Ethernet Version 2 Versus IEEE 802.3 Ethernet

Technote (troubleshooting)


Problem(Abstract)

This document describes the differences between Ethernet standards.

Resolving the problem

This document provides information to network users to assist them in decisions regarding which Ethernet standard to use.

Brief History

In 1980-81, the first Ethernet standard was developed by Digital (DEC-Digital Equipment Corporation), Intel, and Xerox, and combined the first letter of all of their names. This standard is called DIX 1.0, sometimes referred to as Ethernet I. The standard was superseded in 1982, by DIX 2.0, the current Ethernet standard, also known as Ethernet II (or Ethernet Version 2).

In 1983, the IEEE (Institute of Electrical and Electronics Engineers, Inc.) introduced the 802.3 Ethernet standard, named after the committee responsible for its development. The reason for its development was to attempt to standardize the protocol across all pieces of networking equipment, regardless of the manufacturer.

Frame Layout Differences

The primary difference between Ethernet Version 2 and IEEE 802.3 Ethernet standards is the frame layout.

Ethernet Version 2

The frame begins with an 8-byte preamble that is used to alert and synchronize the Ethernet Network Interface Card (NIC) to the incoming data.

The next two fields are the Destination MAC Address and the Source MAC Address. Each field is 6 bytes.

Following the address fields, Ethernet Version 2 uses a 2-byte "type" (or EtherType) field that identifies the unique network protocol type embedded within the data field.

The following EtherTypes are examples used in this field:

hex '0080' - Internet IP (IPv4)
hex '809B' - Appletalk
hex '80D5' - IBM SNA Service over Ethernet
hex '8137' - Novell Netware (IPX/SPX)

The data unit follows the EtherType field. The field 46 to 1500 bytes.

Finally, the frame ends with a 4-byte Frame Check Sequence (FCS) that uses a 32-bit cyclical redundancy check (CRC) for error detection. This is an algorithm that is generated over the address, type, and data fields based on the number of 1s and 0s (bits) in each. The receiving system generates the same CRC and compares that value to the value in the FCS. If they match, the frame is assumed to have been received with no errors. However, if, along the path of transmission, other network equipment strips the FCS and regenerates it, it is still possible that an error has occurred but that the regenerated FCS is accurate, based on the corrupted data. The receiving system then passes the frame up to higher levels (provided the FCS check is accurate) not knowing that the data has been corrupted.

A typical Ethernet Version 2 frame looks like this:

P
P
P
P
P
P
P
P
D
D
D
D
D
D
S
S
S
S
S
S
T
T
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
F
F
F
F

P = Preamble
D = Destination MAC Address
S = Source MAC Address
T = Ether Type
x = data unit
F = Frame Check Sequence (FCS)


IEEE 802.3 Ethernet

The frame begins with a 1-byte preamble followed by a 7-byte start frame delimiter. Combined, these fields are no different than the 8-byte preamble used by Ethernet Version 2.

The next two fields are the Destination MAC Address and the Source MAC Address. Each field is 6 bytes.

Following the address fields, IEEE 802.3 Ethernet uses a 2-byte "length" field that includes the IEEE 802.2 Logical Link Control (LLC) bytes and the data bytes. The length field in IEEE 802.3 Ethernet frames is always less than hex '05DC'. This corresponds to 1500 bytes which is the maximum frame size for Ethernet. NIC cards look at this field to determine which Ethernet standard is being used. If the field is less than '05DC', it represents a length field, and the frame is an IEEE 802.3 Ethernet frame. If the field is greater than '05DC', the frame is an EtherType field and is an Ethernet Version 2 frame.

IEEE 802.2 is not part of the definition of IEEE 802.3; however, it is always bound together with an IEEE 802.3 header.

This field includes:
o A 1-byte Destination Service Access Point (DSAP)
o A 1-byte Source Service Access Point (SSAP)
o A 1- or 2-byte Control Field
The DSAP/SSAP provide information on what network protocol is embedded within the data field.

Common SAP values include:
o hex '04' - IBM SNA (Systems Network Architecture)
o hex '06' - IP (Internet Protocol)
o hex '12' - LAN Printing
o hex 'AA' - SNAP (Sub-Network Access Protocol)
o hex 'BC' - Banyan
o hex 'C8' - HPR (High Performance Routing)
o hex 'E0' - Novell

The rest of the data unit follows the 802.2 LLC fields. This can be from 42 to 1496 bytes.

Finally, the frame ends with a 4-byte FCS field which is the same as that described in the Ethernet Version 2 standard.

A typical IEEE 802.3 Ethernet frame looks like this:

P
R
R
R
R
R
R
R
D
D
D
D
D
D
S
S
S
S
S
S
L
L
d
s
c
c
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
F
F
F
F

P = Preamble
R = STA RT Frame Delimiter
D = Destination MAC Address
S = Source MAC Address
802.2 LLC Header
L = Length
d = DSAP
s = SSAP
c = Control
x = data unit
F = Frame Check Sequence (FCS)


SNAP Header

When the implementation of IEEE 802.3 Ethernet came about, not every proprietary protocol was represented by a designated SAP value. Therefore, certain protocols could not use the IEEE 802.3 Ethernet standard (they were non-compliant), and were limited to the use of Ethernet Version 2. In an effort to allow those protocols to communicate on 802.3 networks (for ease of standardization), the SNAP (Sub-Network Access Protocol) header was designed. This header could be used in conjunction with the 802.2 LLC header and allow those protocols to communicate in 802.3 networks.

When using a SNAP header, the 802.2 LLC header is always the same:

DSAP (1 byte) = hex 'AA'
SSAP (1 byte) = hex 'AA'
Control (1 byte) = hex '03'

The SNAP header is 5 bytes and is included in the frame immediately following the 802.2 LLC header.

The first 3 bytes of the SNAP header are referred to as the Organization Unique Identifier (OUI), or simply the Organization ID. This indicates the company to which the embedded non-compliant protocol belongs.

Common OUI values include:

'00-02-55' - IBM Corporation (along with many other OUIs)
'00-00-0C' - Cisco Systems (along with many other OUIs)
'00-80-C2' - IEEE 802.1 Committee

Note: Most of the time, this field is set to '00-00-00'.

The last 2 bytes of the SNAP header include the EtherType (sometimes called the protocol ID), which indicates the embedded non-compliant protocol. These are the same as the EtherTypes included in the Ethernet Version 2 frame format.

A typical IEEE 802.3 Ethernet frame, including the SNAP header, looks like this:

P
R
R
R
R
R
R
R
D
D
D
D
D
D
S
S
S
S
S
S
L
L
d
s
c
O
O
O
T
T
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
F
F
F
F

P = Preamble
R = STA RT Frame Delimiter
D = Destination MAC Address
S = Source MAC Address
802.2 LLC Header
L = Length
d = DSAP = 'AA'
s = SSAP = 'AA'
c = Control = '03'
SNAP Header
O = Organization Unique Identifier
T = Ether Type (protocol ID)
x = data unit
F = Frame Check Sequence (FCS)


Communications Trace Information

The iSeries system does not include the Preamble, Start Frame Delimiter, or Frame Check Sequence when it formats a communications trace on any of these Ethernet standards. If an error occurs, indicating a FCS error, for example, a communications trace will not be useful in determining what that FCS number was. An external piece of equipment (for example, network sniffer) must be used to read this information.

Determining When to Use a Specific Standard

Many systems and pieces of network equipment do not conform to using the SNAP header found with IEEE 802.3 Ethernet frames. The iSeries system allows the Ethernet adapter cards to support Ethernet Version 2 and IEEE 802.3 Ethernet (with or without the SNAP header) frame types.

When using the CRTLINETH command, the default for the ETHSTD (Ethernet Standard) parameter is *ALL. In TCP/IP networks, the iSeries will send out ARP (Address Resolution Protocol) packets on Ethernet Version 2 and IEEE 802.3 Ethernet standards. Whichever frame type the response comes back on is the frame format that the iSeries will use to communicate to that piece of equipment. For the Ethernet Version 2 frame, the EtherType field is set to '0806'. For an IEEE 802.3 Ethernet frame, the EtherType field in the SNAP header is set to '0806'.

Another default parameter in the CRTLINETH command is the SSAP parameter. If the default value *SYSGEN is specified, the system generates a SSAP list of the following SSAPs:

----Source Service Access Points-----
   SSAP      Maximum Frame    Type    
    04            1496        *SNA    
    12            1496        *NONSNA
    AA            1496        *NONSNA
    C8            1496        *HPR    

The SSAP '06' (IP) is not included, by default, but the SSAP 'AA' (SNAP header) is included. This means that by default, all TCP/IP traffic using IEEE 802.3 Ethernet will use a SNAP header.

If routers or other equipment on the Ethernet network cannot support the SNAP header, the following options are available for the network administrator:
o Delete and recreate the ethernet line, setting ETHSTD(*ETHV2) to use only Ethernet Version 2 Standard
o Add '06' to the SSAP listing and delete 'AA' from the listing in the Ethernet line description.
SNA traffic over an Ethernet line typically uses the IEEE 802.3 Ethernet standard. This takes advantage of the 802.2 LLC flow control commands used in the "Control" field of the 802.2 LLC header. Changing the Ethernet line to use only the Ethernet Version 2 standard might force the flow control to be handled at a higher level than the data link level. This might have an effect on the SNA traffic over the Ethernet LAN. However, deleting the use of the SNAP header and trying to send TCP/IP traffic over an IEEE 802.3/802.2 ethernet frame could also have adverse effects on certain TCP/IP traffic. TCP/IP uses the Ethertype field to differentiate between ARP frames and normal IP frames.

The best solution in this case, if fixing the router is not an option, would be to use Ethernet Version 2 standard for TCP/IP traffic, while still using IEEE 802.3 Ethernet for SNA traffic.

Historical Number

331289258

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Document information


More support for:

IBM i
Communications-SNA

Software version:

5.3.0, 5.3.5, 5.4.0, 5.4.5, 6.1.0

Operating system(s):

IBM i

Reference #:

N1016185

Modified date:

2013-03-23

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