1. Frank wishes to make a Skype call to Jane. As both Frank and Jane are at home
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Question
1. Frank wishes to make a Skype call to Jane. As both Frank and Jane are at home behind their NAT routers, and B respectively, proxies must be used to establish and complete the call. Refer the picture below. Both Frank and Jane maintain "keep alive" connections with Skype server S. When Frank makes the call, the following steps occur: 1) Frank sends S a message requesting a call to Jane, 2) S sends Carol's IP address and a port number to Frank, 3) S sends Carol's IP address and the same port number to Jane, 4) Frank sends a call setup message to Carol, 5) Jane sends a call setup message to Carol completing the call set up, 6) Jane says "Hello" and the corresponding voice frame is sent to Carol, 7) Carol forwards the voice frame to Frank. All communication uses UDP. Frank's IP address is 10.0.0.2 and A's IP address is 44.56.78.90 Jane's IP address is 192.168.0.2 and B's IP address is 88.76.54.32. Carol's IP address is 128.83.83.20. Carol sends and receives all mes sages on port 5001. S's IP address is 22.34.45.56. S sends and receives all messages on port 7001. Frank's next unused local port number prior to step 4 is 3001. Jane's next unused local port number prior Skype server Jane to step 5 is 4001. A's next unused port number in its NAT table Frank prior to step 4 is 6001. B's next unused port number in its NAT Carol table prior to step 5 is 7001. (a) In step 1, what are the source and destination IP addresses in the: (i) packet from Frank to A, (ii) packet (b) In step 4, what is the 4-tuple (Explanation / Answer
Question (a):
In step 1, Frank sends a message to Skype server S requesting to connect a call to Jane.
1. The source IP addresses in the packet from Frank to A – 10.0.0.2.
The destination IP addresses in the packet from Frank to A – 44.56.78.90
2. The source IP addresses in the packet from A to destination S – 44.56.78.90
The destination IP addresses in the packet from A to destination S – 22.34.45.56
Question (b):
In step 4, Frank sends a call setup message to Carol. Therefore,
(i)Packet from Frank to A:
Source IP address: Frank’s IP address i.e. 10.0.0.2
Destination IP address: A’s IP address i.e. 44.56.78.90
Source Port: Frank’s local port i.e. 3001
Destination port: A’s port number i.e. 6001
(ii)Packet from A to Carol:
Source IP address: A’s IP address i.e. 44.56.78.90
Destination IP address: Carol’s IP address i.e. 128.83.83.20
Source port: A’s port i.e. 6001
Destination port: Carol’s port number i.e. 5001
Question (c):
NAT table for A:
IP address, port number
IP address, port number
10.0.0.2, 3001
192.168.0.2, 4001 (for step 3)
22.34.45.56, 7001
128.83.83.20, 5001
Question (d):
In step 7, Carol forwards the frame to Frank. Therefore,
(i)Packet from Carol to A:
Source IP address: Carol’s IP address i.e. 128.83.83.20
Destination IP address: A’s IP address i.e. 44.56.78.90
Source port: Carol’s port number i.e. 5001
Destination port: A’s port i.e. 6001
(ii)Packet from A to Frank:
Source IP address: A’s IP address i.e. 44.56.78.90
Destination IP address: Frank’s IP address i.e. 10.0.0.2
Source port: A’s port number i.e. 6001
Destination port: Frank’s port number i.e. 3001
IP address, port number
IP address, port number
10.0.0.2, 3001
192.168.0.2, 4001 (for step 3)
22.34.45.56, 7001
128.83.83.20, 5001
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