U.S. patent application number 09/982721 was filed with the patent office on 2003-04-24 for content request routing and load balancing for content distribution networks.
Invention is credited to Ajalat, Casey, Fuller, Vincent A. III, Miller, Matthew, Slocombe, Michael.
Application Number | 20030079027 09/982721 |
Document ID | / |
Family ID | 25529453 |
Filed Date | 2003-04-24 |
United States Patent
Application |
20030079027 |
Kind Code |
A1 |
Slocombe, Michael ; et
al. |
April 24, 2003 |
Content request routing and load balancing for content distribution
networks
Abstract
A content distribution mechanism that distributes content of a
content provider at various sites across a network and selects the
site that is nearest a content requestor using an anycast address
that resides at each of the sites. The sites are configured as
nodes (or clusters) and each node includes a content server and a
DNS server. The DNS servers are so associated with the content
servers at their respective nodes as to resolve the name of the
content provider to the IP address of the content servers at the
nodes. The DNS servers each are assigned the anycast address in
addition to a unique address, and the anycast address is advertised
to the network (in particular, the network routing infrastructure)
using Border Gateway Protocol (BGP). Node selection occurs when the
network routing infrastructure selects a shortest path to the
anycast address during DNS name resolution.
Inventors: |
Slocombe, Michael; (Acton,
MA) ; Miller, Matthew; (Plainville, MA) ;
Ajalat, Casey; (North Billerica, MA) ; Fuller,
Vincent A. III; (Palo Alto, CA) |
Correspondence
Address: |
FISH & RICHARDSON PC
225 FRANKLIN ST
BOSTON
MA
02110
US
|
Family ID: |
25529453 |
Appl. No.: |
09/982721 |
Filed: |
October 18, 2001 |
Current U.S.
Class: |
709/229 ;
709/217 |
Current CPC
Class: |
H04L 61/4511 20220501;
H04L 61/5069 20220501; H04L 9/40 20220501; H04L 67/54 20220501;
H04L 67/288 20130101; H04L 67/2895 20130101; H04L 61/50 20220501;
H04L 67/568 20220501 |
Class at
Publication: |
709/229 ;
709/217 |
International
Class: |
G06F 015/16 |
Claims
What is claimed is:
1. A method of content delivery in a network, comprising:
associating devices in a Domain Name System (DNS) with content
server systems located in the network, the content server systems
maintaining and serving content of a content provider, each DNS
device configured to resolve the name of the content provider to an
address for the content server system with which such DNS device is
associated; and assigning to the DNS devices a common address, the
common address being usable to resolve the name of the content
provider such that a request for content of the content provider by
a content requestor is sent to the content server system nearest
the content requestor.
2. The method of claim 1, wherein the common address is an anycast
address.
3. The method of claim 2, wherein routing information associated
with the anycast address is advertised over the network by the DNS
devices in accordance with the Border Gateway Protocol (BGP).
4. The method of claim 1, wherein the content server systems are
geographically distributed across the network.
5. The method of claim 1, wherein the DNS devices are collocated
with the content server systems with which the DNS devices are
associated.
6. The method of claim 1, wherein each content server system and
associated DNS device are located in a different Internet Service
Provider Point of Presence.
7. The method of claim 1, wherein each content server system and
associated DNS device is located at or near an entry point of the
network.
8. The method of claim 1, where the content server systems comprise
cache systems that cache content of the content provider.
9. The method of claim 8, wherein at least one of the cache systems
comprises at least two cache servers connected in a cluster, and
wherein the at least two cache servers are coupled to a switch
usable to select from among the at least two cache servers based on
a selection policy.
10. A computer program product residing on a computer-readable
medium for managing a content delivery network, the computer
program product comprising instructions causing a computer to:
maintain content server systems at different locations in the
network to store content of a content provider and serve content to
a content requester; configure Domain Name System (DNS) devices to
resolve a name of the content provider to an address for a
corresponding one of the content server systems; and assign to each
of the DNS devices a common address, the common address being
usable to allow a content request intended for the content provider
to be sent to the content server system nearest the content
requestor.
11. A content distribution system comprising: content distribution
nodes connected to a content provider, a content provider Domain
Name System (DNS) server and a content requestor DNS server via the
Internet, each content distribution node including a DNS server
coupled to and associated with a content server system, the content
server system operating to store content originating with the
content provider and serve content to a content requester, and each
DNS server being assigned an address common to all of the DNS
servers in the content distribution nodes; the common address being
provided by the content provider DNS server to the content
requestor DNS server in response to a DNS request from the content
requestor DNS server, thereby enabling the content requestor DNS
server to use the Internet to select a path to the content
distribution node nearest the content requestor and forward the DNS
request to the selected node; and the DNS server in the selected
node being configured to resolve the name of the content provider
in the DNS request to an IP address of the associated content
server system and cause the IP address to be returned to the
content requester.
Description
BACKGROUND
[0001] The invention relates generally to information retrieval in
a network and, more particularly, to hosting and distributing
content on a content delivery network such as the Internet.
[0002] The World Wide Web is the Internet's content retrieval
system. In the Web environment, client systems effect transactions
to Web servers using the Hypertext Transfer Protocol (HTTP), which
provides clients with access to files (e.g., text, graphics,
images, sound, video, etc.) using a standard page description
language, for example, Hypertext Markup Language (HTML). A network
path to a server is identified by a so-called Uniform Resource
Locator (URL) having a special syntax for defining a network
connection. Use of a Web browser at a client (end user) system
involves specification of a link via the URL. In response, the
client system makes a request to the server (sometimes referred to
as a "Web site") identified in the link and, in return, receives
content from the Web server. The Web server can return different
content data object types, such as .gif and jpeg files (graphics),
.mpeg files (video), .wav files (audio) and the like.
[0003] As the Web server content provided by World Wide Web has
continued to grow over time, so too has the number of users
demanding access to such content. Unfortunately, the
ever-increasing number of end users requesting Web content from Web
sites has resulted in serious bandwidth and latency issues, which
manifest themselves in delay to the end user.
[0004] To address these problems, many networking product and
service providers have developed solutions that distribute Web site
content across the network in some manner. One class of solutions
involves replicating Web servers at multiple locations and
directing traffic (by modifying the URL and forwarding, or using
HTTP re-direct) to the "best" server based on a predefined
selection policy, e.g., load balancing, network topology. Another
class of solutions distributes content strategically and/or
geographically, and often uses some type of centralized or
hierarchical Domain Name System (DNS)-based site selection. The
distributed sites include servers that perform reverse proxy with
(or without) caching. One such technique routes traffic to a
content distribution site nearest the requestor by modifying URLs
in the top-level Web page. Other DNS techniques use a round robin
traffic distribution to distribute load to the content sites, but
do not take into account the location of the requester relative to
those content sites.
SUMMARY
[0005] In one aspect, the invention provides a method of content
delivery in a network. The method includes associating devices in a
Domain Name System (DNS) with content server systems located in the
network, the content server systems maintaining and serving content
of a content provider, each DNS device configured to resolve the
name of the content provider to an address for the content server
system with which such DNS device is associated. The method further
includes assigning to the DNS devices a common address, the common
address being usable to resolve the name of the content provider
such that a request for content of the content provider by a
content requestor is sent to the content server system nearest the
content requestor.
[0006] Particular implementations of the invention may provide one
or more of the following advantages.
[0007] A performance benefit is gained because a content requestor
can generally retrieve content from a content site closer than the
origin server of the content provider. In addition, because there
are multiple sites serving the content, the load from many end
users is distributed among multiple systems and across different
parts of the network. Also, an end user's DNS request can be routed
to a content site nearest the requestor using pre-existing routing
infrastructure. Because DNS uses a stateless protocol (UDP) for
routing, the solution can handle anycast addressable caching
without the problems associated with anycast service, namely, the
protential packet-by packet load balancing site effects of
protocols like TCP which maintain state information.
[0008] Other features and advantages of the invention will be
apparent from the following detailed description and from the
claims.
DESCRIPTION OF DRAWINGS
[0009] FIG. 1 is a depiction of a prior Web transaction.
[0010] FIG. 2 is a depiction of a prior Web transaction using
reverse-proxy caching.
[0011] FIG. 3 is a block diagram of an exemplary content
distribution network which includes content distribution nodes that
support reverse-proxy caching and anycast address service to
achieve distributed content delivery and load balancing.
[0012] FIG. 4 is a simplified block diagram of routing
configuration software and associated table(s) (provided in the
routing infrastructure of the content distribution network of FIG.
3) used to perform distributed content site selection based on
anycast address routing and Border Gateway Protocol (BGP).
[0013] FIG. 5 is a depiction of an exemplary Web transaction using
the content distribution network shown in FIG. 3.
[0014] Like reference numbers will be used to represent like
elements.
DETAILED DESCRIPTION
[0015] The present invention features a content distribution
mechanism for routing a content request to the nearest content
distribution site in a content distribution network. It also
provides for load sharing across multiple content distribution
sites in a content distribution network. The content distribution
mechanism has particular utility in and is therefore described
within the context of an Internet-based, World Wide Web ("Web")
network infrastructure.
[0016] Hereinafter, the following terminology is used:
[0017] "Content provider" refers to an entity having a Web site on
a network. Generally, the entity produces the content for the Web
site. The entity may operate a Web server system or may use the
services of a hosting provider.
[0018] "End user" refers to a person who wishes to view the Web
content. Typically, an end user uses a Web browser, such as
Netscape Navigator or Internet Explorer, executing on a computer
system, e.g., a personal computer.
[0019] "Domain Name System" (DNS) refers to a collection of systems
available on the public Internet that can resolve a domain name to
a specific Internet Protocol (IP) address, as is known in the
art.
[0020] It will be understood that reference to distance on the
network, such as one server being "closer" to an end user than
another server, refers to a network distance. Thus, a shorter
distance implies a better path based on network criteria, and not
necessarily a shorter geographic distance.
[0021] In the description to follow, a fictitious company "ABCD" is
used as an example of a content provider having a Web site on a
network.
[0022] FIG. 1 shows a conventional content delivery Web transaction
10. The transaction 10 involves a Web server 12 (also referred to
as an "origin server"), an authoritative DNS system 14, an end-user
system 16 and an end-user DNS system 18, all of which are coupled
to a network 20, for example, the public Internet. The Web server
12 is a computer system containing content (e.g., Web pages) for a
content provider, with the ability to provide this content in
response to a Web request via the HTTP protocol. The authoritative
DNS system 14 is a DNS system that can resolve domain names within
the content provider's namespace.
[0023] For example, the DNS system 14 for the company "ABCD" would
have information for host names ending in ".abcd.com" (such as
"www.abcd.com"). Typically, the end-user system 16 is a computer
being operated by an end user to perform Web "browsing" (that is,
view Web pages). The end-user DNS system 18 is a DNS server that
the end-user system 16 uses to resolve domain names to IP
addresses.
[0024] When the end user wishes to view a Web page or object (such
as "www.abcd.com/PriceList"), the transaction 10 occurs as follows.
First, the end user using the end user system 16 enters the name of
a Web page into a browser (not shown) executing on the end user
system 16. The end-user system 16 requests a DNS resolution for the
host name ("www.abcd.com") from the end-user DNS system 18 ("DNS
Req 22"). The end-user DNS system 18 determines which of the DNS
systems that make up the DNS for the network 20 can resolve this
host name by sending a DNS request to the authoritative DNS system
14 ("DNS Req 24"). The authoritative DNS system 14 resolves the
name to an IP address and returns a response containing the IP
address to the end-user 16 system via the end-user DNS system 18
("DNS Resp 26"). The end-user DNS system 18, in turn, communicates
the IP address to the end-user station 16 in a DNS response to the
end-user system 16 ("DNS Resp 27"). The end-user system 16 contacts
the Web server 12 at the specified IP address and requests the Web
object ("www.abcd.comlPriceList") ("HTTP Req 28"). The Web server
12 returns the Web page corresponding to the requested Web object
to the end-user system 16 ("HTTP Resp" 30). The browser running on
the end-user station 16 displays the returned Web page on the
end-user system 16 for viewing by the end user.
[0025] FIG. 2 illustrates a Web transaction with reverse proxy
caching 40. That is, the transaction 40 is the same basic Web
transaction as shown in FIG. 1, but now employs a reverse proxy
content server (shown as a cache server) 42 acting on behalf of the
Web server 12. During reverse-proxy caching, the cache server 42
assumes the identity of the Web server 12 so that a Web request
directed to the Web server 12 (in the example, "www.abcd.com") is
instead directed to the cache server 42. This re-direction is
accomplished by changing the entry for the Web site
("www.abcd.com") in the authoritative DNS system 14 so that the
host name resolves to the address of the cache server 42 instead of
the address of the original Web server 12. A new name is assigned
to the address of the original Web server 12 (e.g.,
"origin.abcd.com"). Thus, 22, 24, 26 and 27 are the same as in FIG.
1. However, the IP address returned by the authoritative DNS system
14 is that of the cache server 42, not the Web server itself, as
was previously described with reference to FIG. 1. Consequently,
instead of sending the subsequent content request to and receiving
a response from the Web server 12 (as was shown in steps 28 and 30
of FIG. 1), the end-user system 16 sends the content request to the
cache server 22 ("HTTP Req 44") and the cache server 22 fetches the
requested content from the Web server ("HTTP Req 46"). The Web
server 12 returns the requested content to the cache server 42
("HTTP Resp 48"). The cache server 42 completes the transaction by
serving the requested content to the end-user system 16 ("HTTP Resp
50") and caching the content so the content will be readily
available to support future requests for the same content
("store"52).
[0026] FIG. 3 shows a content distribution network ("CDN") 60. The
CDN 60 includes at least one Web server, shown as a Web server 62,
a DNS system 64 for Web server 62, and end-user stations 66a and
66b, all connected to a network 72. The network 72 is implemented
as the public Internet. The end-user stations execute Web browsers
68a and 68b, respectively. Each end-user station 66 has an
associated end user DNS, however, only an end user DNS 70a for the
end-user station 66a is shown. Of course, additional end-user
stations may be connected to the network 72. Also connected to the
network 72 are multiple content distribution nodes 76a, 76b and
76c, which support distributed content delivery services for one or
more Web sites on the network 72. The CDN nodes 76 interact with
the origin server 62 containing the original Web content, the
various DNS systems 64, 70 and, of course, the end-user systems
66.
[0027] Each of the CDN nodes 76 includes a DNS system 78 coupled to
and associated with a Web content server system or site 80. In one
embodiment, as described herein, each content server system 80 is
implemented as a cache server system. The techniques described
herein could also apply to other types of content servers, such as
mirrored Web content servers, or Web content servers having
different content (e.g., customized for geographic area). Each DNS
system 78 in each node holds a table that includes an address entry
which the DNS system 78 uses to map the domain name of the content
provider to the IP address of the cache server in that same node.
Although only one such Web site (Web site 62) is shown, it will be
appreciated that other Web sites may be connected to the network 72
and use the DNS and content caching services of the nodes 76, as
will be described. The nodes 76 are deployed at different locations
in the network 72. Preferably, the nodes 76 are geographically
distributed across the network 72.
[0028] Optionally, the CDN 60 may include a CDN manager 82 that can
be used by a network administrator (for example, a CDN node
hardware and/or CDN node service provider) to configure the CDN to
use the CDN nodes.
[0029] The network 72 is intended to represent a simplified view of
the Internet. In the simplified depiction of FIG. 3, the network 72
includes a plurality of interconnected routers or routing networks,
e.g., routers 74a. 74b, . . . , 74g, for routing packets from one
domain to another within network 60. In actuality, the Internet is
made up of many private "routing networks" (networks including one
or more routers, and possibly other types of networking components
as well), e.g., local, regional and centralized Internet Service
Providers (ISPs), some of which are connected to Network Access
Points (NAPs) or exchanges, or each other at public or private
peering points. In the simplified Internet configuration shown in
FIG. 3, routers 74a, 74b, 74c 74d and 74e are located at network
entry points. The end user station 66a and end user DNS system 70a
connect to the network 72 via the router 74a. The end user station
66b is coupled to another router 74i, which connects to the router
74d. The Web Server 62 and associated DNS system 64 are connected
to the network 72 via the router 74b. Preferably, to the extent
possible, and for reasons which will be discussed below, the end
user DNS systems such as system 70a are located near the end user
systems with which they are associated.
[0030] Also, preferably, the geographically dispersed nodes 76 are
located so as to be as close as possible to various network entry
points, exchanges or both. The network entry points each may
correspond to an ISP Point of Presence (POP). In FIG. 3, for
illustrative purposes, the nodes 76a, 76b and 76c are shown as
being connected to entry point routers 74c, 74b and 74d,
respectively, but need not be directly connected to network access
routers in this manner.
[0031] The caching servers 80 have unique IP addresses. The DNS
systems 78 share a common IP address as well as have unique IP
addresses. The end-user DNS systems, e.g., end user DNS system 70a,
resolve to the common address. That is, the end-user DNS system 70a
knows which DNS system (in this example, the DNS system 64) has an
address for a high level domain server, e.g., .com, org, and
maintains tables of all domain names and knows which server
(authoritative DNS server) to consult for the address of the domain
server. Thus, the address lookup table in the DNS system 64 is
configured to indicate that a server corresponding to the common
address can resolve the domain name of the content provider to an
IP address.
[0032] One way to implement this content distribution configuration
is to use an anycast address as the common address. An anycast
address is a unicast address that is used in multiple places.
Various Internet Engineering Task Force (IETF) Internet Requests
for Comments (RFCs) describe implementations of anycast addresses
in IP networks. The IETF is a large open international community of
network designers, operators, vendors, and researchers concerned
with the evolution of the Internet architecture and the smooth
operation of the Internet. The following anycast-related RFCs are
hereby incorporated by reference in their entirety for all
purposes: RFC 1546 (November 1993); RFC 2372 (July 1998); RFC 2373
(August 1998); and RFC 2526 (March 1999).
[0033] As described in RFC 1546, an anycast address may include a
subnet prefix identifier and an anycast identifier. The subnet
prefix may be used to specify the network providing the anycast
addresses. The anycast identifier is used to specify one of many
possible anycast addresses on a particular subnet. A unicast
address, or conventional IP address, specifies a single interface
on a computer network. In contrast, an anycast address may specify
more than one interface. For example, anycast addresses may be used
to specify a group of one or more servers on a computer network.
These servers may provide a redundant service. Routers forward
packets destined to anycast addresses to the closest anycast
destination for a particular address. Thus, anycast addresses
provide a way to distribute load across one or more servers
[0034] The anycast address is advertised to the network 72 from
each node 76 using a dynamic routing protocol, the Border Gateway
Protocol (BGP). The BGP is a routing protocol used to exchange
network reachability information between Internet border routers.
It enables those routers to make intelligent routing decisions as
to the best path. The BGP is used by such routers as their exterior
routing protocol in order to advertise routes to other border
routers. BGP uses TCP as its transport protocol for exchanging
routing information. Adjacent routers running BGP set up a TCP
connection to exchange entire routing tables. Each router has
enough information to determine an optimal next hop to a
destination. The BGP is also described in various RFCs, including
RFC 1267 (October 1991) and RFC 1654 (July 1994), incorporated
herein by reference.
[0035] Referring to FIG. 4, routing configuration support 90 in a
router such as router 74b includes a routing algorithm (software)
94 and a routing table 92. The router 74b receives address path
information from the nodes 76 in accordance with the BGP route
information exchanges and updates 96, including the anycast address
98 and associated paths 100 for each of the nodes, and stores them
in the routing table 92. As a result of the BGP table information
exchanges and updates, routers in the network 72 maintain pointers
(that is, the paths or routes) that allow it to determine the next
hop to every unique address in the network, as well as multiple
pointers to the anycast address. The routing algorithm 94, in
response to receipt of a DNS request packet 102 from an end-user
system 66 for resolution of a DNS name for the content provider,
uses the path information 100 stored in the routing table 92 to a
select a path to the nearest CDN node. Thus, a router may see
multiple connections to the anycast address, but selects the path
that represents the shortest network distance (e.g, the
topologically shortest path). More specifically, as this routing
occurs as part of a DNS resolution, the router selects a route to
the closest DNS system 78. Because the selected DNS system resolves
to the address of the cache server in the DNS system's node, the
DNS anycast routing, in essence, serves to select the local content
site (cache) from which content will be served.
[0036] Referring now to FIG. 5, a Web transaction 90 occurring over
a CDN (such as the CDN 60 shown in FIG. 3) is illustrated for the
running example of the "ABCD" Web site. To provide content
distribution service for this site, the following configurations
are implemented. The original Web server 62 is renamed in the DNS
system 64 to reflect a change in its role from being the primary
Web server to becoming the original content source for the cache
servers 80. The domain "www.abcd.com" is renamed "origin.abcd.com".
In the end-user DNS system 70a, an entry is made to indicate that
the CDN DNS system 78 is the authoritative server for the end user
web domain. For example, in the dns.abcd.com system, the entry for
"www.abcd.com" no longer resolves to a specific address but instead
refers the content requestor to the CDN DNS server 78 at the
anycast address of 4.2.2.19. The CDN DNS server entry resolves to
the address of the associated node's caching server 80. As a
result, each node 76 resolves the Web site name to a different
address. In the above example, DNS server 78a in node 76a (node 1)
resolves the name "www.abcd.com" to 10.3.15.1, whereas in node 76b
(node 2), the DNS server 78b resolves the same address to
10.3.15.2. In this manner, therefore, the entry made in the CDN DNS
server 78 in each node enables distribution of the Web site among
the cache servers 80 in their respective nodes and locations, as
will be described in further detail below.
[0037] Once the appropriate configurations have been completed, it
is assumed that the Web site www.abcd.com is being handled by the
CDN nodes 76. Each node 76 advertises the anycast address of its
own DNS server 78 to the network 72 using the BPG protocol, as
discussed above. The address of the DNS server at each node is
identical. That is, from a network point of view, the network
"thinks" it is connected to a single host at multiple points.
[0038] Referring to FIGS. 3 and 5, the Web transaction operation 90
is as follows. When an end user of the end-user station 66a
requests an object from the now "accelerated" Web site, the
end-user system 66a first resolves the content provider name via
DNS. That is, the end-user station (acting as a content requestor)
sends a request to the local DNS server 70a ("DNS Req 92"). That
server 70a resolves the name, ultimately by sending a request to
the CDN DNS anycast address. The DNS server 70a sends a request to
the ABCD DNS server 64 ("DNS Req 94"), which returns the anycast
address of the CDN nodes 76 to the requesting DNS server 70a (DNS
Resp 96). The DNS server 70a then transmits a DNS name resolution
request addressed to the anycast address and that request is
generally routed (by various ones of the network routers 74a-74h)
to the CDN node nearest the user's DNS system ("DNS Req 96"). In
this instance, because the request enters the network via the
router 74b, the router 74b determines that the shortest path to the
anycast address is the path to the node 76b (node 2). At that node,
the DNS server 78b resolves the name to the cache server address
for that node, that is, IP address 10.3.15.2 assigned to the cache
server 80b, and returns the cache server address to the end-user
DNS server 70a ("DNS Resp 98"). The end-user DNS server 70a
forwards the address to the end-user system 66a ("DNS Resp 100).
From this point on, the remaining steps of the transaction are much
the same as steps 44 through 52, shown in FIG. 2. That is, the
end-user system 70a requests the Web object from the cache server
in the nearby node where the DNS name was resolved, i.e., node 76b,
and that node's cache server 80b, in turn, checks for a cached
version of the object (if cached content already resides on the
cache server). If no content has yet been cached in the server, or
the cached copy is stale or otherwise invalid, the cache server 80b
retrieves the Web object from the origin server 62, and serves the
object to the end-user system that requested it.
[0039] Typically, as is known in the art, Web content can be marked
with certain caching attributes, for example, whether or not the
content is at all cacheable, how long the content may be held in
cache. In the case of the former attribute, if the content is
marked as uncacheable (e.g., dynamic or content containing
sensitive information), the cache server discards the content after
serving it to the requestor. Otherwise, if the content is
cacheable, the cache server will store the content in local storage
and maintain the cached content according to any other cache
attributes. Content can be localized, for example, using ad
insertions with local content. Content in the caches can be
pre-loaded (all the cache servers receiving the same content). That
is, the content can be replicated on all cache servers so that even
the first request will have a fast response time. Preferably, the
caches are not preloaded with content but instead build their
cached content based on user requests/usage over time. Content is
retrieved from the origin server 12 when a first user request is
received, and then stored locally. If subsequent requests are
received for the same content, the cached copy is used if it is
still valid, as was mentioned earlier. Thus, the cache server need
not retrieve the content from the origin server again. Each cache
server contains a translation table so that it knows where to
retrieve any particular Web page from the origin server. For
example, the cache server 80b would know that the page
"www.abcd.com/PriceList" can be retrieved from
"origin.abcd.com/PriceList".
[0040] Preferably, the CDN node contains software to monitor the
load in various parts of the node cache system (disk, CPU, I/O, et
cetera) by determining at least one load metric value (based on
metrics such as utilization, latency, etc.) and comparing each such
metric value to a predefined overload threshold. Upon reaching a
predefined overload threshold, the monitoring software informs the
routing software in the CDN DNS server to withdraw its BGP routing
advertisement.
[0041] Thus, under normal conditions, all CDN nodes are advertising
the address of their DNS servers to the network, and so a DNS
request will be directed to the nearest CDN node. If a node becomes
heavily loaded and detects an overload condition through its
internal monitoring, the node stops advertising its DNS address to
the network so that no further requests will be directed to that
node. Consequently, DNS requests that normally would have been
routed to that node as a first choice are routed to the next
closest active node.
[0042] This overload detection and load balancing mechanism has the
advantage that Web transactions already in progress are not
interrupted by a shift in resources. Any system that has already
resolved a DNS name to the now inactive node will continue using
that node until the DNS name expires. The load in that node will
slowly decrease until such time as the node can start accepting new
clients, at which time it will start advertising its DNS system
address to the network again.
[0043] Other embodiments are contemplated. For example, it is
possible to use an anycast scheme with the cache servers
themselves. With reference to the system shown in FIG. 3, the
network routing directs the DNS request from the end user's DNS
system, such as system 70a, to the closest of the CDN nodes 76.
Although the node is the node closest to the end user's DNS system,
it may not necessarily be the closest to the end-user system. In
those cases where the end user's DNS system is a substantial
distance from the end user, it is possible that the end user system
will use a CDN node that is not the closest one to the end user
system. Allowing the cache servers to use a common address would
ensure that the end user's Web request is indeed routed to the
nearest CDN node. There is a significant drawback associated with
using the anycast-addressable cache server approach, however. The
client/server portion of the transaction uses the TCP protocol,
thus requiring multiple exchanges between the end user and the
cache server to complete a transaction. With anycasting, there is
no guarantee that subsequent packets in a transaction will be
routed to the same server. In cases where the packets are split
between two or more cache servers, a successful transaction cannot
occur. In contrast, although requiring that the end user DNS system
be located in close proximity to the end user system for optimal
CDN performance, the anycast-based DNS resolution is completed
using a single packet exchange with the stateless UDP protocol,
thus eliminating the packet-by-packet load distribution problem
seen with TCP.
[0044] In addition, the Web caches, i.e., the cache systems 80 each
can be implemented to include multiple caches servers connected,
for example, in a cluster configuration. There may be multiple
servers available to support one customer (origin server) or,
alternatively, one or more cache servers available to support
multiple customers' content cached at one node (site). In yet
another alternative, the cache server clusters can include a switch
to select from among the cache servers in a given node/cluster
based on a predetermined selection policy, for example,
content-aware selection (which enables the clustered servers store
different content, and maps requested objects to the appropriate
servers), load balancing, and so forth, using known techniques.
[0045] Other embodiments are within the scope of the following
claims.
* * * * *
References