tbf: Force ACK after the last DL LCC frame has been received

If the protocol layers above LLC (e.g. TCP) need an acknowledgement
to continue, it can take up to 400ms (single TS) until the MS is
polled for Ack/Nack which it can use to request an uplink TBF
quickly. The 400ms result from requesting an DL Ack/Nack every 20 RLC
blocks until all pending LLC frames have been sent.

Especially TCP's slow start mechanism can lead to a high delay at the
start of the connection, since the sender will eventually stop after
having sent the first packets (up to 4 (RFC2581) or 10 (RFC6928)).

This commit modifies append_data() to (re-)start
a timer every time it handles an LLC packet and to request an
Ack/Nack every time it expires. So if the server ceases to send IP
packets, the MS is polled in the assumption, that the server is
waiting for an ACK.

The following VTY commands are added (pcu node):

 - queue idle-ack-delay <1-65535>  timeout in centiseconds
 - no queue idle-ack-delay         disable this feature (default)

A sensible value is 10 (100ms) that at gave promising results when
testing locally.

Sponsored-by: On-Waves ehf

1 files changed, 5 insertions, 0 deletions

diff --git a/src/tbf.h b/src/tbf.h
index 17ba980e..e835ab6f 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -311,6 +311,8 @@ inline time_t gprs_rlcmac_tbf::created_ts() const
 }
 
 struct gprs_rlcmac_dl_tbf : public gprs_rlcmac_tbf {
+	void cleanup();
+
 	/* dispatch Unitdata.DL messages */
 	static int handle(struct gprs_rlcmac_bts *bts,
 		const uint32_t tlli, const char *imsi, const uint8_t ms_class,
@@ -365,6 +367,9 @@ protected:
 	int maybe_start_new_window();
 	bool dl_window_stalled() const;
 	void reuse_tbf(const uint8_t *data, const uint16_t len);
+	void start_llc_timer();
+
+	struct osmo_timer_list m_llc_timer;
 };
 
 struct gprs_rlcmac_ul_tbf : public gprs_rlcmac_tbf {