fix(udp): Properly initialize for the first UDP packet received. (#990)

This fixes a problem where UDP tests between systems with significant
clock skew would register large amounts of jitter at the start of the
test.

Fixes #842.  Analysis done by (and solution inspired by) @davidBar-On.

1 files changed, 16 insertions, 0 deletions

diff --git a/src/iperf_udp.c b/src/iperf_udp.c
index 2245825..ab6be5e 100644
--- a/src/iperf_udp.c
+++ b/src/iperf_udp.c
@@ -66,6 +66,7 @@ iperf_udp_recv(struct iperf_stream *sp)
     uint64_t  pcount;
     int       r;
     int       size = sp->settings->blksize;
+    int       first_packet = 0;
     double    transit = 0, d = 0;
     struct iperf_time sent_time, arrival_time, temp_time;
 
@@ -81,9 +82,19 @@ iperf_udp_recv(struct iperf_stream *sp)
 
     /* Only count bytes received while we're in the correct state. */
     if (sp->test->state == TEST_RUNNING) {
+
+	/*
+	 * For jitter computation below, it's important to know if this
+	 * packet is the first packet received.
+	 */
+	if (sp->result->bytes_received == 0) {
+	    first_packet = 1;
+	}
+
 	sp->result->bytes_received += r;
 	sp->result->bytes_received_this_interval += r;
 
+	/* Dig the various counters out of the incoming UDP packet */
 	if (sp->test->udp_counters_64bit) {
 	    memcpy(&sec, sp->buffer, sizeof(sec));
 	    memcpy(&usec, sp->buffer+4, sizeof(usec));
@@ -168,6 +179,11 @@ iperf_udp_recv(struct iperf_stream *sp)
 
 	iperf_time_diff(&arrival_time, &sent_time, &temp_time);
 	transit = iperf_time_in_secs(&temp_time);
+
+	/* Hack to handle the first packet by initializing prev_transit. */
+	if (first_packet)
+	    sp->prev_transit = transit;
+
 	d = transit - sp->prev_transit;
 	if (d < 0)
 	    d = -d;