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rfc:rfc992
                                                     K. P. Birman (Cornell)

Network Working Group T. A. Joseph (Cornell) Request for Comments: 992 November 1986

     On Communication Support for Fault Tolerant Process Groups
                   K. P. Birman and T. A. Joseph
           Dept. of Computer Science, Cornell University
                         Ithaca, N.Y. 14853
                            607-255-9199

1. Status of this Memo.

 This memo describes a collection of multicast communication primi-
 tives integrated with a mechanism for handling process failure and
 recovery.  These primitives facilitate the implementation of fault-
 tolerant process groups, which can be used to provide distributed
 services in an environment subject to non-malicious crash failures.
 Unlike other process group approaches, such as Cheriton's "host
 groups" (RFC's 966, 988, [Cheriton]), our approach provides powerful
 guarantees about the behavior of the communication subsystem when
 process group membership is changing dynamically, for example due to
 process or site failures, recoveries, or migration of a process from
 one site to another.  Our approach also addresses delivery ordering
 issues that arise when multiple clients communicate with a process
 group concurrently, or a single client transmits multiple multicast
 messages to a group without pausing to wait until each is received.
 Moreover, the cost of the approach is low.  An implementation is be-
 ing undertaken at Cornell as part of the ISIS project.
 Here, we argue that the form of "best effort" reliability provided by
 host groups may not address the requirements of those researchers who
 are building fault tolerant software.  Our basic premise is that re-
 liable handling of failures, recoveries, and dynamic process migra-
 tion are important aspects of programming in distributed environ-
 ments, and that communication support that provides unpredictable
 behavior in the presence of such events places an unacceptable burden
 of complexity on higher level application software.  This complexity
 does not arise when using the fault-tolerant process group alterna-
 tive.
 This memo summarizes our approach and briefly contrasts it with other
 process group approaches.  For a detailed discussion, together with
 figures that clarify the details of the approach, readers are re-
 ferred to the papers cited below.
 Distribution of this memo is unlimited.

Birman & Joseph [Page 1] RFC 992 November 1986

2. Acknowledgments

 This memo was adopted from a paper presented at the Asilomar workshop
 on fault-tolerant distributed computing, March 1986, and summarizes
 material from a technical report that was issued by Cornell Universi-
 ty, Dept. of Computer Science, in August 1985, which will appear in
 ACM Transactions on Computer Systems in February 1987 [Birman-b].
 Copies of these paper, and other relevant papers, are available on
 request from the author: Dept. of Computer Science, Cornell Universi-
 ty, Ithaca, New York 14853. (birman@gvax.cs.cornell.edu).  The ISIS
 project also maintains a mailing list.  To be added to this list,
 contact M. Schmizzi (schiz@gvax.cs.cornell.edu).
 This work was supported by the Defense Advanced Research Projects
 Agency (DoD) under ARPA order 5378, Contract MDA903-85-C-0124, and by
 the National Science Foundation under grant DCR-8412582.  The views,
 opinions and findings contained in this report are those of the au-
 thors and should not be construed as an official Department of De-
 fense position, policy, or decision.

3. Introduction

 At Cornell, we recently completed a prototype of the ISIS system,
 which transforms abstract type specifications into fault-tolerant
 distributed implementations, while insulating users from the mechan-
 isms by which fault-tolerance is achieved.  This version of ISIS, re-
 ported in [Birman-a], supports transactional resilient objects as a
 basic programming abstraction.  Our current work undertakes to pro-
 vide a much broader range of fault-tolerant programming mechanisms,
 including fault-tolerant distributed bulletin boards [Birman-c] and
 fault-tolerant remote procedure calls on process groups [Birman-b].
 The approach to communication that we report here arose as part of
 this new version of the ISIS system.
 Unreliable communication primitives, such as the multicast group com-
 munication primitives proposed in RFC's 966 and 988 and in [Cheri-
 ton], leave some uncertainty in the delivery status of a message when
 failures and other exceptional events occur during communication.
 Instead, a form of "best effort" delivery is provided, but with the
 possibility that some member of a group of processes did not receive
 the message if the group membership was changing just as communica-
 tion took place.  When we tried to use this sort of primitive in our
 original work on ISIS, which must behave reliably in the presence of
 such events, we had to address this aspect at an application level.
 The resulting software was complex, difficult to reason about, and
 filled with obscure bugs, and we were eventually forced to abandon
 the entire approach as infeasible.
 A wide range of reliable communication primitives have been proposed
 in the literature, and we became convinced that by using them, the
 complexity of our software could be greatly reduced.  These range

Birman & Joseph [Page 2] RFC 992 November 1986

 from reliable and atomic broadcast [Chang] [Cristian] [Schneider] to
 Byzantine agreement [Strong].  For several reasons, however, the ex-
 isting work does not solve the problem at hand.  The most obvious is
 that they do not provide a mechanism for sending a message to all the
 members of a group when the membership is changing dynamically (the
 "group addressing" problem).  In addition, one can identify delivery
 ordering issues and questions regarding the detection of communica-
 tion failures that should be handled within the broadcast mechanism.
 These motivate a careful reexamination of the entire reliable broad-
 cast problem.
 The multicast primitives we report here are designed to respect
 several sorts of ordering constraints, and have cost and latency that
 varies depending on the nature of the constraint required [Birman-b]
 [Joseph-a] [Joseph-b].  Failure and recovery are integrated into the
 communication subsystem by treating these events as a special sort of
 multicast issued on behalf of a process that has failed or recovered.
 The primitives are presented in the context of fault tolerant process
 groups: groups of processes that cooperate to implement some distri-
 buted algorithm or service, and which need to see consistent order-
 ings of system events in order to achieve mutually consistent
 behavior.  Such groups are similar to the host groups of the V system
 and the ones described in RFC's 966 and 988, but provide guarantees
 of consistency in just the situations where a host group provides a
 "best effort" delivery which may sometimes be erroneous.
 It is helpful to think of our primitives as providing a logical or
 "virtual" form of reliability: rather than addressing physical
 delivery issues, they ensure that a client will never observe a sys-
 tem state "inconsistent" with the assumption that reliable delivery
 has occurred.  Readers familiar with serializability theory may want
 to think of this as a weaker analog: in serializability, one allows
 interleaved executions of operations provided that the resulting sys-
 tem state is consistent with the assumption that execution was
 sequential.  Similarly, reliable communication primitives permit de-
 viations from the reliable delivery abstraction provided that the
 resulting system state is indistinguishable from one in which reli-
 able delivery actually did occur.
 Using our primitives, the ISIS system achieved both high levels of
 concurrency and suprisingly good performance.  Equally important, its
 structure was made suprisingly simple, making it feasible to reason
 about the correctness of the algorithms that are needed to maintain
 high availability even when failures, recoveries, or process migra-
 tion occurs.  More recently, we have applied the same approach to a
 variety of other problems in distributed computing, and even designed
 a consistent, fault tolerant, distributed bulletin board data struc-
 ture (a generalized version of the blackboards used in artificial in-
 telligence programs), with equally good results [Birman-c].  Thus, we
 feel that the approach has been shown to work in a variety of set-
 tings where unreliable primitives simply could not be used.

Birman & Joseph [Page 3] RFC 992 November 1986

 In the remainder of this memo we summarize the issues and alterna-
 tives that the designer of a distributed system is presented with,
 focusing on two styles of support for fault-tolerant computing: re-
 mote procedure calls coupled with a transactional execution facility,
 such as is used in the ARGUS system [Liskov], and the fault-tolerant
 process group mechanism mentioned above.  We argue that transactional
 interactions are too restrictive to support the sort of mechanism
 needed, and then show how our primitives can be used to provide such
 a mechanism.  We conclude by speculating on future directions in
 which this work might be taken.

4. Issues in fault-tolerance

 The difficulty of constructing fault-tolerant distributed software
 can be traced to a number of interrelated issues.  The list that fol-
 lows is not exhaustive, but attempts to touch on the principal con-
 siderations that must be addressed in any such system:
    [1]Synchronization.  Distributed systems offer the potential for
    large amounts of concurrency, and it is usually desirable to
    operate at as high a level of concurrency as possible.  However,
    when we move from a sequential execution environment to a con-
    current one, it becomes necessary to synchronize actions that may
    conflict in their access to shared data or entail communication
    with overlapping sets of processes.  Thus, a mechanism is needed
    for ordering conflicting events.  Additional problems that can
    arise in this context include deadlock avoidance or detection,
    livelock avoidance, etc.
    [2]Failure detection.  It is usually necessary for a fault-
    tolerant application to have a consistent picture of which com-
    ponents fail, and in what order. Timeout, the most common mechan-
    ism for detecting failure, is unsatisfactory, because there are
    many situations in which a healthy component can timeout with
    respect to one component without this being detected by some
    another.  Failure detection under more rigorous requirements
    requires an agreement protocol that is related to Byzantine agree-
    ment [Strong] [Hadzilacos].  Regardless of how this problem is
    solved, some sort of reliable failure detection mechanism will be
    needed in any fault-tolerant distributed system.
    [3] Consistency.  When a group of processes cooperate in a distri-
    buted system, it is necessary to ensure that the operational
    processes have consistent views of the state of the group as a
    whole.  For example, if process p believes that some property A
    holds, and on the basis of this interacts with process q, the
    state of q should not contradict the fact that p believes A to be
    true.  This problem is closely related to notions of knowledge and
    consistency in distributed systems [Halpern] [Lamport].  In our
    context, A will often be the assertion that a multicast has been
    received by q, or that q saw some sequence of events occur in the

Birman & Joseph [Page 4] RFC 992 November 1986

    same order as did p.  Thus, it is necessary to be able to specify
    the precise consistency constraints on a distributed software sys-
    tem, and system support should be available to facilitate the
    attainment of these constraints.
    [4] Serializability.  Many distributed systems are partitioned
    into data manager processes, which implement shared variables, and
    transaction manager processes, which issue requests to data
    managers [Bernstein].  If transaction managers can execute con-
    currently, it is desirable to ensure that transactions produce
    serializable outcomes [Eswaren] [Papadimitrou].  Serializability
    is increasingly viewed as an important property in "object-
    oriented" distributed systems that package services as abstract
    objects with which clients communicate by remote procedure calls
    (RPC).  On the other hand, there are systems for which serializa-
    bility is either too strong a constraint, or simply inappropriate.
    Thus, one needs a way to achieve serializability in applications
    where it will be needed, without imposing system-wide restrictions
    that would prevent the design of software subsystems for which
    serializability is not needed.
 Jointly, these problems render the design of fault-tolerant distri-
 buted software daunting in the absence of adequate support.  The
 correctness of any proposed design and of its implementation become
 serious, if not insurmountable, concerns.  In Sec. 7, we will show
 how the primitives of Sec. 6 provide simple ways to overcome all of
 these issues.

5. Existing alternatives

 If one rules out "unreliable" communication mechanisms, there are
 basically two fault-tolerant alternatives that can be pursued.
 The first approach is to provide mechanisms for transactional
 interactions between processes that communicate using remote pro-
 cedure calls [Birrell].  This has lead to work on nested transactions
 (due to nested RPC's) [Moss], support for transactions at the
 language level [Liskov], transactions within an operating systems
 kernel [Spector] [Allchin] [Popek] [Lazowska], and transactional
 access to higher-level replicated services, such as resilient objects
 in ISIS or relations in database systems.  The primitives in a tran-
 sactional system provide mechanisms for distributing the request that
 initiates the transaction, accessing data (which may be replicated),
 performing concurrency control, and implementing commit or abort.
 Additional mechanisms are normally needed for orphan termination,
 deadlock detection, etc.  The issue then arises of how these mechan-
 isms should themselves be implemented.
 Our work in ISIS leads us to believe that whereas transactions are
 easily implemented on top of fault-tolerant process groups -- we have
 done so -- the converse is much harder.  Moreover, transactions

Birman & Joseph [Page 5] RFC 992 November 1986

 represent a relatively heavy-weight solution to the problems surveyed
 in the previous section, and might impose an unacceptable overhead on
 subsystems that need to run non-transactionally, for example because
 a pair of concurrent processes needs to interact on a frequent basis.
 (We are not saying that "transactional" mechanisms such as cobegins
 and toplevel actions can't solve this problem, but just that they
 yield a solution that is awkward and costly).  This sort of reasoning
 has lead us to focus on non-transactional interaction mechanisms, and
 to treat transactions as a special class of mechanisms used when
 processes that have been designed to employ a transactional protocol
 interact.
 The second approach involves the provision of a communication primi-
 tive, such as atomic broadcast, which can be used as the framework on
 which higher level algorithms are designed.  Such a primitive seeks
 to deliver messages reliably to some set of destinations, despite the
 possibility that failures might occur during the execution of the
 protocol.  Above, we termed this the fault tolerant process group
 approach, since it lends itself to the organization of cooperating
 processes into groups, as described in the introduction.  Process
 groups are an extremely flexible abstraction, and have been employed
 in the V Kernel [Cheriton] and in UNIX, and more recently in the ISIS
 system.  A proposal to provide Internet support for host groups was
 raised in RFC's 966 and 988.  However, the idea of adapting the pro-
 cess group approach to work reliably in an environment subject to the
 sorts of exception events and concurrency cited in the previous sec-
 tion seems to be new.
 As noted earlier, existing reliable communication protocols do not
 address the requirements of fault-tolerant process groups.  For exam-
 ple, in [Schneider], an implementation of a reliable multicast primi-
 tive is described.  Such a primitive ensures that a designated mes-
 sage will be transmitted from one site to all other operational sites
 in a system; if a failure occurs but any site has received the mes-
 sage, all will eventually do so.  [Chang] and [Cristian] describe
 implementations for atomic broadcast, which is a reliable broadcast
 (sent to all sites in a system) with the additional property that
 messages are delivered in the same order at all overlapping destina-
 tions, and this order preserves the transmission order if messages
 originate in a single site.
 Atomic broadcast is a powerful abstraction, and essentially the same
 behavior is provided by one of the multicast primitives we discuss in
 the next section.  However, it has several drawbacks which made us
 hesitant to adopt it as the only primitive in the system.  Most seri-
 ous is the latency that is incurred in order to satisfy the delivery
 ordering property.  Without delving deeply into the implementations,
 which are based on a token scheme in [Chang] and an acknowledgement
 protocol in [Schneider], we observe that the delaying of certain mes-
 sages is fundamental to the establishment of a unique global delivery
 ordering; indeed, it is easy to prove on knowledge theoretic grounds

Birman & Joseph [Page 6] RFC 992 November 1986

 that this must always be the case.  In [Chang] a primary goal is to
 minimize the number of messages sent, and the protocol given performs
 extremely well in this regard.  However, a delay occurs while waiting
 for tokens to arrive and the delivery latency that results may be
 high.  [Cristian] assumes that clocks are closely synchronized and
 that message transit times are bounded by well-known constants, and
 uses this to derive atomic broadcast protocols tolerant of increas-
 ingly severe classes of failures.  The protocols explicitly delay
 delivery to achieve the desired global ordering on multicasts.  For
 reasons discussed below, this tends to result in high latency in typ-
 ical local networking environments.  An additional drawback of the
 atomic broadcast protocols is that no mechanism is provided for
 ensuring that all processes observe the same sequence of failures and
 recoveries, or for ensuring that failures and recoveries are ordered
 relative to ongoing multicasts.  Since this problem arises in any
 setting where one process monitors another, we felt it should be
 addressed at the same level as the communication protocol.  Finally,
 one wants a group oriented multicast protocol, not a site oriented
 broadcast, and this issue must be resolved too.

6. Our multicast primitives

 We now describe three multicast protocols - GBCAST, ABCAST, and
 CBCAST - for transmitting a message reliably from a sender process to
 some set of destination processes.  Details of the protocols and
 their correctness proofs can be found in [Birman-b].  The protocols
 ensure "all or nothing" behavior: if any destination receives a mes-
 sage, then unless it fails, all destinations will receive it.  Group
 addressing is discussed in Sec. 6.5.
 The failure model that one adopts has a considerable impact on the
 structure of the resulting system.  We adopted the model of fail-stop
 processors [Schneider]: when failures occur, a processor simply stops
 (crashes), as do all the processes executing on it.  We also assume
 that individual processes can crash, and that this is detected when
 it occurs by a monitoring mechanism present at each site.  Further
 assumptions are sometimes made about the availability of synchronized
 realtime clocks.  Here, we adopt the position that although reason-
 ably accurate elapsed-time clocks may be available, closely synchron-
 ized clocks probably will not be.  For example, the 60Hz "line"
 clocks commonly used on current workstations are only accurate to
 16ms.  On the other hand, 4-8ms inter-site message transit times are
 common and 1-2ms are reported increasingly often.  Thus, it is impos-
 sible to synchronize clocks to better than 32-48ms, enough time for a
 pair of sites to exchange between 4 and 50 messages.  Even with
 advancing technology, it seems safe to assume that clock skew will
 remain "large" when compared to inter-site message transmission
 speed.  In particular, this argues against time-based protocols such
 as the one used in [Cristian]

Birman & Joseph [Page 7] RFC 992 November 1986

 6.1 The GBCAST primitive
     GBCAST (group multicast) is the most constrained, and costly, of
     the three primitives.  It is used to transmit information about
     failures and recoveries to members of a process group.  A recov-
     ering member uses GBCAST to inform the operational ones that it
     has become available.  Additionally, when a member fails, the
     system arranges for a GBCAST to be issued to group members on its
     behalf, informing them of its failure.  Arguments to GBCAST are a
     message and a process group identifier, which is translated into
     a set of destinations as described below (Sec. 6.5).
     Our GBCAST protocol ensures that if any process receives a multi-
     cast B before receiving a GBCAST G, then all overlapping destina-
     tions will receive B before G <1> This is true regardless of the
     type of multicast involved.  Moreover, when a failure occurs, the
     corresponding GBCAST message is delivered after any other multi-
     casts from the failed process.  Each member can therefore main-
     tain a VIEW listing the membership of the process group, updating
     it when a GBCAST is received.  Although VIEW's are not updated
     simultaneously in real time, all members observe the same
     sequence of VIEW changes.  Since, GBCAST's are ordered relative
     to all other multicasts, all members receiving a given multicast
     will have the same value of VIEW when they receive it.
     Notice that GBCAST also provides a convenient way to change other
     global properties of a group "atomically".  In our work, we have
     used GBCAST to dynamically change a ranking on the members of a
     group, to request that group members establish checkpoints for
     use if recovery is needed after all failure, and to implement
     process migration.  In each case, the ordering of GBCAST relative
     to other events that makes it possible to perform the desired
     action without running any additional protocol.  Other uses for
     GBCAST will no doubt emerge as our research continues.
     Members of a process group can also use the value of VIEW to pick
     a strategy for processing an incoming request, or to react to
     failure or recovery without having to run any special protocol
     first.  Since the GBCAST ordering is the same everywhere, their
     actions will all be consistent.  Notice that when all the members
     of a process group may have failed, GBCAST also provides an inex-
     pensive way to determine the last site that failed: process group
     members simply log each value of VIEW that becomes defined on
     stable storage before using it; a simplified version of the algo-
     rithm in [Skeen-a] can then be executed when recovering from
     failure.

Birman & Joseph [Page 8] RFC 992 November 1986

 6.2 The ABCAST primitive
     The GBCAST primitive is too costly to be used for general commun-
     ication between process group members.  This motivates the intro-
     duction of weaker (less ordered) primitives, which might be used
     in situations where a total order on multicast messages is not
     necessary.  Our second primitive, ABCAST (atomic multicast),
     satisfies such a weaker constraint.  Specifically, it is often
     desired that if two multicasts are received in some order at a
     common destination site, they be received in that order at all
     other common destinations, even if this order was not predeter-
     mined.  For example, if a process group is being used to maintain
     a replicated queue and ABCAST is used to transmit queue opera-
     tions to all copies, the operations will be done in the same
     order everywhere, hence the copies of the queue will remain mutu-
     ally consistent.  The primitive ABCAST(msg, label, dests) pro-
     vides this behavior.  Two ABCAST's having the same label are
     delivered in the same order at all common destinations.
 6.3 The CBCAST primitive
     Our third primitive, CBCAST (causal multicast), is weakest in the
     sense that it involves less distributed synchronization then
     GBCAST or ABCAST.  CBCAST(msg, dests) atomically delivers msg to
     each operational dest.  The CBCAST protocol ensures that if two
     multicasts are potentially causally dependent on another, then
     the former is delivered after the latter at all overlapping des-
     tinations.  A multicast B' is potentially causally dependent on a
     multicast B if both multicasts originate from the same process,
     and B' is sent after B, or if there exists a chain of message
     transmissions and receptions or local events by which knowledge
     could have been transferred from the process that issued B to the
     process that issued B' [Lamport].  For causally independent mul-
     ticasts, the delivery ordering is not constrained.
     CBCAST is valuable in systems like ISIS, where concurrency con-
     trol algorithms are used to synchronize concurrent computations.
     In these systems, if two processes communicate concurrently with
     the same process the messages are almost always independent ones
     that can be processed in any order: otherwise, concurrency con-
     trol would have caused one to pause until the other was finished.
     On the other hand, order is clearly important within a causally
     linked series of multicasts, and it is precisely this sort of
     order that CBCAST respects.
 6.4 Other multicast primitives
     A weaker multicast primitive is reliable multicast, which pro-
     vides all-or-nothing delivery, but no ordering properties.  The
     formulation of CBCAST in [Birman-b] actually includes a mechanism
     for performing multicasts of this sort, hence no special

Birman & Joseph [Page 9] RFC 992 November 1986

     primitive is needed for the purpose.  Additionally, there may be
     situations in which ABCAST protocols that also satisfy a CBCAST
     ordering property would be valuable.  Our ABCAST primitive could
     be changed to respect such a rule, and we made use of a multicast
     primitive that is simultaneously causal and atomic in our work on
     consistent shared bulletin boards ([Birman-c]).  For simplicity,
     the presentation here assumes that ABCAST is completely orthogo-
     nal to CBCAST, but a simple way to build an efficient "causal
     atomic" multicast is described in our full-length paper.  The
     cost of this protocol is only slightly higher than that of
     ABCAST.
 6.5 Group addressing protocol
     Since group membership can change dynamically, it may be diffi-
     cult for a process to compute a list of destinations to which a
     message should be sent, for example, as is needed to perform a
     GBCAST.  In [Birman-b] we report on a protocol for ensuring that
     a given multicast will be delivered to all members of a process
     group in the same view.  This view is either the view that was
     operative when the message transmission was initiated, or a view
     that was defined subsequently.  The algorithm is a simple itera-
     tive one that costs nothing unless the group membership changes,
     and permits the caching of possibly inaccurate membership infor-
     mation near processes that might want to communicate with a
     group.  Using the protocol, a flexible message addressing scheme
     can readily be supported.
     Iterative addressing is only required when the process transmit-
     ting a message has an inaccurate copy of the process group view.
     In the implementation we are now building, this would rarely be
     the case, and iteration is never needed if the view is known to
     be accurate.  Thus, iterated delivery should be very infrequent.
 6.6 Synchronous versus asynchronous multicast abstractions
     Many systems employ RPC internally, as a lowest level primitive
     for interaction between processes.  It should be evident that all
     of our multicast primitives can be used to implement replicated
     remote procedure calls [Cooper]: the caller would simply pause
     until replies have been received from all the participants
     (observation of a failure constitutes a reply in this case).  We
     term such a use of the primitives synchronous, to distinguish it
     from from an asynchronous multicast in which no replies, or just
     one reply, suffices.
     In our work on ISIS, GBCAST and ABCAST are normally invoked syn-
     chronously, to implement a remote procedure call by one member of
     an object on all the members of its process group.  However,
     CBCAST, which is the most frequently used overall, is almost
     never invoked synchronously.  Asynchronous CBCAST's are the

Birman & Joseph [Page 10] RFC 992 November 1986

     primary source of concurrency in ISIS: although the delivery ord-
     ering is assured, transmission can be delayed to enable a message
     to be piggybacked on another, or to schedule IO within the system
     as a whole.  While the system cannot defer an asynchronous multi-
     cast indefinitely, the ability to defer it a little, without
     delaying some computation by doing so, permits load to be
     smoothed.  Since CBCAST respects the delivery orderings on which
     a computation might depend, and is ordered with respect to
     failures, the concurrency introduced does not complicate higher
     level algorithms.  Moreover, the protocol itself is extremely
     cheap.
     A problem is introduced by our decision to allow asynchronous
     multicasts: the atomic reception property must now be extended to
     address causally related sequences of asynchronous messages.  If
     a failure were to result in some multicasts being delivered to
     all their destinations but others that precede them not being
     delivered anywhere, inconsistency might result even if the desti-
     nations do not overlap.  We therefore extend the atomicity pro-
     perty as follows.  If process t receives a message m from process
     s, and s subsequently fails, then unless t fails as well, all
     messages m' that s received prior to its failure must be
     delivered to their remaining operational destinations.  This is
     because the state of t may now depend on the contents of any such
     m', hence the system state could become inconsistent if the
     delivery of m' were not completed.  The costs of the protocols
     are not affected by this change.
     A second problem arises when the user-level implications of this
     atomicity rule are considered.  In the event of a failure, any
     suffix of a sequence of aysnchronous multicasts could be lost and
     the system state would still be internally consistent.  A process
     that is about to take some action that may leave an externally
     visible side-effect will need a way to pause until it is
     guaranteed that such multicasts have actually been delivered.
     For this purpose, a flush primitive is provided.  Occasional
     calls to flush do not eliminate the benefit of using CBCAST asyn-
     chronously.  Unless the system has built up a considerable back-
     log of undelivered multicast messages, which should be rare,
     flush will only pause while transmission of the last few multi-
     casts complete.

7. Using the primitives

 The reliable communication primitives described above lead to simple
 solutions for the problems cited in Sec. 4:
     [1]  Synchronization.  Many synchronization problems are subsumed
     into the primitives themselves.  For example, consider the use of
     GBCAST to implement recovery.  A recovering process would issue a
     GBCAST to the process group members, requesting that state

Birman & Joseph [Page 11] RFC 992 November 1986

     information be transferred to it.  In addition to sending the
     current state of the group to the recovering process, group
     members update the process group view at this time.  Subsequent
     messages to the group will be delivered to the recovered process,
     with all necessary synchronization being provided by the ordering
     properties of GBCAST.  In situations where other forms of syn-
     chronization are needed, ABCAST provides a simple way to ensure
     that several processes take actions in the same order, and this
     form of low-level synchronization simplifies a number of higher-
     level synchronization problems.  For example, if ABCAST is used
     to do P() and V() operations on a distributed semaphore, the
     order of operations on the semaphore is set by the ABCAST, hence
     all the managers of the semaphore see these operations in a fixed
     order.
     [2]  Failure detection.  Consistent failure (and recovery) detec-
     tion are trivial using our primitives: a process simply waits for
     the appropriate process group view to change.  This facilitates
     the implementation of algorithms in which one processes monitors
     the status of another process.  A process that acts on the basis
     of a process group view change does so with the assurance that
     other group members will (eventually) observe the same event and
     will take consistent actions.
     [3]  Consistency.  We believe that consistency is generally
     expressible as a set of atomicity and ordering constraints on
     message delivery, particularly causal ones of the sort provided
     by CBCAST.  Our primitives permit a process to specify the com-
     munication properties needed to achieve a desired form of con-
     sistency.  Continued research will be needed to understand pre-
     cisely how to pick the weakest primitive in a designated situa-
     tion.
     [4]  Serializability.  To achieve serializability, one implements
     a concurrency control algorithm and then forces computations to
     respect the serialization order that this algorithm choses.  The
     ABCAST primitive, as observed above, is a powerful tool for
     establishing an order between concurrent events, e.g. by lock
     acquisition.  Having established such an order, CBCAST can be
     used to distribute information about the computation and also its
     termination (commit or abort).  Any process that observes the
     commit or abort of a computation will only be able to interact
     with data managers that have received messages preceding the com-
     mit or abort, hence a highly asynchronous transactional execution
     results.  If a process running a computation fails, this is
     detected when a failure GBCAST is received instead of the commit.
     Thus, executions are simple and quite deterministic.
     If commit is conditional, CBCAST would be used to first interro-
     gate participants to learn if they are prepared to commit, and
     then to transmit the commit or abort decision (the usual two-

Birman & Joseph [Page 12] RFC 992 November 1986

     phase commit).  On the other hand, conditional commits can often
     be avoided using our approach.  A method for building transac-
     tions that will roll-forward after failure after failure is dis-
     cussed in more detail in [Birman-a] [Joseph-a] [Joseph-b].  Other
     forms of concurrency control, such as timestamp generation, can
     similarly be implemented using ABCAST and CBCAST.  We view tran-
     sactional data storage as an application-level concern, which can
     be handled using a version stack approach or a multi-version
     store, or any other appropriate mechanism.

8. Implementation

 The communication primitives can be built in layers, starting with a
 bare network providing unreliable Internet datagrams.  The software
 structure is, however, less mature and more complex than the one sug-
 gested in RFC's 966 and 988.  For example, at this stage of our
 research we do not understand how to optimize our protocols to the
 same extent as for the unreliable host multicast approach described
 in those RFC's.  Thus, the implementation we describe here should be
 understood to be a prototype.  A particularly intriguing question,
 which we are investigating actively, concerns the use of a "best
 effort" ethernet or Internet multicast as a tool to optimize the
 implementation of our protocols.
 Our basic approach is to view large area networks as a set of clus-
 ters of sites interconnected by high speed LAN devices and intercon-
 nected by slower long-haul links.  We first provide protocols for use
 within clusters, and then extend them to run between clusters too.
 Network partitioning can be tolerated at all levels of the hierarchy
 in the sense that no incorrect actions can result after network par-
 titioning, although our approach will sometimes block until the par-
 tition is repaired.  Our protocols also tend to block within a clus-
 ter while the list of operational sites for that cluster is being
 changed.  In normal LAN's, this happens infrequently (during site
 failure or recovery), and would not pose a problem.  (In failure
 intensive applications, alternative protocols might be needed to
 address this issue).
 The lowest level of our software uses a site-to-site acknowledgement
 protocol to convert the unreliable packet transport this into a
 sequenced, error-free message abstraction, using timeouts to detect
 apparent failures.  TCP can also be used for this purpose, provided
 that a "filter" is placed on the incoming message stream and certain
 types of messages are handled specially.  An agreement protocol is
 then used to order the site-failures and recoveries consistently.  If
 timeouts cause a failure to be detected erroneously, the protocol
 forces the affected site to undergo recovery.
 Built on this is a layer that supports the primitives themselves.
 CBCAST has a very light-weight implementation, based on the idea of
 flooding the system with copies of a message: Each process buffers

Birman & Joseph [Page 13] RFC 992 November 1986

 copies of any messages needed to ensure the consistency of its view
 of the system.  If message m is delivered to process p, and m is
 potentially causally dependent on a message m prime, then a copy of m
 prime is sent to p as well (duplicates are discarded).  A garbage
 collector deletes superfluous copies after a message has reached all
 its destinations.  By using extensive piggybacking and a simple
 scheduling algorithm to control message transmission, the cost of a
 CBCAST is kept low -- often, less than one packet per destination.
 ABCAST employs a two-phase protocol based on one suggested to us by
 Skeen [Skeen-b].  This protocol has higher latency than CBCAST
 because delivery can only occur during the second phase; ABCAST is
 thus inherently synchronous.  In ISIS, however, ABCAST is used
 rarely; we believe that this would be the case in other systems as
 well.  GBCAST is implemented using a two-phase protocol similar to
 the one for ABCAST, but with an additional mechanism that flushes
 messages from a failed process before delivering the GBCAST announc-
 ing the failure.  Although GBCAST is slower than ABCAST or CBCAST, it
 is used rarely enough so that performance is probably less of an
 issue here -- and in any case, even GBCAST could be tuned to give
 very high throughput.  Preliminary performance figures appear in
 [Birman-b].
 Although satisfactory performance should be possible using an imple-
 mentation that sits on top of a conventional Internet mechanism, it
 should be noted that to achieve really high rates of communication
 the layers of software described above must reside in the kernel,
 because they run on behalf of large numbers of clients, run fre-
 quently, and tend to execute for very brief periods before doing I/O
 and pausing.  A non-kernel implementation will thus incur high
 scheduling and context switching overhead.  Additionally, it is not
 at all clear how to use ethernet style broadcast mechanisms to optim-
 ize the performance of this sort of protocol, although it should be
 possible.  We view this as an interesting area for research.
 A forthcoming paper will describe higher level software that we are
 building on top of the basic fault-tolerant process group mechanism
 described above.

9. Conclusions

 The experience of implementing a substantial fault-tolerant system
 left us with insights into the properties to be desired from a com-
 munication subsystem.  In particular, we became convinced that to
 build a reliable distributed system, one must start with a reliable
 communication subsystem.  The multicast primitives described in this
 memo present a simple interface, achieve a high level of concurrency,
 can be used in both local and wide area networks, and are applicable
 to software ranging from distributed database systems to the fault-
 tolerant objects and bulletin boards provided by ISIS.  Because they
 are integrated with failure handling mechanisms and respect desired
 event orderings, they introduce a desirable form of determinism into

Birman & Joseph [Page 14] RFC 992 November 1986

 distributed computation without compromising efficiency.  A conse-
 quence is that high-level algorithms are greatly simplified, reducing
 the probability of error.  We believe that this is a very promising
 and practical approach to building large fault-tolerant distributed
 systems, and it is the only one we know of that leads to a rigorous
 form of confidence in the resulting software.

NOTES:

 <1> A problem arises if a process p fails without receiving some mes-
 sage after that message has already been delivered to some other pro-
 cess q: q's VIEW when it received the message would show p to be
 operational; hence, q will assume that p received the message,
 although p is physically incapable of doing so.  However, the state
 of the system is now equivalent to one in which p did receive the
 message, but failed before acting on it.  In effect, there exists an
 interpretation of the actual system state that is consistent with q's
 assumption.  Thus, GBCAST satisfies the sort of logical delivery pro-
 perty cited in the introduction.

Birman & Joseph [Page 15] RFC 992 November 1986

10. References

[RFC966] Deering, S. and Cheriton, D. Host groups: A multicast exten-

    sion to the internet protocol.  Stanford University, December
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[RFC988] Deering, S. Host extensions for IP multicasting. Stanford

    University, July 1986.

[Allchin] Allchin, J., McKendry, M. Synchronization and recovery of

    actions.  Proc. 2nd ACM SIGACT/SIGOPS Principles of Distributed
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[Babaoglu] Babaoglu, O., Drummond, R. The streets of Byzantium: Network

    architectures for fast reliable multicast.  IEEE Trans. on
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[Bernstein] Bernstein, P., Goodman, N. Concurrency control algorithms

    for replicated database systems.  ACM Computing Surveys 13, 2
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[Birman-a] Birman, K. Replication and fault-tolerance in the ISIS sys-

    tem.  Proc. 10th ACM SIGOPS Symposium on Operating Systems Princi-
    ples.  Orcas Island, Washington, Dec. 1985, 79-86.

[Birman-b] Birman, K., Joseph, T. Reliable communication in the pres-

    ence of failures.  Dept. of Computer Science, Cornell Univ., TR
    85-694, Aug. 1985.  To appear in ACM TOCS (Feb. 1987).

[Birman-c] Birman, K., Joseph, T., Stephenson, P. Programming with

    fault tolerant bulletin boards in asynchronous distributed sys-
    tems.  Dept. of Computer Science, Cornell Univ., TR 85-788, Aug.
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[Birrell] Birrell, A., Nelson, B. Implementing remote procedure calls.

    ACM Transactions on Computer Systems 2, 1 (Feb. 1984), 39-59.

[Chang] Chang, J., Maxemchuck, M. Reliable multicast protocols. ACM

    TOCS 2, 3 (Aug. 1984), 251-273.

[Cheriton] Cheriton, D. The V Kernel: A software base for distributed

    systems.  IEEE Software 1 12, (1984), 19-43.

[Cooper] Cooper, E. Replicated procedure call. Proc. 3rd ACM Symposium

    on Principles of Distributed Computing., August 1984, 220-232.
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[Cristian] Cristian, F. et al Atomic multicast: From simple diffusion to

    Byzantine agreement.  IBM Technical Report RJ 4540 (48668), Oct.
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Birman & Joseph [Page 16] RFC 992 November 1986

[Eswaren] Eswaren, K.P., et al The notion of consistency and predicate

    locks in a database system.  Comm. ACM 19, 11 (Nov. 1976), 624-
    633.

[Hadzilacos] Hadzilacos, V. Byzantine agreement under restricted types

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[Halpern] Halpern, J., and Moses, Y. Knowledge and common knowledge in

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[Joseph-a] Joseph, T. Low cost management of replicated data. Ph.D.

    dissertation, Dept. of Computer Science, Cornell Univ., Ithaca
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[Joseph-b] Joseph, T., Birman, K. Low cost management of replicated

    data in fault-tolerant distributed systems.  ACM TOCS 4, 1 (Feb
    1986), 54-70.

[Lamport] Lamport, L. Time, clocks, and the ordering of events in a

    distributed system.  CACM 21, 7, July 1978, 558-565.

[Lazowska] Lazowska, E. et al The architecture of the EDEN system.

    Proc. 8th Symposium on Operating Systems Principles, Dec. 1981,
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[Liskov] Liskov, B., Scheifler, R. Guardians and actions: Linguistic

    support for robust, distributed programs.  ACM TOPLAS 5, 3 (July
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[Moss] Moss, E. Nested transactions: An approach to reliable, distri-

    buted computing.  Ph.D. thesis, MIT Dept of EECS, TR 260, April
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[Papadimitrou] Papadimitrou, C. The serializability of concurrent data-

    base updates.  JACM 26, 4 (Oct. 1979), 631-653.

[Popek] Popek, G. et al. Locus: A network transparent, high reliability

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[Schlicting] Schlicting, R, Schneider, F. Fail-stop processors: An

    approach to designing fault-tolerant distributed computing sys-
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[Schneider] Schneider, F., Gries, D., Schlicting, R. Reliable multicast

    protocols.  Science of computer programming 3, 2 (March 1984).

[Skeen-a] Skeen, D. Determining the last process to fail. ACM TOCS 3,

Birman & Joseph [Page 17] RFC 992 November 1986

    1, Feb. 1985, 15-30.

[Skeen-b] Skeen, D. A reliable multicast protocol. Unpublished.

[Spector] Spector, A., et al Distributed transactions for reliable sys-

    tems.  Proc. 10th ACM SIGOPS Symposium on Operating Systems Prin-
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[Strong] Strong, H.R., Dolev, D. Byzantine agreement. Digest of papers,

    Spring Compcon 83, San Francisco, CA, March 1983, 77-81.

Birman & Joseph [Page 18]

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