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Jakob Borg
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protocol/PROTOCOL.md
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protocol/PROTOCOL.md
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Block Exchange Protocol v1.0
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============================
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Introduction and Definitions
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----------------------------
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The BEP is used between two or more _nodes_ thus forming a _cluster_.
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Each node has a _repository_ of files described by the _local model_,
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containing modifications times and block hashes. The local model is sent
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to the other nodes in the cluster. The union of all files in the local
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models, with files selected for most recent modification time, forms the
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_global model_. Each node strives to get it's repository in synch with
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the global model by requesting missing blocks from the other nodes.
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Transport and Authentication
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----------------------------
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The BEP itself does not provide retransmissions, compression, encryption
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nor authentication. It is expected that this is performed at lower
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layers of the networking stack. A typical deployment stack should be
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similar to 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 version
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and type. For BEP v1.0 the Version field is set to zero. Future versions
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with incompatible message formats will increment the Version field. The
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reserved bits must be set to zero.
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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=0 | Message ID | Type | Reserved |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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All data following the message header is in XDR (RFC 1014) encoding.
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The actual data types in use by BEP, in XDR naming convention, are:
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- unsigned int -- unsigned 32 bit integer
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- hyper -- signed 64 bit integer
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- unsigned hyper -- signed 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 encoding of opaque<> and string<> are identical, the distinction is
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solely in interpretation. Opaque data should not be interpreted as such,
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but can be compared bytewise to other opaque data. All strings use the
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UTF-8 encoding.
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### Index (Type = 1)
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The Index message defines the contents of the senders repository. A Index
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message is sent by each peer immediately upon connection and whenever the
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local repository contents changes. However, if a peer has no data to
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advertise (the repository is empty, or it is set to only import data) it
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is allowed but not required to send an empty Index message (a file list of
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zero length). If the repository contents change from non-empty to empty,
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an empty Index message must be sent. There is no response to the Index
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message.
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struct IndexMessage {
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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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BlockInfo Blocks<>;
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}
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struct BlockInfo {
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unsigned int Length;
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opaque Hash<>
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}
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The file name is the part relative to the repository root. The
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modification time is expressed as the number of seconds since the Unix
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Epoch. The hash algorithm is implied by the hash length. Currently, the
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hash must be 32 bytes long and computed by SHA256.
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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 |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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one second past the last know modification time.
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- Bit 0 through 18 are reserved for future use and shall be set to
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zero.
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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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The requested block must correspond exactly to one block seen in the
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peer's Index message. The hash field must be set to the expected value by
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the sender. The receiver may validate that this is actually the case
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before transmitting data. Each Request message must be met with a Response
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message.
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struct RequestMessage {
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string Name<>;
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unsigned hyper Offset;
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unsigned int Length;
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opaque Hash<>;
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}
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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 Message ID in the header must set to a unique value to be able to
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correlate the request with the response message.
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### Response (Type = 3)
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The Response message is sent in response to a Request message. In case the
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requested data was not available (an outdated block was requested, or
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the file has been deleted), the Data field is empty.
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struct ResponseMessage {
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opaque Data<>
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}
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The Message ID in the header is used to correlate requests and
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responses.
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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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struct PingMessage {
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}
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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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struct PongMessage {
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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->
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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 updated Index 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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