413 lines
17 KiB
Markdown
413 lines
17 KiB
Markdown
Block Exchange Protocol v1
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==========================
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Introduction and Definitions
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----------------------------
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BEP is used between two or more _nodes_ thus forming a _cluster_. Each
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node has one or more _repositories_ of files described by the _local
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model_, containing metadata and block hashes. The local model is sent to
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the other nodes in the cluster. The union of all files in the local
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models, with files selected for highest change version, forms the
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_global model_. Each node strives to get it's repositories in sync with
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the global model by requesting missing or outdated blocks from the other
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nodes in the cluster.
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File data is described and transferred in units of _blocks_, each being
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128 KiB (131072 bytes) in size.
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Transport and Authentication
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----------------------------
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BEP itself does not provide retransmissions, compression, encryption nor
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authentication. It is expected that this is performed at lower layers of
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the networking stack. The typical deployment stack is the following:
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+-----------------------------|
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| Block Exchange Protocol |
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|-----------------------------|
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| Compression (RFC 1951) |
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|-----------------------------|
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| Encryption & Auth (TLS 1.0) |
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|-----------------------------|
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| TCP |
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|-----------------------------|
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v v
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The exact nature of the authentication is up to the application.
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Possibilities include certificates signed by a common trusted CA,
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preshared certificates, preshared certificate fingerprints or
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certificate pinning combined with some out of band first verification.
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There is no required order or synchronization among BEP messages - any
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message type may be sent at any time and the sender need not await a
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response to one message before sending another. Responses must however
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be sent in the same order as the requests are received.
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Compression is started directly after a successfull TLS handshake,
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before the first message is sent. The compression is flushed at each
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message boundary.
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Messages
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--------
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Every message starts with one 32 bit word indicating the message
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version, type and ID.
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Ver | Type | Message ID | Reply To |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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For BEP v1 the Version field is set to zero. Future versions with
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incompatible message formats will increment the Version field.
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The Type field indicates the type of data following the message header
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and is one of the integers defined below.
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The Message ID is set to a unique value for each transmitted message. In
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request messages the Reply To is set to zero. In response messages it is
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set to the message ID of the corresponding request.
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All data following the message header is in XDR (RFC 1014) encoding. All
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fields smaller than 32 bits and all variable length data is padded to a
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multiple of 32 bits. The actual data types in use by BEP, in XDR naming
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convention, are:
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- (unsigned) int -- (unsigned) 32 bit integer
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- (unsigned) hyper -- (unsigned) 64 bit integer
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- opaque<> -- variable length opaque data
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- string<> -- variable length string
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The transmitted length of string and opaque data is the length of actual
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data, excluding any added padding. The encoding of opaque<> and string<>
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are identical, the distinction being solely in interpretation. Opaque
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data should not be interpreted but can be compared bytewise to other
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opaque data. All strings use the UTF-8 encoding.
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### Index (Type = 1)
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The Index message defines the contents of the senders repository. An
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Index message is sent by each peer immediately upon connection. A peer
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with no data to advertise (the repository is empty, or it is set to only
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import data) is allowed but not required to send an empty Index message
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(a file list of zero length). If the repository contents change from
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non-empty to empty, an empty Index message must be sent. There is no
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response to the Index message.
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#### Graphical Representation
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IndexMessage Structure:
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Length of Repository |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ Repository (variable length) \
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/ /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Number of Files |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ Zero or more FileInfo Structures \
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/ /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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FileInfo Structure:
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Length of Name |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ Name (variable length) \
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/ /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Flags |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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+ Modified (64 bits) +
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Version |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Number of Blocks |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ Zero or more BlockInfo Structures \
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/ /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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BlockInfo Structure:
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Size |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Length of Hash |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ Hash (variable length) \
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/ /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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#### Fields
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The Repository field identifies the repository that the index message
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pertains to. For single repository implementations an empty repository
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ID is acceptable, or the word "default". The Name is the file name path
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relative to the repository root. The combination of Repository and Name
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uniquely identifies each file in a cluster.
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The Version field is a counter that is initially zero for each file. It
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is incremented each time a change is detected. The combination of
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Repository, Name and Version uniquely identifies the contents of a file
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at a certain point in time.
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The Flags field is made up of the following single bit flags:
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Reserved |I|D| Unix Perm. & Mode |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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- The lower 12 bits hold the common Unix permission and mode bits.
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- Bit 19 ("D") is set when the file has been deleted. The block list
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shall contain zero blocks and the modification time indicates the
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time of deletion or, if deletion time is not reliably determinable,
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the last known modification time and a higher version number.
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- Bit 18 ("I") is set when the file is invalid and unavailable for
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synchronization. A peer may set this bit to indicate that it can
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temporarily not serve data for the file.
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- Bit 0 through 17 are reserved for future use and shall be set to
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zero.
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The hash algorithm is implied by the Hash length. Currently, the hash
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must be 32 bytes long and computed by SHA256.
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The Modified time is expressed as the number of seconds since the Unix
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Epoch. In the rare occasion that a file is simultaneously and
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independently modified by two nodes in the same cluster and thus end up
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on the same Version number after modification, the Modified field is
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used as a tie breaker.
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The Size field is the size of the file, in bytes.
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The Blocks list contains the size and hash for each block in the file.
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Each block represents a 128 KiB slice of the file, except for the last
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block which may represent a smaller amount of data.
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#### XDR
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struct IndexMessage {
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string Repository<>;
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FileInfo Files<>;
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}
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struct FileInfo {
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string Name<>;
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unsigned int Flags;
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hyper Modified;
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unsigned int Version;
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BlockInfo Blocks<>;
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}
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struct BlockInfo {
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unsigned int Size;
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opaque Hash<>;
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}
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### Request (Type = 2)
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The Request message expresses the desire to receive a data block
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corresponding to a part of a certain file in the peer's repository.
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#### Graphical Representation
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RequestMessage Structure:
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Length of Repository |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ Repository (variable length) \
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/ /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Length of Name |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ Name (variable length) \
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/ /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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+ Offset (64 bits) +
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Size |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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#### Fields
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The Repository and Name fields are as documented for the Index message.
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The Offset and Size fields specify the region of the file to be
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transferred. This should equate to exactly one block as seen in an Index
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message.
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#### XDR
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struct RequestMessage {
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string Repository<>;
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string Name<>;
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unsigned hyper Offset;
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unsigned int Size;
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}
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### Response (Type = 3)
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The Response message is sent in response to a Request message.
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#### Graphical Representation
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Length of Data |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ Data (variable length) \
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/ /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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#### Fields
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The Data field contains either a full 128 KiB block, a shorter block in
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the case of the last block in a file, or is empty (zero length) if the
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requested block is not available.
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#### XDR
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struct ResponseMessage {
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opaque Data<>
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}
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### Ping (Type = 4)
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The Ping message is used to determine that a connection is alive, and to
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keep connections alive through state tracking network elements such as
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firewalls and NAT gateways. The Ping message has no contents.
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### Pong (Type = 5)
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The Pong message is sent in response to a Ping. The Pong message has no
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contents, but copies the Message ID from the Ping.
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### Index Update (Type = 6)
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This message has exactly the same structure as the Index message.
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However instead of replacing the contents of the repository in the
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model, the Index Update merely amends it with new or updated file
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information. Any files not mentioned in an Index Update are left
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unchanged.
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### Options (Type = 7)
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This informational message provides information about the client
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configuration, version, etc. It is sent at connection initiation and,
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optionally, when any of the sent parameters have changed. The message is
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in the form of a list of (key, value) pairs, both of string type.
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Key ID:s apart from the well known ones are implementation specific. An
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implementation is expected to ignore unknown keys. An implementation may
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impose limits on key and value size.
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Well known keys:
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- "clientId" -- The name of the implementation. Example: "syncthing".
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- "clientVersion" -- The version of the client. Example: "v1.0.33-47". The
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Following the SemVer 2.0 specification for version strings is
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encouraged but not enforced.
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#### Graphical Representation
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Number of Options |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ Zero or more KeyValue Structures \
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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KeyValue Structure:
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Length of Key |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ Key (variable length) \
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/ /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Length of Value |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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/ /
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\ Value (variable length) \
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/ /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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#### XDR
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struct OptionsMessage {
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KeyValue Options<>;
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}
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struct KeyValue {
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string Key<>;
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string Value<>;
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}
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Example Exchange
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----------------
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A B
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1. Index-> <-Index
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2. Request->
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3. Request->
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4. Request->
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5. Request->
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6. <-Response
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7. <-Response
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8. <-Response
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9. <-Response
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10. Index Update->
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...
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11. Ping->
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12. <-Pong
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The connection is established and at 1. both peers send Index records.
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The Index records are received and both peers recompute their knowledge
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of the data in the cluster. In this example, peer A has four missing or
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outdated blocks. At 2 through 5 peer A sends requests for these blocks.
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The requests are received by peer B, who retrieves the data from the
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repository and transmits Response records (6 through 9). Node A updates
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their repository contents and transmits an Index Update message (10).
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Both peers enter idle state after 10. At some later time 11, peer A
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determines that it has not seen data from B for some time and sends a
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Ping request. A response is sent at 12.
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