Unclear binding between externalReference and Policy Template identifier in M5 Service Access Information

[rjb1000]

Context

According to TS 26.512 V17.6.0:

• A PolicyTemplate is provisioned with a single mandatory externalReference string that is used to correlate with a Service Operation Point declared in a DASH MPD.
• When the Media Session Handler retrieves its Service Access Information from the 5GMS AF at reference point M5, this includes a dynamicPolicyInvocationConfiguration comprising:
  • An array of Policy Template resource identifiers.
  • An array of external references.
• When it sees a Service Operation Point in a DASH MPD, the 5GMS Client instantiates a dynamic policy in the 5GMS AF by citing the Policy Template resource identifier corresponding to the external reference of that Service Operation Point.

Problem description

The set of Policy Template resource identifiers and external references are not explicitly bound together in the OpenAPI YAML specification for M5 Service Access Information.

{
	"provisioningSessionId": "b54a1428-6848-41ee-aa9a-ad12c2bbb05a",
	"provisioningSessionType": "DOWNLINK",
	"streamingAccess": {
		...
	},
	"dynamicPolicyInvocationConfiguration": {
		"serverAddresses": ["http://127.0.0.24:7777/3gpp-m5/v2/"],
		"validPolicyTemplateIds": ["c291c720-6848-41ee-aa9a-ad12c2bbb05a", "ce073c84-6848-41ee-aa9a-ad12c2bbb05a"],
		"sdfMethods":	["5_TUPLE"],
		"externalReferences": ["SD_Basic", "HD_Premium"]
	}
}

The 5GMS Client must use array indices to implicitly match them up into corresponding pairs. This requires the members of the array to be populated in the same order by the 5GMS AF, which is not specified.

This implicit binding is workable, but not a very good API design.

Suggested solution

It would be better to express this binding more explicitly in the OpenAPI YAML specification.

Ideally, this would be an associative array (a.k.a. dictionary or hash map) with the external reference as the key and the Policy Template identifier as the value. However, OpenAPI 3.0 associative arrays are not well supported by 3GPP.

A simpler solution is an array of duples:

{
	"provisioningSessionId": "b54a1428-6848-41ee-aa9a-ad12c2bbb05a",
	"provisioningSessionType": "DOWNLINK",
	"streamingAccess": {
		...
	},
	"dynamicPolicyInvocationConfiguration": {
		"serverAddresses": ["http://127.0.0.24:7777/3gpp-m5/v2/"],
		"policyTemplateBindings": [
			{
				"externalReference": "SD_Basic",
				"policyTemplateId": "c291c720-6848-41ee-aa9a-ad12c2bbb05a"
			},
			{
				"externalReference": "HD_Premium",
				"policyTemplateId": "ce073c84-6848-41ee-aa9a-ad12c2bbb05a"
			}
		],
		"sdfMethods": ["5_TUPLE"]
	}
}

[rjb1000]

@ibouazizi: It is valid to have a Policy Template without an external reference, so we do need to deal with this eventuality.

Could make the externalReference property optional in the binding object.

@ibouazizi: How about changing the name from policyTemplateBindings to simply policyTemplates?

[rjb1000]

Revised design, as discussed on yesterday's joint call between 5G-MAG and 3GPP SA4 MBS:

{
	"provisioningSessionId": "b54a1428-6848-41ee-aa9a-ad12c2bbb05a",
	"provisioningSessionType": "DOWNLINK",
	"streamingAccess": {
		...
	},
	"dynamicPolicyInvocationConfiguration": {
		"serverAddresses": ["http://127.0.0.24:7777/3gpp-m5/v2/"],
		"policyTemplates": [
			{
				"externalReference": "SD_Basic",
				"policyTemplateId": "c291c720-6848-41ee-aa9a-ad12c2bbb05a"
			},
			{
				"externalReference": "HD_Premium",
				"policyTemplateId": "ce073c84-6848-41ee-aa9a-ad12c2bbb05a"
			},
			{
				"policyTemplateId": "1fef53ec-d8db-4c06-9653-3103b3158e5c"
			}
		],
		"sdfMethods": ["5_TUPLE"]
	}
}

Note the third example with no externalReference.

@ibouazizi, @tlohmar: On what basis would a Media Session Handler instantiate such a Policy Template given that it has no other information to go on?

[rjb1000]

Change Requests agreed at SA4#126 (Chicago):

• TS 26 512
  • Rel-17: CR0054r1 "Essential maintenance" in S4-231631.
  • Rel-16: CR0056 "Essential maintenance" in S4-231632.

policyTemplateBindings replaces validPolicyTemplateIds and externalReferences in table 11.2.3.1-1 and clause C.3.8.

[rjb1000]

CR Packs approved at SA#102 (Edinburgh):

• Rel-16: SP-231367.
• Rel-17: SP-231368.

[rjb1000]

• TS 26.512 V16.12.0 published.
• TS 26.512 V17.7.0 published.
• TS 26.512 V18.0.0 published.