Features that are stuck on the side of the FIB. Those that directly use the services that the FIB provides.
In the section on FIB fundamentals it was mentioned that there is a separation between what to match and how to forward. In an IP FIB what to match is the packet's destination address against a table of IP prefixes, and how to forward is described by a list of paths (the fib_path_list_t).
ACL Based Forwarding
ACL Based Forwarding (ABF) is also know as policy based routing (PBR). In ABF what to match is described by an ACL.
ABF uses two VPP services; ACL as a service, as provided by the ACL plugin and FIB path-lists. It just glues them together.
An ABF policy is the combination of an ACL with the forwarding description of a FIB path-list. An ABF attachment is the association of [an ordered set of] ABF policies to an interface. The attachment is consulted on the ingress path of the IP DP (as an input feature). If the ACL matches then the associated forwarding is followed, if not, the packet continues along the DP. Simple.
Layer 3 Cross Connect
An L3 cross-connect (L3XC) matches all packets that ingress the interface and then forwards using the supplied FIB path-list. Naturally it runs as an input feature in the IP path. Super simple.
Matches all IP packets that VPP has punted. Why they are punted is not relevant. All IP punted packets are sent by VPP to the punt feature arc. This feature 'matches' all packets that it receives and forwards using the FIB path-list.
Unicast Reverse Path Forwarding
Unicast Reverse Path Forwarding (uRPF) is the process of ensuring that a packet has a conforming source address. It comes in two flavours:
loose: The source address must be reachable, i.e. FIB must have a route that will forward to the source address. The default route counts as long as it does not drop.
strict: The source address is reachable via the interface on which the packet arrived, i.e. the FIB's route for the source address must include the input interface as an output interface.
The uRPF feature can run on either the input or output IP feature arc. In both cases it serves as an anti-spoofing check, though the semantics are slightly different. On the input arc it enforces that peers on that link are only using source addresses that they should - a network admin should employ at the access edge. On the output arc it enforces that a packet is sourced from a prefix that belongs to the network, i.e. that is has originated from within an SP's network, a network admin could use at its peering points.
To perform a uRPF check, the DP performs an IP FIB lookup on the source address, this always results in a load-balance (LB) object. If the LB has only 1 bucket and that bucket stacks on a drop DPO, then both a loose and strict check will fail, otherwise a loose check will pass. Each LB object has an associated uRPF list object. This object holds the list of interfaces through which the prefix is reachable. To pass the strict check, the input/output interface must be in this list.