[Idr] optional transitive - recognised but out of context

["iLya" <[e-mail]>]

Hi,

yesterday and few days earlier several people have observed their BGP 
sessions being reset after seeing certain prefix. After some evaluation I've 
found that offending prefix (in global internet table) had optional 
transitive ATTR_SET (code 128) attached. Not having first hand access to 
platforms that exhibited session reset I'm speculating that they've 
recognised attribute as such but found it to be inappropriate for given 
context and so treated update as malformed with all consequences.

BGP specs seem to have only one place that could (with some stretching) 
justify such behaviour - from rfc4271#section-6.3 "...If an optional 
attribute is recognized, then the value of this attribute MUST be 
checked...". Wouldn't it be appropriate to add extra text that would clarify 
that optional transitive attributes out of context should not result in 
session reset?

P.S.: as far as I understood problematic BGP implementation is fixed in 
later software releases, but lack of clarity in BGP specs leaves room for 
similar problems in the future

Cheers,
iLya