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-Pack transfer protocols
-=======================
-
-There are two Pack push-pull protocols.
-
-upload-pack (S) | fetch/clone-pack (C) protocol:
-
- # Tell the puller what commits we have and what their names are
- S: SHA1 name
- S: ...
- S: SHA1 name
- S: # flush -- it's your turn
- # Tell the pusher what commits we want, and what we have
- C: want name
- C: ..
- C: want name
- C: have SHA1
- C: have SHA1
- C: ...
- C: # flush -- occasionally ask "had enough?"
- S: NAK
- C: have SHA1
- C: ...
- C: have SHA1
- S: ACK
- C: done
- S: XXXXXXX -- packfile contents.
-
-send-pack | receive-pack protocol.
-
- # Tell the pusher what commits we have and what their names are
- C: SHA1 name
- C: ...
- C: SHA1 name
- C: # flush -- it's your turn
- # Tell the puller what the pusher has
- S: old-SHA1 new-SHA1 name
- S: old-SHA1 new-SHA1 name
- S: ...
- S: # flush -- done with the list
- S: XXXXXXX --- packfile contents.
+Packfile transfer protocols
+===========================
+
+Git supports transferring data in packfiles over the ssh://, git:// 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. All three transports (ssh, git, file) use the same
+protocol to transfer data.
+
+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.
+
+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.
+
+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 paramater, terminated by a NUL byte.
+
+ 0032git-upload-pack /project.git\0host=myserver.com\0
+
+--
+ git-proto-request = request-command SP pathname NUL [ host-parameter NUL ]
+ request-command = "git-upload-pack" / "git-receive-pack" /
+ "git-upload-archive" ; case sensitive
+ pathname = *( %x01-ff ) ; exclude NUL
+ host-parameter = "host=" hostname [ ":" port ]
+--
+
+Only host-parameter is allowed in the git-proto-request. Clients
+MUST NOT attempt to send additional parameters. It 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 \
+ "0039git-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, its 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'"
+
+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 listing of each reference it has (all branches and tags) along
+with the object name that each reference currently points to.
+
+ $ echo -e -n "0039git-upload-pack /schacon/gitbook.git\0host=example.com\0" |
+ nc -v example.com 9418
+ 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
+
+Server SHOULD terminate each non-flush line using LF ("\n") terminator;
+client MUST NOT complain if there is no terminator.
+
+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 its an annotated tag.
+
+----
+ advertised-refs = (no-refs / list-of-refs)
+ flush-pkt
+
+ no-refs = PKT-LINE(zero-id SP "capabilities^{}"
+ NUL capability-list LF)
+
+ list-of-refs = first-ref *other-ref
+ first-ref = PKT-LINE(obj-id SP refname
+ NUL capability-list LF)
+
+ other-ref = PKT-LINE(other-tip / other-peeled)
+ other-tip = obj-id SP refname LF
+ other-peeled = obj-id SP refname "^{}" LF
+
+ 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 (as happens with the ls-remote
+command) or it can enter the negotiation phase, where the client and
+server determine what the minimal packfile necessary for transport is.
+
+Once the client has the initial list of references that the server
+has, as well as the list of capabilities, it will begin telling the
+server what objects it wants and what objects it has, so the server
+can make a packfile that only contains the objects that the client needs.
+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
+ have-list
+ compute-end
+
+ want-list = first-want
+ *additional-want
+ flush-pkt
+
+ first-want = PKT-LINE("want" SP obj-id SP capability-list LF)
+ additional-want = PKT-LINE("want" SP obj-id LF)
+
+ have-list = *have-line
+ have-line = PKT-LINE("have" SP obj-id LF)
+ compute-end = flush-pkt / PKT-LINE("done")
+----
+
+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.
+
+If client is requesting a shallow clone, it will now send a 'deepen'
+line with the depth it is requesting.
+
+Once all the "want"s (and optional 'deepen') are transferred,
+clients MUST send a flush-pkt. If the client has all the references
+on the server, client flushes and disconnects.
+
+TODO: shallow/unshallow response and document the deepen command in the ABNF.
+
+Now the client will send a list of the obj-ids it has using 'have'
+lines. 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.
+
+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 'NACK' 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, its 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 it's objects), then the client will send
+a 'done' command. The 'done' command signals to the server that the client
+is ready to receive it's 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'. 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.
+
+Then the server will start sending it's packfile data.
+
+----
+ server-response = *ack_multi ack / nak
+ ack_multi = PKT-LINE("ACK" SP obj-id ack_status LF)
+ ack_status = "continue" / "common" / "ready"
+ ack = PKT-LINE("ACK SP obj-id LF)
+ nak = PKT-LINE("NAK" LF)
+----
+
+A simple clone may look like this (with no 'have' lines):
+
+----
+ C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d\0multi_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\0multi_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: 003aACK 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 pack-format.txt 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', 'delete-refs' and 'ofs-delta'.
+
+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 and then the packfile that should
+contain all the objects that the server will need to complete the new
+references.
+
+----
+ update-request = command-list [pack-file]
+
+ command-list = PKT-LINE(command NUL capability-list LF)
+ *PKT-LINE(command LF)
+ 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
+
+ pack-file = "PACK" 28*(OCTET)
+----
+
+If the receiving end does not support delete-refs, the sending end MUST
+NOT ask for delete command.
+
+The pack-file MUST NOT be sent if the only command used is 'delete'.
+
+A pack-file 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 pack-file. 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.
+
+Report Status
+-------------
+
+After receiving the pack data from the sender, the receiver sends a
+report if 'report-status' 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 LF)
+ unpack-result = "ok" / error-msg
+
+ command-status = command-ok / command-fail
+ command-ok = PKT-LINE("ok" SP refname LF)
+ command-fail = PKT-LINE("ng" SP refname SP error-msg LF)
+
+ error-msg = 1*(OCTECT) ; where not "ok"
+----
+
+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: 007c74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/local\0report-status delete-refs ofs-delta\n
+ S: 003e7d1665144a3a975c05f1f43902ddaf084e784dbe refs/heads/debug\n
+ S: 003f74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/master\n
+ S: 003f74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/team\n
+ S: 0000
+
+ C: 003e7d1665144a3a975c05f1f43902ddaf084e784dbe 74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/debug\n
+ C: 003e74730d410fcb6603ace96f1dc55ea6196122532d 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a refs/heads/master\n
+ C: 0000
+ C: [PACKDATA]
+
+ S: 000aunpack ok\n
+ S: 0014ok refs/heads/debug\n
+ S: 0026ng refs/heads/master non-fast-forward\n
+----