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GPRS LLC: Add non-standard method of sequence number recovery

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In some situations (like MS reboot without prior DETACH or SGSN reboot
without prior MS detach), the LLC sequence numbers for UI mode could
be different on both sides.

The LLC spec unfortunately doesn't permit us to send something like a
FRMR in this case, but instructs us to silently discard the frame.  At
that time the remote LLC entity will re-transmit the frame with the same
seqeunce number over and over again, which we will drop again and again.

The mthod used now will keep track of the last received UI sequence
number.  If that number is retransmitted for three times in a row, then
we accept this sequence number and recover from that point on.
This commit is contained in:
Harald Welte
2013-06-21 14:06:18 +02:00
parent 22ce59826a
commit abadd54346
2 changed files with 21 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

4
openbsc/include/openbsc/gprs_llc.h
View File

@@ -126,6 +126,10 @@ struct gprs_llc_lle {
uint16_t vu_send;
uint16_t vu_recv;
/* non-standard LLC state */
uint16_t vu_recv_last;
uint16_t vu_recv_duplicates;
/* Overflow Counter for ABM */
uint32_t oc_i_send;
uint32_t oc_i_recv;

18
openbsc/src/gprs/gprs_llc.c
View File

@@ -547,7 +547,23 @@ static int gprs_llc_hdr_rx(struct gprs_llc_hdr_parsed *gph,
"TLLI=%08x dropping UI, N(U=%d) not in window V(URV(UR:%d).\n",
lle->llme ? lle->llme->tlli : -1,
gph->seq_tx, lle->vu_recv);
return -EIO;
/* HACK: non-standard recovery handling. If remote LLE
* is re-transmitting the same sequence number for
* threee times, don't discard the frame but pass it on
* and 'learn' the new sequence number */
if (gph->seq_tx != lle->vu_recv_last) {
lle->vu_recv_last = gph->seq_tx;
lle->vu_recv_duplicates = 0;
} else {
lle->vu_recv_duplicates++;
if (lle->vu_recv_duplicates < 3)
return -EIO;
LOGP(DLLC, LOGL_NOTICE, "TLLI=%08x recovering "
"N(U=%d) after receiving %u duplicates\n",
lle->llme ? lle->llme->tlli : -1,
gph->seq_tx, lle->vu_recv_duplicates);
}
}
/* Increment the sequence number that we expect in the next frame */
lle->vu_recv = (gph->seq_tx + 1) % 512;