从今天开始将陆续翻译Peer-to-Peer (P2P) communication across middleboxes这篇文章,并没有按照章节次序来，请读者见谅。
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
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
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：
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 NATs are cone NATs, or if one is a cone NAT and the other is a symmetric NAT, the application would need to detect beforehand what kind of NAT is involved on either end [STUN] and modify its behavior accordingly, increasing the complexity of the algorithm and the general brittleness of the network.
Finally, port number prediction has no chance of working if either client is behind two or more levels of NAT and the NAT(s) closest to the client are symmetric. For all of these reasons, it is NOT recommended that new applications implement this trick; it is mentioned here for historical and informational purposes.
自从使用这种方法来实践P2P的应用程序以来,在处于 cone NAT 系列的网络环境中这个方法还是实用的；如果有一方为 cone NAT 而另外一方为 symmetric NAT，那么应用程序就应该预先发现另外一方的 NAT 是什么类型，再做出正确的行为来处理通信，这样就增大了算法的复杂度，并且降低了在真实网络环境中的普适性。
最后，如果P2P的一方处在两级或者两级以上的NAT下面，并且这些NATs 接近这个客户端是 symmetric的话，端口号预言 是无效的！
因此，并不推荐使用这个方法来写新的P2P应用程序，这也是历史的经验和教训！
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：
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)！
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.
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