SYNCTHING-BEP(7)

NAME
syncthing-bep - 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 Each device strives to get its folders in sync with the global model by requesting missing or outdated blocks from the other devices in the cluster.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.

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. In this document, in diagrams and text, "bit 0" refers to the most significant bit of a word; "bit 31" is thus the least significant bit of a 32 bit word.

 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 request messages may be outstanding at any given moment. For message types that do not have a corresponding response (Cluster Configuration, Index, etc.) the Message ID field is irrelevant and SHOULD be set to zero.

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=0:

•   The Length field contains the length, in bytes, of the uncompressed message data.

•   The message is not compressed.

    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/.

    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 Device Name                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                 Device Name (variable length)                 \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Length of Client Name                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                 Client Name (variable length)                 \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Length of Client Version                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\               Client Version (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)                      \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                 Label (length + padded data)                  \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       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)                      \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Length of Name                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                    Name (variable length)                     \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Number of Addresses                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Length of Address                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                   Address (variable length)                   \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Compression                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Length of Cert Name                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                  Cert Name (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 (ClusterConfigMessage)

The Device Name is a human readable (configured or auto detected) device name or host name, for the sending device.

The Client Name and Client Version identifies 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 Client Name is "syncthing" and an example Client Version is "v0.7.2". The Client Version field SHOULD follow the patterns laid out in the Semantic Versioning <http://semver.org/> standard.

The Folders field contains the list of folders that will be synchronized over the current connection.

The Options field is a list of options that apply to the current connection. The options are used in an implementation specific manner. The options list is conceptually a map of keys to values, although it is transmitted in the form of a list of key and 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.

  Fields (Folder Structure)

The ID field contains the folder ID, as a human readable string.

The Label field contains the folder label, as human readable name for the folder.

The Devices field is list of devices participating in sharing this folder.

The Flags field contains flags that affect the behavior of the folder. The folder 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                      |D|P|R|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Bit 31 ("R", Read Only)
    is set for folders that the device will accept no updates from the network for.

Bit 30 ("P", Ignore Permissions)
    is set for folders that the device will not accept or announce file permissions for.

Bit 29 ("D", Ignore Deletes)
    is set for folders that the device will ignore deletes for.

    The Options field contains a list of options that apply to the folder.

  Fields (Device Structure)

The device ID field 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.

The Name field is a human readable name assigned to the described device by the sending device. It MAY be empty and it need not be unique.

The list of Addressess is that used by the sending device to connect to the described device.

The Compression field indicates the compression mode in use for this device and folder. The following values are valid:

0   Compress metadata. This enables compression of metadata messages such as Index.

1   Compression disabled. No compression is used on any message.

2   Compress always. Metadata messages as well as Response messages are compressed.

    The Cert Name field indicates the expected certificate name for this device. It is commonly blank, indicating to use the implementation default.

    The 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 Flags field indicates the sharing mode of the folder and other device & folder specific settings. 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 Options field contains a list of options that apply to the device.

  XDR

struct ClusterConfigMessage {
    string DeviceName<64>;
    string ClientName<64>;
    string ClientVersion<64>;
    Folder Folders<1000000>;
    Option Options<64>;
};
struct Folder {
    string ID<256>;
    string Label<256>;
    Device Devices<1000000>;
    unsigned int Flags;
    Option Options<64>;
};
struct Device {
    opaque ID<32>;
    string Name<64>;
    string Addresses<64>;
    unsigned int Compression;
    string CertName<64>;
    hyper MaxLocalVersion;
    unsigned int Flags;
    Option Options<64>;
};
struct Option {
    string Key<64>;
    string Value<1024>;
};

  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)                       +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                   Version (variable length)                   \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                    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 (Index Message)

The Folder field identifies the folder that the index message pertains to.

Files

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.

  Fields (FileInfo Structure)

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 Flags field 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|D|I|R|   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 ("R")
    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 ("D")
    is set when the item represents a directory. The block list SHALL be of length zero.

Bit 16 ("P")
    is set when there is no permission information for the file. This is the case when it originates on a file system which does not support permissions. 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 15 ("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 14 ("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 13
    are reserved for future use and SHALL be set to zero.

    The Modified time is expressed as the number of seconds since the Unix Epoch (1970-01-01 00:00:00 UTC).

    The Version field is a version vector describing the updates performed to a 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 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 hash algorithm is implied by the Hash length. Currently, the hash MUST be 32 bytes long and computed by SHA256.

  XDR

struct IndexMessage {
    string Folder<256>;
    FileInfo Files<1000000>;
    unsigned int Flags;
    Option Options<64>;
};
struct FileInfo {
    string Name<8192>;
    unsigned int Flags;
    hyper Modified;
    Vector Version;
    hyper LocalVersion;
    BlockInfo Blocks<10000000>;
};
struct Vector {
    Counter Counters<>;
};
struct Counter {
    unsigned hyper ID;
    unsigned hyper Value;
};
struct BlockInfo {
    unsigned int Size;
    opaque Hash<64>;
};

  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. A Ping message is sent every 90 seconds, if no other message has been sent in the preceding 90 seconds.

  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 apply any updates from the cluster, but publishes changes of its local folder to the cluster 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 | 512 MiB
Index and Index Update Messages
 | Folder | 64 bytes
 | Number of Files | 1.000.000
 | Name | 8192 bytes
 | Number of Blocks | 10.000.000
 | 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

The currently defined values allow maximum file size of 1220 GiB (10.000.000 x 128 KiB). The maximum message size covers an Index message for the maximum file.

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 5 through 8 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 (9 through 12). Device A updates their folder contents and transmits an Index Update message (13). Both peers enter idle state after 13. At some later time 14, the ping timer on device B expires and a Ping message is sent. The same process occurs for device A at 15.

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

AUTHOR
The Syncthing Authors

COPYRIGHT
2015, The Syncthing Authors