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