<pre>Internet Engineering Task Force (IETF)                  B. Niven-Jenkins
Request for Comments: 6707                      Velocix (Alcatel-Lucent)
Category: Informational                                   F. Le Faucheur
ISSN: 2070-1721                                                    Cisco
                                                                N. Bitar
                                                                 Verizon
                                                          September 2012


 <span class="h1">Content Distribution Network Interconnection (CDNI) Problem Statement</span>

Abstract

   Content Delivery Networks (CDNs) provide numerous benefits for
   cacheable content: reduced delivery cost, improved quality of
   experience for End Users, and increased robustness of delivery.  For
   these reasons, they are frequently used for large-scale content
   delivery.  As a result, existing CDN Providers are scaling up their
   infrastructure, and many Network Service Providers (NSPs) are
   deploying their own CDNs.  It is generally desirable that a given
   content item can be delivered to an End User regardless of that End
   User's location or attachment network.  This is the motivation for
   interconnecting standalone CDNs so they can interoperate as an open
   content delivery infrastructure for the end-to-end delivery of
   content from Content Service Providers (CSPs) to End Users.  However,
   no standards or open specifications currently exist to facilitate
   such CDN Interconnection.

   The goal of this document is to outline the problem area of CDN
   Interconnection for the IETF CDNI (CDN Interconnection) working
   group.

Status of This Memo

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

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc6707">http://www.rfc-editor.org/info/rfc6707</a>.




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Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) 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.

Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
      <a href="#section-1.1">1.1</a>. Terminology ................................................<a href="#page-4">4</a>
      <a href="#section-1.2">1.2</a>. CDN Background .............................................<a href="#page-9">9</a>
   <a href="#section-2">2</a>. CDN Interconnection Use Cases ...................................<a href="#page-9">9</a>
   <a href="#section-3">3</a>. CDN Interconnection Model and Problem Area for IETF ............<a href="#page-11">11</a>
   <a href="#section-4">4</a>. Scoping the CDNI Problem .......................................<a href="#page-15">15</a>
      <a href="#section-4.1">4.1</a>. CDNI Control Interface ....................................<a href="#page-16">16</a>
      <a href="#section-4.2">4.2</a>. CDNI Request Routing Interface ............................<a href="#page-16">16</a>
      <a href="#section-4.3">4.3</a>. CDNI Metadata Interface ...................................<a href="#page-17">17</a>
      <a href="#section-4.4">4.4</a>. CDNI Logging Interface ....................................<a href="#page-17">17</a>
   <a href="#section-5">5</a>. Security Considerations ........................................<a href="#page-17">17</a>
      <a href="#section-5.1">5.1</a>. Security of the CDNI Control Interface ....................<a href="#page-18">18</a>
      <a href="#section-5.2">5.2</a>. Security of the CDNI Request Routing Interface ............<a href="#page-18">18</a>
      <a href="#section-5.3">5.3</a>. Security of the CDNI Metadata Interface ...................<a href="#page-19">19</a>
      <a href="#section-5.4">5.4</a>. Security of the CDNI Logging Interface ....................<a href="#page-19">19</a>
   <a href="#section-6">6</a>. Acknowledgements ...............................................<a href="#page-19">19</a>
   <a href="#section-7">7</a>. Informative References .........................................<a href="#page-20">20</a>
   <a href="#appendix-A">Appendix A</a>. Design Considerations for Realizing the CDNI
               Interfaces ............................................<a href="#page-22">22</a>
     <a href="#appendix-A.1">A.1</a>. CDNI Control Interface .....................................<a href="#page-22">22</a>
     <a href="#appendix-A.2">A.2</a>. CDNI Request Routing Interface .............................<a href="#page-23">23</a>
     <a href="#appendix-A.3">A.3</a>. CDNI Metadata Interface ....................................<a href="#page-25">25</a>
     <a href="#appendix-A.4">A.4</a>. CDNI Logging Interface .....................................<a href="#page-26">26</a>
   <a href="#appendix-B">Appendix B</a>. Additional Material ...................................<a href="#page-27">27</a>
     <a href="#appendix-B.1">B.1</a>. Non-Goals for IETF .........................................<a href="#page-27">27</a>
     B.2. Relationship to Relevant IETF Working Groups and IRTF
          Research Groups ............................................<a href="#page-29">29</a>
       <a href="#appendix-B.2.1">B.2.1</a>. ALTO WG ................................................<a href="#page-29">29</a>
       <a href="#appendix-B.2.2">B.2.2</a>. DECADE WG ..............................................<a href="#page-29">29</a>
       <a href="#appendix-B.2.3">B.2.3</a>. PPSP WG ................................................<a href="#page-31">31</a>
       <a href="#appendix-B.2.4">B.2.4</a>. IRTF P2P Research Group ................................<a href="#page-31">31</a>



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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   The volume of video and multimedia content delivered over the
   Internet is rapidly increasing and expected to continue doing so in
   the future.  In the face of this growth, Content Delivery Networks
   (CDNs) provide numerous benefits for cacheable content: reduced
   delivery cost, improved quality of experience for End Users (EUs),
   and increased robustness of delivery.  For these reasons, CDNs are
   frequently used for large-scale content delivery.  As a result,
   existing CDN Providers are scaling up their infrastructure, and many
   Network Service Providers (NSPs) are deploying their own CDNs.

   It is generally desirable that a given content item can be delivered
   to an EU regardless of that EU's location or the network they are
   attached to.  However, a given CDN in charge of delivering a given
   content may not have a footprint that expands close enough to the
   EU's current location or attachment network, or may not have the
   necessary resources, to realize the user experience and cost benefit
   that a more distributed CDN infrastructure would allow.  This is the
   motivation for interconnecting standalone CDNs so that their
   collective CDN footprint and resources can be leveraged for the
   end-to-end delivery of content from Content Service Providers (CSPs)
   to EUs.  As an example, a CSP could contract with an "authoritative"
   CDN Provider for the delivery of content, and that Authoritative CDN
   Provider could contract with one or more downstream CDN Providers to
   distribute and deliver some or all of the content on behalf of the
   Authoritative CDN Provider.

   A typical end-to-end content delivery scenario would then involve the
   following business arrangements:

   o  A business arrangement between the EU and his CSP, authorizing
      access by the EU to content items controlled by the CSP.

   o  A business arrangement between the CSP and an "authoritative" CDN
      Provider where the CSP mandates that the CDN Provider perform the
      content delivery on behalf of the CSP.

   o  A business arrangement between the Authoritative CDN Provider and
      another (or other) CDN(s) where the Authoritative CDN may delegate
      the actual serving of some of the content delivery requests to the
      other CDN(s).  A particular case is where this other CDN Provider
      happens to also be the Network Service Provider providing network
      access to the EU, in which case there is also a separate and
      independent business relationship between the EU and the NSP for
      the corresponding network access.





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   The formation and details of any business relationships between a CSP
   and a CDN Provider as well as between one CDN Provider and another
   CDN Provider are out of scope of this document.  However, this
   document concerns itself with the fact that no standards or open
   specifications currently exist to facilitate such CDN Interconnection
   from a technical perspective.

   One possible flow for performing an end-to-end content delivery
   across a CDN Interconnection is described below:

   o  The initial content request from an EU's User Agent is first
      received by the authoritative (upstream) CDN, which is the CDN
      with a business arrangement with the CSP.

   o  The authoritative (upstream) CDN may serve the request itself, or
      it may elect to use CDN Interconnection to redirect the request to
      a Downstream CDN that is in a better position to do so (e.g., a
      Downstream CDN that is "closer" to the EU).

   o  The EU's User Agent will "follow" the redirect returned by the
      Authoritative CDN and request the content from the Downstream CDN.
      If required, the Downstream CDN will acquire the requested content
      from the authoritative (upstream) CDN, and if necessary the
      Authoritative CDN will acquire the requested content from the
      Content Service Provider.

   The goal of this document is to outline the problem area of CDN
   Interconnection.  <a href="#section-2">Section 2</a> discusses the use cases for CDN
   Interconnection.  <a href="#section-3">Section 3</a> presents the CDNI model and problem area
   being considered by the IETF.  <a href="#section-4">Section 4</a> describes each CDNI
   interface individually and highlights example candidate protocols
   that could be considered for reuse or leveraging to implement the
   CDNI interfaces.  <a href="#appendix-B.2">Appendix B.2</a> describes the relationships between
   the CDNI problem space and other relevant IETF working groups and
   IRTF research groups.

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Terminology</span>

   This document uses the following terms:

   Authoritative CDN: A CDN that has a direct relationship with a CSP
   for the distribution and delivery of that CSP's content by the
   Authoritative CDN or by Downstream CDNs of the Authoritative CDN.

   CDN Interconnection (CDNI): A relationship between a pair of CDNs
   that enables one CDN to provide content delivery services on behalf
   of another CDN.  A CDN Interconnection may be wholly or partially
   realized through a set of interfaces over which a pair of CDNs



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   communicate with each other in order to achieve the delivery of
   content to User Agents by Surrogates in one CDN (the Downstream CDN)
   on behalf of another CDN (the Upstream CDN).

   CDN Provider: The service provider who operates a CDN and offers a
   service of content delivery, typically used by a Content Service
   Provider or another CDN Provider.  Note that a given entity may
   operate in more than one role.  For example, a company may
   simultaneously operate as a Content Service Provider, a Network
   Service Provider, and a CDN Provider.

   CDNI Metadata: The subset of Content Distribution Metadata that
   has an inter-CDN scope.  For example, CDNI Metadata may include
   geo-blocking information (i.e., information defining geographical
   areas where the content is to be made available or blocked),
   availability windows (i.e., information defining time windows during
   which the content is to be made available or blocked) and access
   control mechanisms to be enforced (e.g., URI signature validation).
   CDNI Metadata may also include information about desired distribution
   policy (e.g., pre-positioned vs dynamic acquisition) and about where/
   how a CDN can acquire the content.

   Content: Any form of digital data.  One important form of Content
   with additional constraints on distribution and delivery is
   continuous media (i.e., where there is a timing relationship between
   source and sink).

   Content Distribution Metadata: The subset of Content Metadata that is
   relevant to the distribution of the content.  This is the metadata
   required by a CDN in order to enable and control content distribution
   and delivery by the CDN.  In a CDN Interconnection environment, some
   of the Content Distribution Metadata may have an intra-CDN scope (and
   therefore need not be communicated between CDNs), while some of the
   Content Distribution Metadata may have an inter-CDN scope (and
   therefore needs to be communicated between CDNs).

   Content Distribution Network (CDN) / Content Delivery Network (CDN):
   Network infrastructure in which the network elements cooperate at
   Layers 4 through 7 for more effective delivery of Content to User
   Agents.  Typically, a CDN consists of a Request Routing system, a
   Distribution system (that includes a set of Surrogates), a Logging
   system, and a CDN Control system.









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   Content Metadata: This is metadata about Content.  Content Metadata
   comprises:

   1.  Metadata that is relevant to the distribution of the content (and
       therefore relevant to a CDN involved in the delivery of that
       content).  We refer to this type of metadata as "Content
       Distribution Metadata".  See also the definition of Content
       Distribution Metadata.

   2.  Metadata that is associated with the actual Content or content
       representation, and not directly relevant to the distribution of
       that Content.  For example, such metadata may include information
       pertaining to the Content's genre, cast, rating, etc. as well as
       information pertaining to the Content representation's
       resolution, aspect ratio, etc.

   Content Service: The service offered by a Content Service Provider.
   The Content Service encompasses the complete service, which may be
   wider than just providing access to items of Content; e.g., the
   Content Service also includes any middleware, key distribution,
   program guide, etc. that may not require any direct interaction with
   the CDN, or CDNs, involved in the distribution and delivery of the
   content.

   Content Service Provider (CSP): Provides a Content Service to End
   Users (which they access via a User Agent).  A CSP may own the
   Content made available as part of the Content Service, or may license
   content rights from another party.

   Control system: The function within a CDN responsible for
   bootstrapping and controlling the other components of the CDN as well
   as for handling interactions with external systems (e.g., handling
   delivery service creation/update/removal requests, or specific
   service provisioning requests).

   Delivery: The function within CDN Surrogates responsible for
   delivering a piece of content to the User Agent.  For example,
   delivery may be based on HTTP progressive download or HTTP adaptive
   streaming.

   Distribution system: The function within a CDN responsible for
   distributing Content Distribution Metadata as well as the Content
   itself inside the CDN (e.g., down to the Surrogates).

   Downstream CDN: For a given End User request, the CDN (within a pair
   of directly interconnected CDNs) to which the request is redirected
   by the other CDN (the Upstream CDN).  Note that in the case of
   successive redirections (e.g., CDN1--&gt;CDN2--&gt;CDN3), a given CDN



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   (e.g., CDN2) may act as the Downstream CDN for a redirection (e.g.,
   CDN1--&gt;CDN2) and as the Upstream CDN for the subsequent redirection
   of the same request (e.g., CDN2--&gt;CDN3).

   Dynamic CDNI Metadata acquisition: In the context of CDN
   Interconnection, dynamic CDNI Metadata acquisition means that a
   Downstream CDN acquires CDNI Metadata for content from the Upstream
   CDN at some point in time after a request for that content is
   delegated to the Downstream CDN by an Upstream CDN (and that specific
   CDNI Metadata is not yet available in the Downstream CDN).  See also
   the definitions for Downstream CDN and Upstream CDN.

   Dynamic content acquisition: Dynamic content acquisition is where a
   CDN acquires content from the content source in response to an End
   User requesting that content from the CDN.  In the context of CDN
   Interconnection, dynamic acquisition means that a Downstream CDN
   acquires the content from content sources (including Upstream CDNs)
   at some point in time after a request for that content is delegated
   to the Downstream CDN by an Upstream CDN (and that specific content
   is not yet available in the Downstream CDN).

   End User (EU): The 'real' user of the system, typically a human but
   maybe some combination of hardware and/or software emulating a human
   (e.g., for automated quality monitoring etc.).

   Logging system: The function within a CDN responsible for collecting
   the measurement and recording of distribution and delivery
   activities.  The information recorded by the Logging system may be
   used for various purposes, including charging (e.g., of the CSP),
   analytics, and monitoring.

   Metadata: Metadata in general is data about data.

   Network Service Provider (NSP): Provides network-based connectivity/
   services to End Users.

   Over-the-top (OTT): A service, e.g., content delivery using a CDN,
   operated by a different operator than the NSP to which the users of
   that service are attached.

   Pre-positioned CDNI Metadata acquisition: In the context of CDN
   Interconnection, CDNI Metadata pre-positioning is where the
   Downstream CDN acquires CDNI Metadata for content prior to, or
   independently of, any End User requesting that content from the
   Downstream CDN.






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   Pre-positioned content acquisition: Content pre-positioning is where
   a CDN acquires content from the content source prior to, or
   independently of, any End User requesting that content from the CDN.
   In the context of CDN Interconnection, the Upstream CDN instructs the
   Downstream CDN to acquire the content from content sources (including
   Upstream CDNs) in advance of, or independently of, any End User
   requesting it.

   Quality of Experience (QoE): As defined in <a href="./rfc6390#section-2.4">Section&nbsp;2.4 of [RFC6390]</a>.

   Request Routing system: The function within a CDN responsible for
   receiving a Content Request from a User Agent, obtaining and
   maintaining necessary information about a set of candidate Surrogates
   or candidate CDNs, and for selecting and redirecting the user to the
   appropriate Surrogate or CDN.  To enable CDN Interconnection, the
   Request Routing system must also be capable of handling User Agent
   Content Requests passed to it by another CDN.

   Surrogate: A device/function (often called a cache) that interacts
   with other elements of the CDN for the control and distribution of
   Content within the CDN and interacts with User Agents for the
   delivery of the Content.  Typically, Surrogates will cache requested
   content so that they can directly deliver the same content in
   response to requests from multiple User Agents (and their End Users),
   avoiding the need for the content to transit multiple times through
   the network core (i.e., from the content origin to the Surrogate).

   Upstream CDN: For a given End User request, the CDN (within a pair of
   directly interconnected CDNs) that redirects the request to the
   other CDN.

   User Agent (UA): Software (or a combination of hardware and software)
   through which the End User interacts with a Content Service.  The
   User Agent will communicate with a Content Service for the selection
   of content and one or more CDNs for the delivery of the Content.
   Such communication is not restricted to HTTP and may be via a variety
   of protocols.  Examples of User Agents (non-exhaustive) are browsers,
   Set Top Boxes (STBs), dedicated content applications (e.g., media
   players), etc.












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<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  CDN Background</span>

   Readers are assumed to be familiar with the architecture, features,
   and operation of CDNs.  For readers less familiar with the operation
   of CDNs, the following resources may be useful:

   o  <a href="./rfc3040">RFC 3040</a> [<a href="./rfc3040" title="&quot;Internet Web Replication and Caching Taxonomy&quot;">RFC3040</a>] describes many of the component technologies
      that are used in the construction of a CDN.

   o  Taxonomy [<a href="#ref-TAXONOMY" title="&quot;A Taxonomy and Survey of Content Delivery Networks&quot;">TAXONOMY</a>] compares the architecture of a number of CDNs.

   o  <a href="./rfc3466">RFC 3466</a> [<a href="./rfc3466" title="&quot;A Model for Content Internetworking (CDI)&quot;">RFC3466</a>] and <a href="./rfc3570">RFC 3570</a> [<a href="./rfc3570" title="&quot;Content Internetworking (CDI) Scenarios&quot;">RFC3570</a>] are the output of the
      IETF Content Distribution Internetworking (CDI) working group,
      which was closed in 2003.

   Note: Some of the terms used in this document are similar to terms
   used in the above referenced documents.  When reading this document,
   terms should be interpreted as having the definitions provided in
   <a href="#section-1.1">Section 1.1</a>.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  CDN Interconnection Use Cases</span>

   An increasing number of NSPs are deploying CDNs in order to deal
   cost-effectively with the growing usage of on-demand video services
   and other content delivery applications.

   CDNs allow caching of content closer to the edge of a network so that
   a given item of content can be delivered by a CDN Surrogate (i.e., a
   cache) to multiple User Agents (and their End Users) without
   transiting multiple times through the network core (i.e., from the
   content origin to the Surrogate).  This contributes to bandwidth cost
   reductions for the NSP and to improved quality of experience for the
   End Users.  CDNs also enable replication of popular content across
   many Surrogates, which enables content to be served to large numbers
   of User Agents concurrently.  This also helps in dealing with
   situations such as flash crowds and denial-of-service attacks.

   The CDNs deployed by NSPs are not just restricted to the delivery of
   content to support the Network Service Provider's own 'walled garden'
   services, such as IP delivery of television services to Set Top
   Boxes, but are also used for delivery of content to other devices,
   including PCs, tablets, mobile phones, etc.

   Some service providers operate over multiple geographies and federate
   multiple affiliate NSPs.  These NSPs typically operate independent
   CDNs.  As they evolve their services (e.g., for seamless support of
   content services to nomadic users across affiliate NSPs), there is a




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   need for interconnection of these CDNs; this represents a first use
   case for CDNI.  However, there are no open specifications, nor common
   best practices, defining how to achieve such CDN Interconnection.

   CSPs have a desire to be able to get (some of) their content to very
   large numbers of End Users, who are often distributed across a number
   of geographies, while maintaining a high quality of experience, all
   without having to maintain direct business relationships with many
   different CDN Providers (or having to extend their own CDN to a large
   number of locations).  Some NSPs are considering interconnecting
   their respective CDNs (as well as possibly over-the-top CDNs) so that
   this collective infrastructure can address the requirements of CSPs
   in a cost-effective manner.  This represents a second use case for
   CDNI.  In particular, this would enable the CSPs to benefit from
   on-net delivery (i.e., within the Network Service Provider's own
   network/CDN footprint) whenever possible and off-net delivery
   otherwise, without requiring the CSPs to maintain direct business
   relationships with all the CDNs involved in the delivery.  Again, CDN
   Providers (NSPs or over-the-top CDN operators) are faced with a lack
   of open specifications and best practices.

   NSPs have often deployed CDNs as specialized cost-reduction projects
   within the context of a particular service or environment.  Some NSPs
   operate separate CDNs for separate services.  For example, there may
   be a CDN for managed IPTV service delivery, a CDN for web-TV
   delivery, and a CDN for video delivery to mobile terminals.  As NSPs
   integrate their service portfolio, there is a need for
   interconnecting these CDNs, representing a third use case for CDNI.
   Again, NSPs face the problem of lack of open interfaces for CDN
   Interconnection.

   For operational reasons (e.g., disaster, flash crowd) or commercial
   reasons, an over-the-top CDN may elect to make use of another CDN
   (e.g., an NSP CDN with on-net Surrogates for a given footprint) for
   serving a subset of the user requests (e.g., requests from users
   attached to that NSP), which results in a fourth use case for CDNI
   because CDN Providers (over-the-top CDN Providers or NSPs) are faced
   with a lack of open specifications and best practices.

   Use cases for CDN Interconnection are further discussed in
   [<a href="#ref-CDNI-USE-CASES">CDNI-USE-CASES</a>].










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<span class="grey"><a href="./rfc6707">RFC 6707</a>          CDN Interconnection Problem Statement   September 2012</span>


<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  CDN Interconnection Model and Problem Area for IETF</span>

   This section discusses the problem area for the IETF work on CDN
   Interconnection.

   Interconnecting CDNs involves interactions among multiple different
   functions and components that form each CDN.  Only some of those
   require additional specification by the IETF.

   Some NSPs have started to perform experiments to explore whether
   their CDN use cases can already be addressed with existing CDN
   implementations.  One set of such experiments is documented in
   [<a href="#ref-CDNI-EXPERIMENTS">CDNI-EXPERIMENTS</a>].  The conclusions of those experiments are that
   while some basic limited CDN Interconnection functionality can be
   achieved with existing CDN technology, the current lack of any
   standardized CDNI interfaces with the necessary level of
   functionality such as those discussed in this document is preventing
   the deployment of CDN Interconnection.

   Listed below are the four interfaces required to interconnect a pair
   of CDNs and that constitute the problem space of CDN Interconnection
   along with the required functionality of each interface for which
   standards do not currently exist.  As part of the development of the
   CDNI interfaces, it will also be necessary to agree on common
   mechanisms for how to identify and name the data objects that are to
   be interchanged between interconnected CDNs.

   The use of the term "interface" is meant to encompass the protocol
   over which CDNI data representations (e.g., CDNI Metadata objects)
   are exchanged as well as the specification of the data
   representations themselves (i.e., what properties/fields each object
   contains, its structure, etc.).

   o  CDNI Control interface: This interface allows the "CDNI Control"
      system in interconnected CDNs to communicate.  This interface may
      support the following:

      *  Allow bootstrapping of the other CDNI interfaces (e.g.,
         interface address/URL discovery and establishment of security
         associations).

      *  Allow configuration of the other CDNI interfaces (e.g.,
         Upstream CDN specifies information to be reported through the
         CDNI Logging interface).

      *  Allow the Downstream CDN to communicate static (or fairly
         static) information about its delivery capabilities and
         policies.



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      *  Allow bootstrapping of the interface between CDNs for content
         acquisition (even if that interface itself is outside the scope
         of the CDNI work).

      *  Allow an Upstream CDN to initiate or request specific actions
         to be undertaken in the Downstream CDN.  For example, to allow
         an Upstream CDN to initiate content or CDNI Metadata
         acquisition (pre-positioning) or to request the invalidation
         or purging of content files and/or CDNI Metadata in a
         Downstream CDN.

   o  CDNI Request Routing interface: This interface allows the Request
      Routing systems in interconnected CDNs to communicate to ensure
      that an End User request can be (re)directed from an Upstream CDN
      to a Surrogate in the Downstream CDN, in particular where
      selection responsibilities may be split across CDNs (for example,
      the Upstream CDN may be responsible for selecting the Downstream
      CDN, while the Downstream CDN may be responsible for selecting the
      actual Surrogate within that Downstream CDN).  In particular, the
      functions of the CDN Request Routing interface may be divided as
      follows:

      *  A CDNI Request Routing Redirection interface, which allows the
         Upstream CDN to query the Downstream CDN at request routing
         time before redirecting the request to the Downstream CDN.

      *  A CDNI Footprint &amp; Capabilities advertisement interface, which
         allows the Downstream CDN to provide to the Upstream CDN
         (static or dynamic) information (e.g., resources, footprint,
         load) to facilitate selection of the Downstream CDN by the
         Upstream CDN Request Routing system when processing subsequent
         Content Requests from User Agents.

   o  CDNI Metadata interface: This interface allows the Distribution
      system in interconnected CDNs to communicate to ensure that CDNI
      Metadata can be exchanged across CDNs.  See <a href="#section-1.1">Section 1.1</a> for the
      definition and examples of CDNI Metadata.

   o  CDNI Logging interface: This interface allows the Logging system
      in interconnected CDNs to communicate the relevant activity logs
      in order to allow log-consuming applications to operate in a
      multi-CDN environment.  For example, an Upstream CDN may collect
      delivery logs from a Downstream CDN in order to perform
      consolidated charging of the CSP or for settlement purposes across
      CDNs.  Similarly, an Upstream CDN may collect delivery logs from a
      Downstream CDN in order to provide consolidated reporting and
      monitoring to the CSP.




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   Note that the actual grouping of functionalities under these four
   interfaces is considered tentative at this stage and may be changed
   after further study (e.g., some subset of functionality may be moved
   from one interface into another).

   The above list covers a significant potential problem space, in part
   because in order to interconnect two CDNs there are several 'touch
   points' that require standardization.  However, it is expected that
   the CDNI interfaces need not be defined from scratch and instead can
   reuse or leverage existing protocols to a very significant extent;
   this is discussed further in <a href="#section-4">Section 4</a>.

   The interfaces that form the CDNI problem area are illustrated in
   Figure 1.





































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     --------
    /        \
    |   CSP  |
    \        /
     --------
         *
         *
         *                         /\
         *                        /  \
     ----------------------      |CDNI|        ----------------------
    /     Upstream CDN     \     |    |       /    Downstream CDN    \
    |      +-------------+ | Control Interface| +-------------+      |
    |*******   Control   |&lt;======|====|========&gt;|   Control   *******|
    |*     +------*----*-+ |     |    |       | +-*----*------+     *|
    |*            *    *   |     |    |       |   *    *            *|
    |*     +------*------+ | Logging Interface| +------*------+     *|
    |* *****   Logging   |&lt;======|====|========&gt;|   Logging   ***** *|
    |* *   +-*-----------+ |     |    |       | +-----------*-+   * *|
    |* *     *         *   | Request Routing  |   *         *     * *|
  .....*...+-*---------*-+ |    Interface     | +-*---------*-+...*.*...
  . |* * *** Req-Routing |&lt;======|====|========&gt;| Req-Routing *** * *| .
  . |* * * +-------------+.|     |    |       | +-------------+ * * *| .
  . |* * *                 .  CDNI Metadata   |                 * * *| .
  . |* * * +-------------+ |.   Interface     | +-------------+ * * *| .
  . |* * * | Distribution|&lt;==.===|====|========&gt;| Distribution| * * *| .
  . |* * * |             | |  .   \  /        | |             | * * *| .
  . |* * * |+---------+  | |   .   \/         | |  +---------+| * * *| .
  . |* * ***| +---------+| |    ....Request......+---------+ |*** * *| .
  . |* *****+-|Surrogate|************************|Surrogate|-+***** *| .
  . |*******  +---------+| |   Acquisition    | |+----------+ *******| .
  . |      +-------------+ |                  | +-------*-----+      | .
  . \                      /                  \         *            / .
  .  ----------------------                    ---------*------------  .
  .                                                     *              .
  .                                                     * Delivery     .
  .                                                     *              .
  .                                                  +--*---+          .
  ...............Request.............................| User |..Request..
                                                     | Agent|
                                                     +------+

  &lt;==&gt;  interfaces inside the scope of CDNI
  ****  interfaces outside the scope of CDNI
  ....  interfaces outside the scope of CDNI

                Figure 1: A Model for the CDNI Problem Area





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   As illustrated in Figure 1, the acquisition of content between
   interconnected CDNs is out of scope for CDNI; this deserves some
   additional explanation.  The consequence of such a decision is that
   the CDNI problem space described in this document is focused on only
   defining the control plane for CDNI, and the CDNI data plane (i.e.,
   the acquisition and distribution of the actual content objects) is
   out of scope.  The rationale for such a decision is that CDNs today
   typically already use standardized protocols such as HTTP, FTP,
   rsync, etc. to acquire content from their CSP customers, and it is
   expected that the same protocols could be used for acquisition
   between interconnected CDNs.  Therefore, the problem of content
   acquisition is considered already solved, and all that is required
   with respect to content acquisition from specifications developed by
   the CDNI working group is to describe within the CDNI Metadata the
   parameters to use to retrieve the content -- for example, the IP
   address/hostname to connect to, what protocol to use to retrieve the
   content, etc.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Scoping the CDNI Problem</span>

   This section outlines how the scope of work addressing the CDNI
   problem space can be constrained through reuse or leveraging of
   existing protocols to implement the CDNI interfaces.  This discussion
   is not intended to preempt any working group decision as to the most
   appropriate protocols, technologies, and solutions to select to
   realize the CDNI interfaces but is intended as an illustration of the
   fact that the CDNI interfaces need not be created in a vacuum and
   that reuse or leverage of existing protocols is likely possible.

   The four CDNI interfaces (CDNI Control interface, CDNI Request
   Routing interface, CDNI Metadata interface, and CDNI Logging
   interface) described in <a href="#section-3">Section 3</a> within the CDNI problem area are
   all control plane interfaces operating at the application layer
   (Layer 7 in the OSI network model).  Firstly, since it is not
   expected that these interfaces would exhibit unique session,
   transport, or network requirements as compared to the many other
   existing applications in the Internet, it is expected that the CDNI
   interfaces will be defined on top of existing session, transport, and
   network protocols.

   Secondly, although a new application protocol could be designed
   specifically for CDNI, our analysis below shows that this is
   unnecessary, and it is recommended that existing application
   protocols be reused or leveraged (HTTP [<a href="./rfc2616" title="&quot;Hypertext Transfer Protocol -- HTTP/1.1&quot;">RFC2616</a>], the Atom Publishing
   Protocol [<a href="./rfc5023" title="&quot;The Atom Publishing Protocol&quot;">RFC5023</a>], the Extensible Messaging and Presence Protocol
   (XMPP) [<a href="./rfc6120" title="&quot;Extensible Messaging and Presence Protocol (XMPP): Core&quot;">RFC6120</a>], for example) to realize the CDNI interfaces.





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<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  CDNI Control Interface</span>

   The CDNI Control interface allows the Control system in
   interconnected CDNs to communicate.  The exact inter-CDN control
   functionality required to be supported by the CDNI Control interface
   is less well defined than the other three CDNI interfaces at this
   time.

   It is expected that for the Control interface, as for the other CDNI
   interfaces, existing protocols can be reused or leveraged.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  CDNI Request Routing Interface</span>

   The CDNI Request Routing interface enables a Request Routing function
   in an Upstream CDN to query a Request Routing function in a
   Downstream CDN to determine if the Downstream CDN is able (and
   willing) to accept the delegated Content Request.  It also allows the
   Downstream CDN to control what should be returned to the User Agent
   in the redirection message by the upstream Request Routing function.

   The CDNI Request Routing interface is therefore a fairly
   straightforward request/response interface and could be implemented
   over any number of request/response protocols.  For example, it may
   be implemented as a WebService using one of the common WebServices
   methodologies (Extensible Markup Language-Remote Procedure Calling
   (XML-RPC), HTTP query to a known URI, etc.).  This removes the need
   for the CDNI working group to define a new protocol for the request/
   response element of the CDNI Request Routing interface.

   Additionally, as discussed in <a href="#section-3">Section 3</a>, the CDNI Request Routing
   interface is also expected to enable a Downstream CDN to provide to
   the Upstream CDN (static or dynamic) information (e.g., resources,
   footprint, load) to facilitate selection of the Downstream CDN by the
   Upstream CDN Request Routing system when processing subsequent
   Content Requests from User Agents.  It is expected that such
   functionality of the CDNI request routing could be specified by the
   CDNI working group with significant leveraging of existing IETF
   protocols supporting the dynamic distribution of reachability
   information (for example, by leveraging existing routing protocols)
   or supporting application-level queries for topological information
   (for example, by leveraging Application-Layer Traffic Optimization
   (ALTO) [<a href="./rfc5693" title="&quot;Application-Layer Traffic Optimization (ALTO) Problem Statement&quot;">RFC5693</a>]).









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<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  CDNI Metadata Interface</span>

   The CDNI Metadata interface enables the Distribution system in a
   Downstream CDN to request CDNI Metadata from an Upstream CDN so that
   the Downstream CDN can properly process and respond to redirection
   requests received over the CDNI Request Routing interface and Content
   Requests received directly from User Agents.

   The CDNI Metadata interface is therefore similar to the CDNI Request
   Routing interface because it is a request/response interface with the
   potential addition that CDNI Metadata search may have more complex
   semantics than a straightforward Request Routing redirection request.
   Therefore, like the CDNI Request Routing interface, the CDNI Metadata
   interface may be implemented as a WebService using one of the common
   WebServices methodologies (XML-RPC, HTTP query to a known URI, etc.)
   or possibly using other existing protocols such as XMPP [<a href="./rfc6120" title="&quot;Extensible Messaging and Presence Protocol (XMPP): Core&quot;">RFC6120</a>].
   This removes the need for the CDNI working group to define a new
   protocol for the request/response element of the CDNI Metadata
   interface.

<span class="h3"><a class="selflink" id="section-4.4" href="#section-4.4">4.4</a>.  CDNI Logging Interface</span>

   The CDNI Logging interface enables details of content distribution
   and delivery activities to be exchanged between interconnected CDNs
   -- for example, the exchange of log records related to the delivery
   of content, similar to the log records recorded in a web server's
   access log.

   Several protocols already exist that could potentially be used to
   exchange CDNI logs between interconnected CDNs, including the Simple
   Network Management Protocol (SNMP), syslog, FTP (and secure
   variants), HTTP POST, etc.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Security Considerations</span>

   Distribution of content by a CDN comes with a range of security
   considerations, such as how to enforce control of access to the
   content by End Users in line with the CSP policy, or how to trust the
   logging information generated by the CDN for the purposes of charging
   the CSP.  These security aspects are already dealt with by CDN
   Providers and CSPs today in the context of standalone CDNs.  However,
   interconnection of CDNs introduces a new set of security
   considerations by extending the trust model to a chain of trust
   (i.e., the CSP "trusts" a CDN that "trusts" another CDN).  The
   mechanisms used to mitigate these risks in multi-CDN environments may
   be similar to those used in the single-CDN case, but their
   suitability in this more complex environment must be validated.




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   The interconnection of CDNs may also introduce additional privacy
   considerations on top of those that apply to the single-CDN case.  In
   a multi-CDN environment, the different CDNs may reside in different
   legal regimes that require differing privacy requirements to be
   enforced.  Such privacy requirements may impact the granularity of
   information that can be exchanged across the CDNI interfaces.  For
   example, the Logging system in a Downstream CDN may need to apply
   some degree of anonymization, obfuscation, or even the complete
   removal of some fields before exchanging log records containing
   details of End User deliveries with an Upstream CDN.

   Maintaining the security of the content itself, its associated
   metadata (including delivery policies), and the CDNs distributing and
   delivering it, are critical requirements for both CDN Providers and
   CSPs, and the CDN Interconnection interfaces must provide sufficient
   mechanisms to maintain the security of the overall system of
   interconnected CDNs as well as the information (content, metadata,
   logs, etc.) distributed and delivered through any set of
   interconnected CDNs.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Security of the CDNI Control Interface</span>

   Information exchanged between interconnected CDNs over this interface
   is of a sensitive nature.  A pair of CDNs use this interface to allow
   bootstrapping of all the other CDNI interfaces, possibly including
   establishment of the mechanisms for securing these interfaces.
   Therefore, corruption of that interface may result in corruption of
   all other interfaces.  Using this interface, an Upstream CDN may
   pre-position or delete content or metadata in a Downstream CDN, a
   Downstream CDN may provide administrative information to an Upstream
   CDN, etc.  All of these operations require that the peer CDNs are
   appropriately authenticated and that the confidentiality and
   integrity of information flowing between them can be ensured.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Security of the CDNI Request Routing Interface</span>

   Appropriate levels of authentication and confidentiality must be used
   in this interface because it allows an Upstream CDN to query the
   Downstream CDN in order to redirect requests, and conversely, allows
   the Downstream CDN to influence the Upstream CDN's Request Routing
   function.

   In the absence of appropriate security on this interface, a rogue
   Upstream CDN could inundate Downstream CDNs with bogus requests or
   have the Downstream CDN send the rogue Upstream CDN private
   information.  Also, a rogue Downstream CDN could influence the





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   Upstream CDN so the Upstream CDN redirects requests to the rogue
   Downstream CDN or another Downstream CDN in order to, for example,
   attract additional delivery revenue.

<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>.  Security of the CDNI Metadata Interface</span>

   This interface allows a Downstream CDN to request CDNI Metadata from
   an Upstream CDN, and therefore the Upstream CDN must ensure that the
   former is appropriately authenticated before sending the data.
   Conversely, a Downstream CDN must authenticate an Upstream CDN before
   requesting metadata to insulate itself from poisoning by rogue
   Upstream CDNs.  The confidentiality and integrity of the information
   exchanged between the peers must be protected.

<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>.  Security of the CDNI Logging Interface</span>

   Logging data consists of potentially sensitive information (which End
   User accessed which media resource, IP addresses of End Users,
   potential names and subscriber account information, etc.).
   Confidentiality of this information must be protected as log records
   are moved between CDNs.  This information may also be sensitive from
   the viewpoint that it can be the basis for charging across CDNs.
   Therefore, appropriate levels of protection are needed against
   corruption, duplication, and loss of this information.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Acknowledgements</span>

   The authors would like to thank Andre Beck, Gilles Bertrand, Mark
   Carlson, Bruce Davie, David Ferguson, Yiu Lee, Kent Leung, Will Li,
   Kevin Ma, Julien Maisonneuve, Guy Meador, Larry Peterson, Emile
   Stephan, Oskar van Deventer, Mahesh Viveganandhan, and Richard Woundy
   for their review comments and contributions to the text.



















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<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Informative References</span>

   [<a id="ref-ALTO-CDN-USE-CASES">ALTO-CDN-USE-CASES</a>]
              Niven-Jenkins, B., Ed., Watson, G., Bitar, N., Medved, J.,
              and S. Previdi, "Use Cases for ALTO within CDNs", Work
              in Progress, June 2012.

   [<a id="ref-ALTO-Charter">ALTO-Charter</a>]
              "IETF ALTO WG Charter",
              &lt;<a href="http://datatracker.ietf.org/wg/alto/charter/">http://datatracker.ietf.org/wg/alto/charter/</a>&gt;.

   [<a id="ref-CDNI-EXPERIMENTS">CDNI-EXPERIMENTS</a>]
              Bertrand, G., Ed., Le Faucheur, F., and L. Peterson,
              "Content Distribution Network Interconnection (CDNI)
              Experiments", Work in Progress, February 2012.

   [<a id="ref-CDNI-USE-CASES">CDNI-USE-CASES</a>]
              Bertrand, G., Ed., Emile, S., Burbridge, T., Eardley, P.,
              Ma, K., and G. Watson, "Use Cases for Content Delivery
              Network Interconnection", Work in Progress, August 2012.

   [<a id="ref-DECADE-Charter">DECADE-Charter</a>]
              "IETF DECADE WG Charter",
              &lt;<a href="http://datatracker.ietf.org/wg/decade/charter/">http://datatracker.ietf.org/wg/decade/charter/</a>&gt;.

   [<a id="ref-P2PRG-CDNI">P2PRG-CDNI</a>]
              Davie, B. and F. Le Faucheur, "Interconnecting CDNs aka
              'Peering Peer-to-Peer'", March 2010,
              &lt;<a href="http://www.ietf.org/proceedings/77/slides/P2PRG-2.pdf">http://www.ietf.org/proceedings/77/slides/P2PRG-2.pdf</a>&gt;.

   [<a id="ref-PPSP-Charter">PPSP-Charter</a>]
              "IETF PPSP WG Charter",
              &lt;<a href="http://datatracker.ietf.org/wg/ppsp/charter/">http://datatracker.ietf.org/wg/ppsp/charter/</a>&gt;.

   [<a id="ref-RFC2616">RFC2616</a>]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", <a href="./rfc2616">RFC 2616</a>, June 1999.

   [<a id="ref-RFC3040">RFC3040</a>]  Cooper, I., Melve, I., and G. Tomlinson, "Internet Web
              Replication and Caching Taxonomy", <a href="./rfc3040">RFC 3040</a>, January 2001.

   [<a id="ref-RFC3466">RFC3466</a>]  Day, M., Cain, B., Tomlinson, G., and P. Rzewski, "A Model
              for Content Internetworking (CDI)", <a href="./rfc3466">RFC 3466</a>,
              February 2003.

   [<a id="ref-RFC3570">RFC3570</a>]  Rzewski, P., Day, M., and D. Gilletti, "Content
              Internetworking (CDI) Scenarios", <a href="./rfc3570">RFC 3570</a>, July 2003.




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   [<a id="ref-RFC5023">RFC5023</a>]  Gregorio, J., Ed., and B. de hOra, Ed., "The Atom
              Publishing Protocol", <a href="./rfc5023">RFC 5023</a>, October 2007.

   [<a id="ref-RFC5693">RFC5693</a>]  Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement", <a href="./rfc5693">RFC 5693</a>,
              October 2009.

   [<a id="ref-RFC6120">RFC6120</a>]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", <a href="./rfc6120">RFC 6120</a>, March 2011.

   [<a id="ref-RFC6390">RFC6390</a>]  Clark, A. and B. Claise, "Guidelines for Considering New
              Performance Metric Development", <a href="https://www.rfc-editor.org/bcp/bcp170">BCP 170</a>, <a href="./rfc6390">RFC 6390</a>,
              October 2011.

   [<a id="ref-TAXONOMY">TAXONOMY</a>] Pathan, A. and R. Buyya, "A Taxonomy and Survey of Content
              Delivery Networks", 2007,
              &lt;<a href="http://www.cloudbus.org/reports/CDN-Taxonomy.pdf">http://www.cloudbus.org/reports/CDN-Taxonomy.pdf</a>&gt;.


































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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Design Considerations for Realizing the CDNI Interfaces</span>

   This section expands on how CDNI interfaces can reuse and leverage
   existing protocols before describing each CDNI interface individually
   and highlighting example candidate protocols that could be considered
   for reuse or leveraging to implement the CDNI interfaces.  However,
   the options discussed here are purely examples and do not present any
   consensus on protocols to be used later on.

<span class="h3"><a class="selflink" id="appendix-A.1" href="#appendix-A.1">A.1</a>.  CDNI Control Interface</span>

   The CDNI Control interface allows the Control system in
   interconnected CDNs to communicate.  The exact inter-CDN control
   functionality required to be supported by the CDNI Control interface
   is less well defined than the other three CDNI interfaces at this
   time.

   However, as discussed in <a href="#section-3">Section 3</a>, the CDNI Control interface may be
   required to support functionality similar to the following:

   o  Allow an Upstream CDN and Downstream CDN to establish, update, or
      terminate their CDNI interconnection.

   o  Allow bootstrapping of the other CDNI interfaces (e.g., protocol
      address discovery and establishment of security associations).

   o  Allow configuration of the other CDNI interfaces (e.g., Upstream
      CDN specifies information to be reported through the CDNI Logging
      interface).

   o  Allow the Downstream CDN to communicate static information about
      its delivery capabilities, resources, and policies.

   o  Allow bootstrapping of the interface between CDNs for content
      acquisition (even if that interface itself is outside the scope of
      the CDNI work).

   It is expected that for the Control interface, as for the other CDNI
   interfaces, existing protocols can be reused or leveraged.  Those
   will be considered once the requirements for the Control interface
   have been refined.










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<span class="h3"><a class="selflink" id="appendix-A.2" href="#appendix-A.2">A.2</a>.  CDNI Request Routing Interface</span>

   The CDNI Request Routing interface enables a Request Routing function
   in an Upstream CDN to query a Request Routing function in a
   Downstream CDN to determine if the Downstream CDN is able (and
   willing) to accept the delegated Content Request and to allow the
   Downstream CDN to control what the upstream Request Routing function
   should return to the User Agent in the redirection message.

   Therefore, the CDNI Request Routing interface needs to offer a
   mechanism for an Upstream CDN to issue a "Redirection Request" to a
   Downstream CDN.  The Request Routing interface needs to be able to
   support scenarios where the initial User Agent request to the
   Upstream CDN is received over DNS as well as over a content-specific
   application protocol (e.g., HTTP, the Real Time Streaming Protocol
   (RTSP), the Real Time Messaging Protocol (RTMP), etc.).

   Therefore, a Redirection Request is expected to contain information
   such as:

   o  The protocol (e.g., DNS, HTTP) over which the Upstream CDN
      received the initial User Agent request.

   o  Additional details of the User Agent request that are required to
      perform effective Request Routing by the Downstream CDN.  For DNS,
      this would typically be the IP address of the DNS resolver making
      the request on behalf of the User Agent.  For requests received
      over content-specific application protocols, the Redirection
      Request could contain significantly more information related to
      the original User Agent request but at a minimum is expected to
      include the User Agent's IP address, the equivalent of the HTTP
      Host header, and the equivalent of the HTTP abs_path as defined in
      [<a href="./rfc2616" title="&quot;Hypertext Transfer Protocol -- HTTP/1.1&quot;">RFC2616</a>].

   It should be noted that the CDNI architecture needs to consider that
   a Downstream CDN may receive requests from User Agents without first
   receiving a Redirection Request from an Upstream CDN for the
   corresponding User Agent request because, for example:

   o  User Agents (or DNS resolvers) may cache DNS or application
      responses from Request Routers.

   o  Responses to Redirection Requests over the Request Routing
      interface may be cacheable.







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   o  Some CDNs may rely on simple coarse policies, e.g., CDN B agrees
      to always serve CDN A's delegated redirection requests, in which
      case the necessary redirection details are exchanged out of band
      (of the CDNI interfaces), e.g., configured.

   On receiving a Redirection Request, the Downstream CDN will use the
   information provided in the request to determine if it is able (and
   willing) to accept the delegated Content Request and needs to return
   the result of its decision to the Upstream CDN.

   Thus, a Redirection Response from the Downstream CDN is expected to
   contain information such as:

   o  Status code indicating acceptance or rejection (possibly with
      accompanying reasons).

   o  Information to allow redirection by the Upstream CDN.  In the case
      of DNS-based request routing, this is expected to include the
      equivalent of a DNS record(s) (e.g., a CNAME) that the Upstream
      CDN should return to the requesting DNS resolver.  In the case of
      application-based request routing, this is expected to include the
      information necessary to construct the application-specific
      redirection response(s) to return to the requesting User Agent.
      For HTTP requests from User Agents, this could include a URI that
      the Upstream CDN could return in an HTTP 3xx response.

   The CDNI Request Routing interface is therefore a fairly
   straightforward request/response interface and could be implemented
   over any number of request/response protocols.  For example, it may
   be implemented as a WebService using one of the common WebServices
   methodologies (XML-RPC, HTTP query to a known URI, etc.).  This
   removes the need for the CDNI working group to define a new protocol
   for the request/response element of the CDNI Request Routing
   interface.  Thus, the CDNI working group would be left only with the
   task of specifying:

   o  The recommended request/response protocol to use along with any
      additional semantics and procedures that are specific to the CDNI
      Request Routing interface (e.g., handling of malformed requests/
      responses).

   o  The syntax (i.e., representation/encoding) of the redirection
      requests and responses.

   o  The semantics (i.e., meaning and expected contents) of the
      redirection requests and responses.





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   Additionally, as discussed in <a href="#section-3">Section 3</a>, the CDNI Request Routing
   interface is also expected to enable a Downstream CDN to provide to
   the Upstream CDN (static or dynamic) information (e.g., resources,
   footprint, load) to facilitate selection of the Downstream CDN by the
   Upstream CDN Request Routing system when processing subsequent
   Content Requests from User Agents.  It is expected that such
   functionality of the CDNI request routing could be specified by the
   CDNI working group with significant leveraging of existing IETF
   protocols supporting the dynamic distribution of reachability
   information (for example, by leveraging existing routing protocols)
   or supporting application-level queries for topological information
   (for example, by leveraging ALTO).

<span class="h3"><a class="selflink" id="appendix-A.3" href="#appendix-A.3">A.3</a>.  CDNI Metadata Interface</span>

   The CDNI Metadata interface enables the Distribution system in a
   Downstream CDN to obtain CDNI Metadata from an Upstream CDN so that
   the Downstream CDN can properly process and respond to:

   o  Redirection Requests received over the CDNI Request Routing
      interface.

   o  Content Requests received directly from User Agents.

   The CDNI Metadata interface needs to offer a mechanism for an
   Upstream CDN to:

   o  Distribute/update/remove CDNI Metadata to a Downstream CDN.

   and/or to allow a Downstream CDN to:

   o  Make direct requests for CDNI Metadata objects.

   o  Make recursive requests for CDNI Metadata -- for example, to
      enable a Downstream CDN to walk down a tree of objects with
      inter-object relationships.

   The CDNI Metadata interface is therefore similar to the CDNI Request
   Routing interface because it is a request/response interface with the
   potential addition that CDNI Metadata search may have more complex
   semantics than a straightforward Request Routing redirection request.
   Therefore, like the CDNI Request Routing interface, the CDNI Metadata
   interface may be implemented as a WebService using one of the common
   WebServices methodologies (XML-RPC, HTTP query to a known URI, etc.)
   or possibly using other existing protocols such as XMPP [<a href="./rfc6120" title="&quot;Extensible Messaging and Presence Protocol (XMPP): Core&quot;">RFC6120</a>].
   This removes the need for the CDNI working group to define a new
   protocol for the request/response element of the CDNI Metadata
   interface.



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   Thus, the CDNI working group would be left only with the task of
   specifying:

   o  The recommended request/response protocol to use along with any
      additional semantics that are specific to the CDNI Metadata
      interface (e.g., handling of malformed requests/responses).

   o  The syntax (i.e., representation/encoding) of the CDNI Metadata
      objects that will be exchanged over the interface.

   o  The semantics (i.e., meaning and expected contents) of the
      individual properties of a Metadata object.

   o  How the relationships between different CDNI Metadata objects are
      represented.

<span class="h3"><a class="selflink" id="appendix-A.4" href="#appendix-A.4">A.4</a>.  CDNI Logging Interface</span>

   The CDNI Logging interface enables details of content distribution
   and delivery activities to be exchanged between interconnected CDNs,
   such as log records related to the delivery of content (similar to
   the log records recorded in a web server's access log).

   Within CDNs today, log records are used for a variety of purposes.
   Specifically, CDNs use logs to generate Call Data Records (CDRs) for
   passing to billing and payment systems and to real-time (and near
   real-time) analytics systems.  Such applications place requirements
   on the CDNI Logging interface to support guaranteed and timely
   delivery of log messages between interconnected CDNs.  It may also be
   necessary to be able to prove the integrity of received log messages.

   Several protocols already exist that could potentially be used to
   exchange CDNI logs between interconnected CDNs, including SNMP traps,
   syslog, FTP, HTTP POST, etc., although it is likely that some of the
   candidate protocols may not be well suited to meet all the
   requirements of CDNI.  For example, SNMP traps pose scalability
   concerns, and SNMP does not support guaranteed delivery of traps and
   therefore could result in log records being lost and the consequent
   CDRs and billing records for that content delivery not being
   produced, as well as that content delivery being invisible to any
   analytics platforms.










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   Although it is not necessary to define a new protocol for exchanging
   logs across the CDNI Logging interface, the CDNI working group would
   still need to specify:

   o  The recommended protocol to use.

   o  A default set of log fields and of their syntax and semantics.
      Today there is no standard set of common log fields across
      different content delivery protocols, and in some cases there is
      not even a standard set of log field names and values for
      different implementations of the same delivery protocol.

   o  A default set of conditions that trigger log records to be
      generated.

<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>.  Additional Material</span>

   This section records related information that was produced as part of
   defining the CDNI problem statement.

<span class="h3"><a class="selflink" id="appendix-B.1" href="#appendix-B.1">B.1</a>.  Non-Goals for IETF</span>

   Listed below are aspects of content delivery that the authors propose
   be kept outside of the scope of the CDNI working group:

   o  The interface between the Content Service Provider and the
      Authoritative CDN (i.e., the Upstream CDN contracted by the CSP
      for delivery by this CDN or by its Downstream CDNs).

   o  The delivery interface between the delivering CDN Surrogate and
      the User Agent, such as streaming protocols.

   o  The request interface between the User Agent and the Request
      Routing system of a given CDN.  Existing IETF protocols (e.g.,
      HTTP, RTSP, DNS) are commonly used by User Agents to request
      content from a CDN and by CDN Request Routing systems to redirect
      the User Agent requests.  The CDNI working group need not define
      new protocols for this purpose.  Note, however, that the CDNI
      control plane interface may indirectly affect some of the
      information exchanged through the request interface (e.g., URI).

   o  The content acquisition interface between CDNs (i.e., the data
      plane interface for actual delivery of a piece of content from one
      CDN to the other).  This is expected to use existing protocols
      such as HTTP or protocols defined in other forums for content
      acquisition between an origin server and a CDN (e.g., HTTP-based
      C2 reference point of the Alliance for Telecommunications Industry
      Solutions IPTV Interoperability Forum Content on Demand service



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      (ATIS IIF CoD)).  The CDN Interconnection problem space described
      in this document may therefore only concern itself with the
      agreement/negotiation aspects of which content acquisition
      protocol is to be used between two interconnected CDNs in view of
      facilitating interoperability.

   o  End User/User Agent Authentication.  End User/User Agent
      authentication and authorization are the responsibility of the
      Content Service Provider.

   o  Content preparation, including encoding and transcoding.  The CDNI
      architecture aims at allowing distribution across interconnected
      CDNs of content treated as opaque objects.  Interpretation and
      processing of the objects, as well as optimized delivery of these
      objects by the Surrogate to the End User, are outside the scope of
      CDNI.

   o  Digital Rights Management (DRM).  DRM is an end-to-end issue
      between a content protection system and the User Agent.

   o  Applications consuming CDNI logs (e.g., charging, analytics,
      reporting, ...).

   o  Internal CDN interfaces and protocols (i.e., interfaces and
      protocols within one CDN).

   o  Scalability of individual CDNs.  While scalability of the CDNI
      interfaces/approach is in scope, how an individual CDN scales is
      out of scope.

   o  Actual algorithms for selection of CDNs or Surrogates by Request
      Routing systems (however, some specific parameters required as
      input to these algorithms may be in scope when they need to be
      communicated across CDNs).

   o  Surrogate algorithms.  For example, caching algorithms and content
      acquisition methods are outside the scope of the CDNI work.
      Content management (e.g., Content Deletion) as it relates to CDNI
      content management policies is in scope, but the internal
      algorithms used by a cache to determine when to no longer cache an
      item of Content (in the absence of any specific metadata to the
      contrary) is out of scope.

   o  Element management interfaces.

   o  Commercial, business, and legal aspects related to the
      interconnections of CDNs.




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<span class="h3"><a class="selflink" id="appendix-B.2" href="#appendix-B.2">B.2</a>.  Relationship to Relevant IETF Working Groups and IRTF Research</span>
      Groups

<span class="h4"><a class="selflink" id="appendix-B.2.1" href="#appendix-B.2.1">B.2.1</a>.  ALTO WG</span>

   As stated in the ALTO working group charter [<a href="#ref-ALTO-Charter">ALTO-Charter</a>]:

      The Working Group will design and specify an Application-Layer
      Traffic Optimization (ALTO) service that will provide applications
      with information to perform better-than-random initial peer
      selection.  ALTO services may take different approaches at
      balancing factors such as maximum bandwidth, minimum cross-domain
      traffic, lowest cost to the user, etc.  The working group will
      consider the needs of BitTorrent, tracker-less P2P, and other
      applications, such as content delivery networks (CDN) and mirror
      selection.

   In particular, the ALTO service can be used by a CDN Request Routing
   system to improve its selection of a CDN Surrogate to serve a
   particular User Agent request (or to serve a request from another
   Surrogate).  [<a href="#ref-ALTO-CDN-USE-CASES">ALTO-CDN-USE-CASES</a>] describes a number of use cases for
   a CDN to be able to obtain network topology and cost information from
   an ALTO server(s) and discusses how CDN Request Routing could be used
   as an integration point of ALTO into CDNs.  It is possible that the
   ALTO service could be used in the same manner in a multi-CDN
   environment based on CDN Interconnection.  For example, an Upstream
   CDN may take advantage of the ALTO service in its decision for
   selecting a Downstream CDN to which a user request should be
   delegated.

   However, the current work of ALTO is complementary to and does not
   overlap with the work described in this document because the
   integration between ALTO and a CDN is an internal decision for a
   specific CDN and is therefore out of scope for the CDNI working
   group.  One area for further study is whether additional information
   should be provided by an ALTO service to facilitate CDNI CDN
   selection.

<span class="h4"><a class="selflink" id="appendix-B.2.2" href="#appendix-B.2.2">B.2.2</a>.  DECADE WG</span>

   The DECADE working group [<a href="#ref-DECADE-Charter">DECADE-Charter</a>] is addressing the problem
   of reducing traffic on the last-mile uplink, as well as backbone and
   transit links caused by peer-to-peer (P2P) streaming and file-sharing
   applications.  It addresses the problem by enabling an application
   endpoint to make content available from an in-network storage service
   and by enabling other application endpoints to retrieve the content
   from there.




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   Exchanging data through the in-network storage service in this
   manner, instead of through direct communication, provides significant
   gain where:

   o  The network capacity/bandwidth from the in-network storage service
      to the application endpoint significantly exceeds the capacity/
      bandwidth from application endpoint to application endpoint (e.g.,
      because of an end-user uplink bottleneck); and

   o  The content is to be accessed by multiple instances of application
      endpoints (e.g., as is typically the case for P2P applications).

   While, as is the case for any other data distribution application,
   the DECADE architecture and mechanisms could potentially be used for
   exchange of CDNI control plane information via an in-network storage
   service (as opposed to directly between the entities terminating the
   CDNI interfaces in the neighbor CDNs), we observe that:

   o  CDNI would operate as a "Content Distribution Application" from
      the DECADE viewpoint (i.e., would operate on top of DECADE).

   o  There do not seem to be obvious benefits in integrating the DECADE
      control plane responsible for signaling information relating to
      control of the in-network storage service itself, and the CDNI
      control plane responsible for application-specific CDNI
      interactions (such as exchange of CDNI Metadata, CDNI request
      redirection, and transfer of CDNI logging information).

   o  There would typically be limited benefits in making use of a
      DECADE in-network storage service because the CDNI interfaces are
      expected to be terminated by a very small number of CDNI clients
      (if not one) in each CDN, and the CDNI clients are expected to
      benefit from high bandwidth/capacity when communicating directly
      to each other (at least as high as if they were communicating via
      an in-network storage server).

   The DECADE in-network storage architecture and mechanisms may
   theoretically be used for the acquisition of the content objects
   themselves between interconnected CDNs.  It is not expected that this
   would have obvious benefits in typical situations where a content
   object is acquired only once from an Upstream CDN to a Downstream CDN
   (and then distributed as needed inside the Downstream CDN).  But it
   might have benefits in some particular situations.  Since the
   acquisition protocol between CDNs is outside the scope of the CDNI
   work, this question is left for further study.






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   The DECADE in-network storage architecture and mechanisms may
   potentially also be used within a given CDN for the distribution of
   the content objects themselves among Surrogates of that CDN.  Since
   the CDNI work does not concern itself with operation within a CDN,
   this question is left for further study.

   Therefore, the work of DECADE may be complementary to, but does not
   overlap with, the CDNI work described in this document.

<span class="h4"><a class="selflink" id="appendix-B.2.3" href="#appendix-B.2.3">B.2.3</a>.  PPSP WG</span>

   As stated in the PPSP working group charter [<a href="#ref-PPSP-Charter">PPSP-Charter</a>]:

      The Peer-to-Peer Streaming Protocol (PPSP) working group develops
      two signaling and control protocols for a peer-to-peer (P2P)
      streaming system for transmitting live and time-shifted media
      content with near real-time delivery requirements...

      ...  The PPSP working group designs a protocol for signaling and
      control between trackers and peers (the PPSP "tracker protocol")
      and a signaling and control protocol for communication among the
      peers (the PPSP "peer protocol").  The two protocols enable peers
      to receive streaming data within the time constraints required by
      specific content items.

   Therefore, PPSP is concerned with the distribution of the streamed
   content itself along with the necessary signaling and control
   required to distribute the content.  As such, it could potentially be
   used for the acquisition of streamed content across interconnected
   CDNs.  But since the acquisition protocol is outside the scope of the
   work proposed for CDNI, we leave this for further study.  Also,
   because of its streaming nature, PPSP is not seen as applicable to
   the distribution and control of the CDNI control plane and CDNI data
   representations.

   Therefore, the work of PPSP may be complementary to, but does not
   overlap with, the work described in this document for CDNI.

<span class="h4"><a class="selflink" id="appendix-B.2.4" href="#appendix-B.2.4">B.2.4</a>.  IRTF P2P Research Group</span>

   Some information on CDN Interconnection motivations and technical
   issues were presented in the P2P research group at IETF 77.  The
   presentation can be found in [<a href="#ref-P2PRG-CDNI">P2PRG-CDNI</a>].








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Authors' Addresses

   Ben Niven-Jenkins
   Velocix (Alcatel-Lucent)
   326 Cambridge Science Park
   Milton Road, Cambridge  CB4 0WG
   UK

   EMail: ben@velocix.com


   Francois Le Faucheur
   Cisco Systems
   Greenside, 400 Avenue de Roumanille
   Sophia Antipolis  06410
   France

   Phone: +33 4 97 23 26 19
   EMail: flefauch@cisco.com


   Nabil Bitar
   Verizon
   60 Sylvan Road
   Waltham, MA  02145
   USA

   EMail: nabil.n.bitar@verizon.com























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