Hi Chris,
Thanks for spending time on the spec. Please see NB> below for some of
the issues you have raised.
------------------------
The description of the nexthop-preference-def (see below) is confusing.
First, I assume there is an error in the text since the example of
downloading primary/standby/tertiary nexthops to the FIB should presumably
select three nexthops, but the text refers to selecting the two resolved
nexthops with the highest preference .
NB> That is a typo IMO.
More fundamentally, this example seems to imply that only two next-hops
will get downloaded to the FIB, whereas one could imagine an implementation
that uses three, four, five or more nexthops of different preferences. If
hardware supports more than two next-hop preferences being installed in the
FIB, then what is the mechanism for the client to learn how many preference
values are supported? This should all be made clearer in the text.
typedef nexthop-preference-def {
type uint8 {
range "1..99";
}
description
"Nexthop-preference is used for protection schemes.
It is an integer value between 1 and 99. A lower
value indicates higher preference. To download a
primary/standby/tertiary group to the FIB, the
nexthops that are resolved and have two highest
preferences are selected.";
}
NB> Would something like this be preferable
"To download N nexthops to the FIB, the N nexthops with the lowest value are
selected².
------------------------
If I understand the yang syntax and semantics correctly, the ³nh-add² RPC
creates a nexthop in a given RIB that is not associated with any match
condition on a route. I assume the intention is to create a nexthop with a
nexthop-id but not associated with any prefix that can then be referenced by
multiple other route match conditions. This seems like a reasonable thing
to do. However, I can see two possible issues with this.
The first issue is that the structure of the data model doesn¹t seem to not
allow this. ³grouping route² uses ³grouping route-prefix² next to
³container next-hop². It appears to me that as currently written ³container
match² in ³grouping route-prefix² is a mandatory node based on section 3.1
of RFC6020 , since the ³mandatory² statements below choice/cases cascade up
to the container. So the current form of the ³nh-add² RPC may not be
consistent with the data model as currently defined.
The second issue is that creating a next-hop without an associated match
appears to differ from the RIB grammar defined in section 6 of
draft-ietf-i2rs-rib-info-model-08. In the RIB info model, it seems that the
only way for a <nexthop> to appear is together with a <match>.
<route> ::= <match> <nexthop>
[<route-attributes>]
[<route-vendor-attributes>]
NB> I¹m not sure if I agree with the creation of nexthop discrepancy. A
nexthop can be created independently and then in the <route>, you can
reference the relevant nexthop-id.
<route> ::= <match> <nexthop>
= <match> <nexthop-base>
= <match> <NEXTHOP_ID>
------------------------
Treating tunnel-decap as a type of next-hop doesn¹t make sense to me. I
assume the desire to include tunnel-decap as well as tunnel-encap is to have
a usable stand-alone data model module which to deal with some use cases
without having to rely on another module that defines tunnel-decap.
However, the result doesn¹t make sense. I think the most common scenario
involving routers would be to install a route on router A for prefix P whose
nexthop is a tunnel-encap whose destination is router B. One router B one
would need to install a corresponding tunnel-decap whose source is router A.
The most common scenario is then that router B does a normal route lookup on
the decapsulated packet independent of the interface it entered on.
I don¹t see how this most common use case would be handled with the
tunnel-decap next-hop in this document.
NB> On router B, <match> condition would be the incoming label and the
³action² to be taken on that would be ³decap² the MPLS label. And you can
chain that with <RIB_NAME> to say continue the route lookup in say ³inet.0².
The draft says, "RIB_NAME: A nexthop pointing to a RIB indicates that the
route
lookup needs to continue in the specified RIB.²
NB> Does that address your concern?
Section 7.2.5. of draft-ietf-i2rs-rib-info-model-08 gives an example usage
for tunnel-decap where the tunnel-decap nexthop is used to pop an MPLS label
and send the traffic out an egress interface next-hop. This example is not
the most common scenario. And if we want to accomplish this scenario of
decapsulating and sending to a particular nexthop, it makes more sense to
treat tunnel-decap as a route match condition similar to an interface-route
in the existing data model. However, I think the model should be able to
handle the more common scenario described above when traffic needs to be
decapsulated and routed based on a normal route lookup.
Treating tunnel-decap as a next-hop type really makes no sense to me. I
think this aspect of the model should be changed.
NB> See above comment.
Thanks
Nitin
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