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Overview

MEF LSO defines a set of specifications and reference points aimed at providing end to end service orchestration across multiple network domains using standardized APIs. One of the reference points in this set is interlude which focuses on providing control related management interactions between service provider and partner (link). The other inter-provider reference point in LSO is SONATA which mainly focuses on the OSS/BSS level business interactions. As ONAP is not specifically on the SONATA level of interactions, the rest of the document focuses mainly on the interlude reference point in MEF and similar specifications in other standard organizations. While interlude is one of the reference point and specification which addresses inter-provider interaction, it is worthwhile to look at the broader scope considering typical operational, business use cases and aspects impacting such interactions in ONAP External API specifically and ONAP in general. MEF Interlude scope is covered in detail in a separate section.

5GExchange (link) as part of 5GPPP is one of the relevant project worth referring which focuses on "cross-domain orchestration of services over multiple administrations or over multi-domain single administrations" (link). 5GEx project defines a multi-domain logical interworking architecture which covers multi-operator interaction and multi-domain interaction within the same operator. As part of the 5GEx project a detailed study is being conducted around inter-domain and inter-provider interactions and results are published here. The 5GEx proposed system consists of multi-domain orchestrator (MdO), domain orchestrators and interactions between MdO (marked as #2 in the diagram below), the interaction between MdO and Domain orchestrators (marked as #3 in the diagram), the interaction between customer and MdO (marked as #1), interaction between Domain Orchestrators and controllers (marked as #5) and interaction between domain orchestrators (marked as #4). Each of these interactions is identified by different types of interfaces. There is also a classification based on business level interactions, management/orchestration level interaction, control level interactions, and data level interaction. Out of these #2 is the one which close matches the MEF interlude reference point, but the scope is slightly different because in 5GEx project business, management/orchestration related interactions are expected to be handled by the same interface (i.e #2). While #3 (between MdO and DO) is quite relevant in the case of External API, this may be an item for future study as domain orchestration concept is currently under discussion in ONAP Tiger team as of September 2018. For the sake of this document, MdO is functionally mapped to the External API component in ONAP as it is providing an end to end service management capability.

Following diagram captures the interface mapping to standards as defined by 5GEx in their functional model found here

Looking at the picture above, it can be observed that 5GEx mostly follows the ETSI MANO specific interfaces for interactions across MdOs or between MdO and DO. But the picture also includes some additional scope as listed below

• Topology distribution across domains for exchanging network topology details that may be used by MLPCE.
• Multi-domain path computation engine interaction across domains for exchanging path specific data
• SLA Management interaction across domains for exchanging business agreement.
• Service Catalogue interactions across domains for exchanging service specifications

Summarizing the scope in5GExproject, its key focus is in virtualized infrastructure with ETSI MANO building blocks with additional scope for exchanging the network topology, network path, business agreement and service specification across different domains. For ONAP Ext-API this may not be quite relevant as it functionsata layer above the NFVO. However, if External API scope is expanded to have cross-layer interaction, i.e MdO of one operator domain interacting with DO of another operator domain 5GEx specific interfaces may be relevant. But what can be learned from the 5GEx project is the concept of SLA Management, Catalogue exchange, Security mechanisms and approaches for supporting use cases such as Slicing.

The topic of inter-provider/inter-domain interaction is being discussed in TMF ODA, ETSI ZSM, but these specifications are still in the early stages of development and may not be relevant in the near future of ONAP development.

Another specification worth referring is ETSI IFA 028 v3.1.1 - wherein MANO architectural options to support multiple administrative domains is being discussed. This specification introduces two concepts - MLPOC (Multiple Logical Point of Contact) andSLPOC (Single Logical Point of Contact) with varying degrees of cross-layer interaction and information abstraction across domains. This specification also defines an Or-Or interface across NFVOs in different administrative domains. Assescribed in the case of 5GEx, ETSI Or-Or level interaction may not be

Subsequent sections in this page cover a comparison of different SDO/OSSP activities around inter-provider APIs.

Multi-domain Interaction

As defined in 5GEx project multi-domain can be multiple network operators or it can be multiple subdomains within a single operator. The scope of interaction might be slightly different within single operator domains and across multiple operators because the latter will be governed by SLAs with strict policies and predefined trust/contract between the two operators. So security and trust are some of the key criteria for interaction across multiple parties. All interaction should be governed, policy controlled based on the trust agreement. Within the same operator domain, there can be multiple administrative domains which can be governed by SLO/OLOs and trust agreement as in the case of inter-provider interaction. But there can also be a model of distributed deployment which may not fit into the purview of multi-domain interaction. For example, geo-redundancy and HA deployments may not be classified as multi-domain interaction, but governed mostly by policies defined between two software components and interaction over internal APIs.

Federation and Delegation

Similar to multi-domain interaction, federation and delegation are two terminologies used for interaction between two logically separated endpoints. While there is no standard terminology defined at the ONAP level, we can assume federation to be the east-west interaction between systems/components at same logical domains- for example between Orchestrators in two administrative domains, or controller in two administrative domains. The delegation terminology can be associated with the interactions between systems/components at different logical domains- for example, an end to end orchestrator interacting with a domain orchestrator. Federation and Delegation can be classified with reference to the diagram above from 5GEx. Interactions marked 2 can be classified in federation and interaction marked 3 and 5 can be classified as delegation. Here the federation is across domains of different operators, whereas delegation is between the same operator domains. Another differentiator is that federation is between logical domains with similar scope whereas delegation is between logical domains with a different scope. The logical separation can be based on the technology abstraction, geographic abstraction or deployment model. One example of the federation model is the interactions in the CCVPN use case an example of the delegation model is interaction possible between the central site and edge site orchestrators in an edge automation use case.

In the ETSIIFA028there are two models of inter-administrative domain interactions -SLPOCand MLPOCSLPOC (Single Logical Point of Contact) and MLPOC (Multi Logical Point of Contact). In SLPOC there is a single interaction point between two administrative domains whereas in MLPOC there are multiple interaction points between administrative domains. In simple terms, it is possible based on ETSI MLPOC model for NFVO in one administrative domain to interact with VNFM or VIM in another administrative domain over the ETSI interfaces. Since External API functions at a layer above NFVO, the current scope of interaction is limited to the federation model described above -i.einteraction between External API in two operator domains.

The delegation model support in External API requires further discussion based on specific use cases. There is also discussion around Recursive Orchestration, Orchestration Hierarchy and Domain Orchestration. The delegation model can be considered within the scope of External API once some concrete decision is made by the Architecture team.

Business Agreement and Policy

MEF Interlude does not have a specific scope for managing the Business Agreement between SP and Partner, however, the interaction between the parties might be governed and controlled based on the predefined business agreement and associated policy rules, security mechanism. 5GEx document on Business and Economic Layer (link) elaborates this aspect in detail but limits the focus on the SLA between parties. Some interesting aspects to be considered for Interlude are as follows

• Roles of the parties (SP and Partner) which will determine the mode of communication, specific controls required at either end, Policies to be enforced, the direction of communication (Some examples given for the 5G case are - Infrastructure Service Provider, Network Service Provider, Communication Service Provider, Over the top service provider, Exchange point service provider . The roles defined by 5GEx are mostly inspired by those defined by 3GPP 28.801.
• Centralized vs Distributed Interaction: The interaction between parties can be centralized i.e coordinated by an aggregator provider acting as an exchange point between parties or it can be one to one. The aggregator model is better as it will avoid need for multiple business agreements while ensuring centralized enforcement SLA and policies
• Coordination model: Consists of two phases - publishing phase where information is exchanged between parties on the offered services and service composition phase when the actual customer request for a service is forwarded from one party to other. In the case of Ext-API this will be translated to a query on the other parties Service Catalog and initiation of service Configuration/Control Request
• Agreement Push vs Pull Model: In Push model the Business Agreement and policies are predetermined/agreed before any interaction between parties over the inter-provider API where as in pull model business agreement is dynamically decided based on the customer request and required SLA, monitoring levels.

The 5GEx project defines Business Agreement in terms of SLA and the document referred at the beginning of this section also gives a template for defining SLA. For ONAP Ext-API this may not be useful as it currently does not have any referenceable entities for defining policies for interaction between parties, which is quite relevant at the interlude level.

Other relevant SDO References for adapting Business Agreement are as follows

• TMF B2B2X Partnering Step by Step Guide (link): Lifecycle model for B2B partner management, templates for B2B agreement mostly focused on the business layer, but the operating agreement may be relevant for defining the interlude policies
• Recent ONS presentation on BlockChain based inter-operator agreement implemented based on HyperLedger (link)

For External-API project a new set of APIs needs to be defined for the Business Application layer to push the policies for interacting with the partner. In the absence of this API, it may be assumed that Ext-API will consult the Policy Engine in ONAP for determining the control mechanisms that need to be established before interacting with the partner over the inter-provider API.

Cross-layer Interaction

IN MEF LSO, the interlude reference point is between SOF in Service Provider domain and Partner Domain

Security

TBD

. If this principle is strictly followed the interaction can be confined to External API level or External API+SO combined. i.e SOF functionality can be assumed to be fulfilled using Ext-API and SO as a unified block. However, in practical deployments, there may be scenarios which might require cross-layer interaction, for example, MLPOC proposed in ETSI IFA028 wherein the Orchestration function in one domain interacts with VIM or VNFM in another domain. The multilayer interaction might also be possible in a hybrid orchestration scenario wherein the Virtualization and Non-Virtualized domains might have to interact at different levels - for example, a MEF LSO compatible system needs to interact with a non-MEF LSO compatible system. One more practical case is the domain orchestration scenario wherein different logical domains interact with each other. This is a wider consideration and decision requiring input from architecture subcommittee and EUAG.

Another aspect of inter-operator cross-layer interaction is cross-layer data reconciliation say at the inventory level or at the assurance level assuming the cross-layer interaction is permissible as per the business agreement and requires for efficiency. But this aspect is outside the scope of the interlude and may be a topic for wider discussions across SDOs.

For the scope of External API in the short term it is assumed that the cross-layer interaction is limited to the interaction between two systems in SP and Partner domains at least one of those is Ext-API component and other one is the entity which is logically equivalent in functionality and having permissible scope and APIs as defined in the MEF interlude specification and compatible with the Agreement between the two parties (SP and Partner)

Security

TBD

Standard APIs

MEF Interlude is a reference point which is expected to accommodate many APIs going forward based on the scope defined. Few example of APIs that may fit as per the scope in MEF 55 are as follows

Information/Data Model

There are multiple models found to be relevant for inter-provider API

• MCM aligned E-Line Service Model defined in MEF Interlude Contribution - Access E-Line Service Control Classes - 5th Draft
• Work in progress MEF Services Common Model (link) - Initial Proposal for Work Item
• CFS/RFS being referenced by the TMF 641 (based on SID) (link) - Currently followed by CCVPN use case

The choice of a specific model will depend on the decision of EUAG, TOSCA Task Force in ONAP. From Ext-API point of view it is expected to leverage the CFS/RFS model being referred by the TMF 641 API . In future as Interlude specific model in MEF and MEF Services Common Model matures appropriate mapping can be incorporated to accommodate specific service characteristics to the TMF APIs. In MEF LSO there is also NRM model being used for the Presto interface (derived from ONF). The NRM model is assumed to be out of scope for Ext-API unless there is a cross-layer interaction between SOF in SP domain and ICM in Partner domain is required.

Open Questions

• Do we need to consider intraoperator multi-administrative domain interaction – i.e communication across different instances of domain orchestrators (ONAP or non-ONAP) belonging to the same operator?
• Do we need to limit the scope of Interlude to Service Control, Activation, and Configuration or include Service Order Management?
• Do we need to come up with ONAP specific terminology? Different SDOs follow different terminology e.g. operator interoperability, domain interoperability, administrative system interoperability etc. ?
• Catalogue and Inventory Management – Strategy for 1) onboarding the catalog with service specification across interdomain boundaries 2) Reconciliation and aggregation of inventory at each domain – Pull vs Push model
• Service Model Impact: Service hierarchy in the Service model – i.e Composite or Nested Service, Constituent Service – How the service model is decomposed and distributed to operator and partner domains? Any pattern to follow? Or based on request attributes?
• Cross-layer access requirement for multi-domain interaction for example Orchestration layer of SP need to interact with VIM of Partner for resource instantiation – Is this model valid (MLPOC as per ETSI IFA028 )
• Federation vs Delegation
• Federation Actors and Roles: What type of provider roles we should consider – (NGMN Actor Roles ? 5GEx Actor Roles etc – Infrastructure provider, Connectivity SP, Partner SP, Master/Slave), Do we also need to consider different layers of partners – infrastructure, connectivity etc.
• Use Cases: What use cases we should consider for the interlude specification? Generic Operational use cases (Service activation, query etc)  or Specific Business use case ? (NaaS, NFaaS, Access E-Line etc) – Short term and Long Term Target?
• Consideration for interaction between ONAP and non-ONAP (Legacy) Management system across operator domains
• Need for including the Business layer interactions within the scope of interlude (for example dependency on Service Policy)
• Strategy for closed-loop control (Assurance) – Who will manage? Partner managed or SP managed
• Resiliency requirements for inter-operator management connectivity – Failover mechanisms
• Do we need to consider interdependency of Interlude and Legato/Sonata interface? (for example contract)