UD Connection Issues
GSI and SMI Are Well-Known UD QPs
A port's GSI (QP1) and SMI (QP0) are specification-defined UD QPs that are used to send and receive GMPs and SMPs, respectively. In other words, they provide management-related services.
CA May Implement Service(s) Using Known UD QPs
In addition to its GSI and SMI UD QPs, a specific CA may implement one or more non-management services using UD QPs with QP numbers that are well-known (e.g., by its device driver). In this case, software can send and receive messages with these QPs in a remote CA (because the destination QPN is known to the driver).
CA May Implement Service(s) Using Unknown UD QPs
General
On the other hand, a CA may provide one or more additional, non-management services through UD QPs with less-known (or unknown) QPNs. In this case, software would not know the destination QPN to specify in a WR posted to an HCA UD QP's SQ. There must therefore be some way to “discover” that a specific service is provided by the remote CA, as well as the identity of the UD QP used to communicate with that service (in other words, what is the QPN assigned to the UD QP that acts as a conduit for the desired service).
Discovering the Service and QPN
The communications establishment procedure can be used to discover:
whether or not the CA provides a desired service, and, if it does,
the QPN and Q_Key of the UD QP in the remote CA that must be used to communicate with that service.
To discover if the remote CA provides the desired service, software takes the following actions:
Software has the HCA's CM send a SIDR_REQ message [Service ID Resolution Request; see “SIDR_REQ (ServiceID Resolution Request) MAD” on page 1108] to the remote CA's CM. The REQ message specifies the following:
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Class = ComMgt.
The GSI that receives the packet on the remote CA will deliver the packet to the CA's CM for processing.
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Method = ComMgtSend.
Informs the remote CA's CM that this is a connection establishment message type.
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Attribute = ServiceIDResReq.
In other words, this is a Service ID Resolution Request message (referred to as a SIDR_REQ message).
- Service ID. Specifies which service to establish a connection with. See “Discovering Services a CA Provides” on page 185.
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Class = ComMgt.
The GSI that receives the packet on the remote CA will deliver the packet to the CA's CM for processing.
Assuming that the requested service is supported, the remote CA's CM returns a SIDR_REP (Service ID Resolution Reply) message [see “SIDR_REP (ServiceID Resolution Reply) MAD” on page 1109] to the HCA's CM containing the QPN and Q_Key of the remote CA's UD QP that must be used to communicate with that service.
The HCA's CM passes the reply message back to software.
If a CA provides services (other than Subnet Management and General Services) using the UD service type at other than fixed QPNs, the CA's CM must support receiving, processing and replying to the SIDR_REQ message.