Initial commit of Sphinx docs

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Signed-off-by: John DeNisco <jdenisco@cisco.com>

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+Feature Arcs
+============
+
+A significant number of vpp features are configurable on a per-interface
+or per-system basis. Rather than ask feature coders to manually
+construct the required graph arcs, we built a general mechanism to
+manage these mechanics.
+
+Specifically, feature arcs comprise ordered sets of graph nodes. Each
+feature node in an arc is independently controlled. Feature arc nodes
+are generally unaware of each other. Handing a packet to "the next
+feature node" is quite inexpensive.
+
+The feature arc implementation solves the problem of creating graph arcs
+used for steering.
+
+At the beginning of a feature arc, a bit of setup work is needed, but
+only if at least one feature is enabled on the arc.
+
+On a per-arc basis, individual feature definitions create a set of
+ordering dependencies. Feature infrastructure performs a topological
+sort of the ordering dependencies, to determine the actual feature
+order. Missing dependencies **will** lead to runtime disorder. See
+<https://gerrit.fd.io/r/#/c/12753> for an example.
+
+If no partial order exists, vpp will refuse to run. Circular dependency
+loops of the form "a then b, b then c, c then a" are impossible to
+satisfy.
+
+Adding a feature to an existing feature arc
+-------------------------------------------
+
+To nobody's great surprise, we set up feature arcs using the typical
+"macro -> constructor function -> list of declarations" pattern:
+
+```c
+    VNET_FEATURE_INIT (mactime, static) =
+    {
+      .arc_name = "device-input",
+      .node_name = "mactime",
+      .runs_before = VNET_FEATURES ("ethernet-input"),
+    };  
+```
+
+This creates a "mactime" feature on the "device-input" arc.
+
+Once per frame, dig up the vnet\_feature\_config\_main\_t corresponding
+to the "device-input" feature arc:
+
+```c
+    vnet_main_t *vnm = vnet_get_main ();
+    vnet_interface_main_t *im = &vnm->interface_main;
+    u8 arc = im->output_feature_arc_index;
+    vnet_feature_config_main_t *fcm;
+
+    fcm = vnet_feature_get_config_main (arc);
+```
+
+Note that in this case, we've stored the required arc index - assigned
+by the feature infrastructure - in the vnet\_interface\_main\_t. Where
+to put the arc index is a programmer's decision when creating a feature
+arc.
+
+Per packet, set next0 to steer packets to the next node they should
+visit:
+
+```c
+    vnet_get_config_data (&fcm->config_main,
+                          &b0->current_config_index /* value-result */, 
+                          &next0, 0 /* # bytes of config data */);
+```
+
+Configuration data is per-feature arc, and is often unused. Note that
+it's normal to reset next0 to divert packets elsewhere; often, to drop
+them for cause:
+
+```c
+    next0 = MACTIME_NEXT_DROP;
+    b0->error = node->errors[DROP_CAUSE];
+```
+
+Creating a feature arc
+----------------------
+
+Once again, we create feature arcs using constructor macros:
+
+```c
+    VNET_FEATURE_ARC_INIT (ip4_unicast, static) =
+    {
+      .arc_name = "ip4-unicast",
+      .start_nodes = VNET_FEATURES ("ip4-input", "ip4-input-no-checksum"),
+      .arc_index_ptr = &ip4_main.lookup_main.ucast_feature_arc_index,
+    };  
+```
+
+In this case, we configure two arc start nodes to handle the
+"hardware-verified ip checksum or not" cases. During initialization,
+the feature infrastructure stores the arc index as shown.
+
+In the head-of-arc node, do the following to send packets along the
+feature arc:
+
+```c
+    ip_lookup_main_t *lm = &im->lookup_main;
+    arc = lm->ucast_feature_arc_index;
+```
+
+Once per packet, initialize packet metadata to walk the feature arc:
+
+```c
+vnet_feature_arc_start (arc, sw_if_index0, &next, b0);
+```
+
+Enabling / Disabling features
+-----------------------------
+
+Simply call vnet_feature_enable_disable to enable or disable a specific
+feature:
+
+```c
+    vnet_feature_enable_disable ("device-input", /* arc name */
+                                 "mactime",      /* feature name */
+           		             sw_if_index,    /* Interface sw_if_index */
+                                 enable_disable, /* 1 => enable */
+                                 0 /* (void *) feature_configuration */, 
+                                 0 /* feature_configuration_nbytes */);
+```
+
+The feature_configuration opaque is seldom used. 
+
+If you wish to make a feature a _de facto_ system-level concept, pass
+sw_if_index=0 at all times. Sw_if_index 0 is always valid, and
+corresponds to the "local" interface.
+
+Related "show" commands
+-----------------------
+
+To display the entire set of features, use "show features [verbose]". The
+verbose form displays arc indices, and feature indicies within the arcs
+
+```
+$ vppctl show features verbose
+Available feature paths
+<snip>
+[14] ip4-unicast:
+  [ 0]: nat64-out2in-handoff
+  [ 1]: nat64-out2in
+  [ 2]: nat44-ed-hairpin-dst
+  [ 3]: nat44-hairpin-dst
+  [ 4]: ip4-dhcp-client-detect
+  [ 5]: nat44-out2in-fast
+  [ 6]: nat44-in2out-fast
+  [ 7]: nat44-handoff-classify
+  [ 8]: nat44-out2in-worker-handoff
+  [ 9]: nat44-in2out-worker-handoff
+  [10]: nat44-ed-classify
+  [11]: nat44-ed-out2in
+  [12]: nat44-ed-in2out
+  [13]: nat44-det-classify
+  [14]: nat44-det-out2in
+  [15]: nat44-det-in2out
+  [16]: nat44-classify
+  [17]: nat44-out2in
+  [18]: nat44-in2out
+  [19]: ip4-qos-record
+  [20]: ip4-vxlan-gpe-bypass
+  [21]: ip4-reassembly-feature
+  [22]: ip4-not-enabled
+  [23]: ip4-source-and-port-range-check-rx
+  [24]: ip4-flow-classify
+  [25]: ip4-inacl
+  [26]: ip4-source-check-via-rx
+  [27]: ip4-source-check-via-any
+  [28]: ip4-policer-classify
+  [29]: ipsec-input-ip4
+  [30]: vpath-input-ip4
+  [31]: ip4-vxlan-bypass
+  [32]: ip4-lookup
+<snip>
+```
+
+Here, we learn that the ip4-unicast feature arc has index 14, and that
+e.g. ip4-inacl is the 25th feature in the generated partial order.
+
+To display the features currently active on a specific interface,
+use "show interface <name> features":
+
+```
+$ vppctl show interface GigabitEthernet3/0/0 features
+Feature paths configured on GigabitEthernet3/0/0...
+<snip>
+ip4-unicast:
+  nat44-out2in
+<snip>
+```
+
+Table of Feature Arcs
+---------------------
+
+Simply search for name-strings to track down the arc definition, location of
+the arc index, etc.
+
+```
+            |    Arc Name      |
+            |------------------|
+            | device-input     |
+            | ethernet-output  |
+            | interface-output |
+            | ip4-drop         |
+            | ip4-local        |
+            | ip4-multicast    |
+            | ip4-output       |
+            | ip4-punt         |
+            | ip4-unicast      |
+            | ip6-drop         |
+            | ip6-local        |
+            | ip6-multicast    |
+            | ip6-output       |
+            | ip6-punt         |
+            | ip6-unicast      |
+            | mpls-input       |
+            | mpls-output      |
+            | nsh-output       |
+```