1. A computer system implementing a first content publisher addressable by a first domain name, the computer system comprising:at least one processor implementing a plurality of software modules embodied on one or more machine-readable media, the modules comprising:
a cookie setter configured to cause a client device communicatively coupled to the first content publisher to store a plurality of tokens under the first domain name, the plurality of tokens specifying state data during a communication session between the client device and the first content publisher, the communication session utilizing a stateless communication protocol, each of the plurality of tokens comprising a respective name-value pair;
a redirection module configured to embed, in content provided by the first content publisher, a uniform resource locator identifying a server-side script hosted by a second content publisher addressable by a second domain name, the redirection module further to embed, in association with the uniform resource locator, data representing the name-value pairs of the plurality of tokens; and
a communication module configured to communicate the content with the embedded uniform resource locator to the client device, selection of the uniform resource locator by a user causing the client device to send a web request for the server-side script to the second content publisher, the web request including the data representing the name-value pairs of the plurality of tokens, execution of the server-side script causing the second content publisher to extract the data representing the name-value pairs of the plurality of tokens and to send a directive to the client device to set, under the second domain name, a token having a name-value pair whose value encodes the name-value pairs of the plurality of tokens previously set under the first domain name.