Computer Networks

Question

" A process can have one or more sockets ".

More than one socket for a process.How?

Can someone explain this concept?

Answer 1

yes , a process could have more that one socket (combination of ip address and port address) but at a particular instance of time it can be only one..

Comments

So a process cannot listen to multiple sockets at the same time?

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It can surely be listen sir.i thnk that is how these threads of a process wrks...but what i meant to say is ..Simply, if I connect to the same web server twice from my client, the two connections will have different source ports from my perspective and destination ports from the web server's. So there is no ambiguity, even though both connections have the same source and destination IP addresses.
Ports are a way to multiplex IP addresses so that different applications can listen on the same IP address/protocol pair. Unless an application defines its own higher-level protocol, there is no way to multiplex a port. If two connections using the same protocol have identical source and destination IPs and identical source and destination ports, they must be the same connection.

Answer 2

This is even more clearer explanation ..

 

Primary key of a socket: A socket is identified by {SRC-IP, SRC-PORT, DEST-IP, DEST-PORT, PROTOCOL} not by {SRC-IP, SRC-PORT, DEST-IP, DEST-PORT} - Protocol is an important part of a socket's definition.

OS Process & Socket mapping: A process can be associated with (can open/can listen to) multiple sockets which might be obvious to many readers.

Example 1: Two clients connecting to same server port means: socket1 {SRC-A, 100, DEST-X,80, TCP} and socket2{SRC-B, 100, DEST-X,80, TCP}. This means host A connects to server X's port 80 and another host B also connects to same server X to the same port 80. Now, how the server handles these two sockets depends on if the server is single threaded or multiple threaded (I'll explain this later). What is important is that one server can listen to multiple sockets simultaneously.

Now , how a server knows which socket to answer. First understand that for a single server process that is listening to same port, there could be more than one sockets (may be from same client or from different clients). Now as long as a server knows which request is associated with which socket, it can always respond to appropriate client using the same socket. Thus a server never needs to open another port in its own node than the original one on which client initially tried to bind connect. If any server allocates different server-ports after a socket is bound, then in my opinion the server is wasting its resource and it must be needing the client to bind connect again to the new port assigned.

A bit more for completeness:

Example 2: It's very interesting question that can a server's two different processes listen to same port. If you do not consider protocol as one of parameter defining socket then the answer is no. Initiatively this is so because we can say that in such case for a single client trying to connect to a server-port will not have any mechanism to mention which of the two listening process the client intends to. This is the same theme asserted by rule (2). However this is WRONG answer because 'protocol' is also a part of the socket definition. Thus two processes in same node can listen to same port only if they are using different protocol. For example two unrelated clients (say one is using TCP and another is using UDP) can bind connect and communicate to the same server node and to the same port but they must be served by two different server-processes.

Server Types - single & multiple:

When a server's processes listening to a port that means multiple sockets can simultaneously connect and communicate with the same server-process. If a server uses only a single child-process to serve all the sockets then the server is called single-process/threaded and if the server uses many sub-processes to serve each socket by one sub-process then the server is called multi-process/threaded server. Note that irrespective of the server's type a server can/should always uses the same initial socket to respond back (no need to allocate another server-port)...

Comments

" a Web server spawns a new process for each connection.
 each of these processes has its own connection socket
through which HTTP requests arrive and HTTP responses are sent. We mention,
however, that there is not always a one-to-one correspondence between connection sockets and processes. In fact, today’s high-performing Web servers often use only one process, and create a new thread with a new connection socket for each new client connection. (A thread can be viewed as a lightweight subprocess.)
 For such a server, at any given time there may be many connection sockets (with different identifiers) attached to the same process. " - book(Kurose & Ross)

by sysadmin1138 on https://serverfault.com/questions/296603/understanding-ports-how-do-multiple-browser-tabs-communicate-at-the-same-time
"The server listening on port 80 doesn't HAVE to handle multiple processes. Simple TCP daemons of elder years could only handle one connection at a time. You can emulate this behavior by having a program like netcat listen on a specific port and try to connect two machines to it. One will get in, the other will bounce off without a connection. These daemons are mostly-useless so you never see them any more.
For something like a web-server, it's listening on the port directly. The thing to keep in mind is that it is sitting on top of the operating-system's socket library. When a new connection is established, the socket library passes the brand new socket to the web-server software. At that point, the web-server software has some options.One possibility is that it passes the socket-object to a new thread in the same process. Whenever communication happens over this socket, this thread will handle it. The parent process mediates which threads are active at any given time, which could be a lot.
Another possibility is that it spins up a new process and passes the socket-object to the process. As I understand it, it's now up to the operating system's socket system mediate communication between these child-processes and their targets. The parent process still has some control over the processes, such as killing hung ones and other inter-process communications.
Which of these approaches is more efficient depends on the operating system. IIRC, Apache can run in either mode.
In essence, the socket library provides a level of parallel processing to the web-server. It can handle multiple simultaneous connections actively transferring data, all while accepting new connections.
For a browser that can spin up multiple connection attempts to a webserver in order to improve load times, parallelism applies on the browser end as well, this is a good and wonderful thing. The browser keeps track of the state of the page as it is loading, and the multiple connection attempts it spins up are all part of the process."

I found these two references helpful.I want to confirm a concept .Please

correct me if I am wrong. 

"The situation is illustrated in Figure 3.5, in which Host C initiates two HTTP sessions to server B, and Host A initiates one HTTP session to B. Hosts A and C and server
B each have their own unique IP address—A, C, and B, respectively. Host C assigns two different source port numbers (26145 and 7532) to its two HTTP connections. Because Host A is choosing source port numbers independently of C, it might also assign a source port of 26145 to its HTTP connection. But this is not a problem—server B will still be able to correctly demultiplex the two connections having the same source port number, since the two connections have different source IP addresses."

Now in this situation many( 3 in this eg.) threads or child process(as mentioned in sysadmin1138 answer)  are running on the server.

1)All these processes use port 80.

2)And it would be wrong to say all these processes are listening on port 80.

Are both the statement 1 and statement 2 are True?