docs: Update FIB documentation
Type: docs
Signed-off-by: Neale Ranns <nranns@cisco.com>
Change-Id: I3dfde4520a48c945ca9707accabbe1735c1a8799
diff --git a/docs/gettingstarted/developers/fib20/missing.rst b/docs/gettingstarted/developers/fib20/missing.rst
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+.. _missing:
+
+Missing Functionality
+---------------------
+
+A list of functionality that the FIB does not currently provide.
+
+
+PIC Edge Backup Paths
+^^^^^^^^^^^^^^^^^^^^^
+
+FIB supports the concept of path 'preference'. Only paths that have
+the best preference contribute to forwarding. Only once all the paths with
+the best preference go down do the paths with the next best preference
+contribute.
+
+In BGP PIC edge, BGP would install the primary paths and the backup
+paths. With expectation that backups are only used once all primaries
+fail; this is the same behaviour that FIB's preference sets provide.
+
+However, in order to get prefix independent convergence, one must be
+able to only modify the path-list's load-balance map (LBM) to choose the
+paths to use. Hence the paths must already be in the map, and
+conversely must be in the fib_entry's load-balance (LB). In other
+words, to use backup paths with PIC, the fib_entry's LB must include
+the backup paths, and the path-lists LBM must map from the backups to
+the primaries.
+
+This is change that is reasonably easy w.r.t. to knowing what to
+change, but hard to get right and hard to test.
+
+
+Loop Free Alternate Paths
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Contrary to the BGP approach for path backups, an IGP could install a
+loop free alternate (LFA) path to achieve fast re-route (FRR).
+
+Because of the way the LFA paths are calculated by the IGP an LFA backup
+path is always paired with a primary. VPP FIB does not support this
+primary-backup pair relationship.
+
+In intent of LFA FRR is/was to get below the magic 50ms mark. To do
+this the expectation is/was that one would need in the forwarding
+graph an object that represents a path's state. This object would be
+checked for each packet being sent. If the path is up, the graph (an
+adjacency since it's the IGP) for the primary path is taken, if it's
+down the graph for the backup is taken. When a path goes down only
+this indirection object needs to be updated to affect all
+routes. Naturally, the indirection would incur a performance cost, but
+we know that there are many performance-convergence trade-offs in a
+FIB design.
+
+Should VPP's FIB support this feature? It all depends on the
+50ms. LFA FRR comes from the era when routers ran on lower performance
+CPUs and interface down was an interrupt. VPP typically has plenty of
+gas but runs as a user space process. So, can it update all routes in
+under 50ms on a meaty CPU and can the OS deliver the interface down
+within the time requirements? I don't have the answers to either
+question.
+
+
+Extranets for Multicast
+^^^^^^^^^^^^^^^^^^^^^^^
+
+When a unicast prefix is present in two different tables, then it
+refers to a different set of devices. When the prefix is imported it
+refers to the same set of devices. If the set of paths to reach the
+prefix is different in the import and export table, it doesn't matter,
+since they both refer to the same devices, so either set can be
+used. Therefore, FIB's usual source preference rules can apply. The
+'import' source is lower priority.
+
+When a multicast prefix is present in two different tables, then it's
+two different flows referring to two different set of receivers. When
+the prefix is imported, then it refers to the same flow and two
+different sets of receivers. In other words, the receiver set in the
+import table needs to be the super set of receivers.
+
+There are two ways one might consider doing this; merging the
+path-lists or replicating the packet first into each table.
+
+
+Collapsing
+^^^^^^^^^^
+
+Read :ref:`fastconvergence`
+
+Collapsing the DPO graph for recursive routes doesn't have to be an
+all or nothing. Easy cases:
+
+
+- A recursive prefix with only one path and a path-list that is not
+ popular, could stack directly on the LB of the via entry.
+- A recursive prefix with only multiple paths and a path-list that is not
+ popular, could construct a new load balance using the choices
+ present in each bucket of its via entries. The choices in the new LB
+ though would need to reflect the relative weighting.
+
+
+The condition of an non-popular path-list means that the LB doesn't
+have an LB map and hence it needs to be updated for convergence to
+occur.
+
+The more difficult cases come when the recursive prefix has labels
+which need to be stack on the via entries' choices.
+
+You might also envision a global configuration that always collapses all
+chains, which could be used in deployments where convergence is not a
+priority.