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Versions: 00 01 02 draft-ietf-nfsv4-migration-issues

NFSv4                                                     D. Noveck, Ed.
Internet-Draft                                                       EMC
Intended status: Informational                                 P. Shivam
Expires: July 19, 2012                                          C. Lever
                                                                B. Baker
                                                                  ORACLE
                                                        January 16, 2012


NFSv4.0 migration: Implementation experience and spec issues to resolve
                draft-dnoveck-nfsv4-migration-issues-02

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 and explores the options available for
   curing the issues via clarification and correction of the NFSv4.0
   specification.

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 July 19, 2012.

Copyright Notice

   Copyright (c) 2012 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.














































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Implementation Experience  . . . . . . . . . . . . . . . . . .  6
     3.1.  Implementation issues  . . . . . . . . . . . . . . . . . .  6
       3.1.1.  Failure to free migrated state on client reboot  . . .  6
       3.1.2.  Server reboots resulting in a confused lease
               situation  . . . . . . . . . . . . . . . . . . . . . .  7
       3.1.3.  Client complexity issues . . . . . . . . . . . . . . .  8
     3.2.  Sources of Protocol difficulties . . . . . . . . . . . . .  9
       3.2.1.  Issues with nfs_client_id4 generation and use  . . . .  9
       3.2.2.  Issues with lease proliferation  . . . . . . . . . . . 11
   4.  Issues to be resolved  . . . . . . . . . . . . . . . . . . . . 12
     4.1.  Possible changes to nfs_client_id4 client-string . . . . . 12
     4.2.  Possible changes to handle differing nfs_client_id4
           string values  . . . . . . . . . . . . . . . . . . . . . . 13
     4.3.  Other issues within migration-state sections . . . . . . . 13
     4.4.  Issues within other sections . . . . . . . . . . . . . . . 14
   5.  Proposed resolution of protocol difficulties . . . . . . . . . 14
     5.1.  Proposed changes: nfs_client_id4 client-string . . . . . . 14
     5.2.  Client-string Models (AS PROPOSED) . . . . . . . . . . . . 15
       5.2.1.  Non-Uniform Client-string Model  . . . . . . . . . . . 16
       5.2.2.  Uniform Client-string Model  . . . . . . . . . . . . . 17
     5.3.  Proposed changes: merged (vs. synchronized) leases . . . . 21
     5.4.  Other proposed changes to migration-state sections . . . . 22
       5.4.1.  Proposed changes: Client ID migration  . . . . . . . . 22
       5.4.2.  Proposed changes: Callback re-establishment  . . . . . 23
       5.4.3.  Proposed changes: NFS4ERR_LEASE_MOVED rework . . . . . 23
     5.5.  Proposed changes to other sections . . . . . . . . . . . . 24
       5.5.1.  Proposed changes: callback update  . . . . . . . . . . 24
       5.5.2.  Proposed changes: clientid4 handling . . . . . . . . . 24
     5.6.  Migration, Replication and State (AS PROPOSED) . . . . . . 26
       5.6.1.  Migration and State  . . . . . . . . . . . . . . . . . 26
       5.6.2.  Replication and State  . . . . . . . . . . . . . . . . 28
       5.6.3.  Notification of Migrated Lease . . . . . . . . . . . . 29
       5.6.4.  Migration and the Lease_time Attribute . . . . . . . . 31
   6.  Results of proposed changes  . . . . . . . . . . . . . . . . . 32
     6.1.  Results: Failure to free migrated state on client
           reboot . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     6.2.  Results: Server reboots resulting in confused lease
           situation  . . . . . . . . . . . . . . . . . . . . . . . . 33
     6.3.  Results: Client complexity issues  . . . . . . . . . . . . 34
     6.4.  Result summary . . . . . . . . . . . . . . . . . . . . . . 35
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 35
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 35
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36



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     10.1. Normative References . . . . . . . . . . . . . . . . . . . 36
     10.2. Informative References . . . . . . . . . . . . . . . . . . 36
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36
















































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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, in the expectation that the working group
   will choose to address these issues, but the choice of exactly how to
   address this is best given effect in a working group document.


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], the current
      pending draft of RFC3530bis expected to become the definitive
      definition of NFSv4.0 once certain procedural steps are taken
      [cur-v4.0-bis], or an eventual RFC3530bis RFC, taking over the
      role of definitive definition of NFSv4.0 from RFC3530.

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




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   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.  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
   model"

   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" value "C-ABC" and 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
      clientid4 C1 are now available for use by client C at server XYZ.
      So far, so good.

   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" value "C-XYZ" and 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.  In the protocol, it



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   really is 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 like
      "C-ABC" and 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 like
      "C-ABC" and 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 are multiple, confirmed client records with
   identical nfs_client_id4 values, there is no way to map the callback
   update request to the correct client record.

   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 }




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   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 can't
   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 clientid's, 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 clientid's 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.

   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 clientid's even after all
      transferred state is gone.





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   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 or 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 clinetid4.

   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.

3.2.  Sources of Protocol difficulties

3.2.1.  Issues with nfs_client_id4 generation and use

   The current definitive definition of the NFSv4.0 protocol [RFC3530],
   and the current pending draft of RFC3530bis [cur-v4.0-bis] both
   agree.  The section entitled "Client ID" says:

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




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   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.

   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 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.



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   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 definition
   of the NFSv4.0 protocol [RFC3530], and the current pending draft of
   RFC3530bis [cur-v4.0-bis] 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 newer instance of the same client, i.e. if the id is
   the same and there is a different 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
   closely connected in that non-uniform client-strings make it
   impossible for the server to appropriately combine leases from the
   same client.  See Section 5.2.1 for a discussion of non-uniform
   client-strings.

   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.




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   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 model for clients
   supporting the migration feature.


4.  Issues to be resolved

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 model always turn out to be cases in which, if the uniform
   model 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 thus not a
   reason to use the non-uniform model but rather an illustration of the
   fact that those using the uniform model must use server behavior to
   determine whether any trunking of IP addresses exists, as is
   described in Section 5.2.2.

   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".



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   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.

   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.







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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, which is part of the
      client's handling of migration, may cause the destination server
      to free existing state.  There needs 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.


5.  Proposed resolution of protocol difficulties

5.1.  Proposed changes: nfs_client_id4 client-string

   We propose replacing client-string-BP3 with the following text and
   adding the following proposed Section 5.2 to provide implementation
   guidance.

   o  The string MAY be different for each server network address that
      the client accesses, rather than common to all server network
      addresses.  The considerations that might influence a client to
      use different strings for each are explained in Section 5.2.

   o  Despite the use of the word "string" for this identifier, and the
      fact that using strings will often be convenient, it should be
      understood that the protocol defines this as opaque data.  In
      particular, those receiving such an id should not assume that it
      will be in UTF-8 format nor should they reject it if it is not.






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5.2.  Client-string Models (AS PROPOSED)

   One particular aspect of the construction of the nfs4_client_id4
   string has proved recurrently troublesome.  The client has a choice
   of:

   o  Presenting the same id string to each server address accessed.
      This is referred to as the "uniform client-string model" and is
      discussed in Section 5.2.2.

   o  Presenting a different id string to each server address accessed.
      This is referred to as the "non-uniform client-string model" and
      is discussed in Section 5.2.1.

   Construction of the client-string has been a troublesome issue
   because of the way in which the NFS protocols have evolved.

   o  NFSv3 as a stateless protocol had no need to identify the state
      shared by a particular client-server pair.  Thus there was no
      occasion to consider the question of whether a set of requests
      come from the same client, or whether two server IP addresses are
      connected to the same server.  As the environment was one in which
      the user supplied the target server IP address as part of
      incorporating the remote filesystem in the client's file name
      space, there was no occasion to take note of server trunking.
      Within a stateless protocol, the situation was symmetrical.  The
      client has no server identity information and the server has no
      client identity information.

   o  NFSv4.1 is a stateful protocol with full support for client and
      server identity determination.  This enables the server to be
      aware when two requests come from the same client (they are on
      sessions sharing a clientid4) and the client to be aware when two
      server IP addresses are connected to the same server (they return
      the same server name in responding to an EXCHANGE_ID).

   NFSv4.0 is unfortunately halfway between these two.  The two client-
   string models have arisen in attempts to deal with the changing
   requirements of the protocol as implementation has proceeded and
   features that were not very substantial in [RFC3530], got more
   substantial.

   o  In the absence of any implementation of the fs_locations-related
      features (replication, referral, and migration), the situation is
      very similar to that of NFSv3, with the addition of state but with
      no concern to provide accurate client and server identity
      determination.  This is the situation that gave rise to the non-
      uniform client-string model.



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   o  In the presence of replication and referrals, the client may have
      occasion to take advantage of knowledge of server trunking
      information.  Even more important, migration, by transferring
      state among servers, causes difficulties for the non-uniform
      client-string model, in that the two different client-strings sent
      to different IP addresses may wind up on the same IP address,
      adding confusion.

   Both models have to deal with the asymmetry in client and server
   identity information between client and server.  Each seeks to make
   the client's and the server's views match.  In the process, each
   encounters some combination of inelegant protocol features and/or
   implementation difficulties.  The choice of which to use is up to the
   client implementer and the sections below try to give some useful
   guidance.

5.2.1.  Non-Uniform Client-string Model

   The non-uniform client-string model is an attempt to handle these
   matters in NFSv4.0 client implementations in as NFSv3-like a way as
   possible.

   For a client using the non-uniform model, all internal recording of
   clientid4 values is to include, whether explicitly or implicitly, the
   server IP address so that one always has an (IP-address, clientid4)
   pair.  Two such pairs from different servers are always distinct even
   when the clientid4 values are the same, as they may occasionally be.
   In this model, such equality is always treated as simple
   happenstance.

   Making the client-string different on different servers means that a
   server has no way of tying together information from the same client
   and so will treat a single client as multiple clients with multiple
   leases for each server network address.  Since there is no way in the
   protocol for the client to determine if two network addresses are
   connected to the same server, the resulting lack of knowledge is
   symmetrical and can result in simpler client implementations in which
   there is a single clientid/lease per server network addresses.

   Support for migration, particularly with transparent state migration,
   is more complex in the case of non-uniform client-strings.  For
   example, migration of a lease can result in multiple leases for the
   same client accessing the same server addresses, vitiating many of
   the advantages of this approach.  Therefore, client implementations
   that support migration with transparent state migration SHOULD NOT
   use the non-uniform client-string model.





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5.2.2.  Uniform Client-string Model

   When the client-string is kept uniform, the server has the basis to
   have a single clientid4/lease for each distinct client.  The problem
   that has to be addressed is the lack of explicit server identity
   information, which is made available in NFSv4.1.

   When the same client-string is given to multiple IP addresses, the
   client can determine whether two IP addresses correspond to a single
   server, based on the server's behavior.  This is the inverse of the
   strategy adopted for the non-uniform model in which different server
   IP addresses are told about different clients, simply to prevent a
   server from manifesting behavior that is inconsistent with there
   being a single server for each IP address, in line with the
   traditions of NFS.  So, to compare:

   o  In the non-uniform model, servers are told about different clients
      because, if the server were to use accurate information as to
      client identity, two IP addresses on the same server would behave
      as if they were talking to the same client, which might prove
      disconcerting to a client not expecting such behavior.

   o  In the uniform model, the servers are told about there being a
      single client, which is, after all, the truth.  Then, when the
      server uses this information, two IP addresses on the same server
      will behave as if they are talking to the same client, and this
      difference in behavior allows the client to infer the server IP
      address trunking configuration, even though NFSv4.0 does not
      explicitly provide this information.

      The approach given below shows one example of how this might be
      done.

   For a client using the uniform model, clientid4 values are treated as
   important information in determining server trunking patterns.  For
   two different IP addresses to return the same clientid4 value is a
   necessary, though not a sufficient condition for them to be
   considered as connected to the same server.  As a result, when two
   different IP addresses return the same clientid4, the client needs to
   determine, using the procedure given below or otherwise, whether the
   IP addresses are connected to the same server.  For such clients, all
   internal recording of clientid4 values needs to include, whether
   explicitly or implicitly, identification of the server from which the
   clientid4 was received so that one always has a (server clientid4)
   pair.  Two such pairs from different servers are always considered
   distinct even when the clientid4 values are the same, as they may
   occasionally be.




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   In order to make this approach work, the client must have accessible,
   for each nfs4_client_id4 used (only one in the uniform model) a list
   of all server IP addresses, together with the associated clientid4
   values.  As a part of the associated data structures, there should be
   the ability to mark a server IP structure as having the same server
   as another and to mark an IP-address as currently unresolved.  One
   way to do this is to a allow each such entry to point to another with
   the pointer value being one of:

   o  A pointer to another entry for an IP address associated with the
      same server, where that IP address is the first one referenced to
      access that server.

   o  A pointer to the current entry if there is no earlier IP address
      associated with the same server, i.e. where the current IP address
      is the first one referenced to access that server.  We'll refer to
      such an IP address as the lead IP address for a given server.

   o  The value NULL if the address's server identity is currently
      unresolved.

   When a SETCLIENTID is done and a clientid4 returned, the data
   structure is searched for a matching clientid4 and processing depends
   on what is found.  We will refer to the IP address on which this
   SETCLIENTID is done as X. The SETCLIENTID will use the common
   nfs_client_id4 and specify X as part of the callback parameters.  We
   call the clientid4 and verifier returned by this operation XC and XV.

   Note that at this point no SETCLIENTID_CONFIRM has yet been done.
   This is because we have either established a new clientid4 on a
   previously unknown server or changed the callback parameters on a
   clientid4 associated with some already known server.  We don't want
   to confirm something that we are not sure we want to happen.

   o  If no matching clientid4 is found, the IP address X and clientid4
      XC are added to the list and considered as having no existing
      known IP addresses trunked with it.  The IP address is marked as a
      lead IP address for a new server.  A SETCLIENTID_CONFIRM is done
      using XC and XV.

   o  If a matching clientid4 is found which is marked unresolved,
      processing on the new IP address is suspended.  In order to
      simplify processing, there can only be one unresolved IP address
      for any given clientid4.

   o  If one or more matching clientid4's is found, none of which is
      marked unresolved, the new IP address in entered and marked
      unresolved.  After applying the steps below to each of the lead IP



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      addresses with a matching clientid4, the address will have been
      resolved: either it will be part of the same server as a new IP
      address to be added to an existing set of IP addresses for a
      server, or it will be recognized as a new server.  At the point at
      which this determination is made, the unresolved indication is
      cleared and any suspended SETCLIENTID processing is restarted

   So for each lead IP address IPn with a clientid4 matching XC, the
   following steps are done.

   o  If the server has an associated stateid S, S is used in a request
      issued on the address X with the fact of whether it is recognized
      on X giving definitive information of X's server identity.

   o  If S is not recognized as valid on X, then X and IPn are
      recognized as distinct and we go on to the next IPn, until we run
      out of them.

   o  If S is recognized as valid on X, then X and IPn are recognized as
      connected to the same server and the entry for X is marked as
      associated with IPn.  The entry is now resolved and processing can
      be restarted for IP addresses whose clientid4 matched XC and whose
      resolution had been deferred.

   o  If there is no such S for IPn, a different procedure is used. a
      SETCLIENTID is done to update the callback parameters to reflect
      the possibility that X will be marked as associated with the
      server whose lead IP address is IPn.  So assume that we do that
      SETCLIENTID and get back verifier Vn.

   o  Note that we don't want this to happen if address X is not
      associated with this server.  So we do a SETCLIENTID_CONFIRM on
      address IPn using verifier Vn.

   o  If the verifier generated on X is accepted on IPn, then X and IPn
      are recognized as connected to the same server and the entry for X
      is marked as associated with IPn.  The entry is now resolved and
      processing can be restarted for IP addresses whose clientid4
      matched XC but whose resolution had been deferred.

   o  If the verifier generated on X is not accepted on IPn, then X and
      IPn are distinct and the callback update will not be confirmed.
      So we go on to the next IPn, until we run out of them.

   The procedure above has made no explicit mention of the possibility
   that server reboot can occur at any time.  To address this
   possibility the client should periodically use the clientid4 XC in
   RENEW operations, directed to both the IP address X and the current



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   lead IP address that is currently being tested for identity.

   o  When XC becomes invalid on X, the resolution process should be
      terminated, subject to being redone later.  Before redoing the
      resolution, XC should be checked on all the lead IP addresses on
      which it was valid.  Once a new clientid4 is established on any
      servers on which XC became invalid, a new clientid4 can be
      established on X and the resolution process for X can be
      restarted.

   o  When XC does not becomes invalid on X, but becomes invalid on the
      current IPn being tested, it should be concluded that X and IPn do
      not match and that it is time to advance to the next IPn, if any.

   o  In the event of a reboot detected on any server lead IP, the set
      of IP addresses associated with the server should not change and
      state should be re-established for the lease as a whole, using all
      available connected server IP addresses.  It is prudent to verify
      connectivity by doing a RENEW using the new clientid4 on each such
      server address before using it, however.

   If we have run out of IPn's without finding a matching server, X is
   considered as having no existing known IP addresses trunked with it.
   The IP address is marked as a lead IP address for a new server.  A
   SETCLIENTID_CONFIRM is done using XC and XV.

   The following are advantages for the implementation of using the
   uniform client-string model:

   o  Clients can take advantage of server trunking (and clustering with
      single-server-equivalent semantics) to increase bandwidth or
      reliability.

   o  There are advantages in state management so that, for example, we
      never have a delegation under one clientid revoked because of a
      reference to the same file from the same client under a different
      clientid.

   o  The uniform client-string model allows the server to do any
      necessary automatic lease merger in connection with migration,
      without requiring any client involvement.  This consideration is
      of sufficient weight to cause us RECOMMEND use of the uniform
      client-string model for clients supporting transparent state
      migration.

   The following implementation considerations might cause issues for
   client implementations.




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   o  This model is considerably different from the non-uniform model,
      which most client implementations have been following.  Until
      substantial implementation experience is obtained with this model,
      reluctance to embrace something so new is to be expected.

   o  Mapping between server network addresses and leases is more
      complicated in that it is no longer a one-to-one mapping.

   How to balance these considerations depends on implementation goals.

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

   The current definitive definition of the NFSv4.0 protocol [RFC3530],
   and the current pending draft of RFC3530bis [cur-v4.0-bis] both
   agree.  The section entitled "Migration and State" says:

      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



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   leases since there is no real help from synchronizing them at a
   single instant.

   For the uniform model, 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 model in order to get
      good migration support.

   o  Servers SHOULD provide automatic lease merger during state
      migration so that clients using the uniform id model 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.

5.4.  Other proposed changes to migration-state sections

5.4.1.  Proposed changes: Client ID migration

   The current definitive definition of the NFSv4.0 protocol [RFC3530],
   and the current pending draft of RFC3530bis [cur-v4.0-bis] 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



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      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.

   We have decided to address this issue as follows, with the relevant
   changes all reflected in Section 5.6.

   o  Make it clear that both clientid4 and nfs_client_id4 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.

5.4.2.  Proposed changes: Callback re-establishment

   The current definitive definition of the NFSv4.0 protocol [RFC3530],
   and the current pending draft of RFC3530bis [cur-v4.0-bis] 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 and the text to do this appears as part of Section 5.6.

5.4.3.  Proposed changes: NFS4ERR_LEASE_MOVED rework

   The current definitive definition of the NFSv4.0 protocol [RFC3530],
   and the current pending draft of RFC3530bis [cur-v4.0-bis] both



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   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 fs's that the client may have state for with
   respect to a given server, the actual process can be done
   efficiently.

   To address these issues we propose replacing the above with the text
   addressing NFS4RR_LEASE_MOVED as given in Section 5.6.3.

5.5.  Proposed changes to other sections

5.5.1.  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.

5.5.2.  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



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      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.  For a discussion of how to address the issue in the
      face of possible trunking of server IP addresses, see Section 5.2.

      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
         client

      *  Client error sending an incorrect clientid4 or 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 and a non-matching
         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.
      See Section 5.6 for details.  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 stateid cease to exist while in



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      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).

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

5.6.  Migration, Replication and State (AS PROPOSED)

   When responsibility for handling a given filesystem is transferred to
   a new server (migration) or the client chooses to use an alternate
   server (e.g., in response to server unresponsiveness) in the context
   of filesystem replication, the appropriate handling of state shared
   between the client and server (i.e., locks, leases, stateids, and
   client IDs) is as described below.  The handling differs between
   migration and replication.

   If a server replica or a server immigrating a filesystem agrees to,
   or is expected to, accept opaque values from the client that
   originated from another server, then it is a wise implementation
   practice for the servers to encode the "opaque" values in network
   byte order.  When doing so, servers acting as replicas or immigrating
   filesystems will be able to parse values like stateids, directory
   cookies, filehandles, etc. even if their native byte order is
   different from that of other servers cooperating in the replication
   and migration of the filesystem.

5.6.1.  Migration and State

   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.

   If transferring stateids from server to server would result in a
   conflict for an existing stateid for the destination server with the
   existing client, transparent state migration MUST NOT happen for that



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   client.  Servers participating in using transparent state migration
   should co-ordinate their stateid assignment policies to make this
   situation unlikely or impossible.  The means by which this might be
   done, like all of the inter-server interactions for migration, are
   not specified by the NFS version 4.0 protocol.

   Handling of clientid values is similar but not identical.  The
   clientid4 and nfs_client_id4 information (id and verifier) will be
   transferred with the rest of the state information and the
   destination server should use that information to determine
   appropriate clientid4 handling.  Although the destination server may
   make state stored under an existing lease available under the
   clientid4 used on the source server, the client should not assume
   that this is always so.  In particular,

   o  If there is an existing lease with an nfs_client_id4 that matches
      a migrated lease (same id and verifier), the server SHOULD merge
      the two, making the union of the sets of stateids available under
      the clientid4 for the existing lease.  As part of the lease
      merger, the expiration time of the lease will reflect renewal done
      within either of the ancestor leases (and so will reflect the
      latest of the renewals).

   o  If there is an existing lease with an nfs_client_id4 that
      partially matches a migrated lease (same id and a different
      verifier), the server MUST eliminate one of the two, possibly
      invalidating one of the ancestor clientid4's.  Since verifiers are
      not ordered, the later lease renewal time will prevail.

   When leases are not merged, the transfer of state should result in
   creation of a confirmed client record with empty callback information
   but matching the {v, x, c} for the transferred client information.
   This should enable establishment of new callback information using
   SETCLIENTID and SETCLIENTID_CONFIRM.

   A client may determine the disposition of migrated state by using a
   stateid associated with the migrated state and in an operation on the
   new server and using the associated clientid4 in a RENEW on the new
   server.

   o  If the stateid is not valid and an error NFS4ERR_BAD_STATEID is
      received, either transparent state migration has not occurred or
      the state was purged due to verifier mismatch.

   o  If the stateid is valid and an error NFS4ERR_STALE_CLIENTID is
      received on the RENEW, transparent state migration has occurred
      and the lease has been merged with an existing lease on the
      destination server.



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   o  If the stateid is valid and the clientid4 is valid, the lease has
      been transferred intact.

   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.

   The servers may choose not to transfer the state information upon
   migration.  However, this choice is discouraged, except where
   specific issues such as stateid conflicts make it necessary.  In the
   case of migration without state transfer, when the client presents
   state information from the original server (e.g. in a RENEW op or a
   READ op of zero length), the client must be prepared to receive
   either NFS4ERR_STALE_CLIENTID or NFS4ERR_STALE_STATEID from the new
   server.  The client should then recover its state information as it
   normally would in response to a server failure.  The new server must
   take care to allow for the recovery of state information as it would
   in the event of server restart.

   When a lease is transferred to a new server (as opposed to being
   merged with a lease already on the new server), 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.

5.6.2.  Replication and State

   Since client switch-over in the case of replication is not under
   server control, the handling of state is different.  In this case,
   leases, stateids and client IDs do not have validity across a
   transition from one server to another.  The client must re-establish
   its locks on the new server.  This can be compared to the re-
   establishment of locks by means of reclaim-type requests after a
   server reboot.  The difference is that the server has no provision to
   distinguish requests reclaiming locks from those obtaining new locks
   or to defer the latter.  Thus, a client re-establishing a lock on the
   new server (by means of a LOCK or OPEN request), may have the
   requests denied due to a conflicting lock.  Since replication is
   intended for read-only use of filesystems, such denial of locks
   should not pose large difficulties in practice.  When an attempt to
   re-establish a lock on a new server is denied, the client should
   treat the situation as if its original lock had been revoked.







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5.6.3.  Notification of Migrated Lease

   In the case of lease renewal, the client may not be submitting
   requests for a filesystem that has been migrated to another server.
   This can occur because of the implicit lease renewal mechanism.  The
   client renews a lease containing state of multiple filesystems when
   submitting a request to any one filesystem at the server.

   In order for the client to schedule renewal of leases that may have
   been relocated to the new server, the client must find out about
   lease relocation before those leases expire.  To accomplish this, all
   operations which implicitly renew leases for a client (such as OPEN,
   CLOSE, READ, WRITE, RENEW, LOCK, and others), will return the error
   NFS4ERR_LEASE_MOVED if responsibility for any of the leases to be
   renewed has been transferred to a new server.  Note that when the
   transfer of responsibility leaves remaining state for that lease on
   the source server, the lease is renewed just as it would have been in
   the NFS4ERR_OK case, despite returning the error.  The transfer of
   responsibility happens when the server receives a
   GETATTR(fs_locations) from the client for each filesystem for which a
   lease has been moved to a new server.  Normally it does this after
   receiving an NFS4ERR_MOVED for an access to the filesystem but the
   server is not required to verify that this happens in order to
   terminate the return of NFS4ERR_LEASE_MOVED.  By convention, the
   compounds containing GETATTR(fs_locations) SHOULD include an appended
   RENEW operation to permit the server to identify the client getting
   the information.

   Note that the NFS4ERR_LEASE_MOVED error is only required when
   responsibility for at least one stateid has been transferred.  In the
   case of a null lease, where the only associated state is a clientid,
   no NFS4ERR_LEASE_MOVED error need be generated.

   Upon receiving the NFS4ERR_LEASE_MOVED error, a client that supports
   filesystem migration MUST perform the necessary GETATTR operation for
   each of the filesystems containing state that have been migrated and
   so give the server evidence that it is aware of the migration of the
   filesystem.  Once the client has done this for all migrated
   filesystems on which the client holds state, the server MUST resume
   normal handling of stateful requests from that client.

   One way in which clients can do this efficiently in the presence of
   large numbers of filesystems is described below.  This approach
   divides the process into two phases, one devoted to finding the
   migrated filesystems and the second devoted to doing the necessary
   GETATTRs.

   The client can find the migrated filesystems by building and issuing



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   one or more COMPOUND requests, each consisting of a set of PUTFH/
   GETFH pairs, each pair using an fh in one of the filesystems in
   question.  All such COMPOUND requests can be done in parallel.  The
   successful completion of such a request indicates that none of the
   fs's interrogated have been migrated while termination with
   NFS4ERR_MOVED indicates that the filesystem getting the error has
   migrated while those interrogated before it in the same COMPOUND have
   not.  Those whose interrogation follows the error remain in an
   uncertain state and can be interrogated by restarting the requests
   from after the point at which NFS4ERR_MOVED was returned or by
   issuing a new set of COMPOUND requests for the filesystems which
   remain in an uncertain state.

   Once the migrated filesystems have been found, all that is needed is
   for client to give evidence to the server that it is aware of the
   migrated status of filesystems found by this process, by
   interrogating the fs_locations attribute for an fh each of the
   migrated filesystems.  The client can do this building and issuing
   one or more COMPOUND requests, each of which consists of a set of
   PUTFH operations, each followed by a GETATTR of the fs_locations
   attribute.  A RENEW follows to help tie the operations to the lease
   returning NFS4ERR_LEASE_MOVED.  Once the client has done this for all
   migrated filesystems on which the client holds state, the server will
   resume normal handling of stateful requests from that client.

   In order to support legacy clients that do not handle the
   NFS4ERR_LEASE_MOVED error correctly, the server SHOULD time out after
   a wait of at least two lease periods, at which time it will resume
   normal handling of stateful requests from all clients.  If a client
   attempts to access the migrated files, the server MUST reply
   NFS4ERR_MOVED.

   When the client receives an NFS4ERR_MOVED error, the client can
   follow the normal process to obtain the new server information
   (through the fs_locations attribute) and perform renewal of those
   leases on the new server.  If the server has not had state
   transferred to it transparently, the client will receive either
   NFS4ERR_STALE_CLIENTID or NFS4ERR_STALE_STATEID from the new server,
   as described above.  The client can then recover state information as
   it does in the event of server failure.

   Aside from recovering from a migration, there are other reasons a
   client may wish to retrieve fs_locations information from a server.
   When a server becomes unresponsive, for example, a client may use
   cached fs_locations data to discover an alternate server hosting the
   same fs data.  A client may periodically request fs_locations data
   from a server in order to keep its cache of fs_locations data fresh.




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   Since a GETATTR(fs_locations) operation would be used for refreshing
   cached fs_locations data, a server could mistake such a request as
   indicating recognition of an NFS4ERR_LEASE_MOVED condition.
   Therefore a compound which is not intended to signal that a client
   has recognized a migrated lease SHOULD be prefixed with a guard
   operation which fails with NFS4ERR_MOVED if the file handle being
   queried is no longer present on the server.  The guard can be as
   simple as a GETFH operation.

   Though unlikely, it is possible that the target of such a compound
   could be migrated in the time after the guard operation is executed
   on the server but before the GETATTR(fs_locations) operation is
   encountered.  When a client issues a GETATTR(fs_locations) operation
   as part of a compound not intended to signal recognition of a
   migrated lease, it SHOULD be prepared to process fs_locations data in
   the reply that shows the current location of the fs is gone.

5.6.4.  Migration and the Lease_time Attribute

   In order that the client may appropriately manage its leases in the
   case of migration, the destination server must establish proper
   values for the lease_time attribute.

   When state is transferred transparently, that state should include
   the correct value of the lease_time attribute.  The lease_time
   attribute on the destination server must never be less than that on
   the source since this would result in premature expiration of leases
   granted by the source server.  Upon migration in which state is
   transferred transparently, the client is under no obligation to re-
   fetch the lease_time attribute and may continue to use the value
   previously fetched (on the source server).

   In the case in which lease merger occurs as part of state transfer,
   the lease_time attribute of the destination lease remains in effect.
   The client can simply renew that lease with its existing lease_time
   attribute.  State in the source lease is renewed at the time of
   transfer so that it cannot expire, as long as the destination lease
   is appropriately renewed.

   If state has not been transferred transparently (i.e., the client
   sees a real or simulated server reboot), the client should fetch the
   value of lease_time on the new (i.e., destination) server, and use it
   for subsequent locking requests.  However the server must respect a
   grace period at least as long as the lease_time on the source server,
   in order to ensure that clients have ample time to reclaim their
   locks before potentially conflicting non-reclaimed locks are granted.
   The means by which the new server obtains the value of lease_time on
   the old server is left to the server implementations.  It is not



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   specified by the NFS version 4.0 protocol.


6.  Results of proposed changes

   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.

   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 the various merger-related
   "SHOULD" clauses to better understand how they have addressed the
   issues seen, we abbreviate these (collectively known as "SHOULD-
   merges") 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 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" value "C" and 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.  So far, so good.

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




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   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

   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 like
      "C-ABC" and verifier v1.  As a result a lease with clientid4 c.i
      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 like
      "C-ABC" and 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 like "C"
      and 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 like "C"
      and verifier v1.  As a result a lease with clientid4 c.j is



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      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

   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 clientid's, 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 model.  In that
      case, each client-server pair could have up to m clientid's 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 clientid's per client.




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   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 clientid's per server.

   The correctness signature for this issue is

      (SHOULD-UF-CID & 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.

6.4.  Result summary

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


7.  Security Considerations

   The current definitive definition of the NFSv4.0 protocol [RFC3530],
   and the current pending draft of RFC3530bis [cur-v4.0-bis] 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.


8.  IANA Considerations

   This document does not require actions by IANA.


9.  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.


10.  References




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10.1.  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.

10.2.  Informative References

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

   [cur-v4.0-bis]
              Haynes, T., Ed. and D. Noveck, Ed., "Network File System
              (NFS) Version 4 Protocol", 2011, <http://www.ietf.org/id/
              draft-ietf-nfsv4-rfc3530bis-16.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








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

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


   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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