From 95f8ae04bd6ece0989c754e16019dcce397aacbe Mon Sep 17 00:00:00 2001
From: Alexander Couzens <lynxis@fe80.eu>
Date: Fri, 11 Nov 2016 16:17:54 +0100
Subject: [PATCH] lapd_switch_altc: allow to switch between timeslot and
 superchannel

a special crafted packet can switch between the modes. The packet look like
0x23004200 as magic and 1byte as value (0 = timeslot, 1 = superchannel)
---
 siu/l2tp/l2tpd_lapd.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/siu/l2tp/l2tpd_lapd.c b/siu/l2tp/l2tpd_lapd.c
index f540b09..1078018 100644
--- a/siu/l2tp/l2tpd_lapd.c
+++ b/siu/l2tp/l2tpd_lapd.c
@@ -205,6 +205,38 @@ int lapd_ehdlc_to_lapd(struct l2tpd_instance *l2i, struct l2tpd_session *l2s, st
 	return 0;
 }
 
+/*!
+ * \brief lapd_switch_altc try to parse the msg, if valid it change the ALTC type to the requested
+ * \param l2i
+ * \param msg
+ * \return 1 if valid and parsed, 0 if should passthrough to the siu
+ */
+int lapd_switch_altc(struct l2tpd_instance *l2i, struct msgb *msg)
+{
+	struct l2tpd_session *l2s = msg->dst;
+
+	/* magic 0x23004200 (4 byte) + value (1 byte) */
+	if (msgb_length(msg) != (4 + 1))
+		return 0;
+
+	/* skip lapd header */
+	if (osmo_load32be(msgb_data(msg)) != 0x23004200)
+		return 0;
+
+	/* pull data pointer to next object */
+	msgb_pull(msg, 4);
+
+	if (msgb_pull_u8(msg)) {
+		LOGP(DL2TP, LOGL_INFO, "ALTCRQ -> SuperChannel requested\n");
+		l2tp_tx_altc_rq_superchannel(l2s->connection);
+	} else {
+		LOGP(DL2TP, LOGL_INFO, "ALTCRQ -> TimeSlot requested\n");
+		l2tp_tx_altc_rq_timeslot(l2s->connection);
+	}
+
+	return 1;
+}
+
 
 /*!
  * \brief rsl_oml_cb called when data arrived on the unix socket
@@ -243,6 +275,12 @@ int unix_rsl_oml_cb(struct osmo_fd *fd)
 		return 1;
 	}
 
+	/* check if this packet is for us */
+	if (lapd_switch_altc(l2i, msg)) {
+		msgb_free(msg);
+		return 0;
+	}
+
 	rc = lapd_lapd_to_ehdlc(l2i, msg);
 	if (rc) {
 		LOGP(DL2TP, LOGL_NOTICE, "lapd_to_ehlc returned != 0: %d.\n", rc);