Data Sheet

Inline Functions—Ethernet Controller I210
293
7.2.2.2.7 IDX (3)
Index into the hardware context table where this context is stored. In the I210 the 2 available register
context sets per queue are accessed using the LSB bit and the two MSB bits are reserved and should
always be 0.
Note: In Qav mode for the SR queues a valid context descriptor should be placed ahead of any
timed packet pointed by a data descriptor and the IDX field is ignored.
7.2.2.2.8 L4LEN (8)
Layer 4 header length. If TSE is set in the data descriptor pointing to this context, this field must be
greater than or equal to 12 and less than or equal to 255. Otherwise, this field is ignored.
7.2.2.2.9 MSS (16)
Controls the Maximum Segment Size (MSS). This specifies the maximum TCP payload segment sent
per frame, not including any header or trailer. The total length of each frame (or section) sent by the
TCP/UDP segmentation mechanism (excluding Ethernet CRC) as follows:
Total length is equal to:
MACLEN + 4(if VLE set) + IPLEN + L4LEN + MSS
The one exception is the last packet of a TCP/UDP segmentation, which is typically shorter.
MSS is ignored when DCMD.TSE is not set.
Note: The headers lengths must meet the following:
MACLEN + IPLEN + L4LEN <= 512
Note: The MSS value should be larger than 0 and the maximum MSS value should not exceed 9216
bytes (9KB) length.
The context descriptor requires valid data only in the fields used by the specific offload options.
Table 7-32 lists the required valid fields according to the different offload options.
Table 7-32. Valid Field in Context vs. Required Offload
Required Offload Valid Fields in Context
TSE TXSM IXSM VLAN
1
1. VLAN field is required only if the VLE bit in Tx descriptor is set.
L4LEN IPLEN MACLEN MSS L4T IPV4
1b
2
2. If TSE is set, TXSM must be set to 1b.
1b X
3
3. X - don’t care.
VLE Yes Yes Yes Yes Yes Yes
0b 1b X
2
VLE No Yes Yes No Yes Yes
0b 0b 1b VLE No Yes Yes No No Yes
0b 0b 0b No context required unless VLE is set.