Nmap Development mailing list archives
Re: New development in host discovery: response rate scaled congestion control
From: David Fifield <david () bamsoftware com>
Date: Wed, 5 Sep 2007 17:26:13 -0600
On Wed, Sep 05, 2007 at 04:14:25PM -0700, Fyodor wrote:
On Wed, Sep 05, 2007 at 04:36:14PM -0600, David Fifield wrote:The idea is that whenever we make a change to the group congestion window, we scale the increment by the inverse of the packet receipt ratio; i.e., the ratio of the packets that have been responded to versus the number that have been sent. This makes the congestion window vary in a healthier manner, as it would with a TCP stream with a steady supply of responses coming in. More information and graphs are here: http://www.bamsoftware.com/wiki/Nmap/ResponseRateScaledCongestionControlHi David. That sounds like a great idea, and it is well presented on the ResponseRateScaledCongestionControl page. This scaling addresses the problem of the congestion window increasing too slowly, but a possible problem is extrapolating too much data based on one response. If we have a 95% drop rate, then one response is weighted such that it counts like 20. That is OK, but what if we have a 99.99% drop rate? We don't want one response to act like 10,000. So I think there should be a limit. Maybe there already is -- I haven't read your patch yet. If I had to pick a limit out of the air, I'd say that 50x is reasonable. Though something different is OK too if it works better. If hitting that limit is causing problems, then maybe we should use more of the confusingly named host discovery pings, where we ping a known live host just to generating timing/drop info.
There is a limit in slow start mode--it's the congestion threshold. (Although that may be scaling a bit too abruptly--see the occasional spikes in the graphs.) There's no limit in congestion avoidance mode, though that's a good idea. Here are statistics for 1/packet_ratio in the 10,000-host test: Min. : 17.29 1st Qu.: 51.19 Median : 58.84 Mean : 57.04 3rd Qu.: 63.30 Max. :105.00 So 50 is probably safe. I'll add that. David Fifield _______________________________________________ Sent through the nmap-dev mailing list http://cgi.insecure.org/mailman/listinfo/nmap-dev Archived at http://SecLists.Org
Current thread:
- Re: New development in host discovery: response rate scaled congestion control, (continued)
- Re: New development in host discovery: response rate scaled congestion control David Fifield (Sep 05)
- Re: New development in host discovery: response rate scaled congestion control Brandon Enright (Sep 05)
- Re: New development in host discovery: response rate scaled congestion control David Fifield (Sep 05)
- Re: New development in host discovery: response rate scaled congestion control Brandon Enright (Sep 05)
- Re: New development in host discovery: response rate scaled congestion control David Fifield (Sep 06)
- Re: New development in host discovery: response rate scaled congestion control Brandon Enright (Sep 06)
- Re: New development in host discovery: response rate scaled congestion control David Fifield (Sep 06)
- Re: New development in host discovery: response rate scaled congestion control Brandon Enright (Sep 06)
- Re: New development in host discovery: response rate scaled congestion control David Fifield (Sep 05)