gprs: Honor GSM 04.64 8.4.2 Receipt of unacknowledged information
GSM 04.64 8.4.2 asks to ignore UI frames if the DLCI is not known,
or if the "(V(UR)- 32) <= N(U) < V(UR)". E.g. if we want to have
V(UR) == 511 and this frame is dropped, we would ignore N(U)'s
0 to 510. Calculate the delta.
The code is based on Jonathan Santos's "LLC UI window" fix but the
issue was discovered independly.
diff --git a/openbsc/tests/gprs/gprs_test.ok b/openbsc/tests/gprs/gprs_test.ok
new file mode 100644
index 0000000..39d37c1
--- /dev/null
+++ b/openbsc/tests/gprs/gprs_test.ok
@@ -0,0 +1,16 @@
+Testing gprs_llc_is_retransmit.
+N(U) = 0, V(UR) = 0 => new
+N(U) = 0, V(UR) = 1 => retransmit
+N(U) = 0, V(UR) = 1 => retransmit
+N(U) = 511, V(UR) = 1 => retransmit
+N(U) = 483, V(UR) = 1 => retransmit
+N(U) = 482, V(UR) = 1 => retransmit
+N(U) = 481, V(UR) = 1 => new
+N(U) = 0, V(UR) = 240 => new
+N(U) = 0, V(UR) = 511 => new
+N(U) = 1, V(UR) = 511 => new
+N(U) = 511, V(UR) = 511 => new
+N(U) = 510, V(UR) = 511 => retransmit
+N(U) = 481, V(UR) = 511 => retransmit
+N(U) = 479, V(UR) = 511 => new
+Done.