osi 七层网络模型

网络层-OSI模型 (Network Layer - OSI Model)

The network Layer controls the operation of the subnet. The main aim of this layer is to deliver packets from source to destination across multiple links (networks). If two computers (system) are connected on the same link, then there is no need for a network layer. It routes the signal through different channels to the other end and acts as a network controller.

网络层控制子网的操作。 该层的主要目的是通过多个链路（网络）将数据包从源传送到目的地。 如果两台计算机（系统）连接在同一链路上，则无需网络层。 它通过不同的通道将信号路由到另一端，并充当网络控制器。

It also divides the outgoing messages into packets and to assemble incoming packets into messages for higher levels.

它还将传出的消息划分为数据包，并将传入的数据包组合为更高级别的消息。

In broadcast networks, the routing problem is simple, so the network layer is often thin or even non-existent.

在广播网络中，路由问题很简单，因此网络层通常很薄，甚至根本不存在。

网络层的功能 (Functions of Network Layer)

1. It translates logical network address into physical address. Concerned with circuit, message or packet switching.
   它将逻辑网络地址转换为物理地址。 与电路，消息或数据包交换有关。
2. Routers and gateways operate in the network layer. Mechanism is provided by Network Layer for routing the packets to final destination.
   路由器和网关在网络层中运行。 网络层提供了将数据包路由到最终目的地的机制。
3. Connection services are provided including network layer flow control, network layer error control and packet sequence control.
   提供的连接服务包括网络层流控制，网络层错误控制和数据包序列控制。
4. Breaks larger packets into small packets.
   将较大的数据包分解为小数据包。

网络层的设计问题 (Design Issues with Network Layer)

• determining how packets are routed from source to destination. Routes can be based on static tables that are wired into the network and rarely changed. They can also be highly dynamic, being determined anew for each packet, to reflect the current network load.确定数据包如何从源路由到目的地 。 路由可以基于连接到网络且很少更改的静态表。 它们还可以是高度动态的（针对每个数据包重新确定），以反映当前的网络负载。
• If too many packets are present in the subnet at the same time, they will get into one another's way, forming bottlenecks. The control of such congestion also belongs to the network layer.
• 如果子网中同时存在太多的数据包 ，它们将互相干扰，形成瓶颈 。 这种拥塞的控制也属于网络层。
• quality of service provided(delay, transmit time, jitter, etc) is also a network layer issue.的服务质量 （延迟，传输时间，抖动等）也是网络层的问题。
• travel from one network to another to get to its destination, many problems can arise such as: 从一个网络到达另一个网络到达目的地时 ，可能会出现许多问题，例如：
  • The addressing used by the second network may be different from the first one.
    第二个网络使用的寻址可能与第一个不同。
  • The second one may not accept the packet at all because it is too large.
    第二个可能根本不接受该数据包，因为它太大了。
  • The protocols may differ, and so on.
    协议可能不同，依此类推。
• It is up to the network layer to overcome all these problems to allow heterogeneous networks to be interconnected.
  网络层要克服所有这些问题，以允许异构网络互连。

翻译自: https://www.studytonight.com/computer-networks/osi-model-network-layer

osi 七层网络模型