gitprotocol-v2(5)

NAME#

gitprotocol-v2 - Git Wire Protocol, Version 2

SYNOPSIS#

<over-the-wire-protocol>

DESCRIPTION#

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

In protocol v2 communication is command oriented. When first contacting a server a list of capabilities will be 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 gitprotocol-pack(5) and gitprotocol-common(5) for more information.

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

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 gitprotocol-pack(5) and gitprotocol-http(5), as well as the GIT_PROTOCOL definition in git.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". The server may need to be configured to allow this environment variable to pass.

HTTP Transport#

When using the http:// or https:// transport a client makes a "smart" info/refs request as described in gitprotocol-http(5) and requests that v2 be used by supplying "version=2" in the Git-Protocol header.

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

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).

Uses the --http-backend-info-refs option to git-upload-pack(1).

The server may need to be configured to pass this header’s contents via the GIT_PROTOCOL variable. See the discussion in git-http-backend.txt.

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
\n
                           capability-list
\n
                           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
\n
                  capability-list
\n
                  delim-pkt
\n
                  command-args
\n
                  flush-pkt
command = PKT-LINE("command=" key LF)
command-args = *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 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
\n
    In addition to the object pointed by it, show the underlying ref
\n
    pointed by it when showing a symbolic ref.
peel
\n
    Show peeled tags.
ref-prefix <prefix>
\n
    When specified, only references having a prefix matching one of
\n
    the provided prefixes are displayed. Multiple instances may be
\n
    given, in which case references matching any prefix will be
\n
    shown. Note that this is purely for optimization; a server MAY
\n
    show refs not matching the prefix if it chooses, and clients
\n
    should filter the result themselves.

If the unborn feature is advertised the following argument can be included in the client’s request.

unborn
\n
    The server will send information about HEAD even if it is a symref
\n
    pointing to an unborn branch in the form "unborn HEAD
\n
    symref-target:<target>".

The output of ls-refs is as follows:

output = *ref
\n
         flush-pkt
obj-id-or-unborn = (obj-id | "unborn")
ref = PKT-LINE(obj-id-or-unborn 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>
\n
    Indicates to the server an object which the client wants to
\n
    retrieve.  Wants can be anything and are not limited to
\n
    advertised objects.

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

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

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

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

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

ofs-delta
\n
    Indicate that the client understands PACKv2 with delta referring
\n
    to its base by position in pack rather than by an oid.  That is,
\n
    they can read OBJ_OFS_DELTA (aka 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>
\n
    A client must notify the server of all commits for which it only
\n
    has shallow copies (meaning that it doesn't have the parents of
\n
    a commit) by supplying a 'shallow <oid>' line for each such
\n
    object so that the server is aware of the limitations of the
\n
    client's history.  This is so that the server is aware that the
\n
    client may not have all objects reachable from such commits.

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

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

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

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

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

filter <filter-spec>
\n
    Request that various objects from the packfile be omitted
\n
    using one of several filtering techniques. These are intended
\n
    for use with partial clone and partial fetch operations. See
\n
    `rev-list` for possible "filter-spec" values. When communicating
\n
    with other processes, senders SHOULD translate scaled integers
\n
    (e.g. "1k") into a fully-expanded form (e.g. "1024") to aid
\n
    interoperability with older receivers that may not understand
\n
    newly-invented scaling suffixes. However, receivers SHOULD
\n
    accept the following suffixes: 'k', 'm', and 'g' for 1024,
\n
    1048576, and 1073741824, respectively.

If the ref-in-want feature is advertised, the following argument can be included in the client’s request as well as the potential addition of the wanted-refs section in the server’s response as explained below.

want-ref <ref>
\n
    Indicates to the server that the client wants to retrieve a
\n
    particular ref, where <ref> is the full name of a ref on the
\n
    server.  It is a protocol error to send want-ref for the
\n
    same ref more than once.

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

sideband-all
\n
    Instruct the server to send the whole response multiplexed, not just
\n
    the packfile section. All non-flush and non-delim PKT-LINE in the
\n
    response (not only in the packfile section) will then start with a byte
\n
    indicating its sideband (1, 2, or 3), and the server may send "0005\2"
\n
    (a PKT-LINE of sideband 2 with no payload) as a keepalive packet.

If the packfile-uris feature is advertised, the following argument can be included in the client’s request as well as the potential addition of the packfile-uris section in the server’s response as explained below. Note that at most one packfile-uris line can be sent to the server.

packfile-uris <comma-separated-list-of-protocols>
\n
    Indicates to the server that the client is willing to receive
\n
    URIs of any of the given protocols in place of objects in the
\n
    sent packfile. Before performing the connectivity check, the
\n
    client should download from all given URIs. Currently, the
\n
    protocols supported are "http" and "https".

If the wait-for-done feature is advertised, the following argument can be included in the client’s request.

wait-for-done
\n
    Indicates to the server that it should never send "ready", but
\n
    should wait for the client to say "done" before sending the
\n
    packfile.

The response of fetch is broken into a number of sections separated by delimiter packets (0001), with each section beginning with its section header. Most sections are sent only when the packfile is sent.

output = acknowledgements flush-pkt |
\n
         [acknowledgments delim-pkt] [shallow-info delim-pkt]
\n
         [wanted-refs delim-pkt] [packfile-uris delim-pkt]
\n
         packfile flush-pkt

acknowledgments = PKT-LINE("acknowledgments" LF)
\n
                  (nak | *ack)
\n
                  (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)
\n
               *PKT-LINE((shallow | unshallow) LF)
shallow = "shallow" SP obj-id
unshallow = "unshallow" SP obj-id

wanted-refs = PKT-LINE("wanted-refs" LF)
\n
              *PKT-LINE(wanted-ref LF)
wanted-ref = obj-id SP refname

packfile-uris = PKT-LINE("packfile-uris" LF) *packfile-uri
packfile-uri = PKT-LINE(40*(HEXDIGIT) SP *%x20-ff LF)

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

acknowledgments section
\n
    * If the client determines that it is finished with negotiations by
\n
      sending a "done" line (thus requiring the server to send a packfile),
\n
      the acknowledgments sections MUST be omitted from the server's
\n
      response.

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

wanted-refs section
\n
    * This section is only included if the client has requested a
\n
      ref using a 'want-ref' line and if a packfile section is also
\n
      included in the response.

packfile-uris section
\n
    * This section is only included if the client sent
\n
      'packfile-uris' and the server has at least one such URI to
\n
      send.

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

The stream code can be one of:
\n
      1 - pack data
\n
      2 - progress messages
\n
      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.

object-format#

The server can advertise the object-format capability with a value X (in the form object-format=X) to notify the client that the server is able to deal with objects using hash algorithm X. If not specified, the server is assumed to only handle SHA-1. If the client would like to use a hash algorithm other than SHA-1, it should specify its object-format string.

session-id=<session-id>#

The server may advertise a session ID that can be used to identify this process across multiple requests. The client may advertise its own session ID back to the server as well.

Session IDs should be unique to a given process. They must fit within a packet-line, and must not contain non-printable or whitespace characters. The current implementation uses trace2 session IDs (see api-trace2[1] for details), but this may change and users of the session ID should not rely on this fact.

object-info#

object-info is the command to retrieve information about one or more objects. Its main purpose is to allow a client to make decisions based on this information without having to fully fetch objects. Object size is the only information that is currently supported.

An object-info request takes the following arguments:

size
Requests size information to be returned for each listed object id.

oid <oid>
Indicates to the server an object which the client wants to obtain
information for.

The response of object-info is a list of the requested object ids and associated requested information, each separated by a single space.

output = info flush-pkt

info = PKT-LINE(attrs) LF)
\n
        *PKT-LINE(obj-info LF)

attrs = attr | attrs SP attrs

attr = "size"

obj-info = obj-id SP obj-size

bundle-uri#

If the bundle-uri capability is advertised, the server supports the ‘bundle-uri’ command.

The capability is currently advertised with no value (i.e. not "bundle-uri=somevalue"), a value may be added in the future for supporting command-wide extensions. Clients MUST ignore any unknown capability values and proceed with the 'bundle-uri` dialog they support.

The bundle-uri command is intended to be issued before fetch to get URIs to bundle files (see git-bundle(1)) to "seed" and inform the subsequent fetch command.

The client CAN issue bundle-uri before or after any other valid command. To be useful to clients it’s expected that it’ll be issued after an ls-refs and before fetch, but CAN be issued at any time in the dialog.

DISCUSSION of bundle-uri

The intent of the feature is optimize for server resource consumption in the common case by changing the common case of fetching a very large PACK during git-clone(1) into a smaller incremental fetch.

It also allows servers to achieve better caching in combination with an uploadpack.packObjectsHook (see git-config(1)).

By having new clones or fetches be a more predictable and common negotiation against the tips of recently produces *.bundle file(s). Servers might even pre-generate the results of such negotiations for the uploadpack.packObjectsHook as new pushes come in.

One way that servers could take advantage of these bundles is that the server would anticipate that fresh clones will download a known bundle, followed by catching up to the current state of the repository using ref tips found in that bundle (or bundles).

PROTOCOL for bundle-uri

A bundle-uri request takes no arguments, and as noted above does not currently advertise a capability value. Both may be added in the future.

When the client issues a command=bundle-uri request, the response is a list of key-value pairs provided as packet lines with value <key>=<value>. Each <key> should be interpreted as a config key from the bundle.* namespace to construct a list of bundles. These keys are grouped by a bundle.<id>. subsection, where each key corresponding to a given <id> contributes attributes to the bundle defined by that <id>. See git-config(1) for the specific details of these keys and how the Git client will interpret their values.

Clients MUST parse the line according to the above format, lines that do not conform to the format SHOULD be discarded. The user MAY be warned in such a case.

bundle-uri CLIENT AND SERVER EXPECTATIONS

URI CONTENTS

The advertised URI may contain a bundle file that git bundle verify would accept. I.e. they MUST contain one or more reference tips for use by the client, MUST indicate prerequisites (in any) with standard "-" prefixes, and MUST indicate their "object-format", if applicable.

The advertised URI may alternatively contain a plaintext file that git config --list would accept (with the --file option). The key-value pairs in this list are in the bundle.* namespace (see git-config(1)).

bundle-uri CLIENT ERROR RECOVERY

Server operators should feel confident in turning on "bundle-uri" and not worry if e.g. their CDN goes down that clones or fetches will run into hard failures. Even if the server bundle(s) are incomplete, or bad in some way the client should still end up with a functioning repository, just as if it had chosen not to use this protocol extension.

All subsequent discussion on client and server interaction MUST keep this in mind.

bundle-uri SERVER TO CLIENT

A server MAY even return bundle(s) that don’t have any direct relationship to the repository being cloned (either through accident, or intentional "clever" configuration), and expect a client to sort out what data they’d like from the bundle(s), if any.

bundle-uri CLIENT TO SERVER

WHEN ADVERTISED BUNDLE(S) REQUIRE NO FURTHER NEGOTIATION

EARLY CLIENT DISCONNECTIONS AND ERROR RECOVERY

I.e. a client might need to re-connect and issue a fetch command, and possibly fall back to not making use of bundle-uri at all.

This "MAY" behavior is specified as such (and not a "SHOULD") on the assumption that a server advertising bundle uris is more likely than not to be serving up a relatively large repository, and to be pointing to URIs that have a good chance of being in working order. A client MAY e.g. look at the payload size of the bundles as a heuristic to see if an early disconnect is worth it, should falling back on a full "fetch" dialog be necessary.

WHEN ADVERTISED BUNDLE(S) REQUIRE FURTHER NEGOTIATION

This allows for aggressive early disconnects from any interactive server dialog. The client blindly trusts that the advertised OID tips are relevant, and issues them as have lines, it then requests any tips it would like (usually from the "ls-refs" advertisement) via want lines. The server will then compute a (hopefully small) PACK with the expected difference between the tips from the bundle(s) and the data requested.

The only connection the client then needs to keep active is to the concurrently downloading static bundle(s), when those and the incremental PACK are retrieved they should be inflated and validated. Any errors at this point should be gracefully recovered from, see above.

bundle-uri PROTOCOL FEATURES

The client constructs a bundle list from the <key>=<value> pairs provided by the server. These pairs are part of the bundle.* namespace as documented in git-config(1). In this section, we discuss some of these keys and describe the actions the client will do in response to this information.

In particular, the bundle.version key specifies an integer value. The only accepted value at the moment is 1, but if the client sees an unexpected value here then the client MUST ignore the bundle list.

As long as bundle.version is understood, all other unknown keys MAY be ignored by the client. The server will guarantee compatibility with older clients, though newer clients may be better able to use the extra keys to minimize downloads.

Any backwards-incompatible addition of pre-URI key-value will be guarded by a new bundle.version value or values in bundle-uri capability advertisement itself, and/or by new future bundle-uri request arguments.

Some example key-value pairs that are not currently implemented but could be implemented in the future include:

This would allow for optimizing the common case of servers who’d like to provide one "big bundle" containing only their "main" branch, and/or incremental updates thereof.

A client receiving such a a response MAY assume that they can skip retrieving the header from a bundle at the indicated URI, and thus save themselves and the server(s) the request(s) needed to inspect the headers of that bundle or bundles.

GIT#

Part of the git(1) suite

NOTES#

1.

api-trace2