.HTML "The Organization of Networks in Plan 9
.TL
The Organization of Networks in Plan 9
.AU
Dave Presotto
Phil Winterbottom
.sp
presotto,philw@plan9.bell-labs.com
.AB
.FS
Originally appeared in
.I
Proc. of the Winter 1993 USENIX Conf.,
.R
pp. 271-280,
San Diego, CA
.FE
In a distributed system networks are of paramount importance. This
paper describes the implementation, design philosophy, and organization
of network support in Plan 9. Topics include network requirements
for distributed systems, our kernel implementation, network naming, user interfaces,
and performance. We also observe that much of this organization is relevant to
current systems.
.AE
.NH
Introduction
.PP
Plan 9 [Pike90] is a general-purpose, multi-user, portable distributed system
implemented on a variety of computers and networks.
What distinguishes Plan 9 is its organization.
The goals of this organization were to
reduce administration
and to promote resource sharing. One of the keys to its success as a distributed
system is the organization and management of its networks.
.PP
A Plan 9 system comprises file servers, CPU servers and terminals.
The file servers and CPU servers are typically centrally
located multiprocessor machines with large memories and
high speed interconnects.
A variety of workstation-class machines
serve as terminals
connected to the central servers using several networks and protocols.
The architecture of the system demands a hierarchy of network
speeds matching the needs of the components.
Connections between file servers and CPU servers are high-bandwidth point-to-point
fiber links.
Connections from the servers fan out to local terminals
using medium speed networks
such as Ethernet [Met80] and Datakit [Fra80].
Low speed connections via the Internet and
the AT&T backbone serve users in Oregon and Illinois.
Basic Rate ISDN data service and 9600 baud serial lines provide slow
links to users at home.
.PP
Since CPU servers and terminals use the same kernel,
users may choose to run programs locally on
their terminals or remotely on CPU servers.
The organization of Plan 9 hides the details of system connectivity
allowing both users and administrators to configure their environment
to be as distributed or centralized as they wish.
Simple commands support the
construction of a locally represented name space
spanning many machines and networks.
At work, users tend to use their terminals like workstations,
running interactive programs locally and
reserving the CPU servers for data or compute intensive jobs
such as compiling and computing chess endgames.
At home or when connected over
a slow network, users tend to do most work on the CPU server to minimize
traffic on the slow links.
The goal of the network organization is to provide the same
environment to the user wherever resources are used.
.NH 
Kernel Network Support
.PP
Networks play a central role in any distributed system. This is particularly
true in Plan 9 where most resources are provided by servers external to the kernel.
The importance of the networking code within the kernel
is reflected by its size;
of 25,000 lines of kernel code, 12,500 are network and protocol related.
Networks are continually being added and the fraction of code
devoted to communications
is growing.
Moreover, the network code is complex.
Protocol implementations consist almost entirely of
synchronization and dynamic memory management, areas demanding 
subtle error recovery
strategies.
The kernel currently supports Datakit, point-to-point fiber links,
an Internet (IP) protocol suite and ISDN data service.
The variety of networks and machines
has raised issues not addressed by other systems running on commercial
hardware supporting only Ethernet or FDDI.
.NH 2
The File System protocol
.PP
A central idea in Plan 9 is the representation of a resource as a hierarchical
file system.
Each process assembles a view of the system by building a
.I "name space
[Needham] connecting its resources.
File systems need not represent disc files; in fact, most Plan 9 file systems have no
permanent storage.
A typical file system dynamically represents
some resource like a set of network connections or the process table.
Communication between the kernel, device drivers, and local or remote file servers uses a
protocol called 9P. The protocol consists of 17 messages
describing operations on files and directories.
Kernel resident device and protocol drivers use a procedural version
of the protocol while external file servers use an RPC form.
Nearly all traffic between Plan 9 systems consists
of 9P messages.
9P relies on several properties of the underlying transport protocol.
It assumes messages arrive reliably and in sequence and
that delimiters between messages
are preserved.
When a protocol does not meet these
requirements (for example, TCP does not preserve delimiters)
we provide mechanisms to marshal messages before handing them
to the system.
.PP
A kernel data structure, the
.I channel ,
is a handle to a file server.
Operations on a channel generate the following 9P messages.
The
.CW session
and
.CW attach
messages authenticate a connection, established by means external to 9P,
and validate its user.
The result is an authenticated
channel
referencing the root of the
server.
The
.CW clone
message makes a new channel identical to an existing channel, much like
the
.CW dup
system call.
A
channel
may be moved to a file on the server using a
.CW walk
message to descend each level in the hierarchy.
The
.CW stat
and
.CW wstat
messages read and write the attributes of the file referenced by a channel.
The
.CW open
message prepares a channel for subsequent
.CW read
and
.CW write
messages to access the contents of the file.
.CW Create
and
.CW remove
perform the actions implied by their names on the file
referenced by the channel.
The
.CW clunk
message discards a channel without affecting the file.
.PP
A kernel resident file server called the
.I "mount driver"
converts the procedural version of 9P into RPCs.
The
.I mount
system call provides a file descriptor, which can be
a pipe to a user process or a network connection to a remote machine, to
be associated with the mount point.
After a mount, operations
on the file tree below the mount point are sent as messages to the file server.
The
mount
driver manages buffers, packs and unpacks parameters from
messages, and demultiplexes among processes using the file server.
.NH 2
Kernel Organization
.PP
The network code in the kernel is divided into three layers: hardware interface,
protocol processing, and program interface.
A device driver typically uses streams to connect the two interface layers.
Additional stream modules may be pushed on
a device to process protocols.
Each device driver is a kernel-resident file system.
Simple device drivers serve a single level
directory containing just a few files;
for example, we represent each UART
by a data and a control file.
.P1
cpu% cd /dev
cpu% ls -l eia*
--rw-rw-rw- t 0 bootes bootes 0 Jul 16 17:28 eia1
--rw-rw-rw- t 0 bootes bootes 0 Jul 16 17:28 eia1ctl
--rw-rw-rw- t 0 bootes bootes 0 Jul 16 17:28 eia2
--rw-rw-rw- t 0 bootes bootes 0 Jul 16 17:28 eia2ctl
cpu%
.P2
The control file is used to control the device;
writing the string
.CW b1200
to
.CW /dev/eia1ctl
sets the line to 1200 baud.
.PP
Multiplexed devices present
a more complex interface structure.
For example, the LANCE Ethernet driver
serves a two level file tree (Figure 1)
providing
.IP \(bu
device control and configuration
.IP \(bu
user-level protocols like ARP
.IP \(bu
diagnostic interfaces for snooping software.
.LP
The top directory contains a
.CW clone
file and a directory for each connection, numbered
.CW 1
to
.CW n .
Each connection directory corresponds to an Ethernet packet type.
Opening the
.CW clone
file finds an unused connection directory
and opens its
.CW ctl
file.
Reading the control file returns the ASCII connection number; the user
process can use this value to construct the name of the proper 
connection directory.
In each connection directory files named
.CW ctl , 
.CW data , 
.CW stats ,
and 
.CW type
provide access to the connection.
Writing the string
.CW "connect 2048"
to the
.CW ctl
file sets the packet type to 2048
and
configures the connection to receive
all IP packets sent to the machine.
Subsequent reads of the file
.CW type
yield the string
.CW 2048 .
The
.CW data
file accesses the media;
reading it
returns the
next packet of the selected type.
Writing the file
queues a packet for transmission after
appending a packet header containing the source address and packet type.
The
.CW stats
file returns ASCII text containing the interface address,
packet input/output counts, error statistics, and general information
about the state of the interface.
.so tree.pout
.PP
If several connections on an interface
are configured for a particular packet type, each receives a
copy of the incoming packets.
The special packet type
.CW -1
selects all packets.
Writing the strings
.CW promiscuous
and
.CW connect
.CW -1
to the
.CW ctl
file
configures a conversation to receive all packets on the Ethernet.
.PP
Although the driver interface may seem elaborate,
the representation of a device as a set of files using ASCII strings for
communication has several advantages.
Any mechanism supporting remote access to files immediately
allows a remote machine to use our interfaces as gateways.
Using ASCII strings to control the interface avoids byte order problems and
ensures a uniform representation for
devices on the same machine and even allows devices to be accessed remotely.
Representing dissimilar devices by the same set of files allows common tools
to serve
several networks or interfaces.
Programs like
.CW stty
are replaced by
.CW echo
and shell redirection.
.NH 2
Protocol devices
.PP
Network connections are represented as pseudo-devices called protocol devices.
Protocol device drivers exist for the Datakit URP protocol and for each of the
Internet IP protocols TCP, UDP, and IL.
IL, described below, is a new communication protocol used by Plan 9 for
transmitting file system RPC's.
All protocol devices look identical so user programs contain no
network-specific code.
.PP
Each protocol device driver serves a directory structure
similar to that of the Ethernet driver.
The top directory contains a
.CW clone
file and a directory for each connection numbered
.CW 0
to
.CW n .
Each connection directory contains files to control one
connection and to send and receive information.
A TCP connection directory looks like this:
.P1
cpu% cd /net/tcp/2
cpu% ls -l
--rw-rw---- I 0 ehg    bootes 0 Jul 13 21:14 ctl
--rw-rw---- I 0 ehg    bootes 0 Jul 13 21:14 data
--rw-rw---- I 0 ehg    bootes 0 Jul 13 21:14 listen
--r--r--r-- I 0 bootes bootes 0 Jul 13 21:14 local
--r--r--r-- I 0 bootes bootes 0 Jul 13 21:14 remote
--r--r--r-- I 0 bootes bootes 0 Jul 13 21:14 status
cpu% cat local remote status
135.104.9.31 5012
135.104.53.11 564
tcp/2 1 Established connect
cpu%
.P2
The files
.CW local ,
.CW remote ,
and
.CW status
supply information about the state of the connection.
The
.CW data
and
.CW ctl
files
provide access to the process end of the stream implementing the protocol.
The
.CW listen
file is used to accept incoming calls from the network.
.PP
The following steps establish a connection.
.IP 1)
The clone device of the
appropriate protocol directory is opened to reserve an unused connection.
.IP 2)
The file descriptor returned by the open points to the
.CW ctl
file of the new connection.
Reading that file descriptor returns an ASCII string containing
the connection number.
.IP 3)
A protocol/network specific ASCII address string is written to the
.CW ctl
file.
.IP 4)
The path of the
.CW data
file is constructed using the connection number.
When the
.CW data
file is opened the connection is established.
.LP
A process can read and write this file descriptor
to send and receive messages from the network.
If the process opens the
.CW listen
file it blocks until an incoming call is received.
An address string written to the
.CW ctl
file before the listen selects the
ports or services the process is prepared to accept.
When an incoming call is received, the open completes
and returns a file descriptor
pointing to the
.CW ctl
file of the new connection.
Reading the
.CW ctl
file yields a connection number used to construct the path of the
.CW data
file.
A connection remains established while any of the files in the connection directory
are referenced or until a close is received from the network.
.NH 2
Streams
.PP
A
.I stream 
[Rit84a][Presotto] is a bidirectional channel connecting a
physical or pseudo-device to user processes.
The user processes insert and remove data at one end of the stream.
Kernel processes acting on behalf of a device insert data at
the other end.
Asynchronous communications channels such as pipes,
TCP conversations, Datakit conversations, and RS232 lines are implemented using
streams.
.PP
A stream comprises a linear list of
.I "processing modules" .
Each module has both an upstream (toward the process) and
downstream (toward the device)
.I "put routine" .
Calling the put routine of the module on either end of the stream
inserts data into the stream.
Each module calls the succeeding one to send data up or down the stream.
.PP
An instance of a processing module is represented by a pair of
.I queues ,
one for each direction.
The queues point to the put procedures and can be used
to queue information traveling along the stream.
Some put routines queue data locally and send it along the stream at some
later time, either due to a subsequent call or an asynchronous
event such as a retransmission timer or a device interrupt.
Processing modules create helper kernel processes to
provide a context for handling asynchronous events.
For example, a helper kernel process awakens periodically
to perform any necessary TCP retransmissions.
The use of kernel processes instead of serialized run-to-completion service routines
differs from the implementation of Unix streams.
Unix service routines cannot
use any blocking kernel resource and they lack a local long-lived state.
Helper kernel processes solve these problems and simplify the stream code.
.PP
There is no implicit synchronization in our streams.
Each processing module must ensure that concurrent processes using the stream
are synchronized.
This maximizes concurrency but introduces the
possibility of deadlock.
However, deadlocks are easily avoided by careful programming; to
date they have not caused us problems.
.PP
Information is represented by linked lists of kernel structures called
.I blocks .
Each block contains a type, some state flags, and pointers to
an optional buffer.
Block buffers can hold either data or control information, i.e., directives
to the processing modules.
Blocks and block buffers are dynamically allocated from kernel memory.
.NH 3
User Interface
.PP
A stream is represented at user level as two files, 
.CW ctl
and
.CW data .
The actual names can be changed by the device driver using the stream,
as we saw earlier in the example of the UART driver.
The first process to open either file creates the stream automatically.
The last close destroys it.
Writing to the
.CW data
file copies the data into kernel blocks
and passes them to the downstream put routine of the first processing module.
A write of less than 32K is guaranteed to be contained by a single block.
Concurrent writes to the same stream are not synchronized, although the
32K block size assures atomic writes for most protocols.
The last block written is flagged with a delimiter
to alert downstream modules that care about write boundaries.
In most cases the first put routine calls the second, the second
calls the third, and so on until the data is output.
As a consequence, most data is output without context switching.
.PP
Reading from the
.CW data
file returns data queued at the top of the stream.
The read terminates when the read count is reached
or when the end of a delimited block is encountered.
A per stream read lock ensures only one process
can read from a stream at a time and guarantees
that the bytes read were contiguous bytes from the
stream.
.PP
Like UNIX streams [Rit84a],
Plan 9 streams can be dynamically configured.
The stream system intercepts and interprets
the following control blocks:
.IP "\f(CWpush\fP \fIname\fR" 15
adds an instance of the processing module 
.I name
to the top of the stream.
.IP \f(CWpop\fP 15
removes the top module of the stream.
.IP \f(CWhangup\fP 15
sends a hangup message
up the stream from the device end.
.LP
Other control blocks are module-specific and are interpreted by each
processing module
as they pass.
.PP
The convoluted syntax and semantics of the UNIX
.CW ioctl
system call convinced us to leave it out of Plan 9.
Instead,
.CW ioctl
is replaced by the
.CW ctl
file.
Writing to the
.CW ctl
file
is identical to writing to a
.CW data
file except the blocks are of type
.I control .
A processing module parses each control block it sees.
Commands in control blocks are ASCII strings, so
byte ordering is not an issue when one system
controls streams in a name space implemented on another processor.
The time to parse control blocks is not important, since control
operations are rare.
.NH 3
Device Interface
.PP
The module at the downstream end of the stream is part of a device interface.
The particulars of the interface vary with the device.
Most device interfaces consist of an interrupt routine, an output
put routine, and a kernel process.
The output put routine stages data for the
device and starts the device if it is stopped.
The interrupt routine wakes up the kernel process whenever
the device has input to be processed or needs more output staged.
The kernel process puts information up the stream or stages more data for output.
The division of labor among the different pieces varies depending on
how much must be done at interrupt level.
However, the interrupt routine may not allocate blocks or call
a put routine since both actions require a process context.
.NH 3
Multiplexing
.PP
The conversations using a protocol device must be
multiplexed onto a single physical wire.
We push a multiplexer processing module
onto the physical device stream to group the conversations.
The device end modules on the conversations add the necessary header
onto downstream messages and then put them to the module downstream
of the multiplexer.
The multiplexing module looks at each message moving up its stream and
puts it to the correct conversation stream after stripping
the header controlling the demultiplexing.
.PP
This is similar to the Unix implementation of multiplexer streams.
The major difference is that we have no general structure that
corresponds to a multiplexer.
Each attempt to produce a generalized multiplexer created a more complicated
structure and underlined the basic difficulty of generalizing this mechanism.
We now code each multiplexer from scratch and favor simplicity over
generality.
.NH 3
Reflections
.PP
Despite five year's experience and the efforts of many programmers,
we remain dissatisfied with the stream mechanism.
Performance is not an issue;
the time to process protocols and drive
device interfaces continues to dwarf the
time spent allocating, freeing, and moving blocks
of data.
However the mechanism remains inordinately
complex.
Much of the complexity results from our efforts
to make streams dynamically configurable, to
reuse processing modules on different devices
and to provide kernel synchronization
to ensure data structures
don't disappear under foot.
This is particularly irritating since we seldom use these properties.
.PP
Streams remain in our kernel because we are unable to
devise a better alternative.
Larry Peterson's X-kernel [Pet89a]
is the closest contender but
doesn't offer enough advantage to switch.
If we were to rewrite the streams code, we would probably statically
allocate resources for a large fixed number of conversations and burn
memory in favor of less complexity.
.NH
The IL Protocol
.PP
None of the standard IP protocols is suitable for transmission of
9P messages over an Ethernet or the Internet.
TCP has a high overhead and does not preserve delimiters.
UDP, while cheap, does not provide reliable sequenced delivery.
Early versions of the system used a custom protocol that was
efficient but unsatisfactory for internetwork transmission.
When we implemented IP, TCP, and UDP we looked around for a suitable
replacement with the following properties:
.IP \(bu
Reliable datagram service with sequenced delivery
.IP \(bu
Runs over IP
.IP \(bu
Low complexity, high performance
.IP \(bu
Adaptive timeouts
.LP
None met our needs so a new protocol was designed.
IL is a lightweight protocol designed to be encapsulated by IP.
It is a connection-based protocol
providing reliable transmission of sequenced messages between machines.
No provision is made for flow control since the protocol is designed to transport RPC
messages between client and server.
A small outstanding message window prevents too
many incoming messages from being buffered;
messages outside the window are discarded
and must be retransmitted.
Connection setup uses a two way handshake to generate
initial sequence numbers at each end of the connection;
subsequent data messages increment the
sequence numbers allowing
the receiver to resequence out of order messages. 
In contrast to other protocols, IL does not do blind retransmission.
If a message is lost and a timeout occurs, a query message is sent.
The query message is a small control message containing the current
sequence numbers as seen by the sender.
The receiver responds to a query by retransmitting missing messages.
This allows the protocol to behave well in congested networks,
where blind retransmission would cause further
congestion.
Like TCP, IL has adaptive timeouts.
A round-trip timer is used
to calculate acknowledge and retransmission times in terms of the network speed.
This allows the protocol to perform well on both the Internet and on local Ethernets.
.PP
In keeping with the minimalist design of the rest of the kernel, IL is small.
The entire protocol is 847 lines of code, compared to 2200 lines for TCP.
IL is our protocol of choice.
.NH
Network Addressing
.PP
A uniform interface to protocols and devices is not sufficient to
support the transparency we require.
Since each network uses a different
addressing scheme,
the ASCII strings written to a control file have no common format.
As a result, every tool must know the specifics of the networks it
is capable of addressing.
Moreover, since each machine supplies a subset
of the available networks, each user must be aware of the networks supported
by every terminal and server machine.
This is obviously unacceptable.
.PP
Several possible solutions were considered and rejected; one deserves
more discussion.
We could have used a user-level file server
to represent the network name space as a Plan 9 file tree. 
This global naming scheme has been implemented in other distributed systems.
The file hierarchy provides paths to
directories representing network domains.
Each directory contains
files representing the names of the machines in that domain;
an example might be the path
.CW /net/name/usa/edu/mit/ai .
Each machine file contains information like the IP address of the machine.
We rejected this representation for several reasons.
First, it is hard to devise a hierarchy encompassing all representations
of the various network addressing schemes in a uniform manner.
Datakit and Ethernet address strings have nothing in common.
Second, the address of a machine is
often only a small part of the information required to connect to a service on
the machine.
For example, the IP protocols require symbolic service names to be mapped into
numeric port numbers, some of which are privileged and hence special.
Information of this sort is hard to represent in terms of file operations.
Finally, the size and number of the networks being represented burdens users with
an unacceptably large amount of information about the organization of the network
and its connectivity.
In this case the Plan 9 representation of a
resource as a file is not appropriate.
.PP
If tools are to be network independent, a third-party server must resolve
network names.
A server on each machine, with local knowledge, can select the best network
for any particular destination machine or service.
Since the network devices present a common interface,
the only operation which differs between networks is name resolution.
A symbolic name must be translated to
the path of the clone file of a protocol
device and an ASCII address string to write to the
.CW ctl
file.
A connection server (CS) provides this service.
.NH 2
Network Database
.PP
On most systems several
files such as
.CW /etc/hosts ,
.CW /etc/networks ,
.CW /etc/services ,
.CW /etc/hosts.equiv ,
.CW /etc/bootptab ,
and
.CW /etc/named.d
hold network information.
Much time and effort is spent
administering these files and keeping
them mutually consistent.
Tools attempt to
automatically derive one or more of the files from
information in other files but maintenance continues to be
difficult and error prone.
.PP
Since we were writing an entirely new system, we were free to
try a simpler approach.
One database on a shared server contains all the information
needed for network administration.
Two ASCII files comprise the main database:
.CW /lib/ndb/local
contains locally administered information and
.CW /lib/ndb/global
contains information imported from elsewhere.
The files contain sets of attribute/value pairs of the form
.I attr\f(CW=\fPvalue ,
where
.I attr
and
.I value
are alphanumeric strings.
Systems are described by multi-line entries;
a header line at the left margin begins each entry followed by zero or more
indented attribute/value pairs specifying
names, addresses, properties, etc.
For example, the entry for our CPU server
specifies a domain name, an IP address, an Ethernet address,
a Datakit address, a boot file, and supported protocols.
.P1
sys=helix
	dom=helix.research.bell-labs.com
	bootf=/mips/9power
	ip=135.104.9.31 ether=0800690222f0
	dk=nj/astro/helix
	proto=il flavor=9cpu
.P2
If several systems share entries such as
network mask and gateway, we specify that information
with the network or subnetwork instead of the system.
The following entries define a Class B IP network and 
a few subnets derived from it.
The entry for the network specifies the IP mask,
file system, and authentication server for all systems
on the network.
Each subnetwork specifies its default IP gateway.
.P1
ipnet=mh-astro-net ip=135.104.0.0 ipmask=255.255.255.0
	fs=bootes.research.bell-labs.com
	auth=1127auth
ipnet=unix-room ip=135.104.117.0
	ipgw=135.104.117.1
ipnet=third-floor ip=135.104.51.0
	ipgw=135.104.51.1
ipnet=fourth-floor ip=135.104.52.0
	ipgw=135.104.52.1
.P2
Database entries also define the mapping of service names
to port numbers for TCP, UDP, and IL.
.P1
tcp=echo	port=7
tcp=discard	port=9
tcp=systat	port=11
tcp=daytime	port=13
.P2
.PP
All programs read the database directly so
consistency problems are rare.
However the database files can become large.
Our global file, containing all information about
both Datakit and Internet systems in AT&T, has 43,000
lines.
To speed searches, we build hash table files for each
attribute we expect to search often.
The hash file entries point to entries
in the master files.
Every hash file contains the modification time of its master
file so we can avoid using an out-of-date hash table.
Searches for attributes that aren't hashed or whose hash table
is out-of-date still work, they just take longer.
.NH 2
Connection Server
.PP
On each system a user level connection server process, CS, translates
symbolic names to addresses.
CS uses information about available networks, the network database, and
other servers (such as DNS) to translate names.
CS is a file server serving a single file,
.CW /net/cs .
A client writes a symbolic name to
.CW /net/cs
then reads one line for each matching destination reachable
from this system.
The lines are of the form
.I "filename message",
where
.I filename
is the path of the clone file to open for a new connection and
.I message
is the string to write to it to make the connection.
The following example illustrates this.
.CW Ndb/csquery
is a program that prompts for strings to write to
.CW /net/cs
and prints the replies.
.P1
% ndb/csquery
> net!helix!9fs
/net/il/clone 135.104.9.31!17008
/net/dk/clone nj/astro/helix!9fs
.P2
.PP
CS provides meta-name translation to perform complicated
searches.
The special network name
.CW net
selects any network in common between source and
destination supporting the specified service.
A host name of the form \f(CW$\fIattr\f1
is the name of an attribute in the network database.
The database search returns the value
of the matching attribute/value pair
most closely associated with the source host.
Most closely associated is defined on a per network basis.
For example, the symbolic name
.CW tcp!$auth!rexauth
causes CS to search for the
.CW auth
attribute in the database entry for the source system, then its
subnetwork (if there is one) and then its network.
.P1
% ndb/csquery
> net!$auth!rexauth
/net/il/clone 135.104.9.34!17021
/net/dk/clone nj/astro/p9auth!rexauth
/net/il/clone 135.104.9.6!17021
/net/dk/clone nj/astro/musca!rexauth
.P2
.PP
Normally CS derives naming information from its database files.
For domain names however, CS first consults another user level
process, the domain name server (DNS).
If no DNS is reachable, CS relies on its own tables.
.PP
Like CS, the domain name server is a user level process providing
one file,
.CW /net/dns .
A client writes a request of the form
.I "domain-name type" ,
where
.I type
is a domain name service resource record type.
DNS performs a recursive query through the
Internet domain name system producing one line
per resource record found.  The client reads
.CW /net/dns 
to retrieve the records.
Like other domain name servers, DNS caches information
learned from the network.
DNS is implemented as a multi-process shared memory application
with separate processes listening for network and local requests.
.NH
Library routines
.PP
The section on protocol devices described the details
of making and receiving connections across a network.
The dance is straightforward but tedious.
Library routines are provided to relieve
the programmer of the details.
.NH 2
Connecting
.PP
The
.CW dial
library call establishes a connection to a remote destination.
It
returns an open file descriptor for the
.CW data
file in the connection directory.
.P1
int  dial(char *dest, char *local, char *dir, int *cfdp)
.P2
.IP \f(CWdest\fP 10
is the symbolic name/address of the destination.
.IP \f(CWlocal\fP 10
is the local address.
Since most networks do not support this, it is
usually zero.
.IP \f(CWdir\fP 10
is a pointer to a buffer to hold the path name of the protocol directory
representing this connection.
.CW Dial
fills this buffer if the pointer is non-zero.
.IP \f(CWcfdp\fP 10
is a pointer to a file descriptor for the
.CW ctl
file of the connection.
If the pointer is non-zero,
.CW dial
opens the control file and tucks the file descriptor here.
.LP
Most programs call
.CW dial
with a destination name and all other arguments zero.
.CW Dial
uses CS to
translate the symbolic name to all possible destination addresses
and attempts to connect to each in turn until one works.
Specifying the special name
.CW net
in the network portion of the destination
allows CS to pick a network/protocol in common
with the destination for which the requested service is valid.
For example, assume the system
.CW research.bell-labs.com
has the Datakit address
.CW nj/astro/research
and IP addresses
.CW 135.104.117.5
and
.CW 129.11.4.1 .
The call
.P1
fd = dial("net!research.bell-labs.com!login", 0, 0, 0, 0);
.P2
tries in succession to connect to
.CW nj/astro/research!login
on the Datakit and both
.CW 135.104.117.5!513
and
.CW 129.11.4.1!513
across the Internet.
.PP
.CW Dial
accepts addresses instead of symbolic names.
For example, the destinations
.CW tcp!135.104.117.5!513
and
.CW tcp!research.bell-labs.com!login
are equivalent
references to the same machine.
.NH 2
Listening
.PP
A program uses
four routines to listen for incoming connections.
It first
.CW announce() s
its intention to receive connections,
then
.CW listen() s
for calls and finally
.CW accept() s
or
.CW reject() s
them.
.CW Announce
returns an open file descriptor for the
.CW ctl
file of a connection and fills
.CW dir
with the
path of the protocol directory
for the announcement.
.P1
int  announce(char *addr, char *dir)
.P2
.CW Addr
is the symbolic name/address announced;
if it does not contain a service, the announcement is for
all services not explicitly announced.
Thus, one can easily write the equivalent of the
.CW inetd
program without
having to announce each separate service.
An announcement remains in force until the control file is
closed.
.LP
.CW Listen
returns an open file descriptor for the
.CW ctl
file and fills
.CW ldir
with the path
of the protocol directory
for the received connection.
It is passed
.CW dir
from the announcement.
.P1
int  listen(char *dir, char *ldir)
.P2
.LP
.CW Accept
and
.CW reject
are called with the control file descriptor and
.CW ldir
returned by
.CW listen.
Some networks such as Datakit accept a reason for a rejection;
networks such as IP ignore the third argument.
.P1
int  accept(int ctl, char *ldir)
int  reject(int ctl, char *ldir, char *reason)
.P2
.PP
The following code implements a typical TCP listener.
It announces itself, listens for connections, and forks a new
process for each.
The new process echoes data on the connection until the
remote end closes it.
The "*" in the symbolic name means the announcement is valid for
any addresses bound to the machine the program is run on.
.P1
.ta 8n 16n 24n 32n 40n 48n 56n 64n
int
echo_server(void)
{
	int dfd, lcfd;
	char adir[40], ldir[40];
	int n;
	char buf[256];

	afd = announce("tcp!*!echo", adir);
	if(afd < 0)
		return -1;

	for(;;){
		/* listen for a call */
		lcfd = listen(adir, ldir);
		if(lcfd < 0)
			return -1;

		/* fork a process to echo */
		switch(fork()){
		case 0:
			/* accept the call and open the data file */
			dfd = accept(lcfd, ldir);
			if(dfd < 0)
				return -1;

			/* echo until EOF */
			while((n = read(dfd, buf, sizeof(buf))) > 0)
				write(dfd, buf, n);
			exits(0);
		case -1:
			perror("forking");
		default:
			close(lcfd);
			break;
		}

	}
}
.P2
.NH
User Level
.PP
Communication between Plan 9 machines is done almost exclusively in
terms of 9P messages. Only the two services
.CW cpu
and
.CW exportfs
are used.
The
.CW cpu
service is analogous to
.CW rlogin .
However, rather than emulating a terminal session
across the network,
.CW cpu
creates a process on the remote machine whose name space is an analogue of the window
in which it was invoked.
.CW Exportfs
is a user level file server which allows a piece of name space to be
exported from machine to machine across a network. It is used by the
.CW cpu
command to serve the files in the terminal's name space when they are
accessed from the
cpu server.
.PP
By convention, the protocol and device driver file systems are mounted in a
directory called
.CW /net .
Although the per-process name space allows users to configure an
arbitrary view of the system, in practice their profiles build
a conventional name space.
.NH 2
Exportfs
.PP
.CW Exportfs
is invoked by an incoming network call.
The
.I listener
(the Plan 9 equivalent of
.CW inetd )
runs the profile of the user
requesting the service to construct a name space before starting
.CW exportfs .
After an initial protocol
establishes the root of the file tree being
exported,
the remote process mounts the connection,
allowing
.CW exportfs
to act as a relay file server. Operations in the imported file tree
are executed on the remote server and the results returned.
As a result
the name space of the remote machine appears to be exported into a
local file tree.
.PP
The
.CW import
command calls
.CW exportfs
on a remote machine, mounts the result in the local name space,
and
exits.
No local process is required to serve mounts;
9P messages are generated by the kernel's mount driver and sent
directly over the network.
.PP
.CW Exportfs
must be multithreaded since the system calls
.CW open,
.CW read
and
.CW write
may block.
Plan 9 does not implement the 
.CW select
system call but does allow processes to share file descriptors,
memory and other resources.
.CW Exportfs
and the configurable name space
provide a means of sharing resources between machines.
It is a building block for constructing complex name spaces
served from many machines.
.PP
The simplicity of the interfaces encourages naive users to exploit the potential
of a richly connected environment.
Using these tools it is easy to gateway between networks.
For example a terminal with only a Datakit connection can import from the server
.CW helix :
.P1
import -a helix /net
telnet ai.mit.edu
.P2
The
.CW import
command makes a Datakit connection to the machine
.CW helix
where
it starts an instance
.CW exportfs
to serve
.CW /net .
The
.CW import
command mounts the remote
.CW /net
directory after (the
.CW -a
option to
.CW import )
the existing contents
of the local
.CW /net
directory.
The directory contains the union of the local and remote contents of
.CW /net .
Local entries supersede remote ones of the same name so
networks on the local machine are chosen in preference
to those supplied remotely.
However, unique entries in the remote directory are now visible in the local
.CW /net 
directory.
All the networks connected to
.CW helix ,
not just Datakit,
are now available in the terminal. The effect on the name space is shown by the following
example:
.P1
philw-gnot% ls /net
/net/cs
/net/dk
philw-gnot% import -a musca /net
philw-gnot% ls /net
/net/cs
/net/cs
/net/dk
/net/dk
/net/dns
/net/ether
/net/il
/net/tcp
/net/udp
.P2
.NH 2
Ftpfs
.PP
We decided to make our interface to FTP
a file system rather than the traditional command.
Our command,
.I ftpfs,
dials the FTP port of a remote system, prompts for login and password, sets image mode,
and mounts the remote file system onto
.CW /n/ftp .
Files and directories are cached to reduce traffic.
The cache is updated whenever a file is created.
Ftpfs works with TOPS-20, VMS, and various Unix flavors
as the remote system.
.NH
Cyclone Fiber Links
.PP
The file servers and CPU servers are connected by
high-bandwidth
point-to-point links.
A link consists of two VME cards connected by a pair of optical
fibers.
The VME cards use 33MHz Intel 960 processors and AMD's TAXI
fiber transmitter/receivers to drive the lines at 125 Mbit/sec.
Software in the VME card reduces latency by copying messages from system memory
to fiber without intermediate buffering.
.NH
Performance
.PP
We measured both latency and throughput
of reading and writing bytes between two processes
for a number of different paths.
Measurements were made on two- and four-CPU SGI Power Series processors.
The CPUs are 25 MHz MIPS 3000s.
The latency is measured as the round trip time
for a byte sent from one process to another and
back again.
Throughput is measured using 16k writes from
one process to another.
.DS C
.TS
box, tab(:);
c s s
c | c | c
l | n | n.
Table 1 - Performance
_
test:throughput:latency
:MBytes/sec:millisec
_
pipes:8.15:.255
_
IL/ether:1.02:1.42
_
URP/Datakit:0.22:1.75
_
Cyclone:3.2:0.375
.TE
.DE
.NH
Conclusion
.PP
The representation of all resources as file systems
coupled with an ASCII interface has proved more powerful
than we had originally imagined.
Resources can be used by any computer in our networks
independent of byte ordering or CPU type.
The connection server provides an elegant means
of decoupling tools from the networks they use.
Users successfully use Plan 9 without knowing the
topology of the system or the networks they use.
More information about 9P can be found in the Section 5 of the Plan 9 Programmer's
Manual, Volume I.
.NH
References
.LP
[Pike90] R. Pike, D. Presotto, K. Thompson, H. Trickey,
``Plan 9 from Bell Labs'',
.I
UKUUG Proc. of the Summer 1990 Conf. ,
London, England,
1990.
.LP
[Needham] R. Needham, ``Names'', in
.I
Distributed systems,
.R
S. Mullender, ed.,
Addison Wesley, 1989.
.LP
[Presotto] D. Presotto, ``Multiprocessor Streams for Plan 9'',
.I
UKUUG Proc. of the Summer 1990 Conf. ,
.R
London, England, 1990.
.LP
[Met80] R. Metcalfe, D. Boggs, C. Crane, E. Taf and J. Hupp, ``The
Ethernet Local Network: Three reports'',
.I
CSL-80-2,
.R
XEROX Palo Alto Research Center, February 1980.
.LP
[Fra80] A. G. Fraser, ``Datakit - A Modular Network for Synchronous
and Asynchronous Traffic'', 
.I
Proc. Int'l Conf. on Communication,
.R
Boston, June 1980.
.LP
[Pet89a] L. Peterson, ``RPC in the X-Kernel: Evaluating new Design Techniques'',
.I
Proc. Twelfth Symp. on Op. Sys. Princ.,
.R
Litchfield Park, AZ, December 1990.
.LP
[Rit84a] D. M. Ritchie, ``A Stream Input-Output System'',
.I
AT&T Bell Laboratories Technical Journal, 68(8),
.R
October 1984.