1GITPROTOCOL-CAPABILITIES(5)       Git Manual       GITPROTOCOL-CAPABILITIES(5)
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NAME

6       gitprotocol-capabilities - Protocol v0 and v1 capabilities
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SYNOPSIS

9       <over-the-wire-protocol>
10

DESCRIPTION

12           Note
13           this document describes capabilities for versions 0 and 1 of the
14           pack protocol. For version 2, please refer to the gitprotocol-v2(5)
15           doc.
16
17       Servers SHOULD support all capabilities defined in this document.
18
19       On the very first line of the initial server response of either
20       receive-pack and upload-pack the first reference is followed by a NUL
21       byte and then a list of space delimited server capabilities. These
22       allow the server to declare what it can and cannot support to the
23       client.
24
25       Client will then send a space separated list of capabilities it wants
26       to be in effect. The client MUST NOT ask for capabilities the server
27       did not say it supports.
28
29       Server MUST diagnose and abort if capabilities it does not understand
30       were sent. Server MUST NOT ignore capabilities that client requested
31       and server advertised. As a consequence of these rules, server MUST NOT
32       advertise capabilities it does not understand.
33
34       The atomic, report-status, report-status-v2, delete-refs, quiet, and
35       push-cert capabilities are sent and recognized by the receive-pack
36       (push to server) process.
37
38       The ofs-delta and side-band-64k capabilities are sent and recognized by
39       both upload-pack and receive-pack protocols. The agent and session-id
40       capabilities may optionally be sent in both protocols.
41
42       All other capabilities are only recognized by the upload-pack (fetch
43       from server) process.
44

MULTI_ACK

46       The multi_ack capability allows the server to return "ACK obj-id
47       continue" as soon as it finds a commit that it can use as a common
48       base, between the client’s wants and the client’s have set.
49
50       By sending this early, the server can potentially head off the client
51       from walking any further down that particular branch of the client’s
52       repository history. The client may still need to walk down other
53       branches, sending have lines for those, until the server has a complete
54       cut across the DAG, or the client has said "done".
55
56       Without multi_ack, a client sends have lines in --date-order until the
57       server has found a common base. That means the client will send have
58       lines that are already known by the server to be common, because they
59       overlap in time with another branch on which the server hasn’t found a
60       common base yet.
61
62       For example suppose the client has commits in caps that the server
63       doesn’t and the server has commits in lower case that the client
64       doesn’t, as in the following diagram:
65
66                 +---- u ---------------------- x
67                /              +----- y
68               /              /
69              a -- b -- c -- d -- E -- F
70                 \
71           +--- Q -- R -- S
72
73       If the client wants x,y and starts out by saying have F,S, the server
74       doesn’t know what F,S is. Eventually the client says "have d" and the
75       server sends "ACK d continue" to let the client know to stop walking
76       down that line (so don’t send c-b-a), but it’s not done yet, it needs a
77       base for x. The client keeps going with S-R-Q, until a gets reached, at
78       which point the server has a clear base and it all ends.
79
80       Without multi_ack the client would have sent that c-b-a chain anyway,
81       interleaved with S-R-Q.
82

MULTI_ACK_DETAILED

84       This is an extension of multi_ack that permits the client to better
85       understand the server’s in-memory state. See gitprotocol-pack(5),
86       section "Packfile Negotiation" for more information.
87

NO-DONE

89       This capability should only be used with the smart HTTP protocol. If
90       multi_ack_detailed and no-done are both present, then the sender is
91       free to immediately send a pack following its first "ACK obj-id ready"
92       message.
93
94       Without no-done in the smart HTTP protocol, the server session would
95       end and the client has to make another trip to send "done" before the
96       server can send the pack. no-done removes the last round and thus
97       slightly reduces latency.
98

THIN-PACK

100       A thin pack is one with deltas which reference base objects not
101       contained within the pack (but are known to exist at the receiving
102       end). This can reduce the network traffic significantly, but it
103       requires the receiving end to know how to "thicken" these packs by
104       adding the missing bases to the pack.
105
106       The upload-pack server advertises thin-pack when it can generate and
107       send a thin pack. A client requests the thin-pack capability when it
108       understands how to "thicken" it, notifying the server that it can
109       receive such a pack. A client MUST NOT request the thin-pack capability
110       if it cannot turn a thin pack into a self-contained pack.
111
112       Receive-pack, on the other hand, is assumed by default to be able to
113       handle thin packs, but can ask the client not to use the feature by
114       advertising the no-thin capability. A client MUST NOT send a thin pack
115       if the server advertises the no-thin capability.
116
117       The reasons for this asymmetry are historical. The receive-pack program
118       did not exist until after the invention of thin packs, so historically
119       the reference implementation of receive-pack always understood thin
120       packs. Adding no-thin later allowed receive-pack to disable the feature
121       in a backwards-compatible manner.
122

SIDE-BAND, SIDE-BAND-64K

124       This capability means that the server can send, and the client can
125       understand, multiplexed progress reports and error info interleaved
126       with the packfile itself.
127
128       These two options are mutually exclusive. A modern client always favors
129       side-band-64k.
130
131       Either mode indicates that the packfile data will be streamed broken up
132       into packets of up to either 1000 bytes in the case of side_band, or
133       65520 bytes in the case of side_band_64k. Each packet is made up of a
134       leading 4-byte pkt-line length of how much data is in the packet,
135       followed by a 1-byte stream code, followed by the actual data.
136
137       The stream code can be one of:
138
139           1 - pack data
140           2 - progress messages
141           3 - fatal error message just before stream aborts
142
143       The "side-band-64k" capability came about as a way for newer clients
144       that can handle much larger packets to request packets that are
145       actually crammed nearly full, while maintaining backward compatibility
146       for the older clients.
147
148       Further, with side-band and its up to 1000-byte messages, it’s actually
149       999 bytes of payload and 1 byte for the stream code. With
150       side-band-64k, same deal, you have up to 65519 bytes of data and 1 byte
151       for the stream code.
152
153       The client MUST send only one of "side-band" and "side- band-64k". The
154       server MUST diagnose it as an error if client requests both.
155

OFS-DELTA

157       The server can send, and the client can understand, PACKv2 with delta
158       referring to its base by position in pack rather than by an obj-id.
159       That is, they can send/read OBJ_OFS_DELTA (aka type 6) in a packfile.
160

AGENT

162       The server may optionally send a capability of the form agent=X to
163       notify the client that the server is running version X. The client may
164       optionally return its own agent string by responding with an agent=Y
165       capability (but it MUST NOT do so if the server did not mention the
166       agent capability). The X and Y strings may contain any printable ASCII
167       characters except space (i.e., the byte range 32 < x < 127), and are
168       typically of the form "package/version" (e.g., "git/1.8.3.1"). The
169       agent strings are purely informative for statistics and debugging
170       purposes, and MUST NOT be used to programmatically assume the presence
171       or absence of particular features.
172

OBJECT-FORMAT

174       This capability, which takes a hash algorithm as an argument, indicates
175       that the server supports the given hash algorithms. It may be sent
176       multiple times; if so, the first one given is the one used in the ref
177       advertisement.
178
179       When provided by the client, this indicates that it intends to use the
180       given hash algorithm to communicate. The algorithm provided must be one
181       that the server supports.
182
183       If this capability is not provided, it is assumed that the only
184       supported algorithm is SHA-1.
185

SYMREF

187       This parameterized capability is used to inform the receiver which
188       symbolic ref points to which ref; for example,
189       "symref=HEAD:refs/heads/master" tells the receiver that HEAD points to
190       master. This capability can be repeated to represent multiple symrefs.
191
192       Servers SHOULD include this capability for the HEAD symref if it is one
193       of the refs being sent.
194
195       Clients MAY use the parameters from this capability to select the
196       proper initial branch when cloning a repository.
197

SHALLOW

199       This capability adds "deepen", "shallow" and "unshallow" commands to
200       the fetch-pack/upload-pack protocol so clients can request shallow
201       clones.
202

DEEPEN-SINCE

204       This capability adds "deepen-since" command to fetch-pack/upload-pack
205       protocol so the client can request shallow clones that are cut at a
206       specific time, instead of depth. Internally it’s equivalent of doing
207       "rev-list --max-age=<timestamp>" on the server side. "deepen-since"
208       cannot be used with "deepen".
209

DEEPEN-NOT

211       This capability adds "deepen-not" command to fetch-pack/upload-pack
212       protocol so the client can request shallow clones that are cut at a
213       specific revision, instead of depth. Internally it’s equivalent of
214       doing "rev-list --not <rev>" on the server side. "deepen-not" cannot be
215       used with "deepen", but can be used with "deepen-since".
216

DEEPEN-RELATIVE

218       If this capability is requested by the client, the semantics of
219       "deepen" command is changed. The "depth" argument is the depth from the
220       current shallow boundary, instead of the depth from remote refs.
221

NO-PROGRESS

223       The client was started with "git clone -q" or something similar, and
224       doesn’t want that side band 2. Basically the client just says "I do not
225       wish to receive stream 2 on sideband, so do not send it to me, and if
226       you did, I will drop it on the floor anyway". However, the sideband
227       channel 3 is still used for error responses.
228

INCLUDE-TAG

230       The include-tag capability is about sending annotated tags if we are
231       sending objects they point to. If we pack an object to the client, and
232       a tag object points exactly at that object, we pack the tag object too.
233       In general this allows a client to get all new annotated tags when it
234       fetches a branch, in a single network connection.
235
236       Clients MAY always send include-tag, hardcoding it into a request when
237       the server advertises this capability. The decision for a client to
238       request include-tag only has to do with the client’s desires for tag
239       data, whether or not a server had advertised objects in the refs/tags/*
240       namespace.
241
242       Servers MUST pack the tags if their referent is packed and the client
243       has requested include-tags.
244
245       Clients MUST be prepared for the case where a server has ignored
246       include-tag and has not actually sent tags in the pack. In such cases
247       the client SHOULD issue a subsequent fetch to acquire the tags that
248       include-tag would have otherwise given the client.
249
250       The server SHOULD send include-tag, if it supports it, regardless of
251       whether or not there are tags available.
252

REPORT-STATUS

254       The receive-pack process can receive a report-status capability, which
255       tells it that the client wants a report of what happened after a
256       packfile upload and reference update. If the pushing client requests
257       this capability, after unpacking and updating references the server
258       will respond with whether the packfile unpacked successfully and if
259       each reference was updated successfully. If any of those were not
260       successful, it will send back an error message. See gitprotocol-pack(5)
261       for example messages.
262

REPORT-STATUS-V2

264       Capability report-status-v2 extends capability report-status by adding
265       new "option" directives in order to support reference rewritten by the
266       "proc-receive" hook. The "proc-receive" hook may handle a command for a
267       pseudo-reference which may create or update a reference with different
268       name, new-oid, and old-oid. While the capability report-status cannot
269       report for such case. See gitprotocol-pack(5) for details.
270

DELETE-REFS

272       If the server sends back the delete-refs capability, it means that it
273       is capable of accepting a zero-id value as the target value of a
274       reference update. It is not sent back by the client, it simply informs
275       the client that it can be sent zero-id values to delete references.
276

QUIET

278       If the receive-pack server advertises the quiet capability, it is
279       capable of silencing human-readable progress output which otherwise may
280       be shown when processing the received pack. A send-pack client should
281       respond with the quiet capability to suppress server-side progress
282       reporting if the local progress reporting is also being suppressed
283       (e.g., via push -q, or if stderr does not go to a tty).
284

ATOMIC

286       If the server sends the atomic capability it is capable of accepting
287       atomic pushes. If the pushing client requests this capability, the
288       server will update the refs in one atomic transaction. Either all refs
289       are updated or none.
290

PUSH-OPTIONS

292       If the server sends the push-options capability it is able to accept
293       push options after the update commands have been sent, but before the
294       packfile is streamed. If the pushing client requests this capability,
295       the server will pass the options to the pre- and post- receive hooks
296       that process this push request.
297

ALLOW-TIP-SHA1-IN-WANT

299       If the upload-pack server advertises this capability, fetch-pack may
300       send "want" lines with object names that exist at the server but are
301       not advertised by upload-pack. For historical reasons, the name of this
302       capability contains "sha1". Object names are always given using the
303       object format negotiated through the object-format capability.
304

ALLOW-REACHABLE-SHA1-IN-WANT

306       If the upload-pack server advertises this capability, fetch-pack may
307       send "want" lines with object names that exist at the server but are
308       not advertised by upload-pack. For historical reasons, the name of this
309       capability contains "sha1". Object names are always given using the
310       object format negotiated through the object-format capability.
311

PUSH-CERT=<NONCE>

313       The receive-pack server that advertises this capability is willing to
314       accept a signed push certificate, and asks the <nonce> to be included
315       in the push certificate. A send-pack client MUST NOT send a push-cert
316       packet unless the receive-pack server advertises this capability.
317

FILTER

319       If the upload-pack server advertises the filter capability, fetch-pack
320       may send "filter" commands to request a partial clone or partial fetch
321       and request that the server omit various objects from the packfile.
322

SESSION-ID=<SESSION ID>

324       The server may advertise a session ID that can be used to identify this
325       process across multiple requests. The client may advertise its own
326       session ID back to the server as well.
327
328       Session IDs should be unique to a given process. They must fit within a
329       packet-line, and must not contain non-printable or whitespace
330       characters. The current implementation uses trace2 session IDs (see
331       api-trace2[1] for details), but this may change and users of the
332       session ID should not rely on this fact.
333

GIT

335       Part of the git(1) suite
336

NOTES

338        1. api-trace2
339           file:///usr/share/doc/git/technical/api-trace2.html
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342
343Git 2.43.0                        11/20/2023       GITPROTOCOL-CAPABILITIES(5)
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