gsm0808: Introduce Osmocom extensions to announce Osmux support

IE GSM0808_IE_OSMO_OSMUX_SUPPORT (T, 1 byte) is sent in AoIP appended to
BSSMAP RESET in order to announce the peer that its MGW supports handling
Osmux streams upon call set up.

IE GSM0808_IE_OSMO_OSMUX_CID (TV, T 1 byte & V 1 byte) is sent in AoIP
during call set up:
* MSC->BSC Assignment Request
* BSC->MSC Assignemnt Complete

The 1 byte value contains the local Osmux CID, aka the recvCID aka CID where the
peer sending the Assign Req/Compl will look for Osmux frames on that
call. Hence, the peer receiving this CID value must use it to send Osmux
frames for that call.

As a result, a given call leg BSC<->MSC can have one different Osmux CID
per direction. For example:
* MS => MGW_BSC ==CID 0==> MGW_MSC
* MS <= MGW_BSC <=CID 1=== MGW_MSC

This allows for setups with 256 call legs per BSC on scenarios where NAT
is not a problem, where MSC can have a pool of 256 CID per MGW_BSC (or
remote peer).

Related: OS#2551
Change-Id: I28f83e2e32b9533c99e65ccc1562900ac2aec74e

5 files changed, 29 insertions, 0 deletions

diff --git a/include/osmocom/gsm/gsm0808_utils.h b/include/osmocom/gsm/gsm0808_utils.h
index 3a7beb70..9cfaea68 100644
--- a/include/osmocom/gsm/gsm0808_utils.h
+++ b/include/osmocom/gsm/gsm0808_utils.h
@@ -100,6 +100,7 @@ uint8_t gsm0808_enc_aoip_trasp_addr(struct msgb *msg,
 				    const struct sockaddr_storage *ss);
 int gsm0808_dec_aoip_trasp_addr(struct sockaddr_storage *ss,
 				const uint8_t *elem, uint8_t len);
+int gsm0808_dec_osmux_cid(uint8_t *cid, const uint8_t *elem, uint8_t len);
 
 uint8_t gsm0808_enc_lcls(struct msgb *msg, const struct osmo_lcls *lcls);
 int gsm0808_dec_lcls(struct osmo_lcls *lcls, const struct tlv_parsed *tp);
diff --git a/include/osmocom/gsm/protocol/gsm_08_08.h b/include/osmocom/gsm/protocol/gsm_08_08.h
index aa01ee5c..9806e083 100644
--- a/include/osmocom/gsm/protocol/gsm_08_08.h
+++ b/include/osmocom/gsm/protocol/gsm_08_08.h
@@ -303,6 +303,10 @@ enum GSM0808_IE_CODING {
 	GSM0808_IE_SELECTED_OPERATOR		= 0x98,
 	GSM0808_IE_PS_REGISTERED_OPERATOR	= 0x99,
 	GSM0808_IE_CS_REGISTERED_OPERATOR	= 0x9a,
+
+	/* Osmocom extensions: */
+	GSM0808_IE_OSMO_OSMUX_SUPPORT		= 0xf0,
+	GSM0808_IE_OSMO_OSMUX_CID		= 0xf1,
 };
 
 /* 3GPP TS 48.008 3.2.3 Signalling Field Element Coding */
diff --git a/src/gsm/gsm0808.c b/src/gsm/gsm0808.c
index 3c77c77a..514d7f22 100644
--- a/src/gsm/gsm0808.c
+++ b/src/gsm/gsm0808.c
@@ -1387,6 +1387,10 @@ static const struct tlv_definition bss_att_tlvdef = {
 		[GSM0808_IE_CN_TO_MS_TRANSP_INFO]	= { TLV_TYPE_TLV },
 		[GSM0808_IE_SELECTED_PLMN_ID]		= { TLV_TYPE_FIXED, 3 },
 		[GSM0808_IE_LAST_USED_EUTRAN_PLMN_ID]	= { TLV_TYPE_FIXED, 3 },
+
+		/* Osmocom extensions */
+		[GSM0808_IE_OSMO_OSMUX_SUPPORT]		= { TLV_TYPE_T },
+		[GSM0808_IE_OSMO_OSMUX_CID]		= { TLV_TYPE_TV },
 	},
 };
 
diff --git a/src/gsm/gsm0808_utils.c b/src/gsm/gsm0808_utils.c
index e8259300..26f7944d 100644
--- a/src/gsm/gsm0808_utils.c
+++ b/src/gsm/gsm0808_utils.c
@@ -172,6 +172,25 @@ int gsm0808_dec_aoip_trasp_addr(struct sockaddr_storage *ss,
 	return (int)(elem - old_elem);
 }
 
+/*! Decode TS 08.08 (Osmocom Extension) Osmux CID
+ *  TV with len(V) == 1, and V is the CID to be used.
+ *  \param[out] cid Caller-provided variable where CID is stored
+ *  \param[in] elem pointer to IE value
+ *  \param[in] len length of \a elem in bytes
+ *  \returns number of bytes parsed */
+int gsm0808_dec_osmux_cid(uint8_t *cid, const uint8_t *elem, uint8_t len)
+{
+	OSMO_ASSERT(cid);
+	if (!elem)
+		return -EINVAL;
+	if (len != 1)
+		return -EINVAL;
+
+	*cid = *elem;
+
+	return 1;
+}
+
 #endif /* HAVE_SYS_SOCKET_H */
 
 /* Helper function for gsm0808_enc_speech_codec()
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 840bac9c..0f4a0db1 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -198,6 +198,7 @@ gsm0808_prepend_dtap_header;
 gsm0808_enc_cause;
 gsm0808_enc_aoip_trasp_addr;
 gsm0808_dec_aoip_trasp_addr;
+gsm0808_dec_osmux_cid;
 gsm0808_enc_speech_codec;
 gsm0808_dec_speech_codec;
 gsm0808_enc_speech_codec_list;