This document provides a summary of known request routing techniques
that are used by the industry before December 2000. Request routing
techniques are generally 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].
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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.
DNS based Request-Routing techniques are common due to the ubiquity
of the DNS system [10][12][13]. 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. In [11] RFC 2782 (DNS SRV)
provides guidance on the use of DNS for load balancing. The RFC
describes some of the limitations and suggests appropriate useage of
DNS based techniques. The next sections provides a summary of some
of the used techniques.
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.
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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.
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.
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-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.
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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.
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.
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 utilizes the service, it 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.
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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.
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.
This section lists some of the limitations of DNS based
Request-Routing techniques.
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.
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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.
DNS based request routing techniques can suffer from serious
limitations. For example, the use of such techniques can overburden
third party DNS servers, which should not be allowed [19]. In [11]
RFC 2782 provides warnings on the use of DNS for load balancing.
Readers are encouraged to read the RFC for better understanding of
the limitations.
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 [20][18][15] 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.
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The overhead associated with transport-layer Request-Routing [21][19]
is 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.
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.
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.
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.
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.
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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.
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.
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.
This technique involves the task of using HTTP [4] such as Cookie,
Language, and User-Agent, in order to select a surrogate. In [20]
some examples of using this technique are provided.
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 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.
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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.
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.
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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 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.
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.
Content modification as a Request-Routing mechanism suffers from many
limitation [23]. For example:
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.
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.
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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.
The main objective of this document is to provide a summary of
current Request-Routing techniques. Such techniques are currently
implemented in the Internet. However, 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 intend to modify requests for content in the Internet.
Some active probing techniques will set off intrusion detection
systems and firewalls. Therefore, it is recommended that
implementers be aware of routing protocol security [25].
It is important to note the impact of TLS [2] on request routing in
CNs. Specifically, when TLS is used the full URL is not visible to
the content network unless it terminates the TLS session. The
current document focuses on HTTP techniques. TLS based techniques
that require the termination of TLS sessions on Content Peering
Gateways [8] are beyond the of scope of this document.
The details of security techniques are also beyond the scope of this
document.
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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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.
Proximity measurements can be used by the Request-Routing system to
direct users to the "closest" surrogate. In this document proximity
means round-trip time. 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 [24].
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 [24].
In order to obtain a set of proximity measurements, a network may
employ active probing techniques.
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.
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 active probing techniques.
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.
It is important to note that the value of BGP AS PATH information can
be meaningless as a good selection metric [24].
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.
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[1] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC
959, October 1985.
[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] Partridge, C., Mendez, T. and W. Milliken, "Host Anycasting
Service", RFC 1546, November 1993.
[6] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform Resource
Locators (URL)", RFC 1738, December 1994.
[7] Schulzrinne, H., Casner, S., Federick, R. and V. Jacobson, "RTP:
A Transport Protocol for Real-Time Applications", RFC 1889,
January 1996.
[8] Day, M., Cain, B., Tomlinson, G. and P. Rzewski, "A Model for
Content Internetworking (CDI)", RFC 3466, February 2003.
[9] Floyd, S. and L. Daigle, "IAB Architectural and Policy
Considerations for Open Pluggable Edge Services", RFC 3238,
January 2002.
[10] Eastlake, D. and A, Panitz, "Reserved Top Level DNS Names", BCP
32, RFC 2606, June 1999.
[11] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2002.
[12] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
[13] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1035, November 1987.
Barbir, et al. Informational [Page 15]
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[14] Elz, R. and R. Bush, "Clarifications to the DNS Specification",
RFC 2181, July 1997.
[15] Awduche, D., Chiu, A., Elwalid, A., Widjaja, I. and X. Xiao,
"Overview and Principles of Internet Traffic Engineering", RFC
3272, May 2002.
[16] Crawley, E., Nair, R., Rajagopalan, B. and H. Sandick, "A
Framework for QoS-based Routing in the Internet", RFC 2386,
August 1998.
[17] Huston, G., "Commentary on Inter-Domain Routing in the
Internet", RFC 3221, December 2001.
[18] M. Welsh et al., "SEDA: An Architecture for Well-Conditioned,
Scalable Internet Services", Proceedings of the Eighteenth
Symposium on Operating Systems Principles (SOSP-18) 2001,
October 2001.
[19] A. Shaikh, "On the effectiveness of DNS-based Server Selection",
INFOCOM 2001, August 2001.
[20] C. Yang et al., "An effective mechanism for supporting content-
based routing in scalable Web server clusters", Proc.
International Workshops on Parallel Processing 1999, September
1999.
[21] R. Liston et al., "Using a Proxy to Measure Client-Side Web
Performance", Proceedings of the Sixth International Web Content
Caching and Distribution Workshop (WCW'01) 2001, August 2001.
[22] W. Jiang et al., "Modeling of packet loss and delay and their
effect on real-time multimedia service quality", Proceedings of
NOSSDAV 2000, June 2000.
[23] K. Johnson et al., "The measured performance of content
distribution networks", Proceedings of the Fifth International
Web Caching Workshop and Content Delivery Workshop 2000, May
2000.
[24] V. Paxson, "End-to-end Internet packet dynamics", IEEE/ACM
Transactions 1999, June 1999.
[25] F. Wang et al., "Secure routing protocols: Theory and Practice",
Technical report, North Carolina State University 1997, May
1997.
Barbir, et al. Informational [Page 16]
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Barbir, et al. Informational [Page 17]
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MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Barbir, et al. Informational [Page 19]