Ethernet driver development based on DWC_ether_qos-MAC frame format introduction

Latest update time：2023-08-28

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I. Introduction

When developing Ethernet drivers, applications, writing and debugging, packet capture analysis is often required. At this time, it is necessary to understand the format of MAC frames for analysis. We refer to the annotation document to organize it.

3. Media access control frame structure of " 802.3-2000_part1.pdf " specifies two frame formats for data communication systems using CSMA/CD MAC :

Basic MAC frame format

An extension of the basic MAC frame format , the Tagged MAC frame , is a frame that carries the QTag prefix.

2. Basic MAC frame format

The basic MAC frame format is as follows, consisting of 9 parts

OCTETS means 8 bits ,

1. The low bit of the byte is sent first , except for the CRC31 checksum field, which is sent high bit first. The bit stream is x 31 , x 30 ,…, x 1 , x 0 , i.e. x0 is the last bit sent of the last byte sent .

2. Multi-byte data is parsed according to the big-endian format , with the high byte first being sent .

Each part is described in detail as follows :

Serial number	Fields	Size ( bytes )	Function	illustrate
1	PREAMBLE	7	Leading field	Used for PLS (PHYSICAL LAYER SIGNALING) to achieve synchronization with the received frame ( even if there is a conflict when sending the preamble and SFD , the entire preamble and SFD will be sent) 10101010 10101010 10101010 10101010 10101010 10101010 is the sequence of 0x55
2	SFD	1	Start of frame delimiter	10101011 is 0xD5
3	DESTINATION ADDRESS (DA)	6	Destination Address Field	The address of the destination station . This can be a unicast or multicast (including broadcast) address. All 1s are the broadcast address. See the MAC address section below for details.
4	SOURCE ADDRESS (SA)	6	Source Address Field	The source address field is the address of the station sending the frame . The CSMA/CD MAC sublayer does not interpret the source address field.
5	LENGTH/TYPE	2	Length / Type Field	If it is less than or equal to maxValidFrame = maxUntaggedFrameSize (1518)-18=1500 , it means the length of MAC CLIENT DATA area If it is greater than or equal to 0x 0600 , it indicates the frame type.
6	MAC CLIENT DATA	/	data	MAC+DATA range: 46~1500 bytes
7	PAD	/	filling	Padding is performed so that DESTINATION ADDRESS ~FCS meets the minimum frame length of 64 bytes.
8	FRAME CHECK SEQUENCE (FCS)	4		CRC32 checksum , check data between DA and PAD area. Generate polynomial. G(x) = x 32 + x 26 + x 23 + x 22 + x 16 + x 12 + x 11 + x 10 + x 8 + x 7 + x 5 + x 4 + x 2 + x + 1 Note that FCS is x31 sent first and x0 sent last.
9	EXTENSION	/	Extended domain	0~( slotTime – minFrameSize) bits long 1000 Mb/s half-duplex only

2.1 MAC address

The definition of a 48 -bit MAC address is as follows

	Bit Field	Function	illustrate
1	I/G	Address Type	LSB is the first bit sent. Distinguish Is it unicast or multicast Address : 0: I individual is a unicast address , 1: G group is the multicast address. Note that this bit is only for the destination address , and is always 0 for the source address.
2	U/L		Distinguish between local and global addresses : 0: U universal is the global address , 1: L locally is the local address. The broadcast address bit is 1.
3	46-BIT ADDRESSS

Address Type :


1	Individual Address Unicast Address		An address corresponds to a specific site in the network.
2	Group Address Multicast Address	Multicast -Group Address	An address associated by a higher-level convention with a group of logically related stations.
2	Group Address Multicast Address	Broadcast Address	A distinguishable predefined multicast address that always represents the set of all stations on a given LAN. All 1s are the broadcast address .

2.2 Invalid Frame

Meet one or more of the following conditions

1. The frame length is inconsistent with the length value specified in the Length / Type field. If the Length / Type field indicates a Type value, the frame length is assumed to be consistent with that field and should not be considered an invalid frame on this basis.

2. The length is not an integer multiple of eight bits .

3. CRC32 checksum error.

4. The contents of an invalid MAC frame shall not be passed to the LLC or MAC control sublayer. The presence of an invalid MAC frame may be communicated to network management.

3. Tagged MAC frame format

The tagged MAC frame is an extension of the normal MAC frame. A 4-byte Qtag is added after the SA. See the standard IEEE P802.1Q

Serial number	Fields	Size ( bytes )	Function	illustrate
1	PREAMBLE	7	Leading field	Same as basic MAC frame
2	SFD	1	Start of frame delimiter	Same as basic MAC frame
3	DESTINATION ADDRESS	6	Destination Address Field	Same as basic MAC frame
4	SOURCE ADDRESS	6	Source Address Field	Same as basic MAC frame
5	LENGTH/TYPE = 802.1QTagType	2		QTag length / type Fixed to 0x8100 , indicating 802.1Q Tag Protocol Type.
6	TAG CONTROL INFORMATION	2		QTag control information a) A 3-bit User Priority field, b) A Canonical Format Indicator (CFI), and c) A 12-bit VLAN Identifier.
7	LENGTH/TYPE	2	Length / Type Field	The original length / type of the MAC frame before QTag is inserted . Same as basic MAC frame
8	MAC CLIENT DATA	/	data	Same as basic MAC frame
9	PAD	/	filling	Same as basic MAC frame For padding , the minimum frame length of 64 bytes may or may not include a 4- byte TAG . If the TAG is not included , the actual frame length is 64 bytes.
10	FRAME CHECK SEQUENCE	4		Same as basic MAC frame
11	EXTENSION	/	Extended domain	Same as basic MAC frame

4. Frame parsing in wireshark

For example, the following is an ARP request packet . Wireshark can parse its different fields . For example, click the data 0806 on the right side of Type on the lower left side to highlight it. Wireshark does not display the preamble, frame start symbol and FCS in the captured packet , because the former two are fixed. If the FCS is wrong, it cannot be received, and the MAC layer will discard it as an invalid frame.

5. Conclusion

When debugging Ethernet, the format of MAC frames must be understood. Wireshark can be used for easy parsing, but sometimes we directly print the raw data in the driver code, so we need to parse it manually.