\chapter{Networking Support}
\label{internet}

\credits{by Mario S. Mommer}

This chapter documents the IPv4 networking and local sockets support
offered by \cmucl{}. It covers most of the basic sockets interface
functionality in a convenient and transparent way.

For reasons of space it would be impossible to include a thorough
introduction to network programming, so we assume some basic knowledge
of the matter.

\section{Byte Order Converters}

These are the functions that convert integers from host byte order to
network byte order (big-endian).

\begin{defun}{extensions:}{htonl}{%
    \args{\var{integer}}}
  
  Converts a $32$ bit integer from host byte order to network byte
  order.

\end{defun}

\begin{defun}{extensions:}{htons}{%
    \args{\var{integer}}}
  
  Converts a $16$ bit integer from host byte order to network byte
  order.

\end{defun}

\begin{defun}{extensions:}{ntohs}{%
    \args{\var{integer}}}
  
  Converts a $32$ bit integer from network byte order to host
  byte order.

\end{defun}

\begin{defun}{extensions:}{ntohl}{%
    \args{\var{integer}}}
  
  Converts a $32$ bit integer from network byte order to host byte
  order.

\end{defun}

\section{Domain Name Services (DNS)}

The networking support of \cmucl{} includes the possibility of doing
DNS lookups. The function 

\begin{defun}{extensions:}{lookup-host-entry}{%
    \args{\var{host}}}
 
  returns a structure of type \var{host-entry} (explained below) for
  the given \var{host}.  If \var{host} is an integer, it will be
  assumed to be the IP address in host (byte-)order. If it is a string,
  it can contain either the host name or the IP address in dotted
  format.
  
  This function works by completing the structure \var{host-entry}.
  That is, if the user provides the IP address, then the structure will
  contain that information and also the domain names. If the user
  provides the domain name, the structure will be complemented with
  the IP addresses along with the any aliases the host might have.

\end{defun}

\newpage

\begin{deftp}{structure}{host-entry}\args{\var{name} \var{aliases}
    \var{addr-type} \var{addr-list}}
  
  This structure holds all information available at request time on a
  given host. The entries are self-explanatory. Aliases is a list of
  strings containing alternative names of the host, and addr-list a
  list of addresses stored in host byte order. The field
  \var{addr-type} contains the number of the address family, as
  specified in {\tt socket.h}, to which the addresses belong. Since
  only addresses of the IPv4 family are currently supported, this slot
  always has the value $2$.

\end{deftp}

\begin{defun}{extensions:}{ip-string}{%
    \args{\var{addr}}}
  
  This function takes an IP address in host order and returns a string
  containing it in dotted format.

\end{defun}

\section{Binding to Interfaces}

In this section, functions for creating sockets bound to an interface
are documented.

\begin{defun}{extensions:}{create-inet-listener}{%
    \args{\var{port} \ampoptional{} \var{kind}  %
      \keys{\kwd{reuse-address} \kwd{backlog} \kwd{host}}}}
                     
  Creates a socket and binds it to a port, prepared to receive
  connections of kind \var{kind} (which defaults to \kwd{stream}),
  queuing up to \var{backlog} of them. If \kwd{reuse-address} \var{T}
  is used, the option SO\_REUSEADDR is used in the call to \var{bind}.
  If no value is given for \kwd{host}, it will try to bind to the
  default IP address of the machine where the Lisp process is running.

\end{defun}

\begin{defun}{extensions:}{create-unix-listener}{%
    \args{\var{path} \ampoptional{} \var{kind} \kwd{reuse-address}
      \kwd{backlog}}}
  
  Creates a socket and binds it to the file name given by \var{path},
  prepared to receive connections of kind \var{kind} (which defaults
  to \kwd{stream}), queuing up to \var{backlog} of them. If
  \kwd{reuse-address} \var{T} is used, then the file given by
  \var{path} is unlinked first.

\end{defun}

\section{Accepting Connections}

Once a socket is bound to its interface, we have to explicitly accept
connections. This task is performed by the functions we document here.

\begin{defun}{extensions:}{accept-tcp-connection}{%
    \args{\var{unconnected}}}
  
  Waits until a connection arrives on the (internet family) socket
  \var{unconnected}. Returns the file descriptor of the connection.
  These can be conveniently encapsulated using file descriptor
  streams; see \ref{sec:fds}.

\end{defun}

\begin{defun}{extensions:}{accept-unix-connection}{%
    \args{\var{unconnected}}}

  Waits until a connection arrives on the (unix family) socket
  \var{unconnected}. Returns the file descriptor of the connection.
  These can be conveniently encapsulated using file descriptor
  streams; see \ref{sec:fds}.

\end{defun}

\section{Connecting}

The task performed by the functions we present next is connecting to
remote hosts.

\begin{defun}{extensions:}{connect-to-inet-socket}{%
    \args{\var{host} \var{port} \ampoptional{} \var{kind}}}
  
  Tries to open a connection to the remote host \var{host} (which may
  be an IP address in host order, or a string with either a host name
  or an IP address in dotted format) on port \var{port}. Returns the
  file descriptor of the connection.  The optional parameter
  \var{kind} can be either \kwd{stream} (the default) or \kwd{datagram}.

\end{defun}

\begin{defun}{extensions:}{connect-to-unix-socket}{%
    \args{\var{path} \ampoptional{} \var{kind}}}
  
  Opens a connection to the unix ``adress'' given by \var{path}.
  Returns the file descriptor of the connection.  The type of
  connection is given by \var{kind}, which can be either \kwd{stream}
  (the default) or \kwd{datagram}.

\end{defun}

\section{Out-of-Band Data}
\label{internet-oob}

Out-of-band data is data transmitted with a higher priority than
ordinary data. This is usually used by either side of the connection
to signal exceptional conditions. Due to the fact that most TCP/IP
implementations are broken in this respect, only single characters can
reliably be sent this way.

\begin{defun}{extensions:}{add-oob-handler}{%
    \args{\var{fd} \var{char} \var{handler}}}
  
  Sets the function passed in \var{handler} as a handler for the
  character \var{char} on the connection whose descriptor is \var{fd}.
  In case this character arrives, the function in \var{handler} is
  called without any argument.

\end{defun}

\begin{defun}{extensions:}{remove-oob-handler}{%
    \args{\var{fd} \var{char}}}
  
  Removes the handler for the character \var{char} from the connection
  with the file descriptor \var{fd}

\end{defun}

\begin{defun}{extensions:}{remove-all-oob-handlers}{%
    \args{\var{fd}}}

  After calling this function, the connection whose descriptor is
  \var{fd} will ignore any out-of-band character it receives.

\end{defun}

\begin{defun}{extensions:}{send-character-out-of-band}{%
    \args{\var{fd} \var{char}}}

  Sends the character \var{char} through the connection \var{fd} out
  of band.

\end{defun}

\section{Unbound Sockets, Socket Options, and Closing Sockets}

These functions create unbound sockets. This is usually not necessary,
since connectors and listeners create their own.

\begin{defun}{extensions:}{create-unix-socket}{%
    \args{\ampoptional{} \var{type}}}
  
  Creates a unix socket for the unix address family, of type
  \var{:stream} and (on success) returns its file descriptor.

\end{defun}

\begin{defun}{extensions:}{create-inet-socket}{%
    \args{\ampoptional{} \var{type}}}
  
  Creates a unix socket for the internet address family, of type
  \var{:stream} and (on success) returns its file descriptor.

\end{defun}

Further, it is desirable to be able to change socket options. This is
performed by the following two functions, which are essentially
wrappers for system calls to {\tt getsockopt} and {\tt setsockopt}.

\begin{defun}{extensions:}{get-socket-option}{%
    \args{\var{socket} \var{level} \var{optname}}}
  
  Gets the value of option \var{optname} from the socket \var{socket}.

\end{defun}

\begin{defun}{extensions:}{set-socket-option}{%
    \args{\var{socket} \var{level} \var{optname} \var{optval}}}
  
  Sets the value of option \var{optname} from the socket \var{socket}
  to the value \var{optval}.

\end{defun}

For information on possible options and values we refer to the
manpages of {\tt getsockopt} and {\tt setsockopt}, and to {\tt
 socket.h}

Finally, the function

\begin{defun}{extensions:}{close-socket}{%
    \args{\var{socket}}}

  Closes the socket given by the file descriptor \var{socket}.

\end{defun}

\section{Unix Datagrams}

Datagram network is supported with the following functions.

\begin{defun}{unix:}{inet-recvfrom}{%
	\args{\var{fd} \var{buffer} \var{size}}
	\keys{\var{flags}}}
   A simple interface to the Unix \code{recvfrom} function.
\end{defun}

\begin{defun}{unix:}{inet-sendto}{%
	\args{\var{fd} \var{buffer} \var{size} \var{port}}
	\keys{\var{flags}}}
   A simple interface to the Unix \code{sendto} function.
\end{defun}

\begin{defun}{unix:}{inet-shutdown}{%
	\args{\var{fd} \var{level}}}

   A simple interface to the Unix \code{shutdown} function.  For
   \code{level}, you may use the following symbols to close one or
   both ends of a socket: \code{shut-rd}, \code{shut-wr},
   \code{shut-rdwr}.

\end{defun}

\section{Errors}

Errors that occur during socket operations signal a
\code{socket-error} condition, a subtype of the \code{error}
condition.  Currently this condition includes just the Unix
\code{errno} associated with the error.
