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 24The 'ofs-delta' capability is sent and recognized by both upload-pack 25and receive-pack protocols. 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 63walking down that line (so don't send c-b-a), but it's not done yet, 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 122Server can send, and client understand PACKv2 with delta referring to 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 171The upload-pack process can receive a 'report-status' capability, 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 184it is capable of accepting a zero-id value as the target 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.