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: GET $GIT_URL/info/refs?service=git-upload-pack HTTP/1.0
  68    C: Git-Protocol: version=2
  69
  70 A v2 server would reply:
  71
  72    S: 200 OK
  73    S: <Some headers>
  74    S: ...
  75    S:
  76    S: 000eversion 2\n
  77    S: <capability-advertisement>
  78
  79 Subsequent requests are then made directly to the service
  80 `$GIT_URL/git-upload-pack`. (This works the same for git-receive-pack).
  81
  82  Capability Advertisement
  83 --------------------------
  84
  85 A server which decides to communicate (based on a request from a client)
  86 using protocol version 2, notifies the client by sending a version string
  87 in its initial response followed by an advertisement of its capabilities.
  88 Each capability is a key with an optional value.  Clients must ignore all
  89 unknown keys.  Semantics of unknown values are left to the definition of
  90 each key.  Some capabilities will describe commands which can be requested
  91 to be executed by the client.
  92
  93     capability-advertisement = protocol-version
  94                                capability-list
  95                                flush-pkt
  96
  97     protocol-version = PKT-LINE("version 2" LF)
  98     capability-list = *capability
  99     capability = PKT-LINE(key[=value] LF)
 100
 101     key = 1*(ALPHA | DIGIT | "-_")
 102     value = 1*(ALPHA | DIGIT | " -_.,?\/{}[]()<>!@#$%^&*+=:;")
 103
 104  Command Request
 105 -----------------
 106
 107 After receiving the capability advertisement, a client can then issue a
 108 request to select the command it wants with any particular capabilities
 109 or arguments.  There is then an optional section where the client can
 110 provide any command specific parameters or queries.  Only a single
 111 command can be requested at a time.
 112
 113     request = empty-request | command-request
 114     empty-request = flush-pkt
 115     command-request = command
 116                       capability-list
 117                       [command-args]
 118                       flush-pkt
 119     command = PKT-LINE("command=" key LF)
 120     command-args = delim-pkt
 121                    *command-specific-arg
 122
 123     command-specific-args are packet line framed arguments defined by
 124     each individual command.
 125
 126 The server will then check to ensure that the client's request is
 127 comprised of a valid command as well as valid capabilities which were
 128 advertised.  If the request is valid the server will then execute the
 129 command.  A server MUST wait till it has received the client's entire
 130 request before issuing a response.  The format of the response is
 131 determined by the command being executed, but in all cases a flush-pkt
 132 indicates the end of the response.
 133
 134 When a command has finished, and the client has received the entire
 135 response from the server, a client can either request that another
 136 command be executed or can terminate the connection.  A client may
 137 optionally send an empty request consisting of just a flush-pkt to
 138 indicate that no more requests will be made.
 139
 140  Capabilities
 141 --------------
 142
 143 There are two different types of capabilities: normal capabilities,
 144 which can be used to to convey information or alter the behavior of a
 145 request, and commands, which are the core actions that a client wants to
 146 perform (fetch, push, etc).
 147
 148 Protocol version 2 is stateless by default.  This means that all commands
 149 must only last a single round and be stateless from the perspective of the
 150 server side, unless the client has requested a capability indicating that
 151 state should be maintained by the server.  Clients MUST NOT require state
 152 management on the server side in order to function correctly.  This
 153 permits simple round-robin load-balancing on the server side, without
 154 needing to worry about state management.
 155
 156  agent
 157 ~~~~~~~
 158
 159 The server can advertise the `agent` capability with a value `X` (in the
 160 form `agent=X`) to notify the client that the server is running version
 161 `X`.  The client may optionally send its own agent string by including
 162 the `agent` capability with a value `Y` (in the form `agent=Y`) in its
 163 request to the server (but it MUST NOT do so if the server did not
 164 advertise the agent capability). The `X` and `Y` strings may contain any
 165 printable ASCII characters except space (i.e., the byte range 32 < x <
 166 127), and are typically of the form "package/version" (e.g.,
 167 "git/1.8.3.1"). The agent strings are purely informative for statistics
 168 and debugging purposes, and MUST NOT be used to programmatically assume
 169 the presence or absence of particular features.
 170
 171  ls-refs
 172 ~~~~~~~~~
 173
 174 `ls-refs` is the command used to request a reference advertisement in v2.
 175 Unlike the current reference advertisement, ls-refs takes in arguments
 176 which can be used to limit the refs sent from the server.
 177
 178 Additional features not supported in the base command will be advertised
 179 as the value of the command in the capability advertisement in the form
 180 of a space separated list of features: "<command>=<feature 1> <feature 2>"
 181
 182 ls-refs takes in the following arguments:
 183
 184     symrefs
 185         In addition to the object pointed by it, show the underlying ref
 186         pointed by it when showing a symbolic ref.
 187     peel
 188         Show peeled tags.
 189     ref-prefix <prefix>
 190         When specified, only references having a prefix matching one of
 191         the provided prefixes are displayed.
 192
 193 The output of ls-refs is as follows:
 194
 195     output = *ref
 196              flush-pkt
 197     ref = PKT-LINE(obj-id SP refname *(SP ref-attribute) LF)
 198     ref-attribute = (symref | peeled)
 199     symref = "symref-target:" symref-target
 200     peeled = "peeled:" obj-id
 201
 202  fetch
 203 ~~~~~~~
 204
 205 `fetch` is the command used to fetch a packfile in v2.  It can be looked
 206 at as a modified version of the v1 fetch where the ref-advertisement is
 207 stripped out (since the `ls-refs` command fills that role) and the
 208 message format is tweaked to eliminate redundancies and permit easy
 209 addition of future extensions.
 210
 211 Additional features not supported in the base command will be advertised
 212 as the value of the command in the capability advertisement in the form
 213 of a space separated list of features: "<command>=<feature 1> <feature 2>"
 214
 215 A `fetch` request can take the following arguments:
 216
 217     want <oid>
 218         Indicates to the server an object which the client wants to
 219         retrieve.  Wants can be anything and are not limited to
 220         advertised objects.
 221
 222     have <oid>
 223         Indicates to the server an object which the client has locally.
 224         This allows the server to make a packfile which only contains
 225         the objects that the client needs. Multiple 'have' lines can be
 226         supplied.
 227
 228     done
 229         Indicates to the server that negotiation should terminate (or
 230         not even begin if performing a clone) and that the server should
 231         use the information supplied in the request to construct the
 232         packfile.
 233
 234     thin-pack
 235         Request that a thin pack be sent, which is a pack with deltas
 236         which reference base objects not contained within the pack (but
 237         are known to exist at the receiving end). This can reduce the
 238         network traffic significantly, but it requires the receiving end
 239         to know how to "thicken" these packs by adding the missing bases
 240         to the pack.
 241
 242     no-progress
 243         Request that progress information that would normally be sent on
 244         side-band channel 2, during the packfile transfer, should not be
 245         sent.  However, the side-band channel 3 is still used for error
 246         responses.
 247
 248     include-tag
 249         Request that annotated tags should be sent if the objects they
 250         point to are being sent.
 251
 252     ofs-delta
 253         Indicate that the client understands PACKv2 with delta referring
 254         to its base by position in pack rather than by an oid.  That is,
 255         they can read OBJ_OFS_DELTA (ake type 6) in a packfile.
 256
 257 If the 'shallow' feature is advertised the following arguments can be
 258 included in the clients request as well as the potential addition of the
 259 'shallow-info' section in the server's response as explained below.
 260
 261     shallow <oid>
 262         A client must notify the server of all commits for which it only
 263         has shallow copies (meaning that it doesn't have the parents of
 264         a commit) by supplying a 'shallow <oid>' line for each such
 265         object so that the server is aware of the limitations of the
 266         client's history.  This is so that the server is aware that the
 267         client may not have all objects reachable from such commits.
 268
 269     deepen <depth>
 270         Requests that the fetch/clone should be shallow having a commit
 271         depth of <depth> relative to the remote side.
 272
 273     deepen-relative
 274         Requests that the semantics of the "deepen" command be changed
 275         to indicate that the depth requested is relative to the client's
 276         current shallow boundary, instead of relative to the requested
 277         commits.
 278
 279     deepen-since <timestamp>
 280         Requests that the shallow clone/fetch should be cut at a
 281         specific time, instead of depth.  Internally it's equivalent to
 282         doing "git rev-list --max-age=<timestamp>". Cannot be used with
 283         "deepen".
 284
 285     deepen-not <rev>
 286         Requests that the shallow clone/fetch should be cut at a
 287         specific revision specified by '<rev>', instead of a depth.
 288         Internally it's equivalent of doing "git rev-list --not <rev>".
 289         Cannot be used with "deepen", but can be used with
 290         "deepen-since".
 291
 292 If the 'filter' feature is advertised, the following argument can be
 293 included in the client's request:
 294
 295     filter <filter-spec>
 296         Request that various objects from the packfile be omitted
 297         using one of several filtering techniques. These are intended
 298         for use with partial clone and partial fetch operations. See
 299         `rev-list` for possible "filter-spec" values.
 300
 301 If the 'ref-in-want' feature is advertised, the following argument can
 302 be included in the client's request as well as the potential addition of
 303 the 'wanted-refs' section in the server's response as explained below.
 304
 305     want-ref <ref>
 306         Indicates to the server that the client wants to retrieve a
 307         particular ref, where <ref> is the full name of a ref on the
 308         server.
 309
 310 If the 'sideband-all' feature is advertised, the following argument can be
 311 included in the client's request:
 312
 313     sideband-all
 314         Instruct the server to send the whole response multiplexed, not just
 315         the packfile section. All non-flush and non-delim PKT-LINE in the
 316         response (not only in the packfile section) will then start with a byte
 317         indicating its sideband (1, 2, or 3), and the server may send "0005\2"
 318         (a PKT-LINE of sideband 2 with no payload) as a keepalive packet.
 319
 320 The response of `fetch` is broken into a number of sections separated by
 321 delimiter packets (0001), with each section beginning with its section
 322 header.
 323
 324     output = *section
 325     section = (acknowledgments | shallow-info | wanted-refs | packfile)
 326               (flush-pkt | delim-pkt)
 327
 328     acknowledgments = PKT-LINE("acknowledgments" LF)
 329                       (nak | *ack)
 330                       (ready)
 331     ready = PKT-LINE("ready" LF)
 332     nak = PKT-LINE("NAK" LF)
 333     ack = PKT-LINE("ACK" SP obj-id LF)
 334
 335     shallow-info = PKT-LINE("shallow-info" LF)
 336                    *PKT-LINE((shallow | unshallow) LF)
 337     shallow = "shallow" SP obj-id
 338     unshallow = "unshallow" SP obj-id
 339
 340     wanted-refs = PKT-LINE("wanted-refs" LF)
 341                   *PKT-LINE(wanted-ref LF)
 342     wanted-ref = obj-id SP refname
 343
 344     packfile = PKT-LINE("packfile" LF)
 345                *PKT-LINE(%x01-03 *%x00-ff)
 346
 347     acknowledgments section
 348         * If the client determines that it is finished with negotiations
 349           by sending a "done" line, the acknowledgments sections MUST be
 350           omitted from the server's response.
 351
 352         * Always begins with the section header "acknowledgments"
 353
 354         * The server will respond with "NAK" if none of the object ids sent
 355           as have lines were common.
 356
 357         * The server will respond with "ACK obj-id" for all of the
 358           object ids sent as have lines which are common.
 359
 360         * A response cannot have both "ACK" lines as well as a "NAK"
 361           line.
 362
 363         * The server will respond with a "ready" line indicating that
 364           the server has found an acceptable common base and is ready to
 365           make and send a packfile (which will be found in the packfile
 366           section of the same response)
 367
 368         * If the server has found a suitable cut point and has decided
 369           to send a "ready" line, then the server can decide to (as an
 370           optimization) omit any "ACK" lines it would have sent during
 371           its response.  This is because the server will have already
 372           determined the objects it plans to send to the client and no
 373           further negotiation is needed.
 374
 375     shallow-info section
 376         * If the client has requested a shallow fetch/clone, a shallow
 377           client requests a fetch or the server is shallow then the
 378           server's response may include a shallow-info section.  The
 379           shallow-info section will be included if (due to one of the
 380           above conditions) the server needs to inform the client of any
 381           shallow boundaries or adjustments to the clients already
 382           existing shallow boundaries.
 383
 384         * Always begins with the section header "shallow-info"
 385
 386         * If a positive depth is requested, the server will compute the
 387           set of commits which are no deeper than the desired depth.
 388
 389         * The server sends a "shallow obj-id" line for each commit whose
 390           parents will not be sent in the following packfile.
 391
 392         * The server sends an "unshallow obj-id" line for each commit
 393           which the client has indicated is shallow, but is no longer
 394           shallow as a result of the fetch (due to its parents being
 395           sent in the following packfile).
 396
 397         * The server MUST NOT send any "unshallow" lines for anything
 398           which the client has not indicated was shallow as a part of
 399           its request.
 400
 401         * This section is only included if a packfile section is also
 402           included in the response.
 403
 404     wanted-refs section
 405         * This section is only included if the client has requested a
 406           ref using a 'want-ref' line and if a packfile section is also
 407           included in the response.
 408
 409         * Always begins with the section header "wanted-refs".
 410
 411         * The server will send a ref listing ("<oid> <refname>") for
 412           each reference requested using 'want-ref' lines.
 413
 414         * The server MUST NOT send any refs which were not requested
 415           using 'want-ref' lines.
 416
 417     packfile section
 418         * This section is only included if the client has sent 'want'
 419           lines in its request and either requested that no more
 420           negotiation be done by sending 'done' or if the server has
 421           decided it has found a sufficient cut point to produce a
 422           packfile.
 423
 424         * Always begins with the section header "packfile"
 425
 426         * The transmission of the packfile begins immediately after the
 427           section header
 428
 429         * The data transfer of the packfile is always multiplexed, using
 430           the same semantics of the 'side-band-64k' capability from
 431           protocol version 1.  This means that each packet, during the
 432           packfile data stream, is made up of a leading 4-byte pkt-line
 433           length (typical of the pkt-line format), followed by a 1-byte
 434           stream code, followed by the actual data.
 435
 436           The stream code can be one of:
 437                 1 - pack data
 438                 2 - progress messages
 439                 3 - fatal error message just before stream aborts
 440
 441  server-option
 442 ~~~~~~~~~~~~~~~
 443
 444 If advertised, indicates that any number of server specific options can be
 445 included in a request.  This is done by sending each option as a
 446 "server-option=<option>" capability line in the capability-list section of
 447 a request.
 448
 449 The provided options must not contain a NUL or LF character.