Verification of NFV Services : Problem Statement and Challenges
draft-shin-nfvrg-service-verification-03

NFVRG                                                           M-K. Shin
Internet-Draft                                                       ETRI
Intended status: Informational                                     K. Nam
Expires: January 19, 2016                                         Friesty
                                                                  S. Pack
                                                                       KU
                                                                   S. Lee
                                                                     ETRI
                                                              R. Krishnan  
                                                                     Dell
                                                                   T. Kim
                                                                    LG U+

                                                            July 19, 2015

    Verification of NFV Services : Problem Statement and Challenges
                draft-shin-nfvrg-service-verification-03

Abstract         

   NFV relocates network functions from dedicated hardware appliances to
   generic servers, so they can run in software. However, incomplete or
   inconsistent configuration of virtualized network functions (VNF) and
   forwarding graph (FG, aka service chain) could cause break-down of
   the supporting infrastructure. In this sense, verification is
   critical for network operators to check their requirements and
   network properties are correctly enforced in the supporting
   infrastructures. Recognizing these problems, we discuss key
   properties to be checked on NFV-enabled services. Also, we present
   architectural challenging issues related to verification in NFV
   environments.  

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
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   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

 

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   This Internet-Draft will expire on January 19, 2016.

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Table of Contents

   1.  Introduction  . . . .  . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Problem statement: Key properties to be checked on NFV services 3
     2.1. Dependencies of network service components in NFV framework .3
     2.2. Invariant and error check in VNF FGs . . . . . . . . . . . . 4 
     2.3. Load Balancing and optimization among VNF Instances  . . . . 4
     2.4. Policy and state consistency on NFV services . . . . . . . . 4
     2.5. Performance  . . . . . . . . . . . . . . . . . . . . . . . . 5 
     2.6. Security . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   3.  Requirements of verification framework  . . . . . . . . . . . . 5
   4.  Open source project related architectural considerations  . . . 6
   5.  Challenging Issues  . . . . . . . . . . . . . . . . . . . . . . 7
     5.1. Consistency check in distributed state .   . . . . . . . . . 7
     5.2. Intent-based service composition . . . . . . . . . . . . . . 7 
     5.3. Finding infinite loops in VNF FGs . .  . . . . . . . . . . . 8
     5.4. Real-time verification . . . . . . . . . . . . . . . . . . . 8
     5.5. Languages and their semantics  . . . . . . . . . . . . . . . 8 
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 8
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

 

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1. Introduction

   NFV is a network architecture concept that proposes using IT
   virtualization related technologies, to virtualize entire classes of
   network service functions into building blocks that may be connected,
   or chained, together to create network services. NFV service is
   defined as a composition of network functions and described by its
   functional and behavioral specification, where network functions
   (i.e., firewall, DPI, SSL, load balancer, NAT, AAA, etc.) are well-
   defined, hence both their functional behavior as well as their
   external interfaces are described in each specifications. 

   In NFV, a VNF is a software package that implements such network
   functions. A VNF can be decomposed into smaller functional modules or
   APIs for scalability, reusability, and/or faster response [ETSI-NFV-
   Arch],[ETSI-NFV-MANO]. This modular updates or composition for a
   network function may lead to many other verification or security
   issues. In addition, a set of ordered network functions which build
   FGs may be connected, or chained, together to create an end-to-end
   network service. Multiple of VNFs can be composed together to reduce
   management and VNF FGs. While autonomic networking techniques could
   be used to automate the configuration process including FG updates,
   it is important to take into account that incomplete and/or
   inconsistent configuration may lead to verification issues. Moreover,
   automation of NFV process with integration of SDN may lead the
   network services to be more error-prone. In this sense, we need to
   identify and verify key properties to be correct before VNFs and FGs
   are physically placed and realized in the supporting infrastructure. 

1.1.  Terminology

   This document draws freely on the terminology defined in [ETSI-NFV-
   Arch].

2. Problem statement : Key properties to be checked on NFV services

   The verification servic should be able to check the following
   properties:

2.1. Dependencies of network service components in NFV framework

   In NFV framework, there exist several network service components
   including NFVI, VNFs, MANO, etc. as well as network controller and
   switches to realize end-to-end network services. Unfortunately, these
   components have intricate dependencies that make operation incorrect.
   In this case, there is inconsistency between states stored and
   managed in VNF FGs and network tables (e.g., flow tables), due to
 

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   communication delays and/or configuration errors. For example, if a
   VNF is replicated into the other same one for the purpose of load
   balance and a new FG is established through the copied one, but all
   the state/DBs replication is not finished yet due to delays, this can
   be lead to unexpected behaviors or errors of the network service.
   Therefore, these dependencies make it difficult to correctly compose
   NFV-enabled end-to-end network services.

2.2. Invariant and error check in VNF FGs  

   In VNF FGs, an infinite loop construction should be avoided and
   verified. Let us consider the example. Two VNF A and VNF B locate in
   the same service node X whereas another VNF C resides in other
   service node Y [SIGCOMM-Gember]. Also, the flow direction is from X
   toY, and the given forwarding rule is A->C->B. In such a case,
   service node Y can receive two ambiguous flows from VNF A: 1) one
   flow processed by VNF A and 2) another flow processed by VNF A, B,
   and C. For the former case, the flow should be processed by VNF C
   whereas the latter flow should be further routed to next service
   nodes. If these two flows cannot be distinguished, service node Y can
   forward the flow to service node X even for the latter case and a
   loop can be formed. To avoid the infinite loop formation, the
   forwarding path over VNF FG should be checked in advance with the
   consideration of physical placement of VNF among service nodes. Also,
   reactive verification may be necessary, since infinite loop formation
   may not be preventable in cases where configuration change is
   happening with live traffic. 

   In addition, isolation between VNFs (e.g. confliction of properties
   or interference between VNFs) and consistent ordering of VNF FGs
   should be always checked and maintained. 

2.3. Load balancing among VNF instances

   In VNF FG, different number of VNF instances can be activated on
   several service nodes to carry out the given task. In such a
   situation, load balancing among the VNF instances is one of the most
   important considerations. In particular, the status in resource usage
   of each service node can be different and thus appropriate amount of
   jobs should be distributed to the VNF instances. To guarantee well-
   balanced load among VNF instances, the correctness of hash functions
   for load balancing needs to be verified. Moreover, when VNF instances
   locate in physically different service nodes, simple verification of
   load balancing in terms of resource usage is not sufficient because
   different service nodes experience diverse network conditions (e.g.,
   different levels of network congestion)[ONS- Gember]. Therefore, it
   is needed to monitor global network condition as well as local
   resource condition to achieve the network-wide load balancing in VNF
 

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   FGs. Also, whether the monitoring function for
   network/compute/storage resources is correctly working should be
   checked.

2.4. Policy and state consistency on NFV services

   In VNF FG, policy to specific users can be dynamically changed. For
   example, a DPI VNF can be applied only in the daytime in order to
   prohibit from watching adult contents while no DPI VNFs applied
   during the nighttime. When the policy is changed, the changed policy
   should be reconfigured in VNF service nodes as soon as possible. If
   the reconfiguration procedure is delayed, inconsistent policies may
   exist in service nodes. Consequently, policy inconsistency or
   confliction needs to be checked. Also in some situations, states for
   VNF instances may be conflicted or inconsistent. Especially when a
   new VNF instance is instantiated for scale-up and multiple VNF
   instances are running, these multiple VNF instances may have
   inconsistent states owing to inappropriate instantiation procedure
   [SIGCOMM-Gember]. In particular, since the internal states of VNF
   instances (e.g., the instantaneous state of CPU, register, and memory
   in virtual machine) are not easily-visible, a new way to check the
   VNF internal states should be devised. 

2.5. Performance 

   In VNF FG, VNF instances can locate in different service nodes and
   these service nodes have different load status and network
   conditions. Consequently, the overall throughput of VNF FG is
   severely affected by the service nodes running VNF instances. For
   example, if a VNF instance locates in a heavily loaded service node,
   the service time at the service node will be increased. In addition,
   when a VNF FG includes a bottleneck link with network congestion, the
   end-to-end performance (e.g., latency and throughput) in the VNF FG
   can be degraded. Therefore, the identification of bottleneck link and
   node is the first step for performance verification or guarantee of
   the VNF FG [ONS-Gember]. After detecting the bottleneck link/node,
   the VNF requiring scale up or down can be identified and the
   relocation of VNF instance among service nodes can be determined

2.6. Security 

   How to verify security holes in VNF FG is another important
   consideration. In terms of security services, authentication, data
   integrity, confidentiality, and replay protection should be provided.
   On the other hand, several VNFs (e.g., NAT) can modify or update
   packet headers and payload. In these environments, it is difficult to
   protect the integrity of flows traversing such VNFs. Another security
   concern in the VNF FG is distributed denial of service (DDoS) to a
 

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   specific service node. If an attacker floods packets to a target
   service node, the target service node cannot perform its functions
   correctly. Therefore, such security attacks in the VNF FG should be
   detected and handled in an efficient manner. In the case of DDoS,
   adding a DDoS appliance as the first element in the service chain
   would help alleviate the problem. Moreover, unknown or unauthorized
   VNFs can run and thus how to identify those problems is another
   security challenge. 

3. Requirements of verification framework

   A verification framework for NFV-based services needs to satisfy the
   following requirements:.

     o R1 : It should be able to check global and local properties and
            invariants. Global properties and invariants relate to the 
            entire VNFs, and local properties and invariants relates to 
            the specific domain or resources that some of the VNFs are   
            using. For example, Loop-freeness and isolation between VNFs 
            can be regarded as global. The policies that are related 
            only to the specific network controllers or devices are 
            local.

     o R2 : It should be able to access to the entire network states
            whenever verification tasks are started. It can directly 
            manage the states of network and NFV-based services 
            through databases or any solution that specializes in 
            dealing with the network topology and configurations, or 
            can utilize the functions provided by NFV M&O and VNFI
            solutions to get or set the states at any time.

     o R3 : It should be independent from specific solutions and
            frameworks, and provide standard APIs.

     o R4 : It should process standard protocols such as NetConf, 
            YANG, OpenFlow, and northbound and southbound interfaces 
            that are related network configurations, and used by OSS.

4. Open source project related architectural considerations 

   Recently, the OPNFV (Open Platform for NFV) community is
   collaborating on a carrier-grade, integrated, open source platform to
   accelerate the introduction of new NFV products and services. Open
   Daylight is also being tightly coupled with this OPNFV platform to
   integrate SDN controller into NFV framework.   

 

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   The verification functions should spreads over the platforms to
   accomplish the requirements mentioned above. The correctness of NFV-
   based services and their network configurations can be checked in the
   NFV MANO layer which has the entire states of the VNFs. Each NFVI
   needs to provide verification layer which composed of policy manager,
   network database and interfaces (e.g. REST APIs). Local properties
   and invariants can be verified inside the specific NFVI, and the
   global properties and invariants can be checked by merging local
   verification results from the related NFVIs.

   The verification service provides verification functions to NFV MANO,
   NFVI, and any other low-level modules such as SDN controllers. For
   the platform independency, it provides standard APIs to process the
   verification tasks. It also uses standard APIs provided by OSS such
   as OpenStack (Neutron) and Open Daylight. The compiler and
   interpreter translate standard description languages and protocols
   into the internal model which optimized to the verification tasks. It
   can process user-defined properties to be checked as well. The
   properties to be checked whether they are user-defined or pre-defined
   invariants are managed by property library. The verifier maintains a
   set of verification algorithms to check the properties. The network
   database inside the verification service manages the global network
   states directly or indirectly.

   A PoC can be implemented using OpenStack (Neutron) and Open Daylight.
   The modules related to verification framework can reside in between
   network virtualization framework (e.g. OpenStack Neutron) and SDN
   controller (e.g. Open Daylight). Neutron and Open Daylight uses
   standard APIs provided by verification service to accomplish
   verification tasks. The initial use case for the PoC could be, in
   particular, any of security, performance, etc as mentioned in clause
   2.

5. Challenging Issues 

   There are emerging challenges that the verification services face
   with.

5.1. Consistency check in distributed state

   Basically, NFV states as well as SDN controllers are distributed.
   writing code that works correctly in a distributed setting is very
   hard. Therefore, distributed state management and consistency check
   has challenging issues. Some open source project such as ONOS offers
   a core set of primitives to manage this complexity. RAFT algorithm
   [RAFT] is used for distribution and replication. Similarly, Open
   daylight project has a clustering concept to management distributed
 

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   state. There is no "one-size-fits-all" solution for control plane
   data consistency.

5.2. Intent-based service composition

   Recently, Intent-based high-level language is newly proposed and
   discussed in open source project. The Intent allows for a descriptive
   way to get what is desired from the infrastructure, unlike the
   current NFV description and SDN interfaces which are based on
   describing how to provide different services. This Intent will
   accommodate orchestration services and network and business oriented
   SDN/NFV applications, including OpenStack Neutron, Service Function
   Chaining, and Group Based Policy. A Intent compiler that translates
   and compiles it into low level instructions (e.g., SDN
   controller/OpenStack primitives) for network service components. In
   this sense, error checking and debugging are critical for reliable
   Intent-based service composition. 

5.3. Finding infinite loops 

   General solutions for the infinite loop can lead to intractable
   problem (e.g. the halting problem). To make the verification
   practical and minimize the complexity, some of the restrictions are
   required. Finding cycle can be processed in polynomial time but the
   restriction could be too much for some cases that service functions
   or network flows requires finite loops.

5.4. Live traffic verification : 

   It is known fact that the complexity of verification tasks for the
   real and big problem is high. A few invariants can be checked in
   real-time but it would be impossible if the size of VNFs increases or
   properties to be checked are complex.

5.5. Languages and their semantics

   For the verification, configurations and states of VNFs need to be
   precisely expressed using formal semantics. There are many languages
   and models, and it is impractical for the verification frameworks to
   support all of the existing languages and models. Languages and
   semantic models optimized to the verification framework need to
   selected or newly developed.

6. Security Considerations

   As already described in clause 2.6, how to verify security holes in
   VNF FG is very important consideration. In terms of security
 

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   services, authentication, data integrity, confidentiality, and replay
   protection should be provided. On the other hand, potential security
   concern should be also carefully checked since several VNFs (e.g.,
   NAT) can modify or update packet headers and payload. 

7. Acknowledgements 

   The authors would like to thank formal methods lab members in Korea
   University for their verification theory support.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

8.2.  Informative References

   [ETSI-NFV-Arch] ETSI, "Architectural Framework v1.1.1," Oct 2013.

   [ETSI-NFV-MANO] ETSI, "Network Function Virtualization (NFV)
              Management and Orchestration,"(Work-in-Progress), 2014.

   [SIGCOMM-Qazi] Z. Qazi, C. Tu, L. Chiang, R. Miao, V. Sekar, and M.
              Yu, "SIMPLE-fying Middlebox Policy Enforcement Using SDN,"
              in Proc. ACM SIGCOMM 2013, August 2013.

   [ONS-Gember] A. Gember, R. Grandl, A. Anand, T. Benson, and A.
              Akella, "Stratos: Virtual Middleboxes as First-Class
              Entities," ONS 2013 and TR.

   [SIGCOMM-Gember] A. Gember, R. Viswanathan, C. Prakash, R. Grandl, J.
              Khalid, S. Das, and A. Akella, "OpenNF: Enabling
              Innovation in Network Function Control," in Proc. ACM
              SIGCOMM 2014, August 2014.

   [RAFT] https://raftconsensus.github.io/.

 

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Authors' Addresses

      Myung-Ki Shin
      ETRI
      161 Gajeong-dong Yuseng-gu
      Daejeon, 305-700
      Korea

      Phone: +82 42 860 4847
      Email: mkshin@etri.re.kr

      Ki-Hyuk Nam
      Friesty

      Email: nam@friesty.com

      Sangheon Pack
      Korea University 

      Email: shpack@korea.ac.kr

      Seungik Lee
      ETRI
      161 Gajeong-dong Yuseng-gu
      Daejeon, 305-700
      Korea

      Phone: +82 42 860 1483
      Email: seungiklee@etri.re.kr

      Ramki Krishnan 
      Dell     

      Email: Ramki_Krishnan@dell.com

      Tae-wan Kim
      LG U+ 

      Phone: +82 10 8080 6603
      Email: dm24ks@lguplus.co.kr

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