resolved: transaction - exponentially increase retry timeouts

Rather than fixing this to 5s for unicast DNS and 1s for LLMNR, start
at a tenth of those values and increase exponentially until the old
values are reached. For LLMNR the recommended timeout for IEEE802
networks (which basically means all of the ones we care about) is 100ms,
so that should be uncontroversial. For unicast DNS I have found no
recommended value. However, it seems vastly more likely that hitting a
500ms timeout is casued by a packet loss, rather than the RTT genuinely
being greater than 500ms, so taking this as a startnig value seems
reasonable to me.

In the common case this greatly reduces the latency due to normal packet
loss. Moreover, once we get support for probing for features, this means
that we can send more packets before degrading the feature level whilst
still allowing us to settle on the correct feature level in a reasonable
timeframe.

The timeouts are tracked per server (or per scope for the multicast
protocols), and once a server (or scope) receives a successfull package
the timeout is reset. We also track the largest RTT for the given
server/scope, and always start our timouts at twice the largest
observed RTT.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/resolve/resolved-dns-server.h b/src/resolve/resolved-dns-server.h
index 06059e8829..10111fd6bd 100644
--- a/src/resolve/resolved-dns-server.h
+++ b/src/resolve/resolved-dns-server.h
@@ -46,6 +46,9 @@ struct DnsServer {
         int family;
         union in_addr_union address;
 
+        usec_t resend_timeout;
+        usec_t max_rtt;
+
         bool marked:1;
 
         LIST_FIELDS(DnsServer, servers);
@@ -62,6 +65,9 @@ int dns_server_new(
 DnsServer* dns_server_ref(DnsServer *s);
 DnsServer* dns_server_unref(DnsServer *s);
 
+void dns_server_packet_received(DnsServer *s, usec_t rtt);
+void dns_server_packet_lost(DnsServer *s, usec_t usec);
+
 DEFINE_TRIVIAL_CLEANUP_FUNC(DnsServer*, dns_server_unref);
 
 extern const struct hash_ops dns_server_hash_ops;