
=head1 NAME

Net::SIP::NATHelper::Base - rewrite SDP and transport RTP for NAT

=head1 DESCRIPTION

This module helps with doing NAT. It is implicitly used in B<do_nat>
from L<Net::SIP::StatelessProxy>.

It cares about the rewriting the SDP bodies, forwarding RTP data for
active sessions and expiring sessions.

=head1 CONSTRUCTOR

=over 4

=item new ( %ARGS )

Creates a new object.
%ARGS can be of:

=over 8

=item max_sockets N

Restricts the maximum number of sockets allocated inside the object to N.

=item max_sockets_in_group N

Restricts the maximum number of sockets allocated for a single socket group
(e.g. a single call to B<get_rtp_sockets>) to N.

=back

=back

=head1 METHODS

=over 4

=item allocate_sockets ( CALLID,CSEQ,IDFROM,IDTO,SIDE,ADDR,MEDIA )

This is called to allocate new local sockets for MEDIA. MEDIA
is a \@list of specifications like you get from B<get_media>
in L<Net::SIP::SDP>. ADDR is the local address, where the sockets
should be allocated.

IDFROM and IDTO represent the sides of the session, while SIDE
helps to pick the right side for allocation, e.g. if SIDE is 0
the sockets will be allocated on the IDFROM side, if it is 1
it will be on the IDTO side. Thus for Requests SIDE will be 0,
while for responses it will be 1.

CALLID and CSEQ are used to identify the SIP transaction, for which
the NAT will be done, while IDSIDE is either IDFROM or IDTO (see below)
depending on the side, where the packet came in.

The methode will return  the \@list of new media in the format
needed by B<replace_media_listen> in L<Net::SIP::SDP>, e.g tuples
of C<[ip,base_port]>.

If the allocation of sockets failed (because of resource constraints)
it will return undef. This will usually cause the caller do simply
not forward the packet and wait for the reetransmit (at least for UDP).

=item activate_session ( CALLID,CSEQ,IDFROM,IDTO,[\%PARAM] )

Activates a session in the transaction defined by CALLID,CSEQ. This
will cause all sessions for older transactions in the same call (identified
by CALLID) to shut down.

IDFROM and IDTO represent the sides of the session, e.g. for the activation
to succeed there had to be an B<allocate_sockets> call for each of these
sides.

PARAM is an optional hash reference. If given it will be added as user
information to the new session and given back in the information hash
returned by L<close_session> etc. 

The method returns two items: The first is information about the session
like returned in L<close_session>, the second is a flag, which is true,
if the activation was for an already astablished session.

The method is usually called whenever a SDP body is given and on ACK requests.

=item close_session ( CALLID,CSEQ,IDFROM,IDTO )

This will cause the close of the session described by the arguments
(which have the same meaning like in B<activate_session>). Usually
called for BYE or CANCEL requests. For CANCEL it will be called with
CSEQ (because it should cancel a specific transaction) while for
BYE it will called with CSEQ undef, because it should end all sessions
in this call.

It will return a list with infos about all closed sessions. Each of
these infos is a reference to a hash with the following items:

=over 8

=item callid

=item cseq

=item idfrom

=item idto

=item bytes_from - number of bytes received on the 'from' side

=item bytes_to - number of bytes received on the 'to' side

=item from - space delimited list of "ip:port/range" for the media on the 'from' side

=item to - same for the 'to' side

=item created - C<time_t> when the session was created

=back


Stateless proxies should forward the packet even if the session did
not exist, because the packet might be a retransmit referring to an
already closed session.

=item expire ( [ %ARGS ] )

This causes the sessions and sockets to expire. 
It will return a list with a hash for each expired session.
See L<close_session> for details on the returned information.

With %ARGS the behavior can be customized:

=over 8

=item time

Current time, can be given to save system call for getting current time.

=item unused

Expire time for unused sockets. Default ist 180, e.g. 3 minutes.

=item active

Expire time for active sessions. Default to 30 seconds. If no data
gets transfered through the session for C<active> seconds the session
will be closed.

=back

=item callbacks

This will return a list (not a reference) of C<< [cbid,fd,callback] >> pairs,
where fd is the file descriptor of the socket and callback is a callback
for B<invoke_callback> in L<Net::SIP::Util>. cbid is a uniq id for each callback
so that one get compare the result from different calls to L<callbacks>
and see, what changed.

Each of these callbacks should be invoked once the file descriptor gets
readable and the callback will then forward the received packet to the
other side of the session.

=item sessions ( [ CALLBACK ] )

This will call CALLBACK on all sessions and return an array containing
the results of the callbacks together. If no CALLBACK is given it will
return all session objects.

To use this function you might need to know the interna of the session
objects (see source code), but in most cases it is enough to know, that
session objects have a method L<info_as_hash> which will return infos 
about the session as hash like described in L<close_session>.

=item get_rtp_sockets ( NEW_ADDR, @MEDIA )

Allocates sockets for on address NEW_ADDR for @MEDIA. @MEDIA is in
the format given by B<get_media> in L<Net::SIP::SDP>.

Returns reference to array of allocated media, each item in the
array is C<< [ ADDR,BASE_PORT,\@SOCKS,\@TARGETS ] >>, where ADDR
is NEW_ADDR, BASE_PORT the base port for the allocated sockets,
@SOCKS the allocated sockets and @TARGETS the C<sockaddr_in> of
the original targets, e.g. where it needs to forward the data
received on the sockets.

If not all necssary sockets could be allocated it will allocate
none and return C<()>.

This method might be redefined to better control resource allocation
or to cache temporally unused resources together with unget_rtp_sockets.

=item unget_rtp_sockets ( NEW_MEDIA )

Will take resources back. NEW_MEDIA is the result from B<get_rtp_sockets>.
Returns nothing.

Together with get_rtp_sockets it could be redefined to not destroy
resources but to cache them for future use.

=item dump

This method is used for debugging only. It will return a string with information
about all calls within this nathelper and all sessions and allocated sockets within
the calls.

=item number_of_calls

Returns the number of calls within the nathelper. This vaalue has not much
association with the result of B<callbacks> because there might be inactive
sockets (not yet or no longer in active session) which don't affect the
result of B<callbacks>.

This value can be used to determine if B<expire> needs to be called at all.

=back
