[alto] Some considerations about path vector design

[Jensen Zhang <[e-mail]>]

Hi all,

I am going to resume the discussion about graph representation and path
vector. After some attempts to adopt the path-vector extension, I have the
following considerations:

1. The current design is inefficient. Since IETF 98, we have agreed on the
design of a two-stage protocol extension (path-vector cost map + unified
property map) for graph representation. It is extensible enough because we
can customize the element (ane) properties. But the two-stage protocol will
conduct the long delay and increase the complexity of state maintenance.

2. The resource dependencies are not clear. The "Dependent Resources"
attribute in ALTO protocol is defined to claim the one-way resource
dependencies. But many resource dependencies should be bidirectional. e.g.
In path vector design, the cost map and the corresponding property map are
bound to the same query.

For 1, my personal idea is to deliver a service to accept a sequence of
queries. In this way, the client can query both cost map and property map
in a single request. And the server will handle the two-stage protocol
efficiently.

Here is a rough example:

request:
- resource: /alto/pv-costmap
  pids:
  - srcs: [xxx]
  - dsts: [yyy]
- resource: /alto/ane-prop-map
  query-id: $resource[0].$response.meta.queryId
  entities: $resource[0].$response.costmap.values.reduce(
            (x,y) => x.values.concat(y.values))

For 2, I have not found a good solution to handle the generic case.

Looking forward to any feedback.

Best regards,
Jensen