Fix nginx quic initial congestion window

pq-tls-benchmark-framework/emulation-exp/code/install-prereqs-ubuntu.sh

@@ -122,6 +122,7 @@ echo "Openssl seems to be installed correctly"
 
 # apply patches to nginx source
 patch ${ROOT}/nginx-${NGINX_VERSION}/src/event/quic/ngx_event_quic_openssl_compat.c < ${ROOT}/../patches/nginx-${NGINX_VERSION}-patches/ngx_event_quic_openssl_compat.c.diff
+patch ${ROOT}/nginx-${NGINX_VERSION}/src/event/quic/ngx_event_quic.c < ${ROOT}/../patches/nginx-${NGINX_VERSION}-patches/ngx_event_quic.c.diff
 
 # build nginx with dynamically linked custom openssl
 # NOTE it may be that the first time nginx is built it is not linked correctly to the custom openssl, but just do it again, it should work

pq-tls-benchmark-framework/emulation-exp/code/patches/nginx-1.26.1-patches/ngx_event_quic.c.diff

+--- ngx_event_quic.c	2024-05-28 15:28:07.000000000 +0200
++++ ngx_event_quic.c	2025-02-04 23:33:38.990662931 +0100
+@@ -308,8 +308,13 @@
+     qc->streams.client_max_streams_uni = qc->tp.initial_max_streams_uni;
+     qc->streams.client_max_streams_bidi = qc->tp.initial_max_streams_bidi;
+ 
+-    qc->congestion.window = ngx_min(10 * qc->tp.max_udp_payload_size,
+-                                    ngx_max(2 * qc->tp.max_udp_payload_size,
++    // qc->tp.max_udp_payload_size is set via a constant in ngx_quic_init_transport_params() to 65527 which is wrong.
++    // It is not set by inspecting the peer's transport parameters, i think this happens at a later time in ngx_quic_apply_transport_params(),
++    // but there the initial congestion window is not recalculated and can't be since the peer's transport parameters are not available at that time.
++    // So for a fix for this experiment, the max_udp_payload_size is set to 1200 for only the calculation of the initial congestion window.
++    const int PATCHED_MAX_UDP_PAYLOAD_SIZE = 1200;
++    qc->congestion.window = ngx_min(10 * PATCHED_MAX_UDP_PAYLOAD_SIZE,
++                                    ngx_max(2 * PATCHED_MAX_UDP_PAYLOAD_SIZE,
+                                             14720));
+     qc->congestion.ssthresh = (size_t) -1;
+     qc->congestion.recovery_start = ngx_current_msec;