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2.1.1 The Fields of the Special File Name

This section 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 section.
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 fields of the special file name. When in doubt, 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, use only the patterns printed in bold letters.

protocol local port host name remote port Resulting connection-level 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.