Implementing Remote Procedure Calls ANDREW D. BIRRELL and BRUCE JAY NELSON Xerox Palo Alto Research Center
Implementing Remote Procedure Calls ANDREW D. BIRRELL and BRUCE JAY NELSON Xerox Palo Alto Research Center
Remote procedure calls (RPC) appear to be a useful paradigm for providing communication across a network between programs written in a high-level language. This paper describes a package providing a remote procedure call facility, the options that face the designer of such a package, and the decisions we made. We describe the overall structure of our RPC mechanism, our facilities for binding RPC clients, the transport level communication protocol, and some performance measurements. We include descriptior,s of some optimizations used to achieve high performance and to minimize the load on server machines that have many clients. CR Categories and Subject Descriptors: C.2.2 [Computer-Communication Networks]: Network Protocols—protocol architecture; C.2.4 [Computer-Communication Networks]: Distributed Sys tems—distributed applications, network operating systems; D.4.4 [Operating Systems]: Communi cations Management—message sending, network communication; D.4.7[Operating Systems]: Or ganization and Design—distributed systems
General Terms: Design, Experimentation, Performance, Security Additional Keywords and Phrases: Remote procedure calls, transport layer protocols, distributed naming and binding, inter-process communication, performance of communication protocols.
1. INTRODUCTION
1.1 Background The idea of remote procedure calls (hereinafter called RPC) is quite simple. It is based on the observation that procedure calls are a well-known and well- understood mechanism for transfer of control and data within a program running on a single computer. Therefore, it is proposed that this same mechanism be extended to provide for transfer of control and data across a communication network. When a remote procedure is invoked, the calling environment is suspended, the parameters are passed across the network to the environment where the procedure is to execute (which we will refer to as the callee), and the desired procedure is executed there. When the procedure finishes and produces its results, the results are passed backed to the calling environment, where execution resumes as if returning from a simple single-machine call. While the calling environment is suspended, other processes on that machine may (possibly)
Authors’ address: Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304. Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the ACM copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Association for Computing Machinery. To copy otherwise, or to republish, requires a fee and/or specific permission. © 1984 ACM 0734-2071/84/0200-0039 $00.75
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40 A. D. Birrell and B. J. Nelson
still execute (depending on the details of the parallelism of that environment and the RPC implementation).
There are many attractive aspects to this idea. One is clean and simple semantics: these should make it easier to build distributed computations, and to get them right. Another is efficiency: procedure calls seem simple enough for the communication to be quite rapid. A third is generality: in single-machine com putations, procedures are often the most important mechanism for communica tion between parts of the algorithm.
The idea of RPC has been around for many years. It has been discussed in the public literature many times since at least as far back as 1976 [15]. Nelson’s doctoral dissertation [13] is an extensive examination of the design possibilities for an RPC system and has references to much of the previous work on RPC. However, full-scale implementations of RPC have been rarer than paper designs. Notable recent efforts include Courier in the Xerox NS family of protocols [4], and current work at MIT [10].
This paper results from the construction of an RPC facility for the Cedar project. We felt, because of earlier work (particularly Nelson’s thesis and asso ciated experiments), that we understood the choices the designer of an RPC facility must make. Our task was to make the choices in light of our particular aims and environment. In practice, we found that several areas were inadequately understood, and we produced a system whose design has several novel aspects. Major issues facing the designer of an RPC facility include: the precise semantics of a call in the presence of machine and communication failures; the semantics of address-containing arguments in the (possible) absence of a shared address space; integration of remote calls into existing (or future) programming systems; binding (how a caller determines the location and identity of the callee); suitable protocols for transfer of data and control between caller and callee; and how to provide data integrity and security (if desired) in an open communication network. In building our RPC package we addressed each of these issues, but it not possible to describe all of them in suitable depth in a single paper. This paper includes a discussion of the issues and our major decisions about them, and describes the overall structure of our solution. We also describe in some detail our binding mechanism and our transport level communication protocol. We plan to produce subsequent papers describing our facilities for encryption-based security, and providing more information about the manufacture of the stub modules (which are responsible for the interpretation of arguments and results of RPC calls) and our experiences with practical use of this facility.
1.2 Environment The remote-procedure-call package we have built was developed primarily for use within the Cedar programming environment, communicating across the Xerox research internetwork. In building such a package, some characteristics of the environment inevitably have an impact on the design, so the environment is summarized here.
Cedar [6] is a large project concerned with developing a programming environ ment that is powerful and convenient for the building of experimental programs and systems. There is an emphasis on uniform, highly interactive user interfaces, and ease of construction and debugging of programs. Cedar is designed to be used ACM Transactions on Computer Systems, Vol. 2, No. 1, February 1984
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on single-user workstations, although it is also used for the construction of servers (shared computers providing common services, accessible through the communication network).
Most of the computers used for Cedar are Dorados [8]. The Dorado is a very powerful machine (e.g., a simple Algol-style call and return takes less than 10 microseconds). It is equipped with a 24-bit virtual address space (of 16-bit words) and an 80-megabyte disk. Think of a Dorado as having the power of an IBM 370/168 processor, dedicated to a single user.
Communication between these computers is typically by means of a 3-megabit- per-second Ethernet [11]. (Some computers are on a 10-megabit-per-second Ethernet [7].) Most of the computers running Cedar are on the same Ethernet, but some are on different Ethernets elsewhere in our research internetwork. The internetwork consists of a large number of 3-megabyte and 10-megabyte Ether nets (presently about 160) connected by leased telephone and satellite links (at data rates of between 4800 and 56000 bps). We envisage that our RPC commu nication will follow the pattern we have experienced with other protocols: most communication is on the local Ethernet (so the much lower data rates of the internet links are not an inconvenience to our users), and the Ethernets are not overloaded (we very rarely see offered loads above 40 percent of the capacity of an Ethernet, and 10 percent is typical).
The PUP family of protocols [3] provides uniform access to any computer on this internetwork. Previous PUP protocols include simple unreliable (but high- probability) datagram service, and reliable flow-controlled byte streams. Between two computers on the same Ethernet, the lower level raw Ethernet packet format is available.
Essentially all programming is in high-level languages. The dominant language is Mesa [12] (as modified for the purposes of Cedar), although Smalltalk and InterLisp are also used. There is no assembly language for Dorados.
1.3 Aims The primary purpose of our RPC project was to make distributed computation easy. Previously, it was observed within our research community that the con struction of communicating programs was a difficult task, undertaken only by members of a select group of communication experts. Even researchers with substantial systems experience found it difficult to acquire the specialized exper tise required to build distributed systems with existing tools. This seemed undesirable. We have available to us a very large, very powerful communication network, numerous powerful computers, and an environment that makes building programs relatively easy. The existing communication mechanisms appeared to be a major factor constraining further development of distributed computing. Our hope is that by providing communication with almost as much ease as local procedure calls, people will be encouraged to build and experiment with distrib uted applications. RPC will, we hope, remove unnecessary difficulties, leaving only the fundamental difficulties of building distributed systems: timing, inde pendent failure of components, and the coexistence of independent execution environments.
We had two secondary aims that we hoped would support our purpose. We wanted to make RPC communication highly efficient (within, say, a factor of
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five beyond the necessary transmission times of the network). This seems important, lest communication become so expensive that application designers strenuously avoid it. The applications that might otherwise get developed would be distorted by their desire to avoid communicating. Additionally, we felt that it was important to make the semantics öf the RPC package as powerful as possible, without loss of simplicity or efficiency. Otherwise, the gains of a single unified communication paradigm would be lost by requiring application programmers to build extra mechanisms on top of the RPC package. An important issue in design is resolving the tension between powerful semantics and efficiency.
Our final major aim was to provide secure communication with RPC. None of the previously implemented protocols had any provision for protecting the data in transit on our networks. This was true even to the extent that passwords were transmitted as clear-text. Our belief was that research on the protocols and mechanisms for secure communication across an open network had reached a stage where it was reasonable and desirable for us to include this protection in our package. In addition, very few (if any) distributed systems had previously provided secure end-to-end communication, and it had never been applied to RPC, So the design might provide useful research insights.
1.4 Fundamental Decisions It is not an immediate consequence of our aims that we should use procedure calls as the paradigm for expressing control and data transfers. For example, message passing might be a plausible alternative. It is our belief that a choice between these alternatives would not make a major difference in the problems faced by this design, nor in the solutions adopted. The problems of reliable and efficient transmission of a message and of its possible reply are quite similar to the problems encountered for remote procedure calls. The problems of passing arguments and results, and of network security, are essentialy unchanged. The overriding consideration that made us choose procedure calls was that they were the major control and data transfer mechanism imbedded in our major language, Mesa.
One might also consider using a more parallel paradigm for our communication, such as some form of remote fork. Since our language already includes a construct for forking parallel computations, we could have chosen this as the point at which to add communication semantics. Again, this would not have changed the major design problems significantly.
We discarded the possibility of emulating some form of shared address space among the computers. Previous work has shown that with sufficient care mod erate efficiency can be achieved in doing this [14]. We do not know whether an approach employing shared addresses is feasible, but two potentially major difficulties spring to mind: first, whether the representation of remote addresses can be integrated into our programming languages (and possibly the underlying machine architecture) without undue upheaval; second, whether acceptable effi ciency can be achieved. For example, a host in the PUP internet is represented by a 16-bit address, so a naive implementation of a shared address space would extend the width of language addresses by 16-bits. On the other hand, it is possible that careful use of the address-mapping mechanisms of our virtual memory hardware could allow shared address space without changing the address ACM Transactions on Computer Systems, Vol. 2, No. 1, February 1984
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width. Even on our 10 megabit Ethernets, the minimum average round trip time for a packe exchange is 120 microseconds [7], so the most likely way to approach this would be to use some form of paging system. In summary, a shared address space between participants in RPC might be feasible, but since we were not willing to undertake that research our subsequent design assumes the absence of shared addresses. Our intuition is that with our hardware the cost of a shared address space would exceed the additional benefits.
A principle that we used several times in making design choices is that the semantics of remote procedure calls should be as close as possible to those of local (single-machine) procedure calls. This principle seems attractive as a way of ensuring that the RPC facility is easy to use, particularly for programmers familiar with single-machine use of our languages and packages. Violation of this principle seemed likely to lead us into the complexities that have made previous communication packages and protocols difficult to use. This principle has occa sionally caused us to deviate from designs that would seem attractive to those more experienced in distributed computing. For example, we chose to have no time-out mechanism limiting the duration of a remote call (in the absence of machine or communication failures), whereas most communication packages consider this a worthwhile feature. Our argument is that local procedure calls have no time-out mechanism, and our languages include mechanisms to abort an activity as part of the parallel processing mechanism. Designing a new time-out arrangement just for RPC would needlessly complicate the programmer’s world. Similarly, we chose the building semantics described below (based closely on the existing Cedar mechanisms) in preference to the ones presented in Nelson’s thesis [13].
