8. Implementation Considerations

The destination address selection algorithm needs information about potential source addresses. One possible implementation strategy is for getaddrinfo() to call down to the network layer with a list of destination addresses, sort the list in the network layer with full current knowledge of available source addresses, and return the sorted list to getaddrinfo(). This is simple and gives the best results, but it introduces the overhead of another system call. One way to reduce this overhead is to cache the sorted address list in the resolver, so that subsequent calls for the same name do not need to re-sort the list.

Another implementation strategy is to call down to the network layer to retrieve source address information and then sort the list of addresses directly in the context of getaddrinfo(). To reduce overhead in this approach, the source address information can be cached, amortizing the overhead of retrieving it across multiple calls to getaddrinfo(). In this approach, the implementation might not have knowledge of the outgoing interface for each destination, so it MAY use a looser definition of the candidate set during destination address ordering.

In any case, if the implementation uses cached and possibly stale information in its implementation of destination address selection or if the ordering of a cached list of destination addresses is possibly stale, then it MUST ensure that the destination address ordering returned to the application is no more than one second out of date. For example, an implementation might make a system call to check if any routing table entries, source address assignments, or prefix policy table entries that might affect these algorithms have changed. Another strategy is to use an invalidation counter that is incremented whenever any underlying state is changed. By caching the current invalidation counter value with derived state and then later comparing against the current value, the implementation could detect if the derived state is potentially stale.