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2. Configuration
NIC/TOE Driver Configuration
Typical cases for using the MTU:
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Server interconnects are typically deployed using jumbo frames. This is the
most efficient configuration for high bandwidth server-to-server
communication, such as Network Attached Storage, iSCSI and database
transactions.
Servers connected to client systems that run desktop operating systems
typically use standard 1500-byte frames. Most desktop systems do not support
jumbo packets.
Servers that need both high performance server-to-server communication and
client access can be configured with jumbo frames with Path MTU Discovery
enabled. Path MTU Discovery is enabled by default in Windows Server, and it
allows TCP connections to negotiate the optimal packet size that avoids IP
fragmentation.
Flow Control
The UCNA supports IEEE 802.3x standard flow control, which uses control packets to
temporarily pause the transmission of packets between two endpoints. These control
messages are point-to-point, they are not forwarded by switches or routers. You must
configure both endpoints for flow control. The UCNA can either respond to flow
control packets (by temporarily pausing transmits) or send flow control PAUSE packets
when the transmitter is overwhelming the system's receive bandwidth. For best
performance, flow control must be enabled on the switches as well as on UCNAs.
Receive and transmit flow control are on by default. Flow control is not available if
using FCoE on a converged network adapter. In this situation, priority pause is
negotiated with the network switch and used only for the FCoE protocol packets.
The NIC function can also use priority pause if supported by the switch. This requires
tagging packets in the operating system with the correct priority value, and enabling
ETS in the driver properties.
Configurations that support multiple PCI functions per port generally configure flow
control from the switch or blade configuration application. Since flow control is an
Ethernet port property, it must be the same for all PCI functions using the same port.
If multiple PCI functions are exposed for a single 10-Gb/s Ethernet port, such as in a
blade configuration, the flow control parameter must be set the same on all adapters for
the port. The results are unpredictable if the setting differs among PCI functions,
because this is a shared property of the 10-Gb/s port.
Examples:
Flow control greatly improves the following situations:

The UCNA is installed in 4x PCI-Express slot or an underpowered server
system.
If the PCI-Express bus does not provide 10 Gb/s of throughput due to chipset
limitations or the bus width, the UCNA cannot maintain 10 Gb/s of incoming
receive data. It starts dropping packets quickly. In this situation it may be
beneficial to enable receive flow control in the UCNA, and enable flow control
in the attached switch for all devices. This helps to slow down the transmitters.
Emulex Drivers for Windows User Manual
P008011-01A Rev. A
62