iodine/doc/proto_00000501.txt

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

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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

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.

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.