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分享请教个问题：当IP数据包小于46字节时为啥网卡没有自动填充
根据IEEE标准，如果以太网帧小于64字节（IP数据包小于46字节），必须自动填充到最小帧长。
可是我在Fedora 15下面用sendto方法送出的一个43字节的UDP数据包，用ethereal抓的结果却是没有填充。是不是我的网卡要做什么设置才行？
期待高手指教!
No. Time Source Destination Protocol Info size
1 12:23:45.607414 192.168.18.137 192.168.18.1 UDP Source port: 5001 Destination port: 5001 57
Frame 1 (57 bytes on wire, 57 bytes captured)
Arrival Time: Aug 7, 2011 12:23:45.607414000
Time delta from previous packet: 0.000000000 seconds
Time since reference or first frame: 0.000000000 seconds
Frame Number: 1
Packet Length: 57 bytes
Capture Length: 57 bytes
Protocols in frame: eth:ip:udp:cpfi:data
Ethernet II, Src: 00:0c:29:0e:b2:89, Dst: 00:50:56:c0:00:08
Destination: 00:50:56:c0:00:08 (Vmware_c0:00:08)
Source: 00:0c:29:0e:b2:89 (Vmware_0e:b2:89)
Type: IP (0x0800) <----这里没有padding/trailer
Internet Protocol, Src Addr: 192.168.18.137 (192.168.18.137), Dst Addr: 192.168.18.1 (192.168.18.1)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
0000 00.. = Differentiated Services Codepoint: Default (0x00)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 43 <---------总长度43byte
Identification: 0x0000 (0)
Flags: 0x04 (Don't Fragment)
0... = Reserved bit: Not set
.1.. = Don't fragment: Set
..0. = More fragments: Not set
可是我在Fedora 15下面用sendto方法送出的一个43字节的UDP数据包，用ethereal抓的结果却是没有填充。是不是我的网卡要做什么设置才行？
期待高手指教!
No. Time Source Destination Protocol Info size
1 12:23:45.607414 192.168.18.137 192.168.18.1 UDP Source port: 5001 Destination port: 5001 57
Frame 1 (57 bytes on wire, 57 bytes captured)
Arrival Time: Aug 7, 2011 12:23:45.607414000
Time delta from previous packet: 0.000000000 seconds
Time since reference or first frame: 0.000000000 seconds
Frame Number: 1
Packet Length: 57 bytes
Capture Length: 57 bytes
Protocols in frame: eth:ip:udp:cpfi:data
Ethernet II, Src: 00:0c:29:0e:b2:89, Dst: 00:50:56:c0:00:08
Destination: 00:50:56:c0:00:08 (Vmware_c0:00:08)
Source: 00:0c:29:0e:b2:89 (Vmware_0e:b2:89)
Type: IP (0x0800) <----这里没有padding/trailer
Internet Protocol, Src Addr: 192.168.18.137 (192.168.18.137), Dst Addr: 192.168.18.1 (192.168.18.1)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
0000 00.. = Differentiated Services Codepoint: Default (0x00)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 43 <---------总长度43byte
Identification: 0x0000 (0)
Flags: 0x04 (Don't Fragment)
0... = Reserved bit: Not set
.1.. = Don't fragment: Set
..0. = More fragments: Not set
...全文
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日立奔腾浪潮微软松下联想 2011-08-22
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填充（如果需要的话）是数据链路层协议自动完成的，如果你数据发送、接收都正确，那就应该没问题。我估计还是你的抓包软件设置的问题（只是没显示填充而已，CRC32不是也没显示出来吗）。
liyanshao2 2011-08-21
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网上查到的以太帧的描述来看，不应该是前导码的问题。是在类型和填充（Data and Pad）这个域的。但不知道这个地方怎么加个Pad.
字段 字段长度（字节） 目的
前导码（Preamble） 7 同步
帧开始符（SFD） 1 标明下一个字节为目的MAC字段
目的MAC地址 6 指明帧的接受者
源MAC地址 6 指明帧的发送者
长度（Length） 2 帧的数据字段的长度（长度或类型）
类型（Type） 2 帧中数据的协议类型（长度或类型）
类型和填充（Data and Pad）注 46~1500 高层的数据，通常为3层协议数据单元。对于TCP/IP是IP数据包
帧校验序列（FCS） 4 对接收网卡提供判断是否传输错误的一种方法，如果发现错误，丢弃此帧
注：如果数据包小于46字节，则要求“填充”，以使这个字段达到46字节。填充是必须的，因为数据字段要求至少46字节长。
字段 字段长度（字节） 目的
前导码（Preamble） 7 同步
帧开始符（SFD） 1 标明下一个字节为目的MAC字段
目的MAC地址 6 指明帧的接受者
源MAC地址 6 指明帧的发送者
长度（Length） 2 帧的数据字段的长度（长度或类型）
类型（Type） 2 帧中数据的协议类型（长度或类型）
类型和填充（Data and Pad）注 46~1500 高层的数据，通常为3层协议数据单元。对于TCP/IP是IP数据包
帧校验序列（FCS） 4 对接收网卡提供判断是否传输错误的一种方法，如果发现错误，丢弃此帧
注：如果数据包小于46字节，则要求“填充”，以使这个字段达到46字节。填充是必须的，因为数据字段要求至少46字节长。
liyanshao2 2011-08-21
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贴个我抓的一个完整的。都是ethereal抓的包，我用的是version 0.10.11，我不知道他用的是哪个版本。
我比较两个包，UDP包和IP包的长度都一样，就是他多了3个byte的padding。所以我的长度就是57，他的就是60。
Frame 1 (57 bytes on wire, 57 bytes captured)
Arrival Time: Aug 7, 2011 12:23:45.607414000
Time delta from previous packet: 0.000000000 seconds
Time since reference or first frame: 0.000000000 seconds
Frame Number: 1
Packet Length: 57 bytes
Capture Length: 57 bytes
Protocols in frame: eth:ip:udp:cpfi:data
Ethernet II, Src: 00:0c:29:0e:b2:89, Dst: 00:50:56:c0:00:08
Destination: 00:50:56:c0:00:08 (Vmware_c0:00:08)
Source: 00:0c:29:0e:b2:89 (Vmware_0e:b2:89)
Type: IP (0x0800) <---- 就是这里少了3个byte的padding
Internet Protocol, Src Addr: 192.168.18.137 (192.168.18.137), Dst Addr: 192.168.18.1 (192.168.18.1)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
0000 00.. = Differentiated Services Codepoint: Default (0x00)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 43 <---- IP包的长度跟他也一样
Identification: 0x0000 (0)
Flags: 0x04 (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 (0x11)
Header checksum: 0x94e7 (correct)
Source: 192.168.18.137 (192.168.18.137)
Destination: 192.168.18.1 (192.168.18.1)
User Datagram Protocol, Src Port: 5001 (5001), Dst Port: 5001 (5001)
Source port: 5001 (5001)
Destination port: 5001 (5001)
Length: 23 <-----UDP的长度跟他一样
Checksum: 0x28c9 (correct)
Cross Point Frame Injector
Header
Word one: 0x01000000
0000 .... .... .... .... .... .... .... = FrmType: 0x00000000
.... 0001 0000 00.. .... .... .... .... = srcTDA: 0x00000040
.... .... .... ..00 0000 0000 .... .... = dstTDA: 0x00000000
.... .... .... .... .... .... 0000 .... = SOFtype: SOFf (0x00000000)
.... .... .... .... .... .... .... 00.. = speed: 1 GBIT (0x00000000)
.... .... .... .... .... .... .... ..0. = OPMerror: False
.... .... .... .... .... .... .... ...0 = fromLCM: False
Word two: 0x01020607
Data (7 bytes)
我比较两个包，UDP包和IP包的长度都一样，就是他多了3个byte的padding。所以我的长度就是57，他的就是60。
Frame 1 (57 bytes on wire, 57 bytes captured)
Arrival Time: Aug 7, 2011 12:23:45.607414000
Time delta from previous packet: 0.000000000 seconds
Time since reference or first frame: 0.000000000 seconds
Frame Number: 1
Packet Length: 57 bytes
Capture Length: 57 bytes
Protocols in frame: eth:ip:udp:cpfi:data
Ethernet II, Src: 00:0c:29:0e:b2:89, Dst: 00:50:56:c0:00:08
Destination: 00:50:56:c0:00:08 (Vmware_c0:00:08)
Source: 00:0c:29:0e:b2:89 (Vmware_0e:b2:89)
Type: IP (0x0800) <---- 就是这里少了3个byte的padding
Internet Protocol, Src Addr: 192.168.18.137 (192.168.18.137), Dst Addr: 192.168.18.1 (192.168.18.1)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
0000 00.. = Differentiated Services Codepoint: Default (0x00)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 43 <---- IP包的长度跟他也一样
Identification: 0x0000 (0)
Flags: 0x04 (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 (0x11)
Header checksum: 0x94e7 (correct)
Source: 192.168.18.137 (192.168.18.137)
Destination: 192.168.18.1 (192.168.18.1)
User Datagram Protocol, Src Port: 5001 (5001), Dst Port: 5001 (5001)
Source port: 5001 (5001)
Destination port: 5001 (5001)
Length: 23 <-----UDP的长度跟他一样
Checksum: 0x28c9 (correct)
Cross Point Frame Injector
Header
Word one: 0x01000000
0000 .... .... .... .... .... .... .... = FrmType: 0x00000000
.... 0001 0000 00.. .... .... .... .... = srcTDA: 0x00000040
.... .... .... ..00 0000 0000 .... .... = dstTDA: 0x00000000
.... .... .... .... .... .... 0000 .... = SOFtype: SOFf (0x00000000)
.... .... .... .... .... .... .... 00.. = speed: 1 GBIT (0x00000000)
.... .... .... .... .... .... .... ..0. = OPMerror: False
.... .... .... .... .... .... .... ...0 = fromLCM: False
Word two: 0x01020607
Data (7 bytes)
liyanshao2 2011-08-21
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贴上人家抓的包
Frame 2 (60 bytes on wire, 60 bytes captured)
Arrival Time: Jul 26, 2011 10:30:57.800924000
Time delta from previous packet: 0.555307000 seconds
Time since reference or first frame: 0.555307000 seconds
Frame Number: 2
Packet Length: 60 bytes
Capture Length: 60 bytes
Protocols in frame: eth:ip:udp:cpfi:data
Ethernet II, Src: 00:0a:eb:fd:7a:00, Dst: 60:eb:69:31:57:23
Destination: 60:eb:69:31:57:23 (192.168.1.40)
Source: 00:0a:eb:fd:7a:00 (192.168.2.254)
Type: IP (0x0800)
Trailer: 000000 <-----这里有3个字节的padding
Internet Protocol, Src Addr: 192.168.2.254 (192.168.2.254), Dst Addr: 192.168.1.40 (192.168.1.40)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
0000 00.. = Differentiated Services Codepoint: Default (0x00)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 43
Identification: 0x0067 (103)
Flags: 0x00
0... = Reserved bit: Not set
.0.. = Don't fragment: Not set
..0. = More fragments: Not set
Fragment offset: 0
Time to live: 128
Protocol: UDP (0x11)
Header checksum: 0xb4e4 (correct)
Source: 192.168.2.254 (192.168.2.254)
Destination: 192.168.1.40 (192.168.1.40)
User Datagram Protocol, Src Port: 5001 (5001), Dst Port: 5001 (5001)
Source port: 5001 (5001)
Destination port: 5001 (5001)
Length: 23
Checksum: 0x492d (correct)
Cross Point Frame Injector
Header
Word one: 0x01000000
0000 .... .... .... .... .... .... .... = FrmType: 0x00000000
.... 0001 0000 00.. .... .... .... .... = srcTDA: 0x00000040
.... .... .... ..00 0000 0000 .... .... = dstTDA: 0x00000000
.... .... .... .... .... .... 0000 .... = SOFtype: SOFf (0x00000000)
.... .... .... .... .... .... .... 00.. = speed: 1 GBIT (0x00000000)
.... .... .... .... .... .... .... ..0. = OPMerror: False
.... .... .... .... .... .... .... ...0 = fromLCM: False
Word two: 0x01020607
Data (7 bytes)
Frame 2 (60 bytes on wire, 60 bytes captured)
Arrival Time: Jul 26, 2011 10:30:57.800924000
Time delta from previous packet: 0.555307000 seconds
Time since reference or first frame: 0.555307000 seconds
Frame Number: 2
Packet Length: 60 bytes
Capture Length: 60 bytes
Protocols in frame: eth:ip:udp:cpfi:data
Ethernet II, Src: 00:0a:eb:fd:7a:00, Dst: 60:eb:69:31:57:23
Destination: 60:eb:69:31:57:23 (192.168.1.40)
Source: 00:0a:eb:fd:7a:00 (192.168.2.254)
Type: IP (0x0800)
Trailer: 000000 <-----这里有3个字节的padding
Internet Protocol, Src Addr: 192.168.2.254 (192.168.2.254), Dst Addr: 192.168.1.40 (192.168.1.40)
Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00)
0000 00.. = Differentiated Services Codepoint: Default (0x00)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
Total Length: 43
Identification: 0x0067 (103)
Flags: 0x00
0... = Reserved bit: Not set
.0.. = Don't fragment: Not set
..0. = More fragments: Not set
Fragment offset: 0
Time to live: 128
Protocol: UDP (0x11)
Header checksum: 0xb4e4 (correct)
Source: 192.168.2.254 (192.168.2.254)
Destination: 192.168.1.40 (192.168.1.40)
User Datagram Protocol, Src Port: 5001 (5001), Dst Port: 5001 (5001)
Source port: 5001 (5001)
Destination port: 5001 (5001)
Length: 23
Checksum: 0x492d (correct)
Cross Point Frame Injector
Header
Word one: 0x01000000
0000 .... .... .... .... .... .... .... = FrmType: 0x00000000
.... 0001 0000 00.. .... .... .... .... = srcTDA: 0x00000040
.... .... .... ..00 0000 0000 .... .... = dstTDA: 0x00000000
.... .... .... .... .... .... 0000 .... = SOFtype: SOFf (0x00000000)
.... .... .... .... .... .... .... 00.. = speed: 1 GBIT (0x00000000)
.... .... .... .... .... .... .... ..0. = OPMerror: False
.... .... .... .... .... .... .... ...0 = fromLCM: False
Word two: 0x01020607
Data (7 bytes)
日立奔腾浪潮微软松下联想 2011-08-21
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你顶楼贴的是完整的吗？怎么比他的结构少很多？是同一软件的同一版本抓的吗？
日立奔腾浪潮微软松下联想 2011-08-18
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Type: IP (0x0800) <----这里没有padding/trailer
===================================================
这里的0800只是标识frame里的数据类型是IP，不是长度，长度最多只到0x05dc（1500），你怎么知道“没有padding/trailer”的？可以把别人抓的包贴上来看看。
另外更正一个错误，64字节的最小frame限制不应该包括7字节的前导码和1字节的frame起始分隔符，所以你贴frame长度57应该是6（mac）+6（mac）+2（type）+43（payload），为什么没有列出来padding和CRC32不清楚，但是应该有的，否则应该不正常的。
===================================================
这里的0800只是标识frame里的数据类型是IP，不是长度，长度最多只到0x05dc（1500），你怎么知道“没有padding/trailer”的？可以把别人抓的包贴上来看看。
另外更正一个错误，64字节的最小frame限制不应该包括7字节的前导码和1字节的frame起始分隔符，所以你贴frame长度57应该是6（mac）+6（mac）+2（type）+43（payload），为什么没有列出来padding和CRC32不清楚，但是应该有的，否则应该不正常的。
liyanshao2 2011-08-17
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这样啊。谢谢DelphiGuy。
我看别人抓的包是有3个byte的trailer/padding，其他的长度跟我一样，就有60个byte。你知道怎么样加上trailer/padding吗？
我看别人抓的包是有3个byte的trailer/padding，其他的长度跟我一样，就有60个byte。你知道怎么样加上trailer/padding吗？
日立奔腾浪潮微软松下联想 2011-08-13
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43是IP包的长度，Frame长度不是写着57吗？前面还有7字节的前导码没计算，总共恰好64字节。
