(gawkinet.info.gz) Special File Fields

(gawkinet.info.gz) Gawk Special Files
(gawkinet.info.gz) Comparing Protocols
 
 2.1.1 The Fields of the Special File Name
 -----------------------------------------
 
 This node explains the meaning of all the other fields, as well as the
 range of values and the defaults.  All of the fields are mandatory.  To
 let the system pick a value, or if the field doesn't apply to the
 protocol, specify it as `0':
 
 NET-TYPE
      This is one of `inet4' for IPv4, `inet6' for IPv6, or `inet' to
      use the system default (which is likely to be IPv4).  For the rest
      of this document, we will use the generic `/inet' in our
      descriptions of how `gawk''s networking works.
 
 PROTOCOL
      Determines which member of the TCP/IP family of protocols is
      selected to transport the data across the network. There are two
      possible values (always written in lowercase): `tcp' and `udp'.
      The exact meaning of each is explained later in this node.
 
 LOCALPORT
      Determines which port on the local machine is used to communicate
      across the network.  Application-level clients usually use `0' to
      indicate they do not care which local port is used--instead they
      specify a remote port to connect to. It is vital for
      application-level servers to use a number different from `0' here
      because their service has to be available at a specific publicly
      known port number. It is possible to use a name from
      `/etc/services' here.
 
 HOSTNAME
      Determines which remote host is to be at the other end of the
      connection. Application-level servers must fill this field with a
      `0' to indicate their being open for all other hosts to connect to
      them and enforce connection level server behavior this way.  It is
      not possible for an application-level server to restrict its
      availability to one remote host by entering a host name here.
      Application-level clients must enter a name different from `0'.
      The name can be either symbolic (e.g., `jpl-devvax.jpl.nasa.gov')
      or numeric (e.g., `128.149.1.143').
 
 REMOTEPORT
      Determines which port on the remote machine is used to communicate
      across the network.  For `/inet/tcp' and `/inet/udp',
      application-level clients _must_ use a number other than `0' to
      indicate to which port on the remote machine they want to connect.
      Application-level servers must not fill this field with a `0'.
      Instead they specify a local port to which clients connect.  It is
      possible to use a name from `/etc/services' here.
 
    Experts in network programming will notice that the usual
 client/server asymmetry found at the level of the socket API is not
 visible here. This is for the sake of simplicity of the high-level
 concept. If this asymmetry is necessary for your application, use
 another language.  For `gawk', it is more important to enable users to
 write a client program with a minimum of code. What happens when first
 accessing a network connection is seen in the following pseudocode:
 
      if ((name of remote host given) && (other side accepts connection)) {
        rendez-vous successful; transmit with getline or print
      } else {
        if ((other side did not accept) && (localport == 0))
          exit unsuccessful
        if (TCP) {
          set up a server accepting connections
          this means waiting for the client on the other side to connect
        } else
          ready
      }
 
    The exact behavior of this algorithm depends on the values of the
 table-inet-components:: gives you the combinations of values and their
 meaning. If this table is too complicated, focus on the three lines
 printed in *bold*. All the examples in  Networking With `gawk'
 Using Networking, use only the patterns printed in bold letters.
 
 PROTOCOL    LOCAL PORT  HOST NAME   REMOTE      RESULTING CONNECTION-LEVEL
                                     PORT        BEHAVIOR
 ------------------------------------------------------------------------------ 
 *tcp*       *0*         *x*         *x*          *Dedicated client, fails if
                                                 immediately connecting to a
                                                             server on the
                                                 other side fails*
 udp         0           x           x           Dedicated client
 *tcp, udp*  *x*         *x*         *x*          *Client, switches to
                                                 dedicated server if
                                                 necessary*
 *tcp, udp*  *x*         *0*         *0*          *Dedicated server*
 tcp, udp    x           x           0           Invalid
 tcp, udp    0           0           x           Invalid
 tcp, udp    x           0           x           Invalid
 tcp, udp    0           0           0           Invalid
 tcp, udp    0           x           0           Invalid
 
 Table 2.1: /inet Special File Components
 
    In general, TCP is the preferred mechanism to use.  It is the
 simplest protocol to understand and to use.  Use UDP only if
 circumstances demand low-overhead.
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