tbf: Use a hysteresis when discarding DL LLC frames

Currently single LLC blocks are discarded when the PDU lifetime
expires. If an IP packet has been fragmented either on the IP or on
the LLC layer and is therefore distributed over several LLC frames,
the kept fragments are transmitted and then discarded by the MS
because of the missing PDU. This can cause massive IP packet loss
when there are many fragmented packets (e.g. when trying 'ping
-s1800' or if the GGSN chops downlink IP packets into several SNDCP
packets).

On the other hand, discarding too many packets might disturb the
congestion handling of TCP. Dropping plain TCP ACKs might also hinder
flow control and congestion avoidance.

This commit adds a hysteresis algorithm to the LLC discard loop. If
an LLC message's age reaches the high water mark, further message's
with an age above the low water mark are discarded, too. This is
aborted, if a GMM, a non-UI, or a small message is detected. In
these cases, that message is kept.

The following VTY commands are added (pcu config node):

- queue hysteresis <1-65535>   set the difference between high
                               (lifetime) and low watermark in
                               centiseconds
- no queue hysteresis          disable this feature (default)

Since the SGSN will most probably send all fragments of a single
N-PDU without much delay between them, a value slightly above the
average transmission delay jitter between SGSN and PCU is probably a
sensible value to discard all fragments of a single IP packet.

This is an experimental feature that might be replaced by more
advanced means of active queue management in the future.

Sponsored-by: On-Waves ehf

1 files changed, 1 insertions, 0 deletions

diff --git a/src/bts.h b/src/bts.h
index 2b43736e..fae3c5e5 100644
--- a/src/bts.h
+++ b/src/bts.h
@@ -118,6 +118,7 @@ struct gprs_rlcmac_bts {
 	uint8_t initial_cs_dl, initial_cs_ul;
 	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;
 	uint8_t t3142;
 	uint8_t t3169;
 	uint8_t t3191;