Internet Engineering Task Force (IETF)
Request for Comments: 6770
Obsoletes: 3570
Category: Informational
ISSN: 2070-1721
G. Bertrand, Ed.
E. Stephan
France Telecom - Orange
T. Burbridge
P. Eardley
K. Ma
Azuki Systems, Inc.
G. Watson
Alcatel-Lucent (Velocix)
November 2012

Use Cases for Content Delivery Network Interconnection


Content Delivery Networks (CDNs) are commonly used for improving the End User experience of a content delivery service while keeping cost at a reasonable level. This document focuses on use cases that correspond to identified industry needs and that are expected to be realized once open interfaces and protocols supporting the interconnection of CDNs are specified and implemented. This document can be used to motivate the definition of the requirements to be supported by CDN Interconnection (CDNI) interfaces. It obsoletes RFC 3570.

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 Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at

Copyright Notice

Copyright © 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( 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

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Abbreviations  . . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Rationale for CDN Interconnection  . . . . . . . . . . . .  4
   2.  Footprint Extension Use Cases  . . . . . . . . . . . . . . . .  6
     2.1.  Geographic Extension . . . . . . . . . . . . . . . . . . .  6
     2.2.  Inter-Affiliates Interconnection . . . . . . . . . . . . .  6
     2.3.  ISP Handling of Third-Party Content  . . . . . . . . . . .  7
     2.4.  Nomadic Users  . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Offload Use Cases  . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Overload Handling and Dimensioning . . . . . . . . . . . .  8
     3.2.  Resiliency . . . . . . . . . . . . . . . . . . . . . . . .  9
       3.2.1.  Failure of Content Delivery Resources  . . . . . . . .  9
       3.2.2.  Content Acquisition Resiliency . . . . . . . . . . . . 10
   4.  Capability Use Cases . . . . . . . . . . . . . . . . . . . . . 11
     4.1.  Device and Network Technology Extension  . . . . . . . . . 11
     4.2.  Technology and Vendor Interoperability . . . . . . . . . . 12
     4.3.  QoE and QoS Improvement  . . . . . . . . . . . . . . . . . 12
   5.  Enforcement of Content Delivery Policy . . . . . . . . . . . . 12
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Appendix A.  Content Service Providers' Delivery Policies  . . . . 14
     A.1.  Content Delivery Policy Enforcement  . . . . . . . . . . . 14
     A.2.  Secure Access  . . . . . . . . . . . . . . . . . . . . . . 15
     A.3.  Branding . . . . . . . . . . . . . . . . . . . . . . . . . 15

1. Introduction

Content Delivery Networks (CDNs) are commonly used for improving the End User experience of a content delivery service while keeping cost at a reasonable level. This document focuses on use cases that correspond to identified industry needs and that are expected to be realized once open interfaces and protocols supporting the interconnection of CDNs are specified and implemented. The document can be used to motivate the definition of the requirements (as documented in [CDNI-REQ]) to be supported by the set of CDN Interconnection (CDNI) interfaces defined in [RFC6707].

[RFC3570] describes slightly different terminologies and models for "Content Internetworking (CDI)". This document obsoletes RFC 3570 to avoid confusion.

This document identifies the main motivations for a CDN Provider to interconnect its CDN:

  • CDN Footprint Extension Use Cases (Section 2)
  • CDN Offload Use Cases (Section 3)
  • CDN Capability Use Cases (Section 4)

Then, the document highlights the need for interoperability in order to exchange and enforce content delivery policies (Section 5).

1.1. Terminology

In this document, the first letter of each CDNI-specific term is capitalized. We adopt the terminology described in [RFC6707].

We extend this terminology with the following term:

Access CDN:

A CDN that includes Surrogates in the same administrative network as the End User. Such a CDN can use accurate information on the End User's network context to provide additional Content Delivery Services to Content Service Providers.

1.2. Abbreviations

  • CDN: Content Delivery Network, also known as Content Distribution Network
  • CSP: Content Service Provider
  • dCDN: downstream CDN
  • DNS: Domain Name System
  • EU: End User
  • ISP: Internet Service Provider
  • NSP: Network Service Provider
  • QoE: Quality of Experience
  • QoS: Quality of Service
  • uCDN: upstream CDN
  • URL: Uniform Resource Locator
  • WiFi: Wireless local area network (WLAN) based on IEEE 802.11

1.3. Rationale for CDN Interconnection

Content Delivery Networks (CDNs) are used to deliver content because they can:

  • improve the experience for the End User; for instance delivery has lower latency (decreased round-trip-time and higher throughput between the user and the delivery server) and better robustness (ability to use multiple delivery servers),
  • reduce the network operator's costs; for instance, lower delivery cost (reduced bandwidth usage) for cacheable content,
  • reduce the Content Service Provider's (CSP) internal infrastructure costs, such as data center capacity, space, and electricity consumption, as popular content is delivered externally through the CDN rather than through the CSP's own servers.

Indeed, many Network Service Providers (NSPs) and Enterprise Service Providers are deploying or have deployed their own CDNs. Despite the potential benefits of interconnecting CDNs, today each CDN is a stand-alone network. The objective of CDN Interconnection is to overcome this restriction; the interconnected CDNs should be able to collectively behave as a single delivery infrastructure.

An example is depicted in Figure 1, where two CDN Providers establish a CDN Interconnection. The Content Service Provider CSP-1 reaches an agreement with CDN Provider 'A' for the delivery of its content. Independently, CDN Provider 'A' and CDN Provider 'B' agree to interconnect their CDNs.

When a given User Agent requests content from CSP-1, CDN-A may consider that delivery by CDN-B is appropriate, for instance, because CDN-B is an Access CDN and the user is directly attached to it. Through the CDN Interconnection arrangements put in place between CDN-A and CDN-B (as a result of the CDN Interconnection agreement established between CDN Provider 'A' and CDN Provider 'B'), CDN-A can redirect the request to CDN-B and the content is actually delivered to the User Agent by CDN-B.

The End User benefits from this arrangement through a better Quality of Experience (QoE, see [RFC6390]), because the content is delivered from a nearby Surrogate (e.g., lower latency, bottlenecks avoided). CDN Provider 'A' benefits because it does not need to deploy such an extensive CDN, while CDN Provider 'B' may receive some compensation for the delivery. CSP-1 benefits because it only needs to make one business agreement and one technical arrangement with CDN Provider 'A', but its End Users get a service quality as though CSP-1 had also gone to the trouble of making a business agreement and technical arrangement with CDN Provider 'B'.

    +-------+ +-------+
    | CSP-1 | | CSP-2 |
    +-------+ +-------+
        |         |
       ,--,--,--./            ,--,--,--.
    ,-'          `-.       ,-'          `-.
   (CDN Provider 'A')=====(CDN Provider 'B')
    `-.  (CDN-A) ,-'       `-. (CDN-B)  ,-'
       `--'--'--'             `--'--'--'
                            | End User |
    === CDN Interconnection

Figure 1

To extend the example, another Content Service Provider, CSP-2, may also reach an agreement with CDN Provider 'A'. However, CSP-2 may not want its content to be distributed by CDN Provider B; for example, CSP-2 may not want to distribute its content in the area where CDN Provider 'B' operates. This example illustrates that policy considerations are an important part of CDNI.

2. Footprint Extension Use Cases

Footprint extension is expected to be a major use case for CDN Interconnection.

2.1. Geographic Extension

In this use case, the CDN Provider wants to extend the geographic distribution that it can offer to its CSPs:

  • without compromising the quality of delivery.
  • without incurring additional transit and other network costs that would result from serving content from geographically or topologically remote Surrogates.
  • without incurring the cost of deploying and operating Surrogates and the associated CDN infrastructure that may not be justified in the corresponding geographic region (e.g., because of relatively low delivery volume, or conversely because of the high investments that would be needed to satisfy the high volume).

If there are several CDN Providers that have a geographically limited footprint (e.g., restricted to one country), or do not serve all End Users in a geographic area, then interconnecting their CDNs enables these CDN Providers to provide their services beyond their own footprint.

As an example, suppose a French CSP wants to distribute its TV programs to End Users located in France and various countries in North Africa. It asks a French CDN Provider to deliver the content. The French CDN Provider's network only covers France, so it makes an agreement with another CDN Provider that covers North Africa. Overall, from the CSP's perspective, the French CDN Provider provides a CDN service for both France and North Africa.

In addition to video, this use case applies to other types of content such as automatic software updates (browser updates, operating system patches, virus database update, etc.).

2.2. Inter-Affiliates Interconnection

The previous section describes the case of geographic extension between CDNs operated by different entities. A large CDN Provider may have several subsidiaries that each operate their own CDN (which may rely on different CDN technologies, see Section 4.2). In certain circumstances, the CDN Provider needs to make these CDNs interoperate to provide consistent service to its customers on the whole collective footprint.

2.3. ISP Handling of Third-Party Content

Consider an ISP carrying to its subscribers a lot of content that comes from a third-party CSP and that is injected into the ISP's network by an Authoritative CDN Provider. There are mutual benefits to the ISP (acting as an Access CDN), the Authoritative CDN, and the CSP that would make a case for establishing a CDNI agreement. For example:

  • allowing the CSP to offer improved QoE and QoE services to subscribers, for example, reduced content startup time or increased video quality and resolution of adaptive streaming content.
  • allowing the Authoritative CDN to reduce hardware capacity and footprint, by using the ISP caching and delivery capacity.
  • allowing the ISP to reduce traffic load on some segments of the network by caching inside of the ISP network.
  • allowing the ISP to influence and/or control the traffic ingress points.
  • allowing the ISP to derive some incremental revenue for transport of the traffic and to monetize QoE services.

2.4. Nomadic Users

In this scenario, a CSP wishes to allow End Users who move between access networks to continue to access their content. The motivation of this case is to allow nomadic End Users to maintain access to content with a consistent QoE across a range of devices and/or geographic regions.

This use case covers situations like:

  • End Users moving between different access networks, which may be located within the same geographic region or different geographic regions.
  • End Users switching between different devices or delivery technologies, as discussed in Section 4.

Consider the following example, illustrated in Figure 2: End User A has a subscription to a broadband service from ISP A, her "home ISP". ISP A hosts CDN-A. Ordinarily, when End User A accesses content via ISP A (her "home ISP"), the content is delivered from CDN-A, which in this example is within ISP A's network.

However, while End User A is not connected to ISP A's network, for example, because it is connected to a WiFi provider or mobile network, End User A can also access the same content. In this case, End User A may benefit from accessing the same content delivered by an alternate CDN (CDN-B), in this case, hosted in the network of the WiFi or mobile provider (ISP B), rather than from CDN-A in ISP A's network.

       ,--,--,--.             ,--,--,--.
    ,-'  ISP A   `-.       ,-'  ISP B   `-.
   (    (CDN-A)     )=====(    (CDN-B)     )
    `-.          ,-'       `-.          ,-'
       `--'--'--'             `--'--'--'
            |                     |
      +------------+      +---------------+
      + EU A (home)|      | EU A (nomadic)|
      +------------+      +---------------+
    === CDN Interconnection

Figure 2

Though the content of CSP A is not accessible by typical End Users of CDN-B, End User A is able to gain access to her "home" content (i.e., the content of CSP A) through the alternate CDN (CDN-B).

Depending on the CSP's content delivery policies (see Appendix A.1), a user moving to a different geographic region may be subject to geo- blocking content delivery restrictions. In this case, he/she may not be allowed to access some pieces of content.

3. Offload Use Cases

3.1. Overload Handling and Dimensioning

A CDN is likely to be dimensioned to support an expected maximum traffic load. However, unexpected spikes in content popularity (flash crowd) may drive load beyond the expected peak. The prime recurrent time peaks of content distribution may differ between two

CDNs. Taking advantage of the different traffic peak times, a CDN may interconnect with another CDN to increase its effective capacity during the peak of traffic. This brings dimensioning savings to the CDNs, as they can use each other's resources during their respective peaks of activity.

Offload also applies to planned situations in which a CDN Provider needs CDN capacity in a particular region during a short period of time. For example, a CDN can offload traffic to another CDN for the duration of a specific maintenance operation or for the distribution of a special event, as in the scenario depicted in Figure 3. For instance, consider a TV channel that is the distributor for a major event, such as a celebrity's wedding or a major sport competition, and this TV channel has contracted particular CDNs for the delivery. The CDNs (CDN-A and CDN-B) that the TV channel uses for delivering the content related to this event are likely to experience a flash crowd during the event and will need to offload traffic, while other CDNs (CDN-C) will support a more typical traffic load and be able to handle the offloaded traffic.

In this use case, the Delivering CDN on which requests are offloaded should be able to handle the offloaded requests. Therefore, the uCDN might require information on the dCDNs to be aware of the amount of traffic it can offload to each dCDN.

     | TV Channel |
         |         \
      ,-,--,-.      \ ,-,--,-.        ,-,--,-.
    ,'        `.    ,'        `.    ,' CDN-C  `.
   (   CDN-A    )  (   CDN-B    )==(  offload   )
    `.        ,'    `.        ,'    `.        ,'
      `-'--'-'        `-'--'-'        `-'--'-'
    === CDN Interconnection

Figure 3

3.2. Resiliency

3.2.1. Failure of Content Delivery Resources

It is important for CDNs to be able to guarantee service continuity during partial failures (e.g., failure of some Surrogates). In partial failure scenarios, a CDN Provider has at least three options:

  1. if possible, use internal mechanisms to redirect traffic onto surviving equipment,
  1. depending on traffic management policies, forward some requests to the CSP's origin servers, and/or
  1. redirect some requests toward another CDN, which must be able to serve the redirected requests.

The last option is a use case for CDNI.

3.2.2. Content Acquisition Resiliency

Source content acquisition may be handled in one of two ways:

  • CSP origin, where a CDN acquires content directly from the CSP's origin server, or
  • CDN origin, where a downstream CDN acquires content from a Surrogate within an upstream CDN.

The ability to support content acquisition resiliency is an important use case for interconnected CDNs. When the content acquisition source fails, the CDN might switch to another content acquisition source. Similarly, when several content acquisition sources are available, a CDN might balance the load between these multiple sources.

Though other server and/or DNS load-balancing techniques may be employed in the network, interconnected CDNs may have a better understanding of origin-server availability, and be better equipped to both distribute load between origin servers and attempt content acquisition from alternate content sources when acquisition failures occur. When normal content acquisition fails, a CDN may need to try other content source options, for example:

  • an upstream CDN may acquire content from an alternate CSP origin server,
  • a downstream CDN may acquire content from an alternate Surrogate within an upstream CDN,
  • a downstream CDN may acquire content from an alternate upstream CDN, or
  • a downstream CDN may acquire content directly from the CSP's origin server.

Though content acquisition protocols are beyond the scope of CDNI, the selection of content acquisition sources should be considered and facilitated.

4. Capability Use Cases

4.1. Device and Network Technology Extension

In this use case, the CDN Provider may have the right geographic footprint, but may wish to extend the supported range of devices and User Agents or the supported range of delivery technologies. In this case, a CDN Provider may interconnect with a CDN that offers services that:

  • the CDN Provider is not willing to provide, or
  • its own CDN is not able to support.

The following examples illustrate this use case:

  1. CDN-A cannot support a specific delivery protocol. For instance, CDN-A may interconnect with CDN-B to serve a proportion of its traffic that requires HTTPS [RFC2818]. CDN-A may use CDN-B's footprint (which may overlap with its own) to deliver HTTPS without needing to deploy its own infrastructure. This case could also be true of other formats, delivery protocols (e.g., the Real Time Messaging Protocol (RTMP), the Real Time Streaming Protocol (RTSP), etc.), and features (specific forms of authorization such as tokens, per session encryption, etc.).
  1. CDN-A has a footprint covering traditional fixed-line broadband and wants to extend coverage to mobile devices. In this case, CDN-A may contract and interconnect with CDN-B, who has both:
  • a physical footprint inside the mobile network,
  • the ability to deliver content over a protocol that is required by specific mobile devices.
  1. CDN-A only supports IPv4 within its infrastructure but wants to deliver content over IPv6. CDN-B supports both IPv4 and IPv6 within its infrastructure. CDN-A interconnects with CDN-B to serve out its content over native IPv6 connections.

These cases can apply to many CDN features that a given CDN Provider may not be able to support or not be willing to invest in, and thus, that the CDN Provider would delegate to another CDN.

4.2. Technology and Vendor Interoperability

A CDN Provider may deploy a new CDN to run alongside its existing CDN as a simple way of migrating its CDN service to a new technology. In addition, a CDN Provider may have a multi-vendor strategy for its CDN deployment. Finally, a CDN Provider may want to deploy a separate CDN for a particular CSP or a specific network. In all these circumstances, CDNI benefits the CDN Provider, as it simplifies or automates some inter-CDN operations (e.g., migrating the request routing function progressively).

4.3. QoE and QoS Improvement

Some CSPs are willing to pay a premium for enhanced delivery of content to their End Users. In some cases, even if the CDN Provider could deliver the content to the End Users, it would not meet the CSP's service-level requirements. As a result, the CDN Provider may establish a CDN Interconnection agreement with another CDN Provider that can provide the expected QoE to the End User, e.g., via an Access CDN that is able to deliver content from Surrogates located closer to the End User and with the required service level.

5. Enforcement of Content Delivery Policy

An important aspect common to all the above use cases is that CSPs typically want to enforce content delivery policies. A CSP may want to define content delivery policies that specify when, how, and/or to whom the CDN delivers content. These policies apply to all interconnected CDNs (uCDNs and dCDNs) in the same or similar way that a CSP can define content delivery policies for content delivered by a single, non-interconnected CDN. Appendix A provides examples of CSP- defined policies.

6. Acknowledgments

The authors would like to thank Kent Leung, Francois Le Faucheur, Ben Niven-Jenkins, and Scott Wainner for lively discussions, as well as for their reviews and comments on the mailing list.

They also thank the contributors of the EU FP7 OCEAN and ETICS projects for valuable inputs.

Finally, the authors acknowledge the work of the former CDI working group. This document obsoletes [RFC3570] to avoid confusion.

7. Security Considerations

This document focuses on the motivational use cases for CDN Interconnection and does not analyze the associated threats. Those threats are discussed in [RFC6707]. Appendix A.2 of this document provides example security policies that CSPs might impose on CDNs to mitigate the threats.

8. References

8.1. Normative References

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

8.2. Informative References

   [CDNI-REQ]  Leung, K. and Y. Lee, "Content Distribution Network
               Interconnection (CDNI) Requirements", Work in Progress,
               June 2012.
   [RFC2818]   Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
   [RFC3570]   Rzewski, P., Day, M., and D. Gilletti, "Content
               Internetworking (CDI) Scenarios", RFC 3570, July 2003.
   [RFC6390]   Clark, A. and B. Claise, "Guidelines for Considering New
               Performance Metric Development", BCP 170, RFC 6390,
               October 2011.

Appendix A. Content Service Providers' Delivery Policies

CSPs commonly apply different delivery policies to given sets of content assets delivered through CDNs. Interconnected CDNs need to support these policies. This appendix presents examples of CSPs' delivery policies and their consequences on CDNI operations.

A.1. Content Delivery Policy Enforcement

The content distribution policies that a CSP attaches to a content asset may depend on many criteria. For instance, distribution policies for audiovisual content often combine constraints of varying levels of complexity and sophistication, for example:

   o  temporal constraints (e.g., available for 24 hours, available 28
      days after DVD release, etc.),
  • user agent platform constraints (e.g., mobile device platforms, desktop computer platforms, set-top-box platforms, etc.),
   o  resolution-based constraints (e.g., high definition vs. standard
      definition encodings),
  • user agent identification or authorization,
   o  access network constraints (e.g., per NSP), and
  • IP geo-blocking constraints (e.g., for a given coverage area).

CSPs may use sophisticated policies in accordance with their business model. However, the enforcement of those policies does not necessarily require that the delivery network understand the policy rationales or how policies apply to specific content assets. Content delivery policies may be distilled into simple rules that can be commonly enforced across all dCDNs. These rules may influence dCDN delegation and Surrogate selection decisions, for instance, to ensure that the specific rules (e.g., time-window, geo-blocking, pre- authorization validation) can indeed be enforced by the Delivering CDN. In turn, this can guarantee to the CSP that content delivery policies are properly applied.

   | CSP |  Policies driven by business (e.g., available only
   +-----+  in the UK and only from July 1st to September 1st)
       \ Translate policies into
        \simple rules (e.g., provide an authorization token)
        | CDN | Apply simple rules (e.g., check an
        +-----+ authorization token and enforce geo-blocking)
             \ Distribute simple rules
            | CDN | Apply simple rules

Figure 4

A.2. Secure Access

Many protocols exist for delivering content to End Users. CSPs may dictate a specific protocol or set of protocols that are acceptable for delivery of their content, especially in the case where a secured content transmission is required (e.g., must use HTTPS). CSPs may also perform a per-request authentication/authorization decision and then have the CDNs enforce that decision (e.g., must validate URL signing, etc.).

A.3. Branding

Preserving the branding of the CSP throughout delivery is often important to the CSP. CSPs may desire to offer content services under their own name, even when the associated CDN service involves other CDN Providers. For instance, a CSP may desire to ensure that content is delivered with URIs appearing to the End Users under the CSP's own domain name, even when the content delivery involves separate CDN Providers. The CSP may wish to prevent the delivery of its content by specific dCDNs that lack support for such branding preservation features.

Analogous cases exist when the uCDN wants to offer CDN services under its own branding even if dCDNs are involved, and so it restricts the delivery delegation to a chain that preserves its brand visibility.

Authors' Addresses

   Gilles Bertrand (editor)
   France Telecom - Orange
   38-40 rue du General Leclerc
   Issy les Moulineaux,   92130
   Phone: +33 1 45 29 89 46
   Stephan Emile
   France Telecom - Orange
   2 avenue Pierre Marzin
   Lannion  F-22307
   Trevor Burbridge
   B54 Room 70, Adastral Park, Martlesham
   Ipswich,   IP5 3RE
   Philip Eardley
   B54 Room 77, Adastral Park, Martlesham
   Ipswich,   IP5 3RE
   Kevin J. Ma
   Azuki Systems, Inc.
   43 Nagog Park
   Acton, MA  01720
   Phone: +1 978-844-5100
   Grant Watson
   Alcatel-Lucent (Velocix)
   3 Ely Road
   Milton, Cambridge  CB24 6AA