Network Working Group
RFC #684
NIC #32252
April 15,1975
A Commentary on Procedure Calling as a Network Protocol
Richard Schantz
BBN-TENEX
Preface_______
This RFC is being issued as a first step in an attempt to stimulate
a dialog on some issues in designing a distributed computing system.
In particular, it considers the approach taken in a design set forth
in RFC #674, commonly known as the "Procedure Call Protocol" (PCP).
In the present document, the concentration is on what we believe to
be the shortcomings of such a design approach.
Note at the outset that this is not the first time we are providing
a critical commentary on PCP. During the earlier PCP design stages,
we met with the PCP designers for a brief period, and suggested
several changes, many of which became part of PCP Version 2. We
hasten to add, however, that the nature of those suggestions stem
from an entirely different point of view than those presented here.
Our original suggestions, and also some subsequent ones, were mainly
addressing details of implementation. In this note the concern is
more with the concepts underlying the PCP design than with the PCP
implementation.
This note is being distributed because we feel that it raises
certain issues which have not been adequately addressed yet. The
PCP designers are to be congratulated for providing a detailed
written description of their ideas, thereby creating a natural
starting point for a discussion of distributed system design
concepts. It is the intent of this note to stimulate an interaction
among individuals involved with distributed computing, which could
perhaps result in systems whose designs don't preclude their use in
projects other than the one for which they were originally
conceived.
The ideas expressed in this RFC have benefited from numerous
discussions with Bob Thomas, BBN-TENEX, who shares the point of view
taken.
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Introduction____________
While the Procedure Call Protocol (PCP) and its use within the
National Software Works (NSW) context attacks many of the problems
associated with integrating independent computing systems to handle
a distributed computation, it is our feeling that its design
contains flaws which should prevent its widespread use, and in our
view, limit its overall utility. We are not voicing our objection
to the use of PCP, in its current definition, as the base level
implementation vehicle for the NSW project. It is already too late
for any such objection, and PCP may, in fact, be very effective for
the NSW implementation, since they are proceeding in parallel and
have probably influenced each other. Rather, we are voicing an
objection to the "PCP philosophy", in the hope of preventing this
type of protocol from becoming the de-facto network standard for
distributed computation, and in the hope of influencing the future
direction of this and similar efforts.
Some of the objectionable aspects of PCP, it can be argued, are
differences of individual preference, and philosophers have often
indicated that you cannot argue about tastes. We have tried to
avoid such arguments in this document. Rather, we consider PCP in
light of our experience in developing distributed systems.
Considered in this way, we feel that PCP and its underlying
philosophy have flaws which make it inappropriate as a general
purpose protocol and virtual programming system for the construction
of distributed software systems. It is our opinion that PCP is
probably complete in the sense that one can probably do anything
that is required using its primitives. A key issue then, is not
whether this function or that function can be supported. Rather, to
us an important question is how easy it is to do the things which
experience has indicated are important to distributed computing. In
addition, a programming discipline dedicated to network applications
should pay particular attention to coercing its users away from
actions which systems programming in general and network programming
in particular have shown to be pitfalls in system implementation.
A Point of View_ _____ __ ____
At the outset, we fully support the aspects of the PCP design
effort that have gone into systematizing the interaction and
agreements between distributed elements to support inter-machine
computing. This includes the definition of the various types of
replies, the standardization of the data structure format for
inter-machine exchange, and the process creation primitives which
extend the machine boundaries. Such notions are basic and must be
part of any distributed system definition. Our main concern is not
with these efforts.
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Rather, we take exception to PCP's underlying premise: that the
procedure calling discipline is the starting point for building
multi-computer systems. This premise leads to a model which has a
central point for the entire algorithm control, rather than a more
natural (in network situations) distributed control accomplished by
cooperating independent entities interacting through common
communication paths. While the procedure call may be an appropriate
basis for certain applications, we believe that it can neither
directly nor accurately model the interactions and control
structures that occur in many distributed multi-computer systems.
Much of what follows may seem to be a pedagogic argument, and
PCP supporters may take the position of "who cares what you call it,
its doing the same thing". Our reply is that it is very important
to achieve a clear and concise model of distributed computation, and
while the PCP model does not require "poor implementation" of
distributed systems, neither does it make "good implementation" any
easier, nor does it prohibit ill-advised programming practices. A
model stressing the dynamic interconnection of somewhat independent
computing entities, we feel, adheres more to the notions of
defensive programming, which we have found to be fundamental to
building usable multi-machine implementations.
The rest of this RFC discusses what we feel to be some of the
shortcomings of a procedure call protocol.
Limitations of Procedure Calling Across Machines___________ __ _________ _______ ______ ________
First and foremost, it is our contention that procedure calling
should not be the basis for multi-machine interactions. We feel
that a request and reply protocol along with suitably manipulated
communication paths between processes forms a model better suited to
the situation in which the network places us. In a network
environment one has autonomous computing entities which have agreed
on their cooperation, rather than a master process forcing execution
of a certain body of code to fulfill its computing needs. In such a
configuration, actions required of a process are best accommodated
indirectly (by request) rather than directly (by procedure call), in
order to maintain the integrity of the constituent processes.
Procedure calling is most often a very primitive operation
whose implementation often requires only a single machine
instruction. In addition, it is usually true that procedure calling
is usually not within the domain of the operating system. [The
Multics intersegment procedure calling mechanism may present an
exception to this, until linkage is complete. In the remote PCP
case, however, linkage can never be complete in the sense of
supporting a fast transfer of control between modules]. Processes
and communication paths between processes, however, are undeniably
operating system constructs. In an environment where local
procedure calling was "cheap", it would be ill-advised to blur the
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distinction between a local (inexpensive in time and effort) and a
remote procedure call, which obviously requires a great deal of
effort by the "PCP system", if not by the PCP user. It also seems
to be the case that the cost of blurring the local/remote
distinction at the procedure call level will be found in the more
frequent use of a less efficient local procedure calling mechanism.
Interprocess communication, on the other hand, (at least with regard
to stream or message oriented channels and not just interrupt
signals) is generally regarded as having a significant cost
associated with it. Message sending is always an interprocess
action, and requires system intervention always. There is not as
substantial a difference between the IPC of local processes and the
IPC of remote processes, as between local and remote procedure
calling. PCP is suggestive of a model in which processes exist that
span machine boundaries to provide inter-machine subroutine calling.
Yet the PCP documentation has not advocated the notion of a process
that spans machine boundaries, and rightfully so since such a
creation would cause innumerable problems. Since procedure calling
is more suitable as an intra-process notion, it seems to be a better
idea to take the interprocess communication framework and extend it
to have a uniform interpretation locally and remotely, rather than
to extend the procedure calling model. It is also our contention
that a model which relies on procedure calling for its basis does
not take into account the special nature of the network environment,
and that such an environment can be more suitably handled in a
message passing model. Furthermore, we feel that programming as a
whole, even purely local computing, will benefit from paying more
attention to such areas as reliability and robustness, which have
been brought to the forefront through experience with an oftentimes
unreliable network and collection of hosts. An IPC model, by
emphasizing the connections between disjoint processes, seems to
reinforce the idea that distributed computing is accomplished by
joining separate entities, and that defensive programming and error
handling techniques are appropriate. Since PCP is, we think, for
distributed system builders, and not for the end user (e.g. an
RSEXEC user), avoiding the network, interconnection issues, and
relative costs, may be counter-productive if the goal is to achieve
usable network systems.
In a similar vein, the entire notion of inter-machine procedure
calling underlies a model which in effect has extended the address
space of a single process. That is, there is a single locus of
algorithm control (although perhaps not a single locus of
execution). While this model may well serve the needs of a "local"
computation where the parts are strongly bound together, our
experience in building working distributed systems has shown the
utility of a model which has multiple loci of control and execution.
In such a model, it is through agreements on the method and type of
information interchange and synchronization, that a computation is
carried out, rather than at the singular direction of a central
entity. In a model that has distributed control and execution, we
feel a process will be in a better position to naturally cope with
the many vagaries that necessarily arise in a network environment.
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The unmistakable trend in systems programming is toward
inviolable (protected) process structures with external
synchronization as a means of coping with complex debugging tasks
and the difficulty of making system changes. This trend is better
supported, we feel, by a message passing rather than a procedural
model of computation. Furthermore, we feel that network programming
techniques should be applied to local computation, not the other way
around.
Some Particulars____ ___________
In the following list, we try to be more specific with respect
to particular situations where we think the PCP concept may be weak
as the basis for a network programming system. For some of these
examples to be meaningful, the reader should be fairly familiar with
the PCP documents issued as RFC 674.
1. Recovery from component malfunction may be very
difficult to handle by a process that is not the central
control (i.e. a process which is being manipulated by
having its procedures executed). Is the situation where
there is network trouble, for example, to be modeled by a
forced procedure call to some error recovery routine? It is
precisely such situations where distributed control serves
as a better model. Consider the act of introducing an
inferior to another acquaintance and then supplying the new
handle as a parameter of a subsequent procedure call in the
inferior. The inferior's blind use of the parameter to
interact with the other process illustrates the manipulative
aspects of a superior. The inferior never really is aware
of a new communication path to a new process. The inferior
environment (as maintained by the PCP "system") has been
changed by the superior, with no active notification of the
inferior. Certainly this makes user coded error recovery
somewhat awkward.
2. Such process manipulation may at times violate the
principles of modular programming. In this vein, it seems
beneficial to be able to debug separately the pieces of a
computation and then worry only about their synchronization
to achieve a totally debugged system. With PCP in its
fullest sense, the danger of error propagation seems greater
because of the power of a process to cause execution of an
arbitrary procedure and to read/write remote data stores
without the active participation of the remote process.
3. Can we assume a proper initialization sequence if our
procedures are called remotely? Must every procedure
contain the code to check for the propriety and correct
sequencing of the call? A model in which each remote process
is an active computing element seems better able to
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conveniently apply protective standards to the code and data
it encompasses.
4. PCP doesn't model long term parallel activity in a
convenient fashion, as is required to handle various
asynchronous producer/consumer process relationships. The
synchronization is geared more to a one-to-one call and
return, rather than to the asynchronous nature and multiple
returns for a single request, as exhibited by many network
services. In addition, low priority, preemptable background
tasks are hard (impossible?) to model in a procedure call
environment.
5. Communication paths are not treated as abstract
objects which are independent from the actual entities they
connect, and hence they cannot be utilized in some useful
ways (e.g. to carry non PCP messages). Also with respect to
treating communication paths as objects, there is no concept
of passing a communication path to an inferior (or an
acquaintance), without having to create a new "connection"
(whether or not this turns out to be a physical channel).
The ability to pass communication paths is often useful in
subcontracting requests to inferior processes. To do this
within PCP requires the cooperation of the calling process
(i.e. to use the new connection handle), which again seems
to violate the concepts of modular programming. The
alternative approach in PCP is to have the superior relay
the subsequent communications to its created inferior, but
the effort involved would probably prohibit the use of this
technique for subcontracting.
6. PCP seems too complicated to be used for the type of
processing which requires periodic but short (i.e. a few
words exchanged) interactions. An example of such
interactions is the way the TIP uses the TENEX accounting
servers (see RFC #672). Furthermore, PCP is probably much
too complex for implementation on a small host. In that
regard, there does not seem to be a definition of what might
constitute a minimum implementation for a host/process which
did/could not handle all of what has been developed.
7. In the PCP model, it may become awkward or resource
consuming for a service program to do such things as queue
operations for execution at a later time (persistence) or at
a more opportune time (priority servicing mechanism). Such
implementations may require dummy returns and modification
of the controlling fork concept, or maintenance of
processing forks over long periods of inactivity.
8. It is not always true that a process connecting
(splicing) to a service should be able to influence the
service process environment in any direct way. How can a
service process in PCP prevent a malicious user fom splicing
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to it and then introducing it to an arbitrary number of
processes, thereby overflowing the table space in that
process. All of that could have been done without ever
executing a single instruction of user written code. This
difficulty is a consequence of the PCP notion of having one
process manipulate the environment of another without its
active participation in such actions.
9. Doesn't the fact that the network PCP process
implementation is so much neater than the TENEX PCP process
implementation (since TENEX doesn't have a general IPC
facility) suggest that message passing and communication
facilities supported by the "system" provides a sound basis
for multi-process implementations, and that perhaps such
facilities should be primitively available to the
distributed system builders who will use PCP?
10. There is a question of whether PCP is an
implementation virtual machine (language), or an application
virtual machine (language). That is, is PCP intended to be
used to implement systems which manage distributed
resources, or as an end product which makes the network
resources themselves easier to use for the every day,
ordinary programmer (e.g. makes the network itself
transparent to users). One gets the feeling that the
designers had both goals, and that neither one is completely
satisfied. If the former goal is taken, we believe that
most of the complexities (e.g. network trouble, broken
connections, etc.) and possibilities (e.g. redundant
implementation, broadcast request, etc.) of network
implementations are not provided for adequately. In this
view, the NSW framework (Works manager, FE) is the
distributed system that utilizes the PCP implementation
language. We do not see how the use of PCP in this context
provides for either an extra-reliable system through
component redundancy, or a persistent system which can
tolerate temporary malfunctions. If one subscribes to this
view, then it doesn't seem right that the objects that run
under the created system (i.e. the tools that run under the
PCP implemented Front End, Works Manager, and TBH monitor)
should also be aware of or use PCP. If one considers the
latter goal, that PCP implements a virtual machine to be
presented to all programmers for making distributed
resources easy to use, then it is clear that PCP with its
manifest concern for object location does not provide for
the desireable properties of network transparency.
Our conclusion is that procedure calling is not the appropriate
basis for distributed multi-computer systems because it can neither
directly nor accurately model the network environment. The PCP
virtual programming system may be inadequate for implementing many
distributed systems because the complexities and possibilities
unique to the network environment are not provided for at this basic
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level.
RFC, FYI, BCP