# Git Wire Protocol, Version 2

This document presents a specification for a version 2 of Git’s wire protocol. Protocol v2 will improve upon v1 in the following ways:

- Instead of multiple service names, multiple commands will be supported by a single service

- Easily extendable as capabilities are moved into their own section of the protocol, no longer being hidden behind a NUL byte and limited by the size of a pkt-line

- Separate out other information hidden behind NUL bytes (e.g. agent string as a capability and symrefs can be requested using ls-refs)

- Reference advertisement will be omitted unless explicitly requested

- ls-refs command to explicitly request some refs

- Designed with http and stateless-rpc in mind. With clear flush semantics the http remote helper can simply act as a proxy

In protocol v2 communication is command oriented. When first contacting a server a list of capabilities will advertised. Some of these capabilities will be commands which a client can request be executed. Once a command has completed, a client can reuse the connection and request that other commands be executed.

## Packet-Line Framing

All communication is done using packet-line framing, just as in v1. See `Documentation/technical/pack-protocol.txt` and `Documentation/technical/protocol-common.txt` for more information.

In protocol v2 these special packets will have the following semantics:

- 0000 Flush Packet (flush-pkt) - indicates the end of a message

- 0001 Delimiter Packet (delim-pkt) - separates sections of a message

## Initial Client Request

In general a client can request to speak protocol v2 by sending `version=2` through the respective side-channel for the transport being used which inevitably sets `GIT_PROTOCOL`. More information can be found in `pack-protocol.txt` and `http-protocol.txt`. In all cases the response from the server is the capability advertisement.

### Git Transport

When using the git:// transport, you can request to use protocol v2 by sending "version=2" as an extra parameter:

    003egit-upload-pack /project.git\0host=myserver.com\0\0version=2\0

### SSH and File Transport

When using either the ssh:// or file:// transport, the GIT_PROTOCOL environment variable must be set explicitly to include "version=2".

### HTTP Transport

When using the http:// or https:// transport a client makes a "smart" info/refs request as described in `http-protocol.txt` and requests that v2 be used by supplying "version=2" in the `Git-Protocol` header.

    C: Git-Protocol: version=2
    C:
    C: GET $GIT_URL/info/refs?service=git-upload-pack HTTP/1.0

A v2 server would reply:

    S: 200 OK
    S: <Some headers>
    S: ...
    S:
    S: 000eversion 2\n
    S: <capability-advertisement>

Subsequent requests are then made directly to the service `$GIT_URL/git-upload-pack`. (This works the same for git-receive-pack).

## Capability Advertisement

A server which decides to communicate (based on a request from a client) using protocol version 2, notifies the client by sending a version string in its initial response followed by an advertisement of its capabilities. Each capability is a key with an optional value. Clients must ignore all unknown keys. Semantics of unknown values are left to the definition of each key. Some capabilities will describe commands which can be requested to be executed by the client.

    capability-advertisement = protocol-version
                               capability-list
                               flush-pkt

    protocol-version = PKT-LINE("version 2" LF)
    capability-list = *capability
    capability = PKT-LINE(key[=value] LF)

    key = 1*(ALPHA | DIGIT | "-_")
    value = 1*(ALPHA | DIGIT | " -_.,?\/{}[]()<>!@#$%^&*+=:;")

## Command Request

After receiving the capability advertisement, a client can then issue a request to select the command it wants with any particular capabilities or arguments. There is then an optional section where the client can provide any command specific parameters or queries. Only a single command can be requested at a time.

    request = empty-request | command-request
    empty-request = flush-pkt
    command-request = command
                      capability-list
                      [command-args]
                      flush-pkt
    command = PKT-LINE("command=" key LF)
    command-args = delim-pkt
                   *command-specific-arg

    command-specific-args are packet line framed arguments defined by
    each individual command.

The server will then check to ensure that the client’s request is comprised of a valid command as well as valid capabilities which were advertised. If the request is valid the server will then execute the command. A server MUST wait till it has received the client’s entire request before issuing a response. The format of the response is determined by the command being executed, but in all cases a flush-pkt indicates the end of the response.

When a command has finished, and the client has received the entire response from the server, a client can either request that another command be executed or can terminate the connection. A client may optionally send an empty request consisting of just a flush-pkt to indicate that no more requests will be made.

## Capabilities

There are two different types of capabilities: normal capabilities, which can be used to to convey information or alter the behavior of a request, and commands, which are the core actions that a client wants to perform (fetch, push, etc).

Protocol version 2 is stateless by default. This means that all commands must only last a single round and be stateless from the perspective of the server side, unless the client has requested a capability indicating that state should be maintained by the server. Clients MUST NOT require state management on the server side in order to function correctly. This permits simple round-robin load-balancing on the server side, without needing to worry about state management.

### agent

The server can advertise the `agent` capability with a value `X` (in the form `agent=X`) to notify the client that the server is running version `X`. The client may optionally send its own agent string by including the `agent` capability with a value `Y` (in the form `agent=Y`) in its request to the server (but it MUST NOT do so if the server did not advertise the agent capability). The `X` and `Y` strings may contain any printable ASCII characters except space (i.e., the byte range 32 < x < 127), and are typically of the form "package/version" (e.g., "git/1.8.3.1"). The agent strings are purely informative for statistics and debugging purposes, and MUST NOT be used to programmatically assume the presence or absence of particular features.

### ls-refs

`ls-refs` is the command used to request a reference advertisement in v2. Unlike the current reference advertisement, ls-refs takes in arguments which can be used to limit the refs sent from the server.

Additional features not supported in the base command will be advertised as the value of the command in the capability advertisement in the form of a space separated list of features: "<command>=<feature 1> <feature 2>"

ls-refs takes in the following arguments:

    symrefs
        In addition to the object pointed by it, show the underlying ref
        pointed by it when showing a symbolic ref.
    peel
        Show peeled tags.
    ref-prefix <prefix>
        When specified, only references having a prefix matching one of
        the provided prefixes are displayed.

The output of ls-refs is as follows:

    output = *ref
             flush-pkt
    ref = PKT-LINE(obj-id SP refname *(SP ref-attribute) LF)
    ref-attribute = (symref | peeled)
    symref = "symref-target:" symref-target
    peeled = "peeled:" obj-id

### fetch

`fetch` is the command used to fetch a packfile in v2. It can be looked at as a modified version of the v1 fetch where the ref-advertisement is stripped out (since the `ls-refs` command fills that role) and the message format is tweaked to eliminate redundancies and permit easy addition of future extensions.

Additional features not supported in the base command will be advertised as the value of the command in the capability advertisement in the form of a space separated list of features: "<command>=<feature 1> <feature 2>"

A `fetch` request can take the following arguments:

    want <oid>
        Indicates to the server an object which the client wants to
        retrieve.  Wants can be anything and are not limited to
        advertised objects.

    have <oid>
        Indicates to the server an object which the client has locally.
        This allows the server to make a packfile which only contains
        the objects that the client needs. Multiple 'have' lines can be
        supplied.

    done
        Indicates to the server that negotiation should terminate (or
        not even begin if performing a clone) and that the server should
        use the information supplied in the request to construct the
        packfile.

    thin-pack
        Request that a thin pack be sent, which is a pack with deltas
        which reference base objects not contained within the pack (but
        are known to exist at the receiving end). This can reduce the
        network traffic significantly, but it requires the receiving end
        to know how to "thicken" these packs by adding the missing bases
        to the pack.

    no-progress
        Request that progress information that would normally be sent on
        side-band channel 2, during the packfile transfer, should not be
        sent.  However, the side-band channel 3 is still used for error
        responses.

    include-tag
        Request that annotated tags should be sent if the objects they
        point to are being sent.

    ofs-delta
        Indicate that the client understands PACKv2 with delta referring
        to its base by position in pack rather than by an oid.  That is,
        they can read OBJ_OFS_DELTA (ake type 6) in a packfile.

If the shallow feature is advertised the following arguments can be included in the clients request as well as the potential addition of the shallow-info section in the server’s response as explained below.

    shallow <oid>
        A client must notify the server of all commits for which it only
        has shallow copies (meaning that it doesn't have the parents of
        a commit) by supplying a 'shallow <oid>' line for each such
        object so that the server is aware of the limitations of the
        client's history.  This is so that the server is aware that the
        client may not have all objects reachable from such commits.

    deepen <depth>
        Requests that the fetch/clone should be shallow having a commit
        depth of <depth> relative to the remote side.

    deepen-relative
        Requests that the semantics of the "deepen" command be changed
        to indicate that the depth requested is relative to the client's
        current shallow boundary, instead of relative to the requested
        commits.

    deepen-since <timestamp>
        Requests that the shallow clone/fetch should be cut at a
        specific time, instead of depth.  Internally it's equivalent to
        doing "git rev-list --max-age=<timestamp>". Cannot be used with
        "deepen".

    deepen-not <rev>
        Requests that the shallow clone/fetch should be cut at a
        specific revision specified by '<rev>', instead of a depth.
        Internally it's equivalent of doing "git rev-list --not <rev>".
        Cannot be used with "deepen", but can be used with
        "deepen-since".

If the filter feature is advertised, the following argument can be included in the client’s request:

    filter <filter-spec>
        Request that various objects from the packfile be omitted
        using one of several filtering techniques. These are intended
        for use with partial clone and partial fetch operations. See
        `rev-list` for possible "filter-spec" values.

The response of `fetch` is broken into a number of sections separated by delimiter packets (0001), with each section beginning with its section header.

    output = *section
    section = (acknowledgments | shallow-info | packfile)
              (flush-pkt | delim-pkt)

    acknowledgments = PKT-LINE("acknowledgments" LF)
                      (nak | *ack)
                      (ready)
    ready = PKT-LINE("ready" LF)
    nak = PKT-LINE("NAK" LF)
    ack = PKT-LINE("ACK" SP obj-id LF)

    shallow-info = PKT-LINE("shallow-info" LF)
                   *PKT-LINE((shallow | unshallow) LF)
    shallow = "shallow" SP obj-id
    unshallow = "unshallow" SP obj-id

    packfile = PKT-LINE("packfile" LF)
               *PKT-LINE(%x01-03 *%x00-ff)

    acknowledgments section
        * If the client determines that it is finished with negotiations
          by sending a "done" line, the acknowledgments sections MUST be
          omitted from the server's response.

- Always begins with the section header "acknowledgments"

- The server will respond with "NAK" if none of the object ids sent as have lines were common.

- The server will respond with "ACK obj-id" for all of the object ids sent as have lines which are common.

- A response cannot have both "ACK" lines as well as a "NAK" line.

- The server will respond with a "ready" line indicating that the server has found an acceptable common base and is ready to make and send a packfile (which will be found in the packfile section of the same response)

- If the server has found a suitable cut point and has decided to send a "ready" line, then the server can decide to (as an optimization) omit any "ACK" lines it would have sent during its response. This is because the server will have already determined the objects it plans to send to the client and no further negotiation is needed.

      shallow-info section
          * If the client has requested a shallow fetch/clone, a shallow
            client requests a fetch or the server is shallow then the
            server's response may include a shallow-info section.  The
            shallow-info section will be included if (due to one of the
            above conditions) the server needs to inform the client of any
            shallow boundaries or adjustments to the clients already
            existing shallow boundaries.

- Always begins with the section header "shallow-info"

- If a positive depth is requested, the server will compute the set of commits which are no deeper than the desired depth.

- The server sends a "shallow obj-id" line for each commit whose parents will not be sent in the following packfile.

- The server sends an "unshallow obj-id" line for each commit which the client has indicated is shallow, but is no longer shallow as a result of the fetch (due to its parents being sent in the following packfile).

- The server MUST NOT send any "unshallow" lines for anything which the client has not indicated was shallow as a part of its request.

- This section is only included if a packfile section is also included in the response.

      packfile section
          * This section is only included if the client has sent 'want'
            lines in its request and either requested that no more
            negotiation be done by sending 'done' or if the server has
            decided it has found a sufficient cut point to produce a
            packfile.

- Always begins with the section header "packfile"

- The transmission of the packfile begins immediately after the section header

- The data transfer of the packfile is always multiplexed, using the same semantics of the side-band-64k capability from protocol version 1. This means that each packet, during the packfile data stream, is made up of a leading 4-byte pkt-line length (typical of the pkt-line format), followed by a 1-byte stream code, followed by the actual data.

      The stream code can be one of:
            1 - pack data
            2 - progress messages
            3 - fatal error message just before stream aborts

### server-option

If advertised, indicates that any number of server specific options can be included in a request. This is done by sending each option as a "server-option=<option>" capability line in the capability-list section of a request.

The provided options must not contain a NUL or LF character.