Consider the following arrangement of hosts A and B and routers R1-R2. fast sloo
ID: 3603507 • Letter: C
Question
Consider the following arrangement of hosts A and B and routers R1-R2.
fast slooow slooow
A------R1--------R2--------B
The A-R1 link is instantaneous. The R1-R2 and R2-B links, however, have low bandwidth, so all data packets take 1 second to go each hop (2 sec total from A to B). ACKs are small, however, so they go from B to A instantaneously. Only one packet can be sent on each link in each direction at a time, although the two directions work independently. Suppose A uses a sliding window protocol, with window size 5 packets, to send 9 packets to B. State what is sent and received by A each second, from T=0 until the 9 packets are sent. Also state what packets are queued at R1; do not count the packet currently transmitting. Nothing should ever be queued at R2. Label the packets D1, D2, ... D9, Ack1, Ack2, ..., Ack9. You may not need all the rows. The path of D1 is done for you.
Time
A gets ACK / sends Data
R1 queues
R1 begins
sending to R2
R2 begins
sending to B
B receives,
instantly sends ACK to A
0
D1--D5
D2-D5
D1
1
waiting
D3-D5
D2
D1
2
receive Ack1 /
send _________
__________
___________
D2
D1
3
D2
4
5
6
7
8
9
10
11
12
13
Time
A gets ACK / sends Data
R1 queues
R1 begins
sending to R2
R2 begins
sending to B
B receives,
instantly sends ACK to A
0
D1--D5
D2-D5
D1
1
waiting
D3-D5
D2
D1
2
receive Ack1 /
send _________
__________
___________
D2
D1
3
D2
4
5
6
7
8
9
10
11
12
13
Explanation / Answer
Time
A gets ACK / sends Data
R1 queues
R1 begins
sending to R2
R2 begins
sending to B
B receives,
instantly sends ACK to A
0
D1--D5
D2-D5
D1
1
waiting
D3-D5
D2
D1
2
receive Ack1 for D1/
send D6
D4-D6
D3
D2
D1
3
receive Ack2 for D2/
send D7
D5-D7
D4
D3
D2
4
receive Ack3 for D3/
send D8
D6-D8
D5
D4
D3
5
receive Ack4 for D4/
send D9
D7-D9
D6
D5
D4
6
receive Ack5 for D5
D8-D9
D7
D6
D5
7
receive Ack6 for D6
D9
D8
D7
D6
8
receive Ack7 for D7
D9
D8
D7
9
receive Ack8 for D8
D9
D8
10
receive Ack9 for D9
D9
Time
A gets ACK / sends Data
R1 queues
R1 begins
sending to R2
R2 begins
sending to B
B receives,
instantly sends ACK to A
0
D1--D5
D2-D5
D1
1
waiting
D3-D5
D2
D1
2
receive Ack1 for D1/
send D6
D4-D6
D3
D2
D1
3
receive Ack2 for D2/
send D7
D5-D7
D4
D3
D2
4
receive Ack3 for D3/
send D8
D6-D8
D5
D4
D3
5
receive Ack4 for D4/
send D9
D7-D9
D6
D5
D4
6
receive Ack5 for D5
D8-D9
D7
D6
D5
7
receive Ack6 for D6
D9
D8
D7
D6
8
receive Ack7 for D7
D9
D8
D7
9
receive Ack8 for D8
D9
D8
10
receive Ack9 for D9
D9