NAT穿越（转）

NAT穿越

原文版权：Copyright (C) The Internet Society (2003).All Rights Reserved.
原文地址：http://midcom-p2p.sourceforge.net/draft-ford-midcom-p2p-01.txt
译文版权申明：请引用此文的作者或网站注明出处：http://blog.csdn.net/hxhbluestar，以尊重译者的劳动成果！

随着IPv6时代的到来，我也一直怀疑，是不是还有必要再去学习NAT技术——因为网络的资源不再如IPv4时代匮乏，而NAT技术正是为解决IP地址的紧缺而存在的，如此，NAT便没有存在的必要了。
但 是，随着这篇文章的翻译，我的怀疑慢慢变成庆幸，渐而又变为肯定，通过翻译所学到的东西，不再仅仅是翻译第一手资料带来的成就感，更多的是通过翻译，去领 悟技术前辈们的智慧与经验，也通过翻译，养成自己从第一手资料获得信息的习惯，从而将视野放得更宽，让理解更为透彻——至少，很多东西都是要经过仔细斟酌 才真正转化为自己思想的一部分的。正是如此，我才坚定的要把这篇文章翻译完，也如之前所提到的，如果时间允许的话，我会用C#来写一些例子，让大家更好的 理解NAT技术，掌握NAT技术（主要涉及到即时通讯、文件对等传输和语音应用三个方面）。

这篇文章主要是介绍一下“代理”机制的起因以及给P2P应用带来的不便，不需要任何基础知识：）

1. Introduction
1、简介

关键词：
middleboxe(s) —— 我翻译成“代理”，也许有更好的翻译
host —— 我翻译成“主机”，希望大家不要理解成服务器了，主机就是一台普通的终端机

Present-day Internet has seen ubiquitous deployment of "middleboxes" such as network address translators(NAT), driven primarily by the ongoing depletion of the IPv4 address space. The asymmetric addressing and connectivity regimes established by these middleboxes, however, have created unique problems for peer-to-peer (P2P) applications and protocols, such as teleconferencing and multiplayer on-line gaming. These issues are likely to persist even into the IPv6 world, where NAT is often used as an IPv4 compatibility mechanism [NAT-PT], and firewalls will still be commonplace even after NAT is no longer required.

在当今的Internet中，普遍存在使用“代理”设备来进行网络地址转换（NAT），导致这种现象的原因是 IPV4 地址空间的资源耗尽危机。虽然不对称 asymmetric 的地址分配和连通性制度已经在代理中被定义，但是却给端对端应用程序和协议制定造成了一些特殊的问题。像电话会议和多媒体网络游戏。这些问题即使在 IPV6世界中还是会存在，因为NAT作为IPV4的一种兼容性机制经常被使用[NAT-PT]，并且防火墙将仍然将普遍存在，即使不再需要NAT技术。

Currently deployed middleboxes are designed primarily around the client/server paradigm, in which relatively anonymous client machines actively initiate connections to well-connected servers having stable IP addresses and DNS names.
Most middleboxes implement an asymmetric communication model in which hosts on the private internal network can initiate outgoing connections to hosts on the public network, but external hosts cannot initiate connections to internal hosts except as specifically configured by the middlebox's administrator. In the common case of NAPT, a client on the internal network does not have a unique IP address on the public Internet, but instead must share a single public IP address, managed by the NAPT, with other hosts on the same private network.The anonymity and inaccessibility of the internal hosts behind a middlebox is not a problem for client software such as web browsers, which only need to initiate outgoing connections. This inaccessibility is sometimes seen as a privacy benefit.

当前使用的“代理”技术主要是为 客户端/服务端 C/S 结构设计的，为了实现那些需要连接但是又没有固定IP地址的客户端能够连接到一台配置好的拥有固定IP和DNS域名的服务器。
大多数的“代理”使用一种 asymmetric 通信模型，即 私网（局域网） 的主机能发起一个“外出”连接去连接公网上的主机。 但是公网上的主机却无法发送信息给私网上的主机（除非对“代理”进行特殊的配置），NAPT（网络地址端口转换）的普通情况是，一个私网客户端不需要一个 公网的固定的IP地址，但是必须要共享一个由NAPT控制的公网的固定IP地址（当然这个NAPT是处于同一个私网内部的）。这样的话，这些匿名的并且看 起来难以触及的藏在NAT之后的内网主机对于像 Web浏览器 这种软件来说就不是一个问题,因为内网的主机只需要发起向外部的连接就可以了。这样一来，无法触及也还是有他的优点的——那就是具有保密性。

In the peer-to-peer paradigm, however, Internet hosts that would normally be considered "clients" need to establish communication sessions directly with each other. The initiator and the responder might lie behind different middleboxes with neither endpoint having any permanent IP address or other form of public network presence. A common on-line gaming architecture, for example, is for the participating application hosts to contact a well-known server for initialization and administration purposes. Subsequent to this, the hosts establish direct connections with each other for fast and efficient propagation of updates during game play.
Similarly, a file sharing application might contact a well-known server for resource discovery or searching, but establish direct connections with peer hosts for data transfer. Middleboxes create problems for peer-to-peer connections because hosts behind a middlebox normally have no permanently usable public ports on the Internet to which incoming TCP or UDP connections from other peers can be directed.
RFC 3235 [NAT-APPL] briefly addresses this issue, but does not offer any general solutions.

然 而，在P2P的应用中，Internet上的“客户机”之间是需要建立一个通信会话直连的。邀请者和响应者也许会处于不同的NAT之后，也许他们都没有固 定IP或者即使有也不是公网的IP地址。举例来说，在一个普通的网络游戏体系结构中，都是通过客户端向一个具有公网固定IP的服务器发起申请进行初始化并 通过验证的。同时，客户端之间也要建立直连，才使网络间传输的速度加快，保证数据即时更新（不然抢不到装备啊，呵呵）。
同样的，一个文件共享应用 程序也必须通过到一个服务器上去查找它想要的资源，然后再到拥有这个数据的主机上去下载（BT网站，走了一个中介），“代理”造成了很多P2P直连的问 题，因为藏在“代理”之后的的主机通常没有固定的端口来使其他的客户端发起的TCP或UDP连接能够最终到达。
RFC 3235[NAT-APPL] 简要的提到了这个问题，但是没有给出任何的解决方案。

In this document we address the P2P/middlebox problem in two ways. First, we summarize known methods by which P2P applications can work around the presence of middleboxes. Second, we provide a set of application design guidelines based on these practices to make P2P applications operate more robustly over currently-deployed middleboxes. Further, we provide design guidelines for future middleboxes to allow them to support P2P applications more effectively. Our focus is to enable immediate and wide deployment of P2P applications requiring to traverse middleboxes.

在这篇文章中，我们用两种方式讨论 P2P/代理 的问题。首先，概要的讲叙已有的P2P应用程序能够在现有的代理机制中的工作原理。然后，我们提供一组应用程序设计指南，基于已有的实践，在现有的配置好 的代理上，来使得P2P应用程序操作更加有条理。最后，我们提供了设计指南，为以后的代理机制能够更方便支持P2P应用程序。讨论的焦点是如何 直接的、广泛的 配置那些需要经过“代理”的P2P应用程序。

2. Terminology
2. 术语

In this section we first summarize some middlebox terms. We focus hereon the two kinds of middleboxes that commonly cause problems for P2P applications.
在这一章节中，首先概要的介绍一下“代理”技术的一些术语。然后集中讨论两种造成P2P应用问题的代理机制。

Firewall
A firewall restricts communication between a private internal network and the public Internet, typically by dropping packets that are deemed unauthorized. A firewall examines but does not modify the IP address and TCP/UDP port information in packets crossing the boundary.
防火墙
防火墙限制了私网与公网的通信，它主要是将（防火墙）认为未经授权的的包丢弃，防火墙只是检验包的数据，并不修改数据包中的IP地址和TCP/UDP端口信息。

Network Address Translator (NAT)
A network address translator not only examines but also modifies the header information in packets flowing across the boundary, allowing many hosts behind the NAT to share the use of a smaller number of public IP addresses (often one). Network address translators in turn have two main varieties:
网络地址转换（NAT）
当有数据包通过时，网络地址转换器不仅检查包的信息，还要将包头中的IP地址和端口信息进行修改。以使得处于NAT之后的机器共享几个仅有的公网IP地址（通常是一个）。网络地址转换器主要有两种类型：

Basic NAT
A Basic NAT maps an internal host's private IP address to a public IP address without changing the TCP/UDP port numbers in packets crossing the boundary. Basic NAT is generally only useful when the NAT has a pool of public IP addresses from which to make address bindings on behalf of internal hosts.
基础NAT
基础NAT 将私网主机的私有IP地址转换成公网IP地址，但并不将TCP/UDP端口信息进行转换。基础NAT一般用在当NAT拥有很多公网IP地址的时候，它将公 网IP地址与内部主机进行绑定，使得外部可以用公网IP地址访问内部主机。（译者注：实际上是只将IP转换，192.168.0.23 <-> 210.42.106.35,这与直接设置IP地址为公网IP还是有一定区别的，特别是对于企业来说，外部的信息都要经过统一防火墙才能到达内部，但是内 部主机又可以使用公网IP）

Network Address/Port Translator (NAPT)
By far the most common, a Network Address/Port Translator examines and modifies both the IP address and the TCP/UDP port number fields of packets crossing the boundary, allowing multiple internal hosts to share a single public IP address simultaneously.
Refer to [NAT-TRAD] and [NAT-TERM] for more general information on NAT taxonomy and terminology. Additional terms that further classify NAPT are defined in more recent work [STUN]. When an internal host opens an outgoing TCP or UDP session through a network address/port translator, the NAPT assigns the session a public IP address and port number so that subsequent response packets from the external endpoint can be received by the NAPT, translated, and forwarded to the internal host. The effect is that the NAPT establishes a port binding between (private IP address, private port number) and (public IP address, public port number).
The port binding defines the address translation the NAPT will perform for the duration of the session. An issue of relevance to P2P applications is how the NAT behaves when an internal host initiates multiple simultaneous sessions from a single (private IP, private port) pair to multiple distinct endpoints on the external network.
网络地址和端口转换 （NAPT）
这是最普遍的情况，网络地址/端口转换器检查、修改包的IP地址和TCP/UDP端口信息，这样，更多的内部主机就可以同时使用一个公网IP地址。
请 参考[NAT-TRAD]和[NAT-TERM]两个文档了解更多的NAT分类和术语信息。另外，关于NAPT的分类和术语，[STUN]在最近做了更多 的定义。当一个内部网主机通过NAT打开一个“外出”的TCP或UDP会话时，NAPT分配给这个会话一个公网IP和端口，用来接收外网的响应的数据包， 并经过转换通知内部网的主机。这样做的效果是，NAPT在 [私有IP:私有端口] 和[公网IP:公网端口]之间建立了一个端口绑定。
端口绑定指定了NAPT将在这个会话的生存期内进行地址转换任务。这中间存在一个这样的问题，如果P2P应用程序从内部网络的一个[私有IP地址:端口]对同时发出多条会话给不同的外网主机，那么NAT会怎样处理呢？请看以下几种方案。

Cone NAT
After establishing a port binding between a (private IP, private port) tuple and a (public IP, public port) tuple, a cone NAT will re-use this port binding for subsequent sessions the application may initiate from the same private IP address and port number, for as long as at least one session using the port binding remains active.
锥形NAT
（译者注：为什么叫做锥形呢？请看以下图形,终端和外部服务器，都通过NAT分派的这个绑定地址对来传送信息，就象一个漏斗一样，筛选并传递信息）



当建立了一个 [私有IP:端口]-[公网IP:端口] 端口绑定之后，对于来自同一个[私有IP:端口]会话，锥形NAT服务器允许发起会话的应用程序 重复使用这个端口绑定，一直到这个会话结束才解除（端口绑定）。

For example, suppose Client A in the diagram below initiates two simultaneous outgoing sessions through a cone NAT, from the same internal endpoint (10.0.0.1:1234) to two different external servers, S1 and S2. The cone NAT assigns just one public endpoint tuple（元组）, 155.99.25.11:62000, to both of these sessions, ensuring that the "identity" of the client's port is maintained across address translation. Since Basic NATs and firewalls do not modify port numbers as packets flow across the middlebox, these types of middleboxes can be viewed as a degenerate form of Cone NAT.

例如，假设 Client A（IP地址信息如上图所示）通过一个 锥形NAT 同时发起两个外出的连接，它使用同一个内部端口（10.0.0.1:1234）给公网的两台不同的服务器，S1和S2。锥形NAT 只分配一个公网IP和端口（155.99.25.11:62000）给这个两个会话，通过地址转换可以 确保 Client使用端口的“同一性”（译者注：即这个Client只使用这个端口）。而基础NATs和防火墙却不能修改经过的数据包端口号，它们可以看作是 锥形NAT的精简版本。

Symmetric NAT
A symmetric NAT, in contrast, does not maintain a consistent port binding between (private IP, private port) and (public IP, public port) across all sessions.
Instead, it assigns a new public port to each new session. For example, suppose Client A initiates two outgoing sessions from the same port as above, one with S1 and one with S2. A symmetric NAT might allocate the public endpoint 155.99.25.11:62000 to session 1, and then allocate a different public endpoint 155.99.25.11:62001, when the application initiates session 2. The NAT is able to differentiate between the two sessions for translation purposes because the external endpoints involved in the sessions (those of S1 and S2) differ, even as the endpoint identity of the client application is lost across the address translation boundary.
对称NAT
对称NAT，与Cone NAT是大不相同的，并不对会话进行端口绑定，而是分配一个全新的 公网端口 给每一个新的会话。
还 是上面那个例子：如果 Client A (10.0.0.1:1234)同时发起两个 "外出" 会话,分别发往S1和S2。对称Nat会分配公共地址155.99.25.11:62000给Session1，然后分配另一个不同的公共地址 155.99.25.11:62001给Session2。对称Nat能够区别两个不同的会话并进行地址转换，因为在 Session1 和 Session2中的外部地址是不同的，正是因为这样，Client端的应用程序就迷失在这个地址转换边界线了，因为这个应用程序每发出一个会话都会使用 一个新的端口，无法保障只使用同一个端口了。

The issue of cone versus symmetric NAT behavior applies equally to TCP and UDP traffic. Cone NAT is further classified according to how liberally the NAT accepts incoming traffic directed to an already-established (publicIP, public port) pair. This classification generally applies only to UDP traffic, since NATs and firewalls reject incoming TCP connection attempts unconditionally unless specifically configured to do otherwise.
在TCP和UDP通信中， （到底是使用同一个端口，还是分配不同的端口给同一个应用程序），锥形NAT和对称NAT各有各的理由。当然锥形NAT在根据如何公平地将NAT接受的连 接直达一个已创建的地址对上有更多的分类。这个分类一般应用在Udp通信（而不是Tcp通信上），因为NATs和防火墙阻止了试图无条件传入的TCP连 接，除非明确设置NAT不这样做。这些分类如下：

Full Cone NAT
After establishing a public/private port binding for a new outgoing session, a full cone NAT will subsequently accept incoming traffic to the corresponding public port from ANY external endpoint on the public network. Full cone NAT is also sometimes called "promiscuous" NAT.
全双工锥形NAT
当内部主机发出一个“外出”的连接会话，就会创建了一个 公网/私网 地址，一旦这个地址对被创建，全双工锥形NAT会接收随后任何外部端口传入这个公共端口地址的通信。因此，全双工锥形NAT有时候又被称为"混杂"NAT。

Restricted Cone NAT
A restricted cone NAT only forwards an incoming packet directed to a public port if its external (source) IP address matches the address of a node to which the internal host has previously sent one or more outgoing packets. A restricted cone NAT effectively refines the firewall principle of rejecting unsolicited incoming traffic, by restricting incoming traffic to a set of "known" external IP addresses.
受限制的锥形NAT
受 限制的锥形NAT会对传入的数据包进行筛选，当内部主机发出“外出”的会话时，NAT会记录这个外部主机的IP地址信息，所以，也只有这些有记录的外部 IP地址，能够将信息传入到NAT内部，受限制的锥形NAT 有效的给防火墙提炼了筛选包的原则——即限定只给那些已知的外部地址“传入”信息到NAT内部。

Port-Restricted Cone NAT
A port-restricted cone NAT, in turn, only forwards an incoming packet if its external IP address AND port number match those of an external endpoint to which the internal host has previously sent outgoing packets. A port-restricted cone NAT provides internal nodes the same level of protection against unsolicited incoming traffic that a symmetric NAT does, while maintaining a private port's identity across translation.
端口受限制的Cone NAT
端口受限制的锥形NAT，与受限制的锥形NAT不同的是：它同时记录了外部主机的IP地址和端口信息，端口受限制的锥形NAT给内部节点提供了同一级别的保护，在维持端口“同一性”过程中，将会丢弃对称NAT传回的信息。

Finally, in this document we define new terms for classifying the P2P-relevant behavior of middleboxes:
最后，在这篇文档里我们将定义一组新的术语 ，以便更好的对P2P代理相关的行为进行分类。

P2P应用程序
P2P应用程序是指，在已有的一个公共服务器的基础上，并分别利用自己的私有地址或者公有地址（或者两者兼备）来建立一个端到端的会话通信。
P2P-Application
P2P-application as used in this document is an application in which each P2P participant registers with a public registration server, and subsequently uses either its private endpoint, or public endpoint, or both, to establish peering sessions.

P2P-Middlebox
A P2P-Middlebox is middlebox that permits the traversal of P2P applications.
P2P代理
P2P代理是一个允许 P2P应用程序进行通信的代理机制

P2P-firewall
A P2P-firewall is a P2P-Middlebox that provides firewall functionality but performs no address translation.
P2P防火墙
P2P防火墙是一个提供了防火墙的功能的P2P代理，但是不进行地址转换.

P2P-NAT
A P2P-NAT is a P2P-Middlebox that provides NAT functionality, and may also provide firewall functionality. At minimum, a P2P-Middlebox must implement Cone NAT behavior for UDP traffic, allowing applications to establish robust P2P connectivity using the UDP hole punching technique.
P2P-NAT
P2P-NAT 是一个 P2P代理,提供了NAT的功能,也提供了防火墙的功能,一个最简的P2P代理必须具有 锥形NAT对Udp通信支持的功能,并允许应用程序利用Udp打洞技术建立强健的P2P连接。

Loopback translation
When a host in the private domain of a NAT device attempts to connect with another host behind the same NAT device using the public address of the host, the NAT device performs the equivalent of a "Twice-nat" translation on the packet as follows. The originating host's private endpoint is translated into its assigned public endpoint, and the target host's public endpoint is translated into its private endpoint, before the packet is forwarded to the target host. We refer the above translation performed by a NAT device as "Loopback translation".
回环转换
当NAT的私网内部机器想通过公共地址来访问同一台局域网内的机器的时，NAT设备等价于做了两次NAT的事情，在包到达目标机器之前，先将私有地址转换为公网地址，然后再将公网地址转换回私有地址。我们把具有上叙转换功能的NAT设备叫做“回环转换”设备。

3、基于代理服务上的P2P通信技术
本章节详细地回顾了当前比较流行的一些基于当前代理设备的点对点通信技术，来源于应用或协议设计者的前瞻。

3.1. Relaying
3.1 转发

最可靠，但又是最低效的点对点通信方法，莫过于将p2p网络通信看作一个C/S结构，通过（服务器来）转发信息。举例来说，如下图，两个客户端A和B，均 与服务器S初始化了一个TCP或UDP连接，服务器S具有公网固定IP地址,两个客户端分布在不同的私网中，这样，他们各自的NAT代理服务器将不允许他 们进行直连。

The most reliable, but least efficient, method of implementing peer-to-peer communication in the presence of a middlebox is to make the peer-to-peer communication look to the network like client/server communication through relaying. For example, suppose two client hosts, A and B, have each initiated TCP or UDP connections with a well-known server S having a permanent IP address. The clients reside on separate private networks, however, and their respective middleboxes prevent either client from directly initiating a connection to the other.

取而代之的方式是，两个客户端可以把服务器S当作信使来转发消息。比如，为了将消息发送到B，A先发送一条信息给服务器S，服务器S再利用初始化时已经建立的连接，将信息转发给B。

Instead of attempting a direct connection, the two clients can simply use the server S to relay messages between them. For example, to send a message to client B, client A simply sends the message to server S along its already-established client/server connection, and server S then sends the message on to client B using its existing client/server connection with B.

4. Application design guidelines
4.程序设计指南
4.1. What works with P2P middleboxes
4.1. P2P代理的现状
对于两端都处于NAT之后的P2P直连，当前最佳解决方案仍然是UDP打洞技术，而在各种NAT系统中这种技术也得到了相当广泛的应用。当程序需要进行有 效的p2p直连的通讯时候，推荐使用UDP打洞技术，当然，当无法建立直连时，也要做好消息转发的处理。
Since UDP hole punching is the most efficient existing method of establishing direct peer-to-peer communication between two nodes that are both behind NATs, and it works with a wide variety of existing NATs, it is recommended that applications use this technique if efficient peer-to-peer communication is required, but be prepared to fall back on simple relaying when direct communication cannot be established.
4.2. Peers behind the same NAT
4.2. 位于同一个NAT后的端与端通信指南
在实际的情况中，还有相当大一部分用户不止两个IP地址（多网卡情况），而是三个或者更多，这种情况下，如果很难决定到底使用哪个地址来注册到服务器，就要应用程序将所有的地址都注册到服务器上去。
In practice there may be a fairly large number of users who have not two IP addresses, but three or more. In these cases, it is hard or impossible to tell which addresses to send to the registration server. The applications should send all its addresses, in such a case.
4.3. Peer discovery
4.3. 主机发现
应用程序发送很多包到网络的几个地址上，用于发现哪个地址对于指定的主机来说是最好的。这样是导致网络“空间浪费”的源头之一，就象是在网络上倒垃圾一 样；端将会选择不正确的路由地址；就像在内部网中一样（例如：11.0.1.1，分配给DOD [DOD还不能确定是什么，查到相关文献是与美国国防部相关的协议] 的）;因此应用程序在发送hello包时，应该小心地练习。(这段话翻译得不是很好，请求指正)
Applications sending packets to several addresses to discover which one is best to use for a given peer may become a significant source of 'space junk' littering the net, as the peer may have chosen to use routable addresses improperly as an internal LAN (e.g. 11.0.1.1, which is assigned to the DOD). Thus applications should exercise caution when sending the speculative hello packets.
4.4. TCP P2P applications
4.4. 基于TCP 的P2P应用程序
套接字API被应用程序开发者广泛地使用，但它其实最初是专门设计用于 C/S模式的应用程序的。由于这个自身原因，就出现了一些限制：一个套接字只能绑定一个TCP或者UDP端口；应用程序不允许多个套接字绑定同一个端口 （TCP或UDP）用于同时与多个外部节点建立会话；也不允许使用一个套接字来监听这个端口的同时，其他套接字通过这个端口发出向外的初始化会话连接。
The sockets API, used widely by application developers, is designed with client-server applications in mind. In its native form, only a single socket can bind to a TCP or UDP port. An application is not allowed to have multiple sockets binding to the same port (TCP or UDP) to initiate simultaneous sessions with multiple external nodes (or) use one socket to listen on the port and the other sockets to initiate outgoing sessions.
上面所说的“单一套接字对应单一端口”绑定约束对于UDP来说并不算一个障碍，因为UDP是一个基于数据报的协议。UDP P2P应用程序设计者可以用recvfrom()和sendto()函数来让一个SOCKET不仅发送而且可以从多个主机上接受数据报文。
The above single-socket-to-port bind restriction is not a problem however with UDP, because UDP is a datagram based protocol. UDP P2P application designers could use a single socket to send as well as receive datagrams from multiple peers using recvfrom() and sendto() calls.
但是TCP就不一样了。TCP中，每个进入和外出的连接都和一个单独的套接字保持关联。Linux 套接字API中使用 SO_REUSEADDR 选项标记了这个问题。在FreeBSD和NetBSD上，这个选项一般来说是无法正常工作的，但是，可以将其改为使用BSD-specific SetReuseAddress call(Linux中并没有这个命令，纯Unix标准中亦不存在），就可以使用了。Win32 API提供了一个等效的SetReuseAddress 命令。使用以上所提到的选项，应用程序就能使用多个套接字用来重用TCP端口。那就是说，打开两个TCP套接字流绑定使用同一个端口，只要使用 listen()在一边并在另外一边使用connect()在另外一端。
This is not the case with TCP. With TCP, each incoming and outgoing connection is to be associated with a separate socket. Linux sockets API addresses this problem with the aid of SO_REUSEADDR option. On FreeBSD and NetBSD, this option does not seem to work; but, changing it to use the BSD-specific SetReuseAddress call (which Linux doesn't have and isn't in the Single Unix Standard) seems to work. Win32 API offers an equivalent SetReuseAddress call. Using any of the above mentioned options, an application could use multiple sockets to reuse a TCP port. Say, open two TCP stream sockets bound to the same port, do a listen() on one and a connect() from the other.
4.5. Use of midcom protocol
4.5. 使用 MidCom 协议
如果应用程序知道它们需要穿越的代理并且这些代理实现Midcom协议，应用程序能使用Midcom协议更容易的穿越代理。
If the applications know the middleboxes they would be traversing and these middleboxes implement the midcom protocol, applications could use the midcom protocol to ease their way through the middleboxes.
例如：P2P应用程序需要NAT代理保持终端端口的绑定状态。假如代理可以支持Midcom，P2P应用程序可以控制修改绑定端口(或者绑定地址)的参 数，例如生存时间，最大空闲时间，因此应用程序不仅可以直接的连接外部主机而且也可以从外部主机接受连接；这样就不需要定期保持端口绑定的状态。当应用程 序不再需要绑定，也可以使用Midcom协议简单的取消绑定。
For example, P2P applications require that NAT middleboxes preserve end-point port bindings. If midcom is supported on the middleboxes, P2P applications can exercise control over port binding (or address binding) parameters such as lifetime, maxidletime, and directionality so the applications can both connect to external peers as well as receive connections from external peers; and do not need to send periodic keep-alives to keep the port binding alive. When the application no longer needs the binding, the application could simply dismantle the binding, also using the midcom protocol.
参考：MidCom方案
MidCom（Middlebox Communications）方案是通过在第三方实体和FW/NAT之间建立中间盒来通信，使FW/NAT设备变为可控的一种新的概念。如图所 示，MidCom包括MidCom Agent和Middlebox，Agent通过MidCom协议通知Middlebox建立相应的NAT映射表项。

一般情况下，Middlebox集成在NAT或FW设备中，Agent可在软交换、代理服务器或终端上实现。
由 于应用业务识别的智能从Middlebox移到外部的MidCom Agent上，因此，根据MidCom的架构，在不需要更改Middlebox基本特性的基础上，通过对MidCom Agent的升级就可以支持更多的新业务。这是相对于NAT/ALG方式的一个很大的优势。
从安全性考虑，MidCom方式支持控制报文和媒体流的加密，因此安全性比较高。

这个方法的优势是：当两个客户端都与服务端保持连接的时候，它将始终如一的正常工作。
但是它的劣势也很明显:它将全面依赖并消耗服务器的资源和性能和网络带宽。两个客户端的通信反应时间将明显增加，即使他们与服务器始终保持着连接。名为 TURN 的协议[TURN]定义了一个利用转发技术进行可靠通信的模型。

This method has the advantage that it will always work as long as both clients have connectivity to the server. Its obvious disadvantages are that it consumes the server's processing power and network bandwidth unnecessarily, and communication latency between the two clients is likely to be increased even if the server is well- connected. The TURN protocol [TURN] defines a method of implementing relaying in a relatively secure fashion.

3.2. Connection reversal
3.2 反向连接

这里介绍第二种技术，但是它只能在通信的两端只有一端处于NAT之后的情况下。举例来说，假设客户端A处于NAT之后，而客户端B有一个公网IP地址，如下图所示

The second technique works if only one of the clients is behind a middlebox. For example, suppose client A is behind a NAT but client B has a globally routable IP address, as in the following diagram:

客户端A的私有IP地址是10.0.0.1，并使用TCP端口1234，客户端A初始化了一个与服务器S（IP=18.181.0.31:1235）的连 接。NAT A（IP=155.99.25.11）分配了一个62000的TCP端口给这个连接。因此，服务器S认为客户端A的IP地址是 155.99.25.11:62000。而因为客户端B拥有固定IP地址138.76.29.7，所以在这个端对端的连接中，客户端B使用TCP端口 1234。

Client A has private IP address 10.0.0.1, and the application is using TCP port 1234. This client has established a connection with server S at public IP address 18.181.0.31 and port 1235. NAT A has assigned TCP port 62000, at its own public IP address 155.99.25.11, to serve as the temporary public endpoint address for A's session with S: therefore, server S believes that client A is at IP address 155.99.25.11 using port 62000. Client B, however, has its own permanent IP address, 138.76.29.7, and the peer-to-peer application on B is accepting TCP connections at port 1234.

现在我们假设客户端B将会与客户端A初始化一个端对端连接会话。B将首先试图
连 接A的任何一个地址——客户端A认为是它自己所有的地址，即10.0.0.1:1234。或者是从服务器S观察到的地址,即 155.99.25.11:62000。然而不论是连接上叙地址中的哪一个，都不可能成功。第一种情况：试图直接连到10.0.0.1肯定会失败，因为 10.0.0.1根本就不是一个可以在公网上路由的IP地址；第二种情况，从B传来的TCP SYN请求将能够到达端口NAT A的端口62000，但NAT A却会拒绝这个连接请求，因为只有外出的连接才允许（进入）。

Now suppose client B would like to initiate a peer-to-peer communication session with client A. B might first attempt to contact client A either at the address client A believes itself to have, namely 10.0.0.1:1234, or at the address of A as observed by server S, namely 55.99.25.11:62000. In either case, however, the connection will fail. In the first case, traffic directed to IP address 10.0.0.1 will simply be dropped by the network because 10.0.0.1 is not a publicly routable IP address. In the second case, the TCP SYN request from B will arrive at NAT A directed to port 62000, but NAT A will reject the connection request because only outgoing connections are allowed.

在所有的尝试都失败之后，客户端B就只能借用服务器S来传递一个到客户端A的请求，请求一个“翻转”的连接到客户端B，而客户端A，在接受了这个通过服务 器S转发的请求之后，将打开一个与客户端B通讯的TCP连接（在B的公网IP地址和端口号上）。NAT A允许这个连接通过，因为这个连接起源于NAT A的内部，并且同时客户端B能够受这个连接因为B并不位于NAT之后。
After attempting and failing to establish a direct connection to A, client B can use server S to relay a request to client A to initiate a "reversed" connection to client B. Client A, upon eceiving this relayed request through S, opens a TCP connection to client B at B's public IP address and port number. NAT A allows the connection to proceed because it is originating inside the firewall, and client B can receive the connection because it is not behind a middlebox.

当前很多p2p系统都使用了这种技术。它的主要限制在于：只能有一端位于NAT之后这个技术才能生效。然而当今真实的情况是，越来越多的客户端两端都处于 NAT之后，那么这个方法就是不可行的。因为逆向连接不是一个通用的解决方案，所以在这里就不推荐使用了。应用程序可以选择尝试做逆向连接，但是有可能消 息会被自动退回——如果另外一端的消息传递机制既不是“正向”也不是“逆向”连接的话。
A variety of current peer-to-peer systems implement this technique. Its main limitation, of course, is that it only works as long as only one of the communicating peers is behind a NAT: in the increasingly common case where both peers are behind NATs, the method fails. Because connection reversal is not a general solution to the problem, it is NOT recommended as a primary strategy. Applications may choose to attempt connection reversal, but should be able to fall back automatically on another mechanism such as relaying if neither a "forward" nor a "reverse" connection can be established.



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3.3. UDP hole punching UDP打洞技术
The third technique, and the one of primary interest in this document, is widely known as "UDP Hole Punching." UDP hole punching relies on the properties of common firewalls and cone NATs to allow appropriately designed peer-to-peer applications to "punch holes" through the middlebox and establish direct connectivity with each other, even when both communicating hosts may lie behind middleboxes. This technique was mentioned briefly in section 5.1 of RFC 3027 [NAT-PROT], and has been informally described elsewhere on the Internet [KEGEL] and used in some recent protocols [TEREDO, ICE]. As the name implies, unfortunately, this technique works reliably only with UDP.

第三种技术，也是这篇文章主要要研究的，就是非常有名的“UDP打洞技术”，UDP打洞技术依赖于由公共防火墙和cone NAT，允许适当的有计划的端对端应用程序通过NAT“打洞”，即使当双方的主机都处于NAT之后。这种技术在 RFC3027的5.1节[NAT PROT] 中进行了重点介绍，并且在Internet[KEGEL]中进行了非正式的描叙，还应用到了最新的一些协议，例如[TEREDO,ICE]协议中。不过， 我们要注意的是，“术”如其名，UDP打洞技术的可靠性全都要依赖于UDP。

We will consider two specific scenarios, and how applications can be designed to handle both of them gracefully. In the first situation, representing the common case, two clients desiring direct peer-to- peer communication reside behind two different NATs. In the second, the two clients actually reside behind the same NAT, but do not necessarily know that they do.

这里将考虑两种典型场景，来介绍连接的双方应用程序如何按照计划的进行通信的，第一种场景，我们假设两个客户端都处于不同的NAT之后；第二种场景，我们假设两个客户端都处于同一个NAT之后，但是它们彼此都不知道(他们在同一个NAT中)。

3.3.1. Peers behind different NATs 处于不同NAT之后的客户端通信

Suppose clients A and B both have private IP addresses and lie behind different network address translators. The peer-to-peer application running on clients A and B and on server S each use UDP port 1234.? A and B have each initiated UDP communication sessions with server S, causing NAT A to assign its own public UDP port 62000 for A's session with S, and causing NAT B to assign its port 31000 to B's session with S, respectively.

我们假设 Client A 和 Client B 都拥有自己的私有IP地址，并且都处在不同的NAT之后，端对端的程序运行于 CLIENT A,CLIENT B,S之间，并且它们都开放了UDP端口1234。 CLIENT A和CLIENT B首先分别与S建立通信会话，这时NAT A把它自己的UDP端口62000分配给CLIENT A与S的会话，NAT B也把自己的UDP端口31000分配给CLIENT B与S的会话。如下图所示：
Server S
18.181.0.31:1234
|
|
+----------------------+----------------------+
| |
NAT A NAT B

Now suppose that client A wants to establish a UDP communication session directly with client B.? If A simply starts sending UDP messages to B's public address, 138.76.29.7:31000, then NAT B will typically discard these incoming messages (unless it is a full cone NAT), because the source address and port number does not match those of S, with which the original outgoing session was established. Similarly, if B simply starts sending UDP messages to A's public address, then NAT A will typically discard these messages.

假如这个时候 CLIENT A 想与 CLIENT B建立一条UDP通信直连，如果 CLIENT A只是简单的发送一个UDP信息到CLIENT B的公网地址138.76.29.7:31000的话，NAT B会不加考虑的将这个信息丢弃（除非NAT B是一个 full cone NAT），因为 这个UDP信息中所包含的地址信息，与CLIENT B和服务器S建立连接时存储在NAT B中的服务器S的地址信息不符。同样的，CLIENT B如果做同样的事情，发送的UDP信息也会被 NAT A 丢弃。

Suppose A starts sending UDP messages to B's public address, however, and simultaneously relays a request through server S to B, asking B to start sending UDP messages to A's public address.? A's outgoing messages directed to B's public address (138.76.29.7:31000) cause NAT A to open up a new communication session between A's private address and B's public address. At the same time, B's messages to A's public address (155.99.25.11:62000) cause NAT B to open up a new communication session between B's private address and A's public address. Once the new UDP sessions have been opened up in each direction, client A and B can communicate with each other directly without further burden on the "introduction" server S.

假如 CLIENT A 开始发送一个 UDP 信息到 CLIENT B 的公网地址上，与此同时，他又通过S中转发送了一个邀请信息给CLIENT B，请求CLIENT B也给CLIENT A发送一个UDP信息到 CLIENT A的公网地址上。这时CLIENT A向CLIENT B的公网IP(138.76.29.7:31000)发送的信息导致 NAT A 打开一个处于 CLIENT A的私有地址和CLIENT B的公网地址之间的新的通信会话，与此同时，NAT B 也打开了一个处于CLIENT B的私有地址和CLIENT A的公网地址(155.99.25.11:62000)之间的新的通信会话。一旦这个新的UDP会话各自向对方打开了，CLIENT A和CLIENT B之间就可以直接通信，而无需S来牵线搭桥了。(这就是所谓的打洞技术)！

155.99.25.11:62000 138.76.29.7:31000
| |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234

The UDP hole punching technique has several useful properties. Once a direct peer-to-peer UDP connection has been established between two clients behind middleboxes, either party on that connection can in turn take over the role of "introducer" and help the other party establish peer-to-peer connections with additional peers, minimizing the load on the initial introduction server S. The application does not need to attempt to detect explicitly what kind of middlebox it is behind, if any [STUN], since the procedure above will establish peer- to-peer communication channels equally well if either or both clients do not happen to be behind a middlebox.? The hole punching technique even works automatically with multiple NATs, where one or both clients are removed from the public Internet via two or more levels of address translation.

UDP打洞技术有很多实用的地方：第一，一旦这种处于NAT之后的端对端的直连建立之后，连接的双方可以轮流担任 对方的“媒人”，把对方介绍给其他的客户端，这样就极大的降低了服务器S的工作量；第二，应用程序不用关心这个NAT是属于cone还是 symmetric，即便要，如果连接的双方有一方或者双方都恰好不处于NAT之后，基于上叙的步骤，他们之间还是可以建立很好的通信通道；第三，打洞技 术能够自动运作在多重NAT之后，不论连接的双方经过多少层NAT才到达Internet，都可以进行通信。


译后小记：本来已经翻译好了，是在网文快捕中翻译的，结果，一个全选把所有翻译的内容全部删除了(网文快捕的Bug?：)，不得不痛苦的再翻一遍。不过，有失必有得，第二次翻译流畅多了，希望大家读来还顺口。

3.3.2. Peers behind the same NAT 客户端都处于相同的NAT之后

Now consider the scenario in which the two clients (probably unknowingly) happen to reside behind the same NAT, and are therefore located in the same private IP address space. Client A has established a UDP session with server S, to which the common NAT has assigned public port number 62000. Client B has similarly established a session with S, to which the NAT has assigned public port number 62001.

现在让我们来考虑一下两个客户端(很有可能不知不觉的就会)同时位于相同的NAT之后，而且是在同一个子网内部的情况， Client A与S之间的会话使用了NAT的62000端口，Client B与S之间的会话使用了62001端口，如下图所示：
Server S
18.181.0.31:1234
|
|
NAT
A-S 155.99.25.11:62000
B-S 155.99.25.11:62001
|
+----------------------+----------------------+
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234

Suppose that A and B use the UDP hole punching technique as outlined above to establish a communication channel using server S as an introducer. Then A and B will learn each other's public IP addresses and port numbers as observed by server S, and start sending each other messages at those public addresses.The two clients will be able to communicate with each other this way as long as the NAT allows hosts on the internal network to open translated UDP sessions with other internal hosts and not just with external hosts. We refer to this situation as "loopback translation," because packets arriving at the NAT from the private network are translated and then "looped back" to the private network rather than being passed through to the public network. For example, when A sends a UDP packet to B's public address, the packet initially has a source IP address and port number of 10.0.0.1:124 and a destination of 155.99.25.11:62001. The NAT receives this packet, translates it to have a source of 155.99.25.11:62000 (A's public address) and a destination of 10.1.1.3:1234, and then forwards it on to B. Even if loopback translation is supported by the NAT, this translation and forwarding step is obviously unnecessary in this situation, and is likely to add latency to the dialog between A and B as well as burdening the NAT.

我们假设，Client A 和 Client B 要使用上一节我们所描述的 “UDP打洞技术”，并通过服务器S这个“媒人”来认识，这样Client A 和Client B首先从服务端S得到了彼此的公网IP地址和端口，然后就往对方的公网IP地址和端口上发送消息。在这种情况下，如果NAT 仅仅允许在 内部网主机与其他内部网主机（处于同一个NAT之后的网络主机）之间打开UDP会话通信通道，而内部网主机与其他外部网主机就不允许的话，那么 Client A 和Client B就可以通话了。我们把这种情形叫做“loopback translation”(“回环转换”)，因为数据包首先从局域网的私有IP发送到NAT转换，然后“绕一圈”，再回到局域网中来，但是这样总比这些数 据通过公网传送好。举例来说，当 Client A发送了一个UDP数据包到 Client B的公网IP地址，这个数据包的报头中就会有一个源地址10.0.0.1:124和一个目标地址155.99.25.11:62001。NAT接收到这个 包以后，就会(进行地址转换)解析出这个包中有一个公网地址源地址155.99.25.11:62000和一个目标地址10.1.1.3:1234，然后 再发送给B，虽说NAT支持“loopback translation”，我们也发现，在这种情形下,这个解析和发送的过程有些多余，并且这个Client A 和Client B 之间的对话可能潜在性地给NAT增加了负担。

The solution to this problem is straightforward, however. When A and B initially exchange address information through server S, they should include their own IP addresses and port numbers as "observed" by themselves, as well as their addresses as observed by S.The clients then simultaneously start sending packets to each other at each of the alternative addresses they know about, and use the first address that leads to successful communication. If the two clients are behind the same NAT, then the packets directed to their private addresses are likely to arrive first, resulting in a direct communication channel not involving the NAT. If the two clients are behind different NATs, then the packets directed to their private addresses will fail to reach each other at all, but the clients will hopefully establish connectivity using their respective public addresses. It is important that these packets be authenticated in some way, however, since in the case of different NATs it is entirely possible for A's messages directed at B's private address to reach some other, unrelated node on A's private network, or vice versa.

其 实，解决这个问题的方案是显而易见的。当 Client A和ClientB 最初通过服务器S交换彼此的地址信息时，它们应该发现了自己的IP地址和端口，也就是自己的 Local IP，同时，加上Server S发现的它们的公网地址和端口（即NAT分配给它们的） 。两个客户端同时的发送 数据包 到对方的公网地址和私有地址上，然后选择首先使得通信成功的那个地址就可以了。如果两个客户端都位于同一个NAT之后，那么发往私有地址的数据包应该先于 发往公网地址的数据包到达，这样就建立了一个不包括NAT的直连通信通道。如果两个客户端位于不同NAT之后，虽然发送到对方私有地址的数据包会毫无疑问 的发送失败，但还是很有可能使用他们各自的公网IP地址来建立一条通信通道的。所以检测这些数据包的方法和工作就变得非常重要，不论如何，只要双方都处于 不同NAT之后，就完全有可能 Client A 想发送到 Client B 的信息会被发到别的无关的地方去，反之亦然（Client B 想发送到 Client A的消息也会被发到别的无关的地方去）。

(最后一句“unrelated node on A's private network”没有完全理解是什么意思，总之，放到整个语境中，应该就是说，Client A 瞄准 Client B的私有地址端口的信息会被NAT转发到别的地方去，因为两者处于不同的NAT之后，NAT A 如果在 内部网络 找到了一个拥有与Client B相同的私有地址的电脑，就会把信息发送过去，这样，就根本不会发送到 Client B 上去)

3.3.3. Peers separated by multiple NATs 客户端分别处于多层NAT之后

In some topologies involving multiple NAT devices, it is not possible for two clients to establish an "optimal" P2P route between them without specific knowledge of the topology. Consider for example the following situation.

在有些网络拓扑中就存在多层NAT设备，如果不熟悉网络拓扑的知识，要想建立一条“理想的”端对端连接基本上是不可能的。让我们来看看下图这种情况：
Server S
18.181.0.31:1234
|
|
NAT X
A-S 155.99.25.11:62000
B-S 155.99.25.11:62001
|
|
+----------------------+----------------------+
| |
NAT A NAT B
192.168.1.1:30000 192.168.1.2:31000
| |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234

Suppose NAT X is a large industrial NAT deployed by an internet service provider (ISP) to multiplex many customers onto a few public IP addresses, and NATs A and B are small consumer NAT gateways deployed independently by two of the ISP's customers to multiplex their private home networks onto their respective ISP-provided IP addresses. Only server S and NAT X have globally routable IP addresses; the "public" IP addresses used by NAT A and NAT B are actually private to the ISP's addressing realm, while client A's and B's addresses in turn are private to the addressing realms of NAT A and B, respectively.
Each client initiates an outgoing connection to server S as before, causing NATs A and B each to create a single public/private translation, and causing NAT X to establish a public/private translation for each session.

假如 NAT X 是由 Internet服务供应商（ISP） 配置的一个 大型工业 NAT，它使用少量的公网IP地址来为一些客户群提供服务；NAT A 和 NAT B 则是为ISP的两个客户群所配置的小一点的独立NAT网关，它们为各自客户群的私人家庭网络提供IP地址。只有 Server S 和NAT X 拥有 公网固定IP地址，而NAT A 和 NAT B所拥有的“公网”IP地址对于ISP的寻址域来说则实际上“私有”的，这时 Client A的地址对于NAT A的寻址领域来说是“私有”的，Client B的地址对于NAT B的寻址域来说同样是“私有”的。
还是跟以前一样，每个客户端都建立了一个“外出”的连接到服务器S，导致NATA 和 NAT B 分别进行一次 公有/私有 转换，并导致 NAT X 为 每个 会话都建立了一个 公有/私有 的转换。（也就是把私有地址转换成为公网地址的过程，NAT的本质工作）

Now suppose clients A and B attempt to establish a direct peer-to- peer UDP connection. The optimal method would be for client A to send messages to client B's public address at NAT B, 192.168.1.2:31000 in the ISP's addressing realm, and for client B to send messages to A's public address at NAT B, namely 192.168.1.1:30000. Unfortunately, A and B have no way to learn these addresses, because server S only sees the "global" public addresses of the clients, 155.99.25.11:62000 and 155.99.25.11:62001.Even if A and B had some way to learn these addresses, there is still no guarantee that they would be usable because the address assignments in the ISP's private addressing realm might conflict with unrelated address assignments in the clients' private realms. The clients therefore have no choice but to use their global public addresses as seen by S for their P2P communication, and rely on NAT X to provide loopback translation.

现在让我们假设 Client A 和 Client B 想要建立一条 端对端 的UDP 直连。理想的方法应该是 Client A 发送一条 信息到 Client B 在NAT B的公网地址192.168.1.2:31000上,这个地址在ISP的寻址域内；同时 Client B也发送一条消息到Client A 在 NAT B的公网地址上，也就是192.168.1.1:30000；如果能这样发的话，问题就解决了。可惜Client A和 Client B根本就不可能知道对方的这个地址，因为Server S只记录了他们真正的公网地址155.99.25.11:62000和155.99.25.11:62001。即使 Client A 和 Client B 通过某种途径得知了这些地址，还是不能够保证这样就能进行通话了，因为这些地址是由ISP的私有寻址域分配的，可能会与私有域所分配的其他无关客户端地址 相冲突因此，如果客户端之间想要进行端对端的通信的话，别无选择，只能通过他们真正的公网地址来进行；并且 NAT X必须还得支持 “loopback translation”才行。

3.3.4. Consistent port bindings 保持端口绑定

The hole punching technique has one main caveat: it works only if both NATs are cone NATs (or non-NAT firewalls), which maintain a consistent port binding between a given (private IP, private UDP) pair and a (public IP, public UDP) pair for as long as that UDP port is in use. Assigning a new public port for each new session, as a symmetric NAT does, makes it impossible for a UDP application to reuse an already-established translation for communication with different external destinations. Since cone NATs are the most widespread, the UDP hole punching technique is fairly broadly applicable; nevertheless a substantial fraction of deployed NATs are symmetric and do not support the technique.

在使用“UDP打洞技术”时有一点必须要注意：它只能在双方的NAT都是cone NAT（或者干脆没有NAT）时才能正常工作；这些NAT在自己的公网UDP端口被使用时保持着端口的绑定——[私有IP，私有UDP端口]对和[公网 IP，公网UDP端口]对的一一对应。如果像 symmetricNAT那样给每个新的会话都分配一个新的公网端口，那么UDP应用程序想要与其他外部客户端进行通话，就无法重复使用已经建立好的通信 转换。
伴随着 cone NAT 的推广，“UDP打洞技术”也被越来越广泛的应用。然而，仍存在一小部分使用 symmetric NAT 的网络，那么在这小部分网络环境中，就不能使用“UDP打洞技术”。

（注：因为我国的国情，网络技术应用得比较晚，所以可以说绝大部分的网络都是cone NAT，所以 UDP打洞技术基本上可以畅通无阻的使用，只是还要注意对NAT是否支持“loopback translation”的测试）

3.4. UDP port number prediction UPD端口号预言

A variant of the UDP hole punching technique discussed above exists that allows P2P UDP sessions to be created in the presence of some symmetric NATs. This method is sometimes called the "N+1" technique [BIDIR] and is explored in detail by Takeda [SYM-STUN]. The method works by analyzing the behavior of the NAT and attempting to predict the public port numbers it will assign to future sessions.
Consider again the situation in which two clients, A and B, each behind a separate NAT, have each established UDP connections with a permanently addressable server S:
让我们来考虑这样一种情况，有两个客户端 A 和 B，他们都藏在不同的NAT后面，他们都开放了一个UDP连接给具有固定IP的Server S：如下图
Server S
18.181.0.31:1234
|
|
+----------------------+----------------------+
| |
Symmetric NAT A Symmetric NAT B


NAT A has assigned its own UDP port 62000 to the communication session between A and S, and NAT B has assigned its port 31000 to the session between B and S. By communicating through server S, A and B learn each other's public IP addresses and port numbers as observed by S. Client A now starts sending UDP messages to port 31001 at address 138.76.29.7 (note the port number increment), and client B simultaneously starts sending messages to port 62001 at address 155.99.25.11. If NATs A and B assign port numbers to new sessions sequentially, and if not much time has passed since the A-S and B-S sessions were initiated, then a working bi-directional communication channel between A and B should result.

A's messages to B cause NAT A to open up a new session, to which NAT A will (hopefully) assign public port number 62001, because 62001 is next in sequence after the port number 62000 it previously assigned to the session between A and S. Similarly, B's messages to A will cause NAT B to open a new session, to which it will (hopefully) assign port number 31001. If both clients have correctly guessed the port numbers each NAT assigns to the new sessions, then a bi-directional UDP communication channel will have been established as shown below.

NAT A 分配了它自己的UDP端口62000，用来保持 客户端A 与 服务器S 的通信会话， NAT B 也分配了31000端口，用来保持 客户端B 与 服务器S 的通信会话。通过与 服务器S的对话，客户端A 和 客户端B 都相互知道了对方所映射的真实IP和端口。
客户端A发送一条UDP消息到 138.76.29.7:31001(请注意到端口号的增加)，同时 客户端B发送一条UDP消息到 155.99.25.11:62001。如果NAT A 和NAT B继续分配端口给新的会话，并且从A-S和B-S的会话时间消耗得并不多的话，那么一条处于客户端A和客户端B之间的双向会话通道就建立了。
客户端A发出的消息送达B导致了NAT A打开了一个新的会话，并且我们希望 NAT A将会指派62001端口给这个新的会话，因为62001是继62000后，NAT会自动指派给 从服务器S到客户端A之间的新会话的端口号；类似的，客户端B发出的消息送达A导致了 NAT B打开了一个新的会话，并且我们希望 NAT B 将会指派31001这个端口给新的会话；如果两个客户端都正确的猜测到了对方新会话被指派的端口号，那么这个 客户端A－客户端B的双向连接就被打通了。其结果如下图所示：
A-S 155.99.25.11:62000 B-S 138.76.29.7:31000
| |
| |
Client A Client B
10.0.0.1:1234 10.1.1.3:1234

Obviously there are many things that can cause this trick to fail. If the predicted port number at either NAT already happens to be in use by an unrelated session, then the NAT will skip over that port number and the connection attempt will fail. If either NAT sometimes or always chooses port numbers non-sequentially, then the trick will fail.

If a different client behind NAT A (or B respectively) opens up a new outgoing UDP connection to any external destination after A (B) establishes its connection with S but before sending its first message to B (A), then the unrelated client will inadvertently "steal" the desired port number. This trick is therefore much less likely to work when either NAT involved is under load.

明显的，有许多因素会导致这个方法失败：如果这个预言的新端口（62001和31001） 恰好已经被一个不相关的会话所使用，那么NAT就会跳过这个端口号，这个连接就会宣告失败；如果两个NAT有时或者总是不按照顺序来生成新的端口号，那么这个方法也是行不通的。

如 果隐藏在NAT A后的一个不同的客户端X（或者在NAT B后）打开了一个新的“外出”UDP 连接，并且无论这个连接的目的如何；只要这个动作发生在 客户端A 建立了与服务器S 的连接之后，客户端A 与 客户端B 建立连接之前；那么这个无关的客户端X 就会趁人不备地“偷” 到这个我们渴望分配的端口。所以，这个方法变得如此脆弱而且不堪一击，只要任何一个NAT方包含以上碰到的问题，这个方法都不会奏效。

Since in practice a P2P application implementing this trick would still need to work if the N