The Telecom Corner: Ethernet Frame Types

Ethernet Frame Types

Introduction

There are a variety of Ethernet Frame Types that may be observed in a Novell LAN environment. This depends upon the version of NetWare, and the applications employed.

NetWare 2.X : Ethernet 802.3 (Raw) Ethernet II NetWare 3.X : Ethernet 802.3 (Raw) Ethernet II Ethernet 802.2 Ethernet SNAP (Sub-Network Access Protocol)

Not all frame types can support all traffic variants. For example, NetWare IPX/SPX packets are usually transmitted by 802.3 (Raw) frames. In fact, the 802.3 frame type can only support IPX/SPX traffic. When TCP traffic is generated, the Ethernet II frame is often used. Some desktop LAN analyzers will employ SNAP frames.

Ethernet Frame Types

Ethernet 802.3 (Raw)

This is the original (and default) frame type used by NetWare. IT CAN ONLY SUPPORT NOVELL IPX/SPX TRAFFIC! The frame is similar to that described in 802.3 except that it does not contain the Logical Link Control (LLC) information in the packet.

Preamble : 8 bytes Destination Address : 6 bytes Source Address : 6 bytes Length Field : 2 bytes Data Field : Between 46 and 1500 bytes Pad Characters : Variable, stuffs data field up to 46 bytes Frame Check Sequence: 4 bytes Min Frame Length : 64 bytes Max Frame Length : 1518 bytes (not including Preamble)

Ethernet 802.2

This frame includes fields from 802.3 and 802.2 (Logical Link Control) and can support the Novell IPX/SPX and FTAM (File Transfer, Access, and Management) protocols. The frame parameters are identical to those listed above, EXCEPT that the first three bytes of the data field are used to indicate 802.2 header (LLC) information.

Preamble : 8 bytes Destination Address : 6 bytes Source Address : 6 bytes Length Field : 2 bytes Data Field : Between 46 and 1500 bytes (including LLC) Pad Characters : Variable, stuffs data field up to 46 bytes Frame Check Sequence: 4 bytes Min Frame Length : 64 bytes Max Frame Length : 1518 bytes (not including Preamble)

The LLC field consists of:

Destination Service Access Point (DSAP): 1 byte (NetWare 0xE0) Source Service Access Point (SSAP) : 1 byte (NetWare 0xE0) Control Field : 1 byte (NetWare 0x03)

NetWare IPX/SPX packets will assign a hexadecimal value of E0 to the DSAP and SSAP fields and a hexadecimal value of 03 to the Control field. The “03” Control value indicates an unnumbered 802.2 layer.

Ethernet II

Again, a similar frame type, EXCEPT that the two-byte Length field has been replaced with a two-byte Type field (Ethertype). Ethernet II frames can support Novell IPX/SPX, TCP/IP, and AppleTalk Phase 1 protocols. Ethernet II frames do not use a LLC header in the data field.

Preamble : 8 bytes Destination Address : 6 bytes Source Address : 6 bytes Ethernet Type : 2 bytes (Novell 0x81-37) Data Field : Between 46 and 1500 bytes Pad Characters : Variable, stuffs data field up to 46 bytes Frame Check Sequence: 4 bytes Min Frame Length : 64 bytes Max Frame Length : 1518 bytes (not including Preamble)

The Ethernet Type field is coded with hexadecimal 8137 for transport of NetWare IPX/SPX packets.

Go here for more information on EtherType variable assignments

Ethernet SNAP

Sub-Network Access Protocol (SNAP) is similar to 802.2, with LLC parameters, but with expanded LLC capabilities. Ethernet SNAP can support IPX/SPX, TCP/IP, and AppleTalk Phase 2 protocols.

The LLC field (the first eight bytes in the data field) consists of:

Destination Service Access Point (DSAP): 1 byte (0xAA) Source Service Access Point (SSAP) : 1 byte (0xAA) Control Field : 1 byte (NetWare 0x03) Organizational Code : 3 bytes (0x00-00-00) Ethernet Type : 2 bytes (NetWare 0x81-37)

Go here for more information on EtherType variable assignments

Hexadecimal AA (decimal 170) values are usually employed for the DSAP and SSAP values. Netware uses a hexadecimal 03 in the SNAP Control field and will use the 0x81-37 Ethernet Type value. Usually, the Organizational Code is coded as all 0s (0x00-00-00) and NetWare is no exception.

Distinguishing Between Frame Types

1) Receive good frame.
2) Analyze frame. Perform the following steps, in order:

If the EtherType/Length value is greater than 0x05-DC (decimal 1500), then process the frame as Ethernet II. Any EtherType value greater than 0x05-DC (such as 0x0800 for IP or 0x81-37 for NetWare IPX/SPX) will be interpreted as an Ethernet II frame.

If the IPX header (0xFF-FF) follows the Length field, the frame is interpreted as a 802.3 (Raw) frame with Netware IPX/SPX traffic. Standard SSAP and DSAP values do not include hexadecimal FF, so the 802.3 (Raw) frame can be distinguished from LLC frames (Ethernet SNAP, 802.2).

Next, the byte following the length field (DSAP) is examined. If the value is 0xAA, the frame is interpreted as a SNAP frame. Otherwise, it is interpreted as a 802.2 frame.