ssb-blobs

Protocol for gossiping "blobs", in no particular order. This is a plugin for secret-stack and such general information about these type of plugins is documented in plugins.md in that repository.

Protocol

When two peers connect, they request the blobs they "want" from each other. They do this by sending a JSON object that is a map of {: -hop_count} (note, the hop count is negative) to the peer. If they have a blob wanted by the peer, they respond with the size they have for that blob: {: size} (note, size is positive - this is how sizes and blobs are distinguished).

Peers can only request blobs they know about. In the legacy scuttlebot/plugins/blobs peers would request blobs that where mentioned in an ssb-feed but this approach means they cannot know about (encrypted) blobs shared in private messages, or about blobs recursively linked from other blobs.

This protocol addresses that by implementing sympathetic wants. If a peer requests a blob that you do not have, you start to want it too. To prevent these requests from flooding the entire network, you signal how much you want a blob with a hop count. If you want it for yourself, you use a hop_count of -1. If you want it for a friend, you use a hop_count of -2 (and so on..)

This allows you to publish a secret blob, without creating a permanent cryptographic record of that blob.

However, this alone would mean that to upload a blob, someone else needs to request it from you, which requires both peers to be online at the same time.

To address this, we have a push method. "pushing" a blob, just makes a peer "pretend" that they want a blob, triggering sympathetic wants from intermediate nodes, ensuring that there are at least some peers that will have the blob. (currently, your peer will continue to pretend they want the blob until at least 3 peers report having it)

API

ssb.blobs.add(blobId, cb) => sink

Creates a pull-stream sink that works well with e.g. pull-files

• blobId String - (optional) the hash id of the blob you're adding. If the hash of what arrives in the stream doesn't match this the add is aborted
• cb Function - a callback which is run which reveives err, blobId

Note: as of 2.0.0 any blob you try to add with size >= max (set in config) will result in an Error. The error message will say something about can't write to tmpfile, but if you look in the stack trace you will see an error about file size

ssb.blobs.push(blobId, cb)

Push a blob out to other peers. This is useful for blobs which are attached to encrypted messages. (Because pubs will generally not know about blobs attached to encrypted messgages so won't replicate) You will keep pushing the blob out until pushy (see config) peers have taken it.

ssb.blobs.has(blobId, cb)

Find out if you have a copy of blob locally

ssb.blobs.want(blobId, cb)

Declare that you want a particular blob. cb will be run once the blob arrives

ssb.blobs.get(blobId) => source

Creates a pull-stream source of a blob

other methods

The following are less commonly used externally but are available:

• ssb.blobs.getSlice
• ssb.blobs.rm
• ssb.blobs.ls
• ssb.blobs.meta
• ssb.blobs.changes
• ssb.blobs.createWants
• ssb.blobs.help

you can read about these in the source code or ./help.js

Configuration

By changing .ssb/config, you can control how generous ssb-blobs will be. These are the default values:

"blobs": {
  "stingy": false,
  "sympathy": 3,
  "pushy": 3,
  "legacy": true,
  "max": 5 * 1024 * 1024 // 5MB 
}

stingy (default false)

Enabling stingy mode will make your pub pretend it does not have blobs unless it wants to push those blobs to the network. (because you have uploaded that file)

sympathy (default 3)

When a peer asks for a blob, if you do not have it, then out of sympathy you'll ask for it from your other peers. The value for sympathy determines how many hops away from you that peer may be. Note that this depends on hops on the current network topology, not the follow graph: ssb-blobs is actually completely independent of the ssb logs.

Set this to 0 to never request a blob that someone else has asked for, unless you want it too.

pushy (default 3)

When you publish a blob, tell everyone about it, until at least this many peers have taken it. Of course for this they will need a setting for sympathy > 0.

legacy (default true)

Whether you support the legacy style blob replication or not. It's probably safe to disable this now since most pubs will have updated by now.

max (default 5MB)

Maximum size of blobs to replicate. Notes:

• blobs of size >= this max cannot be added using blobs.add
• if you request a blob and discover its size is >= max, you will not replicate it from other peers

Usage

This plugin is required by default by ssb-server and doesn't need to be added to your system if you're using a standard Secure Scuttlebutt install. If you're rolling your own, please refer to the documentation in plugins.md in the secret-stack repository for how to create your own peer-to-peer solution that uses this plugin.

License

MIT