After looking at hundreds if not thousands of transactions in trace files, I've discovered that many of the "slow" Web servers are those in which there is a large delay between the get and the first data packet to be returned. Once the data (for that one Web object) is returned, it typically flows reasonably well. In other words, once the server's application retrieves a chunk of data and hands it over to the transport layer (TCP) to return it to the user, the application is pretty much out of the picture.
Thus, a very critical part of ART is that first delay for every transaction -- how fast can the server process the request and return the information? Another interesting behavior I've learned is the TCP stack on the server is typically independent of
It order to measure ART accurately, the network analysis tool must understand the difference between receiving an ACK and receiving the first data packet. Unfortunately, this is not always the case. I've seen products that measure the ACK packets only, leading to very misleading results and interpretations, leading to erroneous response time figures and even expert system reports that miss the real delay.
Such products will accurately measure ART for servers and applications that are behaving normally, but they become inaccurate when there are serious problems, which of course are the very things we network engineers want to detect and troubleshoot! Such delays go totally undetected if the tool does not understand the difference between a command packet, which has been ACKed, vs. versus real data being returned from the server.
ART is not always easy to measure on an automated basis, since the analyzer doing it must separate packets that are the start of commands from those that are simply part of data transfer and are possibly bi-directional. Further, an analyzer that performs ART on all conversations or flows between users and servers should understand all protocols and applications, even if they are not "decoded" or are proprietary.
As network analysts doing our day-to-day job, we on the front line require accurate and verifiable ART. We must build confidence our analytical tools, especially when users are complaining of slow response times. To gain confidence in our tools, we should manually verify the numbers for every application of high business value to our organization, rather than relying on such data at face value.
Here's an example of how to verify ART data. Let's examine the following three packet summaries from a SQL transaction, beginning with a SELECT statement from the client.
Packet Source Destination Delta Time Relative Time Protocol Summary 43039 220.127.116.11 18.104.22.168 0.000000 TDS SELECT * FROM… 43044 22.214.171.124 126.96.36.199 0.122394 0.122394 TCP Flags= .A.... 43874 188.8.131.52 184.108.40.206 12.687890 12.810284 TDS Response STATUS=Last…
ART analysis should tell us that the response time to the SELECT statement is 12.8 seconds, the total time before we saw data from the server relative to the initial request, not the .122 seconds (122 milliseconds) that we see between the request and the return of the first packet, which is merely an ACK packet. Further, an expert system should alert us to this difference -- do not assume that if the ART data is correct, the expert system is, too.
Verifying and paying attention to such detail will give you the confidence that the ART you are looking at is the real deal, and not a forgery.
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This was first published in March 2005