This subsection presents the precedence of SDLI primitives relative to one another. Two queues are used to describe SDLI precedence rules. One queue contains SDL user-originated primitives and corresponds to the STREAMS write queue of the SDL provider. The other queue contains SDL provider-originated primitives and corresponds to the STREAMS read queue of the SDL user. The SDL provider is responsible for determining precedence on its write queue and the SDL user is responsible for determining precedence on its read queue as indicated in the precedence tables below. 

For each precedence table, the rows (labeled PRIM X) correspond to the primitives that are on the given queue and the columns (PRIM Y) correspond to the primitives that are about to be placed on that queue. Each pair of primitives (PRIM X, PRIM Y) may be manipulated resulting in: 

The precedence rules define the allowed manipulations of a pair of SDLI primitives. Whether these actions are performed is the choice of the SDL provider for user-originated primitives and the choice of the SDL user for provider-originated primitives. 

The following table presents the precedence rules for SDL user-originated primitives on the SDL provider's STREAMS write queue. It assumes that only non-local primitives (i.e. those that generate protocol data units to a peer SDL user) are queue by the SDL provider. 

The following table presents the precedence rules for SDL provider-originated primitives on the SDL user's STREAMS read queue. 

If the SDL user is a user-level process, it's read queue is the stream head read queue. Because a user process has no control over the placement of SDL primitives on the stream head queue, a SDL user cannot straighforwardly initiate the actions specified in the following precedence table. The SDL user can ignore the precedence rules defined in the table below. This is equivalent to saying that the SDL user's read queue can contain at most one primitive. 

(tables)