tree 73f8446ef4e95d8aa0085d3059b03d1139d069eb
parent 0a0b5dcb324d3652f55541855426c91f0c9d9536
author Jacob Erlbeck <jerlbeck@sysmocom.de> 1426849362 +0100
committer Jacob Erlbeck <jerlbeck@sysmocom.de> 1427970888 +0200

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