protocol version 0x536b616e

DATA        0x01
HASH        0x02
LIST        0x03
NULL        0x04
TYPE_MASK   0x0f

LENGTH_32   0x00
LENGTH_16   0x10
LENGTH_8    0x20
LENGTH_MASK 0xf0


MESSAGE ENCODING
----------------

When decoding, the entire message length must be known.  If this is
transmitted over a raw stream such as TCP, this is usually encoded
with a 4-byte length followed by the message itself.  If some other
wrapping is used (say as part of a different message structure) the
length of the message must be preserved and included for decoding.

The first 4 bytes of the message is the protocol version encoded
directly as a 4-byte value.  Immediately following this is a HASH
element.  The length of the hash element is the remainder of the
message after subtracting 4 bytes for the protocol version.

This initial HASH is intended to be used by the message routing system
if one is in use.


ITEM TYPES
----------

There are four basic types encoded in this protocol.  A simple data
blob (DATA), a tag-value series (HASH), an ordered list (LIST), and
a NULL type (which is used internally to encode DATA types which are
empty and can be used to indicate existance without data in a hash.)

Each item can be of any type, so a hash of hashes and hashes of lists
are typical.

All multi-byte integers which are encoded in binary are in network
byte order.


ITEM ENCODING
-------------

Each item is preceeded by a single byte which describes that item.
This byte contains the item type and item length encoding:

    Thing             Length    Description
    ----------------  --------  ------------------------------------
    TyLen             1 byte    Item type and length encoding
    Length            variable  Item data blob length
    Item Data         variable  Item data blob

The TyLen field includes both the item data type and the item's
length.  The length bytes are encoded depending on the length of data
portion, and the smallest data encoding type supported should be
used.  Note that this length compression is used just for data
compactness.  It is wasteful to encode the most common length (8-bit
length) as 4 bytes, so this method allows one byte to be used rather
than 4, three of which are nearly always zero.


HASH
----

This is a tag/value pair where each tag is an opaque unique blob and
the data elements are of any type.  Hashes are not encoded in any
specific tag or item order.

The length of the HASH's data area is processed for tag/value pairs
until the entire area is consumed.  Running out of data prematurely
indicates an incorrectly encoded message.

The data area consists of repeated items:

    Thing             Length    Description
    ----------------  --------  ------------------------------------
    Tag Length       1 byte    The length of the tag.
    Tag              Variable  The tag name
    Item             Variable  Encoded item

The Tag Length field is always one byte, which limits the tag name to
255 bytes maximum.  A tag length of zero is invalid.


LIST
----

A LIST is a list of items encoded and decoded in a specific order.
The order is chosen entirely by the source curing encoding.

The length of the LIST's data is consumed by the ITEMs it contains.
Running out of room prematurely indicates an incorrectly encoded
message.

The data area consists of repeated items:

     Thing           Length    Description
     --------------  ------    ----------------------------------------
     Item	     Variable  Encoded item


DATA
----

A DATA item is a simple blob of data.  No further processing of this
data is performed by this protocol on these elements.

The data blob is the entire data area.  The data area can be 0 or more
bytes long.

It is typical to encode integers as strings rather than binary
integers.  However, so long as both sender and recipient agree on the
format of the data blob itself, any blob encoding may be used.


NULL
----

This data element indicates no data is actually present.  This can be
used to indicate that a tag is present in a HASH but no data is
actually at that location, or in a LIST to indicate empty item
positions.

There is no data portion of this type, and the encoded length is
ignored and is always zero.

Note that this is different than a DATA element with a zero length.


EXAMPLE
-------

This is Ruby syntax, but should be clear enough for anyone to read.

Example data encoding:

{
  "from" => "sender@host",
  "to" => "recipient@host",
  "seq" => 1234,
  "data" => {
    "list" => [ 1, 2, nil, "this" ],
    "description" => "Fun for all",
  },
}


Wire-format:

In this format, strings are not shown in hex, but are included "like
this."  Descriptions are written (like this.)

Message Length: 0x64 (100 bytes)
Protocol Version:  0x53 0x6b 0x61 0x6e
(remaining length: 96 bytes)

0x04 "from" 0x21 0x0b "sender@host"
0x02 "to" 0x21 0x0e "recipient@host"
0x03 "seq" 0x21 0x04 "1234"
0x04 "data" 0x22
  0x04 "list" 0x23 
    0x21 0x01 "1"
    0x21 0x01 "2"
    0x04
    0x21 0x04 "this"
  0x0b "description" 0x0b "Fun for all"


MESSAGE ROUTING
---------------

The message routing daemon uses the top-level hash to contain routing
instructions and additional control data.  Not all of these are
required for various control message types; see the individual
descriptions for more information.

    Tag      Description
    -------  ----------------------------------------
    msg      Sender-supplied data
    from     sender's identity
    group    Group name this message is being sent to
    instance Instance in this group
    repl     if present, this message is a reply.
    seq	     sequence number, used in replies
    to	     recipient or "*" for no specific receiver
    type     "send" for a channel message


"type" is a DATA element, which indicates to the message routing
system what the purpose of this message is.


Get Local Name (type "getlname")
--------------------------------

Upon connection, this is the first message to be sent to the control
daemon.  It will return the local name of this client.  Each
connection gets its own unique local name, and local names are never
repeated.  They should be considered opaque strings, in a format
useful only to the message routing system.  They are used in replies
or to send to a specific destination.

To request the local name, the only element included is the
  "type" => "getlname"
tuple.  The response is also a simple, single tuple:
  "lname" => "UTF-8 encoded local name blob"

Until this message is sent, no other types of messages may be sent on
this connection.


Regular Group Messages (type "send")
------------------------------------

When sending a message:

"msg" is the sender supplied data.  It is encoded as per its type.
It is a required field, but may be the NULL type if not needed.
In OpenReg, this was another wire format message, stored as an
ITEM_DATA.  This was done to make it easy to decode the routing
information without having to decode arbitrary application-supplied
data, but rather treat this application data as an opaque blob.

"from" is a DATA element, and its value is a UTF-8 encoded sender
identity.  It MUST be the "local name" supplied by the message
routing system upon connection.  The message routing system will
enforce this, but will not add it.  It is a required field.

"group" is a DATA element, and its value is the UTF-8 encoded group
name this message is being transmitted to.  It is a required field for
all messages of type "send".

"instance" is a DATA element, and its value is the UTF-8 encoded
instance name, with "*" meaning all instances.

"repl" is the sequence number being replied to, if this is a reply.

"seq" is a unique identity per client.  That is, the <lname, seq>
tuple must be unique over the lifetime of the connection, or at least
over the lifetime of the expected reply duration.

"to" is a DATA element, and its value is a UTF-8 encoded recipient
identity.  This must be a specific recipient name or "*" to indicate
"all listeners on this channel."  It is a required field.

When a message of type "send" is received by the client, all the data
is used as above.  This indicates a message of the given type was
received.

A client does not see its own transmissions. (XXXMLG Need to check this)


Group Subscriptions (type "subscribe")
--------------------------------------

A subscription requires the "group", "instance", and a flag to
indicate the subscription type ("sybtype").  If instance is "*" the
instance name will be ignored when decising to forward a message to
this client or not.

"subtype" is a DATA element, and contains "normal" for normal channel
subscriptions, "meonly" for only those messages on a channel with the
recipient specified exactly as the local name, or "promisc" to receive
all channel messages regardless of other filters.  As its name
implies, "normal" is for typical subscriptions, and "promisc" is
intended for channel message debugging.

There is no response to this message.


Group Unsubscribe (type "unsubscribe")
-------------------------------

The fields to be included are "group" and "instance" and have the same
meaning as a "subscribe" message.

There is no response to this message.


Statistics (type "stats")
-------------------------

Request statistics from the message router.  No other fields are
inclued in the request.

The response contains a single element "stats" which is an opaque
element.  This is used mostly for debugging, and its format is
specific to the message router.  In general, some method to simply
dump raw messages would produce something useful during debugging.
