designs/mctp/mctp.md

# OpenBMC platform communication channel: MCTP & PLDM

Author: Jeremy Kerr <jk@ozlabs.org> <jk>

## Problem Description

Currently, we have a few different methods of communication between host
and BMC. This is primarily IPMI-based, but also includes a few
hardware-specific side-channels, like hiomap. On OpenPOWER hardware at
least, we've definitely started to hit some of the limitations of IPMI
(for example, we have need for >255 sensors), as well as the hardware
channels that IPMI typically uses.

This design aims to use the Management Component Transport Protocol
(MCTP) to provide a common transport layer over the multiple channels
that OpenBMC platforms provide. Then, on top of MCTP, we have the
opportunity to move to newer host/BMC messaging protocols to overcome
some of the limitations we've encountered with IPMI.

## Background and References

Separating the "transport" and "messaging protocol" parts of the current
stack allows us to design these parts separately. Currently, IPMI
defines both of these; we currently have BT and KCS (both defined as
part of the IPMI 2.0 standard) as the transports, and IPMI itself as the
messaging protocol.

Some efforts of improving the hardware transport mechanism of IPMI have
been attempted, but not in a cross-implementation manner so far. This
does not address some of the limitations of the IPMI data model.

MCTP defines a standard transport protocol, plus a number of separate
physical layer bindings for the actual transport of MCTP packets. These
are defined by the DMTF's Platform Management Working group; standards
are available at:

  https://www.dmtf.org/standards/pmci

The following diagram shows how these standards map to the areas of
functionality that we may want to implement for OpenBMC. The DSP numbers
provided are references to DMTF standard documents.

![](mctp-standards.svg)

One of the key concepts here is that separation of transport protocol
from the physical layer bindings; this means that an MCTP "stack" may be
using either a I2C, PCI, Serial or custom hardware channel, without the
higher layers of that stack needing to be aware of the hardware
implementation.  These higher levels only need to be aware that they are
communicating with a certain entity, defined by an Entity ID (MCTP EID).
These entities may be any element of the platform that communicates
over MCTP - for example, the host device, the BMC, or any other
system peripheral - static or hot-pluggable.

This document is focused on the "transport" part of the platform design.
While this does enable new messaging protocols (mainly PLDM), those
components are not covered in detail much; we will propose those parts
in separate design efforts. For example, the PLDM design at
[pldm-stack.md].

As part of the design, the references to MCTP "messages" and "packets"
are intentional, to match the definitions in the MCTP standard. MCTP
messages are the higher-level data transferred between MCTP endpoints,
which packets are typically smaller, and are what is sent over the
hardware. Messages that are larger than the hardware Maximum Transmit
Unit (MTU) are split into individual packets by the transmit
implementation, and reassembled at the receive implementation.

## Requirements

Any channel between host and BMC should:

 - Have a simple serialisation and deserialisation format, to enable
   implementations in host firmware, which have widely varying runtime
   capabilities

 - Allow different hardware channels, as we have a wide variety of
   target platforms for OpenBMC

 - Be usable over simple hardware implementations, but have a facility
   for higher bandwidth messaging on platforms that require it.

 - Ideally, integrate with newer messaging protocols

## Proposed Designs

The MCTP infrastrcuture in OpenBMC is implemented in two approaches:

 - A userspace-based approach, using a core library, plus a
   demultiplexing daemon. This is described in [MCTP
   Userspace](mctp-userspace.md).

   This is in use by a few platforms, but should be avoided for new
   designs.

 - A kernel-based approach, using a sockets API for client and server
   applications. This approach is recommended for new designs, and is
   described in [MCTP Kernel](mctp-kernel.md)

Design details for both approaches are covered in their relevant
documents, but both share the same Problem Description, Background and
Requirements, Alternatives and Impacts sections as defined by this
document.

## Alternatives Considered

There have been two main alternatives to an MCTP implementation in
OpenBMC:

Continue using IPMI, but start making more use of OEM extensions to
suit the requirements of new platforms. However, given that the IPMI
standard is no longer under active development, we would likely end up
with a large amount of platform-specific customisations. This also does
not solve the hardware channel issues in a standard manner.

Redfish between host and BMC. This would mean that host firmware needs a
HTTP client, a TCP/IP stack, a JSON (de)serialiser, and support for
Redfish schema. While this may be present in some environments (for
example, UEFI-based firmware), this is may not be feasible for all host
firmware implementations (for example, OpenPOWER). It's possible that we
could run a simplified Redfish stack - indeed, MCTP has a proposal for a
Redfish-over-MCTP channel (DSP0218), which uses simplified serialisation
format and no requirement on HTTP. However, this may involve a large
amount of complexity in host firmware.

## Impacts

Development would be required to implement the MCTP transport, plus any
new users of the MCTP messaging (eg, a PLDM implementation). These would
somewhat duplicate the work we have in IPMI handlers.

We'd want to keep IPMI running in parallel, so the "upgrade" path should
be fairly straightforward.

Design and development needs to involve potential host, management
controllers and managed device implementations.