To follow up my post on USB, I thought I’d talk about MTP over USB.
MTP stands for “Media Transfer Protocol”. It was created as an extension to PTP (Picture Transfer Protocol). PTP was originally created for transfering pictures to and from cameras. MTP extends off of it and is transport-agnostic, but I will just focus on MTP over USB.
MTP is strucctured via “objects”. Each object coresponds to a file or a directory. Each object is assicned a numerical ID. Objects can be sent to and from the device.
Objects are stored on “storages”. Storages just logically separate groups of files. For example, storages on a phone could include internal memory and SD card memory.
Communication is done via transactions during sessions. A session must be opened before most operations can be requested. A transaction consists of three phases, the operation request phase, the data phase, and the response phase. In the operation request phase, an operation code is sent to the device along with the hessionID, transactionID, and arguments. During the data phase, the device may send back binary data depending on the operation. During the response phase, the device sends back a response code and some data (along with the sessionID and transactionID for confirmation). All data is sent in containers that specify the length, type, operation, transactionID, of the payload, along with the payload itself.
Currently, in the MTP project I’m working on for QEMU, we have a subset of the full operation list implemented; Just enought to have a implementation with minimal features. The operations we have currently are:
- GetDeviceInfo
- OpenSession
- CloseSession
- GetStorageIDs
- GetStorageInfo
- GetNumObjects
- GetObjectHandles
- GetObjectInfo
- GetObject
- GetPartialObject
I will be hoping to fix some existing bugs as well as adding SendObjectInfo and SendObject to the list of operations in the coming weeks.