From 084f0287a3f3c277a75424fb1e672a2c06335899 Mon Sep 17 00:00:00 2001
From: Benjamin Valentin <benjamin.valentin@ml-pa.com>
Date: Wed, 19 Oct 2022 15:52:40 +0200
Subject: [PATCH] nanocoap_sock: support handling empty ACKs with separate
 response

---
 sys/include/net/coap.h                    |  8 ++++++++
 sys/net/application_layer/nanocoap/sock.c | 11 ++++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/sys/include/net/coap.h b/sys/include/net/coap.h
index 1d31ccd7e9..34eb47b911 100644
--- a/sys/include/net/coap.h
+++ b/sys/include/net/coap.h
@@ -251,6 +251,14 @@ extern "C" {
 #define CONFIG_COAP_RANDOM_FACTOR_1000      (1500)
 #endif
 
+/**
+ * @brief    Timeout in milliseconds for a separate (deferred) response
+ *           sent after an empty ACK.
+ */
+#ifndef CONFIG_COAP_SEPARATE_RESPONSE_TIMEOUT_MS
+#define CONFIG_COAP_SEPARATE_RESPONSE_TIMEOUT_MS    (10 * MS_PER_SEC)
+#endif
+
 /** @brief   Maximum number of retransmissions for a confirmable request */
 #ifndef CONFIG_COAP_MAX_RETRANSMIT
 #define CONFIG_COAP_MAX_RETRANSMIT     (4)
diff --git a/sys/net/application_layer/nanocoap/sock.c b/sys/net/application_layer/nanocoap/sock.c
index 75ad443983..eb81fee1cd 100644
--- a/sys/net/application_layer/nanocoap/sock.c
+++ b/sys/net/application_layer/nanocoap/sock.c
@@ -224,8 +224,17 @@ ssize_t nanocoap_sock_request_cb(nanocoap_sock_t *sock, coap_pkt_t *pkt,
             case COAP_TYPE_CON:
                 _send_ack(sock, pkt);
                 /* fall-through */
-            case COAP_TYPE_NON:
             case COAP_TYPE_ACK:
+                if (cb && coap_get_code(pkt) == COAP_CODE_EMPTY) {
+                    /* empty ACK, wait for separate response */
+                    state = STATE_RESPONSE_RCVD;
+                    deadline = _deadline_from_interval(CONFIG_COAP_SEPARATE_RESPONSE_TIMEOUT_MS
+                                                     * US_PER_MS);
+                    DEBUG("nanocoap: wait for separate response\n");
+                    continue;
+                }
+                /* fall-through */
+            case COAP_TYPE_NON:
                 /* call user callback */
                 if (cb) {
                     res = cb(arg, pkt);