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 75A thin pack is one with deltas which reference base objects not 76contained within the pack (but are known to exist at the receiving 77end). This can reduce the network traffic significantly, but it 78requires the receiving end to know how to "thicken" these packs by 79adding the missing bases to the pack. 80 81The upload-pack server advertises 'thin-pack' when it can generate 82and send a thin pack. A client requests the 'thin-pack' capability 83when it understands how to "thicken" it, notifying the server that 84it can receive such a pack. A client MUST NOT request the 85'thin-pack' capability if it cannot turn a thin pack into a 86self-contained pack. 87 88Receive-pack, on the other hand, is assumed by default to be able to 89handle thin packs, but can ask the client not to use the feature by 90advertising the 'no-thin' capability. A client MUST NOT send a thin 91pack if the server advertises the 'no-thin' capability. 92 93The reasons for this asymmetry are historical. The receive-pack 94program did not exist until after the invention of thin packs, so 95historically the reference implementation of receive-pack always 96understood thin packs. Adding 'no-thin' later allowed receive-pack 97to disable the feature in a backwards-compatible manner. 98 99 100side-band, side-band-64k 101------------------------ 102 103This capability means that server can send, and client understand multiplexed 104progress reports and error info interleaved with the packfile itself. 105 106These two options are mutually exclusive. A modern client always 107favors 'side-band-64k'. 108 109Either mode indicates that the packfile data will be streamed broken 110up into packets of up to either 1000 bytes in the case of 'side_band', 111or 65520 bytes in the case of 'side_band_64k'. Each packet is made up 112of a leading 4-byte pkt-line length of how much data is in the packet, 113followed by a 1-byte stream code, followed by the actual data. 114 115The stream code can be one of: 116 117 1 - pack data 118 2 - progress messages 119 3 - fatal error message just before stream aborts 120 121The "side-band-64k" capability came about as a way for newer clients 122that can handle much larger packets to request packets that are 123actually crammed nearly full, while maintaining backward compatibility 124for the older clients. 125 126Further, with side-band and its up to 1000-byte messages, it's actually 127999 bytes of payload and 1 byte for the stream code. With side-band-64k, 128same deal, you have up to 65519 bytes of data and 1 byte for the stream 129code. 130 131The client MUST send only maximum of one of "side-band" and "side- 132band-64k". Server MUST diagnose it as an error if client requests 133both. 134 135ofs-delta 136--------- 137 138Server can send, and client understand PACKv2 with delta referring to 139its base by position in pack rather than by an obj-id. That is, they can 140send/read OBJ_OFS_DELTA (aka type 6) in a packfile. 141 142agent 143----- 144 145The server may optionally send a capability of the form `agent=X` to 146notify the client that the server is running version `X`. The client may 147optionally return its own agent string by responding with an `agent=Y` 148capability (but it MUST NOT do so if the server did not mention the 149agent capability). The `X` and `Y` strings may contain any printable 150ASCII characters except space (i.e., the byte range 32 < x < 127), and 151are typically of the form "package/version" (e.g., "git/1.8.3.1"). The 152agent strings are purely informative for statistics and debugging 153purposes, and MUST NOT be used to programatically assume the presence 154or absence of particular features. 155 156shallow 157------- 158 159This capability adds "deepen", "shallow" and "unshallow" commands to 160the fetch-pack/upload-pack protocol so clients can request shallow 161clones. 162 163no-progress 164----------- 165 166The client was started with "git clone -q" or something, and doesn't 167want that side band 2. Basically the client just says "I do not 168wish to receive stream 2 on sideband, so do not send it to me, and if 169you did, I will drop it on the floor anyway". However, the sideband 170channel 3 is still used for error responses. 171 172include-tag 173----------- 174 175The 'include-tag' capability is about sending annotated tags if we are 176sending objects they point to. If we pack an object to the client, and 177a tag object points exactly at that object, we pack the tag object too. 178In general this allows a client to get all new annotated tags when it 179fetches a branch, in a single network connection. 180 181Clients MAY always send include-tag, hardcoding it into a request when 182the server advertises this capability. The decision for a client to 183request include-tag only has to do with the client's desires for tag 184data, whether or not a server had advertised objects in the 185refs/tags/* namespace. 186 187Servers MUST pack the tags if their referrant is packed and the client 188has requested include-tags. 189 190Clients MUST be prepared for the case where a server has ignored 191include-tag and has not actually sent tags in the pack. In such 192cases the client SHOULD issue a subsequent fetch to acquire the tags 193that include-tag would have otherwise given the client. 194 195The server SHOULD send include-tag, if it supports it, regardless 196of whether or not there are tags available. 197 198report-status 199------------- 200 201The receive-pack process can receive a 'report-status' capability, 202which tells it that the client wants a report of what happened after 203a packfile upload and reference update. If the pushing client requests 204this capability, after unpacking and updating references the server 205will respond with whether the packfile unpacked successfully and if 206each reference was updated successfully. If any of those were not 207successful, it will send back an error message. See pack-protocol.txt 208for example messages. 209 210delete-refs 211----------- 212 213If the server sends back the 'delete-refs' capability, it means that 214it is capable of accepting a zero-id value as the target 215value of a reference update. It is not sent back by the client, it 216simply informs the client that it can be sent zero-id values 217to delete references. 218 219quiet 220----- 221 222If the receive-pack server advertises the 'quiet' capability, it is 223capable of silencing human-readable progress output which otherwise may 224be shown when processing the received pack. A send-pack client should 225respond with the 'quiet' capability to suppress server-side progress 226reporting if the local progress reporting is also being suppressed 227(e.g., via `push -q`, or if stderr does not go to a tty). 228 229allow-tip-sha1-in-want 230---------------------- 231 232If the upload-pack server advertises this capability, fetch-pack may 233send "want" lines with SHA-1s that exist at the server but are not 234advertised by upload-pack.