zl50415 Zarlink Semiconductor, zl50415 Datasheet - Page 39

• Manufacturer Part Number: zl50415
• Description: Managed 16-port 10/100 M + 2-port 1 G Ethernet Switch
• Manufacturer: Zarlink Semiconductor

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. What this means is 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.
7.10.1
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.
7.10.2
In unmanaged mode, flow control for multicast frames is triggered by a global buffer counter. When the system
exceeds a programmable threshold of multicast packets Xoff is triggered. Xon is triggered when the system returns
below this threshold.
In managed mode, per-VLAN flow control is used for multicast frames. In this case, flow control is triggered by
congestion at the destination. How so? The ZL50418 checks each destination to which a multicast packet is
headed. For each destination port, the occupancy of the lowest-priority transmission multicast queue (measured in
number of frames) is compared against a programmable congestion threshold. If congestion is detected at even
one of the packet’s destinations, then Xoff is triggered.
In addition, each source port has a 26-bit port map recording which port or ports of the multicast frame’s fanout
were congested at the time Xoff was triggered. All ports are continuously monitored for congestion, and a port is
identified as uncongested when its queue occupancy falls below a fixed threshold. When all those ports that were
originally marked as congested in the port map have become uncongested, then Xon is triggered, and the 26-bit
vector is reset to zero.
The ZL50418 also provides the option of disabling VLAN multicast flow control.
Note: If per-Port flow control is on, QoS performance will be affected.
Unicast Flow Control
Multicast Flow Control
Zarlink Semiconductor Inc.
ZL50418
39
Data Sheet