NVM Express Management Interface 1.0a
17
3 Message Transport
NVMe-MI utilizes MCTP as a reliable in-order message transport between a Management Controller and a
Management Endpoint.
This section summarizes the NVMe-MI MCTP packet and message format. A Management Endpoint
compliant to this specification shall implement all required behaviors detailed in the Management
Component Transport Protocol (MCTP) Base Specification and corresponding transport binding
specification in addition to the requirements outlined in this specification (e.g., the Message Integrity Check
algorithm).
3.1 MCTP Packet
In MCTP, the smallest unit of data transfer is the MCTP packet. One or more packets are combined to
create an MCTP message. A packet always contains at least 1 byte of payload but the total length shall
never exceed the negotiated MCTP Transmission Unit Size. The format of an MCTP packet is shown in
Figure 8.
Figure 8: MCTP Packet Format
Physical Medium Specific Header
7 6 5 4 3 2 1 0
Byte 3
7 6 5 4 3 2 1 0
Byte 2
7 6 5 4 3 2 1 0
Byte 1
7 6 5 4 3 2 1 0
Byte 0
Header
Version
Reserved
Destination
Endpoint ID
Source
Endpoint ID
Pkt
Seq
#
Msg
tag
T
O
E
O
M
S
O
M
Packet Payload
Physical Medium Specific Trailer
Dwords
0 to (N-1)
DWord N
>
>
>
DWords
(N+1)
to
(M-1)
DWords
M
to
(P-1)
>
Physical Medium Specific
MCTP Packet Header
MCTP Packet Payload
MCTP Packet
Header
MCTP specifications use big endian byte ordering while NVM Express specifications use little endian byte
ordering. All figures in this specification are illustrated with little endian byte ordering. Note that this pictorial
representation does not change the order that bytes are sent out on the physical layer.
The Physical Medium-Specific Header and Physical Medium-Specific Trailer are defined by the MCTP
transport binding specification utilized by the port. Refer to the MCTP transport binding specifications.
The Management Component Transport Protocol (MCTP) Base Specification defines the MCTP packet
header (refer to DSP0236 for field descriptions). The fields of an MCTP Packet are shown in Figure 9.
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