Reliability of Internet hosts: a case study from the end user's perspective

This paper presents the results of a reliability study on a set of nearly 1 00 popular Web sites done from an end user's perspective. The study attempt s to address the following issues: (a) What is the (stationary) probability that a user's request to access a Web site succeeds? (b) On average, what percentage of Web sites remain accessible to the user at a given moment? (c ) What are the major causes of access failures as seen by the user? (d) Typ ically, how long could a Web site be unavailable to the user? (e) What para meters could be used to quantitatively describe the behavior of a host on t he Internet? Data for the study was acquired by periodically attempting to fetch an HTML file from each Web site and recording the outcome of such att empts. Analysis of the acquired data revealed: (i) Over 94% of the HTML fil e fetch requests succeed on average. (ii) Over 70% of the failures last les s than 15 min (iii) Network-related outages account for over half of the fa ilures. (iv) Network-related outages can potentially render more than 70% o f the hosts inaccessible to the user. (v) Host-related failures tend to be of a shorter duration than failures that might involve the network. (vi) Ne twork connectivity is good on average, with 93% of the sites being accessib le at any given time. (vii) Mean Availability of the hosts is high (0.993). (viii) On average, a host remains unavailable to a user for about 2.5 days per year. However, the total downtimes exhibited by individual hosts varie d from about 2 h per year to nearly 20 days per year. (C) 1999 Elsevier Sci ence B.V. All rights reserved.