diff --git a/examples/sender.py b/examples/sender.py
index 433251186f1a8cd6ddfb1e647460ed1b172f26ad..fc1b98bd74dc652f7b88dab6aefe65261f333b1d 100644
--- a/examples/sender.py
+++ b/examples/sender.py
@@ -9,5 +9,5 @@ s = prrt.PrrtSocket(("0.0.0.0", localport), maximum_payload_size=150)
 s.connect((host, port))
 
 for i in range(10):
-    s.send("Packet {}".format(i).encode("utf8"))
-s.send("Close".encode("utf8"))
+    s.send_sync("Packet {}".format(i).encode("utf8"))
+s.send_sync("Close".encode("utf8"))
diff --git a/prrt/proto/processes/dataTransmitter.c b/prrt/proto/processes/dataTransmitter.c
index f8356954d6bc0a5b52b44999960c7c90602b36e7..957cbb1df70be016db23bfe22a7fbc81c87f903c 100644
--- a/prrt/proto/processes/dataTransmitter.c
+++ b/prrt/proto/processes/dataTransmitter.c
@@ -181,6 +181,8 @@ static bool send_packet(PrrtSocket *sock_ptr, PrrtPacket *packet) {
     return false;
 }
 
+void block_timeout(void* arg);
+
 typedef struct timer_arg {
     PrrtSocket* socket;
     PrrtBlock* block;
@@ -261,9 +263,10 @@ void PrrtDataTransmitter_transmit(PrrtSocket *sock_ptr, PrrtPacket *packet) {
 
             RetransmissionTimerArgs* args = (RetransmissionTimerArgs*) calloc(1, sizeof(RetransmissionTimerArgs));
             args->block = sock_ptr->receiveBlock;
-            sock_ptr->receiveBlock = NULL;
             args->socket = sock_ptr;
             retransmission_round_handler(args);
+
+            sock_ptr->receiveBlock = NULL;
         }
     } else {
         PrrtPacket_destroy(packet);
diff --git a/prrt/proto/socket.c b/prrt/proto/socket.c
index e5e74938a0a8090b255be40a5e40d57a2b3c069a..457af8eda6ab0f3b1e1d6b52ae33ff2add328ad1 100644
--- a/prrt/proto/socket.c
+++ b/prrt/proto/socket.c
@@ -97,6 +97,8 @@ PrrtSocket *PrrtSocket_create(prrtByteCount_t maximum_payload_size, prrtTimedelt
     atomic_store_explicit(&s->closing, false, memory_order_release);
     s->receiver = NULL;
 
+    s->retransmissionTimer = PrrtTimer_create(0);
+
     uint8_t n_cycle[1] = { N_START - K_START };
     s->codingParameters = PrrtCodingConfiguration_create(K_START, N_START, 1, n_cycle);
     s->coder = PrrtCoder_create(s->codingParameters);
diff --git a/prrt/proto/socket.h b/prrt/proto/socket.h
index 7d86d754eae7e1ae3b2a552a9a13e28aeb8a3414..87a297e3e09ae4d7255dcf7f2edfa5faf67c790a 100644
--- a/prrt/proto/socket.h
+++ b/prrt/proto/socket.h
@@ -100,7 +100,6 @@ typedef struct prrtSocket {
 
     atomic_bool isThreadPinning;
     prrtByteCount_t maximum_payload_size;
-
     PrrtTimer *retransmissionTimer;
 } PrrtSocket;
 
diff --git a/prrt/proto/timer.c b/prrt/proto/timer.c
index 2dc921ced5c8fa7bc52ca1aec652c04b62f12ac4..1ce9a491ab129ef9c95db249495335337b83cf61 100644
--- a/prrt/proto/timer.c
+++ b/prrt/proto/timer.c
@@ -133,7 +133,7 @@ struct prrtTimer {
 	atomic_int           wait;
 	_Atomic(TimerNode *) new;
 	_Atomic(TimerNode *) old;
-	_Atomic(TimerNode *) del;
+	_Atomic(TimerNode *) cur;
 	TimerDateUDiff_t     precision;
 	TimerDateUDiff_t     lcp;
 	TimerDateUDiff_t     osp;
@@ -143,7 +143,7 @@ struct prrtTimer {
 
 typedef struct prrtTimer Timer;
 
-static bool timer_date_is_due(Timer *self, TimerDate *when, const TimerDate *now)
+static bool timer_date_is_due(Timer *self, const TimerDate *when, const TimerDate *now)
 {
 	// TODO: use self->precision to check whether now and *when are similar enough
 	(void) self;
@@ -208,6 +208,8 @@ static void *timer_worker_loop(void *arg)
 		TimerNode *task = atomic_load(&self->new);
 		assert(task != NULL && "task list contains NULL node");
 
+		atomic_store(&self->cur, task);
+
 		if (timer_date_is_inf(&task->date)) {
 			if (!atomic_load_explicit(&self->alive, memory_order_acquire)) {
 				if (task == atomic_load(&self->new))
@@ -260,11 +262,9 @@ static void *timer_worker_loop(void *arg)
 			task->done = true;
 		}
 
-		atomic_store(&self->del, task);
 		TimerNode *next = atomic_load(&task->next);
 		TimerNode *temp = task;
 		atomic_compare_exchange_strong(&self->new, &temp, next);
-		atomic_store(&self->del, NULL);
 
 		if (slept && !learned) {
 			clock_gettime(CLOCK_REALTIME, &td1);
@@ -306,7 +306,7 @@ PrrtTimer *PrrtTimer_create(unsigned int core)
 	atomic_store_explicit(&self->wait,  0,    memory_order_relaxed);
 	atomic_store_explicit(&self->new,   node, memory_order_relaxed);
 	atomic_store_explicit(&self->old,   node, memory_order_relaxed);
-	atomic_store_explicit(&self->del,   NULL, memory_order_relaxed);
+	atomic_store_explicit(&self->cur,   node, memory_order_relaxed);
 
 	self->precision = timer_measure_clock_precision();
 	for (int i = 0; i < OSP_WINDOW_SIZE; i++)
@@ -364,7 +364,7 @@ PrrtTimer *PrrtTimer_create(unsigned int core)
 
 int PrrtTimer_submit(PrrtTimer *self, const TimerDate *when, const PrrtTimerTask *what)
 {
-	TimerNode *iter, *stop, *next;
+	TimerNode *iter, *stop, *next, *hold;
 	TimerNode *node = malloc(sizeof(TimerNode));
 	if (!node)
 		return -1;
@@ -380,13 +380,14 @@ int PrrtTimer_submit(PrrtTimer *self, const TimerDate *when, const PrrtTimerTask
 
 	iter = atomic_load(&self->old);
 	stop = atomic_load(&self->new);
-	while (iter != stop) {
+	hold = atomic_load(&self->cur);
+	while (iter != stop && iter != hold) {
 		next = iter->next;
 		assert(iter->done && "cleanup task that is not marked as done");
 		free(iter);
 		iter = next;
 	}
-	atomic_store(&self->old, stop);
+	atomic_store(&self->old, iter);
 
 	_Atomic(TimerNode *) *addr = &self->old;
 	while (1) {
@@ -407,9 +408,8 @@ int PrrtTimer_submit(PrrtTimer *self, const TimerDate *when, const PrrtTimerTask
 	atomic_store(&node->next, iter);
 	atomic_store(addr, node);
 
-	TimerNode *del = atomic_load(&self->del);
 	TimerNode *tail = atomic_load(&self->new);
-	if (del == tail || timer_date_is_lt(&node->date, &tail->date)) {
+	if (timer_date_is_lt(&node->date, &tail->date) || (addr == &tail->next && atomic_load(&tail->done))) {
 		atomic_store(&self->new, node);
 		timer_wake_worker(self, false);
 	}
@@ -417,6 +417,50 @@ int PrrtTimer_submit(PrrtTimer *self, const TimerDate *when, const PrrtTimerTask
 	return -1;
 }
 
+static void wake_sleeping_thread(void *arg)
+{
+	atomic_int *ip = (atomic_int *) arg;
+	atomic_store_explicit(ip, 1, memory_order_release);
+	futex((int *) ip, FUTEX_WAKE|FUTEX_PRIVATE_FLAG, 1, NULL, NULL, 0);
+}
+
+void PrrtTimer_sleep_until(PrrtTimer *self, const TimerDate *end)
+{
+	atomic_int cond;
+	atomic_store_explicit(&cond, 0, memory_order_release);
+
+	TimerDate now;
+	TimerDate care = *end;
+	PrrtTimerTask what;
+	what.fun = wake_sleeping_thread;
+	what.arg = &cond;
+	timer_date_sub(&care, 2*self->osp);
+	clock_gettime(CLOCK_REALTIME, &now);
+	if (!timer_date_is_due(self, &care, &now)) {
+		PrrtTimer_submit(self, &care, &what);
+		while (!atomic_load(&cond)) {
+			clock_gettime(CLOCK_REALTIME, &now);
+			if (!timer_date_is_due(self, &care, &now))
+				//futex(&cond, FUTEX_WAIT_BITSET|FUTEX_PRIVATE_FLAG|FUTEX_CLOCK_REALTIME, 1, care, NULL, FUTEX_BITSET_MATCH_ANY);
+				futex(&cond, FUTEX_WAIT|FUTEX_PRIVATE_FLAG, 0, NULL, NULL, 0);
+		}
+	}
+
+	while (1) {
+		clock_gettime(CLOCK_REALTIME, &now);
+		if (timer_date_is_due(self, end, &now))
+			break;
+	}
+}
+
+void PrrtTimer_sleep_nanos(PrrtTimer *self, TimerDateUDiff_t nanos)
+{
+	TimerDate when;
+	clock_gettime(CLOCK_REALTIME, &when);
+	timer_date_add(&when, nanos);
+	PrrtTimer_sleep_until(self, &when);
+}
+
 void PrrtTimer_end(PrrtTimer *self)
 {
 	atomic_store_explicit(&self->alive, false, memory_order_release);
diff --git a/prrt/proto/timer.h b/prrt/proto/timer.h
index 232e05ebe5dbdb6a8337a5b86f08db0fd0854b12..0c0570cfd25b594bad0cf2489167df704d2829c9 100644
--- a/prrt/proto/timer.h
+++ b/prrt/proto/timer.h
@@ -10,6 +10,7 @@ typedef void *prrtTimerTaskArg;
 typedef void (*prrtTimerTaskFun)(prrtTimerTaskArg);
 
 typedef struct timespec prrtTimerDate;
+typedef unsigned long long TimerDateUDiff_t;
 
 typedef struct prrtTimerTask {
 	prrtTimerTaskFun fun;
@@ -24,4 +25,7 @@ int PrrtTimer_submit(PrrtTimer *timer, const prrtTimerDate *when, const PrrtTime
 
 void PrrtTimer_end(PrrtTimer *timer);
 
-#endif // PRRT_TIMER_H
\ No newline at end of file
+void PrrtTimer_sleep_until(PrrtTimer *self, const prrtTimerDate *end);
+void PrrtTimer_sleep_nanos(PrrtTimer *self, TimerDateUDiff_t nanos);
+
+#endif // PRRT_TIMER_H
diff --git a/prrt/util/common.c b/prrt/util/common.c
index 37b2397eeaf1dc4bcc6b0ed61c6ac28068f4ec6f..9035ffe90176616b3290fda4f09375e363677760 100644
--- a/prrt/util/common.c
+++ b/prrt/util/common.c
@@ -43,3 +43,17 @@ int pin_thread_to_core(pthread_attr_t *ap, int core)
     return pthread_attr_setaffinity_np(ap, sizeof(cpu_set_t), &cpuset);
 }
 
+struct timespec abstime_from_now(uint32_t wait_time) {
+    struct timespec now;
+    clock_gettime(CLOCK_REALTIME, &now);
+
+    struct timespec deadline;
+    uint32_t diff_s = wait_time / 1000000;
+    uint32_t diff_ns = (wait_time % 1000000) * 1000;
+    __syscall_slong_t sum = diff_ns + now.tv_nsec;
+    __syscall_slong_t carry = (sum) / 1000000000;
+    __syscall_slong_t rest = (sum) % 1000000000;
+    deadline.tv_sec = diff_s + now.tv_sec + carry;
+    deadline.tv_nsec = rest;
+    return deadline;
+}
diff --git a/prrt/util/common.h b/prrt/util/common.h
index 034cd44a89d5b841a84a63235e6afbb2ceb9f22f..1ed95272ffd4cdd5e423157f7d24d6b6d35f0c71 100644
--- a/prrt/util/common.h
+++ b/prrt/util/common.h
@@ -9,6 +9,7 @@
 int print_buffer(const char *buf, const int length);
 void print_gf(const gf *start, const int len);
 int pin_thread_to_core(pthread_attr_t *ap, int core);
+struct timespec abstime_from_now(uint32_t wait_time);
 
 #define PERROR(fmt, args...) \
 printf("PRRT ERROR (" __FILE__ ":%d)\n" fmt, __LINE__, ## args);
diff --git a/prrt/util/time.c b/prrt/util/time.c
index ca191e8145dca9e5a1ff9266b5a6841cb8161378..e647f14a4177dfa7dd4c376be4816355b6244ad3 100644
--- a/prrt/util/time.c
+++ b/prrt/util/time.c
@@ -15,21 +15,6 @@ struct timespec abstime_now() {
     return now;
 }
 
-struct timespec abstime_from_now(uint32_t wait_time) {
-    struct timespec now;
-    clock_gettime(CLOCK_REALTIME, &now);
-
-    struct timespec deadline;
-    uint32_t diff_s = wait_time / 1000000;
-    uint32_t diff_ns = (wait_time % 1000000) * 1000;
-    __syscall_slong_t sum = diff_ns + now.tv_nsec;
-    __syscall_slong_t carry = (sum) / 1000000000;
-    __syscall_slong_t rest = (sum) % 1000000000;
-    deadline.tv_sec = diff_s + now.tv_sec + carry;
-    deadline.tv_nsec = rest;
-    return deadline;
-}
-
 // < 0: a less than b (b is in the future)
 // > 0: a greater b (b is in the past)
 // == 0: a equal b