| SDNRG | E.H. Haleplidis |
| Internet-Draft | S.D. Denazis |
| Intended status: Informational | University of Patras |
| Expires: May 08, 2014 | K.P. Pentikousis |
| EICT | |
| J. Hadi Salim | |
| Mojatatu Networks | |
| O.K. Koufopavlou | |
| University of Patras | |
| November 04, 2013 |
SDN Layers and Architecture Terminology
draft-haleplidis-sdnrg-layer-terminology-02
Software-Defined Networking (SDN) is a new approach for network programmability. Network programmability refers to the ability to control, change, and manage network behavior dynamically through software via open interfaces as opposed to relying on closed boxes and propietary defined interfaces. SDN introduces 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 is the layer structure in an SDN architecture and how do layers interface with each other. This document does not aim to standardize any layers or interfaces but rather aims to answer these questions and provide a concise reference document for SDNRG in particular, and the SDN community in general.
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Software-Defined Networking (SDN) is a relevant new term for the programmable networks paradigm. In short, SDN refers to the ability to use software to program individual network devices dynamically and therefore control the behavior of the network as a whole. A key element in SDN is the introduction of an abstraction between the (traditional) Forwarding and the Control planes in order to separate them and provide applications with the necessary application programming interfaces (APIs) to programmatically control the network. The goal is to leverage on this separation, and the associated programmability, to enable faster innovation at both planes.
Current and earlier research in SDN 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 corelate well with each other. For example, both OpenFlow [OpenFlow] and NETCONF [RFC6241] have been characterized as SDN, but they refer to control and management respectively.
This motivates us to consolidate the defitions of SDN in the literature and correlate them with earlier work in IETF and the research community. Of particular interest, for example, is to determine which layers comprise the SDN architecture and which interfaces are 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 document regarding the SDN layers architecture, which would be useful to upcoming work in SDNRG as well as future discussions within the SDN community as a whole.
This document uses the following terms:
Figure 1 provides a detailed high-level overview of the current SDN architecture abstractions. Note that planes can be collocated with other planes or can be physically separated, as we discuss below.
o--------------------------------o
| |
| +-------------+ +----------+ |
| | Application | | Service | |
| +-------------+ +----------+ |
| Application Plane |
o---------------Y----------------o
|
*-----------------------------Y---------------------------------*
| Service Abstraction Layer (SAL) |
*------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 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
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:
All planes mentioned above are connected via Interfaces (as indicated with "Y" in Figure 1. The Interface may take multiple roles depending on whether 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 co-located on the same device, then the Interface could either be an open/proprietary protocol, an open/proprietary software inter-process communication API, or 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.
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.
This document considers four abstraction layers:
A Network Device is any device on a network that performs a function over a packet that it receives via its input port. The network device could, for example, forward the packet, drop the packet, change and forward the packet, etc. NDs can be implemented in hardware or software and can be either a physical or virtual network element. Each network network device has both 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 Operational Plane is responsible for operational state of the ND, for example, with respect to status of network ports and interfaces.
The Forwarding and the Operational Planes can be exposed via a Device Abstraction Layer (DAL), which may be comprised of one or more abstraction models. Examples of Forwarding Plane abstraction models are the ForCES model [RFC5812] and the OpenFlow switch model [OpenFlow]. Examples of the Operational Plane abstraction model include the ForCES model [RFC5812], the YANG model [RFC6020] and SNMP MIBs [RFC3418].
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.
Note that applications can also reside in a network device. Examples of such applications include event monitoring, and handling (offloading) topology discovery or ARP in the device itself instead of forwarding such traffic to the control plane.
The Control Plane communicates with the Forwarding Plane of devices using a Control Plane Southbound Interface (CPSI) with DAL as a point of reference. The Control Plane is responsible for instructing the Forwarding Plane about how to handle network packets.
Normally the CPSI is a time-critical interface and requires low latency and sometimes high bandwidth in order to perform many operations in short order. 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 interprocess 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 CPSIs.
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, BGP, etc.
Control Plane Services provide access to other Services or Applications above the control plane. Examples include a virtual private LAN service, service tunnels, etc.
The Management Plane communicates with the network device Operational Plane using a Management Plane Southbound Interface (MPSI) with DAL as a point of reference.
Normally MSPI, in contrast to the CPSI, is not a time-critical interface and does not share the CPSI's requirements. The management plane is typically closer to human interaction than the control plane and therefore the MSPI is usually based more on usability than performance. Messages tend to be less frequent than in the CPSI.
The MPSI can range from a protocol, to an API or even interprocess communication. If the Management Plane is not embedded in the network device, the MSPI is certainly a protocol. Examples of MPSIs are ForCES [RFC5810], NETCONF [RFC6241], OVSDB [I-D.pfaff-ovsdb-proto] 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 applications.
Management Plane Services provide access to other services or applications above the Management Plane.
The Service Abstraction Layer (SAL) provides access from services of the control, management and application planes to services and applications of the application plane.
Service Interfaces can take many forms pertaining to their specific requirements. Examples of service interfaces include but are not limited to, RESTful APIs, open or proprietary protocols such as NETCONF, inter-process communications, CORBA interfaces, etc.
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.
We advocate that the SDN southbound interface should encompass both CSPI and MSPI.
The SDN northbound interface is implemented in the Service Abstraction Layer.
The above model can be used to describe in a concise manner all prominent SDN-enabling technologies, as we explain in the following subsections.
The Forwarding and Control Element Separation (ForCES [RFC5810]) is an IETF framework consisting of a model and two protocols. ForCES separates the Forwarding from the Control Plane via an open interface, namely the ForCES protocol which operates on entities of the forwarding plane that have been modeled using the ForCES model.
The ForCES model is based on the fact that a network element is composed of numerous logically separate entities that cooperate to provide a given functionality -such as routing or IP switching- and yet appear as a normal integrated network element to external entities and secondly with a protocol to transport information.
ForCES models the Forwarding Plane using Logical Functional Blocks (LFBs) which are connected in a graph, consisting the Forwarding Element (FE). LFBs are described in an XML language, based on an XML schema.
LFB definitions include:
The ForCES model can be used to define LFBs from fine- to coarse-grained as needed.
The ForCES protocol is agnostic to the model and can be used to monitor, configure and control any ForCES-modeled element. The protocol has very simple commands: Set, Get and Del. ForCES is a protocol designed for high throughput and fast updates.
ForCES [RFC5810] can be mapped to the framework illustrated in Figure 1 as follows:
The Network Configuration Protocol (NETCONF [RFC6241]), is an IETF-standardized network management protocol. NETCONF provides mechanisms to install, manipulate, and delete the configuration of network devices.
NETCONF protocol operations are realized as remote procedure calls (RPCs). The NETCONF protocol uses an Extensible Markup Language (XML) based data encoding for the configuration data as well as the protocol messages.
Additonally, the YANG data modeling language has been developed for specifying NETCONF data models and protocol operations. YANG is a data modeling language used to model configuration and state data manipulated by NETCONF, NETCONF remote procedure calls, and NETCONF notifications.
YANG models the hierarchical organization of data as a tree, in which each node has either a value or a set of child nodes. Additionally, YANG structures data models into modules and submodules allowing reusability and augmentation. YANG models can describe constraints to be enforced on the data. Additionally YANG has a set of base datatype and allows custom defined datatypes as well.
YANG allows the definition of NETCONF RPCs allowing the protocol to have an extensible number of commands. For RPC definition, the operations names, input parameters, and output parameters are defined using YANG data definition statements.
NETCONF can be mapped to the framework illustrated in Figure 1 as follows:
OpenFlow is a framework developed by Standford, currently run by the Open Networking Foundation, initially to provide a way for researchers to run experimental protocols in the network. OpenFlow provides a protocol with which a controller may manage a static model of an OpenFlow switch.
An OpenFlow Switch consists of one or more flow tables which perform packet lookups and forwarding, a group table and an OpenFlow channel to an external controller. The switch communicates with the controller which manages the switch via the OpenFlow protocol.
OpenFlow can be mapped to the framework illustrated in Figure 1 as follows:
I2RS, although still work in progress at the IETF, can be mapped to the framework illustrated in Figure 1 as follows:
The authors would like to acknowledge David Meyer, Salvatore Loreto and Sudhir Modali for their discussion and comments that helped put this document in a better shape.
This memo makes no requests to IANA.
TBD