P2P communication across middleboxes（術語篇）

2. Terminology

2. 術語

In this section we first summarize some middlebox terms. We focus hereon the two kinds of middleboxes that commonly cause problems for P2P applications.


在這一章節中，首先概要的介紹一下“代理”技術的一些術語。然后集中討論兩種造成P2P應用問題的代理機制。



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.

防火墻

防火墻限制了私網與公網的通信，它主要是將（防火墻）認為未經授權的的包丟棄，防火墻只是檢驗包的數據，并不修改數據包中的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:

網絡地址轉換（NAT）

當有數據包通過時，網絡地址轉換器不僅檢查包的信息，還要將包頭中的IP地址和端口信息進行修改。以使得處于NAT之后的機器共享幾個僅有的公網IP地址（通常是一個）。網絡地址轉換器主要有兩種類型：



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.

基礎NAT

基礎NAT 將私網主機的私有IP地址轉換成公網IP地址，但并不將TCP/UDP端口信息進行轉換。基礎NAT一般用在當NAT擁有很多公網IP地址的時候，它將公網IP地址與內部主機進行綁定，使得外部可以用公網IP地址訪問內部主機。（譯者注：實際上是只將IP轉換，192.168.0.23 <-> 210.42.106.35,這與直接設置IP地址為公網IP還是有一定區別的，特別是對于企業來說，外部的信息都要經過統一防火墻才能到達內部，但是內部主機又可以使用公網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.

網絡地址和端口轉換 （NAPT）

這是最普遍的情況，網絡地址/端口轉換器檢查、修改包的IP地址和TCP/UDP端口信息，這樣，更多的內部主機就可以同時使用一個公網IP地址。

請參考[NAT-TRAD]和[NAT-TERM]兩個文檔了解更多的NAT分類和術語信息。另外，關于NAPT的分類和術語，[STUN]在最近做了更多的定義。當一個內部網主機通過NAT打開一個“外出”的TCP或UDP會話時，NAPT分配給這個會話一個公網IP和端口，用來接收外網的響應的數據包，并經過轉換通知內部網的主機。這樣做的效果是，NAPT在 [私有IP:私有端口] 和[公網IP:公網端口]之間建立了一個端口綁定。

端口綁定指定了NAPT將在這個會話的生存期內進行地址轉換任務。這中間存在一個這樣的問題，如果P2P應用程序從內部網絡的一個[私有IP地址:端口]對同時發出多條會話給不同的外網主機，那么NAT會怎樣處理呢？請看以下幾種方案。



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.

錐形NAT

（譯者注：為什么叫做錐形呢？請看以下圖形,終端和外部服務器，都通過NAT分派的這個綁定地址對來傳送信息，就象一個漏斗一樣，篩選并傳遞信息）



    當建立了一個 [私有IP:端口]-[公網IP:端口] 端口綁定之后，對于來自同一個[私有IP:端口]會話，錐形NAT服務器允許發起會話的應用程序 重復使用這個端口綁定，一直到這個會話結束才解除（端口綁定）。



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（元組）, 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，假設 Client A（IP地址信息如上圖所示）通過一個 錐形NAT 同時發起兩個外出的連接，它使用同一個內部端口（10.0.0.1:1234）給公網的兩臺不同的服務器，S1和S2。錐形NAT 只分配一個公網IP和端口（155.99.25.11:62000）給這個兩個會話，通過地址轉換可以 確保 Client使用端口的“同一性”（譯者注：即這個Client只使用這個端口）。而基礎NATs和防火墻卻不能修改經過的數據包端口號，它們可以看作是錐形NAT的精簡版本。

         

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.

對稱NAT

對稱NAT，與Cone NAT是大不相同的，并不對會話進行端口綁定，而是分配一個全新的 公網端口 給每一個新的會話。

還是上面那個例子：如果 Client A (10.0.0.1:1234)同時發起兩個 "外出" 會話,分別發往S1和S2。對稱Nat會分配公共地址155.99.25.11:62000給Session1，然后分配另一個不同的公共地址155.99.25.11:62001給Session2。對稱Nat能夠區別兩個不同的會話并進行地址轉換，因為在 Session1 和 Session2中的外部地址是不同的，正是因為這樣，Client端的應用程序就迷失在這個地址轉換邊界線了，因為這個應用程序每發出一個會話都會使用一個新的端口，無法保障只使用同一個端口了。


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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， （到底是使用同一個端口，還是分配不同的端口給同一個應用程序），錐形NAT和對稱NAT各有各的理由。當然錐形NAT在根據如何公平地將NAT接受的連接直達一個已創建的地址對上有更多的分類。這個分類一般應用在Udp通信（而不是Tcp通信上），因為NATs和防火墻阻止了試圖無條件傳入的TCP連接，除非明確設置NAT不這樣做。這些分類如下：



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.

全雙工錐形NAT

當內部主機發出一個“外出”的連接會話，就會創建了一個 公網/私網 地址，一旦這個地址對被創建，全雙工錐形NAT會接收隨后任何外部端口傳入這個公共端口地址的通信。因此，全雙工錐形NAT有時候又被稱為"混雜"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.

受限制的錐形NAT

受限制的錐形NAT會對傳入的數據包進行篩選，當內部主機發出“外出”的會話時，NAT會記錄這個外部主機的IP地址信息，所以，也只有這些有記錄的外部IP地址，能夠將信息傳入到NAT內部，受限制的錐形NAT 有效的給防火墻提煉了篩選包的原則——即限定只給那些已知的外部地址“傳入”信息到NAT內部。



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

端口受限制的錐形NAT，與受限制的錐形NAT不同的是：它同時記錄了外部主機的IP地址和端口信息，端口受限制的錐形NAT給內部節點提供了同一級別的保護，在維持端口“同一性”過程中，將會丟棄對稱NAT傳回的信息。



Finally, in this document we define new terms for classifying the P2P-relevant behavior of middleboxes:

最后，在這篇文檔里我們將定義一組新的術語 ，以便更好的對P2P代理相關的行為進行分類。



P2P應用程序

P2P應用程序是指，在已有的一個公共服務器的基礎上，并分別利用自己的私有地址或者公有地址（或者兩者兼備）來建立一個端到端的會話通信。

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代理是一個允許 P2P應用程序進行通信的代理機制



P2P-firewall

A P2P-firewall is a P2P-Middlebox that provides firewall functionality but performs no address translation.

P2P防火墻

P2P防火墻是一個提供了防火墻的功能的P2P代理，但是不進行地址轉換.



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 是一個 P2P代理,提供了NAT的功能,也提供了防火墻的功能,一個最簡的P2P代理必須具有 錐形NAT對Udp通信支持的功能,并允許應用程序利用Udp打洞技術建立強健的P2P連接。



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".


回環轉換

當NAT的私網內部機器想通過公共地址來訪問同一臺局域網內的機器的時，NAT設備等價于做了兩次NAT的事情，在包到達目標機器之前，先將私有地址轉換為公網地址，然后再將公網地址轉換回私有地址。我們把具有上敘轉換功能的NAT設備叫做“回環轉換”設備。