docs: note that receive-pack knows side-band-64k capability
[git/git.git] / Documentation / technical / protocol-capabilities.txt
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1Git Protocol Capabilities
2=========================
3
4Servers SHOULD support all capabilities defined in this document.
5
6On the very first line of the initial server response of either
7receive-pack and upload-pack the first reference is followed by
8a NUL byte and then a list of space delimited server capabilities.
9These allow the server to declare what it can and cannot support
10to the client.
11
12Client will then send a space separated list of capabilities it wants
13to be in effect. The client MUST NOT ask for capabilities the server
14did not say it supports.
15
16Server MUST diagnose and abort if capabilities it does not understand
17was sent. Server MUST NOT ignore capabilities that client requested
18and server advertised. As a consequence of these rules, server MUST
19NOT advertise capabilities it does not understand.
20
21The 'report-status' and 'delete-refs' capabilities are sent and
22recognized by the receive-pack (push to server) process.
23
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24The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized
25by both upload-pack and receive-pack protocols.
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26
27All other capabilities are only recognized by the upload-pack (fetch
28from server) process.
29
30multi_ack
31---------
32
33The 'multi_ack' capability allows the server to return "ACK obj-id
34continue" as soon as it finds a commit that it can use as a common
35base, between the client's wants and the client's have set.
36
37By sending this early, the server can potentially head off the client
38from walking any further down that particular branch of the client's
39repository history. The client may still need to walk down other
40branches, sending have lines for those, until the server has a
41complete cut across the DAG, or the client has said "done".
42
43Without multi_ack, a client sends have lines in --date-order until
44the server has found a common base. That means the client will send
45have lines that are already known by the server to be common, because
46they overlap in time with another branch that the server hasn't found
47a common base on yet.
48
49For example suppose the client has commits in caps that the server
50doesn't and the server has commits in lower case that the client
51doesn't, as in the following diagram:
52
53 +---- u ---------------------- x
54 / +----- y
55 / /
56 a -- b -- c -- d -- E -- F
57 \
58 +--- Q -- R -- S
59
60If the client wants x,y and starts out by saying have F,S, the server
61doesn't know what F,S is. Eventually the client says "have d" and
62the server sends "ACK d continue" to let the client know to stop
6a5d0b0a 63walking down that line (so don't send c-b-a), but it's not done yet,
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64it needs a base for x. The client keeps going with S-R-Q, until a
65gets reached, at which point the server has a clear base and it all
66ends.
67
68Without multi_ack the client would have sent that c-b-a chain anyway,
69interleaved with S-R-Q.
70
71thin-pack
72---------
73
74This capability means that the server can send a 'thin' pack, a pack
75which does not contain base objects; if those base objects are available
76on client side. Client requests 'thin-pack' capability when it
77understands how to "thicken" it by adding required delta bases making
78it self-contained.
79
80Client MUST NOT request 'thin-pack' capability if it cannot turn a thin
81pack into a self-contained pack.
82
83
84side-band, side-band-64k
85------------------------
86
87This capability means that server can send, and client understand multiplexed
88progress reports and error info interleaved with the packfile itself.
89
90These two options are mutually exclusive. A modern client always
91favors 'side-band-64k'.
92
93Either mode indicates that the packfile data will be streamed broken
94up into packets of up to either 1000 bytes in the case of 'side_band',
95or 65520 bytes in the case of 'side_band_64k'. Each packet is made up
96of a leading 4-byte pkt-line length of how much data is in the packet,
97followed by a 1-byte stream code, followed by the actual data.
98
99The stream code can be one of:
100
101 1 - pack data
102 2 - progress messages
103 3 - fatal error message just before stream aborts
104
105The "side-band-64k" capability came about as a way for newer clients
106that can handle much larger packets to request packets that are
107actually crammed nearly full, while maintaining backward compatibility
108for the older clients.
109
110Further, with side-band and its up to 1000-byte messages, it's actually
111999 bytes of payload and 1 byte for the stream code. With side-band-64k,
112same deal, you have up to 65519 bytes of data and 1 byte for the stream
113code.
114
115The client MUST send only maximum of one of "side-band" and "side-
116band-64k". Server MUST diagnose it as an error if client requests
117both.
118
119ofs-delta
120---------
121
5d1e3415 122Server can send, and client understand PACKv2 with delta referring to
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123its base by position in pack rather than by an obj-id. That is, they can
124send/read OBJ_OFS_DELTA (aka type 6) in a packfile.
125
126shallow
127-------
128
129This capability adds "deepen", "shallow" and "unshallow" commands to
130the fetch-pack/upload-pack protocol so clients can request shallow
131clones.
132
133no-progress
134-----------
135
136The client was started with "git clone -q" or something, and doesn't
137want that side band 2. Basically the client just says "I do not
138wish to receive stream 2 on sideband, so do not send it to me, and if
139you did, I will drop it on the floor anyway". However, the sideband
140channel 3 is still used for error responses.
141
142include-tag
143-----------
144
145The 'include-tag' capability is about sending annotated tags if we are
146sending objects they point to. If we pack an object to the client, and
147a tag object points exactly at that object, we pack the tag object too.
148In general this allows a client to get all new annotated tags when it
149fetches a branch, in a single network connection.
150
151Clients MAY always send include-tag, hardcoding it into a request when
152the server advertises this capability. The decision for a client to
153request include-tag only has to do with the client's desires for tag
154data, whether or not a server had advertised objects in the
155refs/tags/* namespace.
156
157Servers MUST pack the tags if their referrant is packed and the client
158has requested include-tags.
159
160Clients MUST be prepared for the case where a server has ignored
161include-tag and has not actually sent tags in the pack. In such
162cases the client SHOULD issue a subsequent fetch to acquire the tags
163that include-tag would have otherwise given the client.
164
165The server SHOULD send include-tag, if it supports it, regardless
166of whether or not there are tags available.
167
168report-status
169-------------
170
9a621ad0 171The receive-pack process can receive a 'report-status' capability,
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172which tells it that the client wants a report of what happened after
173a packfile upload and reference update. If the pushing client requests
174this capability, after unpacking and updating references the server
175will respond with whether the packfile unpacked successfully and if
176each reference was updated successfully. If any of those were not
177successful, it will send back an error message. See pack-protocol.txt
178for example messages.
179
180delete-refs
181-----------
182
183If the server sends back the 'delete-refs' capability, it means that
6a5d0b0a 184it is capable of accepting a zero-id value as the target
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185value of a reference update. It is not sent back by the client, it
186simply informs the client that it can be sent zero-id values
187to delete references.