(a) The following figure describes a TCP three-way handshake process. i Three ty

Question

(a) The following figure describes a TCP three-way handshake process. i Three types of packets are used in the figure. What are the three packet types? (3 points) ii. Fill in blanks (5 points) client server Seq 2021 Seq-1232, ACK- SeqACK (b) If one packet from an internal computer comes to NAT with source address as 192.168.0.3 and destination address as 56.21.46.89 i What source and destination addresses will this packet have after it passes NAT? (3 points) ii. Assuming this packet reaches the remote server and the remote server sends a response packet, what source and destination address will the response packet have? (3 points) 192.168.0.3 Gateway/NAT 192.168.0.4 Public IP address: 128.87.55.67 192.168.0.5 institutional network

Explanation / Answer

(5) (a)

The following are the three packet types used in TCP 3-Way handshake process.

Host X sends a TCP SYNchronize packet to Host Y. Host Y receives X's SYN packet.
Host Y sends a SYNchronize-ACKnowledgement. Host X receives Y's SYN-ACK packet.
Host X sends ACKnowledge packet to Y. Host Y receives ACK packet.
TCP connection is established.

Note: SYNchronize and ACKnowledge packets are indicated by the SYN bit, or the ACK bit present in the TCP header.

(b)

Network Address Translation is used by devices to make a mapping between the internal network and the outside of network.
NAT can be configured in many forms, but for our example lets consider overloading NAT.
Overloading NAT : A form of dynamic NAT that maintains mapping between unregistered IP addresses to a single registered IP address, where each one is bound to a different port than other. This is known also as Port Address Translation (PAT).
So in example, Packet from host 192.168.0.3 goes to 56.21.46.89

1. When it passes through NAT, the router will create a table for this packet, mapped to a particular unique port number.
Before NAT,   Src = 192.168.0.3    Dst = 56.21.46.89 port = A
After NAT,      Src = 128.87.55.67 Dst = 56.21.46.89 port = X
And the Router will add an entry in NAT table to filter packets coming to Dst = 128.87.55.67 , port = X and forward them to Dst = 192.168.0.3 port = A (by changing the destination ip and port in the packet).
So after the packet passes NAT, it will have
Src = 128.87.55.67 Dst = 56.21.46.89

2. Similarly when the response comes from the remote server,
it will have Src = 56.21.46.89     Dst = 128.87.55.67 before it comes to NAT

Then NAT will refer its table and change the Destination ip to the host ip (which is mapped in the table).

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