Network Working Group M. Day
Internet-Draft Cisco
Expires: May 13, 2001 B. Cain
Mirror Image Internet
G. Tomlinson
Entera
November 12, 2000
A Model for CDN Peering
draft-day-cdnp-model-03.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 the technology for 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 introduces
CDNs and CDN peering, and proposes elements for such a common
vocabulary.
Notes on Mailing List and Content Alliance
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This document and related documents are discussed on the cdn mailing
list. To join the list, send mail to cdn-request@ops.ietf.org. To
contribute to the discussion, send mail to cdn@ops.ietf.org. The
archives are at ftp://ops.ietf.org/pub/lists/cdn.*
This document is an interim product of work initiated by the Content
Alliance. For more information about the Content Alliance, please
see http://www.content-peering.org.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. CDNs and Other Content Architectures . . . . . . . . . . . . . 4
2.1 Problem Description . . . . . . . . . . . . . . . . . . . . . 4
2.2 Introduction to CDNs . . . . . . . . . . . . . . . . . . . . . 5
2.3 Extending Reach & Scale . . . . . . . . . . . . . . . . . . . 6
3. CDN Model Terms . . . . . . . . . . . . . . . . . . . . . . . 8
4. CDN Examples and Commentary . . . . . . . . . . . . . . . . . 11
4.1 Understanding CDNs . . . . . . . . . . . . . . . . . . . . . . 11
4.2 Understanding content structure . . . . . . . . . . . . . . . 11
5. Peering Model Terms . . . . . . . . . . . . . . . . . . . . . 12
6. Peering Examples and Commentary . . . . . . . . . . . . . . . 14
6.1 Understanding Peering . . . . . . . . . . . . . . . . . . . . 14
6.2 Content Signalling . . . . . . . . . . . . . . . . . . . . . . 14
7. Operational Considerations . . . . . . . . . . . . . . . . . . 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
Content distribution networks, or CDNs, are of increasing importance
to the overall architecture of the Web. This document presents a
vocabulary for use in developing technology for interconnecting
CDNs. By analogy with peering of IP networks, this interconnection
is sometimes called "content peering," or (somewhat more accurately)
"peering of CDNs". Section 2 describes content distribution, CDNs,
and the motivation for peering of CDNs in some more detail. Section
3 introduces the terms used for elements of a CDN and explains how
those terms are used. Section 5 deals with CDN peering, 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 CDN peering.
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-peering-related interactions; [4], which
describes models for accounting and associated issues; and [5],
which gives an overall architecture of the elements for CDN peering.
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2. CDNs and Other Content Architectures
A CDN (content distribution network or content delivery network) is
an architecture of Web-based network elements, arranged for
efficient delivery of digital content. 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. The proliferation of CDNs and CDN
capabilities gives rise to interest in interconnecting CDNs and
finding ways for distinct CDNs to cooperate for better overall
service.
In this section we describe the problem of content distribution, the
use of server farms and server accelerators to improve the
performance of content distribution, the contrast between CDNs and
those solutions, and what makes a CDN valuable.
2.1 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, 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.
All of these techniques are useful, 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 congestion. Caching
proxies can improve performance problems due to 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
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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.
2.2 Introduction to CDNs
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 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 from multiple
diverse locations. The increase in number and diversity of locations
is intended to improve download times and thus improve the user
experience. A CDN has some combination of a mapping infrastructure,
a content-delivery infrastructure, and a distribution
infrastructure. The content-delivery infrastructure consists of a
set of "surrogate" servers [2] that deliver copies of content to
sets of users. The mapping 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. An effective CDN serves
frequently-accessed content from a surrogate that is "best suited"
for a given client.
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2.3 Extending Reach & 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."
In a typical (non-peered) CDN, a single service provider operates
the request mappers, 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.
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.
As used in this document, "peering" is interconnection among two or
more separately-administered content networks. There are several
other potential meanings for "peering" that are not intended. For
example, interconnection of similar-sized networks could be seen as
interconnecting "peers." This document does not mean to imply a
requirement that the interconnected networks be similar in size or
capability. For example, a publisher might choose to have just
enough of a CDN so that they could make use of several other
"industrial strength" CDNs, without any intent of building a global
distribution network themselves. Another example is the distinction
between "peering" and "transit," where the former means a
settlement-free economic relationship and the latter means that one
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network is paying the other. This document does not mean to imply a
requirement for any particular economic or business relationship
among the interconnected networks.
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3. CDN Model Terms
This section consists of the definitions of a number of terms used
to refer to roles, participants, and objects involved in CDNs.
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 CDN.
AUTHORITATIVE MAPPING SYSTEM
The MAPPING SYSTEM that is the correct/final authority for a
particular item of CONTENT.
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
MAPPING SYSTEM, SURROGATES, a DIRECTING 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 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
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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 MAPPING.
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 MAPPING.
DISTRIBUTION SYSTEM
A collection of NETWORK ELEMENTS that support DISTRIBUTION for a
single CDN. The DISTRIBUTION SYSTEM also propagates CONTENT
SIGNALs.
MAPPING
The activity of mapping a REQUEST from a CLIENT to a suitable
SURROGATE.
MAPPING SYSTEM
A collection of NETWORK ELEMENTS that support MAPPING for a
single CDN.
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 MAPPING SYSTEM.
REQUEST
A message identifying a particular item of CONTENT to be
delivered. [Editor Note: Brad Cain recommends distinguishing
MAPPING REQUEST from CONTENT REQUEST. Does this make the model
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too closely tied to DNS-style mapping? To be discussed.]
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].
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4. CDN Examples and Commentary
This section uses the terms of the previous to explain concepts of
CDNs and content.
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 MAPPING
SYSTEM, and the ORIGIN's CONTENT enters a DISTRIBUTION SYSTEM. Note
that the relative timing of these events is unspecified. Both
systems (MAPPING and DISTRIBUTION) converge on SURROGATES, which are
non-ORIGIN servers of CONTENT. Effectively, the DISTRIBUTION SYSTEM
is moving CONTENT out to SURROGATES, and the MAPPING SYSTEM is then
taking advantage of that distribution of CONTENT.
[Editor Note: Could change this description to deal with MAPPING
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 CDN.
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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5. Peering Model Terms
This section consists of the definitions of a number of terms used
to refer to roles, participants, and objects involved in peering
CDNs.
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
See PEERING SYSTEM.
ADVERTISEMENT
Information about available resources, exchanged among PEERING
SYSTEMS. Types of ADVERTISEMENT include MAPPING 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 CDN can be peered with others through one
or more kinds of peering. A CPG may be the point of contact for
DISTRIBUTION PEERING, MAPPING PEERING, and/or ACCOUNTING PEERING,
and thus may incorporate some or all of the corresponding PEERING
SYSTEMs for the CDN.
DISTRIBUTING CDN
A CDN that does not have a NEGOTIATED RELATIONSHIP with the
PUBLISHER for the CONTENT being delivered.
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
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SURROGATES.
DISTRIBUTION PEERING SYSTEM
See PEERING SYSTEM.
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.
MAPPING ADVERTISEMENT
An ADVERTISEMENT from a CDN's MAPPING PEERING SYSTEM describing
the availability of collections of CONTENT via that CDN's MAPPING
SYSTEM.
MAPPING PEERING
Interconnection of two or more MAPPING SYSTEMS so as to increase
the number of REACHABLE SURROGATES for at least one of the
interconnected systems.
MAPPING PEERING SYSTEM
See PEERING SYSTEM.
NEGOTIATED RELATIONSHIP
A relationship whose terms and conditions are partially or
completely established outside the context of CDN peering
protocols.
PEERING SYSTEM
A collection of NETWORK ELEMENTS supporting some form of
interconnected operation among two or more CDNs. Examples (not
separately defined): ACCOUNTING PEERING SYSTEM, DISTRIBUTION
PEERING SYSTEM, MAPPING PEERING SYSTEM.
REMOTE CDN
A CDN able to deliver CONTENT for a particular REQUEST that is
not the AUTHORITATIVE MAPPING SYSTEM for that REQUEST.
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6. Peering Examples and Commentary
This section uses the terms of the previous to explain concepts of
CDN peering.
6.1 Understanding Peering
The model offers a number of ways in which different CDNs can be
interconnected. An arrangement of interconnected MAPPING SYSTEMS is
called MAPPING 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, MAPPING 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 MAPPING or DISTRIBUTION as well?] A MAPPING
ADVERTISEMENT describes a collection of URLs whose content can be
delivered by MAPPING 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.
6.2 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 peered distribution where the peered
systems support such signals, CONTENT SIGNALS must be propagated to
each SURROGATE with a copy of the relevant CONTENT.
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7. 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.]
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8. 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: Include reference to security section of
architecture document rather than restating issues here?]
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9. 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), Barron Housel (Cisco), 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-05.txt (work in progress), June 2000,
<URL:http://www.ietf.org/internet-drafts/draft-ietf-wrec-taxonom
y-05.txt>.
[3] Day, M. and D. Gilletti, "CDN Peering Scenarios",
draft-day-cdnp-scenarios-01.txt (work in progress), Novmber
2000,
<URL:http://www.ietf.org/internet-drafts/draft-day-cdnp-scenario
s-01.txt>.
[4] Gilletti, D., Nair, R. and J. Scharber, "CDN Peering
Authentication, Authorization, and Accounting Requirements",
draft-gilletti-cdnp-aaa-reqs-00.txt (work in progress),
November 2000,
<URL:http://www.ietf.org/internet-drafts/draft-gilletti-cdnp-aaa
-reqs-00.txt>.
[5] Green, M., Cain, B. and G. Tomlinson, "CDN Peering
Architectural Overview", draft-green-cdnp-gen-arch-01.txt (work
in progress), November 2000,
<URL:http://www.ietf.org/internet-drafts/draft-green-cdnp-gen-ar
ch-01.txt>.
Authors' Addresses
Mark S. Day
Cisco Systems
135 Beaver Street
Waltham, MA 02452
US
Phone: +1 781 663 8310
EMail: markday@cisco.com
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Brad Cain
Mirror Image Internet
49 Dragon Court
Woburn, MA 01801
US
Phone: +1 781 276 1904
EMail: brad.cain@mirror-image.com
Gary Tomlinson
Entera, Inc.
40971 Encyclopedia Circle
Fremont, CA 94538
US
Phone: +1 510 580 3726
EMail: garyt@entera.com
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