lease with
      clientid4 c.j established: {v1, "C-ABC", c.j}.

   o  fs_a2 migrates from server ABC to server XYZ.  As part of
      migration the incoming lease is seen to denote same nfs_client_id4
      and so is merged with {v1, "C-ABC, c.i}.

   o  Now server XYZ has only one lease that matches {v1, "C_ABC", *},
      so the problem is solved

   Now let's consider the same scenario in the situation in which
   SHOULD-UF-CID holds and SHOULD-SVR-AM holds as well.

   o  Client C talks to server ABC using an nfs_client_id4 id string "C"
      and boot verifier v1.  As a result a lease with clientid4 c.i is
      established: {v1, "C", c.i}.

   o  fs_a1 migrates from server ABC to server XYZ along with its state.
      Now XYZ also has a lease: {v1, "C", c.i}

   o  Server ABC reboots.

   o  Client C talks to server ABC using an nfs_client_id4 id string "C"
      and boot verifier v1.  As a result a lease with clientid4 c.j is
      established: {v1, "C", c.j}.

   o  fs_a2 migrates from server ABC to server XYZ.  As part of
      migration the incoming lease is seen to denote the same
      nfs_client_id4 and so is merged with {v1, "C", c.i}.

   o  Now server XYZ has only one lease that matches {v1, "C", *}, so
      the problem is solved




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   The correctness signature for this issue is

      SHOULD-SVR-AM

   so if you have clients and servers that obey the SHOULD clauses, the
   problem is gone regardless of the choice on the MAY.

6.3.  Results: Client complexity issues

   Consider the following situation:

   o  There are a set of clients C1 through Cn accessing servers S1
      through Sm.  Each server manages some significant number of
      filesystems with the filesystem count L being significantly
      greater than m.

   o  Each client Cx will access a subset of the servers and so will
      have up to m clientids, which we will call Cxy for server Sy.

   o  Now assume that for load-balancing or other operational reasons,
      numbers of filesystems are migrated among the servers.  As a
      result, depending on how this handled, the number of clientids may
      explode.  See below.

   Now look what will happen under various scenarios:

   o  We have previously (in Section 3.1.3) looked at this in case of
      client following the non-uniform client-string approach.  In that
      case, each client-server pair could have up to m clientids and
      each client will have up to m**2 clientids.  If we add the
      possibility of server reboot, the only bound on a client's
      clientid count is L.

   o  If we look at this in the SHOULD-UF-CID case in which the SHOULD-
      SVR_AM condition holds, the situation is no different.  Although
      the server has the client identity information that could enable
      same-client-same-server leases to be combined, it does not do so.
      We still have up to L clientids per client.

   o  On the other hand, if we look at the SHOULD-UF-CID case in which
      SHOULD-SVR-AM holds, the problem is gone.  There can be no more
      than m clientids per client, and n clientids per server.

   The correctness signature for this issue is

      (SHOULD-UF-CID & SHOULD-SVR-AM)





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   so if you have clients and servers that obey the SHOULD clauses, the
   problem is gone regardless of the choice on the MAY.

6.4.  Result summary

   We have seen that (SHOULD-SVR-AM & SHOULD-UF-CID) are sufficient to
   solve the problems people have experienced.

7.  Issues for NFSv4.1

   Because NFSv4.1 embraces the uniform client-string approach,
   addressing migration issues is simpler.  In the terms of Section 6,
   we already have SHOULD-UF-CID, for NFSv4.1, as advised by section 2.4
   of [RFC5661], simplifying the work to be done.

   Nevertheless, there are some issues that will have to be addressed.
   Some examples:

   o  The other necessary part of addressing migration issues, which we
      call above SHOULD-SVR-AM, is not currently addressed by NFSv4.1
      and changes need to be made to make it clear that state needs to
      be appropriately merged as part of migration, to avoid multiple
      clientids between a client-server pair.

   o  There needs to be some clarification of how migration, and
      particularly transparent state migration, should interact with
      pNFS layouts.

   o  The current discussion (in [RFC5661]), of the possibility of
      server_owner changes is incomplete and confusing.

   Discussion of how to resolve these issues will appear in the sections
   below.

7.1.  Addressing state merger in NFSv4.1

   The existing treatment of state transfer in [RFC5661], has similar
   problems to that in [RFC3530] and [RFC3530bis] in that it assumes
   that the state for multiple filesystems on different servers will not
   be merged to so that it appears under a single common clientid.
   We've already seen the reasons that this is a problem, with regard to
   NFSv4.0.

   Although we don't have the problems stemming from the non-uniform
   client-string approach, there are a number of complexities in the
   existing treatment of state management in the section entitled "Lock
   State and File System Transitions" in [RFC5661] that make this non-
   trivial to address:



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   o  Migration is currently treated together with other sorts of
      filesystem transitions including transitioning between replicas
      without any NFS4ERR_MOVED errors.

   o  There is separate handling and discussion of the cases of matching
      and non-matching server scopes.

   o  In the case of matching server scopes, the text calls for an
      impossible degree of transparency.

   o  In the case of non-matching server scopes, the text does not
      mention transparent state migration at all, resulting in a
      functional regression from NFSV4.0

7.2.  Addressing pNFS relationship with migration

   This is made difficult because, within the PNFS framework, migration
   might mean any of several things:

   o  Transfer of the MDS, leaving DS's alone.

      This would be minimally disruptive to those using layouts but
      would require the pNFS control protocol to support the DS being
      directed to a new MDS.

   o  Transfer of a DS, leaving everything else in place.

      Such a transfer can be handled without using migration at all.
      The server can recall/revoke layouts, as appropriate.

   o  Transfer of the filesystem to a new filesystem with both MDS and
      DS's moving.

      In such a transfer, an entirely different set of DS's will be at
      the target location.  There may even be no pNFS support on the
      destination filesystem at all.

   Migration needs to support both the first and last of these models.

7.3.  Addressing server owner changes in NFSv4.1

   Section 2.10.5 of [RFC5661] states the following.

      The client should be prepared for the possibility that
      eir_server_owner values may be different on subsequent EXCHANGE_ID
      requests made to the same network address, as a result of various
      sorts of reconfiguration events.  When this happens and the
      changes result in the invalidation of previously valid forms of



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      trunking, the client should cease to use those forms, either by
      dropping connections or by adding sessions.  For a discussion of
      lock reclaim as it relates to such reconfiguration events, see
      Section 8.4.2.1.

   While this paragraph is literally true in that such reconfiguration
   events can happen and clients have to deal with them, it is confusing
   in that it can be read as suggesting that clients have to deal with
   them without disruption, which in general is impossible.

   A clearer alternative would be:

      It is always possible that, as a result of various sorts of
      reconfiguration events, eir_server_scope and eir_server_owner
      values may be different on subsequent EXCHANGE_ID requests made to
      the same network address.

      In most cases such reconfiguration events will be disruptive and
      indicate that an IP address formerly connected to one server is
      now connected to an entirely different one.

      Some guidelines on client handling of such situations follow:

      *  When eir_server_scope changes, the client has no assurance that
         any id's it obtained previously (e.g.  file handles) can be
         validly used on the new server, and, even if the new server
         accepts them, there is no assurance that this is not due to
         accident.  Thus it is best to treat all such state as lost/
         stale although a client may assume that the probability of
         inadvertent acceptance is low and treat this situation as
         within the next case.

      *  When eir_server_scope remains the same and
         eir_server_owner.so_major_id changes, the client can use
         filehandles it has and attempt reclaims.  It may find that
         these are now stale but if NFS4ERR_STALE is not received, he
         can proceed to reclaim his opens.

      *  When eir_server_scope and eir_server_owner.so_major_id remain
         the same, the client has to use the now-current values of
         eir_server-owner.so_minor_id in deciding on appropriate forms
         of trunking.









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8.  Security Considerations

   The current definitive definitions of the NFSv4.0 protocol, [RFC3530]
   and [RFC3530bis] both agree.  The section entitled "Security
   Considerations" encourages that clients protect the integrity of the
   SECINFO operation, any GETATTR operation for the fs_locations
   attribute, and the operations SETCLIENTID/SETCLIENTID_CONFIRM.  A
   migration recovery event can use any or all of these operations.  We
   do not recommend any change here.

9.  IANA Considerations

   This document does not require actions by IANA.

10.  Acknowledgements

   The editor and authors of this document gratefully acknowledge the
   contributions of Trond Myklebust of NetApp and Robert Thurlow of
   Oracle.  We also thank Tom Haynes of NetApp and Spencer Shepler of
   Microsoft for their guidance and suggestions.

   Special thanks go to members of the Oracle Solaris NFS team,
   especially Rick Mesta and James Wahlig, for their work implementing
   an NFSv4.0 migration prototype and identifying many of the issues
   documented here.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3530]  Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
              Beame, C., Eisler, M., and D. Noveck, "Network File System
              (NFS) version 4 Protocol", RFC 3530, April 2003.

   [RFC3530bis]
              Haynes, T., Ed. and D. Noveck, Ed., "Network File System
              (NFS) Version 4 Protocol", 2011, <http://www.ietf.org/id/
              draft-ietf-nfsv4-rfc3530bis-25.txt>.

              Work in progress.

   [RFC5661]  Shepler, S., Eisler, M., and D. Noveck, "Network File
              System (NFS) Version 4 Minor Version 1 Protocol", RFC
              5661, January 2010.




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11.2.  Informative References

   [migr-v4.0-update]
              Noveck, D., Ed., Shivam, P., Lever, C., and B. Baker,
              "NFSv4.0 migration: Specification Update", 2013, <http://
              www.ietf.org/id/draft-ietf-nfsv4-rfc3530-migration-
              update-01.txt>.

              Work in progress.

Authors' Addresses

   David Noveck (editor)
   EMC Corporation
   228 South Street
   Hopkinton, MA  01748
   US

   Phone: +1 508 249 5748
   Email: david.noveck@emc.com


   Piyush Shivam
   Oracle Corporation
   5300 Riata Park Ct.
   Austin, TX  78727
   US

   Phone: +1 512 401 1019
   Email: piyush.shivam@oracle.com


   Charles Lever
   Oracle Corporation
   1015 Granger Avenue
   Ann Arbor, MI  48104
   US

   Phone: +1 248 614 5091
   Email: chuck.lever@oracle.com











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   Bill Baker
   Oracle Corporation
   5300 Riata Park Ct.
   Austin, TX  78727
   US

   Phone: +1 512 401 1081
   Email: bill.baker@oracle.com










































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