# Network Protocol

《计算机组成与系统结构》、[《数据结构与算法》](/notes/computer-science/algorithm.md)、《操作系统》、《计算机网络》、《编译原理》是大学计算机的核心课程。

新技术层出不穷，网络协议是你到了 45 岁之后任然有价值的知识。

协议的三要素：

* 语法：一段内容要符合一定的规则和格式。如括号要成对。
* 语义：一段内容要代表某种意义。如数字减去数字是有意义的。
* 顺序。

只有通过网络协议，才能使一大片机器互相协作，共同完成一件事。

起初，TCP/IP 的设计者提出分层，它分为四层。TCP/IP 发明于 1970年代，当时还有很多网络协议，整个网络世界比较混乱。这时候国际标准组织（ISO）想来个大统一，于是设计了 OSI，开放式系统互联通信参考模型（Open System Interconnection Reference Model）。OSI 分为 7 层，设计参考了 TCP/IP 的分层，所以四层和七层有不是很精确的对应关系：

![](/files/-Lo-SBYAlNX_Axv1M4s6)

OSI 在四层以上分的太细，TCP/IP 在实际应用时会话管理、编码转换、压缩等和具体应用紧密联系，很难分开，所以 5、6层就不见了。

| 层级                                                                   | 协议                                                                                                                                                                                                                                                                                                                                  |
| -------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| [应用层](/notes/computer-science/network-protocol/application-layer.md) | [DHCP,](/notes/computer-science/network-protocol/application-layer.md#dhcp) [HTTP](/notes/computer-science/network-protocol/application-layer.md#http), [HTTPS](/notes/computer-science/network-protocol/application-layer.md#https), RTMP, P2P, [DNS](/notes/computer-science/network-protocol/application-layer.md#dns), GTP, RPC |
| [传输层](/notes/computer-science/network-protocol/transport-layer.md)   | [UDP](/notes/computer-science/network-protocol/transport-layer.md#udp), [TCP](/notes/computer-science/network-protocol/transport-layer.md#tcp)                                                                                                                                                                                      |
| [网络层](/notes/computer-science/network-protocol/network-layer.md)     | [ICMP](/notes/computer-science/network-protocol/network-layer.md#icmp), [IP](/notes/computer-science/network-protocol/network-layer.md#ip), [OSPF](/notes/computer-science/network-protocol/network-layer.md#ospf), [BGP](/notes/computer-science/network-protocol/network-layer.md#bgp), IPSec, GRE                                |
| [链路层](/notes/computer-science/network-protocol/data-link-layer.md)   | [ARP](/notes/computer-science/network-protocol/data-link-layer.md#arp), [VLAN](/notes/computer-science/network-protocol/data-link-layer.md#vlan), [STP](/notes/computer-science/network-protocol/data-link-layer.md#stp)                                                                                                            |
| [物理层](/notes/computer-science/network-protocol/pysical-layer.md)     | [网络跳线](/notes/computer-science/network-protocol/pysical-layer.md#8p-8-c), [集线器](/notes/computer-science/network-protocol/pysical-layer.md#hub)                                                                                                                                                                                      |

只要是在网络上跑的包，都是完整的。可以有下层没有上层，绝对不可能有上层没有下层。比如对于 TCP 协议，无论是三次握手、重试，只要想发出去包，就一定有 IP 和 MAC 层。

对于 UDP、TCP 在操作系统的实现，采用 [Socket 编程](/notes/computer-science/network-protocol/transport-layer.md#socket-bian-cheng)。


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