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 72multi_ack_detailed 73------------------ 74This is an extension of multi_ack that permits client to better 75understand the server's in-memory state. See pack-protocol.txt, 76section "Packfile Negotiation" for more information. 77 78thin-pack 79--------- 80 81A thin pack is one with deltas which reference base objects not 82contained within the pack (but are known to exist at the receiving 83end). This can reduce the network traffic significantly, but it 84requires the receiving end to know how to "thicken" these packs by 85adding the missing bases to the pack. 86 87The upload-pack server advertises 'thin-pack' when it can generate 88and send a thin pack. A client requests the 'thin-pack' capability 89when it understands how to "thicken" it, notifying the server that 90it can receive such a pack. A client MUST NOT request the 91'thin-pack' capability if it cannot turn a thin pack into a 92self-contained pack. 93 94Receive-pack, on the other hand, is assumed by default to be able to 95handle thin packs, but can ask the client not to use the feature by 96advertising the 'no-thin' capability. A client MUST NOT send a thin 97pack if the server advertises the 'no-thin' capability. 98 99The reasons for this asymmetry are historical. The receive-pack 100program did not exist until after the invention of thin packs, so 101historically the reference implementation of receive-pack always 102understood thin packs. Adding 'no-thin' later allowed receive-pack 103to disable the feature in a backwards-compatible manner. 104 105 106side-band, side-band-64k 107------------------------ 108 109This capability means that server can send, and client understand multiplexed 110progress reports and error info interleaved with the packfile itself. 111 112These two options are mutually exclusive. A modern client always 113favors 'side-band-64k'. 114 115Either mode indicates that the packfile data will be streamed broken 116up into packets of up to either 1000 bytes in the case of 'side_band', 117or 65520 bytes in the case of 'side_band_64k'. Each packet is made up 118of a leading 4-byte pkt-line length of how much data is in the packet, 119followed by a 1-byte stream code, followed by the actual data. 120 121The stream code can be one of: 122 123 1 - pack data 124 2 - progress messages 125 3 - fatal error message just before stream aborts 126 127The "side-band-64k" capability came about as a way for newer clients 128that can handle much larger packets to request packets that are 129actually crammed nearly full, while maintaining backward compatibility 130for the older clients. 131 132Further, with side-band and its up to 1000-byte messages, it's actually 133999 bytes of payload and 1 byte for the stream code. With side-band-64k, 134same deal, you have up to 65519 bytes of data and 1 byte for the stream 135code. 136 137The client MUST send only maximum of one of "side-band" and "side- 138band-64k". Server MUST diagnose it as an error if client requests 139both. 140 141ofs-delta 142--------- 143 144Server can send, and client understand PACKv2 with delta referring to 145its base by position in pack rather than by an obj-id. That is, they can 146send/read OBJ_OFS_DELTA (aka type 6) in a packfile. 147 148agent 149----- 150 151The server may optionally send a capability of the form `agent=X` to 152notify the client that the server is running version `X`. The client may 153optionally return its own agent string by responding with an `agent=Y` 154capability (but it MUST NOT do so if the server did not mention the 155agent capability). The `X` and `Y` strings may contain any printable 156ASCII characters except space (i.e., the byte range 32 < x < 127), and 157are typically of the form "package/version" (e.g., "git/1.8.3.1"). The 158agent strings are purely informative for statistics and debugging 159purposes, and MUST NOT be used to programatically assume the presence 160or absence of particular features. 161 162shallow 163------- 164 165This capability adds "deepen", "shallow" and "unshallow" commands to 166the fetch-pack/upload-pack protocol so clients can request shallow 167clones. 168 169no-progress 170----------- 171 172The client was started with "git clone -q" or something, and doesn't 173want that side band 2. Basically the client just says "I do not 174wish to receive stream 2 on sideband, so do not send it to me, and if 175you did, I will drop it on the floor anyway". However, the sideband 176channel 3 is still used for error responses. 177 178include-tag 179----------- 180 181The 'include-tag' capability is about sending annotated tags if we are 182sending objects they point to. If we pack an object to the client, and 183a tag object points exactly at that object, we pack the tag object too. 184In general this allows a client to get all new annotated tags when it 185fetches a branch, in a single network connection. 186 187Clients MAY always send include-tag, hardcoding it into a request when 188the server advertises this capability. The decision for a client to 189request include-tag only has to do with the client's desires for tag 190data, whether or not a server had advertised objects in the 191refs/tags/* namespace. 192 193Servers MUST pack the tags if their referrant is packed and the client 194has requested include-tags. 195 196Clients MUST be prepared for the case where a server has ignored 197include-tag and has not actually sent tags in the pack. In such 198cases the client SHOULD issue a subsequent fetch to acquire the tags 199that include-tag would have otherwise given the client. 200 201The server SHOULD send include-tag, if it supports it, regardless 202of whether or not there are tags available. 203 204report-status 205------------- 206 207The receive-pack process can receive a 'report-status' capability, 208which tells it that the client wants a report of what happened after 209a packfile upload and reference update. If the pushing client requests 210this capability, after unpacking and updating references the server 211will respond with whether the packfile unpacked successfully and if 212each reference was updated successfully. If any of those were not 213successful, it will send back an error message. See pack-protocol.txt 214for example messages. 215 216delete-refs 217----------- 218 219If the server sends back the 'delete-refs' capability, it means that 220it is capable of accepting a zero-id value as the target 221value of a reference update. It is not sent back by the client, it 222simply informs the client that it can be sent zero-id values 223to delete references. 224 225quiet 226----- 227 228If the receive-pack server advertises the 'quiet' capability, it is 229capable of silencing human-readable progress output which otherwise may 230be shown when processing the received pack. A send-pack client should 231respond with the 'quiet' capability to suppress server-side progress 232reporting if the local progress reporting is also being suppressed 233(e.g., via `push -q`, or if stderr does not go to a tty). 234 235allow-tip-sha1-in-want 236---------------------- 237 238If the upload-pack server advertises this capability, fetch-pack may 239send "want" lines with SHA-1s that exist at the server but are not 240advertised by upload-pack.