From 9b3467318ce44327066b763c8f83b0cf7410aabf Mon Sep 17 00:00:00 2001
From: aidan garske <aidan@wolfssl.com>
Date: Wed, 15 Jul 2026 11:20:59 -0700
Subject: [PATCH] Retry the ISO TP flow control wait up to N_Bs

---
 src/wolfio.c     | 14 +++++++++++++-
 wolfssl/wolfio.h |  2 ++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/src/wolfio.c b/src/wolfio.c
index f5b91bc5cee..eefdc7c76a3 100644
--- a/src/wolfio.c
+++ b/src/wolfio.c
@@ -3851,9 +3851,21 @@ static int isotp_send_flow_control(struct isotp_wolfssl_ctx *ctx,
 static int isotp_receive_flow_control(struct isotp_wolfssl_ctx *ctx)
 {
     int ret;
+    int waited;
     enum isotp_frame_type type;
     enum isotp_flow_control flow_control;
-    ret = ctx->recv_fn(&ctx->frame, ctx->arg, ISOTP_DEFAULT_TIMEOUT);
+    /* Wait up to N_Bs for flow control rather than a single 100ms poll: a peer
+     * that is momentarily late is normal, and every other ISO-TP receive here
+     * retries. Treating the first miss as fatal aborts healthy transfers. */
+    waited = 0;
+    do {
+        ret = ctx->recv_fn(&ctx->frame, ctx->arg, ISOTP_DEFAULT_TIMEOUT);
+        if (ret != 0) {
+            break;
+        }
+        waited += ISOTP_DEFAULT_TIMEOUT;
+    } while (waited < ISOTP_FLOW_CONTROL_TIMEOUT);
+
     if (ret == 0) {
         return WOLFSSL_CBIO_ERR_TIMEOUT;
     } else if (ret < 0) {
diff --git a/wolfssl/wolfio.h b/wolfssl/wolfio.h
index 4e5a80d5afb..7098e61c48a 100644
--- a/wolfssl/wolfio.h
+++ b/wolfssl/wolfio.h
@@ -906,6 +906,8 @@ WOLFSSL_API void wolfSSL_SetIOWriteFlags(WOLFSSL* ssl, int flags);
 #endif
 #ifdef WOLFSSL_ISOTP
     #define ISOTP_DEFAULT_TIMEOUT 100
+    /* ISO 15765-2 N_Bs: how long a sender waits for flow control */
+    #define ISOTP_FLOW_CONTROL_TIMEOUT 1000
     #define ISOTP_DEFAULT_WAIT_COUNT 3
     #define ISOTP_FIRST_FRAME_DATA_SIZE 6
     #define ISOTP_SINGLE_FRAME_DATA_SIZE 7