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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' and 'side-band-64k' capabilities are 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 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.