A Survey and a Layered Taxonomy of SDN (2)

基于SDN技术的网络质量优化方案研究随着互联网的普及和网络应用的不断发展，网络质量已经成为人们关注的焦点。

像视频会议、在线教育等实时应用的需求日益增加，对网络带宽和延迟要求也越来越高。

如何保证网络的高可用性和高质量成为了一个急需解决的问题。

而基于SDN技术的网络质量优化方案便是当前研究的热点之一。

SDN(软件定义网络)是指将控制平面和数据平面进行分离，使网络管理人员可以通过软件来实现对网络的全局控制。

这种新型的网络架构使得网络可以更加灵活地响应业务需求。

而基于SDN 的网络质量优化方案在此意义下显得特别重要。

下面从以下几个角度来探讨SDN技术在网络质量方面的优化方案：一、网络拓扑结构的优化一个优秀的网络拓扑结构能够提高网络的性能和质量。

而因为SDN架构将网络的控制平面和数据平面分离，它可以更加灵活地进行网络拓扑结构的调整，从而优化网络质量。

比如，在一些有较高流量需求的网络环境下，SDN可以将网络拓扑结构调整为树型或星型，从而更好地满足业务需求。

另外，SDN还可以在网络拓扑结构中加入一些路由器和交换机的备份，以解决意外故障带来的问题。

二、带宽调度和QoS保证基于SDN的网络可以灵活地分配带宽资源，从而更好地满足业务需求。

比如，在视频会议期间需要大量的带宽资源，SDN可以在保证其他业务不受影响的情况下，将额外的带宽优化地分配给视频会议。

此外，通过设置合适的QoS(Quality of Service)，可以保证网络上不同的应用之间互相不会干扰，避免了传统网络中的"拥塞崩溃"现象。

三、流量控制与管理SDN可以通过流表来对网络流量进行控制和管理。

通过控制流表，可以实现对网络上数据包的过滤、分类、转发、修改和统计等操作。

对于不同类型的网络流量，可以根据不同的需求加以处理，比如对实时流量进行优先处理，避免延迟和阻塞等情况。

四、安全性和故障恢复安全性和故障恢复也是网络质量的一个重要方面。

基于SDN 的网络可以借助控制平面来进行安全性和故障恢复等操作。

SDN多控制器架构在改善网络可扩展性和灵活 性的同时，也给SDN多域管理带来了新的问题％如 对SDN网络进行合理分割、分配子域内所包含设备 如交换机和控制器的数量等％上述因素对于增强
基金项目：国家自然科学基金创新研究群体项目(61521003)；国家网络空间安全专项课题(2017YFB0803204)。 作者简介：姚蓝(1982&)，女，博士研究生，主研方向为软件定义网络；兰巨龙，教授、博士生导师；胡涛(通信作者)，硕士研究生 收稿日期：2018-03—8 修回日期：2018-05-21 E-mad：hutaondsc@ 163. com
+ Abstract] Aiming at the problem that sub-domain planning and controller deployment ae unreasonable in Software D+fin+d N+tworking ( SDN) mul ti-domain n+tworks， an+w SDN multi-domain adap tiv+manag+m+n tm+thod is propos+d. Based onthe improved node clustering idea，the SDN Sub-domain Dividing Optimizetion Algorithm ( DDOA) is d+sign+d. Th+nod+s in th+n+twork ar+clus tr+d according to th+d+gr++ofatrac tion and atribution， and th+sub-domain planning iscompl td.For th+dif+r+n tn+twork costsin th+sub-domain， th+Con tro l r Adap tiv+D+ploym+n tAlgori thm ( CADA) isgiv+n， and th+cos tof th+sub-domain n+twork is ob tain+d by cost-w+ightd summa tion to complt th+ contro l r d+ploym+n t. Simula tion r+sultsshow tha tcompar+d with th+contro l r adap tiv+d+ploym+n talgori thm and load balancing algori thm， th+n+twork sub-domain division and contro l r d+ploym+n tar+mor+r+asonabl ， which can sav+ 23 % backup contro l r spac+and improv+th+ov+ralp+rformanc+of th+n+twork. + Key words] Software-Defined Networking ( SDN ) ； sub-domain planning ； node clustering ； conSoller deployment ； adaptiv+manag+m+nt DOI： 10. 196780. issn. 1000-3428.0050981

与SDN控制层对应的控制平台层及相应的基于REST的OpenDaylight api北向接口. 与SDN应用层对应的网络应用、编排和服务层。
电子工业出版社
8.1.4 了解SDN开源平台OpenDaylight 《网络系统规划与部署》（中级篇）
1. OpenDaylight控制器特点 OpenDaylight控制器基于Java语言开发，相比其他控制器，它具有两大技术特点。 一是采用了 OSGi（Open Service Gateway Initiative）体系结构， 实现了众多网络功能的隔离， 极大地增强了控制平面的可扩展性。 二是引入了 SAL （Service Abstraction Layer，服务抽象层），使得控制器可以自动适配包括OF 交换机等不同底层协议的设备，使得开发者可以专注于业务应用的开发。
电子工业出版社
8.1.3 了解Sห้องสมุดไป่ตู้N数控分离技术
3. 了解SDN的应用平面
《网络系统规划与部署》（中级篇）
电子工业出版社
8.1.3 了解SDN数控分离技术
《网络系统规划与部署》（中级篇）
4. 了解SDN的管理平面 SDN的管理平面着重负责一系列静态的工作，这些工作比较适合在 应用、控制、数据平面外实现，比如对网元进行配置、指定SDN Datapath的控制器。同时，负责定义 SDN控制器以及SDN应用能控制 的范围。结果如图所示。
OpenDaylight（简称ODL）是SDN技术重要的开源平台，由微软、爱立信、红帽等11家科技公 司在Lunix基金会下成立的合作项目。ODL架构与ONF（开放网络基金会）的SDN架构类似，主要包 括如下内容。
与SDN基础设施层对应的数据平面网元（例如虚拟交换机、物理设备等），以及相应的南向 接口（例如OpenFlow等标准协议及一些厂商专有的接口）。

软件定义网络（SDN）中的协议架构与控制平面在当今信息技术快速发展的背景下，软件定义网络（Software-Defined Networking，简称SDN）作为一种新兴的网络架构方式，在网络领域引起了广泛关注。

SDN以其简化网络管理、提高网络灵活性和可编程性的优势，成为了网络技术的热门话题。

协议架构和控制平面是SDN的核心组成部分，它们在整个系统中起着重要的作用。

本文将详细介绍SDN中的协议架构与控制平面，以期为读者提供一个全面的了解。

一、SDN的协议架构SDN的协议架构主要分为三层：应用层、控制层和基础设施层。

1. 应用层：应用层是SDN网络的最高层，负责实现各种网络应用。

在SDN中，应用层通过使用特定的编程接口（API）与控制层进行交互，以实现网络的各种功能和服务。

其中，SDN应用可以根据网络流量情况动态调整路由策略，实现优化网络性能的目的。

2. 控制层：控制层是SDN网络的灵魂所在，它负责网络的管理和控制。

在SDN中，控制层主要由控制器组成，控制器是SDN网络的中央节点，负责协调和管理各个网络设备。

控制器通过分析网络流量和拓扑结构的信息，生成网络的路由表，并将其发送给基础设施层的网络设备。

此外，控制层还提供了对网络的编程接口，以便应用层可以通过控制器控制网络。

3. 基础设施层：基础设施层是SDN网络的底层，由各种网络设备组成，如交换机、路由器等。

基础设施层负责接收控制器发送的路由表信息，并根据路由表进行数据包的转发。

在SDN中，基础设施层的网络设备和控制器的通信通常是通过OpenFlow协议来实现的。

二、SDN的控制平面控制平面是SDN中实现网络控制的关键组成部分，它由控制器和与之相关的协议组成。

1. 控制器：控制器是SDN网络的核心，它负责管理和控制网络设备。

控制器能够通过与基础设施层的网络设备通信，获取网络的拓扑信息和流量统计数据，并根据这些信息生成路由表。

在SDN中，常用的控制器包括OpenDaylight、Floodlight等。

一个SDN网络有三个架构层：物理网络、SDN控制器、SDN应用程序。

物理网络：最底层包含网络中构成所有IT基础设施的基础的物理设备。

我们使用“交换机”这个概念，因为OpenFlow改变了以太网交换机工作的方式。

在本文中，你还可以考虑物理基础设施中的虚拟交换机部分。

SDN控制器：SDN控制器是中间件，由服务器作为整个架构的轴心。

控制器必须和网络中所有物理以及虚拟设备整合。

控制器将物理网络设备从与这些设备协同工作的SDN软件中抽象化出来。

控制器和网络设备之间有高度的整合。

在OpenFlow环境中，控制器将使用OpenFlow协议和NETCONF协议来与交换机对话。

(OpenFlow是发送流数据到交换机的API，而NETCONF是网络设置API。

)SDN应用程序：SDN设计中最具有可视性的层是提供服务(比如交换/网络虚拟化、防火墙和流量均衡器)的应用程序。

(注意，基于OpenFlow的负载均衡器被称为流量均衡器。

它们并不是传统负载均衡器，因为它们不能读取数据包内容)这些应用程序与那些软件运行在专门硬件上的情境中的应用程序基本类似或相同。

网络技术中大部分即将到来的创新将发生在SDN应用程序上。

（1）为什么要搞SDN？因特网存在和发展了几十年。

随着服务类型和规模的急剧增加出现了一些问题。

长期以来通过命令行接口的手动配置阻碍了网络虚拟化的前进，操作费用高，网络刷新慢，容易引入差错。

取消把应用联系到特定网络详情，譬如断开和地址，使物理具体事项的改变无需重写应用和手动配置网络设备的时延和费用，也许是一种思路。

路由是一个大问题。

路由器里面的路由表越来越复杂，分散到各地去路由，既做不到最优的路由，又产生许多重复的计算。

从你的PC到一个网站浏览器，可能要经过20-100路由器或交换机。

如果一个包到来，只知道目的地，但不知道怎么走，那只有交给下一跳。

下一跳要是也不知道呢？这么盲目跳下去，怎么就相信会到达目的地呢？那只能靠相邻路由器经常交换信息。

SDN
图 ２ SDN 的接口
SDN 的体系 结 构 分 为 ３ 个 部 分:数 据 层、控 制
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张 敏,王朝阳
(徐州工程学院,江苏 徐州 ２２１０００)
软件定义网络(
述、
网络体系架构、
工作原理和关键技术,
对于研究 SDN 网络有一定的参考意义.
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１ SDN 的概述
传统网络的控制层和数据层都是集中在同一台

SDN（软件定义网络）技术解析随着信息技术的飞速发展，软件定义网络（Software-Defined Networking，SDN）作为一种新兴的网络架构，正在受到越来越多企业和组织的关注和应用。

本文将对SDN技术进行详细解析，包括其基本概念、架构原理、应用场景以及未来发展方向等。

一、基本概念SDN是一种基于软件控制的网络架构，与传统的网络架构相比，它的核心思想是将网络控制平面与数据转发平面进行分离。

传统网络中，网络设备（如交换机、路由器）同时具备控制和数据转发功能，网络管理员通过配置这些设备的命令来控制网络。

而在SDN中，控制器负责决策网络数据的转发路径，将这些决策下发到数据平面设备执行。

这种分离使得网络的管理与控制变得集中化，便于对网络进行统一的管理与维护。

二、架构原理SDN架构主要由三个组件组成：应用层、控制层和基础设施层。

应用层包括各种网络应用，如负载均衡、安全防护等；控制层由控制器组成，负责管理和控制网络中的各种设备；基础设施层则是实际的网络设备，包括交换机、路由器等。

在SDN中，应用层通过与控制层进行交互来获得网络管理的能力。

应用程序可以通过SDN控制器的API接口与其进行通信，通过发送和接收消息来实现网络上的各种功能。

控制层是SDN的核心，它负责对网络进行管理与控制。

控制器通过与基础设施层的网络设备进行通信，提供网络的可编程性和可配置性。

控制器可根据网络策略和管理员的需求，动态地调整网络的配置，并将这些配置下发至网络设备，从而实现对网络的控制。

基础设施层是实际的网络设备，包括交换机、路由器等。

这些设备根据控制器下发的指令来转发数据。

三、应用场景SDN技术在各个领域有着广泛的应用场景。

以下列举几个典型的应用场景：1. 数据中心网络：SDN技术可以对复杂的数据中心网络进行灵活统一的管理。

通过集中化的控制，管理员可以根据实际需求对数据中心网络进行动态配置，提高网络的资源利用率和性能。

2. 广域网（WAN）优化：SDN可以通过对网络流量进行实时监测与调整，提高广域网的带宽利用率和传输效率。

《SDN与NFV技术介绍》知识点摘录导读5G时代，核心网云化可以最大化发挥5G的优势中国移动的目标：建设架构可灵活调整、资源可弹性伸缩、流量可全局调度、能力可全面开放的网络。

一、SDN介绍(一)SDN控制器与SDN相关技术1.NFV和SDN的关系NFV：网元，软硬件分离，电信网元以软硬件形式部署在云上SDN：网络，可以控制物理网元和NFV网元，网络集中控制，把策略下发到网元。

2.数据中心分层架构转发层：租户业务流量转发控制层：租户网络打通云操作系统层：虚拟机创建删除应用层：租户逻辑网络管理3.SDN数据中心Overlay网络：重叠网络，底层网络通过VXLAN技术为用户创建的虚拟网络Underlay网络：底层网络4.VXLAN：隧道封装，24位ID解决了VLAN4096个的限制，增加了UDP、IP等封装头。

VTEP：在VXLAN网络中，用于建立VXLAN隧道的断电设备5.openFlow是SDN南向接口之一，用于转发面和控制面之间，对转发设备编程。

SDN控制器把openFlow流表下发到转发设备，报文匹配到流表就相应转发，否则丢弃。

6.EVPN实现VXLAN隧道的自动建立BGP EVPN扩展*5种类型路由type1-5，实现数据中心内部和之间的互访。

7.SDN数据中心控制方式分两种强控方式：openFlow流表+NETCONF协议配置弱控方式：EVPN学习用户间MAC和ARP+NETCONF8.Aero自研的SDN控制器，可与多厂家转发设备对接，实现SDN南向北向解耦。

9.控制器解耦方案(二)IP相关基础知识1.IP路由基础路由：把一个数据包从一个设备发送到不同网络中的另一设备上去，查路由表匹配，否则丢弃。

路由表通过OSPF、BGP、ISIS、RIP等方式学习得到格式：*学来的协议，目的网段、cost值、出接口、下一跳等分类：静态路由和动态路由静态路由：手工配置，用于小规模网络动态路由：设备间协议交互得到，用于大规模网络自治域内动态路由协议：OSPF、ISIS、RIP自治域AS间动态路由协议：BGP链路状态动态路由协议：OSPF、ISIS距离矢量动态路由协议：RAP、BGP2.OSPF生成发现路由自治域AS内划分多个Area，核心叫做Area0，其他的不直连，而是通过Area0连接。

➢ NORMAL: 代表传统的非OpenFlow流水线。仅可用于为一个输出端口， 使用普通的流水线处理数据包。如果交换机不能转发数据包从 OpenFlow流水线到普通流水线，它必须表明它不支持这一行动。
➢ FLOOD: 表示使用普通流水线处理进行泛洪（见5.1）。可用于作为一 个输出端口，一般可以讲数据包发往所有标准端口，但不能发往入端 口或OFPPS_BLOCKED状态的端口。交换机也可以通过数据包的VLAller和传统分布式的比 较
➢ SDN产业联盟理事长韦乐平：
15
CCSA：openflow交换机设备要求
➢ OpenFlow交换机包含一个或多个流表、一个Group表、一个Meter表以 及一个OpenFlow通道。 其中流表及Group表用于执行报文查找和转发 ，Meter表完成Qos功能，OpenFlow通道用于和外部的控制器通信，控 制器通过OpenFlow协议管理OpenFlow交换机。
➢ CONTROLLER: 表示的OpenFlow控制器的控制通道，它可以用作一个入 端口或作为一个出端口。当用作一个出端口，封装数据包中为数据包 消息，并使用的OpenFlow协议发送。当用作一个入口端口，确认来自 控制器的数据包。
➢ TABLE: 表示openflow流水线的开始。这个端口仅在输出行为的时候有 效，此时交换机提交报文给第一流表使数据包可以通过定期通过 OpenFlow流水线处理。
18
OpenFlow的保留端口
➢ OpenFlow的保留端口指定通用的转发动作，如发送到控制器，泛洪， 或使用非OpenFlow的方法转发，如“正常”交换机处理。
➢ ALL: 表示交换机转发特定数据包到所有端口，它仅可用于为输出端口 。在这种情况下，数据包被复制后发送到所有的标准端口，包括数据 包的入端口，这些端口被配置OFPPC_NO_FWD。

SDN Architecture Overview Version 1.0December 12, 2013DisclaimerTHIS SPECIFICATION IS PROVIDED "AS IS" WITH NO WARR ANTIES WHATSO EVER, INCLUDING AN Y WARR ANTY O F MER CHANTABILITY, NONIN FRIN GEM ENT, FITN ESS FOR ANY PAR TICU LAR PUR POSE, OR ANY WARR ANTY OTHER WISE AR ISING OU T OF ANY PROPOSAL, SPECIFICATION OR SAMP LE.Without limitation, ONF disclaims all liability, including liability for infringement of any proprietary rights, relating to use of information in this specification and to the implementation of this specification, and ONF disclaims all liability for cost of procurement of substitute goods or services, lost profits, loss of use, loss of data or any incidental, consequential, direct, indirect, or special damages, whether under contract, tort, warranty or otherwise, arising in any way out of use or reliance upon this specification or any information herein.No license, express or implied, by estoppel or otherwise, to any Open Networking Foundation or Open Networking Foundation member intellectual property rights is granted herein.Except that a license is hereby granted by ONF to copy and reproduce this specification for internal use only. Contact the Open Networking Foundation at https:// for information on specification licensing through membership agreements.Any marks and brands contained herein are the property of their respective owners.NoteThis document does not claim global authority over terms used in this document and their definition. All the terms are to be considered in relation to the ONFs SDN architecture, void of any claim of applicability outside the ONFs SDN architecture context. However, for the sake of readability and brevity this document does not always prepend the string “ONF” in front of each term.This version of the overview architecture document is intentionally on a high-level and kept simple. Future more detailed versions may and will extend on this architecture. Example extensions include but are not limited to controller federation, slicing/virtualization, hybrid networks and hybrid switch considerations, layer 4-7 considerations, OA&M, security, charging, performance, northbound interface abstraction layers and functional groups, …,CreditsContributors to this document: Stuart Bailey, Deepak Bansal, Linda Dunbar, Dave Hood, Zoltán Lajos Kis, Ben Mack-Crane, Jeff Maguire, Dan Malek, David Meyer, Manuel Paul, Sibylle Schaller, Fabian Schneider, Rob Sherwood, Johann Tonsing, Tina Tsou, Eve Varma1 SDN Architecture OverviewThis document presents the high-level view of the Software-Defined Network (SDN) architecture as seen by the ONF along with key architectural principles of SDN. Precise implementation details allowed within this SDN architecture are provided in more detailed ONF architecture documents. The aim of SDN is to provide open interfaces enabling development of software that can control the connectivity provided by a set of network resources and the flow ofnetwork traffic though them, along with possible inspection and modification of traffic that may be performed in the network. Figure 1 is a graphical representation of the architectural components and their interactions.The original ONF white paper “Software -Defined Networking: The New Norm for Networks”1 shows infrastructure, control and application layers. In this architecture we refer to them as data, control, and application planes. Atbottom, the data plane is comprised of network elements, whose SDN Datapath s expose their capabilities through the Control-Data-Plane Interface (CDPI) Agent . On top, SDN Applications exist in the application plane, andcommunicate their requirements via NorthBound Interface (NBI) Drivers . In the middle, the SDN Controller translates these requirements and exerts low-level control over the SDN Datapaths, while providing relevantinformation up to the SDN Applications. The Management & Admin plane is responsible for setting up the network1 Link to white paper: https:///images/stories/downloads/sdn-resources/white-papers/wp-sdn-newnorm.pdfFigure 1: Overview of Software-Defined Networking Architectureelements, assigning the SDN Datapaths their SDN Controller, and configuring policies defining the scope of control given to the SDN Controller or SDN Application. This SDN network architecture can coexist with a non-SDN network, especially for the purpose of a migration to a fully enabled SDN network.2 Architectural componentsThe following list defines and explains the architectural components in Figure 1. ONF is continuously updating and evolving the terminology, which is tracked in the ONF Glossary project.∙SDN Application (SDN App): SDN Applications are programs that explicitly, directly, and programmatically communicate their network requirements and desired network behavior to the SDNController via NBIs. In addition they may consume an abstracted view of the network for their internaldecision making purposes.An SDN Application consists of one SDN Application Logic and one or more NBI Drivers. SDN Applications may themselves expose another layer of abstracted network control, thus offering one or more higher-level NBI(s) through respective NBI agent(s) (not shown in Figure 1).∙SDN Controller: The SDN Controller is a logically centralized entity in charge of (i) translating the requirements from the SDN Application layer down to the SDN Datapaths and (ii) providing the SDNApplications with an abstract view of the network (which may include statistics and events).An SDN Controller consists of one or more NBI Agents, the SDN Control Logic, and the CDPI driver.Definition as a logically centralized entity neither prescribes nor precludes implementation details such asthe federation of multiple controllers, the hierarchical connection of controllers, communication interfaces between controllers, nor virtualization or slicing of network resources.∙SDN Datapath: The SDN Datapath is a logical network device, which exposes visibility and uncontended control over its advertised forwarding and data processing capabilities. The logical representation mayencompass all or a subset of the physical substrate resources.An SDN Datapath comprises a CDPI agent and a set of one or more traffic forwarding engines and zero ormore traffic processing functions. These engines and functions may include simple forwarding between the datapath’s external interfaces or internal traffic processing or termination functions. One or more SDNDatapaths may be contained in a single (physical) network element—an integrated physical combination of communications resources, managed as a unit. An SDN Datapath may also be defined across multiplephysical network elements. This logical definition neither prescribes nor precludes implementation details such as the logical to physical mapping, management of shared physical resources, virtualization or slicing of the SDN Datapath, interoperability with non-SDN networking, nor the data processing functionality,which can include L4-7 functions.∙SDN Control to Data-Plane Interface (CDPI): The SDN CDPI is the interface defined between an SDN Controller and an SDN Datapath, which provides at least (i) programmatic control of all forwardingoperations, (ii) capabilities advertisement, (iii) statistics reporting, and (iv) event notification.One value of SDN lies in the expectation that the CDPI is implemented in in an open, vendor-neutral andinteroperable way.∙SDN Northbound Interfaces (NBI): SDN NBIs are interfaces between SDN Applications and SDN Controllers and typically provide abstract network views and enable direct expression of networkbehavior and requirements. This may occur at any level of abstraction (latitude) and across different sets of functionality (longitude).One value of SDN lies in the expectation that these interfaces are implemented in in an open, vendor-neutral and interoperable way.∙Interface Drivers & Agents: Each interface is implemented by a driver-agent pair, the agent representing the “southern”, bottom, or infrastructure facing side and the driver representing the “northern”, top, orapplication facing side.∙Management & Admin: The Management plane covers static tasks that are better handled outside the application, control and data planes. Examples include business relationship management between provider and client, assigning resources to clients, physical equipment setup, coordinating reachability andcredentials among logical and physical entities, configuring bootstrapping. Each business entity has its own management entities. Communication among management entities is beyond the scope of this SDNarchitecture. One goal of SDN is to subsume many management tasks known from legacy network into the CDPI.3 Key Principles of this SDN ArchitectureWith SDN, the applications can be network aware, as opposed to traditional networks where the network is application aware (or rather, application ambivalent):∙Traditional (i.e. non-SDN) applications only implicitly and indirectly describe their network requirements, typically involving several human processing steps, e.g., to negotiate if there are sufficient resources andpolicy controls to support the application.∙Traditional networks (e.g. the current Internet and its services like web browsing, media streaming) do not offer a (dynamic) way to express the full range of user requirements, for example throughput, delay, delayvariation or availability. Packet headers can encode priority requests, but network providers typically do not trust user traffic markings. Therefore some networks try to infer the users requirements on their own (e.g.through traffic analysis), which may incur additional cost and sometimes leads to misclassification. SDNoffers the ability for a user to fully specify its needs in the context of a trusted relationship that can bemonetized.∙Traditional (i.e. non-SDN) networks do not expose information and network state to the applications using them. Using an SDN approach, SDN Applications can monitor network state and adapt accordingly.The control plane is (1) logically centralized and (2) decoupled from the data plane. The SDN Controller summarizes the network state for applications and translates application requirements to low-level rules.∙This does not imply that the controller is physically centralized. For performance, scalability, and/or reliability reasons, the logically centralized SDN Controller can be distributed so that several physicalcontroller instances cooperate to control the network and serve the applications.∙Control decisions are made on an up-to-date global view of the network state, rather than distributed in isolated behavior at each network hop. With SDN, the control plane acts as a single, logically centralizednetwork operating system in terms of both scheduling and resolving resource conflicts, as well as abstracting away low-level device details, e.g., electrical vs. optical transmission.The SDN Controller has complete control of the SDN Datapaths, subject to the limit of their capabilities, and thus does not have to compete/contend with other control plane elements, which simplifies scheduling and resource allocation. This allows networks to run with complex and precise policies with greater network resource utilization and quality of service guarantees. This occurs through a well-understood common information model (e.g. as the one defined by OpenFlow).。

DOI 10.1105/tpc.015800; originally published online April 9, 2004;2004;16;S84-S97Plant Cell Anna F. Edlund, Robert Swanson and Daphne PreussPollen and Stigma Structure and Function: The Role of Diversity in PollinationThis information is current as of October 16, 2013References/content/16/suppl_1/S84.full.html#ref-list-1This article cites 139 articles, 55 of which can be accessed free at:Permissionshttps:///ccc/openurl.do?sid=pd_hw1532298X&issn=1532298X&WT.mc_id=pd_hw1532298X eTOCs/cgi/alerts/ctmain Sign up for eTOCs at:CiteTrack Alerts/cgi/alerts/ctmain Sign up for CiteTrack Alerts at:Subscription Information/publications/subscriptions.cfm is available at:Plant Physiology and The Plant Cell Subscription Information for ADVANCING THE SCIENCE OF PLANT BIOLOGY© American Society of Plant BiologistsPollen and Stigma Structure and Function:The Role of Diversity in PollinationAnna F.Edlund,1Robert Swanson,and Daphne Preuss2Howard Hughes Medical Institute,Molecular Genetics and Cell Biology Department,University of Chicago, Chicago,Illinois60637INTRODUCTIONThe ornate surfaces of male and female reproductive cells in flowering plants have long attracted attention for their variety and evolutionary significance.These structures,and the molecules involved in sexual interactions,remain among the most rapidly evolving and diverse characteristics known.As varied as they may be,each element takes part in performing the same functions,protecting pollen and stigma from the environment, delivering and capturing pollen,promoting pollen hydration and germination,allowing the entry of appropriate pollen tubes into the stigma,and guiding the tubes to the ovary(Figure1A).In this review,we(1)survey recent discoveries of pollen and stigma functions both before and after they make contact,and(2) address the great diversity in pollen and stigma structures across taxa,focusing on how they accomplish key tasks in pollination. This system presents an exciting opportunity for the fruitful unification of cell,genetic,and genomic studies of model organ-isms with comparative studies of relationship and evolution. Angiosperm reproduction is highly selective.Female tissues are able to discriminate between pollen grains,recognizing pollen from the appropriate species while rejecting pollen from unrelated species(or from the same plant in self-incompatible species).This selectivity is accompanied by tremendous di-versity in the cell surfaces of male and female reproductive structures.The uniquely rich fossil record of pollen wall structures has been of great benefit.Literally hundreds of years of scientific effort have focused on integrating the diversity of pollen form with angiosperm taxonomy.Linking these morpho-logical differences to functional roles is more challenging, requiring molecular and genetic assays that reveal purpose not only within a species but across diverse taxa.Here,wefirst survey advances in the cellular and molecular understanding of angiosperm pollen and stigma biology,in-cluding pollen–stigma adhesion,pollen hydration and germina-tion,and pollen tube emergence and invasion.By capitalizing on microscopy,molecular,genetic,and genomic resources,it is possible to magnify and dissect cellular functions at the pollen and stigma surfaces both before and after they make contact (Table1).Recent progress has revealed key molecules and mechanisms and has poised thefield for comparative studies across taxa.In the second part of the review,we discuss diversity in pollen and stigma structures,highlighting the coadaptive evolutionary change that supports efficient pollina-tion within a species while restricting pollination between species.Given its combination of facile,genetic model systems, morphological analyses,and well-characterized phylogenies, the study of angiosperm pollination provides an excellent oppor-tunity to unify cell and comparative biology and yields insight beyond pollination to the very mechanisms of evolution and speciation.POLLEN AND STIGMA CELLULAR FUNCTIONSOverview of StructuresMature angiosperm pollen grains are unusual vegetative cells that contain within themselves sperm cells,complete with cell walls and plasma membranes.This arrangement is accom-plished soon after meiosis,when an asymmetric mitotic division produces a large cell that engulfs its diminutive sister,the generative cell(Twell et al.,1998;Yang and Sundaresan,2000). Subsequently,the generative cell undergoes a second mitosis to form the second sperm cell required for double fertilization;‘‘tricellular’’pollen completes this division before it is released from the anther,whereas‘‘bicellular’’pollen undergoes this division only later,within the elongating pollen tube.These categories do not apply to gymnosperm pollen grains,which can contain a score of cells and differ from angiosperm pollen in several other respects(Pacini et al.,1999).Pollen cells are contained within a unique pollen wall,whose construction begins when the meiocyte is newly formed,with the surface layers elaborated over time.At maturity,the pollen surface can be divided into three principal strata,with the relative amount of each varying between species:(1)an outer exine wall,itself multilayered,composed of the chemically resistant polymer sporopollenin and interrupted by openings called apertures;(2) an inner intine,also sometimes multilayered,made primarily of cellulose;and(3)a pollen coat,composed of lipids,proteins, pigments,and aromatic compounds,thatfills the sculptured cavities of the pollen exine(Figure1B).Stigmas,the receptive portions of the female tissues,bind pollen and mediate tube migration into the style.Stigmas generally are classified into two groups:wet stigmas,which are1Current address:Biology Department,Spelman College,Box349,350Spelman Lane,Atlanta,GA30314.2To whom correspondence should be addressed.E-mail dpreuss@;fax773-702-6648.Article,publication date,and citation information can be found at/cgi/doi/10.1105/tpc.015800.The Plant Cell,Vol.16,S84–S97,Supplement2004,ª2004American Society of Plant BiologistsFigure 1.Pollination in Arabidopsis.(A)Diagram of one of the two ovary chambers after removal of the ovary wall (OW).Pollen lands on the stigma,where it hydrates,germinates,and extends a tube that enters the transmitting tract (TT).The tube emerges onto the septum (S),grows up the funiculus (f),and enters the micropyle opening of the ovule (O),where it fertilizes the egg and central cell.The position of the transmitting tract inside the septum is shown in the cross-section at the bottom of the pistil.(B)Transmission electron micrograph showing the point of contact between a pollen grain (P)and a stigma papillus (S),colorized to highlight the pollen coat (pink),intine (peach),exine (green),stigma cell wall (yellow),and stigma cuticle (blue).A foot of lipid-rich material (arrows)collects between the two surfaces.The thin stigma pellicle that covers the cuticle is not distinct here.(C)Diagram of early events on the stigma,including adhesion (1),pollen coat ‘‘foot’’formation and pollen hydration (2),pollen tube emergence from the grain (3),and pollen tube invasion of the papillae cell wall and extension toward the style (4).(D)to (F)Sequence of early events during Arabidopsis pollination.(D)The pollen coat has mobilized to the site of contact between the pollen and the stigma,forming a foot between the two surfaces (arrow),as visualized with the lipid dye FM1-43.(E)The pollen tube (arrow)has emerged from the grain’s exine wall and projects itself into the stigma papillus,as visualized with the cell wall dye Congo red.(F)Pollen tubes (arrow)wind their way toward the style,moving between the cell walls of the papillae,where they are protected from the Congo red dye.Pollen and Stigma Structure and Function S85covered with surface cells that often lyse to release a viscous surface secretion containing proteins,lipids,polysaccharides,and pigments;and dry stigmas,which have intact surface cells that typically protrude as papillae and are covered by a primary cell wall,a waxy cuticle,and a proteinaceous pellicle (Figure 1B).Stigmas have been studied extensively in plants that exhibit self-incompatibility,a process that restricts inbreeding.Self-incompatible stigmas reject self-pollen by inhibiting pollen hydration,germination,and tube invasion;these processes have been reviewed recently (Nasrallah,2000;Silva and Goring,2001;Wheeler et al.,2001;Hiscock and McInnis,2003;Kao and Tsukamoto,2004).Pollen Adhesion to the Stigma:First ContactTo capture pollen grains,stigmas engage biotic and abiotic pollinators (such as insects and wind)and use rapid and strong adhesive interactions to retain pollen grains.The pollen–stigma interface can differ from species to species as a result of the wide variability in the morphology and content of stigma exudates,exine layers,and pollen coats.Several different methods have been devised to investigate and measure pollen–stigma adhe-sion (Stead et al.,1979;Luu et al.,1997a,1997b;Zinkl et al.,1999),including chemical and detergent washes and centrifu-gation-based adhesion assays.Here,we focus on the char-acterization of pollen adhesion on the dry stigmas of the Brassicaceae,particularly Arabidopsis thaliana and Brassica oleracea .In both cases,stigmas adhere poorly to pollen grains from other botanical families,demonstrating a specificity that restricts inappropriate pollen access (Luu et al.,1998;Zinkl et al.,1999).Adhesion during downstream stages (such as between pollen tubes and the style)has been reviewed recently (Lord and Russell,2002;Lord,2003).In Arabidopsis,the nature of the pollen–stigma interface has been shown to change as pollination progresses,becoming considerably stronger over time,with different types of adhesive contacts supplementing and supplanting each other (Luu et al.,1997a,1997b;Zinkl et al.,1999).Zinkl et al.(1999)measured a very rapid ‘‘initial’’adhesion step in Arabidopsis that relies on the pollen exine but not on the pollen coat.This initial adhesion is not likely to be based on protein–protein interactions,because purified exine fragments retained their binding capacity even when washed with a range of compounds,including organic solvents,salts,and reducing and oxidizing agents.Thus,pollen capture in Arabidopsis most likely depends on biophysical and/or chemical interactions between the stigma surface and the polymers of the pollen exine;the nature of these contacts remains unknown.Identification of these selective adhesives requires further analysis of exine structure and characterization of stigma pellicle components.Given the challenge of purifying large quantities of these chemically complex materials,genetic dissection of the pathways required for their synthesis may prove most worthwhile.After exine-mediated adhesion,mobilization of the pollen coat occurs,leading to mixing of lipids and proteins to form a ‘‘foot’’of contact on the stigma surface (Figures 1C and 1D).There is now extensive evidence that the proteins and lipids in the pollen coat,and proteins on the stigma surface,Table 1.Pollen and Stigma Structural Features and Their Roles Structural Feature Proposed Functional Significance Pollen grain size Biotic and abiotic pollinatorpreference and fluid dynamics Number of pollengrains per pollination unit May increase delivery efficiencyPollen coatProtects pollen cells from excess desiccation after dehiscence;protects from UV radiation and pathogen attack;stickiness,color,and aroma may affectinteraction with pollination vectors;protein components involved in adhesion,signaling,andcompatibility;lipids and proteins are necessary for hydrationExine patternInteracts with biotic and abiotic pollination vectors;affects the surface area of the stigmainterface;mediates stigma adhesion;retains pollen coat;affects wall strength and elasticityExine porosityMicrochannels are sites of water egress and ingress during desiccation and hydration;progressive desiccation limitspollen viability and life expectancy Aperture size,number,and complexityAffect environmental vulnerability to desiccation,fungal invasion,and mechanical stress;accelerated desiccation limits pollen viabilityand life expectancy;sites of focused water ingress during hydration;allow extreme volume changes accompanying desiccation and hydration;serve as portals forpollen tube exit during germination IntineThickness and complexity are inversely coordinated with exine and pollen coat characteristics;specialized layers and inclusions at apertures are involved in pollen tube emergence and invasion of the stigma cell wallStigma coatDefines stigmas as uniquelywater-permeant sites on the plant;proteins and lipids are involved in adhesion,hydration,and germination;dry stigmas are pollen compatibility sites,with selective support of pollen hydration and germination,whereas wet stigmas often are covered in exudates from apoptotic cells and blockinappropriate pollination only in later steps;at the point of contact between pollen and stigma,the two coatings may mix,a process that mediates coatconversion,hydration,germination,and stigma invasionS86The Plant Cellalso contribute to adhesion,albeit most likely at a later stage than at the time of initial contact.This second stage was measured and shown to require protein–protein interactions, including interactions between pollen coat proteins and the Brassica S-locus–related protein(SLR1)(Luu et al.,1999; Takayama et al.,2000).SLR1is a stigma-expressed gene related to the S-locus glycoprotein,a protein involved in self-incompatibility(Lalonde et al.,1989).SLR1-mediated adhe-sion requires some time to establish itself,possibly to allow the mixing of stigma and pollen matrices;consequently,the strength of initial pollen binding in SLR1-deficient plants is the same as in wild-type plants(Luu et al.,1999).SLR1binds to pollen coat proteins.This binding,along with the enhanced cell-to-cell adhesion,can be blocked by antibody inhibition of the SLR1proteins(Luu et al.,1999;Takayama et al.,2000). As pollen tubes germinate from the grains,afinal stage of adhesion begins in which the pollen tube passes through the foot into the stigma surface,tethering the emptying pollen grain to the stigma via the tube(Zinkl et al.,1999;Dickinson et al.,2000;Heizmann et al.,2000).When inappropriate pollen grains reach this stage,further access is blocked by inhibiting tube growth.This mode is common in plants with wet stigmas,in which pollen grains typically are bound and hydrated without applying species selectivity(Wheeler et al., 2001).Pollen Hydration:Activating MetabolismMost pollen grains are metabolically quiescent and highly desiccated,ranging from15to35%water content,when released from the anthers(Heslop-Harrison,1979a;Buitink et al.,2000).Water immediately surrounds grains that land on a wet stigma,but those that land on dry stigmas mobilize their lipid-rich pollen coat to form an interface between the two cell surfaces.This interface converts to a histochemically distin-guishable form thought to promote waterflow(Elleman and Dickinson,1986;Elleman et al.,1992).Water,nutrients,and other small molecules are transported rapidly into the grain from the stigma exudate(wet stigmas)or stigma papillae(dry stigmas)by mechanisms that remain unclear.The discovery of aquaporin expression in the stigma has prompted the exciting model that water channels are involved in the rapid and regulated water release from the stigma to the pollen(Dixit et al.,2001).Also,the stigma cuticle may be uniquely perm-eable to water traffic(Lolle et al.,1997,1998;Pruitt et al., 2000).Regardless of the mechanism of transfer,pollen hy-dration often is regulated,both temporally and spatially. Inappropriate hydration can have disastrous consequences, leading to premature germination within the anther(Johnson and McCormick,2001)or germination on the wrong surface (Lolle and Cheung,1993;Lolle et al.,1998).In plants with dry stigmas,regulated pollen hydration provides an effective early barrier to incompatible pollination.This mode is active in self-incompatible crosses(Sarker et al.,1988)and in crosses between species(Lewis and Crowe,1958;Hu¨lskamp et al., 1995).These processes are remarkably localized:the stigma can hydrate a compatible grain while restricting the hydration of foreign or incompatible pollen on the same papillus (Dickinson,1995).An understanding of the precise surface changes that provide this exquisite control will require both molecular dissection of stigma and pollen surface components and genetic analyses of their roles.The genetic and molecular dissection of the lipid-rich matrices found in the pollen coat has progressed considerably in recent years.In Arabidopsis,the pollen coat contains long-and short-chain lipids along with a small set of proteins, including six lipases and eight Gly-rich oleosin proteins that contain a lipid binding domain(Mayfield et al.,2001;Fiebig et al.,2004).The pollen coating of Brassica is structured similarly and also has been shown to contain peptides involved in self-incompatibility(Doughty et al.,1993).Disrupting pollen coat lipids or pollen coat proteins in Brassicaceae species can delay or block pollen hydration.In particular,mutations that impair long-chain lipid synthesis,and consequently the proper assembly of the pollen coating,severely reduce the hydration of Arabidopsis pollen and result in male sterility(Preuss et al., 1993;Hu¨lskamp et al.,1995).Hydraulic contact can be restored to these mutant grains by the addition of purified triacylglycerides(Wolters-Arts et al.,1998).Mutations that affect pollen coat proteins are less extreme,perhaps because of partial functional redundancy.The most abundant protein in the Arabidopsis pollen coat is the Gly-rich oleosin GRP17; seven related proteins have similar oleosin domains but variable hydrophilic tails.Mutations in GRP17delay the onset of pollen hydration and decrease the ability of the mutant pollen to effectively compete with wild-type pollen(Mayfield and Preuss,2000).The sequences of the GRP proteins are widely divergent across the Brassicaceae,supporting a model in which pollen coat diversification may contribute to specia-tion(Mayfield et al.,2001;Fiebig et al.,2004).Unlike the GRP proteins,the pollen coat lipids of most Brassicaceae species are quite similar,making these molecules poor candidates for dictating species specificity(Piffanelli et al.,1998).Conse-quently,experiments that promote pollen hydration by the application of exogenous lipids(Wolters-Arts et al.,1998, 2002)may bypass crucial regulatory steps that normally occur in vivo,such as those restricting the hydration of incompatible or foreign pollen.The lipid-rich stigma exudate of plants with wet stigmas is thought to be functionally analogous,in part,to the pollen coat(Dickinson,1995).Mutations that eliminate this secreted matrix cause female sterility;this defect can be bypassed by adding exogenous lipids(Goldman et al.,1994;Wolters-Arts et al.,1998).The addition of cis-unsaturated triacylglycerides is sufficient for hydration and germination,even on stigmaless tobacco plants(Wolters-Arts et al.,1998).These observations suggest that lipids could play an important role in the permeability of the stigma cuticle to water(Lolle et al.,1998;Pruitt et al.,2000). Infiddlehead,which is defective in a b-ketoacyl-CoA synthase required for long-chain lipid synthesis,leaf cuticle permeability increases and pollen hydration is stimulated on inappropriate cell surfaces.Based on thesefindings,it is possible to propose a model in which the presence of lipids,whether provided by the male or the female surface,modulates water transfer to desiccated pollen,while highly diverse proteins and peptides mediate self and foreign pollen recognition.Pollen and Stigma Structure and Function S87Pollen Polarization and Germination:Preparingfor Pollen Tube GrowthHydration transforms a pollen grain from a nonpolar cell to a highly polarized cell.Whether tubes emerge on a dry stigma surface,from a grain submerged in stigma exudates,or from pollen germinated in vitro,the grain organizes its cytoplasm and cytoskeleton to support the extension of a single tube. These changes occur within minutes after hydration and include the formation offilamentous cytoskeletal structures that wrap around the nuclei,actin cytoskeleton polarization toward the site of tube emergence(Tiwari and Polito,1988; Heslop-Harrison and Heslop-Harrison,1992),reorientation of the large vegetative nucleus so that it enters the extending tube before the generative cells(Heslop-Harrison et al.,1986a; Heslop-Harrison and Heslop-Harrison,1989;Lalanne and Twell,2002),assembly of mitochondria and polysaccharide particles at the site of the elongating tube tip(Cresti et al., 1977,1985;Mazina et al.,2002),and selection of the pollen plasma membrane for secretory vesicle targeting and deposi-tion of callose(b-1!3glycan)at the site of tube emergence (Johnson and McCormick,2001).It is not yet clear how the polarization signal is perceived and subsequently transduced to select a single point for tube emergence.Several candidate signals have been suggested, including water,lipids,and ions(Feijo et al.,1995;Lush et al., 1998;Wolters-Arts et al.,1998).Evidence for water as a polarity signal has come from in vitro experiments with Nicotiana pollen:immersing these grains in purified lipids or in stigma exudates,and providing a nearby aqueous interface,results in polarized growth toward the aqueous medium,mimicking behavior in vivo.The pollen tubes emerge from the aperture closest to the hydrophobic–aqueous interface,suggesting that the source of water provides directional cues that establish polarity(Lush et al.,1998;Wolters-Arts et al.,1998).Not only does this example implicate water,it also suggests a role for lipids in establishing polarity.Additional support for this idea comes from mutants that lack the lipid-rich pollen coating (Preuss et al.,1993;Hu¨lskamp et al.,1995).As described above,such grains are unable to hydrate or germinate on the stigma.Coat-deficient grains do hydrate at increased ambient humidities,but because they lack key axial information,their pollen tubes often emerge at random orientations relative to the stigma(Dickinson,1995).Polarization signals ultimately trigger the recruitment of RHO OF PLANTS1(ROP1),a GTP binding protein involved in F-actin dynamics and the establishment of calcium gradients at tube tips (Gu et al.,2003).ROP1,with its binding partner ROP INTERACT-ING CRIB-CONTAINING1protein,localizes to the tip of the growing pollen tube,where they act to focus secretory vesicle delivery(Kost et al.,1999;Li et al.,1999;Wu et al.,2001).Other proteins that preferentially localize to the growing pollen tube tip may play a role in the initial establishment of cell polarity. One candidate,found at the tips of elongating fern rhizoids, is annexin,a protein believed to be involved in tip-oriented exocytosis events;annexins exhibit calcium-dependent binding to phospholipids,affect cytoskeletal structure,and potentially modulate voltage-dependent Ca21channels(Clark et al.,1995).Pollen activation promises to be a particularly rich subject for studies integrating the external cues and internal signal molecules necessary for global rearrangements and polarized tip growth.Once the cell has established its internal polarity relative to an external signal,the pollen tube must breach the exine wall to emerge from the grain.Depending on the species examined, pollen tubes either grow out of the apertures or break directly through the exine wall.In rye and eucalyptus,tubes emerge strictly at the apertures,by dissolving apertural intine layers, rupturing the thin sporopollenin wall,and displacing the opercula that guard these sites(Heslop-Harrison,1979b;Heslop-Harrison and Heslop-Harrison,1985;Heslop-Harrison et al.,1986b).In Arabidopsis,whose pollen has three distinct apertures,pollen tubes often break directly through interaperture exine walls precisely at the site of contact with the stigma surface(Figure 1E).Regardless of exit site,pollen tube escape requires either(1) exine weakening by enzymatic digestion from the inside or outside of the wall,or(2)exine tearing by local gel-swelling forces or focused turgor pressure.Evidence for the former possibility comes from reports of significant exine remodeling after contact with the stigma(Gherardini and Healey,1969;Dickinson and Lewis,1974).Gel-swelling forces or turgor pressure also could play important roles,but because such forces radiate in all directions,they must be focused at the site of emergence.A combination of mechanisms is most likely,with the increased turgor pressure of the pollen grain contributing to the rupture of a patch of partially degraded exine at the pollen–stigma inter-face.There is evidence that pollen grains are preset to harness biomechanical forces for pollen tube emergence;dead pollen grains are able to hydrate to the same extent as living grains, swell,alter apertural coverings,and even germinate short tubes before the tubes ultimately rupture(Heslop-Harrison,1979b). The challenge of purifying factors that regulate cell polarity and tube emergence calls for the use of complementary genetic strategies.For example,mutations that upset the physical association between the two Arabidopsis sperm cells and the vegetative pollen nucleus have been identified.These mutations disrupt the polarized transport of the male germ unit to the pollen tube tip by(1)separating the vegetative nucleus from the two sperm cells(germ unit malformed1[gum1]and gum2)or(2) mislocalizing an intact male germ unit to the pollen wall(mgu displaced1[mud1]and mud2)(Lalanne and Twell,2002).Other visual screens have identified mutations that allow tubes to grow without emerging from the grain or disrupt the accumulation and deposition of callose before pollen tube emergence(Johnson and McCormick,2001).In addition,screens for pollen gameto-phytic mutations have yielded mutants altered in germination, polarity,and tube emergence(Ryan et al.,1998;Grini et al.,1999; Johnson and McCormick,2001;Procissi et al.,2001;Johnson and Preuss,2002;Lalanne and Twell,2002).Similar approaches have not yet yielded stigma-specific mutations,despite nu-merous screens to isolate sporophytic sterile mutants.This deficiency suggests functional redundancy in the pollen-recog-nition machinery involved.On the other hand,the fertility of Arabidopsisflowers with genetically ablated stigma cells(Kan-dasamy et al.,1993;Thorsness et al.,1993)calls into question the absolute requirement for the tissue.S88The Plant CellPollen Tube Invasion:Growing into the StigmaAfter crossing the exine wall,pollen tubes can only enter the style after transiting the stigma barrier.The details of this process vary considerably from species to species.In plants with open styles, the stigma is covered with an epidermis that is continuous with the style,but in species with closed stigmas,pollen tubes grow through the outer cuticle and cell wall of the stigma papillae to enter the style.For>100years,enzymes secreted by pollen have been proposed to play an important role in pollen tube invasion of the stigma surface(Green,1894).Acid phosphatase,ribonucle-ase,esterase,amylase,and protease activity have been localized to pollen intines and tubes(Knox and Heslop-Harrison, 1970).Esterases,particularly those known as cutinases,are important for breaching the stigma cuticle;they have been identified in the pollen of Brassica,Tropaeolum,and many other taxa(Knox and Heslop-Harrison,1970;Shaykh et al.,1977; Hiscock et al.,1994)as well as in the proteinaceous pellicles of dry stigma surfaces(Heslop-Harrison and Shivanna,1977; Hiscock et al.,2002b).Removing or disrupting the pellicle prevents compatible pollen tubes from entering the stigma, despite normal germination(Heslop-Harrison and Heslop-Harrison,1975;Heslop-Harrison,1977;Heslop-Harrison and Shivanna,1977).In addition,cutinase inhibitors significantly reduce the ability of pollen tubes to penetrate Brassica stigmas (Hiscock et al.,2002a).Hydrolysis of pectin in the stigma cell wall also is necessary,for which pollen expresses genes encoding pectin esterase and pectate lyase(Kim et al.,1996;Wu et al., 1996).A pollen-specific Brassica polygalacturonase also has been detected at the tip of pollen tubes as they enter the stigma papillar cell walls(Dearnaley and Daggard,2001).Enzymatic penetration of the stigma surface is precisely controlled to not expose the pistil to pathogenic or inappropriate invasion.This control likely requires constant communication between the pollen tube and the stigma.Recently,receptor kinases have been identified as candidate mediators of communication between the pollen tube and the stigma. Members of the large Leu-rich repeat receptor kinase family (Lycopersicon esculentum protein receptor kinase1[LePRK1] and LePRK2)localize to the growing pollen tube(Muschietti et al., 1998).Yeast two-hybrid experiments using the extracellular domains of these kinases yielded many candidate ligands, including cell wall–remodeling proteins and Cys-rich proteins implicated in extracellular signaling.One pollen-specific,Cys-rich protein,LATE ANTHER TOMATO52(LAT52),interacts in vivo with LePRK2;LAT52is essential for pollen hydration and germination in vitro and for normal tube growth in vivo.This finding suggests that LAT52and LePRK2are members of an autocrine signaling cascade that may regulate and maintain pollen tube growth(Tang et al.,2002).POLLEN AND STIGMA STRUCTURAL DIVERSITYAlthough the processes discussed above are conserved across angiosperms,they are accomplished by an astounding variety of pollen and stigma structures.Male and female reproductive tissues evolve rapidly;yet,to maintain their interactions,they also evolve coordinately.This evolution is evident in angiosperm pollen grains,which are remarkably diverse,varying in size(from 20to250m m in diameter),in their pigments(Lunau,2000),in their aromas(Dobson and Bergstrom,2000),in their nutritional content for pollinators(Roulston and Cane,2000),and in the patterns of their ornate exine structures(Figures2A to2D).Many of these characteristics are important for pollen delivery and for modulating the adhesion,hydration,and emergence of pollen tubes.Stigma cells are similarly diverse(Figures2E and2F);they vary in form,are either unicellular or multicellular,and differ in wall structure and cuticle pattern while remaining compatible with their pollen counterparts.The taxonomic relationships of a wealth of angiosperms are understood(Qiu et al.,1999).With the availability of model systems that reveal gene functions,the power of comparative genomics can now be harnessed to investigate the role of genetic diversity in plant reproduction.This system represents an opportunity with enormous potential:it is now possible to merge an understanding of cell biology with the immense evolutionary diversity seen in nature.Pollen Walls:Generating Structural DiversityPollen wall patterns are so diverse and often so characteristic of each species that they have long been used for taxonomic classification and even for forensic identifications(Szibor et al., 1998).Complete understanding of the evolution of diversity in pollen wall characteristics calls for the study of the genes and mechanisms behind their development.Species-specific pollen patterns are established early in meiosis(Sheldon and Dickinson, 1983).Microsporogenesis can be divided into two broad classes:(1)simultaneous,in which thefirst meiotic division is followed immediately by the second,with a wall of callose that is deposited subsequently around all four tetrahedrally arranged microspores;and(2)successive,in which a callose wall is deposited around the dyad formed by thefirst meiotic division, before the second division occurs.Simultaneous microsporo-genesis usually is associated with eudicots,whereas successive microsporogenesis is associated with monocots,although to a lesser extent,because of several evolutionary reversals (Furness et al.,2002).Microspore alignment within the callose wall is important on several levels,including internal vegetative and generative cell polarity(Twell et al.,1998),the release of pollen as a monad,tetrad,or cluster,and exine sculpting and aperture position.A primexine matrix,the precursor of exine, typically is deposited between the callose wall and the plasma membrane of the developing microspores.The callose wall later is degraded,enabling the pollen grains to be released as monads (Paxson-Sowders et al.,1997).In some species,including a number of ancientflowering plants,pollen grains instead remain in fused clusters,an arrangement thought to increase pollination efficiency.Mutants defective in the early stages of pollen development may provide insight into genes that are modulated during evolution,resulting in the striking variation of pollen form.For instance,mutations that mimic natural tetrad clusters have been found in Arabidopsis.In quartet mutants (Figure3),callose deposition is disrupted during microspore development,causing the exine layers of pollen tetrads to merge (Preuss,1994;Rhee and Somerville,1998;Rhee et al.,2003). quartet mutations have been useful for genetically dissecting Pollen and Stigma Structure and Function S89。

软件定义网络技术Software Defined Networking，SDN教学大纲课程概述软件定义网络（Software Defined Network，SDN）作为新一代网络架构，近年来得到了学术界和产业界的广泛关注，随着SDN技术的发展及其在产业界的落地，网络工程人才需求已发生重大变化，SDN技术人才出现明显短缺。

本课程顺应网络工程新技术发展与新人才需求，以培养网络工程新工科人才为目标，同时，本课程还支持中国高校计算机大赛网络技术挑战赛，以赛促学。

本课程视频主要内容包括：SDN概述、基本原理、南向接口协议、数据平面、控制平面、北向接口和SDN应用开发等内容；除教学视频外，本课程还设计了实验和课后练习等环节，实验内容包括Mininet应用实践、OpenFlow协议分析、Open vSwitch应用实践、OpenFlow交换机应用实践、数据平面编程实践、POX 与RYU控制器编程实践等内容。

通过本课程的学习，能够将软件定义网络的基础知识、主流技术与方法，用于具体网络工程项目需求的分析；能够形成软件定义网络技术的思想方法，并用于分析或理解复杂网络问题，指导网络的规划与设计；能够利用控制器提供的开放接口，开发网络应用程序。

授课目标能够将软件定义网络的基础知识、主流技术与方法，用于具体网络工程项目需求的分析与理解；能够形成软件定义网络技术的思想方法，并用于分析或理解复杂网络问题，指导软件定义网络的规划与设计；能够利用控制器提供的北向API接口进行网络编程；能够使用数据平面编程语言进行网络编程；能够具有运用英语阅读软件定义网络技术资料的初步能力；课程大纲第一讲概述1.1 软件定义网络技术的产生1.2 软件定义网络技术的发展1.3 软件定义网络技术的标准化1.4 阅读资料第一讲单元测试第二讲基本原理2.1 软件定义网络的基本架构2.2 软件定义网络技术的核心思想2.3 实验一、Mininet应用实践2.4 阅读资料第二讲单元测试第三讲数据平面3.1 SDN数据平面3.2 Openflow概述3.3 OpenFlow 流表3.4 SDN交换机3.5 实验二、Open vSwitch应用实践3.6 阅读资料第三讲单元测试第四讲南向接口协议4.1 南向接口协议概述4.2 OpenFlow协议4.3 OF-CONFIG协议4.4 NetConf协议4.5 实验三、OpenFlow协议分析4.6 阅读资料第四讲单元测试第五讲控制平面5.1 SDN控制平面5.2 开源SDN控制器5.3 控制器编程案例5.4 实验四、POX控制器编程实践5.5 实验五、RYU控制器编程实践5.6 阅读资料第五讲单元测试第六讲北向接口6.1 北向接口概述6.2 什么是REST API6.3 REST API设计规范6.4 阅读材料第六讲单元测试第七讲SDN商用解决方案-DNAC7.1 DNAC 平台7.2 Intent APIs7.3 Events and Notifications7.4 Multivendor SDK7.5 IT Tools Integration7.6 开发资源7.7 阅读资料预备知识先修课程：计算机网络基础、程序设计基础参考资料[1].《软件定义网络技术》，高等教育出版社。

IEEECOMMUNICATIONSSURVEYS&TUTORIALS,VOL.16,NO.3,THIRDQUARTER20141617ASurveyofSoftware-DefinedNetworking:Past,Present,andFutureofProgrammableNetworks

BrunoAstutoA.Nunes,MarcMendonca,Xuan-NamNguyen,KatiaObraczka,andThierryTurlettiAbstract—Theideaofprogrammablenetworkshasrecentlyre-gainedconsiderablemomentumduetotheemergenceoftheSoftware-DefinedNetworking(SDN)paradigm.SDN,oftenreferredtoasa“radicalnewideainnetworking”,promisestodramaticallysimplifynetworkmanagementandenablein-novationthroughnetworkprogrammability.Thispapersurveysthestate-of-the-artinprogrammablenetworkswithanemphasisonSDN.Weprovideahistoricperspectiveofprogrammablenetworksfromearlyideastorecentdevelopments.ThenwepresenttheSDNarchitectureandtheOpenFlowstandardinparticular,discusscurrentalternativesforimplementationandtestingofSDN-basedprotocolsandservices,examinecurrentandfutureSDNapplications,andexplorepromisingresearchdirectionsbasedontheSDNparadigm.

I2RS Diameter
Netflow XML/Snmp/CLI 3. 早期两个相关工作组：转发与控制分离组ForCES（
Existing Routers/Switches
Figure2 I2RS and other existing device interfaces
Forwarding and Control Element Separation）和应用层流 量优化工作组ALTO（Application-layer traffic optimization）
控制层
SDN 控制软件
网络服务
控制层主要负责处理数据转发面资源的抽象信息，可支持网络 拓扑、状态信息的汇总和维护，并基于应用的控制来调用不同 的转发面资源
控制/数据平面接口，例如OpenFlow
基础设施层
网络设备 网络设备 网络设备
网络设备
网络设备
ONF提供的SDN架构
1、OpenFlow协议制定和完善是目前ONF最高优先级的任务 2、 OpenFlow规范已发布多个版本，OpenFlow 2.0预计 2014年推出
2. 将网络价值向服务和软件转型，实现其向IT转型的战略意图 3. Open Daylight形成SDN系统和架构的事实标准，建立SDN生态链
Page 15
ONF和ODL的正面较量
ONF致力Openflow标准定义
董事会中只有互联网公司和运营商，没有设备厂商
应用 控制器 设备
APP
APP
APP
API
移动核心网VAS(增值业务系 统)随MBB流量↑,扩容压力大
2 提升骨干网带宽利用率
通过SDN集中管控机制实现全 局路由优化,提升带宽利用率

OpenFlow
一个简单的类比
应用/市场
系统
硬件
……
Openflow协议介绍
OpenFlow简介和架构
OpenFlow 1.0
OpenFlow 1.1
OpenFlow是SDN架构中定义的一个控制器与转发层之间的通信接口标准，OpenFlow允 许直接访问和操作网络设备的转发平面
OpenFlow的思想是分离控制平面和数据平面，二者之间使用标准的协议通信；数据平面采 用基于流的方式进行转发。
SDN的相关组织二
国际主流运营商发起成立的ETSI： 网络功能虚拟化工作组（Network Function Virtualizetion，NFV）
NFV的目标是利用当前的一些IT虚拟化技术，讲多种网络设备虚拟到大量符合行业 标准的物理服务器、交换机或者存储设备上，然后在这些标准的硬件上运行各种执 行这些网络功能（路由、安全功能、负载均衡、SBC等） Opendaylight 由各软硬件厂商成立目标打造一个开源的基于SDN的平台框架
OpenFlow网络由OpenFlow网络设备（OpenFlow Switch）和控制器（OpenFlow Controller）通过OpenFlow通道（OpenFlow Channel）组成
OpenFlow的几个基本概念
包+
包输入 Packet In
输入端口
入端口+
元数据
表0
表1
动作集 = {}
IDC：SDN市场规模2016年预计将达37亿美元
什么是SDN
SDN是Software Defined Network的缩写，软件定义网络，SDN并不是一个具体的技术，它 是一种网络设计理念，规划了网络的各个组成部分（软件、硬件、转发面和控制面）及相互之间 的互动关系，其特征属性包括下面几点：

能 力 进 行控 制 。 这 种 原 来 与 网络 设 备 紧密耦 合 的控 制 能 力 ， 通 过 可 变 成 的 方 式调 用底 层 设 施 能 力 的 抽 象描 述 ，从 而 方便 将
在数 据 中心 出 口部 署 支 持 Ｓ Ｄ Ｎ技 术 的 路 由设 备 ， 对 链 路 的 带宽 利 用 效 率 以及 应 用 的流 量 进 行 监 控 ，并 将 监 控 结 果 发
用， 能 够 对 网络 对 应 用 的 转 发 和 处 理 的 策略 进 行 管理 和 控 制 ，
据 的转 发 进 行 处 理 ： 基 础 设施 层 则 是 负责数 据 的 处理 、 转 发 和
状 态收 集。
同 时也 支持 对 网 络属 性 的配 置 ：控 制 层 则 主 要 负责 数 据 对 数 效 率 。
【 关键词 】 Ｓ Ｄ Ｎ ； 发展 ； 网络通信 ； 应用 【 中图分类 Ｉ Ｔ Ｎ ９ １ ５ 【 文献标识码 】 Ａ 【 文章编号 】 １ ０ ０ ６ － ４ ２ ２ ２ （ ２ ０ １ ５ １ ｌ ５ 一 Ｏ ｏ ４ ８ ＿ ｏ ２
１ Ｓ Ｄ Ｎ概述及发展 现状
Ｓ ＤＮ 是 一 种 新 型 的 网 络 架 构 ． 实 现 了 网 络 控 制 和 转 发 能
数 据 中心 控 制 器对 各 个 数 据 中心 出 口 网络 能 力 堪称 逻 辑 分 离或 者 虚 拟 化 的 子 网来 满 足 用 户 的 多样 送 到 数 据 中心控 制 器 的路 由设 备进 行 集 中控 制 ． 统一 调 配 多 个数 据 中心 出 １ ３ ＇ 的 链 化 需求 。根 据 Ｏ Ｎ Ｆ组 织 对 Ｓ Ｄ Ｎ的定义， Ｓ Ｄ Ｎ包括应用层、 控 制 层 和 基 础 设 施层 ３个 层 面【 １ 】 。 其 中应 用 层 包括 多种 业 务 和 应 路 和 业 务 的 流 量 流 向 。从 而根 据 实 际 需 求和 链 路 的 实 际情 况 实现 对 链 路 资 源 的 实 时 调 节 ．有 效提 升链 路 带 宽 资 源 的利 用

Organizations that have adopted SD-WAN have reported an enhanced ability to portions of the network to address quality of service issues resulting from ‘peaky’ traffic from apps using voice or video.
Do we have mission-critical applications that are bandwidth-hungry or particularly jittersensitive and which need to be secure and free of congestion or quality of service issues?
现在，网络不久是连通， 更关注面向应用
Private WAN
Regional Hub and Campus
Remote and Branch Office
Carrier Neutral Facilities
Public Internet
2016+
Remote and Branch Office
Mobile
公司广域网例子 – 架构简单，网络清晰
App. teams
IT Ops
Disaster Recovery Site
Data Center
Internet
Regional Hub and Campus
Private WAN
Remote and Branch Office
过去，网络仅仅是网络，只关注连通
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