Documentation/technical/protocol-v2.txt

   1  Git Wire Protocol, Version 2
   2 ==============================
   3
   4 This document presents a specification for a version 2 of Git's wire
   5 protocol.  Protocol v2 will improve upon v1 in the following ways:
   6
   7   * Instead of multiple service names, multiple commands will be
   8     supported by a single service
   9   * Easily extendable as capabilities are moved into their own section
  10     of the protocol, no longer being hidden behind a NUL byte and
  11     limited by the size of a pkt-line
  12   * Separate out other information hidden behind NUL bytes (e.g. agent
  13     string as a capability and symrefs can be requested using 'ls-refs')
  14   * Reference advertisement will be omitted unless explicitly requested
  15   * ls-refs command to explicitly request some refs
  16   * Designed with http and stateless-rpc in mind.  With clear flush
  17     semantics the http remote helper can simply act as a proxy
  18
  19 In protocol v2 communication is command oriented.  When first contacting a
  20 server a list of capabilities will advertised.  Some of these capabilities
  21 will be commands which a client can request be executed.  Once a command
  22 has completed, a client can reuse the connection and request that other
  23 commands be executed.
  24
  25  Packet-Line Framing
  26 ---------------------
  27
  28 All communication is done using packet-line framing, just as in v1.  See
  29 `Documentation/technical/pack-protocol.txt` and
  30 `Documentation/technical/protocol-common.txt` for more information.
  31
  32 In protocol v2 these special packets will have the following semantics:
  33
  34   * '0000' Flush Packet (flush-pkt) - indicates the end of a message
  35   * '0001' Delimiter Packet (delim-pkt) - separates sections of a message
  36
  37  Initial Client Request
  38 ------------------------
  39
  40 In general a client can request to speak protocol v2 by sending
  41 `version=2` through the respective side-channel for the transport being
  42 used which inevitably sets `GIT_PROTOCOL`.  More information can be
  43 found in `pack-protocol.txt` and `http-protocol.txt`.  In all cases the
  44 response from the server is the capability advertisement.
  45
  46  Git Transport
  47 ~~~~~~~~~~~~~~~
  48
  49 When using the git:// transport, you can request to use protocol v2 by
  50 sending "version=2" as an extra parameter:
  51
  52    003egit-upload-pack /project.git\0host=myserver.com\0\0version=2\0
  53
  54  SSH and File Transport
  55 ~~~~~~~~~~~~~~~~~~~~~~~~
  56
  57 When using either the ssh:// or file:// transport, the GIT_PROTOCOL
  58 environment variable must be set explicitly to include "version=2".
  59
  60  HTTP Transport
  61 ~~~~~~~~~~~~~~~~
  62
  63 When using the http:// or https:// transport a client makes a "smart"
  64 info/refs request as described in `http-protocol.txt` and requests that
  65 v2 be used by supplying "version=2" in the `Git-Protocol` header.
  66
  67    C: Git-Protocol: version=2
  68    C:
  69    C: GET $GIT_URL/info/refs?service=git-upload-pack HTTP/1.0
  70
  71 A v2 server would reply:
  72
  73    S: 200 OK
  74    S: <Some headers>
  75    S: ...
  76    S:
  77    S: 000eversion 2\n
  78    S: <capability-advertisement>
  79
  80 Subsequent requests are then made directly to the service
  81 `$GIT_URL/git-upload-pack`. (This works the same for git-receive-pack).
  82
  83  Capability Advertisement
  84 --------------------------
  85
  86 A server which decides to communicate (based on a request from a client)
  87 using protocol version 2, notifies the client by sending a version string
  88 in its initial response followed by an advertisement of its capabilities.
  89 Each capability is a key with an optional value.  Clients must ignore all
  90 unknown keys.  Semantics of unknown values are left to the definition of
  91 each key.  Some capabilities will describe commands which can be requested
  92 to be executed by the client.
  93
  94     capability-advertisement = protocol-version
  95                                capability-list
  96                                flush-pkt
  97
  98     protocol-version = PKT-LINE("version 2" LF)
  99     capability-list = *capability
 100     capability = PKT-LINE(key[=value] LF)
 101
 102     key = 1*(ALPHA | DIGIT | "-_")
 103     value = 1*(ALPHA | DIGIT | " -_.,?\/{}[]()<>!@#$%^&*+=:;")
 104
 105  Command Request
 106 -----------------
 107
 108 After receiving the capability advertisement, a client can then issue a
 109 request to select the command it wants with any particular capabilities
 110 or arguments.  There is then an optional section where the client can
 111 provide any command specific parameters or queries.  Only a single
 112 command can be requested at a time.
 113
 114     request = empty-request | command-request
 115     empty-request = flush-pkt
 116     command-request = command
 117                       capability-list
 118                       [command-args]
 119                       flush-pkt
 120     command = PKT-LINE("command=" key LF)
 121     command-args = delim-pkt
 122                    *command-specific-arg
 123
 124     command-specific-args are packet line framed arguments defined by
 125     each individual command.
 126
 127 The server will then check to ensure that the client's request is
 128 comprised of a valid command as well as valid capabilities which were
 129 advertised.  If the request is valid the server will then execute the
 130 command.  A server MUST wait till it has received the client's entire
 131 request before issuing a response.  The format of the response is
 132 determined by the command being executed, but in all cases a flush-pkt
 133 indicates the end of the response.
 134
 135 When a command has finished, and the client has received the entire
 136 response from the server, a client can either request that another
 137 command be executed or can terminate the connection.  A client may
 138 optionally send an empty request consisting of just a flush-pkt to
 139 indicate that no more requests will be made.
 140
 141  Capabilities
 142 --------------
 143
 144 There are two different types of capabilities: normal capabilities,
 145 which can be used to to convey information or alter the behavior of a
 146 request, and commands, which are the core actions that a client wants to
 147 perform (fetch, push, etc).
 148
 149 Protocol version 2 is stateless by default.  This means that all commands
 150 must only last a single round and be stateless from the perspective of the
 151 server side, unless the client has requested a capability indicating that
 152 state should be maintained by the server.  Clients MUST NOT require state
 153 management on the server side in order to function correctly.  This
 154 permits simple round-robin load-balancing on the server side, without
 155 needing to worry about state management.
 156
 157  agent
 158 ~~~~~~~
 159
 160 The server can advertise the `agent` capability with a value `X` (in the
 161 form `agent=X`) to notify the client that the server is running version
 162 `X`.  The client may optionally send its own agent string by including
 163 the `agent` capability with a value `Y` (in the form `agent=Y`) in its
 164 request to the server (but it MUST NOT do so if the server did not
 165 advertise the agent capability). The `X` and `Y` strings may contain any
 166 printable ASCII characters except space (i.e., the byte range 32 < x <
 167 127), and are typically of the form "package/version" (e.g.,
 168 "git/1.8.3.1"). The agent strings are purely informative for statistics
 169 and debugging purposes, and MUST NOT be used to programmatically assume
 170 the presence or absence of particular features.
 171
 172  ls-refs
 173 ~~~~~~~~~
 174
 175 `ls-refs` is the command used to request a reference advertisement in v2.
 176 Unlike the current reference advertisement, ls-refs takes in arguments
 177 which can be used to limit the refs sent from the server.
 178
 179 Additional features not supported in the base command will be advertised
 180 as the value of the command in the capability advertisement in the form
 181 of a space separated list of features: "<command>=<feature 1> <feature 2>"
 182
 183 ls-refs takes in the following arguments:
 184
 185     symrefs
 186         In addition to the object pointed by it, show the underlying ref
 187         pointed by it when showing a symbolic ref.
 188     peel
 189         Show peeled tags.
 190     ref-prefix <prefix>
 191         When specified, only references having a prefix matching one of
 192         the provided prefixes are displayed.
 193
 194 The output of ls-refs is as follows:
 195
 196     output = *ref
 197              flush-pkt
 198     ref = PKT-LINE(obj-id SP refname *(SP ref-attribute) LF)
 199     ref-attribute = (symref | peeled)
 200     symref = "symref-target:" symref-target
 201     peeled = "peeled:" obj-id