[Idr] Re: BGP issues

Hi Iljitsch,

>>> In addition to the fact that it's possible to configure iBGP such 
>>> that you can have routing loops?
> 
>> Could you provide an example ?
> 
> R1e - R2i - R3i - R4e
> 
> If R1 and R4 both prefer their eBGP path towards a certain destination 
> and then R2 prefers the path over R4 while R3 prefers the path over R1, 
> packets will loop between R2 and R3. Note that this configuration only 
> happens when applying policy to iBGP sessios or in unfortunate route 
> reflector placement.

Altering best path selection on IBGP routers is a bad idea when you have 
hop by hop switching.

>> BGP can converge today in subseconds.
> 
> Sure, but it can also take many minutes, especially with a 
> min-route-advertisement timer in effect in many places and if the update 
> is a withdrawl. Now I've argued that this shouldn't be too problematic 
> because if you're revoking your advertisement that means you're 
> unreachable anyway, but not everyone agrees with that. (Not good if you 
> are revoking a more specific and you want the aggregate to attract the 
> traffic.)
> 
> One reason that BGP is getting chattier is because updates tend to be 
> multiplied if there are several interconnects between two ASes.

Just to clarify here .. what I call convergence is in fact reachability 
restoration. And that is in vast majority of cases just the thing within 
your AS or at most to your peering AS.

As long as reachability is restored (even if temporarily in an 
suboptimal fashion) does not impact user's traffic. Then BGP can take as 
long as it likes to "converge" to an optimal path as long as 
reachability is in place. That IMHO should be the focus.

>> Not true. There is proposal for BGP aggregate withdraw.
> 
> I'm talking about what's out there, not about what's proposed. Very 
> little of what is proposed makes it to wide deployment.

If you say that very little on what is proposed or what vendors ship 
makes it to the actual wide deployment what could we say about something 
which is not even proposed yet ? Should we sit and do nothing for years 
to come ???

>> Accommodating policies is not a BGP protocol requirement but it comes 
>> from operators using this protocol to fit their operational needs.
> 
> Sure, but in my opinion, BGP takes it a bit too far, it's too easy to 
> shoot yourself in the foot. Or, in other words: a routing protocol that 
> allows loops is doing something wrong.

See the problem is that loops can form with bad policies. And it would 
not be that hard to detect them algorithmically. The problem is that no 
operators wants to share his polices to others. So while you could quite 
easy come out with the intra-domain policy verifier .. doing the same 
inter-domain is a non technical challenge. I don't think BGP or any 
other protocol would do any magic if there is no data at hand to work on.

>> What other metrics would you recommend to add to BGP and how that 
>> would that reduce deaggregation ?
> 
> I would like to see a value that is increased by every AS (twice, for 
> good measure: once on reception, once on transmission), but not by one, 
> but by a value (say) between 1 and 127, with a default in the middle 
> (11). So where you'd see an AS path length 3 today, you'd see a metric 
> value 66 if nobody in the path changed the default. But if someone wants 
> to make this path just a little better or a little worse, they can make 
> it 65 or 67. This allows finer grained control that the AS path so it 
> reduces the need for deaggregation for TE purposes.

Hmmmm that's an idea ... Perhaps you should write a brief draft on it ?

>> IMHO this is a feature not a bug. In fact for convergence reasons this 
>> is a very useful feature to have fast IGP.
> 
> It also adds tons of complexity, especially if you don't want to have a 
> full iBGP mesh. I'm not saying you shouldn't have an IGP, I'm saying you 
> shouldn't _have_ to have an IGP.

IMHO recursive protocol should recurse via something. And for 
intra-domain reachability restoration bgp is not the best choice. I saw 
in the past some attempts to use iBGP as an IGP but it is IMHO a very 
bad idea limited to very few specific topologies.

The issue is that intra and inter domain requirements for routing 
protocol are very different and diverse. It is a challenge to come up 
with a huge protocol to do all .. as such would not do anything very 
good. On that grounds I do believe that we should have precise tools for 
  different jobs. For the same reason I have a big workshop in my garage 
not to have bend wires, cut wood or unscrew car bolts all with one set 
of pliers.

>> When the ability to send more then best path is added true .. paths 
>> would have to be explicitly revoked. Any other proposal in RRG does 
>> the very same.
> 
> It would be better if implementations wouldn't have to keep track of 
> what they sent where. However, this is probably relatively easy to fix 
> ("start now" .... "forget all the prefixes that I haven't refreshed 
> since "start now"") but the fix will probably be hard to deploy.

Then you would loose the huge benefit of BGP to be an incremental protocol !

Cheers,
R.

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