Internet Research Task Force (IRTF) E. Haleplidis, Ed. Request for Comments: 7426 University of Patras Category: Informational K. Pentikousis, Ed. ISSN: 2070-1721 EICT S. Denazis University of Patras J. Hadi Salim Mojatatu Networks D. Meyer Brocade O. Koufopavlou University of Patras January 2015 Software-Defined Networking (SDN): Layers and Architecture Terminology Abstract Software-Defined Networking (SDN) refers to a new approach for network programmability, that is, the capacity to initialize, control, change, and manage network behavior dynamically via open interfaces. SDN emphasizes the role of software in running networks through the introduction of an abstraction for the data forwarding plane and, by doing so, separates it from the control plane. This separation allows faster innovation cycles at both planes as experience has already shown. However, there is increasing confusion as to what exactly SDN is, what the layer structure is in an SDN architecture, and how layers interface with each other. This document, a product of the IRTF Software-Defined Networking Research Group (SDNRG), addresses these questions and provides a concise reference for the SDN research community based on relevant peer- reviewed literature, the RFC series, and relevant documents by other standards organizations.
Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Research Task Force (IRTF). The IRTF publishes the results of Internet-related research and development activities. These results might not be suitable for deployment. This RFC represents the consensus of the Software- Defined Networking Research Group of the Internet Research Task Force (IRTF). Documents approved for publication by the IRSG are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7426. Copyright Notice Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.
Table of Contents 1. Introduction ....................................................4 2. Terminology .....................................................5 3. SDN Layers and Architecture .....................................7 3.1. Overview ...................................................9 3.2. Network Devices ...........................................12 3.3. Control Plane .............................................13 3.4. Management Plane ..........................................14 3.5. Discussion of Control and Management Planes ...............16 3.5.1. Timescale ..........................................16 3.5.2. Persistence ........................................16 3.5.3. Locality ...........................................16 3.5.4. CAP Theorem Insights ...............................17 3.6. Network Services Abstraction Layer ........................18 3.7. Application Plane .........................................19 4. SDN Model View .................................................19 4.1. ForCES ....................................................19 4.2. NETCONF/YANG ..............................................20 4.3. OpenFlow ..................................................21 4.4. Interface to the Routing System ...........................21 4.5. SNMP ......................................................22 4.6. PCEP ......................................................23 4.7. BFD .......................................................23 5. Summary ........................................................24 6. Security Considerations ........................................24 7. Informative References .........................................25 Acknowledgements ..................................................33 Contributors ......................................................34 Authors' Addresses ................................................34
1. Introduction "Software-Defined Networking (SDN)" is a term of the programmable networks paradigm [PNSurvey99] [OF08]. In short, SDN refers to the ability of software applications to program individual network devices dynamically and therefore control the behavior of the network as a whole [NV09]. Boucadair and Jacquenet [RFC7149] point out that SDN is a set of techniques used to facilitate the design, delivery, and operation of network services in a deterministic, dynamic, and scalable manner. A key element in SDN is the introduction of an abstraction between the (traditional) forwarding and control planes in order to separate them and provide applications with the means necessary to programmatically control the network. The goal is to leverage this separation, and the associated programmability, in order to reduce complexity and enable faster innovation at both planes [A4D05]. The historical evolution of the research and development area of programmable networks is reviewed in detail in [SDNHistory] [SDNSurvey], starting with efforts dating back to the 1980s. As documented in [SDNHistory], many of the ideas, concepts, and concerns are applicable to the latest research and development in SDN (and SDN standardization) and have been under extensive investigation and discussion in the research community for quite some time. For example, Rooney, et al. [Tempest] discuss how to allow third-party access to the network without jeopardizing network integrity or how to accommodate legacy networking solutions in their (then new) programmable environment. Further, the concept of separating the control and forwarding planes, which is prominent in SDN, has been extensively discussed even prior to 1998 [Tempest] [P1520] in SS7 networks [ITUSS7], Ipsilon Flow Switching [RFC1953] [RFC2297], and ATM [ITUATM]. SDN research often focuses on varying aspects of programmability, and we are frequently confronted with conflicting points of view regarding what exactly SDN is. For instance, we find that for various reasons (e.g., work focusing on one domain and therefore not necessarily applicable as-is to other domains), certain well-accepted definitions do not correlate well with each other. For example, both OpenFlow [OpenFlow] and the Network Configuration Protocol (NETCONF) [RFC6241] have been characterized as SDN interfaces, but they refer to control and management, respectively. This motivates us to consolidate the definitions of SDN in the literature and correlate them with earlier work at the IETF and the research community. Of particular interest is, for example, to determine which layers comprise the SDN architecture and which
interfaces and their corresponding attributes are best suited to be used between them. As such, the aim of this document is not to standardize any particular layer or interface but rather to provide a concise reference that reflects current approaches regarding the SDN layer architecture. We expect that this document would be useful to upcoming work in SDNRG as well as future discussions within the SDN community as a whole. This document addresses the work item in the SDNRG charter titled "Survey of SDN approaches and Taxonomies", fostering better understanding of prominent SDN technologies in a technology-impartial and business-agnostic manner but does not constitute a new IETF standard. It is meant as a common base for further discussion. As such, we do not make any value statements nor discuss the applicability of any of the frameworks examined in this document for any particular purpose. Instead, we document their characteristics and attributes and classify them, thus providing a taxonomy. This document does not intend to provide an exhaustive list of SDN research issues; interested readers should consider reviewing [SLTSDN] and [SDNACS]. In particular, Jarraya, et al. [SLTSDN] provide an overview of SDN-related research topics, e.g., control partitioning, which is related to the Consistency, Availability and Partitioning (CAP) theorem discussed in Section 3.5.4. This document has been extensively reviewed, discussed, and commented by the vast majority of SDNRG members, a community that certainly exceeds 100 individuals. It is the consensus of SDNRG that this document should be published in the IRTF stream of the RFC series [RFC5743]. The remainder of this document is organized as follows. Section 2 explains the terminology used in this document. Section 3 introduces a high-level overview of current SDN architecture abstractions. Finally, Section 4 discusses how the SDN layer architecture relates to prominent SDN-enabling technologies. 2. Terminology This document uses the following terms: o Software-Defined Networking (SDN) - A programmable networks approach that supports the separation of control and forwarding planes via standardized interfaces. o Resource - A physical or virtual component available within a system. Resources can be very simple or fine-grained (e.g., a port or a queue) or complex, comprised of multiple resources (e.g., a network device).
o Network Device - A device that performs one or more network operations related to packet manipulation and forwarding. This reference model makes no distinction whether a network device is physical or virtual. A device can also be considered as a container for resources and can be a resource in itself. o Interface - A point of interaction between two entities. When the entities are placed at different locations, the interface is usually implemented through a network protocol. If the entities are collocated in the same physical location, the interface can be implemented using a software application programming interface (API), inter-process communication (IPC), or a network protocol. o Application (App) - An application in the context of SDN is a piece of software that utilizes underlying services to perform a function. Application operation can be parameterized, for example, by passing certain arguments at call time, but it is meant to be a standalone piece of software; an App does not offer any interfaces to other applications or services. o Service - A piece of software that performs one or more functions and provides one or more APIs to applications or other services of the same or different layers to make use of said functions and returns one or more results. Services can be combined with other services, or called in a certain serialized manner, to create a new service. o Forwarding Plane (FP) - The collection of resources across all network devices responsible for forwarding traffic. o Operational Plane (OP) - The collection of resources responsible for managing the overall operation of individual network devices. o Control Plane (CP) - The collection of functions responsible for controlling one or more network devices. CP instructs network devices with respect to how to process and forward packets. The control plane interacts primarily with the forwarding plane and, to a lesser extent, with the operational plane. o Management Plane (MP) - The collection of functions responsible for monitoring, configuring, and maintaining one or more network devices or parts of network devices. The management plane is mostly related to the operational plane (it is related less to the forwarding plane). o Application Plane - The collection of applications and services that program network behavior.
o Device and resource Abstraction Layer (DAL) - The device's resource abstraction layer based on one or more models. If it is a physical device, it may be referred to as the Hardware Abstraction Layer (HAL). DAL provides a uniform point of reference for the device's forwarding- and operational-plane resources. o Control Abstraction Layer (CAL) - The control plane's abstraction layer. CAL provides access to the Control-Plane Southbound Interface. o Management Abstraction Layer (MAL) - The management plane's abstraction layer. MAL provides access to the Management-Plane Southbound Interface. o Network Services Abstraction Layer (NSAL) - Provides service abstractions that can be used by applications and services. 3. SDN Layers and Architecture Figure 1 summarizes the SDN architecture abstractions in the form of a detailed, high-level schematic. Note that in a particular implementation, planes can be collocated with other planes or can be physically separated, as we discuss below. SDN is based on the concept of separation between a controlled entity and a controller entity. The controller manipulates the controlled entity via an interface. Interfaces, when local, are mostly API invocations through some library or system call. However, such interfaces may be extended via some protocol definition, which may use local inter-process communication (IPC) or a protocol that could also act remotely; the protocol may be defined as an open standard or in a proprietary manner. Day [PiNA] explores the use of IPC as the mainstay for the definition of recursive network architectures with varying degrees of scope and range of operation. The Recursive InterNetwork Architecture [RINA] outlines a recursive network architecture based on IPC that capitalizes on repeating patterns and structures. This document does not propose a new architecture -- we simply document previous work through a taxonomy. Although recursion is out of the scope of this work, Figure 1 illustrates a hierarchical model in which layers can be stacked on top of each other and employed recursively as needed.
o--------------------------------o | | | +-------------+ +----------+ | | | Application | | Service | | | +-------------+ +----------+ | | Application Plane | o---------------Y----------------o | *-----------------------------Y---------------------------------* | Network Services Abstraction Layer (NSAL) | *------Y------------------------------------------------Y-------* | | | Service Interface | | | o------Y------------------o o---------------------Y------o | | Control Plane | | Management Plane | | | +----Y----+ +-----+ | | +-----+ +----Y----+ | | | Service | | App | | | | App | | Service | | | +----Y----+ +--Y--+ | | +--Y--+ +----Y----+ | | | | | | | | | | *----Y-----------Y----* | | *---Y---------------Y----* | | | Control Abstraction | | | | Management Abstraction | | | | Layer (CAL) | | | | Layer (MAL) | | | *----------Y----------* | | *----------Y-------------* | | | | | | | o------------|------------o o------------|---------------o | | | CP | MP | Southbound | Southbound | Interface | Interface | | *------------Y---------------------------------Y----------------* | Device and resource Abstraction Layer (DAL) | *------------Y---------------------------------Y----------------* | | | | | o-------Y----------o +-----+ o--------Y----------o | | | Forwarding Plane | | App | | Operational Plane | | | o------------------o +-----+ o-------------------o | | Network Device | +---------------------------------------------------------------+ Figure 1: SDN Layer Architecture
3.1. Overview This document follows a network-device-centric approach: control mostly refers to the device packet-handling capability, while management typically refers to aspects of the overall device operation. We view a network device as a complex resource that contains and is part of multiple resources similar to [DIOPR]. Resources can be simple, single components of a network device, for example, a port or a queue of the device, and can also be aggregated into complex resources, for example, a network card or a complete network device. The reader should keep in mind that we make no distinction between "physical" and "virtual" resources or "hardware" and "software" realizations in this document, as we do not delve into implementation or performance aspects. In other words, a resource can be implemented fully in hardware, fully in software, or any hybrid combination in between. Further, we do not distinguish whether a resource is implemented as an overlay or as a part/component of some other device. In general, network device software can run on so- called "bare metal" or on a virtualized substrate. Finally, this document does not discuss how resources are allocated, orchestrated, and released. Indeed, orchestration is out of the scope of this document. SDN spans multiple planes as illustrated in Figure 1. Starting from the bottom part of the figure and moving towards the upper part, we identify the following planes: o Forwarding Plane - Responsible for handling packets in the data path based on the instructions received from the control plane. Actions of the forwarding plane include, but are not limited to, forwarding, dropping, and changing packets. The forwarding plane is usually the termination point for control-plane services and applications. The forwarding plane can contain forwarding resources such as classifiers. The forwarding plane is also widely referred to as the "data plane" or the "data path". o Operational Plane - Responsible for managing the operational state of the network device, e.g., whether the device is active or inactive, the number of ports available, the status of each port, and so on. The operational plane is usually the termination point for management-plane services and applications. The operational plane relates to network device resources such as ports, memory, and so on. We note that some participants of the IRTF SDNRG have a different opinion in regards to the definition of the operational plane. That is, one can argue that the operational plane does not constitute a "plane" per se, but it is, in
practice, an amalgamation of functions on the forwarding plane. For others, however, a "plane" allows one to distinguish between different areas of operations; therefore, the operational plane is included as a "plane" in Figure 1. We have adopted this latter view in this document. o Control Plane - Responsible for making decisions on how packets should be forwarded by one or more network devices and pushing such decisions down to the network devices for execution. The control plane usually focuses mostly on the forwarding plane and less on the operational plane of the device. The control plane may be interested in operational-plane information, which could include, for instance, the current state of a particular port or its capabilities. The control plane's main job is to fine-tune the forwarding tables that reside in the forwarding plane, based on the network topology or external service requests. o Management Plane - Responsible for monitoring, configuring, and maintaining network devices, e.g., making decisions regarding the state of a network device. The management plane usually focuses mostly on the operational plane of the device and less on the forwarding plane. The management plane may be used to configure the forwarding plane, but it does so infrequently and through a more wholesale approach than the control plane. For instance, the management plane may set up all or part of the forwarding rules at once, although such action would be expected to be taken sparingly. o Application Plane - The plane where applications and services that define network behavior reside. Applications that directly (or primarily) support the operation of the forwarding plane (such as routing processes within the control plane) are not considered part of the application plane. Note that applications may be implemented in a modular and distributed fashion and, therefore, can often span multiple planes in Figure 1. [RFC7276] has defined the data, control, and management planes in terms of Operations, Administration, and Maintenance (OAM). This document attempts to broaden the terms defined in [RFC7276] in order to reflect all aspects of an SDN architecture. All planes mentioned above are connected via interfaces (indicated with "Y" in Figure 1. An interface may take multiple roles depending on whether the connected planes reside on the same (physical or virtual) device. If the respective planes are designed so that they do not have to reside in the same device, then the interface can only take the form of a protocol. If the planes are collocated on the
same device, then the interface could be implemented via an open/ proprietary protocol, an open/proprietary software inter-process communication API, or operating system kernel system calls. Applications, i.e., software programs that perform specific computations that consume services without providing access to other applications, can be implemented natively inside a plane or can span multiple planes. For instance, applications or services can span both the control and management planes and thus be able to use both the Control-Plane Southbound Interface (CPSI) and Management-Plane Southbound Interface (MPSI), although this is only implicitly illustrated in Figure 1. An example of such a case would be an application that uses both [OpenFlow] and [OF-CONFIG]. Services, i.e., software programs that provide APIs to other applications or services, can also be natively implemented in specific planes. Services that span multiple planes belong to the application plane as well. While not shown explicitly in Figure 1, services, applications, and entire planes can be placed in a recursive manner, thus providing overlay semantics to the model. For example, application-plane services can be provided to other applications or services through NSAL. Additional examples include virtual resources that are realized on top of a physical resources and hierarchical control- plane controllers [KANDOO]. Note that the focus in this document is, of course, on the north/ south communication between entities in different planes. But this, clearly, does not exclude entity communication within any one plane. It must be noted, however, that in Figure 1, we present an abstract view of the various planes, which is devoid of implementation details. Many implementations in the past have opted for placing the management plane on top of the control plane. This can be interpreted as having the control plane acting as a service to the management plane. Further, in many networks, especially in Internet routers and Ethernet switches, the control plane has been usually implemented as tightly coupled with the network device. When taken as a whole, the control plane has been distributed network-wide. On the other hand, the management plane has been traditionally centralized and has been responsible for managing the control plane and the devices. However, with the adoption of SDN principles, this distinction is no longer so clear-cut.
Additionally, this document considers four abstraction layers: o The Device and resource Abstraction Layer (DAL) abstracts the resources of the device's forwarding and operational planes to the control and management planes. Variations of DAL may abstract both planes or either of the two and may abstract any plane of the device to either the control or management plane. o The Control Abstraction Layer (CAL) abstracts the Control-Plane Southbound Interface and the DAL from the applications and services of the control plane. o The Management Abstraction Layer (MAL) abstracts the Management- Plane Southbound Interface and the DAL from the applications and services of the management plane. o The Network Services Abstraction Layer (NSAL) provides service abstractions for use by applications and other services. At the time of this writing, SDN-related activities have begun in other SDOs. For example, at the ITU, work on architectural [ITUSG13] and signaling requirements and protocols [ITUSG11] has commenced, but the respective study groups have yet to publish their documents, with the exception of [ITUY3300]. The views presented in [ITUY3300] as well as in [ONFArch] are well aligned with this document. 3.2. Network Devices A network device is an entity that receives packets on its ports and performs one or more network functions on them. For example, the network device could forward a received packet, drop it, alter the packet header (or payload), forward the packet, and so on. A network device is an aggregation of multiple resources such as ports, CPU, memory, and queues. Resources are either simple or can be aggregated to form complex resources that can be viewed as one resource. The network device is in itself a complex resource. Examples of network devices include switches and routers. Additional examples include network elements that may operate at a layer above IP (such as firewalls, load balancers, and video transcoders) or below IP (such as Layer 2 switches and optical or microwave network elements). Network devices can be implemented in hardware or software and can be either physical or virtual. As has already been mentioned before, this document makes no such distinction. Each network device has a presence in a forwarding plane and an operational plane.
The forwarding plane, commonly referred to as the "data path", is responsible for handling and forwarding packets. The forwarding plane provides switching, routing, packet transformation, and filtering functions. Resources of the forwarding plane include but are not limited to filters, meters, markers, and classifiers. The operational plane is responsible for the operational state of the network device, for instance, with respect to status of network ports and interfaces. Operational-plane resources include, but are not limited to, memory, CPU, ports, interfaces, and queues. The forwarding and the operational planes are exposed via the Device and resource Abstraction Layer (DAL), which may be expressed by one or more abstraction models. Examples of forwarding-plane abstraction models are Forwarding and Control Element Separation (ForCES) [RFC5812], OpenFlow [OpenFlow], YANG model [RFC6020], and SNMP MIBs [RFC3418]. Examples of the operational-plane abstraction model include the ForCES model [RFC5812], the YANG model [RFC6020], and SNMP MIBs [RFC3418]. Note that applications can also reside in a network device. Examples of such applications include event monitoring and handling (offloading) topology discovery or ARP [RFC0826] in the device itself instead of forwarding such traffic to the control plane. 3.3. Control Plane The control plane is usually distributed and is responsible mainly for the configuration of the forwarding plane using a Control-Plane Southbound Interface (CPSI) with DAL as a point of reference. CP is responsible for instructing FP about how to handle network packets. Communication between control-plane entities, colloquially referred to as the "east-west" interface, is usually implemented through gateway protocols such as BGP [RFC4271] or other protocols such as the Path Computation Element (PCE) Communication Protocol (PCEP) [RFC5440]. These corresponding protocol messages are usually exchanged in-band and subsequently redirected by the forwarding plane to the control plane for further processing. Examples in this category include [RCP], [SoftRouter], and [RouteFlow]. Control-plane functionalities usually include: o Topology discovery and maintenance o Packet route selection and instantiation o Path failover mechanisms
The CPSI is usually defined with the following characteristics: o time-critical interface that requires low latency and sometimes high bandwidth in order to perform many operations in short order o oriented towards wire efficiency and device representation instead of human readability Examples include fast- and high-frequency of flow or table updates, high throughput, and robustness for packet handling and events. CPSI can be implemented using a protocol, an API, or even inter- process communication. If the control plane and the network device are not collocated, then this interface is certainly a protocol. Examples of CPSIs are ForCES [RFC5810] and the OpenFlow protocol [OpenFlow]. The Control Abstraction Layer (CAL) provides access to control applications and services to various CPSIs. The control plane may support more than one CPSI. Control applications can use CAL to control a network device without providing any service to upper layers. Examples include applications that perform control functions, such as OSPF, IS-IS, and BGP. Control-plane service examples include a virtual private LAN service, service tunnels, topology services, etc. 3.4. Management Plane The management plane is usually centralized and aims to ensure that the network as a whole is running optimally by communicating with the network devices' operational plane using a Management-Plane Southbound Interface (MPSI) with DAL as a point of reference. Management-plane functionalities are typically initiated, based on an overall network view, and traditionally have been human-centric. However, lately, algorithms are replacing most human intervention. Management-plane functionalities [FCAPS] typically include: o Fault and monitoring management o Configuration management In addition, management-plane functionalities may also include entities such as orchestrators, Virtual Network Function Managers (VNF Managers) and Virtualised Infrastructure Managers, as described in [NFVArch]. Such entities can use management interfaces to
operational-plane resources to request and provision resources for virtual functions as well as instruct the instantiation of virtual forwarding functions on top of physical forwarding functions. The possibility of a common abstraction model for both SDN and Network Function Virtualization (NFV) is explored in [SDNNFV]. Note, however, that these are only examples of applications and services in the management plane and not formal definitions of entities in this document. As has been noted above, orchestration and therefore the definition of any associated entities is out of the scope of this document. The MPSI, in contrast to the CPSI, is usually not a time-critical interface and does not share the CPSI requirements. MPSI is typically closer to human interaction than CPSI (cf. [RFC3535]); therefore, MPSI usually has the following characteristics: o It is oriented more towards usability, with optimal wire performance being a secondary concern. o Messages tend to be less frequent than in the CPSI. As an example of usability versus performance, we refer to the consensus of the 2002 IAB Workshop [RFC3535]: the key requirement for a network management technology is ease of use, not performance. As per [RFC6632], textual configuration files should be able to contain international characters. Human-readable strings should utilize UTF-8, and protocol elements should be in case-insensitive ASCII, which requires more processing capabilities to parse. MPSI can range from a protocol, to an API or even inter-process communication. If the management plane is not embedded in the network device, the MPSI is certainly a protocol. Examples of MPSIs are ForCES [RFC5810], NETCONF [RFC6241], IP Flow Information Export (IPFIX) [RFC7011], Syslog [RFC5424], Open vSwitch Database (OVSDB) [RFC7047], and SNMP [RFC3411]. The Management Abstraction Layer (MAL) provides access to management applications and services to various MPSIs. The management plane may support more than one MPSI. Management applications can use MAL to manage the network device without providing any service to upper layers. Examples of management applications include network monitoring, fault detection, and recovery applications.
Management-plane services provide access to other services or applications above the management plane. 3.5. Discussion of Control and Management Planes The definition of a clear distinction between "control" and "management" in the context of SDN received significant community attention during the preparation of this document. We observed that the role of the management plane has been earlier largely ignored or specified as out-of-scope for the SDN ecosystem. In the remainder of this subsection, we summarize the characteristics that differentiate the two planes in order to have a clear understanding of the mechanics, capabilities, and needs of each respective interface. 3.5.1. Timescale A point has been raised regarding the reference timescales for the control and management planes regarding how fast the respective plane is required to react to, or how fast it needs to manipulate, the forwarding or operational plane of the device. In general, the control plane needs to send updates "often", which translates roughly to a range of milliseconds; that requires high-bandwidth and low- latency links. In contrast, the management plane reacts generally at longer time frames, i.e., minutes, hours, or even days; thus, wire efficiency is not always a critical concern. A good example of this is the case of changing the configuration state of the device. 3.5.2. Persistence Another distinction between the control and management planes relates to state persistence. A state is considered ephemeral if it has a very limited lifespan and is not deemed necessary to be stored on non-volatile memory. A good example is determining routing, which is usually associated with the control plane. On the other hand, a persistent state has an extended lifespan that may range from hours to days and months, is meant to be used beyond the lifetime of the process that created it, and is thus used across device reboots. Persistent state is usually associated with the management plane. 3.5.3. Locality As mentioned earlier, traditionally, the control plane has been executed locally on the network device and is distributed in nature whilst the management plane is usually executed in a centralized manner, remotely from the device. However, with the advent of SDN centralizing, or "logically centralizing", the controller tends to muddle the distinction of the control and management plane based on locality.
3.5.4. CAP Theorem Insights The CAP theorem views a distributed computing system as composed of multiple computational resources (i.e., CPU, memory, storage) that are connected via a communications network and together perform a task. The theorem, or conjecture by some, identifies three characteristics of distributed systems that are universally desirable: o Consistency, meaning that the system responds identically to a query no matter which node receives the request (or does not respond at all). o Availability, i.e., that the system always responds to a request (although the response may not be consistent or correct). o Partition tolerance, namely that the system continues to function even when specific nodes or the communications network fail. In 2000, Eric Brewer [CAPBR] conjectured that a distributed system can satisfy any two of these guarantees at the same time but not all three. This conjecture was later proven by Gilbert and Lynch [CAPGL] and is now usually referred to as the CAP theorem [CAPFN]. Forwarding a packet through a network correctly is a computational problem. One of the major abstractions that SDN posits is that all network elements are computational resources that perform the simple computational task of inspecting fields in an incoming packet and deciding how to forward it. Since the task of forwarding a packet from network ingress to network egress is obviously carried out by a large number of forwarding elements, the network of forwarding devices is a distributed computational system. Hence, the CAP theorem applies to forwarding of packets. In the context of the CAP theorem, if one considers partition tolerance of paramount importance, traditional control-plane operations are usually local and fast (available), while management- plane operations are usually centralized (consistent) and may be slow. The CAP theorem also provides insights into SDN architectures. For example, a centralized SDN controller acts as a consistent global database and specific SDN mechanisms ensure that a packet entering the network is handled consistently by all SDN switches. The issue of tolerance to loss of connectivity to the controller is not addressed by the basic SDN model. When an SDN switch cannot reach its controller, the flow will be unavailable until the connection is restored. The use of multiple non-collocated SDN controllers has
been proposed (e.g., by configuring the SDN switch with a list of controllers); this may improve partition tolerance but at the cost of loss of absolute consistency. Panda, et al. [CAPFN] provide a first exploration of how the CAP theorem applies to SDN. 3.6. Network Services Abstraction Layer The Network Services Abstraction Layer (NSAL) provides access from services of the control, management, and application planes to other services and applications. We note that the term "SAL" is overloaded, as it is often used in several contexts ranging from system design to service-oriented architectures; therefore, we explicitly add "Network" to the title of this layer to emphasize that this term relates to Figure 1, and we map it accordingly in Section 4 to prominent SDN approaches. Service interfaces can take many forms pertaining to their specific requirements. Examples of service interfaces include, but are not limited to, RESTful APIs, open protocols such as NETCONF, inter- process communication, CORBA [CORBA] interfaces, and so on. The two leading approaches for service interfaces are RESTful interfaces and Remote Procedure Call (RPC) interfaces. Both follow a client-server architecture and use XML or JSON to pass messages, but each has some slightly different characteristics. RESTful interfaces, designed according to the representational state transfer design paradigm [REST], have the following characteristics: o Resource identification - Individual resources are identified using a resource identifier, for example, a URI. o Manipulation of resources through representations - Resources are represented in a format like JSON, XML, or HTML. o Self-descriptive messages - Each message has enough information to describe how the message is to be processed. o Hypermedia as the engine of application state - A client needs no prior knowledge of how to interact with a server, as the API is not fixed but dynamically provided by the server. Remote procedure calls (RPCs) [RFC5531], e.g., XML-RPC and the like, have the following characteristics: o Individual procedures are identified using an identifier. o A client needs to know the procedure name and the associated parameters.
3.7. Application Plane Applications and services that use services from the control and/or management plane form the application plane. Additionally, services residing in the application plane may provide services to other services and applications that reside in the application plane via the service interface. Examples of applications include network topology discovery, network provisioning, path reservation, etc.