Subject: [rbridge] Critical Bits Text

From: "Eastlake III Donald-LDE008" <[email-masked]>
Date: Sat, 8 Dec 2007 17:59:08 -0500

Here is an example of some text on critical summary bits cut and pasted
from an internal version of the protocol document that was never posted:

In the subpart of Section 3 on options:

 =======================================================================

   To ensure backward compatible safe operation, when Op-Length is non-
   zero, indicating that options are present, the top two bits of the
   first byte of the options area are specified as follows:

                      +-----+-----+---+---+---+---+---+---+
                      | HbH | ItE |       Reserved        |
                      +-----+-----+---+---+---+---+---+---+

                 Figure 6. Options Area Initial Flags Byte

   If the HbH bit is one, one or more critical hop-by-hop options are
   present so, for example, transit RBridges that support no options
   MUST drop the frame. If the HbH bit is zero, the frame is safe, from
   the point of view of options, for a transit RBridge. A transit
   RBridge that supports no options and forwards a frame MUST
   transparently forward the options present.

   If the ItE bit is a one, one or more critical ingress-to-egress
   options are present. If it is zero, no such options are present.  If
   either HbH or ItE is non-zero, egress RBridges that support no
   options MUST drop the frame.  If both HbH and ItE are zero, the frame
   is safe, from the point of view of options, for any egress RBridge to
   process.

 =======================================================================

It is the case the if the options format is definite so that the top bit
of the first byte of the option indicates it is a critical hop-by-hop
option and the options are ordered so that such options have to appear
first, then at least transit Rbridges that support no options would be
checking exactly the same bit regardless of whether these summary bits
were present or not.

Thanks,
Donald

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---

From: James Carlson <[email-masked]>
Date: Wed, 12 Dec 2007 10:54:20 -0500

> from an internal version of the protocol document that was never posted:
[...]
>    first byte of the options area are specified as follows:

I'm not at all wild about forcing the nodes that manipulate options to
maintain the state of those two bits (particularly the ItE bit) -- it
looks like a good place to have an accident -- but this looks
sufficient, and I think something like it should find its way into the
document.

-- 
James Carlson, Solaris Networking              <[email-masked]>
Sun Microsystems / 35 Network Drive        71.232W   Vox +1 781 442 2084
MS UBUR02-212 / Burlington MA 01803-2757   42.496N   Fax +1 781 442 1677
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