Provided by: git-man_2.43.0-1ubuntu7_all 
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/html/technical/api-trace2.html