Content Distribution Network CDN Performance

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Content Distribution Network (CDN) Performance

• Punit Shah (pshah_at_cse.ogi.edu)
• CSE581 Internet Technologies
• OGI, OHSU
• 2002, Jan 16th

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Papers

• CDN, CDN Performance
• The measured performance of CDNs.
• On the use and performance of CDNs.
• Analytical model for CDN performance in
  multi-level caching.
• Web caching and content distribution A view from
  the interior.

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What is CDN ?

• The CDNs are means to offload some or all of the
  (mainly static content) content delivery burden
  from the origin server. A replica server, which
  delivers content on behalf of the origin server
  is called a CDN server.
• Aimed to address
• Client perceived latency (e.g. web browsers).
• Capacity management of the server.
• Caching as a side-effect.

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Request Redirection

• Primarily two ways to redirect request to the CDN
  servers.
• DNS redirection
• Authoritative DNS server is controlled by the
  CDN infrastructure. Distributes the load to the
  various CDN servers depending whatever policy
  (e.g. round-robin, least loaded CDN server,
  geographical distance etc.) using DNS trick.
• URL rewriting
• Main page still comes from the origin server,
  but URL for the embedded objects, e.g. images,
  clips are rewritten, which points to a any of the
  CDN server. Some vendors rewrite using hostname
  and some uses IP addr directly.
• Some vendors do employ a combination of these two
  methods.
• Not simple to find a nearest CDN server (in terms
  of latency).

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Full Site DNS redirection example
Origin Server
111.222.100.1
10.20.30.1
www.yahoo.com/GET index.html
10.20.30.4
10.20.30.2
CDN controlled DNS Server
10.20.30.3
Vendors Adero(Full), Akami and Digital Island
(Partial)
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Partial DNS redirect/URL rewriting example
index.html ltHTMLgt ltBODYgt ltA HREF/about_us.htmlgt
About Us lt/Agt ltIMG SRCwww.clearway1.net/www.yah
oo.com/img1.gifgt ltIMG SRCwww.clearway2.net/www.
yahoo.com/img2.gifgt ltIMG SRC10.20.30.2/www.yaho
o.com/img3.gifgt lt/BODYgt lt/HTMLgt
Vendors Clearway (URL RW)
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CDN performance elements

• Client perceived latency.
• Thats what most of the papers focused, as an
  outsider.

• Load balancing among the CDN servers.

• Number of request offloaded from an origin
  server.

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Analytical Model

• Gadde et al. derives CDN cachable ratio as (Cni -
  Cnl)/(1 - Cnl)
• where
• Cni CDN hit ratio for client population of size
  ni who forwards to this CDN server for some
  fixed object x
• Cnl cache hit ratio at leaf node (e.g. proxy)
  serving client population of size nl

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Model Performance

• More clients, less CDN cache hit ratio.
• If number of clients increased further, curve
  take a bell shape, indicating cache thrashing.
• Model validated with the NLANR cache hierarchy at
  the root level (considering all root level
  cache as an unified cache). 32 cache hit ratio
  in Oct 1999.

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CDN Server Selection

• Primary paper Johnson et al. focuses on how
  good (good minimal client latency) CDN
  server is selected by the Akamai and Digital
  Island. Both of these uses partial site DNS
  lookup.
• Used three distinct client locations in the US.
  Two east coast and one western state. Clients
  were running different OS and different internet
  bandwidth.
• Test Procedure
• Determine set of CDN servers (hostnames) used by
  the particular CDN.
• Obtain IP address of the CDN servers.
• Identify a GIF file (3-4KB), and retrieve this
  GIF from each of the CDN servers 25 times. Record
  time taken. Notice that DNS lookup time is not a
  factor, as IP addrs are used.This test was
  conducted at all three client sites.
• Fetch same GIF via CDN server identified by
  contacting an origin server. Record time taken.
  Modified gethostbyname()? or /etc/resolv.conf
  order. Because TTL was quite small (10s of
  seconds). This tests were also conducted at all
  three client sites for both of these vendors.

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Results

• Both vendors demonstrated identical results.
• Not very best CDN is chosen at some locations.
• Performance is highly location dependent. Some
  location performed much better than the others.
  Indicating CDN server placement.
• However gt90 times reasonably good server, with
  respect to particular location is chosen.
• For around 10 of times, rather random choice
  would done better.
• Conclusion Doesnt choose an optimal CDN server,
  but avoids notably bad CDN server.

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Another location
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Other Results

• Focus is to compare Sep 2000 and Jan 2001
  results.
• CDN server selection test results are identical
  to the what we saw earlier.
• HTTP/1.1 results are better than HTTP/1.0
  parallel connection. V1.1 pipeline is faster than
  serial.

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Load balancing and DNS Lookup Overhead

• Till now we ignored DNS lookup time to focus on
  measuring quality of the CDN server chosen.
• However not an insignificant overhead. Esp.
  considering very small download time and TTL,
  e.g. Adero 10sec, Akamai and Digital Island
  20sec. TTL for non-CDN origin site, cnn.com
  15min, espn.com 6hours.
• Bala et al. conducted a test to measure DNS
  lookup overhead (and latency) introduced by the
  CDN load balancing mechanism.
• Test procedure
• Store (fixed) IP addr for each CDN server at
  every 8 hours.
• During this 8 hours period, at every 30 mins.,
  compare new IP returned with previously retrieved
  (fixed) IP addr.
• Access DNS lookup time and download time for new
  IP addr returned.
• Compare download time with fixed IP addr.

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Results

• In Jan 2001, 15 (Fasttide) to 70(Digital
  Island) time new IP is same as fixed.
• In above cases a new IP download time is
  identical to the fixed IP, but DNS lookup
  overhead undermines overall performance.
• 10 of times, download from new IP addr is
  faster, but again DNS lookup
• 30-40(Akamai) times new download time is more
  then a fix IP addr, again DNS lookup ...
• New download time are more than fixed IP addr
  download time.
• Overall redirection is not efficient.

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Some Facts ...

• CDN mainly used for image files (static
  contents).
• Content server by the CDN is a static in the
  nature. Only 0.3 content changed for existing
  URLs and at the most 13 new URLs were
  introduced.
• Black-box performance testing. So no data about
  load-balancing, only latency.
• Large increase in deployment in the CDN between
  Nov 99 (only 1-2 of top 670 sites) and Dec 2000
  (25 of the popular sites).
• Akamai seems to be most popular CDN vendor.
• Images are 96-98 of the CDN served contents. But
  only 40-46 of the CDN-served bytes. Rest is
  dynamic content ?
• CDN images cache-hit rate is 30-80. Only 25-60
  for non-CDN served.
• Needs to map IP addrs with the geography for
  better CDN server selection.
• CDNs can not used for something that involves
  authentication etc.