Re: Thinking differently about the site local problem (was: RE: site local addresses (was Re: Fw: Welcome to the InterNAT...))

[John Stracke <[e-mail]>]

Tony Hain wrote:

>Margaret Wasserman wrote:
>  
>
>>Of course, in the case of site-local addresses, you don't 
>>know for sure that you reached the _correct_ peer, unless you 
>>know for sure that the node you want to reach is in your 
>>site.  
>>    
>>
>
>Since the address block is ambiguous, routing will assure that if you
>reach a node it is the correct one.
>
That's backwards: Since the address block is ambiguous, routing *cannot* 
assure that if you reach a node it is the correct one.  Nobody can, 
because we equate addresses with identities.

Consider a  peer-to-peer conferencing session, with three participants 
A, B, and C.  A and B are at the same site; C is at a separate site; 
both sites use the same range of site-local addresses.  Each has two 
addresses, AG, BG, CG and AL, BL, CL (Global and Local).  A initiates 
the session by connecting to B and C (assume for the moment that this is 
not a problem).  B and C provide A with their addresses; to complete the 
mesh, A tells B to connect to C at CG or CL.  Now, B isn't going to 
connect to *both*, so it'll have some heuristic to pick one.  Suppose it 
picks CL (*).  But, whoops, B's site has some host D, with DL==CL.  So B 
winds up connecting to the wrong host, and doesn't realize it.

(*) Not an unreasonable supposition.  If the app is looking at the 
addresses, it might well notice that CL is on a locally attached subnet, 
and use that.  Or the app might connect to both in parallel 
(non-blocking connect()), and use the address it reaches first, as a 
first cut at discovering the most efficient path (that's what I did when 
I implemented this some time back).  Being on the same network, D will 
probably respond before C.

-- 
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|John Stracke      |[e-mail]                     |
|Principal Engineer|http://www.centive.com                   |
|Centive           |My opinions are my own.                  |
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|"God does not play games with His loyal servants." "Whoo-ee,|
|where have you *been*?" --_Good Omens_                      |
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