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Software / code / clix

Wire Protocol RPC

**Experimental and under development**

This document describe's Mercurial's transport-agnostic remote procedure call (RPC) protocol which is used to perform interactions with remote servers. This protocol is also referred to as "hgrpc".

The protocol has the following high-level features:

  • Concurrent request and response support (multiple commands can be issued simultaneously and responses can be streamed simultaneously).
  • Supports half-duplex and full-duplex connections.
  • All data is transmitted within *frames*, which have a well-defined header and encode their length.
  • Side-channels for sending progress updates and printing output. Text output from the remote can be localized locally.
  • Support for simultaneous and long-lived compression streams, even across requests.
  • Uses CBOR for data exchange.

The protocol is not specific to Mercurial and could be used by other applications.

High-level Overview

To operate the protocol, a bi-directional, half-duplex pipe supporting ordered sends and receives is required. That is, each peer has one pipe for sending data and another for receiving. Full-duplex pipes are also supported.

All data is read and written in atomic units called *frames*. These are conceptually similar to TCP packets. Higher-level functionality is built on the exchange and processing of frames.

All frames are associated with a *stream*. A *stream* provides a unidirectional grouping of frames. Streams facilitate two goals: content encoding and parallelism. There is a dedicated section on streams below.

The protocol is request-response based: the client issues requests to the server, which issues replies to those requests. Server-initiated messaging is not currently supported, but this specification carves out room to implement it.

All frames are associated with a numbered request. Frames can thus be logically grouped by their request ID.

Frames

Frames begin with an 8 octet header followed by a variable length payload: 

+------------------------------------------------+
|                 Length (24)                    |
+--------------------------------+---------------+
|         Request ID (16)        | Stream ID (8) |
+------------------+-------------+---------------+
| Stream Flags (8) |
+-----------+------+
| Type (4)  |
+-----------+
| Flags (4) |
+===========+===================================================|
|                     Frame Payload (0...)                    ...
+---------------------------------------------------------------+

The length of the frame payload is expressed as an unsigned 24 bit little endian integer. Values larger than 65535 MUST NOT be used unless given permission by the server as part of the negotiated capabilities during the handshake. The frame header is not part of the advertised frame length. The payload length is the over-the-wire length. If there is content encoding applied to the payload as part of the frame's stream, the length is the output of that content encoding, not the input.

The 16-bit "Request ID" field denotes the integer request identifier, stored as an unsigned little endian integer. Odd numbered requests are client-initiated. Even numbered requests are server-initiated. This refers to where the *request* was initiated - not where the *frame* was initiated, so servers will send frames with odd "Request ID" in response to client-initiated requests. Implementations are advised to start ordering request identifiers at "1" and "0", increment by "2", and wrap around if all available numbers have been exhausted.

The 8-bit "Stream ID" field denotes the stream that the frame is associated with. Frames belonging to a stream may have content encoding applied and the receiver may need to decode the raw frame payload to obtain the original data. Odd numbered IDs are client-initiated. Even numbered IDs are server-initiated.

The 8-bit "Stream Flags" field defines stream processing semantics. See the section on streams below.

The 4-bit "Type" field denotes the type of frame being sent.

The 4-bit "Flags" field defines special, per-type attributes for the frame.

The sections below define the frame types and their behavior.

Command Request ("0x01")

This frame contains a request to run a command.

The payload consists of a CBOR map defining the command request. The bytestring keys of that map are:

name
Name of the command that should be executed (bytestring).
args
Map of bytestring keys to various value types containing the named arguments to this command.

Each command defines its own set of argument names and their expected types.

redirect (optional)
(map) Advertises client support for following response *redirects*.

This map has the following bytestring keys:

targets
(array of bytestring) List of named redirect targets supported by this client. The names come from the targets advertised by the server's *capabilities* message.
hashes
(array of bytestring) List of preferred hashing algorithms that can be used for content integrity verification.

See the *Content Redirects* section below for more on content redirects.

This frame type MUST ONLY be sent from clients to servers: it is illegal for a server to send this frame to a client.

The following flag values are defined for this type:

0x01
New command request. When set, this frame represents the beginning of a new request to run a command. The "Request ID" attached to this frame MUST NOT be active.
0x02
Command request continuation. When set, this frame is a continuation from a previous command request frame for its "Request ID". This flag is set when the CBOR data for a command request does not fit in a single frame.
0x04
Additional frames expected. When set, the command request didn't fit into a single frame and additional CBOR data follows in a subsequent frame.
0x08
Command data frames expected. When set, command data frames are expected to follow the final command request frame for this request.

"0x01" MUST be set on the initial command request frame for a "Request ID".

"0x01" or "0x02" MUST be set to indicate this frame's role in a series of command request frames.

If command data frames are to be sent, "0x08" MUST be set on ALL command request frames.

Command Data ("0x02")

This frame contains raw data for a command.

Most commands can be executed by specifying arguments. However, arguments have an upper bound to their length. For commands that accept data that is beyond this length or whose length isn't known when the command is initially sent, they will need to stream arbitrary data to the server. This frame type facilitates the sending of this data.

The payload of this frame type consists of a stream of raw data to be consumed by the command handler on the server. The format of the data is command specific.

The following flag values are defined for this type:

0x01
Command data continuation. When set, the data for this command continues into a subsequent frame.
0x02
End of data. When set, command data has been fully sent to the server. The command has been fully issued and no new data for this command will be sent. The next frame will belong to a new command.

Command Response Data ("0x03")

This frame contains response data to an issued command.

Response data ALWAYS consists of a series of 1 or more CBOR encoded values. A CBOR value may be using indefinite length encoding. And the bytes constituting the value may span several frames.

The following flag values are defined for this type:

0x01
Data continuation. When set, an additional frame containing response data will follow.
0x02
End of data. When set, the response data has been fully sent and no additional frames for this response will be sent.

The "0x01" flag is mutually exclusive with the "0x02" flag.

Error Occurred ("0x05")

Some kind of error occurred.

There are 3 general kinds of failures that can occur:

  • Command error encountered before any response issued
  • Command error encountered after a response was issued
  • Protocol or stream level error

This frame type is used to capture the latter cases. (The general command error case is handled by the leading CBOR map in "Command Response" frames.)

The payload of this frame contains a CBOR map detailing the error. That map has the following bytestring keys:

type
(bytestring) The overall type of error encountered. Can be one of the following values:
protocol
A protocol-level error occurred. This typically means someone is violating the framing protocol semantics and the server is refusing to proceed.
server
A server-level error occurred. This typically indicates some kind of logic error on the server, likely the fault of the server.
command
A command-level error, likely the fault of the client.
message
(array of maps) A richly formatted message that is intended for human consumption. See the "Human Output Side-Channel" frame section for a description of the format of this data structure.

Human Output Side-Channel ("0x06")

This frame contains a message that is intended to be displayed to people. Whereas most frames communicate machine readable data, this frame communicates textual data that is intended to be shown to humans.

The frame consists of a series of *formatting requests*. Each formatting request consists of a formatting string, arguments for that formatting string, and labels to apply to that formatting string.

A formatting string is a printf()-like string that allows variable substitution within the string. Labels allow the rendered text to be *decorated*. Assuming use of the canonical Mercurial code base, a formatting string can be the input to the "i18n._" function. This allows messages emitted from the server to be localized. So even if the server has different i18n settings, people could see messages in their *native* settings. Similarly, the use of labels allows decorations like coloring and underlining to be applied using the client's configured rendering settings.

Formatting strings are similar to "printf()" strings or how Python's "%" operator works. The only supported formatting sequences are "%s" and "%%". "%s" will be replaced by whatever the string at that position resolves to. "%%" will be replaced by "%". All other 2-byte sequences beginning with "%" represent a literal "%" followed by that character. However, future versions of the wire protocol reserve the right to allow clients to opt in to receiving formatting strings with additional formatters, hence why "%%" is required to represent the literal "%".

The frame payload consists of a CBOR array of CBOR maps. Each map defines an *atom* of text data to print. Each *atom* has the following bytestring keys:

msg
(bytestring) The formatting string. Content MUST be ASCII.
args (optional)
Array of bytestrings defining arguments to the formatting string.
labels (optional)
Array of bytestrings defining labels to apply to this atom.

All data to be printed MUST be encoded into a single frame: this frame does not support spanning data across multiple frames.

All textual data encoded in these frames is assumed to be line delimited. The last atom in the frame SHOULD end with a newline ("\n"). If it doesn't, clients MAY add a newline to facilitate immediate printing.

Progress Update ("0x07")

This frame holds the progress of an operation on the peer. Consumption of these frames allows clients to display progress bars, estimated completion times, etc.

Each frame defines the progress of a single operation on the peer. The payload consists of a CBOR map with the following bytestring keys:

topic
Topic name (string)
pos
Current numeric position within the topic (integer)
total
Total/end numeric position of this topic (unsigned integer)
label (optional)
Unit label (string)
item (optional)
Item name (string)

Progress state is created when a frame is received referencing a *topic* that isn't currently tracked. Progress tracking for that *topic* is finished when a frame is received reporting the current position of that topic as "-1".

Multiple *topics* may be active at any given time.

Rendering of progress information is not mandated or governed by this specification: implementations MAY render progress information however they see fit, including not at all.

The string data describing the topic SHOULD be static strings to facilitate receivers localizing that string data. The emitter MUST normalize all string data to valid UTF-8 and receivers SHOULD validate that received data conforms to UTF-8. The topic name SHOULD be ASCII.

Sender Protocol Settings ("0x08")

This frame type advertises the sender's support for various protocol and stream level features. The data advertised in this frame is used to influence subsequent behavior of the current frame exchange channel.

The frame payload consists of a CBOR map. It may contain the following bytestring keys:

contentencodings
(array of bytestring) A list of content encodings supported by the sender, in order of most to least preferred.

Peers are allowed to encode stream data using any of the listed encodings.

See the "Content Encoding Profiles" section for an enumeration of supported content encodings.

If not defined, the value is assumed to be a list with the single value "identity", meaning only the no-op encoding is supported.

Senders MAY filter the set of advertised encodings against what it knows the receiver supports (e.g. if the receiver advertised encodings via the capabilities descriptor). However, doing so will prevent servers from gaining an understanding of the aggregate capabilities of clients. So clients are discouraged from doing so.

When this frame is not sent/received, the receiver assumes default values for all keys.

If encountered, this frame type MUST be sent before any other frame type in a channel.

The following flag values are defined for this frame type:

0x01
Data continuation. When set, an additional frame containing more protocol settings immediately follows.
0x02
End of data. When set, the protocol settings data has been completely sent.

The "0x01" flag is mutually exclusive with the "0x02" flag.

Stream Encoding Settings ("0x09")

This frame type holds information defining the content encoding settings for a *stream*.

This frame type is likely consumed by the protocol layer and is not passed on to applications.

This frame type MUST ONLY occur on frames having the *Beginning of Stream* "Stream Flag" set.

The payload of this frame defines what content encoding has (possibly) been applied to the payloads of subsequent frames in this stream.

The payload consists of a series of CBOR values. The first value is a bytestring denoting the content encoding profile of the data in this stream. Subsequent CBOR values supplement this simple value in a profile-specific manner. See the "Content Encoding Profiles" section for more.

In the absence of this frame on a stream, it is assumed the stream is using the "identity" content encoding.

The following flag values are defined for this frame type:

0x01
Data continuation. When set, an additional frame containing more encoding settings immediately follows.
0x02
End of data. When set, the encoding settings data has been completely sent.

The "0x01" flag is mutually exclusive with the "0x02" flag.

Stream States and Flags

Streams can be in two states: *open* and *closed*. An *open* stream is active and frames attached to that stream could arrive at any time. A *closed* stream is not active. If a frame attached to a *closed* stream arrives, that frame MUST have an appropriate stream flag set indicating beginning of stream. All streams are in the *closed* state by default.

The "Stream Flags" field denotes a set of bit flags for defining the relationship of this frame within a stream. The following flags are defined:

0x01
Beginning of stream. The first frame in the stream MUST set this flag. When received, the "Stream ID" this frame is attached to becomes "open".
0x02
End of stream. The last frame in a stream MUST set this flag. When received, the "Stream ID" this frame is attached to becomes "closed". Any content encoding context associated with this stream can be destroyed after processing the payload of this frame.
0x04
Apply content encoding. When set, any content encoding settings defined by the stream should be applied when attempting to read the frame. When not set, the frame payload isn't encoded.

TODO consider making stream opening and closing communicated via explicit frame types (e.g. a "stream state change" frame) rather than flags on all frames. This would make stream state changes more explicit, as they could only occur on specific frame types.

Streams

Streams - along with "Request IDs" - facilitate grouping of frames. But the purpose of each is quite different and the groupings they constitute are independent.

A "Request ID" is essentially a tag. It tells you which logical request a frame is associated with.

A *stream* is a sequence of frames grouped for the express purpose of applying a stateful encoding or for denoting sub-groups of frames.

Unlike "Request ID"s which span the request and response, a stream is unidirectional and stream IDs are independent from client to server.

There is no strict hierarchical relationship between "Request IDs" and *streams*. A stream can contain frames having multiple "Request IDs". Frames belonging to the same "Request ID" can span multiple streams.

One goal of streams is to facilitate content encoding. A stream can define an encoding to be applied to frame payloads. For example, the payload transmitted over the wire may contain output from a zstandard compression operation and the receiving end may decompress that payload to obtain the original data.

The other goal of streams is to facilitate concurrent execution. For example, a server could spawn 4 threads to service a request that can be easily parallelized. Each of those 4 threads could write into its own stream. Those streams could then in turn be delivered to 4 threads on the receiving end, with each thread consuming its stream in near isolation. The *main* thread on both ends merely does I/O and encodes/decodes frame headers: the bulk of the work is done by worker threads.

In addition, since content encoding is defined per stream, each *worker thread* could perform potentially CPU bound work concurrently with other threads. This approach of applying encoding at the sub-protocol / stream level eliminates a potential resource constraint on the protocol stream as a whole (it is common for the throughput of a compression engine to be smaller than the throughput of a network).

Having multiple streams - each with their own encoding settings - also facilitates the use of advanced data compression techniques. For example, a transmitter could see that it is generating data faster and slower than the receiving end is consuming it and adjust its compression settings to trade CPU for compression ratio accordingly.

While streams can define a content encoding, not all frames within that stream must use that content encoding. This can be useful when data is being served from caches and being derived dynamically. A cache could pre-compressed data so the server doesn't have to recompress it. The ability to pick and choose which frames are compressed allows servers to easily send data to the wire without involving potentially expensive encoding overhead.

Content Encoding Profiles

Streams can have named content encoding *profiles* associated with them. A profile defines a shared understanding of content encoding settings and behavior.

Profiles are described in the following sections.

identity

The "identity" profile is a no-op encoding: the encoded bytes are exactly the input bytes.

This profile MUST be supported by all peers.

In the absence of an identified profile, the "identity" profile is assumed.

zstd-8mb

Zstandard encoding (RFC 8478). Zstandard is a fast and effective lossless compression format.

This profile allows decompressor window sizes of up to 8 MB.

zlib

zlib compressed data (RFC 1950). zlib is a widely-used and supported lossless compression format.

It isn't as fast as zstandard and it is recommended to use zstandard instead, if possible.

Command Protocol

A client can request that a remote run a command by sending it frames defining that command. This logical stream is composed of 1 or more "Command Request" frames and and 0 or more "Command Data" frames.

All frames composing a single command request MUST be associated with the same "Request ID".

Clients MAY send additional command requests without waiting on the response to a previous command request. If they do so, they MUST ensure that the "Request ID" field of outbound frames does not conflict with that of an active "Request ID" whose response has not yet been fully received.

Servers MAY respond to commands in a different order than they were sent over the wire. Clients MUST be prepared to deal with this. Servers also MAY start executing commands in a different order than they were received, or MAY execute multiple commands concurrently.

If there is a dependency between commands or a race condition between commands executing (e.g. a read-only command that depends on the results of a command that mutates the repository), then clients MUST NOT send frames issuing a command until a response to all dependent commands has been received. TODO think about whether we should express dependencies between commands to avoid roundtrip latency.

A command is defined by a command name, 0 or more command arguments, and optional command data.

Arguments are the recommended mechanism for transferring fixed sets of parameters to a command. Data is appropriate for transferring variable data. Thinking in terms of HTTP, arguments would be headers and data would be the message body.

It is recommended for servers to delay the dispatch of a command until all argument have been received. Servers MAY impose limits on the maximum argument size. TODO define failure mechanism.

Servers MAY dispatch to commands immediately once argument data is available or delay until command data is received in full.

Once a "Command Request" frame is sent, a client must be prepared to receive any of the following frames associated with that request: "Command Response", "Error Response", "Human Output Side-Channel", "Progress Update".

The *main* response for a command will be in "Command Response" frames. The payloads of these frames consist of 1 or more CBOR encoded values. The first CBOR value on the first "Command Response" frame is special and denotes the overall status of the command. This CBOR map contains the following bytestring keys:

status
(bytestring) A well-defined message containing the overall status of this command request. The following values are defined:
ok
The command was received successfully and its response follows.
error
There was an error processing the command. More details about the error are encoded in the "error" key.
redirect
The response for this command is available elsewhere. Details on where are in the "location" key.
error (optional)
A map containing information about an encountered error. The map has the following keys:
message
(array of maps) A message describing the error. The message uses the same format as those in the "Human Output Side-Channel" frame.
location (optional)
(map) Presence indicates that a *content redirect* has occurred. The map provides the external location of the content.

This map contains the following bytestring keys:

url
(bytestring) URL from which this content may be requested.
mediatype
(bytestring) The media type for the fetched content. e.g. "application/mercurial-*".

In some transports, this value is also advertised by the transport. e.g. as the "Content-Type" HTTP header.

size (optional)
(unsigned integer) Total size of remote object in bytes. This is the raw size of the entity that will be fetched, minus any non-Mercurial protocol encoding (e.g. HTTP content or transfer encoding.)
fullhashes (optional)
(array of arrays) Content hashes for the entire payload. Each entry is an array of bytestrings containing the hash name and the hash value.
fullhashseed (optional)
(bytestring) Optional seed value to feed into hasher for full content hash verification.
serverdercerts (optional)
(array of bytestring) DER encoded x509 certificates for the server. When defined, clients MAY validate that the x509 certificate on the target server exactly matches the certificate used here.
servercadercerts (optional)
(array of bytestring) DER encoded x509 certificates for the certificate authority of the target server. When defined, clients MAY validate that the x509 on the target server was signed by CA certificate in this set.

# TODO support for giving client an x509 certificate pair to be used as a # client certificate.

# TODO support common authentication mechanisms (e.g. HTTP basic/digest # auth).

# TODO support custom authentication mechanisms. This likely requires # server to advertise required auth mechanism so client can filter.

# TODO support chained hashes. e.g. hash for each 1MB segment so client # can iteratively validate data without having to consume all of it first.

TODO formalize when error frames can be seen and how errors can be recognized midway through a command response.

Content Redirects

Servers have the ability to respond to ANY command request with a *redirect* to another location. Such a response is referred to as a *redirect response*. (This feature is conceptually similar to HTTP redirects, but is more powerful.)

A *redirect response* MUST ONLY be issued if the client advertises support for a redirect *target*.

A *redirect response* MUST NOT be issued unless the client advertises support for one.

Clients advertise support for *redirect responses* after looking at the server's *capabilities* data, which is fetched during initial server connection handshake. The server's capabilities data advertises named *targets* for potential redirects.

Each target is described by a protocol name, connection and protocol features, etc. The server also advertises target-agnostic redirect settings, such as which hash algorithms are supported for content integrity checking. (See the documentation for the *capabilities* command for more.)

Clients examine the set of advertised redirect targets for compatibility. When sending a command request, the client advertises the set of redirect target names it is willing to follow, along with some other settings influencing behavior.

For example, say the server is advertising a "cdn" redirect target that requires SNI and TLS 1.2. If the client supports those features, it will send command requests stating that the "cdn" target is acceptable to use. But if the client doesn't support SNI or TLS 1.2 (or maybe it encountered an error using this target from a previous request), then it omits this target name.

If the client advertises support for a redirect target, the server MAY substitute the normal, inline response data for a *redirect response* - one where the initial CBOR map has a "status" key with value "redirect".

The *redirect response* at a minimum advertises the URL where the response can be retrieved.

The *redirect response* MAY also advertise additional details about that content and how to retrieve it. Notably, the response may contain the x509 public certificates for the server being redirected to or the certificate authority that signed that server's certificate. Unless the client has existing settings that offer stronger trust validation than what the server advertises, the client SHOULD use the server-provided certificates when validating the connection to the remote server in place of any default connection verification checks. This is because certificates coming from the server SHOULD establish a stronger chain of trust than what the default certification validation mechanism in most environments provides. (By default, certificate validation ensures the signer of the cert chains up to a set of trusted root certificates. And if an explicit certificate or CA certificate is presented, that greadly reduces the set of certificates that will be recognized as valid, thus reducing the potential for a "bad" certificate to be used and trusted.)