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diff --git a/docs/gettingstarted/developers/fib20/missing.rst b/docs/gettingstarted/developers/fib20/missing.rst deleted file mode 100644 index c5aa51f6106..00000000000 --- a/docs/gettingstarted/developers/fib20/missing.rst +++ /dev/null @@ -1,110 +0,0 @@ -.. _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. |