Network Working Group M. Day Internet-Draft Cisco Expires: September 2, 2001 B. Cain Cereva G. Tomlinson CacheFlow P. Rzewski Inktomi March 2, 2001 A Model for Content Internetworking draft-day-cdnp-model-05.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 2, 2001. Copyright Notice Copyright (C) The Internet Society (2001). All Rights Reserved. Abstract There is wide interest in the technology for interconnecting Content Networks, variously called "Content Peering" or "Content Internetworking". A common vocabulary helps the process of discussing such interconnection and interoperation. This document introduces Content Networks and Content Internetworking, and proposes elements for such a common vocabulary. Day, et. al. Expires September 2, 2001 [Page 1] Internet-Draft CDI Model March 2001 Table of Contents 1. Introduction................................................ 3 2. Content Networks............................................ 3 2.1 Problem Description......................................... 4 2.2 Forward Proxy Caching....................................... 5 2.3 Server Farms................................................ 6 2.4 Content Distribution Networks............................... 7 2.4.1 Historic Evolution of CDNs.................................. 9 2.4.2 Describing CDN Value: Reach and Scale....................... 9 3. Content Network Model Terms.................................10 4. Content Network Examples and Commentary.....................12 4.1 Understanding CDNs..........................................12 4.2 Understanding content structure.............................12 5. Content Internetworking.....................................13 6. Content Internetworking Model Terms.........................13 7. Content Internetworking Examples and Commentary.............15 7.1 Understanding Content Internetworking.......................15 7.2 Content Signaling...........................................16 8. Operational Considerations..................................16 9. Security Considerations.....................................16 10. Acknowledgements............................................16 References..................................................16 Authors' Addresses..........................................17 Full Copyright Statement....................................18 Day, et. al. Expires September 2, 2001 [Page 2] Internet-Draft CDI Model March 2001 1. Introduction Content Networks, such as CDNs, are of increasing importance to the overall architecture of the Web. This document presents a vocabulary for use in developing technology for interconnecting Content Networks. By analogy with peering of IP networks, this interconnection is sometimes called "content peering" or "content internetworking". Section 2 provides background on Content Networks. Section 3 introduces the terms used for elements of a Content Network and explains how those terms are used. Section 5 deals with Content Internetworking, introducing the terms and explaining how those terms are used. The remainder of the document notes various operational and security considerations that are relevant to Content Internetworking. The terminology in this document builds from the previous taxonomy of web caching and replication [2]. In particular, we have attempted to avoid the use of the common terms "proxies" or "caches" in favor of the better-defined terms "caching proxy," "reverse caching proxy," and "server accelerator." The sections defining terms are organized alphabetically, which is appropriate for reference but which makes them difficult to read the first time. Rather than reading the document from beginning to end, the authors recommend that the first-time reader skip past the sections defining terms to the following sections with examples, referring back to the definitions as necessary. The interested reader is also referred to [3], which enumerates scenarios for Content-Internetworking-related interactions; [4], which describes requirements for accounting and associated issues; [5], which gives an overall architecture of the elements for CDN peering; and [6], which summarizes known mechanisms for request- routing. It should be noted that previous versions of this document and other working drafts of CDI appear to be more specifically focused on "Content Distribution Networks" (CDNs) and "CDN Peering". The use of the more general terms "Content Networks" and "Content Internetworking" are currently favored because they imply a wider variety of real-life scenarios that may be encompassed in CDI's work. Also, we are attempting to take the emphasis off use of the word "peering" because the term is often taken to imply a settlement-free arrangement (as is common with bandwidth peering). 2. Content Networks The past several years have seen the evolution of technologies centered around "content". Protocols, appliances, and entire markets Day, et. al. Expires September 2, 2001 [Page 3] Internet-Draft CDI Model March 2001 have been created exclusively for the location, download, and usage- tracking tracking of content. Some sample technologies in this area have included web caching proxies, content management tools, intelligent "web switches", and advanced log analysis tools. When used together, these tools form new types of networks, dubbed "Content Networks". Whereas network infrastructures previously have traditionally occupied layers 1 through 3 of the OSI stack, Content Networks include network infrastructure that exists in layers 4 through 7. Whereas lower-layer network infrastructures revolved around the routing, forwarding, and switching of frames and packets, Content Networks deal with the routing and forwarding of requests and responses for content. The units of transported data in Content Networks, such as web pages, movies, or songs, are often very large and may span hundreds or thousands of packets. Content Networks can be seen as a new virtual overlay to the OSI stack: a "Content Layer", to enable richer services that rely on underlying elements from all 7 layers of the stack. Whereas traditional applications, such as file transfer (FTP), relied on underlying protocols such as TCP/IP for transport, overlay services in Content Networks rely on layer 7 protocols such as HTTP or RTSP for transport. The proliferation of Content Networks and Content Networking capabilities gives rise to interest in interconnecting Content Networks and finding ways for distinct Content Networks to cooperate for better overall service. 2.1 Problem Description Content Networks typically play some role in solving the "content distribution problem". Abstractly, the goal in solving this problem is to arrange a rendezvous between a content source at an origin server and a content sink at a viewer's client. In the trivial case, the rendezvous mechanism is that every client sends every request directly to the origin server named in the host part of the URL identifying the content. As the audience for the content source grows, so do the demands on the origin server. There are a variety of ways in which the trivial system can be modified for better performance. The single logical server may in fact be a large "farm" of server machines behind a switch. Both caching proxies and reverse caching proxies can be deployed between the client and server, so that requests can be satisfied by some cache instead of by the server. For the sake of background, several sample Content Networks are described in the following sections that each attempt to address this problem. Day, et. al. Expires September 2, 2001 [Page 4] Internet-Draft CDI Model March 2001 2.2 Forward Proxy Caching A type of Content Network that has been in use for several years is a forward proxy cache deployment. Such a network might typically be employed by an ISP for the benefit of end users accessing the Internet, such as through dial or cable modem. In the interest of improving performance and reducing bandwidth utilization, caching proxies are deployed close to the end users. These end users are encouraged to route their web requests through the caches rather than directly to origin servers, such as by configuring their browsers to do so. Note that when this is done, the end user's entire browsing session goes through a specific proxy cache. That proxy cache will therefore contain the "hot set" of all Internet content being viewed by the totality of access users utilizing that proxy cache. When a request is being handled a caching proxy on behalf of a user, other routing decisions may be made, such as: o A provider that deploys access caches in many geographically diverse locations may also deploy regional parent caches to further aggregate user requests and responses. This may provide additional performance improvement and bandwidth savings. When parents are included, this is known as hierarchical caching. o Using rich parenting protocols, redundant parents may be deployed such that a failure in a primary parent is detected and a backup is used instead. o Using similar parenting protocols, requests may be partitioned such that requests for certain content domains are sent to a specific primary parent. This can help to maximize the efficient use of caching proxy resources. Day, et. al. Expires September 2, 2001 [Page 5] Internet-Draft CDI Model March 2001 The following diagram depicts a hierarchical cache deployment as described above: ^ ^ | | requests to | | origin servers | | -------- -------- |parent| |parent| |cache | |cache | |proxy | |proxy | -------- -------- ^ ^ requests for \ / requests for foo.com \ / bar.com content \ / content \ / ------- ------- ------- ------- |edge | |edge | |edge | |edge | |cache| |cache| |cache| |cache| |proxy| |proxy| |proxy| |proxy| ------- ------- ------- ------- ^ | all content | requests | for this | client | -------- |client| -------- 2.3 Server Farms Another type of Content Network that has been in widespread use for several years is a server farm. A typical server farm makes use of an intelligent switch that acts as a dispatcher for content requests. The switch then routes requests among a (potentially large) group of servers. Some of the goals of a a server farm include: o Creating the impression that the group of servers is actually a single origin site. o Load-balancing of requests across all servers in the group. o Automatically routing of requests away from servers that fail. o Routing all requests for a particular client's session to the same server, in order to preserve session state. Day, et. al. Expires September 2, 2001 [Page 6] Internet-Draft CDI Model March 2001 The following diagram depicts a simple server farm deployment: --------- --------- --------- --------- |content| |content| |content| |content| |server | |server | |server | |server | | | | | | | | | --------- --------- --------- --------- ^ ^ request from \ / request from client A \ / client B \ / ------------- |intelligent| | switch | ------------- ^ ^ / \ / \ / \ request from request from client A client B A similar type of Content Network may be constructed by replacing the switch with a server accelerator [2]. 2.4 Content Distribution Networks Both hierarchical caching and server farms are useful techniques, but have limits. Server farms and server accelerators can improve the scalability of the origin server. However, since the multiple servers and server accelerators are typically deployed near the origin server, they do little to improve performance problems that are due to network congestion. Caching proxies can improve performance problems due to network congestion (since they are situated near the clients) but they cache objects based on client demand -- so they may not help the distribution load of a given origin server. Thus, a content provider with a popular content source can find that it has to invest in large server farms, load balancing, and high- bandwidth connections to keep up with demand. Even with those investments, the user experience for viewers may still be relatively poor due to congestion in the network as a whole. To address these limitations. another type of Content Network that has been deployed in increasing numbers in recent years is the Content Distribution Network, or CDN. A CDN essentially combines the cache-management approach of reverse caching proxies with the network placement of (forward) caching proxies. A CDN has multiple replicas of each content item being hosted. A request from a browser Day, et. al. Expires September 2, 2001 [Page 7] Internet-Draft CDI Model March 2001 for a single content item is directed to a "good" replica, where "good" usually means that the item is served to the client quickly compared to the time it would take fetch it from the origin server, with appropriate integrity and consistency. Static information about geographic locations and network connectivity is usually not sufficient to do a good job of choosing a replica. Instead, a CDN typically incorporates dynamic information about network conditions and load on the replicas, directing requests so as to balance the load. Compared to using servers and caches in a single data center, a CDN is a relatively complex system encompassing multiple points of presence, in locations that may be geographically far apart. Operating a CDN is not easy for a content provider, since a content provider wants to focus its resources on developing high-value content, not on managing network infrastructure. Instead, a more typical configuration is that a network service provider builds and operates a CDN, offering a content distribution service to a number of content providers. A CDN enables a service provider to act on behalf of the content provider to deliver copies of origin server content to clients from multiple diverse locations. The increase in number and diversity of location is intended to improve download times and thus improve the user experience. A CDN has some combination of a request-routing infrastructure, a content-delivery infrastructure, a distribution infrastructure, and an accounting infrastructure. The content- delivery infrastructure consists of a set of "surrogate" servers [2] that deliver copies of content to sets of users. The request-routing infrastructure consists of mechanisms that move a client toward a rendezvous with a surrogate. The distribution infrastructure consists of mechanisms that move content from the origin server to the surrogates. Finally, the accounting infrastructure tracks and collects data on request-routing, distribution, and delivery functions within the CDN. Day, et. al. Expires September 2, 2001 [Page 8] Internet-Draft CDI Model March 2001 The following diagram depicts a simple CDN as described above: ---------- ---------- |request-| |request-| |routing | |routing | | system | | system | ---------- ---------- ^ | (1) client's | | (2) response content | | indicating request | | location of ----------- | | content |surrogate| | | ----------- ----------- | | |surrogate| | | ----------- ----------- | | |surrogate| | | ----------- | | ^ | v / (3) client opens client--- connection to retrieve content Because CDNs are arguably the most complicated form of Content Network currently deployed, they warrant further description. 2.4.1 Historic Evolution of CDNs The first important use of CDNs was for the distribution of heavily- requested graphic files (such as GIF files on the home pages of popular servers). However, both in principle and increasingly in practice, a CDN can support the delivery of any digital content -- including various forms of streaming media. A number of CDN services have been built and offered commercially. In addition, a number of hardware and software vendors have developed products that enable the construction of a CDN with "off- the-shelf" parts. 2.4.2 Describing CDN Value: Reach and Scale There are two fundamental elements that give a CDN value: outsourcing infrastructure and improved content delivery. A CDN allows multiple surrogates to act on behalf of an orgin server, therefore removing the delivery of content from a centralized site to multiple and (usually) highly distributed sites. We refer to increased aggregate infrastructure size as "scale." In addition, a CDN can be constructed with copies of content near to end users, overcoming issues of network size, network congestion, and network failures. We refer to increased diversity of content locations as "reach." Day, et. al. Expires September 2, 2001 [Page 9] Internet-Draft CDI Model March 2001 In a typical (non-internetworked) CDN, a single service provider operates the request-routers, the surrogates, and the content distributors. In addition, that service provider establishes (business) relationships with content publishers and acts on behalf of their origin sites to provide a distributed delivery system. The value of that CDN to a content provider is a combination of its scale and its reach. 3. Content Network Model Terms This section consists of the definitions of a number of terms used to refer to roles, participants, and objects involved in Content Networks. ACCOUNTING Measurement and recording of DISTRIBUTION and DELIVERY activities, especially when the information recorded is ultimately used as a basis for the subsequent transfer of money, goods, or obligations. ACCOUNTING SYSTEM A collection of NETWORK ELEMENTS that supports ACCOUNTING for a single CONTENT NETWORK. AUTHORITATIVE REQUEST-ROUTING SYSTEM The REQUEST-ROUTING SYSTEM that is the correct/final authority for a particular item of CONTENT. CDN Content Delivery Network or Content Distribution Network. A type of CONTENT NETWORK in which the NETWORK ELEMENTS are arranged for more effective delivery of CONTENT to CLIENTS. Typically a CDN consists of a REQUEST-ROUTING SYSTEM, SURROGATES, a DISTRIBUTION SYSTEM, and an ACCOUNTING SYSTEM. CLIENT The origin of a REQUEST and the destination of the corresponding delivered CONTENT. CONTENT Digital data resources. [Editor note: discussion is currently active about correct alignment between resource/entity/variant model of HTTP and "content".] One important form of CONTENT with additional constraints on DISTRIBUTION and DELIVERY is CONTINUOUS MEDIA. CONTENT NETWORK A collection of NETWORK ELEMENTS that assist in the location, download, and usage-tracking tracking of CONTENT. CONTENT SIGNAL Day, et. al. Expires September 2, 2001 [Page 10] Internet-Draft CDI Model March 2001 A message delivered through a DISTRIBUTION SYSTEM that specifies information about an item of CONTENT. For example, a CONTENT SIGNAL can indicate that the ORIGIN has a new version of some piece of CONTENT. CONTINUOUS MEDIA CONTENT where there is a timing relationship between source and sink; that is, the sink must reproduce the timing relationship that existed at the source. The most common examples of CONTINUOUS MEDIA are audio and motion video. CONTINUOUS MEDIA can be real-time (interactive), where there is a "tight" timing relationship between source and sink, or streaming (playback), where the relationship is less strict. DELIVERY The activity of presenting a PUBLISHER's CONTENT for consumption by a CLIENT. Contrast with DISTRIBUTION and REQUEST-ROUTING. DISTRIBUTION The activity of moving a PUBLISHER's CONTENT from its ORIGIN to one or more SURROGATEs. DISTRIBUTION can happen either in anticipation of a SURROGATE receiving a REQUEST (pre-positioning) or in response to a SURROGATE receiving a REQUEST (fetching on demand). Contrast with DELIVERY and REQUEST-ROUTING. DISTRIBUTION SYSTEM A collection of NETWORK ELEMENTS that support DISTRIBUTION for a single CONTENT NETWORK. The DISTRIBUTION SYSTEM also propagates CONTENT SIGNALs. MAPPING See REQUEST-ROUTING. Some earlier versions of this document and others used the term MAPPING, but REQUEST-ROUTING is now preferred. NETWORK ELEMENT A device or system that affects the processing of network messages. ORIGIN The point at which CONTENT first enters a DISTRIBUTION SYSTEM. The ORIGIN for any item of CONTENT is the server or set of servers at the "core" of the distribution, holding the "master" or "authoritative" copy of that CONTENT. PUBLISHER The party that ultimately controls the content and its distribution. REACHABLE SURROGATES The collection of SURROGATES that can be contacted via a particular DISTRIBUTION SYSTEM or REQUEST-ROUTING SYSTEM. Day, et. al. Expires September 2, 2001 [Page 11] Internet-Draft CDI Model March 2001 REQUEST A message identifying a particular item of CONTENT to be delivered. [Editor Note: Brad Cain recommends distinguishing REQUEST-ROUTING REQUEST from CONTENT REQUEST. Does this make the model too closely tied to DNS-style request-routing? To be discussed.] REQUEST-ROUTING The activity of steering or directing a REQUEST from a CLIENT to a suitable SURROGATE. REQUEST-ROUTING SYSTEM A collection of NETWORK ELEMENTS that support REQUEST-ROUTING for a single CONTENT NETWORK. SURROGATE A delivery server, other than the ORIGIN. Receives a mapped REQUEST and delivers the corresponding CONTENT. Note: This definition has a narrower semantic context than the more generally used term defined in [2]. 4. Content Network Examples and Commentary This section uses the terms of the previous to explain concepts of CONTENT NETWORKs and CONTENT. Because CDNs contain all the major components of Content Networking (i.e. REQUEST-ROUTING, DISTRIBUTION, DELIVERY, ACCOUNTING), the example described is a CDN. 4.1 Understanding CDNs With the elements defined so far, we can outline the operation of a "typical" CDN at a high level. The CLIENT's REQUEST enters a REQUEST-ROUTING SYSTEM, and the ORIGIN's CONTENT enters a DISTRIBUTION SYSTEM. Note that the relative timing of these events is unspecified. Both systems (REQUEST-ROUTING and DISTRIBUTION) converge on SURROGATES, which are non-ORIGIN servers of CONTENT. Effectively, the DISTRIBUTION SYSTEM is moving CONTENT out to SURROGATES, and the REQUEST-ROUTING SYSTEM is then taking advantage of that distribution of CONTENT. [Editor Note: Could change this description to deal with REQUEST- ROUTING REQUESTS and CONTENT REQUESTS.] 4.2 Understanding content structure The model defines CONTENT as well as a subsidiary concept: CONTINUOUS MEDIA. Any identifiable resource of digital data is an item of CONTENT. So CONTENT is the most generic description of what is transported and served up by a CONTENT NETWORK. Day, et. al. Expires September 2, 2001 [Page 12] Internet-Draft CDI Model March 2001 In many cases, an item of CONTENT can be delivered by a CONTENT NETWORK without concern about maintaining timing relationships. However, there are some forms of CONTENT where it is critical that some timing relationships be met. The model refers to those forms of CONTENT as CONTINUOUS MEDIA. 5. Content Internetworking There are limits to how large any one network's scale and reach can be. Increasing either scale or reach is ultimately limited by the cost of equipment, the space available for deploying equipment, and/or the demand for that scale/reach of infrastructure. Sometimes a particular audience is tied to a single service provider or a small set of providers by constraints of technology, economics, or law. Other times, a network provider may be able to manage surrogates and a distribution system, but may have no direct relationship with content providers. Such a provider wants to have a means of affiliating their delivery and distribution infrastructure with other parties who have content to distribute. Content Internetworking allows different Content Networks to share resources so as to provide larger scale and/or reach to each participant than they could otherwise achieve. By using commonly defined protocols for Content Internetworking, each Content Network can treat neighboring Content Networks as "black boxes", allowing them to hide internal details from each other. 6. Content Internetworking Model Terms This section consists of the definitions of a number of terms used to refer to roles, participants, and objects involved in internetworking Content Networks. ACCOUNTING ADVERTISEMENT ADVERTISEMENT from a CONTENT NETWORK's ACCOUNTING PEERING SYSTEM about the collections of CONTENT for which that CONTENT NETWORK requires ACCOUNTING information. ACCOUNTING PEERING Interconnection of two or more ACCOUNTING SYSTEMS so as to enable the exchange of information between them. The form of ACCOUNTING PEERING required may depend on the nature of the NEGOTIATED RELATIONSHIP between the peering parties -- in particular, on the value of the economic exchanges anticipated. ACCOUNTING PEERING SYSTEM See PEERING SYSTEM. ADVERTISEMENT Information about available resources, exchanged among PEERING SYSTEMS. Types of ADVERTISEMENT include REQUEST-ROUTING Day, et. al. Expires September 2, 2001 [Page 13] Internet-Draft CDI Model March 2001 ADVERTISEMENTS, DISTRIBUTION ADVERTISEMENTS and ACCOUNTING ADVERTISEMENTS. BILLING ORGANIZATION An entity that operates an ACCOUNTING SYSTEM to support billing within a NEGOTIATED RELATIONSHIP with a PUBLISHER. CONTENT PEERING GATEWAY (CPG) A point through which a CONTENT NETWORK can be peered with others through one or more kinds of peering. A CPG may be the point of contact for DISTRIBUTION PEERING, REQUEST-ROUTING PEERING, and/or ACCOUNTING PEERING, and thus may incorporate some or all of the corresponding PEERING SYSTEMs for the CONTENT NETWORK. DISTRIBUTING CONTENT NETWORK A CONTENT NETWORK that does not have a NEGOTIATED RELATIONSHIP with the PUBLISHER for the CONTENT being delivered. DISTRIBUTION ADVERTISEMENT An ADVERTISEMENT from a CONTENT NETWORK's DISTRIBUTION PEERING SYSTEM describing the availability of collections of CONTENT via the CONTENT NETWORK's DISTRIBUTION SYSTEM. DISTRIBUTION PEERING Interconnection of two or more DISTRIBUTION SYSTEMS so as to propagate CONTENT SIGNALS and copies of CONTENT to groups of SURROGATES. DISTRIBUTION PEERING SYSTEM See PEERING SYSTEM. INJECTION A "send-only" form of DISTRIBUTION PEERING that takes place from an ORIGIN to a peer CONTENT NETWORK. INTER- Describes activity that involves more than one CONTENT NETWORK (e.g. INTER-CDN). Contrast with INTRA-. INTRA- Describes activity within a single CONTENT NETWORK (e.g. INTRA- CDN). Contrast with INTER-. NEGOTIATED RELATIONSHIP A relationship whose terms and conditions are partially or completely established outside the context of CONTENT NETWORK peering protocols. PEERING SYSTEM A collection of NETWORK ELEMENTS supporting some form of interconnected operation among two or more CDNs. Examples (not Day, et. al. Expires September 2, 2001 [Page 14] Internet-Draft CDI Model March 2001 separately defined): ACCOUNTING PEERING SYSTEM, DISTRIBUTION PEERING SYSTEM, REQUEST-ROUTING PEERING SYSTEM. REMOTE CONTENT NETWORK A CONTENT NETWORK able to deliver CONTENT for a particular REQUEST that is not the AUTHORITATIVE REQUEST-ROUTING SYSTEM for that REQUEST. REQUEST-ROUTING ADVERTISEMENT An ADVERTISEMENT from a CONTENT NETWORK's REQUEST-ROUTING PEERING SYSTEM describing the availability of collections of CONTENT via that CONTENT NETWORK's REQUEST-ROUTING SYSTEM. REQUEST-ROUTING PEERING Interconnection of two or more REQUEST-ROUTING SYSTEMS so as to increase the number of REACHABLE SURROGATES for at least one of the interconnected systems. REQUEST-ROUTING PEERING SYSTEM See PEERING SYSTEM. 7. Content Internetworking Examples and Commentary This section uses the terms of the previous to explain concepts of CONTENT NETWORK peering. Because CDNs contain all the major components of Content Networking (i.e. REQUEST-ROUTING, DISTRIBUTION, DELIVERY, ACCOUNTING), the example describes internetworking among CDNs. 7.1 Understanding Content Internetworking The model offers a number of ways in which different CDNs can be interconnected. An arrangement of interconnected REQUEST-ROUTING SYSTEMS is called REQUEST-ROUTING PEERING. Analogously, interconnected DISTRIBUTION SYSTEMS give rise to DISTRIBUTION PEERING, and interconnected ACCOUNTING SYSTEMS give rise to ACCOUNTING PEERING. The communicating elements on each side are referred to as PEERING SYSTEMS. So when two or more DISTRIBUTION SYSTEMS may be interconnected by PEERING, it is actually the DISTRIBUTION PEERING SYSTEMS that are communicating with each other to accomplish the exchange of information required. A CONTENT PEERING GATEWAY (CPG) is a generic term used in the model for one or more PEERING SYSTEMS when it is not important to distinguish the PEERING SYSTEM or form of PEERING involved. CPGs exchange ADVERTISEMENTS. There are three main kinds of ADVERTISEMENT: ACCOUNTING ADVERTISEMENTS, REQUEST-ROUTING ADVERTISEMENTS, and DISTRIBUTION ADVERTISEMENTS. An ACCOUNTING ADVERTISEMENT describes a collection of URLs for which a given ACCOUNTING SYSTEM wants to receive accounting information when the content is delivered. [Editor note: is accounting information potentially collected for REQUEST-ROUTING or DISTRIBUTION (for Day, et. al. Expires September 2, 2001 [Page 15] Internet-Draft CDI Model March 2001 purposes other than tracking operational health) as well?] A REQUEST-ROUTING ADVERTISEMENT describes a collection of URLs whose content can be delivered by REQUEST-ROUTING through the corresponding CDN. A DISTRIBUTION ADVERTISEMENT describes the service level(s) available from a CDN's SURROGATES (as a whole) to some collection of CLIENT addresses. 7.2 Content Signaling CDNs operate on behalf of PUBLISHERs and ORIGINs and therefore must provide accurate, up-to-date copies of CONTENT. A CDN DISTRIBUTION SYSTEM may deliver CONTENT SIGNALS to relevant SURROGATES when appropriate. In the presence of peered distribution where the peered systems support such signals, CONTENT SIGNALS must be propagated to each SURROGATE with a copy of the relevant CONTENT. 8. Operational Considerations [Editor's Note: Consider problem of incorrect advertisements of content or service levels. Need to ensure that there are means within the protocol or recommended practices so that CDNs aren't encouraged to pull traffic they can't really handle.] 9. Security Considerations Content Internetworking raises some security-related issues, and a detailed discussion of those issues appears in [5]. 10. Acknowledgements The definition of CONTINUOUS MEDIA is adapted from RFC 2326. The authors acknowledge the contributions and comments of Fred Douglis (AT&T), Don Gilletti (CacheFlow), Markus Hoffmann (Lucent), Barron Housel (Cisco), Barbara Liskov (Cisco), John Martin (Network Appliance), Raj Nair (Cisco), Hilarie Orman (Novell), Doug Potter (Cisco), Oliver Spatscheck (AT&T), and Nalin Mistry (Nortel). References [1] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999, . [2] Cooper, I., Melve, I. and G. Tomlinson, "Internet Web Replication and Caching Taxonomy", RFC3040, January 2001, . [3] Day, M., Gilletti, D., and P. Rzewski, "Content Internetworking Scenarios", draft-day-cdnp-scenarios-03.txt (work in progress), March 2001, Day, et. al. Expires September 2, 2001 [Page 16] Internet-Draft CDI Model March 2001 . [4] Gilletti, D., Nair, R., Scharber, J., and J. Guha, "CDN-I Internetworking Authentication, Authorization, and Accounting Requirements", draft-gilletti-cdnp-aaa-reqs-01.txt (work in progress), March 2001, . [5] Green, M., Cain, B., Tomlinson, G., Thomas, S., and P. Rzewski, "Content Internetworking Architectural Overview", draft-green- cdnp-gen-arch-03.txt (work in progress), March 2001, . [6] Barbir, A., Cain, B., Douglis, F., Green, M., Hoffmann, M., Nair, R., Potter, D. and O. Spatscheck, "Known CDN Request- Routing Mechanisms", draft-cain-cdnp-known-request-routing- 01.txt (work in progress), February 2001, . Authors' Addresses Mark S. Day Cisco Systems 135 Beaver Street Waltham, MA 02452 US Phone: +1 781 663 8310 EMail: markday@cisco.com Brad Cain Cereva Networks Email: bcain@cereva.com Gary Tomlinson CacheFlow Inc. 12034 134th Ct. NE Suite 201 Redmond, WA 98052 US Phone: +1 425 820 3009 EMail: garyt@cacheflow.com Phil Rzewski Day, et. al. Expires September 2, 2001 [Page 17] Internet-Draft CDI Model March 2001 Inktomi 4100 East Third Avenue MS FC1-4 Foster City, CA 94404 US Phone +1 650 653 2487 Email: philr@inktomi.com Full Copyright Statement Copyright (C) The Internet Society (2001). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. 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Acknowledgement Funding for the RFC editor function is currently provided by the Internet Society. Day, et. al. Expires September 2, 2001 [Page 18]