docs/gettingstarted/developers/fib20/dataplane.rst

.. _dataplane:

The Data Plane
---------------

The data-plane data model is a directed, acyclic [#f16]_ graph of heterogeneous objects.
A packet will forward walk the graph as it is switched. Each object describes
the actions to perform on the packet. Each object type has an associated VLIB
graph node. For a packet to forward walk the graph is therefore to move from one
VLIB node to the next, with each performing the required actions. This is the
heart of the VPP model. 

The data-plane graph is composed of generic data-path objects (DPOs). A parent
DPO is identified by the tuple:{type,index,next_node}. The *next_node* parameter
is the index of the VLIB node to which the packets should be sent next, this is
present to maximise performance - it is important to ensure that the parent does
not need to be read [#f17]_ whilst processing the child. Specialisations [#f18]_ of the DPO
perform distinct actions. The most common DPOs and briefly what they represent are:

- Load-balance: a choice in an ECMP set. 
- Adjacency:  apply a rewrite and forward through an interface
- MPLS-label: impose an MPLS label.
- Lookup: perform another lookup in a different table.

The data-plane graph is derived from the control-plane graph by the objects
therein 'contributing' a DPO to the data-plane graph. Objects in the data-plane
contain only the information needed to switch a packet, they are therefore
simpler, and in memory terms smaller, with the aim to fit one DPO on a single
cache-line. The derivation from the control plane means that the data-plane
graph contains only object whose current state can forward packets. For example,
the difference between a *fib_path_list_t* and a *load_balance_t* is that the former
expresses the control-plane's desired state, the latter the data-plane available
state. If some paths in the path-list are unresolved or down, then the
load-balance will not include them in the forwarding choice.

.. figure:: /_images/fib20fig8.png

Figure 8: DPO contributions for a non-recursive route

Figure 8 shows a simplified view of the control-plane graph indicating those
objects that contribute DPOs. Also shown are the VLIB node graphs at which the DPO is used.

Each *fib_entry_t* contributes it own *load_balance_t*, for three reasons;

- The result of a lookup in a IPv[46] table is a single 32 bit unsigned integer. This is an index into a memory pool. Consequently the object type must be the same for each result. Some routes will need a load-balance and some will not, but to insert another object in the graph to represent this choice is a waste of cycles, so the load-balance object is always the result. If the route does not have ECMP, then the load-balance has only one choice. 

- In order to collect per-route counters, the lookup result must in some way uniquely identify the *fib_entry_t*. A shared load-balance (contributed by the path-list) would not allow this.
- In the case the *fib_entry_t* has MPLS out labels, and hence a *fib_path_ext_t*, then the load-balance must be per-prefix, since the MPLS labels that are its parents are themselves per-fib_entry_t.

.. figure:: /_images/fib20fig9.png

Figure 9: DPO contribution for a recursive route.

Figure 9 shows the load-balance objects contributed for a recursive route.

.. figure:: /_images/fib20fig10.png

Figure 10: DPO Contributions from labelled recursive routes.

Figure 10 shows the derived data-plane graph for a labelled recursive route.
There can be as many MPLS-label DPO instances as there are routes multiplied by
the number of paths per-route. For this reason the mpls-label DPO should be as
small as possible [#f19]_.

The data-plane graph is constructed by 'stacking' one
instance of a DPO on another to form the child-parent relationship. When this
stacking occurs, the necessary VLIB graph arcs are automatically constructed
from the respected DPO type's registered graph nodes.

The diagrams above show that for any given route the full data-plane graph is
known before any packet arrives. If that graph is composed of n objects, then the
packet will visit n nodes and thus incur a forwarding cost of approximately n
times the graph node cost. This could be reduced if the graph were *collapsed*
into fewer DPOs and nodes. There are two ways we might consider doing
this:

- write custom DPOs/nodes for combinded functions, e.g. pop MPLS label
  and lookup in v4 table. This has the disadvantage that the number of
  such nodes would be, well, combinatorial, and resolving a path via
  a combined DPO would be more difficult as it would involve a
  forward walk of the graph to determine what the combination
  is. However, VPP power users might consider this option for a
  limited set of their use cases where performance is truely king.
- collapse multiple levels of load-balancing into one. For example,
  if there were two levels of load-balancing each with two choices,
  this could equally be represented by one level with 4 choices.

In either case a disadvantage to collapsing the graph is that it
removes the indirection objects that provide fast convergence (see
section Fast Convergence). To collapse is then a trade-off between
faster forwarding and fast convergence; VPP favours the latter.


.. rubric:: Footnotes:

.. [#f16] Directed implies it cannot be back-walked. It is acyclic even in the presence of a recursion loop.
.. [#f17] Loaded into cache, and hence potentially incurring a d-cache miss.
.. [#f18] The engaged reader is directed to vnet/vnet/dpo/*
.. [#f19] i.e. we should not re-use the adjacency structure.