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<body class="manpage">
<div id="header">
<h1>gitprotocol-pack(5) Manual Page</h1>
<h2 id="_name">NAME</h2>
<div class="sectionbody">
<p>gitprotocol-pack - How packs are transferred over-the-wire</p>
</div>
</div>
<div id="content">
<div class="sect1">
<h2 id="_synopsis">SYNOPSIS</h2>
<div class="sectionbody">
<div class="verseblock">
<pre class="content">&lt;over-the-wire-protocol&gt;</pre>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_description">DESCRIPTION</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Git supports transferring data in packfiles over the ssh://, git://, http:// and
file:// transports.  There exist two sets of protocols, one for pushing
data from a client to a server and another for fetching data from a
server to a client.  The three transports (ssh, git, file) use the same
protocol to transfer data. http is documented in <a href="gitprotocol-http.html">gitprotocol-http(5)</a>.</p>
</div>
<div class="paragraph">
<p>The processes invoked in the canonical Git implementation are <em>upload-pack</em>
on the server side and <em>fetch-pack</em> on the client side for fetching data;
then <em>receive-pack</em> on the server and <em>send-pack</em> on the client for pushing
data.  The protocol functions to have a server tell a client what is
currently on the server, then for the two to negotiate the smallest amount
of data to send in order to fully update one or the other.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_pkt_line_format">pkt-line Format</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The descriptions below build on the pkt-line format described in
<a href="gitprotocol-common.html">gitprotocol-common(5)</a>. When the grammar indicates <code>PKT-LINE</code>(<code>...</code>), unless
otherwise noted the usual pkt-line LF rules apply: the sender SHOULD
include a LF, but the receiver MUST NOT complain if it is not present.</p>
</div>
<div class="paragraph">
<p>An error packet is a special pkt-line that contains an error string.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>  error-line     =  PKT-LINE("ERR" SP explanation-text)</pre>
</div>
</div>
<div class="paragraph">
<p>Throughout the protocol, where <code>PKT-LINE</code>(<code>...</code>) is expected, an error packet MAY
be sent. Once this packet is sent by a client or a server, the data transfer
process defined in this protocol is terminated.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_transports">Transports</h2>
<div class="sectionbody">
<div class="paragraph">
<p>There are three transports over which the packfile protocol is
initiated.  The Git transport is a simple, unauthenticated server that
takes the command (almost always <em>upload-pack</em>, though Git
servers can be configured to be globally writable, in which <em>receive-
pack</em> initiation is also allowed) with which the client wishes to
communicate and executes it and connects it to the requesting
process.</p>
</div>
<div class="paragraph">
<p>In the SSH transport, the client just runs the <em>upload-pack</em>
or <em>receive-pack</em> process on the server over the SSH protocol and then
communicates with that invoked process over the SSH connection.</p>
</div>
<div class="paragraph">
<p>The file:// transport runs the <em>upload-pack</em> or <em>receive-pack</em>
process locally and communicates with it over a pipe.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_extra_parameters">Extra Parameters</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The protocol provides a mechanism in which clients can send additional
information in its first message to the server. These are called "Extra
Parameters", and are supported by the Git, SSH, and HTTP protocols.</p>
</div>
<div class="paragraph">
<p>Each Extra Parameter takes the form of <em>&lt;key&gt;</em><code>=</code><em>&lt;value&gt;</em> or <em>&lt;key&gt;</em>.</p>
</div>
<div class="paragraph">
<p>Servers that receive any such Extra Parameters MUST ignore all
unrecognized keys. Currently, the only Extra Parameter recognized is
"version" with a value of <em>1</em> or <em>2</em>.  See <a href="gitprotocol-v2.html">gitprotocol-v2(5)</a> for more
information on protocol version 2.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_git_transport">Git Transport</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The Git transport starts off by sending the command and repository
on the wire using the pkt-line format, followed by a NUL byte and a
hostname parameter, terminated by a NUL byte.</p>
</div>
<div class="literalblock">
<div class="content">
<pre>0033git-upload-pack /project.git\0host=myserver.com\0</pre>
</div>
</div>
<div class="paragraph">
<p>The transport may send Extra Parameters by adding an additional NUL
byte, and then adding one or more NUL-terminated strings:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>003egit-upload-pack /project.git\0host=myserver.com\0\0version=1\0</pre>
</div>
</div>
<div class="openblock">
<div class="content">
<div class="literalblock">
<div class="content">
<pre>git-proto-request = request-command SP pathname NUL
      [ host-parameter NUL ] [ NUL extra-parameters ]
request-command   = "git-upload-pack" / "git-receive-pack" /
      "git-upload-archive"   ; case sensitive
pathname          = *( %x01-ff ) ; exclude NUL
host-parameter    = "host=" hostname [ ":" port ]
extra-parameters  = 1*extra-parameter
extra-parameter   = 1*( %x01-ff ) NUL</pre>
</div>
</div>
</div>
</div>
<div class="paragraph">
<p>host-parameter is used for the
git-daemon name based virtual hosting.  See --interpolated-path
option to git daemon, with the %H/%CH format characters.</p>
</div>
<div class="paragraph">
<p>Basically what the Git client is doing to connect to an <em>upload-pack</em>
process on the server side over the Git protocol is this:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>$ echo -e -n \
  "003agit-upload-pack /schacon/gitbook.git\0host=example.com\0" |
  nc -v example.com 9418</pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_ssh_transport">SSH Transport</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Initiating the upload-pack or receive-pack processes over SSH is
executing the binary on the server via SSH remote execution.
It is basically equivalent to running this:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>$ ssh git.example.com "git-upload-pack '/project.git'"</pre>
</div>
</div>
<div class="paragraph">
<p>For a server to support Git pushing and pulling for a given user over
SSH, that user needs to be able to execute one or both of those
commands via the SSH shell that they are provided on login.  On some
systems, that shell access is limited to only being able to run those
two commands, or even just one of them.</p>
</div>
<div class="paragraph">
<p>In an ssh:// format URI, it&#8217;s absolute in the URI, so the <em>/</em> after
the host name (or port number) is sent as an argument, which is then
read by the remote git-upload-pack exactly as is, so it&#8217;s effectively
an absolute path in the remote filesystem.</p>
</div>
<div class="literalblock">
<div class="content">
<pre>   git clone ssh://user@example.com/project.git
  |
  v
ssh user@example.com "git-upload-pack '/project.git'"</pre>
</div>
</div>
<div class="paragraph">
<p>In a "user@host:path" format URI, it&#8217;s relative to the user&#8217;s home
directory, because the Git client will run:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>   git clone user@example.com:project.git
    |
    v
ssh user@example.com "git-upload-pack 'project.git'"</pre>
</div>
</div>
<div class="paragraph">
<p>The exception is if a <em>~</em> is used, in which case
we execute it without the leading <em>/</em>.</p>
</div>
<div class="literalblock">
<div class="content">
<pre>   ssh://user@example.com/~alice/project.git,
    |
    v
ssh user@example.com "git-upload-pack '~alice/project.git'"</pre>
</div>
</div>
<div class="paragraph">
<p>Depending on the value of the <code>protocol.version</code> configuration variable,
Git may attempt to send Extra Parameters as a colon-separated string in
the GIT_PROTOCOL environment variable. This is done only if
the <code>ssh.variant</code> configuration variable indicates that the ssh command
supports passing environment variables as an argument.</p>
</div>
<div class="paragraph">
<p>A few things to remember here:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>The "command name" is spelled with dash (e.g. git-upload-pack), but
this can be overridden by the client;</p>
</li>
<li>
<p>The repository path is always quoted with single quotes.</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_fetching_data_from_a_server">Fetching Data From a Server</h2>
<div class="sectionbody">
<div class="paragraph">
<p>When one Git repository wants to get data that a second repository
has, the first can <em>fetch</em> from the second.  This operation determines
what data the server has that the client does not then streams that
data down to the client in packfile format.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_reference_discovery">Reference Discovery</h2>
<div class="sectionbody">
<div class="paragraph">
<p>When the client initially connects the server will immediately respond
with a version number (if "version=1" is sent as an Extra Parameter),
and a listing of each reference it has (all branches and tags) along
with the object name that each reference currently points to.</p>
</div>
<div class="literalblock">
<div class="content">
<pre> $ echo -e -n "0045git-upload-pack /schacon/gitbook.git\0host=example.com\0\0version=1\0" |
    nc -v example.com 9418
 000eversion 1
 00887217a7c7e582c46cec22a130adf4b9d7d950fba0 HEAD\0multi_ack thin-pack
side-band side-band-64k ofs-delta shallow no-progress include-tag
 00441d3fcd5ced445d1abc402225c0b8a1299641f497 refs/heads/integration
 003f7217a7c7e582c46cec22a130adf4b9d7d950fba0 refs/heads/master
 003cb88d2441cac0977faf98efc80305012112238d9d refs/tags/v0.9
 003c525128480b96c89e6418b1e40909bf6c5b2d580f refs/tags/v1.0
 003fe92df48743b7bc7d26bcaabfddde0a1e20cae47c refs/tags/v1.0^{}
 0000</pre>
</div>
</div>
<div class="paragraph">
<p>The returned response is a pkt-line stream describing each ref and
its current value.  The stream MUST be sorted by name according to
the C locale ordering.</p>
</div>
<div class="paragraph">
<p>If HEAD is a valid ref, HEAD MUST appear as the first advertised
ref.  If HEAD is not a valid ref, HEAD MUST NOT appear in the
advertisement list at all, but other refs may still appear.</p>
</div>
<div class="paragraph">
<p>The stream MUST include capability declarations behind a NUL on the
first ref. The peeled value of a ref (that is "ref^{}") MUST be
immediately after the ref itself, if presented. A conforming server
MUST peel the ref if it&#8217;s an annotated tag.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>  advertised-refs  =  *1("version 1")
                      (no-refs / list-of-refs)
                      *shallow
                      flush-pkt

  no-refs          =  PKT-LINE(zero-id SP "capabilities^{}"
                      NUL capability-list)

  list-of-refs     =  first-ref *other-ref
  first-ref        =  PKT-LINE(obj-id SP refname
                      NUL capability-list)

  other-ref        =  PKT-LINE(other-tip / other-peeled)
  other-tip        =  obj-id SP refname
  other-peeled     =  obj-id SP refname "^{}"

  shallow          =  PKT-LINE("shallow" SP obj-id)

  capability-list  =  capability *(SP capability)
  capability       =  1*(LC_ALPHA / DIGIT / "-" / "_")
  LC_ALPHA         =  %x61-7A</pre>
</div>
</div>
<div class="paragraph">
<p>Server and client MUST use lowercase for obj-id, both MUST treat obj-id
as case-insensitive.</p>
</div>
<div class="paragraph">
<p>See protocol-capabilities.txt for a list of allowed server capabilities
and descriptions.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_packfile_negotiation">Packfile Negotiation</h2>
<div class="sectionbody">
<div class="paragraph">
<p>After reference and capabilities discovery, the client can decide to
terminate the connection by sending a flush-pkt, telling the server it can
now gracefully terminate, and disconnect, when it does not need any pack
data. This can happen with the ls-remote command, and also can happen when
the client already is up to date.</p>
</div>
<div class="paragraph">
<p>Otherwise, it enters the negotiation phase, where the client and
server determine what the minimal packfile necessary for transport is,
by telling the server what objects it wants, its shallow objects
(if any), and the maximum commit depth it wants (if any).  The client
will also send a list of the capabilities it wants to be in effect,
out of what the server said it could do with the first <em>want</em> line.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>  upload-request    =  want-list
                       *shallow-line
                       *1depth-request
                       [filter-request]
                       flush-pkt

  want-list         =  first-want
                       *additional-want

  shallow-line      =  PKT-LINE("shallow" SP obj-id)

  depth-request     =  PKT-LINE("deepen" SP depth) /
                       PKT-LINE("deepen-since" SP timestamp) /
                       PKT-LINE("deepen-not" SP ref)

  first-want        =  PKT-LINE("want" SP obj-id SP capability-list)
  additional-want   =  PKT-LINE("want" SP obj-id)

  depth             =  1*DIGIT

  filter-request    =  PKT-LINE("filter" SP filter-spec)</pre>
</div>
</div>
<div class="paragraph">
<p>Clients MUST send all the obj-ids it wants from the reference
discovery phase as <em>want</em> lines. Clients MUST send at least one
<em>want</em> command in the request body. Clients MUST NOT mention an
obj-id in a <em>want</em> command which did not appear in the response
obtained through ref discovery.</p>
</div>
<div class="paragraph">
<p>The client MUST write all obj-ids which it only has shallow copies
of (meaning that it does not have the parents of a commit) as
<em>shallow</em> lines so that the server is aware of the limitations of
the client&#8217;s history.</p>
</div>
<div class="paragraph">
<p>The client now sends the maximum commit history depth it wants for
this transaction, which is the number of commits it wants from the
tip of the history, if any, as a <em>deepen</em> line.  A depth of 0 is the
same as not making a depth request. The client does not want to receive
any commits beyond this depth, nor does it want objects needed only to
complete those commits. Commits whose parents are not received as a
result are defined as shallow and marked as such in the server. This
information is sent back to the client in the next step.</p>
</div>
<div class="paragraph">
<p>The client can optionally request that pack-objects omit various
objects from the packfile using one of several filtering techniques.
These are intended for use with partial clone and partial fetch
operations. An object that does not meet a filter-spec value is
omitted unless explicitly requested in a <em>want</em> line. See <code>rev-list</code>
for possible filter-spec values.</p>
</div>
<div class="paragraph">
<p>Once all the <em>want&#8217;s and 'shallow&#8217;s (and optional 'deepen</em>) are
transferred, clients MUST send a flush-pkt, to tell the server side
that it is done sending the list.</p>
</div>
<div class="paragraph">
<p>Otherwise, if the client sent a positive depth request, the server
will determine which commits will and will not be shallow and
send this information to the client. If the client did not request
a positive depth, this step is skipped.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>  shallow-update   =  *shallow-line
                      *unshallow-line
                      flush-pkt

  shallow-line     =  PKT-LINE("shallow" SP obj-id)

  unshallow-line   =  PKT-LINE("unshallow" SP obj-id)</pre>
</div>
</div>
<div class="paragraph">
<p>If the client has requested a positive depth, the server will compute
the set of commits which are no deeper than the desired depth. The set
of commits starts at the client&#8217;s wants.</p>
</div>
<div class="paragraph">
<p>The server writes <em>shallow</em> lines for each
commit whose parents will not be sent as a result. The server writes
an <em>unshallow</em> line for each commit which the client has indicated is
shallow, but is no longer shallow at the currently requested depth
(that is, its parents will now be sent). The server MUST NOT mark
as unshallow anything which the client has not indicated was shallow.</p>
</div>
<div class="paragraph">
<p>Now the client will send a list of the obj-ids it has using <em>have</em>
lines, so the server can make a packfile that only contains the objects
that the client needs. In multi_ack mode, the canonical implementation
will send up to 32 of these at a time, then will send a flush-pkt. The
canonical implementation will skip ahead and send the next 32 immediately,
so that there is always a block of 32 "in-flight on the wire" at a time.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>  upload-haves      =  have-list
                       compute-end

  have-list         =  *have-line
  have-line         =  PKT-LINE("have" SP obj-id)
  compute-end       =  flush-pkt / PKT-LINE("done")</pre>
</div>
</div>
<div class="paragraph">
<p>If the server reads <em>have</em> lines, it then will respond by ACKing any
of the obj-ids the client said it had that the server also has. The
server will ACK obj-ids differently depending on which ack mode is
chosen by the client.</p>
</div>
<div class="paragraph">
<p>In multi_ack mode:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>the server will respond with <em>ACK obj-id continue</em> for any common
commits.</p>
</li>
<li>
<p>once the server has found an acceptable common base commit and is
ready to make a packfile, it will blindly ACK all <em>have</em> obj-ids
back to the client.</p>
</li>
<li>
<p>the server will then send a <em>NAK</em> and then wait for another response
from the client - either a <em>done</em> or another list of <em>have</em> lines.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>In multi_ack_detailed mode:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>the server will differentiate the ACKs where it is signaling
that it is ready to send data with <em>ACK obj-id ready</em> lines, and
signals the identified common commits with <em>ACK obj-id common</em> lines.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>Without either multi_ack or multi_ack_detailed:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>upload-pack sends "ACK obj-id" on the first common object it finds.
After that it says nothing until the client gives it a "done".</p>
</li>
<li>
<p>upload-pack sends "NAK" on a flush-pkt if no common object
has been found yet.  If one has been found, and thus an ACK
was already sent, it&#8217;s silent on the flush-pkt.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>After the client has gotten enough ACK responses that it can determine
that the server has enough information to send an efficient packfile
(in the canonical implementation, this is determined when it has received
enough ACKs that it can color everything left in the --date-order queue
as common with the server, or the --date-order queue is empty), or the
client determines that it wants to give up (in the canonical implementation,
this is determined when the client sends 256 <em>have</em> lines without getting
any of them ACKed by the server - meaning there is nothing in common and
the server should just send all of its objects), then the client will send
a <em>done</em> command.  The <em>done</em> command signals to the server that the client
is ready to receive its packfile data.</p>
</div>
<div class="paragraph">
<p>However, the 256 limit <strong>only</strong> turns on in the canonical client
implementation if we have received at least one "ACK %s continue"
during a prior round.  This helps to ensure that at least one common
ancestor is found before we give up entirely.</p>
</div>
<div class="paragraph">
<p>Once the <em>done</em> line is read from the client, the server will either
send a final <em>ACK obj-id</em> or it will send a <em>NAK</em>. <em>obj-id</em> is the object
name of the last commit determined to be common. The server only sends
ACK after <em>done</em> if there is at least one common base and multi_ack or
multi_ack_detailed is enabled. The server always sends NAK after <em>done</em>
if there is no common base found.</p>
</div>
<div class="paragraph">
<p>Instead of <em>ACK</em> or <em>NAK</em>, the server may send an error message (for
example, if it does not recognize an object in a <em>want</em> line received
from the client).</p>
</div>
<div class="paragraph">
<p>Then the server will start sending its packfile data.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>  server-response = *ack_multi ack / nak
  ack_multi       = PKT-LINE("ACK" SP obj-id ack_status)
  ack_status      = "continue" / "common" / "ready"
  ack             = PKT-LINE("ACK" SP obj-id)
  nak             = PKT-LINE("NAK")</pre>
</div>
</div>
<div class="paragraph">
<p>A simple clone may look like this (with no <em>have</em> lines):</p>
</div>
<div class="listingblock">
<div class="content">
<pre>   C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d multi_ack \
     side-band-64k ofs-delta\n
   C: 0032want 7d1665144a3a975c05f1f43902ddaf084e784dbe\n
   C: 0032want 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a\n
   C: 0032want 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01\n
   C: 0032want 74730d410fcb6603ace96f1dc55ea6196122532d\n
   C: 0000
   C: 0009done\n

   S: 0008NAK\n
   S: [PACKFILE]</pre>
</div>
</div>
<div class="paragraph">
<p>An incremental update (fetch) response might look like this:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>   C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d multi_ack \
     side-band-64k ofs-delta\n
   C: 0032want 7d1665144a3a975c05f1f43902ddaf084e784dbe\n
   C: 0032want 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a\n
   C: 0000
   C: 0032have 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01\n
   C: [30 more have lines]
   C: 0032have 74730d410fcb6603ace96f1dc55ea6196122532d\n
   C: 0000

   S: 003aACK 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01 continue\n
   S: 003aACK 74730d410fcb6603ace96f1dc55ea6196122532d continue\n
   S: 0008NAK\n

   C: 0009done\n

   S: 0031ACK 74730d410fcb6603ace96f1dc55ea6196122532d\n
   S: [PACKFILE]</pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_packfile_data">Packfile Data</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Now that the client and server have finished negotiation about what
the minimal amount of data that needs to be sent to the client is, the server
will construct and send the required data in packfile format.</p>
</div>
<div class="paragraph">
<p>See <a href="gitformat-pack.html">gitformat-pack(5)</a> for what the packfile itself actually looks like.</p>
</div>
<div class="paragraph">
<p>If <em>side-band</em> or <em>side-band-64k</em> capabilities have been specified by
the client, the server will send the packfile data multiplexed.</p>
</div>
<div class="paragraph">
<p>Each packet starting with the packet-line length of the amount of data
that follows, followed by a single byte specifying the sideband the
following data is coming in on.</p>
</div>
<div class="paragraph">
<p>In <em>side-band</em> mode, it will send up to 999 data bytes plus 1 control
code, for a total of up to 1000 bytes in a pkt-line.  In <em>side-band-64k</em>
mode it will send up to 65519 data bytes plus 1 control code, for a
total of up to 65520 bytes in a pkt-line.</p>
</div>
<div class="paragraph">
<p>The sideband byte will be a <em>1</em>, <em>2</em> or a <em>3</em>. Sideband <em>1</em> will contain
packfile data, sideband <em>2</em> will be used for progress information that the
client will generally print to stderr and sideband <em>3</em> is used for error
information.</p>
</div>
<div class="paragraph">
<p>If no <em>side-band</em> capability was specified, the server will stream the
entire packfile without multiplexing.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_pushing_data_to_a_server">Pushing Data To a Server</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Pushing data to a server will invoke the <em>receive-pack</em> process on the
server, which will allow the client to tell it which references it should
update and then send all the data the server will need for those new
references to be complete.  Once all the data is received and validated,
the server will then update its references to what the client specified.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_authentication">Authentication</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The protocol itself contains no authentication mechanisms.  That is to be
handled by the transport, such as SSH, before the <em>receive-pack</em> process is
invoked.  If <em>receive-pack</em> is configured over the Git transport, those
repositories will be writable by anyone who can access that port (9418) as
that transport is unauthenticated.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_reference_discovery_2">Reference Discovery</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The reference discovery phase is done nearly the same way as it is in the
fetching protocol. Each reference obj-id and name on the server is sent
in packet-line format to the client, followed by a flush-pkt.  The only
real difference is that the capability listing is different - the only
possible values are <em>report-status</em>, <em>report-status-v2</em>, <em>delete-refs</em>,
<em>ofs-delta</em>, <em>atomic</em> and <em>push-options</em>.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_reference_update_request_and_packfile_transfer">Reference Update Request and Packfile Transfer</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Once the client knows what references the server is at, it can send a
list of reference update requests.  For each reference on the server
that it wants to update, it sends a line listing the obj-id currently on
the server, the obj-id the client would like to update it to and the name
of the reference.</p>
</div>
<div class="paragraph">
<p>This list is followed by a flush-pkt.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>  update-requests   =  *shallow ( command-list | push-cert )

  shallow           =  PKT-LINE("shallow" SP obj-id)

  command-list      =  PKT-LINE(command NUL capability-list)
                       *PKT-LINE(command)
                       flush-pkt

  command           =  create / delete / update
  create            =  zero-id SP new-id  SP name
  delete            =  old-id  SP zero-id SP name
  update            =  old-id  SP new-id  SP name

  old-id            =  obj-id
  new-id            =  obj-id

  push-cert         = PKT-LINE("push-cert" NUL capability-list LF)
                      PKT-LINE("certificate version 0.1" LF)
                      PKT-LINE("pusher" SP ident LF)
                      PKT-LINE("pushee" SP url LF)
                      PKT-LINE("nonce" SP nonce LF)
                      *PKT-LINE("push-option" SP push-option LF)
                      PKT-LINE(LF)
                      *PKT-LINE(command LF)
                      *PKT-LINE(gpg-signature-lines LF)
                      PKT-LINE("push-cert-end" LF)

  push-option       =  1*( VCHAR | SP )</pre>
</div>
</div>
<div class="paragraph">
<p>If the server has advertised the <em>push-options</em> capability and the client has
specified <em>push-options</em> as part of the capability list above, the client then
sends its push options followed by a flush-pkt.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>  push-options      =  *PKT-LINE(push-option) flush-pkt</pre>
</div>
</div>
<div class="paragraph">
<p>For backwards compatibility with older Git servers, if the client sends a push
cert and push options, it MUST send its push options both embedded within the
push cert and after the push cert. (Note that the push options within the cert
are prefixed, but the push options after the cert are not.) Both these lists
MUST be the same, modulo the prefix.</p>
</div>
<div class="paragraph">
<p>After that the packfile that
should contain all the objects that the server will need to complete the new
references will be sent.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>  packfile          =  "PACK" 28*(OCTET)</pre>
</div>
</div>
<div class="paragraph">
<p>If the receiving end does not support delete-refs, the sending end MUST
NOT ask for delete command.</p>
</div>
<div class="paragraph">
<p>If the receiving end does not support push-cert, the sending end
MUST NOT send a push-cert command.  When a push-cert command is
sent, command-list MUST NOT be sent; the commands recorded in the
push certificate is used instead.</p>
</div>
<div class="paragraph">
<p>The packfile MUST NOT be sent if the only command used is <em>delete</em>.</p>
</div>
<div class="paragraph">
<p>A packfile MUST be sent if either create or update command is used,
even if the server already has all the necessary objects.  In this
case the client MUST send an empty packfile.   The only time this
is likely to happen is if the client is creating
a new branch or a tag that points to an existing obj-id.</p>
</div>
<div class="paragraph">
<p>The server will receive the packfile, unpack it, then validate each
reference that is being updated that it hasn&#8217;t changed while the request
was being processed (the obj-id is still the same as the old-id), and
it will run any update hooks to make sure that the update is acceptable.
If all of that is fine, the server will then update the references.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_push_certificate">Push Certificate</h2>
<div class="sectionbody">
<div class="paragraph">
<p>A push certificate begins with a set of header lines.  After the
header and an empty line, the protocol commands follow, one per
line. Note that the trailing LF in push-cert PKT-LINEs is <em>not</em>
optional; it must be present.</p>
</div>
<div class="paragraph">
<p>Currently, the following header fields are defined:</p>
</div>
<div class="dlist">
<dl>
<dt class="hdlist1"><code>pusher</code> ident</dt>
<dd>
<p>Identify the GPG key in "Human Readable Name &lt;email@address&gt;"
format.</p>
</dd>
<dt class="hdlist1"><code>pushee</code> url</dt>
<dd>
<p>The repository URL (anonymized, if the URL contains
authentication material) the user who ran <code>git</code> <code>push</code>
intended to push into.</p>
</dd>
<dt class="hdlist1"><code>nonce</code> nonce</dt>
<dd>
<p>The <em>nonce</em> string the receiving repository asked the
pushing user to include in the certificate, to prevent
replay attacks.</p>
</dd>
</dl>
</div>
<div class="paragraph">
<p>The GPG signature lines are a detached signature for the contents
recorded in the push certificate before the signature block begins.
The detached signature is used to certify that the commands were
given by the pusher, who must be the signer.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_report_status">Report Status</h2>
<div class="sectionbody">
<div class="paragraph">
<p>After receiving the pack data from the sender, the receiver sends a
report if <em>report-status</em> or <em>report-status-v2</em> capability is in effect.
It is a short listing of what happened in that update.  It will first
list the status of the packfile unpacking as either <em>unpack ok</em> or
<em>unpack [error]</em>.  Then it will list the status for each of the references
that it tried to update.  Each line is either <em>ok [refname]</em> if the
update was successful, or <em>ng [refname] [error]</em> if the update was not.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>  report-status     = unpack-status
                      1*(command-status)
                      flush-pkt

  unpack-status     = PKT-LINE("unpack" SP unpack-result)
  unpack-result     = "ok" / error-msg

  command-status    = command-ok / command-fail
  command-ok        = PKT-LINE("ok" SP refname)
  command-fail      = PKT-LINE("ng" SP refname SP error-msg)

  error-msg         = 1*(OCTET) ; where not "ok"</pre>
</div>
</div>
<div class="paragraph">
<p>The <em>report-status-v2</em> capability extends the protocol by adding new option
lines in order to support reporting of reference rewritten by the
<em>proc-receive</em> hook.  The <em>proc-receive</em> hook may handle a command for a
pseudo-reference which may create or update one or more references, and each
reference may have different name, different new-oid, and different old-oid.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>  report-status-v2  = unpack-status
                      1*(command-status-v2)
                      flush-pkt

  unpack-status     = PKT-LINE("unpack" SP unpack-result)
  unpack-result     = "ok" / error-msg

  command-status-v2 = command-ok-v2 / command-fail
  command-ok-v2     = command-ok
                      *option-line

  command-ok        = PKT-LINE("ok" SP refname)
  command-fail      = PKT-LINE("ng" SP refname SP error-msg)

  error-msg         = 1*(OCTET) ; where not "ok"

  option-line       = *1(option-refname)
                      *1(option-old-oid)
                      *1(option-new-oid)
                      *1(option-forced-update)

  option-refname    = PKT-LINE("option" SP "refname" SP refname)
  option-old-oid    = PKT-LINE("option" SP "old-oid" SP obj-id)
  option-new-oid    = PKT-LINE("option" SP "new-oid" SP obj-id)
  option-force      = PKT-LINE("option" SP "forced-update")</pre>
</div>
</div>
<div class="paragraph">
<p>Updates can be unsuccessful for a number of reasons.  The reference can have
changed since the reference discovery phase was originally sent, meaning
someone pushed in the meantime.  The reference being pushed could be a
non-fast-forward reference and the update hooks or configuration could be
set to not allow that, etc.  Also, some references can be updated while others
can be rejected.</p>
</div>
<div class="paragraph">
<p>An example client/server communication might look like this:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>   S: 006274730d410fcb6603ace96f1dc55ea6196122532d refs/heads/local\0report-status delete-refs ofs-delta\n
   S: 003e7d1665144a3a975c05f1f43902ddaf084e784dbe refs/heads/debug\n
   S: 003f74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/master\n
   S: 003d74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/team\n
   S: 0000

   C: 00677d1665144a3a975c05f1f43902ddaf084e784dbe 74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/debug\n
   C: 006874730d410fcb6603ace96f1dc55ea6196122532d 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a refs/heads/master\n
   C: 0000
   C: [PACKDATA]

   S: 000eunpack ok\n
   S: 0018ok refs/heads/debug\n
   S: 002ang refs/heads/master non-fast-forward\n</pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_git">GIT</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Part of the <a href="git.html">git(1)</a> suite</p>
</div>
</div>
</div>
</div>
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<div id="footer-text">
Last updated 2023-10-23 14:43:46 -0700
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