[tcpm] hystart++ -03 review
Mark Allman <mallman@icir.org> Tue, 12 May 2020 19:54 UTC
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From: Mark Allman <mallman@icir.org>
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Subject: [tcpm] hystart++ -03 review
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I read version -03 of the HyStart++ document and my view differs from many, I guess. IMHO, this document is not ready for adoption. The draft is pretty anemic and I am not sure there is enough there to actually judge whether it is or is not a good idea. It seems like perhaps the document could get there, but at this point I think the authors have some work to do. Review below. allman Section 1: - Nit: RFC 793 does not define slow start, remove ref. (Could add a ref to original Jacobson paper.) - HyStart++ is "widely deployed", but is it widely used? Is it enabled or just there for the enabling? The document makes it sound like we have a lot of experience with this somehow, but none of that is shared (see below). - Nit: Spell out "LSS" on first use. Section 3: - Nit: Totally fine to crib the definitions from RFC 5681, but it'd be good form to note that is where they came from. - Nit: Also, since you use rwnd in one of the definitions and you're copying things, you might as well copy that one, too. Section 4: - There is so much unexplained magic in here. When setting RttThresh, what is magic about 1/8-th of the lastRoundMinRtt? Why clamp this between 4msec and 16msec? Would that really be reasonable for all networks? We need magic numbers sometimes and as long as they're resasonably chosen and motivated we have to live with having them around. But, the draft really needs to explain these choices, how they were made and how broadly applicable they might be. Further, the document suggests one MAY vary the constants, but it'd be great if it gave some sort of advice on how one might decide if different constants were OK or not OK. - The algorithm (series of equations) is here and clear. But, the document really needs to give us a flavor of the why. Talk us through what is happening and why in here, please. General / meta: - The draft offers a little intuition on why this is a good idea (i.e., to prevent massive slow start over-shoot). But, there is no sketch of evidence that shows it is effective. I see this was previously mentioned on the mailing list and Praveen said: We presented lab measurement data at the last IETF. We are in the process of doing some A/B experiments at scale with real world workloads. We did fairness comparisons in our lab measurements between Hystart and non-Hystart and we didn't see any problems. The bar ought to be higher than "didn't see any problems". It is great that there is some testing done and planned. I'd like to see this all sketched in the document itself. I am not even close to convinced that this is a good idea. - In some sense, this idea makes TCP more conservative (i.e., we stop exponential increase sooner than RFC5681 would suggest). So, the bar is likely not huge for saying this is "OK". But, I think we should have some notion that this helps *something* before agreeing to suggest it as somehow a reasonable thing to do in an RFC. Right now this document doesn't concretely show that HyStart++ is useful in any way.
- [tcpm] hystart++ -03 review Mark Allman
- Re: [tcpm] [EXTERNAL] hystart++ -03 review Praveen Balasubramanian