EP80579 Datasheet, PDF (1382/1916 Pages) Intel Corporation – Intel® EP80579 Integrated Processor Product Line

Intel^® EP80579 Integrated Processor
value when writing this field. The TSPMT register is also used to program the threshold
padding overhead. This padding is necessary due to the indeterminate nature of the
MTU and the associated headers.
37.5.7.3
TCP Segmentation Performance
Performance improvements for a hardware implementation of TCP Segmentation off
load include:
• The stack does not need to partition the block to fit the MTU size, saving CPU
cycles.
• The stack only computes one Ethernet, IP, and TCP header per segment, saving
CPU cycles.
• The Stack interfaces with the device driver only once per block transfer, instead of
once per frame.
• Larger internal bus bursts are used which improves bus efficiency (i.e. lowering
transaction overhead).
• Interrupts are easily reduced to one per TCP message instead of one per packet.
• Fewer I/O accesses are required to command the hardware.
37.5.7.4 Packet Format
Typical TCP/IP transmit window size is 8760 bytes (about 6 full size frames). Today the
average size on corporate Intranets is 12-14KB, and normally the maximum window
size allowed is 64KB. A TCP message can be as large as 64KB and is generally
fragmented across multiple pages in host memory. The GbE partitions the data packet
into standard Ethernet frames prior to transmission. The GbE also supports calculating
the Ethernet, IP, TCP, and even UDP headers, including checksum, on a frame by frame
basis.
Figure 37-31.TCP/IP Packet Format
Ethernet
IP
TCP/UDP
Data
FCS
Frame formats supported by the GbE include:
• Ethernet 802.3,
• IEEE 802.1q VLAN (Ethernet 802.3ac),
• Ethernet Type 2,
• Ethernet SNAP,
• IPv4 headers with options,
• IPv6 headers with IP option next headers,
• IPv6 packet tunneled in IPv4,
• TCP with options, and
• UDP with options.
VLAN tag insertion is also handled by hardware.
Note:
UDP (unlike TCP) is not a “reliable protocol”, and fragmentation is not supported at the
UDP level. UDP messages that are larger than the MTU size of the given network
medium are normally fragmented at the IP layer. This is different from TCP, where large
TCP messages can be fragmented at either the IP or TCP layers depending on the
software implementation. The GbE has the ability to segment UDP traffic (in addition to
TCP traffic), but because UDP packets are generally fragmented at the IP layer the GbE
“TCP Segmentation” feature will normally not be conducive to handling UDP traffic.
Intel^® EP80579 Integrated Processor Product Line Datasheet
1382
August 2009
Order Number: 320066-003US