Supporting Technology, 802.2, Ethernet and Token Ring
Although NetBIOS is often encapsulated, it can be implemented "on the wire". This chapter
looks at the implementation of NetBIOS on two popular networking technologies, Ethernet
and Token Ring as well as the 802.2 Logical Link Control layer used with these technologies.
This documentation looks at the technologies in relation to NetBIOS rather than attempting
to provide a full description of the protocols; there are many excellent books on 802.2,
Ethernet and Token Ring that describe those subjects in detail.
IEEE 802.2 Logical Link Control
In the OSI Reference Model, the Datalink layer sits above the Physical layer and below
the Network layer. When considering IEEE LAN technology the situation is a little more complex.
There are a number of LAN systems such as Token Ring and Ethernet and the physical
characteristics of these are defined in the Physical Layer of the OSI model.
Characteristics such as the frame format for systems such as Token Ring and Ethernet
are defined in the Datalink layer in standards such as 802.3, 802.5 etc. A common
interface was required between these standards and the protocols in layer 3 and this is
implemented in 802.2.
A full description of IEEE 802.2 Logical Link Control is beyond the scope of this
document; a brief overview is given below.
IEEE 802.2 Logical Link Control frames often provide the data link layer support
for implementations of NetBIOS. This is the case when NetBIOS frames are being
carried "on the wire" rather than encapsulated in another protocol. The
relationship is illustrated in the
Open Systems Interconnection (OSI) Reference Model
802.2 supports both connection-oriented and connection-less oriented communications.
The Logical Link Control offers services to the Network layer through Service
Access Points (SAPs). The SAP is used to identify the process at the Network
layer.
IEEE 802.2 frames have the following form:
DSAP 1 byte
Destination Service Access Point
SSAP 1 byte
source Service Access Point
Control 1 or 2 bytes
field length depends on the service
Information
This variable length field carries any data
Some examples of DSAP and SSAP values are given below.
For IPX (the network protocol traditionally used with NetWare networks),
DSAP = 0xE0 (224), SSAP = 0xE0 and Control is 1 byte 0x03 which denotes
the 802.2 unnumbered format.
For SNAP (Sub-Network Access Protocol), DSAP = 0xAA (170), SSAP = 0xAA
For NetBIOS, DSAP = 0xF0 (240) , SSAP = 0xF0
Some IEEE 802.2 Numbers of interest can be found at
The Internet Assigned Numbers Authority
web site, Protocol Numbers and Assignment Services
in IEEE 802 Numbers
:
http://www.iana.org/assignments/ieee-802-numbers
In 1985 IBM implemented NetBIOS over Token Ring and established the way in
which NetBIOS frames would map to 802.2 frames.
When NetBIOS is implemented over Token Ring, the NetBIOS frames are mapped directly
on to the 802.2 frames; the NetBIOS frame is contained in the information field of the
802.2 frame:
DSAP 1 byte Destination Service Access Point 0xF0
SSAP 1 byte source Service Access Point 0xF0
Control 1 or 2 bytes field length depends on the service
Information:
NetBIOS header
Optional data
The above scheme is general to implementations of NetBIOS over 802.2 where
other underlying technologies are used such as Ethernet.
Token Ring
Token Ring is becoming less popular with many organizations moving to
Ethernet. Token Ring is discussed here because of it's importance in the
history of NetBIOS and understanding of NetBIOS.
When IBM introduced Token-Ring, an emulator for NetBIOS was produced.
The NetBIOS Extended User Interface (NetBEUI) was introduced in 1985.
NetBIOS was no longer implemented only on a set of propriety protocols, but
also on 802.2 frames. The implementation on Token-Ring was the first
implementation over 802.2 and provides a reference model. Detailed information
can be found in the IBM manual:
IBM LAN Technical Reference, see
IBM LAN Technical Reference IEEE 802.2 and NetBIOS Application Program
Interfaces.
A full description of Token Ring is beyond the scope of this document; some
basic information on Token Ring and its use with NetBIOS is given below.
There are two kinds of Token Ring frames: Media Access Control (MAC) frames and
Non-MAC frames. MAC frames carry Token Ring management information between
nodes, Non-MAC frames carry user data. The non-MAC frames contain
IEEE 802.2 Logical Link Control frames which in turn can contain NetBIOS frames.
Non-MAC Frame Structure
Non-MAC Token Ring Frame Structure
Token Ring frame
802.2 Frame detail
NBF frame
Start Delimiter (SDEL) 1 octet
Access Control (AC) 1 octet
Frame Control (FC) 1 octet
Destination Address 6 octets
Source Address 6 octets
IEEE 802.2 Logical Link Control
DSAP 2 octets
SSAP 2 octets
Control 1 or 2 octets
Data 46-1500 octets
NetBIOS header
Optional data
Frame Check sequence (FCS) 4 octets
End Delimiter (EDEL) 1 octet
Frame Status (FS) Check sequence 1 octet
Further information
Many manuals and documents describe Token-Ring in detail including
Novell's Guide to NetWare LAN Analysis, see
Ethernet
A full description of Ethernet is beyond the scope of this document; some
basic information on Ethernet and its use with NetBIOS is given below.
Ethernet is widely used today and well documented.
Four types of Ethernet frames have been in common use. For convenience the
notation used by Novell is used to describe the four Ethernet frame types:
Ethernet_802.3
Known as Ethernet 802.3 raw, this frame type
is used in NetWare networks and was the default type in NetWare v2.x and v3.x
Ethernet_II
Known as Ethernet DIX (Developed by Digital Intel Xerox)
Ethernet_802.2
IEEE Ethernet
Ethernet_SNAP
SNAP (Sub-Network Access Protocol) derived from the
Ethernet 802.2 structure
Ethernet_802.3
Known as Ethernet 802.3 raw, this frame type
is used in NetWare networks and was the default type in NetWare v2.x and v3.x
Because of the nature of these frames they are unlikely to carry NBF frames,
unless encapsulated in IPX.
Ethernet_802.3 Frame Structure
Preamble 7 octets
Start frame deliminator 1 octet
Destination Address 6 octets
Source Address 6 octets
Length 2 octets
Data 46-1500 octets
Frame Check sequence 4 octets
Ethernet_802.2
NBF frames are found in Ethernet_802.2 frames more than in other
Ethernet frames when NBF is implemented "on the wire" rather than
encapsulated.
Ethernet_802.2 frames are also used with IPX/SPX and FTAM (File Transfer
Access and Management) protocol.
Ethernet_802.2 Frame Structure
Ethernet frame
802.2 Frame detail
NBF frame
Preamble 7 octets
Start frame deliminator 1 octet
Destination Address 6 octets
Source Address 6 octets
Length 2 octets
IEEE 802.2 Logical Link Control
DSAP 2 octets
SSAP 2 octets
Control 1 or 2 octets
Data 46-1500 octets
NetBIOS header
Optional data
Frame Check sequence 4 octets
Ethernet_SNAP
Ethernet_SNAP frames are used by IPX/SPX, TCP/IP and AppleTalk
Phase II.
Ethernet_SNAP Frame Structure
Preamble 7 octets
Start frame deliminator 1 octet
Destination Address 6 octets
Source Address 6 octets
Length 2 octets
DSAP 2 octets value AA
SSAP 2 octets value AA
Control 1 octets
Organizational code 3 octets
Ethernet Type 2 octets
Data 46-1500 octets
Frame Check sequence 4 octets
Ethernet_II
Ethernet_II frames are used with IPX/SPX TCP/IP AppleTalk Phase I
Following the source address, is an Ethernet frame type. Information on
Ethernet frame types can be found at:
http://www.iana.org/assignments/ethernet-numbers
and at:
http://www.cavebear.com/CaveBear/Ethernet/type.html
For SNA (Systems Network Architecture)
communications the value registered for the type is 0x80D5.
This value of 0x80D5 is also used for other systems using the IEEE 802.2 API
including NetBIOS
Ethernet_II Frame Structure
Preamble 8 octets
Destination Address 6 octets
Source Address 6 octets
Ethernet Type 2 octets
Data 46-1500 octets
Frame Check sequence 4 octets
Further information
Many manuals and documents describe Ethernet in detail including
Novell's Guide to NetWare LAN Analysis, see