Re: [rohc] IPR... again...
Alan Kennington <ak1.rohc@topology.org> Tue, 02 September 2003 14:26 UTC
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Date: Tue, 02 Sep 2003 23:52:54 +0930
From: Alan Kennington <ak1.rohc@topology.org>
To: ROHC mailing list <rohc@ietf.org>
Subject: Re: [rohc] IPR... again...
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Reply-To: Alan Kennington <ak1.rohc@topology.org>
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In-Reply-To: <Pine.WNT.4.56.0308312241130.1348@maw-laptop.roke.co.uk>; from mark.a.west@roke.co.uk on Sun, Aug 31, 2003 at 11:00:30PM +0100
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On Sun, Aug 31, 2003 at 11:00:30PM +0100, West, Mark wrote: > [....] > > Oh, and before anyone points out the fact: yes, I do know that IPR is > claimed on computing the static / dynamic part of the CRC. Hmmmm... > sounds like an implementation issue to me. Ok -- let's try this: > everyone raise your right hand & repeat after me: "I do solemnly swear > that I shall not stoop to any nasty implementation enhancements like > caching the static part of the CRC computation without first consulting my > tame patent attorney." There, that should fix it..? > Mark, Personally I think it's bad engineering to cache the static CRC. In my implementation, I didn't even think of doing this. It's asking for trouble because you have to keep track of whether it might change and make sure that the CRC is always synched with the static header fields. It's very much simpler and more reliable to CRC everything every time it's needed. After all, you do a CRC to ensure robustness. So why would you introduce the danger of software error in CRC calculation? Today's CPUs are so fast that there is absolutely no reason to cache the CRC over a few bytes. I don't even bother to use an 8-bit look-up table (which I usually do with CRCs) to accelerate CRC calculation because CPUs are so fast that you can do individual bit-shift CRC calculation. It's also a tiny proportion of the overall CPU burden. When a 400 MHz CPU is doing GSM voice encoding and decoding and all of the ROHC encoding/decoding and much more (like logging 1000 bytes of information to disk for every packet), I find that the CPU usage is about 6%. Conclusion: the static/dynamic CRC calculation split is really perplexing. When I first saw that, I couldn't work out why someone would bother which such a minor optimisation when there are so many other things that could have been optimized. Theoretically, implementation issues that have no influence on the coding "on the wire" have no place in comms standards. So the static CRC caching is in the category of "implementation hints". This should really be in an appendix. However, RFC 3095 contains numerous "implementation hints" of this sort. For example the "compression state" variable is in this category. It does not appear on the wire, and can be dispensed with completely. ------------------------------ Concerning the idea of using a CRC to detect errors in decompressed data, surely this is as old as the history of CRCs and compression. Isn't this just ordinary communications engineering? Question: Could someone please explain what _serious_ IPR issues there might be in RFC 3095? Standard everyday communications engineering surely cannot be patented. Does someone have a patent on the idea of using X module N to communicate X when X is slowly varying? That's as old as using 03 to mean 2003. Cheers, Alan. _______________________________________________ Rohc mailing list Rohc@ietf.org https://www1.ietf.org/mailman/listinfo/rohc
- [rohc] IPR... again... West, Mark
- Re: [rohc] IPR... again... Alan Kennington
- RE: [rohc] IPR... again... West, Mark
- Re: [rohc] IPR... again... Alan Kennington