Quizz 3

Quizz 3

by Alexandre Pierre-Olivier Philippe Tellier -
Number of replies: 2

In the practice quizz 2020 its written that we canno't have 2 packets with same destination adress and same destination port not going to the same end system. But in router configuration we can have a rule on source adress (see attachement), so why the response is like that ?


Screenshot from router configuration where we can see "Source adress field"

In reply to Alexandre Pierre-Olivier Philippe Tellier

Re: Quizz 3

by Katerina Argyraki -
Alexandre, I will need a bit more information to answer your question, if you don't mind.

The statement you are asking about is "we cannot have two packets with the same destination IP address and port number not going to the same end-system." In other words, the destination IP address and destination port number of a packet uniquely identify the target process that is meant to receive this packet. Before we discuss the NAT config you attached, do you not agree with the statement? Do you think that it is possible for two packets with the same destination IP address and port number to go to different end-systems? In what situation would that occur?

Thanks!
In reply to Alexandre Pierre-Olivier Philippe Tellier

Re: Quizz 3

by Katerina Argyraki -
Here is my best guess about the config you are showing:

There's a part of the config you are not showing that specifies the destination IP address (say X) and port number (say Y) of an incoming packet.

The part that you show specifies how to rewrite the source IP address, source port number, destination IP address, and destination port number of the packet.

I.e., the rule overall says: if a packet arrives with destination IP address X and port number Y, then rewrite its source IP address, source port number, destination IP address, and destination port number in this way.

Am I guessing right?