Quality of Service Measurement

[ARM instrumentation of a web browser]ARM instrumentation of a web browser   [r]There are many different ways Service Level Agreements might be defined. Typical representatives are the availability of IP connectivity and the time needed by a server to fulfill a user request. In the given scenario there is the need to measure the response times of web servers in the service areas of both the provider and the customer. The transactions of interest are the download of web pages by the dealers. In order to learn about the actual response times the user experiences, a web client (Lynx) was instrumented with calls to the ARM API. Figure shows the transactions to be monitored.

Most important is transaction A which spans the whole process of loading the page. To allow the manager the localization of failures or bottlenecks two subtransactions are defined. Subtransaction B covers the part of contacting the DNS for hostname resolution and subtransaction C covers the actual download of the page. The measurement agent forwards the results to the FMA which can do some preprocessing on the data and transmits it to the manager whenever the link is up. If the FMA receives information about timed-out connections or if the reported response times exceed given thresholds, it can take some further action to identify the problem. In our example it does a ping to the local router, to the PoP and to the server that the user tried to contact. The results can be used to determine if the problem is located in the dealer's LAN or in the service area of the customer. Both would mean that the provider cannot be held responsible for the malfunction. Through an ARM instrumentation of the servers and a correlation of server transactions with client transactions even more accurate information can be collected. Lynx was chosen as an example because it is a simple browser and the source code is available, so an instrumentation could be done in the course of the project. Of course, typical dealers will not be using Lynx but as ARM is gaining more and more momentum many browsers might come instrumented in the near future.