Relia eliable ble Broadc oadcas ast t (et (et al.) al.) Hoa - - PowerPoint PPT Presentation

NETWORK EMBEDDED SYSTEMS

Relia eliable ble Broadc

• adcas

ast t (et (et al.) al.)

Hoa Do

OVERVIEW

Motivation and Definition Best Effort Broadcast Regular Reliable Broadcast

 Lazy Reliable Broadcast  Eager Reliable Broadcast

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MOTIVATION

Client-Server Computing Point-to point communication protocols Extremely useful is protocol is reliable

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DEFINITION

Reliability

in context of broadcast means, that “Message exchanged between two ore more processors are not lost or duplicated, and are delivered in the order in which they were sent.”

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DEFINITION

Broadcast

“The sending of one transmission to multiple users in a defined group”

 Best-effort: delivery among all correct processes if sender does not fail  Regular Reliable: all-or-nothing delivery semantics, even if sender fails  Uniform Reliable: set of message delivered by fault processes is always

a subset of messages delivered by correct processes

 Totally ordered: delivery message follows the same global order  Termination: processes either deliver a message or eventually are

aware that they should never deliver the message.

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DEFINITION

Multicast

“Multicast is a subset of broadcast that extends the broadcast concept of one to many by allowing "the sending of one transmission to many users in a defined group, but not necessarily to all users in that group."

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BEST EFFORT BROADCAST

 Weak form of reliability  Process sends a message to all processes

(including itself)

 One-shot operation  Reliability is in charge of sender  Remaining processes do not have to be concerned

with enforcing the reliability of received messages

 No delivery guarantees if sender fails  Uses perfect links

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BEST EFFORT BROADCAST

Implementation of BestEffortBroadcast (beb):

 Events:  Request:

 < bebBroadcast | m >: Used to broadcast message m to all

processes

 Indication

 < bebDeliver| src, m >: Used to deliver message m broadcast

by process src

 Uses:  PerfectPointToPointLinks (pp2p)

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BEST EFFORT BROADCAST

Pseudo Code Implementation:

upon event < bebBroadcast | m > do forall pi  ∏ do trigger < pp2pSend | pi,m >;

upon event < pp2pDeliver | pi,m > do

trigger < bebDeliver| pi, m >:

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BEST EFFORT BROADCAST

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BEST EFFORT BROADCAST

PROPERTIES

 BEB1. Validity: If pi and pj are correct, then every

message broadcast by pi is eventually delivered by pj

 BEB2. No duplication: No message is delivered more

than once

 BEB3. No creation: No message is delivered unless it

was broadcast

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BEST EFFORT BROADCAST

Proofs:

 Correctness:

derived from the properties of the underlying perfect point-to- point links (PL1: Reliable delivery)

 Validity

derived from PL1 and the fact that the sender sends the message to every other process in the system

 No Duplication

derived from PL2: No duplication

 No Creation

derived from PL3: No creation

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BEST EFFORT BROADCAST

Performance:

Requires a single communication step Exchange of N messages (where N is the nummer of processes)

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REGULAR RELIABLE BROADCAST

 Stronger form of reliability  Agreement is ensured even if sender fails  Correct processes agree on the set of messages

they deliver, even when the senders of the message crash during the transmission

 Sender may fail before being able to transmit the

message → no process will deliver it

 Uses perfect links  Uses perfect failure detector (optional)

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REGULAR RELIABLE BROADCAST

Implementation of (regular) ReliableBroadcast (rb)

 Events:  Request:

 < rbBroadcast | m >: Used to broadcast message m

 Indication

 < rbDeliver| src, m >: Used to deliver message m broadcast by

process src

 Uses  BestEffortBroadcast (beb)

(only Lazy Reliable Broadcast:

 PerfectFailureDetector  )

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REGULAR RELIABLE BROADCAST

2 Implementations:

 Lazy Reliable Broadcast  Eager Reliable Broadcast

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LAZY RELIABLE BROADCAST

 Uses Best Effort Broadcast to disseminate

message

 Adds some protocol headers to the message

exchanged

 If sender does not crash, message will be delivered

to all correct processes.

 If sender crashes, other process can detect that

crash and relays the message to all

 Relays only a copy of the message and not the

message itself

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LAZY RELIABLE BROADCAST

 Algorithm is said to be lazy in the sense that it

retransmits a message only if the original sender has been detected to have crashed Two kinds of events can force a process to retransmit a message:

 Process detects the crash of the source  Process delivers a message and realizes that the

source has already been detected to have crashed

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LAZY RELIABLE BROADCAST

Implementation of (Lazy) ReliableBroadcast (rb):

upon event < Init > do: delivered := Ø; correct := ∏; forall pi  ∏ do from [pi] := Ø; upon event < rbBroadcast | m > do trigger < bebBroadcast | [DATA, self, m] >; upon event < bebDeliver | pi, [DATA, sm, m] > do if ( m deliver ) then deliver := deliver  {m}; trigger < rbDeliver | sm,m >; from [pi] := from [pi]  {(sm,m )}; if (pi  correct) then trigger < bebBroadcast | [DATA, sm, m] >; upon event <crash | pi > do correct := correct \ {pi}; forall (sm,m)  from [pi] do trigger <bebBroadcast | [DATA, sm, m] >;

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LAZY RELIABLE BROADCAST

PROPERTIES

 RB1 = BEB1. Validity: If pi and pj are correct, then every

message broadcast by pi is eventually delivered by pj

 RB2 = BEB2. No duplication: No message is delivered more

than once

 RB3 = BEB3. No creation: No message is delivered unless it

was broadcast

 RB4. Agreement: For any message m, if a correct

process delivers m, then every correct process delivers m

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LAZY RELIABLE BROADCAST

Performance:

 If sender does not crash, the algorithm requires

a single communication step and N messages

 Otherwise N2 messages and in the worst case

(process crashes in sequence) N steps are required

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EAGER RELIABLE BROADCAST

 If accuracy property of the failure detector is not

satisfied, process might be relaying messages when it is not really necessary → Waste of resource but does not impact correctness

 If completeness property of the failure detector is

not satisfied, process might not be relaying messages that they should be relaying → Might violate agreement Circumvent the need of a failure detector by adopting an eager scheme

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EAGER RELIABLE BROADCAST

 Every process that gets a message relays it

immediately

 We consider the worst case → sender might have

crashed → we relay every message

 This relaying phase is exactly what guarantees the

agreement property of reliable broadcast. The resulting algorithm is called Eager Reliable Broadcast.

 Algorithm assumes fail silent model and does not

use any failure detector.

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EAGER RELIABLE BROADCAST

Implementation of (Eager) ReliableBroadcast (rb): upon event < Init > do: delivered := Ø; upon event < rbBroadcast | m > do deliver := deliver  {m}; trigger < rbDeliver | self, m >; trigger <bebBroadcast | [DATA, self, m] >; upon event < bebDeliver | pi, [DATA, self, m] > do if m deliver do deliver := deliver  {m}; trigger <rbDeliver | sm, m] >; trigger < bebBroadcast | [DATA, sm, m] >;

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EAGER RELIABLE BROADCAST PROPERTIES

 RB1 = BEB1.  RB2 = BEB2.  RB3 = BEB3.  RB4: Agreement: For any message m, if a correct process

delivers m, then every correct process delivers m

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EAGER RELIABLE BROADCAST PROPERTIES

Performance:

In best case algorithm requires a single communication step and N2 messages In worst case (process crashes in sequence) N steps and N2 messages are required

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COMPARISON

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 Reliability in charge if

sender

 only ensures delivery

among all correct processors

 Uses perfect links  All-or-nothing semantic  Agreement is ensured

even if the sender fails

 Uses perfect failure

detector

Best Effort Broadcast Regular Reliable Broadcast

THE END

Thank you for your attention! Questions?

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