PTP-IP and USB transport

After the camera is reachable, control, live view and media share one PTP session. Framing is the PTP-IP packet; this page covers the sockets and sessions around it.

Two sockets

PTP-IP (CIPA DC-005) uses two TCP connections to port 15740:

1. Command socket. Send Init_Command_Request with the app’s initiator GUID and name. The camera answers Init_Command_Ack with a connection number and its own name, or Init_Fail.
2. Event socket. Send Init_Event_Request carrying that connection number; wait for Init_Event_Ack.

If either step fails, both sockets close. The initiator GUID is stable per install: Nikon bodies key their pairing profiles to it, so a new GUID on every launch would force re-pairing. Once stored it is never replaced.

Transactions

Every operation, whether a live-view frame, a property read or write, an event poll, a media chunk or a keep-alive, is one serial transaction on the command socket behind a FIFO gate. There is no pipelining. That is why poll cadence and chunk size matter to the live picture (live view).

Deadline	Value
Command transaction	Short whole-transaction deadline; a wedged or dribbling camera cannot hold the gate
First live-view frame	10 s
Steady-state live-view frame	6 s
USB first command	Long, because iOS may still be cataloguing a full card

A blocked socket read cannot be cancelled, so an expired deadline closes the command socket and recovery takes over. An idle DeviceReady keeps a quiet command channel warm.

Events

The camera pushes events on the event socket (record started 0xC10A, record complete 0xC108, record interrupted 0xC105, DevicePropChanged 0x4006). The app drains that socket continuously: an unread channel backs up the camera’s send buffer and can stall the session deep into a long take. It also answers Probe_Request there.

Nikon bodies deliver capture events (ObjectAdded, CaptureComplete) and many DevicePropChanged announcements through the GetEventEx poll (0x941C, parameter 0 clears the queue) rather than the socket. The app polls it between live-view frames in every monitor mode. An announcement carries no value, so it only schedules an authoritative read; a bounded batch is re-read per poll, oldest first.

Recovery

A dropped link keeps the monitor on the last frame while bounded, jittered retries run (SessionRecovery.swift, ReconnectBackoff.swift). When the budget is spent, or reconnects keep succeeding but die young, automatic recovery stops and the operator chooses. Rapid session churn can wedge a Nikon body until its battery is pulled, so the app sends CloseSession, waits a settle interval before a fresh Init, and never probes a just-dropped camera with an extra handshake. Background: connection reliability audit.

USB-C

The same PTP operations, properties and datasets ride PIMA 15740 generic containers on the USB bulk pipe.

Platform	USB path
iOS / iPadOS	ImageCaptureCore: ICDeviceBrowser discovery and authorization, requestSendPTPCommand for each whole transaction, ptpEventHandler into the event drain
Android	USB Host finds a complete PTP interface, asks per-device consent, and hands raw bulk and interrupt bytes to the Swift facade, which owns container framing and the session

Differences from Wi-Fi:

• GetDeviceInfo is sent before OpenSession. On iOS, ImageCaptureCore often has the session open already; Session_Already_Open (0x201E) counts as success.
• There is no pairing. The pairing operations are absent from the USB operation set; the cable is the trust boundary.
• The ZR boots USB sessions in PC-camera mode and refuses app control until ChangeCameraMode remote mode is set. The body then shows its connected to computer screen.
• Saved USB cameras use a local usb: device key, never a raw USB serial or a network address, and reconnect silently when plugged in.

The CameraTransport boundary

Wi-Fi and USB share one session layer behind the transaction-level CameraTransport protocol (CameraTransport.swift). One call runs a whole transaction: request, optional data-out, any data-in, and the response. It cannot sit lower, because ImageCaptureCore only exposes whole transactions on iOS. Conformers assign transaction IDs and serialize internally.

Layer	iOS	Android
Wi-Fi transport	OpenCine Sources/OpenCineNikon/PTPIPTransport.swift (BSD TCP sockets)	Kotlin socket adapter, Swift facade session
USB transport	OpenCine Sources/OpenCineNikon/USBCameraTransport.swift	Android USB Host adapter, Swift facade session
Session orchestration	NativeCameraSession	OpenCine Sources/OpenCineNikonAndroid/PTPIPClientSession.swift

Kotlin never builds PTP operations. Decisions live in the portable core; see ARCHITECTURE.md.