Internet Draft B. Cain
Cereva Networks
O. Spatscheck
AT&T Labs
M. May
Activia Networks
A. Barbir
Nortel Networks
July 2001
Request-Routing Requirements for Content Internetworking
draft-cain-request-routing-req-02.txt
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Abstract
Request-routing systems (RRS) are components of Content Distribution
Networks (CDNs) that direct client requests to an available copy of
content based on one or more metrics. To enable the interconnection of
CDNs [MODEL], it is necessary for their request-routing systems to
interconnect and exchange information such that client requests can be
routed between CDNs. This document provides an overview of
request-routing systems and specifies the requirements for their
interconnection.
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1. Introduction
Request-routing systems (RRS) are components of Content Distribution
Networks that direct client requests to an available copy of content
based on one or more metrics. In order too enable the
interconnection of CDNs [MODEL], it is necessary for request-routing
systems to interconnect and exchange information such that requests
can be routed between domains. This document provides an overview of
request-routing systems and specifies the requirements to
interconnect request-routing systems.
1.1 Document Organization
This document is organized as follows. Section 1 presents an
introduction to request-routing systems. Section 2 presents the
details of request-routing system components and protocols. Section
3 presents detailed requirements for each component, sub-component or
protocol from sections 1 and 2.
1.2 Overview of Request-Routing Systems
Request-routing systems (RRS) direct client requests to surrogates
that can "best" service the request [MODEL]. Request-routing
decisions are based on a set of metrics that may include for example
network proximity and server load. The basic functionality of a
request-routing system can be summarized by the following:
1. It directs clients to surrogates that are able to serve their
requests.
2. It directs clients to surrogates that (per a set of metrics) are
able to provide the "best" service.
For the sake of clarity, we now reiterate several important
assumptions from [ARCH] [MODEL]:
1. Each CDN is a "black box" to other networks to which it is
interconnected. We use the term "neighbor" to refer to a directly
interconnected CDN.
2. Content is served by surrogates that act on behalf of an origin
server that holds the "master" or "authoritative" copy of content.
Surrogates are part of a distribution system.
3. A request-routing system is responsible for directing/servicing
requests for one or more distribution systems.
4. Each distribution system may have its own internal (or intra-
domain) request-routing system that is not exposed to other
interconnected networks.
5. Request-routing systems interconnect through Content Peering
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Gateways (CPG) that implement standards based interconnection
protocols. A CDN's CPG is the only "visible" element to other
interconnected CDNs.
1.3 Generic Request-Routing System Architecture
This section presents a generic architecture of a request-routing
system to assist in understanding request-routing systems as well as
the requirements for their interconnection. In Figure 1, a
conceptual view of a request-routing system is presented; it consists
of the following components: Content Topology Exchange, Content
Topology Database and Route Computation. A brief summary of these
components is provided below:
____________________________________________
| ___________ ________ ________ |
| |Routing | |Content | |Content | | Request-Routing
| |Computation| |Topology|<->|Topology| |<->Information
| |___________| |Database| |Exchange| | Exchange
| |________| |________| | Protocol
|__________________________________________|
Figure 1.
1. Routing Computation: Responsible for computing the content route
based on information stored in the Content Topology Data Base,
route computing algorithm and policies. This decision may be based
on a variety of internal or external metrics.
2. Content Topology Database: The topology database includes
detailed topology information received from neighbors and
associated metrics exchanged during Content Information Exchange.
3. Content Information Exchange: This functional block is
responsible for implementing the Information Exchange protocol.
4. Information Exchange Protocol: Protocol used to communicate
capabilities, content advertisements, and network advertisements.
1.4 Interconnecting Request-Routing Systems
Within a single CDN, a request-routing system is used to direct
client requests to surrogates that are part of its own distribution
system. However, when request-routing systems are interconnected, a
request-router has the ability to redirect client requests to
neighbor CDNs. That is, when neighbor CDN has a better set of metric
for a set of client(s), it may be desirable to direct requests to
that neighbor CDN. In order to determine which CDN may best serve a
client request, one or more protocols may be required to exchange
various types of information and associated metrics.
This document describes the components of request-routing systems and
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requirements for interconnecting them.
2. Overview of Request-Routing System Components and Protocols
This section provides a detailed description of the basic components
of a request-routing system. Section 3 provides a description of the
specific requirements for each component.
2.1 Request-Routing System Types
The methods in which a client request is directed may be different
depending on the architecture of the request-routing system.
Currently, there are two well-known types of request-routing systems.
These two types are described below:
1. DNS-based Request-Routing Systems: The Domain Name System (DNS)
is used for the direction of client requests. In this approach,
one or more domain names are assigned to the request-routing
system; these names are then used as part of a URI reference to
direct client requests [DNSMAP]. The limitations of DNS-based
systems are described [KNOWN_MECH] and in section 2.1.1.
2. "In-Line" Request-Routing Systems: These request-routing
systems are "in-line" to client requests. Examples of in-line
request-routing systems are those that may be implemented within a
proxy or a layer-7 router. In-line request-routing systems have
full visibility into content requests (e.g. full URL) as well as
visibility of the client's IP address [note: this isn't always true
if transparent proxies are in place].
The distinction between these request-routing system types is
important because of the differences in:
- The view of the content identifier (partial vs. whole).
- The view of the client (e.g. client's IP vs. client's local DNS).
- The implementation requirements of the two types (e.g. DNS
caching).
2.1.1 DNS-Based Request-Routing Systems
In DNS-based request-routing systems [ARCH] [DNSMAP], only aggregate
sets of content may be "directed" because a domain name (e.g.
images.blah.com) can only (reasonably) represent a larger set of
content. A DNS request-routing system works well in scenarios where
many surrogates share large sets of content.
DNS-based request-routing systems suffer from the following
limitations:
- The request-routing system knows only the domain name of the
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requested content. This precludes the RRS from knowing the full
content path (e.g. URI) and the content type (e.g. HTTP, RTSP).
- The request-routing system knows the client's local DNS server,
not the client itself.
- The request-routing system responses may be cached in DNS
servers. The result is that a client request may not be
individually directed by the request-routing system.
2.1.1.1 DNS Example
CDN-A is authoritative for http://images.blah.com (or CNAMEs are used
to ultimately force a resolution of this name to CDN A). Assume that
DNS-based request-router R is part of CDN-A and is also a CPG-A for
CDN-A. When R receives a client DNS request for images.blah.com, it
makes a request-routing decision. This decision may be to direct the
request to its own surrogates or to direct the request to another
CDN. This decision is based on the routing computation by CPG-A that
in turn is based on "area" and/or "content" advertisements received
from neighbors. For example, CPG-A can make a request-routing
decision based on the following:
1. Information contained in area advertisements that have been
received from interconnected CDNs. An example may be an IP prefix
advertised with an associated metric.
2. The ability of interconnected CDNs to support the (content) type
of the request.
3. Information contained in content advertisements that may
include: content metrics, availability of content, etc. With DNS-
based request-routing systems, content specific information is only
relevant to the DNS name (e.g. in a URI).
4. Local request-routing policy.
If the choice is made to direct the request to another CDN, the
appropriate CNAME is used to direct the client's DNS to the chosen
neighbor CDN. The process then continues.
2.1.2 "In-Line" Request-Routing Systems
A Layer-7 router or Proxy situated close to a client may be used as
an "in-line" request-routing system. Such a RRS is capable of
directing client requests based on individual full content requests.
This is possible because layer-7 information (e.g. HTTP headers) is
exposed to the layer-7 router or proxy. In this type of RRS, a
surrogate can be chosen based on, for example, a full URL. Another
example of in-line request-routing is when an origin server (or
reverse proxy) performs a layer-7 redirection by "URL-rewriting".
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There are three major differences between an "in-line" request-
routing system and a DNS-based request-routing system. The first is
that the full content request is exposed (e.g. a full URL). The
second is that the content type of the request is exposed (again from
the full URL). The third is that all client requests can be received
by the request-routing system; this is in contrast to DNS-based
systems where caching may prevent this.
2.1.2.1 "In-Line" Example
Assume client X is configured to forward its requests to layer-7
router R.. Furthermore assume that CDN-A's request-router R is also
a CPG for CDN-A. When a request from client X is received, request-
router R makes a request-routing decision based on its internal
routing database constructed from information communicated from other
CDNs. If request-router R can serve the request as part of its own
distribution system then a request is made to a surrogate that is
part of that system. If router R's decides to direct the client to
another CDN, a redirect is sent to the client to direct the cilent to
another layer-7 router in a neighboring CDN. In summary, a request-
routing decision is made based on the following:
1. Information contained in area advertisements that have been
received from interconnected CDNs. An example may be an IP prefix
advertised with an associated metric.
2. The ability of interconnected CDNs to support the content type
of the request. An example may be a set of content types
supported.
3. Information contained in content advertisements that may
include: content metrics, availability of content, etc. For "in-
line" request-routing systems this may include full URLs or URL
sets.
4. Local request-routing policy.
2.2 Request-Routing Interconnection Model
Request-routing systems present a "black-box" view of their
associated distribution systems. Since in such an environment nobody
possesses a global view of all networks, the request-routing system
must also rely on a peer-to-peer model in which each request-routing
system is only aware of its direct neighbor. [Note: A direct
neighbor of the request-routing systems does not have to be a direct
neighbor at Layer-3].
There are two methods for routing a request between two
interconnected request-routing systems. The first method is an
iterative method where a RRS directs the request to the next-best
RRS. This continues until a surrogate is finally selected. The
second method is recursive where a RRS directs a request to the next-
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best RRS but expects an answer to return to the client. These two
methods are analogous to recursive vs. iterative DNS lookups.
An example of how requests can be directed between domains is through
the use of DNS CNAMEs. When request-routing systems are
interconnected using CNAMEs, a clients a clients DNS resolution is
redirected using a DNS CNAME record to another DNS-based request-
routing system until a surrogate is found that is appropriate
(according to a set of metrics) to serve the content. The drawbacks
of CNAME based request-routing are discussed in [KNOWN MECH].
2.3 CDN Capabilities
Request-routing systems are associated with one or more distribution
systems. When a request-routing system directs a client request it
must ensure that:
1. The client request type can be serviced by the distribution
system (e.g. HTTP vs. RTSP).
2. The distribution system to which a client is directed has the
capacity to service the request.
In order to ensure that an interconnected (neighbor) CDN can service
a request, a request-routing system is required to have the following
information about neighbor CDNs:
1. Request-routing system types.
2. Content types that can be served by the CDN.
3. Sets of metrics that are used for direction.
This information maybe obtained manually (off-line) or through the
use of dynamic (on-line) information exchange protocols.
2.4 Request-Routing Information Exchange
Interconnected request-routing systems need to exchange information
in order to make direction (or content routing) decisions. The two
request-routing system types presented in section 1.2 have slightly
different requirements with respect to the types of information
exchanged. In summary, interconnected request-routing systems need
to exchange two basic types of information:
1. Area Advertisements: This type of information is related to
aggregate network coverage, network capacity, network health, and
other related metrics.
2. Content Advertisements: This type of information is related to
content (e.g. URLs) and may include for example: content types,
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distribution model, authoritative delivery root, etc.
Request-routing information exchange follows the model of layer-3
routing protocols. That is, advertisements are sent to neighbors and
each request-routing system makes its own decisions. The design of
an information exchange protocol must take the following into
consideration:
- Information exchange may occur over highly unreliable networks.
- Information exchange protocols may be required to exchange large
sets of advertisement information.
- Information exchange may occur over insecure networks.
- Arbitrary meshed topologies may exist for information exchange
protocols.
2.5 Request-Routing Decision
Request-routing systems make decisions based on one or more
advertisement types and their associated metrics. Both content
advertisements and area advertisements may be used to construct a
request-routing table. This table is used to determine how requests
should be directed. The request-routing decision process is complex
for the following reasons:
- Content delivery networks are overlay networks which inherently
makes decision processes more complex.
- There are many possible metrics; if multiple metrics are
exchanged, loop prevention may be difficult.
- Request-routing systems may have specific policies with respect
to direction.
- Request-routing decisions are independent; therefore request-
routing loops must be prevented.
2.6 Request-Routing Protocol Design
Request-routing systems will require the use of protocols for the
exchange of information. These protocols are designed to operate in
an inter-domain context and therefore have the following
considerations:
- Protocol sessions will need to be debugged across CDN boundaries.
- Large sets of information may be exchanged between CDNs.
- Policy based request-routing is needed in many scenarios.
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- Protocol designs should be "Internet" scalable.
3. Request-Routing System and Protocol Requirements
3.1 General Requirements
In the following section we describe the general requirements for
interconnection of request-routing systems.
- Request-routing protocols MUST use an administrative identity to
identify themselves in protocol exchanges.
- Request-routing protocols SHOULD support arbitrary direction
topologies; this means "peer-to-peer" design.
- Request-routing protocols MUST treat other networks as "black
boxes"; that is, a given CDN A does not normally posses direct
visibility into another neighbor CDN B.
- Request-routing protocols MUST support methods to determine the
authoritative request-routing system for content.
- Request-routing protocols SHOULD be compatible with existing
applications and protocols.
3.2 Request-Routing System Type Requirements
The following section describes the requirements that apply to both
DNS-based and in-line request-routing systems.
- Request-routing protocols SHOULD support DNS-based and in-line
request-routing systems.
- Request-routing protocols MUST communicate their request-routing
system type to neighbors (e.g. DNS-based).
- Request-routing protocols MAY allow for utilization of more than
one request-routing system type for content.
- Request-routing protocols MUST be able to identify content types
for content.
3.2.1 DNS-Based Request-Routing Requirements
The following section describes requirements that apply to DNS-based
request-routing system types.
- Request-routing protocols MUST support CNAME based DNS
redirection.
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- Request-routing protocols MUST be able to map content types to
CNAMEs in order to make proper direction decisions.
3.2.2 In-Line Based Request-Routing Requirements
The following section describes requirements that apply to in-line
based request-routing system types.
- Request-routing protocols MUST support application layer
redirection (e.g. HTTP redirection).
- Request-routing protocols SHOULD support explicitly configured
application gateways and proxies.
3.3 Request-Routing Interconnection Model Requirements
- Request-routing protocols MUST allow for delegation of requests
to another request-routing system.
- Request-routing protocols SHOULD support both iterative and
recursive redirection models.
- Request-routing protocols SHOULD require that content have only
one authoritative request-routing system.
- Request-routing protocols MUST verify that neighbor CDNs have the
ability to deliver content before directing requests to that
neighbor.
3.4 Request-Routing Capabilities Requirements
- Request-routing protocols MUST support the advertisement of
content type information between neighbors.
- Request-routing protocols SHOULD have primitive methods for
capability advertisement.
3.5 Request-Routing Information Exchange Requirements
3.5.1 General Information Exchange Requirements
- Request-routing protocols MUST define standardized methods for
identifying an atomic unit of content.
- Request-routing protocols MUST define standardized methods for
identifying distribution system capabilities (e.g. content types,
layer-3 coverage, etc).
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- Request-routing protocol MUST not preclude request-routing
systems from implementing policy based routing decisions.
- Request-routing protocols MUST support the exchange of multiple
basic information types (e.g. area and content advertisements).
- Request-routing protocols MUST be able to associate multiple (and
optional) metrics with each basic information types.
- Request-routing protocols MUST exchange information sufficient to
avoid looping of information advertisements.
- Request-routing protocols MAY exchange information sufficient to
prevent request-routing loops.
3.5.2 Specific Information Exchange Requirements
- Request-routing protocols MUST support the exchange of AREA
advertisements (e.g. IP prefixes) between request-routing systems.
- Request-routing protocol AREA advertisements MUST support the
inclusion of multiple capabilities and metrics (e.g. X Mbps, Y CIDR
blocks, Z static http).
- Request-routing protocols SHOULD define a minimum set of metrics
for AREA advertisements.
- Request-routing protocols MUST support the exchange of CONTENT
UNIT advertisements (e.g. URIs) between request-routing systems.
- Request-routing protocol CONTENT UNIT advertisements MUST support
the inclusion of multiple metrics.
- Request-routing protocol CONTENT UNIT advertisements MUST support
the ability to advertise the availability of content.
- Request-routing protocol CONTENT UNIT advertisements SHOULD
identify the authoritative request-routing system.
- Request-routing protocols SHOULD define a minimum set of metrics
for CONTENT UNIT advertisements.
- Request-routing protocols MUST accommodate hierarchy and
aggregation in CONTENT UNIT and AREA advertisements.
3.6 Request-Routing Decision and Policy Requirements
- Request-routing protocols MUST be "policy friendly" (e.g. support
additional neighbor-to-neighbor extensible attributes).
- Request-routing protocols SHOULD support exchange of information
sufficient to prevent routing loops.
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- Request-routing protocols MAY support multiple metrics for
direction decisions as long as routing decisions can be guaranteed
loop free.
3.7 Request-Routing Information Exchange Protocol Design Requirements
This section describes several specific protocol requirements that
have been identified for an inter-domain request-routing exchange
protocol. Note that some of these requirements are redundant with
other sections; we repeat them here for organization.
- Request-routing protocols MUST use a reliable transport protocol.
- Request-routing protocols MUST make use of existing IETF
developed security mechanisms for encryption and authentication.
- Request-routing protocols MUST include protocol notifications for
protocol error conditions.
- Request-routing protocols SHOULD be connection oriented.
- Request-routing protocols MUST provide mechanisms to prevent
looping of advertisement information.
- Request-routing protocols MUST have extensible packet formats.
- Request-routing protocols MUST properly identify neighbors.
- Request-routing protocols MUST properly authenticate neighbors.
- Request-routing protocols MUST scale to accommodate the exchange
of large sets of CONTENT and AREA advertisements.
- Request-routing protocols MUST support (at a minimum) a simple
capability exchange/advertisement.
- Request-routing protocols MUST NOT exchange policy information.
- Request-routing protocols MUST accommodate policy based request-
routing systems.
4. Security Considerations
TBD
5. References
[MODEL] Day, M., Cain, B. and G. Tomlinson, "A Model for CDN
Peering", draft-day-cdnp-model-03.txt (work in progress), November
2000.
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[KNOWN MECH] Cain, B., Douglis, F., Green, M., Hofmann, M., Nair, R.,
Potter, D. and O. Spatscheck, "Known CDN Request-Routing Mechanisms",
draft-cain-cdnp-known-req-route-00.txt (work in progress), November
2000.
[ARCH] Green, M., Cain, B. and G. Tomlinson, "CDN Peering
Architectural Overview", draft-green-cdnp-gen-arch-02.txt (work in
progress), November 2000.
[DNSMAP] Deleuze, C., Gautier, L., and M. Hallgren, "A DNS Based
Mapping Peering System for Peering CDNs", draft-deleuze-cdnp-dnsmap-
peer-00.txt (work in progress), November 2000.
6. Author's Address:
Brad Cain
Cereva Networks
bcain@cereva.com
Oliver Spatscheck
AT&T Labs
spatsch@research.att.com
Martin May
Activia Networks
Martin.May@activia.net
Abbie Barbir
Nortel Networks
abbieb@nortelnetworks.com
7. Acknowledgements
Thanks to the following people for their contributions: John Martin,
Nalin Mistry, Mark Day, Stephen Thomas, Hillary Orman, Phil Rzewski,
and Fred Douglis.
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document. For more information consult the online list of claimed
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