行业趋势，SND是未来. P4 是未来。
SDN is inevitably， and P4 is inevitably。
P4 = Programming Protocol-Independent Packet Processors
http://arxiv.org/pdf/1312.1719.pdf
P4 is a high-level language for programming protocol-independent packet processors.
P4 属于高级语言， 用来对 协议无关的网络包处理器 编程。
P4 works in conjunction with SDN control protocols like OpenFlow.
P4 用于SDN控制协议（如OpenFlow）
In its current form, OpenFlow explicitly specifies protocol headers on which it operates.
在目前格式，OpenFlow 明确指定了其要操作的协议头。
This set has grown from 12 to 41 fields in a few years, increasing the complexity of the specification while still not providing the flexibility to add new headers.
随着规范的复杂性的提高， 这个 set(语法集？) 已经从12个域增加到41个域， 但是仍然不能提供 灵活的增加新头部。
In this paper we propose P4 as a strawman proposal for how OpenFlow should evolve in the future.
在这个论文中，我们建议P4作为一个稻草人提议，用来促进OpenFlow的进化。
We have three goals:
有3个目标
(1)Reconfigurability in the field（域可重构性）:
Programmers should be able to change the way switches process packets once they are deployed.
一旦部署，程序员能够改变交换机处理包的方式。
(2) Protocol independence（协议无关）:
Switches should not be tied to any specific network protocols.
交换机不应该绑定任何网络协议。
(3) Target independence（目标无关）:
Programmers should be able to describe packet processing functionality independently of the specifics of the underlying hardware.
程序员能够描述 与底层硬件规范无关的 包处理功能，
As an example, we describe how to use P4 to configure a switch to add a new hierarchical label.
例子 描述 P4 怎么配置 交换机 增加一个新的分层标签。
原文：http://www.cnblogs.com/shaohef/p/4467911.html