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git/Documentation/gitprotocol-v2.txt

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gitprotocol-v2(5)
=================
NAME
----
gitprotocol-v2 - Git Wire Protocol, Version 2
SYNOPSIS
--------
[verse]
<over-the-wire-protocol>
DESCRIPTION
-----------
This document presents a specification for a version 2 of Git's wire
protocol. Protocol v2 will improve upon v1 in the following ways:
* Instead of multiple service names, multiple commands will be
supported by a single service
* Easily extendable as capabilities are moved into their own section
of the protocol, no longer being hidden behind a NUL byte and
limited by the size of a pkt-line
* Separate out other information hidden behind NUL bytes (e.g. agent
string as a capability and symrefs can be requested using 'ls-refs')
* Reference advertisement will be omitted unless explicitly requested
* ls-refs command to explicitly request some refs
* Designed with http and stateless-rpc in mind. With clear flush
semantics the http remote helper can simply act as a proxy
In protocol v2 communication is command oriented. When first contacting a
server a list of capabilities will advertised. Some of these capabilities
will be commands which a client can request be executed. Once a command
has completed, a client can reuse the connection and request that other
commands be executed.
Packet-Line Framing
-------------------
All communication is done using packet-line framing, just as in v1. See
linkgit:gitprotocol-pack[5] and linkgit:gitprotocol-common[5] for more information.
In protocol v2 these special packets will have the following semantics:
* '0000' Flush Packet (flush-pkt) - indicates the end of a message
* '0001' Delimiter Packet (delim-pkt) - separates sections of a message
* '0002' Response End Packet (response-end-pkt) - indicates the end of a
response for stateless connections
Initial Client Request
----------------------
In general a client can request to speak protocol v2 by sending
`version=2` through the respective side-channel for the transport being
used which inevitably sets `GIT_PROTOCOL`. More information can be
found in linkgit:gitprotocol-pack[5] and linkgit:gitprotocol-http[5], as well as the
`GIT_PROTOCOL` definition in `git.txt`. In all cases the
response from the server is the capability advertisement.
Git Transport
~~~~~~~~~~~~~
When using the git:// transport, you can request to use protocol v2 by
sending "version=2" as an extra parameter:
003egit-upload-pack /project.git\0host=myserver.com\0\0version=2\0
SSH and File Transport
~~~~~~~~~~~~~~~~~~~~~~
When using either the ssh:// or file:// transport, the GIT_PROTOCOL
environment variable must be set explicitly to include "version=2".
The server may need to be configured to allow this environment variable
to pass.
HTTP Transport
~~~~~~~~~~~~~~
When using the http:// or https:// transport a client makes a "smart"
info/refs request as described in linkgit:gitprotocol-http[5] and requests that
v2 be used by supplying "version=2" in the `Git-Protocol` header.
C: GET $GIT_URL/info/refs?service=git-upload-pack HTTP/1.0
C: Git-Protocol: version=2
A v2 server would reply:
S: 200 OK
S: <Some headers>
S: ...
S:
S: 000eversion 2\n
S: <capability-advertisement>
Subsequent requests are then made directly to the service
`$GIT_URL/git-upload-pack`. (This works the same for git-receive-pack).
Uses the `--http-backend-info-refs` option to
linkgit:git-upload-pack[1].
The server may need to be configured to pass this header's contents via
the `GIT_PROTOCOL` variable. See the discussion in `git-http-backend.txt`.
Capability Advertisement
------------------------
A server which decides to communicate (based on a request from a client)
using protocol version 2, notifies the client by sending a version string
in its initial response followed by an advertisement of its capabilities.
Each capability is a key with an optional value. Clients must ignore all
unknown keys. Semantics of unknown values are left to the definition of
each key. Some capabilities will describe commands which can be requested
to be executed by the client.
capability-advertisement = protocol-version
capability-list
flush-pkt
protocol-version = PKT-LINE("version 2" LF)
capability-list = *capability
capability = PKT-LINE(key[=value] LF)
key = 1*(ALPHA | DIGIT | "-_")
value = 1*(ALPHA | DIGIT | " -_.,?\/{}[]()<>!@#$%^&*+=:;")
Command Request
---------------
After receiving the capability advertisement, a client can then issue a
request to select the command it wants with any particular capabilities
or arguments. There is then an optional section where the client can
provide any command specific parameters or queries. Only a single
command can be requested at a time.
request = empty-request | command-request
empty-request = flush-pkt
command-request = command
capability-list
delim-pkt
command-args
flush-pkt
command = PKT-LINE("command=" key LF)
command-args = *command-specific-arg
command-specific-args are packet line framed arguments defined by
each individual command.
The server will then check to ensure that the client's request is
comprised of a valid command as well as valid capabilities which were
advertised. If the request is valid the server will then execute the
command. A server MUST wait till it has received the client's entire
request before issuing a response. The format of the response is
determined by the command being executed, but in all cases a flush-pkt
indicates the end of the response.
When a command has finished, and the client has received the entire
response from the server, a client can either request that another
command be executed or can terminate the connection. A client may
optionally send an empty request consisting of just a flush-pkt to
indicate that no more requests will be made.
Capabilities
------------
There are two different types of capabilities: normal capabilities,
which can be used to convey information or alter the behavior of a
request, and commands, which are the core actions that a client wants to
perform (fetch, push, etc).
Protocol version 2 is stateless by default. This means that all commands
must only last a single round and be stateless from the perspective of the
server side, unless the client has requested a capability indicating that
state should be maintained by the server. Clients MUST NOT require state
management on the server side in order to function correctly. This
permits simple round-robin load-balancing on the server side, without
needing to worry about state management.
agent
~~~~~
The server can advertise the `agent` capability with a value `X` (in the
form `agent=X`) to notify the client that the server is running version
`X`. The client may optionally send its own agent string by including
the `agent` capability with a value `Y` (in the form `agent=Y`) in its
request to the server (but it MUST NOT do so if the server did not
advertise 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.
ls-refs
~~~~~~~
`ls-refs` is the command used to request a reference advertisement in v2.
Unlike the current reference advertisement, ls-refs takes in arguments
which can be used to limit the refs sent from the server.
Additional features not supported in the base command will be advertised
as the value of the command in the capability advertisement in the form
of a space separated list of features: "<command>=<feature 1> <feature 2>"
ls-refs takes in the following arguments:
symrefs
In addition to the object pointed by it, show the underlying ref
pointed by it when showing a symbolic ref.
peel
Show peeled tags.
ref-prefix <prefix>
When specified, only references having a prefix matching one of
the provided prefixes are displayed. Multiple instances may be
given, in which case references matching any prefix will be
shown. Note that this is purely for optimization; a server MAY
show refs not matching the prefix if it chooses, and clients
should filter the result themselves.
If the 'unborn' feature is advertised the following argument can be
included in the client's request.
unborn
The server will send information about HEAD even if it is a symref
pointing to an unborn branch in the form "unborn HEAD
symref-target:<target>".
The output of ls-refs is as follows:
output = *ref
flush-pkt
obj-id-or-unborn = (obj-id | "unborn")
ref = PKT-LINE(obj-id-or-unborn SP refname *(SP ref-attribute) LF)
ref-attribute = (symref | peeled)
symref = "symref-target:" symref-target
peeled = "peeled:" obj-id
fetch
~~~~~
`fetch` is the command used to fetch a packfile in v2. It can be looked
at as a modified version of the v1 fetch where the ref-advertisement is
stripped out (since the `ls-refs` command fills that role) and the
message format is tweaked to eliminate redundancies and permit easy
addition of future extensions.
Additional features not supported in the base command will be advertised
as the value of the command in the capability advertisement in the form
of a space separated list of features: "<command>=<feature 1> <feature 2>"
A `fetch` request can take the following arguments:
want <oid>
Indicates to the server an object which the client wants to
retrieve. Wants can be anything and are not limited to
advertised objects.
have <oid>
Indicates to the server an object which the client has locally.
This allows the server to make a packfile which only contains
the objects that the client needs. Multiple 'have' lines can be
supplied.
done
Indicates to the server that negotiation should terminate (or
not even begin if performing a clone) and that the server should
use the information supplied in the request to construct the
packfile.
thin-pack
Request that a thin pack be sent, which is a pack 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.
no-progress
Request that progress information that would normally be sent on
side-band channel 2, during the packfile transfer, should not be
sent. However, the side-band channel 3 is still used for error
responses.
include-tag
Request that annotated tags should be sent if the objects they
point to are being sent.
ofs-delta
Indicate that the client understands PACKv2 with delta referring
to its base by position in pack rather than by an oid. That is,
they can read OBJ_OFS_DELTA (aka type 6) in a packfile.
If the 'shallow' feature is advertised the following arguments can be
included in the clients request as well as the potential addition of the
'shallow-info' section in the server's response as explained below.
shallow <oid>
A client must notify the server of all commits for which it only
has shallow copies (meaning that it doesn't have the parents of
a commit) by supplying a 'shallow <oid>' line for each such
object so that the server is aware of the limitations of the
client's history. This is so that the server is aware that the
client may not have all objects reachable from such commits.
deepen <depth>
Requests that the fetch/clone should be shallow having a commit
depth of <depth> relative to the remote side.
deepen-relative
Requests that the semantics of the "deepen" command be changed
to indicate that the depth requested is relative to the client's
current shallow boundary, instead of relative to the requested
commits.
deepen-since <timestamp>
Requests that the shallow clone/fetch should be cut at a
specific time, instead of depth. Internally it's equivalent to
doing "git rev-list --max-age=<timestamp>". Cannot be used with
"deepen".
deepen-not <rev>
Requests that the shallow clone/fetch should be cut at a
specific revision specified by '<rev>', instead of a depth.
Internally it's equivalent of doing "git rev-list --not <rev>".
Cannot be used with "deepen", but can be used with
"deepen-since".
If the 'filter' feature is advertised, the following argument can be
included in the client's request:
filter <filter-spec>
Request that various objects from the packfile be omitted
using one of several filtering techniques. These are intended
for use with partial clone and partial fetch operations. See
`rev-list` for possible "filter-spec" values. When communicating
with other processes, senders SHOULD translate scaled integers
(e.g. "1k") into a fully-expanded form (e.g. "1024") to aid
interoperability with older receivers that may not understand
newly-invented scaling suffixes. However, receivers SHOULD
accept the following suffixes: 'k', 'm', and 'g' for 1024,
1048576, and 1073741824, respectively.
If the 'ref-in-want' feature is advertised, the following argument can
be included in the client's request as well as the potential addition of
the 'wanted-refs' section in the server's response as explained below.
want-ref <ref>
Indicates to the server that the client wants to retrieve a
particular ref, where <ref> is the full name of a ref on the
server.
If the 'sideband-all' feature is advertised, the following argument can be
included in the client's request:
sideband-all
Instruct the server to send the whole response multiplexed, not just
the packfile section. All non-flush and non-delim PKT-LINE in the
response (not only in the packfile section) will then start with a byte
indicating its sideband (1, 2, or 3), and the server may send "0005\2"
(a PKT-LINE of sideband 2 with no payload) as a keepalive packet.
If the 'packfile-uris' feature is advertised, the following argument
can be included in the client's request as well as the potential
addition of the 'packfile-uris' section in the server's response as
explained below.
packfile-uris <comma-separated list of protocols>
Indicates to the server that the client is willing to receive
URIs of any of the given protocols in place of objects in the
sent packfile. Before performing the connectivity check, the
client should download from all given URIs. Currently, the
protocols supported are "http" and "https".
If the 'wait-for-done' feature is advertised, the following argument
can be included in the client's request.
wait-for-done
Indicates to the server that it should never send "ready", but
should wait for the client to say "done" before sending the
packfile.
The response of `fetch` is broken into a number of sections separated by
delimiter packets (0001), with each section beginning with its section
header. Most sections are sent only when the packfile is sent.
output = acknowledgements flush-pkt |
[acknowledgments delim-pkt] [shallow-info delim-pkt]
[wanted-refs delim-pkt] [packfile-uris delim-pkt]
packfile flush-pkt
acknowledgments = PKT-LINE("acknowledgments" LF)
(nak | *ack)
(ready)
ready = PKT-LINE("ready" LF)
nak = PKT-LINE("NAK" LF)
ack = PKT-LINE("ACK" SP obj-id LF)
shallow-info = PKT-LINE("shallow-info" LF)
*PKT-LINE((shallow | unshallow) LF)
shallow = "shallow" SP obj-id
unshallow = "unshallow" SP obj-id
wanted-refs = PKT-LINE("wanted-refs" LF)
*PKT-LINE(wanted-ref LF)
wanted-ref = obj-id SP refname
packfile-uris = PKT-LINE("packfile-uris" LF) *packfile-uri
packfile-uri = PKT-LINE(40*(HEXDIGIT) SP *%x20-ff LF)
packfile = PKT-LINE("packfile" LF)
*PKT-LINE(%x01-03 *%x00-ff)
acknowledgments section
* If the client determines that it is finished with negotiations by
sending a "done" line (thus requiring the server to send a packfile),
the acknowledgments sections MUST be omitted from the server's
response.
* Always begins with the section header "acknowledgments"
* The server will respond with "NAK" if none of the object ids sent
as have lines were common.
* The server will respond with "ACK obj-id" for all of the
object ids sent as have lines which are common.
* A response cannot have both "ACK" lines as well as a "NAK"
line.
* The server will respond with a "ready" line indicating that
the server has found an acceptable common base and is ready to
make and send a packfile (which will be found in the packfile
section of the same response)
* If the server has found a suitable cut point and has decided
to send a "ready" line, then the server can decide to (as an
optimization) omit any "ACK" lines it would have sent during
its response. This is because the server will have already
determined the objects it plans to send to the client and no
further negotiation is needed.
shallow-info section
* If the client has requested a shallow fetch/clone, a shallow
client requests a fetch or the server is shallow then the
server's response may include a shallow-info section. The
shallow-info section will be included if (due to one of the
above conditions) the server needs to inform the client of any
shallow boundaries or adjustments to the clients already
existing shallow boundaries.
* Always begins with the section header "shallow-info"
* If a positive depth is requested, the server will compute the
set of commits which are no deeper than the desired depth.
* The server sends a "shallow obj-id" line for each commit whose
parents will not be sent in the following packfile.
* The server sends an "unshallow obj-id" line for each commit
which the client has indicated is shallow, but is no longer
shallow as a result of the fetch (due to its parents being
sent in the following packfile).
* The server MUST NOT send any "unshallow" lines for anything
which the client has not indicated was shallow as a part of
its request.
wanted-refs section
* This section is only included if the client has requested a
ref using a 'want-ref' line and if a packfile section is also
included in the response.
* Always begins with the section header "wanted-refs".
* The server will send a ref listing ("<oid> <refname>") for
each reference requested using 'want-ref' lines.
* The server MUST NOT send any refs which were not requested
using 'want-ref' lines.
packfile-uris section
* This section is only included if the client sent
'packfile-uris' and the server has at least one such URI to
send.
* Always begins with the section header "packfile-uris".
* For each URI the server sends, it sends a hash of the pack's
contents (as output by git index-pack) followed by the URI.
* The hashes are 40 hex characters long. When Git upgrades to a new
hash algorithm, this might need to be updated. (It should match
whatever index-pack outputs after "pack\t" or "keep\t".
packfile section
* This section is only included if the client has sent 'want'
lines in its request and either requested that no more
negotiation be done by sending 'done' or if the server has
decided it has found a sufficient cut point to produce a
packfile.
* Always begins with the section header "packfile"
* The transmission of the packfile begins immediately after the
section header
* The data transfer of the packfile is always multiplexed, using
the same semantics of the 'side-band-64k' capability from
protocol version 1. This means that each packet, during the
packfile data stream, is made up of a leading 4-byte pkt-line
length (typical of the pkt-line format), 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
server-option
~~~~~~~~~~~~~
If advertised, indicates that any number of server specific options can be
included in a request. This is done by sending each option as a
"server-option=<option>" capability line in the capability-list section of
a request.
The provided options must not contain a NUL or LF character.
object-format
~~~~~~~~~~~~~~~
The server can advertise the `object-format` capability with a value `X` (in the
form `object-format=X`) to notify the client that the server is able to deal
with objects using hash algorithm X. If not specified, the server is assumed to
only handle SHA-1. If the client would like to use a hash algorithm other than
SHA-1, it should specify its object-format string.
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
link:technical/api-trace2.html[api-trace2] for details), but this may change
and users of the session ID should not rely on this fact.
object-info
~~~~~~~~~~~
`object-info` is the command to retrieve information about one or more objects.
Its main purpose is to allow a client to make decisions based on this
information without having to fully fetch objects. Object size is the only
information that is currently supported.
An `object-info` request takes the following arguments:
size
Requests size information to be returned for each listed object id.
oid <oid>
Indicates to the server an object which the client wants to obtain
information for.
The response of `object-info` is a list of the requested object ids
and associated requested information, each separated by a single space.
output = info flush-pkt
info = PKT-LINE(attrs) LF)
*PKT-LINE(obj-info LF)
attrs = attr | attrs SP attrs
attr = "size"
obj-info = obj-id SP obj-size
bundle-uri
~~~~~~~~~~
If the 'bundle-uri' capability is advertised, the server supports the
`bundle-uri' command.
The capability is currently advertised with no value (i.e. not
"bundle-uri=somevalue"), a value may be added in the future for
supporting command-wide extensions. Clients MUST ignore any unknown
capability values and proceed with the 'bundle-uri` dialog they
support.
The 'bundle-uri' command is intended to be issued before `fetch` to
get URIs to bundle files (see linkgit:git-bundle[1]) to "seed" and
inform the subsequent `fetch` command.
The client CAN issue `bundle-uri` before or after any other valid
command. To be useful to clients it's expected that it'll be issued
after an `ls-refs` and before `fetch`, but CAN be issued at any time
in the dialog.
DISCUSSION of bundle-uri
^^^^^^^^^^^^^^^^^^^^^^^^
The intent of the feature is optimize for server resource consumption
in the common case by changing the common case of fetching a very
large PACK during linkgit:git-clone[1] into a smaller incremental
fetch.
It also allows servers to achieve better caching in combination with
an `uploadpack.packObjectsHook` (see linkgit:git-config[1]).
By having new clones or fetches be a more predictable and common
negotiation against the tips of recently produces *.bundle file(s).
Servers might even pre-generate the results of such negotiations for
the `uploadpack.packObjectsHook` as new pushes come in.
One way that servers could take advantage of these bundles is that the
server would anticipate that fresh clones will download a known bundle,
followed by catching up to the current state of the repository using ref
tips found in that bundle (or bundles).
PROTOCOL for bundle-uri
^^^^^^^^^^^^^^^^^^^^^^^
A `bundle-uri` request takes no arguments, and as noted above does not
currently advertise a capability value. Both may be added in the
future.
When the client issues a `command=bundle-uri` request, the response is a
list of key-value pairs provided as packet lines with value
`<key>=<value>`. Each `<key>` should be interpreted as a config key from
the `bundle.*` namespace to construct a list of bundles. These keys are
grouped by a `bundle.<id>.` subsection, where each key corresponding to a
given `<id>` contributes attributes to the bundle defined by that `<id>`.
See linkgit:git-config[1] for the specific details of these keys and how
the Git client will interpret their values.
Clients MUST parse the line according to the above format, lines that do
not conform to the format SHOULD be discarded. The user MAY be warned in
such a case.
bundle-uri CLIENT AND SERVER EXPECTATIONS
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
URI CONTENTS::
The content at the advertised URIs MUST be one of two types.
+
The advertised URI may contain a bundle file that `git bundle verify`
would accept. I.e. they MUST contain one or more reference tips for
use by the client, MUST indicate prerequisites (in any) with standard
"-" prefixes, and MUST indicate their "object-format", if
applicable.
+
The advertised URI may alternatively contain a plaintext file that `git
config --list` would accept (with the `--file` option). The key-value
pairs in this list are in the `bundle.*` namespace (see
linkgit:git-config[1]).
bundle-uri CLIENT ERROR RECOVERY::
A client MUST above all gracefully degrade on errors, whether that
error is because of bad missing/data in the bundle URI(s), because
that client is too dumb to e.g. understand and fully parse out bundle
headers and their prerequisite relationships, or something else.
+
Server operators should feel confident in turning on "bundle-uri" and
not worry if e.g. their CDN goes down that clones or fetches will run
into hard failures. Even if the server bundle(s) are
incomplete, or bad in some way the client should still end up with a
functioning repository, just as if it had chosen not to use this
protocol extension.
+
All subsequent discussion on client and server interaction MUST keep
this in mind.
bundle-uri SERVER TO CLIENT::
The ordering of the returned bundle uris is not significant. Clients
MUST parse their headers to discover their contained OIDS and
prerequisites. A client MUST consider the content of the bundle(s)
themselves and their header as the ultimate source of truth.
+
A server MAY even return bundle(s) that don't have any direct
relationship to the repository being cloned (either through accident,
or intentional "clever" configuration), and expect a client to sort
out what data they'd like from the bundle(s), if any.
bundle-uri CLIENT TO SERVER::
The client SHOULD provide reference tips found in the bundle header(s)
as 'have' lines in any subsequent `fetch` request. A client MAY also
ignore the bundle(s) entirely if doing so is deemed worse for some
reason, e.g. if the bundles can't be downloaded, it doesn't like the
tips it finds etc.
WHEN ADVERTISED BUNDLE(S) REQUIRE NO FURTHER NEGOTIATION::
If after issuing `bundle-uri` and `ls-refs`, and getting the header(s)
of the bundle(s) the client finds that the ref tips it wants can be
retrieved entirely from advertised bundle(s), the client MAY disconnect
from the Git server. The results of such a 'clone' or 'fetch' should be
indistinguishable from the state attained without using bundle-uri.
EARLY CLIENT DISCONNECTIONS AND ERROR RECOVERY::
A client MAY perform an early disconnect while still downloading the
bundle(s) (having streamed and parsed their headers). In such a case
the client MUST gracefully recover from any errors related to
finishing the download and validation of the bundle(s).
+
I.e. a client might need to re-connect and issue a 'fetch' command,
and possibly fall back to not making use of 'bundle-uri' at all.
+
This "MAY" behavior is specified as such (and not a "SHOULD") on the
assumption that a server advertising bundle uris is more likely than
not to be serving up a relatively large repository, and to be pointing
to URIs that have a good chance of being in working order. A client
MAY e.g. look at the payload size of the bundles as a heuristic to see
if an early disconnect is worth it, should falling back on a full
"fetch" dialog be necessary.
WHEN ADVERTISED BUNDLE(S) REQUIRE FURTHER NEGOTIATION::
A client SHOULD commence a negotiation of a PACK from the server via
the "fetch" command using the OID tips found in advertised bundles,
even if's still in the process of downloading those bundle(s).
+
This allows for aggressive early disconnects from any interactive
server dialog. The client blindly trusts that the advertised OID tips
are relevant, and issues them as 'have' lines, it then requests any
tips it would like (usually from the "ls-refs" advertisement) via
'want' lines. The server will then compute a (hopefully small) PACK
with the expected difference between the tips from the bundle(s) and
the data requested.
+
The only connection the client then needs to keep active is to the
concurrently downloading static bundle(s), when those and the
incremental PACK are retrieved they should be inflated and
validated. Any errors at this point should be gracefully recovered
from, see above.
bundle-uri PROTOCOL FEATURES
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The client constructs a bundle list from the `<key>=<value>` pairs
provided by the server. These pairs are part of the `bundle.*` namespace
as documented in linkgit:git-config[1]. In this section, we discuss some
of these keys and describe the actions the client will do in response to
this information.
In particular, the `bundle.version` key specifies an integer value. The
only accepted value at the moment is `1`, but if the client sees an
unexpected value here then the client MUST ignore the bundle list.
As long as `bundle.version` is understood, all other unknown keys MAY be
ignored by the client. The server will guarantee compatibility with older
clients, though newer clients may be better able to use the extra keys to
minimize downloads.
Any backwards-incompatible addition of pre-URI key-value will be
guarded by a new `bundle.version` value or values in 'bundle-uri'
capability advertisement itself, and/or by new future `bundle-uri`
request arguments.
Some example key-value pairs that are not currently implemented but could
be implemented in the future include:
* Add a "hash=<val>" or "size=<bytes>" advertise the expected hash or
size of the bundle file.
* Advertise that one or more bundle files are the same (to e.g. have
clients round-robin or otherwise choose one of N possible files).
* A "oid=<OID>" shortcut and "prerequisite=<OID>" shortcut. For
expressing the common case of a bundle with one tip and no
prerequisites, or one tip and one prerequisite.
+
This would allow for optimizing the common case of servers who'd like
to provide one "big bundle" containing only their "main" branch,
and/or incremental updates thereof.
+
A client receiving such a a response MAY assume that they can skip
retrieving the header from a bundle at the indicated URI, and thus
save themselves and the server(s) the request(s) needed to inspect the
headers of that bundle or bundles.
GIT
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