Ethernet

• Minimum ethernet frame is 64 bytes
• Measured in byte

Frame

• Size: 64 - 1518 bytes
  • Minimum: an 18-byte header and a 46-byte payload.
  • Maximum: a 1,500-byte payload.

1. Preamble: Informs the receiving system that a frame is starting and enables synchronization.
2. Start Frame Delimiter (SFD): Signifies that the Destination MAC Address field begins with the next byte.
3. Destination MAC Address: The MAC address of the destination device.
4. Source MAC Address: The MAC address of the source device.
5. EtherType (in Ethernet II): In Ethernet II, this field identifies the protocol carried in the payload of the frame (e.g., IPv4, IPv6, or ARP).
   • A value of 0x0800 indicates that the payload is an IP packet
   • A value of 0x0806 indicates that the payload is an ARP packet.
6. Data and Pad: Contains the payload data. Padding data is added to meet the minimum length requirement for this field (46 bytes).
7. Frame Check Sequence (FCS): Contains a 32-bit Cyclic Redundancy Check (CRC) which allows detection of corrupted data

Links

• Minimum ethernet frame is 64 bytes, Why the payload must be padded to at least 46 bytes - Network Engineering Stack Exchange
• Ethernet frame - Wikipedia
• EtherType - Wikipedia

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CSMA/CD

1. 准备发送： 适配器从网络层获得一个分组，加上首部和尾部，组成以太网帧，放入适配器的缓存中；
2. 检测信道： 若检测到信道忙，则不停检测，直到信道空闲。若检测到信道空闲，并在 96 比特时间内仍保持空闲，则发送帧；
3. 在发送过程中不停检测：
   • 发送成功： 在争用期内未检测到碰撞，则帧肯定发送成功。发送后，回到（1）
   • 发送失败： 在争用期内检测到碰撞，停止发送数据，发送干扰信号。执行二进制 指数退避 算法，等待 r 倍 512 比特后，返回步骤（2），继续检测信道。若重传 16 次仍不成功，则停止重传并向上报错误。

• 争用期长度为 2t，即端到端传播时延的两倍。检测到碰撞后不发送干扰信号。