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+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: 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).
+
+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 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. 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.
+
+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 | wanted-refs | 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
+
+    wanted-refs = PKT-LINE("wanted-refs" LF)
+		  *PKT-LINE(wanted-ref LF)
+    wanted-ref = obj-id SP refname
+
+    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.
+
+    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 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.