# 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.  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.