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Jakob Borg
2015-09-22 23:30:39 +02:00
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_static/syncthing.css
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index.rst
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Introduction <dev/intro>
dev/*
.. toctree::
:caption: Specifications
:maxdepth: 1
:glob:
specs/*
.. _`contribution guidelines`: https://github.com/syncthing/syncthing/blob/master/CONTRIBUTING.md
.. _Github: https://github.com/syncthing/docs
.. _`issue tracker`: https://github.com/syncthing/syncthing/issues
specs/bep-v1.rst
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Block Exchange Protocol v1
==========================
Introduction and Definitions
----------------------------
BEP is used between two or more *devices* thus forming a *cluster*. Each
device has one or more *folders* of files described by the *local
model*, containing metadata and block hashes. The local model is sent to
the other devices in the cluster. The union of all files in the local
models, with files selected for highest change version, forms the
*global model*. Each device strives to get its folders in sync with the
global model by requesting missing or outdated blocks from the other
devices in the cluster.
File data is described and transferred in units of *blocks*, each being
128 KiB (131072 bytes) in size.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
Transport and Authentication
----------------------------
BEP is deployed as the highest level in a protocol stack, with the lower
level protocols providing encryption and authentication.
::
+-----------------------------|
| Block Exchange Protocol |
|-----------------------------|
| Encryption & Auth (TLS 1.2) |
|-----------------------------|
| TCP |
|-----------------------------|
v ... v
The encryption and authentication layer SHALL use TLS 1.2 or a higher
revision. A strong cipher suite SHALL be used, with "strong cipher
suite" being defined as being without known weaknesses and providing
Perfect Forward Secrecy (PFS). Examples of strong cipher suites are
given at the end of this document. This is not to be taken as an
exhaustive list of allowed cipher suites but represents best practices
at the time of writing.
The exact nature of the authentication is up to the application, however
it SHALL be based on the TLS certificate presented at the start of the
connection. Possibilities include certificates signed by a common
trusted CA, preshared certificates, preshared certificate fingerprints
or certificate pinning combined with some out of band first
verification. The reference implementation uses preshared certificate
fingerprints (SHA-256) referred to as "Device IDs".
There is no required order or synchronization among BEP messages except
as noted per message type - any message type may be sent at any time and
the sender need not await a response to one message before sending
another. Responses MUST however be sent in the same order as the
requests are received.
The underlying transport protocol MUST be TCP.
Messages
--------
Every message starts with one 32 bit word indicating the message
version, type and ID, followed by the length of the message. The header
is in network byte order, i.e. big endian.
::
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ver | Message ID | Type | Reserved |C|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For BEP v1 the Version field is set to zero. Future versions with
incompatible message formats will increment the Version field. A message
with an unknown version is a protocol error and MUST result in the
connection being terminated. A client supporting multiple versions MAY
retry with a different protocol version upon disconnection.
The Message ID is set to a unique value for each transmitted request
message. In response messages it is set to the Message ID of the
corresponding request message. The uniqueness requirement implies that
no more than 4096 messages may be outstanding at any given moment. The
ordering requirement implies that a response to a given message ID also
means that all preceding messages have been received, specifically those
which do not otherwise demand a response. Hence their message ID:s may
be reused.
The Type field indicates the type of data following the message header
and is one of the integers defined below. A message of an unknown type
is a protocol error and MUST result in the connection being terminated.
The Compression bit "C" indicates the compression used for the message.
For C=1:
- The Length field contains the length, in bytes, of the compressed
message data plus a four byte uncompressed length field.
- The compressed message data is preceeded by a 32 bit field denoting
the length of the uncompressed message.
- The message data is compressed using the LZ4 format and algorithm
described in http://www.lz4.org/.
For C=0:
- The Length field contains the length, in bytes, of the uncompressed
message data.
- The message is not compressed.
All data within the message (post decompression, if compression is in
use) MUST be in XDR (RFC 1014) encoding. All fields shorter than 32 bits
and all variable length data MUST be padded to a multiple of 32 bits.
The actual data types in use by BEP, in XDR naming convention, are the
following:
:(unsigned) int: (unsigned) 32 bit integer
:(unsigned) hyper: (unsigned) 64 bit integer
:opaque<>: variable length opaque data
:string<>: variable length string
The transmitted length of string and opaque data is the length of actual
data, excluding any added padding. The encoding of opaque<> and string<>
are identical, the distinction being solely one of interpretation.
Opaque data should not be interpreted but can be compared bytewise to
other opaque data. All strings MUST use the Unicode UTF-8 encoding,
normalization form C.
Cluster Config (Type = 0)
~~~~~~~~~~~~~~~~~~~~~~~~~
This informational message provides information about the cluster
configuration as it pertains to the current connection. A Cluster Config
message MUST be the first message sent on a BEP connection. Additional
Cluster Config messages MUST NOT be sent after the initial exchange.
Graphical Representation
^^^^^^^^^^^^^^^^^^^^^^^^
::
ClusterConfigMessage 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 ClientName |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ ClientName (variable length) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of ClientVersion |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ ClientVersion (variable length) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Folders |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Zero or more Folder Structures \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Zero or more Option Structures \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Folder 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 Devices |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Zero or more Device Structures \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Zero or more Option 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) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Max Local Version (64 bits) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Zero or more Option Structures \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option 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 Key |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Key (variable length) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Value (variable length) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields
^^^^^^
The ClientName and ClientVersion fields identify the implementation. The
values SHOULD be simple strings identifying the implementation name, as
a user would expect to see it, and the version string in the same
manner. An example ClientName is "syncthing" and an example
ClientVersion is "v0.7.2". The ClientVersion field SHOULD follow the
patterns laid out in the `Semantic Versioning <http://semver.org/>`__
standard.
The Folders field lists all folders that will be synchronized over the
current connection. Each folder has a list of participating Devices,
Flags and Options. Currently no flags are defined so the field MUST be
set to all zeroes. The Options field is implementation specific and
described below.
The Device ID is a 32 byte number that uniquely identifies the device.
For instance, the reference implementation uses the SHA-256 of the
device X.509 certificate.
Each device has an associated Flags field to indicate the sharing mode
of that device for the folder in question. See the discussion on Sharing
Modes. The Device Flags field contains the following single bit flags:
::
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Pri| Reserved |I|R|T|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
:Bit 31 ("T", Trusted): is set for devices that participate in trusted
mode.
:Bit 30 ("R", Read Only): is set for devices that participate in read
only mode.
:Bit 29 ("I", Introducer): is set for devices that are trusted as
cluster introducers.
:Bits 16 through 28: are reserved and MUST be set to zero.
:Bits 14-15 ("Pri): indicate the device's upload priority for this
folder. Possible values are:
:00: The default. Normal priority.
:01: High priority. Other devices SHOULD favour requesting files
from this device over devices with normal or low priority.
:10: Low priority. Other devices SHOULD avoid requesting files from
this device when they are available from other devices.
:11: Sharing disabled. Other devices SHOULD NOT request files from
this device.
:Bits 0 through 14: are reserved and MUST be set to zero.
Exactly one of the T and R bits MUST be set.
The per device Max Local Version field contains the highest local file
version number of the files already known to be in the index sent by
this device. If nothing is known about the index of a given device, this
field MUST be set to zero. When receiving a Cluster Config message with
a non-zero Max Local Version for the local device ID, a device MAY elect
to send an Index Update message containing only files with higher local
version numbers in place of the initial Index message.
The Options field contain option values to be used in an implementation
specific manner. The options list is conceptually a map of Key => Value
items, although it is transmitted in the form of a list of (Key, Value)
pairs, both of string type. Key ID:s are implementation specific. An
implementation MUST ignore unknown keys. An implementation MAY impose
limits on the length keys and values. The options list may be used to
inform devices of relevant local configuration options such as rate
limiting or make recommendations about request parallelism, device
priorities, etc. An empty options list is valid for devices not having
any such information to share. Devices MAY NOT make any assumptions
about peers acting in a specific manner as a result of sent options.
XDR
^^^
::
struct ClusterConfigMessage {
string ClientName<>;
string ClientVersion<>;
Folder Folders<>;
Option Options<>;
}
struct Folder {
string ID<64>;
Device Devices<>;
unsigned int Flags;
Option Options<64>;
}
struct Device {
opaque ID<32>;
hyper MaxLocalVersion;
unsigned int Flags;
Option Options<64>;
}
struct Option {
string Key<>;
string Value<>;
}
Index (Type = 1) and Index Update (Type = 6)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The Index and Index Update messages define the contents of the senders
folder. An Index message represents the full contents of the folder and
thus supersedes any previous index. An Index Update amends an existing
index with new information, not affecting any entries not included in
the message. An Index Update MAY NOT be sent unless preceded by an
Index, unless a non-zero Max Local Version has been announced for the
given folder by the peer device.
Graphical Representation
^^^^^^^^^^^^^^^^^^^^^^^^
::
IndexMessage 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 Folder |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Folder (variable length) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Files |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Zero or more FileInfo Structures \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Zero or more Option Structures \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FileInfo 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 Name |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Name (variable length) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Modified (64 bits) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Vector Structure \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Local Version (64 bits) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Blocks |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Zero or more BlockInfo Structures \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Vector 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Counters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Zero or more Counter Structures \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Counter 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ ID (64 bits) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Value (64 bits) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
BlockInfo 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of Hash |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Hash (variable length) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields
^^^^^^
The Folder field identifies the folder that the index message pertains
to. For single folder implementations the device MUST use the string
"default".
The Name is the file name path relative to the folder root. Like all
strings in BEP, the Name is always in UTF-8 NFC regardless of operating
system or file system specific conventions. The Name field uses the
slash character ("/") as path separator, regardless of the
implementation's operating system conventions. The combination of Folder
and Name uniquely identifies each file in a cluster.
The Version field is a version vector describing the updates performed
to file by all members in the cluster. Each counter in the version
vector is an ID-Value tuple. The ID is used the first 64 bits of the
device ID. The Value is a simple incrementing counter, starting at zero.
The combination of Folder, Name and Version uniquely identifies the
contents of a file at a given point in time.
The Local Version field is the value of a device local monotonic clock
at the time of last local database update to a file. The clock ticks on
every local database update.
The Flags field (per FileInfo) is made up of the following single bit
flags:
::
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |U|S|P|I|D| Unix Perm. & Mode |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
:The lower 12 bits: hold the common Unix permission and mode bits. An
implementation MAY ignore or interpret these as is suitable on the
host operating system.
:Bit 19 ("D"): is set when the file has been deleted. The block list
SHALL be of length zero and the modification time indicates the time
of deletion or, if the time of deletion is not reliably determinable,
the last known modification time.
:Bit 18 ("I"): is set when the file is invalid and unavailable for
synchronization. A peer MAY set this bit to indicate that it can
temporarily not serve data for the file.
:Bit 17 ("P"): is set when there is no permission information for the
file. This is the case when it originates on a non-permission-
supporting file system. Changes to only permission bits SHOULD be
disregarded on files with this bit set. The permissions bits MUST be
set to the octal value 0666.
:Bit 16 ("S"): is set when the file is a symbolic link. The block list
SHALL be of one or more blocks since the target of the symlink is
stored within the blocks of the file.
:Bit 15 ("U"): is set when the symbolic links target does not exist. On
systems where symbolic links have types, this bit being means that
the default file symlink SHALL be used. If this bit is unset bit 19
will decide the type of symlink to be created.
:Bit 0 through 14: are reserved for future use and SHALL be set to
zero.
The hash algorithm is implied by the Hash length. Currently, the hash
MUST be 32 bytes long and computed by SHA256.
The Modified time is expressed as the number of seconds since the Unix
Epoch (1970-01-01 00:00:00 UTC).
In the rare occasion that a file is simultaneously and independently
modified by two devices in the same cluster and thus end up on the same
Version number after modification, the Modified field is used as a tie
breaker (higher being better), followed by the hash values of the file
blocks (lower being better).
The Blocks list contains the size and hash for each block in the file.
Each block represents a 128 KiB slice of the file, except for the last
block which may represent a smaller amount of data.
The Flags field (in IndexMessage) is reserved for future use and MUST
currently be set to zero. The Options list is implementation defined and
as described in the ClusterConfig message section.
XDR
^^^
::
struct IndexMessage {
string Folder<>;
FileInfo Files<>;
unsigned int Flags;
Option Options<64>;
}
struct FileInfo {
string Name<8192>;
unsigned int Flags;
hyper Modified;
Vector Version;
hyper LocalVersion;
BlockInfo Blocks<>;
}
struct Vector {
Counter Counters<>
}
struct Counter {
unsigned hyper ID
unsigned hyper Value
}
struct BlockInfo {
unsigned int Size;
opaque Hash<>;
}
Request (Type = 2)
~~~~~~~~~~~~~~~~~~
The Request message expresses the desire to receive a data block
corresponding to a part of a certain file in the peer's folder.
Graphical Representation
^^^^^^^^^^^^^^^^^^^^^^^^
::
RequestMessage 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 Folder |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Folder (variable length) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of Name |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Name (variable length) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Offset (64 bits) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length of Hash |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Hash (variable length) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Zero or more Option Structures \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields
^^^^^^
The Folder and Name fields are as documented for the Index message. The
Offset and Size fields specify the region of the file to be transferred.
This SHOULD equate to exactly one block as seen in an Index message.
The Hash field MAY be set to the expected hash value of the block, or
may be left empty (zero length). If set, the other device SHOULD ensure
that the transmitted block matches the requested hash. The other device
MAY reuse a block from a different file and offset having the same size
and hash, if one exists.
The Flags field is reserved for future use and MUST currently be set to
zero. The Options list is implementation defined and as described in the
ClusterConfig message section.
XDR
^^^
::
struct RequestMessage {
string Folder<64>;
string Name<8192>;
hyper Offset;
int Size;
opaque Hash<64>;
unsigned int Flags;
Option Options<64>;
}
Response (Type = 3)
~~~~~~~~~~~~~~~~~~~
The Response message is sent in response to a Request message.
Graphical Representation
^^^^^^^^^^^^^^^^^^^^^^^^
ResponseMessage 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 Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Data (variable length) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields
^^^^^^
The Data field contains either a full 128 KiB block, a shorter block in
the case of the last block in a file, or is empty (zero length) if the
requested block is not available.
The Code field contains an error code describing the reason a Request
could not be fulfilled, in the case where a zero length Data was
returned. The following values are defined:
:0: No Error (Data should be present)
:1: Generic Error
:2: No Such File (the requested file does not exist, or the offset is
outside the acceptable range for the file)
:3: Invalid (file exists but has invalid bit set or is otherwise
unavailable)
XDR
^^^
::
struct ResponseMessage {
opaque Data<>;
int Code;
}
Ping (Type = 4)
~~~~~~~~~~~~~~~
The Ping message is used to determine that a connection is alive, and to
keep connections alive through state tracking network elements such as
firewalls and NAT gateways. The Ping message has no contents.
Pong (Type = 5)
~~~~~~~~~~~~~~~
The Pong message is sent in response to a Ping. The Pong message has no
contents, but copies the Message ID from the Ping.
Close (Type = 7)
~~~~~~~~~~~~~~~~
The Close message MAY be sent to indicate that the connection will be
torn down due to an error condition. A Close message MUST NOT be
followed by further messages.
Graphical Representation
^^^^^^^^^^^^^^^^^^^^^^^^
::
CloseMessage 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 Reason |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Reason (variable length) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields
^^^^^^
The Reason field contains a human description of the error condition,
suitable for consumption by a human. The Code field is for a machine
readable error code. Codes are reserved for future use and MUST
currently be set to zero.
::
struct CloseMessage {
string Reason<1024>;
int Code;
}
Sharing Modes
-------------
Trusted
~~~~~~~
Trusted mode is the default sharing mode. Updates are exchanged in both
directions.
::
+------------+ Updates /---------\
| | -----------> / \
| Device | | Cluster |
| | <----------- \ /
+------------+ Updates \---------/
Read Only
~~~~~~~~~
In read only mode, a device does not synchronize the local folder to the
cluster, but publishes changes to its local folder contents as usual.
The local folder can be seen as a "master copy" that is never affected
by the actions of other cluster devices.
::
+------------+ Updates /---------\
| | -----------> / \
| Device | | Cluster |
| | \ /
+------------+ \---------/
Message Limits
--------------
An implementation MAY impose reasonable limits on the length of messages
and message fields to aid robustness in the face of corruption or broken
implementations. These limits, if imposed, SHOULD NOT be more
restrictive than the following. An implementation should strive to keep
messages short and to the point, favouring more and smaller messages
over fewer and larger. For example, favour a smaller Index message
followed by one or more Index Update messages rather than sending a very
large Index message.
=================== =================== =============
Message Type Field Limit
=================== =================== =============
**All Messages**
-----------------------------------------------------
| Total length 64 MiB
**Index and Index Update Messages**
-----------------------------------------------------
| Folder 64 bytes
| Number of Files 1.000.000
| Name 8192 bytes
| Number of Blocks 1.000.000
| Hash 64 bytes
| Number of Counters 1.000.000
**Request Messages**
-----------------------------------------------------
| Folder 64 bytes
| Name 8192 bytes
**Response Messages**
-----------------------------------------------------
| Data 256 KiB
**Cluster Config Message**
-----------------------------------------------------
| Number of Folders 1.000.000
| Number of Devices 1.000.000
| Number of Options 64
| Key 64 bytes
| Value 1024 bytes
=================== =================== =============
Example Exchange
----------------
=== ======================= ======================
# A B
=== ======================= ======================
1 ClusterConfiguration-> <-ClusterConfiguration
2 Index-> <-Index
3 IndexUpdate-> <-IndexUpdate
4 IndexUpdate->
5 Request->
6 Request->
7 Request->
8 Request->
9 <-Response
10 <-Response
11 <-Response
12 <-Response
13 Index Update->
...
14 <-Ping
15 Ping->
=== ======================= ======================
The connection is established and at 1. both peers send
ClusterConfiguration messages and then Index records. The Index records
are received and both peers recompute their knowledge of the data in the
cluster. In this example, peer A has four missing or outdated blocks. At
2 through 5 peer A sends requests for these blocks. The requests are
received by peer B, who retrieves the data from the folder and transmits
Response records (6 through 9). Device A updates their folder contents
and transmits an Index Update message (10). Both peers enter idle state
after 10. At some later time 11, peer A determines that it has not seen
data from B for some time and sends a Ping request. A response is sent
at 12.
Examples of Strong Cipher Suites
--------------------------------
====== =========================== ==================================
ID Name Description
====== =========================== ==================================
0x009F DHE-RSA-AES256-GCM-SHA384 TLSv1.2 DH RSA AESGCM(256) AEAD
0x006B DHE-RSA-AES256-SHA256 TLSv1.2 DH RSA AES(256) SHA256
0xC030 ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 ECDH RSA AESGCM(256) AEAD
0xC028 ECDHE-RSA-AES256-SHA384 TLSv1.2 ECDH RSA AES(256) SHA384
0x009E DHE-RSA-AES128-GCM-SHA256 TLSv1.2 DH RSA AESGCM(128) AEAD
0x0067 DHE-RSA-AES128-SHA256 TLSv1.2 DH RSA AES(128) SHA256
0xC02F ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 ECDH RSA AESGCM(128) AEAD
0xC027 ECDHE-RSA-AES128-SHA256 TLSv1.2 ECDH RSA AES(128) SHA256
====== =========================== ==================================
specs/localdisco-v3.rst
+145
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Local Discovery Protocol v3
===========================
Mode of Operation
-----------------
Each participating device periodically sends an Announcement packet. It also
keeps a table of the announcements it has seen. There is no way to solicit a
reply; the only message type is Announcement.
On multihomed hosts the announcement packets should be sent on each interface
on which Syncthing will accept connections.
For IPv4, the Announcement packet is broadcasted either to the link-specific
broadcast address, or to the generic link-local broadcast address
``255.255.255.255``, with destination port 21027.
For IPv6, the Announcement packet is multicasted to the transient link-local
multicast address ``[ff12::8384]``, with destination port 21027.
It is recommended that local discovery Announcement packets be sent on a 30 to
60 second interval, possibly with immediate transmissions when a previously
unknown device is discovered.
Device ID
---------
The device ID is the SHA-256 (32 bytes) of the device X.509 certificate. See
:ref:`device-ids` in the Syncthing documentation.
Announcement packet
-------------------
The Announcement packet has the following structure::
Announce 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Device Structure \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Extra |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ 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 \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of Relays |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ Zero or more Relay 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 URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ URL (variable length) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Relay 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 URL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
\ URL (variable length) \
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Latency |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The corresponding XDR representation is as follows (see
`RFC4506 <http://tools.ietf.org/html/rfc4506>`__ for the XDR format):
::
struct Announcement {
unsigned int Magic;
Device This;
Device Extra<>;
}
struct Device {
opaque ID<32>;
Address Addresses<16>;
Relay Relays<16>;
}
struct Address {
string URL<2083>;
}
struct Relay {
string URL<2083>;
int Latency;
}
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 ``Device`` structure, field ``DeviceID`` is the SHA-256 (32
bytes) of the device X.509 certificate, as explained in section *Device
ID*.
For each ``Address`` and ``Relay`` the ``URL`` field contains the actual
target address. For direction connections (the ``Address`` list) these will
typically have the ``tcp://`` scheme. Relay connections will typically use the
``relay://`` scheme.
The ``Latency`` field contains the approximate latency for a TCP handshake
(i.e. three packet round trips) between the other device and the relay, in
milliseconds.