Re: IETF OSPF YANG and BFD Configuration

"Acee Lindem (acee)" <acee@cisco.com> Tue, 20 June 2017 14:14 UTC

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From: "Acee Lindem (acee)" <acee@cisco.com>
To: Jeffrey Haas <jhaas@pfrc.org>, Mahesh Jethanandani <mjethanandani@gmail.com>
CC: "Reshad Rahman (rrahman)" <rrahman@cisco.com>, Jeffrey Haas <jhaas@juniper.net>, OSPF WG List <ospf@ietf.org>, "rtg-bfd@ietf.org" <rtg-bfd@ietf.org>, "draft-ietf-bfd-yang@ietf.org" <draft-ietf-bfd-yang@ietf.org>, "draft-ietf-ospf-yang@ietf.org" <draft-ietf-ospf-yang@ietf.org>
Subject: Re: IETF OSPF YANG and BFD Configuration
Thread-Topic: IETF OSPF YANG and BFD Configuration
Thread-Index: AQHS6Syo68r6z3GylU2z1PR9VYBctaItArCAgAENr4D//7xOAA==
Date: Tue, 20 Jun 2017 14:14:28 +0000
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Hi Jeff, Mahesh, 

See a couple inlines.

On 6/20/17, 10:16 AM, "Jeffrey Haas" <jhaas@pfrc.org>; wrote:

>Mahesh,
>
>On Mon, Jun 19, 2017 at 03:11:25PM -0700, Mahesh Jethanandani wrote:
>> > On Jun 19, 2017, at 11:57 AM, Jeffrey Haas <jhaas@pfrc.org>; wrote:
>> > Where we run into some issues are the cases highlighted: when the
>>sessions
>> > don't share common properties, how should the protocol pick what BFD
>>session
>> > to use?  
>> 
>> The issue that I hear most is the timer granularity. Is there something
>>else?
>
>Potentially mode (async vs. echo) and authentication.  However, I believe
>timer granularity is the biggest one.
>
>> > The current BFD yang model only permits a single IP single-hop session
>> > to be configured.  (Key is interface/dst-ip)  This means that if
>>different
>> > parameters *were* desired, the BFD model won't permit it today.
>>However,
>> > BFD sessions for many protocols tend not to be configured, but may
>>spring
>> > forth from protocol state, such as IGP adjacencies.  Thus, it's not
>> > "configured" - it's solely operational state.  However, the BFD yang
>>model
>> > doesn't really make good provision for that as an "on”.
>> 
>> The idea is that a BFD session is configured a priori and before a IGP
>>session is configured with the most aggressive timer. IGP sessions then
>>refer to the BGP session configured. If a IGP session is added that
>>requires a more aggressive timer, we would have to renegotiate the more
>>aggressive timer value.
>
>Consider a broadcast network segment such as Ethernet.
>Consider a few dozen routers on such a segment.
>
>Is it your expectation that an IGP would require each of those routers to
>be
>manually configured in the Yang module a priori?  That is, after all, much
>of the point of an IGP: automatic discovery.

I think the BFD model should support a wildcard for destination IP address
to avoid this problem.

>
>> > Where all endpoint state is known a priori, config state makes better
>>sense.
>> > 
>> > To pick the example of Juniper's configuration, if OSPF and eBGP were
>>using
>> > BFD, both can choose differing timers.  This represents two pieces of
>> > configuration state for the same endpoints.  Additionally, only one
>>BFD
>> > session is formed using the most aggressive timers.
>> 
>> That is what we are suggesting also.
>
>The distinguishing point is configuration vs. operational state.  The
>current model doesn't permit more than one set of parameters to be
>provisioned even if the implementation may choose to instantiate exactly
>one
>session.

This could be supported with extensions to the BFD model.

Thanks,
Acee 
>
>> > I partially point out the situation of multiple timers since there
>>have been
>> > prior list discussions on the situation where clients have different
>>timing
>> > requirements.  I don't think we handle this operationally in the BFD
>> > protocol in the cleanest fashion right now - the session will go to
>>Down
>> > when the aggressive timers fail and there's no clean way to
>>renegotiate to
>> > the less aggressive timers.
>> 
>> A BFD session would fail more likely because there is a real network
>>failure than because the timer was more aggressive than what IGP had
>>requested. 
>
>Please note that I raise this point mostly because of prior discussion.
>I'm
>well aware of the headaches this currently causes:
>
>Different protocols have different survivability requirements.  An IGP may
>very well want sub-second timers, potentially for repair behaviors.  BGP
>may
>want fast failover, but may be fine with second level granularity.  This
>is
>particularly true since the cost of too aggressively flapping BGP is of
>significantly greater impact to the network and the router.
>
>But moving to what *is* rather than what should be, if there are two
>different timing setups for the same endpoint: If you deprovision the more
>aggressive timer, the session should likely renegotiate to the less
>aggressive one rather than drop.
>
>-- Jeff