If have a webserver running at port 80 and someone connects from a client using randomly assigned port x, then the server knows which port to reply to. However, at that time on, does the communication to the server continue on port 80 from then on (assigned a file descriptor to socket-pair ip:x), or does the server also delegate further communication onto another randomly assigned port of itself; y?
So what I am really asking is: When the server replies -does it reply with a source port of 80 back for further communication?
If have a webserver running at port 80 and someone connects from a client using randomly assigned port x
At the client end.
then the server knows on what port to reply.
The server replies via the same connection it received the request on. What happens below that is up to TCP. It isn't 'knowledge' of the server application.
However, at that time on, does the communication to the server continue on port 80 from then on
Yes.
(assigned a file descriptor to socket-pair ip:x)
To the socket quad {local-IP, local-port, remote-IP, remote port}.
or does the server also delegate further communication onto another randomly assigned port of itself;
No.
So what I am really asking is: When the server replies -does it reply with a source port of 80 back for further communication?
Yes.
Related
(This question is inspired by a response to this thread: How WebSocket server handles multiple incoming connection requests?)
My understanding is this way:
Assume client IP = 1.1.1.1, server IP = 9.9.9.9
Browser choose a random local available port, say 5555, and initiate a connection to server's port 80. So on client, the socketfd_client should represent an IP connection like (1.1.1.1:5555, 9.9.9.9:80, TCP).
Server calls accept() on its port 80 and identified the connection request from client. Then server picks a random local available port, say 8888, to fulfill that connection request. So on server, the socketfd_server should represent an IP connection like (1.1.1.1:5555, 9.9.9.9:8888, TCP).
My question is:
If my above understanding is correct, socektfd_client and socketfd_server have different server port. Client has 80 while server has 8888. How could the communication be carried out? I think client should change to use the server port 8888 as well, but when and how?
Browser choose a random local available port, say 5555
No. The operating system does that: specifically, the TCP part of the network stack.
and initiate a connection to server's port 80. So on client, the socketfd_client should represent an IP connection like (1.1.1.1:5555, 9.9.9.9:80, TCP).
Correct.
Server calls accept() on its port 80 and identified the connection request from client.
Correct.
Then server picks a random local available port, say 8888
No.
to fulfill that connection request.
No.
So on server, the socketfd_server should represent an IP connection like (1.1.1.1:5555, 9.9.9.9:8888, TCP).
No. The connection at both ends is represented by {1.1.1.1:5555, 9.9.9.9:80}. There is no new port at the server end.
My question is:
If my above understanding is correct
It isn't.
socektfd_client and socketfd_server have different server port.
No.
Client has 80 while server has 8888. How could the communication be carried out? I think client should change to use the server port 8888 as well, but when and how?
Never.
Given that you have multiple web browsers running, all which obviously listen on port 80, how would a browser figure if an incoming HTTP response was originated by itself? And whether or not catch the response and show it?
As part of the connection process a TCP/IP connection is assigned a client port. Browsers do not "listen on port 80"; rather a browser/clients initiate a request to port 80 on the server and waits for a reply on the client port from the server's IP.
After the client port is assigned (locally), each client [TCP/IP] connection is uniquely identified by (server IP, server port, client IP, client port) and the connection (and response sent over such) can be "connected back" to the correct browser. This same connection-identifying tuple is how a server doesn't confuse multiple requests coming from the same client/IP1
HTTP sits on top of the TCP/IP layer and doesn't have to concern itself with mixing up connection streams. (HTTP/2 introduces multiplexing, but that is a different beast and only affects connection from the same browser.)
See The Ephemeral Port Range for an overview:
A TCP/IPv4 connection consists of two endpoints, and each endpoint consists of an IP address and a port number. Therefore, when a client user connects to a server computer, an established connection can be thought of as the 4-tuple of (server IP, server port, client IP, client port). Usually three of the four are readily known -- client machine uses its own IP address and when connecting to a remote service, the server machine's IP address and service port number are required [leaving only the client port unknown and to be automatically assigned].
What is not immediately evident is that when a connection is established that the client side of the connection uses a port number. Unless a client program explicitly requests a specific port number, the port number used is an ephemeral port number. Ephemeral ports are temporary ports assigned by a machine's IP stack, and are assigned from a designated range of ports for this purpose. When the connection terminates, the ephemeral port is available for reuse, although most IP stacks won't reuse that port number until the entire pool of ephemeral ports have been used. So, if the client program reconnects, it will be assigned a different ephemeral port number for its side of the new connection.
See TCP/IP Client (Ephemeral) Ports and Client/Server Application Port Use for an additional gentle explanation:
To know where to send the reply, the server must know the port number the client is using. This [client port] is supplied by the client as the Source Port in the request, and then used by the server as the destination port to send the reply. Client processes don't use well-known or registered ports. Instead, each client process is assigned a temporary port number for its use. This is commonly called an ephemeral port number.
1 If there are multiple client computers (ie. different TCP/IP stacks each assigning possibly-duplicate ephemeral ports) using the same external IP then something like Network Address Translation must be used so the server still has a unique tuple per connection:
Network address translation (NAT) is a methodology of modifying network address information in Internet Protocol (IP) datagram packet headers while they are in transit across a traffic routing device for the purpose of remapping one IP address space into another.
thank you all for answers.
the hole listening thing over port 80 was my bad,I must have been dizzy last night :D
anyway,as I have read HTTP is connectionless.
browser initiates an HTTP request and after a request is made, the client disconnects from >the server and waits for a response. The server process the request and re-establish the >connection with the client to send response back.
therefor the browser does not maintain connection waiting for a response.so the answer is not that easy to just send the response back to the open socket.
here's the source
Pay attention browesers aren't listening on specific port to receive HTTP response. Web server listening on specific ports (usually 80 or 443). Browser open connection to web server, and send HTTP request to web server. Browser don't close connection before receive HTTP response. Web server writes HTTP response on opened connection.
Given that you have multiple web browsers running, all which obviously listen on port 80
Not obvious: just wrong. The HTTP server listens on port 80. The browsers connect to port 80.
how would a browser figure if an incoming HTTP response was originated by itself?
Because it comes back on the same connection and socket that was used to send the request.
And whether or not catch the response and show it?
Anything that comes back on the connected socket belongs to the guy who connected the socket.
And in any case all this is the function of TCP, not the browser.
I heard that for TCP connection, server will listen on 1 port, and use another port for sending data.
For example, web server listen on port 80. Whenever client connects to it, this server will use another port (say 9999) to send data (web content) to client. If multiple clients connect to it, does this server create multiple "another ports" to send data back ?
And does the client uses 2 ports (listening port and sending-data port) as the server does ?
Am I right? I heard many people said that and I can't find any good books or articles about this
Please explain
I heard that for TCP connection, server will listen on 1 port, and use another port for sending data.
No. The same port is used for both listening and for accepted connections, and the same connection is normally used for both sending and receiving. TCP connections are bidirectional. FTP is an exception to this, as it uses two connections: one for commands and one for data.
For example, web server listen on port 80. Whenever client connects to it, this server will use another port (say 9999) to send data (web content) to client.
No.
If multiple clients connect to it, does this server create multiple "another ports" to send data back ?
No.
And does the client uses 2 ports (listening port and sending-data port) as the server does ?
No.
Am I right?
No.
I heard many people said that
They were all wrong.
and I can't find any good books or articles about this
There aren't any. Only a very bad book or article would say any such thing.
Here is a simple example.
When the browser in your laptop sends a request to a web server which listens on port 443, your OS uses an ephemeral port (e.g. tcp/3105) as source port and Port tcp/443 of the web server as the destination port.
Once the web server sends back the response, it will use the source port as tcp/443 and the destination port as tcp/3105.
Here is a link to an AWS documentation.
https://docs.aws.amazon.com/vpc/latest/userguide/vpc-network-acls.html
When TCP client requests conn'n on server's listening port, server will accept it and create a new port meant for this conn'n with this client. Hence forth the client will communicate with server on this new port.
if the above statement is true and possible, how server conveys the newly generated port to client. In reply to the conn'n request the packet from server to client will have what port as source port (Server's listening port OR New port generated by server for client).
Will Client accept this port and take into use or it will give error ? I need this to implement an architecture having 2 clients and one server in an embedded system using lwip stack.
regards,
ED
The server doesn't create a new port. It creates a new TCP connection and it sends its reply packets to the IP and port the client sent its connection request from. (A TCP connection has an IP address and port on each side.)
When you connect to a server, you get a port number yourself, which is assigned to you by the system (unless you bind the socket before connecting). When the network stack of the server replies to your connection request, the "source" port is the new port number of the server, and the "destination" port of the message is your port. That's how the network stack on the client side knows what port the server has.
The new port number on the server used for your connection can not be set or changed by the actual server program, it's the network stack on the server machine that just grabs an available port number.
Edit: You might also want to read up a little on how connections are established, a.k.a. the three-way handshake.
If I understand right, applications sometimes use HTTP to send messages, since using other ports is liable to cause firewall problems. But how does that work without conflicting with other applications such as web-browsers? In fact how do multiple browsers running at once not conflict? Do they all monitor the port and get notified... can you share a port in this way?
I have a feeling this is a dumb question, but not something I ever thought of before, and in other cases I've seen problems when 2 apps are configured to use the same port.
There are 2 ports: a source port (browser) and a destination port (server). The browser asks the OS for an available source port (let's say it receives 33123) then makes a socket connection to the destination port (usually 80/HTTP, 443/HTTPS).
When the web server receives the answer, it sends a response that has 80 as source port and 33123 as destination port.
So if you have 2 browsers concurrently accessing stackoverflow.com, you'd have something like this:
Firefox (localhost:33123) <-----------> stackoverflow.com (69.59.196.211:80)
Chrome (localhost:33124) <-----------> stackoverflow.com (69.59.196.211:80)
Outgoing HTTP requests don't happen on port 80. When an application requests a socket, it usually receives one at random. This is the Source port.
Port 80 is for serving HTTP content (by the server, not the client). This is the Destination port.
Each browser uses a different Source to generate requests. That way, the packets make it back to the correct application.
It is the 5-tuple of (IP protocol, local IP address, local port, remote IP address, remote port) that identifies a connection. Multiple browsers (or in fact a single browser loading multiple pages simultaneously) will each use destination port 80, but the local port (which is allocated by the O/S) is distinct in each case. Therefore there is no conflict.
Clients usually pick a port between 1024 and 65535.
It depends on the operating system how to handle this. I think Windows Clients increment the value for each new connection, Unix Clients pick a random port no.
Some services rely on a static client port like NTP (123 UDP)
A browser is a client application that you use in order to see content on a web server which is usually on a different machine.
The web server is the one listening on port 80, not the browser on the client.
You need to be careful in making the distinction between "listening on port 80" and "connecting to port 80".
When you say "applications sometimes use HTTP to send messages, since using other ports is liable to cause firewall problems", you actually mean "applications sometimes send messages to port 80".
The server is listening on port 80, and can accept multiple connections on that port.
Port 80 you're talking about here is the remote port on the server, locally browser opens high port for each connection established.
Each connection has port numbers on both ends, one is called local port, other remote port.
Firewall will allow traffic to high port for browser, because it knows that connection has been established from you computer.