=head1 The VPE Protocol

=head2 Anatomy of a VPN packet

The exact layout and field lengths of a VPN packet is determined at
compiletime and doesn't change. The same structure is used for all
protocols, be it rawip or tcp.

 +------+------+--------+------+
 | HMAC | TYPE | SRCDST | DATA |
 +------+------+--------+------+

The HMAC field is present in all packets, even if not used (e.g. in
authentification packets), in which case it is set to all zeroes. The
checksum itself is over the TYPE, SRCDST and DATA fields in all cases.

The TYPE field is a single byte and determines the purpose of the packet
(e.g. RESET, COMPRESSED/UNCOMPRESSED DATA, PING, AUTH REQUEST/RESPONSE,
CONNECT REQUEST/INFO etc.).

SRCDST is a three byte field which contains the source and destination
node ids (12 bits each). The protocol does not yet scale well beyond 30+
hosts, since all hosts connect to each other on startup. But if restarts
are rare or tolerable and most connections are on demand, larger networks
are possible.

The DATA portion differs between each packet type, naturally, and is the
only part that can be encrypted encrypted. Data packets contain more
fields, as shown:

 +------+------+--------+------+-------+------+
 | HMAC | TYPE | SRCDST | RAND | SEQNO | DATA |
 +------+------+--------+------+-------+------+

RAND is a sequence of fully random bytes, used to increase the entropy of the data
for encryption purposes.

SEQNO is a 32-bit sequence number. It is negotiated at every connection
initialization and starts at some random value.

=head2 The authentification protocol

Before hosts can exchange packets, they need to establish authenticity of
the other side and a key. Every host has a private RSA key and the public
RSA keys of all other hosts.

A host establishes a simplex connection by sending the other host a RSA
challenge containing the random digest and encryption keys (different)
to use when sending packets, plus more randomness plus some PKCS1_OAEP
padding plus a random 16 byte id. The destination host will respond by
replying with an (unencrypted) RIPEMD160 hash of the decrypted data, which
will authentify that host. The destination host will also set the outgoing
encryption parameters as given in the packet.

When the source host receives a correct auth reply (by verifying the
hash and the id, which will expire after 20 seconds). it will start to
accept data packets from the destination host. The protocol is completely
symmetric, so to be able to send packets the destination host must send a
challenge in the exact same way as already described.

=head2 Retrying

When there is no response to an auth request, the host will send auth
requests in bursts with an exponential backoff. After some time it will
resort to PING packets, which are very small (8 byte) and lightweight (no
RSA operations). A host that receives ping requests from an unconnected
peer will respond by trying to create a connection.

In addition to the exponential backoff, there is a global rate-limit on
a per-ip base. It allows long bursts but will limit total packet rate to
something like one control packet every ten seconds, to avoid accidental
floods due to protocol problems (like a rsa key file mismatch between two
hosts).

=head2 Routing and Protocol translation

... not yet written, please bug me ...