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RFC 5716 - Requirements for Federated File Systems


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Internet Engineering Task Force (IETF)                        J. Lentini
Request for Comments: 5716                                   C. Everhart
Category: Informational                                           NetApp
ISSN: 2070-1721                                                D. Ellard
                                                        BBN Technologies
                                                               R. Tewari
                                                                 M. Naik
                                                             IBM Almaden
                                                            January 2010

                Requirements for Federated File Systems

Abstract

   This document describes and lists the functional requirements of a
   federated file system and defines related terms.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5716.

Copyright Notice

   Copyright (c) 2010 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
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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1. Overview ........................................................3
      1.1. Requirements Language ......................................4
   2. Purpose .........................................................5
   3. Examples and Discussion .........................................5
      3.1. Create a Fileset and Its FSL(s) ............................5
           3.1.1. Creating a Fileset and an FSN .......................6
           3.1.2. Adding a Replica of a Fileset .......................6
      3.2. Junction Resolution ........................................7
      3.3. Junction Creation ..........................................9
   4. Glossary ........................................................9
   5. Proposed Requirements ..........................................11
      5.1. Basic Assumptions .........................................11
      5.2. Requirements ..............................................14
   6. Non-Requirements ...............................................20
   7. Security Considerations ........................................21
   8. References .....................................................22
      8.1. Normative References ......................................22
      8.2. Informative References ....................................23
   Appendix A.  Acknowledgments ......................................25

1.  Overview

   This document describes and lists the functional requirements of a
   federated file system and defines related terms.

   We do not describe the mechanisms that might be used to implement
   this functionality except in cases where specific mechanisms, in our
   opinion, follow inevitably from the requirements.  Our focus is on
   the interfaces between the entities of the system, not on the
   protocols or their implementations.

   Today, there are collections of fileservers that inter-operate to
   provide a single namespace comprised of filesystem resources provided
   by different members of the collection, joined together with inter-
   filesystem references.  The namespace can either be assembled at the
   fileservers, the clients, or by an external namespace service, and is
   often not easy or uniform to manage.  The requirements in this
   document are meant to lead to a uniform server-based namespace that
   is capable of spanning a whole enterprise and that is easy to manage.

   We define some terms to better describe the solution space.  A
   "fileset" is the abstract view of a filesystem in a uniform
   namespace, and may be implemented behind that abstraction by one or
   more physical filesystems at any given time.  Each fileset has a name
   called an "FSN" (fileset name), and each physical filesystem has a
   fileset location ("FSL").  A fileset is a directory tree containing
   files and directories, and it may also contain references to other
   filesets.  These references are called "junctions".  To provide
   location independence, a junction does not contain information about
   the location of the real resource(s), but instead contains an FSN
   that can be used to look up the location information.  The service
   that can be used to map from the FSN to the FSL(s) is called a
   namespace database (NSDB) service.  The NSDB provides a level of
   indirection from the virtual paths in the uniform namespace to the
   actual locations of files.  By design, the NSDB does not store the
   junctions.  This allows junction administration and NSDB
   administration to be separate roles.

   The servers direct clients to the proper locations by existing
   mechanisms (e.g., the referrals mechanism within [RFC3530] and
   [RFC5661]).  Updates to the locations make it possible to support
   migration and replication of physical filesystems that comprise the
   namespace, in a way that is transparent to filesystem applications.

   Figure 1 shows an example of a federation.  This federation has two
   members, named ALPHA and BETA.  Federation members may contain an
   arbitrary number of fileservers and NSDB nodes; in this illustration,
   ALPHA and BETA each have three servers, one NSDB node, and are
   administered separately.

      +----------------------+       +----------------------+
      |  Federation Member   |       |  Federation Member   |
      |        ALPHA         |       |         BETA         |
      |                      |       |                      |
      |                      |       |                      |
      |    +------------+    |       |    +------------+    |
      |    |    NSDB    |    |       |    |    NSDB    |    |
      |    |            |    |       |    |            |    |
      |    +------------+    |       |    +------------+    |
      |                      |       |                      |
      |                      |       |                      |
      |                      |       |                      |
      |         +----------+ |       |         +----------+ |
      |         |          | |       |         |          | |
      |     +-- | Servers  | |       |     +-- | Servers  | |
      |     |   |          | |       |     |   |          | |
      | +-- |   |          | |       | +-- |   |          | |
      | |   |   +----------+ |       | |   |   +----------+ |
      | |   |          |     |       | |   |          |     |
      | |   +----------+     |       | |   +----------+     |
      | |          |         |       | |          |         |
      | +----------+         |       | +----------+         |
      +----------------------+       +----------------------+

                                 Figure 1

1.1.  Requirements Language

   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].

   Note that this is a requirements document, and in many instances
   where these words are used in this document they refer to qualities
   of a specification for a system that satisfies the document, or
   requirements of a system that matches that specification.  These
   cases are distinguished when there is potential for ambiguity.

2.  Purpose

   Our objective is to specify a set of protocols by which fileservers
   or collections of fileservers, with different administrators, can
   form a federation of fileservers and NSDB nodes that provides a
   namespace composed of the filesets hosted on the different
   fileservers and fileserver collections.

   It should be possible, using a system that implements the protocols,
   to share a common namespace across all the fileservers in the
   federation.  It should also be possible for different fileservers in
   the federation to project different namespaces and enable clients to
   traverse them.

   Such a federation may contain an arbitrary number of NSDB nodes, each
   belonging to a different administrative entity, and each providing
   the mappings that define a part of a namespace.  Such a federation
   may also have an arbitrary number of administrative entities, each
   responsible for administering a subset of the fileservers and NSDB
   nodes.  Acting in concert, the administrators should be able to build
   and administer this multi-fileserver, multi-collection namespace.

   It is not the intent of the federation to guarantee namespace
   consistency across all client views.  Since different parts of the
   namespace may be administered by different entities, it is possible
   that a client could be accessing a stale area of the namespace
   managed by one entity because a part of the namespace above it,
   managed by another entity, has changed.

3.  Examples and Discussion

   In this section we provide examples and discussion of the basic
   operations facilitated by the federated file system protocol:
   creating a fileset, adding a replica of a fileset, resolving a
   junction, and creating a junction.

3.1.  Create a Fileset and Its FSL(s)

   A fileset is the abstraction of a set of files and the directory tree
   that contains them.  The fileset abstraction is the fundamental unit
   of data management in the federation.  This abstraction is
   implemented by an actual directory tree whose root location is
   specified by a fileset location (FSL).

   In this section, we describe the basic requirements for starting with
   a directory tree and creating a fileset that can be used in the
   federation protocols.  Note that we do not assume that the process of
   creating a fileset requires any transformation of the files or the

   directory hierarchy.  The only thing that is required by this process
   is assigning the fileset a fileset name (FSN) and expressing the
   location(s) of the implementation of the fileset as FSL(s).

   There are many possible variations to this procedure, depending on
   how the FSN that binds the FSL is created, and whether other replicas
   of the fileset exist, are known to the federation, and need to be
   bound to the same FSN.

   It is easiest to describe this in terms of how to create the initial
   implementation of the fileset, and then describe how to add replicas.

3.1.1.  Creating a Fileset and an FSN

   1.  Choose the NSDB node that will keep track of the FSL(s) and
       related information for the fileset.

   2.  Request that the NSDB node register a new FSN for the fileset.

       The FSN may either be chosen by the NSDB node or by the server.
       The latter case is used if the fileset is being restored, perhaps
       as part of disaster recovery, and the server wishes to specify
       the FSN in order to permit existing junctions that reference that
       FSN to work again.

       At this point, the FSN exists, but its location is unspecified.

   3.  Send the FSN, the local volume path, the export path, and the
       export options for the local implementation of the fileset to the
       NSDB node.  Annotations about the FSN or the location may also be
       sent.

       The NSDB node records this information and creates the initial
       FSL for the fileset.

3.1.2.  Adding a Replica of a Fileset

   Adding a replica is straightforward: the NSDB node and the FSN are
   already known.  The only remaining step is to add another FSL.

   Note that the federation protocols do not include methods for
   creating or managing replicas: this is assumed to be a platform-
   dependent operation (at least at this time).  The only requirement is
   that these fileset replicas be registered and unregistered with the
   NSDB.

3.2.  Junction Resolution

   A fileset may contain references to other filesets.  These references
   are represented by junctions.  If a client requests access to a
   fileset object that is a junction, the server resolves the junction
   to discover the FSL(s) that implements the referenced fileset.

   There are many possible variations to this procedure, depending on
   how the junctions are represented and how the information necessary
   to perform resolution is represented by the server.

   Step 4 is the only step that interacts directly with the federation
   protocols.  The rest of the steps may use platform-specific
   interfaces.

   1.  The server determines that the object being accessed is a
       junction.

   2.  Using the junction, the server does a local lookup to find the
       FSN of the target fileset.

   3.  Using the FSN, the server finds the NSDB node responsible for the
       target object.

   4.  The server contacts that NSDB node and asks for the set of FSLs
       that implement the target FSN.  The NSDB node responds with a set
       of FSLs.

   5.  The server converts one or more of the FSLs to the location type
       used by the client (e.g., a Network File System (NFSv4)
       fs_location, as described in [RFC3530]).

   6.  The server redirects (in whatever manner is appropriate for the
       client) the client to the location(s).

   These steps are illustrated in Figure 2.  The client sends request 1
   to server X, in federation member ALPHA, in an attempt to reference
   an object (which appears to the client as a directory).  Server X
   recognizes that the referenced object is actually a junction that
   refers to a directory in a different fileset.  Server X finds, from
   the FSN in the junction, that the NSDB responsible for knowing the
   location of the target of the junction is the NSDB of federation
   member BETA.  Server X sends request 2 to the NSDB of BETA, asking
   for the current location of the directory.  The NSDB sends response 3
   to server X, telling the server that the directory is located on
   server Y.  Server X sends response 4 to the client, indicating that
   the directory is in a "new" location on server Y.  The client then
   sends request 5 to server Y, repeating the initial request.

   Given the current requirements and definitions, this resolution
   method MUST work.  However, there is no requirement that this is the
   only resolution method that can be used.  This method may be used as
   the fallback when all else fails (or, for a simple implementation, it
   could be the only method).  This is a degenerate implementation of
   the NSDB service as a simple composition of NSDB nodes; we expect
   that large federations will use more sophisticated methods to share
   the FSN and FSL information among multiple NSDB nodes.

          +---------------+
          |               |
          |    Client     | >--------------------------+
          |               |                            |
          +---------------+                            |
            v   ^                                      |
      +-----+---+-------------+      +-----------------+-----+
      |     |   |   Federation|      |Federation       |     |
      |     |   |   member    |      |member           |     |
      |     |   |   ALPHA     |      |BETA             |     |
      |     |   |             |      |                 |     |
      |     |   |             |      |                 |     |
      |     |   |             |      |                 |     |
      |     |   |             |      |                 |     |
      |     |   |             |      |   +---------+   |     |
      |     |   |   +---------+------+-> |         |   |     |
      |     |   |   |         |      |   | NSDB Y  |   |     |
      |     |   |   |   +-----+------+-< |         |   |     |
      |     |   |   |   |     |      |   +---------+   |     |
      |     |   |   |   |     |      |                 |     |
      |     |   |   |   |     |      |                 |     |
      |     |   |   |   |     |      |                 |     |
      |    1|  4|  2|  3|     |      |                5|     |
      |     v   ^   ^   v     |      |                 v     |
      |   +---------------+   |      |   +---------------+   |
      |   |               |   |      |   |               |   |
      |   |   Server X    |   |      |   |   Server Y    |   |
      |   |               |   |      |   |               |   |
      |   +---------------+   |      |   +---------------+   |
      |                       |      |                       |
      +-----------------------+      +-----------------------+

                                 Figure 2

3.3.  Junction Creation

   Given a local path and the FSN of a remote fileset, an administrator
   can create a junction from the local path to the remote fileset.

   There are many possible variations to this procedure, depending on
   how the junctions are represented and how the information necessary
   to perform resolution is represented by the server.

   Step 1 is the only step that uses the federation interfaces.  The
   remaining step may use platform-specific interfaces.

   1.  The administrator requests the server create a junction to the
       FSN of the remote fileset at the given path.

   2.  The server inserts the junction to the FSN, at the given path,
       into the local filesystem.

4.  Glossary

   Administrator:  user with the necessary authority to initiate
      administrative tasks on one or more servers.

   Admin Entity:  A server or agent that administers a collection of
      fileservers and persistently stores the namespace information.

   Client:  Any client that accesses the fileserver data using a
      supported filesystem access protocol.

   Federation:  A set of server collections and singleton servers that
      use a common set of interfaces and protocols in order to provide
      to their clients a federated namespace accessible through a
      filesystem access protocol.

   Fileserver:  A server exporting a filesystem via a network filesystem
      access protocol.

   Fileset:  The abstraction of a set of files and the directory tree
      that contains them.  A fileset is the fundamental unit of data
      management in the federation.

      Note that all files within a fileset are descendants of one
      directory, and that filesets do not span filesystems.

   Filesystem:  A self-contained unit of export for a fileserver, and
      the mechanism used to implement filesets.  The fileset does not
      need to be rooted at the root of the filesystem, nor at the export
      point for the filesystem.

      A single filesystem MAY implement more than one fileset, if the
      client protocol and the fileserver permit this.

   Filesystem Access Protocol:  A network filesystem access protocol
      such as NFSv2 [RFC1094], NFSv3 [RFC1813], NFSv4 [RFC3530], or CIFS
      (Common Internet File System) [MS-SMB] [MS-SMB2] [MS-CIFS].

   FSL (Fileset Location):  The location of the implementation of a
      fileset at a particular moment in time.  An FSL MUST be something
      that can be translated into a protocol-specific description of a
      resource that a client can access directly, such as an fs_location
      (for NFSv4), or share name (for CIFS).  Note that not all FSLs
      need to be explicitly exported as long as they are contained
      within an exported path on the fileserver.

   FSN (Fileset Name):  A platform-independent and globally unique name
      for a fileset.  Two FSLs that implement replicas of the same
      fileset MUST have the same FSN, and if a fileset is migrated from
      one location to another, the FSN of that fileset MUST remain the
      same.

   Junction:  A filesystem object used to link a directory name in the
      current fileset with an object within another fileset.  The
      server-side "link" from a leaf node in one fileset to the root of
      another fileset.

   Namespace:  A filename/directory tree that a sufficiently authorized
      client can observe.

   NSDB (Namespace Database) Service:  A service that maps FSNs to FSLs.
      The NSDB may also be used to store other information, such as
      annotations for these mappings and their components.

   NSDB Node:  The name or location of a server that implements part of
      the NSDB service and is responsible for keeping track of the FSLs
      (and related info) that implement a given partition of the FSNs.

   Referral:  A server response to a client access that directs the
      client to evaluate the current object as a reference to an object
      at a different location (specified by an FSL) in another fileset,
      and possibly hosted on another fileserver.  The client re-attempts
      the access to the object at the new location.

   Replica:  A replica is a redundant implementation of a fileset.  Each
      replica shares the same FSN, but has a different FSL.

      Replicas may be used to increase availability or performance.
      Updates to replicas of the same fileset MUST appear to occur in
      the same order, and therefore each replica is self-consistent at
      any moment.

      We do not assume that updates to each replica occur
      simultaneously.  If a replica is offline or unreachable, the other
      replicas may be updated.

   Server Collection:  A set of fileservers administered as a unit.  A
      server collection may be administered with vendor-specific
      software.

      The namespace provided by a server collection could be part of the
      federated namespace.

   Singleton Server:  A server collection containing only one server; a
      stand-alone fileserver.

5.  Proposed Requirements

   The phrase "USING THE FEDERATION INTERFACES" implies that the
   subsequent requirement must be satisfied, in its entirety, via the
   federation interfaces.

   Note that the requirements are described in terms of correct behavior
   by all entities.  We do not address the requirements of the system in
   the presence of faults.

5.1.  Basic Assumptions

   Several of the requirements are so fundamental that we treat them as
   basic assumptions; if any of these assumptions are violated, the rest
   of the requirements must be reviewed in their entirety.

   A1:  The federation protocols do not require any changes to existing
        client-facing protocols, and MAY be extended to incorporate new
        client-facing protocols.

   A2:  A client SHOULD NOT require any a priori knowledge of the
        general structure or composition of the federation.

        The client may require some specific knowledge in order to find
        and access an instance of the fileset that defines the root of
        its view of the namespace.  As the client traverses the
        namespace, the client discovers the information it needs in
        order to locate the filesets it accesses.

   A3:  All requirements MUST be satisfiable via the federation
        protocols and the standard protocols used by the fileservers
        (i.e., NFS, CIFS, DNS, etc.).

        USING THE FEDERATION INTERFACES, a federation operation that
        requires an interaction between two (or more) entities that are
        members of the federation MUST be possible without requiring any
        proprietary protocols.

   A4:  All the entities participating in a federation operation MUST be
        able to authenticate each other.

        All principals (clients, users, administrator of a singleton or
        server collection, hosts, NSDB nodes, etc.) that can assume a
        role defined by the federation protocol can identify themselves
        to each other via an authentication mechanism.  This mechanism
        is not defined or further described in this document.

        The authority of a principal to request that a second principal
        perform a specific operation is ultimately determined by the
        second.  Authorization may be partitioned by server collection
        or set of servers as well as by operation.  For example, if a
        user has administrative privileges on one server in the
        federation, this does not imply that they have administrative
        privileges (or, for that matter, any privileges whatsoever) on
        any other server in the federation.

        In order to access the functionality provided by the federation
        interfaces, it may be necessary to have elevated privileges or
        authorization.  The authority required by different operations
        may be different.  For example, the authority required to query
        the NSDB about the FSLs bound to an FSN may be different than
        the authority required to change the bindings of that FSN.

        An operation attempted by an unauthorized entity MUST fail in a
        manner that indicates that the failure was due to insufficient
        authorization.

        This document does not enumerate the authorization necessary for
        any operation.

   A5:  The federation protocols MUST NOT require changes to existing
        authentication/authorization mechanisms in use at the
        fileservers for client-facing protocols.

        A user's view of the namespace may be limited by the
        authentication and authorization privileges it has on the
        different fileservers in the federation.  As such, users may
        only be able to traverse the parts of the namespace to which
        they have access.

        The federation protocols do not impose any restrictions on how
        users are represented within the federation.  For example, a
        single enterprise could employ a common identity for users
        across the federation.  A grid environment could utilize user
        mapping or translations across different administrative domains.

   A6:  In a federated system, we assume that an FSN MUST express, or
        can be used to discover, the following two pieces of
        information:

        1.  The location of the NSDB node that is responsible for
            knowing the filesystem location(s) (FSLs) of the named
            fileset.

            The NSDB node must be specified because there may be many
            NSDB nodes in a federation.  We do not assume that any
            single entity knows the location of all of the NSDB nodes,
            and therefore exhaustive search is not an option.

            There are several ways in which a fileserver can locate the
            NSDB node responsible for a given fileset.  One approach,
            given a DNS infrastructure, is to specify the location of
            the NSDB node by the Fully-Qualified Domain Name (FQDN) of
            the server hosting the NSDB node.  Another approach is to
            use a separate DNS-style hierarchy to resolve the location
            of the NSDB node.

        2.  The FSN identifier.

            The FSN identifier is the index used by the NSDB node to
            identify the target fileset.

            There are several ways to represent FSN identifiers.  One
            approach could use 128-bit Universally Unique IDentifiers
            (UUIDs) as described in [RFC4122].

        As an example, an FSN could be represented by a URL of the form
        nsdb://nsdb.example.com/UUID where nsdb is the scheme name,
        nsdb.example.com is the FQDN of the server hosting the NSDB
        node, and UUID is the string representation of the identifier.

        Note that it is not assumed that it is always required for a
        server to contact the NSDB node specified by the FSN in order to
        find the FSLs.  The relevant information stored in that NSDB
        node may also be cached local to the server or on a proxy NSDB
        node "near" the server.

   A7:  All federation servers and NSDB nodes are assumed to execute the
        federation protocols correctly.  The behavior of the federation
        is undefined in the case of Byzantine behavior by any federation
        server or NSDB node.

   A8:  The locations of federation services (such as NSDBs and FSLs)
        can be specified in a manner such that they can be correctly
        interpreted by all members of the federation that will access
        them.

        For example, if an NSDB node is specified by an FQDN, then this
        implies that every member of the federation that needs to access
        this NSDB node can resolve this FQDN to an IP address for that
        NSDB node.  (It is not necessary that the FQDN always resolve to
        the same address; the same service may appear at different
        addresses on different networks.)

        It is the responsibility of each federation member to ensure
        that the resources it wishes to expose have accessible network
        locations and that the necessary resolution mechanisms (i.e.,
        DNS) are given the necessary data to perform the resolution
        correctly.

5.2.  Requirements

   R1:   Requirements of each FSN:

         a.  Each FSN MUST be unique within the scope of its NSDB (so
             that the FSN is globally unique).

         b.  The FSN MUST be sufficiently descriptive to locate an
             instance of the fileset it names within the federation at
             any time.

         c.  All FSNs MUST be invariant when their underlying
             filesystems move or are replicated; only mappings from FSN
             to FSL(s) change under these transformations.

         d.  All files accessible from the global namespace MUST be part
             of a fileset that has an assigned FSN.

         Not all filesets in the federation are required to have an FSN
         or be reachable by an FSL.  Only those filesets that are the
         target of a junction (as described in R3) are required to have
         an FSN.

         The FSN format MAY be of variable size.  If the format is
         variable in size, fileserver implementations MAY have a maximum
         supported FSN size.  By bounding the FSN size, some fileserver
         implementations might be able to efficiently organize FSNs in
         stable storage.  For interoperability, the federation protocols
         SHOULD define an FSN size that all fileservers support.

   R2:   USING THE FEDERATION INTERFACES, it MUST be possible to create
         an FSN for a fileset, and it must be possible to bind an FSL to
         that FSN.  These operations are NSDB operations and do not
         require any action on the part of a file server.

         It is possible to create an FSN for a fileset that has not
         actually been created.  It is also possible to bind a
         nonexistent FSL to an FSN.  It is also possible to create a
         fileset without assigning it an FSN.  The binding between an
         FSN and an FSL is defined entirely within the context of the
         NSDB; the servers do not "know" whether the filesets they host
         have been assigned FSNs (or, if so, what those FSNs are).

         The requirement that filesets can exist prior to being assigned
         an FSN and the requirement that FSNs can exist independent of
         filesets are intended to simplify the construction of the
         namespace in a convenient manner.  For example, they permit an
         admin to assign FSNs to existing filesets and thereby
         incorporate existing filesets into the namespace.  They also
         permit the structure of the namespace to be defined prior to
         creation of the component filesets.  In either case, it is the
         responsibility of the entity updating the NSDB with FSNs and
         FSN-to-FSL mappings to ensure that the namespace is constructed
         in a consistent manner.  (The simplest way to accomplish this
         is to ensure that the FSN and FSN-to-FSL mappings are always
         recorded in the NSDB prior to the creation of any junctions
         that refer to that FSN.)

         a.  An administrator MAY specify the entire FSN (including both
             the NSDB node location and the identifier) of the newly
             created FSL, or the administrator MAY specify only the NSDB
             node and have the system choose the identifier.

             The admin can choose to specify the FSN explicitly in order
             to recreate a lost fileset with a given FSN (for example,
             as part of disaster recovery).  It is an error to assign an
             FSN that is already in use by an active fileset.

             Note that creating a replica of an existing filesystem is
             NOT accomplished by assigning the FSN of the filesystem you
             wish to replicate to a new filesystem.

         b.  USING THE FEDERATION INTERFACES, it MUST be possible to
             create a federation FSL by specifying a specific local
             volume, path, export path, and export options.

   R3:   USING THE FEDERATION INTERFACES, and given the FSN of a target
         fileset, it MUST be possible to create a junction to that
         fileset at a named place in another fileset.

         After a junction has been created, clients that access the
         junction transparently interpret it as a reference to the
         FSL(s) that implement the FSN associated with the junction.

         a.  It SHOULD be possible to have more than one junction whose
             target is a given fileset.  In other words, it SHOULD be
             possible to mount a fileset at multiple named places.

         b.  If the fileset in which the junction is created is
             replicated, then the junction MUST eventually appear in all
             of its replicas.

             The operation of creating a junction within a fileset is
             treated as an update to the fileset, and therefore obeys
             the general rules about updates to replicated filesets.

   R4:   USING THE FEDERATION INTERFACES, it MUST be possible to delete
         a specific junction from a fileset.

         If a junction is deleted, clients who are already viewing the
         fileset referred to by the junction after traversing the
         junction MAY continue to view the old namespace.  They might
         not discover that the junction no longer exists (or has been
         deleted and replaced with a new junction, possibly referring to
         a different FSN).

         After a junction is deleted, another object with the same name
         (another junction, or an ordinary filesystem object) may be
         created.

         The operation of deleting a junction within a fileset is
         treated as an update to the fileset, and therefore obeys the
         general rules about updates to replicated filesets.

   R5:   USING THE FEDERATION INTERFACES, it MUST be possible to
         invalidate an FSN.

         a.  If a junction refers to an FSN that is invalid, attempting
             to traverse the junction MUST fail.

         An FSN that has been invalidated MAY become valid again if the
         FSN is recreated (i.e., as part of a disaster recovery
         process).

         If an FSN is invalidated, clients who are already viewing the
         fileset named by the FSN MAY continue to view the old
         namespace.  They might not discover that the FSN is no longer
         valid until they try to traverse a junction that refers to it.

   R6:   USING THE FEDERATION INTERFACES, it MUST be possible to
         invalidate an FSL.

         a.  An invalid FSL MUST NOT be returned as the result of
             resolving a junction.

         An FSL that has been invalidated MAY become valid again if the
         FSL is recreated (i.e., as part of a disaster recovery
         process).

         If an FSL is invalidated, clients who are already viewing the
         fileset implemented by the FSL MAY continue to use that FSL.
         They might not discover that the FSL is no longer valid until
         they try to traverse a junction that refers to the fileset
         implemented by the FSL.

         Note that invalidating an FSL does not imply that the
         underlying export or share (depending on the file access
         protocol in use) is changed in any way -- it only changes the
         mappings from FSNs to FSLs on the NSDB.

   R7:   It MUST be possible for the federation of servers to provide
         multiple namespaces.

   R8:   USING THE FEDERATION INTERFACES:

         a.  It MUST be possible to query the fileserver named in an FSL
             to discover whether a junction exists at a given path
             within that FSL.

         b.  It MAY be possible to query the fileserver named in an FSL
             to discover the junctions, if any, in that FSL.  If this
             feature is implemented, the fileserver SHOULD report each
             junction's path within the FSL and the targeted FSN.

   R9:   The projected namespace (and the objects named by the
         namespace) MUST be accessible to clients via at least one of
         the following standard filesystem access protocols:

         a.  The namespace SHOULD be accessible to clients via versions
             of the CIFS (Common Internet File System) protocol as
             described in [MS-SMB] [MS-SMB2] [MS-CIFS].

         b.  The namespace SHOULD be accessible to clients via the NFSv4
             protocol as described in [RFC3530].

         c.  The namespace SHOULD be accessible to clients via the NFSv3
             protocol as described in [RFC1813].

         d.  The namespace SHOULD be accessible to clients via the NFSv2
             protocol as described in [RFC1094].

         It must be understood that some of these protocols, such as
         NFSv3 and NFSv2, have no innate ability to access a namespace
         of this kind.  Where such protocols have been augmented with
         other protocols and mechanisms (such as autofs or amd for
         NFSv3) to provide an extended namespace, we propose that these
         protocols and mechanisms may be used, or extended, in order to
         satisfy the requirements given in this document, and different
         clients may use different mechanisms.

   R10:  USING THE FEDERATION INTERFACES, it MUST be possible to modify
         the NSDB mapping from an FSN to a set of FSLs to reflect the
         migration from one FSL to another.

   R11:  FSL migration SHOULD have little or no impact on the clients,
         but this is not guaranteed across all federation members.

         Whether FSL migration is performed transparently depends on
         whether the source and destination servers are able to do so.
         It is the responsibility of the administrator to recognize
         whether or not the migration will be transparent, and advise
         the system accordingly.  The federation, in turn, MUST advise
         the servers to notify their clients, if necessary.

         For example, on some systems, it may be possible to migrate a
         fileset from one system to another with minimal client impact
         because all client-visible metadata (inode numbers, etc.) are
         preserved during migration.  On other systems, migration might
         be quite disruptive.

   R12:  USING THE FEDERATION INTERFACES, it MUST be possible to modify
         the NSDB mapping from an FSN to a set of FSLs to reflect the
         addition/removal of a replica at a given FSL.

   R13:  Replication SHOULD have little or no negative impact on the
         clients.

         Whether FSL replication is performed transparently depends on
         whether the source and destination servers are able to do so.
         It is the responsibility of the administrator initiating the
         replication to recognize whether or not the replication will be
         transparent, and advise the federation accordingly.  The
         federation MUST advise the servers to notify their clients, if
         necessary.

         For example, on some systems, it may be possible to mount any
         FSL of an FSN read/write, while on other systems, there may be
         any number of read-only replicas but only one FSL that can be
         mounted as read/write.

   R14:  USING THE FEDERATION INTERFACES, it SHOULD be possible to
         annotate the objects and relations managed by the federation
         protocol with arbitrary name/value pairs.

         These annotations are not used by the federation protocols --
         they are intended for use by higher-level protocols.  For
         example, an annotation that might be useful for a system
         administrator browsing the federation would be the "owner" of
         each FSN (i.e., "this FSN is for the home directory of Joe
         Smith").  As another example, the annotations may express hints
         used by the clients (such as priority information for NFSv4.1).

         Both FSNs and FSLs may be annotated.  For example, an FSN
         property might be "This is Joe Smith's home directory", and an
         FSL property might be "This instance of the FSN is at the
         remote backup site".

         a.  USING THE FEDERATION INTERFACES, it MUST be possible to
             query the system to find the annotations for a junction.

         b.  USING THE FEDERATION INTERFACES, it MUST be possible to
             query the system to find the annotations for an FSN.

         c.  USING THE FEDERATION INTERFACES, it MUST be possible to
             query the system to find the annotations for an FSL.

   R15:  It MUST be possible for the federation to project a namespace
         with a common root.

         a.  It SHOULD be possible to define a root fileset that is
             exported by one or more fileservers in the federation as
             the top level of a namespace.  (Corollary: There is one
             root fileset per namespace and it is possible to support
             multiple namespaces per federation.)

         b.  It SHOULD be possible for a fileserver to locate an NSDB
             that stores the layout of a root fileset.

         c.  It SHOULD be possible to access, store, and update
             information related to a root fileset using the federation
             protocols.

         d.  It SHOULD be possible to replicate root fileset information
             across multiple repositories.

         e.  If a root fileset is defined, it SHOULD be possible to
             enable a fileserver to export that root fileset for client
             access.

         f.  If a root fileset is defined, it SHOULD be possible for
             multiple fileservers to project a common root with defined
             consistency semantics.

         g.  If a root fileset is defined, it SHOULD be stored using a
             compact representation that is compatible with
             heterogeneous fileserver implementations.  The root
             fileset's internal format SHOULD contain enough information
             to generate any attributes, including referrals, required
             by the standard filesystem access protocols in R9.

6.  Non-Requirements

   N1:  It is not necessary for the namespace to be known by any
        specific fileserver.

        In the same manner that clients do not need to have a priori
        knowledge of the structure of the namespace or its mapping onto
        federation members, the projected namespace can exist without
        individual fileservers knowing the entire organizational
        structure, or, indeed, without knowing exactly where in the
        projected namespace the filesets they host exist.

        Fileservers do need to be able to handle referrals from other
        fileservers, but they do not need to know what path the client
        was accessing when the referral was generated.

   N2:  It is not necessary for updates and accesses to the NSDB data to
        occur in transaction or transaction-like contexts.

        One possible requirement that is omitted from our current list
        is that updates and accesses to the data stored in the NSDB (or
        individual NSDB nodes) occur within a transaction context.  We
        were not able to agree whether the benefits of transactions are
        worth the complexity they add (both to the specification and its
        eventual implementation), but this topic is open for discussion.

        Below is the draft of a proposed requirement that provides
        transactional semantics:

           There MUST be a way to ensure that sequences of operations,
           including observations of the namespace (including finding
           the locations corresponding to a set of FSNs) and changes to
           the namespace or related data stored in the system (including
           the creation, renaming, or deletion of junctions, and the
           creation, altering, or deletion of mappings between FSN and
           filesystem locations), can be performed in a manner that
           provides predictable semantics for the relationship between
           the observed values and the effect of the changes.

           It MUST be possible to protect sequences of operations by
           transactions with NSDB-wide or server-wide Atomicity,
           Consistency, Isolation, and Durability (ACID) semantics.

7.  Security Considerations

   Assuming the Internet threat model, the federated resolution
   mechanism described in this document MUST be implemented in such a
   way to prevent loss of CONFIDENTIALITY, DATA INTEGRITY, and PEER
   ENTITY AUTHENTICATION, as described in [RFC3552].

   CONFIDENTIALITY may be violated if an unauthorized party is able to
   eavesdrop on the communication between authorized servers and NSDB
   nodes and thereby learn the locations or other information about FSNs
   that they would not be authorized to discover via direct queries.
   DATA INTEGRITY may be compromised if a third party is able to
   undetectably alter the contents of the communication between servers
   and NSDB nodes.  PEER ENTITY AUTHENTICATION is defeated if one server
   can masquerade as another server without proper authority, or if an
   arbitrary host can masquerade as a NSDB node.

   Well-established techniques for providing authenticated channels may
   be used to defeat these attacks, and the protocol MUST support at
   least one of them.

   For example, if Lightweight Directory Access Protocol (LDAP) is used
   to implement the query mechanism [RFC4510], then Transport Layer
   Security (TLS) may be used to provide both authentication and
   integrity [RFC5246] [RFC4513].  If the query protocol is implemented
   on top of Open Network Computing / Remote Procedure Call (ONC/RPC),
   then RPCSEC_GSS may be used to fill the same role [RFC2203]
   [RFC2743].

   A federation could contain multiple Public Key Infrastructure (PKI)
   trust anchors [RFC5280].  The federation protocols SHOULD define a
   mechanism for managing a fileserver's NSDB trust anchors
   [TA-MGMT-REQS].  A general purpose trust anchor management protocol
   [TAMP] would be appropriate, though it might be desirable for the
   federation protocols to facilitate trust anchor management by, for
   example, using trust anchor interchange formats [TA-FORMAT].

   It is useful to note that the requirements described in this document
   lead naturally to a system with distributed authorization, which has
   scalability and manageability benefits.

   FSNs are likely to be long-lived resources.  Therefore, the privilege
   to create FSNs SHOULD be carefully controlled.  To assist in
   determining if an FSN is referenced by a junction somewhere in the
   federation, the NSDB records SHOULD include non-authoritative
   informational annotations recording the locations of any such
   junctions.  These annotations are non-authoritative because a

   junction might be created, deleted, or modified by an individual that
   does not have permission to modify the NSDB records.

8.  References

8.1.  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.

   [RFC3552]       Rescorla, E. and B. Korver, "Guidelines for Writing
                   RFC Text on Security Considerations", BCP 72,
                   RFC 3552, July 2003.

   [RFC4122]       Leach, P., Mealling, M., and R. Salz, "A Universally
                   Unique IDentifier (UUID) URN Namespace", RFC 4122,
                   July 2005.

   [RFC4510]       Zeilenga, K., "Lightweight Directory Access Protocol
                   (LDAP): Technical Specification Road Map", RFC 4510,
                   June 2006.

   [RFC5280]       Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
                   Housley, R., and W. Polk, "Internet X.509 Public Key
                   Infrastructure Certificate and Certificate Revocation
                   List (CRL) Profile", RFC 5280, May 2008.

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

8.2.  Informative References

   [MS-CIFS]       Microsoft Corporation, "Common Internet File System
                   (CIFS) Protocol Specification", MS-CIFS 2.0,
                   November 2009.

   [MS-SMB]        Microsoft Corporation, "Server Message Block (SMB)
                   Protocol Specification", MS-SMB 17.0, November 2009.

   [MS-SMB2]       Microsoft Corporation, "Server Message Block (SMB)
                   Version 2 Protocol Specification", MS-SMB2 19.0,
                   November 2009.

   [RFC1094]       Nowicki, B., "NFS: Network File System Protocol
                   specification", RFC 1094, March 1989.

   [RFC1813]       Callaghan, B., Pawlowski, B., and P. Staubach, "NFS
                   Version 3 Protocol Specification", RFC 1813,
                   June 1995.

   [RFC2203]       Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS
                   Protocol Specification", RFC 2203, September 1997.

   [RFC2743]       Linn, J., "Generic Security Service Application
                   Program Interface Version 2, Update 1", RFC 2743,
                   January 2000.

   [RFC4513]       Harrison, R., "Lightweight Directory Access Protocol
                   (LDAP): Authentication Methods and Security
                   Mechanisms", RFC 4513, June 2006.

   [RFC5246]       Dierks, T. and E. Rescorla, "The Transport Layer
                   Security (TLS) Protocol Version 1.2", RFC 5246,
                   August 2008.

   [TA-FORMAT]     Housley, R., Ashmore, S., and C. Wallace, "Trust
                   Anchor Format", Work in Progress, October 2009.

   [TA-MGMT-REQS]  Reddy, R. and C. Wallace, "Trust Anchor Management
                   Requirements", Work in Progress, September 2009.

   [TAMP]          Housley, R., Ashmore, S., and C. Wallace, "Trust
                   Anchor Management Protocol (TAMP)", Work in Progress,
                   December 2009.

Appendix A.  Acknowledgments

   We would like to thank Robert Thurlow of Sun Microsystems for helping
   to author this document.

   We would also like to thank Peter McCann and Nicolas Williams for
   their comments and suggestions.

Authors' Addresses

   James Lentini
   NetApp
   1601 Trapelo Rd, Suite 16
   Waltham, MA  02451
   US

   Phone: +1 781-768-5359
   EMail: jlentini@netapp.com

   Craig Everhart
   NetApp
   7301 Kit Creek Rd
   Research Triangle Park, NC  27709
   US

   Phone: +1 919-476-5320
   EMail: everhart@netapp.com

   Daniel Ellard
   BBN Technologies
   10 Moulton Street
   Cambridge, MA  02138
   US

   Phone: +1 617-873-8000
   EMail: dellard@bbn.com

   Renu Tewari
   IBM Almaden
   650 Harry Rd
   San Jose, CA  95120
   US

   EMail: tewarir@us.ibm.com

   Manoj Naik
   IBM Almaden
   650 Harry Rd
   San Jose, CA  95120
   US

   EMail: manoj@almaden.ibm.com

 

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