intro – introduction to the Plan 9 File Protocol, 9P
A Plan 9 server is an agent that provides one or more hierarchical
file systems -- file trees -- that may be accessed by Plan 9 processes.
A server responds to requests by clients to navigate the hierarchy,
and to create, remove, read, and write files. The prototypical
server is a separate machine that stores large numbers
of user files on permanent media; such a machine is called, somewhat
confusingly, a file server. Another possibility for a server is
to synthesize files on demand, perhaps based on information on
data structures inside the kernel; the proc(3) kernel device is
a part of the Plan 9 kernel that does this. User programs can
also act as servers.
A connection to a server is a bidirectional communication path from the client to the server. There may be a single client or multiple clients sharing the same connection. A server's file tree is attached to a process group's name space by bind(2) and mount calls; see intro(2). Processes in the group are then clients of the server: system calls operating on files are translated into requests and responses transmitted on the connection to the appropriate service.
The Plan 9 File Protocol, 9P, is used for messages between clients and servers. A client transmits requests (T–messages) to a server, which subsequently returns replies (R–messages) to the client. The combined acts of transmitting (receiving) a request of a particular type, and receiving (transmitting) its reply is called a transaction of that type.
Each message consists of a sequence of bytes. Two–, four–, and eight–byte fields hold unsigned integers represented in little–endian order (least significant byte first). Data items of larger or variable lengths are represented by a two–byte field specifying a count, n, followed by n bytes of data. Text strings are represented this way, with the text itself stored as a UTF–8 encoded sequence of Unicode characters (see utf(6)). Text strings in 9P messages are not NUL–terminated: n counts the bytes of UTF–8 data, which include no final zero byte. The NUL character is illegal in all text strings in 9P, and is therefore excluded from file names, user names, and so on.
Each 9P message begins with a four–byte size field specifying the
length in bytes of the complete message including the four bytes
of the size field itself. The next byte is the message type, one
of the constants in the enumeration in the include file <fcall.h>.
The next two bytes are an identifying tag, described
below. The remaining bytes are parameters of different sizes.
In the message descriptions, the number of bytes in a field is
given in brackets after the field name. The notation parameter[n]
where n is not a constant represents a variable–length parameter:
n followed by n bytes of data forming the parameter. The
notation string[s] (using a literal s character) is shorthand
for s followed by s bytes of UTF–8 text. (Systems may choose
to reduce the set of legal characters to reduce syntactic problems,
for example to remove slashes from name components, but the protocol
has no such restriction. Plan 9 names may contain any
printable character (that is, any character outside hexadecimal
00–1F and 80–9F) except slash.) Messages are transported in byte
form to allow for machine independence; fcall(2) describes routines
that convert to and from this form into a machine–dependent C structure.
The type of an R–message will either be one greater than the type of the corresponding T–message or Rerror, indicating that the request failed. In the latter case, the ename field contains a string describing the reason for failure.
The version message identifies the version of the protocol and indicates the maximum message size the system is prepared to handle. It also initializes the connection and aborts all outstanding I/O on the connection. The set of messages between version requests is called a session.
Most T–messages contain a fid, a 32–bit unsigned integer that the client uses to identify a ``current file'' on the server. Fids are somewhat like file descriptors in a user process, but they are not restricted to files open for I/O: directories being examined, files being accessed by stat(2) calls, and so on -- all files being manipulated by the operating system -- are identified by fids. Fids are chosen by the client. All requests on a connection share the same fid space; when several clients share a connection, the agent managing the sharing must arrange that no two clients choose the same fid.
The fid supplied in an attach message will be taken by the server to refer to the root of the served file tree. The attach identifies the user to the server and may specify a particular file tree served by the server (for those that supply more than one).
Permission to attach to the service is proven by providing a special fid, called afid, in the attach message. This afid is established by exchanging auth messages and subsequently manipulated using read and write messages to exchange authentication information not defined explicitly by 9P. Once the authentication protocol is complete, the afid is presented in the attach to permit the user to access the service.
A walk message causes the server to change the current file associated with a fid to be a file in the directory that is the old current file, or one of its subdirectories. Walk returns a new fid that refers to the resulting file. Usually, a client maintains a fid for the root, and navigates by walks from the root fid.
A client can send multiple T–messages without waiting for the corresponding R–messages, but all outstanding T–messages must specify different tags. The server may delay the response to a request and respond to later ones; this is sometimes necessary, for example when the client reads from a file that the server synthesizes from external events such as keyboard characters.
Replies (R–messages) to auth, attach, walk, open, and create requests convey a qid field back to the client. The qid represents the server's unique identification for the file being accessed: two files on the same server hierarchy are the same if and only if their qids are the same. (The client may have multiple fids pointing to a single file on a server and hence having a single qid.) The thirteen–byte qid fields hold a one–byte type, specifying whether the file is a directory, append–only file, etc., and two unsigned integers: first the four–byte qid version, then the eight–byte qid path. The path is an integer unique among all files in the hierarchy. If a file is deleted and recreated with the same name in the same directory, the old and new path components of the qids should be different. The version is a version number for a file; typically, it is incremented every time the file is modified.
An existing file can be opened, or a new file may be created in the current (directory) file. I/O of a given number of bytes at a given offset on an open file is done by read and write.
A client should clunk any fid that is no longer needed. The remove transaction deletes files.
The stat transaction retrieves information about the file. The stat field in the reply includes the file's name, access permissions (read, write and execute for owner, group and public), access and modification times, and owner and group identifications (see stat(2)). The owner and group identifications are textual names. The wstat transaction allows some of a file's properties to be changed.
A request can be aborted with a flush request. When a server receives a Tflush, it should not reply to the message with tag oldtag (unless it has already replied), and it should immediately send an Rflush. The client must wait until it gets the Rflush (even if the reply to the original message arrives in the interim), at which point oldtag may be reused.
Because the message size is negotiable and some elements of the protocol are variable length, it is possible (although unlikely) to have a situation where a valid message is too large to fit within the negotiated size. For example, a very long file name may cause a Rstat of the file or Rread of its directory entry to be too large to send. In most such cases, the server should generate an error rather than modify the data to fit, such as by truncating the file name. The exception is that a long error string in an Rerror message should be truncated if necessary, since the string is only advisory and in some sense arbitrary.
Most programs do not see the 9P protocol directly; instead calls
to library routines that access files are translated by the mount
driver, mnt(3), into 9P messages.
Directories are created by create with DMDIR set in the permissions
argument (see stat(5)). The members of a directory can be found
with read(5). All directories must support walks to the directory
.. (dot–dot) meaning parent directory, although by convention directories
contain no explicit entry for .. or .
(dot). The parent of the root directory of a server's tree is
Each file server maintains a set of user and group names. Each
user can be a member of any number of groups. Each group has a
group leader who has special privileges (see stat(5) and users(6)).
Every file request has an implicit user id (copied from the original
attach) and an implicit set of groups (every group of
which the user is a member).
Each file has an associated owner and group id and three sets of permissions: those of the owner, those of the group, and those of ``other'' users. When the owner attempts to do something to a file, the owner, group, and other permissions are consulted, and if any of them grant the requested permission, the operation is allowed. For someone who is not the owner, but is a member of the file's group, the group and other permissions are consulted. For everyone else, the other permissions are used. Each set of permissions says whether reading is allowed, whether writing is allowed, and whether executing is allowed. A walk in a directory is regarded as executing the directory, not reading it. Permissions are kept in the low–order bits of the file mode: owner read/write/execute permission represented as 1 in bits 8, 7, and 6 respectively (using 0 to number the low order). The group permissions are in bits 5, 4, and 3, and the other permissions are in bits 2, 1, and 0.
The file mode contains some additional attributes besides the
permissions. If bit 31 (DMDIR) is set, the file is a directory;
if bit 30 (DMAPPEND) is set, the file is append–only (offset is
ignored in writes); if bit 29 (DMEXCL) is set, the file is exclusive–use
(only one client may have it open at a time); if bit 27 (DMAUTH)
set, the file is an authentication file established by auth messages;
if bit 26 (DMTMP) is set, the contents of the file (or directory)
are not included in nightly archives. (Bit 28 is skipped for historical
reasons.) These bits are reproduced, from the top bit down, in
the type byte of the Qid: QTDIR, QTAPPEND, QTEXCL,
(skipping one bit) QTAUTH, and QTTMP. The name QTFILE, defined
to be zero, identifies the value of the type for a plain file.