tbf: Implement delayed release of a downlink TBF

Currently a DL TBF is immediately closed, when the LLC queue is
drained. This will lead to a new DL assignment if data is received
afterwards. In addition, it is not possible to keep the PACCH open
to poll the MS for UL establishment requests there.

GSM 44.060, 9.3.1a suggests to delay the release of an inactive TBF
for some time (max 5s).

This commit mainly changes create_new_bsn() to send LLC dummy
commands as filler if no LLC data is available until keep_open()
returns false. The keep_open() functions returns true unless a
configurable time has passed after the LLC data store drained. By
default, that time is not set which causes keep_open() to always
return false, so that delayed release is effectively disabled.

The following VTY commands are added:
  - dl-tbf-idle-time <1-5000>    to set the delay in ms
  - no dl-tbf-idle-time          to disable delayed release

Ticket: #556
Sponsored-by: On-Waves ehf

1 files changed, 41 insertions, 5 deletions

diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 9baa1cd0..65b164eb 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -27,6 +27,8 @@
 #include <gprs_bssgp_pcu.h>
 #include <decoding.h>
 
+#include "pcu_utils.h"
+
 extern "C" {
 #include <osmocom/core/msgb.h>
 #include <osmocom/core/talloc.h>
@@ -74,6 +76,8 @@ int gprs_rlcmac_dl_tbf::append_data(const uint8_t ms_class,
 	} else if (!have_data()) {
 		m_llc.put_frame(data, len);
 		bts->llc_frame_sched();
+		/* it is no longer drained */
+		m_last_dl_drained_fn = -1;
 		tbf_update_ms_class(this, ms_class);
 	} else {
 		/* the TBF exists, so we must write it in the queue
@@ -354,14 +358,19 @@ struct msgb *gprs_rlcmac_dl_tbf::create_new_bsn(const uint32_t fn, const uint8_t
 			 * space-1 octets */
 			m_llc.put_dummy_frame(space - 1);
 
+			/* The data just drained, store the current fn */
+			if (m_last_dl_drained_fn < 0)
+				m_last_dl_drained_fn = fn;
+
 			/* It is not clear, when the next real data will
 			 * arrive, so request a DL ack/nack now */
 			request_dl_ack();
 
 			LOGP(DRLCMACDL, LOGL_DEBUG,
 				"-- Empty chunk, "
-				"added LLC dummy command of size %d\n",
-				m_llc.frame_length());
+				"added LLC dummy command of size %d, "
+				"drained_since=%d\n",
+				m_llc.frame_length(), frames_since_last_drain(fn));
 		}
 
 		chunk = m_llc.chunk_size();
@@ -380,7 +389,8 @@ struct msgb *gprs_rlcmac_dl_tbf::create_new_bsn(const uint32_t fn, const uint8_t
 			break;
 		}
 		/* if FINAL chunk would fit precisely in space left */
-		if (chunk == space && llist_empty(&m_llc.queue)) {
+		if (chunk == space && llist_empty(&m_llc.queue) && !keep_open(fn))
+		{
 			LOGP(DRLCMACDL, LOGL_DEBUG, "-- Chunk with length %d "
 				"would exactly fit into space (%d): because "
 				"this is a final block, we don't add length "
@@ -424,6 +434,7 @@ struct msgb *gprs_rlcmac_dl_tbf::create_new_bsn(const uint32_t fn, const uint8_t
 			/* return data block as message */
 			break;
 		}
+
 		LOGP(DRLCMACDL, LOGL_DEBUG, "-- Chunk with length %d is less "
 			"than remaining space (%d): add length header to "
 			"to delimit LLC frame\n", chunk, space);
@@ -456,17 +467,19 @@ struct msgb *gprs_rlcmac_dl_tbf::create_new_bsn(const uint32_t fn, const uint8_t
 			m_llc.put_frame(msg->data, msg->len);
 			bts->llc_frame_sched();
 			msgb_free(msg);
+			m_last_dl_drained_fn = -1;
 		}
 		/* if we have more data and we have space left */
-		if (space > 0 && m_llc.frame_length()) {
+		if (space > 0 && (m_llc.frame_length() || keep_open(fn))) {
 			li->m = 1; /* we indicate more frames to follow */
 			continue;
 		}
 		/* if we don't have more LLC frames */
-		if (!m_llc.frame_length()) {
+		if (!m_llc.frame_length() && !keep_open(fn)) {
 			LOGP(DRLCMACDL, LOGL_DEBUG, "-- Final block, so we "
 				"done.\n");
 			li->e = 1; /* we cannot extend */
+
 			rh->fbi = 1; /* we indicate final block */
 			request_dl_ack();
 			set_state(GPRS_RLCMAC_FINISHED);
@@ -771,3 +784,26 @@ int gprs_rlcmac_dl_tbf::frames_since_last_poll(unsigned fn) const
 		return wrapped - 2715648;
 }
 
+int gprs_rlcmac_dl_tbf::frames_since_last_drain(unsigned fn) const
+{
+	unsigned wrapped;
+	if (m_last_dl_drained_fn < 0)
+		return -1;
+
+	wrapped = (fn + 2715648 - m_last_dl_drained_fn) % 2715648;
+	if (wrapped < 2715648/2)
+		return wrapped;
+	else
+		return wrapped - 2715648;
+}
+
+bool gprs_rlcmac_dl_tbf::keep_open(unsigned fn) const
+{
+	int keep_time_frames;
+
+	if (bts_data()->dl_tbf_idle_msec <= 0)
+		return false;
+
+	keep_time_frames = msecs_to_frames(bts_data()->dl_tbf_idle_msec);
+	return frames_since_last_drain(fn) <= keep_time_frames;
+}