syncthing/protocol/PROTOCOL.md

Block Exchange Protocol v1

Introduction and Definitions

BEP is used between two or more nodes thus forming a cluster. Each node has one or more repositories of files described by the local model, containing metadata and block hashes. The local model is sent to the other nodes in the cluster. The union of all files in the local models, with files selected for highest change version, forms the global model. Each node strives to get it's repositories in sync with the global model by requesting missing or outdated blocks from the other nodes in the cluster.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 compression, encryption and authentication.

+-----------------------------|
|   Block Exchange Protocol   |
|-----------------------------|
|   Compression (RFC 1951)    |
|-----------------------------|
| Encryption & Auth (TLS 1.2) |
|-----------------------------|
|             TCP             |
|-----------------------------|
v             ...             v

Compression is started directly after a successfull TLS handshake, before the first message is sent. The compression is flushed at each message boundary. Compression SHALL use the DEFLATE format as specified in RFC 1951.

The encryption and authentication layer SHALL use TLS 1.2 or a higher revision. A strong cipher suite SHALL be used, with "string 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 "Node IDs".

There is no required order or synchronization among BEP messages - 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.

 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  |  Type |       Message ID      |        Reply To       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

For BEP v1 the Version field is set to zero. Future versions with incompatible message formats will increment the Version field.

The Type field indicates the type of data following the message header and is one of the integers defined below.

The Message ID is set to a unique value for each transmitted message. In request messages the Reply To is set to zero. In response messages it is set to the message ID of the corresponding request. 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.

All data following the message header 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.

Index (Type = 1)

The Index message defines the contents of the senders repository. An Index message MUST be sent by each node immediately upon connection. A node with no data to advertise MUST send an empty Index message (a file list of zero length). If the repository contents change from non-empty to empty, an empty Index message MUST be sent. There is no response to the Index message.

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 Repository                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                 Repository (variable length)                  \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Number of Files                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\               Zero or more FileInfo 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 (64 bits)                       +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Number of Blocks                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\               Zero or more BlockInfo Structures               \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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 Repository field identifies the repository that the index message pertains to. For single repository implementations the node MAY send an empty repository ID or use the string "default".

The Name is the file name path relative to the repository 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 Repository and Name uniquely identifies each file in a cluster.

The Version field is the value of a cluster wide Lamport clock indicating when the change was detected. The clock ticks on every detected and received change. The combination of Repository, Name and Version uniquely identifies the contents of a file at a given point in time.

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             |I|D|   Unix Perm. & Mode   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

• The lower 12 bits hold the common Unix permission and mode bits. An implemention 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 0 through 17 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 nodes 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.

XDR

struct IndexMessage {
    string Repository<>;
    FileInfo Files<>;
}

struct FileInfo {
    string Name<>;
    unsigned int Flags;
    hyper Modified;
    unsigned hyper Version;
    BlockInfo Blocks<>;
}

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 repository.

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 Repository                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                 Repository (variable length)                  \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Length of Name                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\                    Name (variable length)                     \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
+                       Offset (64 bits)                        +
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Size                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields

The Repository 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.

XDR

struct RequestMessage {
    string Repository<>;
    string Name<>;
    unsigned hyper Offset;
    unsigned int Size;
}

Response (Type = 3)

The Response message is sent in response to a Request message.

Graphical Representation

 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)                     \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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.

XDR

struct ResponseMessage {
    opaque Data<>
}

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.

Index Update (Type = 6)

This message has exactly the same structure as the Index message. However instead of replacing the contents of the repository in the model, the Index Update merely amends it with new or updated file information. Any files not mentioned in an Index Update are left unchanged.

Options (Type = 7)

This informational message provides information about the client configuration, version, etc. It is sent at connection initiation and, optionally, when any of the sent parameters have changed. The message is in the form of a list of (key, value) pairs, both of string type.

Key ID:s apart from the well known ones are implementation specific. An implementation is expected to ignore unknown keys. An implementation may impose limits on key and value size.

Well known keys:

• "clientId" -- The name of the implementation. Example: "syncthing".

• "clientVersion" -- The version of the client. Example: "v1.0.33-47". The Following the SemVer 2.0 specification for version strings is encouraged but not enforced.

Graphical Representation

 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 Options                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/                                                               /
\               Zero or more KeyValue Structures                \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

KeyValue 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)                    \
/                                                               /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

XDR

struct OptionsMessage {
    KeyValue Options<>;
}

struct KeyValue {
    string Key<>;
    string Value<>;
}

Message Limits

An implementation MAY impose reasonable limits on the length of 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:

Index and Index Update Messages

• Repository: 64 bytes
• Number of Files: 100.000
• Name: 1024 bytes
• Number of Blocks: 100.000
• Hash: 64 bytes

Request Messages

• Repository: 64 bytes
• Name: 1024 bytes

Response Messages

• Data: 256 KiB

Options Message

• Number of Options: 64
• Key: 64 bytes
• Value: 1024 bytes

Example Exchange

      A            B
 1. Index->      <-Index
 2. Request->
 3. Request->
 4. Request->
 5. Request->
 6.         <-Response
 7.         <-Response
 8.         <-Response
 9.         <-Response
10. Index Update->
    ...
11. Ping->
12.            <-Pong

The connection is established and at 1. both peers send 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 repository and transmits Response records (6 through 9). Node A updates their repository 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

• 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)