Re: [netmod] comments on draft-ietf-netmod-routing-cfg-04

[t.petch <[e-mail]>]

----- Original Message -----
From: "Ladislav Lhotka" <[e-mail]>
To: "t.petch" <[e-mail]>; "Yi Yang (yiya)" <[e-mail]>
Cc: <[e-mail]>
Sent: Friday, August 03, 2012 12:10 AM
> "t.petch" <[e-mail]> writes:
> > <tp>
> > In a sense I agree with you, in a sense I think the exact opposite.
> >
> > This topic of what tables there are in a box that routes packets has
> > surfaced before.  I see, in a typical I-D produced about routing,
> > reference to FIB and RIB, without any need to define or explain the
> > terms, they are that well known, they are what routers and other
boxes
> > have as a matter or course.
>
> Yes, it's been my problem that I couldn't find a precise definition of
these acronyms, and I understand most routing experts use the definition
"you know it when you see it".
>
> >
> > For me, it is the FIB that is always present; if you do not know
where
> > to send a packet, how to route it, then you are not an IP box of any
> > shape or form - you must have a FIB..  In simple boxes, like Linux
or
> > Windows, then that is all you have.
> >
> > In more sophisticated boxes, 'proper' routers, for some meaning of
the
> > word proper, then you need much more data, which may closely model
the
> > FIB, as with RIP, or may be utterly different, as with OSPF, and for
> > this, I see the term RIB used, initially with BGP but latterly with
any
> > routing protocol.  RIBs vary widely with the routing protocol and so
are
> > properly part of routing protocol extensions.
>
> So at the level of the present core routing model we are dealing with
FIBs, but as soon as a BGP module augments routes with that
sophisticated stuff a FIB gets promoted to a RIB, right? But this may be
quite confusing from the point of view of the I-D text, and in this case
I would probably prefer to avoid these acronyms entirely and use only
"routing table".
>
> >
> > So I think that this I-D gets the terminology quite wrong; as I say,
> > this has surfaced before without any visible change to the I-D.  I
was
> > surprised that this topic attracted no comment from the Routing
> > Directorate.
>
> Can you suggest what to do in order to get the terminology right?
>
> Anyway, I think that Yi Yang's comments were about "forwarding table",
i.e. a (simplified) routing table which by definition contains only
active routes.

I think this a misuse of forwarding table:-(

RIB is defined in RFC1771 (BGP) and later BGP RFC such as RFC2439 (Nov
1998) and RFC4771; it is clearly not the routing table, containing
everything including the kitchen sink and its structure is specific to
BGP.  But I see the term applied generically to what is learnt by an
instance of a routing protocol, and so could be applied to the
equivalent structures of EIGRP or IS-IS, which are very different in
kind.  Sometimes I see it used generically to include everything learnt
by all instances of all routing protocols but I think that that is
wrong.

FIB appears in RFC1812
"The goal of the next-hop selection process is to examine the entries
   in the router's Forwarding Information Base (FIB) and select the best
   route (if there is one) for the packet from those available in the
   FIB."
which is what, in popular parlance, is a routing table.  For example,
Moy, writing on OSPF, says
"A router's routing table instructs the router how to forward  packets.
Given a packet, the router performs a routing table lookup, using the
packet's IP destination address as a key.  This lookup returns the best
matching routing table entry ..."

I would not expect the term FIB to include all the baggage that routing
protocols provide nor for it to include any route which is not
available, as far as the host or router knows, for immediate use.

But the terms FIB and RIB are a bit specialised; I-Ds such as
draft-uttaro-idr-bgp-persistence-01
and
draft-ietf-grow-simple-va-11
use them without any explanation as do posters on many if not most of
the WG lists in Routing and parts of Operations Areas, but that is
probably not appropriate for this I-D, just as the terms are not used
in, eg, RFC2096, which is entitled 'IP Forwarding Table MIB' but then
defines 'IP CIDR Route Table', the DESCRIPTION of which is "This
entity's IP Routing table."; oh dear, you can't win!

So I think that conceptually, you should be defining the FIB, of usable
routes from which the next hop or interface of a packet is selected on
the basis of longest match, but probably calling it a route or routing
table since that is the term most widely used.  Where individual router
manufactuers do clever things on line cards, then that is best modelled
by clever router manufacturers, even if they are all doing something
similar - the speed and rate of change of those data structures probably
makes them unsuitable for MIB, yang or anything else.

You should allow for augmentation, or some such;  BGP revolves around
peers and path attributes (RFC4273) and avoids all mention of FIB, RIB
or the IP Route Table.  OSPF (RFC4750) covers interfaces, neighbors,
areas and link states, and has a reference to the Forwarding Table.
RIP(RFC1724) has interfaces and peers and refers to the  "the IP Route
Database by RIP"

Yup, you can't win, but referring to RFC1812 is unlikely to lose.

Tom Petch

> Thanks, Lada
>
> >
> > Tom Petch
> > </tp>
> > Yi
> >