# 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 and delete-refs capabilities are sent and recognized by the receive-pack (push to server) process.

The ofs-delta capability is sent and recognized by both upload-pack and receive-pack 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.

## 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 upload-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.