Frames

Frames are the lowest level data format in the OP Stack protocol.

Where Frames fit in the OP Stack

Transactions posted to the data availability layer of the rollup contain one or multiple Frames. Frames are chunks of raw data that belong to a given Channel, the next, higher up data format in the OP Stack protocol. Importantly, a given transaction can contain a variety of frames from different channels, allowing maximum flexibility when breaking up channels into batcher transactions.

Contents of a Frame

A Frame is comprised of the following items.

  • A ChannelId which is a 16 byte long identifier for the channel that the given frame belongs to.
  • A number that identifies the index of the frame within the channel. Frames are 0-indexed and are bound to u16 size limit.
  • data contains the raw data within the frame.
  • is_last marks if the frame is the last within the channel.

Frame Encoding

When frames are posted through a batcher transaction, they are encoded as a contiguous list with a single byte prefix denoting the derivation version. The encoding can be represented as the following concatenated bytes.

encoded = DERIVATION_VERSION_0 ++ encoded_frame_0 ++ encoded_frame_1 ++ ..

Where DERIVATION_VERSION_0 is a single byte (0x00) indicating the derivation version including how the frames are encoded. Currently, the only supported derivation version is 0.

encoded_frame_0, encoded_frame_1, and so on, are all Frames encoded as raw bytes. A single encoded Frame can be represented by the following concatenation of it's fields.

encoded_frame = channel_id ++ frame_number ++ frame_data_length ++ frame_data ++ is_last

Where ++ represents concatenation. The frame's fields map to it's encoding.

  • channel_id is the 16 byte long Frame::id.
  • frame_number is the 2 byte long (or u16) Frame::number.
  • frame_data_length and frame_data provide the necessary details to decode the Frame::data, where frame_data_length is 4 bytes long (or u32).
  • is_last is a single byte Frame::is_last.

maili's Frame Type

maili-protocol provides the Frame type with a few useful methods. Frames can be encoded and decoded using the Frame::encode and Frame::decode methods. Given the raw batcher transaction data or blob data containing the concatenated derivation version and contiguous list of encoded frames, the Frame::parse_frame and Frame::parse_frames methods provide ways to decode single and multiple frames, respectively.