这是我所在组织的一位软件工程师提出的问题。我感兴趣的是最广义的定义。


当前回答

这个问题已经有了理论上的答案。我想举一个实际的例子来解释这个问题,让大家对Socket和Port有一个更清晰的理解。

我在这里找到的

This example will walk you thru the process of connecting to a website, such as Wiley. You would open your web browser (like Mozilla Firefox) and type www.wiley.com into the address bar. Your web browser uses a Domain Name System (DNS) server to look up the name www.wiley.com to identify its IP address is. For this example, the address is 192.0.2.100. Firefox makes a connection to the 192.0.2.100 address and to the port where the application layer web server is operating. Firefox knows what port to expect because it is a well-known port . The well-known port for a web server is TCP port 80. The destination socket that Firefox attempts to connect is written as socket:port, or in this example, 192.0.2.100:80. This is the server side of the connect, but the server needs to know where to send the web page you want to view in Mozilla Firefox, so you have a socket for the client side of the connection also. The client side connection is made up of your IP address, such as 192.168.1.25, and a randomly chosen dynamic port number. The socket associated with Firefox looks like 192.168.1.25:49175. Because web servers operate on TCP port 80, both of these sockets are TCP sockets, whereas if you were connecting to a server operating on a UDP port, both the server and client sockets would be UDP sockets.

其他回答

A socket is a communication endpoint. A socket is not directly related to the TCP/IP protocol family, it can be used with any protocol your system supports. The C socket API expects you to first get a blank socket object from the system that you can then either bind to a local socket address (to directly retrieve incoming traffic for connection-less protocols or to accept incoming connection requests for connection-oriented protocols) or that you can connect to a remote socket address (for either kind of protocol). You can even do both if you want to control both, the local socket address a socket is bound to and the remote socket address a socket is connected to. For connection-less protocols connecting a socket is even optional but if you don't do that, you'll have to also pass the destination address with every packet you want to send over the socket as how else would the socket know where to send this data to? Advantage is that you can use a single socket to send packets to different socket addresses. Once you have your socket configured and maybe even connected, consider it to be a bi-directional communication pipe. You can use it to pass data to some destination and some destination can use it to pass data back to you. What you write to a socket is send out and what has been received is available for reading.

Ports on the other hand are something that only certain protocols of the TCP/IP protocol stack have. TCP and UDP packets have ports. A port is just a simple number. The combination of source port and destination port identify a communication channel between two hosts. E.g. you may have a server that shall be both, a simple HTTP server and a simple FTP server. If now a packet arrives for the address of that server, how would it know if that is a packet for the HTTP or the FTP server? Well, it will know so as the HTTP server will run on port 80 and the FTP server on port 21, so if the packet arrives with a destination port 80, it is for the HTTP server and not for the FTP server. Also the packet has a source port since without such a source port, a server could only have one connection to one IP address at a time. The source port makes it possible for a server to distinguish otherwise identical connections: they all have the same destination port, e.g. port 80, the same destination IP (the IP of the server), and the same source IP, as they all come from the same client, but as they have different source ports, the server can distinguish them from each other. And when the server sends back replies, it will do so to the port the request came from, that way the client can also distinguish different replies it receives from the same server.

套接字= IP地址+端口(数字地址) 它们一起标识机器上网络连接的端点。(我刚刚是不是网络基础课不及格?)

总结

TCP套接字是在特定TCP连接或监听状态的上下文中由IP地址和端口定义的端点实例。

端口是定义服务端点的虚拟化标识符(与服务实例端点又名会话标识符不同)。

TCP套接字不是一个连接,而是一个特定连接的端点。

可以存在到服务端点的并发连接,因为连接由其本地和远程端点标识,从而允许将流量路由到特定的服务实例。

对于给定的地址/端口组合,只能有一个侦听器套接字。

博览会

这是一个有趣的问题,它迫使我重新审视一些我自以为已经完全了解的东西。你可能会认为“插座”这样的名字是不言自明的:选择它显然是为了唤起你插入网线的端点的形象,有很强的功能相似之处。然而,在网络用语中,“套接字”这个词承载了如此多的负担,以至于有必要仔细地重新检查。

在最广泛的意义上,端口是一个入口或出口点。虽然不在网络环境中使用,但法语单词porte的字面意思是门或网关,进一步强调了端口是运输端点的事实,无论您运送数据还是大型钢铁集装箱。

为了本讨论的目的,我将只考虑TCP-IP网络的上下文。OSI模型非常好,但从未完全实现,更不用说在高流量、高压力条件下广泛部署。

IP地址和端口的组合严格地称为端点,有时称为套接字。这种用法起源于RFC793,即最初的TCP规范。

TCP连接由两个端点定义,也就是套接字。

端点(套接字)由网络地址和端口标识符的组合定义。请注意,address/port并不能完全标识一个套接字(稍后会详细介绍)。

端口的目的是区分给定网络地址上的多个端点。您可以说端口是一个虚拟端点。这种虚拟化使单个网络接口上的多个并发连接成为可能。

它是套接字对(4元组) 包括客户端IP地址, 客户端端口号,服务器IP地址, 和服务器端口号)指定 这两个端点唯一 对象中的每个TCP连接 互联网。(TCP-IP插图卷1,W. Richard Stevens)

在大多数c派生语言中,TCP连接是使用Socket类实例上的方法建立和操作的。虽然在更高的抽象级别上操作是常见的,通常是NetworkStream类的实例,但这通常会公开对套接字对象的引用。对于编码器来说,这个套接字对象似乎表示连接,因为连接是使用套接字对象的方法创建和操作的。

在c#中,要建立一个TCP连接(到一个现有的侦听器),首先要创建一个TcpClient。如果您没有指定TcpClient构造函数的端点,它将使用默认值—以某种方式定义本地端点。然后调用Connect 方法。此方法需要描述另一个端点的参数。

所有这些都有点令人困惑,并导致您相信套接字是一个连接,这是胡说八道。在理查德·多尔曼提出这个问题之前,我一直在这个误解中苦苦挣扎。

在进行了大量阅读和思考之后,我现在确信,使用带有两个参数LocalEndpoint和RemoteEndpoint的构造函数的类TcpConnection会更有意义。当本地端点的默认值是可接受的时,您可能会支持单个参数RemoteEndpoint。这在多址计算机上是不明确的,但是可以使用路由表通过选择到远程端点的最短路由的接口来解决这种不明确。

在其他方面,透明度也将得到提高。套接字不能通过IP地址和端口的组合来识别:

[...]TCP demultiplexes incoming segments using all four values that comprise the local and foreign addresses: destination IP address, destination port number, source IP address, and source port number. TCP cannot determine which process gets an incoming segment by looking at the destination port only. Also, the only one of the [various] endpoints at [a given port number] that will receive incoming connection requests is the one in the listen state. (p255, TCP-IP Illustrated Volume 1, W. Richard Stevens)

正如您所看到的,网络服务有许多具有相同地址/端口的套接字,但在特定地址/端口组合上只有一个侦听器套接字,这不仅是可能的,而且很有可能。典型的库实现提供了一个套接字类,该类的实例用于创建和管理连接。这是非常不幸的,因为它引起了混乱,并导致了两个概念的广泛合并。

Hagrawal不相信我(见评论),所以这里有一个真实的例子。我把一个浏览器连接到http://dilbert.com,然后运行netstat -an -p tcp。输出的最后六行包含两个示例,说明地址和端口不足以唯一地标识一个套接字。在192.168.1.3(我的工作站)和54.252.94.236:80(远程HTTP服务器)之间有两个不同的连接。

  TCP    192.168.1.3:63240      54.252.94.236:80       SYN_SENT
  TCP    192.168.1.3:63241      54.252.94.236:80       SYN_SENT
  TCP    192.168.1.3:63242      207.38.110.62:80       SYN_SENT
  TCP    192.168.1.3:63243      207.38.110.62:80       SYN_SENT
  TCP    192.168.1.3:64161      65.54.225.168:443      ESTABLISHED

由于套接字是连接的端点,因此有两个套接字的地址/端口组合为207.38.110.62:80,另外两个套接字的地址/端口组合为54.252.94.236:80。

我认为哈格拉瓦尔的误解源于我对“认同”一词的谨慎使用。我的意思是“完全的,明确的,唯一的识别”。在上面的示例中,有两个端点的地址/端口组合为54.252.94.236:80。如果你只有地址和端口,你没有足够的信息来区分这些套接字。这些信息不足以识别套接字。

齿顶高

RFC793第2.7节第2段说

连接完全由两端的一对套接字指定。一个 本地套接字可能参与到不同外部的多个连接 套接字。

从编程的角度来看,socket的这个定义没有帮助,因为它与socket对象不同,后者是特定连接的端点。对于程序员(这个问题的大多数读者都是程序员)来说,这是一个至关重要的功能差异。

@plugwash提出了一个显著的观察。

最根本的问题是TCP RFC对socket的定义与所有主流操作系统和库所使用的socket的定义相冲突。

根据定义,RFC是正确的。当一个标准库误用术语时,它不会取代RFC。相反,它给该库的用户带来了一种责任负担,即理解两种解释,并小心用词和上下文。如果RFC不一致,则最新的和最直接适用的RFC优先。

参考文献

TCP-IP图解卷1协议,W. Richard Stevens, 1994 Addison Wesley RFC793,信息科学研究所,南加州大学DARPA RFC147,插座的定义,Joel M. Winett,林肯实验室

A socket represents a single connection between two network applications. These two applications nominally run on different computers, but sockets can also be used for interprocess communication on a single computer. Applications can create multiple sockets for communicating with each other. Sockets are bidirectional, meaning that either side of the connection is capable of both sending and receiving data. Therefore a socket can be created theoretically at any level of the OSI model from 2 upwards. Programmers often use sockets in network programming, albeit indirectly. Programming libraries like Winsock hide many of the low-level details of socket programming. Sockets have been in widespread use since the early 1980s.

端口表示网络通信的端点或“通道”。端口号允许同一计算机上的不同应用程序在不相互干扰的情况下利用网络资源。端口号最常出现在网络编程中,尤其是套接字编程中。但是,有时端口号对普通用户是可见的。例如,一个人在因特网上访问的一些网站使用如下URL:

http://www.mairie-metz.fr:8080/在本例中,数字8080指Web浏览器连接到Web服务器所使用的端口号。通常,Web站点使用端口号80,该端口号不需要包含在URL中(尽管可以包含)。

在IP组网中,端口号理论上可以在0到65535之间。但是,大多数流行的网络应用程序使用范围较低的端口号(例如HTTP的80)。

注意:术语端口还指网络技术的其他几个方面。端口可以指外部设备的物理连接点,如串口、并口和USB端口。术语端口也指某些以太网连接点,例如集线器、交换机或路由器上的连接点。

ref http://compnetworking.about.com/od/basicnetworkingconcepts/l/bldef_port.htm

ref http://compnetworking.about.com/od/itinformationtechnology/l/bldef_socket.htm

港口定义

文顿·g·瑟夫和罗伯特·e·卡恩(1974年5月)。分组网络互通协议。IEEE通讯汇刊,第22卷,第5号。IEEE。

端口是“一对(实体)在一段时间内交换一条或多条消息”的单元。

“……我们可以看到一个端口产生的消息序列,就好像它嵌入在一个无限长的字节流中……我们强调,与给定数据包相关联的序列号仅对通信的端口对是唯一的…对到达的数据包进行检查,以确定它们要发送到哪个端口。目标进程应该指定它愿意侦听特定端口或‘任何’端口。”

一个“端口只是一个的指示器……双工…消息流……[在一个或多个]信息流中…与流程相关联。”

信息科学研究所:南加州大学(1981年9月)。RFC 793:传输控制协议:DARPA互联网程序协议规范。

端口是一个或多个实体的一个实体,进程通过一个或多个通信流与一个或多个其他进程通信。

由于一个进程可能需要区分它自己和另一个进程(或多个进程)之间的几个通信流,我们设想每个进程都可能有若干端口,通过这些端口与其他进程通信。

“这样做的目的是,只允许在具有完全相同的安全性和隔间值的端口之间进行连接,并且在两个端口要求的优先级中具有更高的优先级。” 注意,此检查放在顺序检查之后,以防止来自这些具有不同安全性或优先级的端口之间的旧连接的段导致当前连接中断。

端口是一个地址,它指定进程的哪个逻辑输入或输出通道与[数据流]相关联。 为了允许单个主机内的多个进程同时使用TCP通信设施,TCP在每台主机内提供了一组地址或端口。

套接字中指定进程的哪个逻辑输入或输出通道与数据相关联的部分。

Socket的定义

甲骨文(2020)。类套接字。Java平台,标准版7 API规范。

套接字是两台机器之间通信的端点。

信息科学研究所:南加州大学(1981年9月)。RFC 793:传输控制协议:DARPA互联网程序协议规范。

A socket is a string consisting of an Internet address [i.e., the first eight-bit number (e.g., 123) of a network address (e.g., 123.45.78.0), a period, the second eight-bit number (e.g., 45) of the network address, a period, the third eight-bit number (e.g., 78) of the network address, a period, and a host address (e.g., 90)], a colon, and a TCP port (e.g., 1234). A socket is a unit of “A pair of [entities that] uniquely identify [a] connection[, and that] may be simultaneously used in multiple connections.”

"To allow for many processes within a single Host to use TCP communication facilities simultaneously, the TCP provides a set of addresses or ports within each host. Concatenated with the network and host addresses from the internet communication layer, this forms a socket. A pair of sockets uniquely identifies each connection. That is, a socket may be simultaneously used in multiple connections." “To provide for unique addresses within each TCP, we concatenate an internet address identifying the TCP with a port identifier to create a socket which will be unique throughout all networks connected together.”

上面描述的可靠性和流量控制机制要求tcp初始化和维护每个数据流的特定状态信息。这些信息(包括套接字、序列号和窗口大小)的组合称为连接。每个连接都由一对识别其两端的套接字唯一指定。”