Reorganize source tree to use single autotools instance

Change-Id: I7b51f88292e057c6443b12224486f2d0c9f8ae23
Signed-off-by: Damjan Marion <damarion@cisco.com>

1 files changed, 652 insertions, 0 deletions

diff --git a/src/vnet/fib/fib.h b/src/vnet/fib/fib.h
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+/*
+ * Copyright (c) 2016 Cisco and/or its affiliates.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+/**
+ * \brief
+ * A IP v4/6 independent FIB.
+ *
+ * The main functions provided by the FIB are as follows;
+ *
+ *  - source priorities
+ *
+ *   A route can be added to the FIB by more than entity or source. Sources
+ * include, but are not limited to, API, CLI, LISP, MAP, etc (for the full list
+ * see fib_entry.h). Each source provides the forwarding information (FI) that
+ * is has determined as required for that route. Since each source determines the
+ * FI using different best  path and loop prevention algorithms, it is not
+ * correct for the FI of multiple sources to be combined. Instead the FIB must
+ * choose to use the FI from only one source. This choose is based on a static
+ * priority assignment. For example;
+ * IF a prefix is added as a result of interface configuration:
+ *    set interface address 192.168.1.1/24 GigE0
+ * and then it is also added from the CLI
+ *    ip route 192.168.1.1/32 via 2.2.2.2/32
+ * then the 'interface' source will prevail, and the route will remain as
+ * 'local'.
+ * The requirement of the FIB is to always install the FI from the winning
+ * source and thus to maintain the FI added by losing sources so it can be
+ * installed should the winning source be withdrawn.
+ *
+ *  - adj-fib maintenance
+ *
+ *   When ARP or ND discover a neighbour on a link an adjacency forms for the
+ * address of that neighbour. It is also required to insert a route in the
+ * appropriate FIB table, corresponding to the VRF for the link, an entry for
+ * that neighbour. This entry is often referred to as an adj-fib. Adj-fibs
+ * have a dedicated source; 'ADJ'.
+ * The priority of the ADJ source is lower than most. This is so the following
+ * config;
+ *    set interface address 192.168.1.1/32 GigE0
+ *    ip arp 192.168.1.2 GigE0 dead.dead.dead
+ *    ip route add 192.168.1.2 via 10.10.10.10 GigE1
+ * will forward traffic for 192.168.1.2 via GigE1. That is the route added
+ * by the control plane is favoured over the adjacency discovered by ARP.
+ * The control plane, with its associated authentication, is considered the
+ * authoritative source.
+ * To counter the nefarious addition of adj-fib, through the nefarious injection
+ * of adjacencies, the FIB is also required to ensure that only adj-fibs whose
+ * less specific covering prefix is connected are installed in forwarding. This
+ * requires the use of 'cover tracking', where a route maintains a dependency
+ * relationship with the route that is its less specific cover. When this cover
+ * changes (i.e. there is a new covering route) or the forwarding information
+ * of the cover changes, then the covered route is notified.
+ *
+ * Overlapping sub-nets are not supported, so no adj-fib has multiple paths.
+ * The control plane is expected to remove a prefix configured for an interface
+ * before the interface changes VRF.
+ * So while the following config is accepted:
+ *    set interface address 192.168.1.1/32 GigE0
+ *    ip arp 192.168.1.2 GigE0 dead.dead.dead
+ *    set interface ip table GigE0 2
+ * it does not result in the desired behaviour.
+ *
+ *  - attached export.
+ *
+ * Further to adj-fib maintenance above consider the following config:
+ *    set interface address 192.168.1.1/24 GigE0
+ *    ip route add table 2 192.168.1.0/24 GigE0
+ * Traffic destined for 192.168.1.2 in table 2 will generate an ARP request
+ * on GigE0. However, since GigE0 is in table 0, all adj-fibs will be added in
+ * FIB 0. Hence all hosts in the sub-net are unreachable from table 2. To resolve
+ * this, all adj-fib and local prefixes are exported (i.e. copied) from the 
+ * 'export' table 0, to the 'import' table 2. There can be many import tables
+ * for a single export table.
+ *
+ *  - recursive route resolution
+ *
+ *   A recursive route is of the form:
+ *       1.1.1.1/32 via 10.10.10.10
+ * i.e. a route for which no egress interface is provided. In order to forward
+ * traffic to 1.1.1.1/32 the FIB must therefore first determine how to forward
+ * traffic to 10.10.10.10/32. This is recursive resolution.
+ * Recursive resolution, just like normal resolution, proceeds via a longest
+ * prefix match for the 'via-address' 10.10.10.10. Note it is only possible
+ * to add routes via an address (i.e. a /32 or /128) not via a shorter mask
+ * prefix. There is no use case for the latter.
+ * Since recursive resolution proceeds via a longest prefix match, the entry
+ * in the FIB that will resolve the recursive route, termed the via-entry, may
+ * change as other routes are added to the FIB. Consider the recursive
+ * route shown above, and this non-recursive route:
+ *       10.10.10.0/24 via 192.168.16.1 GigE0
+ * The entry for 10.10.10.0/24 is thus the resolving via-entry. If this entry is
+ * modified, to say;
+ *       10.10.10.0/24 via 192.16.1.3 GigE0
+ * Then packet for 1.1.1.1/32 must also be sent to the new next-hop.
+ * Now consider the addition of;
+ *       10.10.10.0/28 via 192.168.16.2 GigE0
+ * The more specific /28 is a better longest prefix match and thus becomes the
+ * via-entry. Removal of the /28 means the resolution will revert to the /24.
+ * The tracking to the changes in recursive resolution is the requirement of
+ * the FIB. When the forwarding information of the via-entry changes a back-walk
+ * is used to update dependent recursive routes. When new routes are added to
+ * the table the cover tracking feature provides the necessary notifications to
+ * the via-entry routes.
+ * The adjacency constructed for 1.1.1.1/32 will be a recursive adjacency
+ * whose next adjacency will be contributed from the via-entry. Maintaining
+ * the validity of this recursive adjacency is a requirement of the FIB.
+ *
+ *  - recursive loop avoidance
+ *
+ * Consider this set of routes:
+ *     1.1.1.1/32 via 2.2.2.2
+ *     2.2.2.2/32 via 3.3.3.3
+ *     3.3.3.3/32 via 1.1.1.1
+ * this is termed a recursion loop - all of the routes in the loop are
+ * unresolved in so far as they do not have a resolving adjacency, but each
+ * is resolved because the via-entry is known. It is important here to note
+ * the distinction between the control-plane objects and the data-plane objects
+ * (more details in the implementation section). The control plane objects must
+ * allow the loop to form (i.e. the graph becomes cyclic), however, the
+ * data-plane absolutely must not allow the loop to form, otherwise the packet
+ * would loop indefinitely and never egress the device - meltdown would follow.
+ * The control plane must allow the loop to form, because when the loop breaks,
+ * all members of the loop need to be updated. Forming the loop allows the
+ * dependencies to be correctly setup to allow this to happen.
+ * There is no limit to the depth of recursion supported by VPP so:
+ *    9.9.9.100/32 via 9.9.9.99
+ *    9.9.9.99/32  via 9.9.9.98
+ *    9.9.9.98/32  via 9.9.9.97
+ *      ... turtles, turtles, turtles ...
+ *    9.9.9.1/32 via 10.10.10.10 Gig0
+ * is supported to as many layers of turtles is desired, however, when
+ * back-walking a graph (in this case from 9.9.9.1/32 up toward 9.9.9.100/32)
+ * a FIB needs to differentiate the case where the recursion is deep versus
+ * the case where the recursion is looped. A simple method, employed by VPP FIB,
+ * is to limit the number of steps. VPP FIB limit is 16. Typical BGP scenarios
+ * in the wild do not exceed 3 (BGP Inter-AS option C).
+ * 
+ * - Fast Convergence
+ * 
+ * After a network topology change, the 'convergence' time, is the time taken
+ * for the router to complete a transition to forward traffic using the new
+ * topology. The convergence time is therefore a summation of the time to;
+ *  - detect the failure.
+ *  - calculate the new 'best path' information
+ *  - download the new best paths to the data-plane.
+ *  - install those best best in data-plane forwarding.
+ * The last two points are of relevance to VPP architecture. The download API is
+ * binary and batch, details are not discussed here. There is no HW component to
+ * programme, installation time is bounded by the memory allocation and table
+ * lookup and insert access times.
+ *
+ * 'Fast' convergence refers to a set of technologies that a FIB can employ to
+ * completely or partially restore forwarding whilst the convergence actions
+ * listed above are ongoing. Fast convergence technologies are further
+ * sub-divided into Prefix Independent Convergence (PIC) and Loop Free
+ * Alternate path Fast re-route (LFA-FRR or sometimes called IP-FRR) which
+ * affect recursive and non-recursive routes respectively.
+ *
+ * LFA-FRR
+ *
+ * Consider the network topology below:
+ *
+ *          C
+ *        /   \
+ *  X -- A --- B - Y
+ *       |     |
+ *       D     F
+ *        \   /
+ *          E
+ *
+ * all links are equal cost, traffic is passing from X to Y. the best path is
+ * X-A-B-Y. There are two alternative paths, one via C and one via E. An
+ * alternate path is considered to be loop free if no other router on that path
+ * would forward the traffic back to the sender. Consider router C, its best
+ * path to Y is via B, so if A were to send traffic destined to Y to C, then C
+ * would forward that traffic to B - this is a loop-free alternate path. In
+ * contrast consider router D. D's shortest path to Y is via A, so if A were to
+ * send traffic destined to Y via D, then D would send it back to A; this is
+ * not a loop-free alternate path. There are several points of note;
+ *   - we are considering the pre-failure routing topology
+ *   - any equal-cost multi-path between A and B is also a LFA path.
+ *   - in order for A to calculate LFA paths it must be aware of the best-path
+ *     to Y from the perspective of D. These calculations are thus limited to
+ *     routing protocols that have a full view of the network topology, i.e.
+ *     link-state DB protocols like OSPF or an SDN controller. LFA protected
+ *     prefixes are thus non-recursive.
+ *
+ * LFA is specified as a 1 to 1 redundancy; a primary path has only one LFA
+ * (a.k.a. backup) path. To my knowledge this limitation is one of complexity
+ * in the calculation of and capacity planning using a 1-n redundancy. 
+ *
+ * In the event that the link A-B fails, the alternate path via C can be used.
+ * In order to provide 'fast' failover in the event of a failure, the control
+ * plane will download both the primary and the backup path to the FIB. It is
+ * then a requirement of the FIB to perform the failover (a.k.a cutover) from
+ * the primary to the backup path as quickly as possible, and particularly
+ * without any other control-plane intervention. The expectation is cutover is
+ * less than 50 milli-seconds - a value allegedly from the VOIP QoS. Note that
+ * cutover time still includes the fault detection time, which in a vitalised
+ * environment could be the dominant factor. Failure detection can be either a
+ * link down, which will affect multiple paths on a multi-access interface, or
+ * via a specific path heartbeat (i.e. BFD). 
+ * At this time VPP does not support LFA, that is it does not support the
+ * installation of a primary and backup path[s] for a route. However, it does
+ * support ECMP, and VPP FIB is designed to quickly remove failed paths from
+ * the ECMP set, however, it does not insert shared objects specific to the
+ * protected resource into the forwarding object graph, since this would incur
+ * a forwarding/performance cost. Failover time is thus route number dependent.
+ * Details are provided in the implementation section below.
+ *
+ * PIC
+ *
+ * PIC refers to the concept that the converge time should be independent of
+ * the number of prefixes/routes that are affected by the failure. PIC is
+ * therefore most appropriate when considering networks with large number of
+ * prefixes, i.e. BGP networks and thus recursive prefixes. There are several
+ * flavours of PIC covering different locations of protection and failure
+ * scenarios. An outline is given below, see the literature for more details:
+ *
+ * Y/16 - CE1 -- PE1---\
+ *                | \   P1---\
+ *                |  \        PE3 -- CE3 - X/16
+ *                |   - P2---/
+ * Y/16 - CE2 -- PE2---/
+ *
+ * CE = customer edge, PE = provider edge. external-BGP runs between customer
+ * and provider, internal-BGP runs between provider and provider.
+ *
+ * 1) iBGP PIC-core: consider traffic from CE1 to X/16 via CE3. On PE1 there is
+ *    are routes;
+ *       X/16 (and hundreds of thousands of others like it)
+ *         via PE3
+ *    and
+ *      PE3/32 (its loopback address)
+ *        via 10.0.0.1 Link0 (this is P1)
+ *        via 10.1.1.1 Link1 (this is P2)
+ * the failure is the loss of link0 or link1
+ * As in all PIC scenarios, in order to provide prefix independent convergence
+ * it must be that the route for X/16 (and all other routes via PE3) do not
+ * need to be updated in the FIB. The FIB therefore needs to update a single
+ * object that is shared by all routes - once this shared object is updated,
+ * then all routes using it will be instantly updated to use the new forwarding
+ * information. In this case the shared object is the resolving route via PE3.
+ * Once the route via PE3 is updated via IGP (OSPF) convergence, then all
+ * recursive routes that resolve through it are also updated. VPP FIB
+ * implements this scenario via a recursive-adjacency. the X/16 and it sibling
+ * routes share a recursive-adjacency that links to/points at/stacks on the
+ * normal adjacency contributed by the route for PE3. Once this shared
+ * recursive adj is re-linked then all routes are switched to using the new
+ * forwarding information. This is shown below;
+ *
+ * pre-failure;
+ *   X/16 --> R-ADJ-1 --> ADJ-1-PE3 (multi-path via P1 and P2)
+ *
+ * post-failure:
+ *   X/16 --> R-ADJ-1 --> ADJ-2-PE3 (single path via P1)
+ *
+ * note that R-ADJ-1 (the recursive adj) remains in the forwarding graph,
+ * therefore X/16 (and all its siblings) is not updated.
+ * X/16 and its siblings share the recursive adj since they share the same
+ * path-list. It is the path-list object that contributes the recursive-adj
+ * (see next section for more details)
+ *
+ *
+ * 2) iBGP PIC-edge; Traffic from CE3 to Y/16. On PE3 there is are routes;
+ *      Y/16  (and hundreds of thousands of others like it)
+ *        via PE1
+ *        via PE2 
+ *  and
+ *     PE1/32 (PE1's loopback address)
+ *       via 10.0.2.2 Link0 (this is P1)
+ *     PE2/32 (PE2's loopback address)
+ *       via 10.0.3.3 Link1 (this is P2)
+ *
+ * the failure is the loss of reachability to PE2. this could be either the
+ * loss of the link P2-PE2 or the loss of the node PE2. This is detected either
+ * by the withdrawal of the PE2's loopback route or by some form of failure
+ * detection (i.e. BFD).
+ * VPP FIB again provides PIC via the use of the shared recursive-adj. Y/16 and
+ * its siblings will again share a path-list for the list {PE1,PE2}, this
+ * path-list will contribute a multi-path-recursive-adj, i.e. a multi-path-adj
+ * with each choice therein being another adj;
+ *
+ *  Y/16 -> RM-ADJ --> ADJ1 (for PE1)
+ *                 --> ADJ2 (for PE2)
+ *
+ * when the route for PE1 is withdrawn then the multi-path-recursive-adjacency
+ * is updated to be;
+ *
+ * Y/16 --> RM-ADJ --> ADJ1 (for PE1)
+ *                 --> ADJ1 (for PE1)
+ *
+ * that is both choices in the ECMP set are the same and thus all traffic is
+ * forwarded to PE1. Eventually the control plane will download a route update
+ * for Y/16 to be via PE1 only. At that time the situation will be:
+ *
+ * Y/16 -> R-ADJ --> ADJ1 (for PE1)
+ *
+ * In the scenario above we assumed that PE1 and PE2 are ECMP for Y/16. eBGP
+ * PIC core is also specified for the case were one PE is primary and the other
+ * backup - VPP FIB does not support that case at this time.
+ *
+ * 3) eBGP PIC Edge; Traffic from CE3 to Y/16. On PE1 there is are routes;
+ *      Y/16 (and hundreds of thousands of others like it)
+ *         via CE1 (primary)
+ *         via PE2 (backup)
+ *   and
+ *     CE1 (this is an adj-fib)
+ *       via 11.0.0.1 Link0 (this is CE1) << this is an adj-fib
+ *     PE2 (PE2's loopback address)
+ *       via 10.0.5.5 Link1 (this is link PE1-PE2)
+ * the failure is the loss of link0 to CE1. The failure can be detected by FIB
+ * either as a link down event or by the control plane withdrawing the connected
+ * prefix on the link0 (say 10.0.5.4/30). The latter works because the resolving
+ * entry is an adj-fib, so removing the connected will withdraw the adj-fib, and
+ * hence the recursive path becomes unresolved. The former is faster,
+ * particularly in the case of Inter-AS option A where there are many VLAN
+ * sub-interfaces on the PE-CE link, one for each VRF, and so the control plane
+ * must remove the connected prefix for each sub-interface to trigger PIC in
+ * each VRF. Note though that total PIC cutover time will depend on VRF scale
+ * with either trigger.
+ * Primary and backup paths in this eBGP PIC-edge scenario are calculated by
+ * BGP. Each peer is configured to always advertise its best external path to
+ * its iBGP peers. Backup paths therefore send traffic from the PE back into the
+ * core to an alternate PE. A PE may have multiple external paths, i.e. multiple
+ * directly connected CEs, it may also have multiple backup PEs, however there
+ * is no correlation between the two, so unlike LFA-FRR, the redundancy model is
+ * N-M; N primary paths are backed-up by M backup paths - only when all primary
+ * paths fail, then the cutover is performed onto the M backup paths. Note that
+ * PE2 must be suitably configured to forward traffic on its external path that
+ * was received from PE1. VPP FIB does not support external-internal-BGP (eiBGP)
+ * load-balancing.
+ *
+ * As with LFA-FRR the use of primary and backup paths is not currently
+ * supported, however, the use of a recursive-multi-path-adj, and a suitably
+ * constrained hashing algorithm to choose from the primary or backup path sets,
+ * would again provide the necessary shared object and hence the prefix scale
+ * independent cutover.
+ *
+ * Astute readers will recognise that both of the eBGP PIC scenarios refer only
+ * to a BGP free core.
+ *
+ * Fast convergence implementation options come in two flavours:
+ *  1) Insert switches into the data-path. The switch represents the protected
+ *     resource. If the switch is 'on' the primary path is taken, otherwise
+ *     the backup path is taken. Testing the switch in the data-path comes with
+ *     an associated performance cost. A given packet may encounter more than
+ *     one protected resource as it is forwarded. This approach minimises
+ *     cutover times as packets will be forwarded on the backup path as soon
+ *     as the protected resource is detected to be down and the single switch
+ *     is tripped. However, it comes at a performance cost, which increases
+ *     with each shared resource a packet encounters in the data-path.
+ *     This approach is thus best suited to LFA-FRR where the protected routes
+ *     are non-recursive (i.e. encounter few shared resources) and the
+ *     expectation on cutover times is more stringent (<50msecs).
+ *  2) Update shared objects. Identify objects in the data-path, that are
+ *     required to be present whether or not fast convergence is required (i.e.
+ *     adjacencies) that can be shared by multiple routes. Create a dependency
+ *     between these objects at the protected resource. When the protected
+ *     resource fails, each of the shared objects is updated in a way that all
+ *     users of it see a consistent change. This approach incurs no performance
+ *     penalty as the data-path structure is unchanged, however, the cutover
+ *     times are longer as more work is required when the resource fails. This
+ *     scheme is thus more appropriate to recursive prefixes (where the packet
+ *     will encounter multiple protected resources) and to fast-convergence
+ *     technologies where the cutover times are less stringent (i.e. PIC).
+ *
+ * Implementation:
+ * ---------------
+ *
+ * Due to the requirements outlined above, not all routes known to FIB
+ * (e.g. adj-fibs) are installed in forwarding. However, should circumstances
+ * change, those routes will need to be added. This adds the requirement that
+ * a FIB maintains two tables per-VRF, per-AF (where a 'table' is indexed by
+ * prefix); the forwarding and non-forwarding tables.
+ *
+ * For DP speed in VPP we want the lookup in the forwarding table to directly 
+ * result in the ADJ. So the two tables; one contains all the routes (a 
+ * lookup therein yields a fib_entry_t), the other contains only the forwarding 
+ * routes (a lookup therein yields an ip_adjacency_t). The latter is used by the
+ * DP. 
+ * This trades memory for forwarding performance. A good trade-off in VPP's
+ * expected operating environments.
+ *
+ * Note these tables are keyed only by the prefix (and since there 2 two
+ * per-VRF, implicitly by the VRF too). The key for an adjacency is the
+ * tuple:{next-hop, address (and it's AF), interface, link/ether-type}.
+ * consider this curious, but allowed, config;
+ *
+ *   set int ip addr 10.0.0.1/24 Gig0
+ *   set ip arp Gig0 10.0.0.2 dead.dead.dead
+ *   # a host in that sub-net is routed via a better next hop (say it avoids a
+ *   # big L2 domain)
+ *   ip route add 10.0.0.2 Gig1 192.168.1.1
+ *   # this recursive should go via Gig1
+ *   ip route add 1.1.1.1/32 via 10.0.0.2
+ *   # this non-recursive should go via Gig0
+ *   ip route add 2.2.2.2/32 via Gig0 10.0.0.2
+ *
+ * for the last route, the lookup for the path (via {Gig0, 10.0.0.2}) in the
+ * prefix table would not yield the correct result. To fix this we need a
+ * separate table for the adjacencies.
+ *
+ *  - FIB data structures;
+ *
+ * fib_entry_t:
+ *   - a representation of a route.
+ *     - has a prefix.
+ *    - it maintains an array of path-lists that have been contributed by the
+ *      different sources
+ *    - install an adjacency in the forwarding table contributed by the best
+ *      source's path-list.
+ *
+ * fib_path_list_t:
+ *   - a list of paths
+ *   - path-lists may be shared between FIB entries. The path-lists are thus
+ *     kept in a DB. The key is the combined description of the paths. We share
+ *     path-lists  when it will aid convergence to do so. Adding path-lists to
+ *     this DB that are never shared, or are not shared by prefixes that are
+ *     not subject to PIC, will increase the size of the DB unnecessarily and
+ *     may lead to increased search times due to hash collisions.
+ *   - the path-list contributes the appropriate adj for the entry in the 
+ *     forwarding table. The adj can be 'normal', multi-path or recursive,
+ *     depending on the number of paths and their types.
+ *   - since path-lists are shared there is only one instance of the multi-path 
+ *     adj that they [may] create. As such multi-path adjacencies do not need a
+ *     separate DB.
+ * The path-list with recursive paths and the recursive adjacency that it
+ * contributes forms the backbone of the fast convergence architecture (as 
+ * described previously). 
+ *
+ * fib_path_t:
+ *   - a description of how to forward the traffic (i.e. via {Gig1, K}).
+ *   - the path describes the intent on how to forward. This differs from how 
+ *     the path resolves. I.e. it might not be resolved at all (since the
+ *     interface is deleted or down).
+ *   - paths have different types, most notably recursive or non-recursive.
+ *   - a fib_path_t will contribute the appropriate adjacency object. It is from
+ *     these contributions that the DP graph/chain for the route is built.
+ *   - if the path is recursive and a recursion loop is detected, then the path
+ *     will contribute the special DROP adjacency. This way, whilst the control
+ *     plane graph is looped, the data-plane graph does not.
+ *
+ * we build a graph of these objects;
+ *
+ *  fib_entry_t -> fib_path_list_t -> fib_path_t -> ...
+ *
+ * for recursive paths:
+ *
+ *   fib_path_t -> fib_entry_t -> ....
+ *
+ * for non-recursive paths
+ *
+ *  fib_path_t -> ip_adjacency_t -> interface
+ *
+ * These objects, which constitute the 'control plane' part of the FIB are used
+ * to represent the resolution of a route. As a whole this is referred to as the
+ * control plane graph. There is a separate DP graph to represent the forwarding
+ * of a packet. In the DP graph each object represents an action that is applied
+ * to a packet as it traverses the graph. For example, a lookup of a IP address
+ * in the forwarding table could result in the following graph:
+ *
+ *    recursive-adj --> multi-path-adj --> interface_A
+ *                                     --> interface_B
+ *
+ * A packet traversing this FIB DP graph would thus also traverse a VPP node
+ * graph of:
+ *
+ *    ipX_recursive --> ipX_rewrite --> interface_A_tx --> etc
+ *
+ * The taxonomy of objects in a FIB graph is as follows, consider;
+ *
+ *   A -->  
+ *   B --> D
+ *   C -->
+ *
+ * Where A,B and C are (for example) routes that resolve through D. 
+ *  parent; D is the parent of A, B, and C.
+ *  children: A, B, and C are children of D. 
+ *  sibling: A, B and C are siblings of one another.
+ *
+ * All shared objects in the FIB are reference counted. Users of these objects
+ * are thus expected to use the add_lock/unlock semantics (as one would
+ * normally use malloc/free).
+ *
+ * WALKS
+ *
+ * It is necessary to walk/traverse the graph forwards (entry to interface) to
+ * perform a collapse or build a recursive adj and backwards (interface
+ * to entry) to perform updates, i.e. when interface state changes or when
+ * recursive route resolution updates occur.
+ * A forward walk follows simply by navigating an object's parent pointer to
+ * access its parent object. For objects with multiple parents (e.g. a 
+ * path-list), each parent is walked in turn.
+ * To support back-walks direct dependencies are maintained between objects,
+ * i.e. in the relationship, {A, B, C} --> D, then object D will maintain a list
+ * of 'pointers' to its children {A, B, C}. Bare C-language pointers are not 
+ * allowed, so a pointer is described in terms of an object type (i.e. entry,
+ * path-list, etc) and index - this allows the object to be retrieved from the
+ * appropriate pool. A list is maintained to achieve fast convergence at scale.
+ * When there are millions or recursive prefixes, it is very inefficient to
+ * blindly walk the tables looking for entries that were affected by a given
+ * topology change. The lowest hanging fruit when optimising is to remove
+ * actions that are not required, so all back-walks only traverse objects that
+ * are directly affected by the change.
+ *
+ * PIC Core and fast-reroute rely on FIB reacting quickly to an interface
+ * state change to update the multi-path-adjacencies that use this interface.
+ * An example graph is shown below:
+ *
+ *    E_a -->
+ *    E_b --> PL_2 --> P_a --> Interface_A
+ *    ...          --> P_c -\
+ *    E_k -->                \
+ *                            Interface_K
+ *                            /
+ *    E_l -->                /
+ *    E_m --> PL_1 --> P_d -/ 
+ *    ...          --> P_f --> Interface_F
+ *    E_z -->
+ *
+ * E  = fib_entry_t
+ * PL = fib_path_list_t
+ * P  = fib_path_t 
+ * The subscripts are arbitrary and serve only to distinguish object instances.
+ * This CP graph result in the following DP graph:
+ *
+ *     M-ADJ-2 --> Interface_A
+ *             \
+ *              -> Interface_K
+ *             / 
+ *     M-ADJ-1 --> Interface_F
+ *
+ * M-ADJ = multi-path-adjacency.
+ *
+ * When interface K goes down a back-walk is started over its dependants in the
+ * control plane graph. This back-walk will reach PL_1 and PL_2 and result in
+ * the calculation of new adjacencies that have interface K removed. The walk
+ * will continue to the entry objects and thus the forwarding table is updated
+ * for each prefix with the new adjacency. The DP graph then becomes:
+ *
+ *    ADJ-3 --> Interface_A
+ *
+ *    ADJ-4 --> Interface_F
+ * 
+ * The eBGP PIC scenarios described above relied on the update of a path-list's
+ * recursive-adjacency to provide the shared point of cutover. This is shown
+ * below
+ *
+ *    E_a -->
+ *    E_b --> PL_2 --> P_a --> E_44 --> PL_a --> P_b --> Interface_A
+ *    ...          --> P_c -\
+ *    E_k -->                \
+ *                            \
+ *                           E_1 --> PL_k -> P_k --> Interface_K
+ *                            /
+ *    E_l -->                /
+ *    E_m --> PL_1 --> P_d -/ 
+ *    ...          --> P_f --> E_55 --> PL_e --> P_e --> Interface_E
+ *    E_z -->
+ *
+ * The failure scenario is the removal of entry E_1 and thus the paths P_c and
+ * P_d become unresolved. To achieve PIC the two shared recursive path-lists,
+ * PL_1 and PL_2 must be updated to remove E_1 from the recursive-multi-path-
+ * adjacencies that they contribute, before any entry E_a to E_z is updated.
+ * This means that as the update propagates backwards (right to left) in the
+ * graph it must do so breadth first not depth first. Note this approach leads
+ * to convergence times that are dependent on the number of path-list and so
+ * the number of combinations of egress PEs - this is desirable as this
+ * scale is considerably lower than the number of prefixes.
+ *
+ * If we consider another section of the graph that is similar to the one
+ * shown above where there is another prefix E_2 in a similar position to E_1
+ * and so also has many dependent children. It is reasonable to expect that a
+ * particular network failure may simultaneously render E_1 and E_2 unreachable.
+ * This means that the update to withdraw E_2 is download immediately after the
+ * update to withdraw E_1. It is a requirement on the FIB to not spend large
+ * amounts of time in a back-walk whilst processing the update for E_1, i.e. the
+ * back-walk must not reach as far as E_a and its siblings. Therefore, after the
+ * back-walk has traversed one generation (breadth first) to update all the
+ * path-lists it should be suspended/back-ground and further updates allowed
+ * to be handled. Once the update queue is empty, the suspended walks can be
+ * resumed. Note that in the case that multiple updates affect the same entry
+ * (say E_1) then this will trigger multiple similar walks, these are merged,
+ * so each child is updated only once.
+ * In the presence of more layers of recursion PIC is still a desirable
+ * feature. Consider an extension to the diagram above, where more recursive
+ * routes (E_100 -> E_200) are added as children of E_a:
+ *
+ * E_100 -->
+ * E_101 --> PL_3 --> P_j-\
+ * ...                     \
+ * E_199 -->               E_a -->
+ *                         E_b --> PL_2 --> P_a --> E_44 --> ...etc..
+ *                         ...          --> P_c -\
+ *                         E_k                    \
+ *                                                E_1 --> ...etc..
+ *                                                 /
+ *                         E_l -->                /
+ *                         E_m --> PL_1 --> P_d -/ 
+ *                         ...          --> P_e --> E_55 --> ...etc..
+ *                         E_z -->
+ *
+ * To achieve PIC for the routes E_100->E_199, PL_3 needs to be updated before
+ * E_b -> E_z, a breadth first traversal at each level would not achieve this.
+ * Instead the walk must proceed intelligently. Children on PL_2 are sorted so
+ * those Entry objects that themselves have children appear first in the list,
+ * those without later. When an entry object is walked that has children, a
+ * walk of its children is pushed to the front background queue. The back
+ * ground queue is a priority queue. As the breadth first traversal proceeds
+ * across the dependent entry object E_a to E_k, when the first entry that does
+ * not have children is reached (E_b), the walk is suspended and placed at the
+ * back of the queue. Following this prioritisation method shared path-list
+ * updates are performed before all non-resolving entry objects.
+ * The CPU/core/thread that handles the updates is the same thread that handles
+ * the back-walks. Handling updates has a higher priority than making walk
+ * progress, so a walk is required to be interruptable/suspendable when new
+ * updates are available.
+ * !!! TODO - this section describes how walks should be not how they are !!!
+ *
+ * In the diagram above E_100 is an IP route, however, VPP has no restrictions
+ * on the type of object that can be a dependent of a FIB entry. Children of
+ * a FIB entry can be (and are) GRE & VXLAN tunnels endpoints, L2VPN LSPs etc.
+ * By including all object types into the graph and extending the back-walk, we
+ * can thus deliver fast convergence to technologies that overlay on an IP
+ * network.
+ *
+ * If having read all the above carefully you are still thinking;  'i don't need
+ * all this %&$* i have a route only I know about and I just need to jam it in',
+ * then fib_table_entry_special_add() is your only friend.
+ */
+
+#ifndef __FIB_H__
+#define __FIB_H__
+
+#include <vnet/fib/fib_table.h>
+#include <vnet/fib/fib_entry.h>
+#include <vnet/fib/ip4_fib.h>
+#include <vnet/fib/ip6_fib.h>
+
+#endif