Question 1 (4 marks)

a) What is the TCP port number used by your computer to transfer the file?

Source port: 63548

b) What is the TCP port number used by the server to receive the file?

Destination port: 80

Question 2 (4 marks)

a) What is the sequence number of the TCP SYN segment that establishes the TCP connection

between your computer and the server?

SN: 0

b) What indicates that the segment is a SYN segment?

Only SYN flag is set.

Question 3 (6 marks)

a) What is the sequence number of the SYN/ACK segment that the server responds the SYN with?

SN: 0

b) How about its acknowledgement number? How does the server determine this value?

ACK: 1. This value is determined by the sequence number of SYN segment + 1.

c) What indicates that the segment is a SYN/ACK segment?

ACK and SYN flag are set.

Question 4 (4 marks)

a) What are the sequence numbers of the data-containing segments in the TCP connection related

to the file transfer?

Length > 0 for data-containing segments.

Sequence numbers: 1, 1461, 2921, 4381, 5841, 7301, 8761, 10221

b) What is the length of each of them?

SN 1 1461 2921 4381 5841 7301 8761 10221

length 1460 1460 1460 1460 1460 1460 1460 454

Question 5 (2 marks)

What is the RTT of each of the data-containing segments?

SN 1 1461 2921 4381 5841 7301 8761 10221

RTT 0.000328 0.000546 0.000245 0.000254 0.000239 0.000243 0.000221 0.000221

Question 6 (6 marks)

a) What is the payload size of the TCP data packet sent from the server?

137 bytes

b) What is the TCP/IP overhead?

TCP/IP overhead = (TCP header size + IP header size)/payload size = (20 +20)/137 = 29%

(corrected to nearest integer)

c) Assume the sizes of the TCP and IP headers remain unchanged. How much more TCP/IP data

would the server need to transmit if piggybacking is not used?

40 bytes. As it will send one more pure-ACK packet, 20 bytes on IP header, 20 bytes on TCP

header.

Question 7 (6 marks)

a) How many TCP RST packets are injected by the GFW?

4 RST packets are injected.

b) What is the difference of the sequence numbers between each subsequent TCP RST packet?

What is special about this number in TCP?

The difference is 1460. It is a common maximum segment size (MSS) specifies the largest

amount of data a packet can carry.

c) Why does the GFW inject TCP RST packets with these specific sequence numbers?

GFW injects serval RST packets because if the undesired server responds faster than GFW, the

RST packets from GFW with lower sequence number will be ignored. To maximize the sequence

number of GFW RST packets, 1460, a usual MSS, is added to overcome the packets from

undesired server. It increases the blocking chance of every packet and overall connection.