Directive s=pw sets structure to phrase if term includes blank(s).

[yaz-moved-to-github.git] / doc / comstack.xml

<!-- $Header: /home/cvsroot/yaz/doc/comstack.xml,v 1.1 2001-01-04 13:36:24 adam Exp $ -->
<chapter><title id="comstack">The COMSTACK Module</title>

<sect1><title>Synopsis (blocking mode)</title>

<programlisting>

COMSTACK *stack;
char *buf = 0;
int size = 0, length_incoming;
char *protocol_package; 
int protocol_package_length;
char server_address[] = "myserver.com:2100";
int status;

stack = cs_create(tcpip_type, 1, PROTO_Z3950);
if (!stack) {
    perror("cs_create");  /* note use of perror() here since we have no stack yet */
    exit(1);
}

status = cs_connect(stack, server_address);
if (status != 0) {
    cs_perror(stack, "cs_connect");
    exit(1);
}

status = cs_put(stack, protocol_package, protocol_package_length);
if (status) {
    cs_perror(stack, "cs_put");
    exit(1);
}

/* Now get a response */

length_incoming = cs_get(stack, &amp;buf, &amp;size);
if (!length_incoming) {
    fprintf(stderr, "Connection closed\n");
    exit(1);
} else if (length_incoming < 0) {
    cs_perror(stack, "cs_get");
    exit(1);
}

/* Do stuff with buf here */

/* clean up */
cs_close(stack);
if (buf)
    free(buf);

</programlisting>

</sect1>
<sect1><title>Introduction</title>

<para>
The &comstack;
subsystem provides a transparent interface to different types of transport
stacks for the exchange of BER-encoded data. At present, the
RFC1729 method (BER over TCP/IP), and Peter Furniss' XTImOSI
stack are supported, but others may be added in time. The philosophy of the
module is to provide a simple interface by hiding unused options and
facilities of the underlying libraries. This is always done at the risk
of losing generality, and it may prove that the interface will need
extension later on.
</para>

<para>
The interface is implemented in such a fashion that only the
sub-layers constructed to the transport methods that you wish to
use in your application are linked in.
</para>

<para>
You will note that even though simplicity was a goal in the design,
the interface is still orders of magnitudes more complex than the
transport systems found in many other packages. One reason is that
the interface needs to support the somewhat different requirements of
the different lower-layer communications stacks; another important reason is
that the interface seeks to provide a more or less industrial-strength
approach to asynchronous event-handling. When no function is allowed
to block, things get more complex - particularly on the server
side. We urge you to have a look at the demonstration client and server
provided with the package. They are meant to be easily readable and
instructive, while still being at least moderately useful.
</para>

</sect1>
<sect1><title>Common Functions</title>

<sect2><title>Managing Endpoints</title>

<synopsis>
  COMSTACK cs_create(CS_TYPE type, int blocking, int protocol);
</synopsis>

<para>
Creates an instance of the protocol stack - a communications endpoint.
The <literal>type</literal> parameter determines the mode of communication.
At present, the values
<literal>tcpip_type</literal>
and
<literal>mosi_type</literal>
are recognized. The function returns a null-pointer if a system error
occurs. The <literal>blocking</literal> parameter should be one if you wish
the association to operate in blocking mode, zero otherwise. The
<literal>protocol</literal> field should be one of
<literal>PROTO_SR</literal> or <literal>PROTO_Z3950</literal>.
</para>


<synopsis>
int cs_close(COMSTACK handle);
</synopsis>

<para>
Closes the connection (as elegantly as the lower layers will permit),
and releases the resouces pointed to by the
<literal>handle</literal>
parameter. The
<literal>handle</literal>
should not be referenced again after this call.
</para>

<note>
<para>
We really need a soft disconnect, don't we?
</para>
</note>
</sect2>

<sect2><title>Data Exchange</title>

<synopsis>
  (COMSTACK handle, char *buf, int len);
</synopsis>

<para>
Sends
<literal>buf</literal>
down the wire. In blocking mode, this function will return only when a
full buffer has been written, or an error has occurred. In nonblocking
mode, it's possible that the function will be unable to send the full
buffer at once, which will be indicated by a return value of 1. The
function will keep track of the number of octets already written; you
should call it repeatedly with the same values of <literal>buf</literal>
and <literal>len</literal>, until the buffer has been transmitted.
When a full buffer has been sent, the function will return 0 for
success. -1 indicates an error condition (see below).
</para>

<synopsis>
  int cs_get(COMSTACK handle, char **buf, int *size);
</synopsis>

<para>
Receives a PDU from the peer. Returns the number of bytes
read. In nonblocking mode, it is possible that not all of the packet can be
read at once. In this case, the function returns 1. To simplify the
interface, the function is
responsible for managing the size of the buffer. It will be reallocated
if necessary to contain large packages, and will sometimes be moved
around internally by the subsystem when partial packages are read. Before
calling
<function>cs_get</function>
for the fist time, the buffer can be initialized to the null pointer,
and the length should also be set to 0 - cs_get will perform a
<function>malloc(2)</function>
on the buffer for you. When a full buffer has been read, the size of
the package is returned (which will always be greater than 1). -1
indicates an error condition.
</para>

<para>
See also the <function>cs_more()</function> function below.
</para>

<synopsis>
  int cs_more(COMSTACK handle);
</synopsis>

<para>
The <function>cs_more()</function> function should be used in conjunction
with <function>cs_get</function> and
<function>select(2)</function>.
The <function>cs_get()</function> function will sometimes
(notably in the TCP/IP mode) read more than a single protocol package
off the network. When this happens, the extra package is stored
by the subsystem. After callig <function>cs_get()</function>, and before
waiting for more input, You should always call
<function>cs_more()</function>
to check if there's a full protocol package already read. If
<function>cs_more()</function>
returns 1,
<function>cs_get()</function>
can be used to immediately fetch the new package. For the
mOSI
subsystem, the function should always return 0, but if you want your
stuff to be protocol independent, you should use it.
</para>

<note>
<para>
The <function>cs_more()</function>
function is required because the RFC1729-method
does not provide a way of separating individual PDUs, short of
partially decoding the BER. Some other implementations will carefully
nibble at the packet by calling
<function>read(2)</function>
several times. This was felt to be too inefficient (or at least
clumsy) - hence the call for this extra function.
</para>
</note>

<synopsis>
  int cs_look(COMSTACK handle);
</synopsis>

<para>
This function is useful when you're operating in nonblocking
mode. Call it when
<function>select(2)</function>
tells you there's something happening on the line. It returns one of
the following values:
</para>

<variablelist>
<varlistentry><term>CS_NONE</term><listitem><para>
No event is pending. The data found on the line was not a complete package.
</para></listitem></varlistentry>

<varlistentry><term>CS_CONNECT</term><listitem><para>
A response to your connect request has been received. Call
<function>cs_rcvconnect</function>
to process the event and to finalize the connection establishment.
</para></listitem></varlistentry>

<varlistentry><term>CS_DISCON</term><listitem><para>
The other side has closed the connection (or maybe sent a disconnect
request - but do we care? Maybe later). Call
<function>cs_close</function> to close your end of the association as well.
</para></listitem></varlistentry>

<varlistentry><term>CS_LISTEN</term><listitem><para>
A connect request has been received. Call <function>cs_listen</function>
to process the event.
</para></listitem></varlistentry>

<varlistentry><term>CS_DATA</term><listitem><para>
There's data to be found on the line. Call <function>cs_get</function>
to get it.
</para></listitem></varlistentry>
</variablelist>

<note>
<para>
You should be aware that even if
<function>cs_look()</function>
tells you that there's an event event pending, the corresponding
function may still return and tell you there was nothing to be found.
This means that only part of a package was available for reading. The
same event will show up again, when more data has arrived.
</para>
</note>

<synopsis>
  int cs_fileno(COMSTACK h);
</synopsis>

<para>
Returns the file descriptor of the association. Use this when
file-level operations on the endpoint are required
(<function>select(2)</function> operations, specifically).
</para>
</sect2>

</sect1>

<sect1><title>Client Side</title>

<synopsis>
  int cs_connect(COMSTACK handle, void *address);
</synopsis>

<para>
Initiate a connection with the target at <literal>address</literal>
(more onaddresses below). The function will return 0 on success, and 1 if
the operation does not complete immediately (this will only
happen on a nonblocking endpoint). In this case, use
<function>cs_rcvconnect</function> to complete the operation,
when <function>select(2)</function> reports input pending on the
association.
</para>

<synopsis>
  int cs_rcvconnect(COMSTACK handle);
</synopsis>

<para>
Complete a connect operation initiated by <function>cs_connect()</function>.
It will return 0 on success; 1 if the operation has not yet completed (in
this case, call the function again later); -1 if an error has occured.
</para>

</sect1>

<sect1><title>Server Side</title>

<para>
To establish a server under the <application>inetd</application> server, you
can use
</para>

<synopsis>
  COMSTACK cs_createbysocket(int socket, CS_TYPE type, int blocking,
                             int protocol);
</synopsis>

<para>
The <literal>socket</literal> parameter is an established socket (when
your application is invoked from <application>inetd</application>, the
socket will typically be 0.
The following parameters are identical to the ones for
<function>cs_create</function>.
</para>

<synopsis>
  int cs_bind(COMSTACK handle, void *address, int mode)
</synopsis>

<para>
Binds a local address to the endpoint. Read about addresses below. The
<literal>mode</literal> parameter should be either
<literal>CS_CLIENT</literal> or <literal>CS_SERVER</literal>.
</para>

<synopsis>
  int cs_listen(COMSTACK handle, char *addr, int *addrlen);
</synopsis>

<para>
Call this to process incoming events on an endpoint that has been
bound in listening mode. It will return 0 to indicate that the connect
request has been received, 1 to signal a partial reception, and -1 to
indicate an error condition.
</para>

<synopsis>
  COMSTACK cs_accept(COMSTACK handle);
</synopsis>

<para>
This finalises the server-side association establishment, after
cs_listen has completed successfully. It returns a new connection
endpoint, which represents the new association. The application will
typically wish to fork off a process to handle the association at this
point, and continue listen for new connections on the old
<literal>handle</literal>.
</para>

<para>
You can use the call
</para>

<synopsis>
  char *cs_addrstr(COMSTACK);
</synopsis>

<para>
on an established connection to retrieve the hostname of the remote host.
</para>

<note>
<para>You may need to use this function with some care if your
name server service is slow or unreliable
</para>
</note>

</sect1>
<sect1><title>Addresses</title>

<para>
The low-level format of the addresses are different depending on the
mode of communication you have chosen. A function is provided by each
of the lower layers to map a user-friendly string-form address to the
binary form required by the lower layers.
</para>

<synopsis>
  struct sockaddr_in *tcpip_strtoaddr(char *str);

  struct netbuf *mosi_strtoaddr(char *str);
</synopsis>

<para>
The format for TCP/IP addresses is straightforward:
</para>

<synopsis>
&lt;host> &lsqb; ':' &lt;portnum> &rsqb;
</synopsis>

<para>
The <literal>hostname</literal> can be either a domain name or an IP address.
The port number, if omitted, defaults to 210.
</para>

<para>
For OSI, the format is
</para>

<synopsis>
&lsqb; &lt;t-selector> '/' &rsqb; &lt;host> &lsqb; ':' &lt;port> &rsqb;
</synopsis>

<para>
The transport selector is given as an even number of hex digits.
</para>

<para>
You'll note that the address format for the OSI mode are just a subset
of full presentation addresses. We use presentation addresses because
xtimosi doesn't, in itself, allow access to the X.500 Directory
service. We use a limited form, because we haven't yet come across an
implementation that used more of the elements of a full p-address. It
is a fairly simple matter to add the rest of the elements to the
address format as needed, however: Xtimosi <emphasis>does</emphasis>
support the full P-address structure.
</para>

<para>
In both transport modes, the special hostname &quot;@&quot; is mapped
to any local address (the manifest constant <literal>INADDR_ANY</literal>).
It is used to establish local listening endpoints in the server role.
</para>

<para>
When a connection has been established, you can use
</para>

<synopsis>
  char cs_addrstr(COMSTACK h);
</synopsis>

<para>
to retrieve the host name of the peer system. The function returns a pointer
to a static area, which is overwritten on the next call to the function.
</para>

<note>
<para>
We have left the issue of X.500 name-to-address mapping open, for the
moment. It would be a simple matter to provide a table-based mapping,
if desired. Alternately, we could use the X.500 client-function that
is provided with the ISODE (although this would defeat some of the
purpose of using ThinOSI in the first place. We have been told that it
should be within the realm of the possible to implement a lightweight
implementation of the necessary X.500 client capabilities on top of
ThinOSI. This would be the ideal solution, we feel. On the other hand, it
still remains to be seen just what role the Directory will play in a world
populated by ThinOSI and other pragmatic solutions.
</para>
</note>

</sect1>

<sect1><title>Diagnostics</title>

<para>
All functions return -1 if an error occurs. Typically, the functions
will return 0 on success, but the data exchange functions
(<function>cs_get</function>, <function>cs_put</function>,
<function>cs_more</function>) follow special rules. Consult their
descriptions.
</para>

<para>
When a function (including the data exchange functions) reports an
error condition, use the function
<function>cs_errno()</function> to determine the cause of the
problem. The function
</para>

<synopsis>
  void cs_perror(COMSTACK handle char *message);
</synopsis>

<para>
works like <function>perror(2)</function> and prints the
<literal>message</literal> argument, along with a system message, to
<literal>stderr</literal>. Use the character array
</para>

<synopsis>
  extern const char *cs_errlist&lsqb;&rsqb;;
</synopsis>

<para>
to get hold of the message, if you want to process it differently.
The function
</para>

<synopsis>
  const char *cs_stackerr(COMSTACK handle);
</synopsis>

<para>
Returns an error message from the lower layer, if one has been
provided.
</para>
</sect1>

<sect1><title>Enabling OSI Communication</title>

<sect2><title>Installing Xtimosi</title>
<para>
Although you will have to download Peter Furniss' XTI/mOSI
implementation for yourself, we've tried to make the integration as
simple as possible.
</para>

<para>
The latest version of xtimosi will generally be under
</para>

<screen>
ftp://pluto.ulcc.ac.uk/ulcc/thinosi/xtimosi/
</screen>

<para>
When you have downloaded and unpacked the archive, it will (we assume)
have created a directory called <literal>xtimosi</literal>.
We suggest that you place this directory <emphasis>in the same
directory</emphasis> where you unpacked the &yaz;
distribution. This way, you shouldn't have to fiddle with the
makefiles of &yaz; beyond uncommenting a few lines.
</para>

<para>
Go to <literal>xtimosi/src</literal>, and type &quot;make libmosi.a/&quot;.
This should generally create the library, ready to use.
</para>

<note>
<para>
The currently available release of xtimosi has some inherent
problems that make it disfunction on certain platforms - eg. the
Digital OSF/1 workstations. It is supposedly primarily a
compiler problem, and we hope to see a release that is generally
portable. While we can't guarantee that it can be brought to work
on your platform, we'll be happy to talk to you about problems
that you might see, and relay information to the author of the
software. There are some signs that the <application>gcc</application>
compiler is more likely to produce a fully functional library, but this
hasn't been verified (we think that the problem is limited to the use
of hexadecimal escape-codes used in strings, which are silently
ignored by some compilers).
</para>
<para>
A problem has been encountered in the communication with
ISODE-based applications. If the ISODE presentation-user calls
<function>PReadRequest()</function> with a timeout value different
from <literal>OK</literal> or <literal>NOTOK</literal>,
he will get an immediate TIMEOUT abort when receiving large (&gt;2041
bytes, which is the SPDU-size that the ISODE likes to work with) packages
from an xtimosi-based implementation (probably most
other implementations as well, in fact). It seems to be a flaw in the
ISODE API, and the workaround (for ISODE users) is to either not
use an explicit timeout (switching to either blocking or
nonblocking mode), or to check that the timer really has expired
before closing the connection.
</para>
</note>

<para>
The next step in the installation is to modify the makefile in the toplevel
&yaz;
directory. The place to change is in the top of the file, and is
clearly marked with a comment.
</para>

<para>
Now run <literal>make</literal> in the &yaz; toplevel directory (do a
<literal>make clean</literal> first, if the system has been previously
made without OSI support). Use the &yaz;
<application>yaz-ztest</application> and <application>yaz-client</application>
demo programs to verify that OSI communication works OK. Then, you can go
ahead and try to talk to other implementations.
</para>

<note>
<para>
Our interoperability experience is limited to version
7 of the Nordic SR-Nett package, which has had several
protocol errors fixed from the earlier releases. If you have
problems or successes in interoperating with other
implementations, we'd be glad to hear about it, or to help
you make things work, as our resources allow.
</para>
</note>

<para>
If you write your own applications based on &yaz;, and you wish to
include OSI support, the procedure is equally simple. You should
include the <filename>xmosi.h</filename> header file in addition to
<filename>comstack.h</filename>. <filename>xmosi.h</filename>
will define the manifest constant <literal>mosi_type</literal>, which you
should pass to the <function>cs_create()</function> function. In
addition, you should use the function <function>mosi_strtoaddr()</function>
rather than <function>tcpip_strtoaddr()</function> when you need to
prepare an address.
</para>

<para>
When you link your application, you should include (after the
<filename>libyaz.a</filename> library) the <literal>libmosi.a</literal>
library, and the <filename>librfc.a</filename> library provided with
&yaz; (for OSI transport).
</para>
<para>
As always, it can be very useful, if not essential, to have a look at the
example applications to see how things are done.
</para>

</sect2>
<sect2><title>OSI Transport</title>

<para>
Xtimosi requires an implementation of the OSI transport service under
the X/OPEN XTI API. We provide an implementation of the RFC1006
encapsulation of OSI/TP0 in TCP/IP (through the Berkeley Sockets API),
as an independent part of &yaz; (it's found under the
<filename>rfc1006</filename> directory).
If you have access to an OSI transport provider under XTI,
you should be able to make that work too, although it may require
tinkering with the <function>mosi_strtoaddr()</function> function.
</para>
</sect2>

<sect2><title>Presentation Context Management</title>

<para>
To simplify the implementation, we use Peter Furniss' alternative (PRF)
option format
for the Control of the presentation negotiation phase. This format
is enabled by default when you
compile xtimosi.
</para>

<para>
The current version of &yaz; does <emphasis>not</emphasis> support
presentation-layer negotiation of response record formats. The primary
reason is that we have had access to no other SR or Z39.50
implementations over OSI that used this
method. Secondarily, we believe that the EXPLAIN facility is a superior
mechanism for relaying target capabilities in this respect. This is not to
say that we have no intentions of supporting presentation context
negotiation - we have just hitherto given it a lower priority than other
aspects of the protocol.
</para>
<para>
One thing is certain: The addition of this capability to &yaz; should
have only a minimal impact on existing applications, and on the
interface to the software in general. Most likely, we will add an extra
layer of interface to the processing of EXPLAIN records, which will
convert back and forth between <literal>oident</literal> records (see
section <link linkend="oid">Object Identifiers</link>) and direct or
indirect references, given the current association setup. Implementations
based on any of the higher-level interfaces will most likely not have to
be changed at all.
</para>
</sect2>
</sect1>
<sect1><title>Summary and Synopsis</title>

<synopsis>
#include &lt;comstack.h>

#include &lt;tcpip.h>      /* this is for TCP/IP support   */
#include &lt;xmosi.h>      /* and this is for mOSI support */

COMSTACK cs_create(CS_TYPE type, int blocking, int protocol);

COMSTACK cs_createbysocket(int s, CS_TYPE type, int blocking,
                              int protocol);

int cs_bind(COMSTACK handle, int mode);

int cs_connect(COMSTACK handle, void *address);

int cs_rcvconnect(COMSTACK handle);

int cs_listen(COMSTACK handle);

COMSTACK cs_accept(COMSTACK handle);

int cs_put(COMSTACK handle, char *buf, int len);

int cs_get(COMSTACK handle, char **buf, int *size);

int cs_more(COMSTACK handle);

int cs_close(COMSTACK handle);

int cs_look(COMSTACK handle);

struct sockaddr_in *tcpip_strtoaddr(char *str);

struct netbuf *mosi_strtoaddr(char *str);

extern int cs_errno;

void cs_perror(COMSTACK handle char *message);

const char *cs_stackerr(COMSTACK handle);

extern const char *cs_errlist[];
</synopsis>
</sect1>

</chapter>

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