Data Packet Check

General

Refer to Figure 25-20 on page 659. The order in which the checks are shown indicates the error precedence for reporting, not necessarily the order in which the errors are detected.

As an example, most implementations would detect an invalid VL shortly after the packet starts (because the LRH:VL field is close to the start of the packet), while a CRC error cannot be detected until the end of the packet. The CRCs are shown first in the illustration because if both of these errors occur, the CRC error indicates that the packet was damaged and the CRC error is reported rather than the VL error.

When a switch or a router port is the destination port (e.g., a MAD is targeting that port), the ingress port's Link Layer must perform the same checks as a CA.

When a packet only fails a test that requires the packet be discarded or corrupted:

• A CA must discard the packet.

• A switch or router can either discard the packet or corrupt and forward it.

DLID Check

CA and router ports are each assigned a base LID address by the SM. In addition, the SM may choose to assign a range of LID addresses to the port using the port's PortInfo.LMC attribute element. See “Assigning LID Address Range to Port” on page 137.

When a CA or router port's Link Layer checks that a unicast LRH:DLID in a received packet is a valid DLID for that port, it must mask the number of low- order bits indicated by port's PortInfo.LMC before comparing the DLID.

Switch or Router May Skip ICRC Check

The Link Layer of a switch or router is only required to check the ICRC if the switch or router is the final destination of the packet. Otherwise, this check can be skipped.

Definition of Received Length and Minimum Packet Length

Received length is the number of bytes between the SDP and EGP. In the figure, MTU is the PortInfo.MTUCap (i.e., the maximum MTU size supported by this port).

The minimum packet length is five dwords (20 bytes) for raw packets and six dwords (32 bytes) for IBA transport packets.