[dpdk-dev] [PATCH 2/2] doc: add programmer's guide for the FIB library

[Vladimir Medvedkin <vladimir.medvedkin@intel.com>]

Currently, programmer's guide for the FIB library is missing.
This commit adds it.

Signed-off-by: Vladimir Medvedkin <vladimir.medvedkin@intel.com>
---
 doc/guides/prog_guide/fib_lib.rst          | 139 +++++++++++++++++++++
 doc/guides/prog_guide/img/dir_24_8_alg.svg | 136 ++++++++++++++++++++
 doc/guides/prog_guide/index.rst            |   1 +
 doc/guides/prog_guide/rib_lib.rst          |   2 +-
 4 files changed, 277 insertions(+), 1 deletion(-)
 create mode 100644 doc/guides/prog_guide/fib_lib.rst
 create mode 100644 doc/guides/prog_guide/img/dir_24_8_alg.svg

diff --git a/doc/guides/prog_guide/fib_lib.rst b/doc/guides/prog_guide/fib_lib.rst
new file mode 100644
index 0000000000..4aa02e1ed9
--- /dev/null
+++ b/doc/guides/prog_guide/fib_lib.rst
@@ -0,0 +1,139 @@
+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright(c) 2021 Intel Corporation.
+
+.. _FIB_Library:
+
+FIB Library
+===========
+
+The FIB library provides a fast Longest Prefix Match (LPM) search for 32-bit
+keys or 128-bit for IPv6. It can be used in a variety of applications,
+the most typical of which is IPv4/IPv6 forwarding.
+
+.. note::
+
+   The API and implementation are very similar for IPv4's ``fib`` library and
+   IPv6's ``fib6`` library, therefore only the ``fib`` library will be discussed here.
+   Everything within this document except for the size of the prefixes applies
+   to the ``rib6`` library.
+
+FIB API Overview
+----------------
+
+The main configuration struct contains:
+
+* Type of :ref:`dataplane algorithm <fib_dataplane_algorithms>`.
+
+* Default next hop ID.
+
+* The maximum number of routes.
+
+* Settings for the data algorithm (:ref:`will be discussed later <fib_dataplane_algorithms>`).
+
+A FIB rule consists of a prefix and an associated next hop ID. The prefix consists
+of an IPv4 network address (``uint32_t``) and the corresponding prefix length.
+The prefix serves as the key and the next hop ID as the value while doing an LPM
+search within FIB. The size of the next hop ID is variable and must be configured
+during initialization.
+
+The main methods within the ``fib`` library are:
+
+* ``rte_fib_add()``: Add a new route with a corresponding next hop ID to the
+  table or update the next hop ID if the prefix already exists in a table.
+
+* ``rte_fib_delete()``: Delete an existing route from the table.
+
+* ``rte_fib_lookup_bulk()``: Provides a bulk Longest Prefix Match (LPM) lookup function
+  for a set of IP addresses, it will return a set of corresponding next hop IDs.
+
+
+Implementation details
+----------------------
+
+Internally FIB contains the ``rte_rib`` data struct to help maintain the dataplane struct.
+The dataplane struct is opaque, so that users can add their own algorithm implementations.
+
+
+.. _fib_dataplane_algorithms:
+
+Dataplane Algorithms
+--------------------
+
+Dummy
+~~~~~
+
+This algorithm uses ``rte_rib`` as a dataplane struct. Lookups are relatively slow,
+but extra memory isn't used for the dataplane struct. This algorithm should only
+be used for testing and debugging purposes.
+
+This algorithm will be used if the ``RTE_FIB_DUMMY`` type is configured as the
+dataplane algorithm on FIB creation.
+
+
+DIR-24-8
+~~~~~~~~
+
+The current implementation of this algorithm uses a variation of the DIR-24-8
+algorithm that trades memory usage for improved LPM lookup speed.
+This algorithm allows the lookup operation to be performed using only a single
+memory read access in most cases. In the statistically rare case where the best
+match rule has a depth larger than 24, the lookup operation will require two
+memory read accesses.
+
+This algorithm will be used if the ``RTE_FIB_DIR24_8`` type is configured as the
+dataplane algorithm on FIB creation.
+
+The main FIB configuration struct stores the dataplane parameters inside ``dir24_8``
+within the ``rte_fib_conf`` and it consists of:
+
+* ``nh_sz``: The size of the entry containing the next hop ID.
+  This could be 1, 2, 4 or 8 bytes long.
+  Note that all available bits except one are used to store the actual next hop ID.
+
+* ``num_tbl8``: The number of tbl8 groups, each group consists of 256 entries
+  corresponding to the ``nh_sz`` size.
+
+The main elements of the dataplane struct for the DIR-24-8 algorithm are:
+
+* TBL24: An array with 2\ :sup:`24` entries, corresponding to the ``nh_sz`` size.
+
+* TBL8: An array of ``num_tbl8`` tbl8 groups.
+
+The lookup algorithms logic can be seen in :numref:`figure_dir_24_8_alg`:
+
+.. _figure_dir_24_8_alg:
+
+.. figure:: img/dir_24_8_alg.*
+
+   DIR-24-8 Lookup algorithm
+
+The first table ``tbl24``, is indexed using the first 24 bits of the IP address to be looked up,
+while the second table(s) ``tbl8``, is indexed using the last 8 bits of the IP address.
+This means that depending on the outcome of trying to match the IP address of an incoming packet
+to a rule stored in the tbl24 we might need to continue the lookup process in the second level.
+
+Since every entry of the tbl24 can potentially point to a tbl8,
+ideally we would have 2\ :sup:`24` tbl8s, which would be the same as having a
+single table with 2\ :sup:`32` entries. This is not feasible due to resource restrictions.
+Instead, this approach takes advantage of the fact that rules longer than 24 bits are very rare.
+By splitting the process into two different tables/levels and limiting the number of tbl8s,
+we can greatly reduce memory consumption while maintaining a very good lookup speed.
+This method generally results in one memory access per lookup.
+
+An entry in a tbl8 contains the following fields:
+
+* The next hop ID.
+
+* 1 bit indicating if the lookup should proceed inside the tbl8.
+
+Use cases
+---------
+
+The FIB library is useful for any use cases that rely on the Longest Prefix Match (LPM)
+algorithm such as IP forwarding or packet classification.
+
+More complex use cases are also possible, as it is possible to have next hop IDs
+which are 63 bits long (using ``RTE_FIB_DIR24_8_8B`` as a next hop size).
+These use cases could include storing two next hop IDs inside the 63 bits of the next hop.
+This may be useful to provide a fallback next hop ID, ASN or forwarding class
+corresponding to a given prefix without having to perform an additional lookup.
diff --git a/doc/guides/prog_guide/img/dir_24_8_alg.svg b/doc/guides/prog_guide/img/dir_24_8_alg.svg
new file mode 100644
index 0000000000..17725b7b97
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diff --git a/doc/guides/prog_guide/index.rst b/doc/guides/prog_guide/index.rst
index 0a53ec71c8..42c4a690c6 100644
--- a/doc/guides/prog_guide/index.rst
+++ b/doc/guides/prog_guide/index.rst
@@ -38,6 +38,7 @@ Programmer's Guide
     member_lib
     lpm_lib
     lpm6_lib
+    fib_lib
     rib_lib
     flow_classify_lib
     packet_distrib_lib
diff --git a/doc/guides/prog_guide/rib_lib.rst b/doc/guides/prog_guide/rib_lib.rst
index e8bb35a2b6..0a6c3d7063 100644
--- a/doc/guides/prog_guide/rib_lib.rst
+++ b/doc/guides/prog_guide/rib_lib.rst
@@ -7,7 +7,7 @@ RIB Library
 The Routing Information Base (RIB) library provides a data store for routing information.
 This library is intended for use in control or management plane applications.
 There are more suitable libraries for use in data plane applications such as
-:ref:`LPM <LPM_Library>` or FIB.
+:ref:`LPM <LPM_Library>` or :ref:`FIB <FIB_Library>`.
 
 
 Implementation details
-- 
2.25.1