Birman Schiper Stephenson Protocol

Last Updated : 10 Oct, 2022

This algorithm is used to maintain the causal ordering of the messages i.e. the message which is sent first should be received first. If send (M1)-->send(M2) then for all processes which receive the messages M1 and M2 should receive M1 before M2. Features :

Broadcast based messaging.
Size of the messages are small.
More no. of messages are sent.
Limited state information.

Key Points :

Each process increases its vector clock by 1 upon sending of messages.
Message is delivered to a process if the process has received all the messages preceding to it.
Else buffer the message.
Update the vector clock for the process.

Reference :

Process: P_i
Event: e_ij, where i:process is number & j: jth event in ith process.
T_m:vector time stamp for message m.
C_i vector clock associated with process P_i; j^th element is C_i[j] and contains P_i's latest value for the current time in process P_j

Protocol : P_i sending a message to P_j -

P_i increments C_i[i] and sets the timestamp t_m = C_i[i] for message m.

P_j receiving a message from P_i -

When P_j, j != i, receives m with timestamp t_m, it delays the message's delivery until both these conditions are met:

C_j[i] = tm[i] - 1; and  for all k <= n and k != i, Cj[k] >= tm[k].

When the message is delivered to P_j, update P_j's vector clock.
Check buffer to see if any can be delivered.

Example -

Initial state for all the processes are 000.
M1 is broadcasted from P3 to P1 and P2. e31 updates the vector clock to (001) and sends P1 and P2.
P2 accepts the M1 with timestamp (001) because when it compares it with its initial timestamp i.e. (000) it finds that M1 is the 1st message it is receiving.
Now we consider that before M1 could reach P1, P2 sends M2 to P1 and P3 with time stamp (011).
P1 could not accept M2 because upon comparing the timestamp of M2 with its initial timestamp a discrepancy is found because P1 has no message with timestamp (001) received earlier, so M2 is stored in buffer.
Now M1 is received by P1 and accepted.
M2 is removed from buffer and accepted by P1.
M2 is accepted by P3 as there is no discrepancy in the time stamp.

Birman Schiper Stephenson Protocol

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Birman Schiper Stephenson Protocol

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