[Tsvwg] Re: [rdma] a proposal for a different No-Data-Touch framer for TCP
"John Hufferd" <hufferd@us.ibm.com> Tue, 12 February 2002 01:21 UTC
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To: Stephen Bailey <steph@cs.uchicago.edu>
Cc: tsvwg@ietf.org, rdma@yahoogroups.com
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From: John Hufferd <hufferd@us.ibm.com>
Date: Mon, 11 Feb 2002 17:03:27 -0800
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Subject: [Tsvwg] Re: [rdma] a proposal for a different No-Data-Touch framer for TCP
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Steph, How small do you think the segmentation will be, and will they generally be on a word boundary? The value I used for 361 or 362 (lets call it the segmentation-point-number) was based on the possibility that the segmentation could occur on any word boundary in a normal 1460 byte payload. If this segment will never get smaller then say 500 bytes, then clearly the number will change and the probability will be even lower, on the other hand if the segmentation can be on a byte level, the segmentation-point-number will be larger and the probability of false positive would be higher. So some guidance on the techniques of fragmentation and resultant sizes would be useful. I had assumed, based on what I thought I read in your document, that as soon as you detected the Re-Segmentation, you would stop TUFing, since I thought that the re-segmenting was likely to last for a long time. Then, when you thought things had reach a steady state again, you would re-probe for the PMTU, and use that for your new Frame Size and begin again. If I had that right, then, the shut down would be immediate, and only one False Positive Event would need to be detected. In that single event case the probabilities I calculated would be (approximately) correct. Now, given a single segmentation, and the other assumptions I used, we were able to get a probability of a single occurrence. However, if this re-segmentation and the resulting needed detection, was to continue for some period of time, then that would clearly change (raise) the probability very quickly. . . . John L. Hufferd Senior Technical Staff Member (STSM) IBM/SSG San Jose Ca Main Office (408) 256-0403, Tie: 276-0403, eFax: (408) 904-4688 Home Office (408) 997-6136, Cell: (408) 499-9702 Internet address: hufferd@us.ibm.com Stephen Bailey <steph@cs.uchicago.edu> on 02/11/2002 02:41:26 PM To: John Hufferd/San Jose/IBM@IBMUS cc: tsvwg@ietf.org, rdma@yahoogroups.com Subject: Re: [rdma] a proposal for a different No-Data-Touch framer for TCP John, Thanks for the exhausting analysis. As Paul points out, you do not want to scan the received segment to recover synchronization with either of these techniques, since it's too risky. The TUF draft specifically makes this point. Basically, you get one event per segment. However, if you want to be really pessemistic you could say the stream IS segmented into tiny frames that almost result in scanning. The TUF draft also says that the use of a TUF-derived PDU containment property should be discontinued after repeated failure of PDU containment. Given Julian's repeated statements that TUF should be analyzed against an infinite stream, I can conclude that the draft does not make this point strongly or clearly enough. The TUF assumption is that there are two distinct, quasi-stable scenarios: segmentation is preserved or it is not. Obviously if segmentation is not preserved, there's no point in trying to use the PDU containment property because it will never hold. If a receiver detect that the PDU containment property does not hold for longer than would represent a transient condition like PMTU change, it gives up and stops using it. Julian's argument that TUF needs to be analyzed against an infinite stream is a straw man. You can pick the threshold at which a TUF receiver gives up to be any number that makes you comfortable (10e6 packets, 10e4, whatever). I'm not clear on how your analysis incorporates worst case data pattern distribution assumptions. There is a ULP running inside the framing protocol, and if we assume the ULP is efficient, MOST of the data IT carries is actually controlled by the user of the ULP. This user data can just as well be a `search and destroy' sequence that tries every `eye catcher' in a well-formed context. This technique boils down to a blind search for a 32-bit integer, and there's the additional probability of hitting any particular element of the blind search pattern (basically like your ex and ey). Julian's extra 64 bits (salt & digest) are not doing anything for you in this case. It's as simple as the probability of the resegmented stream hitting the correct 32-bit number. Thanks again for your work on this. Steph > Steph, Julian, > The following is an attempt to determine the probability of a false > positive, both with the TUF approach as defined in > draft-ietf-tsvwg-tcp-ulp-frame-01.txt, and the semi draft that Julian > proposed. > _______________________________________________ tsvwg mailing list tsvwg@ietf.org https://www1.ietf.org/mailman/listinfo/tsvwg
- [Tsvwg] a proposal for a different No-Data-Touch … Julian Satran
- RE: [Tsvwg] a proposal for a different No-Data-To… Douglas Otis
- [Tsvwg] RE: [rdma] a proposal for a different No-… Julian Satran
- [Tsvwg] RE: [rdma] a proposal for a different No-… Culley, Paul
- [Tsvwg] RE: [rdma] a proposal for a different No-… Julian Satran
- Re: [Tsvwg] RE: [rdma] a proposal for a different… Lloyd Wood
- [Tsvwg] RE: [rdma] a proposal for a different No-… Julian Satran
- [Tsvwg] Re: [rdma] a proposal for a different No-… Stephen Bailey
- Re: [Tsvwg] RE: [rdma] a proposal for a different… Julian Satran
- [Tsvwg] Re: [rdma] a proposal for a different No-… Julian Satran
- [Tsvwg] Re: [rdma] a proposal for a different No-… Stephen Bailey
- [Tsvwg] Re: [rdma] a proposal for a different No-… John Hufferd
- [Tsvwg] Re: [rdma] a proposal for a different No-… Julian Satran
- [Tsvwg] Re: [rdma] a proposal for a different No-… John Hufferd
- [Tsvwg] Re: [rdma] a proposal for a different No-… John Hufferd
- [Tsvwg] Re: [rdma] a proposal for a different No-… Julian Satran
- [Tsvwg] Re: [rdma] a proposal for a different No-… Stephen Bailey
- RE: [Tsvwg] a proposal for a different No-Data-To… Douglas Otis
- [Tsvwg] Re: [rdma] a proposal for a different No-… John Hufferd
- [Tsvwg] Re: [rdma] a proposal for a different No-… Julian Satran
- RE: [Tsvwg] a proposal for a different No-Data-To… Douglas Otis
- [Tsvwg] Re: [rdma] a proposal for a different No-… Stephen Bailey
- [Tsvwg] Re: [rdma] a proposal for a different No-… Stephen Bailey