# Route Representation
Open/R provides flexible ways of expressing reachability information. It allows
a network operator to express `Preference`, `Performance`, and `Policy Metadata`
with every route announcement. This document describes the route representation
in detail.
### Design Rationale
Simplicity and flexibility are the two main criteria that we considered into the
design of Open/R route. Traditional protocols like BGP, OSPF, and ISIS
influenced the design. The key takeaways
- Numeric metrics to indicate route preference
- Customizable forwarding behaviors within Area
- Meta-data for influencing route policing at Area Border
- Expressing performance requirements
- Path tracing
### Definitions
#### `transitive`
Attributes that are preserved during route re-distribution at Area Border. Note
that not all `transitive` attributes are `mutable` by the policy.
`non-transitive` is the opposite of `transitive`.
#### `mutable`
The route attribute that can be altered by the policy during route
re-distribution at Area Border. `immutable` is the opposite of `mutable`.
### Attributes
#### > `prefix`
`transitive`, `immutable` Express Network Layer Reachability Information (NLRI)
for IPv4 or IPv6. This is a key attribute of the advertised route.
#### > `metrics`
Expresses the relative preference of route among all received advertisements.
The metrics can be tuned to prefer one network path over another or prefer
certain applications over another in-case of anycast addresses. Open/R supports
3 dimensions of metrics to express a preference. The design allows us to add
more metrics in the future based on the use-case. All metrics are numeric by
definition, as it greatly simplifies the understanding and implementation.
**`metrics.path_preference`** `numeric`, `transitive`, `mutable`,
`prefer-higher`, `1st criterion` Network path preference for this route. This is
set at the origination point. It is either retained (transitive) or updated as
the route traverses the network. e.g. LIVE-PATH=1000, DRAINED-PATH=500
**`metrics.source_preference`** `numeric`, `transitive`, `immutable`,
`prefer-higher`, `2nd criterion` User or Application preference of the route.
Set at the origination point and is never modified as route propagates in the
network. e.g. HIGH-PRI=200, LOW-PRI=100
**`metrics.distance`** `numeric`, `transitive`, `mutable`, `prefer-lower`,
`3rd criterion` Cost to reach the originating node from the current area border.
By default incremented by `igp_cost` of the route within the area during route
re-distribution across area. Should never decrease if manipulated via policy to
ensure loop-free routing. This resembles AS-Path length of BGP
> Take a look at [Decision Route Computation](../Protocol_Guide/Decision.md) for
> understanding best route selection process
#### > `tags`
`set[string]`, `transitive`, `mutable` Meta-data associated with the route to
facilitate route policing as the route propagates through the network. Encoded
as `set` and hence there is no ordering and duplicates. This can be used
to express various things like origination node name, propagation scopes, route
type, etc. This analogous to BGP Communities, except this, is much more flexible
as there is no byte limit.
#### > `area_stack`
`list[string]`, `transitive`, `immutable` An ordered list of areas the route has
traversed so far. The entry at the front indicates the `originating area`, while
entry at the back indicates the `peer area` re-distributing the route. This
helps in preventing route looping and debugging.
You can draw similar to BGP AS-Path. However, note that it is not used in best
route selection and can't be modified by the policy.
#### > `forwarding-algorithm`
`non-transitive`, `mutable` Link-state algorithm for route computaion. Open/R
supports two forwarding algorithm, `SP_ECMP` (Shortest Path ECMP) and
`KSP2_ED_ECMP` (K-Shortest Path). The algorithm with the lowest value is chosen
in-case of conflicting advertisements.
#### > `forwarding-type`
`non-transitive`, `mutable` Data plane forwarding mechanism to use. Open/R
supports two forwarding types, `IP` (Usual IP routing) and `SR_MPLS` (Source
Routing with MPLS data plane). The type with the lowest value is chosen in-case
of conflicting advertisements.
> NOTE: `KSP2_ED_ECMP` is only compatible with `SR_MPLS` forwarding type.
#### > `min-nexthops`
`optional[numeric]`, `non-transitive`, `mutable` Expresses the minimum number of
next-hops on a computed route. The route is programmed and hence re-distributed
if min-next-hop criteria is met. Kind of a performance requirement. This feature
helps to avoid funneling in the CLOS-Fabric networks when the capacity of one of
the planes is reduced greatly compared to others because of maintenance or
device failure.
### Thrift Struct
Refer to
[struct PrefixEntry in openr/if/Types.thrift](https://github.com/facebook/openr/blob/master/openr/if/Types.thrift)