82583V Datasheet, PDF (148/374 Pages) Intel Corporation – Intel® 82583V GbE Controller

82583V GbE Controller—Inline Functions
7.2.9.3.1
Header Length - HDRLEN
HDRLEN is used to specify the length (in bytes) of the header to be used for each frame
of a TCP segmentation operation. The first HDRLEN bytes fetched from data descriptor
are stored internally and are used as a prototype header. The prototype header is
updated for each packet and is prepended to the packet payload. For UDP packets this
will normally be equal to UDP checksum offset + 2. For TCP messages it will normally
be equal to TCP checksum offset + 4 + TCP header option bytes. This field is ignored if
TDESC.TSE is not set.
Maximum limits for the HDRLEN and MSS fields are dictated by the lengths variables.
However, there is a further restriction that for any TCP segmentation operation, the
hardware must be capable of storing a complete framed fragment (completely-built
frames) in the transmit FIFO prior to transmission. Therefore, the output TX FIFO
(packet buffer) should at least have (MSS + HDRLEN) space available. In addition MSS
must be set to a value more than 0x10 and HDRLEN must be smaller than 256 bytes.
7.2.9.4
Note:
Payload - PAYLEN
The Packet Length field (PAYLEN) is the total number of payload bytes for this TCP
segmentation offload (for example, the total number of payload bytes includes those
that are distributed across multiple frames after TCP segmentation is performed).
Following the fetch of the prototype header, PAYLEN specifies the length of data that is
fetched next from data descriptor. This field is also used to determine when last-frame
processing needs to be performed. The PAYLEN specification does not include any
header bytes. This field is ignored if TDESC.TSE is not set.
There is no restriction on the overall PAYLEN specification with respect to the transmit
FIFO size, once the MSS and HDRLEN specifications are legal.
7.2.9.5
Descriptor Type - DTYP
Setting the descriptor type (TDESC.DTYP) field to 0x0000 identifies this descriptor as a
context descriptor.
7.2.9.6
Table 41.
Command - TUCMD
The command field (TDESC.TUCMD) provides options that control the checksum
offloading and TCP segmentation features, along with some of the generic descriptor
processing functions. Table 41 lists the bit definitions for the TDESC.TUCMD field. The
IDE, DEXT, and RS bits are valid regardless of the state of TSE. All other bits are
ignored if TSE=0b.
Command TUCMD Fields
7
6
5
4
3
2
IDE
SNAP
DEXT
Rsv
RS
TSE
1
0
IP
TCP
148