fixes #269 nngcat unreliable with ZeroTier transport

This does a few things.  First it closes a case where a dropped
message could prevent subsequent connection attempts from getting through.

Second, it changes the rate at which we retry, and the timeout, to be
a lot more aggressive when attempting to establish a connection.  We
retry every 500 ms, for up to 2 minutes, before giving up.  This gives
a lot more resilience in the face of message loss that is typical of
ZeroTier in some environments when first establishing communication.

Third, makes the values for the connection attempts *tunable*, so
that applications can adjust for different deployment scenarios.

Fourth, it includes the ability to get the UDP socket name.  This was
needed during some debug, and may be useful for a real UDP transport
later, so we're keeping it.

Finally, we added documentation for the above items.

1 files changed, 10 insertions, 2 deletions

diff --git a/src/transport/zerotier/zerotier.h b/src/transport/zerotier/zerotier.h
index 9083f665..a191b106 100644
--- a/src/transport/zerotier/zerotier.h
+++ b/src/transport/zerotier/zerotier.h
@@ -78,7 +78,7 @@
 // is presented as an ASCIIZ string.
 #define NNG_OPT_ZT_NETWORK_NAME "zt:network-name"
 
-// NNG_OPT_ZT_PING_TIME and NNG_OPT_ZT_PING_COUNT are used to send ping
+// NNG_OPT_ZT_PING_TIME and NNG_OPT_ZT_PING_TRIES are used to send ping
 // requests when a connection appears to be idled.  If a logical session
 // has not received traffic from it's peer for ping-time, then a ping packet
 // is sent.  This will be done up to ping-count times.  If no traffic from
@@ -88,7 +88,15 @@
 // NNG_OPT_ZT_PING_COUNT is an integer.)  This ping process can be disabled
 // by setting either ping-time or ping-count to zero.
 #define NNG_OPT_ZT_PING_TIME "zt:ping-time"
-#define NNG_OPT_ZT_PING_COUNT "zt:ping-count"
+#define NNG_OPT_ZT_PING_TRIES "zt:ping-tries"
+
+// NNG_OPT_ZT_CONN_TIME and NNG_OPT_ZT_CONN_TRIES are used to control
+// the interval between connection attempts, and the maximum number of
+// connection attempts to make before assuming that the peer is absent
+// (and returning NNG_ETIMEDOUT).  The NNG_OPT_ZT_CONN_TIME is a duration,
+// the NNG_OPT_ZT_CONN_TRIES is an integer.
+#define NNG_OPT_ZT_CONN_TIME "zt:conn-time"
+#define NNG_OPT_ZT_CONN_TRIES "zt:conn-tries"
 
 // NNG_OPT_ZT_MTU is a read-only size_t and contains the ZeroTier virtual
 // network MTU (i.e. the L2 payload MTU). Messages that are larger than this