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

From: "Julian Satran" <[email-masked]>
Date: Sat, 9 Feb 2002 06:55:00 +0200

Steph,

The mathematics behind the proposal are simple. You have two random 
numbers "against" a finite-length PDU.
In you original proposal you have single random number (the marker) 
against a quasi infinite stream. The chance to miss is 1.
With any decent digest (e.g. a CRC) you have a small (2**-64) chance to 
miss per PDU word (for every new PDU you have a new pattern).  All the 
tricks like alignment etc do not decrease the cardinality of stream - 
i.e., it is still infinite and then a constant framer is no good.
The cost of the new scheme is also tolerable.

Julo

Stephen Bailey <[email-masked]>
08-02-02 05:17

        To:     Julian Satran/Haifa/IBM@IBMIL
        cc:     [email-masked], [email-masked]
        Subject:        Re: [rdma] a proposal for a different No-Data-Touch framer for TCP

Julian,

I was also initially excited by your proposal, but I've come to a
similar conclusion to Paul.

The key question is what property is probability in the framing
mechanism providing.  I claim it's for safety.

A safety analysis should assume a worst possible data stream.  But not
worst `impossible'---bits that are knowable are knowable, and bits
that are unknowable can't be used to construct a worst possible
stream).

I do not think your proposal provides better safety than the current
TUF.  In fact, it might be worse.  I agree that both are difficult to
analyze, in any case.

Bad data streams in your proposal would be something like groups of 3
words, with an incrementing first word (the supposed marker), a 0
salt and the corresponding digest.  If you got clever, I bet
you could even interleave so that the digest from the first candidate
is is the marker for the second.  Really clever would be to use the
salt of the first as the marker for the second.  I imagine that a
`good digest' would have the property that you could cover all the
marker possibilities this way in only 2^32 words.

As far as I can tell, ceasing PDU containment validation attempts long
before you've checked many false candidates is of primary importance
to maximizing safety in EITHER approach.  I don't buy your claim that
safety is not a function of the number of tests of false candidates in
your approach.  The probability that must be controlled in either
case, is the chance of actually testing a false candidate.

Steph

> Paul,
> 
> The salt is a random number that is changing on every PDU. That changes
> radically the false detection probability in face on an infinite stream
> form certainty to a value that is acceptable from engineering point of
> view. Neither protects against malicious user. However the probability 
of
> random good framer in the data becomes nil.
> 
> Julo

---

From: Stephen Bailey <[email-masked]>
Date: Sat, 09 Feb 2002 11:35:17 -0500

For a proper safety analysis, you can not assume that the the contents
of the the stream (or the PDU) are randomly distributed.  You have to
assume the stream has worst-case contents.  Defining `worst case' is
the hard part.

Steph

> The cost of the new scheme is also tolerable.

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