Using the Bracket Protocol

Bracket protocol gives the SLU and the PLU context control of the data transmission, indicating that a session concerns a single transaction. Bracket protocol protects a current session from interruption by a concurrent transaction. A bracket encompasses the duration of a transaction.

The first message in the bracket contains a begin-bracket (BB) indicator, and the last message in the bracket contains an end-bracket (EB) indicator. A single message can be a bracket if it contains both indicators.

For a bracket session, the BIND command specifies one LU as the first speaker, and the other LU as the bidder. The first speaker can begin a bracket without permission from the other LU. The bidder, however, must request and receive permission from the first speaker to begin a bracket.

A BID command is a normal-flow request that is issued by the bidder to request permission to begin a bracket. A positive response to a BID command indicates that the first speaker will not begin a bracket but will wait for the bidder to begin a bracket. A negative response to a BID command indicates that the first speaker denies permission for the bidder to begin a bracket. The first speaker can send a Ready-to-Receive (RTR) command when permission is granted to start a bracket.

Instead of sending a BID command followed by a first-in-chain FMD with a BB indicator, the bidder can attempt to initiate a bracket by sending a first-in-chain FMD with a BB indicator. The first speaker can grant the attempt with a positive response or it can refuse the attempt with a negative response that indicates either of the negative response codes. However, if the bidder stops the chain that carries the BB indicator by sending the CANCEL command, the bracket is not initiated, regardless of the response. The RTR command can be issued by the first speaker either to grant permission to the bidder to begin a bracket or to find out if the bidder wants to begin a bracket.

A positive response to an RTR command indicates that the bidder will initiate the next bracket. If the bidder does not want to initiate a bracket, the bidder issues a negative response with the RTR-Not-Required sense code.