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
git-htmldocs/technical/api-trace2.html
Git 2.48.1 2025-01-13 GITPROTOCOL-CAPABILITIES(5)
