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我用 C 语言创建了一个程序，它使用 UDP 数据包发送数据。

套接字制作不要使用

int optval = IP_PMTUDISC_DO;
if(setsockopt(sd,IPPROTO_IP,IP_MTU_DISCOVER,&optval,sizeof(int))!=0)
{
    perror("setsocketopt()");
    return 0;
}

在服务器上使用 TSHARK 检查（debian 8 KVM 虚拟化），所有数据包都没有分段：

服务器嗅探器

但是在客户端，接收的大数据包是零散的！！

客户端wireshark

然后我想出了一些更有线的东西。IPv4 ID 字段已设置为 0！

我认为这可能是因为不分段效应（因为没有数据包会被分段）。

然后我启动 Openvpn 程序并在服务器上嗅探它的数据包，但数据包的 IPv4 ID ！= 0 而没有设置分段。

你们知道为什么这会发生在我身上吗？！

编辑：这是服务器副本中的另一个大数据包示例，并从 tshark 结果粘贴。

Frame 1049: 1514 bytes on wire (12112 bits), 1514 bytes captured (12112 bits) on interface 0
Ethernet II, Src: server_mac, Dst: gateway_mac
Internet Protocol Version 4, Src: server_ip, Dst: client_ip
Version: 4
Header Length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00: Not-ECT (Not ECN-Capable Transport))
    0000 00.. = Differentiated Services Codepoint: Default (0x00)
    .... ..00 = Explicit Congestion Notification: Not-ECT (Not ECN-Capable Transport) (0x00)
Total Length: 1500
Identification: 0x0000 (0)
Flags: 0x02 (Don't Fragment)
    0... .... = Reserved bit: Not set
    .1.. .... = Don't fragment: Set
    ..0. .... = More fragments: Not set
Fragment offset: 0
Time to live: 64
Protocol: UDP (17)
Header checksum: 0xa52d [validation disabled]
    [Good: False]
    [Bad: False]
Source: server_ip
Destination: client_ip
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
User Datagram Protocol, Src Port: 7554 (7554), Dst Port: 45376 (45376)
Data (1472 bytes)

如您所见，没有设置分段，而所有大于 PMTU 的数据包都将分段接收到客户端。

这是同一服务器上 openvpn 的数据包跟踪示例。就像你看到的那样，至少计算了 IPv4 ID！

Frame 3749: 1455 bytes on wire (11640 bits), 1455 bytes captured (11640 bits) on interface 0
Ethernet II, Src: SERVER_MAC, Dst: GATEWAY_MAC
Internet Protocol Version 4, Src: SERVER_IP, Dst: CLIENT_IP
Version: 4
Header Length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00: Not-ECT (Not ECN-Capable Transport))
    0000 00.. = Differentiated Services Codepoint: Default (0x00)
    .... ..00 = Explicit Congestion Notification: Not-ECT (Not ECN-Capable Transport) (0x00)
Total Length: 1441
Identification: 0xcc96 (52374)
Flags: 0x02 (Don't Fragment)
    0... .... = Reserved bit: Not set
    .1.. .... = Don't fragment: Set
    ..0. .... = More fragments: Not set
Fragment offset: 0
Time to live: 64
Protocol: UDP (17)
Header checksum: 0xe252 [validation disabled]
    [Good: False]
    [Bad: False]
Source: SERVER_IP
Destination: CLIENT_IP
[Source GeoIP: Unknown]
[Destination GeoIP: Unknown]
User Datagram Protocol, Src Port: 19234 (19234), Dst Port: 46921 (46921)
Data (1413 bytes)
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