[CN notes] SDN (Part 1)

CN notes 前情提要：傳送門

目錄：
• What spurred the development of Software Defined Networking (SDN)?
• What are the three phases in the history of SDN?
• Summarize each phase in the history of SDN.
• What is the function of the control and data planes?
• Why separate the control from the data plane?
• Why did the SDN lead to opportunities in various areas such as data centers, routing, enterprise
networks, and research networks?
• What is the relationship between forwarding and routing?
• What is the difference between a traditional and SDN approach in terms of coupling of control and data plane?
• What are the main components of an SDN network and their responsibilities?
• What are the four defining features of an SDN architecture?
• What are the three layers of SDN controllers?

What spurred the development of Software Defined Networking (SDN)?

SDN arose as part of the process to make computer networks more programmable . Due to a diversity of equipment and the requirement to handle different proprietary technologies from different vendors for different network devices, SDN works to redesign networks to make them more manageable.

What are the three phases in the history of SDN?

SDN的歷史可以分為三個階段：

1. Active networks

2. Control and data plane separation

3. OpenFlow API and network operating systems

Summarize each phase in the history of SDN.

1. Packaging of code in network packets to create flows / QoS for different packet types. Most of this occurred at the edge of the network. Code was executed inside of Virtual Machines. Backbone network managers were not comfortable with arbitrary code execution by any developer on network nodes.
2. Packet forwarding was implemented directly in the hardware. This era tailored to administrators being able to control / manage routing decisions. Emphasized programmability in the control domain rather than the data domain.
3. The vision of fully programmable networks for research is realized, and then actually used by businesses / ISPs. Abstracted away the specifics of vendor software / proprietary products and allowed the entire network to be treated as one programmable entity. Lead to the rise of network operating systems and distributed state management.

What is the function of the control and data planes?

• The control plane contains the logic that controls the forwarding behavior of routers such as routing protocols and network middlebox configurations.
• The data plane performs the actual forwarding as dictated by the control plane. For example, IP forwarding and Layer 2 switching are functions of the data plane.

Why separate the control from the data plane?

• Independent evolution and development — 在傳統方法中，路由器負責路由和轉發功能。這意味著對其中任何一個功能的更改都需要升級硬件。在這種新方法中路由器只關注轉發。因此，此設計中的創新可以獨立於其他路由考慮因素進行。類似地，可以在不影響任何現有路由器的情況下改進路由算法。通過限制這兩個功能之間的相互作用，我們可以更輕鬆地開發它們。
• Control from high-level software program — 在 SDN 中我們使用軟件來計算轉發表。因此，我們可以很容易地使用高階程序來控制路由器的行為。功能的解耦使調試和檢查網絡的行為更加容易。

Why did the SDN lead to opportunities in various areas such as data centers, routing, enterprise networks, and research networks?

• Data centers — 考慮擁有數千台服務器和虛擬機的大型數據中心。管理如此龐大的網絡並不容易。SDN 有助於簡化網絡管理。
• Routing — 今天使用的域間路由協議 BGP 限制了路由。對入站和出站流量的控制有限。路線選擇需要遵循一套程序。此外，很難使用多個標準做出路由決策。使用 SDN，更新路由器的狀態更容易，而且 SDN 可以提供對路徑選擇的更多控制。
• Enterprise networks — SDN可以提高企業網絡的安全應用。例如如果我們在網絡的戰略位置丟棄攻擊流量，則使用 SDN 可以更輕鬆地保護網絡免受 DDoS 等容量攻擊。
• Research networks — SDN 允許研究網絡與生產網絡共存。

What is the relationship between forwarding and routing?

• In forwarding, the router inspects the packet coming in from a port, consults the routing table, and sends the packet to the output port dictated by the forwarding table. Forwarding is a function of the data plane.
• In routing, routers use routing algorithms to generate the forwarding table, implementing policies and using these algorithms to determine the best path for traffic. Routing is a function of the control plane.

What is the difference between a traditional and SDN approach in terms of coupling of control and data plane?

In the traditional approach, the control and data planes are closely coupled. The router runs routing algorithms and generates the forwarding table . In the SDN approach, the remote controller computers and distributes forwarding tables. The controller is physically separated from the router.

What are the main components of an SDN network and their responsibilities?

• SND-controlled network elements — infrastructure layer, responsible for forwarding traffic using rules computed by the SDN control plane.
• SDN controller — logically centralized entity that behaves as an interface between the network elements and the network-control applications.
• Network-control applications — programs that manage the underlying network by collecting information about the network elements with the help of the SDN controller.

What are the four defining features of an SDN architecture?

• Flow-base forwarding — rules for packets can be computed based on any number of header field values in various layers (transport, network, link).
• Separation of data and control planes
• Network control functions — controller maintains up-to-date network state information and provides it to the network-control applications. This enables network-control applications to monitor and control network devices.
• Programmable network — enables the implementation of sophisticated routing applications to include network management, traffic engineering, security, automation, analytics, etc.

What are the three layers of SDN controllers?

• Communication layer — comms between the controller and the network elements
• Network-wide state-management layer — stores information about the network state
• Interface to the network-control application layer — communication between controller and network-control applications