The git-annex P2P protocol is a custom protocol that git-annex uses to
communicate between peers.

There's a common line-based serialization of the protocol, but other
serializations are also possible. The line-based serialization is spoken
by [[git-annex-shell], and by git-annex over tor.

One peer is known as the client, and is the peer that initiates the
connection and sends commands. The other peer is known as the server, and
is the peer that the client connects to. It's possible for two connections
to be run at the same time between the same two peers, in different
directions.

## Errors

Either the client or the server may send an error message at any
time. 

When the client sends an ERROR, the server will close the connection.

If the server sends an ERROR in response to the client's
request, the connection will remain open, and the client can make
another request.

	ERROR this repository is read-only; write access denied

## Authentication

The protocol generally starts with authentication. However, if
authentication already occurs on another layer, as is the case with
git-annex-shell, authentication will be skipped.

The client starts by sending an authentication command to the server,
along with its UUID. The AuthToken is some arbitrary token that has been
agreed upon beforehand.

	AUTH UUID AuthToken

The server responds with either its own UUID when authentication
is successful. Or, it can fail the authentication, and close the
connection.

	AUTH_SUCCESS UUID
	AUTH_FAILURE

Note that authentication does not guarantee that the client is talking to
who they expect to be talking to. This, and encryption of the connection,
are handled at a lower level.

## Protocol version

The default protocol version is 0. The client can choose to
negotiate a new version with the server. This must come after
any authentication.

The client sends the highest protocol version it supports:

	VERSION 2

The server responds with the highest protocol version it supports
that is less than or equal to the version the client sent:

	VERSION 1

Now both client and server should use version 1.

## Binary data

The protocol allows raw binary data to be sent. This is done
using a DATA message. In the line-based serialization, this comes
on its own line, followed by a newline and the binary data.
The Len value tells how many bytes of data to read.

	DATA 3
	foo1

Note that there is no newline after the binary data; the next protocol
message will come immediately after it.

If the sender finds itself unable to send as many bytes of data as it
promised (perhaps because a file got truncated while it was being sent),
its only option is to close the protocol connection.

And if the receiver finds itself unable to receive all the data for some
reason (eg, out of disk space), its only option is to close the protocol
connection.

## Checking if content is present

To check if a key is currently present on the server, the client sends:

	CHECKPRESENT Key

The server responds with either SUCCESS or FAILURE.

## Locking content

To lock content on the server, preventing it from being removed,
the client sends:

	LOCKCONTENT Key

The server responds with either SUCCESS or FAILURE.
The former indicates the content is locked. It will remain
locked until the connection is broken, or the client
sends:

	UNLOCKCONTENT Key

The server makes no response to that.

## Removing content

To remove a key's content from the server, the client sends:

	REMOVE Key

The server responds with either SUCCESS or FAILURE.

## Storing content on the server

To store content on the server, the client sends:

	PUT AssociatedFile Key

Here AssociatedFile may be the name of a file in the git
repository, for information purposes only. Or it can be the
empty string. It will always have unix directory separators.

(Note that in the line-based serialization. AssociatedFile may not contain any
spaces, since it's not the last token in the line. Use '%' to indicate
whitespace.)

The server may respond with ALREADY-HAVE if it already
had the conent of that key. Otherwise, it responds with:

	PUT-FROM Offset

Offset is the number of bytes into the file that the server wants
the client to start. This allows resuming transfers.

The client then sends a DATA message with content of the file from
the offset to the end of file.

In protocol version 1, after the data, the client sends an additional
message, to indicate if the content of the file has changed while it
was being sent.

	INVALID
	VALID

If the server successfully receives the data and stores the content,
it replies with SUCCESS. Otherwise, FAILURE.

## Getting content from the server

To get content from the server, the client sends:

	GET Offset AssociatedFile Key

The Offset is the number of bytes into the file that the client wants
the server to skip, which allows resuming transfers.
See description of AssociatedFile above.

The server then sends a DATA message with the content of the file
from the offset to end of file.

In protocol version 1, after the data, the server sends an additional
message, to indicate if the content of the file has changed while it
was being sent.

	INVALID
	VALID

The client replies with SUCCESS or FAILURE.

## Connection to services

This is used to connect to services like git-upload-pack and
git-receive-pack that speak their own protocol.

The client sends a message to request the connection.
Service is the name of the service, eg "git-upload-pack".

	CONNECT Service

Both client and server may now exchange DATA messages in any order,
encapsulating the service's protocol.

When the service exits, the server indicates this by telling the client
its exit code.

	CONNECTDONE ExitCode

## Change notification

The client can request to be notified when a ref in 
the git repository on the server changes.

	NOTIFYCHANGE

The server will block until at least
one of the refs changes, and send a list of changed
refs.

	CHANGED ChangedRefs

For example:

	CHANGED refs/heads/master refs/heads/git-annex

Some servers may not support this command.
