From: koning@koning.enet.dec.com (Paul Koning)
Subject: Re: What's difference between Scrub & Strip
Reply-To: koning@koning.enet.dec.com
Organization: Digital Equipment Co., distributed systems architecture
Date: Thu, 10 Sep 1992 15:20:47 GMT


In article <1992Sep9.162240.25518@webo.dg.com>, mike@vulcan.webo.dg.com (Mike Fugere) writes:
|>I am reasonably sure that the new FDDI chipset offering from
|>Motorola can be set to automatically perform frame scrubbing.
|>This feature can also be disabled.  If I am wrong, perhaps
|>someone can correct me on this.... It would then be up to
|>the hardware and firmware guys designing the FDDI network
|>adaptor to utilize this feature in their products or not.
|>
|>Mike Fugere
|>Design Engineer/Aviion Systems Development
|>Data General Corporation
|>Westborough, MA
|>"Of course, my employer has absolutely nothing to do with anything I say..."

Unfortunately, there are a number of similar-sounding terms around to 
describe very different functions.  You were caught by one of them.

What the Motorola chipset can do is "ring purging".  This function -- usually
enabled in Digital products -- indeed removes fragments left by the Strip
function.  Its main purpose, though, is to remove "no owner" frames, which
are things that can cause real trouble.  You can find more information about
the ring purger in the following paper:

	Proceedings of 16th Conf on LCN, Oct 14-17, 1991, 
	"A Ring Purger for the FDDI Token Ring", by Henry Yang and KK
	Ramakrishnan, page 503-514.

Fragments are harmless things, since they consume no ring bandwidth.  (This may
seem strange at first.  You can prove it to yourself by drawing diagrams of
where they are found on the ring with respect to the token.  This will show
that the space taken up by fragments does not take away from real traffic;
instead, that space would be filled with Idles if it weren't filled with
a fragment.)  

About the only annoyance caused by fragments is that they MAY consume some
adapter, and maybe even host bus, bandwidth.  Typical FDDI chipsets will
internally buffer and discard "short" fragments -- those caused by stripping.
Fragments can also be caused by bit errors; such fragments may be any length.
If they are longer than whatever the FIFO size is between the FDDI chipsets
and memory, the data from the fragment will start to move into memory.
Depending on the adapter design, that may or may not matter at all.

Incidentally, there is another function that will eliminate fragments:
the Smoother (in the PHY) will remove the last symbol or byte from a fragment
being repeated.  (At least this is required in some cases, and some 
implementations do it even where it is not required.)  This causes fragments
to shrink and eventually disappear.

So what is scrubbing?  It's neither stripping nor purging; it is a function
of the PHY, used at station insertion and removal.  The purpose of scrubbing
is to ensure that a configuration change doesn't leave any no-owner frames
on the ring.  One reason this is important is to make sure that such a frame
isn't received over and over by the station(s) it is addressed to.  A second,
more subtle, reason is to put a bound on packet lifetime, which is important
to the correct operation of higher layer protocols.

There are several ways to do scrubbing; a commonly used one is for the PHY
to send Idle symbols for a time period sufficient to ensure that nothing old
is left on the ring.  Sending for D_Max will do that. 

	paul

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