Re: [Idr] RFC-4893 handling malformed AS4_PATH attributes

["John G. Scudder" <[e-mail]>]

On Dec 17, 2008, at 3:13 AM, Paul Jakma wrote:
> On Tue, 16 Dec 2008, John G. Scudder wrote:
>
>> 1. All the ASes in the path are 4-byte capable, in which case the  
>> issue does not exist.
>> 2. At least one AS in the path is not 4-byte capable. In that case,  
>> if there is a loop it will eventually be caught by the regular 2- 
>> byte AS_PATH.
>>
>> In neither case is what you describe needed, AFAICT.
>
> We're talking about the case where the AS4_PATH has been removed by  
> a preceding NEW speaker though.
>
> Your 2 seems to suggest that in such paths, loop-detection should  
> only be done by the OLD speakers (and NEW speakers preceding it may  
> happily install the path).

That's more or less correct, modulo the adverb :-).  The point is that  
as long as the loop gets broken, regardless of by whom (in this case  
the OLD speaker preceding the NEW speaker which removed the AS4_PATH),  
it will proceed to unwind.  That is to say, it's only a transient loop.

Keep in mind this behavior SHOULD never be seen in the wild since it's  
predicated on broken behavior along the path someplace to begin with;  
with luck this is just a chalkboard exercise.  I for one find it  
heartening that the protocol appears to be robust even in the face of  
certain types of mangling of the AS4_PATH.

--John
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