docs: FIB 2.0 start

Change-Id: I87cd2eae133c9f5b9f7764a0f7a41bcc28523e4c
Signed-off-by: jdenisco <jdenisco@cisco.com>

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+.. _graphwalks:
+
+Graph Walks
+^^^^^^^^^^^^
+
+All FIB object types are allocated from a VPP memory pool [#f13]_. The objects are thus
+susceptible to memory re-allocation, therefore the use of a bare "C" pointer to refer
+to a child or parent is not possible. Instead there is the concept of a *fib_node_ptr_t*
+which is a tuple of type,index. The type indicates what type of object it is
+(and hence which pool to use) and the index is the index in that pool. This allows
+for the safe retrieval of any object type. 
+
+When a child resolves via a parent it does so knowing the type of that parent. The
+child to parent relationship is thus fully known to the child, and hence a forward
+walk of the graph (from child to parent) is trivial. However, a parent does not choose
+its children, it does not even choose the type. All object types that form part of the
+FIB control plane graph all inherit from a single base class14; *fib_node_t*. A *fib_node_t*
+indentifies the object's index and its associated virtual function table provides the
+parent a mechanism to Զisitՠthat object during the walk. The reason for a back-walk
+is to inform all children that the state of the parent has changed in some way, and
+that the child may itself need to update.
+
+To support the many to one, child to parent, relationship a parent must maintain a
+list of its children. The requirements of this list are;
+
+- O(1) insertion and delete time. Several child-parent relationships are made/broken during route addition/deletion.
+- Ordering. High priority children are at the front, low priority at the back (see section Fast Convergence)
+- Insertion at arbitrary locations.
+
+To realise these requirements the child-list is a doubly linked-list, where each element
+contains a *fib_node_ptr_t*. The VPP pool memory model applies to the list elements, so
+they are also identified by an index. When a child is added to a list it is returned the
+index of the element. Using this index the element can be removed in constant time.
+The list supports 'push-front' and 'push-back' semantics for ordering. To walk the children
+of a parent is then to iterate of this list.
+
+A back-walk of the graph is a depth first search where all children in all levels of the
+hierarchy are visited. Such walks can therefore encounter all object instances in the
+FIB control plane graph, numbering in the millions. A FIB control-plane graph is cyclic
+in the presence of a recursion loop, so the walk implementation has mechanisms to detect
+this and exit early.
+
+A back-walk can be either synchronous or asynchronous. A synchronous walk will visit the
+entire section of the graph before control is returned to the caller, an asynchronous
+walk will queue the walk to a background process, to run at a later time, and immediately
+return to the caller. To implement asynchronous walks a *fib_walk_t* object it added to
+the front of the parent's child list. As children are visited the *fib_walk_t* object
+advances through the list. Since it is inserted in the list, when the walk suspends
+and resumes, it can continue at the correct location. It is also safe with respect to
+the deletion of children from the list. New children are added to the head of the list,
+and so will not encounter the walk, but since they are new, they already have the up to
+date state of the parent.
+
+A VLIB process 'fib-walk' runs to perform the asynchronous walks. VLIB has no priority
+scheduling between respective processes, so the fib-walk process does work in small
+increments so it does not block the main route download process. Since the main download
+process effectively has priority numerous asynchronous back-walks can be started on the
+same parent instance before the fib-walk process can run. FIB is a 'final state' application.
+If a parent changes n times, it is not necessary for the children to also update n
+times, instead it is only necessary that this child updates to the latest, or final,
+state. Consequently when multiple walks on a parent (and hence potential updates to a
+child) are queued, these walks can be merged into a single walk. 
+
+Choosing between a synchronous and an asynchronous walk is therefore a trade-off between
+time it takes to propagate a change in the parent to all of its children, versus the
+time it takes to act on a single route update. For example, if a route update where to
+affect millions of child recursive routes, then the rate at which such updates could be
+processed would be dependent on the number of child recursive route Рwhich would not be
+good. At the time of writing FIB2.0 uses synchronous walk in all locations except when
+walking the children of a path-list, and it has more than 32 [#f15]_ children. This avoids the
+case mentioned above.
+
+.. rubric:: Footnotes:
+
+.. [#f13] Fast memory allocation is crucial to fast route update times.
+.. [#f14] VPP may be written in C and not C++ but inheritance is still possible.
+.. [#f15] The value is arbitrary and yet to be tuned.