CDNI Request Routing with ALTO
draft-seedorf-cdni-request-routing-alto-04

CDNI                                                          J. Seedorf
Internet-Draft                                                       NEC
Intended status: Informational                             July 15, 2013
Expires: January 16, 2014

                     CDNI Request Routing with ALTO
               draft-seedorf-cdni-request-routing-alto-04

Abstract

   Network Service Providers (NSPs) are currently considering to deploy
   Content Delivery Networks (CDNs) within their networks.  As a
   consequence of this development, there is a need for interconnecting
   these local CDNs.  The necessary interfaces for inter-connecting CDNs
   are currently being defined in the Content Delivery Networks
   Interconnection (CDNI) WG.  This document focuses on the CDNI
   Footprint & Capabilities Advertisement interface (FCI).
   Specifically, this document outlines how the solutions currently
   being defined in the Application Layer Traffic Optimization (ALTO) WG
   can facilitate Footprint & Capabilities Advertisement in a CDNI
   context, i.e. how the CDNI FCI can be realised with the ALTO
   protocol.  Concrete examples of how ALTO can be integrated within
   CDNI request routing and in particular in the process of selecting a
   downstream CDN are given.  The examples in this document are based on
   the use cases and examples currently being discussed in the CDNI WG.

Status of This Memo

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Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  ALTO within CDNI Request Routing  . . . . . . . . . . . . . .   3
   3.  Assumptions and High-Level Design Considerations  . . . . . .   4
     3.1.  General Assumptions and Consideration . . . . . . . . . .   4
     3.2.  Semantics for Footprint/Capabilities Advertisment . . . .   5
   4.  Selection of a Downstream CDN with ALTO . . . . . . . . . . .   7
     4.1.  Footprint Advertisement with ALTO Network Map . . . . . .   7
     4.2.  Capabilities Advertisement with ALTO Network Maps . . . .   8
     4.3.  Conveying additional information with ALTO Cost Maps  . .   8
     4.4.  Example of Selecting a Downstream CDN based on ALTO Maps    9
     4.5.  Advantages of using ALTO  . . . . . . . . . . . . . . . .  10
   5.  Useful ALTO extensions for CDNI Request Routing . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  Summary and Outlook . . . . . . . . . . . . . . . . . . . . .  12
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Many Network Service Providers (NSPs) are currently considering or
   have already started to deploy Content Delivery Networks (CDNs)
   within their networks.  As a consequence of this development, there
   is a need for interconnecting these local CDNs.  Content Delivery
   Networks Interconnection (CDNI) has the goal of standardizing
   protocols to enable such interconnection of CDNs [RFC6707].

   The CDNI problem statement [RFC6707] envisions four interfaces to be
   standardized within the IETF for CDN interconnection:

   o  CDNI Request Routing Interface

   o  CDNI Metadata Interface

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   o  CDNI Logging Interface

   o  CDNI Control Interface

   This document focuses solely on the CDNI Request Routing Interface.
   This interface can further be divided into two interfaces (see
   [RFC6707] for a detailed description): the CDNI Request Routing
   Redirection interface (RI), and the CDNI Footprint & Capabilities
   Advertisement interface (FCI).  This document argues that ALTO is a
   good candidate as a protocol for CDNI Footprint & Capabilities
   Advertisement.  Concrete examples of how the CDNI FCI can be
   implemented with the ALTO protocol [I-D.ietf-alto-protocol] are
   given.  The examples used in this document are based on the use cases
   and request routing proposals currently being discussed in the CDNI
   WG [RFC6770] [I-D.peterson-CDNI-strawman] and in the ALTO WG
   [I-D.jenkins-alto-cdn-use-cases].  The overall rationale of this
   document is that the ALTO protocol [I-D.ietf-alto-protocol]
   constitutes a good solution for the CDNI FCI.

   A previous version of this document [I-D.seedorf-alto-for-cdni]
   contained detailed examples of actual request routing and surrogate
   selection with ALTO, i.e. how ALTO could be used for implementing the
   CDNI Request Routing Redirection interface (RI).  This version solely
   focuses on implementing the CDNI Footprint & Capabilities
   Advertisement interface (FCI) with ALTO, i.e. the selection of a
   downstream CDN and how ALTO can support such downstream CDN
   selection.

   Throughout this document, we use the terminology for CDNI defined in
   [I-D.ietf-cdni-problem-statement].

2.  ALTO within CDNI Request Routing

   The main purpose of the CDNI Request Routing Interface is described
   in [RFC6707] as follows: "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."  On a high level, the scope of the CDNI Request
   Routing Interface therefore contains two main tasks:

   o  A) Determining if the downstream CDN is willing to accept a
      delegated content request

   o  B) Redirecting the content request coming from an upstream CDN to
      the proper entry point or entity in the downstream CDN

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   More precisely, in [I-D.ietf-cdni-framework] the request routing
   interface is broadly divided into two functionalities:

   o  1) the asynchronous advertisement of footprint and capabilities by
      a dCDN that allows a uCDN to decide whether to redirect particular
      user requests to that dCDN (the CDNI FCI)

   o  2) the synchronous operation of actually redirecting a user
      request (the CDNI RI)

   Application Layer Traffic Optimization (ALTO) is an approach for
   guiding the resource provider selection process in distributed
   applications that can choose among several candidate resources
   providers to retrieve a given resource.  By conveying network layer
   (topology) information, an ALTO server can provide important
   information to "guide" the resource provider selection process in
   distributed applications.  Usually, it is assumed that an ALTO server
   conveys information these applications cannot measure themselves
   [RFC5693].

   Originally, ALTO was motivated by the huge amount of cross-ISP
   traffic generated by P2P applications [RFC5693].  Recently, however,
   ALTO is also being considered for improving the request routing in
   CDNs [I-D.jenkins-alto-cdn-use-cases].  In this context, it has also
   been proposed to use ALTO for selecting an entry-point in a
   downstream NSP's network (see section 3.4 "CDN delivering Over-The-
   Top of a NSP's network" in [I-D.jenkins-alto-cdn-use-cases]).  Also,
   the CDNI problem statement explicitly mentions ALTO as a candidate
   protocol for "algorithms for selection of CDN or Surrogate by
   Request-Routing systems" [I-D.ietf-cdni-problem-statement].  Yet,
   there have not been concrete proposals so far on how to use ALTO in
   the context of CDN interconnection.  This document tries to close
   this gap by giving some examples on how ALTO could be used within
   CDNI request routing.

3.  Assumptions and High-Level Design Considerations

   In this section we list some assumptions and design issues to be
   considered when using ALTO for the CDNI Footprint and Capabilities
   Advertisement interface

3.1.  General Assumptions and Consideration

   Below we list some general assumptions and considerations:

   o  As explicitly being out-of-scope for CDNI
      [I-D.ietf-cdni-problem-statement], the examples used in this
      document assume that ingestion of content or acquiring content

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      across CDNs is not part of request routing as considered within
      CDNI standardization work.  The focus of using ALTO (as considered
      in this document) is hence on request routing only, assuming that
      the content (desired by the end user) is available in the
      downstream CDN (or can be aquired by the downstream CDN by some
      means).

   o  Federation Model: "Footprint and Capabilities Advertisement" and
      in general CDN request routing depends on the federation model
      among the CDN providers.  Designing a suitable solution thus
      depends on whether a solution is needed for different settings,
      where CDNs consist of both NSP CDNs (serving individual ASes) and
      general, traditional CDNs (such as Akamai).  We assume that CDNI
      is not designed for a setting where only NSP CDNs each serve a
      single AS only.

   o  In this document, we assume that the upstream CDN (uCDN) makes the
      decision on selecting a downstream CDN, based on information that
      each downstream CDN has made available to the upstream CDN.
      Further, we assume that in principle more than one dCDN may be
      suitable for a given end-user request (i.e. different dCDNs may
      claim "overlapping" footprints).  The uCDN hence potentially has
      to select among several candidate downstream CDNs for a given end
      user request.

   o  It is not clear what kind(s) of business, contract, and
      operational relationships two peering CDNs may form.  For the
      Internet, we see provider-customer and peering as two main
      relations; providers may use different charging models (e.g.,
      95-percentile, total volume) and may provide different SLAs.
      Given such unknown characteristics of CDN peering business
      agreements, we should design the protocol to support as much
      diverse potential business and operational models as possible.

3.2.  Semantics for Footprint/Capabilities Advertisment

   The CDNI document on "Footprint and Capabilities Semantics"
   [I-D.spp-cdni-rr-foot-cap-semantics] defines the semantics for the
   CDNI FCI.  It thus provides guidance on what Footprint and
   Capabilities mean in a CDNI context and how a protocol solution
   should in principle look like.  Here we briefly summarize the key
   points of the semantics of Footprint and Capabilities (for a detailed
   discussion, the reader is referred to
   [I-D.spp-cdni-rr-foot-cap-semantics]):

   o  Often, footprint and capabilities are tied together and cannot be
      interpreted independently from each other.  In such cases, i.e.
      where capabilities must be expressed on a per footprint basis, it

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      may be beneficial to combine footprint and capabilities
      advertisement.

   o  Given that a large part of Footprint and Capabilities
      Advertisement will actually happen in contractual agreements, the
      semantics of CDNI Footprint and Capabilities advertisement refer
      to answering the following question: what exactly still needs to
      be advertised by the CDNI FCI?  For instance, updates about
      temporal failures of part of a footprint can be useful information
      to convey via the CDNI request routing interface.  Such
      information would provide updates on information previously agreed
      in contracts between the participating CDNs.  In other words, the
      CDNI FCI is a means for a dCDN to provide changes/updates
      regarding a footprint and/or capabilities it has prior agreed to
      serve in a contract with a uCDN.

   o  It seems clear that "coverage/reachability" types of footprint
      must be supported within CDNI.  The following such types of
      footprint are mandatory and must be supported by the CDNI FCI:

      *  List of ISO Country Codes

      *  List of AS numbers

      *  Set of IP-prefixes

      A 'set of IP-prefixes' must be able to contain full IP addresses,
      i.e., a /32 for IPv4 and a /128 for IPv6, and also IP prefixes
      with an arbitrary prefix length.  There must also be support for
      multiple IP address versions, i.e., IPv4 and IPv6, in such a
      footprint.

   o  For all of these mandatory-to-implement footprint types,
      footprints can be viewed as constraints for delegating requests to
      a dCDN: A dCDN footprint advertisement tells the uCDN the
      limitations for delegating a request to the dCDN.  For IP prefixes
      or ASN(s), the footprint signals to the uCDN that it should
      consider the dCDN a candidate only if the IP address of the
      request routing source falls within the prefix set (or ASN,
      respectively).  The CDNI specifications do not define how a given
      uCDN determines what address ranges are in a particular ASN.
      Similarly, for country codes a uCDN should only consider the dCDN
      a candidate if it covers the country of the request routing
      source.  The CDNI specifications do not define how a given uCDN
      determines the country of the request routing source.  Multiple
      footprint constraints are additive, i.e. the advertisement of
      different types of footprint narrows the dCDN candidacy
      cumulatively.

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   o  The following capabilities seem useful as 'base' capabilities,
      i.e. ones that are needed in any case and therefore constitute
      mandatory capabilities to be supported by the CDNI FCI:

      *  Delivery Protocol (e.g., HTTP vs. RTMP)

      *  Acquisition Protocol (for aquiring content from a uCDN)

      *  Redirection Mode (e.g., DNS Redirection vs. HTTP Redirection as
         discussed in [I-D.ietf-cdni-framework])

      *  Capabilities related to CDNI Logging (e.g., supported logging
         mechanisms)

      *  Capabilities related to CDNI Metadata (e.g., authorization
         algorithms or support for proprietary vendor metadata)

4.  Selection of a Downstream CDN with ALTO

   Under the considerations stated in Section 3, ALTO can help the
   upstream CDN provider to select a proper downstream CDN provider for
   a given end user request as follows: Each downstream CDN provider
   hosts an ALTO server which provides ALTO information (i.e. ALTO
   network maps and ALTO cost maps [I-D.ietf-alto-protocol]) to an ALTO
   client at the upstream CDN provider.  A network map provided by each
   of several candidate downstream CDNs can provide information to the
   upstream CDN provider about each dCDN's "coverage/reachability"
   footprint, e.g. regarding geographical coverage, the IP-prefix ranges
   the dCDN claims it can cover with agreed upon delivery quality, or
   similar.

   Additional ALTO network maps or cost maps can provide an upstream CDN
   provider additional information about the capabilities each
   individual dCDN offers.  Potentially, such capabilities can be tied
   to a given (partial) footprint of the dCDN.  ALTO 'costs' in this
   context is a generic term; many types of costs are possible and can
   be useful in the context of the CDNI FCI whenever numeric or metric
   types of capabilities need to be expressed.

4.1.  Footprint Advertisement with ALTO Network Map

   An ALTO network map contains a "set of Network Location groupings"
   [I-D.ietf-alto-protocol].  The groupings are defined in the form of
   so-called "PIDs".  A PID is an identifier to group network location
   endpoints, e.g. IP-addresses in the form of prefixes (see section 4
   in [I-D.ietf-alto-protocol] for details).

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   The concept of an ALTO network map (and the PIDs contained therein)
   is a natural and straightforward candidate for CDNI Fooprint
   Advertisement: The downstream CDN provider groups the IP-addresses in
   its footprint into PIDs and makes these groupings available to an
   upstream CDN via an ALTO network map.  With such a network map, the
   upstream CDN provider can easily match a given end user request with
   the footprint of the downstream CDN provider to see if a given
   downstream CDN can in principle provide "coverage" for the IP-address
   of the end user.  Whenever the footprint changes, the downstream CDN
   creates an updated network map and makes it available via its ALTO
   server.

4.2.  Capabilities Advertisement with ALTO Network Maps

   ALTO network maps can convey all mandatory capabilities (see
   Section 3.2) via PID names: Additional network maps provided by a
   dCDN can group the dCDN's coverage footprint into several PIDs, where
   each PID name has a certain 'capability' semantic.  For instance, one
   mandatory type of capability is the 'Delivery Protocol' (see further
   the discussion in [I-D.spp-cdni-rr-foot-cap-semantics]) supported by
   a dCDN for a given footprint.  For each supported delivery protocol,
   the dCDN would provide an ALTO PID in a network map that contains all
   IP-prefixes that support this delivery protocol.

4.3.  Conveying additional information with ALTO Cost Maps

   An ALTO cost map contains costs between defined groupings of a
   corresponding network map (i.e. costs between PIDs): "An ALTO Cost
   Map defines Path Costs pairwise amongst sets of source and
   destination Network Locations" [I-D.ietf-alto-protocol].  This
   concept enables the provider of a cost map to express (and quantify)
   preferences of a destination network location with respect to a given
   source network location.

   In the context of CDNI, the ALTO cost map concept is an extensive
   tool to convey additional information about the footprint or
   capabilties of a downstream CDN.  The cost map concept provides a
   means for a downstream CDN provider to convey numeric values
   associated with a PID, e.g. in order to convey metrics associated
   with a footprint or a capability.  This may be useful for future,
   non-mandatory types of footprint or capabilties.

   One way to use ALTO cost maps would have these maps of the type
   N-to-1, i.e. 'costs' are expressed for each of N end user source PIDs
   to 1 single dCDN request router PID.  Semantically, the source PID in
   a CDNI ALTO cost map is thus the end user location, whereas the
   destination is the request router to which the uCDN redirects the end
   user request.  Note that this perspective is driven by the CDNI

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   request routing.  An alternative way - seen from the perspective of
   content retrieval - would be to have a 1-to-N cost map where the
   source is always the dCDN and the destination is the end user (with
   the semantic "if the source dCDN would deliver content to an end user
   in the destination PID, the costs would be the following).

4.4.  Example of Selecting a Downstream CDN based on ALTO Maps

   In the following, we will outline an example of dCDN selection by a
   uCDN based on ALTO maps provided by each dCDN.  In the example, an
   ALTO network map "NM_cov" is used to express the overall "coverage"
   footprint of each dCDN.  In addition, each dCDN provides one or more
   ALTO network maps regarding capabilities "CM_1", "CM_2", ..., "CM_n"
   to express the supported capabilities, corresponding with the
   "NM_cov" coverage footprint network map.

   Consider the following example: An upstream CDN (uCDN) has agreed on
   CDN interconnection with several downstream CDNs (dCDN-a, dCDN-b, and
   dCDN-c).  Each of these downstream CDNs runs an ALTO server to
   provide information about what locations it can deliver content to
   (coverage footprint) by means of a network map "NM_cov" and
   additionally capability network maps "CM_1", "CM_2", ..., "CM_n".
   uCDN has downloaded from each candidate downstream CDN "NM_cov" and
   one or more ALTO network maps regarding capabilities (e.g. by using
   the "Filtered Cost Map" option and different "cost-types" as
   specified in 7.7.3.2. of [I-D.ietf-alto-protocol]).  The ALTO network
   map provides "coverage" (footprint) for each downstream CDN as
   aggregated network locations in the form of ALTO PIDs.  The additonal
   capability network maps provide the upstream CDN information
   regarding the support for capabilities each individual downstream CDN
   would imply depending on the given location of an end user request.

   Whenever the upstream CDN receives a request from an end user and has
   determined that this request is best served by an interconnected
   dCDN, the uCDN uses ALTO maps to make a redirection decision.  For a
   given request, assume that only the ALTO network maps provided by
   dCDN-a and dCDN-c, "NM_cov(dCDN-a)" and "NM_cov(dCDN-c)", indicate
   that these downstream CDNs can deliver content to the location of the
   request.  In this case, additonal ALTO network maps received from
   dCDN-a and dCDN-c provide useful additional information to the
   upstream CDN in order to make a selection decision regarding either
   dCDN-a or dCDN-c. For instance, if both downstream CDNs have provided
   two ALTO capbility network maps "CM_delivery_protocol" and
   "CM_redirect_mode", uCDN can make a downstream CDN selection based on
   its needs or preference.  This enables the upstream CDN to make a
   well-considered downstream CDN selection.

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4.5.  Advantages of using ALTO

   The following reasons make ALTO a suitable candidate protocol for
   downstream CDN selection as part of CDNI request routing and in
   particular for a FCI protocol:

   o  CDN request routing is done at the application layer.  ALTO is a
      protocol specifically designed to improve application layer
      traffic (and application layer connections among hosts on the
      Internet) by providing additonal information to applications that
      these applications could not easily retrieve themselves.  For
      CDNI, this is exactly the case: a uCDN wants to improve
      application layer CDN request routing by using dedicated
      information (provided by a dCDN) that the uCDN could not easily
      obtain otherwise.

   o  The semantics of an ALTO network are an exact match for the needed
      information to convey a footprint by a downstream CDN, in
      particular if such a footprint is being expressed by IP-prefix
      ranges.

   o  ALTO cost maps are suitable to express various types of numeric
      values and can hence be used by an upstream CDN to obtain metrics
      for capabilities associated with a given dCDN for a given
      foorprint.  Further, an ALTO cost map could also convey relevant
      network topology information other than simply routing hops or
      reachability.  This facilitiates advanced and more sophisticated
      selection of a downstream CDN based on various metrics by the
      upstream CDN and increases flexibility to cover different use
      cases and business models for CDN interconnection.

   o  Flexible granularity: The concept of the PID and ALTO network/cost
      maps allows for different degrees of granularity.  This enables a
      dCDN to differentiate the delivery quality for serving an end user
      request on a fine granularity depending on the end user location
      (and not only express delivery quality e.g. on an AS-level).  It
      remains at the discretion of each dCDN how fine-granular the ALTO
      network and cost maps are that it publishes.

   o  ALTO maps can be signed and hence provide inherent integrity
      protection (see Section 6)

5.  Useful ALTO extensions for CDNI Request Routing

   It is envisioned that yet-to-be-defined ALTO extensions will be
   standardized that make the ALTO protocol more suitable and useful for
   applications other than the originally considered P2P use case
   [I-D.marocco-alto-next].  Some of these extensions to the ALTO

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   protocol would be useful for ALTO to be used as a protocol within
   CDNI request routing, and in particular within the "Footprint and
   Capabilities Advertisment" part of the CDNI request routing
   interface.

   The following proposed extensions to ALTO would be beneficial to
   facilitate CDNI request routing with ALTO as outlined in Section 4:

   o  Server-initiated Notifications and Incremental Updates: In case
      the footprint or the capabilities of a downstream CDN change
      abruptly (i.e. unexpectedly from the perspective of an upstream
      CDN), server initiated notifications would enable a dCDN to
      directly inform an upstream CDN about such changes.  Consider the
      case where - due to failure - part of the footprint of the dCDN is
      not functioning, i.e. the CDN cannot serve content to such clients
      with reasonable QoS.  Without server-initiated notifications, the
      uCDN might still use a very recent network and cost map from dCDN,
      and therefore redirect request to dCDN which it cannot serve.
      Similarly, the possibility for incremental updates would enable
      efficient conveyance of the aforementioned (or similar) status
      changes by the dCDN to the uCDN.  A proposal for server-initiated
      ALTO updates can be found in [I-D.marocco-alto-ws].  A discussion
      of incremental ALTO updates can be found in
      [I-D.schwan-alto-incr-updates].

   o  Content Availability on Hosts: A dCDN might want to express CDN
      capabilties in terms of certain content types (e.g. codecs/
      formats, or content from certain content providers).  A new
      endpoint property for ALTO that would be able to express such
      "content availability" would enable a dCDN to make available such
      information to an upstream CDN.  This would enable a uCDN to
      determine if a given dCDN actually has the capabilities for a
      given request with respect to the type of content requested.

   o  Resource Availability on Hosts or Links: The capabilities on links
      (e.g. maximum bandwidth) or caches (e.g. average load) might be
      useful information for an upstream CDN for optimized dowmstream
      CDN selection.  For instance, if a uCDN receives a streaming
      request for content with a certain bitrate, it needs to know if it
      is likely that a dCDN can fulfill such stringent application-level
      requirements (i.e. can be expected to have enough consistent
      bandwidth) before it redirects the request.  In general, if ALTO
      could convey such information via new endpoint properties, it
      would enable more sophisticated means for downstream CDN selection
      with ALTO.

6.  Security Considerations

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   One important security consideration is the proper authentication of
   advertisement information provided by a downstream CDN.  The ALTO
   protocol provides a specification for a signature of ALTO maps (see
   8.2.2. of [I-D.ietf-alto-protocol].  ALTO thus provides a proper
   means for protecting the integrity of footprint advertisment
   information.

   More Security Considerations will be discussed in a future version of
   this document.

7.  Summary and Outlook

   This document presented conrete examples of how ALTO can be used
   within the downstream CDN selection of CDNI Request Routing.
   Further, the document provides arguments why ALTO is a meaningful
   protocol in this context.  Essentially, ALTO network and cost maps
   are a means to provide detailed and various types of information to
   an upstream CDN, in order to facilitate well-considered downstream
   CDN selection.

   The intention of this document is to find consensus in the CDNI WG
   that ALTO is a useful protocol for CDNI request routing, and that
   ALTO has many benefits for proper selection of a downstream CDN.  The
   overall objective is to form agreement on how ALTO should be used
   within the CDNI request routing protocol.  It is the intention to
   capture the outcome of such continuing discussions in future versions
   of this document.

8.  Acknowledgements

   Jan Seedorf is partially supported by the CHANGE project (CHANGE:
   Enabling Innovation in the Internet Architecture through Flexible
   Flow-Processing Extensions, http://www.change-project.eu/), a
   research project supported by the European Commission under its 7th
   Framework Program (contract no. 257422).  The views and conclusions
   contained herein are those of the authors and should not be
   interpreted as necessarily representing the official policies or
   endorsements, either expressed or implied, of the CHANGE project or
   the European Commission.

   Jan Seedorf has been partially supported by the COAST project
   (COntent Aware Searching, retrieval and sTreaming, http://www.coast-
   fp7.eu), a research project supported by the European Commission
   under its 7th Framework Program (contract no.  248036).  The views
   and conclusions contained herein are those of the authors and should
   not be interpreted as necessarily representing the official policies
   or endorsements, either expressed or implied, of the COAST project or
   the European Commission.

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   Thanks to Richard Yang for providing valuable comments, and for
   contributiong some design considerations and assumptions.

9.  References

9.1.  Normative References

   [RFC5693]  Seedorf, J. and E. Burger, "Application-Layer Traffic
              Optimization (ALTO) Problem Statement", RFC 5693, October
              2009.

   [RFC6707]  Niven-Jenkins, B., Le Faucheur, F., and N. Bitar, "Content
              Distribution Network Interconnection (CDNI) Problem
              Statement", RFC 6707, September 2012.

   [RFC6770]  Bertrand, G., Stephan, E., Burbridge, T., Eardley, P., Ma,
              K., and G. Watson, "Use Cases for Content Delivery Network
              Interconnection", RFC 6770, November 2012.

9.2.  Informative References

   [I-D.peterson-CDNI-strawman]
              Peterson, L. and J. Hartman, "Content Distribution Network
              Interconnection (CDNI) Problem Statement", draft-peterson-
              CDNI-strawman-01 (work in progress), May 2011.

   [I-D.ietf-cdni-problem-statement]
              Niven-Jenkins, B., Faucheur, F., and N. Bitar, "Content
              Distribution Network Interconnection (CDNI) Problem
              Statement", draft-ietf-cdni-problem-statement-08 (work in
              progress), June 2012.

   [I-D.marocco-alto-next]
              Marocco, E. and V. Gurbani, "Extending the Application-
              Layer Traffic Optimization (ALTO) Protocol", draft-
              marocco-alto-next-00 (work in progress), January 2012.

   [I-D.ietf-alto-protocol]
              Alimi, R., Penno, R., and Y. Yang, "ALTO Protocol", draft-
              ietf-alto-protocol-17 (work in progress), July 2013.

   [I-D.ietf-cdni-requirements]
              Leung, K. and Y. Lee, "Content Distribution Network
              Interconnection (CDNI) Requirements", draft-ietf-cdni-
              requirements-09 (work in progress), July 2013.

   [I-D.ietf-cdni-use-cases]

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              Bertrand, G., Emile, S., Burbridge, T., Eardley, P., Ma,
              K., and G. Watson, "Use Cases for Content Delivery Network
              Interconnection", draft-ietf-cdni-use-cases-10 (work in
              progress), August 2012.

   [I-D.marocco-alto-ws]
              Marocco, E. and J. Seedorf, "WebSocket-based server-to-
              client notifications for the Application-Layer Traffic
              Optimization (ALTO) Protocol", draft-marocco-alto-ws-01
              (work in progress), July 2012.

   [I-D.schwan-alto-incr-updates]
              Schwan, N. and B. Roome, "ALTO Incremental Updates",
              draft-schwan-alto-incr-updates-02 (work in progress), July
              2012.

   [I-D.jenkins-alto-cdn-use-cases]
              Niven-Jenkins, B., Watson, G., Bitar, N., Medved, J., and
              S. Previdi, "Use Cases for ALTO within CDNs", draft-
              jenkins-alto-cdn-use-cases-03 (work in progress), June
              2012.

   [I-D.seedorf-alto-for-cdni]
              Seedorf, J., "ALTO for CDNi Request Routing", draft-
              seedorf-alto-for-cdni-00 (work in progress), October 2011.

   [I-D.ietf-cdni-framework]
              Peterson, L. and B. Davie, "Framework for CDN
              Interconnection", draft-ietf-cdni-framework-03 (work in
              progress), February 2013.

   [I-D.liu-cdni-cost]
              Liu, H., "A Cost Perspective on Using Multiple CDNs",
              draft-liu-cdni-cost-00 (work in progress), October 2011.

   [I-D.spp-cdni-rr-foot-cap-semantics]
              Seedorf, J., Peterson, J., Previdi, S., Brandenburg, R.,
              and K. Ma, "CDNI Request Routing: Footprint and
              Capabilities Semantics", draft-spp-cdni-rr-foot-cap-
              semantics-04 (work in progress), February 2013.

Author's Address

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   Jan Seedorf
   NEC Laboratories Europe, NEC Europe Ltd.
   Kurfuersten-Anlage 36
   Heidelberg  69115
   Germany

   Phone: +49 (0) 6221 4342 221
   Email: jan.seedorf@neclab.eu
   URI:   http://www.neclab.eu

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