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.\" ========================================================================
.\"
.IX Title "VPE.PROTOCOL 7"
.TH VPE.PROTOCOL 7 "2003-04-15" "0.9" "Virtual Private Ethernet"
.SH "The VPE Protocol"
.IX Header "The VPE Protocol"
.Sh "Anatomy of a \s-1VPN\s0 packet"
.IX Subsection "Anatomy of a VPN packet"
The exact layout and field lengths of a \s-1VPN\s0 packet is determined at
compiletime and doesn't change. The same structure is used for all
protocols, be it rawip or tcp.
.PP
.Vb 3
\& +------+------+--------+------+
\& | HMAC | TYPE | SRCDST | DATA |
\& +------+------+--------+------+
.Ve
.PP
The \s-1HMAC\s0 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 \s-1TYPE\s0, \s-1SRCDST\s0 and \s-1DATA\s0 fields in all cases.
.PP
The \s-1TYPE\s0 field is a single byte and determines the purpose of the packet
(e.g. \s-1RESET\s0, \s-1COMPRESSED/UNCOMPRESSED\s0 \s-1DATA\s0, \s-1PING\s0, \s-1AUTH\s0 \s-1REQUEST/RESPONSE\s0,
\&\s-1CONNECT\s0 \s-1REQUEST/INFO\s0 etc.).
.PP
\&\s-1SRCDST\s0 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.
.PP
The \s-1DATA\s0 portion differs between each packet type, naturally, and is the
only part that can be encrypted encrypted. Data packets contain more
fields, as shown:
.PP
.Vb 3
\& +------+------+--------+------+-------+------+
\& | HMAC | TYPE | SRCDST | RAND | SEQNO | DATA |
\& +------+------+--------+------+-------+------+
.Ve
.PP
\&\s-1RAND\s0 is a sequence of fully random bytes, used to increase the entropy of the data
for encryption purposes.
.PP
\&\s-1SEQNO\s0 is a 32\-bit sequence number. It is negotiated at every connection
initialization and starts at some random value.
.Sh "The authentification protocol"
.IX Subsection "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 \s-1RSA\s0 key and the public
\&\s-1RSA\s0 keys of all other hosts.
.PP
A host establishes a simplex connection by sending the other host a \s-1RSA\s0
challenge containing the random digest and encryption keys (different)
to use when sending packets, plus more randomness plus some \s-1PKCS1_OAEP\s0
padding plus a random 16 byte id. The destination host will respond by
replying with an (unencrypted) \s-1RIPEMD160\s0 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.
.PP
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.
.Sh "Retrying"
.IX Subsection "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 \s-1PING\s0 packets, which are very small (8 byte) and lightweight (no
\&\s-1RSA\s0 operations). A host that receives ping requests from an unconnected
peer will respond by trying to create a connection.
.PP
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).
.Sh "Routing and Protocol translation"
.IX Subsection "Routing and Protocol translation"
The vpe routing algorithm is easy: there isn't any routing. Vped always
tries to establish direct connections, if the protocol abilities of the
two hosts allow it.
.PP
If the two hosts should be able to reach each other (common protocol, ip
and port all known), but cannot (network down), then there will be no
connection, point.
.PP
A host can usually declare itself unreachable directly by setting it's
port number(s) to zero. It can declare other hosts as unreachable by using
a config-file that disables all protocols for these other hosts.
.PP
If two hosts cannot connect to each other because their \s-1IP\s0 address(es)
are not known (such as dialup hosts), one side will send a connection
request to a router (routers must be configured to act as routers!), which
will send both the originating and the destination host a connection info
request with protocol information and \s-1IP\s0 address of the other host (if
known). Both hosts will then try to establish a connection to the other
peer, which is usually possible even when both hosts are behind a \s-1NAT\s0
gateway.
.PP
If the hosts cannot reach each other because they have no common protocol,
the originator instead use the router with highest priority and matching
protocol as peer. Since the \s-1SRCDST\s0 field is not encrypted, the router host
can just forward the packet to the destination host. Since each host uses
it's own private key, the router will not be able to decrypt or encrypt
packets, it will just act as a simple router and protocol translator.
.PP
When no router is connected, the host will aggressively try to connect to
all routers, and if a router is asked for an unconnected host it will try
to ask another router to establish the connection.
.PP
\&... more not yet written about the details of the routing, please bug me
\&...