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4 Broadcom Gigabit Ethernet Teaming Services 139
E
Checksum Offload
Checksum Offload is a property of the Broadcom network adapters that allows the TCP/IP/UDP
checksums for send and receive traffic to be calculated by the adapter hardware rather than by the
host CPU. In high-traffic situations, this can allow a system to handle more connections more
efficiently than if the host CPU were forced to calculate the checksums. This property is inherently a
hardware property and would not benefit from a software-only implementation. An adapter that
supports Checksum Offload advertises this capability to the operating system so that the checksum
does not need to be calculated in the protocol stack. Checksum Offload is only supported for IPv4 at
this time.
IEEE 802.1p QoS Tagging
The IEEE 802.1p standard includes a 3-bit field (supporting a maximum of 8 priority levels), which
allows for traffic prioritization. The BASP intermediate driver does not support IEEE 802.1p QoS
tagging.
Large Send Offload
Large Send Offload (LSO) is a feature provided by Broadcom network adapters that prevents an
upper level protocol such as TCP from breaking a large data packet into a series of smaller packets
with headers appended to them. The protocol stack need only generate a single header for a data
packet as large as 64 KB, and the adapter hardware breaks the data buffer into appropriately-sized
Ethernet frames with the correctly sequenced header (based on the single header originally provided).
Jumbo Frames
The use of jumbo frames was originally proposed by Alteon Networks, Inc. in 1998 and increased the
maximum size of an Ethernet frame to a maximum size of 9000 bytes. Though never formally
adopted by the IEEE 802.3 Working Group, support for jumbo frames has been implemented in this
product. The BASP intermediate driver supports jumbo frames, provided that all of the physical
adapters in the team also support jumbo frames and the same size is set on all adapters in the team.
IEEE 802.1Q VLANs
In 1998, the IEEE approved the 802.3ac standard, which defines frame format extensions to support
Virtual Bridged Local Area Network tagging on Ethernet networks as specified in the IEEE 802.1Q
specification. The VLAN protocol permits insertion of a tag into an Ethernet frame to identify the
VLAN to which a frame belongs. If present, the 4-byte VLAN tag is inserted into the Ethernet frame
between the source MAC address and the length/type field. The first 2-bytes of the VLAN tag consist
of the IEEE 802.1Q tag type, whereas the second 2 bytes include a user priority field and the VLAN
identifier (VID). Virtual LANs (VLANs) allow the user to split the physical LAN into logical
subparts. Each defined VLAN behaves as its own separate network, with its traffic and broadcasts
isolated from the others, thus increasing bandwidth efficiency within each logical group. VLANs also
enable the administrator to enforce appropriate security and quality of service (QoS) policies. The
BASP supports the creation of 64 VLANs per team or adapter: 63 tagged and 1 untagged. The
operating system and system resources, however, limit the actual number of VLANs. VLAN support
is provided according to IEEE 802.1Q and is supported in a teaming environment as well as on a
single adapter. Note that VLANs are supported only with homogeneous teaming and not in a
multivendor teaming environment. The BASP intermediate driver supports VLAN tagging. One or
more VLANs may be bound to a single instance of the intermediate driver.