XMPP interleaved binary
----

S: <stream:features>
S:   <interleaved xmlns='ns:interleaved'/>
S: </stream:features>

C: <enable xmlns='ns:interleaved'/>

S: <enabled xmlns='ns:interleaved'/>

Starting after the final '>' character, all data exchanged with the server
must use this framing format:

  [1-bit type] [15-bits length] [value bytes]

The length is a 15-bit unsigned integer, and the bits are ordered starting
with the most significant bit.  The length indicates the number of value
bytes that follow.  If the number of value bytes to send is larger than can
be represented in 15 bits, then the data should be sent using more than one
frame.

For example:

  00 0B 3C 70 72 65 73 65 6E 63 65 2F 3E

Would mean:

  Type: 0
  Value: <presence/>

Type 0 is XML, type 1 is binary.  The binary type can be used in place
of base64 encoded data.

The following two transmissions are equivalent (whitespace added for clarity)

C0: <iq from='romeo@montague.net/orchard' to='juliet@capulet.com/balcony'
C0:     type='set' id='ibb1'>
C0:   <data xmlns='http://jabber.org/protocol/ibb' sid='mySID' seq='0'>
C0:     SGVsbG8gd29ybGQ=
C0:   </data>
C0: </iq>

C0: <iq from='romeo@montague.net/orchard' to='juliet@capulet.com/balcony'
C0:     type='set' id='ibb1'>
C0:   <data xmlns='http://jabber.org/protocol/ibb' sid='mySID' seq='0'>
C1:     Hello world
C0:   </data>
C0: </iq>

The framing protocol sits under the XML stream, however it is always above
all other layer negotiations.  For example, if TLS or compression is
negotiated, then the entire interleaved stream is encrypted or compressed,
not simply the type 0 XML data.  These layer negotiation protocols have an
indicator for where the new layer should begin (for example, with TLS, this
is after the final '>' of '<proceed/>').  When using interleaved, the new
layer begins after the type 0 frame containing this indicator.  Since the
interleaved framing protocol has no footer, this point is mostly moot (the
last byte in the frame containing '<proceed/>' would be '>', so it works out
the same whether or not interleaved framing is used).