# Bosco: One-Step Byzantine Asynchronous Consensus.

**ABSTRACT** Asynchronous Byzantine consensus algorithms are an important primitive for building Byzantine fault-tolerant systems. Algorithms for Byzantine consensus typically require at least two communication steps for decision; in many systems, this imposes a significant performance overhead. In this paper, we show that it is possible to design Byzantine fault-tolerant consensus algorithms that decide in one message latency under contention-free scenarios and still provide strong consistency guarantees when contention occurs. We define two variants of one-step asynchronous Byzantine consensus and show a lower bound on the number of processors needed for each. We present a Byzantine consensus algorithm, Bosco, for asynchronous networks that meets these bounds, even in the face of a strong network adversary.

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**ABSTRACT:**The paper considers the consensus problem in a partially synchronous system with Byzantine faults. It turns out that, in the partially synchronous system, all deterministic algorithms that solve consensus with Byzantine faults are leader-based. This is not the case of benign faults, which raises the following fundamental question: is it possible to design a deterministic Byzantine consensus algorithm for a partially synchronous system that is not leader-based? The paper gives a positive answer to this question, and presents a leader-free algorithm that is resilient-optimal and signature-free.02/2010; - SourceAvailable from: Achour Mostéfaoui07/2014;
- SourceAvailable from: Achour Mostéfaoui[Show abstract] [Hide abstract]

**ABSTRACT:**Provide application processes with strong agreement guarantees despite failures is a fundamental problem of fault-tolerant distributed computing. Correct processes have not to be “polluted” by the erroneous behavior of faulty processes. This paper considers the consensus agreement problem in a setting where some processes can behave arbitrarily (Byzantine behavior). In such a context it is possible that Byzantine processes collude to direct the correct processes to decide on a “bad” value (a value proposed only by faulty processes). The paper has several contributions. It presents a family of consensus algorithms in which no bad value is ever decided by correct processes. These processes always decide a value they have proposed (and this is always the case when they all propose the same value) or a default value ?. These algorithms are called intrusion-free consensus algorithms. To that end, each consensus algorithm is based on an appropriate underlying broadcast algorithm. One of these abstractions, called validated broadcast is new and allows the design of a resilience-optimal consensus algorithm (i.e., it copes with up to t < n/3 faulty processes where n is the total number of processes). All proposed consensus algorithms assume the underlying system is enriched with additional computational power provided by a binary Byzantine consensus algorithm. The paper presents also a resilience-optimal randomized binary consensus algorithm based on the validated broadcast abstraction. An important feature of all these algorithms lies in the fact that they are signature-free (and hence particularly efficient).01/2010;

Page 1

Bosco: One-Step Byzantine Asynchronous

Consensus?

Yee Jiun Song and Robbert van Renesse

Cornell University, Ithaca, NY 14850, USA

Abstract. Asynchronous Byzantine consensus algorithms are an impor-

tant primitive for building Byzantine fault-tolerant systems. Algorithms

for Byzantine consensus typically require at least two communication

steps for decision; in many systems, this imposes a significant perfor-

mance overhead. In this paper, we show that it is possible to design

Byzantine fault-tolerant consensus algorithms that decide in one mes-

sage latency under contention-free scenarios and still provide strong con-

sistency guarantees when contention occurs. We define two variants of

one-step asynchronous Byzantine consensus and show a lower bound on

the number of processors needed for each. We present a Byzantine con-

sensus algorithm, Bosco, for asynchronous networks that meets these

bounds, even in the face of a strong network adversary.

1 Introduction

Informally, the consensus problem is the task of getting a set of processors to

agree on a common value. This simple primitive can be used to implement atomic

broadcast, replicated state machines, and view synchrony, thus making consensus

an important building block in distributed systems.

Many variants of the consensus problem have been proposed. The differences

between them lie mainly in the failure assumptions and the synchronicity as-

sumptions. In this paper, we are concerned with Byzantine consensus in an

asynchronous environment, i.e., faulty processors can behave in an arbitrary

manner and there are no assumptions about the relative speed of processors nor

about the timely delivery of messages.

Consensus algorithms allow processors to converge on a value by exchanging

messages. Previous results have shown that algorithms that solve asynchronous

Byzantine consensus must have correct executions that require at least two com-

munication steps even in the absence of faults [1], where a single communication

step is defined as a period of time where each processor can i) send messages;

ii) receive messages; and iii) do local computations, in that order. However, this

?The authors were supported by AFRL award FA8750-06-2-0060 (CASTOR), NSF

award 0424422 (TRUST), AFOSR award FA9550-06-1-0244 (AF-TRUST), DHS

award 2006-CS-001-000001 (I3P), as well as by ISF, ISOC, CCR, and Intel Cor-

poration. The views and conclusions herein are those of the authors.

G. Taubenfeld (Ed.): DISC 2008, LNCS 5218, pp. 438–450, 2008.

c ? Springer-Verlag Berlin Heidelberg 2008

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Bosco: One-Step Byzantine Asynchronous Consensus 439

does not mean that such algorithms must always take two or more communica-

tion steps. We show that when there is no contention, it is possible for processors

to decide a value in one communication step.

One-step decisions can improve performance for applications where contention

is rare. Consider a replicated state machine: if a client broadcasts its operation

to all machines, and there is no contention with other clients, then all correct

machines propose the same operation and can respond to the client immediately.

Thus an operation completes in just two message latencies, the same as for a

Remote Procedure Call to an unreplicated service.

Previously such one-step asynchronous consensus algorithms have been pro-

posed for crash failure assumptions [2,3,4,5,6,7]; Friedman et al. proposed a

common coin-based one-step consensus algorithm that tolerates Byzantine fail-

ures and terminates with probability 1 but requires that the network scheduler

has no knowledge of the common coin oracle [8]. In this paper, we consider

one-step algorithms for Byzantine asynchronous consensus in the presence of a

strong network adversary. We define two different notions of one-step Byzantine

asynchronous algorithms and prove a lower bound for the number of processors

that are required for each. Next we show that the lower bounds are tight by

extending the work presented in [2] to handle Byzantine failures, resulting in

Bosco, an algorithm that meets these bounds.

The rest of the paper is organized as follows: Section 2 defines the model and

the Byzantine consensus problem; Section 3 proves lower bounds for the two

versions of one-step Byzantine consensus; Section 4 describes Bosco, a one-step

consensus algorithm; Section 5 discusses some properties of Bosco; Section 6

presents a brief survey of some related work; finally, Section 7 concludes.

2 The Byzantine Consensus Problem

The Byzantine consensus problem was first posed in [9], albeit for a synchronous

environment. In this paper we focus on an asynchronous environment.

In this problem, there is a set of n processors P = {p,q,...} each of which have

an initial value, 0 or 1. An unknown subset T of P contains faulty processors.

These faulty processors may exhibit arbitrary (aka Byzantine) behavior, and

may collude maliciously. Processors in P − T are correct and behave according

to some protocol. Processors communicate with each other by sending messages

via a network. The network is assumed to be fully asynchronous but reliable, that

is, messages may be arbitrarily delayed but between two correct processors, will

be eventually be delivered. Links between processors are private so a Byzantine

processor cannot forge a message from a correct processor.

In addition, we assume a strong network adversary. By this, we mean that the

network is controlled by an adversary that, with full knowledge of the contents

of messages, may choose to arbitrarily delay messages as long as between any

two correct processes, messages are eventually delivered.

Page 3

440Y.J. Song and R. van Renesse

The goal of a Byzantine consensus protocol is to allow all correct processors

to eventually decide some value. Specifically, a protocol that solves Byzantine

consensus must satisfy:

Definition 1. Agreement. If two correct processors decide, then they decide the

same value. Also, if a correct processor decides more than once, it decides the

same value each time.

Definition 2. Unanimity. If all correct processors have the same initial value

v, then a correct processor that decides must decide v.

Definition 3. Validity. If a correct processor decides v, then v was the initial

value of some processor.

Definition 4. Termination. All correct processors must eventually decide.

Note that algorithms that satisfy all of the above requirements are not possible

in asynchronous environments when even a single crash failure must be toler-

ated [10]. In practice, algorithms circumvent this limitation by assuming some

limitation in the extent of asynchrony in the system, or by relaxing the Termina-

tion property to a probabilistic one where all correct processors terminate with

probability 1.

Unanimity requires that the outcome be predetermined when the initial values

of all correct processors are unanimous. A one-step algorithm takes advantage

of such favorable initial conditions to allow correct processors to decide in one

communication step.

We define two notions of one-step protocols:

Definition 5. Strongly one-step. If all correct processors have the same initial

value v, a strongly one-step Byzantine consensus algorithm allows all correct

processors to decide v in one communication step.

Definition 6. Weakly one-step. If there are no faulty processors in the system

and all processors have the same initial value v, a weakly one-step Byzantine

consensus algorithm allows all correct processors to decide v in one communica-

tion step.

While both can decide in one step, strongly one-step algorithms make fewer as-

sumptions about the required conditions and in particular cannot be slowed down

by Byzantine failures when all correct processors have the same initial value.

Strongly one-step algorithms optimize for the case where some processors may

be faulty, but there is no contention among correct processors, and weakly one-

step algorithms optimize for cases that are both contention-free and failure-free.

3 Lower Bounds

We show that a Byzantine consensus algorithm that tolerates t Byzantine fail-

ures among n processors requires n > 7t to be strongly one-step and n > 5t to be

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Bosco: One-Step Byzantine Asynchronous Consensus 441

weakly one-step.1These results are for the best case scenario in which each

correct processor broadcasts its initial value to all other processors in the first

communication step and thus they hold for any algorithm.

3.1 Lower Bound for Strongly One-Step Byzantine Consensus

Lemma 1. A strongly one-step Byzantine consensus algorithm must allow a

correct processor to decide v after receiving the same initial value v from n −2t

processors.

Proof. Assume otherwise, that there exists a run in which a strongly one-step

ByzantinealgorithmAdoesnotallowacorrectprocessorptodecidev afterreceiv-

ing the same initial value v from n−2t processors. Since A is a strongly one-step

algorithm, the fact that processor p does not decide after the first round implies

that some correct processor q has an initial value v?, v??= v. Now consider a sec-

ond run, in which all correct processors do have the same initial value v. Without

blocking, p can wait for messages from at most n − t processors. Among these, t

can be Byzantine and send arbitrary initial values. This means that processor p is

only guaranteed to receive n−2t messages indicating that n−2t processors have

the initial value v. Given that A is a strongly one-step algorithm, p must decide

v at this point. However, from the point of view of p, this second run is indistin-

guishable from the first run. This is a contradiction.

? ?

Theorem 1. Any strongly one-step Byzantine consensus protocol that tolerates

t failures requires at least 7t + 1 processors.

Proof. Assume that there exists a strongly one-step Byzantine consensus algo-

rithm A that tolerates up to t Byzantine faults and requires only 7t processors.

We divide the processors into three groups: G0and G1each contain 3t proces-

sors, of which the correct processors have initial values 0 and 1 respectively; G∗

contain the remaining t processors.

Now consider the following configurations C0and C1. In C0, t of the processors

in G1are Byzantine, and processors in G∗have the initial value 0. Assume that

Byzantine processors act as if they are correct processors with initial value 0

when communicating with processors in G∗, and initial value 1 when communi-

cating with processors not in G∗. Now consider that a correct processor p0∈ G∗

collects messages from n − t processors in the first communication step. Given

that the network adversary controls the order of message delivery, p0 can be

made to receive messages from all processors in G0and G∗, and the t Byzantine

processors in G1. p0 thus receives n − 2t messages indicating that the n − 2t

senders have initial value 0. By Lemma 1, p0must decide 0 after that first com-

munication step. In order to satisfy Agreement, A must ensure that any correct

processor that ever decides in C0decides 0. We say that C0is 0-valent.

In C1, t of the processors in G0 are Byzantine, and processors in G∗ have

the initial value 1. In addition, Byzantine processors act as if they are correct

1These results are for threshold quorum systems, but may be generalized to use

arbitrary quorum systems.

Page 5

442 Y.J. Song and R. van Renesse

processors with initial value 1 when communicating with processors from G∗

and initial value 0 when communicating with processors not in G∗. A correct

processor p1∈ G∗collects messages from n − t in the first communication step.

Suppose that the network adversary chooses to deliver messages from G1 and

G∗, as well as from the t Byzantine processors. Now p1collects n −2t messages

indicating that n−2t senders have initial value 1. By Lemma 1, p1must decide 1

after the first communication step. In order to satisfy Agreement, A must ensure

that any correct processor that ever decides in C1decides 1. We say that C1is

1-valent.

Further assume that for both configurations, messages from any processor in

G∗ to any processor not in G∗ are arbitrarily delayed such that in any asyn-

chronous round, when a processor that is not in G∗ awaits n − t messages, it

receives messages from every processor that is not in G∗. Now, any correct pro-

cess q0 / ∈ G∗executing A in C0will be communicating with 3t processors that

behave as if they are correct processors with initial value 0 and 3t processors that

behave as if they are correct processors with initial value 1. As we have shown

above, C0is a 0-valent configuration, so A must ensure that q0 decides 0, if it

ever decides. Similarly, a correct processor q1 / ∈ G∗executing A in C1will also be

communicating with 3t processors that behave as if they are correct processors

with initial value 0 and 3t processors that behave as if they are correct proces-

sors with initial value 1. However, since we have shown that C1 is a 1-valent

configuration, A must ensure that q1decides 1, even though it sees exactly the

same inputs as q0. This is a contradiction.

? ?

3.2 Lower Bound for Weakly One-Step Byzantine Consensus

We now show the corresponding lower bound for weakly one-step algorithms.

The lower bound for weakly one-step algorithms happens to be identical to that

for two-step algorithms. The bound for two-step algorithms was shown in [11].

We show a corresponding bound for weakly one-step algorithm for completeness,

but note that this is not a new result.

We weaken the requirement on Lemma 1 as follows:

Lemma 2. A weakly one-step Byzantine consensus algorithm must allow a pro-

cessor to decide v after learning that n−t processors have the same initial value v.

Proof. A processor can only wait for messages from n − t processors without

risking having to wait indefinitely. Since a weakly one-step Byzantine consensus

algorithm must decide in one communication step if all correct processors have

the same initial value and there are no Byzantine processors, it must decide if

all of the n − t messages claim the same initial value.

Theorem 2. A weakly one-step Byzantine consensus protocol that tolerates t

failures requires at least 5t + 1 processors.

? ?

Proof. We provide only a sketch of the proof since it is similar to that of The-

orem 1. Proof by contradiction. Assume that a Byzantine consensus algorithm

Page 6

Bosco: One-Step Byzantine Asynchronous Consensus443

A is weakly one-step and requires only 5t processors. We divide the 5t pro-

cessors into three groups, G0, G1, and G∗, containing 2t, 2t, and t processors

respectively. All correct processors in G0have the initial value 0 and all correct

processors in G1have the initial value 1.

As in the proof of Theorem 1, we construct two configurations C0and C1. In

C0, processors in G∗have the initial value 0 and t processors in G1are Byzantine.

Correspondingly, in C1, processors in G∗have the initial value 1 and t processors

in G0are Byzantine. These Byzantine processors behave as they do in the proof

of Theorem 1. It is thus possible for processors in G∗ to decide 0 and 1 in C0

and C1respectively. Therefore, correct processors in G0and G1must not decide

any value other than 0 and 1 respectively. However, if all messages from any

processor in G∗to any processor not in G∗are delayed, then correct processors

in C0and C1see exactly the same inputs. This is a contradiction.

? ?

4Bosco

We now present Bosco (Byzantine One-Step COnsensus), an algorithm that

meets the bounds presented in the previous section. To the best of our knowledge,

Bosco is the first strongly one-step algorithm that solves asynchronous Byzantine

consensus with optimal resilience. The idea behind Bosco is simple, and resembles

the one presented in [2]. We simply extend the results of [2] to handle Byzantine

failures. The Bosco algorithm is shown in Algorithm 1.

Algorithm 1. Bosco: a one-step asynchronous Byzantine consensus algo-

rithm

Input: vp

broadcast ?VOTE,vp? to all processors

wait until n − t VOTE messages have been received

if more than

2

VOTE messages contain the same value v then

3

DECIDE(v)

4

if more than

2

VOTE messages contain the same value v,

5

and there is only one such value v then

6

vp ← v

Underlying-Consensus(vp)

8

1

2

n+3t

n−t

7

Bosco is an asynchronous Byzantine consensus algorithm that satisfies Agree-

ment, Unanimity, Validity, and Termination. Bosco requires n > 3t, where n

is the number of processors in the system, and t is the maximum number of

Byzantine failures that can be tolerated, in order to provide these correctness

properties. In addition, Bosco is weakly one-step when n > 5t and strongly

one-step when n > 7t.

The main idea behind Bosco is that if all processors have the same initial

value, then given enough processors in the system, a correct processor is able to

observe sufficient information to safely decide in the first communication round.

Additional mechanisms ensure that if such an early decision ever happens, all

Page 7

444 Y.J. Song and R. van Renesse

correct processors must either i) early decide the same value; or ii) set their local

estimates to the value that has been decided.

When the algorithm starts, each processor p receives an input value vp, that

is the value that the processor is trying to get decided and the value that it

will use for its local estimate. Each processor broadcasts this initial value in a

VOTE message, and then waits for VOTE messages from n − t processors (likely

including itself). Since at most t processors can fail, votes from n−t processors

will eventually be delivered to each correct processor.

Among the votes that are collected, each processor checks two thresholds: if

more thann+3t

2

of the votes are for some value v, then a processor decides v; if

more thann−t

2

of the votes are for some value v, then a processor sets its local

estimate to v. Each processor then invokes Underlying-Consensus, a protocol

that solves asynchronous Byzantine consensus (satisfies Agreement, Unanimity,

Validity, and Termination), but is not necessarily one-step.

We first prove that Bosco satisfies Agreement, Unanimity, Validity, and Ter-

mination, when n > 3t.

Lemma 3. If two correct processors p and q decide values v and v?in line 4,

then v = v?.

Proof. Assume otherwise, that two correct processors p and q decide values v

and v?in line 4 such that v ?= v?. p and q must have collected more thann+3t

votes for v and v?each. Since there are only n processors in the system, these

two sets of votes share more than3t

senders can be Byzantine, more of

Since a correct processor must send the same vote to all processors (in line 1),

v = v?. This is a contradiction.

2

2common senders. Given that only t of these

t

2of these senders are correct processors.

? ?

Lemma 4. If a correct processor p decides a value v in line 4, then any correct

processor q must set its local estimate to v in line 6.

Proof. Assume otherwise, that a correct processor p decides a value v in line 4,

and a correct processor q does not set its local estimate to v in line 6. Since

processor p decides in line 4, it must have collected more thann+3t

in line 2. Since processor q does not set its local estimate to v in line 6, it must

have collected no more thann−t

2

votes for v, or collected more thann−t

for some value v?, v??= v. For the first case, consider that since there are only

n processors in the system, processor q must have collected votes from at least

n − 2t of the senders that processor p collected from. Among these, more than

n+t

2

sent a vote for v to q. Since at most t of these processors can be Byzantine,

processor q must have received more thann−t

For the second case, if q collects more thann−t

then more than t of these senders must be among those that sent a vote for v to

processor q. This is a contradiction, since, no more than t of the processors in

the system can be Byzantine.

2

votes for v

2

votes

2

votes for v. This is a contradiction.

votes for some value v?, v??= v,

2

? ?

Page 8

Bosco: One-Step Byzantine Asynchronous Consensus 445

Theorem 3. Bosco satisfies Agreement.

Proof. There are two cases to consider. In the first case, no processor collects suf-

ficient votes containing the same value to decide in line 4. This means that all de-

cisions occur in Underlying-Consensus. Since Underlying-Consensus satisfies

Agreement, Bosco satisfies Agreement. In the second case, some correct processor

p decides some value v in line 4. By Lemma 3, any other processor that decides

in line 4 must decide the same value. By Lemma 4, all correct processors must

change their local estimates to v in line 6. Therefore, all correct processors will

invoke Underlying-Consensus with the value v. Since Underlying-Consensus

satisfies Unanimity, all correct processors that decide in Underlying-Consensus

must also decide v.

? ?

Theorem 4. Bosco satisfies Unanimity.

Proof. Proof by contradiction. Suppose a processor p decides v?, but all correct

processors have the same initial value v, v??= v. Since only t Byzantine processors

can broadcast vote messages that contain v ?= v?, no correct processor can collect

sufficient votes to either decide in line 4 or to set its local estimate in line 6.

Therefore, in order for a processor to decide v, Underlying-Consensus must

allow correct processors to decide v even though all correct processors start

Underlying-Consensus with the initial value v?. This is a contradiction since

Underlying-Consensus satisfies Unanimity.

? ?

Theorem 5. Bosco satisfies Validity.

Proof. If a processor decides v in line 4, more thann+3t

more thann+t

2

of these processors are correct and had initial value v. Similarly,

if a processor sets its local estimate to v in line 6, more than

voted v and more thann−3t

2

of these processors are correct and had initial value

v. Combined with the fact that Underlying-Consensus satisfies Validity, Bosco

satisfies Validity.

2

processors voted v and

n−t

2

processors

? ?

Note that satisfying Validity in general in a consensus protocol is non-trivial,

particularly if the range of initial values is large. A thorough examination of

the hardness of satisfying Validity is beyond the scope of this paper; we simply

assume that Underlying-Consensus satisfies Validity for the range of initial

values that it allows.

Theorem 6. Bosco satisfies Termination.

Proof. Since each processor awaits messages from n − t processors in line 2, and

there can only be t failures, line 2 is guaranteed not to block forever. Each proces-

sor will therefore invoke the underlying consensus protocol at some point. There-

fore, Bosco inherits the Termination property of Underlying-Consensus.

? ?

Next, we show that Bosco offers strongly and weakly one-step properties when

n > 7t and n > 5t respectively.

Page 9

446 Y.J. Song and R. van Renesse

Theorem 7. Bosco is Strongly One-Step if n > 7t.

Proof. Assume that all correct processors have the same initial value v. Now

consider any correct processor that collects n − t votes in line 2. At most t of

these votes can be from Byzantine processors and contain values other than v.

Therefore, all correct processors must obtain at least n − 2t votes for v. Since

n > 7t, 2n − 4t > n + 3t. This means that n − 2t >n+3t

processors will collect sufficient votes and decide in line 4.

2

. Therefore, all correct

? ?

Theorem 8. Bosco is Weakly One-Step if n > 5t.

Proof. Assume that there are no failures in the system and that all processors

have the same initial value v. Then any correct processor must collect n−t votes

that contain v in line 2. Given that n > 5t, 2n − 2t > n + 3t. This means that

n − t >

decide in line 4.

n+3t

2. Therefore, all correct processors will collect sufficient votes and

? ?

5 Discussion

One important feature of Bosco, from which it draws its simplicity, is its depen-

dence on an underlying consensus protocol that it invokes as a subroutine. This

allows the specification of Bosco to be free of complicated mechanisms typically

found in consensus protocols to ensure correctness. While it is clear that any

Byzantine fault-tolerant consensus protocol that provides Agreement, Unanim-

ity, Validity, and Termination can be used for the subroutine in Bosco, the FLP

impossibility result [10] states that such a protocol cannot actually exist! Two

common approaches have been used to sidestep the FLP result: assuming par-

tial synchrony or relaxing the termination property to a probabilistic termination

property. Thankfully, such algorithms can be used as subroutines to Bosco, re-

sulting in one-step algorithms that either require partial synchrony assumptions,

or provide probabilistic termination properties (or both). An example of an al-

gorithm that can be used as a subroutine in Bosco is the Ben-Or algorithm [12].

Algorithms that do not provide validity, such as PBFT [13], cannot be used by

Bosco.

While abstracting away the underlying consensus protocol simplifies the speci-

fication and correctness proof of Bosco, for practical purposes it may be advanta-

geous to unroll the subroutine. This potentially allows piggybacking of messages

and improves the efficiency of implementations. As an example, Algorithm 2

shows RS-Bosco, a randomized strongly one-step version of Bosco which does

not depend on any underlying consensus protocol. RS-Bosco is strongly one-step

and requires that n > 7t. It does not satisfy Termination as defined in section 2,

but instead provides Probabilistic Termination:

Definition 7. Probabilistic Termination. All correct processors decide with prob-

ability 1.

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Bosco: One-Step Byzantine Asynchronous Consensus 447

Algorithm 2. RS-Bosco: a randomized strongly one-step asynchronous

Byzantine consensus algorithm

Initialization

xp ← vp

rp ← 0

Round rp

Broadcast ?VOTE,rp,xp? to all processors

Collect n − t ?VOTE,rp,∗? messages

if more than

2

VOTE msgs contain v then

DECIDE(v)

if more than

2

VOTE msgs contain v then

Broadcast ?CANDIDATE,rp,v?

else

Broadcast ?CANDIDATE,rp,⊥?

end

Collect n − t ?CANDIDATE,rp,∗? messages

if at least t + 1 msgs are NOT of the form ?CANDIDATE,rp,xp? then

xp ←RANDOM() // pick randomly from {0,1}

rp ← rp+ 1

1

2

3

4

5

6

n+3t

7

8

n−t

9

10

11

12

13

14

15

16

17

For brevity, the proof of correctness of RS-Bosco is omitted. We note that RS-

Bosco suffers from two limitations as currently constructed. First, RS-Bosco

solves only binary consensus. Second, RS-Bosco uses a local coin to randomly

update local estimates when a threshold of identical votes cannot be obtained.

This mechanism is similar to that in the Ben-Or algorithm and causes the algo-

rithm to require an exponential number of rounds for decision when contention

is present. We believe that these limitations can be overcome in practical imple-

mentations, but a thorough discussion is beyond the scope of this paper.

6 Related Work

One-step consensus algorithms for crash failures have previously been studied.

Brasileiro et al. [2] proposed a general technique for converting any crash-tolerant

consensus algorithm into a crash-tolerant consensus algorithm that terminates

in one communication step if all correct processors have the same initial value.

Bosco is an extension of the ideas presented in that work to handle Byzantine

failures. The key difference between handling crashed failures and Byzantine

failures is that when Byzantine failures need to be tolerated, equivocation must

be handled correctly.

A simple and elegant crash-tolerant consensus algorithm of the same fla-

vor, One-Third-Rule, appears in [4]. This work has been extended to handle

Byzantine faults by considering transmission faults where messages can be cor-

rupted in addition to being dropped [14]. The algorithms in [4,14] differ from the

Page 11

448 Y.J. Song and R. van Renesse

algorithms we have presented because they use a different failure model, where

failures are attributed to message transmissions, rather than to processors.

Friedman et al. [8] proposed a weakly one-step algorithm that tolerates Byzan-

tine faults and terminates with probability 1 but does not tolerate a strong net-

work adversary. In particular, their protocol is dependent on a common coin

oracle and assumes that the network adversary has no access to this common

coin; a strong network adversary with access to the common coin can prevent

termination. In comparison, Bosco does not explicitly depend on any oracles,

although the subroutine invoked by Bosco may have such dependencies. With a

judicious choice of the consensus subroutine, Bosco can tolerate a strong network

adversary that can arbitrarily re-order messages and collude with Byzantine pro-

cessors. In particular, RS-Bosco does not require any oracles and tolerates strong

network adversaries.

Zielinski [15] presents a framework for expressing various consensus proto-

cols using an abstraction called Optimistically Terminating Consensus (OTC).

Among the algorithms constructed by Zielinski are two Byzantine consensus al-

gorithms with one-step characteristics that require n > 5t and n > 3t. The

first of these algorithms is a weakly one-step algorithm that requires partial

synchrony; the second algorithm, while appearing to violate the lower bounds

we have shown in this paper, is neither weakly nor strongly one-step because

processors can only decide in the first communication step when, in addition to

the system being failure-free and contention-free, all processors are fast enough

that the timeout mechanism in the algorithm is not triggered.

Many techniques have been proposed to improve the performance and reduce

the overhead of providing Byzantine fault tolerance. Abd-El-Malek et al. [16]

proposed the optimistic use of quorums rather than agreement protocols to ob-

tain higher throughput. However, in the face of contention, optimistic quorum

systems perform poorly. HQ combines the use of quorums and consensus tech-

niques to provide high performance during normal operation and minimize over-

head during periods of contention [17]. Probabilistic techniques have also been

proposed to reduce the overhead of using quorum systems to provide Byzantine

fault-tolerance [18,19]. Hendricks et al. [20] proposed the use of erasure coding

to minimize the overhead of a Byzantine fault-tolerant storage system. Zyzzyva,

another recently proposed Byzantine fault-tolerant system, uses optimistic spec-

ulation to decrease the latency observed by clients [21]. In comparison, the one-

step Byzantine consensus algorithms presented in this paper aims to improve

performance by exploiting contention-free and failure-free situations to provide

decisions in one communication step.

Lamport [5] presents lower bounds for the number of message delays and the

number of processors needed for several kinds of asynchronous non-Byzantine

consensus algorithm in; in particular, Fast Learning algorithms are one-step

algorithms for non-Byzantine settings. A one-step version of Paxos [22], Fast

Paxos, is presented in [3,6]. Fast Paxos tolerates only crash failures, although [6]

alludes to the possibility of a Byzantine fault-tolerant version of Fast Paxos.

Page 12

Bosco: One-Step Byzantine Asynchronous Consensus 449

7 Conclusion

Byzantine fault tolerance has drawn significant interest from both academia and

the industry recently. While Byzantine fault tolerance aims to provide resilience

against arbitrary failures, in many applications, failures and contention are not

the norm. This paper explores optimization opportunities in contention-free and

failure-free situations.

Overall, this paper makes three contributions: 1) we provide two definitions of

one-step asynchronous Byzantine consensus algorithms that provide low latency

performance in favorable conditions while guaranteeing strong consistency when

failures and contention occur; 2) we prove lower bounds in the number of proces-

sors required for such algorithms; and 3) we present Bosco, a one-step algorithm

for Byzantine asynchronous consensus that meets these bounds.

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