Internet Engineering Task Force (IETF) A. Knauf

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.

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Copyright Notice

Copyright (c) 2017 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.

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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

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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.

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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.

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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.

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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.

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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.

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+------------------------------------------------------+
| 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;

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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

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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.

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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.

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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.

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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:

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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.

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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.

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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.

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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.

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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

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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>.

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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.

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Authors' Addresses

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

Phone: +4940428758067
Email: [email protected]

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

Email: [email protected]
URI: http://inet.haw-hamburg.de/members/schmidt

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

Phone: +493043004344
Email: [email protected]

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

Email: [email protected]
URI: http://www.inf.fu-berlin.de/~waehl

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