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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 14:47:53 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 14:47:53 +0000
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+gitprotocol-v2(5)
+=================
+
+NAME
+----
+gitprotocol-v2 - Git Wire Protocol, Version 2
+
+SYNOPSIS
+--------
+[verse]
+<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:
+
+ * 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
+linkgit:gitprotocol-pack[5] and linkgit:gitprotocol-common[5] 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
+ * '0002' Response End Packet (response-end-pkt) - indicates the end of a
+ response for stateless connections
+
+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 linkgit:gitprotocol-pack[5] and linkgit: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 linkgit: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
+linkgit: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
+ 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
+ delim-pkt
+ command-args
+ 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
+ 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. Multiple instances may be
+ given, in which case references matching any prefix will be
+ shown. Note that this is purely for optimization; a server MAY
+ show refs not matching the prefix if it chooses, and clients
+ should filter the result themselves.
+
+If the 'unborn' feature is advertised the following argument can be
+included in the client's request.
+
+ unborn
+ The server will send information about HEAD even if it is a symref
+ pointing to an unborn branch in the form "unborn HEAD
+ symref-target:<target>".
+
+The output of ls-refs is as follows:
+
+ output = *ref
+ 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>
+ 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 (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>
+ 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. When communicating
+ with other processes, senders SHOULD translate scaled integers
+ (e.g. "1k") into a fully-expanded form (e.g. "1024") to aid
+ interoperability with older receivers that may not understand
+ newly-invented scaling suffixes. However, receivers SHOULD
+ accept the following suffixes: 'k', 'm', and 'g' for 1024,
+ 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>
+ Indicates to the server that the client wants to retrieve a
+ particular ref, where <ref> is the full name of a ref on the
+ server.
+
+If the 'sideband-all' feature is advertised, the following argument can be
+included in the client's request:
+
+ sideband-all
+ Instruct the server to send the whole response multiplexed, not just
+ the packfile section. All non-flush and non-delim PKT-LINE in the
+ response (not only in the packfile section) will then start with a byte
+ indicating its sideband (1, 2, or 3), and the server may send "0005\2"
+ (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.
+
+ packfile-uris <comma-separated list of protocols>
+ Indicates to the server that the client is willing to receive
+ URIs of any of the given protocols in place of objects in the
+ sent packfile. Before performing the connectivity check, the
+ client should download from all given URIs. Currently, the
+ 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
+ Indicates to the server that it should never send "ready", but
+ should wait for the client to say "done" before sending the
+ 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 |
+ [acknowledgments delim-pkt] [shallow-info delim-pkt]
+ [wanted-refs delim-pkt] [packfile-uris delim-pkt]
+ packfile flush-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
+
+ wanted-refs = PKT-LINE("wanted-refs" LF)
+ *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)
+ *PKT-LINE(%x01-03 *%x00-ff)
+
+ acknowledgments section
+ * If the client determines that it is finished with negotiations by
+ sending a "done" line (thus requiring the server to send a packfile),
+ 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.
+
+ wanted-refs section
+ * This section is only included if the client has requested a
+ ref using a 'want-ref' line and if a packfile section is also
+ included in the response.
+
+ * Always begins with the section header "wanted-refs".
+
+ * The server will send a ref listing ("<oid> <refname>") for
+ each reference requested using 'want-ref' lines.
+
+ * The server MUST NOT send any refs which were not requested
+ using 'want-ref' lines.
+
+ packfile-uris section
+ * This section is only included if the client sent
+ 'packfile-uris' and the server has at least one such URI to
+ send.
+
+ * Always begins with the section header "packfile-uris".
+
+ * For each URI the server sends, it sends a hash of the pack's
+ contents (as output by git index-pack) followed by the URI.
+
+ * The hashes are 40 hex characters long. When Git upgrades to a new
+ hash algorithm, this might need to be updated. (It should match
+ whatever index-pack outputs after "pack\t" or "keep\t".
+
+ 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.
+
+ 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
+link:technical/api-trace2.html[api-trace2] 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)
+ *PKT-LINE(obj-info LF)
+
+ attrs = attr | attrs SP attrs
+
+ attr = "size"
+
+ obj-info = obj-id SP obj-size
+
+GIT
+---
+Part of the linkgit:git[1] suite