RFC 简介

STUN（ Simple Traversal of User Datagram Protocol (UDP) Through Network

Address Translators (NATs)）是一种轻量级的协议，它允许应用程序发现它们和公网之间是否存在NATs和防火墙，并确定NATs和防火墙的类型。它也可以让应用程序确定NAT分配给它们的公网IP地址和端口号。STUN是一种client-server的协议，也是一种request-response的协议，STUN的默认端口是3478。

NAT的类型：

Full Cone NAT:

所有从同一个内网IP和端口号发送过来的请求都会被映射成同一个外网IP地址和端口号。而且任何一个外网主机都可以通过这个映射的外网IP和端口号向这台内网主机发关怉。

Restricted Cone:

它也是所有从同一个内网IP和端口号发送过来的请求都会被映射成同一个外网IP和端口号。和Full-Cone NAT不同的是，一个拥有IP地址X的外网主机如果想要给内网主机发送数据包，必须是这台内网主机之前给IP地址X发送过数据包才可以。

Port Restricted Cone:

它和Restricted Cone很相似，只不过它包括端口号，也就是，一台IP地址X和端口P的外网主机想给内网主机发送数据包，必须是这台内网主机之前给这个IP地址X和端口P发送过数据包才可以。

Symmetric:

对称NAT就是，所有从同一个内网IP和端口发送到一个特定的目的IP和端口的请求，都会被映射到同一个IP和端口。如果同一台主机用相同的源地址和端口号发送数据包，但是发往不同的目的地，NAT将会使用不同的映射。进一步说，只有当外网主机收到内网主机发送来的数据包之后才能向内网主机往回发送数据包。

STUN Binding Request使用UDP协议发送到STUN服务器，当Binding Request消息到达服务器的时候它可能经过了一个或者多个NAT。结果是STUN服务器收到的request消息的源IP地址被映射成最靠近STUN服务器的NAT的IP地址，STUN服务器把这个源IP地址和端口号复制到一个Bind Response消息中，通过发送回拥有这个IP地址和端口号的客户端，对于上面提到的所有类型的NAT，这个消息都会到达客户端。

Network Working Group E. Rescorla

Request for Comments: 3552 RTFM, Inc.

BCP: 72 B. Korver

Category: Best Current Practice Xythos Software

Internet Architecture Board

IAB

July 2003

Guidelines for Writing RFC Text on Security Considerations

Status of this Memo

This document specifies an Internet Best Current Practices for the

Internet Community, and requests discussion and suggestions for

improvements. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

All RFCs are required to have a Security Considerations section.

Historically, such sections have been relatively weak. This document

provides guidelines to RFC authors on how to write a good Security

Considerations section.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3

Network Working Group D. Mills

Request for Comments: 2030 University of Delaware

Obsoletes: 1769 October 1996

Category: Informational

Simple Network Time Protocol (SNTP) Version 4

for IPv4, IPv6 and OSI

Status of this Memo

This memo provides information for the Internet community. This memo

does not specify an Internet standard of any kind. Distribution of

this memo is unlimited.

Abstract

This memorandum describes the Simple Network Time Protocol (SNTP)

Version 4, which is an adaptation of the Network Time Protocol (NTP)

used to synchronize computer clocks in the Internet. SNTP can be used

when the ultimate performance of the full NTP implementation

described in RFC-1305 is not needed or justified. When operating with

current and previous NTP and SNTP versions, SNTP Version 4 involves

no changes to the NTP specification or known implementations, but

Network Working Group

Request for Comments: 1662

STD: 51

Obsoletes: 1549

Category: Standards Track W. Simpson, Editor

Daydreamer

July 1994

PPP in HDLC-like Framing

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and

requests discussion and suggestions for improvements. Please refer to the current edition of the

"Internet Official Protocol Standards" (STD 1) for the standardization state and status of this

protocol. Distribution of this memo is unlimited.

Abstract

The Point-to-Point Protocol (PPP) [1] provides a standard method for transporting multi-protocol

datagrams over point-to-point links.

This document describes the use of HDLC-like framing for PPP encapsulated packets.

Table of Contents

1. Introduction .......................................... 1