One document matched: draft-day-cdnp-model-00.txt
Network Working Group M. Day
Internet-Draft Cisco
Expires: March 26, 2001 B. Cain
Mirror Image Internet
G. Tomlinson
Entera
September 25, 2000
A Model for Content Peering
draft-day-cdnp-model-00.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
There is wide interest in interconnecting content distribution
networks (CDNs), variously called "content peering" or "CDN
peering". A common vocabulary helps the process of discussing such
interconnection and interoperation. This document proposes elements
for a such a common vocabulary.
Note on Content Alliance
This document is an interim product of work in progress within the
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Content Alliance. For information about the Content Alliance, see
www.content-peering.org. Please send comments about this document to
cpwg-discussion@content-peering.org.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Description . . . . . . . . . . . . . . . . . . . . . 4
2.1 Extending Reach & Scale . . . . . . . . . . . . . . . . . . . 5
3. Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Examples and Commentary . . . . . . . . . . . . . . . . . . . 11
4.1 Understanding CDNs . . . . . . . . . . . . . . . . . . . . . . 11
4.2 Understanding content structure . . . . . . . . . . . . . . . 11
4.3 Understanding Peering . . . . . . . . . . . . . . . . . . . . 12
4.4 Content Signalling . . . . . . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
This document presents a vocabulary for use in developing "content
peering," or more accurately "peering of CDNs". Section 2 describes
content distribution, CDNs, and the motivation for peering of CDNs
in some more detail. Section 3 consists of the actual definitions
that constitute the model, and is intended to be the authoritative
part of the document. All other sections contain examples and
introduction. Section 4 uses the definitions to further explain how
aspects of peering are likely to work.
Section 3 is organized alphabetically, which is appropriate for
reference but which makes it difficult to read the first time.
Rather than reading the document from beginning to end, the authors
recommend that the first-time reader read sections 2 and 4, looking
at definitions in section 3 as necessary.
The interested reader is also referred to [3] which enumerates a
large number of scenarios for content-peering-related interactions,
and [4] which gives the overall architecture of the elements for CDN
peering.
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2. Problem Description
Abstractly, the "content distribution 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 (conventional
web access without a CDN), the rendezvous mechanism is that every
client sends every request directly to the origin server.
This trivial approach to content distribution suffers from scaling
and manageability problems. As the audience for the content source
grows, so do the demands on the origin server and its associated
infrastructure. 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.
A content provider wants to focus its resources on developing
high-value content, not on managing network infrastructure; so there
is an opportunity for a service provider in this situation. A
service provider can build a CDN and offer a content distribution
service to a content provider.
A CDN enables a service provider to act on behalf of the content
provider to deliver copies of origin server content from multiple
diverse locations. The increase in number and diversity of locations
is intended to speed download times and thus improve the user
experience. A nontrivial CDN has some combination of a redirection
infrastructure, a content-delivery infrastructure, and a
distribution infrastructure. The content-delivery infrastructure
consists of a set of "surrogate" servers that deliver copies of
content to sets of users. The redirection infrastructure consists of
mechanisms that move a client toward a rendezvous with a content
server. The distribution infrastructure consists of mechanisms that
move content from the origin server to the surrogates. An effective
CDN serves frequently-accessed content from a surrogate that is
"best suited" for a given client.
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.
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2.1 Extending Reach & Scale
In a typical (non-peered) CDN, a single service provider operates
the request routers, the surrogates, and the content distributors.
In addition, that service provider establishes (business)
relationships withe 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.
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.
CDN peering allows different CDNs to share resources so as to
provide larger scale and/or reach to each participant than they
could otherwise achieve.
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3. Model
This section consists of the definitions of a number of terms used
to refer to roles, participants, and objects involved in Content
Distribution Networks and CDN Peering.
This section and the references section are normative.
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 ADVERTISEMENT
ADVERTISEMENT from a CDN's ACCOUNTING PEERING SYSTEM about the
collections of CONTENT for which that CDN 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
A collection of NETWORK ELEMENTS that support ACCOUNTING PEERING
with other CDNs.
ACCOUNTING SYSTEM
A collection of NETWORK ELEMENTS that supports ACCOUNTING for a
single CDN.
ADVERTISEMENT
Information about available resources, exchanged among PEERING
SYSTEMS. Types of ADVERTISEMENT include REDIRECTION
ADVERTISEMENTS, DISTRIBUTION ADVERTISEMENTS and ACCOUNTING
ADVERTISEMENTS.
AUTHORITATIVE REDIRECTION SYSTEM
The REDIRECTION SYSTEM that is the correct/final authority for a
particular item of CONTENT. Typically operated by the BILLING
CDN.
BILLING CDN
The single CDN that has a NEGOTIATED RELATIONSHIP with the
PUBLISHER making that CDN responsible for DISTRIBUTION of some
particular collection of CONTENT. Contrast with DISTRIBUTING CDN.
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CDN
Content Delivery Network or Content Distribution Network. A
collection of NETWORK ELEMENTS arranged for more effective
delivery of CONTENT to CLIENTS. Typically a CDN consists of a
DISTRIBUTION SYSTEM, SURROGATES, a REDIRECTION SYSTEM, and an
ACCOUNTING SYSTEM.
CLIENT
The origin of a REQUEST and the destination of the corresponding
delivered CONTENT.
CONTENT
Digital data resources. CONTENT ultimately consists of CONTENT
DATA UNITs whose internal structure is unknown by the CDN.
However, some CONTENT may be in a format known to the CDN, so
that the CDN does have some ability to parse, analyze, or
rearrange the collection of CONTENT DATA UNITs. One important
form of CONTENT with additional constraints on DISTRIBUTION and
DELIVERY is CONTINUOUS MEDIA.
CONTENT DATA UNIT
A single collection of CONTENT bits, where any internal structure
or semantics are unknown to the CDN. Thus, a CDN can succeed or
fail at transporting a CONTENT DATA UNIT, but it cannot parse,
analyze, or rearrange it.
CONTENT PEERING GATEWAY (CPG)
A point through which a CDN can be peered with others through one
or more kinds of peering. A CPG may be the point of contact for
DISTRIBUTION PEERING, REDIRECTION PEERING, and/or ACCOUNTING
PEERING, and thus may incorporate some or all of the
corresponding PEERING SYSTEMs for the CDN.
CONTENT SIGNAL
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
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The activity of presenting a PUBLISHER's CONTENT for consumption
by a CLIENT. Contrast with DISTRIBUTION and REDIRECTION.
DISTRIBUTING CDN
A CDN delivering CONTENT that does not have a NEGOTIATED
RELATIONSHIP with the PUBLISHER. Contrast with BILLING CDN.
DISTRIBUTION
The activity of moving a PUBLISHER's CONTENT from its ORIGIN to
one or more SURROGATEs. Contrast with DELIVERY and REDIRECTION.
DISTRIBUTION ADVERTISEMENT
An ADVERTISEMENT from a CDN's DISTRIBUTION PEERING SYSTEM
describing the availability of collections of CONTENT via the
CDN'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
A collection of NETWORK ELEMENTS that support DISTRIBUTION
PEERING with other CDNs.
DISTRIBUTION SYSTEM
A collection of NETWORK ELEMENTS that support DISTRIBUTION for a
single CDN. The DISTRIBUTION SYSTEM also propagates CONTENT
SIGNALs.
FIRST-REDIRECTION CDN
The CDN whose REDIRECTION SYSTEM first receives a particular
REQUEST. The FIRST-REDIRECTION CDN is able to decide how that
REQUEST should be redirected, if at all. Contrast with REMOTE CDN.
INTER-CDN
Related to an activity that involves more than one CDN. Contrast
with INTRA-CDN.
INTRA-CDN
Related to an activity within a single CDN. Contrast with
INTER-CDN.
NEGOTIATED RELATIONSHIP
A relationship whose terms and conditions are established outside
the context of CDN peering protocols.
NETWORK ELEMENT
A device or system that affects the processing of network
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messages.
ORIGIN
The point at which CONTENT enters the DISTRIBUTION SYSTEM of the
BILLING CDN. 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.
PEERING SYSTEM
A collection of NETWORK ELEMENTS supporting some form of
interconnected operation among two or more CDNs. See ACCOUNTING
PEERING SYSTEM, DISTRIBUTION PEERING SYSTEM, REDIRECTION PEERING
SYSTEM.
PUBLISHER
The party that ultimately controls the content and its
distribution. The PUBLISHER is the other party to the NEGOTIATED
RELATIONSHIP with the BILLING CDN.
REACHABLE SURROGATES
The collection of SURROGATES that can be contacted via a
particular DISTRIBUTION SYSTEM or REDIRECTION SYSTEM.
REDIRECTING CDN
A CDN whose REDIRECTION SYSTEM participates in the REDIRECTION of
a particular REQUEST.
REDIRECTION
The activity of directing a REQUEST from a CLIENT to a suitable
SURROGATE.
REDIRECTION ADVERTISEMENT
An ADVERTISEMENT from a CDN's REDIRECTION PEERING SYSTEM
describing the availability of collections of CONTENT via that
CDN's REDIRECTION SYSTEM.
REDIRECTION PEERING
Interconnection of two or more REDIRECTION SYSTEMS so as to
increase the number of REACHABLE SURROGATES for at least one of
the interconnected systems.
REDIRECTION PEERING SYSTEM
A collection of NETWORK ELEMENTS that support REDIRECTION PEERING
with other CDNs.
REDIRECTION SYSTEM
A collection of NETWORK ELEMENTS that support REDIRECTION for a
single CDN.
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REMOTE CDN
A CDN able to deliver CONTENT for a particular REQUEST that is
not the FIRST-REDIRECTION CDN for that REQUEST.
REQUEST
A message identifying a particular item of CONTENT to be
delivered. [Editor Note: Brad Cain recommends distinguishing
REDIRECTION REQUEST from CONTENT REQUEST. Does this make the
model too closely tied to DNS-style redirection? To be discussed.]
SURROGATE
A delivery server, other than the ORIGIN. Receives a redirected
REQUEST and delivers the corresponding CONTENT. Note: This
definition has a narrower semantic context than the more
generally used term defined in [2].
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4. Examples and Commentary
This section further describes the model of Section 3 to explain
concepts of CDNs and CDN peering. Note that these are only
illustrative examples. If there are conflicts between the way a term
is used here and the way it is defined in Section 3, the definition
in Section 3 is authoritative.
4.1 Understanding CDNs
The first use of the model, before peering CDNs, is simply to
describe a single CDN at an abstract level. The model describes
CLIENTS that issue REQUESTS for CONTENT. Each item of CONTENT
starts from some ORIGIN.
In the absence of a CDN, each REQUEST could simply go to an
appropriate ORIGIN, which would deliver the corresponding CONTENT to
the CLIENT.
With a CDN, the picture is a little more elaborate. The CLIENT's
REQUEST enters a REDIRECTION SYSTEM, and the ORIGIN's CONTENT enters
a DISTRIBUTION SYSTEM. Both systems converge on SURROGATES, which
are non-ORIGIN servers of CONTENT. Effectively, the DISTRIBUTION
SYSTEM is moving CONTENT out to SURROGATES, and the REDIRECTION
SYSTEM is then taking advantage of that distribution of CONTENT.
[Editor Note: Could change this description to deal with REDIRECTION
REQUESTS and CONTENT REQUESTS.]
4.2 Understanding content structure
The model defines CONTENT as well as two subsidiary but orthogonal
concepts: CONTENT DATA UNIT and 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 CDN. Although an item of CONTENT may have some
internal structure that is known to the CDN, there is some unit
(possibly as small as a bit) that is opaque to the CDN. These opaque
elements are CONTENT DATA UNITs.
In many cases, an item of CONTENT can be delivered by a CDN 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.
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4.3 Understanding Peering
The model offers a number of ways in which different CDNs can be
interconnected. An arrangement of interconnected REDIRECTION
SYSTEMS is called REDIRECTION 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: REDIRECTION ADVERTISEMENTS, DISTRIBUTION
ADVERTISEMENTS, and ACCOUNTING ADVERTISEMENTS. A REDIRECTION
ADVERTISEMENT describes a collection of URLs whose content can be
delivered by redirection through the corresponding CDN. A
DISTRIBUTION ANNOUNCEMENT describes the service level(s) available
from a CDN's SURROGATES (as a whole) to some collection of CLIENT
addresses. An ACCOUNTING ANNOUNCEMENT a collection of CLIENT
addresses, and the level of service that it can offer for delivering
content to those CLIENTS.
4.4 Content Signalling
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 peering, CONTENT SIGNALS must be
propagated to the each SURROGATE with a copy of the relevant
CONTENT.
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5. Security Considerations
[Editor's Note: Discuss the issues of delegated authority and trust
between CDNs and Origin Servers.]
[Editor's Note: Discuss man-in-the-middle and denial-of-service
attacks on peered CDNs.]
[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.]
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6. 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 (Entera), Barbara Liskov (Cisco), John Martin
(Network Appliance), Raj Nair (Cisco), and Doug Potter (Cisco).
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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,
<URL:http://www.rfc-editor.org/rfc/rfc2616.txt>.
[2] Cooper, I., Melve, I. and G. Tomlinson, "Internet Web
Replication and Caching Taxonomy",
draft-ietf-wrec-taxonomy-04.txt (work in progress), June 2000,
<URL:http://www.ietf.org/internet-drafts/draft-ietf-wrec-taxonom
y-04.txt>.
[3] Day, M. and D. Gilletti, "Content Peering Scenarios",
draft-day-cdnp-scenarios-00.txt (work in progress), September
2000,
<URL:http://www.ietf.org/internet-drafts/draft-day-cdnp-scenario
s-00.txt>.
[4] Green, M., Cain, B. and G. Tomlinson, "Content Peering
Architectural Overview", draft-green-cdnp-gen-arch-txt (work in
progress), September 2000,
<URL:http://www.ietf.org/internet-drafts/draft-green-cdnp-gen-ar
ch-00.txt>.
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
Mirror Image Internet
49 Dragon Court
Woburn, MA 01801
US
Phone: +1 781 276 1904
EMail: brad.cain@mirror-image.com
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Gary Tomlinson
Entera, Inc.
40971 Encyclopedia Circle
Freemont, CA 94538
US
Phone: +1 510 580 3726
EMail: garyt@entera.com
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