IEEE 802.3 SNAP以太帧格式

IEEE 802.3 SNAP以太帧，是IEEE为保证在802.3 LLC上支持更多的上层协议同时更好的支持IP协议而发布的标准。

一、 IEEE 802.3 SNAP帧格式

            +-----------+-----------+-----------+----------+----------+----------+---------+---------+------------------+----------+
            |   DMAC    |   SMAC    |   Length  |   DSAP   |   SSAP   |   Ctrl   |   OUI   |   Type  |        Data      |   FCS    |
            |  6 Bytes  |  6 Bytes  |  2 Bytes  |  1 Bytes |  1 Bytes |  1 Bytes | 3 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   |
            +--------------------+-----------------+-------------+---------------------+

802.3 SNAP以太帧的链路层各字段含义
字段	长度	含义
DMAC	6字节	目的MAC地址，该字段标识帧的接收者。
SMAC	6字节	源MAC地址，该字段标识帧的发送者。
Length	2字节	指后续数据的字节长度，但不包括FCS字段。
DSAP	1字节	目的服务访问点，若后面类型为IP帧值设为0x06。服务访问点的功能类似于Ethernet II帧中的Type字段或TCP/UDP传输协议中的端口号。
SSAP	1字节	源服务访问点，若后面类型为IP帧值设为0x06。
Ctrl	1字节	该字段值通常设为0x03，表示无连接服务的IEEE 802.2无编号数据格式。
SNAP-ID	5字节	由OUI和Type两部分组成。
OUI	3字节	3字节的组织唯一标识符（Organizationally Unique Identifier），其值通常等于MAC地址的前3字节，即网络适配器厂商代码。
Type	2字节	标识以太网帧所携带的上层数据类型。根据RFC1042标准，OUI使用一个特殊的数值0x00-00-00，当这个字段为0x00-00-00时，Type字段与Ethernet II封装中的Type具有相同的含义。
Data	38～1492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38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38字节的数据字段。如果填入该字段的信息少于38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1492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。 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）。

802.3 SNAP以太帧的物理层各字段含义
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作必要的处理（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。

二、IEEE 802.3 SNAP以太帧帧示例

Frame 1: 300 bytes on wire (2400 bits), 300 bytes captured (2400 bits)
    Arrival Time: Nov 20, 2006 22:49:06.739806000
    Epoch Time: 1164034146.739806000 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: 300 bytes (2400 bits)
    Capture Length: 300 bytes (2400 bits)
    [Frame is marked: False]
    [Frame is ignored: False]
    [Protocols in frame: eth:llc:cdp:data]
    [Coloring Rule Name: Routing]
    [Coloring Rule String: hsrp || eigrp || ospf || bgp || cdp || vrrp || gvrp || igmp || ismp]
IEEE 802.3 Ethernet 
    Destination: CDP/VTP/DTP/PAgP/UDLD (01:00:0c:cc:cc:cc)
        Address: CDP/VTP/DTP/PAgP/UDLD (01:00:0c:cc:cc:cc)
        .... ...1 .... .... .... .... = IG bit: Group address (multicast/broadcast)
        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
    Source:  d5:d5:15 (00:e0:1e:d5:d5:15)
        Address:  d5:d5:15 (00:e0:1e:d5:d5:15)
        .... ...0 .... .... .... .... = IG bit: Individual address (unicast)
        .... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
    Length: 286
Logical-Link Control
    DSAP: SNAP (0xaa)
    IG Bit: Individual
    SSAP: SNAP (0xaa)
    CR Bit: Command
    Control field: U, func=UI (0x03)
        000. 00.. = Command: Unnumbered Information (0x00)
        .... ..11 = Frame type: Unnumbered frame (0x03)
    Organization Code:   (0x00000c)
    PID: CDP (0x2000)
  Discovery Protocol
    Version: 1
    TTL: 180 seconds
    Checksum: 0xdff0 [correct]
        [Good: True]
        [Bad: False]
    Device ID: R1
        Type: Device ID (0x0001)
        Length: 6
        Device ID: R1
    Addresses
        Type: Addresses (0x0002)
        Length: 17
        Number of addresses: 1
        IP address: 192.168.10.1
            Protocol type: NLPID
            Protocol length: 1
            Protocol: IP
            Address length: 4
            IP address: 192.168.10.1
    Port ID: Ethernet0
        Type: Port ID (0x0003)
        Length: 13
        Sent through Interface: Ethernet0
    Capabilities
        Type: Capabilities (0x0004)
        Length: 8
        Capabilities: 0x00000001
            .... .... .... .... .... .... .... ...1 = Is  a Router
            .... .... .... .... .... .... .... ..0. = Not a Transparent Bridge
            .... .... .... .... .... .... .... .0.. = Not a Source Route Bridge
            .... .... .... .... .... .... .... 0... = Not a Switch
            .... .... .... .... .... .... ...0 .... = Not a Host
            .... .... .... .... .... .... ..0. .... = Not IGMP capable
            .... .... .... .... .... .... .0.. .... = Not a Repeater
    Software Version
        Type: Software version (0x0005)
        Length: 216
        Software Version:   Internetwork Operating System Software 
                          IOS (tm) 1600 Software (C1600-NY-L), Version 11.2(12)P, RELEASE SOFTWARE (fc1)
                          Copyright (c) 1986-1998 by   Systems, Inc.
                          Compiled Tue 03-Mar-98 06:33 by dschwart
    Platform:   1601
        Type: Platform (0x0006)
        Length: 14
        Platform:   1601


三、IEEE 802.3 SNAP以太帧参考标准

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

IEEE 802.3 SNAP以太帧格式

IEEE 802.3 SNAP以太帧格式

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