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我遇到了带有 SO_REUSEADDR 选项集的 UDP 套接字的有趣行为......

如果我通过 IP/UDP 发送 1472 个字节，我会在一帧中得到所有内容 - 这是预期的......

但是对于 1473 碎片，有/没有该选项会有所不同。有谁知道为什么会发生这种情况（我使用的是 Debian 2.6.32-5-amd64）？

SO_REUSEADDR 启用：

框架#1

Internet Protocol Version 4, Src: 173.59.3.22 (173.59.3.22), Dst: 173.70.1.5 (173.70.1.5)
    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: **1476**
    Identification: 0x214d (8525)
    Flags: 0x01 (More Fragments)
        0... .... = Reserved bit: Not set
        .0.. .... = Don't fragment: Not set
        ..1. .... = More fragments: Set
    Fragment offset: 0
    Time to live: 64
    Protocol: UDP (17)
    Header checksum: 0xd53f [correct]
        [Good: True]
        [Bad: False]
    Source: 173.59.3.22 (173.59.3.22)
    Destination: 173.70.1.5 (173.70.1.5)
    [Source GeoIP: Unknown]
    [Destination GeoIP: Unknown]
User Datagram Protocol (**8** bytes)
    Source port: icl-twobase1 (25000)
    Destination port: 5000 (5000)
    Length: **1481**
    Checksum: 0x3cac [unchecked, not all data available]
        [Good Checksum: False]
        [Bad Checksum: False]
Data (**1448** bytes)

框架#2

Internet Protocol Version 4, Src: 173.59.3.22 (173.59.3.22), Dst: 173.70.1.5 (173.70.1.5)br/>
    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: **45**
    Identification: 0x214d (8525)
    Flags: 0x00
        0... .... = Reserved bit: Not set
        .0.. .... = Don't fragment: Not set
        ..0. .... = More fragments: Not set
    Fragment offset: 1456
    Time to live: 64
    Protocol: UDP (17)
    Header checksum: 0xfa20 [correct]
        [Good: True]
        [Bad: False]
    Source: 173.59.3.22 (173.59.3.22)
    Destination: 173.70.1.5 (173.70.1.5)
    [Source GeoIP: Unknown]
    [Destination GeoIP: Unknown]
Data (**25** bytes)

SO_REUSEADDR 已禁用：

框架#1

Internet Protocol Version 4, Src: 173.59.3.22 (173.59.3.22), Dst: 173.70.1.5 (173.70.1.5)
    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: 0x214a (8522)
    Flags: 0x01 (More Fragments)
        0... .... = Reserved bit: Not set
        .0.. .... = Don't fragment: Not set
        ..1. .... = More fragments: Set
    Fragment offset: 0
    Time to live: 64
    Protocol: UDP (17)
    Header checksum: 0xd52a [correct]
        [Good: True]
        [Bad: False]
    Source: 173.59.3.22 (173.59.3.22)
    Destination: 173.70.1.5 (173.70.1.5)
    [Source GeoIP: Unknown]
    [Destination GeoIP: Unknown]

User Datagram Protocol (**8** bytes)
    Source port: icl-twobase1 (25000)
    Destination port: 5000 (5000)
    Length: **1481**
    Checksum: 0x3cac [unchecked, not all data available]
        [Good Checksum: False]
        [Bad Checksum: False]
Data (**1472** bytes)

框架#2

Internet Protocol Version 4, Src: 173.59.3.22 (173.59.3.22), Dst: 173.70.1.5 (173.70.1.5)
    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: **21**
    Identification: 0x214a (8522)
    Flags: 0x00
        0... .... = Reserved bit: Not set
        .0.. .... = Don't fragment: Not set
        ..0. .... = More fragments: Not set
    Fragment offset: 1480
    Time to live: 64
    Protocol: UDP (17)
    Header checksum: 0xfa38 [correct]
        [Good: True]
        [Bad: False]
    Source: 173.59.3.22 (173.59.3.22)
    Destination: 173.70.1.5 (173.70.1.5)
    [Source GeoIP: Unknown]
    [Destination GeoIP: Unknown]
Data (**1** byte)
4

1 回答 1

1

答案是否定的——SO_REUSEADDR 不会影响分段……


线索是MTU 发现和IP 标头中的不要分段标志。
如果在 MTU Discovery = ON 的情况下发送 1472 个字节：
在 Don't Frag Flag ON => 的情况下发送 1500 个字节我得到 ICMP：

Internet 控制消息协议
类型：3（目标不可达）
代码：4（需要分段）
 校验和：0x90db [正确]
下一跳的 MTU：1480

之后，发送方对所有数据包进行分段以适应 1480 MTU，同时为该接收方保留 route\arp 缓存信息（路由 5 分钟，arp 30 秒 - 我的系统上配置的默认值）。

这就是为什么我看到意外的 1473 字节碎片（两帧中的 1456 + 25 字节）（SO_REUSEADDR 在该实验中为 ON）...

下次我尝试发送相同的 1473 字节（SO_REUSEADDR 已关闭）时，route\arp 缓存释放了有关接收器的信息，我看到了不同的混乱碎片（两帧中的 1480 + 1 个字节）。

很抱歉让大家感到困惑......
但是，我深入研究这种行为并澄清我观察到的事情对我来说是相当认知的。这里没有奇迹。

于 2013-03-28T07:44:47.103 回答