S3: 32 KB TCP window, SACK, 1 s RTO, WiFi/lwIP buffers from PSRAM -- the 5760-byte window backed off for 40 s at a time on a weak link

Author: bdbarnett

esp32_partitions/ESP32_GENERIC_S3_SPIRAM_OCT.sdkconfig

@@ -32,3 +32,17 @@ CONFIG_LWIP_MAX_SOCKETS=16
 # DRAM. This makes the offending free abort where the damage is, instead of
 # a later walk aborting where it is not.
 CONFIG_HEAP_POISONING_LIGHT=y
+
+# TCP over a weak link. The port defaults gave lwIP a 5760-byte receive window
+# and a 1.5 s minimum RTO; on the T-Embed at -77 dBm a burst of losses backed the
+# retransmit timer off until chunk data stopped for 40 s at a time (2026-09-16
+# 22:00, ring log). A 32 KB window, selective ACK and a 1 s RTO, with the WiFi
+# and lwIP buffers taken from PSRAM so internal DRAM is not the price.
+CONFIG_LWIP_TCP_WND_DEFAULT=32768
+CONFIG_LWIP_TCP_SND_BUF_DEFAULT=32768
+CONFIG_LWIP_TCP_RECVMBOX_SIZE=16
+CONFIG_LWIP_TCP_RTO_TIME=1000
+CONFIG_LWIP_TCP_SACK_OUT=y
+CONFIG_SPIRAM_TRY_ALLOCATE_WIFI_LWIP=y
+CONFIG_ESP_WIFI_RX_BA_WIN=16
+CONFIG_ESP_WIFI_DYNAMIC_RX_BUFFER_NUM=48