Design and Implementation of TinyGALS: A Programming Model for ...

and pulls packets from its input. Click is a pure polling system; the device never interrupts
the processor.
Timers are another way of activating an element besides tasks. An element can have any
number of active timers, where each timer calls an arbitrary method when it fires. Timers
are implemented using Linux timer queues.
FromDevice
receive
packet p
Null Null ToDevice
enqueue p
dequeue p
and return it
ready to
transmit
send p
push(p) push(p)
return
return
pull()
pull()
return p return p
Figure 22: A simple Click configuration with sequence diagram.
Figure 22 shows a simple Click router configuration with a push chain (FromDevice
and Null) and a pull chain (Null and ToDevice). The two chains are separated by a Queue
element. The Null element simply passes a packet from its input port to its output port;
it performs no processing on the packet. Note that in the sequence diagram in Figure 22,
time moves downwards. Control flow moves forward during a push sequence, and moves
backward during a pull sequence. Data flow (in this case, the packet p) always moves
forwards.
Figure 23 illustrates the basic execution sequence of Figure 22. When the task corre-
sponding to FromDevice is activated, the element polls the receive DMA ring for a packet.
FromDevice calls push() on its output port, which calls the push() method of Null. The
push() method of Null calls push() on its output port, which calls the push() method
of the Queue. The Queue element enqueues the packet if its queue is not full; otherwise
it drops the packet. The calls to push() then return in the reverse order. Later, the task
corresponding to ToDevice is activated. If there is an empty slot in its transmit DMA ring,
ToDevice calls pull() on its input port, which calls the pull() method of Null. The
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