+++ /dev/null
-.. _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.
Code Review / vpp.git / blobdiff