1. A computer-implemented method comprising: creating a message in an Iframe in a first domain for communication to a second domain, the act of creating being performed by a web page that includes an associated listener Iframe in the first domain that is configured to receive messages from the second domain; using the associated listener Iframe as a target window for cross domain communication between the first and second domains, the cross domain communicating being performed without round tripping to a server and comprising: communicating, using a post back URL, relationship information associated with a user between the first and second domains such that the relationship information associated with the user remains protected; manipulating a URL associated with the Iframe, wherein the manipulating comprises: appending the created message to a URL of the form: scheme://authority/path?query#[message], such that: the authority comprises a name or IP address; the path specifies a location in some hierarchical structure; the query expresses parameters of a dynamic query to a database, program or script; and a fragment occurring after the hash (“#”) identifies a portion of a resource; detecting a change in the URL at the Iframe and parsing the URL to access the message; and associating a unique, incremental ID for each new message from a specific URL; using a message counter implemented as a field in a URL associated with a target recipient of the message to ensure that messages that are communicated between the different domains are not missed, wherein if the message counter is off by one or more increments, requesting a missing message from a sender; and processing the message.