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tinc/src/protocol.h
Etienne Dechamps b1421b9190 Add MTU_INFO protocol message. 
In this commit, nodes use MTU_INFO messages to provide MTU information.

The issue this code is meant to address is the non-trivial problem of
finding the proper MTU when UDP SPTPS relays are involved. Currently,
tinc has no idea what the MTU looks like beyond the first relay, and
will arbitrarily use the first relay's MTU as the limit. This will fail
miserably if the MTU decreases after the first relay, forcing relays to
fall back to TCP. More generally, one should keep in mind that relay
paths can be arbitrarily complex, resulting in packets taking "epic
journeys" through the graph, switching back and forth between UDP (with
variable MTUs) and TCP multiple times along the path.

A solution that was considered consists in sending standard MTU probes
through the relays. This is inefficient (if there are 3 nodes on one
side of relay and 3 nodes on the other side, we end up with 3*3=9 MTU
discoveries taking place at the same time, while technically only
3+3=6 are needed) and would involve eyebrow-raising behaviors such as
probes being sent over TCP.

This commit implements an alternative solution, which consists in
the packet receiver sending MTU_INFO messages to the packet sender.
The message contains an MTU value which is set to maximum when the
message is originally sent. The message gets altered as it travels
through the metagraph, such that when the message arrives to the
destination, the MTU value contained in the message can be used to
send packets while making sure no relays will be forced to fall back to
TCP to deliver them.

The operating principles behind such a protocol message are similar to
how the UDP_INFO message works, but there is a key difference that
prevents us from simply reusing the same message: the UDP_INFO message
only cares about relay-to-relay links (i.e. it is sent between static
relays and the information it contains only makes sense between two
adjacent static relays), while the MTU_INFO cares about the end-to-end
MTU, including the entire relay path. Therefore, UDP_INFO messages stop
when they encounter static relays, while MTU_INFO messages don't stop
until they get to the original packet sender.

Note that, technically, the MTU that is obtained through this mechanism
can be slightly pessimistic, because it can be lowered by an
intermediate node that is not being used as a relay. Since nodes have no
way of knowing whether they'll be used as dynamic relays or not (and
have no say in the matter), this is not a trivial problem. That said,
this is highly unlikely to result in noticeable issues in realistic
scenarios.
2015-03-14 13:39:05 +00:00

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/*
protocol.h -- header for protocol.c
Copyright (C) 1999-2005 Ivo Timmermans,
2000-2013 Guus Sliepen <guus@tinc-vpn.org>
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License along
with this program; if not, write to the Free Software Foundation, Inc.,
51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*/
#ifndef __TINC_PROTOCOL_H__
#define __TINC_PROTOCOL_H__
#include "ecdsa.h"
/* Protocol version. Different major versions are incompatible. */
#define PROT_MAJOR 17
#define PROT_MINOR 6 /* Should not exceed 255! */
/* Silly Windows */
#ifdef ERROR
#undef ERROR
#endif
/* Request numbers */
typedef enum request_t {
ALL = -1, /* Guardian for allow_request */
ID = 0, METAKEY, CHALLENGE, CHAL_REPLY, ACK,
STATUS, ERROR, TERMREQ,
PING, PONG,
ADD_SUBNET, DEL_SUBNET,
ADD_EDGE, DEL_EDGE,
KEY_CHANGED, REQ_KEY, ANS_KEY,
PACKET,
/* Tinc 1.1 requests */
CONTROL,
REQ_PUBKEY, ANS_PUBKEY,
REQ_SPTPS,
UDP_INFO, MTU_INFO,
LAST /* Guardian for the highest request number */
} request_t;
typedef struct past_request_t {
const char *request;
time_t firstseen;
} past_request_t;
extern bool tunnelserver;
extern bool strictsubnets;
extern bool experimental;
extern ecdsa_t *invitation_key;
/* Maximum size of strings in a request.
* scanf terminates %2048s with a NUL character,
* but the NUL character can be written after the 2048th non-NUL character.
*/
#define MAX_STRING_SIZE 2049
#define MAX_STRING "%2048s"
#include "edge.h"
#include "net.h"
#include "node.h"
#include "subnet.h"
/* Basic functions */
extern bool send_request(struct connection_t *, const char *, ...) __attribute__ ((__format__(printf, 2, 3)));
extern void forward_request(struct connection_t *, const char *);
extern bool receive_request(struct connection_t *, const char *);
extern void init_requests(void);
extern void exit_requests(void);
extern bool seen_request(const char *);
/* Requests */
extern bool send_id(struct connection_t *);
extern bool send_metakey(struct connection_t *);
extern bool send_metakey_ec(struct connection_t *);
extern bool send_challenge(struct connection_t *);
extern bool send_chal_reply(struct connection_t *);
extern bool send_ack(struct connection_t *);
extern bool send_status(struct connection_t *, int, const char *);
extern bool send_error(struct connection_t *, int, const char *);
extern bool send_termreq(struct connection_t *);
extern bool send_ping(struct connection_t *);
extern bool send_pong(struct connection_t *);
extern bool send_add_subnet(struct connection_t *, const struct subnet_t *);
extern bool send_del_subnet(struct connection_t *, const struct subnet_t *);
extern bool send_add_edge(struct connection_t *, const struct edge_t *);
extern bool send_del_edge(struct connection_t *, const struct edge_t *);
extern void send_key_changed(void);
extern bool send_req_key(struct node_t *);
extern bool send_ans_key(struct node_t *);
extern bool send_tcppacket(struct connection_t *, const struct vpn_packet_t *);
extern bool send_udp_info(struct node_t *, struct node_t *);
extern bool send_mtu_info(struct node_t *, struct node_t *, int);
/* Request handlers */
extern bool id_h(struct connection_t *, const char *);
extern bool metakey_h(struct connection_t *, const char *);
extern bool challenge_h(struct connection_t *, const char *);
extern bool chal_reply_h(struct connection_t *, const char *);
extern bool ack_h(struct connection_t *, const char *);
extern bool status_h(struct connection_t *, const char *);
extern bool error_h(struct connection_t *, const char *);
extern bool termreq_h(struct connection_t *, const char *);
extern bool ping_h(struct connection_t *, const char *);
extern bool pong_h(struct connection_t *, const char *);
extern bool add_subnet_h(struct connection_t *, const char *);
extern bool del_subnet_h(struct connection_t *, const char *);
extern bool add_edge_h(struct connection_t *, const char *);
extern bool del_edge_h(struct connection_t *, const char *);
extern bool key_changed_h(struct connection_t *, const char *);
extern bool req_key_h(struct connection_t *, const char *);
extern bool ans_key_h(struct connection_t *, const char *);
extern bool tcppacket_h(struct connection_t *, const char *);
extern bool control_h(struct connection_t *, const char *);
extern bool udp_info_h(struct connection_t *, const char *);
extern bool mtu_info_h(struct connection_t *, const char *);
#endif /* __TINC_PROTOCOL_H__ */
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