VIA Technologies, Inc.
Preliminary VT6508 Datasheet
-23-
XOFF_WINDOW[0] == 1
XOFF[0]
XON[0]
XOFF[0]
It will trigger
sending a
XOFF flow
control frame
XOFF_WINDOW[0] == 0
XOFF_WINDOW[0] == 0
XON[0]
It is used to
make sure
leaving
XOFF, not to
trigger Tx.
Figure 6. XON/XOFF Window Concept.
Congestion-Control Operations
The starvation control threshold is 9 in VT6508. Let
Φ be the Free Memory Count. Let
Ω be the Virtual Free Buffer Count, where Ω = Φ - 9. In congestion control, decisions are
based on
Ω. Within XOFF window, the corresponding congestion-control operations are as
follows:
(1) flow control operation
As a unicast or broadcast packet, enqueued by Input Control, violates the congestion
control constraints, Queue Control will assert a trigger to its source port’s TMAC to
send out a flow control frame with pause time = FFFFH. There are two congestion
conditions: (1)
{
}
28
,
max
Ψ
≤
Φ
and
0
=
k
R
, (2) any port is in XOFF window and
0
=
k
R
.
Note that even a source port is within XOFF window, an XOFF triggering signal from
Queue Control will also make the TMAC to send out a flow control frame of pause time
= FFFFH. Although it costs more bandwidth to perform the flow control operation, this
duplicated transmission can guarantee the XOFF flow control frame could be received
by the party successfully.
(2) backpressure for half-duplex ports
As a non-local incoming packet is received, Input Control will assert a signal of
“Non_Local” to notify its TMAC sub-module about the status. TMAC will collide this
packet if it is a half-duplex port and it is within XOFF window.
(3) drop control for private buffer reservation
As an incoming packet, coming from a port within DROP_ON window and destining to
a port within CONGESTION window, is detected by Forwarding Control in lookup
operation, Forwarding Control will unmask its port mask to make it to be dropped by
Input Control. Note that the DROP_ON window indicates the time duration in which
the drop feature to the specific port is enabled, that is determined by TMAC. Note that
the CONGESTION window of port k is turned on when any port is in XOFF window
and
0
=
k
R
, that is determined by Queue Control.
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