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, 1 insertions, 7 deletions

diff --git a/src/resolve/resolved-dns-transaction.h b/src/resolve/resolved-dns-transaction.h
index a8f4267bc8..007f7e5959 100644
--- a/src/resolve/resolved-dns-transaction.h
+++ b/src/resolve/resolved-dns-transaction.h
@@ -58,6 +58,7 @@ struct DnsTransaction {
         DnsAnswer *cached;
         int cached_rcode;
 
+        usec_t start_usec;
         sd_event_source *timeout_event_source;
         unsigned n_attempts;
 
@@ -95,12 +96,6 @@ void dns_transaction_complete(DnsTransaction *t, DnsTransactionState state);
 const char* dns_transaction_state_to_string(DnsTransactionState p) _const_;
 DnsTransactionState dns_transaction_state_from_string(const char *s) _pure_;
 
-/* After how much time to repeat classic DNS requests */
-#define DNS_TRANSACTION_TIMEOUT_USEC (5 * USEC_PER_SEC)
-
-/* After how much time to repeat LLMNR requests, see RFC 4795 Section 7 */
-#define LLMNR_TRANSACTION_TIMEOUT_USEC (1 * USEC_PER_SEC)
-
 /* LLMNR Jitter interval, see RFC 4795 Section 7 */
 #define LLMNR_JITTER_INTERVAL_USEC (100 * USEC_PER_MSEC)
 
@@ -110,5 +105,4 @@ DnsTransactionState dns_transaction_state_from_string(const char *s) _pure_;
 /* Maximum attempts to send LLMNR requests, see RFC 4795 Section 2.7 */
 #define LLMNR_TRANSACTION_ATTEMPTS_MAX 3
 
-#define TRANSACTION_TIMEOUT_USEC(p) (p == DNS_PROTOCOL_LLMNR ? LLMNR_TRANSACTION_TIMEOUT_USEC : DNS_TRANSACTION_TIMEOUT_USEC)
 #define TRANSACTION_ATTEMPTS_MAX(p) (p == DNS_PROTOCOL_LLMNR ? LLMNR_TRANSACTION_ATTEMPTS_MAX : DNS_TRANSACTION_ATTEMPTS_MAX)