| Git Protocol Capabilities | |
| ========================= | |
| Servers SHOULD support all capabilities defined in this document. | |
| On the very first line of the initial server response of either | |
| receive-pack and upload-pack the first reference is followed by | |
| a NUL byte and then a list of space delimited server capabilities. | |
| These allow the server to declare what it can and cannot support | |
| to the client. | |
| Client will then send a space separated list of capabilities it wants | |
| to be in effect. The client MUST NOT ask for capabilities the server | |
| did not say it supports. | |
| Server MUST diagnose and abort if capabilities it does not understand | |
| was sent. Server MUST NOT ignore capabilities that client requested | |
| and server advertised. As a consequence of these rules, server MUST | |
| NOT advertise capabilities it does not understand. | |
| The 'report-status', 'delete-refs', and 'quiet' capabilities are sent and | |
| recognized by the receive-pack (push to server) process. | |
| The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized | |
| by both upload-pack and receive-pack protocols. The 'agent' capability | |
| may optionally be sent in both protocols. | |
| All other capabilities are only recognized by the upload-pack (fetch | |
| from server) process. | |
| multi_ack | |
| --------- | |
| The 'multi_ack' capability allows the server to return "ACK obj-id | |
| continue" as soon as it finds a commit that it can use as a common | |
| base, between the client's wants and the client's have set. | |
| By sending this early, the server can potentially head off the client | |
| from walking any further down that particular branch of the client's | |
| repository history. The client may still need to walk down other | |
| branches, sending have lines for those, until the server has a | |
| complete cut across the DAG, or the client has said "done". | |
| Without multi_ack, a client sends have lines in --date-order until | |
| the server has found a common base. That means the client will send | |
| have lines that are already known by the server to be common, because | |
| they overlap in time with another branch that the server hasn't found | |
| a common base on yet. | |
| For example suppose the client has commits in caps that the server | |
| doesn't and the server has commits in lower case that the client | |
| doesn't, as in the following diagram: | |
| +---- u ---------------------- x | |
| / +----- y | |
| / / | |
| a -- b -- c -- d -- E -- F | |
| \ | |
| +--- Q -- R -- S | |
| If the client wants x,y and starts out by saying have F,S, the server | |
| doesn't know what F,S is. Eventually the client says "have d" and | |
| the server sends "ACK d continue" to let the client know to stop | |
| walking down that line (so don't send c-b-a), but it's not done yet, | |
| it needs a base for x. The client keeps going with S-R-Q, until a | |
| gets reached, at which point the server has a clear base and it all | |
| ends. | |
| Without multi_ack the client would have sent that c-b-a chain anyway, | |
| interleaved with S-R-Q. | |
| thin-pack | |
| --------- | |
| This capability means that the server can send a 'thin' pack, a pack | |
| which does not contain base objects; if those base objects are available | |
| on client side. Client requests 'thin-pack' capability when it | |
| understands how to "thicken" it by adding required delta bases making | |
| it self-contained. | |
| Client MUST NOT request 'thin-pack' capability if it cannot turn a thin | |
| pack into a self-contained pack. | |
| side-band, side-band-64k | |
| ------------------------ | |
| This capability means that server can send, and client understand multiplexed | |
| progress reports and error info interleaved with the packfile itself. | |
| These two options are mutually exclusive. A modern client always | |
| favors 'side-band-64k'. | |
| Either mode indicates that the packfile data will be streamed broken | |
| up into packets of up to either 1000 bytes in the case of 'side_band', | |
| or 65520 bytes in the case of 'side_band_64k'. Each packet is made up | |
| of a leading 4-byte pkt-line length of how much data is in the packet, | |
| followed by a 1-byte stream code, followed by the actual data. | |
| The stream code can be one of: | |
| 1 - pack data | |
| 2 - progress messages | |
| 3 - fatal error message just before stream aborts | |
| The "side-band-64k" capability came about as a way for newer clients | |
| that can handle much larger packets to request packets that are | |
| actually crammed nearly full, while maintaining backward compatibility | |
| for the older clients. | |
| Further, with side-band and its up to 1000-byte messages, it's actually | |
| 999 bytes of payload and 1 byte for the stream code. With side-band-64k, | |
| same deal, you have up to 65519 bytes of data and 1 byte for the stream | |
| code. | |
| The client MUST send only maximum of one of "side-band" and "side- | |
| band-64k". Server MUST diagnose it as an error if client requests | |
| both. | |
| ofs-delta | |
| --------- | |
| Server can send, and client understand PACKv2 with delta referring to | |
| its base by position in pack rather than by an obj-id. That is, they can | |
| send/read OBJ_OFS_DELTA (aka type 6) in a packfile. | |
| agent | |
| ----- | |
| The server may optionally send a capability of the form `agent=X` to | |
| notify the client that the server is running version `X`. The client may | |
| optionally return its own agent string by responding with an `agent=Y` | |
| capability (but it MUST NOT do so if the server did not mention the | |
| agent capability). The `X` and `Y` strings may contain any printable | |
| ASCII characters except space (i.e., the byte range 32 < x < 127), and | |
| are typically of the form "package/version" (e.g., "git/1.8.3.1"). The | |
| agent strings are purely informative for statistics and debugging | |
| purposes, and MUST NOT be used to programatically assume the presence | |
| or absence of particular features. | |
| shallow | |
| ------- | |
| This capability adds "deepen", "shallow" and "unshallow" commands to | |
| the fetch-pack/upload-pack protocol so clients can request shallow | |
| clones. | |
| no-progress | |
| ----------- | |
| The client was started with "git clone -q" or something, and doesn't | |
| want that side band 2. Basically the client just says "I do not | |
| wish to receive stream 2 on sideband, so do not send it to me, and if | |
| you did, I will drop it on the floor anyway". However, the sideband | |
| channel 3 is still used for error responses. | |
| include-tag | |
| ----------- | |
| The 'include-tag' capability is about sending annotated tags if we are | |
| sending objects they point to. If we pack an object to the client, and | |
| a tag object points exactly at that object, we pack the tag object too. | |
| In general this allows a client to get all new annotated tags when it | |
| fetches a branch, in a single network connection. | |
| Clients MAY always send include-tag, hardcoding it into a request when | |
| the server advertises this capability. The decision for a client to | |
| request include-tag only has to do with the client's desires for tag | |
| data, whether or not a server had advertised objects in the | |
| refs/tags/* namespace. | |
| Servers MUST pack the tags if their referrant is packed and the client | |
| has requested include-tags. | |
| Clients MUST be prepared for the case where a server has ignored | |
| include-tag and has not actually sent tags in the pack. In such | |
| cases the client SHOULD issue a subsequent fetch to acquire the tags | |
| that include-tag would have otherwise given the client. | |
| The server SHOULD send include-tag, if it supports it, regardless | |
| of whether or not there are tags available. | |
| report-status | |
| ------------- | |
| The receive-pack process can receive a 'report-status' capability, | |
| which tells it that the client wants a report of what happened after | |
| a packfile upload and reference update. If the pushing client requests | |
| this capability, after unpacking and updating references the server | |
| will respond with whether the packfile unpacked successfully and if | |
| each reference was updated successfully. If any of those were not | |
| successful, it will send back an error message. See pack-protocol.txt | |
| for example messages. | |
| delete-refs | |
| ----------- | |
| If the server sends back the 'delete-refs' capability, it means that | |
| it is capable of accepting a zero-id value as the target | |
| value of a reference update. It is not sent back by the client, it | |
| simply informs the client that it can be sent zero-id values | |
| to delete references. | |
| quiet | |
| ----- | |
| If the receive-pack server advertises the 'quiet' capability, it is | |
| capable of silencing human-readable progress output which otherwise may | |
| be shown when processing the received pack. A send-pack client should | |
| respond with the 'quiet' capability to suppress server-side progress | |
| reporting if the local progress reporting is also being suppressed | |
| (e.g., via `push -q`, or if stderr does not go to a tty). | |
| allow-tip-sha1-in-want | |
| ---------------------- | |
| If the upload-pack server advertises this capability, fetch-pack may | |
| send "want" lines with SHA-1s that exist at the server but are not | |
| advertised by upload-pack. |