以太帧格式 Ethernet Ⅱ以太帧
Ethernet Ⅱ帧，也称为Ethernet V2帧，是如今局域网里最常见的以太帧，是以太网事实标准。如今大多数的TCP/IP应用（如HTTP、FTP、SMTP、POP3等）都是采用Ethernet II帧承载。
1、以太帧格式
+-----------+-----------+-------------+--------------------+----------+
| DMAC | SMAC | Type | Data | FCS |
| 6 Bytes | 6 Bytes | 2 Bytes | Variable length | 4 Bytes |
+-----------+-----------+-------------+--------------------+----------+
| |
| |
| |
| |
| |
| |
+-------------+-----------+----------------+-----------------+
| 帧间隙 |前同步码 | 帧开始定界符 | Ethernet Frame |
|至少12Bytes | 7 Bytes | 1 Byte | Variable length |
+-------------+-----------+----------------+-----------------+
Ethernet Ⅱ帧格式
|
字段 |
长度 |
含义 |
|---|---|---|
|
DMAC |
6字节 |
目的MAC地址，IPV4为6字节，该字段标识帧的接收者。 |
|
SMAC |
6字节 |
源MAC地址，IPV4为6字节，该字段标识帧的发送者。 |
|
Type |
2字节 |
协议类型。表3列出了链路直接封装的协议类型。 |
|
Data |
46～1500字节 |
数据字段，标识帧的负载（可能包含填充位）。 数据字段的最小长度必须为46字节以保证帧长至少为64字节，这意味着传输1字节信息也必须使用46字节的数据字段。 如果填入该字段的信息少于46字节，该字段的其余部分也必须进行填充。数据字段的最大长度为1500字节。 以太帧的长度必须为整数字节，因此帧的负载长度不足整数字节，需插入填充字段以保证数据帧的长度为整数字节。 |
|
FCS |
4字节 |
帧校验序列FCS（Frame Check Sequence）是为接收者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。 FCS只是通用叫法，具体的FCS还可以细分多种校验方法。在以太帧中，FCS通常采用循环冗余码校验CRC（Cyclical Redundancy Check）。 |
|
字段 |
长度 |
含义 |
|---|---|---|
|
帧间隙 |
至少12字节 |
每个以太帧之间都要有帧间隙（Inter Frame Gap），即每发完一个帧后要等待一段时间才能再发另外一个帧，以便让帧接收者对接收的帧做必要的处理（如调整缓存的指针、更新计数、通知对报文进行处理等等）。在以太网标准中规定最小帧间隙是12个字节，其数据为全1。对于个别的接口，可减少到64(GE)或40比特(10GE)，其他的接口都不应该小于12字节。 |
|
前同步码 |
7字节 |
以太网标准中规定前导码为10101010 10101010 10101010 10101010 10101010 10101010 10101010（二进制），共7字节。 |
|
帧开始定界符 |
1字节 |
以太网标准中规定帧开始定界符为10101011（二进制），共1字节。 |
2、以太帧格式常见Type值
|
值 |
协议 |
|---|---|
|
0x0800 |
Internet Protocol Version 4 (IPv4) |
|
0x0801 |
X.75 Internet |
|
0x0805 |
X.25 Level 3 |
|
0x0806 |
Address Resolution Protocol (ARP) |
|
0x0808 |
Frame Relay ARP |
|
0x22F3 |
TRILL |
|
0x22F4 |
L2-IS-IS |
|
0x6558 |
Trans Ether Bridging |
|
0x6559 |
Raw Frame Relay |
|
0x8035 |
Reverse Address Resolution Protocol (RARP) |
|
0x809b |
Appletalk |
|
0x8100 |
IEEE Std 802.1Q - Customer VLAN Tag Type (C-Tag, formerly called the Q-Tag) (initially Wellfleet) |
|
0x8137 |
Novell NetWare IPX/SPX (old) |
|
0x8138 |
Novell, Inc. |
|
0x8100 |
IEEE Std 802.1Q - Customer VLAN Tag Type |
|
0x814C |
SNMP over Ethernet |
|
0x86DD |
IP Protocol version 6 (IPv6) |
|
0x876B |
TCP/IP Compression |
|
0x876C |
IP Autonomous Systems |
|
0x876D |
Secure Data |
|
0x8808 |
IEEE Std 802.3 - Ethernet Passive Optical Network (EPON) |
|
0x880B |
Point-to-Point Protocol (PPP) |
|
0x880C |
General Switch Management Protocol (GSMP) |
|
0x8847 |
MPLS (multiprotocol label switching) |
|
0x8848 |
MPLS with upstream-assigned label |
|
0x8863 |
PPP over Ethernet (PPPoE) Discovery Stage |
|
0x8864 |
PPP over Ethernet (PPPoE) Session Stage |
|
0x888E |
IEEE Std 802.1X - Port-based network access control |
|
0x88A8 |
IEEE Std 802.1Q - Service VLAN tag identifier (S-Tag) |
|
0x88B7 |
IEEE Std 802 - OUI Extended Ethertype |
|
0x88C7 |
IEEE Std 802.11 - Pre-Authentication (802.11i) |
|
0x88CC |
IEEE Std 802.1AB - Link Layer Discovery Protocol (LLDP) |
|
0x88E5 |
IEEE Std 802.1AE - Media Access Control Security |
|
0x88F5 |
IEEE Std 802.1Q - Multiple VLAN Registration Protocol (MVRP) |
|
0x88F6 |
IEEE Std 802.1Q - Multiple Multicast Registration Protocol (MMRP) |
|
0x893B |
TRILL Fine Grained Labeling (FGL) |
|
0x8946 |
TRILL RBridge Channel |
3、以太帧格式帧示例
Frame 1: 54 bytes on wire (432 bits), 54 bytes captured (432 bits)
Arrival Time: May 4, 2008 18:15:17.630001000
Epoch Time: 1209896117.630001000 seconds
[Time delta from previous captured frame: 0.000000000 seconds]
[Time delta from previous displayed frame: 0.000000000 seconds]
[Time since reference or first frame: 0.000000000 seconds]
Frame Number: 1
Frame Length: 54 bytes (432 bits)
Capture Length: 54 bytes (432 bits)
[Frame is marked: False]
[Frame is ignored: False]
[Protocols in frame: eth:ip:tcp]
[Coloring Rule Name: TCP]
[Coloring Rule String: tcp]
Ethernet II, Src: CompalEl_df:d0:05 (00:0f:b0:df:d0:05), Dst: HughesNe_a9:4f:87 (00:80:ae:a9:4f:87)
Destination: HughesNe_a9:4f:87 (00:80:ae:a9:4f:87)
Address: HughesNe_a9:4f:87 (00:80:ae:a9:4f:87)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
Source: CompalEl_df:d0:05 (00:0f:b0:df:d0:05)
Address: CompalEl_df:d0:05 (00:0f:b0:df:d0:05)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
Type: IP (0x0800)
Internet Protocol Version 4, Src: 192.168.128.101 (192.168.128.101), Dst: 172.25.4.24 (172.25.4.24)
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: 40
Identification: 0xc17e (49534)
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: 128
Protocol: TCP (6)
Header checksum: 0x4812 [correct]
[Good: True]
[Bad: False]
Source: 192.168.128.101 (192.168.128.101)
Destination: 172.25.4.24 (172.25.4.24)
Transmission Control Protocol, Src Port: tht-treasure (1832), Dst Port: icon-discover (2799), Seq: 1, Ack: 1, Len: 0
Source port: tht-treasure (1832)
Destination port: icon-discover (2799)
[Stream index: 0]
Sequence number: 1 (relative sequence number)
Acknowledgement number: 1 (relative ack number)
Header length: 20 bytes
Flags: 0x10 (ACK)
000. .... .... = Reserved: Not set
...0 .... .... = Nonce: Not set
.... 0... .... = Congestion Window Reduced (CWR): Not set
.... .0.. .... = ECN-Echo: Not set
.... ..0. .... = Urgent: Not set
.... ...1 .... = Acknowledgement: Set
.... .... 0... = Push: Not set
.... .... .0.. = Reset: Not set
.... .... ..0. = Syn: Not set
.... .... ...0 = Fin: Not set
Window size value: 17520
[Calculated window size: 17520]
[Window size scaling factor: -1 (unknown)]
Checksum: 0xf893 [validation disabled]
[Good Checksum: False]
[Bad Checksum: False]
4、以太帧格式参考标准
|
标准 |
描述 |
|---|---|
|
IEEE 802.3 |
Carrier sense multiple access with collision detection (CSMA/CD) access method and physical layer specifications |
|
IEEE 802.3ae |
Media Access Control (MAC) Parameters, Physical Layers, and Management parameters for 10Gb/s Operation |
|
RFC 894 |
A Standard for the Transmission of IP Datagrams over Ethernet Networks |
|
RFC 1042 |
A Standard for the Transmission of IP Datagrams over IEEE 802 Networks |
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