Lesson 4 Client Server Flashcards
What is ICANN?
ICANN (Internet Corporation for Assigned Names and Numbers) is the private, non-government, nonprofit corporation with responsibility for Internet Protocol (IP) address space allocation, protocol parameter assignment, domain name system (DNS) management and root server system management functions.
IPV4 vs IPv6
2^32 address space
2^64 address space
What is a socket?
A socket is one endpoint of a two-way communication link between two programs running on the network. A socket is bound to a port number so that the TCP layer can identify the application that data is destined to be sent to.
An endpoint is a combination of an IP address and a port number. Every TCP connection can be uniquely identified by its two endpoints. That way you can have multiple connections between your host and the server.
Client server connection
Normally, a server runs on a specific computer and has a socket that is bound to a specific port number. The server just waits, listening to the socket for a client to make a connection request.
On the client-side: The client knows the hostname of the machine on which the server is running and the port number on which the server is listening. To make a connection request, the client tries to rendezvous with the server on the server’s machine and port. The client also needs to identify itself to the server so it binds to a local port number that it will use during this connection. This is usually assigned by the system.
A client’s connection request
If everything goes well, the server accepts the connection. Upon acceptance, the server gets a new socket bound to the same local port and also has its remote endpoint set to the address and port of the client. It needs a new socket so that it can continue to listen to the original socket for connection requests while tending to the needs of the connected client.
The connection is made
On the client side, if the connection is accepted, a socket is successfully created and the client can use the socket to communicate with the server.
3 way handshake
Step 1: A connection between server and client is established
Target server must have open ports that can accept and initiate new connections. The client node sends a SYN (Synchronize Sequence Number) data packet over an IP network to a server on the same or an external network.
This SYN packet is a random sequence number that the client wants to use for the communication (for example, X). The objective of this packet is to ask/infer if the server is open for new connections.
Step 2: The server receives the SYN packet from the client node
When the server receives the SYN packet from the client node, it responds and returns a confirmation receipt – the ACK (Acknowledgement Sequence Number) packet or SYN/ACK packet. This packet includes two sequence numbers.
The first one is ACK one, which is set by the server to one more than the sequence number it received from the client (e.g. X+1).
The second one is the SYN sent by the server, which is another random sequence number (for example, Y).
This sequence indicates that the server correctly acknowledged the client’s packet, and that is sending its own to be acknowledged as well.
Step 3: Client node receives the SYN/ACK from the server and responds with an ACK packet
The client node receives the SYN/ACK from the server and responds with an ACK packet. Once again, each side must acknowledge the sequence number received by incrementing it by one.
So now it’s the turn of the client to acknowledge the server’s packet by adding one to the sequence number (in this case, Y+1), and resend it to the server.
Upon completion of this process, the connection is created and the host and server can communicate.
All these steps are necessary to verify the serial numbers originated by both sides, guaranteeing the stability of the connection.
Since both hosts must acknowledge the connection parameters of the other side, a missing or out-of-order segment can be quickly detected before the actual data transfer process is initiated.
