gvpe/doc/vpe.protocol.7.pod

=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

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.

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.

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.

If two hosts cannot connect to each other because their IP 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 IP 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 NAT
gateway.

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 SRCDST 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.

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.

... more not yet written about the details of the routing, please bug me
...

Revision:	1.2
Committed:	Tue Apr 15 04:54:44 2003 UTC (21 years, 1 month ago) by pcg
Branch:	MAIN
CVS Tags:	VPE_1_0
Changes since 1.1:	+34 -1 lines
Log Message:	* empty log message *
#	User	Rev	Content
1	pcg	1.1	=head1 The VPE Protocol
2
3			=head2 Anatomy of a VPN packet
4
5			The exact layout and field lengths of a VPN packet is determined at
6			compiletime and doesn't change. The same structure is used for all
7			protocols, be it rawip or tcp.
8
9			+------+------+--------+------+
10			\| HMAC \| TYPE \| SRCDST \| DATA \|
11			+------+------+--------+------+
12
13			The HMAC field is present in all packets, even if not used (e.g. in
14			authentification packets), in which case it is set to all zeroes. The
15			checksum itself is over the TYPE, SRCDST and DATA fields in all cases.
16
17			The TYPE field is a single byte and determines the purpose of the packet
18			(e.g. RESET, COMPRESSED/UNCOMPRESSED DATA, PING, AUTH REQUEST/RESPONSE,
19			CONNECT REQUEST/INFO etc.).
20
21			SRCDST is a three byte field which contains the source and destination
22			node ids (12 bits each). The protocol does not yet scale well beyond 30+
23			hosts, since all hosts connect to each other on startup. But if restarts
24			are rare or tolerable and most connections are on demand, larger networks
25			are possible.
26
27			The DATA portion differs between each packet type, naturally, and is the
28			only part that can be encrypted encrypted. Data packets contain more
29			fields, as shown:
30
31			+------+------+--------+------+-------+------+
32			\| HMAC \| TYPE \| SRCDST \| RAND \| SEQNO \| DATA \|
33			+------+------+--------+------+-------+------+
34
35			RAND is a sequence of fully random bytes, used to increase the entropy of the data
36			for encryption purposes.
37
38			SEQNO is a 32-bit sequence number. It is negotiated at every connection
39			initialization and starts at some random value.
40
41			=head2 The authentification protocol
42
43			Before hosts can exchange packets, they need to establish authenticity of
44			the other side and a key. Every host has a private RSA key and the public
45			RSA keys of all other hosts.
46
47			A host establishes a simplex connection by sending the other host a RSA
48			challenge containing the random digest and encryption keys (different)
49			to use when sending packets, plus more randomness plus some PKCS1_OAEP
50			padding plus a random 16 byte id. The destination host will respond by
51			replying with an (unencrypted) RIPEMD160 hash of the decrypted data, which
52			will authentify that host. The destination host will also set the outgoing
53			encryption parameters as given in the packet.
54
55			When the source host receives a correct auth reply (by verifying the
56			hash and the id, which will expire after 20 seconds). it will start to
57			accept data packets from the destination host. The protocol is completely
58			symmetric, so to be able to send packets the destination host must send a
59			challenge in the exact same way as already described.
60
61			=head2 Retrying
62
63			When there is no response to an auth request, the host will send auth
64			requests in bursts with an exponential backoff. After some time it will
65			resort to PING packets, which are very small (8 byte) and lightweight (no
66			RSA operations). A host that receives ping requests from an unconnected
67			peer will respond by trying to create a connection.
68
69			In addition to the exponential backoff, there is a global rate-limit on
70			a per-ip base. It allows long bursts but will limit total packet rate to
71			something like one control packet every ten seconds, to avoid accidental
72			floods due to protocol problems (like a rsa key file mismatch between two
73			hosts).
74
75			=head2 Routing and Protocol translation
76
77	pcg	1.2	The vpe routing algorithm is easy: there isn't any routing. Vped always
78			tries to establish direct connections, if the protocol abilities of the
79			two hosts allow it.
80
81			If the two hosts should be able to reach each other (common protocol, ip
82			and port all known), but cannot (network down), then there will be no
83			connection, point.
84
85			A host can usually declare itself unreachable directly by setting it's
86			port number(s) to zero. It can declare other hosts as unreachable by using
87			a config-file that disables all protocols for these other hosts.
88
89			If two hosts cannot connect to each other because their IP address(es)
90			are not known (such as dialup hosts), one side will send a connection
91			request to a router (routers must be configured to act as routers!), which
92			will send both the originating and the destination host a connection info
93			request with protocol information and IP address of the other host (if
94			known). Both hosts will then try to establish a connection to the other
95			peer, which is usually possible even when both hosts are behind a NAT
96			gateway.
97
98			If the hosts cannot reach each other because they have no common protocol,
99			the originator instead use the router with highest priority and matching
100			protocol as peer. Since the SRCDST field is not encrypted, the router host
101			can just forward the packet to the destination host. Since each host uses
102			it's own private key, the router will not be able to decrypt or encrypt
103			packets, it will just act as a simple router and protocol translator.
104
105			When no router is connected, the host will aggressively try to connect to
106			all routers, and if a router is asked for an unconnected host it will try
107			to ask another router to establish the connection.
108
109			... more not yet written about the details of the routing, please bug me
110			...
111	pcg	1.1