types. The Ethernet II frame is usually used for transmission of IP datagrams.
Ethernet 802.3 was developed by the IEEE from the original Ethernet standard in 1983. IEEE Ethernet defines two layers; the lower MAC layer in 802.3 and an upper LLC (logical link control) layer in 802.2. These are sublayers of the OSI data link layer (Layer 2). The two layers were defined separately to provide additional link control features and so that common LLC frames could be used for different media types, such as Ethernet, Token Ring and FDDI. This allows bridging at Layer 2 between the different media types.
There are three different 802.3 formats that were used for older protocols such as Novel Netware’s IPX and Apple Computer’s Appletalk protocols and OSI protocols. Today, these formats are rarely used. The Alcatel- Lucent 7750 SR uses the 802.3 for the transmission of IS-IS routing updates; however, it uses Ethernet II for other traffic such as IP and MPLS.
Alcatel-Lucent Scalable IP Networks v2.01 Module 3 | 26 All rights reserved © 2008 Alcatel-Lucent
General Ethernet Frame Format
Fixed sequence to alert the receiver (8 bytes)
(0x55555555555555D5), start frame delimiter
Destination MAC address (6 bytes) Source MAC address (6 bytes)
Frame length or type information (2 bytes) Payload: Internet layer
Frame check sequence (4 bytes)
SFD
Preamble DA SA Length/type P a y l o a d (46 to 1500 bytes) FCS
The frame consists of a set of bits organized into several fields. These fields include address fields, a variable- size data field that carries from 46 to 1500 bytes of data, and an error checking field that checks the integrity of the bits in the frame to make sure that the frame has arrived intact. The original Ethernet standards defined the minimum frame size as 64 bytes and the maximum as 1518 bytes. These numbers include all bytes from the destination MAC address field to the frame check sequence field. The preamble and the start frame delimiter fields are not included when quoting the size of a frame. The IEEE 802.3ac standard released in 1998 extended the maximum allowable frame size to 1522 bytes to allow for a VLAN tag to be inserted into the Ethernet frame format. Gigabit Ethernet and 10 gigabit Ethernet ports may support jumbo frames, which can be 9000 bytes.
Preamble: A stream of bits that allows the transmitter and receiver to synchronize their communication. The preamble is a 56-bit long pattern of alternating ones and zeroes. The preamble is immediately followed by the Start Frame Delimiter.
Start Frame Delimiter (SFD): Always 10101011 and is used to indicate the beginning of the frame information. Destination MAC (DA): The MAC address of the machine receiving data.
Source MAC (SA): The MAC address of the machine transmitting data.
Length/Type: The payload length or type field, (also known as Ethertype). If the Ethernet frame is in the 802.3 format, this field is interpreted as length. If the Ethernet frame is in the Ethernet II or original DIX format, the field is interpreted as type, or Ethertype. The numeric value in this field determines whether the frame is an 802.3 frame or Ethernet II frame. If the value is less than 1536, it is an 802.3 frame. If the value is 1536 or greater it is an Ethernet II frame.
Alcatel-Lucent Scalable IP Networks v2.01 Module 3 | 27 All rights reserved © 2008 Alcatel-Lucent
General Ethernet Frame Format
Fixed sequence to alert the receiver (8 bytes)
(0x55555555555555D5), start frame delimiter
Destination MAC address (6 bytes) Source MAC address (6 bytes)
Frame length or type information (2 bytes) Payload: Internet layer
Frame check sequence (4 bytes)
SFD
Preamble DA SA Length/type P a y l o a d (46 to 1500 bytes) FCS
(. . . continued from slide 21)
Data/Padding (also known as Payload): Where the IP header and data are placed if you are running IP over Ethernet. This field contains IPX information if you are running IPX/SPX (Novell). Contained within the payload section of an IEEE 802.2 frame are four specific fields:
DSAP - Destination Service Access Point
SSAP - Source Service Access Point
CTRL - Control bits for Ethernet communication
NLI - Network Layer Interface
An Ethernet frame must be a minimum of 64 bytes long. Therefore, if the data field is less than 46 bytes in length, padding is included to bring the frame length to 64 bytes.
Frame Check Sequence (FCS): A part of the frame that verifies that the information each frame contains is not damaged during transmission. If a frame is damaged during transmission, the FCS on the frame will not match with the recipient's calculated FCS. The FCS is calculated by the sender based on the entire contents of the frame. The recipient calculates an expected FCS value on the frame that it receives. Any frames that do not match the calculated FCS are discarded.
Alcatel-Lucent Scalable IP Networks v2.01 Module 3 | 28 All rights reserved © 2008 Alcatel-Lucent
Ethernet II Frame Capture
0000 00 11 43 45 61 23 00 e0 52 d4 a5 00 08 00 45 00 ..CEa#..R...E. 0010 01 21 0e ab 00 00 40 06 ea a8 8a 78 35 fe 8a 78 [email protected] 0020 35 95 00 17 09 55 98 09 6c 96 8e 7b 67 a7 50 18 5....U..l..{g.P. 0030 40 00 bc 0e 00 00 ff fb 03 0d 0a 64 65 76 69 63 @...devic 0040 65 3a 20 20 73 54 57 33 32 66 62 69 38 32 0d 0a e: sTW32fbi82.. 0050 0d 0a 41 6c 63 61 74 65 6c 20 4e 65 74 77 6f 72 ..Alcatel Networ 0060 6b 73 20 43 61 6e 61 64 61 2c 20 36 30 30 20 4d ks Canada, 600 M 0070 61 72 63 68 20 52 6f 61 64 2c 20 4b 61 6e 61 74 arch Road, Kanat 0080 61 2c 20 4f 6e 74 61 72 69 6f 0d 0a 55 6e 61 75 a, Ontario..Unau 0090 74 68 6f 72 69 7a 65 64 20 61 63 63 65 73 73 20 thorized access 00a0 70 72 6f 68 69 62 69 74 65 64 2e 20 20 41 63 63 prohibited. Acc 00b0 65 73 73 20 74 6f 20 74 68 69 73 20 64 65 76 69 ess to this dev Destination address Source address Ether type L3/IP information TCP information
This slide shows an actual sniffer trace of an Ethernet packet. Details of this trace are as follows: Frame 234 (303 bytes on wire, 303 bytes captured)
Ethernet II, Src: FoundryN_d4:a5:00 (00:e0:52:d4:a5:00), Dst: Dell_45:61:23 (00:11:43:45:61:23) Destination: Dell_45:61:23 (00:11:43:45:61:23)
Source: FoundryN_d4:a5:00 (00:e0:52:d4:a5:00) Type: IP (0x0800)
Internet Protocol, Src: 138.120.53.254 (138.120.53.254), Dst: 138.120.53.149 (138.120.53.149) Version: 4
Header length: 20 bytes
Differentiated Services Field: 0x00 (DSCP 0x00: Default; ECN: 0x00) Total Length: 289 Identification: 0x0eab (3755) Flags: 0x00 Fragment offset: 0 Time to live: 64 Protocol: TCP (0x06)
Header checksum: 0xeaa8 [correct] Source: 138.120.53.254 (138.120.53.254) Destination: 138.120.53.149 (138.120.53.149)
Transmission Control Protocol, Src Port: 23 (23), Dst Port: 2389 (2389), Seq: 4, Ack: 1, Len: 249 Source port: 23 (23)
Destination port: 2389 (2389)
Sequence number: 4 (relative sequence number) Next sequence number: 253 (relative sequence number) Acknowledgement number: 1 (relative ack number) Header length: 20 bytes
Flags: 0x0018 (PSH, ACK) Window size: 16384
Checksum: 0xbc0e [correct] Telnet
Alcatel-Lucent Scalable IP Networks v2.01 Module 3 | 29 All rights reserved © 2008 Alcatel-Lucent
LLC – Interface to the L3 protocol
MAC – L2 addressing, data transfer, sync, error control, and data flow