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RDMA protocol overview

A series of protocols are layered to perform RDMA over TCP (Figure 3). The top three layers form the

iWARP family of protocols that provide high-speed internet operability. The RDMA protocol converts

RDMA write, RDMA read, and sends into Direct Data Placement (DDP) messages. The DDP protocol

segments outbound DDP messages into one or more DDP segments, and reassembles one or more

DDP segments into a DDP message. The marker-based, protocol-data-unit-aligned (MPA) protocol

adds a backward marker at a fixed interval to DDP segments and also adds a length and cyclical

redundancy check (CRC) to each MPA segment. TCP schedules outbound TCP segments and satisfies

delivery guarantees. IP adds necessary network routing information.

Figure 3. Protocol layers for RDMA over TCP

RDMA

DDP

MPA

TCP

TCP uses a stream of 8-bit fields (octets) to communicate data, while DDP uses fixed protocol data

units (PDUs). To enable RDMA, DDP needs a framing mechanism for the TCP transport protocol. MPA

is a marker-based PDU-aligned framing mechanism for the TCP protocol.

MPA provides a generalized framing mechanism that enables a network adapter using DDP to locate

the DDP header. The adapter can then place the data directly in the application buffer, based on the

control information carried in the header. MPA enables this capability even when the packets arrive

out of order. By enabling DDP, MPA avoids the memory copy overhead and reduces the memory

requirement for handling out-of-order packets and dropped packets.

MPA creates a framed PDU (FPDU) by prefixing a header, inserting markers, and appending a CRC

after the DDP segment. MPA delivers the FPDU to TCP. The MPA-aware TCP sender puts the FPDUs

into the TCP stream and segments the TCP stream so that each TCP segment contains a single FPDU.

The MPA receiver locates and assembles complete FPDUs within the stream, verifies their integrity,

and removes information that is no longer necessary. MPA then provides the complete DDP segment

to DDP.

Converts RDMA write, RDMA read, and sends into DDP

messages

Segments outbound (TX) DDP messages into one or more

DDP segments; reassembles one or more DDP segments

into a DDP message

Adds a backward marker at a fixed interval to DDP

segments; also adds a length and CRC to each MPA

segment

Schedules outbound TCP segments and satisfies delivery

guarantees

Adds necessary network routing information