http-protocol.txt

HTTP transfer protocols
=======================

Git supports two HTTP based transfer protocols.  A "dumb" protocol
which requires only a standard HTTP server on the server end of the
connection, and a "smart" protocol which requires a Git aware CGI
(or server module).  This document describes both protocols.

As a design feature smart clients can automatically upgrade "dumb"
protocol URLs to smart URLs.  This permits all users to have the
same published URL, and the peers automatically select the most
efficient transport available to them.


URL Format
----------

URLs for Git repositories accessed by HTTP use the standard HTTP
URL syntax documented by RFC 1738, so they are of the form:

  http://<host>:<port>/<path>

Within this documentation the placeholder $GIT_URL will stand for
the http:// repository URL entered by the end-user.

Both the "smart" and "dumb" HTTP protocols used by Git operate
by appending additional path components onto the end of the user
supplied $GIT_URL string.

Clients MUST strip a trailing '/', if present, from the user supplied
$GIT_URL string to prevent empty path tokens ('//') from appearing
in any URL sent to a server.  Compatible clients must expand
'$GIT_URL/info/refs' as 'foo/info/refs' and not 'foo//info/refs'.


Authentication
--------------

Standard HTTP authentication is used if authentication is required
to access a repository, and MAY be configured and enforced by the
HTTP server software.

Because Git repositories are accessed by standard path components
server administrators MAY use directory based permissions within
their HTTP server to control repository access.

Clients SHOULD support Basic authentication as described by RFC 2616.
Servers SHOULD support Basic authentication by relying upon the
HTTP server placed in front of the Git server software.

Servers MUST NOT require HTTP cookies for the purposes of
authentication or access control.

Clients and servers MAY support other common forms of HTTP based
authentication, such as Digest authentication.


SSL
---

Clients and servers SHOULD support SSL, particularly to protect
passwords when relying on Basic HTTP authentication.


Session State
-------------

The Git over HTTP protocol (much like HTTP itself) is stateless
from the perspective of the HTTP server side.  All state must be
retained and managed by the client process.  This permits simple
round-robin load-balancing on the server side, without needing to
worry about state mangement.

Clients MUST NOT require state management on the server side in
order to function correctly.

Servers MUST NOT require HTTP cookies in order to function correctly.
Clients MAY store and forward HTTP cookies during request processing
as described by RFC 2616 (HTTP/1.1).  Servers SHOULD ignore any
cookies sent by a client.


pkt-line Format
---------------

Much (but not all) of the payload is described around pkt-lines.

A pkt-line is a variable length binary string.  The first four bytes
of the line indicates the total length of the line, in hexadecimal.
The total length includes the 4 bytes used to denote the length.
A line SHOULD BE terminated by an LF, which if present MUST be
included in the total length.

A pkt-line MAY contain binary data, so implementors MUST ensure all
pkt-line parsing/formatting routines are 8-bit clean.  The maximum
length of a pkt-line's data is 65532 bytes (65536 - 4).

Examples (as C-style strings):

  pkt-line          actual value
  ---------------------------------
  "0006a\n"         "a\n"
  "0005a"           "a"
  "000bfoobar\n"    "foobar\n"
  "0004"            ""

A pkt-line with a length of 0 ("0000") is a special case and MUST
be treated as a message break or terminator in the payload.


General Request Processing
--------------------------

Except where noted, all standard HTTP behavior SHOULD be assumed
by both client and server.  This includes (but is not necessarily
limited to):

If there is no repository at $GIT_URL, the server MUST respond with
the '404 Not Found' HTTP status code.

If there is a repository at $GIT_URL, but access is not currently
permitted, the server MUST respond with the '403 Forbidden' HTTP
status code.

Servers SHOULD support both HTTP 1.0 and HTTP 1.1.
Servers SHOULD support chunked encoding for both
request and response bodies.

Clients SHOULD support both HTTP 1.0 and HTTP 1.1.
Clients SHOULD support chunked encoding for both
request and response bodies.

Servers MAY return ETag and/or Last-Modified headers.

Clients MAY revalidate cached entities by including If-Modified-Since
and/or If-None-Match request headers.

Servers MAY return '304 Not Modified' if the relevant headers appear
in the request and the entity has not changed.  Clients MUST treat
'304 Not Modified' identical to '200 OK' by reusing the cached entity.

Clients MAY reuse a cached entity without revalidation if the
Cache-Control and/or Expires header permits caching.  Clients and
servers MUST follow RFC 2616 for cache controls.


Discovering References
----------------------

All HTTP clients MUST begin either a fetch or a push exchange by
discovering the references available on the remote repository.

Dumb Clients
~~~~~~~~~~~~

HTTP clients that only support the "dumb" protocol MUST discover
references by making a request for the special info/refs file of
the repository.

Dumb HTTP clients MUST NOT include search/query parameters when
fetching the info/refs file.  (That is, '?' must not appear in the
requested URL.)

	C: GET $GIT_URL/info/refs HTTP/1.0

	S: 200 OK
	S:
	S: 95dcfa3633004da0049d3d0fa03f80589cbcaf31	refs/heads/maint
	S: d049f6c27a2244e12041955e262a404c7faba355	refs/heads/master
	S: 2cb58b79488a98d2721cea644875a8dd0026b115	refs/tags/v1.0
	S: a3c2e2402b99163d1d59756e5f207ae21cccba4c	refs/tags/v1.0^{}

The Content-Type of the returned info/refs entity SHOULD be
"text/plain; charset=utf-8", but MAY be any content type.
Clients MUST NOT attempt to validate the returned Content-Type.
Dumb servers MUST NOT return a return type starting with
"application/x-git-".

Cache-Control headers MAY be returned to disable caching of the
returned entity.

When examining the response clients SHOULD only examine the HTTP
status code.  Valid responses are '200 OK', or '304 Not Modified'.

The returned content is a UNIX formatted text file describing
each ref and its known value.  The file SHOULD be sorted by name
according to the C locale ordering.  The file SHOULD NOT include
the default ref named 'HEAD'.

	info_refs     = *( ref_record )
	ref_record    = any_ref | peeled_ref

	any_ref       = id HT name LF
	peeled_ref    = id HT name LF
	                id HT name "^{}" LF
	id            = 40*HEX

	HEX           = "0".."9" | "a".."f"
	LF            = <US-ASCII LF, linefeed (10)>
	HT            = <US-ASCII HT, horizontal-tab (9)>

Smart Clients
~~~~~~~~~~~~~

HTTP clients that support the "smart" protocol (or both the
"smart" and "dumb" protocols) MUST discover references by making
a paramterized request for the info/refs file of the repository.

The request MUST contain exactly one query parameter,
'service=$servicename', where $servicename MUST be the service
name the client wishes to contact to complete the operation.
The request MUST NOT contain additional query parameters.

	C: GET $GIT_URL/info/refs?service=git-upload-pack HTTP/1.0

	dumb server reply:
	S: 200 OK
	S:
	S: 95dcfa3633004da0049d3d0fa03f80589cbcaf31	refs/heads/maint
	S: d049f6c27a2244e12041955e262a404c7faba355	refs/heads/master
	S: 2cb58b79488a98d2721cea644875a8dd0026b115	refs/tags/v1.0
	S: a3c2e2402b99163d1d59756e5f207ae21cccba4c	refs/tags/v1.0^{}

	smart server reply:
	S: 200 OK
	S: Content-Type: application/x-git-upload-pack-advertisement
	S: Cache-Control: no-cache
	S:
	S: ....# service=git-upload-pack
	S: ....95dcfa3633004da0049d3d0fa03f80589cbcaf31 refs/heads/maint\0 multi_ack
	S: ....d049f6c27a2244e12041955e262a404c7faba355 refs/heads/master
	S: ....2cb58b79488a98d2721cea644875a8dd0026b115 refs/tags/v1.0
	S: ....a3c2e2402b99163d1d59756e5f207ae21cccba4c refs/tags/v1.0^{}

Dumb Server Response
^^^^^^^^^^^^^^^^^^^^
Dumb servers MUST respond with the dumb server reply format.

See the prior section under dumb clients for a more detailed
description of the dumb server response.

Smart Server Response
^^^^^^^^^^^^^^^^^^^^^
Smart servers MUST respond with the smart server reply format.

If the server does not recognize the requested service name, or the
requested service name has been disabled by the server administrator,
the server MUST respond with the '403 Forbidden' HTTP status code.

Cache-Control headers SHOULD be used to disable caching of the
returned entity.

The Content-Type MUST be 'application/x-$servicename-advertisement'.
Clients SHOULD fall back to the dumb protocol if another content
type is returned.  When falling back to the dumb protocol clients
SHOULD NOT make an additional request to $GIT_URL/info/refs, but
instead SHOULD use the response already in hand.  Clients MUST NOT
continue if they do not support the dumb protocol.

Clients MUST validate the status code is either '200 OK' or
'304 Not Modified'.

Clients MUST validate the first five bytes of the response entity
matches the regex "^[0-9a-f]{4}#".  If this test fails, clients
MUST NOT continue.

Clients MUST parse the entire response as a sequence of pkt-line
records.

Clients MUST verify the first pkt-line is "# service=$servicename".
Servers MUST set $servicename to be the request parameter value.
Servers SHOULD include an LF at the end of this line.
Clients MUST ignore an LF at the end of the line.

Servers MUST terminate the response with the magic "0000" end
pkt-line marker.

The returned response is a pkt-line stream describing each ref and
its known value.  The stream SHOULD be sorted by name according to
the C locale ordering.  The stream SHOULD include the default ref
named 'HEAD' as the first ref.  The stream MUST include capability
declarations behind a NUL on the first ref.

	smart_reply    = PKT-LINE("# service=$servicename" LF)
	                 ref_list
	                 "0000"
	ref_list       = empty_list | populated_list

	empty_list     = PKT-LINE(id SP "capabilities^{}" NUL cap_list LF)

	non_empty_list = PKT-LINE(id SP name NUL cap_list LF)
	                 *ref_record

	cap_list      = *(SP capability) SP
	ref_record    = any_ref | peeled_ref

	any_ref       = PKT-LINE(id SP name LF)
	peeled_ref    = PKT-LINE(id SP name LF)
	                PKT-LINE(id SP name "^{}" LF
	id            = 40*HEX

	HEX           = "0".."9" | "a".."f"
	NL            = <US-ASCII NUL, null (0)>
	LF            = <US-ASCII LF,  linefeed (10)>
	SP            = <US-ASCII SP,  horizontal-tab (9)>


Smart Service git-upload-pack
------------------------------
This service reads from the remote repository.

Clients MUST first perform ref discovery with
'$GIT_URL/info/refs?service=git-upload-pack'.

	C: POST $GIT_URL/git-upload-pack HTTP/1.0
	C: Content-Type: application/x-git-upload-pack-request
	C:
	C: ....want 0a53e9ddeaddad63ad106860237bbf53411d11a7
	C: ....have 441b40d833fdfa93eb2908e52742248faf0ee993
	C: 0000

	S: 200 OK
	S: Content-Type: application/x-git-upload-pack-result
	S: Cache-Control: no-cache
	S:
	S: ....ACK %s, continue
	S: ....NAK

Clients MUST NOT reuse or revalidate a cached reponse.
Servers MUST include sufficient Cache-Control headers
to prevent caching of the response.

Servers SHOULD support all capabilities defined here.

Clients MUST send at least one 'want' command in the request body.
Clients MUST NOT reference an id in a 'want' command which did not
appear in the response obtained through ref discovery.

	compute_request   = want_list
	                    have_list
	                    request_end
	request_end       = "0000" | "done"

	want_list         = PKT-LINE(want NUL cap_list LF)
	                    *(want_pkt)
	want_pkt          = PKT-LINE(want LF)
	want              = "want" SP id
	cap_list          = *(SP capability) SP

	have_list         = *PKT-LINE("have" SP id LF)

	command           = create | delete | update
	create            = 40*"0" SP new_id SP name
	delete            = old_id SP 40*"0" SP name
	update            = old_id SP new_id SP name

TODO: Document this further.
TODO: Don't use uppercase for variable names below.

Capability include-tag
~~~~~~~~~~~~~~~~~~~~~~

When packing an object that an annotated tag points at, include the
tag object too.  Clients can request this if they want to fetch
tags, but don't know which tags they will need until after they
receive the branch data.  By enabling include-tag an entire call
to upload-pack can be avoided.

Capability thin-pack
~~~~~~~~~~~~~~~~~~~~

When packing a deltified object the base is not included if the base
is reachable from an object listed in the COMMON set by the client.
This reduces the bandwidth required to transfer, but it does slightly
increase processing time for the client to save the pack to disk.

The Negotiation Algorithm
~~~~~~~~~~~~~~~~~~~~~~~~~
The computation to select the minimal pack proceeds as follows
(c = client, s = server):

 init step:
 (c) Use ref discovery to obtain the advertised refs.
 (c) Place any object seen into set ADVERTISED.

 (c) Build an empty set, COMMON, to hold the objects that are later
     determined to be on both ends.
 (c) Build a set, WANT, of the objects from ADVERTISED the client
     wants to fetch, based on what it saw during ref discovery.

 (c) Start a queue, C_PENDING, ordered by commit time (popping newest
     first).  Add all client refs.  When a commit is popped from
     the queue its parents should be automatically inserted back.
     Commits MUST only enter the queue once.

 one compute step:
 (c) Send one $GIT_URL/git-upload-pack request:

	C: 0032want <WANT #1>...............................
	C: 0032want <WANT #2>...............................
	....
	C: 0032have <COMMON #1>.............................
	C: 0032have <COMMON #2>.............................
	....
	C: 0032have <HAVE #1>...............................
	C: 0032have <HAVE #2>...............................
	....
	C: 0000

     The stream is organized into "commands", with each command
     appearing by itself in a pkt-line.  Within a command line
     the text leading up to the first space is the command name,
     and the remainder of the line to the first LF is the value.
     Command lines are terminated with an LF as the last byte of
     the pkt-line value.

     Commands MUST appear in the following order, if they appear
     at all in the request stream:

       * want
       * have

     The stream is terminated by a pkt-line flush ("0000").

     A single "want" or "have" command MUST have one hex formatted
     SHA-1 as its value.  Multiple SHA-1s MUST be sent by sending
     multiple commands.

     The HAVE list is created by popping the first 32 commits
     from C_PENDING.  Less can be supplied if C_PENDING empties.

     If the client has sent 256 HAVE commits and has not yet
     received one of those back from S_COMMON, or the client has
     emptied C_PENDING it should include a "done" command to let
     the server know it won't proceed:

	C: 0009done

  (s) Parse the git-upload-pack request:

      Verify all objects in WANT are directly reachable from refs.

	  The server MAY walk backwards through history or through
      the reflog to permit slightly stale requests.

      If no WANT objects are received, send an error:

TODO: Define error if no want lines are requested.

      If any WANT object is not reachable, send an error:

TODO: Define error if an invalid want is requested.

     Create an empty list, S_COMMON.

     If 'have' was sent:

     Loop through the objects in the order supplied by the client.
     For each object, if the server has the object reachable from
     a ref, add it to S_COMMON.  If a commit is added to S_COMMON,
     do not add any ancestors, even if they also appear in HAVE.

  (s) Send the git-upload-pack response:

     If the server has found a closed set of objects to pack or the
     request ends with "done", it replies with the pack.

TODO: Document the pack based response
	S: PACK...

     The returned stream is the side-band-64k protocol supported
     by the git-upload-pack service, and the pack is embedded into
     stream 1.  Progress messages from the server side may appear
     in stream 2.

     Here a "closed set of objects" is defined to have at least
     one path from every WANT to at least one COMMON object.

     If the server needs more information, it replies with a
     status continue response:

TODO: Document the non-pack response

  (c) Parse the upload-pack response:

TODO: Document parsing response

      Do another compute step.


Smart Service git-receive-pack
------------------------------
This service modifies the remote repository.

Clients MUST first perform ref discovery with
'$GIT_URL/info/refs?service=git-receive-pack'.

	C: POST $GIT_URL/git-receive-pack HTTP/1.0
	C: Content-Type: application/x-git-receive-pack-request
	C:
	C: ....0a53e9ddeaddad63ad106860237bbf53411d11a7 441b40d833fdfa93eb2908e52742248faf0ee993 refs/heads/maint\0 report-status
	C: 0000
	C: PACK....

	S: 200 OK
	S: Content-Type: application/x-git-receive-pack-result
	S: Cache-Control: no-cache
	S:
	S: ....

Clients MUST NOT reuse or revalidate a cached reponse.
Servers MUST include sufficient Cache-Control headers
to prevent caching of the response.

Servers SHOULD support all capabilities defined here.

Clients MUST send at least one command in the request body.
Within the command portion of the request body clients SHOULD send
the id obtained through ref discovery as old_id.

	update_request    = command_list
	                    "PACK" <binary data>

	command_list      = PKT-LINE(command NUL cap_list LF)
	                    *(command_pkt)
	command_pkt       = PKT-LINE(command LF)
	cap_list          = *(SP capability) SP

	command           = create | delete | update
	create            = 40*"0" SP new_id SP name
	delete            = old_id SP 40*"0" SP name
	update            = old_id SP new_id SP name

TODO: Document this further.


References
----------

link:http://www.ietf.org/rfc/rfc1738.txt[RFC 1738: Uniform Resource Locators (URL)]
link:http://www.ietf.org/rfc/rfc2616.txt[RFC 2616: Hypertext Transfer Protocol -- HTTP/1.1]