[PATCH] QUIC: fixed rttvar on subsequent RTT samples (ticket #2505)

[Sergey Kandaurov]

# HG changeset patch
# User Sergey Kandaurov <pluknet at nginx.com>
# Date 1686598736 -14400
#      Mon Jun 12 23:38:56 2023 +0400
# Node ID a32905d6fc108962a73b4affdb38eba5cc6fd549
# Parent  262c0178256623c59a54bea81fcf3f92d12d75b6
QUIC: fixed rttvar on subsequent RTT samples (ticket #2505).

Previously, computing rttvar used an updated smoothed_rtt value as per
RFC 9002, section 5.3, which appears to be specified in a wrong order.
A technical errata ID 7539 is reported.

diff --git a/src/event/quic/ngx_event_quic_ack.c b/src/event/quic/ngx_event_quic_ack.c
--- a/src/event/quic/ngx_event_quic_ack.c
+++ b/src/event/quic/ngx_event_quic_ack.c
@@ -207,9 +207,9 @@ ngx_quic_rtt_sample(ngx_connection_t *c,
             adjusted_rtt -= ack_delay;
         }
 
-        qc->avg_rtt += (adjusted_rtt >> 3) - (qc->avg_rtt >> 3);
         rttvar_sample = ngx_abs((ngx_msec_int_t) (qc->avg_rtt - adjusted_rtt));
         qc->rttvar += (rttvar_sample >> 2) - (qc->rttvar >> 2);
+        qc->avg_rtt += (adjusted_rtt >> 3) - (qc->avg_rtt >> 3);
     }
 
     ngx_log_debug4(NGX_LOG_DEBUG_EVENT, c->log, 0,