syncthing/protocol/DISCOVERY.md

Device Discovery Protocol v2

Mode of Operation

There are two distinct modes: "local discovery", performed on a LAN segment (broadcast domain) and "global discovery" performed over the Internet in general with the support of a well known server.

Local discovery does not use Query packets. Instead Announcement packets are sent periodically and each participating device keeps a table of the announcements it has seen. On multihomed hosts the announcement packets should be sent on each interface that syncthing will accept connections.

It is recommended that local discovery Announcement packets are sent on a 30 to 60 second interval, possibly with forced transmissions when a previously unknown device is discovered.

Global discovery is made possible by periodically updating a global server using Announcement packets indentical to those transmitted for local discovery. The device performing discovery will transmit a Query packet to the global server and expect an Announcement packet in response. In case the global server has no knowledge of the queried device ID, there will be no response. A timeout is to be used to determine lookup failure.

There is no message to unregister from the global server; instead registrations are forgotten after 60 minutes. It is recommended to send Announcement packets to the global server on a 30 minute interval.

Packet Formats

The Announcement packet has the following structure:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Magic (0x9D79BC39)                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                       Device Structure                        \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Number of Extra Devices                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                Zero or more Device Structures                 \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Device Structure:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Length of ID                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                     ID (variable length)                      \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Number of Addresses                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                Zero or more Address Structures                \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Address Structure:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Length of IP                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                     IP (variable length)                      \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             Port              |            0x0000             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

This is the XDR encoding of:

struct Announcement {
    unsigned int Magic;
    Device This;
    Device Extra<>;
}

struct Device {
    string ID<>;
    Address Addresses<>;
}

struct Address {
    opaque IP<>;
    unsigned short Port;
}

The first Device structure contains information about the sending device. The following zero or more Extra devices contain information about other devices known to the sending device.

In the Address structure, the IP field can be of three differnt kinds;

• A zero length indicates that the IP address should be taken from the source address of the announcement packet, be it IPv4 or IPv6. The source address must be a valid unicast address. This is only valid in the first device structure, not in the list of extras.

• A four byte length indicates that the address is an IPv4 unicast address.

• A sixteen byte length indicates that the address is an IPv6 unicast address.

The Query packet has the following structure:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Magic Number (0x2CA856F5)                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Length of Device ID                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                  Device ID (variable length)                  \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

This is the XDR encoding of:

struct Announcement {
    unsigned int MagicNumber;
    string DeviceID<>;
}