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Ethernet

Media Access Control​

MAC Header

Switch​

交换机的作用为包的转发, 它自己不会成为发送方或者接收方, 各个端口不具有 MAC 地址.

Switch

交换机内部有一张 MAC 地址与对应网线端口的 MAC 地址表:

  • 当交换机接收到包时, 会将相应的端口号与发送方 MAC 地址写入表中, 交换机可以根据该地址表进行包的转发.
  • 当交换机发现一个包要发回到原端口时, 会直接丢弃该包, 防止接收方收到重复包.
  • 当地址表中找不到指定的 MAC 地址时, 交换机会将包转发到除了源端口之外的所有端口. 目标设备收到包后会返回响应包, 交换机此时可以更新地址表, 下次转发时无需广播发包.

Switch Circuit

Hub​

以太网集线器 (Ethernet Hub):

  • 中继式集线器: 广播以太帧, 接收方网卡根据 MAC 地址决定接收或丢弃报文.
  • 交换式集线器: 信号只会根据 MAC 地址流到指定设备, 不会到达其他设备.

ARP​

Address resolution protocol:

  • 通过 ARP 广播查询对方 MAC 地址, 填写至 MAC 头部, 传入网卡 (link layer).
  • generates mappings between link layer and network layer addresses cached in nodes
  • request-reply protocol: who has network address X => I have network address X
  • request sent to link layer broadcast address, reply sent to requesting address
  • when request to dest ARP packet header with empty DEST HARDWARE ADDRESS field and opcode 1(request)
  • when reply to src ARP packet header with dest hardware address as SRC HARDWARE ADDRESS field, src hardware address as DEST HARDWARE ADDRESS field and opcode 2(reply)
  • if A and B aren't in same sub-net, they delivery ARP broadcast with third public gateway