If suppose multiple TCP connections are there in a network , then how will I idnetfy a single TCP connecton uniquely ?
Thanks
This sounds like a homework question.... from networking 101.
In any case, TCP connections are uniquely identified by the following properties: source IP address, source port, destination address, and destination port.
Do an internet search for "TCP 4-tuple" or "TCP 5-tuple" for more details.
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I have to design a program that reads all the TCP segments incoming of the same connection. How can I distinguish which connection a packet belongs to, among many packets that I receive? Is it sufficient to discriminate on the basis of the DESTINATION PORT field in the header?
No. Using just the destination port isn't enough. A TCP connection is defined by a combination of the following 4 values:
Source port
Source ip address
Destination port
Destination ip address
I am learning how the IP and TCP headers work. The following is my question:
Can a source address and destination address in an IP header be the same? or doesnt it make any sense?
Similarly, can a source and destination port be the same within a TCP header?
I tried searching google but I didnt find anything specific to my question, but rather how the protocols work.
Sure, source and destination IP address can be the same. That simply represents a connection between client and server (or 2 peers) on the same host.
Source and destination port can also be the same. If that happens, it would usually happen by coincidence.
But source and destination IP address being the same and source and destination port being the same? That doesn't make sense. That would represent a TCP connection connected to itself. Even if you wanted to do it (a kind of loopback connection), the protocol wouldn't be able to distinguish packets in one direction from packets in the other direction.
How should one map a sent packet and its response from server ? Is it safe to use port ?
e.g I captured a tcp packet , from X.X.X.X:45621 to Y.Y.Y.Y:993 , soon another tcp packet , which come from Y.Y.Y.Y:993 , and to X.X.X.X:45621.
Would it be safe to say , the second packet is the response of the first one ?
Is the following two situation causing problems ?
1) Port re-using
2) What if (is that possible ?) multiple request come from one port to the same remote machine ?
A TCP connection (or UDP pseudo-connection) is identified by 4 things: the local IP address, the local port number, the remote IP address, and the remote port number. If all 4 of those things are the same from one packet to the next then it's the same connection. If any of them are different, it's a different connection. Note that the "local" IP address and port appear as the source IP address and port of outgoing packets and as the destination IP address and port of incoming packets, and vice verse for "remote".
All of this is valid during the lifetime of the connection. After the connection is closed, the same 4-tuple might be reused for a new connection.
1) Q: Port re-using
A: many connections can use the same port. The connections will be differentiated by AT LEAST one of the other 3 members of the 4-tuple.
2) Q: What if multiple request come from one port to the same remote machine?
A: If multiple connections arrive destined to the same port of the same IP address and furthermore they have the same source port, that's OK as long as they come from different source hosts: source IP address will be different. This case won't happen for multiple connections coming from the same host destined to the same port on the same host because no two sockets on the same source host could have been bound to the same source port.
I understand that UDP resides on the transport layer and IP on the internet layer. I also get that they're both connectionless and unreliable. Then what is the point of UDP when we already have IP? The distinction is not very clear. Any help on this is greatly appreciated. Thanks!
Then what is the point of UDP when we already have IP?
To multiplex services. The UDP port number can differentiate between multiple services on the same host, using the same L3 identification. Using IP only it wouldn't be possible to host multiple services on the same station and easily differentiate between them.
Also, consider the case of UDP over IPv6. Since IPv6 doesn't have error-checking somebody has to perform it: the Checksum field of UDP is not optional.
Once a packet reaches a host using its IP address, the packet needs to be given to one of the applications on this machine. To determine which application should get the packet, it needs demultiplexing logic, which is based on ports. UDP has port information which is used by IP to deliver the packet to appropriate application.
If a station simultaneously maintains three TCP connections,can use the same TCP port for the three connections? If yes how it could distinguish packets?
Each connection is made of the below parameters.
(source ip,source port destination ip, destination port, protocol).
Thus, the station can distinguish packets.
Yes, multiple connections to the same IP and port are possible. They're distinguished by the IP and port on the other end of each connection.