What is the Standard for Two - way WebService Communication between two services C# .NET

If you are using VS2008, use WCF callbacks:
http://idunno.org/archive/2008/05/29/wcf-callbacks-a-beginners-guide.aspx
http://www.devx.com/dotnet/Article/38814
http://msdn.microsoft.com/en-us/magazine/cc163537.aspx
Even though the examples are not web services, it is just a matter of hosting the WCF library on the web service and define the binding as "WsDualHttpBinding".

Note: Only .net web services/clients can use the callback mechanism. Web services by default do not support callbacks.

Why do you have all the extra moving parts and complexity?  Could you solve your problem without web services?  What problem is the web service solving that you can'd do any other way?

In my world there seem to be two architectures that get a lot of attention: SOAP and REST.  SOAP is complicated and difficult; REST is simple and easy to implement.  Here are links to a quick overview of those things.
http://en.wikipedia.org/wiki/SOAP
http://en.wikipedia.org/wiki/REST

In the RESTful way of web services, every request is atomic and complete (HTTP is a RESTful architecture) and there is no call-to-call dependency.  REST is stateless.  Examples of RESTful web services include the elegant and well-documented Yahoo Geocoder.  Though it is deprecated for unrelated reasons, it is one of the best examples of a successful web service ever written.
http://developer.yahoo.com/maps/rest/V1/geocode.html

mas - I will look

ted billy - Architecture restrictions

Ray = is rest implementable by .Net applications/C# ?  Deprecated sounds like there may be some dirty little secrets

Ray- do you know of any examples being used by .net I can look at for REST >?

mas - is this available in 2.0 ???

The Yahoo Geocoder (linked above) is deprecated for unrelated reasons - they offer a new technology.  The DESIGN PATTERN is what we're after here, and it is a perfect example of a RESTful implementation.  There are no dirty secrets.

Yes, REST is implementable in almost any imaginable language.  I'm not a .NET guy, but maybe you can see through the PHP code here in my teaching example of a REST service.  In its simplest design, you send URL arguments (GET strings) and read the browser output stream.  The answers might be formatted with XML or JSON, or just presented as simple strings.

<?php // RAY_REST_get_last_name.php
error_reporting(E_ALL);



// DEMONSTRATE HOW A RESTFUL WEB SERVICE WORKS - INPUT FIRST NAME, OUTPUT LAST NAME
// CALLING EXAMPLE:
// file_get_contents('http://laprbass.com/RAY_REST_get_last_name.php?key=ABC&resp=XML&name=Ray');



// OUR DATA MODEL CONTAINS ALL THE ANSWERS - THIS COULD BE A DATA BASE - AS SIMPLE OR COMPLEX AS NEEDED
$dataModel
= array
( 'Brian'   => 'Portlock'
, 'Ray'     => 'Paseur'
, 'Richard' => 'Quadling'
, 'Dave'    => 'Baldwin'
)
;


// SHOULD RESPONSE BE PLAIN TEXT OR XML FORMAT
$alpha = '';
$omega = '';
if ( (isset($_GET["resp"])) && ($_GET["resp"] == 'XML') )
{
    $alpha = '<response>';
    $omega = '</response>';
}



// TEST THE API KEY - THE ASSUMPTION IS FAILURE
$key = FALSE;

// IF THE URL CONTAINS THE KEY
if (isset($_GET["key"])) $key = $_GET["key"];

// IF THE KEY FAILS TO MATCH OUR OUR REQUIRED 'ABC'
if ($key !== 'ABC') die($alpha . 'BOGUS API KEY' . $omega);



// LOOK UP THE LAST NAME - THE ASSUMPTION IS FAILURE
$name="?";

// IF THE URL CONTAINS THE FIRST NAME
if (isset($_GET["name"])) $name = $_GET["name"];

// IF THE URL NAME IS FOUND IN THE DATA MODEL
if (array_key_exists($name, $dataModel))
{
    // RETURNS THE LAST NAME FROM THE DATA MODEL
    die($alpha . "$dataModel[$name]" . $omega);
}

// RETURNS THE UNKNOWN NAME INDICATOR
else die($alpha . 'UNKNOWN' . $omega);

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If use are uisng .Net 3.5, you can use Duplex Mode of MEP(Message Exchange Pattern) in WCF.