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Introduction to Distributed Algorithms

Gerard Tel

Chapter 3

Communication Protocols - all with Video Answers

Educators

Chapter Questions

Problem 1

Show that the balanced shding-window protocol does not satisfy the eventual delivery requtrement if, of the fairness assumptions F1 and F2, orly F2 holds.

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09:10

Problem 2

Prove that if $L=1$ in the balanced sliding-window protocol and $a_p$ and $a_q$ are initialized to $-l_q$ and $-l_p$, then $a_p+l_q=s_p$ and $a_q+b_p=s_q$ always hold.

Sandip Ranjan
Sandip Ranjan
Numerade Educator

Problem 3

In the trmer-based protocol the sender may report a word as possibly lost, when in fact the word was delivered correctly by the receiver.
(1) Describe an execution of the protocol where this phenomenon occurs.
(2) Is it possible to design a protocol in which the sender generates an error report within a bounded time, if and only if the word us not delivered by the receiver?

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01:03

Problem 4

Assutue that, due to a falling clock device, the recenver may farl to close its connection in time. Describe a computation of the timer-based protocol in which a word is lost without being reported by the sender.

James Kiss
James Kiss
Numerade Educator

Problem 5

Describe a computation of the timer-based protocol in which the receiver opens a connection upon recespt of a packet wath a sequence number greater than zero.

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Problem 6

The Time- $\epsilon$ action does not model drift in the remanning packet lifetime of packets. Why not?

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Problem 7

Prove Theorem 3.16.

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Problem 8

A network engineer wants to use the tumer-based protocol, but wants to allow possibly lost words to be reported earlier with the following modification of $\mathbf{E}_p$.
$$
\begin{aligned}
\mathrm{E}_{\mathrm{p}}: & \left({ }^*\right. \text { Generate error report for possibly lost word *) } \\
& \{U t[B+\text { Low }]<0\} \\
& \text { begin error }[B+\text { Low }]:=\text { true } ; \text { Low }:=\text { Low }+1 \text { end }
\end{aligned}
$$
Does the protocol thus modified still sat?sfy the No Loss and Ordering requirements or must other changes be made as well?
Give an advantage and a disadvantage of this modification.

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