oracular (5) gitprotocol-pack.5.gz
NAME
gitprotocol-pack - How packs are transferred over-the-wire
SYNOPSIS
<over-the-wire-protocol>
DESCRIPTION
Git supports transferring data in packfiles over the ssh://, git://, http:// and file:// transports.
There exist two sets of protocols, one for pushing data from a client to a server and another for
fetching data from a server to a client. The three transports (ssh, git, file) use the same protocol to
transfer data. http is documented in gitprotocol-http(5).
The processes invoked in the canonical Git implementation are upload-pack on the server side and
fetch-pack on the client side for fetching data; then receive-pack on the server and send-pack on the
client for pushing data. The protocol functions to have a server tell a client what is currently on the
server, then for the two to negotiate the smallest amount of data to send in order to fully update one or
the other.
PKT-LINE FORMAT
The descriptions below build on the pkt-line format described in gitprotocol-common(5). When the grammar indicates PKT-LINE(...), unless otherwise noted the usual pkt-line LF rules apply: the sender SHOULD include a LF, but the receiver MUST NOT complain if it is not present. An error packet is a special pkt-line that contains an error string. error-line = PKT-LINE("ERR" SP explanation-text) Throughout the protocol, where PKT-LINE(...) is expected, an error packet MAY be sent. Once this packet is sent by a client or a server, the data transfer process defined in this protocol is terminated.
TRANSPORTS
There are three transports over which the packfile protocol is initiated. The Git transport is a simple,
unauthenticated server that takes the command (almost always upload-pack, though Git servers can be
configured to be globally writable, in which receive- pack initiation is also allowed) with which the
client wishes to communicate and executes it and connects it to the requesting process.
In the SSH transport, the client just runs the upload-pack or receive-pack process on the server over the
SSH protocol and then communicates with that invoked process over the SSH connection.
The file:// transport runs the upload-pack or receive-pack process locally and communicates with it over
a pipe.
EXTRA PARAMETERS
The protocol provides a mechanism in which clients can send additional information in its first message
to the server. These are called "Extra Parameters", and are supported by the Git, SSH, and HTTP
protocols.
Each Extra Parameter takes the form of <key>=<value> or <key>.
Servers that receive any such Extra Parameters MUST ignore all unrecognized keys. Currently, the only
Extra Parameter recognized is "version" with a value of 1 or 2. See gitprotocol-v2(5) for more
information on protocol version 2.
GIT TRANSPORT
The Git transport starts off by sending the command and repository on the wire using the pkt-line format,
followed by a NUL byte and a hostname parameter, terminated by a NUL byte.
0033git-upload-pack /project.git\0host=myserver.com\0
The transport may send Extra Parameters by adding an additional NUL byte, and then adding one or more
NUL-terminated strings:
003egit-upload-pack /project.git\0host=myserver.com\0\0version=1\0
git-proto-request = request-command SP pathname NUL
[ host-parameter NUL ] [ NUL extra-parameters ]
request-command = "git-upload-pack" / "git-receive-pack" /
"git-upload-archive" ; case sensitive
pathname = *( %x01-ff ) ; exclude NUL
host-parameter = "host=" hostname [ ":" port ]
extra-parameters = 1*extra-parameter
extra-parameter = 1*( %x01-ff ) NUL
host-parameter is used for the git-daemon name based virtual hosting. See --interpolated-path option to
git daemon, with the %H/%CH format characters.
Basically what the Git client is doing to connect to an upload-pack process on the server side over the
Git protocol is this:
$ echo -e -n \
"003agit-upload-pack /schacon/gitbook.git\0host=example.com\0" |
nc -v example.com 9418
SSH TRANSPORT
Initiating the upload-pack or receive-pack processes over SSH is executing the binary on the server via
SSH remote execution. It is basically equivalent to running this:
$ ssh git.example.com "git-upload-pack '/project.git'"
For a server to support Git pushing and pulling for a given user over SSH, that user needs to be able to
execute one or both of those commands via the SSH shell that they are provided on login. On some systems,
that shell access is limited to only being able to run those two commands, or even just one of them.
In an ssh:// format URI, it’s absolute in the URI, so the / after the host name (or port number) is sent
as an argument, which is then read by the remote git-upload-pack exactly as is, so it’s effectively an
absolute path in the remote filesystem.
git clone ssh://user@example.com/project.git
|
v
ssh user@example.com "git-upload-pack '/project.git'"
In a "user@host:path" format URI, it’s relative to the user’s home directory, because the Git client will
run:
git clone user@example.com:project.git
|
v
ssh user@example.com "git-upload-pack 'project.git'"
The exception is if a ~ is used, in which case we execute it without the leading /.
ssh://user@example.com/~alice/project.git,
|
v
ssh user@example.com "git-upload-pack '~alice/project.git'"
Depending on the value of the protocol.version configuration variable, Git may attempt to send Extra
Parameters as a colon-separated string in the GIT_PROTOCOL environment variable. This is done only if the
ssh.variant configuration variable indicates that the ssh command supports passing environment variables
as an argument.
A few things to remember here:
• The "command name" is spelled with dash (e.g. git-upload-pack), but this can be overridden by the
client;
• The repository path is always quoted with single quotes.
FETCHING DATA FROM A SERVER
When one Git repository wants to get data that a second repository has, the first can fetch from the
second. This operation determines what data the server has that the client does not then streams that
data down to the client in packfile format.
REFERENCE DISCOVERY
When the client initially connects the server will immediately respond with a version number (if
"version=1" is sent as an Extra Parameter), and a listing of each reference it has (all branches and
tags) along with the object name that each reference currently points to.
$ echo -e -n "0045git-upload-pack /schacon/gitbook.git\0host=example.com\0\0version=1\0" |
nc -v example.com 9418
000eversion 1
00887217a7c7e582c46cec22a130adf4b9d7d950fba0 HEAD\0multi_ack thin-pack
side-band side-band-64k ofs-delta shallow no-progress include-tag
00441d3fcd5ced445d1abc402225c0b8a1299641f497 refs/heads/integration
003f7217a7c7e582c46cec22a130adf4b9d7d950fba0 refs/heads/master
003cb88d2441cac0977faf98efc80305012112238d9d refs/tags/v0.9
003c525128480b96c89e6418b1e40909bf6c5b2d580f refs/tags/v1.0
003fe92df48743b7bc7d26bcaabfddde0a1e20cae47c refs/tags/v1.0^{}
0000
The returned response is a pkt-line stream describing each ref and its current value. The stream MUST be
sorted by name according to the C locale ordering.
If HEAD is a valid ref, HEAD MUST appear as the first advertised ref. If HEAD is not a valid ref, HEAD
MUST NOT appear in the advertisement list at all, but other refs may still appear.
The stream MUST include capability declarations behind a NUL on the first ref. The peeled value of a ref
(that is "ref^{}") MUST be immediately after the ref itself, if presented. A conforming server MUST peel
the ref if it’s an annotated tag.
advertised-refs = *1("version 1")
(no-refs / list-of-refs)
*shallow
flush-pkt
no-refs = PKT-LINE(zero-id SP "capabilities^{}"
NUL capability-list)
list-of-refs = first-ref *other-ref
first-ref = PKT-LINE(obj-id SP refname
NUL capability-list)
other-ref = PKT-LINE(other-tip / other-peeled)
other-tip = obj-id SP refname
other-peeled = obj-id SP refname "^{}"
shallow = PKT-LINE("shallow" SP obj-id)
capability-list = capability *(SP capability)
capability = 1*(LC_ALPHA / DIGIT / "-" / "_")
LC_ALPHA = %x61-7A
Server and client MUST use lowercase for obj-id, both MUST treat obj-id as case-insensitive.
See protocol-capabilities.txt for a list of allowed server capabilities and descriptions.
PACKFILE NEGOTIATION
After reference and capabilities discovery, the client can decide to terminate the connection by sending
a flush-pkt, telling the server it can now gracefully terminate, and disconnect, when it does not need
any pack data. This can happen with the ls-remote command, and also can happen when the client already is
up to date.
Otherwise, it enters the negotiation phase, where the client and server determine what the minimal
packfile necessary for transport is, by telling the server what objects it wants, its shallow objects (if
any), and the maximum commit depth it wants (if any). The client will also send a list of the
capabilities it wants to be in effect, out of what the server said it could do with the first want line.
upload-request = want-list
*shallow-line
*1depth-request
[filter-request]
flush-pkt
want-list = first-want
*additional-want
shallow-line = PKT-LINE("shallow" SP obj-id)
depth-request = PKT-LINE("deepen" SP depth) /
PKT-LINE("deepen-since" SP timestamp) /
PKT-LINE("deepen-not" SP ref)
first-want = PKT-LINE("want" SP obj-id SP capability-list)
additional-want = PKT-LINE("want" SP obj-id)
depth = 1*DIGIT
filter-request = PKT-LINE("filter" SP filter-spec)
Clients MUST send all the obj-ids it wants from the reference discovery phase as want lines. Clients MUST
send at least one want command in the request body. Clients MUST NOT mention an obj-id in a want command
which did not appear in the response obtained through ref discovery.
The client MUST write all obj-ids which it only has shallow copies of (meaning that it does not have the
parents of a commit) as shallow lines so that the server is aware of the limitations of the client’s
history.
The client now sends the maximum commit history depth it wants for this transaction, which is the number
of commits it wants from the tip of the history, if any, as a deepen line. A depth of 0 is the same as
not making a depth request. The client does not want to receive any commits beyond this depth, nor does
it want objects needed only to complete those commits. Commits whose parents are not received as a result
are defined as shallow and marked as such in the server. This information is sent back to the client in
the next step.
The client can optionally request that pack-objects omit various objects from the packfile using one of
several filtering techniques. These are intended for use with partial clone and partial fetch operations.
An object that does not meet a filter-spec value is omitted unless explicitly requested in a want line.
See rev-list for possible filter-spec values.
Once all the want’s and 'shallow’s (and optional 'deepen) are transferred, clients MUST send a flush-pkt,
to tell the server side that it is done sending the list.
Otherwise, if the client sent a positive depth request, the server will determine which commits will and
will not be shallow and send this information to the client. If the client did not request a positive
depth, this step is skipped.
shallow-update = *shallow-line
*unshallow-line
flush-pkt
shallow-line = PKT-LINE("shallow" SP obj-id)
unshallow-line = PKT-LINE("unshallow" SP obj-id)
If the client has requested a positive depth, the server will compute the set of commits which are no
deeper than the desired depth. The set of commits starts at the client’s wants.
The server writes shallow lines for each commit whose parents will not be sent as a result. The server
writes an unshallow line for each commit which the client has indicated is shallow, but is no longer
shallow at the currently requested depth (that is, its parents will now be sent). The server MUST NOT
mark as unshallow anything which the client has not indicated was shallow.
Now the client will send a list of the obj-ids it has using have lines, so the server can make a packfile
that only contains the objects that the client needs. In multi_ack mode, the canonical implementation
will send up to 32 of these at a time, then will send a flush-pkt. The canonical implementation will skip
ahead and send the next 32 immediately, so that there is always a block of 32 "in-flight on the wire" at
a time.
upload-haves = have-list
compute-end
have-list = *have-line
have-line = PKT-LINE("have" SP obj-id)
compute-end = flush-pkt / PKT-LINE("done")
If the server reads have lines, it then will respond by ACKing any of the obj-ids the client said it had
that the server also has. The server will ACK obj-ids differently depending on which ack mode is chosen
by the client.
In multi_ack mode:
• the server will respond with ACK obj-id continue for any common commits.
• once the server has found an acceptable common base commit and is ready to make a packfile, it will
blindly ACK all have obj-ids back to the client.
• the server will then send a NAK and then wait for another response from the client - either a done or
another list of have lines.
In multi_ack_detailed mode:
• the server will differentiate the ACKs where it is signaling that it is ready to send data with ACK
obj-id ready lines, and signals the identified common commits with ACK obj-id common lines.
Without either multi_ack or multi_ack_detailed:
• upload-pack sends "ACK obj-id" on the first common object it finds. After that it says nothing until
the client gives it a "done".
• upload-pack sends "NAK" on a flush-pkt if no common object has been found yet. If one has been found,
and thus an ACK was already sent, it’s silent on the flush-pkt.
After the client has gotten enough ACK responses that it can determine that the server has enough
information to send an efficient packfile (in the canonical implementation, this is determined when it
has received enough ACKs that it can color everything left in the --date-order queue as common with the
server, or the --date-order queue is empty), or the client determines that it wants to give up (in the
canonical implementation, this is determined when the client sends 256 have lines without getting any of
them ACKed by the server - meaning there is nothing in common and the server should just send all of its
objects), then the client will send a done command. The done command signals to the server that the
client is ready to receive its packfile data.
However, the 256 limit only turns on in the canonical client implementation if we have received at least
one "ACK %s continue" during a prior round. This helps to ensure that at least one common ancestor is
found before we give up entirely.
Once the done line is read from the client, the server will either send a final ACK obj-id or it will
send a NAK. obj-id is the object name of the last commit determined to be common. The server only sends
ACK after done if there is at least one common base and multi_ack or multi_ack_detailed is enabled. The
server always sends NAK after done if there is no common base found.
Instead of ACK or NAK, the server may send an error message (for example, if it does not recognize an
object in a want line received from the client).
Then the server will start sending its packfile data.
server-response = *ack_multi ack / nak
ack_multi = PKT-LINE("ACK" SP obj-id ack_status)
ack_status = "continue" / "common" / "ready"
ack = PKT-LINE("ACK" SP obj-id)
nak = PKT-LINE("NAK")
A simple clone may look like this (with no have lines):
C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d multi_ack \
side-band-64k ofs-delta\n
C: 0032want 7d1665144a3a975c05f1f43902ddaf084e784dbe\n
C: 0032want 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a\n
C: 0032want 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01\n
C: 0032want 74730d410fcb6603ace96f1dc55ea6196122532d\n
C: 0000
C: 0009done\n
S: 0008NAK\n
S: [PACKFILE]
An incremental update (fetch) response might look like this:
C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d multi_ack \
side-band-64k ofs-delta\n
C: 0032want 7d1665144a3a975c05f1f43902ddaf084e784dbe\n
C: 0032want 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a\n
C: 0000
C: 0032have 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01\n
C: [30 more have lines]
C: 0032have 74730d410fcb6603ace96f1dc55ea6196122532d\n
C: 0000
S: 003aACK 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01 continue\n
S: 003aACK 74730d410fcb6603ace96f1dc55ea6196122532d continue\n
S: 0008NAK\n
C: 0009done\n
S: 0031ACK 74730d410fcb6603ace96f1dc55ea6196122532d\n
S: [PACKFILE]
PACKFILE DATA
Now that the client and server have finished negotiation about what the minimal amount of data that needs
to be sent to the client is, the server will construct and send the required data in packfile format.
See gitformat-pack(5) for what the packfile itself actually looks like.
If side-band or side-band-64k capabilities have been specified by the client, the server will send the
packfile data multiplexed.
Each packet starting with the packet-line length of the amount of data that follows, followed by a single
byte specifying the sideband the following data is coming in on.
In side-band mode, it will send up to 999 data bytes plus 1 control code, for a total of up to 1000 bytes
in a pkt-line. In side-band-64k mode it will send up to 65519 data bytes plus 1 control code, for a total
of up to 65520 bytes in a pkt-line.
The sideband byte will be a 1, 2 or a 3. Sideband 1 will contain packfile data, sideband 2 will be used
for progress information that the client will generally print to stderr and sideband 3 is used for error
information.
If no side-band capability was specified, the server will stream the entire packfile without
multiplexing.
PUSHING DATA TO A SERVER
Pushing data to a server will invoke the receive-pack process on the server, which will allow the client
to tell it which references it should update and then send all the data the server will need for those
new references to be complete. Once all the data is received and validated, the server will then update
its references to what the client specified.
AUTHENTICATION
The protocol itself contains no authentication mechanisms. That is to be handled by the transport, such
as SSH, before the receive-pack process is invoked. If receive-pack is configured over the Git transport,
those repositories will be writable by anyone who can access that port (9418) as that transport is
unauthenticated.
REFERENCE DISCOVERY
The reference discovery phase is done nearly the same way as it is in the fetching protocol. Each
reference obj-id and name on the server is sent in packet-line format to the client, followed by a
flush-pkt. The only real difference is that the capability listing is different - the only possible
values are report-status, report-status-v2, delete-refs, ofs-delta, atomic and push-options.
REFERENCE UPDATE REQUEST AND PACKFILE TRANSFER
Once the client knows what references the server is at, it can send a list of reference update requests.
For each reference on the server that it wants to update, it sends a line listing the obj-id currently on
the server, the obj-id the client would like to update it to and the name of the reference.
This list is followed by a flush-pkt.
update-requests = *shallow ( command-list | push-cert )
shallow = PKT-LINE("shallow" SP obj-id)
command-list = PKT-LINE(command NUL capability-list)
*PKT-LINE(command)
flush-pkt
command = create / delete / update
create = zero-id SP new-id SP name
delete = old-id SP zero-id SP name
update = old-id SP new-id SP name
old-id = obj-id
new-id = obj-id
push-cert = PKT-LINE("push-cert" NUL capability-list LF)
PKT-LINE("certificate version 0.1" LF)
PKT-LINE("pusher" SP ident LF)
PKT-LINE("pushee" SP url LF)
PKT-LINE("nonce" SP nonce LF)
*PKT-LINE("push-option" SP push-option LF)
PKT-LINE(LF)
*PKT-LINE(command LF)
*PKT-LINE(gpg-signature-lines LF)
PKT-LINE("push-cert-end" LF)
push-option = 1*( VCHAR | SP )
If the server has advertised the push-options capability and the client has specified push-options as
part of the capability list above, the client then sends its push options followed by a flush-pkt.
push-options = *PKT-LINE(push-option) flush-pkt
For backwards compatibility with older Git servers, if the client sends a push cert and push options, it
MUST send its push options both embedded within the push cert and after the push cert. (Note that the
push options within the cert are prefixed, but the push options after the cert are not.) Both these lists
MUST be the same, modulo the prefix.
After that the packfile that should contain all the objects that the server will need to complete the new
references will be sent.
packfile = "PACK" 28*(OCTET)
If the receiving end does not support delete-refs, the sending end MUST NOT ask for delete command.
If the receiving end does not support push-cert, the sending end MUST NOT send a push-cert command. When
a push-cert command is sent, command-list MUST NOT be sent; the commands recorded in the push certificate
is used instead.
The packfile MUST NOT be sent if the only command used is delete.
A packfile MUST be sent if either create or update command is used, even if the server already has all
the necessary objects. In this case the client MUST send an empty packfile. The only time this is likely
to happen is if the client is creating a new branch or a tag that points to an existing obj-id.
The server will receive the packfile, unpack it, then validate each reference that is being updated that
it hasn’t changed while the request was being processed (the obj-id is still the same as the old-id), and
it will run any update hooks to make sure that the update is acceptable. If all of that is fine, the
server will then update the references.
PUSH CERTIFICATE
A push certificate begins with a set of header lines. After the header and an empty line, the protocol
commands follow, one per line. Note that the trailing LF in push-cert PKT-LINEs is not optional; it must
be present.
Currently, the following header fields are defined:
pusher ident
Identify the GPG key in "Human Readable Name <email@address[1]>" format.
pushee url
The repository URL (anonymized, if the URL contains authentication material) the user who ran git
push intended to push into.
nonce nonce
The nonce string the receiving repository asked the pushing user to include in the certificate, to
prevent replay attacks.
The GPG signature lines are a detached signature for the contents recorded in the push certificate before
the signature block begins. The detached signature is used to certify that the commands were given by the
pusher, who must be the signer.
REPORT STATUS
After receiving the pack data from the sender, the receiver sends a report if report-status or
report-status-v2 capability is in effect. It is a short listing of what happened in that update. It will
first list the status of the packfile unpacking as either unpack ok or unpack [error]. Then it will list
the status for each of the references that it tried to update. Each line is either ok [refname] if the
update was successful, or ng [refname] [error] if the update was not.
report-status = unpack-status
1*(command-status)
flush-pkt
unpack-status = PKT-LINE("unpack" SP unpack-result)
unpack-result = "ok" / error-msg
command-status = command-ok / command-fail
command-ok = PKT-LINE("ok" SP refname)
command-fail = PKT-LINE("ng" SP refname SP error-msg)
error-msg = 1*(OCTET) ; where not "ok"
The report-status-v2 capability extends the protocol by adding new option lines in order to support
reporting of reference rewritten by the proc-receive hook. The proc-receive hook may handle a command for
a pseudo-reference which may create or update one or more references, and each reference may have
different name, different new-oid, and different old-oid.
report-status-v2 = unpack-status
1*(command-status-v2)
flush-pkt
unpack-status = PKT-LINE("unpack" SP unpack-result)
unpack-result = "ok" / error-msg
command-status-v2 = command-ok-v2 / command-fail
command-ok-v2 = command-ok
*option-line
command-ok = PKT-LINE("ok" SP refname)
command-fail = PKT-LINE("ng" SP refname SP error-msg)
error-msg = 1*(OCTET) ; where not "ok"
option-line = *1(option-refname)
*1(option-old-oid)
*1(option-new-oid)
*1(option-forced-update)
option-refname = PKT-LINE("option" SP "refname" SP refname)
option-old-oid = PKT-LINE("option" SP "old-oid" SP obj-id)
option-new-oid = PKT-LINE("option" SP "new-oid" SP obj-id)
option-force = PKT-LINE("option" SP "forced-update")
Updates can be unsuccessful for a number of reasons. The reference can have changed since the reference
discovery phase was originally sent, meaning someone pushed in the meantime. The reference being pushed
could be a non-fast-forward reference and the update hooks or configuration could be set to not allow
that, etc. Also, some references can be updated while others can be rejected.
An example client/server communication might look like this:
S: 006274730d410fcb6603ace96f1dc55ea6196122532d refs/heads/local\0report-status delete-refs ofs-delta\n
S: 003e7d1665144a3a975c05f1f43902ddaf084e784dbe refs/heads/debug\n
S: 003f74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/master\n
S: 003d74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/team\n
S: 0000
C: 00677d1665144a3a975c05f1f43902ddaf084e784dbe 74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/debug\n
C: 006874730d410fcb6603ace96f1dc55ea6196122532d 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a refs/heads/master\n
C: 0000
C: [PACKDATA]
S: 000eunpack ok\n
S: 0018ok refs/heads/debug\n
S: 002ang refs/heads/master non-fast-forward\n
GIT
Part of the git(1) suite
NOTES
1. email@address
mailto:email@address