The git-annex assistant uses XMPP to communicate between peers that
cannot directly talk to one-another. A typical scenario is two users
who share a repository, that is stored in the [[cloud]].

### TODO

* Do git-annex clients sharing an account with regular clients cause confusing
  things to happen? 
  See <http://git-annex.branchable.com/design/assistant/blog/day_114__xmpp/#comment-aaba579f92cb452caf26ac53071a6788>
* Support registering with XMPP provider from within the webapp, 
  as clients like pidgin are able to do.
* At least some XMPP servers are lossy (does XMPP guarantee delivery)?
  I have seen a log where a push's packet 1 and 3 arrived, but 2 did not.
  To deal with this, need at least a 1 packet buffer and ACK or resend
  request implemented over top of XMPP. Essentially, TCP over XMPP. :(

## design goals

1. Avoid user-visible messages. dvcs-autosync uses XMPP similarly, but
   sends user-visible messages. Avoiding user-visible messages lets
   the user configure git-annex to use his existing XMPP account
   (eg, Google Talk).

2. Send notifications to buddies. dvcs-autosync sends only self-messages,
   but that requires every node have the same XMPP account configured.
   git-annex should support that mode, but it should also send notifications
   to a user's buddies. (This will also allow for using XMPP for pairing
   in the future.)

3. Don't make account appear active. Just because git-annex is being an XMPP
   client, it doesn't mean that it wants to get chat messages, or make the
   user appear active when he's not using his chat program.

## protocol

To avoid relying on XMPP extensions, git-annex communicates
using presence messages, and chat messages (with empty body tags,
so clients don't display them).

git-annex sets a negative presence priority, to avoid any regular messages
getting eaten by its clients. It also sets itself extended away.
Note that this means that chat messages always have to be directed at
specific git-annex clients.

To the presence and chat messages, it adds its own tag as
[extended content](http://xmpp.org/rfcs/rfc6121.html#presence-extended).
The xml namespace is "git-annex" (not an URL because I hate wasting bandwidth).

To indicate it's pushed changes to a git repo with a given UUID, a message
that is sent to all buddies and other clients using the account (no
explicit pairing needed), it uses a broadcast presence message containing:

	<git-annex xmlns='git-annex' push="uuid[,uuid...]" />

Multiple UUIDs can be listed when multiple clients were pushed. If the
git repo does not have a git-annex UUID, an empty string is used.

To query if other git-annex clients are around, a presence message is used,
containing:

	<git-annex xmlns='git-annex' query="" />

For pairing, a chat message is sent to every known git-annex client,
containing:

	<git-annex xmlns='git-annex' pairing="PairReq|PairAck|PairDone myuuid" />

### git push over XMPP

To indicate that we could push over XMPP, a chat message is sent,
to each known client of each XMPP remote.

	<git-annex xmlns='git-annex' canpush="myuuid" shas="sha1 sha1" />

The shas are omitted by old clients. If present, they are the git shas of
the head and git-annex branches that are available to be pushed. This lets
the receiver check if it's already got them.

To request that a remote push to us, a chat message can be sent.

	<git-annex xmlns='git-annex' pushrequest="myuuid" />

When replying to an canpush message, this is directed at the specific
client that indicated it could push. To solicit pushes from all clients,
the message has to be sent directed individually to each client.

When a peer is ready to send a git push, it sends:

	<git-annex xmlns='git-annex' startingpush="myuuid" />

The receiver runs `git receive-pack`, and sends back its output in
one or more chat messages, directed to the client that is pushing:

	<git-annex xmlns='git-annex' rp="N">
	007b27ca394d26a05d9b6beefa1b07da456caa2157d7 refs/heads/git-annex report-status delete-refs side-band-64k quiet ofs-delta
	</git-annex>

The sender replies with the data from `git push`, in 
one or more chat messages, directed to the receiver:

	<git-annex xmlns='git-annex' sp="N">
	data
	</git-annex>

The value of rp and sp used to be empty, but now it's a sequence number.
This indicates the sequence of this packet, counting from 1. The receiver
and sender each have their own sequence numbers.

When `git receive-pack` exits, the receiver indicates its exit
status with a chat message, directed at the sender:

	<git-annex xmlns='git-annex' rpdone="0" />

### security

Data git-annex sends over XMPP will be visible to the XMPP account's
buddies, and to the XMPP server (and any attacker who has access to the
XMPP server). So it's important to consider the security exposure of using
it.

Even if git-annex sends only a single bit notification, this lets attackers
know when the user is active and changing files. Although the assistant's other
syncing activities can somewhat mask this.

As soon as git-annex does anything unlike any other client, an attacker can
see how many clients are connected for a user, and fingerprint the ones
running git-annex, and determine how many clients are running git-annex.

An attacker can observe the UUIDs used for pushes and pairing, and determine
how many different remotes are being used.

An attacker could replay push notification messages, reusing UUIDs it's
observed. This would make clients pull repeatedly, perhaps as a DOS.

The XMPP server, or an attacker with access to it can reconstruct the git 
repository from data sent in pushes, in part or in whole.
