1618 IEEE COMMUNICATIONS SURVEYS TUTORIALS.VOL.16.NO.3.THIRD QUARTER 2014 SDN paradigm that laid the foundation for many of the ideas separation between the routing decision logic and the pro- we are seeing today. tocols governing the interaction between network elements a)Open Signaling:The Open Signaling (OPENSIG) It proposed giving the "decision"plane a global view of the working group began in 1995 with a series of workshops network,serviced by a“dissemination'”'and“discovery”plane, dedicated to "making ATM,Internet and mobile networks for control of a"data"plane for forwarding traffic.These ideas more open,extensible,and programmable"[6].They believed provided direct inspiration for later works such as NOX [17]. that a separation between the communication hardware and which proposed an "operating system for networks"in the control software was necessary but challenging to realize;this context of an OpenFlow-enabled network. is mainly due to vertically integrated switches and routers, e)NETCONF:In 2006,the IETF Network Configu- whose closed nature made the rapid deployment of new ration Working Group proposed NETCONF [18]as a man- network services and environments impossible.The core of agement protocol for modifying the configuration of network their proposal was to provide access to the network hardware devices.The protocol allowed network devices to expose an via open,programmable network interfaces;this would allow API through which extensible configuration data could be sent the deployment of new services through a distributed program- and retrieved. ming environment. Another management protocol,widely deployed in the past Motivated by these ideas,an IETF working group was and used until today,is the SNMP [19].SNMP was proposed created,which led to the specification of the General Switch in the late 80's and proved to be a very popular network Management Protocol (GSMP)[7],a general purpose pro- management protocol,which uses the Structured Management tocol to control a label switch.GSMP allows a controller Interface (SMI)to fetch data contained in the Management to establish and release connections across the switch,add Information Base (MIB).It could be used as well to change and delete leaves on a multicast connection,manage switch variables in the MIB in order to modify configuration settings. ports,request configuration information,request and delete It later became apparent that in spite of what it was originally reservation of switch resources,and request statistics.The intended for,SNMP was not being used to configure network working group is officially concluded and the latest standards equipment,but rather as a performance and fault monitoring proposal,GSMPv3,was published in June 2002. tool.Moreover,multiple shortcomings were detected in the b)Active Networking:Also in the mid 1990s,the conception of SNMP,the most notable of which was its lack Active Networking [8],[9]initiative proposed the idea of of strong security.This was addressed in a later version of the a network infrastructure that would be programmable for protocol. customized services.There were two main approaches being NETCONF,at the time it was proposed by IETF,was considered,namely:(1)user-programmable switches,with in- seen by many as a new approach for network management band data transfer and out-of-band management channels; that would fix the aforementioned shortcomings in SNMP. and (2)capsules,which were program fragments that could Although the NETCONF protocol accomplishes the goal of be carried in user messages;program fragments would then simplifying device (re)configuration and acts as a building be interpreted and executed by routers.Despite considerable block for management,there is no separation between data activity it motivated.Active Networking never gathered crit-and control planes.The same can be stated about SNMP. ical mass and transferred to widespread use and industry A network with NETCONF should not be regarded as fully deployment,mainly due to practical security and performance programmable as any new functionality would have to be concerns [10]. implemented at both the network device and the manager so c)DCAN:Another initiative that took place in the that any new functionality can be provided;furthermore,it is mid 1990s is the Devolved Control of ATM Networks designed primarily to aid automated configuration and not for (DCAN)[11].The aim of this project was to design and enabling direct control of state nor enabling quick deployment develop the necessary infrastructure for scalable control and of innovative services and applications.Nevertheless,both management of ATM networks.The premise is that con- NETCONF and SNMP are useful management tools that trol and management functions of the many devices (ATM may be used in parallel on hybrid switches supporting other switches in the case of DCAN)should be decoupled from the solutions that enable programmable networking devices themselves and delegated to external entities dedicated The NETCONF working group is currently active and the to that purpose,which is basically the concept behind SDNs. latest proposed standard was published in June 2011. DCAN assumes a minimalist protocol between the manager f)Ethane:The immediate predecessor to OpenFlow was and the network,in the lines of what happens today in the SANE Ethane project [20],which,in 2006,defined proposals such as OpenFlow.More on the DCAN project can a new architecture for enterprise networks.Ethane's focus be found at [12]. was on using a centralized controller to manage policy and Still in the lines of SDNs and the proposed decoupling of security in a network.A notable example is providing identity- control and data plane over ATM networks,amongst others, based access control.Similar to SDN,Ethane employed two in the work proposed in [13]multiple heterogeneous control components:a controller to decide if a packet should be architectures are allowed to run simultaneously over single forwarded,and an Ethane switch consisting of a flow table physical ATM network by partitioning the resources of that and a secure channel to the controller. switch between those controllers. Ethane laid the foundation for what would become d)4D Project:Starting in 2004,the 4D project [14], Software-Defined Networking.To put Ethane in the context of [15],[16]advocated a clean slate design that emphasized today's SDN paradigm,Ethane's identity-based access control1618 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 16, NO. 3, THIRD QUARTER 2014 SDN paradigm that laid the foundation for many of the ideas we are seeing today. a) Open Signaling: The Open Signaling (OPENSIG) working group began in 1995 with a series of workshops dedicated to “making ATM, Internet and mobile networks more open, extensible, and programmable” [6]. They believed that a separation between the communication hardware and control software was necessary but challenging to realize; this is mainly due to vertically integrated switches and routers, whose closed nature made the rapid deployment of new network services and environments impossible. The core of their proposal was to provide access to the network hardware via open, programmable network interfaces; this would allow the deployment of new services through a distributed program￾ming environment. Motivated by these ideas, an IETF working group was created, which led to the specification of the General Switch Management Protocol (GSMP) [7], a general purpose pro￾tocol to control a label switch. GSMP allows a controller to establish and release connections across the switch, add and delete leaves on a multicast connection, manage switch ports, request configuration information, request and delete reservation of switch resources, and request statistics. The working group is officially concluded and the latest standards proposal, GSMPv3, was published in June 2002. b) Active Networking: Also in the mid 1990s, the Active Networking [8], [9] initiative proposed the idea of a network infrastructure that would be programmable for customized services. There were two main approaches being considered, namely: (1) user-programmable switches, with in￾band data transfer and out-of-band management channels; and (2) capsules, which were program fragments that could be carried in user messages; program fragments would then be interpreted and executed by routers. Despite considerable activity it motivated, Active Networking never gathered crit￾ical mass and transferred to widespread use and industry deployment, mainly due to practical security and performance concerns [10]. c) DCAN: Another initiative that took place in the mid 1990s is the Devolved Control of ATM Networks (DCAN) [11]. The aim of this project was to design and develop the necessary infrastructure for scalable control and management of ATM networks. The premise is that con￾trol and management functions of the many devices (ATM switches in the case of DCAN) should be decoupled from the devices themselves and delegated to external entities dedicated to that purpose, which is basically the concept behind SDNs. DCAN assumes a minimalist protocol between the manager and the network, in the lines of what happens today in proposals such as OpenFlow. More on the DCAN project can be found at [12]. Still in the lines of SDNs and the proposed decoupling of control and data plane over ATM networks, amongst others, in the work proposed in [13] multiple heterogeneous control architectures are allowed to run simultaneously over single physical ATM network by partitioning the resources of that switch between those controllers. d) 4D Project: Starting in 2004, the 4D project [14], [15], [16] advocated a clean slate design that emphasized separation between the routing decision logic and the pro￾tocols governing the interaction between network elements. It proposed giving the “decision” plane a global view of the network, serviced by a “dissemination” and “discovery” plane, for control of a “data” plane for forwarding traffic. These ideas provided direct inspiration for later works such as NOX [17], which proposed an “operating system for networks” in the context of an OpenFlow-enabled network. e) NETCONF: In 2006, the IETF Network Configu￾ration Working Group proposed NETCONF [18] as a man￾agement protocol for modifying the configuration of network devices. The protocol allowed network devices to expose an API through which extensible configuration data could be sent and retrieved. Another management protocol, widely deployed in the past and used until today, is the SNMP [19]. SNMP was proposed in the late 80’s and proved to be a very popular network management protocol, which uses the Structured Management Interface (SMI) to fetch data contained in the Management Information Base (MIB). It could be used as well to change variables in the MIB in order to modify configuration settings. It later became apparent that in spite of what it was originally intended for, SNMP was not being used to configure network equipment, but rather as a performance and fault monitoring tool. Moreover, multiple shortcomings were detected in the conception of SNMP, the most notable of which was its lack of strong security. This was addressed in a later version of the protocol. NETCONF, at the time it was proposed by IETF, was seen by many as a new approach for network management that would fix the aforementioned shortcomings in SNMP. Although the NETCONF protocol accomplishes the goal of simplifying device (re)configuration and acts as a building block for management, there is no separation between data and control planes. The same can be stated about SNMP. A network with NETCONF should not be regarded as fully programmable as any new functionality would have to be implemented at both the network device and the manager so that any new functionality can be provided; furthermore, it is designed primarily to aid automated configuration and not for enabling direct control of state nor enabling quick deployment of innovative services and applications. Nevertheless, both NETCONF and SNMP are useful management tools that may be used in parallel on hybrid switches supporting other solutions that enable programmable networking. The NETCONF working group is currently active and the latest proposed standard was published in June 2011. f) Ethane: The immediate predecessor to OpenFlow was the SANE / Ethane project [20], which, in 2006, defined a new architecture for enterprise networks. Ethane’s focus was on using a centralized controller to manage policy and security in a network. A notable example is providing identity￾based access control. Similar to SDN, Ethane employed two components: a controller to decide if a packet should be forwarded, and an Ethane switch consisting of a flow table and a secure channel to the controller. Ethane laid the foundation for what would become Software-Defined Networking. To put Ethane in the context of today’s SDN paradigm, Ethane’s identity-based access control