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
The LLC dummy command is needed for RLC block stuffing, e.g. when a
TBF should be kept open if no LLC data is available. The RLC block
headers do not support stuffing, only the last block of a TBF can be
used partially. LLC dummy commands are discarded by the receiver
immediately, because the have an invalid FCS checksum.
This commit adds the function put_dummy_frame, which puts a LLC dummy
command into the frame buffer. The requested length is given as an
argument, but the real length might be adjusted according to the
specification (see GSM 44.064, 6.4.2.2).
Sponsored-by: On-Waves ehf
Use a formula like it is used with TCP. This can help to make
decisions about if frames should be dropped or not at the time
we enqueue them.
This code will store two timeval structs in fron the of the
actual data and compute the average at the time of the dequeue.
* Make append_data, remaining_space and fits_in_current.. work
on m_length and not the index. This ways things can't overflow.
* The current API consumer was moving the m_index so it should have
worked okay.
This does not mean that they have been successfully transferred
to the SGSN/MS but at least that they have reached a certain point
in the message flow.
Jacob Erlbeck
Daniel Willmann