General question about use of ports when programming with sockets

Discussion in 'Server Networking' started by 0to60, Sep 30, 2005.

  1. 0to60

    0to60 Guest

    I have a question about socket programming in general.

    Exactly what happens behind the scenes when I one socket connects to a
    different socket in listen mode? Using the dotnet framework, I create a
    socket, bind it to a port, put it in listen mode, and then n sockets can
    connect to it. The code:

    Socket newSocket = listeningSocket.Accept();

    returns a socket. I can communicate on newSocket, and listeningSocket goes
    back to listen mode. Is newSocket using the port that I bound
    listeningSocket to? If 5 clients connect to listeningSocket, and you check
    the remote endpoint on each of them, they will all point to
    listeningSocket's IP address and the same port. The 5 resulting newSockets
    local endpoint properties all show the same port that listeningSocket is
    bound to. Can many sockets all use the same port?

    If this is the case, how come when I try to manually bind a some other
    socket to the port that listeningSocket is listening on, I get a "port in
    use" error?

    So what happens behind the scenes? Does the newSocket use the same port
    that listeningSocket is bound to? Or does it newSocket get "assigned" a
    free port by the WSA code wrapped by the Socket class? When I check
    newSocket's local endpoint property, it says the same port that
    listeningSocket is bound to. How can multiple sockets be using the same
    0to60, Sep 30, 2005
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  2. Each socket must be unique by 4 values:
    IP Local, Port Local
    IP Remote, Port Remote

    When the listener accepts a new socket, it fills (among other things) that
    socket object with those four values. That means no other socket can use
    those same values, at least one must be different. So the same client can
    not connect to the same server and port using the same local IP and port.
    Clients normally get dynamic ports so that allows a client to connect to the
    same server/port more then once. On the listening side, when you try to
    listen to the same IP and port more then once, you get an error unless you
    allow duplicates which is another can of worms. HTH
    William Stacey [MVP], Sep 30, 2005
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  3. Arkady Frenkel, Oct 9, 2005
  4. I saw this little line at the end of that article:
    "Per development: The connection limit refers to the number of
    redirector-based connections and is enforced for any file, print, named
    pipe, or mail slot session. The TCP connection limit is not enforced, but it
    may be bound by legal agreement to not permit more than 10 clients. "

    So from my read, this does not apply to sockets, but to redirector-based
    connections. Is this true? Easy to test, but have not done so myself.
    William Stacey [MVP], Oct 10, 2005
  5. Due to the next ( that that KB )
    "All logical drive, logical printer, and transport level connections
    combined from a single computer are considered to be one session; therefore,
    these connections only count as one connection in the ten- connection limit.
    For example, if a user establishes two logical drive connections, two
    Windows sockets, and one logical printer connection to a Windows XP system,
    one session is established. As a result, there will be only one less
    connection that can be made to the Windows XP system, even though three
    logical connections have been established." that not only SMB connections
    but winsock too , but I didn't check that too ( have to bother 11 buddies
    for that :) )
    Arkady Frenkel, Oct 10, 2005
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