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参数资料
| 型号： | ZL50418/GKC |
| 厂商： | ZARLINK SEMICONDUCTOR INC |
| 元件分类： | 网络接口 |
| 英文描述： | DATACOM, LAN SWITCHING CIRCUIT, PBGA553 |
| 封装： | 37.50 X 37.50 MM, 2.33 MM HEIGHT, MS-034, HSBGA-553 |
| 文件页数： | 117/155页 |
| 文件大小： | 1928K |
| 代理商： | ZL50418/GKC |
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ZL50418
Data Sheet
64
Zarlink Semiconductor Inc.
8.9.1
Dropping When Buffers Are Scarce
Summarizing the two examples of local dropping discussed earlier in this chapter:
If a queue is a delay-bounded queue, we have a multi-level WRED drop scheme designed to control delay
and partition bandwidth in case of congestion.
If a queue is a WFQ-scheduled queue, we have a multi-level WRED drop scheme designed to prevent
congestion.
In addition to these reasons for dropping, we also drop frames when global buffer space becomes scarce. The
function of buffer management is to make sure that such dropping causes as little blocking as possible.
8.10
Flow Control Basics
Because frame loss is unacceptable for some applications, the ZL50418 provides a flow control option. When flow
control is enabled, scarcity of buffer space in the switch may trigger a flow control signal; this signal tells a source
port that is sending a packet to this switch, to temporarily hold off.
While flow control offers the clear benefit of no packet loss, it also introduces a problem for quality of service. When
a source port receives an Ethernet flow control signal, all microflows originating at that port, well-behaved or not,
are halted. A single packet destined for a congested output can block other packets destined for uncongested
outputs. The resulting head-of-line blocking phenomenon means that quality of service cannot be assured with high
confidence when flow control is enabled.
In the ZL50418, each source port can independently have flow control enabled or disabled. For flow control
enabled ports, by default all frames are treated as lowest priority during transmission scheduling. This is done so
that those frames are not exposed to the WRED Dropping scheme. Frames from flow control enabled ports feed to
only one queue at the destination, the queue of lowest priority. This means that if flow control is enabled for a given
source port then we can guarantee that no packets originating from that port will be lost but at the possible expense
of minimum bandwidth or maximum delay assurances. In addition, these “downgraded” frames may only use the
shared pool or the per-source reserved pool in the FDB; frames from flow control enabled sources may not use
reserved FDB slots for the highest six classes (P2-P7).
The ZL50418 does provide a system-wide option of permitting normal QoS scheduling (and buffer use) for frames
originating from flow control enabled ports. When this programmable option is active, it is possible that some
packets may be dropped even though flow control is on. The reason is that intelligent packet dropping is a major
component of the ZL50418’s approach to ensuring bounded delay and minimum bandwidth for high priority flows.
8.10.1
Unicast Flow Control
For unicast frames, flow control is triggered by source port resource availability. Recall that the ZL50418’s buffer
management scheme allocates a reserved number of FDB slots for each source port. If a programmed number of a
source port’s reserved FDB slots have been used then flow control Xoff is triggered.
Xon is triggered when a port is currently being flow controlled and all of that port’s reserved FDB slots have been
released.
Note that the ZL50418’s per-source-port FDB reservations assure that a source port that sends a single frame to a
congested destination will not be flow controlled.
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