A Usage for Shared Resources in RELOAD (ShaRe)

Internet Engineering Task Force (IETF)                          A. Knauf
Request for Comments: 8076                               T. Schmidt, Ed.
Category: Standards Track                                    HAW Hamburg
ISSN: 2070-1721                                                  G. Hege
                                                             daviko GmbH
                                                            M. Waehlisch
                                                    link-lab & FU Berlin
                                                              March 2017


             A Usage for Shared Resources in RELOAD (ShaRe)

Abstract

   This document defines a REsource LOcation And Discovery (RELOAD)
   Usage for managing shared write access to RELOAD Resources.  Shared
   Resources in RELOAD (ShaRe) form a basic primitive for enabling
   various coordination and notification schemes among distributed
   peers.  Access in ShaRe is controlled by a hierarchical trust
   delegation scheme maintained within an access list.  A new
   USER-CHAIN-ACL access policy allows authorized peers to write a
   Shared Resource without owning its corresponding certificate.  This
   specification also adds mechanisms to store Resources with a variable
   name that is useful whenever peer-independent rendezvous processes
   are required.

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/rfc8076.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Shared Resources in RELOAD  . . . . . . . . . . . . . . . . .   5
     3.1.  Mechanisms for Isolating Stored Data  . . . . . . . . . .   6
   4.  Access Control List Definition  . . . . . . . . . . . . . . .   7
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Data Structure  . . . . . . . . . . . . . . . . . . . . .   9
   5.  Extension for Variable Resource Names . . . . . . . . . . . .  10
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Data Structure  . . . . . . . . . . . . . . . . . . . . .  11
     5.3.  Overlay Configuration Document Extension  . . . . . . . .  12
   6.  Access Control to Shared Resources  . . . . . . . . . . . . .  13
     6.1.  Granting Write Access . . . . . . . . . . . . . . . . . .  13
     6.2.  Revoking Write Access . . . . . . . . . . . . . . . . . .  14
     6.3.  Validating Write Access through an ACL  . . . . . . . . .  14
     6.4.  Operations of Storing Peers . . . . . . . . . . . . . . .  15
     6.5.  Operations of Accessing Peers . . . . . . . . . . . . . .  16
     6.6.  USER-CHAIN-ACL Access Policy  . . . . . . . . . . . . . .  16
   7.  ACCESS-CONTROL-LIST Kind Definition . . . . . . . . . . . . .  17
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
     8.1.  Resource Exhaustion . . . . . . . . . . . . . . . . . . .  17
     8.2.  Malicious or Misbehaving Storing Peer . . . . . . . . . .  18
     8.3.  Trust Delegation to a Malicious or Misbehaving Peer . . .  18
     8.4.  Privacy Issues  . . . . . . . . . . . . . . . . . . . . .  18
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Access Control Policy . . . . . . . . . . . . . . . . . .  19
     9.2.  Data Kind-ID  . . . . . . . . . . . . . . . . . . . . . .  19
     9.3.  XML Namespace Registration  . . . . . . . . . . . . . . .  19
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     10.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   [RFC6940] defines the base protocol for REsource LOcation And
   Discovery (RELOAD), which allows for application-specific extensions
   by Usages.  The present document defines such a RELOAD Usage for
   managing shared write access to RELOAD Resources and a mechanism to
   store Resources with variable names.  The Usage for Shared Resources
   in RELOAD (ShaRe) enables overlay users to share their exclusive
   write access to specific Resource/Kind pairs with others.  Shared
   Resources form a basic primitive for enabling various coordination
   and notification schemes among distributed peers.  Write permission
   is controlled by an Access Control List (ACL) Kind that maintains a
   chain of Authorized Peers for a particular Shared Resource.  A newly
   defined USER-CHAIN-ACL access control policy enables shared write
   access in RELOAD.

   The Usage for Shared Resources in RELOAD is designed for jointly
   coordinated group applications among distributed peers (e.g., third-
   party registration, see [RFC7904], or distributed conferencing).  Of
   particular interest are rendezvous processes, where a single
   identifier is linked to multiple, dynamic instances of a distributed
   cooperative service.  Shared write access is based on a trust
   delegation mechanism that transfers the authorization to write a
   specific Kind data by storing logical Access Control Lists.  An ACL
   contains the ID of the Kind to be shared and contains trust
   delegations from one authorized to another (previously unauthorized)
   user.

   Shared write access augments the RELOAD security model, which is
   based on the restriction that peers are only allowed to write
   resources at a small set of well-defined locations (Resource-IDs) in
   the overlay.  Using the standard access control rules in RELOAD,
   these locations are bound to the username or Node-ID in the peer's
   certificate.  This document extends the base policies to enable a
   controlled write access for multiple users to a common Resource-ID.

   Additionally, this specification defines an optional mechanism to
   store Resources with a variable Resource Name.  It enables the
   storage of Resources whose name complies to a specific pattern.
   Definition of the pattern is arbitrary, but it must contain the
   username of the Resource creator.

2.  Terminology

   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].


   This document uses the terminology and definitions from the RELOAD
   base [RFC6940] and [RFC7890], in particular the RELOAD Usage,
   Resource, and Kind.  Additionally, the following terms are used:

   Shared Resource:  The term "Shared Resource" in this document defines
      a RELOAD Resource with its associated Kinds that can be written or
      overwritten by multiple RELOAD users following the specifications
      in this document.

   Access Control List:  The term "Access Control List" in this document
      defines a logical list of RELOAD users allowed to write a specific
      RELOAD Resource/Kind pair by following the specifications in this
      document.  The list items are stored as Access Control List Kinds
      that map trust delegations from user A to user B, where A is
      allowed to write a Shared Resource and the Access Control List,
      while B is a user that obtains write access to specified Kinds
      from A.

   Resource Owner:  The term "Resource Owner" in this document defines a
      RELOAD peer that initially stored a Resource to be shared.  The
      Resource Owner possesses the RELOAD certificate that grants write
      access to a specific Resource/Kind pair using the RELOAD
      certificate-based access control policies.

   Authorized Peer:  The term "Authorized Peer" in this document defines
      a RELOAD peer that was granted write access to a Shared Resource
      by permission of the Resource Owner or another Authorized Peer.

3.  Shared Resources in RELOAD

   A RELOAD user that owns a certificate for writing at a specific
   overlay location can maintain one or more RELOAD Kinds that are
   designated for a non-exclusive write access shared with other RELOAD
   users.  The mechanism to share those Resource/Kind pairs with a group
   of users consists of two basic steps:

   1.  Storage of the Resource/Kind pairs to be shared.

   2.  Storage of an Access Control List (ACL) associated with those
       Kinds.

   ACLs are created by the Resource Owner and contain ACL items, each
   delegating the permission of writing the shared Kind to a specific
   user called the "Authorized Peer".  For each shared Kind data, its
   Resource owner stores a root item that initiates an Access Control
   List.  Trust delegation to the Authorized Peer can include the right
   to further delegate the write permission, enabling a tree of trust
   delegations with the Resource Owner as trust anchor at its root.

   The Resource/Kind pair to be shared can be any RELOAD Kind that
   complies to the following specifications:

   Isolated Data Storage:  To prevent concurrent writing from race
      conditions, each data item stored within a Shared Resource SHALL
      be exclusively maintained by the RELOAD user who created it.
      Hence, Usages that allow the storage of Shared Resources are
      REQUIRED to use either the array or dictionary data model and
      apply additional mechanisms for isolating data as described in
      Section 3.1.

   Access Control Policy:  To ensure write access to Shared Resource by
      Authorized Peers, each Usage MUST use the USER-CHAIN-ACL access
      policy as described in Section 6.6.

   Resource Name Extension:  To enable Shared Resources to be stored
      using a variable resource name, this document defines an optional
      ResourceNameExtension structure.  It contains the Resource Name of
      the Kind data to be stored and allows any receiver of a shared
      data to validate whether the Resource Name hashes to the Resource-
      ID.  The ResourceNameExtension is made optional by configuration.
      The ResourceNameExtension field is only present in the Kind data
      structure when configured in the corresponding kind-block of the
      overlay configuration document (for more details, see
      Section 5.3).  If the configuration allows variable resource
      names, a Kind using the USER-CHAIN-ACL policy MUST use the
      ResourceNameExtension as the initial field within the Kind data
      structure definition.  Otherwise, the Kind data structure does not
      contain the ResourceNameExtension structure.

3.1.  Mechanisms for Isolating Stored Data

   This section defines mechanisms to avoid race conditions while
   concurrently writing an array or dictionary of a Shared Resource.

   If a dictionary is used in the Shared Resource, the dictionary key
   MUST be the Node-ID of the certificate that will be used to sign the
   stored data.  Thus, data access is bound to the unique ID holder, and
   write concurrency does not occur.

   If the data model of the Shared Resource is an array, each Authorized
   Peer that chooses to write data SHALL obtain its exclusive range of
   the array indices.  The following algorithm will generate an array
   indexing scheme that avoids collisions:

   1.  Obtain the Node-ID of the certificate that will be used to sign
       the stored data.

   2.  Take the least significant 24 bits of that Node-ID to prefix the
       array index.

   3.  Append an 8-bit individual index value to those 24 bits of the
       Node-ID.

   The resulting 32-bit long integer MUST be used as the index for
   storing an array entry in a Shared Resource.  The 24 bits of the
   Node-ID serve as a collision-resistant identifier.  The 8-bit
   individual index remains under the control of a single Peer and can
   be incremented individually for further array entries.  In total,
   each Peer can generate 256 distinct entries for application-specific
   use.

   The mechanism to create the array index inherits collision-resistance
   from the overlay hash function in use (e.g., SHA-1).  It is designed
   to work reliably for small sizes of groups as applicable to resource
   sharing.  In the rare event of a collision, the Storing Peer will
   refuse to (over-)write the requested array index and protect indexing
   integrity as defined in Section 6.1.  A Peer could rejoin the overlay
   with a different Node-ID in such a case.

4.  Access Control List Definition

4.1.  Overview

   An Access Control List (ACL) is a (self-managed) Shared Resource that
   contains a list of AccessControlListItem structures as defined in
   Section 4.2.  Each entry delegates write access for a specific Kind
   data to a single RELOAD user.  An ACL enables the RELOAD user who is
   authorized to write a specific Resource-ID to delegate his exclusive
   write access to a specific Kind to further users of the same RELOAD
   overlay.  Therefore, each Access Control List data structure carries
   the information about who obtains write access, the Kind-ID of the
   Resource to be shared, and whether delegation includes write access
   to the ACL itself.  The latter condition grants the right to delegate
   write access further for the Authorized Peer.  Access Control Lists
   are stored at the same overlay location as the Shared Resource and
   use the RELOAD array data model.  They are initially created by the
   Resource Owner.

   Figure 1 shows an example of an Access Control List.  We omit the
   res_name_ext field to simplify illustration.  The array entry at
   index 0x123abc01 displays the initial creation of an ACL for a Shared
   Resource of Kind-ID 1234 at the same Resource-ID.  It represents the
   root item of the trust delegation tree for this shared RELOAD Kind.
   The root entry MUST contain the username of the Resource owner in the
   "to_user" field and can only be written by the owner of the public
   key certificate associated with this Resource-ID.  The
   allow_delegation (ad) flag for a root ACL item is set to 1 by
   default.  The array index is generated by using the mechanism for
   isolating stored data as described in Section 3.1.  Hence, the most
   significant 24 bits of the array index (0x123abc) are the least
   significant 24 bits of the Node-ID of the Resource Owner.

   The array item at index 0x123abc02 represents the first trust
   delegation to an Authorized Peer that is thus permitted to write to
   the Shared Resource of Kind-ID 1234.  Additionally, the Authorized
   peer Alice is also granted write access to the ACL as indicated by
   the allow_delegation flag (ad) set to 1.  This configuration
   authorizes Alice to store further trust delegations to the Shared
   Resource, i.e., add items to the ACL.  On the contrary, index
   0x456def01 illustrates trust delegation for Kind-ID 1234, in which
   the Authorized Peer Bob is not allowed to grant access to further
   peers (ad = 0).  Each Authorized Peer signs its ACL items by using
   its own signer identity along with its own private key.  This allows
   other peers to validate the origin of an ACL item and makes ownership
   transparent.

   To manage Shared Resource access of multiple Kinds at a single
   location, the Resource Owner can create new ACL entries that refer to
   another Kind-ID as shown in array entry index 0x123abc03.  Note that
   overwriting existing items in an Access Control List with a change in
   the Kind-ID revokes all trust delegations in the corresponding
   subtree (see Section 6.2).  Authorized Peers are only enabled to
   overwrite existing ACL item they own.  The Resource Owner is allowed
   to overwrite any existing ACL item, but should be aware of its
   consequences on the trust delegation chain.

         +------------------------------------------------------+
         |                Access Control List                   |
         +-----------+------------------------------+-----------+
         |  #Index   |       Array Entries          | signed by |
         +-----------+------------------------------+-----------+
         | 123abc01  | to_user:Owner Kind:1234 ad:1 |   Owner   |
         +-----------+------------------------------+-----------+
         | 123abc02  | to_user:Alice Kind:1234 ad:1 |   Owner   |
         +-----------+------------------------------+-----------+
         | 123abc03  | to_user:Owner Kind:4321 ad:1 |   Owner   |
         +-----------+------------------------------+-----------+
         | 123abc04  | to_user:Carol Kind:4321 ad:0 |   Owner   |
         +-----------+------------------------------+-----------+
         |    ...    |           ...                |    ...    |
         +-----------+------------------------------+-----------+
         | 456def01  | to_user:Bob   Kind:1234 ad:0 |   Alice   |
         +-----------+------------------------------+-----------+
         |    ...    |           ...                |    ...    |
         +-----------+------------------------------+-----------+

     Figure 1: Simplified Example of an Access Control List, Including
      Entries for Two Different Kind-IDs and Varying Delegation (AD)
                              Configurations

   Implementors of ShaRe should be aware that the trust delegation in an
   Access Control List need not be loop free.  Self-contained circular
   trust delegation from A to B and B to A are syntactically possible,
   even though not very meaningful.

4.2.  Data Structure

   The Kind data structure for the Access Control List is defined as
   follows:


   struct {
        /* res_name_ext is optional, see documentation */
        ResourceNameExtension  res_name_ext;
        opaque                 to_user<0..2^16-1>;
        KindId                 kind;
        Boolean                allow_delegation;
   } AccessControlListItem;

   The AccessControlListItem structure is composed of:

   res_name_ext:  This optional field contains the Resource Name of a
      ResourceNameExtension (see Section 5.2) to be used by a Shared
      Resource with a variable resource name.  This name is used by the
      storing peer for validating, whether a variable resources name
      matches one of the predefined naming pattern from the
      configuration document for this Kind.  The presence of this field
      is bound to a variable resource name element in the corresponding
      kind-block of the configuration document whose "enable" attribute
      is set to true (see Section 5.3).  Otherwise, if the "enable"
      attribute is false, the res_name_ext field SHALL NOT be present in
      the Kind data structure.

   to_user:  This field contains the username of the RELOAD peer that
      obtains write permission to the Shared Resource.

   kind:  This field contains the Kind-ID of the Shared Resource.

   allow_delegation:  If true, this Boolean flag indicates that the
      Authorized Peer in the 'to_user' field is allowed to add
      additional entries to the ACL for the specified Kind-ID.

5.  Extension for Variable Resource Names

5.1.  Overview

   In certain use cases, such as conferencing, it is desirable to
   increase the flexibility of a peer in using Resource Names beyond
   those defined by the username or Node-ID fields in its certificate.
   For this purpose, this document presents the concept for variable
   Resources Names that enables providers of RELOAD instances to define
   relaxed naming schemes for overlay Resources.

   Each RELOAD node uses a certificate to identify itself using its
   username (or Node-ID) while storing data under a specific Resource-ID
   (see Section 7.3 in [RFC6940]).  The specifications in this document
   scheme adhere to this paradigm, but enable a RELOAD peer to store
   values of Resource Names that are derived from the username in its
   certificate.  This is done by using a Resource Name with a variable
   substring that still matches the username in the certificate using a
   pattern defined in the overlay configuration document.  Thus, despite
   being variable, an allowable Resource Name remains tied to the
   Owner's certificate.  A sample pattern might be formed as follows:

   Example Pattern:
   .*-conf-$USER@$DOMAIN
   When defining the pattern, care must be taken to avoid conflicts
   arising from two usernames of which one is a substring of the other.
   In such cases, the holder of the shorter name could threaten to block
   the resources of the longer-named peer by choosing the variable part
   of a Resource Name to contain the entire longer username.  For
   example, a "*$USER" pattern would allow user EVE to define a resource
   with name "STEVE" and to block the resource name for user STEVE
   through this.  This problem can easily be mitigated by delimiting the
   variable part of the pattern from the username part by some fixed
   string, that by convention is not part of a username (e.g., the
   "-conf-" in the above Example).

5.2.  Data Structure

   This section defines the optional ResourceNameExtension structure for
   every Kind that uses the USER-CHAIN-ACL access control policy.

   enum { pattern(1), (255)} ResourceNameType;

   struct {
     ResourceNameType type;
     uint16           length;

     select(type) {
         case pattern:
           opaque     resource_name<0..2^16-1>;

         /* Types can be extended */
     };
   } ResourceNameExtension;

   The content of the ResourceNameExtension consists of:

   length:  This field contains the length of the remaining data
      structure.  It is only used to allow for further extensions to
      this data structure.

   The content of the rest of the data structure depends of the
   ResourceNameType.  Currently, the only defined type is "pattern".

   If the type is "pattern", then the following data structure contains
   an opaque <0..2^16-1> field containing the Resource Name of the Kind
   being stored.  The type "pattern" further indicates that the Resource
   Name MUST match to one of the variable resource name patterns defined
   for this Kind in the configuration document.

   The ResourceNameType enum and the ResourceNameExtension structure can
   be extended by further Usages to define other naming schemes.

5.3.  Overlay Configuration Document Extension

   This section extends the overlay configuration document by defining
   new elements for patterns relating resource names to usernames.  It
   is noteworthy that additional constraints on the syntax and semantic
   of names can apply according to specific Usages.  For example,
   Address of Record (AOR) syntax restrictions apply when using P2PSIP
   [RFC7904], while a more general naming is feasible in plain RELOAD.

   The <variable-resource-names> element serves as a container for one
   or multiple <pattern> sub-elements.  It is an additional parameter
   within the kind-block and has a boolean "enable" attribute that
   indicates, if true, that the overlay provider allows variable
   resource names for this Kind.  The default value of the "enable"
   attribute is "false".  In the absence of a <variable-resource-names>
   element for a Kind using the USER-CHAIN-ACL access policy (see
   Section 6.6), implementors MUST assume this default value.

   A <pattern> element MUST be present if the "enabled" attribute of its
   parent element is set to true.  Each <pattern> element defines a
   pattern for constructing extended resource names for a single Kind.
   It is of type xsd:string and interpreted as a regular expression
   according to "POSIX Extended Regular Expression" (see the
   specifications in [IEEE-Posix]).  In this regular expression, $USER
   and $DOMAIN are used as variables for the corresponding parts of the
   string in the certificate username field (with $USER preceding and
   $DOMAIN succeeding the '@').  Both variables MUST be present in any
   given pattern definition.  Furthermore, variable parts in <pattern>
   elements defined in the overlay configuration document MUST remain
   syntactically separated from the username part (e.g., by a dedicated
   delimiter) to prevent collisions with other names of other users.  If
   no pattern is defined for a Kind, if the "enable" attribute is false,
   or if the regular expression does not meet the requirements specified
   in this section, the allowable Resource Names are restricted to the
   username of the signer for Shared Resource.

   The RELAX NG Grammar for the Variable Resource Names Extension reads:

   # VARIABLE RESOURCE URN SUB-NAMESPACE

   namespace share = "urn:ietf:params:xml:ns:p2p:config-base:share"

   # VARIABLE RESOURCE NAMES ELEMENT

   kind-parameter &= element share:variable-resource-names {

       attribute enable { xsd:boolean },

       # PATTERN ELEMENT

       element share:pattern { xsd:string }*
   }?

   Whitespace and case processing follows the rules of [OASIS.relax_ng]
   and XML Schema Datatypes [W3C.REC-xmlschema-2-20041028].

6.  Access Control to Shared Resources

6.1.  Granting Write Access

   Write access to a Kind that is intended to be shared with other
   RELOAD users can be initiated solely by the Resource Owner.  A
   Resource Owner can share RELOAD Kinds by using the following
   procedure:

   o  The Resource Owner stores an ACL root item at the Resource-ID of
      the Shared Resource.  The root item contains the
      ResourceNameExtension field (see Section 5.2), the username of the
      Resource Owner and Kind-ID of the Shared Resource.  The
      allow_delegation flag is set to 1.  The index of array data
      structure MUST be generated as described in Section 3.1.

   o  Further ACL items for this Kind-ID stored by the Resource Owner
      MAY delegate write access to Authorized Peers.  These ACL items
      contain the same resource name extension field, the username of
      the Authorized Peer, and the Kind-ID of the Shared Resource.
      Optionally, the Resource Owner sets the "ad" to 1 (the default
      equals 0) to enable the Authorized Peer to further delegate write
      access.  For each succeeding ACL item, the Resource Owner
      increments its individual index value by one (see Section 3.1) so
      that items can be stored in the numerical order of the array index
      starting with the index of the root item.

   An Authorized Peer with delegation allowance ("ad"=1) can extend the
   access to an existing Shared Resource as follows:

   o  An Authorized Peer can store additional ACL items at the Resource-
      ID of the Shared Resource.  These ACL items contain the resource
      name extension field, the username of the newly Authorized Peer,
      and the Kind-ID of the Shared Resource.  Optionally, the "ad" flag
      is set to 1 for allowing the newly Authorized Peer to further
      delegate write access.  The array index MUST be generated as
      described in Section 3.1.  Each succeeding ACL item can be stored
      in the numerical order of the array index.

   A store request by an Authorized Peer that attempts to overwrite any
   ACL item signed by another Peer is unauthorized and causes an
   Error_Forbidden response from the Storing Peer.  Such access
   conflicts could be caused by an array index collision.  However, the
   probability of a collision of two or more identical array indices
   will be negligibly low using the mechanism for isolating stored data
   (see Section 3.1).

6.2.  Revoking Write Access

   Write permissions are revoked by storing a nonexistent value (see
   [RFC6940], Section 7.2.1) at the corresponding item of the Access
   Control List.  Revoking a permission automatically invalidates all
   delegations performed by that user including all subsequent
   delegations.  This allows the invalidation of entire subtrees of the
   delegations tree with only a single operation.  Overwriting the root
   item with a nonexistent value of an Access List invalidates the
   entire delegations tree.

   An existing ACL item MUST only be overwritten by the user who
   initially stored the corresponding entry, or by the Resource Owner
   that is allowed to overwrite all ACL items for revoking write access.

   To protect the privacy of the users, the Resource Owner SHOULD
   overwrite all subtrees that have been invalidated.

6.3.  Validating Write Access through an ACL

   Access Control Lists are used to transparently validate authorization
   of peers for writing a data value at a Shared Resource.  Thereby, it
   is assumed that the validating peer is in possession of the complete
   and most recent ACL for a specific Resource/Kind pair.  The
   corresponding procedure consists of recursively traversing the trust
   delegation tree with strings compared as binary objects.  It proceeds
   as follows:

   1.  Obtain the username of the certificate used for signing the data
       stored at the Shared Resource.  This is the user who requested
       the write operation.

   2.  Validate that an item of the corresponding ACL (i.e., for this
       Resource/Kind pair) contains a "to_user" field whose value equals
       the username obtained in step 1.  If the Shared Resource under
       examination is an Access Control List Kind, further validate if
       the "ad" flag is set to 1.

   3.  Select the username of the certificate that was used to sign the
       ACL item obtained in the previous step.

   4.  Validate that an item of the corresponding ACL contains a
       "to_user" field whose value equals the username obtained in step
       3.  Additionally, validate that the "ad" flag is set to 1.

   5.  Repeat steps 3 and 4 until the "to_user" value is equal to the
       username of the signer of the ACL in the selected item.  This
       final ACL item is expected to be the root item of this ACL, which
       MUST be further validated by verifying that the root item was
       signed by the owner of the ACL Resource.

   The trust delegation chain is valid if and only if all verification
   steps succeed.  In this case, the creator of the data value of the
   Shared Resource is an Authorized Peer.

   Note that the ACL validation procedure can be omitted whenever the
   creator of data at a Shared Resource is the Resource Owner itself.
   The latter can be verified by its public key certificate as defined
   in Section 6.6.

6.4.  Operations of Storing Peers

   Storing peers, at which Shared Resource and ACL are physically
   stored, are responsible for controlling storage attempts to a Shared
   Resource and its corresponding Access Control List.  To assert the
   USER-CHAIN-ACL access policy (see Section 6.6), a storing peer MUST
   perform the access validation procedure described in Section 6.3 on
   any incoming store request using the most recent Access Control List
   for every Kind that uses the USER-CHAIN-ACL policy.  It SHALL further
   ensure that only the Resource Owner stores new ACL root items for
   Shared Resources.

6.5.  Operations of Accessing Peers

   Accessing peers, i.e., peers that fetch a Shared Resource, can
   validate that the originator of a Shared Resource was authorized to
   store data at this Resource-ID by processing the corresponding ACL.
   To enable an accessing peer to perform the access validation
   procedure described in Section 6.3, it first needs to obtain the most
   recent Access Control List in the following way:

   1.  Send a Stat request to the Resource-ID of the Shared Resource to
       obtain all array indexes of stored ACL Kinds (as per [RFC6940],
       Section 7.4.3.).

   2.  Fetch all indexes of existing ACL items at this Resource-ID by
       using the array ranges retrieved in the Stat request answer.

   Peers can cache previously fetched Access Control Lists up to the
   maximum lifetime of an individual item.  Since stored values could
   have been modified or invalidated prior to their expiration, an
   accessing peer SHOULD use a Stat request to check for updates prior
   to using the data cache.

6.6.  USER-CHAIN-ACL Access Policy

   This document specifies an additional access control policy to the
   RELOAD base document [RFC6940].  The USER-CHAIN-ACL policy allows
   Authorized Peers to write a Shared Resource, even though they do not
   own the corresponding certificate.  Additionally, the USER-CHAIN-ACL
   allows the storage of Kinds with a variable resource name that are
   following one of the specified naming patterns.  Hence, on an inbound
   store request on a Kind that uses the USER-CHAIN-ACL access policy,
   the following rules MUST be applied:

   In the USER-CHAIN-ACL policy, a given value MUST NOT be written or
   overwritten, if neither one of USER-MATCH or USER-NODE-MATCH
   (mandatory if the data model is dictionary) access policies of the
   base document [RFC6940] applies.

   Additionally, the store request MUST be denied if the signer's
   certificate does not contain a username that matches to the user and
   domain portion in one of the variable resource name patterns (cf.
   Section 5) specified in the configuration document or if the hashed
   Resource Name does not match the Resource-ID.  The Resource Name of
   the Kind to be stored MUST be taken from the mandatory
   ResourceNameExtension field in the corresponding Kind data structure.

   If the access rights cannot be verified according to the ACL
   validation procedure described in Section 6.3, the store request MUST
   also be denied.

   Otherwise, the store request can be processed further.

7.  ACCESS-CONTROL-LIST Kind Definition

   This section defines the ACCESS-CONTROL-LIST Kind previously
   described in this document.

   Name:  ACCESS-CONTROL-LIST

   Kind IDs:  The Resource Name for ACCESS-CONTROL-LIST Kind-ID is the
      Resource Name of the Kind that will be shared by using the ACCESS-
      CONTROL-LIST Kind.

   Data Model:  The data model for the ACCESS-CONTROL-LIST Kind-ID is
      array.  The array indexes are formed by using the mechanism for
      isolated stored data as described in Section 3.1.

   Access Control:  USER-CHAIN-ACL (see Section 6.6).

8.  Security Considerations

   In this section, we discuss security issues that are relevant to the
   usage of Shared Resources in RELOAD [RFC6940].

8.1.  Resource Exhaustion

   Joining a RELOAD overlay inherently poses a certain resource load on
   a peer, because it has to store and forward data for other peers.  In
   common RELOAD semantics, each Resource-ID and thus position in the
   overlay, may only be written by a limited set of peers -- often even
   only a single peer, which limits this burden.  In the case of Shared
   Resources, a single resource may be written by multiple peers who may
   even write an arbitrary number of entries (e.g., delegations in the
   ACL).  This leads to an enhanced use of resources at individual
   overlay nodes.  The problem of resource exhaustion can easily be
   mitigated for Usages based on the ShaRe-Usage by imposing
   restrictions on size, i.e., <max-size> element for a certain Kind in
   the configuration document.

8.2.  Malicious or Misbehaving Storing Peer

   The RELOAD overlay is designed to operate despite the presence of a
   small set of misbehaving peers.  This is not different for Shared
   Resources since a small set of malicious peers does not disrupt the
   functionality of the overlay in general, but may have implications
   for the peers needing to store or access information at the specific
   locations in the ID space controlled by a malicious peer.  A storing
   peer could withhold stored data, which results in a denial of service
   to the group using the specific resource.  But it could not return
   forged data, since the validity of any stored data can be
   independently verified using the attached signatures.

8.3.  Trust Delegation to a Malicious or Misbehaving Peer

   A Resource Owner that erroneously delegated write access to a Shared
   Resource for a misbehaving peer enables this malicious member of the
   overlay to interfere with the corresponding group application in
   several unwanted ways.  Examples of destructive interferences range
   from exhausting shared storage to dedicated application-specific
   misuse.  Additionally, a bogus peer that was granted delegation
   rights may authorize further malicious collaborators to writing the
   Shared Resource.

   It is the obligation of the Resource Owner to bind trust delegation
   to apparent trustworthiness.  Additional measures to monitor proper
   behavior may be applied.  In any case, the Resource Owner will be
   able to revoke the trust delegation of an entire tree in a single
   overwrite operation.  It further holds the right to overwrite any
   malicious contributions to the shared resource under misuse.

8.4.  Privacy Issues

   All data stored in the Shared Resource is readable by any node in the
   overlay; thus, applications requiring privacy need to encrypt the
   data.  The ACL needs to be stored unencrypted; thus, the list members
   of a group using a Shared Resource will always be publicly visible.

9.  IANA Considerations

9.1.  Access Control Policy

   IANA has registered the following entry in the "RELOAD Access Control
   Policies" registry (cf. [RFC6940]) to represent the USER-CHAIN-ACL
   Access Control Policy, as described in Section 6.6.

                     +-------------------+----------+
                     | Access Policy     |      RFC |
                     +-------------------+----------+
                     | USER-CHAIN-ACL    | RFC 8076 |
                     +-------------------+----------+

9.2.  Data Kind-ID

   IANA has registered the following code point in the "RELOAD Data
   Kind-ID" registry (cf. [RFC6940]) to represent the ShaRe ACCESS-
   CONTROL-LIST kind, as described in Section 7.

             +----------------------+------------+----------+
             | Kind                 |    Kind-ID |      RFC |
             +----------------------+------------+----------+
             | ACCESS-CONTROL-LIST  |        0x4 | RFC 8076 |
             +----------------------+------------+----------+

9.3.  XML Namespace Registration

   This document registers the following URI for the config XML
   namespace in the IETF XML registry defined in [RFC3688].

   URI:  urn:ietf:params:xml:ns:p2p:config-base:share

   Registrant Contact:  The IESG

   XML:  N/A, the requested URI is an XML namespace

10.  References

10.1.  Normative References

   [IEEE-Posix]
              "IEEE Standard for Information Technology - Portable
              Operating System Interface (POSIX) - Part 2: Shell and
              Utilities (Vol.  1)", IEEE Std 1003.2-1992, ISBN
              1-55937-255-9, DOI 10.1109/IEEESTD.1993.6880751, January
              1993, <http://ieeexplore.ieee.org/document/6880751/>.

   [OASIS.relax_ng]
              Clark, J. and M. Murata, "RELAX NG Specification",
              December 2001.

   [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>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <http://www.rfc-editor.org/info/rfc3688>.

   [RFC6940]  Jennings, C., Lowekamp, B., Ed., Rescorla, E., Baset, S.,
              and H. Schulzrinne, "REsource LOcation And Discovery
              (RELOAD) Base Protocol", RFC 6940, DOI 10.17487/RFC6940,
              January 2014, <http://www.rfc-editor.org/info/rfc6940>.

   [W3C.REC-xmlschema-2-20041028]
              Malhotra, A. and P. Biron, "XML Schema Part 2: Datatypes
              Second Edition", World Wide Web Consortium Recommendation
              REC-xmlschema-2-20041028, October 2004,
              <http://www.w3.org/TR/2004/REC-xmlschema-2-20041028>.

10.2.  Informative References

   [RFC7890]  Bryan, D., Matthews, P., Shim, E., Willis, D., and S.
              Dawkins, "Concepts and Terminology for Peer-to-Peer SIP
              (P2PSIP)", RFC 7890, DOI 10.17487/RFC7890, June 2016,
              <http://www.rfc-editor.org/info/rfc7890>.

   [RFC7904]  Jennings, C., Lowekamp, B., Rescorla, E., Baset, S.,
              Schulzrinne, H., and T. Schmidt, Ed., "A SIP Usage for
              REsource LOcation And Discovery (RELOAD)", RFC 7904,
              DOI 10.17487/RFC7904, October 2016,
              <http://www.rfc-editor.org/info/rfc7904>.

Acknowledgments

   This work was stimulated by fruitful discussions in the P2PSIP
   working group and the SAM research group.  We would like to thank all
   active members for their constructive thoughts and feedback.  In
   particular, the authors would like to thank (in alphabetical order)
   Emmanuel Baccelli, Ben Campbell, Alissa Cooper, Lothar Grimm, Russ
   Housley, Cullen Jennings, Matt Miller, Peter Musgrave, Joerg Ott,
   Marc Petit-Huguenin, Peter Pogrzeba, and Jan Seedorf.  This work was
   partly funded by the German Federal Ministry of Education and
   Research, projects HAMcast, Mindstone, and SAFEST.

Authors' Addresses

   Alexander Knauf
   HAW Hamburg
   Berliner Tor 7
   Hamburg  D-20099
   Germany

   Phone: +4940428758067
   Email: alexanderknauf@gmail.com


   Thomas C. Schmidt
   HAW Hamburg
   Berliner Tor 7
   Hamburg  D-20099
   Germany

   Email: t.schmidt@haw-hamburg.de
   URI:   http://inet.haw-hamburg.de/members/schmidt


   Gabriel Hege
   daviko GmbH
   Schillerstr. 107
   Berlin  D-10625
   Germany

   Phone: +493043004344
   Email: hege@daviko.com


   Matthias Waehlisch
   link-lab & FU Berlin
   Hoenower Str. 35
   Berlin  D-10318
   Germany

   Email: mw@link-lab.net
   URI:   http://www.inf.fu-berlin.de/~waehl