Date: Tue, 18 Aug 2020 10:47:39 -0700 (PDT) From: "Rodney W. Grimes" <freebsd-rwg@gndrsh.dnsmgr.net> To: Ryan Stone <rysto32@gmail.com> Cc: freebsd-net <freebsd-net@freebsd.org> Subject: Re: Is anybody using ng_pipe? Message-ID: <202008181747.07IHldYZ066440@gndrsh.dnsmgr.net> In-Reply-To: <CAFMmRNxgoSNX2%2BLd=eEXRH0q7-XFzSF=b2GPzZgNW1LDCvq5Xw@mail.gmail.com>
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> I recently needed to be able to simulate a lossy, high-latency network > in an environment where dummynet wasn't possible. I gave ng_pipe a > try, and hit some major issues > > 1. Instead of configuring a packet drop rate, you configure a bit > error rate, which I found significantly less intuitive >From your background being packet network centric perhaps? Those of us who have line oriented, aka telecom, centric backgrounds BER is a very meaningful and useful metric. > 2. The use of BER makes for a very inconvenient implementation, as > ng_pipe has to maintain a table of packet drop rates for every > possible packet size Hum, that sounds like a poor implementation indeed. It seems like it would be easy to convert a BER into a packet drop probability based on bytes that have passed through the pipe. It should be easy to covert a BER into a packet drop rate, but doing the converse leads to quantization errors. I would rather see us keep the BER as the metric and fix what is broken rather than convert this to a packet drop rate.. > 3. The table implementation isn't sized right for LRO or TSO, leading > to ng_pipe going out of bounds of the array and panicking the system Code predates LRO and TSO, so not unexpected. > 4. The table calculation had two integer truncation bugs and used the > wrong formula. I'm reasonably sure it would never calculate a > probability other than 0 due a 64-bit constant being truncated to > 32-bits. You retracted this. > I'd like to dump all of this and just implement a packet loss rate, > which would simplify all this immensely. Is anybody using ng_pipe > with a non-zero BER who would object to this? Given this litany of > issues I doubt it, but I thought that I'd be sure. My gut instinc is that statistically BER leads to a more realistic model. -- Rod Grimes rgrimes@freebsd.org
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