Detailed specification of protocol in version 00000501
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Note: work in progress!!

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1. DNS protocol
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CMC = 2 byte Cache Miss Counter, increased every time it is used

Version:
Client sends:
	First byte v or V
	Rest encoded with base32:
	4 bytes big endian protocol version
	CMC
Server replies:
	4 chars:
		VACK (version ok), followed by login challenge
		VNAK (version differs), followed by server protocol version
		VFUL (server has no free slots), followed by max users
	4 byte value: means login challenge/server protocol version/max users
	1 byte userid of the new user, or any byte if not VACK
	
Login:
Client sends:
	First byte l or L
	Rest encoded with base32:
	1 byte userid
	16 bytes MD5 hash of: (first 32 bytes of password) xor (8 repetitions of login challenge)
	CMC
Server replies:
	LNAK means not accepted
	x.x.x.x-y.y.y.y-mtu-netmask means accepted (server ip, client ip, mtu, netmask bits)

IP Request:
Client sends:
	First byte i or I
	5 bits coded as Base32 char, meaning userid
	CMC
Server replies
	BADIP if bad userid, or
	I and then 4 bytes network order external IP address of iodined server

Case check:
Client sends: 
	First byte z or Z
	Lots of data that should not be decoded
Server replies:
	The requested domain copied raw

Switch codec:
Client sends:
	First byte s or S
	5 bits coded as Base32 char, meaning userid
	5 bits coded as Base32 char, with value 5 or 6, representing number of raw
	bits per encoded byte
	CMC
Server sends:
	Name of codec if accepted. After this all upstream data packets must 
	be encoded with the new codec.
	BADCODEC if not accepted. Client must then revert to Base32
	BADLEN if length of query is too short

Options:
Client sends:
	First byte o or O
	5 bits coded as Base32 char, meaning userid
	1 char, meaning option
Server sends:
	Full name of option if accepted. After this, option immediately takes
	effect in server.
	BADCODEC if not accepted. Previous situation remains.
	All options affect only the requesting client.

	Option chars:
	t or T: Downstream encoding Base32, for TXT/CNAME/A/MX (default)
	s or S: Downstream encoding Base64, for TXT/CNAME/A/MX
	r or R: Downstream encoding Raw, for TXT/NULL (default for NULL)
	If codec unsupported for request type, server will use Base32; note
	that server will answer any mix of request types that a client sends.
	Server may disregard this option; client must always use the downstream
	encoding type indicated in every downstream DNS packet.

	l or L: Lazy mode, server will keep one request unanswered until the
	next one comes in. Applies only to data transfer; handshake is always
	answered immediately.
	i or I: Immediate (non-lazy) mode, server will answer all requests
	(nearly) immediately.

Probe downstream fragment size:
Client sends:
	First byte r or R
	15 bits coded as 3 Base32 chars: UUUUF FFFFF FFFFF
		meaning 4 bits userid, 11 bits fragment size
	Then follows a long random query which contents does not matter
Server sends:
	Requested number of bytes as a response. The first two bytes contains
	the requested length. Rest of message can be any data.
	BADFRAG if requested length not accepted.

Set downstream fragment size:
Client sends:
	First byte n or N
	Rest encoded with base32:
	1 byte userid
	2 bytes new downstream fragment size
	CMC
Server sends:
	2 bytes new downstream fragment size. After this all downstream
	payloads will be max (fragsize + 2) bytes long.
	BADFRAG if not accepted.

Data:
Upstream data header:
	 3210 432 10 43 210 4321 0
	+----+---+--+--+---+----+-+
	|UUUU|SSS|FF|FF|DDD|GGGG|L|
	+----+---+--+--+---+----+-+

Downstream data header:
	 7 654 3210 765 4321 0
	+-+---+----+---+----+-+
	|C|SSS|FFFF|DDD|GGGG|L|
	+-+---+----+---+----+-+

UUUU = Userid
L = Last fragment in packet flag
SS = Upstream packet sequence number
FFFF = Upstream fragment number
DDD = Downstream packet sequence number
GGGG = Downstream fragment number
C = Compression enabled for downstream packet

Upstream data packet starts with 1 byte ASCII hex coded user byte, then 3 bytes 
Base32 encoded header, then comes the payload data, encoded with chosen codec.

Downstream data starts with 2 byte header. Then payload data, which may be
compressed.

In NULL responses, downstream data is always raw. In all other response types,
downstream data is encoded (see Options above).
Encoding type is indicated by 1 prefix char:
TXT:
	End result is always DNS-chopped (series of len-prefixed strings
	<=255 bytes)
	t or T: Base32	 encoded before chop, decoded after un-chop
	s or S: Base64	 encoded before chop, decoded after un-chop
	r or R: Raw	 no encoding, only DNS-chop
CNAME/A/MX:
	h or H: Hostname encoded with Base32
	i or I: Hostname encoded with Base64

Ping:
Client sends:
	First byte p or P
	Rest encoded with Base32:
	1 byte with 4 bits userid
	1 byte with:
		3 bits downstream seqno
		4 bits downstream fragment
	CMC

The server response to Ping and Data packets is a DNS NULL type response:
If server has nothing to send, data length is 0 bytes.
If server has something to send, it will send a downstream data packet, 
prefixed with 2 bytes header as shown above.
 
 
"Lazy-mode" operation
=====================

Client-server DNS traffic sequence has been reordered to provide increased
(interactive) performance and greatly reduced latency.

Idea taken from Lucas Nussbaum's slides (24th IFIP International Security
Conference, 2009) at http://www.loria.fr/~lnussbau/tuns.html. Current
implementation is original to iodine, no code or documentation from any other
project was consulted during development.

Server:
Upstream data is acked immediately*, to keep the slow upstream data flowing
as fast as possible (client waits for ack to send next frag).

Upstream pings are answered _only_ when 1) downstream data arrives from tun,
OR 2) new upstream ping/data arrives from client.
In most cases, this means we answer the previous DNS query instead of the
current one. The current query is kept in queue and used as soon as
downstream data has to be sent.

*: upstream data ack is usually done as reply on the previous ping packet,
and the upstream-data packet itself is kept in queue.
 
Client:
Downstream data is acked immediately, to keep it flowing fast (includes a
ping after last downstream frag).

Also, after all available upstream data is sent & acked by the server (which
in some cases uses up the last query), send an additional ping to prime the
server for the next downstream data.


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2. Raw UDP protocol
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All Raw UDP protcol messages start with a 3 byte header: 0x10d19e
This is not the start of a valid DNS message so it is easy to identify.
The fourth byte contains the command and the user id.

	 7654 3210
	+----+----+
	|CCCC|UUUU|
	+----+----+

Login message (command = 1):
The header is followed by a MD5 hash with the same password as in the DNS
login. The client starts the raw mode by sending this message, and uses
the login challenge +1, and the server responds using the login challenge -1.
After the login message has been exchanged, both the server and the client
switch to raw udp mode for the rest of the connection.

Data message (command = 2):
After the header comes the payload data, which may be compressed.

Ping message (command = 3):
Sent from client to server and back to keep session open. Has no payload.