diff --git a/CHANGELOG b/CHANGELOG
index 04ba03f..09e3cb6 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,8 @@
+2018-05-21
+
+    - [API Change] Add optional callback to call when handshake is done
+    - [API Change, BUGFIX] After send failure, wait until transport available
+
 2018-05-18
 
     - [API] Expose useful lsquic_ver2str[] in lsquic.h
diff --git a/EXAMPLES.txt b/EXAMPLES.txt
index 9e1f7a6..143955d 100644
--- a/EXAMPLES.txt
+++ b/EXAMPLES.txt
@@ -97,6 +97,13 @@ LSQUIC_CUBIC_SAMPLING_RATE
 
     Only available in debug builds.
 
+LSQUIC_RANDOM_SEND_FAILURE
+
+    Frequency with which sending of packets fails: one out of this many
+    times on average.
+
+    Only available when compiled with -DLSQUIC_RANDOM_SEND_FAILURE=1
+
 Control Network-Related Stuff
 -----------------------------
 
@@ -141,3 +148,9 @@ More Compilation Options
 
     Add relatively expensive run-time sanity checks
 
+-DLSQUIC_RANDOM_SEND_FAILURE=1
+
+    Simulate failure to send packets to test send resumption logic.  When
+    this flag is specified, sending of packets will randomly fail, about
+    one out of every 10 attempts.  Set environment variable
+    LSQUIC_RANDOM_SEND_FAILURE to change this frequency.
diff --git a/include/lsquic.h b/include/lsquic.h
index cf74361..4ff4847 100644
--- a/include/lsquic.h
+++ b/include/lsquic.h
@@ -143,6 +143,14 @@ struct lsquic_stream_if {
     void (*on_read)     (lsquic_stream_t *s, lsquic_stream_ctx_t *h);
     void (*on_write)    (lsquic_stream_t *s, lsquic_stream_ctx_t *h);
     void (*on_close)    (lsquic_stream_t *s, lsquic_stream_ctx_t *h);
+    /**
+     * When handshake is completed, this callback is called.  `ok' is set
+     * to true if handshake was successful; otherwise, `ok' is set to
+     * false.
+     *
+     * This callback is optional.
+     */
+    void (*on_hsk_done)(lsquic_conn_t *c, int ok);
 };
 
 /**
@@ -428,7 +436,9 @@ struct lsquic_out_spec
 
 /**
  * Returns number of packets successfully sent out or -1 on error.  -1 should
- * only be returned if no packets were sent out.
+ * only be returned if no packets were sent out.  If -1 is returned,
+ * no packets will be attempted to be sent out until
+ * @ref lsquic_engine_send_unsent_packets() is called.
  */
 typedef int (*lsquic_packets_out_f)(
     void                          *packets_out_ctx,
@@ -519,6 +529,10 @@ lsquic_engine_has_unsent_packets (lsquic_engine_t *engine);
 /**
  * Send out as many unsent packets as possibe: until we are out of unsent
  * packets or until @ref ea_packets_out() fails.
+ *
+ * If @ref ea_packets_out() does fail (that is, it returns an error), this
+ * function must be called to signify that sending of packets is possible
+ * again.
  */
 void
 lsquic_engine_send_unsent_packets (lsquic_engine_t *engine);
diff --git a/src/liblsquic/lsquic_engine.c b/src/liblsquic/lsquic_engine.c
index f60d0a4..3ac2ff2 100644
--- a/src/liblsquic/lsquic_engine.c
+++ b/src/liblsquic/lsquic_engine.c
@@ -302,6 +302,7 @@ lsquic_engine_new (unsigned flags,
         return NULL;
     }
     engine->pub.enp_ver_tags_len = tag_buf_len;
+    engine->pub.enp_flags = ENPUB_CAN_SEND;
 
     engine->flags           = flags;
     engine->stream_if       = api->ea_stream_if;
@@ -951,7 +952,9 @@ send_batch (lsquic_engine_t *engine, struct conns_out_iter *conns_iter,
         LSQ_DEBUG("packets out returned %d (out of %u)", n_sent, n_to_send);
     else
     {
+        engine->pub.enp_flags &= ~ENPUB_CAN_SEND;
         LSQ_DEBUG("packets out returned an error: %s", strerror(errno));
+        EV_LOG_GENERIC_EVENT("cannot send packets");
         n_sent = 0;
     }
     if (n_sent > 0)
@@ -1147,6 +1150,12 @@ lsquic_engine_send_unsent_packets (lsquic_engine_t *engine)
 
     STAILQ_INIT(&closed_conns);
     reset_deadline(engine, lsquic_time_now());
+    if (!(engine->pub.enp_flags & ENPUB_CAN_SEND))
+    {
+        LSQ_DEBUG("can send again");
+        EV_LOG_GENERIC_EVENT("can send again");
+        engine->pub.enp_flags |= ENPUB_CAN_SEND;
+    }
 
     send_packets_out(engine, &closed_conns);
 
@@ -1199,7 +1208,8 @@ process_connections (lsquic_engine_t *engine, conn_iter_f next_conn,
             STAILQ_INSERT_TAIL(&ticked_conns, conn, cn_next_ticked);
     }
 
-    if (lsquic_engine_has_unsent_packets(engine))
+    if ((engine->pub.enp_flags & ENPUB_CAN_SEND)
+                        && lsquic_engine_has_unsent_packets(engine))
         send_packets_out(engine, &closed_conns);
 
     while ((conn = STAILQ_FIRST(&closed_conns))) {
diff --git a/src/liblsquic/lsquic_engine_public.h b/src/liblsquic/lsquic_engine_public.h
index 629936a..880e8eb 100644
--- a/src/liblsquic/lsquic_engine_public.h
+++ b/src/liblsquic/lsquic_engine_public.h
@@ -21,6 +21,7 @@ struct lsquic_engine_public {
         ENPUB_PROC  = (1 << 0), /* Being processed by one of the user-facing
                                  * functions.
                                  */
+        ENPUB_CAN_SEND = (1 << 1),
     }                               enp_flags;
     unsigned char                   enp_ver_tags_buf[ sizeof(lsquic_ver_tag_t) * N_LSQVER ];
     unsigned                        enp_ver_tags_len;
diff --git a/src/liblsquic/lsquic_full_conn.c b/src/liblsquic/lsquic_full_conn.c
index ed0afb3..b277a22 100644
--- a/src/liblsquic/lsquic_full_conn.c
+++ b/src/liblsquic/lsquic_full_conn.c
@@ -3033,6 +3033,8 @@ full_conn_ci_handshake_ok (lsquic_conn_t *lconn)
         lconn->cn_flags |= LSCONN_HANDSHAKE_DONE;
     else
         conn->fc_flags |= FC_ERROR;
+    if (conn->fc_stream_ifs[STREAM_IF_STD].stream_if->on_hsk_done)
+        conn->fc_stream_ifs[STREAM_IF_STD].stream_if->on_hsk_done(lconn, 1);
 }
 
 
@@ -3043,6 +3045,8 @@ full_conn_ci_handshake_failed (lsquic_conn_t *lconn)
     LSQ_DEBUG("handshake failed");
     lsquic_alarmset_unset(&conn->fc_alset, AL_HANDSHAKE);
     conn->fc_flags |= FC_HSK_FAILED;
+    if (conn->fc_stream_ifs[STREAM_IF_STD].stream_if->on_hsk_done)
+        conn->fc_stream_ifs[STREAM_IF_STD].stream_if->on_hsk_done(lconn, 0);
 }
 
 
@@ -3389,7 +3393,8 @@ full_conn_ci_is_tickable (lsquic_conn_t *lconn)
     if (!TAILQ_EMPTY(&conn->fc_pub.service_streams))
         return 1;
 
-    if (lsquic_send_ctl_can_send(&conn->fc_send_ctl)
+    if ((conn->fc_enpub->enp_flags & ENPUB_CAN_SEND)
+        && lsquic_send_ctl_can_send(&conn->fc_send_ctl)
         && (should_generate_ack(conn) ||
             !lsquic_send_ctl_sched_is_blocked(&conn->fc_send_ctl)))
     {
diff --git a/test/http_client.c b/test/http_client.c
index d04e450..3164ad2 100644
--- a/test/http_client.c
+++ b/test/http_client.c
@@ -147,6 +147,13 @@ http_client_on_conn_closed (lsquic_conn_t *conn)
 }
 
 
+static void
+http_client_on_hsk_done (lsquic_conn_t *conn, int ok)
+{
+    LSQ_INFO("handshake %s", ok ? "completed successfully" : "failed");
+}
+
+
 struct lsquic_stream_ctx {
     lsquic_stream_t     *stream;
     struct http_client_ctx   *client_ctx;
@@ -398,6 +405,7 @@ const struct lsquic_stream_if http_client_if = {
     .on_read                = http_client_on_read,
     .on_write               = http_client_on_write,
     .on_close               = http_client_on_close,
+    .on_hsk_done            = http_client_on_hsk_done,
 };
 
 
diff --git a/test/prog.c b/test/prog.c
index 593ffa2..ac815e2 100644
--- a/test/prog.c
+++ b/test/prog.c
@@ -419,3 +419,24 @@ prog_is_stopped (void)
 {
     return prog_stopped != 0;
 }
+
+
+static void
+send_unsent (evutil_socket_t fd, short what, void *arg)
+{
+    struct prog *const prog = arg;
+    assert(prog->prog_send);
+    event_del(prog->prog_send);
+    event_free(prog->prog_send);
+    prog->prog_send = NULL;
+    lsquic_engine_send_unsent_packets(prog->prog_engine);
+}
+
+
+void
+prog_sport_cant_send (struct prog *prog, int fd)
+{
+    assert(!prog->prog_send);
+    prog->prog_send = event_new(prog->prog_eb, fd, EV_WRITE, send_unsent, prog);
+    event_add(prog->prog_send, NULL);
+}
diff --git a/test/prog.h b/test/prog.h
index b58010b..89b2a69 100644
--- a/test/prog.h
+++ b/test/prog.h
@@ -23,6 +23,7 @@ struct prog
     int                             prog_version_cleared;
     struct event_base              *prog_eb;
     struct event                   *prog_timer,
+                                   *prog_send,
                                    *prog_usr1;
     struct sport_head              *prog_sports;
     struct lsquic_engine           *prog_engine;
@@ -83,4 +84,7 @@ prog_is_stopped (void);
 void
 prog_process_conns (struct prog *);
 
+void
+prog_sport_cant_send (struct prog *, int fd);
+
 #endif
diff --git a/test/test_common.c b/test/test_common.c
index aaf1641..7870dd8 100644
--- a/test/test_common.c
+++ b/test/test_common.c
@@ -875,6 +875,23 @@ send_packets_one_by_one (const struct lsquic_out_spec *specs, unsigned count)
     if (0 == count)
         return 0;
 
+#if LSQUIC_RANDOM_SEND_FAILURE
+    {
+        const char *freq_str = getenv("LSQUIC_RANDOM_SEND_FAILURE");
+        int freq;
+        if (freq_str)
+            freq = atoi(freq_str);
+        else
+            freq = 10;
+        if (rand() % freq == 0)
+        {
+            assert(count > 0);
+            sport = specs[0].peer_ctx;
+            LSQ_NOTICE("sending \"randomly\" fails");
+            goto random_send_failure;
+        }
+    }
+#endif
 
     for (n = 0; n < count; ++n)
     {
@@ -931,7 +948,13 @@ send_packets_one_by_one (const struct lsquic_out_spec *specs, unsigned count)
     if (n > 0)
         return n;
     else if (s < 0)
+    {
+#if LSQUIC_RANDOM_SEND_FAILURE
+  random_send_failure:
+#endif
+        prog_sport_cant_send(sport->sp_prog, sport->fd);
         return -1;
+    }
     else
         return 0;
 }