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Fast Broadcast in High-Speed Networks. Ajei Gopal, Inder Gopal and Shay Kutten. Presented By Nuthan M. Sarpangala Divya Rajesh Urmil Shah. Why Flooding protocols are unsuitable for High-Speed Networks ?. Each node - Maintains a software history of prior messages it received.

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fast broadcast in high speed networks

Fast Broadcast in High-Speed Networks

Ajei Gopal, Inder Gopal and Shay Kutten

Presented By

Nuthan M. Sarpangala

Divya Rajesh

Urmil Shah

slide2

Why Flooding protocols are unsuitable for High-Speed Networks ?

  • Each node -
  • Maintains a software history of prior messages it received.
  • Performs a software search to determine whether or not to forward the message it received.

-With increasing network speeds, software search becomes a bottleneck.

Fast Broadcast in High-Speed Networks

getting around the bottleneck
Getting around the bottleneck
  • Implement the search directly in very high-speed hardware.
  • Devise a new broadcast protocol that does not require a software search.

Fast Broadcast in High-Speed Networks

objectives
Objectives
  • Design fast distributed Broadcast protocols for a
  • high-speed arbitrary-topology point to point network which
  • Broadcasts control messages at hardware speed.
  • Is tolerant of failure involving loss of message.

Fast Broadcast in High-Speed Networks

problem rationale
Problem Rationale
  • Links are typically reliable.
  • Nodes have instantaneous knowledge of the state and loading conditions of links in the network.
  • Events like link failure, recovery & congestion occur relatively infrequently.
  • Dissemination time is critical in ensuring a stable network.

Fast Broadcast in High-Speed Networks

slide6

Assumptions

  • No conflicting Broadcasts.
  • Upper bound on the link delay is τ.
  • Failures are due to transient network conditions and not due to permanent removal of a node or link.
  • Links are FIFO and cannot generate or reorder messages.

Fast Broadcast in High-Speed Networks

slide7

Problem Formalization

  • Reliable Broadcasts: Ensures that all messages are delivered reliably and that the message traffic caused by every broadcast rapidly quiesces even under circumstances of failure.
  • Control Broadcasts: Weaker than reliable broadcasting, as it ensures reliable message delivery only in the absence of failure.

Fast Broadcast in High-Speed Networks

slide8
Reliable Broadcast
  • Reliable-Delivery: If a node broadcasts a message m at some time t, then all nodes deliver m by time t + L, where L is a constant called the delivery bound.
  • Quiescence: If m is the last message broadcast, m is broadcast at time t, then no node sends a message after time t + Q, where Q is a constant called the quiescence bound.

Fast Broadcast in High-Speed Networks

control broadcast
Control Broadcast
  • Delivery: If a node broadcasts a message m at some time t, and there are no failures between t and t + L, then all nodes deliver m by time t + L, where L is a constant called the delivery bound.
  • It also satisfies the quiescence property under all conditions.
  • If delivery is critical to a higher layer, it is ensured by a higher layer protocol.

Fast Broadcast in High-Speed Networks

slide10

Network Model

Node U

Node V

Network Control Unit - NCUU

Network Control Unit - NCUV

Switching Subsystem - SSU

Switching Subsystem - SS V

Bi-directional Communication Links

Fast Broadcast in High-Speed Networks

what do ss ncu do
What do SS & NCU do?

SS

  • Message forward function is implemented in hardware.

When SSu forwards m at time t, it does the following:

    • For all nodes v ε Neighbor(u), SSu sends m to SSv at time t.
    • SSusends m to NCUu at time t.
  • SS maintains a hardware clock and a boolean register state

that can take two values – normal or exception

NCU

  • On a request from higher level S/W to broadcast it send m to SS.
  • On receipt of m from SS delivers it to higher level S/W

Fast Broadcast in High-Speed Networks

slide12

Higher-level

Software

Network Control Unit (NCU)

Broadcast a message

Deliver a message

Switching Subsystem (SS)

Communication within a node and between nodes

Fast Broadcast in High-Speed Networks

protocols
Protocols
  • BASIC Protocol: -Hardware implemented
  • -Ensures rapid delivery and quiescence in absence of failure.
  • TIMER Protocol: A Control Broadcast that
    • - behaves like BASIC in absence of failure
    • - rapidly quiesces under conditions of failure.
  • RESPONSE Protocol: A Reliable Broadcast that guarantees
    • Quiescence property in case of failure & conflicting broadcasts
    • Ensures rapid delivery in all cases, provided network is connected.

Fast Broadcast in High-Speed Networks

slide14

Protocol BASIC

Part executed by NCU

To broadcast a message m: NCU sends m to SS

On receipt of a message m´: NCU delivers m´

Part executed by SS

On receipt of broadcast message m and state = normal

SS forwards m

state := exception

SS sets timeout to expire in 2 time

On expiration of the timeout

state := normal

Fast Broadcast in High-Speed Networks

slide15

v1

v2

v3

v4

0

2

3

4

5

6

7

8

9

10

Fast Broadcast in High-Speed Networks

slide16

NOTE:

  • Under failure free conditions protocol BASIC works because it ensures a wave of exception state nodes is created from the source outwards, preventing a broadcast message from looping back and reentering a previously visited node.
  • In the non terminating broadcast , SS forwards m a second time breaking the “wave” of exception state nodes, and a broadcast message is permitted to re-enter a previously visited node
  • If each node stays in the exception state for long enough - until every one of its neighbors has entered the exception state - the “wave” is preserved and the broadcast quiesces.
  • Protocol TIMER and Protocol RESPONSE both implement this intuition.

Fast Broadcast in High-Speed Networks

slide17

Lemma 3:

Protocol BASIC does not ensure the quiescence property when processes may fail

u4

u2

u1

u3

Fast Broadcast in High-Speed Networks

slide18

Lemma1:

Suppose there are no failures and no conflicting broadcasts, Protocol BASIC guarantees that no node forwards the same message twice

Theorem 2:

Suppose there are no failures and no conflicting broadcasts, Protocol BASIC guarantees both quiescence and delivery within (D+2) time

Fast Broadcast in High-Speed Networks

slide19

Protocol TIMER

  • Addresses the problem of non terminating broadcasts by forcing each node to remain in the exception state for a period of time that is larger than c where c is the number of nodes in the largest simple cycle that contains the node
  • c depends on a specific execution and hence is a priori unknown to each node.
  • Problem: node should find a appropriate approximation to c.
  • optimal solution :
    • first guess 2 and let each subsequent guess be twice as large as the previous one, for a total cost less than 2c units (in log c guesses)

Fast Broadcast in High-Speed Networks

slide20

Part executed by NCU

To broadcast a message m:

NCU sends (m,t) to SS, where t is the current clock value

On receipt of a message (m,t*):

T = {t´/(m,t´) was received earlier}

if (T = )

NCU delivers m

set a timeout to expire in the SS at time(t* +2)

else

t´ = max{T}

set a timeout to expire in the SS at time (t* +2(t*-t´))

Fast Broadcast in High-Speed Networks

slide21

Part executed by SS

On receipt of broadcast message(m,-)at t and (state = normal):

SS forwards (m,t)

state := exception

On expiration of the timeout:

state := normal

Fast Broadcast in High-Speed Networks

slide22

Lemma 4:

Suppose SS forwards m for the ith time, i>0 at time t. Then SS enters the exception state at time t and reverts to the normal state no sooner than t+2

Theorem 5:

Suppose there are no failures and no conflicting broadcasts, Protocol TIMER guarantees both quiescence and delivery within(D+2) time.

Fast Broadcast in High-Speed Networks

slide23

Theorem 6:

Protocol TIMER always quiesces

Fast Broadcast in High-Speed Networks

slide24

v1

v2

v3

v4

0

2

3

4

5

6

7

8

9

10

Fast Broadcast in High-Speed Networks

slide25

PERFORMANCE OF TIMER

Lemma 7: For any broadcast, v  V such that:

1) The maximum number of times this node is visited during the broadcast is O (log2(n));

2) The maximum possible time interval between the first visit by the broadcast to that node and the end of the broadcast is O(nτ)

Lemma 11: The maximum number of times a broadcast may visit a particular node is O(nlogn).The time for the longest broadcast is

O(nτlogn)

Fast Broadcast in High-Speed Networks

slide26

Conflicting Broadcasts:

Protocol TIMER cannot handle conflicting broadcasts. The concept of protocol Timer is extended to handle conflicting broadcasts as follows:

1. Each node maintains a set of Boolean Control Registers, ‘state-set’ instead of single register and it recognizes a set of message headers ‘fwd-set’ where |state-set| = |fwd-set|

Thus by associating fwd[i] with the ith element of the fwd-set with state[i], permitting up to |state-set| conflicting broadcasts.

2. There are some context bits stored in the SS and in the header of the message ‘m’.

Fast Broadcast in High-Speed Networks

slide27

cont.

  • Now when the SS is in ‘Exception’ state,and it receives a new message, it checks for the context bits.
  • If the context bits were different then it would alert the software.
  • SS instead of rejecting this message it forwards it to the NCU
  • NCU checks whether this was a duplicate message or it is a new broadcast and instructs the SS to forward or rebroadcast in the case of a new message.
  • Thus duplicate message is never broadcast again.

Fast Broadcast in High-Speed Networks

slide28

FUNCTION Of NCU:

To broadcast a message m: NCUu sends m to SSu

On First Receipt of a message m\'

State’u’:= Exception

NCUu delivers m\'

NCUu sends m\' to all neighbors using a reliable data link protocol

On receipt of a message m\' from NCUv, v Neighbour(u);

if for all w Neighbour(u), NCUu received m\' from NCUw by

the data link protocol

State’u’:= Normal

Fast Broadcast in High-Speed Networks

slide29

Function of SS:

On receipt of broadcast message m and state’u’= Normal:

SSu forwards m

State’u’ := Exception

Fast Broadcast in High-Speed Networks

slide30

NODE

NCU

SS

Fast Broadcast in High-Speed Networks

slide31

PROTOCOL RESPONSE

  • More Reliable protocol.It guarantees quiescence in the case of failure or conflicting broadcasts.
  • It has a stronger model than the protocol BASIC and TIMER
  • NCU has more CONTROL on the State Registers in this protocol and dictates when the timer expires and state should change.
  • When the SSu first receives a message m from its neighbor and it is in the normal state , then it FORWARDS ‘m’ to its neighbors and also to its NCUu.

Fast Broadcast in High-Speed Networks

slide32

Cont.

  • when NCUu receives the first copy of the broadcast message it too sends a copy of this to all its neighbors using a reliable data link protocol.
  • NCUu now waits for all its neighbors to give the copy of the message it had previously sent.
  • Here all the nodes sends the RECEIPT of ‘m’ instead of the complete message they received.
  • Only when the NCUu does not receive a RECEIPT from a particular node in a given TIME FRAME, it sends a copy of ‘m’ to that PARTICULAR NODE ONLY.
  • When it RECEIVES all the RECEIPT then it tells the SSu to go in a ‘normal’ State.

Fast Broadcast in High-Speed Networks

slide33

Cont.

  • Here the SSu forwards the message ‘m’ at hardware speed and the involvement of NCU comes in picture after the message has been forwarded.
  • Thus the primary objective of sending message at hardware speed is accomplished. Although there is an overhead required during normal flooding algorithm.
  • Theorem 14: If there are no failures and no conflicting broadcasts, protocol RESPONSE guarantees delivery within (D+2)τ time

Fast Broadcast in High-Speed Networks

slide34

CONCLUSION:

  • Protocol BASIC:
  • guarantees quiescence and delivery under failure free conditions
  • Protocol TIMER:
  • guarantees quiescence and delivery under failure free conditions
  • guarantees quiescence under failure conditions
  • handles non-terminating broadcast.
  • Both the above protocols can not handle conflicting broadcasts.

Fast Broadcast in High-Speed Networks

slide35

CONCLUSION:

  • Protocol RESPONSE:
  • It has a more stronger model as compared to BASIC and TIMER
  • It guarantees quiescence and delivery in failure conditions and even in conflicting broadcasts.

Fast Broadcast in High-Speed Networks

slide36

LIMITATIONS:

* Many ASSUMPTIONS at the start make this as a weak model and is not practical to implement in the present network.

* For Protocol RESPONSE, though the paper mentions about dealing with the conflicting broadcasts, it has not explained how it deals with such broadcasts.

* After all such models, RESPONSE protocol looks good and stronger but then again we come to the point from where we started.

- NCU has to maintain the history of all its neighboring nodes.

Fast Broadcast in High-Speed Networks

slide37

- It has to set an individual timer for all the nodes to which it is sending the message, so after time out if it does not receive a copy it will transmit again

-NCU has to process all the details and this involves lot of time and overhead.

Fast Broadcast in High-Speed Networks

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