Documentation/technical/protocol-capabilities.txt

   1 Git Protocol Capabilities
   2 =========================
   3
   4 NOTE: this document describes capabilities for versions 0 and 1 of the pack
   5 protocol. For version 2, please refer to the link:protocol-v2.html[protocol-v2]
   6 doc.
   7
   8 Servers SHOULD support all capabilities defined in this document.
   9
  10 On the very first line of the initial server response of either
  11 receive-pack and upload-pack the first reference is followed by
  12 a NUL byte and then a list of space delimited server capabilities.
  13 These allow the server to declare what it can and cannot support
  14 to the client.
  15
  16 Client will then send a space separated list of capabilities it wants
  17 to be in effect. The client MUST NOT ask for capabilities the server
  18 did not say it supports.
  19
  20 Server MUST diagnose and abort if capabilities it does not understand
  21 was sent.  Server MUST NOT ignore capabilities that client requested
  22 and server advertised.  As a consequence of these rules, server MUST
  23 NOT advertise capabilities it does not understand.
  24
  25 The 'atomic', 'report-status', 'delete-refs', 'quiet', and 'push-cert'
  26 capabilities are sent and recognized by the receive-pack (push to server)
  27 process.
  28
  29 The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized
  30 by both upload-pack and receive-pack protocols.  The 'agent' capability
  31 may optionally be sent in both protocols.
  32
  33 All other capabilities are only recognized by the upload-pack (fetch
  34 from server) process.
  35
  36 multi_ack
  37 ---------
  38
  39 The 'multi_ack' capability allows the server to return "ACK obj-id
  40 continue" as soon as it finds a commit that it can use as a common
  41 base, between the client's wants and the client's have set.
  42
  43 By sending this early, the server can potentially head off the client
  44 from walking any further down that particular branch of the client's
  45 repository history.  The client may still need to walk down other
  46 branches, sending have lines for those, until the server has a
  47 complete cut across the DAG, or the client has said "done".
  48
  49 Without multi_ack, a client sends have lines in --date-order until
  50 the server has found a common base.  That means the client will send
  51 have lines that are already known by the server to be common, because
  52 they overlap in time with another branch that the server hasn't found
  53 a common base on yet.
  54
  55 For example suppose the client has commits in caps that the server
  56 doesn't and the server has commits in lower case that the client
  57 doesn't, as in the following diagram:
  58
  59        +---- u ---------------------- x
  60       /              +----- y
  61      /              /
  62     a -- b -- c -- d -- E -- F
  63        \
  64         +--- Q -- R -- S
  65
  66 If the client wants x,y and starts out by saying have F,S, the server
  67 doesn't know what F,S is.  Eventually the client says "have d" and
  68 the server sends "ACK d continue" to let the client know to stop
  69 walking down that line (so don't send c-b-a), but it's not done yet,
  70 it needs a base for x. The client keeps going with S-R-Q, until a
  71 gets reached, at which point the server has a clear base and it all
  72 ends.
  73
  74 Without multi_ack the client would have sent that c-b-a chain anyway,
  75 interleaved with S-R-Q.
  76
  77 multi_ack_detailed
  78 ------------------
  79 This is an extension of multi_ack that permits client to better
  80 understand the server's in-memory state. See pack-protocol.txt,
  81 section "Packfile Negotiation" for more information.
  82
  83 no-done
  84 -------
  85 This capability should only be used with the smart HTTP protocol. If
  86 multi_ack_detailed and no-done are both present, then the sender is
  87 free to immediately send a pack following its first "ACK obj-id ready"
  88 message.
  89
  90 Without no-done in the smart HTTP protocol, the server session would
  91 end and the client has to make another trip to send "done" before
  92 the server can send the pack. no-done removes the last round and
  93 thus slightly reduces latency.
  94
  95 thin-pack
  96 ---------
  97
  98 A thin pack is one with deltas which reference base objects not
  99 contained within the pack (but are known to exist at the receiving
 100 end). This can reduce the network traffic significantly, but it
 101 requires the receiving end to know how to "thicken" these packs by
 102 adding the missing bases to the pack.
 103
 104 The upload-pack server advertises 'thin-pack' when it can generate
 105 and send a thin pack. A client requests the 'thin-pack' capability
 106 when it understands how to "thicken" it, notifying the server that
 107 it can receive such a pack. A client MUST NOT request the
 108 'thin-pack' capability if it cannot turn a thin pack into a
 109 self-contained pack.
 110
 111 Receive-pack, on the other hand, is assumed by default to be able to
 112 handle thin packs, but can ask the client not to use the feature by
 113 advertising the 'no-thin' capability. A client MUST NOT send a thin
 114 pack if the server advertises the 'no-thin' capability.
 115
 116 The reasons for this asymmetry are historical. The receive-pack
 117 program did not exist until after the invention of thin packs, so
 118 historically the reference implementation of receive-pack always
 119 understood thin packs. Adding 'no-thin' later allowed receive-pack
 120 to disable the feature in a backwards-compatible manner.
 121
 122
 123 side-band, side-band-64k
 124 ------------------------
 125
 126 This capability means that server can send, and client understand multiplexed
 127 progress reports and error info interleaved with the packfile itself.
 128
 129 These two options are mutually exclusive. A modern client always
 130 favors 'side-band-64k'.
 131
 132 Either mode indicates that the packfile data will be streamed broken
 133 up into packets of up to either 1000 bytes in the case of 'side_band',
 134 or 65520 bytes in the case of 'side_band_64k'. Each packet is made up
 135 of a leading 4-byte pkt-line length of how much data is in the packet,
 136 followed by a 1-byte stream code, followed by the actual data.
 137
 138 The stream code can be one of:
 139
 140  1 - pack data
 141  2 - progress messages
 142  3 - fatal error message just before stream aborts
 143
 144 The "side-band-64k" capability came about as a way for newer clients
 145 that can handle much larger packets to request packets that are
 146 actually crammed nearly full, while maintaining backward compatibility
 147 for the older clients.
 148
 149 Further, with side-band and its up to 1000-byte messages, it's actually
 150 999 bytes of payload and 1 byte for the stream code. With side-band-64k,
 151 same deal, you have up to 65519 bytes of data and 1 byte for the stream
 152 code.
 153
 154 The client MUST send only maximum of one of "side-band" and "side-
 155 band-64k".  Server MUST diagnose it as an error if client requests
 156 both.
 157
 158 ofs-delta
 159 ---------
 160
 161 Server can send, and client understand PACKv2 with delta referring to
 162 its base by position in pack rather than by an obj-id.  That is, they can
 163 send/read OBJ_OFS_DELTA (aka type 6) in a packfile.
 164
 165 agent
 166 -----
 167
 168 The server may optionally send a capability of the form `agent=X` to
 169 notify the client that the server is running version `X`. The client may
 170 optionally return its own agent string by responding with an `agent=Y`
 171 capability (but it MUST NOT do so if the server did not mention the
 172 agent capability). The `X` and `Y` strings may contain any printable
 173 ASCII characters except space (i.e., the byte range 32 < x < 127), and
 174 are typically of the form "package/version" (e.g., "git/1.8.3.1"). The
 175 agent strings are purely informative for statistics and debugging
 176 purposes, and MUST NOT be used to programmatically assume the presence
 177 or absence of particular features.
 178
 179 object-format
 180 -------------
 181
 182 This capability, which takes a hash algorithm as an argument, indicates
 183 that the server supports the given hash algorithms.  It may be sent
 184 multiple times; if so, the first one given is the one used in the ref
 185 advertisement.
 186
 187 When provided by the client, this indicates that it intends to use the
 188 given hash algorithm to communicate.  The algorithm provided must be one
 189 that the server supports.
 190
 191 If this capability is not provided, it is assumed that the only
 192 supported algorithm is SHA-1.
 193
 194 symref
 195 ------
 196
 197 This parameterized capability is used to inform the receiver which symbolic ref
 198 points to which ref; for example, "symref=HEAD:refs/heads/master" tells the
 199 receiver that HEAD points to master. This capability can be repeated to
 200 represent multiple symrefs.
 201
 202 Servers SHOULD include this capability for the HEAD symref if it is one of the
 203 refs being sent.
 204
 205 Clients MAY use the parameters from this capability to select the proper initial
 206 branch when cloning a repository.
 207
 208 shallow
 209 -------
 210
 211 This capability adds "deepen", "shallow" and "unshallow" commands to
 212 the  fetch-pack/upload-pack protocol so clients can request shallow
 213 clones.
 214
 215 deepen-since
 216 ------------
 217
 218 This capability adds "deepen-since" command to fetch-pack/upload-pack
 219 protocol so the client can request shallow clones that are cut at a
 220 specific time, instead of depth. Internally it's equivalent of doing
 221 "rev-list --max-age=<timestamp>" on the server side. "deepen-since"
 222 cannot be used with "deepen".
 223
 224 deepen-not
 225 ----------
 226
 227 This capability adds "deepen-not" command to fetch-pack/upload-pack
 228 protocol so the client can request shallow clones that are cut at a
 229 specific revision, instead of depth. Internally it's equivalent of
 230 doing "rev-list --not <rev>" on the server side. "deepen-not"
 231 cannot be used with "deepen", but can be used with "deepen-since".
 232
 233 deepen-relative
 234 ---------------
 235
 236 If this capability is requested by the client, the semantics of
 237 "deepen" command is changed. The "depth" argument is the depth from
 238 the current shallow boundary, instead of the depth from remote refs.
 239
 240 no-progress
 241 -----------
 242
 243 The client was started with "git clone -q" or something, and doesn't
 244 want that side band 2.  Basically the client just says "I do not
 245 wish to receive stream 2 on sideband, so do not send it to me, and if
 246 you did, I will drop it on the floor anyway".  However, the sideband
 247 channel 3 is still used for error responses.
 248
 249 include-tag
 250 -----------
 251
 252 The 'include-tag' capability is about sending annotated tags if we are
 253 sending objects they point to.  If we pack an object to the client, and
 254 a tag object points exactly at that object, we pack the tag object too.
 255 In general this allows a client to get all new annotated tags when it
 256 fetches a branch, in a single network connection.
 257
 258 Clients MAY always send include-tag, hardcoding it into a request when
 259 the server advertises this capability. The decision for a client to
 260 request include-tag only has to do with the client's desires for tag
 261 data, whether or not a server had advertised objects in the
 262 refs/tags/* namespace.
 263
 264 Servers MUST pack the tags if their referrant is packed and the client
 265 has requested include-tags.
 266
 267 Clients MUST be prepared for the case where a server has ignored
 268 include-tag and has not actually sent tags in the pack.  In such
 269 cases the client SHOULD issue a subsequent fetch to acquire the tags
 270 that include-tag would have otherwise given the client.
 271
 272 The server SHOULD send include-tag, if it supports it, regardless
 273 of whether or not there are tags available.
 274
 275 report-status
 276 -------------
 277
 278 The receive-pack process can receive a 'report-status' capability,
 279 which tells it that the client wants a report of what happened after
 280 a packfile upload and reference update.  If the pushing client requests
 281 this capability, after unpacking and updating references the server
 282 will respond with whether the packfile unpacked successfully and if
 283 each reference was updated successfully.  If any of those were not
 284 successful, it will send back an error message.  See pack-protocol.txt
 285 for example messages.
 286
 287 delete-refs
 288 -----------
 289
 290 If the server sends back the 'delete-refs' capability, it means that
 291 it is capable of accepting a zero-id value as the target
 292 value of a reference update.  It is not sent back by the client, it
 293 simply informs the client that it can be sent zero-id values
 294 to delete references.
 295
 296 quiet
 297 -----
 298
 299 If the receive-pack server advertises the 'quiet' capability, it is
 300 capable of silencing human-readable progress output which otherwise may
 301 be shown when processing the received pack. A send-pack client should
 302 respond with the 'quiet' capability to suppress server-side progress
 303 reporting if the local progress reporting is also being suppressed
 304 (e.g., via `push -q`, or if stderr does not go to a tty).
 305
 306 atomic
 307 ------
 308
 309 If the server sends the 'atomic' capability it is capable of accepting
 310 atomic pushes. If the pushing client requests this capability, the server
 311 will update the refs in one atomic transaction. Either all refs are
 312 updated or none.
 313
 314 push-options
 315 ------------
 316
 317 If the server sends the 'push-options' capability it is able to accept
 318 push options after the update commands have been sent, but before the
 319 packfile is streamed. If the pushing client requests this capability,
 320 the server will pass the options to the pre- and post- receive hooks
 321 that process this push request.
 322
 323 allow-tip-sha1-in-want
 324 ----------------------
 325
 326 If the upload-pack server advertises this capability, fetch-pack may
 327 send "want" lines with object names that exist at the server but are not
 328 advertised by upload-pack. For historical reasons, the name of this
 329 capability contains "sha1". Object names are always given using the
 330 object format negotiated through the 'object-format' capability.
 331
 332 allow-reachable-sha1-in-want
 333 ----------------------------
 334
 335 If the upload-pack server advertises this capability, fetch-pack may
 336 send "want" lines with object names that exist at the server but are not
 337 advertised by upload-pack. For historical reasons, the name of this
 338 capability contains "sha1". Object names are always given using the
 339 object format negotiated through the 'object-format' capability.
 340
 341 push-cert=<nonce>
 342 -----------------
 343
 344 The receive-pack server that advertises this capability is willing
 345 to accept a signed push certificate, and asks the <nonce> to be
 346 included in the push certificate.  A send-pack client MUST NOT
 347 send a push-cert packet unless the receive-pack server advertises
 348 this capability.
 349
 350 filter
 351 ------
 352
 353 If the upload-pack server advertises the 'filter' capability,
 354 fetch-pack may send "filter" commands to request a partial clone
 355 or partial fetch and request that the server omit various objects
 356 from the packfile.