GITPROTOCOL-CAPABILITIES(5) Git Manual GITPROTOCOL-CAPABILITIES(5)
NAME
gitprotocol-capabilities - Protocol v0 and v1 capabilities
SYNOPSIS
<over-the-wire-protocol>
DESCRIPTION
Note
this document describes capabilities for versions 0 and 1 of the
pack protocol. For version 2, please refer to the gitprotocol-v2(5)
doc.
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
were 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, report-status-v2, 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 and session-id
capabilities 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 on which the server hasn't found a
common base 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 the client to better
understand the server's in-memory state. See gitprotocol-pack(5),
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 the server can send, and the client can
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 one of "side-band" and "side- band-64k". The
server MUST diagnose it as an error if client requests both.
OFS-DELTA
The server can send, and the client can 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.
OBJECT-FORMAT
This capability, which takes a hash algorithm as an argument, indicates
that the server supports the given hash algorithms. It may be sent
multiple times; if so, the first one given is the one used in the ref
advertisement.
When provided by the client, this indicates that it intends to use the
given hash algorithm to communicate. The algorithm provided must be one
that the server supports.
If this capability is not provided, it is assumed that the only
supported algorithm is SHA-1.
SYMREF
This parameterized capability is used to inform the receiver which
symbolic ref points to which ref; for example,
"symref=HEAD:refs/heads/master" tells the receiver that HEAD points to
master. This capability can be repeated to represent multiple symrefs.
Servers SHOULD include this capability for the HEAD symref if it is one
of the refs being sent.
Clients MAY use the parameters from this capability to select the
proper initial branch when cloning a repository.
SHALLOW
This capability adds "deepen", "shallow" and "unshallow" commands to
the fetch-pack/upload-pack protocol so clients can request shallow
clones.
DEEPEN-SINCE
This capability adds "deepen-since" command to fetch-pack/upload-pack
protocol so the client can request shallow clones that are cut at a
specific time, instead of depth. Internally it's equivalent of doing
"rev-list --max-age=<timestamp>" on the server side. "deepen-since"
cannot be used with "deepen".
DEEPEN-NOT
This capability adds "deepen-not" command to fetch-pack/upload-pack
protocol so the client can request shallow clones that are cut at a
specific revision, instead of depth. Internally it's equivalent of
doing "rev-list --not <rev>" on the server side. "deepen-not" cannot be
used with "deepen", but can be used with "deepen-since".
DEEPEN-RELATIVE
If this capability is requested by the client, the semantics of
"deepen" command is changed. The "depth" argument is the depth from the
current shallow boundary, instead of the depth from remote refs.
NO-PROGRESS
The client was started with "git clone -q" or something similar, 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 referent 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 gitprotocol-pack(5)
for example messages.
REPORT-STATUS-V2
Capability report-status-v2 extends capability report-status by adding
new "option" directives in order to support reference rewritten by the
"proc-receive" hook. The "proc-receive" hook may handle a command for a
pseudo-reference which may create or update a reference with different
name, new-oid, and old-oid. While the capability report-status cannot
report for such case. See gitprotocol-pack(5) for details.
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.
PUSH-OPTIONS
If the server sends the push-options capability it is able to accept
push options after the update commands have been sent, but before the
packfile is streamed. If the pushing client requests this capability,
the server will pass the options to the pre- and post- receive hooks
that process this push request.
ALLOW-TIP-SHA1-IN-WANT
If the upload-pack server advertises this capability, fetch-pack may
send "want" lines with object names that exist at the server but are
not advertised by upload-pack. For historical reasons, the name of this
capability contains "sha1". Object names are always given using the
object format negotiated through the object-format capability.
ALLOW-REACHABLE-SHA1-IN-WANT
If the upload-pack server advertises this capability, fetch-pack may
send "want" lines with object names that exist at the server but are
not advertised by upload-pack. For historical reasons, the name of this
capability contains "sha1". Object names are always given using the
object format negotiated through the object-format capability.
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.
FILTER
If the upload-pack server advertises the filter capability, fetch-pack
may send "filter" commands to request a partial clone or partial fetch
and request that the server omit various objects from the packfile.
SESSION-ID=<SESSION ID>
The server may advertise a session ID that can be used to identify this
process across multiple requests. The client may advertise its own
session ID back to the server as well.
Session IDs should be unique to a given process. They must fit within a
packet-line, and must not contain non-printable or whitespace
characters. The current implementation uses trace2 session IDs (see
api-trace2[1] for details), but this may change and users of the
session ID should not rely on this fact.
GIT
Part of the git(1) suite
NOTES
1. api-trace2
file:///usr/share/doc/git/technical/api-trace2.html
Git 2.43.5 05/31/2024 GITPROTOCOL-CAPABILITIES(5)
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