gvpe/doc/gvpe.protocol.7.pod

=head1 The GNU-VPE Protocols

=head1 Overview

GVPE can make use of a number of protocols. One of them is the GNU VPE
protocol which is used to authenticate tunnels and send encrypted data
packets. This protocol is described in more detail the second part of this
document.

The first part of this document describes the transport protocols which
are used by GVPE to send it's data packets over the network.

=head1 PART 1: Tansport protocols

=head2 RAW IP

=head2 ICMP

=head2 UDP

=head2 TCP

=head2 DNS

=head1 PART 2: The GNU VPE protocol

This section, unfortunately, is not yet finished, although the protocol
is stable (until bugs in the cryptography are found, which will likely
completely change the following description). Nevertheless, it should give
you some overview over the 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
transort 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 auth
request packets), in which case it is set to all zeroes. The checksum
itself is calculated 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 must connect to each other once on startup. But if
restarts are rare or tolerable and most connections are on demand, much
larger networks are feasible.

The DATA portion differs between each packet type, naturally, and is the
only part that can be 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 31 bit value. VPE currently uses
a sliding window of 512 packets/sequence numbers to detect reordering,
duplication and reply attacks.

=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 encrypted challenge containing a random challenge (consisting of
the encryption key to use when sending packets, more random data and
PKCS1_OAEP padding) and a random 16 byte "challenge-id" (used to detect
duplicate auth packets). The destination host will respond by replying
with an (unencrypted) RIPEMD160 hash of the decrypted challenge, 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 120 seconds), it will start to
accept data packets from the destination host.

This means that a host can only initate a simplex connection, telling the
other side the key it has to use when it sends packets. The challenge
reply is only used to set the current IP address of the other side and
protocol parameters.

This 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 (so, in essence, two simplex connections are created per host
pair).

=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 bytes) and lightweight
(no RSA operations required). 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 gvpe routing algorithm is easy: there isn't any routing. GVPE 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 Mar 15 19:23:33 2005 UTC (19 years, 2 months ago) by pcg
Branch:	MAIN
Changes since 1.1:	+48 -18 lines
Log Message:	* empty log message *
#	User	Rev	Content
1	pcg	1.2	=head1 The GNU-VPE Protocols
2
3			=head1 Overview
4
5			GVPE can make use of a number of protocols. One of them is the GNU VPE
6			protocol which is used to authenticate tunnels and send encrypted data
7			packets. This protocol is described in more detail the second part of this
8			document.
9
10			The first part of this document describes the transport protocols which
11			are used by GVPE to send it's data packets over the network.
12
13			=head1 PART 1: Tansport protocols
14
15			=head2 RAW IP
16
17			=head2 ICMP
18
19			=head2 UDP
20
21			=head2 TCP
22
23			=head2 DNS
24
25			=head1 PART 2: The GNU VPE protocol
26
27			This section, unfortunately, is not yet finished, although the protocol
28			is stable (until bugs in the cryptography are found, which will likely
29			completely change the following description). Nevertheless, it should give
30			you some overview over the protocol.
31	pcg	1.1
32			=head2 Anatomy of a VPN packet
33
34			The exact layout and field lengths of a VPN packet is determined at
35			compiletime and doesn't change. The same structure is used for all
36	pcg	1.2	transort protocols, be it RAWIP or TCP.
37	pcg	1.1
38			+------+------+--------+------+
39			\| HMAC \| TYPE \| SRCDST \| DATA \|
40			+------+------+--------+------+
41
42			The HMAC field is present in all packets, even if not used (e.g. in auth
43			request packets), in which case it is set to all zeroes. The checksum
44	pcg	1.2	itself is calculated over the TYPE, SRCDST and DATA fields in all cases.
45	pcg	1.1
46			The TYPE field is a single byte and determines the purpose of the packet
47			(e.g. RESET, COMPRESSED/UNCOMPRESSED DATA, PING, AUTH REQUEST/RESPONSE,
48			CONNECT REQUEST/INFO etc.).
49
50			SRCDST is a three byte field which contains the source and destination
51			node ids (12 bits each). The protocol does not yet scale well beyond 30+
52	pcg	1.2	hosts, since all hosts must connect to each other once on startup. But if
53			restarts are rare or tolerable and most connections are on demand, much
54			larger networks are feasible.
55	pcg	1.1
56			The DATA portion differs between each packet type, naturally, and is the
57			only part that can be encrypted. Data packets contain more fields, as
58			shown:
59
60			+------+------+--------+------+-------+------+
61			\| HMAC \| TYPE \| SRCDST \| RAND \| SEQNO \| DATA \|
62			+------+------+--------+------+-------+------+
63
64			RAND is a sequence of fully random bytes, used to increase the entropy of
65			the data for encryption purposes.
66
67			SEQNO is a 32-bit sequence number. It is negotiated at every connection
68			initialization and starts at some random 31 bit value. VPE currently uses
69	pcg	1.2	a sliding window of 512 packets/sequence numbers to detect reordering,
70			duplication and reply attacks.
71	pcg	1.1
72			=head2 The authentification protocol
73
74			Before hosts can exchange packets, they need to establish authenticity of
75			the other side and a key. Every host has a private RSA key and the public
76			RSA keys of all other hosts.
77
78			A host establishes a simplex connection by sending the other host a
79			RSA encrypted challenge containing a random challenge (consisting of
80			the encryption key to use when sending packets, more random data and
81			PKCS1_OAEP padding) and a random 16 byte "challenge-id" (used to detect
82			duplicate auth packets). The destination host will respond by replying
83			with an (unencrypted) RIPEMD160 hash of the decrypted challenge, which
84			will authentify that host. The destination host will also set the outgoing
85			encryption parameters as given in the packet.
86
87			When the source host receives a correct auth reply (by verifying the
88			hash and the id, which will expire after 120 seconds), it will start to
89			accept data packets from the destination host.
90
91			This means that a host can only initate a simplex connection, telling the
92			other side the key it has to use when it sends packets. The challenge
93	pcg	1.2	reply is only used to set the current IP address of the other side and
94			protocol parameters.
95	pcg	1.1
96	pcg	1.2	This protocol is completely symmetric, so to be able to send packets the
97			destination host must send a challenge in the exact same way as already
98			described (so, in essence, two simplex connections are created per host
99			pair).
100	pcg	1.1
101			=head2 Retrying
102
103			When there is no response to an auth request, the host will send auth
104			requests in bursts with an exponential backoff. After some time it will
105	pcg	1.2	resort to PING packets, which are very small (8 bytes) and lightweight
106			(no RSA operations required). A host that receives ping requests from an
107			unconnected peer will respond by trying to create a connection.
108	pcg	1.1
109			In addition to the exponential backoff, there is a global rate-limit on
110	pcg	1.2	a per-IP base. It allows long bursts but will limit total packet rate to
111	pcg	1.1	something like one control packet every ten seconds, to avoid accidental
112	pcg	1.2	floods due to protocol problems (like a RSA key file mismatch between two
113	pcg	1.1	hosts).
114
115			=head2 Routing and Protocol translation
116
117			The gvpe routing algorithm is easy: there isn't any routing. GVPE always
118			tries to establish direct connections, if the protocol abilities of the
119			two hosts allow it.
120
121			If the two hosts should be able to reach each other (common protocol, ip
122			and port all known), but cannot (network down), then there will be no
123			connection, point.
124
125			A host can usually declare itself unreachable directly by setting it's
126			port number(s) to zero. It can declare other hosts as unreachable by using
127			a config-file that disables all protocols for these other hosts.
128
129			If two hosts cannot connect to each other because their IP address(es)
130			are not known (such as dialup hosts), one side will send a connection
131			request to a router (routers must be configured to act as routers!), which
132			will send both the originating and the destination host a connection info
133			request with protocol information and IP address of the other host (if
134			known). Both hosts will then try to establish a connection to the other
135			peer, which is usually possible even when both hosts are behind a NAT
136			gateway.
137
138			If the hosts cannot reach each other because they have no common protocol,
139			the originator instead use the router with highest priority and matching
140			protocol as peer. Since the SRCDST field is not encrypted, the router host
141			can just forward the packet to the destination host. Since each host uses
142			it's own private key, the router will not be able to decrypt or encrypt
143			packets, it will just act as a simple router and protocol translator.
144
145			When no router is connected, the host will aggressively try to connect to
146			all routers, and if a router is asked for an unconnected host it will try
147			to ask another router to establish the connection.
148
149			... more not yet written about the details of the routing, please bug me
150			...
151