TCP can two different sockets share a port

Question

This might be a very basic question but it confuses me  Can two different connected sockets share a port  I m writing an application server that should be able to handle more than 100k concurrent connections  and we know that the number of ports available on a system is around 60k  16bit   A connected socket is assigned to a new  dedicated  port  so it means that the number of concurrent connections is limited by the number of ports  unless multiple sockets can share the same port  So the question

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No  It is not possible to share the same port at a particular instant  But you can make your application such a way that it will make the port access at different instant

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A server socket listens on a single port   All established client connections on that server are associated with that same listening port on the server side of the connection   An established connection is uniquely identified by the combination of client-side and server-side IP Port pairs   Multiple connections on the same server can share the same server-side IP Port pair as long as they are associated with different client-side IP Port pairs  and the server would be able to handle as many clients as available system resources allow it to   On the client-side  it is common practice for new outbound connections to use a random client-side port  in which case it is possible to run out of available ports if you make a lot of connections in a short amount of time

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Theoretically  yes  Practice  not  Most kernels  incl  linux  doesn t allow you a second bind   to an already allocated port  It weren t a really big patch to make this allowed   Conceptionally  we should differentiate between socket and port  Sockets are bidirectional communication endpoints  i e   things  where we can send and receive bytes  It is a conceptional thing  there is no such field in a packet header named  socket    Port is an identifier which is capable to identify a socket  In case of the TCP  a port is a 16 bit integer  but there are other protocols as well  for example  on unix sockets  a  port  is essentially a string    The main problem is the following  if an incoming packet arrives  the kernel can identify its socket by its destination port number  It is a most common way  but it is not the only possibility    Sockets can be identified by the destination IP of the incoming packets  This is the case  for example  if we have a server using two IPs simultanously  Then we can run  for example  different webservers on the same ports  but on the different IPs  Sockets can be identified by their source port and ip as well  This is the case in many load balancing configurations    Because you are working on an application server  it will be able to do that

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A connected socket is assigned to a new  dedicated  port   That s a common intuition  but it s incorrect   A connected socket is not assigned to a new dedicated port   The only actual constraint that the TCP stack must satisfy is that the tuple of  local address  local port  remote address  remote port  must be unique for each socket connection   Thus the server can have many TCP sockets using the same local port  as long as each of the sockets on the port is connected to a different remote location   See the  Socket Pair  paragraph at   http   books google com books id ptSC4LpwGA0C amp lpg PA52 amp dq socket 20pair 20tuple amp pg PA52 v onepage amp q socket 20pair 20tuple amp f false

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TCP   HTTP Listening On Ports  How Can Many Users Share the Same Port  So  what happens when a server listen for incoming connections on a TCP port  For example  let s say you have a web-server on port 80  Let s assume that your computer has the public IP address of 24 14 181 229 and the person that tries to connect to you has IP address 10 1 2 3  This person can connect to you by opening a TCP socket to 24 14 181 229 80  Simple enough   Intuitively  and wrongly   most people assume that it looks something like this       Local Computer      Remote Computer     --------------------------------      lt local ip gt  80        lt foreign ip gt  80         not actually what happens  but this is the conceptual model a lot of people have in mind    This is intuitive  because from the standpoint of the client  he has an IP address  and connects to a server at IP PORT  Since the client connects to port 80  then his port must be 80 too  This is a sensible thing to think  but actually not what happens  If that were to be correct  we could only serve one user per foreign IP address  Once a remote computer connects  then he would hog the port 80 to port 80 connection  and no one else could connect    Three things must be understood   1   On a server  a process is listening on a port  Once it gets a connection  it hands it off to another thread  The communication never hogs the listening port    2   Connections are uniquely identified by the OS by the following 5-tuple   local-IP  local-port  remote-IP  remote-port  protocol   If any element in the tuple is different  then this is a completely independent connection   3   When a client connects to a server  it picks a random  unused high-order source port  This way  a single client can have up to  64k connections to the server for the same destination port   So  this is really what gets created when a client connects to a server       Local Computer     Remote Computer             Role     -----------------------------------------------------------     0 0 0 0 80          lt none gt                       LISTENING     127 0 0 1 80       10 1 2 3  lt random port gt       ESTABLISHED   Looking at What Actually Happens  First  let s use netstat to see what is happening on this computer  We will use port 500 instead of 80  because a whole bunch of stuff is happening on port 80 as it is a common port  but functionally it does not make a difference        netstat -atnp   grep -i   500     As expected  the output is blank  Now let s start a web server       sudo python3 -m http server 500   Now  here is the output of running netstat again       Proto Recv-Q Send-Q Local Address           Foreign Address         State       tcp        0      0 0 0 0 0 500             0 0 0 0                 LISTEN      -    So now there is one process that is actively listening  State  LISTEN  on port 500  The local address is 0 0 0 0  which is code for  listening for all ip addresses   An easy mistake to make is to only listen on port 127 0 0 1  which will only accept connections from the current computer  So this is not a connection  this just means that a process requested to bind   to port IP  and that process is responsible for handling all connections to that port  This hints to the limitation that there can only be one process per computer listening on a port  there are ways to get around that using multiplexing  but this is a much more complicated topic   If a web-server is listening on port 80  it cannot share that port with other web-servers   So now  let s connect a user to our machine       quicknet -m tcp -t localhost 500 -p Test payload    This is a simple script  https   github com grokit quickweb  that opens a TCP socket  sends the payload   Test payload   in this case   waits a few seconds and disconnects  Doing netstat again while this is happening displays the following       Proto Recv-Q Send-Q Local Address           Foreign Address         State       tcp        0      0 0 0 0 0 500             0 0 0 0                 LISTEN      -     tcp        0      0 192 168 1 10 500        192 168 1 13 54240      ESTABLISHED -   If you connect with another client and do netstat again  you will see the following       Proto Recv-Q Send-Q Local Address           Foreign Address         State       tcp        0      0 0 0 0 0 500             0 0 0 0                 LISTEN      -     tcp        0      0 192 168 1 10 500        192 168 1 13 26813      ESTABLISHED -       that is  the client used another random port for the connection  So there is never confusion between the IP addresses

[sockets] TCP: can two different sockets share a port?

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