Basic Operational Characteristics

In Figure 20-1 on page 525, refer to QPs 4 and 5 in one CA and QP 2 in the other CA. The UD service has the following basic characteristics:

• No connection setup. No initial connection setup with a remote QP is necessary prior to sending or receiving messages.

• QP attached to specific port. The QP is associated with a specific local CA port through which it sends and receives messages with remote UD QPs.

• Port association may limit destinations. The QP can only send and receive messages with remote UD QPs that can be reached through the local CA port with which the QP is associated.

• The destination is specified in each WR. Each WR posted to the QP's SQ can target a different remote UD QP.

• Message size. Each message must fit in the data payload field of a single packet (and cannot exceed PMTU bytes in size).

• No Ack/Nak protocol. There is no Ack/Nak protocol, so there is no guarantee that packets are delivered.

• Software completion notification. There are no responses returned by the remote QP's RQ Logic when using the UD transport service type. That being the case, software on the sender's side is alerted that a message transfer has completed immediately upon the transmission of the last request packet of the Send. RDMA Writes, RDMA Reads, and Atomic requests are not supported.

• No packet PSN checking. Although there is a PSN in each packet, it's not meaningful because the entire message is encapsulated in a single packet.

• CRCs protect each packet.

• Support mandatory. Because management messages (referred to as Management Datagrams, or MADs) are sent using UD, all CAs, switches and routers must support the UD protocol on the QPs used to send and receive management messages. The data payload field of a MAD is always exactly 256 bytes in length.

• Operations supported. Only supports the Send message transfer operation (using either the “Send Only” or “Send Only With Immediate” opcode).