RAS> Date: Tue, 28 May 2002 14:43:25 -0400 RAS> From: Richard A Steenbergen RAS> Theoretically, ICMP Echo should be less intrusive for RAS> performance measuring since it is clearly only for this RAS> purpose, whereas doing an actual TCP handshake could easily And less accurate for asymmetric traffic, under-representing the return flow. RAS> be mistaken for a port scan. But for so many network admins, RAS> all they know is "ICMP bad". That'll be the day when someone calls abuse saying "I'm being attacked by ICMP unreachables!" ;-) RAS> Besides, if you thought ICMP was inaccurate, a TCP handshake RAS> is far worse. First, you have no way of knowing if the TCP RAS> replies are the originals, or if they have been RAS> retransmitted, so any single sample is pretty useless for RAS> measuring reply time. Second, for most cases with TCP opens, RAS> you are measuring the interaction time of the application RAS> process, not response time from the kernel. Depending on how Ah, yes. Reminds me of the "benchmark" where a company that writes mailserver software tested HELO response time. Gee, my MXen spend more time actually sending and receiving messages! RAS> the application is written, and what event model it uses, it RAS> is very likely to be doing other things before and after it RAS> gets notified that there is a new connection, then does the RAS> accept(). If it gets that far. I suspect that accept filters would give artificially fast results. Then there are issues such as delayed ACK. And if one is measuring throughput, does the session ever make it past slow start? And just how many frames do they send during slow start? (Tunable under FreeBSD...) Once ECN becomes more common, does one consider those points outliers or useful data? RAS> Internet path measurement isn't simple, you're probably best RAS> off trying to get as much data as possible passively. :) Less intrusive, and chances are that anything too small for statistical validity is insignificant, anyway. -- Eddy Brotsman & Dreger, Inc. - EverQuick Internet Division Phone: +1 (316) 794-8922 Wichita/(Inter)national Phone: +1 (785) 865-5885 Lawrence ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Date: Mon, 21 May 2001 11:23:58 +0000 (GMT) From: A Trap <blacklist@brics.com> To: blacklist@brics.com Subject: Please ignore this portion of my mail signature. These last few lines are a trap for address-harvesting spambots. Do NOT send mail to <blacklist@brics.com>, or you are likely to be blocked.