141 lines
4.7 KiB
Markdown
141 lines
4.7 KiB
Markdown
# Specification
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We use this document to detail the internal specification of Yamux.
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This is used both as a guide for implementing Yamux, but also for
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alternative interoperable libraries to be built.
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# Framing
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Yamux uses a streaming connection underneath, but imposes a message
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framing so that it can be shared between many logical streams. Each
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frame contains a header like:
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* Version (8 bits)
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* Type (8 bits)
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* Flags (16 bits)
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* StreamID (32 bits)
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* Length (32 bits)
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This means that each header has a 12 byte overhead.
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All fields are encoded in network order (big endian).
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Each field is described below:
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## Version Field
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The version field is used for future backward compatibility. At the
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current time, the field is always set to 0, to indicate the initial
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version.
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## Type Field
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The type field is used to switch the frame message type. The following
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message types are supported:
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* 0x0 Data - Used to transmit data. May transmit zero length payloads
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depending on the flags.
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* 0x1 Window Update - Used to updated the senders receive window size.
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This is used to implement per-session flow control.
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* 0x2 Ping - Used to measure RTT. It can also be used to heart-beat
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and do keep-alives over TCP.
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* 0x3 Go Away - Used to close a session.
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## Flag Field
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The flags field is used to provide additional information related
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to the message type. The following flags are supported:
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* 0x1 SYN - Signals the start of a new stream. May be sent with a data or
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window update message. Also sent with a ping to indicate outbound.
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* 0x2 ACK - Acknowledges the start of a new stream. May be sent with a data
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or window update message. Also sent with a ping to indicate response.
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* 0x4 FIN - Performs a half-close of a stream. May be sent with a data
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message or window update.
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* 0x8 RST - Reset a stream immediately. May be sent with a data or
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window update message.
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## StreamID Field
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The StreamID field is used to identify the logical stream the frame
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is addressing. The client side should use odd ID's, and the server even.
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This prevents any collisions. Additionally, the 0 ID is reserved to represent
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the session.
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Both Ping and Go Away messages should always use the 0 StreamID.
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## Length Field
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The meaning of the length field depends on the message type:
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* Data - provides the length of bytes following the header
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* Window update - provides a delta update to the window size
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* Ping - Contains an opaque value, echoed back
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* Go Away - Contains an error code
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# Message Flow
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There is no explicit connection setup, as Yamux relies on an underlying
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transport to be provided. However, there is a distinction between client
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and server side of the connection.
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## Opening a stream
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To open a stream, an initial data or window update frame is sent
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with a new StreamID. The SYN flag should be set to signal a new stream.
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The receiver must then reply with either a data or window update frame
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with the StreamID along with the ACK flag to accept the stream or with
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the RST flag to reject the stream.
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Because we are relying on the reliable stream underneath, a connection
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can begin sending data once the SYN flag is sent. The corresponding
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ACK does not need to be received. This is particularly well suited
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for an RPC system where a client wants to open a stream and immediately
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fire a request without waiting for the RTT of the ACK.
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This does introduce the possibility of a connection being rejected
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after data has been sent already. This is a slight semantic difference
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from TCP, where the conection cannot be refused after it is opened.
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Clients should be prepared to handle this by checking for an error
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that indicates a RST was received.
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## Closing a stream
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To close a stream, either side sends a data or window update frame
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along with the FIN flag. This does a half-close indicating the sender
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will send no further data.
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Once both sides have closed the connection, the stream is closed.
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Alternatively, if an error occurs, the RST flag can be used to
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hard close a stream immediately.
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## Flow Control
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When Yamux is initially starts each stream with a 256KB window size.
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There is no window size for the session.
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To prevent the streams from stalling, window update frames should be
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sent regularly. Yamux can be configured to provide a larger limit for
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windows sizes. Both sides assume the initial 256KB window, but can
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immediately send a window update as part of the SYN/ACK indicating a
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larger window.
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Both sides should track the number of bytes sent in Data frames
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only, as only they are tracked as part of the window size.
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## Session termination
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When a session is being terminated, the Go Away message should
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be sent. The Length should be set to one of the following to
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provide an error code:
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* 0x0 Normal termination
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* 0x1 Protocol error
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* 0x2 Internal error
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