Define the Re6st protocol in docs

docs/re6st-protocol

+                             Re6st Protocol
+                             --------------
+
+Table of Contents
+-----------------
+
+1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
+2. Handshake  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
+3. Re6st packet format  . . . . . . . . . . . . . . . . . . . . . . . . . . .
+4. Network Version Message  . . . . . . . . . . . . . . . . . . . . . . . . .
+5. Address Message  . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
+6. Tunnel Destruction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
+
+1. Introduction
+---------------
+
+The re6st protocol is defined for communication between nodes of a re6st
+network relative to the network itself. Nodes communicate to cooperate
+together on the creation and destruction of tunnels.
+
+2. Handshake
+-----------------
+
+The first step for two nodes to communicate is to establish
+a connection, which requires a handshake. The purpose of the handshake
+is to authenticate the nodes between each other by exchanging their public
+certificate, and initialize a secret.
+
+Three types of exchanges are defined for the handshake protocol:
+
+| Seqno |                             Content                               |
+| ----- | ----------------------------------------------------------------- |
+|   0   | pub(sender)                                                       |
+|   1   | sender protocol, fingerprint(receiver), pub(sender)               |
+|   2   | sender protocol, X = encrypt(receiver, secret), sign(sender, X)   |
+
+During a handshake, two or more exchanges may happen, depending on lost
+packets, the cache's content, etc...
+
+The handshake proceeds as follows:
+
+Beginning of handshake:
+  If the node already has a certificate corresponding to the peer node's
+  IP, he sends seqno 1, else he sends seqno 0.
+
+Reception of seqno 0:
+  The node adds the certificate to its list of known certificates, and sends
+  seqno 1.
+
+Reception of seqno 1:
+  The received certificate is added
+  If the fingerprint is correct:
+    - a new secret strictly greater than the previous one is generated
+    - seqno 2 is sent.
+  Else, the node goes back to the beginning
+
+Reception of seqno 2:
+  If the signature verification or if the secret is less or equal to the secret
+  node previously had, the handshake fails.
+  Else, the node sends its network version.
+
+3. Re6st packet format
+----------------
+
+ | Field           | Length        |
+ | --------------- | ---------     |
+ | Sequence number | 4  octets     |
+ | Code            | 1  octet      |
+ | Message         | Variable      |
+ | HMAC            | 20 octets     |
+
+The sequence number is increased at each message, starting from 2 after
+the handshake.
+
+The code defines the type of message:
+
+  000 : Network Version
+  001 : Address
+  010 : Tunnel Destruction Request
+  011 : Tunnel Destruction Response
+  ... : Reserved
+
+The message contents depends on the code, and can vary in size. It must
+be small enough for the packet to not exceed the default minimum MTU
+of IPv6 (1280) bytes.
+
+The HMAC field contains HMAC(secret, data), where HMAC is a HMAC function
+using sha1 and the secret created during the handshake, and data is the
+packet contents from the sequence number to the message.
+
+4. Network Version Message
+--------------------------
+
+The Network Version Message contains the signed network version. It is
+encoded such that each new version message is strictly greater than the
+previous.
+
+5. Address Message
+------------------
+
+Each node has a list (potentially empty) of addresses to advertise to other
+nodes. An address is a tuple of three values: IP, port and protocol.
+For instance: (10.0.0.2, 1194, TCP), (2001:db8:42:4::1, 1195, UDPv6)
+
+In order for nodes to make tunnels between each other, they need to know
+each other's advertised list of address. There are two ways for nodes
+to learn about this:
+
+- Asking the registry with the Remote Procedure Call getBootstrapPeer:
+    this is done only if a node doesn't know any node's addresses yet
+- Sending a re6st packet with code 001 to a node
+
+The address list is sent as a string, each address is separated by a
+semicolon, and each address contains an ip, port and protocol separated
+by a comma.
+
+"ip1,port1,proto1;ip2,port2,proto2;ip3,port3,proto3"
+
+6. Tunnel Destruction
+---------------------
+
+The destruction of a tunnel requires multiple stages:
+
+- soft locking: the node orders babel to raise the cost of its route to
+the peer
+- hard locking: the route is marked as unusable
+- locking request: a request to close the tunnel is sent to the peer,
+corresponding to an empty code 2 message
+- locking response: the peer answers with an empty code 3 message