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

From: "Julian Satran" <[email-masked]>
Date: Tue, 12 Feb 2002 09:01:42 +0200

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 <[email-masked]>
12-02-02 00:41

        To:     "John Hufferd" <[email-masked]>
        cc:     [email-masked], [email-masked]
        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.
> 

------------------------ Yahoo! Groups Sponsor ---------------------~-->
Sponsored by VeriSign - The Value of Trust
Do you need to encrypt all your online transactions? Find
the perfect solution in this FREE Guide from VeriSign.
http://us.click.yahoo.com/vCuuSA/UdiDAA/yigFAA/W6uqlB/TM
---------------------------------------------------------------------~->

To unsubscribe from this group, send an email to:
[email-masked]

Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/

---

From: "Douglas Otis" <[email-masked]>
Date: Tue, 12 Feb 2002 04:48:33 -0800

Julian,

An application using DDP must continuously communicate with a steering layer
added to the transport.  Placement is an intimate aspect of transport
function and the transport serves as the interface to the application.  It
is
inappropriate to designate this as an IP steering scheme, however a scheme
incorporated into TCP can offer common conventions and structures.

Doug

> 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.

_______________________________________________
tsvwg mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/tsvwg

---

From: Stephen Bailey <[email-masked]>
Date: Tue, 12 Feb 2002 15:19:13 -0500

> without a TCP alignment scheme.

I see it clearly.  However, this seems to be a case where we can't
have everything.  If we want to touch the data we can have a
synchronization technique which can also be used to determine PDU
alignment (COWS).  If we don't want to touch the data, we can have PDU
alignment (TUF or NDT) OR synchronization (FIM), but not both.

The TUF draft specifically says an implementation must not use the TUF
in-stream information to recover synchronization in an arbitrary
stream.  It can only be used to verify PDU containment.  That means
there is only one toss of the dice per PDU rather than PDU length
worth.

If you are proposing that your No-Data-Touch framer CAN be used this
way, I strongly disagree.  You drastically reduce the safety of the
scheme if you do this.

Steph