Re: RFC 2370 Update and a Proposed Change to Stub Area Behavior

Acee Lindem <acee@CISCO.COM> Mon, 15 August 2005 00:29 UTC

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From: Acee Lindem <acee@CISCO.COM>
Subject: Re: RFC 2370 Update and a Proposed Change to Stub Area Behavior
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Mitchell,
Although we are in complete disagreement, this debate is good.
I encourage others to chime in. See my responses inline.

Erblichs wrote:

>Acee,
>
>	Yes, I understand your points, however...
>
>	If ONLY one router understands the new type,
>	  I don't see the benefit of "fully accepting"
>	  the new type into stub areas.
>
>	What prevents a SIGNIFICANT FLOOD of specialized 
>	unknown types?
>  
>
If the one router that understands the new LSA type is connected to all the
other routers in the stub or NSSA - then the LSA is in everyone's database
anyway. It isn't as if these LSAs are coming from some distance galaxy they
are in the stub or NSSA area in question and are presumably under a single
administrative control.

>	
>	Even if a small percentage of routers in the area
>	understands these unknown types, what do we gain?
>  
>
Simplicity and deterministic behavior. Also, it could ease migration if 
the application
of the new LSA isn't dependent on all routers in the area understanding it.

>	It would make sense if we had a notifcation type
>	message that could be generated to reject these
>	unknown types.
>  
>
I certainly wouldn't want to go there. This defeats the whole purpose of 
OSPFv3
unknown type handling.

>	I thought the reasoning for specialized areas was
>	 to generate a homogeneous handing of message
>	 types. Unknown types to some routers in the area
>	 violates this.
>
>	If we don't explicitly state acceptance criteria
>	  for unknown types for stub area then it is a
>	  defacto-standard that we have this restriction.
>	  Wouldn't changing this value promote heterogeneous
>	  handling of unknown types? Murphy says, by the time
>	  unknown type A becomes generally supported, we
>	  will have introduced unknown type B..
>
>  
>
>>>>->As long as there is an intra-area spanning tree of routers that
>>>>understand the LSA type - The LSA will be in everyones database
>>>>
>>>>        
>>>>
>	Ok, then why not add a mechanism for filtering these
>	unknowns from the LSDBs? 
>  
>
Filtering techniques have been implemented. They would be difficult to 
standardize
since they break some basic OSPF principles (e.g., routers within an 
area have an
identical database).


>>>>->No corresponding rule for opaque LSAs
>>>>        
>>>>
>
>	Should we then add a defacto-standard rule?
>  
>
De facto?? This implies that it is already being used. In fact, the opposite
is true - RFC 2370 has been a PS document since July 1998 (and an 
implemented
draft for years long) and I've never heard of anyone having a requirement to
prevent unknown opaque types from being flooded in stub/NSSA areas.


>	Mitchell Erblich
>	--------------------
>
>	
>Acee Lindem wrote:
>  
>
>>Hi Mitchell,
>>Thanks for reponding - I was hoping someone would initiate some
>>discussion (the
>>ADs always ask whether a change was discussed on the mailing list :^).
>>Anyway, what I'm proposing is no worse that OSPFv2. Consider the
>>following points:
>>
>>- The unknown LSA types in question are link or area scoped. Hence,
>>this implies that at least one router in the stub or NSSA area
>>understands them (at
>>least one would hope an implementation would not originate an LSA it didn't
>>understand :^).
>>- The OSPFv2 analogy is a link or area scoped opaque LSA with unknown type.
>>There is no such restriction for these LSAs in OSPFv2 stub or NSSA areas.
>>- The mechanism is topology dependent. As long as there is a spanning
>>tree of
>>OSPFv3 routers in the stub or NSSA understanding the LSA type, it will be
>>flooded to all routers in the area. If there is a real requirement to
>>limit the
>>flooding domain, customers and vendors will implement filters which will
>>deterministically limit flooding.
>>- Experience has shown that the introducion of new OSPFv3 LSA types is
>>extremely slow. Hence, I don't see the need to limit the flooding to limit
>>database size (if there a need, see the previous point).
>>
>>Thanks,
>>Acee
>>
>>Erblichs wrote:
>>
>>    
>>
>>>Group,
>>>
>>>      I vote NOT to remove the restriction on the
>>>flooding of unknown LSAs into stub area. I vote for
>>>#2 or #3. Sorry, I have not spent any major time looking
>>>at the pros / cons between the later 2.
>>>
>>>Why?
>>>   1) The primary reason is that some of the these LSAs
>>>      are unknown to a percentage of the routers within
>>>      the stub area. Even the "attempt" to limit them
>>>      would follow the reason to limit the database size,
>>>      memory requirements, sizing of OSPF control packets,
>>>      etc... This limit is suggested in the 1st paragraph
>>>      of every OSPF v2 RFC. A copy is in the middle of this
>>>      email.
>>>
>>>    2) What is an unknown LSA? What LSA type greater than
>>>      X is an unknown? What help is it by having just 1
>>>      router understand it? Can they equal in number
>>>      over time External-LSAs and be totally useless in
>>>      our env? Where should we put the older routers that
>>>      we want to isolate from our network?
>>>
>>>    3) Is is possible to have 30% - 90% of LSAs in a router's
>>>      db be present in a stub area, be unknown LSAs? Shouldn't
>>>      their be an attempt to limit this percentage?
>>>
>>>      3b) Could we be using / spending a large percentage
>>>          of our OSPF control packet time / resources
>>>          handling unknown LSAs?
>>>
>>>    4) Backward compatibility.. I would assume that most
>>>      environments would not like to just start seeing
>>>      something new in their network just show up.
>>>
>>>      "An area can be configured as a stub when there is a single exit
>>>       point from the area, or when the choice of exit point need not
>>>       be made on a per-external-destination basis."
>>>
>>>      Lets look at the third word, can. It would be different
>>>      if we used the word SHOULD or MUST.
>>>
>>>      Thus, if a area that CAN be configured as a stub wishes
>>>      to process unknown LSAs, then why not configure the
>>>      without the STUB area identification? Wouldn't this allow
>>>      for backward capability? Yes, we then allow AS-external-LSAs
>>>      in this non named stubby area.
>>>
>>>      Or create a new "stubby area" type that accepts or
>>>      not accept, xyz type LSAs. This new area type would then be
>>>      allowed to accept new LSAs as they show up? The diff
>>>      would be that "unknown LSAs" have no restrictions and
>>>      could consume the majority of the router's LSDB.
>>>
>>>
>>>      Mitchell Erblich
>>>      -----------------------
>>>
>>>
>>>RFC 1247, 1583, 2178, 2328 : OSPFv2.
>>>---------
>>>3.6 Supporting stub areas
>>>
>>>In some Autonomous Systems, the majority of the topological database may
>>>consist of external advertisements.  An OSPF external advertisement is
>>>usually flooded throughout the entire AS.  However, OSPF allows certain
>>>areas to be configured as "stub areas".  External advertisements are not
>>>flooded into/throughout stub areas; routing to AS external destinations
>>>in these areas is based on a (per-area) default only.  This reduces the
>>>topological database size, and therefore the memory requirements, for a
>>>stub area's internal routers.
>>>
>>>
>>>==========================
>>>
>>>========================
>>>
>>>Acee Lindem wrote:
>>>
>>>
>>>      
>>>
>>>>At the 63rd IETF in Paris, I proposed that we remove the restriction on the
>>>>flooding of unknown LSAs into stub areas. Here is an excerpt from the
>>>>presentation:
>>>>
>>>>- Section 2.9 mandates that an OSPFv3 router should NOT advertise an
>>>>unknown LSA if the U bit is set to “1” – flood as if known.
>>>>->Should be removed in RFC 2740 respin.
>>>>->Limits backward compatibility for new LSA types
>>>>->No corresponding rule for opaque LSAs
>>>>->Fact that LSA is flooded at all implies one router is stub/NSSA
>>>>understands it.
>>>>->Ineffective/non-deterministic database limit
>>>>->As long as there is an intra-area spanning tree of routers that
>>>>understand the LSA type - The LSA will be in everyones database
>>>>
>>>>Comments? Speak now if you wish to retain the current stub area restriction.
>>>>My intent is to deprecate it with an appendix documenting it's removal.
>>>>
>>>>Thanks,
>>>>Acee
>>>>
>>>>Acee Lindem wrote:
>>>>
>>>>
>>>>
>>>>        
>>>>
>>>>>In the evolution of the OSPFv2 protocol specification
>>>>>(RFC 1247->RFC 1583 -> RFC 2178 -> RFC 2328) numerous
>>>>>bugs were fixed and some protocol behaviors were altered. Examples
>>>>>include the metric cost for area ranges and the selection of the
>>>>>ASBR for AS external route computation.
>>>>>
>>>>>In the context of documenting the OSPFv3 NSSA differences I've
>>>>>looked again at section 2.10 and I really think the idea of not flooding
>>>>>unknown LSA types with the U-bit set to 1 is broken. I think it breaks
>>>>>the whole idea of being able to introduce new LSA types in a backward
>>>>>compatible fashion. Furthermore, it won't stop the leakage of these
>>>>>unknown LSAs when some routers understand them and others do not -
>>>>>it all depends on whether you have a spanning tree of routers that
>>>>>understand them. Since the LSAs in question are area scoped or link
>>>>>scoped, it implies that at least one router (the originator)
>>>>>understands the
>>>>>new type and you will have a mixture. IMHO, this is broken. I've had
>>>>>some discussions with others who agree. At this juncture,
>>>>>we have 3 alternatives:
>>>>>
>>>>>1) Remove the restriction for that unknown LSAs with the U-bit
>>>>>set to 0 for stub areas.
>>>>>2) Extend the broken restriction to NSSAs in the update.
>>>>>3) Limit the damage to stub areas and only restrict AS scoped LSAs
>>>>>          
>>>>>
>>>>>from NSSAs.
>>>>        
>>>>
>>>>>Of course, I'd vote for #1 or I wouldn't be sending this E-mail.
>>>>>
>>>>>Thanks,
>>>>>Acee
>>>>>
>>>>>
>>>>>
>>>>>          
>>>>>
>>>
>>>      
>>>
>
>  
>