Caching
 Temporary storage of frequently accessed
data (duplicating original data stored
somewhere else)
 Reduces access time/latency for clients
 Reduces bandwidth usage
 Reduces load on a server
1
Web cache types
 Browser cache – for a single user
 Shared cache (forward and reverse) – same
principle for multiple users
2
Forward proxy cache
• Cache located closer to the client
• Usually deployed by an ISP
• Decreases bandwidth usage (ISP to the Internet link
in the example below)
3
Reverse proxy cache
• Aka gateway proxy or web accelerators
• Cache proxy located closer to the origin web server
• Usually deployed by an Web hosting ISP
• Decreases load on the web server
• Several reverse proxy caches implemented together
can form a Content Delivery Network
4
How a typical cache works
• Freshness – how long the document stays “fresh” or
can be used from cache without rechecking the
origin server
• Validation – compare the cached document to the
origin document once it’s not “fresh” anymore
5
HTML tags vs HTTP headers
• HTML Meta tags - part of the document; mostly for
browser cache (that parses HTML); most Proxy
caches do not look inside the document
• HTTP headers are sent before HTML document; are
seen by both browser and proxy caches
HTTP/1.1 200 OK
Date: Fri, 30 Oct 1998 13:19:41 GMT
Server: Apache/1.3.3 (Unix)
Cache-Control: max-age=3600, must-revalidate
Expires: Fri, 30 Oct 1998 14:19:41 GMT
Last-Modified: Mon, 29 Jun 1998 02:28:12 GMT
ETag: "3e86-410-3596fbbc"
Content-Length: 1040
Content-Type: text/html
6
HTTP headers
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max-age=[seconds] — specifies the maximum amount of time that an
representation will be considered fresh.
s-maxage=[seconds] — similar to max-age, except that it only applies to shared
(e.g., proxy) caches.
public — marks authenticated responses as cacheable; normally, if HTTP
authentication is required, responses are automatically private.
private — allows caches that are specific to one user (e.g., in a browser) to store
the response; shared caches (e.g., in a proxy) may not.
no-cache — forces caches to submit the request to the origin server for
validation before releasing a cached copy, every time.
no-store — instructs caches not to keep a copy of the representation under any
conditions.
must-revalidate — tells caches that they must obey any freshness information
you give them about a representation.
proxy-revalidate — similar to must-revalidate, except that it only applies to proxy
caches.
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Validators
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Are used by caches to compare the cached document to the original document
for changes
If validator is not present and no freshness information is available, the
document won’t be cached
Last-Modified HTTP header
E-Tag
8
Proxy Server software examples
•
Squid (Unix/Linux and Windows)
•
Varnish (web accelerator)
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Apache proxy module and cache module
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NGINX (HTTP (reverse) and email proxy
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Interception caching
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To avoid configuring each client to point to cache proxy
Can be accomplished using inline cache, layer 4 switch, WCCP, policy-based
routing
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Content Delivery Networks
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Network of computers that deliver content on the web.
Content pushed-out/delivered “closer” to the clients
Designed to improve Internet performance (i.e. decrease latency for clients,
decrease bandwidth use)
Consists of origin server, surrogate (edge servers)
Caching and server load balancing techniques are used
ESI (Edge-Side Includes) – open standard markup language to augment HTML
for help with dynamic delivery and assembly of Web documents
11
Content distribution networks
 challenge: how to stream content (selected
from millions of videos) to hundreds of
thousands of simultaneous users?
 option 1: single, large “mega-server”
 single point of failure
 point of network congestion
 long path to distant clients
 multiple copies of video sent over outgoing link
….quite simply: this solution doesn’t scale
Content distribution networks
 challenge: how to stream content (selected
from millions of videos) to hundreds of
thousands of simultaneous users?
 option 2: store/serve multiple copies of videos
at multiple geographically distributed sites
(CDN)


enter deep: push CDN servers deep into many access
networks
• close to users
• used by Akamai, 1700 locations
bring home: smaller number (10’s) of larger clusters in POPs
near (but not within) access networks
• used by Limelight
Content Delivery Networks
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CDN: “simple” content access
scenario
Bob (client) requests video http://netcinema.com/6Y7B23V

video stored in CDN at http://KingCDN.com/NetC6y&B23V
1. Bob gets URL for for video
http://netcinema.com/6Y7B23V
2. resolve http://netcinema.com/6Y7B23V
from netcinema.com
2 via Bob’s local DNS
web page
1
6. request video from 5
4&5. Resolve
KINGCDN server,
http://KingCDN.com/NetC6y&B23
streamed via HTTP
via KingCDN’s authoritative DNS,
netcinema.com 3. netcinema’s DNS returns URL
4 which returns IP address of KIingCDN
http://KingCDN.com/NetC6y&B23V
server with video
3
netcinema’s
authorative DNS
KingCDN.com
KingCDN
authoritative DNS
CDN cluster selection strategy
 challenge: how does CDN DNS select “good”
CDN node to stream to client
pick CDN node geographically closest to client
 pick CDN node with shortest delay (or min # hops)
to client (CDN nodes periodically ping access
ISPs, reporting results to CDN DNS)
 IP anycast

 alternative: let client decide - give client a list
of several CDN servers
client pings servers, picks “best”
 Netflix approach

Case study: Netflix
 30% downstream US traffic in 2011
 owns very little infrastructure, uses 3rd party services:
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own registration, payment servers
Amazon (3rd party) cloud services:
• Netflix uploads studio master to Amazon cloud
• create multiple version of movie (different
endodings) in cloud
• upload versions from cloud to CDNs
• Cloud hosts Netflix web pages for user
browsing
three 3rd party CDNs host/stream Netflix content:
Akamai, Limelight, Level-3
Case study: Netflix
Amazon cloud
Netflix registration,
accounting servers
2. Bob browses
Netflix video 2
3
upload copies of
multiple versions of
video to CDNs
3. Manifest file
returned for
requested video
Akamai CDN
Limelight CDN
1
1. Bob manages
Netflix account
4. DASH
streaming
Level-3 CDN