From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Samuel Subject: Re: how reliable is ping? Date: Mon, 13 Dec 2004 14:28:32 -0800 Message-ID: <41BE1790.7020204@bcgreen.com> References: <200412061019.19099.fluca1978@infinito.it> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <200412061019.19099.fluca1978@infinito.it> Sender: linux-admin-owner@vger.kernel.org List-Id: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Luca Ferrari Cc: linux-admin@vger.kernel.org Luca Ferrari wrote: > Hi, > in my company we installed a new DSL line, supposed to be faster than the > previous one. And in fact it seems to be, in interactive use, but if I use > ping to test connectivity against the old line I can see higher times. So my > questions is: why does ping show high times and then the line goes faster? Is > ping reliable enough to test network speed, or should I use another tool? (at > last, a chrono and a file transfer!) There are many things that test network reactivity, and ping only checks one or two of them. Ping times only indicate how long it takes a packet to make it from my box to yours, and back. The command is mostly intended to just check to make sure that the machine is up. Note that some routers give ping packets lower priority, aince they're (almost) never sending any real data. Cable connections often have a beter intrinsic bandwidth availability but they're more sporadic because they're shared among some number of users in your neighborhood -- in other words, you might get really good bandwith at 4AM and sorse-than-modem bandwidth at 7PM. In truth, ping times don't predict bandwidth. If you have two machines at the same colo site, one which is bandwidth limited to 18Kb/s and the other with an unlimited 10Mbit connection, they may both give you the same ping times, but vastly different download speeds. Similarly, a modem user in your city may give a slightly better ping time than a well-connect box across the country (or even across the ocean), but still give far worse downloads. Snother difference between cable and DSL is that, because DSL is shared, the ping times are less *predictable*. If you're playing counterstrike, very low average latency with short bursts of high-latency can be far more bothersome than a slightly higher average latency that is more stable. -- Stephen Samuel +1(604)876-0426 samuel@bcgreen.com http://www.bcgreen.com/~samuel/ Powerful committed communication. Transformation touching the jewel within each person and bringing it to light.