Unlocking Board‑Level Communication: A Practical Guide to MCTP and AF_MCTP on Linux

MCTP Overview

MCTP (Management Component Transport Protocol) is a media‑agnostic transport protocol used for communication between board components such as BMC, NIC, and HBA/RAID cards. It works over various physical links (USB, I2C, PCIe, RMII) and defines an Endpoint Identifier (EID) similar to an IP address for device identification.

Common hardware bindings include PCIe VDM, I2C, and I3C, with specifications for message transmission over SMBus (including PEC checks). MCTP provides a lightweight network‑like model for board‑level devices.

AF_MCTP Introduction

AF_MCTP is a Linux address family that exposes MCTP through the standard socket API, similar to AF_UNIX or AF_INET. Kernel versions 5.15 and 5.18 implement the core MCTP protocol and the I2C hardware binding driver respectively, enabling socket‑based communication with board components.

AF_MCTP Advantages

Application code can ignore hardware‑binding details and simply specify the target EID.

Programming is simplified and more flexible.

Built‑in debugging allows packet capture and analysis just like network debugging.

Configuration Steps

Configure Driver

Kernel must be version > 5.18 and enable the following options:

<code>CONFIG_MCTP=y
CONFIG_MCTP_TRANSPORT_I2C=y</code>

Device‑tree example:

<code>&amp;i2c2 {
    status = "okay";
    multi-master;
    mctp-controller;
    mctp@10 {
        compatible = "mctp-i2c-controller";
        reg = &lt;(0x10 | I2C_OWN_SLAVE_ADDRESS)&gt;;
    };
}</code>

Configure Network

Assign an EID to the local MCTP interface:

<code>mctp addr add 8 dev mctpi2c3</code>

Use Socket

<code>/* create the MCTP socket */
sd = socket(AF_MCTP, SOCK_DGRAM, 0);
/* populate the remote address information */
addr.smctp_family = AF_MCTP;  /* MCTP family */
addr.smctp_addr.s_addr = 9;   /* remote endpoint ID */
addr.smctp_type = 2;          /* encapsulated protocol type, e.g., NCSI */
addr.smctp_tag = MCTP_TAG_OWNER; /* kernel allocates a tag */
/* send the MCTP message */
rc = sendto(sd, buf, sizeof(buf), 0, (struct sockaddr *)&addr, sizeof(addr));
/* receive a message */
len = recvfrom(sd, buf, sizeof(buf), MSG_TRUNC, (struct sockaddr *)&addr, &addrlen);
</code>

Debugging AF_MCTP

Packet capture with tcpdump :

<code>tcpdump -i mctpi2c3 -w /tmp/mctp.pcap</code>

Analysis can be performed in Wireshark, which currently supports MCTP Control, NVME‑MI over MCTP, and NCSI over MCTP protocols.

AF_MCTP Applications

NIC/optical module temperature monitoring

NIC asset information management

Link status monitoring

HBA/HDD temperature monitoring

HBA/HDD asset management

Future work includes integrating PLDM for out‑of‑band firmware updates.

Component Initiative

Current challenges in component management include non‑standard command implementations, OEM inconsistencies, and varied out‑of‑band protocols. The initiative promotes unified command sets and standardized testing to reduce integration effort and improve compatibility.