eaab6b4ac723c9f5d7dde9d70fe7e01d234e7734
[git/git.git] / Documentation / technical / protocol-capabilities.txt
1 Git Protocol Capabilities
2 =========================
3
4 Servers SHOULD support all capabilities defined in this document.
5
6 On the very first line of the initial server response of either
7 receive-pack and upload-pack the first reference is followed by
8 a NUL byte and then a list of space delimited server capabilities.
9 These allow the server to declare what it can and cannot support
10 to the client.
11
12 Client will then send a space separated list of capabilities it wants
13 to be in effect. The client MUST NOT ask for capabilities the server
14 did not say it supports.
15
16 Server MUST diagnose and abort if capabilities it does not understand
17 was sent. Server MUST NOT ignore capabilities that client requested
18 and server advertised. As a consequence of these rules, server MUST
19 NOT advertise capabilities it does not understand.
20
21 The 'atomic', 'report-status', 'delete-refs', 'quiet', and 'push-cert'
22 capabilities are sent and recognized by the receive-pack (push to server)
23 process.
24
25 The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized
26 by both upload-pack and receive-pack protocols. The 'agent' capability
27 may optionally be sent in both protocols.
28
29 All other capabilities are only recognized by the upload-pack (fetch
30 from server) process.
31
32 multi_ack
33 ---------
34
35 The 'multi_ack' capability allows the server to return "ACK obj-id
36 continue" as soon as it finds a commit that it can use as a common
37 base, between the client's wants and the client's have set.
38
39 By sending this early, the server can potentially head off the client
40 from walking any further down that particular branch of the client's
41 repository history. The client may still need to walk down other
42 branches, sending have lines for those, until the server has a
43 complete cut across the DAG, or the client has said "done".
44
45 Without multi_ack, a client sends have lines in --date-order until
46 the server has found a common base. That means the client will send
47 have lines that are already known by the server to be common, because
48 they overlap in time with another branch that the server hasn't found
49 a common base on yet.
50
51 For example suppose the client has commits in caps that the server
52 doesn't and the server has commits in lower case that the client
53 doesn't, as in the following diagram:
54
55 +---- u ---------------------- x
56 / +----- y
57 / /
58 a -- b -- c -- d -- E -- F
59 \
60 +--- Q -- R -- S
61
62 If the client wants x,y and starts out by saying have F,S, the server
63 doesn't know what F,S is. Eventually the client says "have d" and
64 the server sends "ACK d continue" to let the client know to stop
65 walking down that line (so don't send c-b-a), but it's not done yet,
66 it needs a base for x. The client keeps going with S-R-Q, until a
67 gets reached, at which point the server has a clear base and it all
68 ends.
69
70 Without multi_ack the client would have sent that c-b-a chain anyway,
71 interleaved with S-R-Q.
72
73 multi_ack_detailed
74 ------------------
75 This is an extension of multi_ack that permits client to better
76 understand the server's in-memory state. See pack-protocol.txt,
77 section "Packfile Negotiation" for more information.
78
79 no-done
80 -------
81 This capability should only be used with the smart HTTP protocol. If
82 multi_ack_detailed and no-done are both present, then the sender is
83 free to immediately send a pack following its first "ACK obj-id ready"
84 message.
85
86 Without no-done in the smart HTTP protocol, the server session would
87 end and the client has to make another trip to send "done" before
88 the server can send the pack. no-done removes the last round and
89 thus slightly reduces latency.
90
91 thin-pack
92 ---------
93
94 A thin pack is one with deltas which reference base objects not
95 contained within the pack (but are known to exist at the receiving
96 end). This can reduce the network traffic significantly, but it
97 requires the receiving end to know how to "thicken" these packs by
98 adding the missing bases to the pack.
99
100 The upload-pack server advertises 'thin-pack' when it can generate
101 and send a thin pack. A client requests the 'thin-pack' capability
102 when it understands how to "thicken" it, notifying the server that
103 it can receive such a pack. A client MUST NOT request the
104 'thin-pack' capability if it cannot turn a thin pack into a
105 self-contained pack.
106
107 Receive-pack, on the other hand, is assumed by default to be able to
108 handle thin packs, but can ask the client not to use the feature by
109 advertising the 'no-thin' capability. A client MUST NOT send a thin
110 pack if the server advertises the 'no-thin' capability.
111
112 The reasons for this asymmetry are historical. The receive-pack
113 program did not exist until after the invention of thin packs, so
114 historically the reference implementation of receive-pack always
115 understood thin packs. Adding 'no-thin' later allowed receive-pack
116 to disable the feature in a backwards-compatible manner.
117
118
119 side-band, side-band-64k
120 ------------------------
121
122 This capability means that server can send, and client understand multiplexed
123 progress reports and error info interleaved with the packfile itself.
124
125 These two options are mutually exclusive. A modern client always
126 favors 'side-band-64k'.
127
128 Either mode indicates that the packfile data will be streamed broken
129 up into packets of up to either 1000 bytes in the case of 'side_band',
130 or 65520 bytes in the case of 'side_band_64k'. Each packet is made up
131 of a leading 4-byte pkt-line length of how much data is in the packet,
132 followed by a 1-byte stream code, followed by the actual data.
133
134 The stream code can be one of:
135
136 1 - pack data
137 2 - progress messages
138 3 - fatal error message just before stream aborts
139
140 The "side-band-64k" capability came about as a way for newer clients
141 that can handle much larger packets to request packets that are
142 actually crammed nearly full, while maintaining backward compatibility
143 for the older clients.
144
145 Further, with side-band and its up to 1000-byte messages, it's actually
146 999 bytes of payload and 1 byte for the stream code. With side-band-64k,
147 same deal, you have up to 65519 bytes of data and 1 byte for the stream
148 code.
149
150 The client MUST send only maximum of one of "side-band" and "side-
151 band-64k". Server MUST diagnose it as an error if client requests
152 both.
153
154 ofs-delta
155 ---------
156
157 Server can send, and client understand PACKv2 with delta referring to
158 its base by position in pack rather than by an obj-id. That is, they can
159 send/read OBJ_OFS_DELTA (aka type 6) in a packfile.
160
161 agent
162 -----
163
164 The server may optionally send a capability of the form `agent=X` to
165 notify the client that the server is running version `X`. The client may
166 optionally return its own agent string by responding with an `agent=Y`
167 capability (but it MUST NOT do so if the server did not mention the
168 agent capability). The `X` and `Y` strings may contain any printable
169 ASCII characters except space (i.e., the byte range 32 < x < 127), and
170 are typically of the form "package/version" (e.g., "git/1.8.3.1"). The
171 agent strings are purely informative for statistics and debugging
172 purposes, and MUST NOT be used to programmatically assume the presence
173 or absence of particular features.
174
175 shallow
176 -------
177
178 This capability adds "deepen", "shallow" and "unshallow" commands to
179 the fetch-pack/upload-pack protocol so clients can request shallow
180 clones.
181
182 no-progress
183 -----------
184
185 The client was started with "git clone -q" or something, and doesn't
186 want that side band 2. Basically the client just says "I do not
187 wish to receive stream 2 on sideband, so do not send it to me, and if
188 you did, I will drop it on the floor anyway". However, the sideband
189 channel 3 is still used for error responses.
190
191 include-tag
192 -----------
193
194 The 'include-tag' capability is about sending annotated tags if we are
195 sending objects they point to. If we pack an object to the client, and
196 a tag object points exactly at that object, we pack the tag object too.
197 In general this allows a client to get all new annotated tags when it
198 fetches a branch, in a single network connection.
199
200 Clients MAY always send include-tag, hardcoding it into a request when
201 the server advertises this capability. The decision for a client to
202 request include-tag only has to do with the client's desires for tag
203 data, whether or not a server had advertised objects in the
204 refs/tags/* namespace.
205
206 Servers MUST pack the tags if their referrant is packed and the client
207 has requested include-tags.
208
209 Clients MUST be prepared for the case where a server has ignored
210 include-tag and has not actually sent tags in the pack. In such
211 cases the client SHOULD issue a subsequent fetch to acquire the tags
212 that include-tag would have otherwise given the client.
213
214 The server SHOULD send include-tag, if it supports it, regardless
215 of whether or not there are tags available.
216
217 report-status
218 -------------
219
220 The receive-pack process can receive a 'report-status' capability,
221 which tells it that the client wants a report of what happened after
222 a packfile upload and reference update. If the pushing client requests
223 this capability, after unpacking and updating references the server
224 will respond with whether the packfile unpacked successfully and if
225 each reference was updated successfully. If any of those were not
226 successful, it will send back an error message. See pack-protocol.txt
227 for example messages.
228
229 delete-refs
230 -----------
231
232 If the server sends back the 'delete-refs' capability, it means that
233 it is capable of accepting a zero-id value as the target
234 value of a reference update. It is not sent back by the client, it
235 simply informs the client that it can be sent zero-id values
236 to delete references.
237
238 quiet
239 -----
240
241 If the receive-pack server advertises the 'quiet' capability, it is
242 capable of silencing human-readable progress output which otherwise may
243 be shown when processing the received pack. A send-pack client should
244 respond with the 'quiet' capability to suppress server-side progress
245 reporting if the local progress reporting is also being suppressed
246 (e.g., via `push -q`, or if stderr does not go to a tty).
247
248 atomic
249 ------
250
251 If the server sends the 'atomic' capability it is capable of accepting
252 atomic pushes. If the pushing client requests this capability, the server
253 will update the refs in one atomic transaction. Either all refs are
254 updated or none.
255
256 allow-tip-sha1-in-want
257 ----------------------
258
259 If the upload-pack server advertises this capability, fetch-pack may
260 send "want" lines with SHA-1s that exist at the server but are not
261 advertised by upload-pack.
262
263 allow-reachable-sha1-in-want
264 ----------------------------
265
266 If the upload-pack server advertises this capability, fetch-pack may
267 send "want" lines with SHA-1s that exist at the server but are not
268 advertised by upload-pack.
269
270 push-cert=<nonce>
271 -----------------
272
273 The receive-pack server that advertises this capability is willing
274 to accept a signed push certificate, and asks the <nonce> to be
275 included in the push certificate. A send-pack client MUST NOT
276 send a push-cert packet unless the receive-pack server advertises
277 this capability.