[rbridge] Summary of "critical bits" issues
touch at ISI.EDU
Thu Dec 13 10:37:38 PST 2007
IMO, here are the important elements:
1. option order
if you care about short-circuit option processing,
which is reasonable, then we should put HBH options
before E2E ones, and required options (i.e., discard
if not supported) before voluntary options (i.e.,
pass if not supported)
2. option groups
examined at all rbridges
examined ONLY at ingress and egress rbridges
if not supported, the device discards the segment
if not supported, the device passes the segment
if not supported, the device acts as indicated
with no further action (e.g., discard or pass)
if not supported, the device sends a signal segment
indicating the failure upstream towards the offending
segment source; this MAY occur periodically as an
NB: that means THREE flags in every option
Some on this list argue that the common case will be devices that
support no options, thus it would be useful to also know the following:
3. summary flag in the main header
ALL SILENT, ALL PASS
This would be a logical OR of all equivalent bits in the options.
The cost of doing this is that:
1. space in the main header
2. adding/removing any option MUST update the summary flag
COST to add: easy; just OR the extra bits
COST to delete: expensive; MUST scan other headers
to determine whether the summary flag changes
i.e., every option delete is O(entire option space) in cost
Whether this summary flag represents HBH only or includes E2E, or
whether there are two flags - one for each - is also up in the air.
A key point here is that the summary flag makes sense ONLY if there are
SILENT, PASS options. IMO, they're useless - if an rbridge can ignore an
option and be equivalently compliant, why would it ever implement that?
I.e., I would argue that any SILENT, PASS options are thus implicitly
optional, and should be omitted from consideration anyway.
As a result, it makes no sense to me to include the summary bit(s).
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