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rfc:rfc7861

Internet Engineering Task Force (IETF) W. Adamson Request for Comments: 7861 NetApp Updates: 5403 N. Williams Category: Standards Track Cryptonector ISSN: 2070-1721 November 2016

           Remote Procedure Call (RPC) Security Version 3

Abstract

 This document specifies version 3 of the Remote Procedure Call (RPC)
 security protocol (RPCSEC_GSS).  This protocol provides support for
 multi-principal authentication of client hosts and user principals to
 a server (constructed by generic composition), security label
 assertions for multi-level security and type enforcement, structured
 privilege assertions, and channel bindings.  This document updates
 RFC 5403.

Status of This Memo

 This is an Internet Standards Track document.
 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).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 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/rfc7861.

Copyright Notice

 Copyright (c) 2016 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
 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.

Adamson & Williams Standards Track [Page 1] RFC 7861 NFSv4 RPC Security November 2016

Table of Contents

 1. Introduction and Motivation .....................................2
    1.1. Requirements Language ......................................3
    1.2. Added Functionality ........................................4
    1.3. XDR Code Extraction ........................................5
 2. The RPCSEC_GSSv3 Protocol .......................................6
    2.1. Compatibility with RPCSEC_GSSv2 ............................6
    2.2. Version Negotiation ........................................6
    2.3. New Reply Verifier .........................................7
    2.4. XDR Code Preliminaries .....................................8
    2.5. RPCSEC_GSS_BIND_CHANNEL Operation .........................10
    2.6. New auth_stat Values ......................................10
    2.7. New Control Procedures ....................................10
         2.7.1. New Control Procedure - RPCSEC_GSS_CREATE ..........12
         2.7.2. New Control Procedure - RPCSEC_GSS_LIST ............20
    2.8. Extensibility .............................................21
 3. Operational Recommendation for Deployment ......................21
 4. Security Considerations ........................................21
 5. IANA Considerations ............................................22
    5.1. New RPC Authentication Status Numbers .....................22
    5.2. Structured Privilege Name Definitions .....................23
         5.2.1. Initial Registry ...................................24
         5.2.2. Updating Registrations .............................24
 6. References .....................................................25
    6.1. Normative References ......................................25
    6.2. Informative References ....................................26
 Acknowledgments ...................................................26
 Authors' Addresses ................................................26

1. Introduction and Motivation

 The original Remote Procedure Call (RPC) security protocol
 (RPCSEC_GSS) [RFC2203] provided for authentication of RPC clients and
 servers to each other using the Generic Security Service Application
 Programming Interface (GSS-API) [RFC2743].  The second version of
 RPCSEC_GSS [RFC5403] added support for channel bindings [RFC5056].
 Existing GSS-API mechanisms are insufficient for communicating
 certain authorization and authentication information to a server.
 The GSS-API and its mechanisms certainly could be extended to address
 this shortcoming.  However, it is addressed here at the application
 layer, i.e., in RPCSEC_GSS.
 A major motivation for version 3 of RPCSEC_GSS (RPCSEC_GSSv3) is to
 add support for multi-level (labeled) security and server-side copy
 for NFSv4.

Adamson & Williams Standards Track [Page 2] RFC 7861 NFSv4 RPC Security November 2016

 Multi-Level Security (MLS) is a traditional model where subjects
 (processes) are given a security level (Unclassified, Secret,
 Top Secret, etc.) and objects (files) are given security labels that
 mandate the access of the subject to the object (see Section 9.1 of
 [RFC7862]).
 Labeled NFS (see Section 9 of [RFC7862]) uses an MLS policy with
 Mandatory Access Control (MAC) systems as defined in [RFC4949].
 Labeled NFS stores MAC file object labels on the NFS server and
 enables client Guest Mode MAC as described in Section 9.5.3 of
 [RFC7862].  RPCSEC_GSSv3 label assertions assert client MAC process
 subject labels to enable Full Mode MAC when combined with Labeled NFS
 as described in Section 9.5.1 of [RFC7862].
 A traditional inter-server file copy entails the user gaining access
 to a file on the source, reading it, and writing it to a file on the
 destination.  In secure NFSv4 inter-server server-side copy (see
 Section 4 of [RFC7862]), the user first secures access to both source
 and destination files and then uses NFSv4.2-defined RPCSEC_GSSv3
 structured privileges to authorize the destination to copy the file
 from the source on behalf of the user.
 Multi-principal authentication can be used to address shared cache
 poisoning attacks (see Section 9 of [AFS-RXGK]) on the client cache
 by a user.  As described in Section 7 of [AFS-RXGK], multi-user
 machines with a single cache manager can fetch and cache data on a
 user's behalf and re-display it for another user from the cache
 without refetching the data from the server.  The initial data
 acquisition is authenticated by the first user's credentials, and if
 only that user's credentials are used, it may be possible for a
 malicious user or users to "poison" the cache for other users by
 introducing bogus data into the cache.
 Another use of the multi-principal assertion is the secure conveyance
 of privilege information for processes running with more (or even
 with less) privilege than the user normally would be accorded.

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 RFC 2119 [RFC2119].

Adamson & Williams Standards Track [Page 3] RFC 7861 NFSv4 RPC Security November 2016

1.2. Added Functionality

 RPCSEC_GSS version 3 (RPCSEC_GSSv3) is the same as RPCSEC_GSSv2
 [RFC5403], except that the following assertions of authority have
 been added:
 o  Security labels for Full Mode security type enforcement, and other
    labeled security models (see Section 9.5.1 of [RFC7862]).
 o  Application-specific structured privileges.  These allow an RPC
    application client to pass structured information to the
    corresponding application code in a server to control the use of
    the privilege and/or the conditions in which the privilege may be
    exercised.  For an example, see server-side copy as described in
    [RFC7862].
 o  Multi-principal authentication of the client host and user to the
    server, done by binding two RPCSEC_GSS handles.
 o  Simplified channel binding.
 Assertions of labels and privileges are evaluated by the server,
 which may then map the asserted values to other values, all according
 to server-side policy.  See [RFC7862].
 An option for enumerating server-supported Label Format Specifiers
 (LFSs) is provided.  See Section 9.1 of [RFC7862].
 Note that there is no RPCSEC_GSS_CREATE payload that is REQUIRED to
 implement.  RPCSEC_GSSv3 implementations are feature driven.  Besides
 implementing the RPCSEC_GSS_CREATE operation and payloads for the
 desired features, all RPCSEC_GSSv3 implementations MUST implement:
 o  The new RPCSEC_GSS version number (Section 2.2).
 o  The new reply verifier (Section 2.3).
 o  The new auth_stat values (Section 2.6).

Adamson & Williams Standards Track [Page 4] RFC 7861 NFSv4 RPC Security November 2016

 RPCSEC_GSSv3 targets implementing a desired feature MUST also
 implement the RPCSEC_GSS_LIST operation, and the RPCSEC_GSS_CREATE
 operation replies for unsupported features as follows:
 o  For label assertions, the target indicates no support by returning
    the new RPCSEC_GSS_LABEL_PROBLEM auth_stat value (see
    Section 2.7.1.3).
 o  For structured privilege assertions, the target indicates no
    support by returning the new RPCSEC_GSS_UNKNOWN_MESSAGE auth_stat
    value (see Section 2.7.1.4).
 o  For multi-principal authentication (Section 2.7.1.1), the target
    indicates no support by not including an rgss3_gss_mp_auth value
    in the rgss3_create_res.
 o  For channel bindings (Section 2.7.1.2), the target indicates no
    support by not including an rgss3_chan_binding value in the
    rgss3_create_res.

1.3. XDR Code Extraction

 This document contains the External Data Representation (XDR)
 [RFC4506] definitions for the RPCSEC_GSSv3 protocol.  The XDR
 description is provided in this document in a way that makes it
 simple for the reader to extract it into a form that is ready to
 compile.  The reader can feed this document in the following shell
 script to produce the machine-readable XDR description of
 RPCSEC_GSSv3:
 <CODE BEGINS>
 #!/bin/sh
 grep "^  *///" | sed 's?^  */// ??' | sed 's?^  *///$??'
 <CODE ENDS>
 That is, if the above script is stored in a file called "extract.sh"
 and this document is in a file called "spec.txt", then the reader
 can do:
 <CODE BEGINS>
  sh extract.sh < spec.txt > rpcsec_gss_v3.x
 <CODE ENDS>

Adamson & Williams Standards Track [Page 5] RFC 7861 NFSv4 RPC Security November 2016

 The effect of the script is to remove leading white space from each
 line, plus a sentinel sequence of "///".

2. The RPCSEC_GSSv3 Protocol

 RPCSEC_GSS version 3 (RPCSEC_GSSv3) is very similar to RPCSEC_GSS
 version 2 (RPCSEC_GSSv2) [RFC5403].  The difference is that the new
 support for assertions and channel bindings is implemented via a
 different mechanism.
 The entire RPCSEC_GSSv3 protocol is not presented here.  Only the
 differences between RPCSEC_GSSv3 and RPCSEC_GSSv2 are shown.
 RPCSEC_GSSv3 is implemented as follows:
 o  A client uses an existing RPCSEC_GSSv3 context handle established
    in the usual manner (see Section 5.2 of [RFC2203]) to protect
    RPCSEC_GSSv3 exchanges; this will be termed the "parent" handle.
 o  The server issues a "child" RPCSEC_GSSv3 handle in the
    RPCSEC_GSS_CREATE response, which uses the underlying GSS-API
    security context of the parent handle in all subsequent exchanges
    that use the child handle.
 o  An RPCSEC_GSSv3 child handle MUST NOT be used as the parent handle
    in an RPCSEC_GSS3_CREATE control message.

2.1. Compatibility with RPCSEC_GSSv2

 The functionality of RPCSEC_GSSv2 [RFC5403] is fully supported by
 RPCSEC_GSSv3, with the exception of the RPCSEC_GSS_BIND_CHANNEL
 operation, which is not supported when RPCSEC_GSSv3 is in use (see
 Section 2.5).

2.2. Version Negotiation

 An initiator that supports version 3 of RPCSEC_GSS simply issues an
 RPCSEC_GSS request with the rgc_version field set to
 RPCSEC_GSS_VERS_3.  If the target does not recognize
 RPCSEC_GSS_VERS_3, the target will return an RPC error per
 Section 5.1 of [RFC2203].
 The initiator MUST NOT attempt to use an RPCSEC_GSS handle returned
 by version 3 of a target with version 1 or version 2 of the same
 target.  The initiator MUST NOT attempt to use an RPCSEC_GSS handle
 returned by version 1 or version 2 of a target with version 3 of the
 same target.

Adamson & Williams Standards Track [Page 6] RFC 7861 NFSv4 RPC Security November 2016

2.3. New Reply Verifier

 A new reply verifier is needed for RPCSEC_GSSv3 because of a
 situation that arises from the use of the same GSS context by child
 and parent handles.  Because the RPCSEC_GSSv3 child handle uses the
 same GSS context as the parent handle, a child and parent
 RPCSEC_GSSv3 handle could have the same RPCSEC_GSS sequence numbers.
 Since the reply verifier of previous versions of RPCSEC_GSS computes
 a Message Integrity Code (MIC) on just the sequence number, this
 provides opportunities for man-in-the-middle attacks.
 This issue is addressed in RPCSEC_GSS version 3 by computing the
 verifier using exactly the same input as the information used to
 compute the request verifier, except that the mtype is changed from
 CALL to REPLY.  The new reply verifier computes a MIC over the
 following RPC reply header data:
   unsigned int xid;
   msg_type mtype;    /* set to REPLY */
   unsigned int rpcvers;
   unsigned int prog;
   unsigned int vers;
   unsigned int proc;
   opaque_auth  cred; /* binds the RPCSEC_GSS handle */

Adamson & Williams Standards Track [Page 7] RFC 7861 NFSv4 RPC Security November 2016

2.4. XDR Code Preliminaries

 The following code fragment replaces the corresponding preliminary
 code shown in Figure 1 of [RFC5403].  The values in the code fragment
 in Section 2.6 are additions to the auth_stat enumeration.
 Subsequent code fragments are additions to the code for version 2
 that support the new procedures defined in version 3.
 <CODE BEGINS>
    ///  /*
    ///   * Copyright (c) 2016 IETF Trust and the persons
    ///   * identified as the authors.  All rights reserved.
    ///   *
    ///   * The authors of the code are identified in RFC 2203,
    ///   * RFC 5403, and RFC 7861.
    ///   *
    ///   * Redistribution and use in source and binary forms,
    ///   * with or without modification, are permitted
    ///   * provided that the following conditions are met:
    ///   *
    ///   * o Redistributions of source code must retain the above
    ///   *   copyright notice, this list of conditions and the
    ///   *   following disclaimer.
    ///   *
    ///   * o Redistributions in binary form must reproduce the
    ///   *   above copyright notice, this list of
    ///   *   conditions and the following disclaimer in
    ///   *   the documentation and/or other materials
    ///   *   provided with the distribution.
    ///   *
    ///   * o Neither the name of Internet Society, IETF or IETF
    ///   *   Trust, nor the names of specific contributors, may be
    ///   *   used to endorse or promote products derived from this
    ///   *   software without specific prior written permission.
    ///   *
    ///   *   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS
    ///   *   AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED
    ///   *   WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
    ///   *   IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
    ///   *   FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO
    ///   *   EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
    ///   *   LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
    ///   *   EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
    ///   *   NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
    ///   *   SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
    ///   *   INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
    ///   *   LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,

Adamson & Williams Standards Track [Page 8] RFC 7861 NFSv4 RPC Security November 2016

    ///   *   OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
    ///   *   IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
    ///   *   ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
    ///   */
    ///
    ///  /*
    ///   * This code was derived from RFC 2203, RFC 5403,
    ///   * and RFC 7861.  Please reproduce this note if possible.
    ///   */
    ///
    ///  enum rpc_gss_service_t {
    ///          /* Note: The enumerated value for 0 is reserved. */
    ///          rpc_gss_svc_none         = 1,
    ///          rpc_gss_svc_integrity    = 2,
    ///          rpc_gss_svc_privacy      = 3,
    ///          rpc_gss_svc_channel_prot = 4
    ///  };
    ///
    ///  enum rpc_gss_proc_t {
    ///           RPCSEC_GSS_DATA          = 0,
    ///           RPCSEC_GSS_INIT          = 1,
    ///           RPCSEC_GSS_CONTINUE_INIT = 2,
    ///           RPCSEC_GSS_DESTROY       = 3,
    ///           RPCSEC_GSS_BIND_CHANNEL  = 4, /* Not used */
    ///           RPCSEC_GSS_CREATE        = 5, /* New */
    ///           RPCSEC_GSS_LIST          = 6  /* New */
    ///  };
    ///
    ///  struct rpc_gss_cred_vers_1_t {
    ///          rpc_gss_proc_t    gss_proc; /* Control procedure */
    ///          unsigned int      seq_num;  /* Sequence number */
    ///          rpc_gss_service_t service;  /* Service used */
    ///          opaque            handle<>; /* Context handle */
    ///  };
    ///
    ///  const RPCSEC_GSS_VERS_1 = 1;
    ///  const RPCSEC_GSS_VERS_2 = 2;
    ///  const RPCSEC_GSS_VERS_3 = 3; /* New */
    ///
    ///  union rpc_gss_cred_t switch (unsigned int rgc_version) {
    ///  case RPCSEC_GSS_VERS_1:
    ///  case RPCSEC_GSS_VERS_2:
    ///  case RPCSEC_GSS_VERS_3: /* New */
    ///          rpc_gss_cred_vers_1_t rgc_cred_v1;
    ///  };
    ///
 <CODE ENDS>

Adamson & Williams Standards Track [Page 9] RFC 7861 NFSv4 RPC Security November 2016

 As seen above, the RPCSEC_GSSv3 credential has the same format as the
 RPCSEC_GSSv1 [RFC2203] and RPCSEC_GSSv2 [RFC5403] credential.
 Setting the rgc_version field to 3 indicates that the initiator and
 target support the new RPCSEC_GSSv3 control procedures.

2.5. RPCSEC_GSS_BIND_CHANNEL Operation

 RPCSEC_GSSv3 provides a channel-binding assertion that replaces the
 RPCSEC_GSSv2 RPCSEC_GSS_BIND_CHANNEL operation.
 The RPCSEC_GSS_BIND_CHANNEL operation is not supported on RPCSEC_GSS
 version 3 handles.  If a server receives an RPCSEC_GSS_BIND_CHANNEL
 operation on an RPCSEC_GSSv3 handle, it MUST return a reply status of
 MSG_ACCEPTED with an accept_stat of PROC_UNAVAIL [RFC5531].

2.6. New auth_stat Values

 RPCSEC_GSSv3 requires the addition of several values to the auth_stat
 enumerated type definition.  The use of these new auth_stat values is
 explained throughout this document.
            enum auth_stat {
                    ...
                    /*
                     * RPCSEC_GSSv3 errors
                     */
                    RPCSEC_GSS_INNER_CREDPROBLEM = 15,
                    RPCSEC_GSS_LABEL_PROBLEM     = 16,
                    RPCSEC_GSS_PRIVILEGE_PROBLEM = 17,
                    RPCSEC_GSS_UNKNOWN_MESSAGE   = 18
            };

2.7. New Control Procedures

 There are two new RPCSEC_GSSv3 control procedures: RPCSEC_GSS_CREATE
 and RPCSEC_GSS_LIST.
 The RPCSEC_GSS_CREATE procedure binds any combination of assertions
 -- multi-principal authentication, labels, structured privileges, or
 channel bindings -- to a new RPCSEC_GSSv3 context returned in the
 rgss3_create_res rcr_handle field.
 The RPCSEC_GSS_LIST procedure queries the target for supported
 assertions.

Adamson & Williams Standards Track [Page 10] RFC 7861 NFSv4 RPC Security November 2016

 RPCSEC_GSS version 3 control messages are similar to the RPCSEC_GSS
 version 1 and version 2 RPCSEC_GSS_DESTROY control message (see
 Section 5.4 of [RFC2203]) in that the sequence number in the request
 must be valid and the header checksum in the verifier must be valid.
 As in RPCSEC_GSS version 1 and version 2, the RPCSEC_GSS version 3
 control messages may contain call data following the verifier in the
 body of the NULLPROC procedure.  In other words, they look a lot like
 an RPCSEC_GSS data message with the header procedure set to NULLPROC.
 The client MUST use one of the following security services to protect
 the RPCSEC_GSS_CREATE or RPCSEC_GSS_LIST control message:
 o  rpc_gss_svc_integrity
 o  rpc_gss_svc_privacy
 Specifically, the client MUST NOT use rpc_gss_svc_none.
 RPCSEC_GSS_LIST can also use rpc_gss_svc_channel_prot (see
 RPCSEC_GSSv2 [RFC5403]) if the request is sent using an RPCSEC_GSSv3
 child handle with channel bindings enabled as described in
 Section 2.7.1.2.

Adamson & Williams Standards Track [Page 11] RFC 7861 NFSv4 RPC Security November 2016

2.7.1. New Control Procedure - RPCSEC_GSS_CREATE

 <CODE BEGINS>
    ///  struct rgss3_create_args {
    ///          rgss3_gss_mp_auth    *rca_mp_auth;
    ///          rgss3_chan_binding   *rca_chan_bind_mic;
    ///          rgss3_assertion_u    rca_assertions<>;
    ///  };
    ///
    ///  struct rgss3_create_res {
    ///          opaque               rcr_handle<>;
    ///          rgss3_gss_mp_auth    *rcr_mp_auth;
    ///          rgss3_chan_binding   *rcr_chan_bind_mic;
    ///          rgss3_assertion_u    rcr_assertions<>;
    ///  };
    ///
    ///  enum rgss3_assertion_type {
    ///          LABEL = 0,
    ///          PRIVS = 1
    ///  };
    ///
    ///  union rgss3_assertion_u
    ///        switch (rgss3_assertion_type atype) {
    ///  case LABEL:
    ///          rgss3_label  rau_label;
    ///  case PRIVS:
    ///          rgss3_privs  rau_privs;
    ///  default:
    ///          opaque       rau_ext<>;
    ///  };
    ///
 <CODE ENDS>
 The call data for an RPCSEC_GSS_CREATE request consists of an
 rgss3_create_args, which binds one or more items of several kinds to
 the returned rcr_handle RPCSEC_GSSv3 context handle (the child
 handle):
 o  Multi-principal authentication: another RPCSEC_GSS context handle
 o  A channel binding
 o  Authorization assertions: labels and/or privileges

Adamson & Williams Standards Track [Page 12] RFC 7861 NFSv4 RPC Security November 2016

 The reply to this message consists of either an error or an
 rgss3_create_res structure.  As noted in Sections 2.7.1.3 and
 2.7.1.4, successful rgss3_assertions are enumerated in rcr_assertions
 and are REQUIRED to be enumerated in the same order as they appeared
 in the rca_assertions argument.
 Upon a successful RPCSEC_GSS_CREATE, both the client and the server
 need to associate the resultant child rcr_handle context handle with
 the parent context handle in their GSS context caches so as to be
 able to reference the parent context given the child context handle.
 RPCSEC_GSSv3 child handles MUST be destroyed upon the destruction of
 the associated parent handle.
 Server implementation and policy MAY result in labels, privileges,
 and identities being mapped to concepts and values that are local to
 the server.  Server policies should take into account the identity of
 the client and/or user as authenticated via the GSS-API.

2.7.1.1. Multi-Principal Authentication

 <CODE BEGINS>
    ///
    ///  struct rgss3_gss_mp_auth {
    ///          opaque          rgmp_handle<>; /* Inner handle */
    ///          opaque          rgmp_rpcheader_mic<>;
    ///  };
    ///
 <CODE ENDS>
 RPCSEC_GSSv3 clients MAY assert a multi-principal authentication of
 the RPC client host principal and a user principal.  This feature is
 needed, for example, when an RPC client host wishes to use authority
 assertions that the server may only grant if a user and an RPC client
 host are authenticated together to the server.  Thus, a server may
 refuse to grant requested authority to a user acting alone (e.g., via
 an unprivileged user-space program) or to an RPC client host acting
 alone (e.g., when an RPC client host is acting on behalf of a user)
 but may grant requested authority to an RPC client host acting on
 behalf of a user if the server identifies the user and trusts the RPC
 client host.

Adamson & Williams Standards Track [Page 13] RFC 7861 NFSv4 RPC Security November 2016

 It is assumed that an unprivileged user-space program would not have
 access to RPC client host credentials needed to establish a GSS-API
 security context authenticating the RPC client host to the server;
 therefore, an unprivileged user-space program could not create an
 RPCSEC_GSSv3 RPCSEC_GSS_CREATE message that successfully binds an RPC
 client host and a user security context.
 In addition to the parent handle (Section 2), the multi-principal
 authentication call data has an RPCSEC_GSS version 3 handle
 referenced via the rgmp_handle field termed the "inner" handle.
 Clients using RPCSEC_GSSv3 multi-principal authentication MUST use an
 RPCSEC_GSSv3 context handle that corresponds to a GSS-API security
 context that authenticates the RPC client host for the parent handle.
 The inner context handle of the multi-principal authentication
 assertion MUST use an RPCSEC_GSSv3 context handle that corresponds to
 a GSS-API security context that authenticates the user.  The reverse
 (parent handle authenticates user, inner context handle authenticates
 an RPC client host) MUST NOT be used.  Other multi-principal parent
 and inner context handle uses might eventually make sense, but they
 would need to be introduced in a new revision of the RPCSEC_GSS
 protocol.
 The child context handle returned by a successful multi-principal
 assertion binds the inner RPCSEC_GSSv3 context handle to the parent
 RPCSEC_GSS context handle and MUST be treated by servers as
 authenticating the GSS-API initiator principal authenticated by the
 inner context handle's GSS-API security context.  This principal may
 be mapped to a server-side notion of user or principal.
 Multi-principal binding is done by including an assertion of type
 rgss3_gss_mp_auth in the RPCSEC_GSS_CREATE rgss3_create_args call
 data.  The inner context handle is placed in the rgmp_handle field.
 A MIC of the RPC header, up to and including the credential, is
 computed using the GSS-API security context associated with the inner
 context handle and is placed in the rgmp_rpcheader_mic field.  Note
 that the rgmp_rpcheader_mic only identifies the client host GSS
 context by its context handle (the parent context handle) in the RPC
 header.
 An RPCSEC_GSS_CREATE control procedure with a multi-principal
 authentication payload MUST use the rpc_gss_svc_privacy security
 service for protection.  This prevents an attacker from intercepting
 the RPCSEC_GSS_CREATE control procedure, reassigning the (parent)
 context handle, and stealing the user's identity.

Adamson & Williams Standards Track [Page 14] RFC 7861 NFSv4 RPC Security November 2016

 The target verifies the multi-principal authentication by first
 confirming that the parent context used is an RPC client host
 context; the target then verifies the rgmp_rpcheader_mic using the
 GSS-API security context associated with the rgmp_handle field.
 On successful verification, the rgss3_gss_mp_auth field in the
 rgss3_create_res reply MUST be filled in with the inner RPCSEC_GSSv3
 context handle as the rgmp_handle and a MIC computed over the RPC
 reply header (see Section 2.3) using the GSS-API security context
 associated with the inner handle.
 On failure, the rgss3_gss_mp_auth field is not sent
 (rgss3_gss_mp_auth is an optional field).  A MSG_DENIED reply to the
 RPCSEC_GSS_CREATE call is formulated as usual.
 As described in Section 5.3.3.3 of [RFC2203], the server maintains a
 list of contexts for the clients that are currently in session with
 it.  When a client request comes in, there may not be a context
 corresponding to its handle.  When this occurs on an
 RPCSEC_GSS3_CREATE request processing of the parent handle, the
 server rejects the request with a reply status of MSG_DENIED with the
 reject_stat of AUTH_ERROR and with an auth_stat value of
 RPCSEC_GSS_CREDPROBLEM.
 A new value, RPCSEC_GSS_INNER_CREDPROBLEM, has been added to the
 auth_stat type.  With a multi-principal authorization request, the
 server must also have a context corresponding to the inner context
 handle.  When the server does not have a context handle corresponding
 to the inner context handle of a multi-principal authorization
 request, the server sends a reply status of MSG_DENIED with the
 reject_stat of AUTH_ERROR and with an auth_stat value of
 RPCSEC_GSS_INNER_CREDPROBLEM.
 When processing the multi-principal authentication request, if the
 GSS_VerifyMIC() call on the rgmp_rpcheader_mic fails to return
 GSS_S_COMPLETE, the server sends a reply status of MSG_DENIED with
 the reject_stat of AUTH_ERROR and with an auth_stat value of
 RPCSEC_GSS_INNER_CREDPROBLEM.

Adamson & Williams Standards Track [Page 15] RFC 7861 NFSv4 RPC Security November 2016

2.7.1.2. Channel Binding

 <CODE BEGINS>
    ///
    ///  typedef opaque rgss3_chan_binding<>;
    ///
 <CODE ENDS>
 RPCSEC_GSSv3 provides a different way to do channel binding than
 RPCSEC_GSSv2 [RFC5403].  Specifically:
 a.  RPCSEC_GSSv3 builds on RPCSEC_GSSv1 by reusing existing,
     established context handles rather than providing a different RPC
     security flavor for establishing context handles.
 b.  Channel-bindings data is not hashed because there is now general
     agreement that it is the secure channel's responsibility to
     produce channel-bindings data of manageable size.
 (a) is useful in keeping RPCSEC_GSSv3 simple in general, not just for
 channel binding.  (b) is useful in keeping RPCSEC_GSSv3 simple
 specifically for channel binding.
 Channel binding is accomplished as follows.  The client prefixes the
 channel-bindings data octet string with the channel type as described
 in [RFC5056]; then, the client calls GSS_GetMIC() to get a MIC of the
 resulting octet string, using the parent RPCSEC_GSSv3 context
 handle's GSS-API security context.  The MIC is then placed in the
 rca_chan_bind_mic field of RPCSEC_GSS_CREATE arguments
 (rgss3_create_args).
 If the rca_chan_bind_mic field of the arguments of an
 RPCSEC_GSS_CREATE control message is set, then the server MUST verify
 the client's channel-binding MIC if the server supports this feature.
 If channel-binding verification succeeds, then the server MUST
 generate a new MIC of the same channel bindings and place it in the
 rcr_chan_bind_mic field of the RPCSEC_GSS_CREATE rgss3_create_res
 results.  If channel-binding verification fails or the server doesn't
 support channel binding, then the server MUST indicate this in its
 reply by not including an rgss3_chan_binding value in
 rgss3_create_res (rgss3_chan_binding is an optional field).
 The client MUST verify the result's rcr_chan_bind_mic value by
 calling GSS_VerifyMIC() with the given MIC and the channel-bindings
 data (including the channel-type prefix).  If client-side channel-
 binding verification fails, then the client MUST call

Adamson & Williams Standards Track [Page 16] RFC 7861 NFSv4 RPC Security November 2016

 RPCSEC_GSS_DESTROY.  If the client requested channel binding but the
 server did not include an rcr_chan_binding_mic field in the results,
 then the client MAY continue to use the resulting context handle as
 though channel binding had never been requested.  If the client
 considers channel binding critical, it MUST call RPCSEC_GSS_DESTROY.
 As per RPCSEC_GSSv2 [RFC5403]:
    Once a successful [channel-binding] procedure has been performed
    on an [RPCSEC_GSSv3] context handle, the initiator's
    implementation may map application requests for rpc_gss_svc_none
    and rpc_gss_svc_integrity to rpc_gss_svc_channel_prot credentials.
    And if the secure channel has privacy enabled, requests for
    rpc_gss_svc_privacy can also be mapped to
    rpc_gss_svc_channel_prot.
 Any RPCSEC_GSSv3 child context handle that has been bound to a secure
 channel in this way SHOULD be used only with the
 rpc_gss_svc_channel_prot and SHOULD NOT be used with rpc_gss_svc_none
 or rpc_gss_svc_integrity -- if the secure channel does not provide
 privacy protection, then the client MAY use rpc_gss_svc_privacy where
 privacy protection is needed or desired.

2.7.1.3. Label Assertions

 <CODE BEGINS>
    ///  struct rgss3_label {
    ///          rgss3_lfs       rl_lfs;
    ///          opaque          rl_label<>;
    ///  };
    ///
    ///  struct rgss3_lfs {
    ///          unsigned int rlf_lfs_id;
    ///          unsigned int rlf_pi_id;
    ///  };
    ///
 <CODE ENDS>
 The client discovers, via the RPCSEC_GSS_LIST control message, which
 LFSs the server supports.  Full Mode MAC is enabled when an
 RPCSEC_GSS version 3 process subject label assertion is combined with
 a file object label provided by the NFSv4.2 sec_label attribute.
 Label encoding is specified to mirror the NFSv4.2 sec_label attribute
 described in Section 12.2.4 of [RFC7862].  The LFS is an identifier
 used by the client to establish the syntactic format of the security

Adamson & Williams Standards Track [Page 17] RFC 7861 NFSv4 RPC Security November 2016

 label and the semantic meaning of its components.  The Policy
 Identifier (PI) is an optional part of the definition of an LFS that
 allows clients and the server to identify specific security policies.
 The opaque label field (rgss3_label) is dependent on the MAC model to
 interpret and enforce.
 If a label itself requires privacy protection (i.e., requires that
 the user can assert that the label is a secret), then the client MUST
 use the rpc_gss_svc_privacy protection service for the
 RPCSEC_GSS_CREATE request.
 RPCSEC_GSSv3 clients MAY assert a set of subject security labels in
 some LFS by binding a label assertion to the RPCSEC_GSSv3 child
 context handle.  This is done by including an assertion of type
 rgss3_label in the RPCSEC_GSS_CREATE rgss3_create_args rca_assertions
 call data.  The label assertion payload is the set of subject labels
 asserted by the calling NFS client process.  The resultant child
 context is used for NFS requests asserting the client process subject
 labels.  The NFS server process that handles such requests then
 asserts the (client) process subject label(s) as it attempts to
 access a file that has associated Labeled NFS object labels.
 Servers that support labeling in the requested LFS MAY map the
 requested subject label to a different subject label as a result of
 server-side policy evaluation.
 The labels that are accepted by the target and bound to the
 RPCSEC_GSSv3 context MUST be enumerated in the rcr_assertions field
 of the rgss3_create_res RPCSEC_GSS_CREATE reply.
 Servers that do not support labeling or that do not support the
 requested LFS reject the label assertion with a reply status of
 MSG_DENIED, a reject_status of AUTH_ERROR, and an auth_stat of
 RPCSEC_GSS_LABEL_PROBLEM.

Adamson & Williams Standards Track [Page 18] RFC 7861 NFSv4 RPC Security November 2016

2.7.1.4. Structured Privilege Assertions

 <CODE BEGINS>
    ///
    ///  typedef opaque utf8string<>;   /* UTF-8 encoding */
    ///  typedef utf8string utf8str_cs; /* Case-sensitive UTF-8 */
    ///
    ///  struct rgss3_privs {
    ///          utf8str_cs      rp_name<>;
    ///          opaque          rp_privilege<>;
    ///  };
 <CODE ENDS>
 A structured privilege is a capability defined by a specific RPC
 application.  To support the assertion of this privilege, by a client
 using the application, in a server that also supports the
 application, the application may define a private data structure that
 is understood by clients and servers implementing the RPC
 application.
 RPCSEC_GSSv3 clients MAY assert a structured privilege by binding the
 privilege to the RPCSEC_GSSv3 context handle.  This is done by
 including an assertion of type rgss3_privs in the RPCSEC_GSS_CREATE
 rgss3_create_args rca_assertions call data.
 The privilege is identified by the description string that is used by
 RPCSEC_GSSv3 to identify the privilege and communicate the private
 data between the relevant RPC application-specific code without
 needing to be aware of the details of the structure used.  Thus, as
 far as RPCSEC_GSSv3 is concerned, the defined structure is passed
 between client and server as opaque data encoded in the
 rpc_gss3_privs rp_privilege field.
 Encoding, server verification, and any server policies for structured
 privileges are described by the RPC application definition.  The
 rp_name field of rpc_gss3_privs carries the description string used
 to identify and list the privilege.  The utf8str_cs definition is
 from [RFC7530].
 A successful structured privilege assertion MUST be enumerated in the
 rcr_assertions field of the rgss3_create_res RPCSEC_GSS_CREATE reply.
 If a server receives a structured privilege assertion that it does
 not recognize, the assertion is rejected with a reply status of
 MSG_DENIED, a reject_status of AUTH_ERROR, and an auth_stat of
 RPCSEC_GSS_UNKNOWN_MESSAGE.

Adamson & Williams Standards Track [Page 19] RFC 7861 NFSv4 RPC Security November 2016

 It is assumed that a client asserting more than one structured
 privilege to be bound to a context handle would not require all the
 privilege assertions to succeed.
 The server MUST NOT reject RPCSEC_GSS_CREATE requests containing
 supported structured privilege assertions, even if some of those
 assertions are rejected (e.g., for local policy reasons).
 If a server receives an RPCSEC_GSS_CREATE request containing one or
 more unsupported structured privilege assertions, the request MUST be
 rejected with a reply status of MSG_DENIED, a reject_status of
 AUTH_ERROR, and an auth_stat of RPCSEC_GSS_PRIVILEGE_PROBLEM.
 Section 4.9.1.1 of [RFC7862] ("Inter-Server Copy via ONC RPC with
 RPCSEC_GSSv3") shows an example of structured privilege definition
 and use.

2.7.2. New Control Procedure - RPCSEC_GSS_LIST

 <CODE BEGINS>
    ///  enum rgss3_list_item {
    ///          LABEL = 0,
    ///          PRIVS = 1
    ///  };
    ///
    ///  struct rgss3_list_args {
    ///          rgss3_list_item      rla_list_what<>;
    ///  };
    ///
    ///  union rgss3_list_item_u
    ///        switch (rgss3_list_item itype) {
    ///  case LABEL:
    ///          rgss3_label          rli_labels<>;
    ///  case PRIVS:
    ///          rgss3_privs          rli_privs<>;
    ///  default:
    ///          opaque               rli_ext<>;
    ///  };
    ///
    ///  typedef rgss3_list_item_u rgss3_list_res<>;
    ///
 <CODE ENDS>
 The call data for an RPCSEC_GSS_LIST request consists of a list of
 integers (rla_list_what) indicating what assertions are to be listed,
 and the reply consists of an error or the requested list.

Adamson & Williams Standards Track [Page 20] RFC 7861 NFSv4 RPC Security November 2016

 The result of requesting a list of rgss3_list_item LABEL objects is a
 list of LFSs supported by the server.  The client can then use the
 LFS list to assert labels via the RPCSEC_GSS_CREATE label assertions.
 See Section 2.7.1.3.

2.8. Extensibility

 Assertion types may be added in the future by adding arms to the
 "rgss3_assertion_u" union (Section 2.7.1) and the "rgss3_list_item_u"
 union (Section 2.7.2).  Examples of other potential assertion types
 include:
 o  Client-side assertions of identity:
  • Primary client/user identity.
  • Supplementary group memberships of the client/user, including

support for specifying deltas to the membership list as seen on

       the server.

3. Operational Recommendation for Deployment

 RPCSEC_GSSv3 is a superset of RPCSEC_GSSv2 [RFC5403], which in turn
 is a superset of RPCSEC_GSSv1 [RFC2203], and so can be used in all
 situations where RPCSEC_GSSv2 is used, or where RPCSEC_GSSv1 is used
 and channel-bindings functionality is not needed.  RPCSEC_GSSv3
 should be used when the new functionality is needed.

4. Security Considerations

 This entire document deals with security issues.
 The RPCSEC_GSSv3 protocol allows for client-side assertions of data
 that is relevant to server-side authorization decisions.  These
 assertions must be evaluated by the server in the context of whether
 the client and/or user are authenticated, whether multi-principal
 authentication was used, whether the client is trusted, what ranges
 of assertions are allowed for the client and the user (separately or
 together), and any relevant server-side policy.
 The security semantics of assertions carried by RPCSEC_GSSv3 are
 application protocol-specific.
 Note that RPCSEC_GSSv3 is not a complete solution for labeling: it
 conveys the labels of actors but not the labels of objects.  RPC
 application protocols may require extending in order to carry object
 label information.

Adamson & Williams Standards Track [Page 21] RFC 7861 NFSv4 RPC Security November 2016

 There may be interactions with NFSv4's callback security scheme and
 NFSv4.1's [RFC5661] GSS SSV (Secret State Verifier) mechanisms.
 Specifically, the NFSv4 callback scheme requires that the server
 initiate GSS-API security contexts, which does not work well in
 practice; in the context of client-side processes running as the same
 user but with different privileges and security labels, the NFSv4
 callback security scheme seems particularly unlikely to work well.
 NFSv4.1 has the server use an existing, client-initiated RPCSEC_GSS
 context handle to protect server-initiated callback RPCs.  The
 NFSv4.1 callback security scheme lacks all the problems of the NFSv4
 scheme; however, it is important that the server pick an appropriate
 RPCSEC_GSS context handle to protect any callbacks.  Specifically, it
 is important that the server use RPCSEC_GSS context handles that
 authenticate the client to protect any callbacks related to server
 state initiated by RPCs protected by RPCSEC_GSSv3 contexts.
 As described in Section 2.10.10 of [RFC5661], the client is permitted
 to associate multiple RPCSEC_GSS handles with a single SSV GSS
 context.  RPCSEC_GSSv3 handles will work well with SSV in that the
 man-in-the-middle attacks described in Section 2.10.10 of [RFC5661]
 are solved by the new reply verifier (Section 2.3).  Using an
 RPCSEC_GSSv3 handle backed by a GSS-SSV mechanism context as a parent
 handle in an RPCSEC_GSS_CREATE call, while permitted, is complicated
 by the lifetime rules of SSV contexts and their associated RPCSEC_GSS
 handles.

5. IANA Considerations

 This section uses terms that are defined in [RFC5226].

5.1. New RPC Authentication Status Numbers

 The following new RPC Authentication Status Numbers have been added
 to the IANA registry:
 o  RPCSEC_GSS_INNER_CREDPROBLEM (15)  "No credentials for
    multi-principal assertion inner context user".  See
    Section 2.7.1.1.
 o  RPCSEC_GSS_LABEL_PROBLEM (16)  "Problem with label assertion".
    See Section 2.7.1.3.
 o  RPCSEC_GSS_PRIVILEGE_PROBLEM (17)  "Problem with structured
    privilege assertion".  See Section 2.7.1.4.
 o  RPCSEC_GSS_UNKNOWN_MESSAGE (18)  "Unknown structured privilege
    assertion".  See Section 2.7.1.4.

Adamson & Williams Standards Track [Page 22] RFC 7861 NFSv4 RPC Security November 2016

5.2. Structured Privilege Name Definitions

 IANA has created a registry called the "RPCSEC_GSS Structured
 Privilege Names Registry".
 Structured privilege assertions (Section 2.7.1.4) are defined by a
 specific RPC application.  The namespace identifiers for these
 assertions (the rp_name) are defined as string names.  The
 RPCSEC_GSSv3 protocol does not define the specific assignment of the
 namespace for these structured privilege assertion names.  The IANA
 registry promotes interoperability where common interests exist.
 While RPC application developers are allowed to define and use
 structured privileges as needed, they are encouraged to register
 structured privilege assertion names with IANA.
 The registry is to be maintained using the Standards Action policy as
 defined in Section 4.1 of [RFC5226].
 Under the RPCSEC_GSS version 3 specification, the name of a
 structured privilege can in theory be up to 2^32 - 1 bytes in length,
 but in practice RPC application clients and servers will be unable to
 handle a string that long.  IANA should reject any assignment request
 with a structured privilege name that exceeds 128 UTF-8 characters.
 To give the IESG the flexibility to set up bases of assignment of
 Experimental Use, the prefix "EXPE" is Reserved.  The structured
 privilege with a zero-length name is Reserved.
 The prefix "PRIV" is allocated for Private Use.  A site that wants to
 make use of unregistered named attributes without risk of conflicting
 with an assignment in IANA's registry should use the prefix "PRIV" in
 all of its structured privilege assertion names.
 Because some RPC application clients and servers have case-
 insensitive semantics, the fifteen additional lower-case and mixed-
 case permutations of each of "EXPE" and "PRIV" are Reserved (e.g.,
 "expe", "expE", and "exPe" are Reserved).  Similarly, IANA must not
 allow two assignments that would conflict if both structured
 privilege names were converted to a common case.

Adamson & Williams Standards Track [Page 23] RFC 7861 NFSv4 RPC Security November 2016

 The registry of structured privilege names is a list of assignments,
 each containing three fields for each assignment.
 1.  A US-ASCII string name that is the actual name of the structured
     privilege.  This name must be unique.  This string name can be 1
     to 128 UTF-8 characters long.
 2.  A reference to the specification of the RPC-application-defined
     structured privilege.  The reference can consume up to 256 bytes
     (or more if IANA permits).
 3.  The point of contact of the registrant.  The point of contact can
     consume up to 256 bytes (or more if IANA permits).

5.2.1. Initial Registry

 The initial registry consists of the three structured privileges
 defined in [RFC7862].
 1.  NAME: copy_to_auth, REFERENCE: RFC 7862, CONTACT: William
     A.(Andy) Adamson, andros@netapp.com
 2.  NAME: copy_from_auth, REFERENCE: RFC 7862, CONTACT: William
     A.(Andy) Adamson, andros@netapp.com
 3.  NAME: copy_confirm_auth, REFERENCE: RFC 7862, CONTACT: William
     A.(Andy) Adamson, andros@netapp.com

5.2.2. Updating Registrations

 The registrant is always permitted to update the point of contact
 field.  To make any other change will require Expert Review or IESG
 Approval.

Adamson & Williams Standards Track [Page 24] RFC 7861 NFSv4 RPC Security November 2016

6. References

6.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC2203]  Eisler, M., Chiu, A., and L. Ling, "RPCSEC_GSS Protocol
            Specification", RFC 2203, DOI 10.17487/RFC2203,
            September 1997, <http://www.rfc-editor.org/info/rfc2203>.
 [RFC2743]  Linn, J., "Generic Security Service Application Program
            Interface Version 2, Update 1", RFC 2743,
            DOI 10.17487/RFC2743, January 2000,
            <http://www.rfc-editor.org/info/rfc2743>.
 [RFC4506]  Eisler, M., Ed., "XDR: External Data Representation
            Standard", STD 67, RFC 4506, DOI 10.17487/RFC4506,
            May 2006, <http://www.rfc-editor.org/info/rfc4506>.
 [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
            Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
            <http://www.rfc-editor.org/info/rfc5056>.
 [RFC5403]  Eisler, M., "RPCSEC_GSS Version 2", RFC 5403,
            DOI 10.17487/RFC5403, February 2009,
            <http://www.rfc-editor.org/info/rfc5403>.
 [RFC5661]  Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
            "Network File System (NFS) Version 4 Minor Version 1
            Protocol", RFC 5661, DOI 10.17487/RFC5661, January 2010,
            <http://www.rfc-editor.org/info/rfc5661>.
 [RFC7530]  Haynes, T., Ed., and D. Noveck, Ed., "Network File System
            (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
            March 2015, <http://www.rfc-editor.org/info/rfc7530>.
 [RFC7862]  Haynes, T., "Network File System (NFS) Version 4 Minor
            Version 2 Protocol", RFC 7862, DOI 10.17487/RFC7862,
            November 2016, <http://www.rfc-editor.org/info/rfc7862>.

Adamson & Williams Standards Track [Page 25] RFC 7861 NFSv4 RPC Security November 2016

6.2. Informative References

 [AFS-RXGK]
            Wilkinson, S. and B. Kaduk, "Integrating rxgk with AFS",
            Work in Progress, draft-wilkinson-afs3-rxgk-afs-08,
            May 2015.
 [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
            FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
            <http://www.rfc-editor.org/info/rfc4949>.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            DOI 10.17487/RFC5226, May 2008,
            <http://www.rfc-editor.org/info/rfc5226>.
 [RFC5531]  Thurlow, R., "RPC: Remote Procedure Call Protocol
            Specification Version 2", RFC 5531, DOI 10.17487/RFC5531,
            May 2009, <http://www.rfc-editor.org/info/rfc5531>.

Acknowledgments

 Andy Adamson would like to thank NetApp, Inc. for its funding of his
 time on this project.
 We thank Lars Eggert, Mike Eisler, Ben Kaduk, Bruce Fields, Tom
 Haynes, and Dave Noveck for their most helpful reviews.

Authors' Addresses

 William A. (Andy) Adamson
 NetApp
 3629 Wagner Ridge Ct.
 Ann Arbor, MI  48103
 United States of America
 Phone: +1 734 665 1204
 Email: andros@netapp.com
 Nico Williams
 cryptonector.com
 13115 Tamayo Dr.
 Austin, TX  78729
 United States of America
 Email: nico@cryptonector.com

Adamson & Williams Standards Track [Page 26]

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