RC Basic Operational Characteristics

“Reliable Connected Service Type” on page 64 provided a very basic overview of the RC transport service's operational characteristics. For convenience sake, that information is repeated here:

• Connection needed. Before any messages may be transferred, a private connection must be established between the two RC QPs in the two CAs, and the QP Contexts of the two QPs are each programmed with the identity of the remote QP as well as the address of the port behind which the remote QP resides.

• Private communications channel. The two RC QPs may then be used to send messages to each other (but not to any other QP in the same or any other target adapter).

• Message size. Each message transfer WR can specify a message transfer anywhere from zero to 2GB in size. Large messages (larger than the path's PMTU) are segmented into multi-packet transfers.

• An Ack/Nak protocol permits the requester (i.e., the QP SQ Logic sending the message) to verify that all packets are delivered to the responder QP's RQ Logic.

• Packet PSN. Each request packet contains a PSN that the responder QP's RQ Logic uses to verify that all request packets are received in order (and that each is only processed once, even if it should be received multiple times; more on this later).

• Two CRC fields in each packet are used to verify the integrity of the packet.

• High-traffic. Because the responder QP's RQ Logic must Ack or Nak each request packet received, this service class generates considerably more network traffic than the “unreliable” service types.

• Completes on receipt of final Ack. The SQ Logic cannot create a CQE on the SCQ (Send Completion Queue) to signal the completion of a message transfer until the Ack for the final packet of the message transfer has been received.

• Operations supported. Supports the following types of message transfer operations:

  - RDMA Read support is required.

  - RDMA Write support is required.

  - It is optional whether or not the CI supports Atomic operations. If it does, then it must support both the Atomic Fetch and Add and Atomic Compare and Swap If Equal operations on the RC transport service type.

  - Send support is required.

  - Bind Memory Window support is required.