Communication

• Sockets
• Socket programming
• Handling multiple connections

OSI model

7 layer

• Layer 7 Application
• Layer 6 Presentation (TLS)
• Layer 5 Session (RPC)
• Layer 4 Transport (TCP))
• Layer 3 Network (IP)
• Layer 2 Datalink
• Layer 1 Physical

Internet model

Merge

• Layer 1-2
• Layer 5-7

IPv6

128-bit addresses

Endianness 字节序

Layer 2 often provides a byte-granular interface

16/32/

Port numbers

typically in the 0-1023 range

'Ephemeral' ports

Naming in the internet

The Domain Name System (DNS) can be used to map names to IP addresses

gethostbyname()

Socket

In practice, some protocols are typically implemented in the kernel

Protected

“逃离”内核

虽然经典的 TCP/UDP 在内核，但一些新兴协议或高性能场景正在转向用户态：

• QUIC (HTTP/3 的基础)：由于 TCP 在内核中更新太慢（操作系统升级周期长），Google 将 QUIC 实现在了用户态。这样开发者可以快速迭代协议特性，而不需要等待内核更新。
• 高性能场景 (DPDK/RDMA)：在金融交易或超大规模数据中心，内核的开销被认为太重了。工程师会使用 DPDK (Data Plane Development Kit) 绕过内核（Kernel Bypass），让用户态程序直接控制网卡，实现极高的吞吐量。

Definition

An endpoint of a connection

Identified by IP address, port number, protocol

API

The kernel provides an API for interacting with sockets

Types

• stream sockets
• datagram sockets

File descriptor

• Client
• Server listen

socket()

Creates a new socket

connect()

Tries to connect to another socket

read() and write()

At first glance, works

Datagram socket

• sent as messages, not as a stream of bytes
• no need to establish and terminate a connection
  • no reliable data streaming

sendto() and recvfrom()

Server architectures

• Polling-based
• Thread-based
• Event-driven

Polling

Many system calls can be made nonblocking

Idea: let's use only nonblocking calls

• server just rotates among its open connections ("polling") and keeps calling nonblocking sys calls until they succeed

Event-driven programming

Threads vs. events