8/13/2019 EX_4a

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Problem 1

Consider a robust ECN-capable TCP connection to transmit a file from sender S to

receiver R. The size of the file is 16 segments (packets), numbered 1 through 16.

Assume the unit of time is one round trip time and the transmission starts at time t=1

(i.e., t=1 is the beginning of the 1st round trip). The receiver is a cheating receiver and is

only interested in reaching quickly to the end of the file so that it can send a FIN signal

to close the connection. The receiver always cheats and falsely acknowledges the receipt

response to CE (congested experienced) notification . The transmission in the first three

round trips (with cwnd = 1, 2, and 4 segments, respectively) went smoothly without any

segment loss. In the fourth round trip (cwnd = 8), the last three segments of the window

(i.e., segments 13, 14, an 15) were ost ( roppe y a non-ECN router) T e

punishment policy adopted by the sender for cheating receivers is as follows. When the

sender discovers a wrong nonce sum, it delays sending segments for two round trips

then resumes retransmission in the third round tri startin with the se ment associated

with the wrong nonce sum, i.e., the sender assumes the segment is lost and retransmits

it. Furthermore, the sender freezes the value of cwnd at 1 until the end of the file.

Assume that after the loss of segments 13, 14 and 15 in the fourth round trip, the

rans er o e e wen smoo y w ou any ur er osses.

8/13/2019 EX_4a

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Problem 1

a) What is the minimum (earliest) time at which the sender closes

the connection after receiving correct ACKs for all 16

segmen s a s e pro a y a s m n mum me s

achieved? Assume the connection started at time t=1.

b What is the maximum worst case time at which the sender

closes the connection after receiving correct ACKs for all 16

segments? What is the probability that this maximum time

c) What is the average time at which the sender closes the

connection after receiving correct ACKs for all 16 segments?