Identifying the Data Inside an Ethernet Frame

Each data-link header has a field in its header with a code that defines the type of protocol header that follows. For example, in the first frame in Figure 3-13, the Destination Service Access Point (DSAP) field has a value of E0, which means that the next header is a Novell IPX header. Why is that? Well, when the IEEE created 802.2, it saw the need for a protocol type field that identified what was inside the field called "data" in an IEEE Ethernet frame.

Figure 3-13 802.2 SAP and SNAP Type Fields

14

802.3

DSAP

SSAP

CTL

IPX Data

802.3

802.2

SNAP

802.3

AA DSAP

AA SSAP

03 CTL

OUI

0800 Type

IP Data

802.3

The IEEE called its Type field the destination service access point (DSAP). When the IEEE first created the 802.2 standard, anyone with a little cash could register favorite protocols with the IEEE and receive a reserved value with which to identify those favorite protocols in the DSAP field. For instance, Novell registered IPX and was assigned hex E0 by the IEEE. However, the IEEE did not plan for a large number of protocols—and it was wrong. As it turns out, the 1-byte-long DSAP field is not big enough to number all the protocols.

To accommodate more protocols, the IEEE allowed the use of an extra header, called a Subnetwork Access Protocol (SNAP) header. In the second frame of Figure 3-13, the DSAP field is AA, which implies that a SNAP header follows the 802.2 header, and the SNAP header includes a 2-byte protocol type field. The SNAP protocol type field is used for the same purpose as the DSAP field, but because it is 2 bytes long, all the possible protocols can be identified. For instance, in Figure 3-13, the SNAP type field has a value of 0800, signifying that the next header is an IP header. RFC 1700, "Assigned Numbers" (www.isi.edu/in-notes/ rfc1700.txt), lists the SAP and SNAP Type field values and the protocol types that they imply.

Table 3-6 summarizes the fields that are used for identifying the types of data contained in a frame.

Table 3-6 Protocol Type Fields in LAN Headers

Field Name

Length

LAN Type

Ethernet Type

2 bytes

DIX Ethernet

802.2 DSAP and SSAP

1 byte each

IEEE Ethernet, IEEE Token Ring, ANSI FDDI

SNAP Protocol

2 bytes

IEEE Ethernet, IEEE Token Ring, ANSI FDDI

Some examples of values in the Ethernet Type and SNAP Protocol fields are 0800 for IP and 8137 for NetWare. Examples of IEEE SAP values are E0 for NetWare, 04 for SNA, and AA for SNAP. Interestingly, the IEEE does not have a reserved DSAP value for TCP/IP; SNAP headers must be used to support TCP/IP over IEEE Ethernet.

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