| 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 'atomic', 'report-status', 'delete-refs', 'quiet', and 'push-cert' |
| 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. |
| |
| multi_ack_detailed |
| ------------------ |
| This is an extension of multi_ack that permits client to better |
| understand the server's in-memory state. See pack-protocol.txt, |
| section "Packfile Negotiation" for more information. |
| |
| no-done |
| ------- |
| This capability should only be used with the smart HTTP protocol. If |
| multi_ack_detailed and no-done are both present, then the sender is |
| free to immediately send a pack following its first "ACK obj-id ready" |
| message. |
| |
| Without no-done in the smart HTTP protocol, the server session would |
| end and the client has to make another trip to send "done" before |
| the server can send the pack. no-done removes the last round and |
| thus slightly reduces latency. |
| |
| thin-pack |
| --------- |
| |
| A thin pack is one with deltas which reference base objects not |
| contained within the pack (but are known to exist at the receiving |
| end). This can reduce the network traffic significantly, but it |
| requires the receiving end to know how to "thicken" these packs by |
| adding the missing bases to the pack. |
| |
| The upload-pack server advertises 'thin-pack' when it can generate |
| and send a thin pack. A client requests the 'thin-pack' capability |
| when it understands how to "thicken" it, notifying the server that |
| it can receive such a pack. A client MUST NOT request the |
| 'thin-pack' capability if it cannot turn a thin pack into a |
| self-contained pack. |
| |
| Receive-pack, on the other hand, is assumed by default to be able to |
| handle thin packs, but can ask the client not to use the feature by |
| advertising the 'no-thin' capability. A client MUST NOT send a thin |
| pack if the server advertises the 'no-thin' capability. |
| |
| The reasons for this asymmetry are historical. The receive-pack |
| program did not exist until after the invention of thin packs, so |
| historically the reference implementation of receive-pack always |
| understood thin packs. Adding 'no-thin' later allowed receive-pack |
| to disable the feature in a backwards-compatible manner. |
| |
| |
| 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 programmatically 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). |
| |
| atomic |
| ------ |
| |
| If the server sends the 'atomic' capability it is capable of accepting |
| atomic pushes. If the pushing client requests this capability, the server |
| will update the refs in one atomic transaction. Either all refs are |
| updated or none. |
| |
| 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. |
| |
| allow-reachable-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. |
| |
| push-cert=<nonce> |
| ----------------- |
| |
| The receive-pack server that advertises this capability is willing |
| to accept a signed push certificate, and asks the <nonce> to be |
| included in the push certificate. A send-pack client MUST NOT |
| send a push-cert packet unless the receive-pack server advertises |
| this capability. |
