hi all,
I have fixed the known request routing draft to reflect the latest changes after the 51 IETF,
I do think the terminology is aligned with the Model draft.
Furtheremore, I have added a small section that talks about security.
I would appreciate your feedback on the draft. Please note that this is the last call.
I will post the draft to the IETF list by Thursday.
Note for authors, please make sure that your names and address are correct/complete.
regards
abbie
Network Working Group A. Barbir
Internet-Draft Nortel Networks
Expires: May 1, 2002 B. Cain
Cereva Networks
F. Douglis
AT&T Labs
M. Green
CacheFlow
M. Hofmann
Lucent
R. Nair
D. Potter
Cisco
O. Spatscheck
AT&T Labs
November 1, 2001
Known CDN Request-Routing Mechanisms
draft-cain-cdnp-known-request-routing-03.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 May 1, 2001.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
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Abstract
The work presents a summary of Request-Routing techniques that are used
to direct client requests to surrogates based on various policies and
possible set of metrics. In this memo the term Request-Routing represents
techniques that are commonly called content routing or content redirection.
In principle, Request-Routing techniques can be classified under: DNS
Request-Routing, Transport-layer Request-Routing, and Application-layer
Request-Routing.
1. Introduction
The document provides a summary of current known techniques that could
be used to direct client requests to surrogates based on various policies
and possible set of metrics. The task of directing clients' requests to
surrogates is also called Request-Routing, Content Routing or Content
Redirection.
Request-Routing techniques are commonly used in Content Networks (also
known as Content Delivery Networks)[5]. Content Networks include network
infrastructure that exists in layers 4 through 7. Content Networks deal
with the routing and forwarding of requests and responses for content.
Content Networks rely on layer 7 protocols such as HTTP [4] OR RTP [8]
for transport.
Request-Routing techniques are generally used to direct client requests
for objects to a surrogate or a set of surrogates that could best serve that
content. Request-Routing mechanisms could be used to direct client requests
to surrogates that are within a Content Delivery Network (CDN) [5]. An
example of a Request-Routing system occurs when a Content Provider relies
on a Content Delivery Networks (CDNs) using DNS Request-Routing to
distribute some or all of its content.
Request-Routing techniques are used as a vehicle to extend the reach and
scale of Content Delivery Networks. There exist multiple Request-Routing
mechanisms. At a high-level, these may be classified under: DNS
Request-Routing, transport-layer Request-Routing, and application-layer
Request-Routing.
A request routing system uses a set of metrics in an attempt to direct
users to surrogate that can best serve the request. For example, the
choice of the surrogate could be based on network proximity, bandwidth
availability, surrogate load and availability of content. Appendix A
provides a summary of metrics and measurement techniques that could be used
in the selection of the best surrogate.
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The memo is organized as follows: Section 2 provides a summary of known
DNS based Request-Routing techniques. Section 3 discusses transport-layer
Request-Routing methods. In section 4 application-layer Request-Routing
mechanisms are explored. Section 5 provides insight on combining the
various methods that were discussed in the earlier sections in order to
optimize the performance of the Request-Routing System. Appendix A provides
a summary of possible metrics and measurements techniques that could be
used by the Request-Routing system to choose a given surrogate.
2. DNS based Request-Routing Mechanisms
DNS based Request-Routing techniques are common due to the ubiquity
of DNS as a directory service. In DNS based Request-Routing techniques,
a specialized DNS server is inserted in the DNS resolution process.
The server is capable of returning a different set of A, NS or CNAME
records based on user defined policies, or metrics or combination of both.
2.1 Single Reply
In this approach, the DNS server is authoritative for the entire DNS
domain or a sub domain. The DNS server returns the IP address of
the best surrogate in an A record to the requesting DNS server. The IP
address of the surrogate could also be a virtual IP(VIP) address of
the best set of surrogates for requesting DNS server.
2.2 Multiple Replies
In this approach, the Request-Routing DNS server returns multiple replies
such as several A records for various surrogates. Common implementations
of client site DNS server's cycles through the multiple replies in a
Round-Robin fashion. The order in which the records are returned can be
used to direct multiple clients using a single client site DNS server.
2.3 Multi-Level Resolution
In this approach multiple Request-Routing DNS servers can be involved in
a single DNS resolution. The rational of utilizing multiple Request-
Routing DNS servers in a single DNS resolution is to allow one to
distribute more complex decisions from a single server to multiple,
more specialized, Request-Routing DNS servers. The most common mechanisms
used to insert multiple Request-Routing DNS servers in a single DNS
resolution is the use of NS and CNAME records. An example would be the
case where a higher level DNS server operates within a territory,
directing the DNS lookup to a more specific DNS server within that
territory to provide a more accurate resolution.
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2.3.1 NS Redirection
A DNS server can use NS records to redirect the authority of the
next level domain to another Request-Routing DNS server. The,
technique allows multiple DNS server to be involved in the name
resolution process. For example, a client site DNS server resolving
a.b.c.com would eventually request a resolution of a.b.c.com from the
name server authoritative for c.com. The name server authoritative for
this domain might be a Request-Routing DNS server. In this case the
Request-Routing DNS server can either return a set of A records
or can redirect the resolution of the request a.b.c.com to the DNS
server that is authoritative for b.c.com using NS records.
One drawback of using NS records is that the number of Request-Routing
DNS servers are limited by the number of parts in the DNS name. This
problem results from DNS policy that causes a client site DNS server
to abandon a request if no additional parts of the DNS name are resolved
in an exchange with an authoritative DNS server.
A second drawback is that the last DNS server can determine the TTL
of the entire resolution process. Basically, the last DNS server can
return in the authoritative section of its response its own NS record.
Basically, the last DNS server solely determines the TTL for this record.
The client will use this cached NS record for further request resolutions
until it expires.
Another drawback is that some implementations of bind voluntarily
cause timeouts to simplify their implementation in cases in which a
NS level redirect points to a name server for which no valid A
record is returned or cached. This is especially a problem if the
domain of the name server does not match the domain currently resolved,
since in this case the A records, which might be passed in the DNS
response, are discarded for security reasons. Another drawback is the
added delay in resolving the request due to the use of multiple DNS
servers.
2.3.2 CNAME Redirection
In this scenario, the Request-Routing DNS server returns a CNAME record to
direct resolution to an entirely new domain. In principle, the new domain
might employ a new set of Request-Routing DNS servers.
One disadvantage of this approach is the additional overhead of resolving
the new domain name. The main advantage of this approach is that the number
of Request-Routing DNS servers is independent of the format the domain name.
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2.6 Anycast
Anycast [6] is an inter-network service that is applicable to networking
situations where a host, application, or user wishes to locate a host
which supports a particular service but, if several servers support the
service, does not particularly care which server is used. In an anycast
service, a host transmits a datagram to an anycast address and the
inter-network is responsible for providing best effort delivery of the
datagram to at least one, and preferably only one, of the servers that
accept datagrams for the anycast address.
The motivation for anycast is that it considerably simplifies the task
of finding an appropriate server. For example, users, instead of consulting
a list of servers and choosing the closest one, could simply type the
name of the server and be connected to the nearest one. By using anycast,
DNS resolvers would no longer have to be configured with the IP addresses
of their servers, but rather could send a query to a well-known DNS anycast
address.
Furthermore, to combine measurement and redirection, the Request-Routing
DNS server can advertise an anycast address as its IP address. The same
address, is used by multiple physical DNS servers. In this scenario, the
Request-Routing DNS server that is the closest to the client site DNS server
in terms of OSPF and BGP routing will receive the packet containing the DNS
resolution request. The server can use this information to make a
Request-Routing decision. Drawbacks of this approach are listed below:
* The DNS server may not be the closest server in terms of routing
to the client.
* Typically, routing protocols are not load sensitive. Hence,
the closest server may not be the one with the least network
latency.
* The server load is not considered during the Request-Routing
process.
2.7 Object Encoding
Since only DNS names are visible during the DNS Request-Routing, some
solutions encode the object type, object hash or similar information into
the DNS name. This might vary from a simple division of objects based on
object type (such as images.a.b.c.com and streaming.a.b.c.com) to a
sophisticate schema in which the domain name contains a unique identifier
(such as a hash) of the object. The obvious advantage is that object
information is available at resolution time. The disadvantage is that the
client site DNS server has to perform multiple resolutions to retrieve a
single Web page, which might increase rather than decrease the overall
latency.
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2.8 DNS Request-Routing Limitations
Some limitations of DNS based Request-Routing techniques are described below:
1. DNS only allows resolution at the domain level. However, an
ideal request resolution system should service requests
per object level.
2. In DNS based Request-Routing systems servers may be required to
return DNS entries with a short time-to-live (TTL)values. This may
be needed in order to be able to react quickly in the face of
hanging conditions. This in return may increase the volume of
requests to DNS servers.
3. Some DNS implementations do not always adhere to DNS standards. For
example, many DNS implementations do not honor the DNS TTL field.
4. DNS Request-Routing is based only on knowledge of the local
DNS server, as client addresses are not relayed within DNS
requests. This limits the ability of the Request-Routing system
to determine client's proximity to the surrogate.
5. DNS servers can request and allow recursive resolution of DNS
names. For recursive resolution of requests, the Request-Routing
DNS server will not be exposed to the IP address of the client's
site DNS server. In this case, the Request-Routing DNS server will be
exposed to the address of the DNS server that is recursively
requesting the information on behalf of the client's site DNS server.
For example, imgs.company.com might be resolved by a CDN, but the
request for the resolution might come from dns1.company.com as a
result of the recursion.
6. Users that share a single client site DNS server will be
redirected to the same set of IP addresses during the TTL
interval. This might lead to overloading of the surrogate
during a flash crowd.
7. Some implementations of bind can cause DNS timeouts to occur
while handling exceptional situations. For example, timeouts
can occur for NS redirections to unknown domains.
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3. Transport-Layer Request-Routing
At the transport-layer finer levels of granularity can be achieved
by the close inspection of client's requests. In this approach, the
Request-Routing system inspects the information available in the first
packet of the client's request to make surrogate selection decisions. The
inspection of the client's requests provides data about the client's IP
address, port information, and layer 4 protocol. The acquired data could
be used in combination of user-defined policies and other metrics to
determine the selection of a surrogate that is better suited to serve the
request. The techniques that are used to hand off the session to a more
appropriate surrogate are beyond the scope of this document.
In general, the forward-flow traffic (client to newly selected surrogate)
will flow through the surrogate originally chosen by DNS. The reverse-flow
(surrogate to client) traffic, which normally transfers much more data
than the forward flow, would typically takes the direct path.
The overhead associated with transport-layer Request-Routing makes it
better suited for long-lived sessions such as FTP [1]and RTSP [3]. However,
it also could be used to direct clients away from overloaded surrogates.
In general, transport-layer Request-Routing can be combined with DNS
based techniques. As stated earlier, DNS based methods resolve clients
requests based on domains or sub domains with exposure to the client's DNS
server IP address. Hence, the DNS based methods could be used as a first
step in deciding on an appropriate surrogate with more accurate refinement
made by the transport-layer Request-Routing system.
4. Application-Layer Request-Routing
Application-layer Request-Routing systems perform deeper examination
of client's packets beyond the transport layer header. Deeper examination
of client's packets provides fine-grained Request-Routing control down
to the level of individual objects. The process could be performed in
real time at the time of the object request. The exposure to the client's
IP address combined with the fine-grained knowledge of the requested
objects enable application-layer Request-Routing systems to provide better
control over the selection of the best surrogate.
4.1 Header Inspection
Applications such as HTTP [4], RTSP [3], and SSL [2] provide hints
in the initial portion of the session about how the client request
must be directed. These hints may come from the URL of the content
or other parts of the MIME request header such as Cookies.
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4.1.1 URL-Based Request-Routing
A Uniform Resource Locator (URL) [7] consists of a naming scheme followed
by a string whose format is a function of the naming scheme. The string
must start with a constant prefix "URL:". Within the URL of a object,
the first element is the name of the scheme, separated from the rest
of the object by a colon. The rest of the URL follows the colon in a
format depending on the scheme. For example, an FTP URL may optionally
specify the FTP data transfer type by which an object is to be retrieved.
Most of the methods correspond to the FTP Data Types such as ASCII and
IMAGE for the retrieval of a document, as specified in FTP by the "TYPE"
Command. The data type is specified by a suffix to the URL.
In a similar fashion, HTTP and RTSP content requests describe the requested
content by its URL. In many cases, this information is sufficient to
disambiguate the content and suitably direct the request. In most cases,
it may be sufficient to make Request-Routing decision just by examining
the prefix or suffix of the URL.
4.1.1.1 302 Redirection
In this approach, the client's request is first resolved to a virtual
surrogate. Consequently, the surrogate returns an application-specific
code such as the 302 (in the case of HTTP or RTSP) to redirect the
client to the actual delivery node.
This technique is relatively simple to implement. However, the
main drawback of this method is the additional latency involved in
sending the redirect message back to the client.
4.1.1.2 In-Path Element
In this technique, an In-Path element is present in the network in the
forwarding path of the client's request. The In-Path element provides
transparent interception of the transport connection. The In-Path element
examines the client's content requests and performs Request-Routing
decisions.
The In-Path element then splices the client connection to a connection
with the appropriate delivery node and passes along the content request.
In general, the return path would go through the In-Path element. However,
it is possible to arrange for a direct return by passing the address
translation information to the surrogate or delivery node through some
proprietary means.
The primary disadvantage with this method is the performance implications of
URL-parsing in the path of the network traffic. However, it is generally the
case that the return traffic is much larger than the forward traffic.
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The technique allows for the possibility of portioning the traffic among a
set of delivery nodes by content objects identified by URLs. This allows
object-specific control of server loading. For example, requests for
non-cacheable objects may be directed away from a cache.
4.1.2 Mime Header-Based Request-Routing
This technique involves the task of using MIME-headers such as Cookie,
Language, and User-Agent, in order to select a surrogate.
Cookies can be used to identify a customer or session by a web site. Cookie
based Request-Routing provides content service differentiation based on the
client. This approach works provided that the cookies belong to the client.
In addition, it is possible to direct a connection from a multi-session
transaction to be directed to the same server to achieve session-level
persistence.
The language header can be used to direct traffic to a language-specific
delivery node. The user-agent header helps identify the type of client
device. For example, a voice-browser, PDA, or cell phone can indicate
the type of delivery node that has content specialized to handle the
content request.
4.1.3 Site-Specific Identifiers
Site-specific identifiers help authenticate and identify a session from a
specific user. This information may be used to direct a content request.
An example of a site-specific identifier is the SSL Session Identifier.
This identifier is generated by a web server and used by the web client in
succeeding sessions to identify itself and avoid an entire security
authentication exchange. In order to inspect the session identifier, an
In-Path element would observe the responses of the web server and determine
the session identifier which is then used to associate the session to a
specific server. The remaining sessions are directed based on the stored
session identifier.
4.2 Content Modification
This technique enables a content provider to take direct control over
Request-Routing decisions without the need for specific switching devices
or directory services in the path between the client and the origin server.
Basically, a content provider can directly communicate to the client the best
surrogate that can serve the request. Decisions about the best surrogate can
be made on a per-object basis or it can depend on a set of metrics. The
overall goal is to improve scalability and the performance for delivering the
modified content, including all embedded objects.
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In general, the method takes advantage of content objects that consist
of basic structure that includes references to additional, embedded objects.
For example, most web pages, consist of an HTML document that contains plain
text together with some embedded objects, such as GIF or JPEG images. The
embedded objects are referenced using embedded HTML directives. In general,
embedded HTML directives direct the client to retrieve the embedded objects
from the origin server. A content provider can now modify references to
embedded objects such that they could be fetched from the best surrogate.
This technique is also known as URL rewriting. The basic types of URL
rewriting are discussed in the following subsections.
4.2.1 A-priori URL Rewriting
In this scheme, a content provider rewrites the embedded URLs before the
content is positioned on the origin server. In this case, URL rewriting can
be done either manually or by using a software tools that parse the content
and replace embedded URLs.
A-priori URL rewriting alone does not allow consideration of client specifics
for Request-Routing. However, it can be used in combination with DNS Request-
Routing to direct related DNS queries into the domain name space of the
service provider. Dynamic Request-Routing based on client specifics are then
done using the DNS approach.
4.2.2 On-Demand URL Rewriting
On-Demand or dynamic URL rewriting, modifies the content when the client
request reaches the origin server. At this time, the identity of the
client is known and can be considered when rewriting the embedded URLs. In
particular, an automated process can determine, on-demand, which surrogate
would serve the requesting client best. The embedded URLs can then be
rewritten to direct the client to retrieve the objects from the best
surrogate rather than from the origin server.
4.2.3 Content Modification Limitations
Content modification as a Request-Routing mechanism suffers from the
following limitations:
1. The first request from a client to a specific site must be served
from the origin server.
2. Content that has been modified to include references to nearby
surrogates rather than to the origin server should be marked as non-
cacheable. Alternatively, such pages can be marked to be cacheable
only for a relatively short period of time. Rewritten URLs on cached
pages can cause problems, because they can get outdated and point to
surrogates that are no longer available or no longer good choices.
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5. Combination of Multiple Mechanisms
There are environments in which a combination of different mechanisms can be
beneficial and advantageous over using one of the proposed mechanisms alone.
The following example illustrates how the mechanisms can be used in
combination.
A basic problem of DNS Request-Routing is the resolution granularity that
allows resolution on a per-domain level only. A per-object redirection cannot
easily be achieved. However, content modification can be used together with
DNS Request-Routing to overcome this problem. With content modification,
references to different objects on the same origin server can be rewritten to
point into different domain name spaces. Using DNS Request-Routing, requests
for those objects can now dynamically be directed to different surrogates.
6. Security Considerations
The main objective of this document is to provide a summary of current Request-Routing techniques. Such techniques are currently implemented in the Internet. The document acknowledges that security must be addressed by any entity that implements any technique that redirects client's requests. However, the details of such techniques are beyond the scope of this document.
7. Acknowledgements
The authors acknowledge the contributions and comments of Ian Cooper
(Equinex), Nalin Mistry, Wayne Ding (Nortel) and Eric Dean (CrystalBall).
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Appendix A
Measurements
Request-Routing systems can use a variety of metrics in order
to determine the best surrogate that can serve a client's request.
In general, these metrics are based on network measurements and
feedback from surrogates. It is possible to combine multiple metrics
using both proximity and surrogate feedback for best surrogate
selection. The following sections describe several well known metrics
as well as the major techniques for obtaining them.
A.1 Proximity Measurements
Proximity measurements can be used by the Request-Routing system to
direct users to the "closest" surrogate. In a DNS Request-Routing
system, the measurements are made to the client's local DNS server.
However, when the IP address of the client is accessible more accurate
proximity measurements can be obtained.
Furthermore, proximity measurements can be exchanged between surrogates and
the requesting entity. In many cases, proximity measurements are "one-way"
in that they measure either the forward or reverse path of packets from the
surrogate to the requesting entity. This is important as many paths in the
Internet are asymmetric.
In order to obtain a set of proximity measurements, a network may employ
active probing techniques and/or passive measurement techniques. The
following sections describe these two techniques.
A.1.1 Active Probing
Active probing is when past or possible requesting entities are probed
using one or more techniques to determine one or more metrics from each
surrogate or set of surrogates. An example of a probing technique is an
ICMP ECHO Request that is periodically sent from each surrogate or set of
surrogates to a potential requesting entity.
In any active probing approach, a list of potential requesting entities need
to be obtained. This list can be generated dynamically. Here, as requests
arrive, the requesting entity addresses can be cached for later probing.
Another potential solution is to use an algorithm to divide address space
into blocks and to probe those blocks. Limitations of active probing
techniques include:
1. Measurements can only be taken periodically.
2. Firewalls and NATs disallow probes.
3. Probes often cause security alarms to be triggered on
intrusion detection systems.
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A.1.2 Passive Measurement
Passive measurements could be obtained when a client performs data
transfers to or from a surrogate. Here, a bootstrap mechanism is
used to direct the client to a bootstrap surrogate. Once the client
connects, the actual performance of the transfer is measured. This
data is then fed back into the Request-Routing system.
An example of passive measurement is to watch the packet loss from a
client to a surrogate by observing TCP behavior. Latency measurements can
also be learned by observing TCP behavior. The limitations of passive
measurement approach are directly related to the bootstrapping mechanism.
Basically, a good mechanism is needed to ensure that not every surrogate is
tested per client in order to obtain the data.
A.1.3 Metric Types
The following sections list some of the metrics, which can be used
for proximity calculations.
* Latency: Network latency measurements metrics are used to
determine the surrogate (or set of surrogates) that has the
least delay to the requesting entity. These measurements can
be obtained using either an active probing approach or a
passive network measurement system.
* Packet Loss: Packet loss measurements can be used as a
selection metric. A passive measurement approach can easily
obtain packet loss information from TCP header information.
Active probing can periodically measure packet loss from
probes.
* Hop Counts: Router hops from the surrogate to the requesting
entity can be used as a proximity measurement.
* BGP Information: BGP AS PATH and MED attributes can be used to
determine the "BGP distance" to a given prefix/length pair.
In order to use BGP information for proximity measurements, it
must be obtained at each surrogate site/location.
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A.2 Surrogate Feedback
The Request-Routing system can use feedback from surrogates in order
to select a "least-loaded" delivery node. Feedback can be delivered
from each surrogate or can be aggregated by site or by location.
A.2.1 Probing
Feedback information may be obtained by periodically probing a surrogate
by issuing an HTTP request and observing the behavior. The problems with
probing for surrogate information are:
1. It is difficult to obtain "real-time" information.
2. Non-real-time information may be inaccurate.
Consequently, feedback information can be obtained by agents that reside on
surrogates that can communicate a variety of metrics about their nodes.
A.2.2 Well Known Metrics
The following provides a brief summary of several of the popular metrics
that is used for surrogate feedback:
* Surrogate CPU Load.
* Interface Load / Dropped packets.
* Number of connections being served.
* Storage I/O Load.
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7. References
[1] Postel, J., "File Transfer Protocol", RFC 765, June 1980,
[2] Dierks, T. and C. Allen, "The TLS Protocol Version 1", RFC
2246, January 1999.
[3] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
Protocol", RFC 2326, April 1998.
[4] 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.
[5] Day, M., Cain, B. and G. Tomlinson, "A Model for CDN Peering",
http://www.ietf.org/internet-drafts/draft-day-cdnp-model-08.txt
(Group Last Call).
[6] Partridge, C., Mendez, T., Milliken W., "Host Anycasting Service",
RFC 1546, November, 1993.
[7] Berners-Lee, T., Masinter, L., McCahill, M., "Uniform Resource Locator
(URL), RFC 1738,May 1994.
[8] H. Schulzrinne, H., Fokus, G., Casner, S., Frederick, R.,
Jacobson, V., "RTP: A Transport Protocol for Real-Time
Applications", RFC 1889, January 1996.
8. Authors' Addresses
Abbie Barbir
Nortel Networks
3500 Carling Avenue, Nepean Ontario K2H 8E9 Canada
EMail: abbieb@nortelnetworks.com
Brad Cain
Cereva Networks
EMail: bcain@cereva.com
Fred Douglis
AT&T Labs
Room B137
180 Park Ave, Bldg 103
Florham Park, NJ 07932, US
EMail: douglis@research.att.com
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Mark Green
CacheFlow
650 Almanor Avenue
Sunnyvale, CA 94086, US
EMail: markg@cacheflow.com
Markus Hofmann
Lucent Technologies
Room 4F-513
101 Crawfords Corner Rd.
Holmdel, NJ 07733, US
EMail: hofmann@bell-labs.com
Raj Nair
Cisco Systems
50 Nagog Park
Acton, MA 01720, US
EMail: rnair@cisco.com
Doug Potter
Cisco Systems
50 Nagog Park
Acton, MA 01720, US
EMail: dougpott@cisco.com
Oliver Spatscheck
AT&T Labs
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EMail: spatsch@research.att.com
Cain, et. al. Expires May 1, 2002 [Page 16]
Internet-Draft Known CDN Request-Routing Mechanisms May 2002
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