[Tsvwg] Re: [rdma] a proposal for a different No-Data-Touch framer for TCP

["Julian Satran" <[e-mail]>]

Steph,

You fail to see that a good synchronization scheme should work with or 
without a TCP alignment scheme.
If the instream alignment can be infered by the ULP the so it can by a 
generic DDP. Alignment can somewhat improve performance
but it is not mandatory neither for a generic nor for a protocol specific 
placement scheme.  In other words - steering every TCP packet is the 
ultimate effectivenss but not necessarily the only steering scheme.  An IP 
steering scheme would work unchanged on every transport.

Regards,
Julo





Stephen Bailey <[e-mail]>
12-02-02 00:41

 
        To:     "John Hufferd" <[e-mail]>
        cc:     [e-mail], [e-mail]
        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.
> 

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