File Transfer Protocol

The file transfer protocol is implemented by both the file transfer service and clients interacting with the service. This protocol uses a content-addressable methodology similar to Git for storing and chunking files. This document covers the content-addressable storage, all messages used in the protocol, and includes diagrams of common use cases.

APIs

• File Protocol
• CBOR Protocol

Content-Addressable Storage

The file protocol uses a content-addressable system to store file data. All files are broken up into chunks prior to sending. This chunking is initiated either by an export or import message. A local storage folder is created by the file transfer service and client for storing the content-addressable information. Inside of this directory, each file has its own folder. The folder name is a 16-bit BLAKE2 hash of the file’s contents. This folder is created as part of the import/export process.

Inside of each file’s folder there is a meta file and numbered chunk files. Each meta file contains metadata describing the file (currently only the number of chunks). Each chunk file is named with its chunk number. Each chunk file contains the raw contents of that chunk.

Here is an example content-addressable storage structure containing an eleven chunk file:

storage/
└── 852f1630f4ed2c0bc934d71ada618974/ <- BLAKE2 hash of file
    ├── 0 <- Each of these are file chunks
    ├── 1
    ├── 2
    ├── 3
    ├── 4
    ├── 5
    ├── 6
    ├── 7
    ├── 8
    ├── 9
    ├── 10
    └── meta <- Contains `{ "num_chunks" : 11 }` in CBOR

Messages

All messages in the file protocol are encoded as CBOR arrays and are sent in UDP packets.

The first value in the encoded list is the channel_id for request/response type messages or the hash for content-addressable messages.

• The channel_id parameter is used to indicate a group of messages associated with a particular file protocol transaction.
• The hash parameter is the BLAKE2 hash for the corresponding file which is being transferred.

Name	Syntax
Metadata	{ hash, num_chunks }
Export Request	{ channel_id, export, hash, path, mode }
Import Request	{ channel_id, import, path }
File Chunk	{ hash, chunk_index, data }
Acknowledge (ACK)	{ hash, true, num_chunks }
Negative Acknowledge (NAK)	{ hash, false, x_start, x_end, y_start, y_end, ... }
Request Success	{ channel_id, true, ..`values` }
Request Failure	{ channel_id, false, error_message }

Metadata

This message is sent to inform the message receiver of the metadata of a particular file. If the hash directory for that file already exists in the receiver’s temporary storage, then the metadata file will be updated with the new information. If the hash directory does not exist, then it will be created, along with the accompanying meta file.

This message should be sent prior to an export request to ensure the expected number of chunks is known.

{ hash, num_chunks }

Export Request

This message is sent to initiate the process of transferring a file from the message sender to the message receiver. It contains the channel id, the string “export”, the file’s hash, the target path for the file and file’s permissions mode.

The message receiver will begin waiting for file chunks after receiving this message. Once the timeout triggers it will attempt to export the file locally. If the file is incomplete then the receiver will request any missing chunks. Upon receiving all chunks it will attempt to verify and export the file to the local filesystem. This message is sent after the sync command as part of the export process.

{ channel_id, "export", hash, path, mode }

Import Request

This message is sent to initiate the process of transferring a file to the message sender from the message receiver. It contains the channel ID, the string “import”, and the requested file’s path.

Upon receiving, the message receiver will import the requested file into the managed content-addressable storage and send a success message to the sender. This success message will contain the file`s hash and allow the original message sender to determine which file chunks are required.

{ channel_id, "import", path }

File Chunk

This message is sent as part of the file import or export process. It contains the file hash, chunk index, and raw chunk data.

By default, each raw chunk is 4KB in size. Individual chunk messages will not get an immediate reply. However, if no chunks are received within the timeout window then an ACK or NAK will be sent depending on whether all the chunks have been received or not.

{ hash, chunk_index, data }

Note

Chunk size configuration is not currently available, but will be added in a future release.

Acknowledge (ACK)

This message is sent to inform the message receiver that the message sender has all chunks for a given file. It contains the file’s hash, the boolean value true, and the number of chunks in the file.

{ hash, true, num_chunks }

Negative Acknowledge (NAK)

This message is sent to inform the message receiver that the message sender does not have all chunks for a given file. It contains the file’s hash, the boolean value false, and a list of missing chunk ranges. The ranges are pairs of numbers where the first number is inclusive and the second is exclusive. For example 0, 2 means the first two chunks, 0 and 1, are missing.

A NAK may be sent after receiving an export request message, after receiving a succes message in reply to an import request message, or after a timeout during a file import or export operation. The message sender should expect the message receiver to send the missing file chunks upon receipt of a NAK.

{ hash, false, 1, 4, 6, 7 }

The above example NAK indicates that chunks 1-3 and 6 are missing.

Request Success

This message is sent as part of the import or export process. It contains the channel ID and the boolean value true.

When this message is sent as part of the export process, it will be sent at the very end, indicating that all file chunks were successfully transmitted to the requester.

{ channel_id, true }

When this message is sent as part of the import process, it will be sent after receiving the initial import request, once the receiver has successfully prepared the file for transfer. The requester will then need to send a NAK to begin the transfer process.

In this case, the message will also contain file’s hash, number of chunks, and mode.

{ channel_id, true, hash, num_chunks, mode }

Request Failure

This message is sent if there as an error in the import or export process. It contains the channel ID, the boolean false and the error message.

{ channel_id, false, error_message }

Common Protocol Usages

Uploading a single chunk file from a ground station to an OBC:

Downloading a single chunk file from an OBC to a ground station:

Uploading a three chunk file from ground station with a chunk re-request: