Change in osmo-pcu[master]: decoding: Fix implicit ACK due to BeginOfWindow=1 on rx EGPRS PKT DL ...

[pespin]

Hello Jenkins Builder, 

I'd like you to reexamine a change. Please visit

    https://gerrit.osmocom.org/c/osmo-pcu/+/21122

to look at the new patch set (#2).

Change subject: decoding: Fix implicit ACK due to BeginOfWindow=1 on rx EGPRS PKT DL ACK/NACK
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decoding: Fix implicit ACK due to BeginOfWindow=1 on rx EGPRS PKT DL ACK/NACK

According to sepcs, if BeginOfWindow=1, then SSN=V(Q)+1 mod 2048.
As a result, all unacked BSN before SSN can be marked as ACKED since
they are implicitly ACKED (because the window advanced).
The calculation, however was wrong. In specs V(A) points to last acked
BSN, but in our implementation, V(A) actually refers to the first
unacked BSN (see how Window object is initialized to v_s=0 and v_a=0).
Hence, spec "V(A) = v_a - 1" in our implementation.
Therefore, the calculation on the amount of blocks to ACK would be:
V(Q) - (v_a - 1) = (SSN - 1) - (v_a - 1)
Which can be simplified to:
SSN - v_a

Example:
PCU sends DL block for BSN=0..20 included, with BSN=20 containing RRBP
to ACK them.
EGPRS MS would send EGPRS PKT DL ACK/NACK with BEginofWindow=1 and
SSN=21 to ACK all of them.
Before this patch, PCU would only mark BSN0..19 as ACKED and BSN20 as
UNACKED, retransmitting BSN20.

Reference: 3GPP TS 44.060 12.3.1 EGPRS Ack/Nack Description
Change-Id: Ic71326fc956491bb7e5e50896a6ba8f5354c20ec
---
M src/decoding.cpp
M tests/tbf/TbfTest.err
2 files changed, 3 insertions(+), 3 deletions(-)


  git pull ssh://gerrit.osmocom.org:29418/osmo-pcu refs/changes/22/21122/2
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Gerrit-Project: osmo-pcu
Gerrit-Branch: master
Gerrit-Change-Id: Ic71326fc956491bb7e5e50896a6ba8f5354c20ec
Gerrit-Change-Number: 21122
Gerrit-PatchSet: 2
Gerrit-Owner: pespin <pespin at sysmocom.de>
Gerrit-Reviewer: Jenkins Builder
Gerrit-MessageType: newpatchset
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