Token-Ring frame headers

There are five fields that comprise the medium access control (MAC) header for the token-ring adapter.

Table 1. Token-ring MAC header
Field Length Definition
AC 1 byte Access control. The value in this field x`00' gives the header priority 0.
FC 1 byte Field control. The value in this field x`40' specifies the Logical Link Control frame.
DA 6 bytes Destination address.
SA 6 bytes Source address. If bit 0 of this field is set to 1, it indicates that routing information (RI) is present.
RI 18 bytes Routing information. The valid fields are discussed below.

The MAC header consists of two routing information fields of two bytes each: routing control (RC) and segment numbers. A maximum of eight segment numbers can be used to specify recipients of a limited broadcast. RC information is contained in bytes 0 and 1 of the RI field. The settings of the first two bits of the RC field have the following meanings:

Item Description
bit (0) = 0 Use the nonbroadcast route specified in the RI field.
bit (0) = 1 Create the RI field and broadcast to all rings.
bit (1) = 0 Broadcast through all bridges.
bit (1) = 1 Broadcast through limited bridges.

The logical link control (LLC) header is composed of five fields, as shown in the following LLC header table.

Table 2. 802.3 LLC header
Field Length Definition
DSAP 1 byte Destination service access point. The value in this field is x`aa'.
SSAP 1 byte Source service access point. The value in this field is x`aa'.
CONTROL 1 byte Determines the LLC commands and responses. The three possible values for this field are discussed below.
PROT_ID 3 bytes Protocol ID. This field is reserved. It has a value of x`0'.
TYPE 2 bytes Specifies whether the packet is IP or ARP.