Problem statement for Verification of Network Service Chains
draft-lee-nsc-verification-problem-statement-01

Abstract

This document addresses the possible conflicts between service overlays in the network service chaining. These conflicts are due to overlapping in classification rules and resource sharing of service overlays. The verification of service chains provides a method for network administrators to detect such conflicts and correct a problematic service chain before applying it on the real network.

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Table of Contents

• 1. Introduction
• 2. Problem Areas
• 3. Verification of Service Chains
• 4. Security Considerations
• 5. IANA Considerations
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Authors' Addresses

1. Introduction

The current network service model is bound to static topologies and manually configured resources. This has motivated a more flexible deployment model which orchestrates the service delivery separated from the network. Network service chaining (NSC) [I-D.quinn-nsc-problem-statement] [I-D.boucadair-network-function-chaining] provides a new network service model that delivers the traffic along the predefined logical paths of network services (i.e., service overlays or service chains). The service overlay provides a specific order of network services with no regard of network topologies. The traffic is classified with a set of rules in different granularity to select a target service overlay.

The service overlays are configured to be isolated from each other with virtualization of the network resources and different traffic classification rules. However, the service overlays can share the physical network resources (i.e., network services); and the traffic classification rules can overlap each other. This may cause unexpected QoS degradation in a composite network service due to network service overload; and service failure due to loops or interventions of the service overlays. In order to these conflicts of service overlays over network resources and classification rules, it is required to verify the newly added service overlays before applying them on the real network.

This document formulates the problems in network service chaining for the verification of service overlays to avoid any conflicts between them.

2. Problem Areas

The main reasons why service chains may bring conflicts between each other are as follows:

1.

Sharing of network services:

The service overlay provides the identifiers of network services; and invocation orders and logical links between them. The network service is instantiated with the identifier so that one or more physical network service nodes are located for it. While the network service instantiation can be orchestrated by NSC functions in a load balanced manner, the computing resource for the network service is limited and dynamic so that it is not avoidable for different service chains to share the same network service instances. This brings uncertainty in QoS of the network service chains because they cannot see which service chains share the same network services. Thus, the network administrator should carefully check the conflict over the network resources before adding a new service chain to the real network for its stability.

2.

Overlapping of classification rules:

An incoming packet (or traffic) is classified according to the classification rules to determine which service overlay will handle it. The classification is based on the contents of one or more packet header fields so that the classification rule may vary in different granularity. This may bring a problematic case that an incoming packet matches two or more classification rules with different service chains, which can result in a service chain loop or intervention. Different priorities of the rules can help the problem but it is not easy to predict which rules may be in a conflict. Moreover, the service chains of low priorities may be unreachable but not intended to. Thus, the network administrator should carefully check the conflict of the classification rules between service chains before adding a new one to the real network for its consistency.

3. Verification of Service Chains

The service chain verification function provides an ability to check whether there is any conflict between a new service chain and the existing ones in the network before applying the new service chain in the network. The aforementioned problems arise from the rule or resource conflicts between service chains. Thus, the verification targets are the classification rules and network resources used for a new service chain.

As a result of the rule verification, the classification rules whose target packets are a subset or a superset of the ones of the new rule are presented out of the existing rules in the network. In the similar way, the shared network services between the new service chain and the existing ones are listed with their frequencies of being shared as a result of resource verification. The verification results are provided to network administrators so that they can easily anticipate the possible problematic cases and determine if the service chain is required to be corrected or not.

The verification procedure above is performed in an off-line manner. In other words, it is a formal verification method which checks the conflicts of configurations at design time. This method is relatively simple and can test a set of service chains in an exhaustive manner. However, dynamic state of network resources and topologies cannot be considered at the verification.

4. Security Considerations

TBD.

5. IANA Considerations

TBD.

6. References

6.1. Normative References

6.2. Informative References

Authors' Addresses