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hnbgw: Avoid allocating SCCP conn id >0x00fffffe

This breaks use of M3UA/SCCP after 2**24 connection IDs have been
allocated.

Related: SYS#6211
Change-Id: I03bad960f65fbff6e467def5bba60fefb328f962
---
M src/osmo-hnbgw/context_map.c
1 file changed, 14 insertions(+), 1 deletion(-)

git pull ssh://gerrit.osmocom.org:29418/osmo-hnbgw refs/changes/83/30283/1

diff --git a/src/osmo-hnbgw/context_map.c b/src/osmo-hnbgw/context_map.c
index c069651..81f2d11 100644
--- a/src/osmo-hnbgw/context_map.c
+++ b/src/osmo-hnbgw/context_map.c
@@ -58,8 +58,21 @@
 	uint32_t i;
 	uint32_t id;
 
-	for (i = 0; i < 0xffffffff; i++) {
+	/* SUA: RFC3868 sec 3.10.4:
+	 *	The source reference number is a 4 octet long integer.
+	 *	This isallocated by the source SUA instance.
+	 * M3UA/SCCP: ITU-T Q.713 sec 3.3:
+	 *	The "source local reference" parameter field is a three-octet field containing a reference number
+	 *	which is generated and used by the local node to identify the connection section after the connection
+	 *	section is set up.
+	 *	The coding "all ones" is reserved for future use.
+	 * Hence, let's simply use 24 bit ids to fit all link types (excluding 0x00ffffff).
+	 */
+
+	for (i = 0; i < 0x00ffffff; i++) {
 		id = cn->next_conn_id++;
+		if (cn->next_conn_id == 0x00ffffff)
+			cn->next_conn_id = 0;
 		if (!cn_id_in_use(cn, id)) {
 			*id_out = id;
 			return 1;

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