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IXP400 Software
Access-Layer Components: Ethernet Database (IxEthDB) API
April 2005 IXP400 Software Version 2.0 Programmer’s Guide
174 Document Number: 252539, Revision: 007
In 802.3 frames, there is a 2-byte Length/Type field, the interpretation of which depends on
whether its value is smaller than 0x0600. When the value of this field is less than 0x0600, it is
interpreted as Length, and the first 8 bytes of the MAC client data field is always the LLC/SNAP
header, as defined in 802.2. Such frames are also known as “8802 frames”. When the value of the
Length/Type field is greater than or equal to 0x600, it is interpreted as Type, and there is no LLC/
SNAP header in the frame. Such frames are also known as “Ethernet frames”. Typically, IP
packets are conveyed via Ethernet frames.
In 802.11 frames, there is always a LLC/SNAP header. This LLC/SNAP header always occupies
the first 8 bytes of the Frame Body field (see Table 30). In addition to its dependence on the source
and destination types, the process of converting from 802.3 frame headers to 802.11 frame headers
also involves the complexity of LLC/SNAP sub-layer conversion. The appropriate conversion is
handled by the NPE automatically.
10.3.6.2 How the 802.3 / 802.11 Frame Conversion Feature Works
IxEthDb maintains two information structures for use in the 802.3/802.11 Frame Conversion
feature:
• WiFi Header Conversion Database. This database contains the MAC addresses of 802.11
destination devices and their type (Access Point or Station).
• Additional 802.11 information that is specific to the Access Point being hosted by the IXP400
software. This information includes the global Frame Control, Duration ID, and BSSID data
for all frames that will be converted. These three elements are referred to as the 802.11 Host
Station Parameters.
The above information is downloaded to each NPE performing 802.3/802.11 conversion via the
ixEthDBWiFiConversionTableDownload() API, and is stored in an NPE 802.3/802.11 Conversion
Table.
Receive Path
For every received 802.3 frame, once it passes all other checking, classification and validation, the
NPE microcode will check the frame to see if the frame needs to be converted to the IEEE802.11
frame format. The NPE does this by comparing the destination MAC address against MAC
addresses of the ultimate destination in the NPE 802.3/802.11 Conversion Table. If no match is
found in the table, the frame will be delivered to the client without conversion.
If a match is found, the NPE microcode inspects the matched table entry to determine whether the
frame is “from AP to STA” or “from AP to AP” and then takes action accordingly. The existing
802.3 header and VLAN tag, if any, are removed and a new 802.11 header is created using the rules
and information listed in Table 32 on page 175. The NPE Ethernet firmware sets the
ixp_ne_flags.link_prot field in the buffer header to indicate the format of the converted frame
header.