oracular (5) gitprotocol-capabilities.5.gz
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