Re: IETF OSPF YANG and BFD Configuration

[Jeffrey Haas <[e-mail]>]

Greg,

On Tue, Jun 20, 2017 at 12:55:58PM -0500, Greg Mirsky wrote:
> Hi Jeff,
> I'd like to hear from others who are familiar with implementations of BFD
> that supports per protocol single-hop BFD multi-sessions between the same
> pair of BFD systems. RFC 5881 does allow per protocol single-hop sessions
> on the same interface, logical or physical, between the same pair of
> systems. But it is not clear, in my opinion, how BFD does demultiplexing if
> Your Discriminator == 0 per protocol (RFC 5881, section 3, last sentence)
> if systems use the same IP addresses. And hence the question, Even though
> it is allowed to have BFD single-hop sessions per application/protocol on
> the same interface between the same pair of systems, is this real,
> practical requirement?
> Or am I missing the point completely?

Correct, 5881 procedures do not currently permit this.
As I mentioned, we've had prior discussions about this being potentially
problematic for clients that may have differing timing requirements.

Part of the point of this exercise is to provide necessary future-proofing
against the need to re-issue the Yang modules if such a mechanism is
developed.  (Somewhat analogous to the instructions to the design team that
MPLS-TP was out of scope for the module, but that its structure shouldn't
prevent its implementation in the future.)

Thus far, two points have sprung from the discussion that are likely
actionable:
- The single-hop configuration in the current BFD module isn't suitable for
  use by the IGPs.  Some form of wildcard of templating behavior is
  necessary.
- Multiple clients that might desire to configure a session with different
  parameters are unable to do so today.  This precludes some implementations
  that permit such configuration, potentially at protocol scope.  It also
  doesn't fit the future-proofing thought.


-- Jeff

> 
> Regards,
> Greg
> 
> On Mon, Jun 19, 2017 at 1:57 PM, Jeffrey Haas <[e-mail]> wrote:
> 
> > [Long delayed response.]
> >
> > Reshad picked up the key points: Some things may make sense in the
> > per-client (protocol) users of BFD, some things perhaps do not.  And some
> > come down to questions for timer granularity.
> >
> > The OSPF and ISIS models both make use of BFD simply by providing a boolean
> > that says "I'm using BFD or not".
> >
> > 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 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".
> >
> > 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.
> >
> > 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.
> >
> > -- Jeff
> >
> >
> >
> >
> >
> >
> > On Fri, May 19, 2017 at 02:31:38AM +0000, Reshad Rahman (rrahman) wrote:
> > > We started off with the intent of having BFD parameters in the
> > applications/protocols which make use of BFD. For timer/multiplier this is
> > pretty straight-forward, although the discussion of what to do when not all
> > applications have the same BFD parameters for the same session (e.g. Go
> > with most aggressive etc). Then we started looking at authentication
> > parameters and having BFD authentication parms in OSPF/ISIS etc is not
> > intuitive. And what do we do if applications have different BFD
> > authentication parms. We concluded that the BFD authentication parms were
> > better off in BFD. And once we did that, the timer/multiplier followed....
> > >
> > > I may not recall all the details/discussons, but I do recall that we
> > went back and forth on this and it took some time to make the decision.
> > >
> > > Regards,
> > > Reshad (as individual contributor).
> > >
> > > From: Mahesh Jethanandani <[e-mail]<mailto:
> > [e-mail]>>
> > > Date: Thursday, May 18, 2017 at 5:34 PM
> > > To: "Acee Lindem (acee)" <[e-mail]<mailto:[e-mail]>>
> > > Cc: Jeffrey Haas <[e-mail]<mailto:[e-mail]>>, OSPF WG
> > List <[e-mail]<mailto:[e-mail]>>, "[e-mail]<
> > mailto:[e-mail]>" <[e-mail]<
> > mailto:[e-mail]>>, "[e-mail]
> > <mailto:[e-mail]>" <[e-mail]
> > <mailto:[e-mail]>>, "[e-mail]<mailto:rtg-
> > [e-mail]>" <[e-mail]<mailto:[e-mail]>>
> > > Subject: Re: IETF OSPF YANG and BFD Configuration
> > > Resent-From: <[e-mail]<mailto:[e-mail]>>
> > > Resent-To: <[e-mail]<mailto:[e-mail]>>,
> > Reshad <[e-mail]<mailto:[e-mail]>>, <
> > [e-mail]<mailto:[e-mail]>>, <
> > [e-mail]<mailto:[e-mail]>>, <
> > [e-mail]<mailto:[e-mail]>>
> > > Resent-Date: Thursday, May 18, 2017 at 5:40 PM
> > >
> > > Resending with correct BFD WG address.
> > >
> > > On May 18, 2017, at 2:33 PM, Mahesh Jethanandani <
> > [e-mail]<mailto:[e-mail]>> wrote:
> > >
> > > Agree with Acee's assessment. After much debate, we decided that we
> > should leave BFD parameter configuration in the BFD model itself, and have
> > any IGP protocol reference the BFD instance in BFD itself. This makes sense
> > specially if multiple protocols fate-share the BFD session.
> > >
> > > Cheers.
> > >
> > > On May 18, 2017, at 12:27 PM, Acee Lindem (acee) <[e-mail]<mailto:
> > [e-mail]>> wrote:
> > >
> > > Hi Jeff,
> > >
> > > At the OSPF WG Meeting in Chicago, you suggested that we may want to
> > provide configuration of BFD parameters within the OSPF model
> > (ietf-ospf.yang). We originally did have this configuration. However, after
> > much discussion and coordination with the BFD YANG design team, we agreed
> > to leave the BFD session parameters in BFD and only enable BFD within the
> > OSPF and IS-IS models.
> > >
> > > We did discuss the fact that vendors (notably Cisco IOS-XR and Juniper
> > JUNOS) do allow configuration within the IGPs. However, the consensus was
> > to leave the BFD configuration in the BFD model. The heuristics to
> > determine what parameters to use when the same BFD endpoint was configured
> > with different parameters in different protocols were proprietary and
> > somewhat of a hack.
> > >
> > > I may have not remembered all the details so I'd encourage others to
> > chime in.
> > >
> > > Thanks,
> > > Acee
> > >
> > > Mahesh Jethanandani
> > > [e-mail]<mailto:[e-mail]>
> > >
> > >
> > >
> > >
> > > Mahesh Jethanandani
> > > [e-mail]<mailto:[e-mail]>
> > >
> > >
> > >
> >