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CS603 Clock Synchronization. February 4, 2002. What is the best we can do? Lundelius and Lynch ‘84. Assumptions: No failures No drift Fully connected network of n nodes Uncertainty of ε in message delivery time Best guarantee: ε (1 – 1/ n ) This is a tight lower bound.

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what is the best we can do lundelius and lynch 84
What is the best we can do?Lundelius and Lynch ‘84
  • Assumptions:
    • No failures
    • No drift
    • Fully connected network of n nodes
    • Uncertainty of ε in message delivery time
  • Best guarantee:
    • ε(1 – 1/n)
    • This is a tight lower bound
lower bound proof
Lower bound proof
  • Idea: Based on view of each node
    • Views indistinguishable even if real time not the same
    • Shift execution of a node relative to real time
  • Shift of global view and local view equivalent if message delays changed
    • Can always shift by at least ε(1 – 1/n) without changing local views
proof induction
Proof: Induction
  • Clocks synchronized to within γ
  • Assume messages one way take time μ, return takes time μ+ε (e1)
  • Induction: Assume node i-1 sends with delay μ, receives with delay μ+ε
    • Shift processes < i by ε
  • Let V1,…,Vn be local times at termination of e1.
    • In e1, Vn ≤ V1 + γ
    • In ei, Vi-1 ≤ Vi + y – ε
  • ∑ Vi ≤ ∑ Vi+nγ – (n-1) ε
    • (n-1) nγ
    • γ ≥ ε(1-1/n)
synchronization with faulty clocks dolev halpern strong 84
Synchronization with Faulty Clocks(Dolev, Halpern, Strong ‘84)
  • Problem: What if some sites are really bad?
    • Bad clocks
    • Don’t follow protocol
  • Notation
    • C: Logical clock
    • D: Physical clock
    • TAR: Time Adjustment Register
      • C = D + TAR
    • Δ: Uncertainty in message delay
    • C(t), D(t) – value of clock at REAL time t
assumptions
Assumptions
  • Fully connected, but not necessarily complete
  • Recipient knows source of message
  • Given nodes p,q; H(p,q) and L(p,q) are upper/lower bounds on transmission time
    • ρ is min(H/L)
  • A real time frame (not directly observable)
  • Correct physical clock has bounded drift rate: R such that time u>v, (1/R)(u-v) ≤ D(u)-D(v) ≤ R(U-v)
  • Correct processor has correct clock, implements algorithm
  • No assumptions on behavior of faulty processor
    • Don’t care if faulty processor knows correct time
  • All processors start within time B (can easily show B ≤ R(n-1)H)
weak synchronization
Weak Synchronization
  • Weak Clock Synchronization Condition: Constants PER, DMAX, ADJ such that:
    • TAR changes only at times that are multiples of PER by amount less than ADJ
    • Difference between clocks bounded by DMAX
  • Theorem: There is an algorithm that achieves WCSC, independent of faults, for which C(t) is unbounded
  • Proof: Set TAR(t’) = logPER(D(t))-D(t)
real clock synchronization
Real clock synchronization
  • Clock Synchronization Condition: Add
    • PER > ADJ
    • Changes occur only first time C reads iPER
      • If change when C(t)=iPER, then C(t’) ≠ iPER  t’<t
  • Gives Linear Envelope Synchronization:
    • at+b < C(t) < ct+d, a>0
  • Theorem:Linear Envelope Synchronization impossible if  1/3 processors faulty
proof sketch
Proof Sketch
  • Construct algorithm that forces a correct processor to run at rate greater than aρn
  • Idea: faulty processor p uses one algorithm for processor q, other for others
    • Two-faced behavior
    • Can’t tell which is two-faced
    • Correct processor caught in the middle – follow fast clock or slow clock?
three processor case p q r
Three-processor case (p, q, r)
  • Assume algorithm A synchronizes in time N and tolerates one fault
  • F0 = A
  • Fm+1: p pretends its clock runs at ρ times q’s rate
  • p pretends r sends messages so Cp(t) > aρmDp(t)+b-mDMAX
    • Fm gives these messages
  • q cannot distinguish from case where p’s clock is fast, r is sending p messages according to Fm
  • Cq(t) > Cp(t) – DMAX > aρmDp(t) + b – (m+1) DMAX = aρm+1Dq(t)+b-(m+1) DMAX (since Dp(t) = ρDq(t)
possibility fischer lynch merritt
Possibility(Fischer, Lynch, Merritt)
  • If no uncertainty in message delay, f faulty, can do with 2f+1 processors
    • Send messages to all neighbors
    • Send all messages back
    • Round trip gives time
    • Faulty processor will be detected if it tries to be worse than round-trip time
      • Messages out of order
possibility dolev halpern simons strong
Possibility(Dolev Halpern Simons Strong)
  • We CAN do better
    • Requires authentication
  • Assumptions:
    • Messages will be received with bounded delay
    • Bounded drift
    • Digital signature
    • If p has set of messages M at time t with more than f distinct signers, one signer was correct at time signed
    • 2ρ(f+1) < 1
  • Key: Synchronization time known in advance
    • At time, send signed “time is now”
    • If receive f+1 messages saying “time is now” before getting to that time, update local time
recruiting bulletin
Recruiting Bulletin
  • Harris Corporation is in the CS lobby until 3pm today
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