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RFC 8006

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Content Delivery Network Interconnection (CDNI) Metadata

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Internet Engineering Task Force (IETF)                  B. Niven-Jenkins
Request for Comments: 8006                                     R. Murray
Category: Standards Track                                          Nokia
ISSN: 2070-1721                                             M. Caulfield
                                                           Cisco Systems
                                                                   K. Ma
                                                                Ericsson
                                                           December 2016


        Content Delivery Network Interconnection (CDNI) Metadata

Abstract

   The Content Delivery Network Interconnection (CDNI) Metadata
   interface enables interconnected Content Delivery Networks (CDNs) to
   exchange content distribution metadata in order to enable content
   acquisition and delivery.  The CDNI Metadata associated with a piece
   of content provides a downstream CDN with sufficient information for
   the downstream CDN to service content requests on behalf of an
   upstream CDN.  This document describes both a base set of CDNI
   Metadata and the protocol for exchanging that metadata.

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/rfc8006.

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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.

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Table of Contents

   1. Introduction ....................................................5
      1.1. Terminology ................................................5
      1.2. Supported Metadata Capabilities ............................6
   2. Design Principles ...............................................7
   3. CDNI Metadata Object Model ......................................8
      3.1. HostIndex, HostMatch, HostMetadata, PathMatch,
           PatternMatch, and PathMetadata Objects .....................9
      3.2. Generic CDNI Metadata Objects .............................11
      3.3. Metadata Inheritance and Override .........................14
   4. CDNI Metadata Objects ..........................................15
      4.1. Definitions of the CDNI Structural Metadata Objects .......16
           4.1.1. HostIndex ..........................................16
           4.1.2. HostMatch ..........................................17
           4.1.3. HostMetadata .......................................18
           4.1.4. PathMatch ..........................................19
           4.1.5. PatternMatch .......................................20
           4.1.6. PathMetadata .......................................21
           4.1.7. GenericMetadata ....................................23
      4.2. Definitions of the Initial Set of CDNI
           GenericMetadata Objects ...................................24
           4.2.1. SourceMetadata .....................................24
                  4.2.1.1. Source ....................................25
           4.2.2. LocationACL Metadata ...............................26
                  4.2.2.1. LocationRule ..............................28
                  4.2.2.2. Footprint .................................29
           4.2.3. TimeWindowACL ......................................30
                  4.2.3.1. TimeWindowRule ............................31
                  4.2.3.2. TimeWindow ................................32
           4.2.4. ProtocolACL Metadata ...............................33
                  4.2.4.1. ProtocolRule ..............................34
           4.2.5. DeliveryAuthorization Metadata .....................35
           4.2.6. Cache ..............................................35
           4.2.7. Auth ...............................................37
           4.2.8. Grouping ...........................................38
      4.3. CDNI Metadata Simple Data Type Descriptions ...............39
           4.3.1. Link ...............................................39
                  4.3.1.1. Link Loop Prevention ......................40
           4.3.2. Protocol ...........................................40
           4.3.3. Endpoint ...........................................40
           4.3.4. Time ...............................................41
           4.3.5. IPv4CIDR ...........................................41
           4.3.6. IPv6CIDR ...........................................42
           4.3.7. ASN ................................................42
           4.3.8. Country Code .......................................42
   5. CDNI Metadata Capabilities .....................................42

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   6. CDNI Metadata Interface ........................................43
      6.1. Transport .................................................44
      6.2. Retrieval of CDNI Metadata Resources ......................44
      6.3. Bootstrapping .............................................45
      6.4. Encoding ..................................................46
      6.5. Extensibility .............................................46
      6.6. Metadata Enforcement ......................................47
      6.7. Metadata Conflicts ........................................47
      6.8. Versioning ................................................48
      6.9. Media Types ...............................................49
      6.10. Complete CDNI Metadata Example ...........................50
   7. IANA Considerations ............................................54
      7.1. CDNI Payload Types ........................................54
           7.1.1. CDNI MI HostIndex Payload Type .....................54
           7.1.2. CDNI MI HostMatch Payload Type .....................55
           7.1.3. CDNI MI HostMetadata Payload Type ..................55
           7.1.4. CDNI MI PathMatch Payload Type .....................55
           7.1.5. CDNI MI PatternMatch Payload Type ..................55
           7.1.6. CDNI MI PathMetadata Payload Type ..................55
           7.1.7. CDNI MI SourceMetadata Payload Type ................56
           7.1.8. CDNI MI Source Payload Type ........................56
           7.1.9. CDNI MI LocationACL Payload Type ...................56
           7.1.10. CDNI MI LocationRule Payload Type .................56
           7.1.11. CDNI MI Footprint Payload Type ....................56
           7.1.12. CDNI MI TimeWindowACL Payload Type ................57
           7.1.13. CDNI MI TimeWindowRule Payload Type ...............57
           7.1.14. CDNI MI TimeWindow Payload Type ...................57
           7.1.15. CDNI MI ProtocolACL Payload Type ..................57
           7.1.16. CDNI MI ProtocolRule Payload Type .................57
           7.1.17. CDNI MI DeliveryAuthorization Payload Type ........58
           7.1.18. CDNI MI Cache Payload Type ........................58
           7.1.19. CDNI MI Auth Payload Type .........................58
           7.1.20. CDNI MI Grouping Payload Type .....................58
      7.2. "CDNI Metadata Footprint Types" Registry ..................58
      7.3. "CDNI Metadata Protocol Types" Registry ...................59
   8. Security Considerations ........................................60
      8.1. Authentication and Integrity ..............................60
      8.2. Confidentiality and Privacy ...............................60
      8.3. Securing the CDNI Metadata Interface ......................61
   9. References .....................................................62
      9.1. Normative References ......................................62
      9.2. Informative References ....................................63
   Acknowledgments ...................................................65
   Contributors ......................................................65
   Authors' Addresses ................................................66

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1.  Introduction

   Content Delivery Network Interconnection (CDNI) [RFC6707] enables a
   downstream Content Delivery Network (dCDN) to service content
   requests on behalf of an upstream CDN (uCDN).

   The CDNI Metadata interface (MI) is discussed in [RFC7336] along with
   four other interfaces that can be used to compose a CDNI solution
   (the CDNI Control interface, the CDNI Request Routing Redirection
   interface, the CDNI Footprint & Capabilities Advertisement interface
   (FCI), and the CDNI Logging interface).  [RFC7336] describes each
   interface and the relationships between them.  The requirements for
   the CDNI Metadata interface are specified in [RFC7337].

   The CDNI Metadata associated with a piece of content (or with a set
   of content) provides a dCDN with sufficient information for servicing
   content requests on behalf of a uCDN, in accordance with the policies
   defined by the uCDN.

   This document defines a CDNI Metadata interface that enables a dCDN
   to obtain CDNI Metadata from a uCDN so that the dCDN can properly
   process and respond to:

   o  Redirection requests received over the CDNI Request Routing
      Redirection interface [RFC7975].

   o  Content requests received directly from User Agents.

   Specifically, this document defines:

   o  A data structure for mapping content requests and redirection
      requests to CDNI Metadata objects (Sections 3 and 4.1).

   o  An initial set of CDNI GenericMetadata objects (Section 4.2).

   o  An HTTP web service for the transfer of CDNI Metadata (Section 6).

1.1.  Terminology

   This document reuses the terminology defined in [RFC6707].

   Additionally, the following terms are used throughout this document
   and are defined as follows:

   o  Object - a collection of properties.

   o  Property - a key and value pair where the key is a property name
      and the value is the property value or another object.

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   This document uses the phrase "[Object] A contains [Object] B" for
   simplicity when a strictly accurate phrase would be "[Object] A
   contains or references (via a Link object) [Object] B".

   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].

1.2.  Supported Metadata Capabilities

   Only the metadata for a small set of initial capabilities is
   specified in this document.  This set provides the minimum amount of
   metadata for basic CDN interoperability while still meeting the
   requirements set forth by [RFC7337].

   The following high-level functionality can be configured via the CDNI
   Metadata objects specified in Section 4:

   o  Acquisition Source: Metadata for allowing a dCDN to fetch content
      from a uCDN.

   o  Delivery Access Control: Metadata for restricting (or permitting)
      access to content based on any of the following factors:

      *  Location

      *  Time window

      *  Delivery protocol

   o  Delivery Authorization: Metadata for authorizing dCDN User Agent
      requests.

   o  Cache Control: Metadata for controlling cache behavior of
      the dCDN.

   The metadata encoding described by this document is extensible in
   order to allow for future additions to this list.

   The set of metadata specified in this document covers the initial
   capabilities above.  It is only intended to support CDNI for the
   delivery of content by a dCDN using HTTP/1.1 [RFC7230] and for a dCDN
   to be able to acquire content from a uCDN using either HTTP/1.1 or
   HTTP/1.1 over Transport Layer Security (TLS) [RFC2818].

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   Supporting CDNI for the delivery of content using unencrypted HTTP/2
   [RFC7540] (as well as for a dCDN to acquire content using unencrypted
   HTTP/2 or HTTP/2 over TLS) requires the registration of these
   protocol names in the "CDNI Metadata Protocol Types" registry
   (Section 7.3).

   Delivery of content using HTTP/1.1 over TLS or HTTP/2 over TLS SHOULD
   follow the guidelines set forth in [RFC7525].  Offline configuration
   of TLS parameters between CDNs is beyond the scope of this document.

2.  Design Principles

   The CDNI Metadata interface was designed to achieve the following
   objectives:

   1.  Cacheability of CDNI Metadata objects;

   2.  Deterministic mapping from redirection requests and content
       requests to CDNI Metadata properties;

   3.  Support for DNS redirection as well as application-specific
       redirection (for example, HTTP redirection);

   4.  Minimal duplication of CDNI Metadata; and

   5.  Leveraging of existing protocols.

   Cacheability can decrease the latency of acquiring metadata while
   maintaining its freshness and can therefore decrease the latency of
   serving content requests and redirection requests, without
   sacrificing accuracy.  The CDNI Metadata interface uses HTTP and its
   existing caching mechanisms to achieve CDNI Metadata cacheability.

   Deterministic mapping from content to metadata properties eliminates
   ambiguity and ensures that policies are applied consistently by all
   dCDNs.

   Support for both HTTP and DNS redirection ensures that the CDNI
   Metadata meets the same design principles for both HTTP-based and
   DNS-based redirection schemes.

   Minimal duplication of CDNI Metadata improves storage efficiency in
   the CDNs.

   Leveraging existing protocols avoids reinventing common mechanisms
   such as data structure encoding (by leveraging I-JSON (Internet JSON)
   [RFC7493]) and data transport (by leveraging HTTP [RFC7230]).

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3.  CDNI Metadata Object Model

   The CDNI Metadata object model describes a data structure for mapping
   redirection requests and content requests to metadata properties.
   Metadata properties describe how to acquire content from a uCDN,
   authorize access to content, and deliver content from a dCDN.  The
   object model relies on the assumption that these metadata properties
   can be grouped based on the hostname of the content and subsequently
   on the resource path (URI) of the content.  The object model
   associates a set of CDNI Metadata properties with a hostname to form
   a default set of metadata properties for content delivered on behalf
   of that hostname.  That default set of metadata properties can be
   overridden by properties that apply to specific paths within a URI.

   Different hostnames and URI paths will be associated with different
   sets of CDNI Metadata properties in order to describe the required
   behavior when a dCDN Surrogate or request router is processing User
   Agent requests for content at that hostname and URI path.  As a
   result of this structure, significant commonality could exist between
   the CDNI Metadata properties specified for different hostnames,
   different URI paths within a hostname, and different URI paths on
   different hostnames.  For example, the definition of which User Agent
   IP addresses should be grouped together into a single network or
   geographic location is likely to be common for a number of different
   hostnames; although a uCDN is likely to have several different
   policies configured to express geo-blocking rules, it is likely that
   a single geo-blocking policy could be applied to multiple hostnames
   delivered through the CDN.

   In order to enable the CDNI Metadata for a given hostname and URI
   path to be decomposed into reusable sets of CDNI Metadata properties,
   the CDNI Metadata interface splits the CDNI Metadata into separate
   objects.  Efficiency is improved by enabling a single CDNI Metadata
   object (that is shared across hostnames and/or URI paths) to be
   retrieved and stored by a dCDN once, even if it is referenced by the
   CDNI Metadata for multiple hostnames and/or URI paths.

   Important Note: Any CDNI Metadata object A that contains another CDNI
   Metadata object B can include a Link object specifying a URI that can
   be used to retrieve object B, instead of embedding object B within
   object A.  The remainder of this document uses the phrase "[Object] A
   contains [Object] B" for simplicity when a strictly accurate phrase
   would be "[Object] A contains or references (via a Link object)
   [Object] B".  It is generally a deployment choice for the uCDN
   implementation to decide when to embed CDNI Metadata objects and when
   to reference separate resources via Link objects.

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   Section 3.1 introduces a high-level description of the HostIndex,
   HostMatch, HostMetadata, PathMatch, PatternMatch, and PathMetadata
   objects, and describes the relationships between them.

   Section 3.2 introduces a high-level description of the CDNI
   GenericMetadata object, which represents the level at which CDNI
   Metadata override occurs between HostMetadata and PathMetadata
   objects.

   Section 4 describes in detail the specific CDNI Metadata objects and
   properties specified by this document that can be contained within a
   CDNI GenericMetadata object.

3.1.  HostIndex, HostMatch, HostMetadata, PathMatch, PatternMatch, and
      PathMetadata Objects

   The relationships between the HostIndex, HostMatch, HostMetadata,
   PathMatch, PatternMatch, and PathMetadata objects are described in
   Figure 1.

   +---------+      +---------+      +------------+
   |HostIndex+-(*)->|HostMatch+-(1)->|HostMetadata+-------(*)------+
   +---------+      +---------+      +------+-----+                |
                                            |                      |
                                           (*)                     |
                                            |                      V
   --> Contains or references               V         *****************
   (1) One and only one                +---------+    *GenericMetadata*
   (*) Zero or more               +--->|PathMatch|    *     Objects   *
                                  |    +----+---++    *****************
                                  |         |   |                  ^
                                 (*)       (1) (1) +------------+  |
                                  |         |   +->|PatternMatch|  |
                                  |         V      +------------+  |
                                  |  +------------+                |
                                  +--+PathMetadata+-------(*)------+
                                     +------------+

           Figure 1: Relationships between CDNI Metadata Objects
                         (Diagram Representation)

   A HostIndex object (see Section 4.1.1) contains an array of HostMatch
   objects (see Section 4.1.2) that contain hostnames (and/or IP
   addresses) for which content requests might be delegated to the dCDN.
   The HostIndex is the starting point for accessing the uCDN CDNI
   Metadata data store.  It enables the dCDN to deterministically
   discover which CDNI Metadata objects it requires in order to deliver
   a given piece of content.

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   The HostIndex links hostnames (and/or IP addresses) to HostMetadata
   objects (see Section 4.1.3) via HostMatch objects.  A HostMatch
   object defines a hostname (or IP address) to match against a
   requested host and contains a HostMetadata object.

   HostMetadata objects contain the default GenericMetadata objects (see
   Section 4.1.7) required to serve content for that host.  When looking
   up CDNI Metadata, the dCDN looks up the requested hostname (or IP
   address) against the HostMatch entries in the HostIndex; from there,
   it can find HostMetadata, which describes the default metadata
   properties for each host as well as PathMetadata objects (see
   Section 4.1.6), via PathMatch objects (see Section 4.1.4).  PathMatch
   objects define patterns, contained inside PatternMatch objects (see
   Section 4.1.5), to match against the requested URI path.
   PatternMatch objects contain the pattern strings and flags that
   describe the URI path to which a PathMatch applies.  PathMetadata
   objects contain the GenericMetadata objects that apply to content
   requests matching the defined URI path pattern.  PathMetadata
   properties override properties previously defined in HostMetadata or
   less-specific PathMatch paths.  PathMetadata objects can contain
   additional PathMatch objects to recursively define more-specific URI
   paths to which GenericMetadata properties might be applied.

   A GenericMetadata object contains individual CDNI Metadata objects
   that define the specific policies and attributes needed to properly
   deliver the associated content.  For example, a GenericMetadata
   object could describe the source from which a CDN can acquire a piece
   of content.  The GenericMetadata object is an atomic unit that can be
   referenced by HostMetadata or PathMetadata objects.

   For example, if "example.com" is a content provider, a HostMatch
   object could include an entry for "example.com" with the URI of the
   associated HostMetadata object.  The HostMetadata object for
   "example.com" describes the metadata properties that apply to
   "example.com" and could contain PathMatches for
   "example.com/movies/*" and "example.com/music/*", which in turn
   reference corresponding PathMetadata objects that contain the
   properties for those more-specific URI paths.  The PathMetadata
   object for "example.com/movies/*" describes the properties that apply
   to that URI path.  It could also contain a PathMatch object for
   "example.com/movies/hd/*", which would reference the corresponding
   PathMetadata object for the "example.com/movies/hd/" path prefix.

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   The relationships in Figure 1 are also represented in tabular format
   in Table 1 below.

   +--------------+----------------------------------------------------+
   | Data Object  | Objects it contains or references                  |
   +--------------+----------------------------------------------------+
   | HostIndex    | 0 or more HostMatch objects.                       |
   |              |                                                    |
   | HostMatch    | 1 HostMetadata object.                             |
   |              |                                                    |
   | HostMetadata | 0 or more PathMatch objects.  0 or more            |
   |              | GenericMetadata objects.                           |
   |              |                                                    |
   | PathMatch    | 1 PatternMatch object.  1 PathMetadata object.     |
   |              |                                                    |
   | PatternMatch | Does not contain or reference any other objects.   |
   |              |                                                    |
   | PathMetadata | 0 or more PathMatch objects.  0 or more            |
   |              | GenericMetadata objects.                           |
   +--------------+----------------------------------------------------+

           Table 1: Relationships between CDNI Metadata Objects
                          (Table Representation)

3.2.  Generic CDNI Metadata Objects

   The HostMetadata and PathMetadata objects contain other CDNI Metadata
   objects that contain properties that describe how User Agent requests
   for content should be processed -- for example, where to acquire the
   content from, authorization rules that should be applied,
   geo-blocking restrictions, and so on.  Each such CDNI Metadata object
   is a specialization of a CDNI GenericMetadata object.  The
   GenericMetadata object abstracts the basic information required for
   metadata override and metadata distribution, from the specifics of
   any given property (i.e., property semantics, enforcement options,
   etc.).

   The GenericMetadata object defines the properties contained within it
   as well as whether or not the properties are "mandatory-to-enforce".
   If the dCDN does not understand or support a mandatory-to-enforce
   property, the dCDN MUST NOT serve the content.  If the property is
   not mandatory-to-enforce, then that GenericMetadata object can be
   safely ignored and the content request can be processed in accordance
   with the rest of the CDNI Metadata.

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   Although a CDN MUST NOT serve content to a User Agent if a
   mandatory-to-enforce property cannot be enforced, it could still be
   safe to redistribute that metadata (the "safe-to-redistribute"
   property) to another CDN without modification.  For example, in the
   cascaded CDN case, a transit CDN (tCDN) could convey
   mandatory-to-enforce metadata to a dCDN.  For metadata that does not
   require customization or translation (i.e., metadata that is
   safe-to-redistribute), the data representation received off the wire
   MAY be stored and redistributed without being understood or supported
   by the tCDN.  However, for metadata that requires translation,
   transparent redistribution of the uCDN metadata values might not be
   appropriate.  Certain metadata can be safely, though perhaps not
   optimally, redistributed unmodified.  For example, a source
   acquisition address might not be optimal if transparently
   redistributed, but it might still work.

   Redistribution safety MUST be specified for each GenericMetadata
   property.  If a CDN does not understand or support a given
   GenericMetadata property that is not safe-to-redistribute, the CDN
   MUST set the "incomprehensible" flag to true for that GenericMetadata
   object before redistributing the metadata.  The "incomprehensible"
   flag signals to a dCDN that the metadata was not properly transformed
   by the tCDN.  A CDN MUST NOT attempt to use metadata that has been
   marked as "incomprehensible" by a uCDN.

   tCDNs MUST NOT change the value of mandatory-to-enforce or
   safe-to-redistribute when propagating metadata to a dCDN.  Although a
   tCDN can set the value of "incomprehensible" to true, a tCDN MUST NOT
   change the value of "incomprehensible" from true to false.

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   Table 2 describes the action to be taken by a tCDN for the different
   combinations of mandatory-to-enforce ("MtE") and safe-to-redistribute
   ("StR") properties when the tCDN either does or does not understand
   the metadata in question:

   +-------+-------+------------+--------------------------------------+
   | MtE   | StR   | Metadata   | Action                               |
   |       |       | Understood |                                      |
   |       |       | by tCDN    |                                      |
   +-------+-------+------------+--------------------------------------+
   | False | True  | True       | Can serve and redistribute.          |
   |       |       |            |                                      |
   | False | True  | False      | Can serve and redistribute.          |
   |       |       |            |                                      |
   | False | False | False      | Can serve.  MUST set                 |
   |       |       |            | "incomprehensible" to true when      |
   |       |       |            | redistributing.                      |
   |       |       |            |                                      |
   | False | False | True       | Can serve.  Can redistribute after   |
   |       |       |            | transforming the metadata (if the    |
   |       |       |            | CDN knows how to do so safely);      |
   |       |       |            | otherwise, MUST set                  |
   |       |       |            | "incomprehensible" to true when      |
   |       |       |            | redistributing.                      |
   |       |       |            |                                      |
   | True  | True  | True       | Can serve and redistribute.          |
   |       |       |            |                                      |
   | True  | True  | False      | MUST NOT serve but can redistribute. |
   |       |       |            |                                      |
   | True  | False | True       | Can serve.  Can redistribute after   |
   |       |       |            | transforming the metadata (if the    |
   |       |       |            | CDN knows how to do so safely);      |
   |       |       |            | otherwise, MUST set                  |
   |       |       |            | "incomprehensible" to true when      |
   |       |       |            | redistributing.                      |
   |       |       |            |                                      |
   | True  | False | False      | MUST NOT serve.  MUST set            |
   |       |       |            | "incomprehensible" to true when      |
   |       |       |            | redistributing.                      |
   +-------+-------+------------+--------------------------------------+

               Table 2: Action to Be Taken by a tCDN for the
             Different Combinations of MtE and StR Properties

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   Table 3 describes the action to be taken by a dCDN for the different
   combinations of mandatory-to-enforce and "incomprehensible" (Incomp)
   properties, when the dCDN either does or does not understand the
   metadata in question:

   +-------+--------+--------------+-----------------------------------+
   | MtE   | Incomp | Metadata     | Action                            |
   |       |        | Understood   |                                   |
   |       |        | by dCDN      |                                   |
   +-------+--------+--------------+-----------------------------------+
   | False | False  | True         | Can serve.                        |
   |       |        |              |                                   |
   | False | True   | True         | Can serve but MUST NOT            |
   |       |        |              | interpret/apply any metadata      |
   |       |        |              | marked as "incomprehensible".     |
   |       |        |              |                                   |
   | False | False  | False        | Can serve.                        |
   |       |        |              |                                   |
   | False | True   | False        | Can serve but MUST NOT            |
   |       |        |              | interpret/apply any metadata      |
   |       |        |              | marked as "incomprehensible".     |
   |       |        |              |                                   |
   | True  | False  | True         | Can serve.                        |
   |       |        |              |                                   |
   | True  | True   | True         | MUST NOT serve.                   |
   |       |        |              |                                   |
   | True  | False  | False        | MUST NOT serve.                   |
   |       |        |              |                                   |
   | True  | True   | False        | MUST NOT serve.                   |
   +-------+--------+--------------+-----------------------------------+

               Table 3: Action to Be Taken by a dCDN for the
            Different Combinations of MtE and Incomp Properties

3.3.  Metadata Inheritance and Override

   In the metadata object model, a HostMetadata object can contain
   multiple PathMetadata objects (via PathMatch objects).  Each
   PathMetadata object can in turn contain other PathMetadata objects.
   HostMetadata and PathMetadata objects form an inheritance tree where
   each node in the tree inherits or overrides the property values set
   by its parent.

   GenericMetadata objects of a given type override all GenericMetadata
   objects of the same type previously defined by any parent object in
   the tree.  GenericMetadata objects of a given type previously defined
   by a parent object in the tree are inherited when no object of the
   same type is defined by the child object.  For example, if

Top      ToC       Page 15 
   HostMetadata for the host "example.com" contains GenericMetadata
   objects of types LocationACL and TimeWindowACL (where "ACL" means
   "Access Control List") while a PathMetadata object that applies to
   "example.com/movies/*" defines an alternate GenericMetadata object of
   type TimeWindowACL, then:

   o  The TimeWindowACL defined in the PathMetadata would override the
      TimeWindowACL defined in the HostMetadata for all User Agent
      requests for content under "example.com/movies/", and

   o  The LocationACL defined in the HostMetadata would be inherited for
      all User Agent requests for content under "example.com/movies/".

   A single HostMetadata or PathMetadata object MUST NOT contain
   multiple GenericMetadata objects of the same type.  If an array of
   GenericMetadata contains objects of duplicate types, the receiver
   MUST ignore all but the first object of each type.



(page 15 continued on part 2)

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