Understanding Linux Network Internals

Interface Between the DST and Calling Protocols

The DST cache is an independent subsystem; it has, for instance, its own garbage collection mechanism. As a subsystem, it provides a set of functions that various protocols can use to change or tune its behavior. When external subsystems need to interact with the routing cache, such as to notify it of an event or read the value of one of its parameters, they do it via a set of DST routines defined in the files net/core/dst.c and include/net/dst.h. These routines are wrappers around a set of functions made available by the L3 protocol that owns the cache, by initializing an instance of a dst_ops VFT, as shown in Figure 33-6.

Figure 33-6. dst_ops interface

The key structure presented by DST to higher layers is dst_entry; protocol-specific structures such as rtable are merely wrappers for this structure. IP owns the routing cache, but other protocols often keep references to routing cache elements. All of those references refer to dst_entry, not to its rtable wrapper. The sk_buff buffers also keep a reference to the dst_entry structure, not to the rtable structure. This reference is used to store the result of the routing lookup.

The dst_entry and dst_ops structures are described in detail in the associated sections in Chapter 36. There is an instance of dst_ops for each protocol; for example, IPv4 uses ipv4_dst_ops, initialized in net/ipv4/route.c ...