commit c1278288b5d4a5dbcfcf4ab615cbf75193222762
author Pau Espin Pedrol <pespin@sysmocom.de> Tue Jul 25 17:43:36 2023 +0200
committer Pau Espin Pedrol <pespin@sysmocom.de> Wed Jul 26 17:03:53 2023 +0200
tree 789d9f9fd5d5960ea3eba48109920d1cbfee718e
parent ff33597e4bec2ab97440d3ff860013412e5d62a5

gprs_rlc_ul_window: Mark received BSNs falling out of the V(N)/RBB when V(R) is raised

This is a port of libosmo-gprs.git commit 49535bec37caddcea5718ed4e526c477c71f1b3a applied to osmo-pcu.

Problem this patch is fixing:
The current RLCMAC code is never invalidating the BSNs which have
been received after they are not needed.
This is somehow workaround by the following check when eg BSN wraps
around 127->0 and the BSN=0 is received:
""""
rcv_data_block_acknowledged()
	m_window.m_v_n.is_received(rdbi->bsn)
		/* Offset to the end of the received window */
		offset_v_r = (m_v_r - 1 - bsn) & mod_sns();
		return m_v_n.is_received(bsn) && offset_v_r < ws();
"""

The first m_v_n_is_received() would return true because the BSN has
never been invalidated, which is wrong, but fortunately the extra check
"offset_v_r < ws()" returns > ws() since:
v_r=0, bsn=0: "(0 - 1 - 0) & 127" ----> 0xffff & 0x007f ---> 0x007f > 64

So thanks to the above the BSN is considered not received, but looking
at the V(N) array would say the opposite. Hence, this commit aims at
fixing the V(N) array to contain proper values (marked "invalid") for
BSNs becoming out of the window when it rolls forward.

Explanation of the solution:
The V(N) array contains the status of the previous WS (64 in GPRS) data
blocks. This array is used to construct the RRB signaled to the peer
during PKT UL ACK/NACK messages together with the SSN (start sequence
number), which in our case is mainly V(R), aka one block higher than the
highest received block in the rx window.
Hence, whenever PKT UL ACK/NACK is transmitted, it contains an RRB
ranging [V(R)-1,...V(R)-WS)] mod SNS (SNS=128 in GPRS).

The receive window is basically [V(Q) <= BSN < V(R)] mod SNS, as is of
size 64.
The V(R) is increased whenever a highest new block arrives which is in the
receive window (guaranteeing it will be increased to at most V(Q)+64.

Since we are only announcing state of blocks from V(R)..V(R)-WS, and
blocks received which are before that BSN are dropped since don't fall
inside the rx window, we can securely mark as invalid those blocks
falling behind V(R)-WS whenever we increase V(R).

Related: OS#6102
Change-Id: I5ef4dcb0c5eac07a892114897f9e4f565b1dcc2c