Dell Emulex Family of Adapters - Manual (Page 654)

Emulex Drivers for Windows User Manual P010077-01A Rev. A
3. Configuration
NIC Driver Configuration
654
Ethernet frame header or the trailing CRC. The standard MTU is 1500 bytes,
corresponding to a 1514-byte packet size plus a 4-byte trailing CRC. Historically, any
1514-byte frame is a standard packet, while any frame larger than 1514 bytes is called a
jumbo packet. Windows Server attempts to standardize the terminology across vendors
so the jumbo packet parameter refers to the byte size of the packet.
The Windows Server driver supports several jumbo packet values. The larger packet
size provides better throughput and CPU usage. Typically, all devices on the network,
including switches, must be configured for the larger size. The drawbacks of using
jumbo packets are interoperability and increased memory usage on the server.
To set a jumbo packet value, go to the Advanced Properties page in Windows Device
Manager. For information on how to configure the options through the Advanced
Property page, see “Modifying Advanced Properties” on page 607
The path MTU is the maximum MTU that can be used before IP fragmentation occurs,
taking into account the MTU for the endpoints and all routers between the endpoints.
To verify the path MTU, ping a remote target with an increasing payload size.
Eventually, the IP packet length exceeds the path MTU, and the packet fragments. This
can be seen by using a packet sniffing application, such as Ethereal, Wireshark, or
Microsoft Network Monitor.
IP fragmentation degrades performance dramatically, because all fragments must be
received and reassembled before delivering the network packet to the upper layer
protocol. In many cases, IP fragmentation may lead to a 10x performance degradation.
The MTU parameter should be modified on all systems to avoid IP fragmentation for
optimal network throughput.
Typical cases for using the MTU:

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 the Windows Server, and
it allows TCP connections to negotiate the optimal packet size that avoids IP
fragmentation.
Flow Control
The adapter 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 adapter 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.