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', 'delete-refs', and 'quiet' capabilities are sent and 22recognized by the receive-pack (push to server) process. 23 24The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized 25by both upload-pack and receive-pack protocols. The 'agent' capability 26may optionally be sent in both protocols. 27 28All other capabilities are only recognized by the upload-pack (fetch 29from server) process. 30 31multi_ack 32--------- 33 34The 'multi_ack' capability allows the server to return "ACK obj-id 35continue" as soon as it finds a commit that it can use as a common 36base, between the client's wants and the client's have set. 37 38By sending this early, the server can potentially head off the client 39from walking any further down that particular branch of the client's 40repository history. The client may still need to walk down other 41branches, sending have lines for those, until the server has a 42complete cut across the DAG, or the client has said "done". 43 44Without multi_ack, a client sends have lines in --date-order until 45the server has found a common base. That means the client will send 46have lines that are already known by the server to be common, because 47they overlap in time with another branch that the server hasn't found 48a common base on yet. 49 50For example suppose the client has commits in caps that the server 51doesn't and the server has commits in lower case that the client 52doesn't, as in the following diagram: 53 54 +---- u ---------------------- x 55 / +----- y 56 / / 57 a -- b -- c -- d -- E -- F 58 \ 59 +--- Q -- R -- S 60 61If the client wants x,y and starts out by saying have F,S, the server 62doesn't know what F,S is. Eventually the client says "have d" and 63the server sends "ACK d continue" to let the client know to stop 64walking down that line (so don't send c-b-a), but it's not done yet, 65it needs a base for x. The client keeps going with S-R-Q, until a 66gets reached, at which point the server has a clear base and it all 67ends. 68 69Without multi_ack the client would have sent that c-b-a chain anyway, 70interleaved with S-R-Q. 71 72thin-pack 73--------- 74 75This capability means that the server can send a 'thin' pack, a pack 76which does not contain base objects; if those base objects are available 77on client side. Client requests 'thin-pack' capability when it 78understands how to "thicken" it by adding required delta bases making 79it self-contained. 80 81Client MUST NOT request 'thin-pack' capability if it cannot turn a thin 82pack into a self-contained pack. 83 84 85side-band, side-band-64k 86------------------------ 87 88This capability means that server can send, and client understand multiplexed 89progress reports and error info interleaved with the packfile itself. 90 91These two options are mutually exclusive. A modern client always 92favors 'side-band-64k'. 93 94Either mode indicates that the packfile data will be streamed broken 95up into packets of up to either 1000 bytes in the case of 'side_band', 96or 65520 bytes in the case of 'side_band_64k'. Each packet is made up 97of a leading 4-byte pkt-line length of how much data is in the packet, 98followed by a 1-byte stream code, followed by the actual data. 99 100The stream code can be one of: 101 102 1 - pack data 103 2 - progress messages 104 3 - fatal error message just before stream aborts 105 106The "side-band-64k" capability came about as a way for newer clients 107that can handle much larger packets to request packets that are 108actually crammed nearly full, while maintaining backward compatibility 109for the older clients. 110 111Further, with side-band and its up to 1000-byte messages, it's actually 112999 bytes of payload and 1 byte for the stream code. With side-band-64k, 113same deal, you have up to 65519 bytes of data and 1 byte for the stream 114code. 115 116The client MUST send only maximum of one of "side-band" and "side- 117band-64k". Server MUST diagnose it as an error if client requests 118both. 119 120ofs-delta 121--------- 122 123Server can send, and client understand PACKv2 with delta referring to 124its base by position in pack rather than by an obj-id. That is, they can 125send/read OBJ_OFS_DELTA (aka type 6) in a packfile. 126 127agent 128----- 129 130The server may optionally send a capability of the form `agent=X` to 131notify the client that the server is running version `X`. The client may 132optionally return its own agent string by responding with an `agent=Y` 133capability (but it MUST NOT do so if the server did not mention the 134agent capability). The `X` and `Y` strings may contain any printable 135ASCII characters except space (i.e., the byte range 32 < x < 127), and 136are typically of the form "package/version" (e.g., "git/1.8.3.1"). The 137agent strings are purely informative for statistics and debugging 138purposes, and MUST NOT be used to programatically assume the presence 139or absence of particular features. 140 141shallow 142------- 143 144This capability adds "deepen", "shallow" and "unshallow" commands to 145the fetch-pack/upload-pack protocol so clients can request shallow 146clones. 147 148no-progress 149----------- 150 151The client was started with "git clone -q" or something, and doesn't 152want that side band 2. Basically the client just says "I do not 153wish to receive stream 2 on sideband, so do not send it to me, and if 154you did, I will drop it on the floor anyway". However, the sideband 155channel 3 is still used for error responses. 156 157include-tag 158----------- 159 160The 'include-tag' capability is about sending annotated tags if we are 161sending objects they point to. If we pack an object to the client, and 162a tag object points exactly at that object, we pack the tag object too. 163In general this allows a client to get all new annotated tags when it 164fetches a branch, in a single network connection. 165 166Clients MAY always send include-tag, hardcoding it into a request when 167the server advertises this capability. The decision for a client to 168request include-tag only has to do with the client's desires for tag 169data, whether or not a server had advertised objects in the 170refs/tags/* namespace. 171 172Servers MUST pack the tags if their referrant is packed and the client 173has requested include-tags. 174 175Clients MUST be prepared for the case where a server has ignored 176include-tag and has not actually sent tags in the pack. In such 177cases the client SHOULD issue a subsequent fetch to acquire the tags 178that include-tag would have otherwise given the client. 179 180The server SHOULD send include-tag, if it supports it, regardless 181of whether or not there are tags available. 182 183report-status 184------------- 185 186The receive-pack process can receive a 'report-status' capability, 187which tells it that the client wants a report of what happened after 188a packfile upload and reference update. If the pushing client requests 189this capability, after unpacking and updating references the server 190will respond with whether the packfile unpacked successfully and if 191each reference was updated successfully. If any of those were not 192successful, it will send back an error message. See pack-protocol.txt 193for example messages. 194 195delete-refs 196----------- 197 198If the server sends back the 'delete-refs' capability, it means that 199it is capable of accepting a zero-id value as the target 200value of a reference update. It is not sent back by the client, it 201simply informs the client that it can be sent zero-id values 202to delete references. 203 204quiet 205----- 206 207If the receive-pack server advertises the 'quiet' capability, it is 208capable of silencing human-readable progress output which otherwise may 209be shown when processing the received pack. A send-pack client should 210respond with the 'quiet' capability to suppress server-side progress 211reporting if the local progress reporting is also being suppressed 212(e.g., via `push -q`, or if stderr does not go to a tty). 213 214allow-tip-sha1-in-want 215---------------------- 216 217If the upload-pack server advertises this capability, fetch-pack may 218send "want" lines with SHA-1s that exist at the server but are not 219advertised by upload-pack.