11.1. A.1. Finding the First (Best) Target
Assuming the client supports 1 protocol for a particular application
service, the following pseudocode outlines the expected process to
find the first (best) target for the client, using S-NAPTR.
target = [initial domain]
naptr-done = false
while (not naptr-done)
{
NAPTR-RRset = [DNSlookup of NAPTR RRs for target]
[sort NAPTR-RRset by ORDER,and PREF within each ORDER]
rr-done = false
cur-rr = [first NAPTR RR]
while (not rr-done)
if ([SERVICE field of cur-rr contains desired application
service and application protocol])
rr-done = true
target= [REPLACEMENT target of NAPTR RR]
else
cur-rr = [next rr in list]
if (not empty [FLAG in cur-rr])
naptr-done = true
}
port = -1
if ([FLAG in cur-rr is "S"])
{
SRV-RRset = [DNSlookup of SRV RRs for target]
[sort SRV-RRset based on PREF]
target = [target of first RR of SRV-RRset]
port = [port in first RR of SRV-RRset]
}
; now, whether it was an "S" or an "A" in the NAPTR, we
; have the target for an A record lookup
host = [DNSlookup of target]
return (host, port)
11.2. A.2. Finding Subsequent Targets
The pseudocode in Appendix A is crafted to find the first, most
preferred host-port pair for a particular application service and
protocol. If, for any reason, that host-port pair did not work
(connection refused, application-level error), the client is expected
to try the next host-port in the S-NAPTR tree.
The pseudocode above does not permit retries -- once complete, it
sheds all context of where in the S-NAPTR tree it finished.
Therefore, client software writers could
o entwine the application-specific protocol with the DNS lookup and
RRset processing described in the pseudocode and continue the S-
NAPTR processing if the application code fails to connect to a
located host-port pair;
o use callbacks for the S-NAPTR processing; or
o use an S-NAPTR resolution routine that finds *all* valid servers
for the required application service and protocol from the
originating domain and that provides them in a sorted order for
the application to try.
