Caching and Content Distribution Networks

1
Caching and Content Distribution Networks
2
Web Caching

• As an example, we use the web to illustrate
  caching and other related issues

3
Web Browser Caching

• Web browsers have their own caches. When a page
  is downloaded from a site the web page is put
  into the browser cache.
• This is especially useful in those cases when the
  back button is pressed.
• If a new copy is needed then a refresh can be
  done.
• No page stays permanently in the cache. There is
  limited room.
• A replacement algorithm is needed to determine
  which cached page should be purged.

4
Why Web Server Caching

• Latency
• Reduce latency
• Request does not require going to the server
• Request is served from the client side which
  means that network communication is avoided
• Reduce traffic

5
Consistency

• What if the page changes after saved in the
  cache?
• This means that cached copy is out of date
• The copy and the original are not consistent
• There are different strategies for dealing with
  this

6
Web Browser Caching

• Client pull
• The server provides the content with instructions
  on when the client should ask for a refreshed
  copy of the content or if the content should be
  cached.
• Server push
• The server transmits page information to the
  screen.
• The browser application displays the information
  and leaves the connection to the server open.
• With an open connection, the server can continue
  to push updated pages for your screen to display
  on an ongoing basis. You can close the connection
  by closing the page.
• The server is in control
• Browser caches are different from proxy caches
  (discussed next).

7
Web Caching

• Proxy caches (also called proxy server)
• Intercepts HTTP requests from client
• Serves object if in its cache and the date is
  still valid
• If not go to objects home server
• On behalf of user, gets the object and possibly
  deposits in its cache before returning to user
• Usually deployed at edges of a network
• Wide area bandwidth savings, improved response
  time and increased availability of static
  web-based objects
• A browser may have to be configured to point to
  the proxy server.
• Usually a proxy cache is purchased and installed
  by an organization

8
Web Caching

• Not all web pages can be cached
• If the Last-Modified tag then page can be cached
• Refresh is often done when
• There is a request and
• Expiry time has passed

9
Cooperative Caching

• Caching infrastructure can have multiple web
  proxies
• Proxies can be arranged in a hierarchy or other
  structures
• Proxies can cooperate with one another
• Answer client requests
• Propagate server notifications
• Uses a combination of HTTP and ICP (Internet
  Caching Protocol).
• ICP can be used by one cache to quickly ask
  another cache if it has an object.
• HTTP is used to actually retrieve the object.

10
Problems

• Caching proxies do not serve all Internet users
• Content providers (say, Web servers) cannot rely
  on existence and correct implementation of
  caching proxies.
• Accounting issues with caching proxies
• Example www.cnn.com needs to know the number of
  hits to the advertisements displayed on the web
  page.

11
Content Distribution Networks (CDN)

• Business Model A content provider such as
  www.cnn.com or Yahoo pays a CDN company (such as
  Akamai) to get its content to the requesting
  users with short delays.
• A CDN provides a mechanism for
• Replicating content on multiple servers in the
  Internet
• Providing clients with a means to determine the
  servers that can deliver the content fastest.

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Terminology

• Content Any publicly accessible combination of
  text, images, applets, frames, MP3, video, flash,
  virtual reality objects, etc.
• Content Provider Any individual, organization,
  or company that has content that it wishes to
  make available to users.
• Origin Server Content providers server , where
  the content is first uploaded.
• Surrogate Server (sometimes called edge server)
  Content distributors server, where the
  replicated content is kept.

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Players
Yahoo, MSNBC, CNN CBC
Content Provider
Send content
Akamai,
Content Distributor
Sells servers
Install servers
H/W and S/W Vendor

• Cisco,
• Oracle-
• Sun

Hosting Provider
Bell
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CDN Distribution
14

• Content providers are CDN customers
• Content replication
• CDN company installs thousands of servers
  throughout Internet
• In large datacenters
• Or, close to users
• CDN replicates customers content
• When provider updates content, CDN updates
  servers

origin server in North America
CDN distribution node
CDN server in S. America
CDN server in Asia
CDN server in Europe
15
CDN Functional Components

• Distribution Service
• Redirection Service
• Accounting and Billing system

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CDNDistribution Service

• The content provider determines which of its
  objects it wants the CDN to distribute.
• The content provider tags and then pushes this
  content to a CDN node, which in turn replicates
  and pushes the content to all its CDN servers.

17
CDN Redirection

• When a browser in a users host is instructed to
  retrieve a specific object (specified using a
  URL), how does the browser determine whether it
  should retrieve the object from the origin server
  or from one of the CDN servers?
• As an example, suppose the hostname of the
  content provider is www.cnn.com

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How Akamai Works
18
cnn.com (content provider)
DNS root server
GET index.html
Akamai cluster
Akamai global DNS server
http//a.73.g.akamai.net/7/23/cnn.com/af/cnn.com/f
oo.jpg
1
2
HTTP
Akamai regional DNS server
Nearby Akamai cluster

• End-user

19
CDN Redirection

• Users get an html document from www.cnn.com this
  could be index.html
• The file index.html uses a modified URL for
  content that has been replicated.
• Example If the jpeg files are what has been
  replicated then
• ltimg srchttp//cnn.com/af/foo.jpggt
• may be modified as follows
• ltimg srchttp//a73.g.akamai.net/7/23/cnn.com/af/f
  oo.jpggt
• The browser needs to resolve a73.g.akamai.net
  hostname for replicated content.

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CDN Redirection

• What does this mean?
• ltimg srchttp//a73.g.akamai.net/7/23/cnn.com/af/f
  oo.jpggt
• host part a73.g.akamai.net
• Akamai control part /7/23
• Content URL /af/foo.jpg

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CDN Redirection

• DNS is configured so that all queries about
  g.akamai.net that arrive at a DNS server are sent
  to an authoritative DNS server for g.akamai.net.
• This is referred to as a Akamai DNS server
  (authoritative DNS server)

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How Akamai Works
cnn.com (content provider)
DNS root server
DNS lookup cache.cnn.com
Akamai cluster
Akamai global DNS server
3
1
2
4
ALIAS g.akamai.net
Akamai regional DNS server
Nearby Akamai cluster

• End-user

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CDN Redirection

• DNS is configured so that all queries about
  g.akamai.net that arrive at a DNS server are sent
  to an authoritative DNS server for g.akamai.net.
  This is referred to as a Akamai DNS server
  (authoritative DNS server)
• When the Akamai DNS server receives the query, it
  extracts the IP address of the requesting
  browser.
• .

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How Akamai Works
P
cnn.com (content provider)
DNS root server
DNS lookup g.akamai.net
Akamai cluster
Akamai global DNS server
5
3
1
2
6
4
Akamai regional DNS server
ALIAS a73.g.akamai.net
Nearby Akamai cluster

• End-user

25
CDN Redirection

• Based on the IP address and information that it
  has about the Internet (called a map), the IP
  address of an Akamai regional server is returned
  to the requesting browser based on policy
• e.g., select the server that is the fewest hops
  away.
• The regional server may choose a surrogate server
  for content retrieval

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How Akamai Works
HTTP
cnn.com (content provider)
DNS root server
Akamai cluster
Akamai global DNS server
5
3
1
2
6
4
Akamai regional DNS server
7
DNS a73.g.akamai.net
8
Address 1.2.3.4
Nearby Akamai cluster

• End-user

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How Akamai Works
HTTP
cnn.com (content provider)
DNS root server
Akamai cluster
Akamai global DNS server
5
3
1
2
6
4
Akamai regional DNS server
7
8
Nearby Akamai cluster
9

• End-user

GET /foo.jpg Host cache.cnn.com
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How Akamai Works
HTTP
cnn.com (content provider)
DNS root server
GET foo.jpg
11
12
Akamai cluster
Akamai global DNS server
5
3
1
2
6
4
Akamai regional DNS server
7
8
Nearby Akamai cluster
9

• End-user

GET /foo.jpg Host cache.cnn.com
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CDN Redirection

• The Akamai DNS server IP address is now in the
  cache of the local DNS server.
• This implies that it is not always necessary to
  go to the root DNS server.
• The TTL associated with the IP address of an
  Akamai server(surrogate) is relatively small.
• This is done for performance reasons.
• Akamai content distribution servers are caches

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CDN Redirection

• What if content is not there?
• If the request content is not found then the
  surrogate will ask other surrogates within a
  specified region for information.
• If requested information is still not found or is
  stale, then a request is made to the original web
  site.

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CDN Selection

• The tricky issue is selecting which local content
  server to use for a particular request
• Want to spread load evenly
• Want minimal impact if server is added or
  removed.
• In Akamai, each surrogate server sends
  measurement results to the Network Operations
  Communications Center (NOCC).
• Measurement results include number of active TCP
  connections, HTTP request arrival rate, bandwidth
  availability, etc
• This information is used by the Akamai DNS
  server.

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Accounting Mechanism

• Accounting mechanisms collect and track
  information related to request routing,
  distribution and delivery.
• Information is gathered in real time and put into
  log files for each CDN component.
• This gets sent to the Network Operations
  Communications Center (NOCC).

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Full Site Delivery vs. Partial Site Delivery

• Full Site Delivery All the contents are
  delivered by the CDN (including HTML, images,
  and other objects).
• Partial Site delivery Only images, streaming
  media and other bandwidth intensive objects
  delivered by the CDN.

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Current Akamai Customers
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Summary

• We have examined replication and issues related
  to the design and implementation of a replicated
  system.
• Many choices and tradeoffs to consider