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Lec 21-22: Network Programming

Linux 哲学：一切皆文件

文件类型标识 文件类型
- 普通文件
d 目录文件
l 符号链接
c（伪文件） 字符设备（character device）
b（伪文件） 块设备（block device）
s（伪文件） 套接字文件（socket）
p（伪文件） 命名管道文件（pipe）

如上图，Linux 的哲学就是，一切皆文件。

其中：

  • 字符设备和块设备

  • 字符设备只能以字节为最小单位访问，而块设备以块为单位访问，例如 512 字节，1024 字节等。

  • 块设备可以随机访问，但是字符设备不可以

  • 套接字文件

  • 使用套接字（socket）除了可以实现网络间不同主机间的通信外，还可以实现同一主机的不同进程间的通信，且建立的通信是双向的通信。

I/O 通路硬件实现

image-20240222163949477

如图，网络信息通过网络适配器进行解译，并通过 I/O 总线传到 I/O 桥，进而进入内存/CPU。

Internet Protocol

image-20240222190759219

How is it possible to send bits across incompatible LANs and WANs?

Solution: protocol software running on each host and router

  • Protocol is a set of rules that governs how hosts and routers should cooperate when they transfer data from network to network.
  • Smooths out the differences between the different networks

What does an internet protocol do?

  • Provides a naming scheme

  • An internet protocol defines a uniform format for host addresses

  • Each host (and router) is assigned at least one of these internet addresses that uniquely identifies it

  • Provides a delivery mechanism

  • An internet protocol defines a standard transfer unit (packet)

  • Packet consists of header and payload

    • Header: contains info such as packet size, source and destination addresses
    • Payload: contains data bits sent from source host

Global IP Internet

  • Most famous example of an internet
  • Based on the TCP/IP protocol family
  • IP (Internet Protocol):
    • Provides basic naming scheme and unreliable delivery capability of packets (datagrams) from host-to-host
  • UDP (Unreliable Datagram Protocol):
    • Uses IP to provide unreliable datagram delivery from process-to-process
  • TCP (Transmission Control Protocol):
    • Uses IP to provide reliable byte streams from process-to-process over connections
  • Accessed via a mix of Unix file I/O and functions from the sockets interface

Internet Connections

  • Clients and servers communicate by sending streams of bytes over connections. Each connection is:
  • Point-to-point: connects a pair of processes.
  • Full-duplex: data can flow in both directions at the same time,
  • Reliable: stream of bytes sent by the source is eventually received by the destination in the same order it was sent.
  • Note: A connection must be RELIABLE (e.g. socket via TCP), otherwise it's not a connection (e.g. socket via UDP, ICMP, etc).
  • A socket is an endpoint of a connection
  • Socket address is an IPaddress:port pair
  • A port is a 16-bit integer that identifies a process:
  • Ephemeral port: Assigned automatically by client kernel when client makes a connection request.
  • Well-known port: Associated with some service provided by a server (e.g., port 80 is associated with Web servers)

Anatomy of a Connection

A connection is uniquely identified by the socket addresses of its endpoints (socket pair)

  • (cliaddr:cliport, servaddr:servport)

image-20240222180003192

Socket Interface

image-20240222182704558

如图，这就是 socket 的一般执行流程。

其中

  • getaddrinfo 用于 DNS lookup
  • socket 用于建立一个抽象的套接字
  • 对于内核而言，是建立了一个 endpoint of communication
  • 对于用户而言，是建立了一个 socket 文件，以便用户直接通过文件 I/O 进行网络通信
  • bind 用于将该套接字文件和对应的套接字 addr bind 在一起
  • 也就是设置这个 socket 的参数
  • listen 用于将该套接字文件设置为 listen 模式，从而阻塞进程，直到接收到 connection request