Plan B: Boxes for networked resources
Francisco J. Ballesteros, Gorka Guardiola Muzquiz,
Katia Leal Algara, Enrique Soriano
Pedro de las Heras Quiros, Eva M. Castro,
Andres Leonardo, and Sergio Arevalo
Laboratorio de Sistemas
Universidad Rey Juan Carlos
Madrid, Spain.
[email protected]
resources available to a human or an application using
the system greatly vary upon time because the devices
can move. Besides, other resources like printers, scanners, etc. should be used or not depending on the
location of the user and their operational status (they
go oine some times).
To pick up an example to illustrate the need of a
new operating system, consider what a user or an application1 has to do for printing a le when using the
pocket-pc. We would like to be able to say just \copy
this le to a printer" and let the system discover if
there is a printer at hand willing to accept print jobs.
If there are several printers available, we would like the
system to choose any one that understands the format
of the le to be printed. If no printer understands the
le format but there is a program to convert the data
to the printer format, we would like the system to run
that program and then queue the result for printing.
This is not the case with current operating systems.
Furthermore, existing systems are likely to send the
le from the le server to the pocket-pc (which can
be connected through a very slow link) just to send
it again to the printer. This means that once more
the user has to make the job of the operating system
by selecting a machine well-connected to the printer
where to execute the print command.
We believe that we have enough technology to be
able to do all this with just a single command like
Nowadays computing environments are made of
heterogeneous networked resources, but unlike environments used a decade ago, the current environments
are highly dynamic. During a computing session, new
resources are likely to appear and some are likely to go
oine or to move to some other place. The operating
system is supposed to hide most of the complexity of
such environments and make it easy to write applications using them. However, that is not the case with
our current operating systems. Plan B is a new operating system that attempts to allow the applications
and their programmers select and use whatever resources are available without forcing them to deal with
the problems created by their dynamic distributed and
heterogeneous environments. It does so by using constraints along with a new abstraction used to replace
the traditional le abstraction.
Keywords: Distributed systems, Operating Systems, Adaptability, Pervasive computing.
1 Introduction
The computing environment used to write this paper is made of three dierent network technologies
(ethernet, wireless ethernet, and serial links) that interconnect a number of dierent devices including laptops, hand-held pocket PCs, desktop PCs and a le
server. Some of these machines have large displays,
some do not. The same happens with keyboards, audio devices, disks, and other resources. Furthermore,
but in the operating systems we use it turns out
to be much more complex. This may be a symptom
that existing operating systems are not supplying appropriate services to handle our computing resources.
This work nanced in part by Spanish MCYT TIC-20011586-C03-01 and URJC PPR-2003-40.
1 In what follows we use the term \user" to refer both to users
and to applications using the system.
cp /this/file /any/printer
To dene the problem more clearly, we can say that
existing operating systems are not helping much their
users to select which resources to use. Note that this
problem, which could be named the \resource selection problem" is dierent from the problems of both
locating and discovering resources, and is actually a
simplied version of the problem of taking context into
account while considering user requests.
This problem can be seen in plenty of dierent examples, whenever the user is selecting a particular resource among the ones available in the network. For
example, we would like to say \execute a program"
without taking care of which binaries are available for
the program, which architectures they can execute on,
and which processors of such architectures are available. Programs like editors would like to ask the operating system to \save data to a temporary le", and let
the system discover whether to use a local le system
(if any), or the department's le server (if available),
or any nearby disk willing to accept requests for temporary storage from our machine (when available). If
we are lucky, our operating system would allow us to
use resources from the network but it would still leave
up to the user the task of selecting which ones to use
even when the choice is obvious.2
A dierent, but related, problem is that once the
resources are selected, we may change our mind. For
example, many of us have wanted to be able to use
our mouse for a while to help a colleague sitting in
the next desk, instead of having to stand up and use
his/her mouse. We would also like to use a keyboard
from a desktop machine to type on a networked pocket
PC with no keyboard. Although the application considered has already a mouse or a keyboard to receive
events or characters, we may still want to make it
use dierent devices for a while. Another instance of
the same problem is that, due to changes in the network, an application using a network connection may
be forced to switch to a dierent connection to stay
connected (e.g. switching from a tcp stream to an infrared connection). We refer to this problem as the
\resource redirection problem".
The objective of this work is to provide a computing system where applications could use the plethora
of networked resources without dealing with the complexity of the environment by themselves; more precisely, to build a system that addresses both the resource selection and redirection problems on behalf of
the applications. The system has been built and is
2 It is a matter of taste, but for the author this case is when all
machines involved run either Plan 9 or Inferno. Both operating
systems exportall resources to the network using a le interface,
which at least is more than other systems do.
named \Plan B".
In what follows, section 2 shows the main ideas behind Plan B. Sections 3 and 4 give an overview of the
system and its main elements. Then we show how
such elements are used to address the problems faced
in sections 5 to 7. Sections 8 to 10 discuss how we
address some important issues like heterogeneity, failures, garbage collection, and protection. Section 11
shows some implementation details. Section 12 discusses more examples to show how the system works.
Section 13 explains the lessons we learned while building and using the system. Sections 14 and 15 discuss
related and future work.
2 Plan B
The main new idea in Plan B is the introduction of
a new abstraction, the box [3], designed to let the application use networked resources in an easy way. But
we believe that its main contribution is not any idea in
particular, but how the combination of its design principles makes up an environment that is more simple
to use for today and upcoming computing resources.
These are the principles Plan B is built on:
All resources (processes, devices, etc.) are perceived as a single abstraction, the box [3]. Boxes
are typed data containers that are operated using a copy operation (instead of the traditional
read/write interface used for les) and have constraints that determine how they can be used together. This lets Plan B know which resources
are being requested to be used together (a binary is copied to a processor, a le to the printer,
etc.) and which constraints must be considered
when using them (binaries must match the processor architecture, le formats must match those
understood by the printers, etc.). The box abstraction is further discussed in the next section.
Note that traditional read and write operations
can still be performed by copying into or out of ondemand created boxes that represent the application's memory. This is further illustrated later.
The system operates on both local and remote
boxes through the same protocol, called Op. Any
server on the network implementing this protocol
can provide boxes (i.e. services) to be used from
a Plan B process.
Name spaces bind names to boxes. Each application has its own name space and can customize
it. Customization is done by dening names for
boxes, as well as the order in which they should
be searched. More than one box may be bound
to the same name.
Box operations use names instead of descriptors. Therefore, applications keep no connections
to resources, they use the network to send selfcontained requests. This is important to better
tolerate changes in the network.
Boxes can be advertised to the network as they
become available. Applications can instruct their
name spaces to automatically import (i.e. bind)
new boxes as soon as their advertisements are received. Applications use this mechanism to learn
of resource availability and to adapt the name
space to the actual environment.
These principles lead to a simple system with just
14 system calls that includes all the functionality of a
typical operating system. The complete list of system
calls is as follows:
System call Purpose
Denes a type conversion
Changes the current box name
Modies metadata for a box
Copies one box to another
Deletes a box
Retrieves the current box name
Forgets about an imported box
Denes a new name for a box
Returns metadata for a box
Returns the box type/constraints
Links one box to another
Makes a box
Returns inner box names
Forgets about a type conversion
Plan B owes much to the design of Plan 9 [14], which
also uses a single abstraction, the le, to export all
resources to the network. But unlike les in Plan 9,
boxes permit Plan B to take over the task of selecting
and combining the resources needed by the user, at
least most of the times.
3 The Box
A box [3] is an abstraction which is meant to replace the more traditional le abstraction and tries to
capture enough of high-level data semantics and relationships to solve the problems faced.
A box contains data and may also contain inner
boxes, leading to a tree structure. Boxes in the tree
are named using path names similar to the ones used
in le systems, with components separated by slashes.
There is no dierence between boxes that contain inner
boxes and those that do not. All boxes are operated
with the same set of operations. Most of the Plan B
system calls are just the box operations shown in this
There is no open operation in Plan B. Box operations use box names, which means that a box name
is likely to be resolved every time a box is used. Although this adds some performance penalty, this is
crucial in that it allows Plan B to decide to which resource the name should be resolved on each operation.
The most important operations to handle boxes are
copy and link. The rst one conceptually copies data
from a box to another. By supplying copy instead of
read and write, the interface permits the system to be
aware of which two resources are being combined. The
second one, link, is used to tie two boxes together.
Link was introduced because it is a convenient way to
express that a box should be like another one. The implementation of link determines that the box linked
is either a reference or a replica of another box. The
exact implementation (reference or replica) of link
depends on the boxes linked. References are useful
to address the resource redirection problem, replicas
are useful to maintain copies of resources at dierent
locations so that the system could select among them.
The operations are atomic with respect to other
operations on the same box. This means that multiple
operations on the same box do not overlap.
We have seen most of the box interface; other operations are delete, which removes a box, info, which
retrieves box metadata, chinfo, which updates box
metadata, and selectors, which asks for the names
of inner boxes to a given box. Since their names give
a good approximation to what they do, they are not
discussed on this paper.3
Box types and constraints
Boxes represent resources, and resources have properties. To provide for a representation of resource
properties, each box has an associated type and constraint set. A box type is dened to Plan B by means
of a string with the type name. A constraint set is
dened by a sequence of values for each property of
interest for the resource. Each value is a string. Values for dierent properties are separated by the \!"
3 The delete system call may also be used to remove previously established links.
character. The type and constraint set are specied
together using a string of the form \a!b!c!. . . ". For
example, the box containing a program binary to be
run on a Plan B system on a PC may have the type and
constraints bin!386!wave. In this example the type is
bin (i.e. \binary") and 386!wave are the constraints.
Here, there are two constraint elements with values
386 and wave. Constraint elements are positional so
that the rst value in any constraint set gives a value
for the same constraint. The convention in our system
is that the rst constraint refers to the architecture of
the machine and the second refers to a network where
the machine is attached (when the machine is attached
to more than one network, the second constraint refers
to the one used to reach the box).
The mechanism used by the kernel to select resources by means of constraints is a constraint resolution algorithm which is inspired by the unication
mechanism of the Prolog programming language. The
algorithm is as follows, although its meaning will be
more clear when we discuss later how Plan B uses it
to select resources.
/* Unifies two sets
* of N and M constraints
Unify(set1[1-n], set2[1-m]) {
if for i = 1 to max(n,m)
unifyval(set1[i], set2[i])!=fail
set of unifyval(set1[i], set2[i])
return fail
/* Unifies two constraints
Unifyval(v1, v2) {
if v1 = "" or v2 = ""
return ""
if v1 = "\%"
return v2
if v2 = "\%"
return v1
if v1 = v2
return v1
return fail
The algorithm determines if two constraint sets can
be unied or not. If they do, the algorithm returns a
constraint that is the unied version of them, and we
say that the constraints match.
We have to say that the kernel itself does not rely
on which constraints are used. This means that resource providers and system users can dene any set
of constraints desired. If a new property is considered
of interest, a new position in the constraint set can be
agreed upon to represent that property.
4 Name spaces and resources
The name space glues together the trees in the forest of boxes that are of interest for the application,
i.e. the set of resources of interest. Plan B refers to
names that have boxes bound to them as prexes ; the
implementation of the name space is in fact a prex
table [21]. The name space enables the automatic selection of resources by allowing multiple bindings to
the same name. It also enables adaptation to environment changes by allowing automatic bindings of
resources that show up in the network while the application runs. The next three sections show how this
is done and how the problems discussed before are
5 Handling the resource selection and
redirection problems
The lack of open introduces an indirection that permits applications to use dierent instances of a given
resource at dierent points in time, depending on resource availability. It also permits the application to
be unaware of the actual location of the resource on
the network, and permits changes in the name space
to take immediate eect in all the applications using
the names aected. For example, network connections
in Plan B are handled by using boxes; the box name
\/b/con/nautilus:80" represents a stream connection
to the service named \80" at the machine named \nautilus". The application resolves the name each time
the connection is used. By using the name space and
binding appropriate resources (e.g. tcp connections
or serial lines) to that name the application can stay
connected despite changes in connectivity.
By using binary operations like copy and link (instead of read and write), the system is aware of which
resources are to be combined. This is quite important
since it permits the system to:
1. Transfer the data from the source to the destination through the best path available. Unlike
traditional le copying, a copy interface permits
data to go straight from the source to the target
box without placing the client process doing the
copy in the middle of the data path.
2. Select an appropriate pair of resources. This can
be done only if the system knows both the source
and the destination. The mechanism used in Plan
B is the resolution of a set of constraints associated with each resource and is discussed later.
Regarding the rst point, the optimization of the path
used to transfer the data may introduce signicant
performance improvements. Since the implementation
of copy asks the destination box to retrieve data from
the source, caching techniques can still be applied to
avoid unnecessary data transfers. When the network
connection between the machine executing the operation and the rest of the system is poor, the use of copy
can make the dierence between being able to perform
the copy and not being able to do it at all.
Regarding the second point, the selection of the resources involved in a system call is performed by unifying constraints on the set of boxes named in the system
call arguments. System calls using a single box (e.g.
info, which retrieves metadata for boxes) unify the
constraints supplied by the user with the constraints
of the boxes with the given name. The rst box found
for which the constraints unify is the one used. On
the other hand, system calls using two boxes together
(e.g. copy, which copies a box to another) try to nd
a pair of boxes such that: the rst one unies with the
constraints supplied for it by the user; the second one
unies with the constraints supplied for it by the user;
and the results of both unications unify.
Figure 1: A network with processors and binaries
To show an example, gure 1 depicts a scenario
where three dierent processors are bound to the name
\/b/proc" and three dierent boxes containing inner boxes with program binaries are bound to the
name \/bin". In this example, two boxes exist with
name /bin/lb, one with constraints p98!ether and
the other with constraints p95!wave. The convention is that the rst constraint species the architecture of the machine involved and the second species the network where the machine is attached at (or
one of them if there are many). In this example, p98
and p95 are two dierent architectures (i586/PC and
sa1110/iPaq). Ether means that the machine is exporting the box through our fast ethernet and wave
means that it is exported through a radio ethernet.
Now consider the system call
make("/b/proc/lb!\%!ether", "/bin/lb")
which requests the creation of a new box (a new
process) named /b/proc/lb such that its constraints
match with those of a box named /bin/lb and also
match the constraints %!ether. The system searches
the name space for a pair of boxes named /b/proc
(since we are making a box, we search for a container
where to create it) and /bin/lb. It may nd rst
the processor box bound to /b/proc with constraints
p98!wave. Such constraint set does not unify with
%!ether and therefore the system keeps on searching until it nds the box bound to /b/proc with constraints p98!ether. Since that unies with %!ether
the system has a candidate for /b/proc. The result of
the unication has the constraints p98!ether.
Now the system searches for boxes bound to
/bin/lb such that its constraint set unies with that
given by the user and also with the result for other
argument (p98!ether). Since the user said nothing
regarding constraints for /bin/lb, the rst box found
whose constraints match p98!ether is considered a
candidate for /bin/lb. In fact, there is a /bin/lb
box whose constraints are exactly p98!ether. That is
the one used. In few words, the user asked the system
to create a process to execute a program and the system searched for an appropriate processor and binary
program pair.
We also need some means to ask the system to
choose a dierent resource for a given resource name
and application, i.e. to perform a \resource redirection". In Plan B, this operation is link. Consider as
an example the redirection of \standard output" for
a process. In Plan B each process is represented by a
box (e.g. /b/proc/lb) and has an inner box named
io1 (e.g. /b/proc/lb/io1). The convention is that
this box is used by the process to serve the same purpose of UNIX's standard output. A link from a box
to io1 would make io1 point to such box. The next
time the process copies something to io1 data will be
sent to the linked box instead. The code needed for
the application to redirect the output of our example
process is quite simple:
Note that the process executing the call might be
at a dierent machine and that the process involved
(/b/proc/lb) may be already running.
Link also helps to keep multiple resource instances
to let the system choose among them. This leads to the
second meaning of link, namely, to maintain replicas
of resources. A clear example is the design for storage boxes. A storage box is roughly the equivalent of a
traditional le system on secondary storage. A storage
box considers links as a means to replicate data on different storage areas, which is clearly dierent from the
meaning of links under /b/proc. If the home for user
\nemo" is kept both at his laptop and at his department's le server, it is reasonable to link both boxes
to let the system know. If both instances of \nemo's"
are bound to the name \/usr/nemo", any path starting at \/usr/nemo" can be resolved to either of the
instances depending on the state of the environment.
The techniques needed to keep both copies coherent
have been studied by systems like Coda [18].
6 Box conversion
Boxes must be of the same type to be operated (e.g.
copied) together. All Plan B name spaces include a
set of conversion denitions along with the prex table. The converter set is made of entries that specify
a program that converts data of a particular type to a
dierent type (a null program may be specied to dene subtypes and constraints can be given along with
the program name to constraint where should it execute). New entries can be added with the cast system
call, and removed later with the uncast system call.
The /b/sys/casts box holds a textual representation
of the conversions dened, to let the user know.
When the name space searches for compatible boxes
it considers the set of conversions dened, and may
cause the execution of the converter program on behalf
of the user.
7 Adapting to changes
The name space can be instructed to pay attention
to announces or advertisements in the network stating
that a particular resource is available. The system call
used is import, usually using the network address any
(which means that we don't care about the resource
location). For example,
import /b/proc any /proc proc!p98 b
arranges for a new prex /b/proc to be entered
in the name space below (b) existing ones. Initially,
there is no box bound to that prex. However, any
box advertised in the network under the name /proc
with a constraint matching proc!p98 will be bound
to the name as soon as the advertisement is received.
Plan B uses an advertisement protocol along with the
Op protocol (which is used to operate on boxes across
the network).
When it is found that an advertised bounded box
is no longer accessible, it is removed from the name
space. Currently, this may only happen after an operation is attempted on the box. Although it has not
been tested yet, constraints could be used to provide
hints about the expected lease time for a new resource
to stay. Besides, the forget system call can be used
to forget about a previously imported box.
8 Heterogeneity
Heterogeneity of architectures and networks is dealt
with by combining several tools:
1. Presenting all resources as a single abstraction
and using conventions to organize the name
spaces. This helps to deal with resources independently of their architecture and inner structure, because at least the interface is always the
2. Using constraints to express restrictions and features of the resources. This helps to know which
resources to import to the name space and which
ones can be combined.
3. Using the converter set to automate data translations between heterogeneous formats.
For example, the standard Plan B graphics device
uses the Plan 9 image format for images. When an
application uses a dierent format for output images,
a conversion can be dened to let the application use
dierent output devices. Furthermore, should the program used to convert the image format require a fast
CPU, a constraint can be dened to determine if a
/proc box (a processor) is considered as either fast or
slow. Thus when the system tries to execute the converter, it will select a CPU considered as fast. Despite
the simplicity of the constraints mechanism, this example illustrates how it can be used to deal with not
so simple problems.
9 System and network failures
Plan B does not try at all to provide fault tolerance,
since we believe that the user of a networked environment would experiment failures anyway. For example,
a user willing to use a friend's laptop to execute a
process must be aware that such process could die if
the friend leaves and shuts down the laptop. In Plan
B the user is responsible for calling import, supplying appropriate constraints, to instruct his/her name
space about which kinds of resources are acceptable
and which ones are not.
Network failures
Using boxes to represent network connections
makes it easier to adapt to network failures. For example, a name space may have both a tcp protocol stack
and an infrared protocol stack bound to the name
/b/con, which is the conventional prex for stream
links. An application may be sending data to service 20 at machine nautilus by copying data to a box
named /b/con/nautilus:20. That name is meaningful to both stacks and the rst time used would issue a
connection request to the remote machine. If the rst
network fails, the second (also bound to /b/con) may
still take over.
One problem that may still happen is that outstanding messages going through a failing network
would be lost despite the supposed reliability of the
connection. In this case, the protocol used by the application over the network link is still required to provide a means to keep the connection reliable (e.g. by
using serial numbers on requests and/or retrying the
requests). The Op protocol used to perform box operations does so, which means that applications do not
need to care about this issue when using boxes from
the network. The implementation is greatly simplied
because, if we ignore the possible error status in the
result, box operations are idempotent in most cases.
The lack of a \le oset" concept makes it easier to
achieve this because updates are atomic and refer to
an entire box, not to a part of a box.
Garbage collection
Box servers are almost stateless because box requests are self-contained. If a client crashes or gets
disconnected from the system, outstanding requests
are completed and the server forgets about that particular client. However, the client might have created
boxes to be used just during its life (e.g. windows,
temporary storage, etc.) and the server should be able
to delete them when the client can no longer use them
due to a crash or a network disconnection.
In this case, garbage collection is done by means of
a lease permission bit. Clients that create boxes that
should be subject of garbage collection upon client failure are supposed to set the lease permission bit on such
boxes. Any box with that bit set would be deleted by
the server if the client does not access the box during
an interval of time specied by the server.
Another mechanism useful for garbage collection is
the deldie (delete on die ) permission bit. When set,
the box involved is removed by the creator process
while it exits. This helps to delete resources no longer
in use when a process crashes (but the machine where
it runs must be alive and connected to perform the
delete operation on the box).
The link operation leads to data replication when
the resource implementation prefers to do so, for example, on storage boxes. This introduces the problem of maintaining data consistent between the set of
replicas. Since the system is built considering that
disconnections are usual, dealing with consistency is
more complex.
We assume that most of the time, the user establishes links to replicate coarse grain data like a
set of system binaries, a home box (what would be
a home directory on other systems) and similar resources. Moreover, we assume that users are responsible for establishing a sensible set of links and that
conicts due to concurrent updates on replicated boxes
are rare. With this set of assumptions, we can borrow
results from systems like Coda [18] to maintain the set
of replicas.
Each server holding a replica accepts updates and is
responsible for propagating them to remaining replicas
as soon as it is feasible. When a conict is detected, a
text message is sent to the user to notify of the conict
and permissions are adjusted to avoid further updates.
When the user resolves the conict, he/she can change
permissions to allow further updates.
10 Protection
Protection is based on authenticated access checking through access control lists. Plan B checks permissions each time a box is used, which would probably not happen on a system using le descriptors.
Each box has a set of permission bits (matching the
operations in the Op protocol) to determine which operations can be performed by the box owner and by
others. There are no user groups in Plan B.
The authentication protocol is left out of the system and it is assumed that box clients and servers
will be authenticated before speaking Op. This is the
approach used by Plan 9 [14]. In fact, the current implementation has a very naive authentication protocol
because we plan to borrow such protocol from Plan 9.
Binary box operations like copy and link, that are
performed on the destination node on behalf of the
client issuing the operation, require the destination
node to be able to speak for the client regarding the
operation being performed. This is achieved by issuing one-shot tickets from the client to the node that
performs the operation. Such tickets are used just to
authenticate the user, not to perform access checking
(which is done with access control lists).
Name space
Op mux
Figure 2: Overview of the Plan B architecture
11 Implementation
As of today, we have a complete implementation
of Plan B, including a shell and utilities going from
a \list boxes" (lb) program to a graphics program to
set the volume on the current audio box. We rewrote
the utilities from their Plan 9 counterparts to use the
system in a real setting, so we could exercise the system. We hope that the examples shown in this paper
will illustrate what we learned using the system. The
implementation, user manual, and system description
are available for download at the Plan B web site [2].
Plan B is just a box multiplexor that allows applications to build a name space for boxes in the network
and tries to help them to select which of them to use
on each case. The implementation follows the architecture shown in gure 2. The kernel performs each
system call by resolving the box names involved and issuing Op RPCs to perform the appropriate operations
on the boxes selected.
At the beginning of each system call, the kernel
creates a Boxref structure for each box involved.
Boxrefs are handles for local and remote boxes. A
boxref carries the box name and constraints as specied by the user and keeps track of which part of the
name (and constraints) are resolved and which part is
yet to be resolved. During the system call, the boxref
may point to dierent boxes while the name space is
being searched for a box that matches the user supplied name and constraints. Once a matching box is
found, the boxref is said to be bound and will not
change any further. Once the system call has completed, the boxrefs used are destroyed. We found
that this is important to make the system more reliable to changes in the environment, since it makes
system calls self-contained. Although it may look inecient, delays introduced are not appreciable for the
Plan B user.
While bound, a boxref contains both the address
of the box server and the box name as known by
the server. The format used to store the address is
a string machine:service and corresponds to a name
for a network connection (/b/con/machine:service ).
Since this name may also correspond to dierent network links/protocols depending on the environment
state, the boxref switches to dierent connections
upon changes in the network (connections are established/terminated automatically by the implementation of the /b/con/ boxes).
The most complex part of the implementation is the
name space, due to the complexities added by the constraint and type checking mechanisms. After a series
of changes to our initial implementation of the name
space, the current implementation performs searches
by unifying constraints as said before in this paper.
But the name space semantics was not obvious from
the beginning and it required experience with the system to reach a satisfactory implementation state. For
example, the constraints may refer to the box or to its
container depending on the system call (e.g. make),
and the type should be checked or not depending on
the system call (once more, make is creating a box and
therefore the type is specied to determine the type
for the new box, not to do a type check).
Furthermore, since a name space lookup might be
retried several times during the search for boxes with
matching constraints, it is necessary to undo the unication of constraints before each attempt to nd a
matching box. This also happens in Prolog while performing unication of expressions, and that was precisely the reference used to reach a sensible implementation.
Binary system calls (those using two box names) resolve the names and constraints by following the code
shown below, where nsselect resolves the prex and
opselect binds the handle to the box. Note that each
call to nsselect selects a dierent name space entry
where to try the binding. The code shown is a simplied version of the real one, which is 58 lines of C
finds a pair that can be
Returns the converter to
be used when needed.
just to perform the operation, but also to learn of the
types and constraints of boxes named by the user.
The current implementation works hosted on top of
Plan 9. The machine dependent part of the kernel uses
Plan 9 processes and les (including network connections and rio windows) to supply services to the machine independent part. The current kernel supplies
boxes for processes, les, network connections, memory, graphics, sound and some other miscellaneous system boxes. A shell and several command line programs have been implemented and used to exercise
the system and gain experience with its interface.
Our experience with the system shows that the performance is reasonable, although no performance tuning has been done yet. Since the merit of any performance numbers is of the underlying Plan 9 system, no
measures have been made yet. A native port would be
tried before. Nevertheless, measures made to the /net
Plan B kernel
Plan B process
Plan B process
// values for op
can(Boxref* sbp, Boxref* dbp, int op)
if (!opselect(sbp))
if (!opselect(dbp))
if (!unify(sbp, dbp))
if (same types)
return found;
if (has converter)
return converter;
Below the name space stands the Op multiplexor. This multiplexor is in charge of implementing
opselect to resolve the sux of the name not specied by the name binding (i.e. the part of the name
determined by the box hierarchy in the server). Besides, the Op multiplexor performs the RPC call to
the appropriate box server requesting the execution of
the box operation. Most system calls issue RPCs not
Plan 9 kernel
Figure 3: Implementation on top of Plan 9
framework of Plan B show that the overhead of the
name space is 1second that corresponds to the lookup
of a name in the prex table. This overhead remains
almost constant for reasonably sized name spaces, and
has been measured on a 2.4GHz Pentium 4 PC running Plan B hosted on a Plan 9 system. This is the
price paid for the lack of binding. Of course, every
time a box server fails, there is an overhead for the
detection of the failure and the reconnection to the
new server. This overhead depends on the particular network protocol used and would be the same for
any other operating system willing to switch from one
server to another.
The approach used to implement Plan B (see gure 3) on a host environment like Plan 9 is to employ
a separate plan 9 process for the Plan B kernel, and
then one Plan 9 process per each Plan B process. More
precisely, the Plan 9 process used for the Plan B kernel has one thread (one Plan 9 process actually) per
Plan B process. Each thread services system calls for
the corresponding Plan B process. System calls are
made using RPCs through a pipe between the user
and kernel processes.
The source code is 3731 lines of machine dependent
code in C, and 7753 lines of portable code in the same
language. Lines are counted using wc on all C source
les under the /src/b/9 and /src/b/port directories.
This is just a hint of the simplicity of the system but is
not to be taken too literally because we must account
for the fact that Plan 9 processes and other resources
are being used to implement Plan B boxes.
simple mechanism, newly started processes canfollow
the user and use appropriate I/O devices depending
on the state of the human using the system. Furthermore, should the user change his mind, for example,
to borrow a friend's keyboard for a while to type to
an already started process, all the user has to do is to
link a new set of I/O devices for the involved process.
13 Lessons learned
The Op protocol
The Op protocol leads to stateless box servers and
is simple enough to permit implementations with tiny
memory and processor usage. The protocol denes an
RPC for each box operation as well as two operations
called getmem and putmem. The rst one is used to
retrieve memory contents from a box and the second
is used to update memory in the box. Unlike read and
write, they transfer all the box contents since there
are no \le osets" in Plan B. When the maximum
message size is not enough to perform the transfer of
the box contents, a series of getmem/putmem messages
is sent; the whole series is considered as a single operation. Note that by avoiding \le osets" and updating
the entire box contents at a time we reduce the problems caused by using dierent replicas of resources as
well as those introduced by the concurrent access from
clients to network resources. A failure in the middle of
a series of putmem RPCs for a single copy is handled
like a crash in the middle of an operation, i.e. servers
should try to keep the old box contents intact until
the series is completed. When this is not feasible due
to the size of the box, the box will be left in an inconsistent state (very much like when the system crashes
in the middle of a single putmem).
12 Other examples
Despite the lack of read and write, applications
can still copy data from their memory to the outside
world and vice-versa. In Plan B each address space is
a box that has inner boxes representing portions of the
application's memory. For example, the equivalent of
a traditional write would be a copy from a memory
box to some other box, like in
copy("/vm/0xf1000:0xf2000", "/some/box");
The /vm box synthesizes inner boxes on demand
just for the system call that uses the inner box name.
As a nal example, there is a usr box in Plan B
that represents a human user. Among other things, it
contains a set of I/O devices preferred by a user. Conventionally, the shell links its I/O devices to those preferred by the user who started the shell. Through this
The constraint mechanism is very powerful when
combined with copy. On its own, constraints seem to
allow the application to select resources given a set of
properties considered. However, by using the system
we found most of the time that the set of properties
desired for a resource depends heavily on the characteristics of another resource (e.g. we don't know which
/bin/lb binary we want without considering which
processors are available). Therefore, the combination
with copy makes a real dierence when compared with
name services that simply resolve a single name given
a set of constraints or properties. We hope that the
examples in the paper had illustrated this, and the
experience of using Plan B.
The lack of connections and le descriptors seems
to make garbage collection harder. However, we found
that this is not exactly true because despite connections, servers must still use either a leasing mechanism
or a timeout based one to determine if a connection
is just slow or the client at the other end is broken.
Nevertheless, we found that this approach makes the
system more resilient to failures.
Most of the times, we are more worried about performance than we should be. We always thought that
the name space implementation would be the bottleneck, but experience said otherwise. Humans using
the system noticed no change when a hardwired implementation that did not use constraints nor searched
the name space multiple times was used instead. The
real performance problem was found in the initial version of the Op protocol (not shown in this paper). This
version issued copy requests to the data source and
not to the data destination, which was preventing the
use of data caches to avoid unnecessary data transfers.
A \don't cache" permission bit was added to permit
the user to request for a box to be transferred on all
Building a new system by starting with a hosted
version and not a native implementation is a very good
approach. No OS toolkit has been used to implement
Plan B, yet it is beneting from all sort of facilities
provided by a host system. We saved a huge amount
of time compared to our earlier experience of building system kernels using a native environment; even
though we built them using facilities supplied by the
OSkit toolkit [7].
14 Related work
Plan 9 [14] is a distributed system that is built by
exporting all resources as les and allowing those les
to be accessed through the network. Plan B borrows
many of the Plan 9 ideas. There are some important
dierences though. Unlike Plan 9, Plan B uses boxes
instead of les permitting data to ow without the controlling client intervention. This is important to permit clients with bad connectivity to control the transfer of huge amounts of information. Besides, along
with the box abstraction comes the use of constraints
(to determine box selection according to the expected
usage for the resources), the lack of le descriptors
(providing better tolerance of network failures), and
the ability to listen for new resources in the network
(to adapt to environment changes).
File systems using typed les like that of Nemesis [5]
are obviously ancestors of the box abstraction. They
do not consider how resources are used together in order to help with resource selection. Moreover, since
their operating systems rely on les and le descriptors, they do not provide a means to perform resource
redirection nor to adapt to environment changes.
This applies also to facilities like the BeOS le system [11], the Semantic File system [8] and the name
service in Globe [12]. They are able to select resources
that present a set of properties by means of attributes.
However, they consider resources on their own, without paying attention to how are they used together.
Furthermore, such systems do not optimize the paths
used for data transfers and it is not clear how they permit the application to automatically adapt to changes
in the network.
Some systems permit exible access to network resources, such as Odissey [13] and Khazana [4]. Although some of them consider disconnected operation
and adapt to changes in the connection status of the
client machine, it is not clear how they adapt to other
changes in the network, for example, changes in the
links used by the servers. This is important since mobile devices are likely to be \servers" for I/O devices.
The problem of considering context to improve applications is addressed by systems like the Context
Toolkit [17] and Gaia [15]. Although some of the problems raised by the willingness to exploit context information during user requests can be addressed by using
constraints, Plan B is not designed as a system for active spaces.
Middleware systems like Globe [19] and WebOS [20]
are targeted to solve a dierent problem. They are
more concerned with scalability and interoperability of
existing systems than they are with rethinking which
services such systems could provide to make things
Many mechanisms have been built (usually as middleware) to address the problems we address on this
paper. Resilient overlay networks [1] and Iceberg's automatic path creation service [22] describe dierent
means to let applications switch to dierent network
links, which is a concrete example of what we named
the resource redirection problem. The Placeless Documents framework [6] provides a means to select instances of documents in the network and automate
conversions between document format; PAST [16] provides a le system that supports replication and permits selection of appropriate le replicas. Such mechanisms are either designed for a concrete kind of application, or supply one of the multiple services that an
operating system is expected to provide. Plan B differs in that it provides a general purpose environment
to build and execute applications.
Other systems, including Ninja [9] (whose architecture for services is called SEDA) and One.world
[10] are designed to provide services by interconnecting small special-purpose devices through the internet.
Although Plan B considers that there might be many
small devices exporting services, it has been built as a
general purpose computing environment.
15 Future work
In the near future we will port more applications
from Plan 9 to Plan B, to get a complete development environment. By using the system for daily work
its shortcomings and strengths will be better noticed.
Later, a native port to run on Intel based PCs will
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