From 072edb36a8309995a8a8be98b71fe147eb6cd79c Mon Sep 17 00:00:00 2001
From: d00f <8052722+DrDoof@users.noreply.github.com>
Date: Fri, 18 Sep 2026 02:39:52 +0200
Subject: [PATCH] httpd: account for the bytes actually sent

uip_mss() is the limit on the next segment, not a record of the last
one: uIP recomputes it from the window the peer advertises in every
incoming segment, before the application is called. The ACK path
subtracted it from slen and added it to o_idx, and the retransmit path
handed it to uip_send(), so whenever the window moved between a segment
going out and its ACK coming back, the file being served lost or
repeated the difference. A window shrinking from 1460 to 600 between the
two repeats 860 bytes under fresh sequence numbers, which a browser
writes into the file; a window opening the same way skips 860.

Remember the length in tx_send() and use it in both places. The
retransmit path then repeats exactly the segment uIP still counts as
outstanding, which is what uip_connr->len makes it put on the wire.
---
 httpd/httpd.c | 34 +++++++++++++++++++---------------
 1 file changed, 19 insertions(+), 15 deletions(-)

diff --git a/httpd/httpd.c b/httpd/httpd.c
index aba33e40..1d5fadf8 100644
--- a/httpd/httpd.c
+++ b/httpd/httpd.c
@@ -37,6 +37,7 @@ __xdata uint8_t outbuf[TCP_OUTBUF_SIZE];
 __xdata uint8_t entry;
 __xdata uint16_t slen;
 __xdata uint16_t o_idx;
+__xdata uint16_t sent_len;
 __xdata uint16_t len_left;
 __xdata uint16_t cont_len;
 __xdata uint32_t cont_addr;
@@ -826,6 +827,13 @@ void handle_post(void)
 }
 
 
+static void tx_send(uint16_t len)
+{
+	sent_len = len;
+	uip_send(outbuf + o_idx, len);
+}
+
+
 void httpd_appcall(void)
 {
 	__xdata struct httpd_state * __xdata s = &(uip_conn->appstate);
@@ -861,9 +869,9 @@ void httpd_appcall(void)
 		}
 	} else if (uip_acked() && s->tstate == TSTATE_TX) {
 		dbg_string("ACK\n");
-		if (slen > uip_mss()) {
-			slen -= uip_mss();
-			o_idx += uip_mss();
+		if (slen > sent_len) {
+			slen -= sent_len;
+			o_idx += sent_len;
 		} else {
 			slen = 0;
 		}
@@ -872,11 +880,11 @@ void httpd_appcall(void)
 
 		if (slen > uip_mss()) {
 			dbg_string("Sending A: "); dbg_short(slen); dbg_char('\n');
-			uip_send(outbuf + o_idx, uip_mss());
+			tx_send(uip_mss());
 			s->tstate = TSTATE_TX;
 		} else if (slen > 0) {
 			dbg_string("Sending B: "); dbg_short(slen); dbg_char('\n');
-			uip_send(outbuf + o_idx, slen);
+			tx_send(slen);
 			s->tstate = TSTATE_TX;
 		} else if (cont_len) {
 			dbg_string("CONT cont_len: "); dbg_short(cont_len);
@@ -887,7 +895,7 @@ void httpd_appcall(void)
 			flash_region.len = slen;
 			flash_read_bulk(outbuf);
 			o_idx = 0;
-			uip_send(outbuf + o_idx, slen);
+			tx_send(slen);
 			cont_len -= slen;
 			cont_addr += slen;
 			s->tstate = TSTATE_TX;
@@ -1045,24 +1053,20 @@ do_send:
 		o_idx = 0;
 		if (slen > uip_mss()) {
 			dbg_string("Sending a: "); dbg_short(slen); dbg_char('\n');
-			uip_send(outbuf + o_idx, uip_mss());
+			tx_send(uip_mss());
 			dbg_string("Sending a done\n");
 		} else {
 			dbg_string("Sending b: "); dbg_short(slen); dbg_char('\n');
-			uip_send(outbuf + o_idx, slen);
+			tx_send(slen);
 			dbg_string("Sending b done\n");
 		}
 		s->tstate = TSTATE_TX;
 	} else if (uip_rexmit()) { // Connection established, need to rexmit?
 		dbg_string("RETRANSMIT requested\n");
-		if (slen > uip_mss()) {
-			dbg_string("Sending C: "); dbg_short(slen); dbg_char('\n');
-			uip_send(outbuf + o_idx, uip_mss());
+		if (sent_len) {
+			dbg_string("Sending C: "); dbg_short(sent_len); dbg_char('\n');
+			tx_send(sent_len);
 			dbg_string("Sending C done\n");
-		} else if (slen > 0) {
-			dbg_string("Sending D: "); dbg_short(slen); dbg_char('\n');
-			uip_send(outbuf + o_idx, slen);
-			dbg_string("Sending D done\n");
 		}
 		s->tstate = TSTATE_TX;
 		uip_len = 0;