Re: [Idr] new thread regarding capabilities handling

[Enke Chen <[e-mail]>]

Hi, Jeff:

On 4/27/17 1:17 PM, Jeffrey Haas wrote:
> On Thu, Apr 27, 2017 at 08:14:07AM +0200, Mikael Abrahamsson wrote:
>>>  https://www.ietf.org/archive/id/draft-ietf-idr-dynamic-cap-14.txt
>>
>> So this is a draft first posted in 2001, with a last revision in 2011.
>>
>> What's the back story here? This is obviously a well-known
>> understood problem then for 16 years already, why don't we still
>> have this in 2017?
> 
> Part of the issue with dynamic caps was that they renegotiated capabilities
> in general.  The edge cases of having to rebuild everything dynamically that
> you could negotiate and still keep a peering session up was... ugly.
> 
> A simple example would be what would happen if you added or deleted the
> add-paths capability.

That is a reasonable summary.

> 
> I know there was discussion at one point to simplify the proposal to simply
> to allow new AFI/SAFI to be re-negotiated.

If there is sufficient interest, we can limit the capability to just AFI/SAFI
without much work.  With GR though the interest seems to have dropped.

Thanks.  -- Enke

> 
> The BGP multisession proposal tried to cover one other piece of the
> headache.  Today, if you get a failure in one AFI/SAFI, we tear down BGP.
> (You're allowed to "shut it down", but such well behaved failures are
> relatively few.)  By having different sessions for the AFI/SAFI, you get
> structural segregation of those classes of packet formatting errors.
> 
> The issues with multisession, aside from burning precious sockets, is that
> there's state that is really shared among the family types.  The question
> then becomes how you share that state appropriately when the sessions can
> move at different asynchronous paths.
> 
> -- Jeff
>