Introduce Gn interface (GTPv1C) Support to PGW (#1351)

* [CORE] tlv: Store mode in ogs_tlv_t

This allows specifying the format of the IE for each individual IE,
hence allowing messages containing IEs formatted in different ways.

This is needed in order to support parsing GTPv1-C, since messages
contain IEs with different structure (TLV vs TV). Hence, this is a
preparation patch to add support for parsing TVs in ogs-tlv.c/.h.

* [CORE] tlv: Support parsing msg with both TLV and TV in it

IEs of type TV are sometimes used in GTPv1-C. Current tlv parser/builder
doesn't provide with ways to parse messages which contain TV formatted
IEs. This patch adds the relevant types and ways to encode/decode them.

Furthermore, the current parser/builder allows parsing/building messages
containing the exact same format in all its IEs. A new parser function
is added which allows parsing messages of different types (TV, TLV)
mixed in the same message. In order to be able to do so, it uses the
general msg_mode passed to it in order to know the general TLV format
(in essence, the length of the Tag field, and also the length of the
Length field if applicable each IE).

Looking up the instance in the TLV description is left undone and
hadcoded to 0, since the only user so far requiring this API is GTPv1-C,
which has no instances.

* [CORE] tlv: Support repeated tag+instance parsing TLV message

In GTPv2C, repeated IEs (same tag) are easily differentiated by the
Instance byte, which provides info to match different decoded
structures. In GTPv1C though, there's no Instance byte, and we still
encounter repeated IEs (like GSN Address in Create PDP Context Request).
Hence, the TLV decoder needs to be updated to track count of IEs found
(identified by tag+instance, where instance is always 0 in GTPv1C) and
get the proper description index + offset into the decoded structure.

* [GTP]: Move GTPv2-C specifics to its own libgtp subdir

This will allow adding GTPv1-C code by the side. Most GTPv2 code is left
in this patch as "gtp" instead of renaming it to "gtp2" in order to
avoid massive changes. It can be done at a later stage if wanted.

* [GTP] Support generating GTPv1-C messages

* [SMF] Add Gn interface support

This patch introduces GTPv1C support to open5gs-smfd. With it,
open5gs-becomes a GGSN too, where SGSN can connect to, hence supporting
GERAN and UTRAN networks.

lib/gtp/v2/support/cache/tlv-msg-65.py

@@ -0,0 +1,7 @@
+ies = []
+ies.append({ "ie_type" : "Cause", "ie_value" : "Cause", "presence" : "M", "instance" : "0", "comment" : ""})
+ies.append({ "ie_type" : "Recovery", "ie_value" : "Recovery", "presence" : "C", "instance" : "0", "comment" : "This IE shall be included on the S5/S8, S4/S11 and S2a/S2b interfaces if contacting the peer for the first time"})
+ies.append({ "ie_type" : "Indication", "ie_value" : "Indication Flags", "presence" : "CO", "instance" : "0", "comment" : "This IE shall be included if any one of the applicable flags is set to 1.Applicable flags are:Associate OCI with PGW nodes identity: The PGW shall set this flag to 1 on the S5/S8 interface or S2a/S2b interface if it has included the PGWs Overload Control Information and if this information is to be associated with the node identity (i.e. FQDN or the IP address received from the HSS or DNS during the PGW selection) of the serving PGW. The SGW shall set this flag on the S11/S4 interface if it supports the overload control feature and if the flag is set on the S5/S8 interface.Associate OCI with SGW nodes identity: The SGW shall set this flag to 1 on the S11/S4 interface if it has included the SGWs Overload Control Information and if this information is to be associated with the node identity (i.e. FQDN or the IP address received from the DNS during the SGW selection) of the serving SGW."})
+ies.append({ "ie_type" : "Overload Control Information", "ie_value" : "PGW's Overload Control Information", "presence" : "O", "instance" : "0", "comment" : "During an overload condition, the PGW may include this IE on the S5/S8 or S2a/S2b interface, if the overload control feature is supported by the PGW and is activated for the PLMN to which the access network node, i.e. MME/S4-SGSN for 3GPP access based network, ePDG/TWAN for non-3GPP access based network, belongs (see clause 12.3.11).When present, the PGW shall provide node level overload control, in one instance of this IE; and/orAPN level overload control , in one or more instances of this IE, up to maximum of 10, with the same type and instance value, each representing the overload information for a list of APN(s).See NOTE 1, NOTE 2."})
+ies.append({ "ie_type" : "Overload Control Information", "ie_value" : "SGW's Overload Control Information", "presence" : "O", "instance" : "1", "comment" : "During an overload condition, the SGW may include this IE over the S11/S4 interface if the overload control feature is supported by the SGW and and is activated in the network (see clause 12.3.11).When present, the SGW shall provide only one instance of this IE, representing its overload information."})
+msg_list[key]["ies"] = ies