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Content Distribution Network (CDN) Performance. Punit Shah ( CSE581 Internet Technologies OGI, OHSU 2002, Jan 16th. Papers. CDN, CDN Performance The measured performance of CDNs. On the use and performance of CDNs.

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Content Distribution Network (CDN) Performance

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Content distribution network cdn performance

Content Distribution Network (CDN) Performance

Punit Shah (

CSE581 Internet Technologies


2002, Jan 16th



  • 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.

CSE581, Winter 2002 | Punit Shah (

What is cdn

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.

CSE581, Winter 2002 | Punit Shah (

Request redirection

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).

CSE581, Winter 2002 | Punit Shah (

Full site dns redirection example

GET index.html

<HTML> … <HTML> (not

IP for

Full Site DNS redirection example

Origin Server index.html

CDN controlled DNS Server

Vendors: Adero(Full), Akami and Digital Island (Partial)

CSE581, Winter 2002 | Punit Shah (

Partial dns redirect url rewriting example

Partial DNS redirect/URL rewriting example




<A HREF=“/about_us.html”> About Us </A>

<IMG SRC=“”>

<IMG SRC=“”>

<IMG SRC=“”>



Vendors:Clearway (URL RW)

CSE581, Winter 2002 | Punit Shah (

Cdn performance elements

CDN performance elements

  • Client perceived latency.

    • That’s what most of the papers focused, as an outsider.

  • Load balancing among the CDN servers.

  • Number of request offloaded from an origin server.

CSE581, Winter 2002 | Punit Shah (

Analytical model

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’

CSE581, Winter 2002 | Punit Shah (

Model performance

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.

CSE581, Winter 2002 | Punit Shah (

Cdn server selection

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.

CSE581, Winter 2002 | Punit Shah (



  • 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 >90% times reasonably good server, with respect to particular location is chosen.

  • For around 10% of times, rather random choice would done better.

  • Conclusion: Doesn’t choose an optimal CDN server, but avoids notably bad CDN server.

CSE581, Winter 2002 | Punit Shah (

Another location

Another location

CSE581, Winter 2002 | Punit Shah (

Other results

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.

CSE581, Winter 2002 | Punit Shah (

Load balancing and dns lookup overhead

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, 15min, 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.

CSE581, Winter 2002 | Punit Shah (



  • 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.

CSE581, Winter 2002 | Punit Shah (

Some facts

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.

CSE581, Winter 2002 | Punit Shah (

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