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, 1 insertions, 0 deletions

diff --git a/src/bts.h b/src/bts.h
index fae3c5e5..4b7f7e86 100644
--- a/src/bts.h
+++ b/src/bts.h
@@ -119,6 +119,7 @@ struct gprs_rlcmac_bts {
 	uint8_t force_cs;	/* 0=use from BTS 1=use from VTY */
 	uint16_t force_llc_lifetime; /* overrides lifetime from SGSN */
 	uint32_t llc_discard_csec;
+	uint32_t llc_idle_ack_csec;
 	uint8_t t3142;
 	uint8_t t3169;
 	uint8_t t3191;