Network Working Group B. Cain
Internet-Draft Storigen Systems
Expires: May 2, 2003 A. Barbir
Nortel Networks
R. Nair
Cisco
O. Spatscheck
AT&T
November 2002
Known CN Request-Routing Mechanisms
draft-ietf-cdi-known-request-routing-02.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
The work presents a summary of Request-Routing techniques that are
used to direct client requests to surrogates based on various
policies and a 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,
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Transport-layer Request-Routing, and Application-layer Request-
Routing.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. DNS based Request-Routing Mechanisms . . . . . . . . . . . . 4
2.1 Single Reply . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Multiple Replies . . . . . . . . . . . . . . . . . . . . . . 4
2.3 Multi-Level Resolution . . . . . . . . . . . . . . . . . . . 4
2.3.1 NS Redirection . . . . . . . . . . . . . . . . . . . . . . . 4
2.3.2 CNAME Redirection . . . . . . . . . . . . . . . . . . . . . 5
2.4 Anycast . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.5 Object Encoding . . . . . . . . . . . . . . . . . . . . . . 6
2.6 DNS Request-Routing Limitations . . . . . . . . . . . . . . 6
3. Transport-Layer Request-Routing . . . . . . . . . . . . . . 8
4. Application-Layer Request-Routing . . . . . . . . . . . . . 9
4.1 Header Inspection . . . . . . . . . . . . . . . . . . . . . 9
4.1.1 URL-Based Request-Routing . . . . . . . . . . . . . . . . . 9
4.1.2 Header-Based Request-Routing . . . . . . . . . . . . . . . . 10
4.1.3 Site-Specific Identifiers . . . . . . . . . . . . . . . . . 10
4.2 Content Modification . . . . . . . . . . . . . . . . . . . . 11
4.2.1 A-priori URL Rewriting . . . . . . . . . . . . . . . . . . . 11
4.2.2 On-Demand URL Rewriting . . . . . . . . . . . . . . . . . . 12
4.2.3 Content Modification Limitations . . . . . . . . . . . . . . 12
5. Combination of Multiple Mechanisms . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . 14
7. Additional Authors and Acknowledgements . . . . . . . . . . 15
Normative References . . . . . . . . . . . . . . . . . . . . 16
Informative References . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 17
A. Measurements . . . . . . . . . . . . . . . . . . . . . . . . 18
A.1 Proximity Measurements . . . . . . . . . . . . . . . . . . . 18
A.1.1 Active Probing . . . . . . . . . . . . . . . . . . . . . . . 18
A.1.2 Passive Measurement . . . . . . . . . . . . . . . . . . . . 19
A.1.3 Metric Types . . . . . . . . . . . . . . . . . . . . . . . . 19
A.1.4 Surrogate Feedback . . . . . . . . . . . . . . . . . . . . . 20
Full Copyright Statement . . . . . . . . . . . . . . . . . . 21
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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 a 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) [8]. 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] 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
Network (CN) [8].
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.
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.
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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,
metrics, or a 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 rationale 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.
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.example.com [10] would eventually request a resolution of
a.b.example.com from the name server authoritative for example.com.
The name server authoritative for this domain might be a Request-
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Routing NS 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.example.com to the DNS server that is authoritative
for example.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. 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 of the domain name.
2.4 Anycast
Anycast [5] 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
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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:
o The DNS server may not be the closest server in terms of routing
to the client.
o Typically, routing protocols are not load sensitive. Hence, the
closest server may not be the one with the least network latency.
o The server load is not considered during the Request-Routing
process.
2.5 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.example.com and
streaming.a.b.example.com) to a sophisticated 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.
2.6 DNS Request-Routing Limitations
Some limitations of DNS based Request-Routing techniques are
described below:
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o DNS only allows resolution at the domain level. However, an ideal
request resolution system should service requests per object
level.
o 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
outages. This in return may increase the volume of requests to
DNS servers.
o Some DNS implementations do not always adhere to DNS standards.
For example, many DNS implementations do not honor the DNS TTL
field.
o DNS Request-Routing is based only on knowledge of the client DNS
server, as client addresses are not relayed within DNS requests.
This limits the ability of the Request-Routing system to determine
a client's proximity to the surrogate.
o 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.example.com might be resolved
by a CN, but the request for the resolution might come from
dns1.example.com as a result of the recursion.
o 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.
o 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 with 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 take
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.
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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
Some application level protocols 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.
4.1.1 URL-Based Request-Routing
Application level protocols such as HTTP and RTSP describe the
requested content by its URL [6]. 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 [4] or RTSP [3])
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
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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.
The technique allows for the possibility of partitioning 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 object types may be directed away from a
cache.
4.1.2 Header-Based Request-Routing
This technique involves the task of using HTTP [4] 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
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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 witching
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.
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.
Content modification techniques must not violate the architectural
concepts of the Internet [9]. Special considerations must be made to
ensure that the task of modifying the content is performed in a
manner that is consistent with RFC 3238 [9] that specifies the
architectural considerations for intermediaries that perform
operations or modifications on content.
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.
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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:
o The first request from a client to a specific site must be served
from the origin server.
o 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.
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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. In [9] RFC 3238 addresses the main
requirements for entities that intent to modify requests for content
in the Internet.
The details of security techniques are beyond the scope of this
document.
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7. Additional Authors and Acknowledgements
The following people have contributed to the task of authoring this
document: Fred Douglis (IBM Research), Mark Green, Markus Hofmann
(Lucent), Doug Potter.
The authors acknowledge the contributions and comments of Ian Cooper,
Nalin Mistry (Nortel), Wayne Ding (Nortel) and Eric Dean
(CrystalBall).
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Normative References
[1] Postel, J.l, "File Transfer Protocol", RFC 765, June 1980.
[2] T. Dierks et. al, "The TLS Protocol Version 1", RFC 846, July
1999.
[3] H. Schulzrinne et. al, "Real Time Streaming Protocol", RFC 2326,
April 1998.
[4] R. Fielding et al, "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2616, June 1999.
[5] C. Partridge et al., "Host Anycasting Service", RFC 1546,
November 1993.
[6] T. Berners-Lee et al, "Uniform Resource Locators (URL)", RFC
1738, May 1994.
[7] H. Schulzrinneet al, "RTP: A Transport Protocol for Real-Time
Applications", RFC 1889, January 1996.
[8] M. Day et al, "A Model for Content Internetworking (CDI)",
Internet-Draft: http://www.ietf.org/internet-drafts/ draft-ietf-
cdi-model-02.txt (groups Last Call), May 2002.
[9] S. Floyd et al, "IAB Architectural and Policy Considerations for
Open Pluggable Edge Services", RFC 3238, January 2002.
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Informative References
[10] D. Eastlake et al, "Reserved Top Level DNS Names", RFC 2606,
June 1999.
Authors' Addresses
Brad Cain
Storigen Systems
650 Suffolk Street
Lowell, MA 01854
USA
Phone: +1 978-323-4454
EMail: bcain@storigen.com
Abbie Barbir
Nortel Networks
3500 Carling Avenue
Nepean, Ontario K2H 8E9
Canada
Phone: +1 613 763 5229
EMail: abbieb@nortelnetworks.com
Raj Nair
Cisco
50 Nagog Park
Acton, MA 01720
USA
EMail: rnair@cisco.com
Oliver Spatscheck
AT&T
180 Park Ave, Bldg 103
Florham Park, NJ 07932
USA
EMail: spatsch@research.att.com
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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 random addresses within those blocks. Limitations of
active probing techniques include:
o Measurements can only be taken periodically.
o Firewalls and NATs disallow probes.
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o Probes often cause security alarms to be triggered on intrusion
detection systems.
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.
o 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.
o 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.
o Hop Counts: Router hops from the surrogate to the requesting
entity can be used as a proximity measurement.
o 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.1.4 Surrogate Feedback
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.1.4.1 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:
o It is difficult to obtain "real-time" information.
o 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.1.4.2 Well Known Metrics
The following provides a list of several of the popular metrics that
are used for surrogate feedback:
o Surrogate CPU Load.
o Interface Load/Dropped packets.
o Number of connections being served.
o Storage I/O Load.
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