Presently disclosed is a method and apparatus for monitoring and diagnosing a content delivery network (CDN) by examining received content elements that have been marked with one or more identifiers. Diagnosing is accomplished by using all or part of one or more of the identifiers associated with errored or corrupted content elements to determine which network elements or connection paths are faulty. The identifiers may contain content-, server-, or receiver-specific data that denotes, respectively, the source of the content, the links associated with one or more of the servers utilized in the network, and/or data associated with the receiving computer system. Both marking and diagnosing may be performed at any location within the CDN or at any host computer connected to the CDN or receiver. The marking may also be dynamically reconfigured in order to facilitate diagnosis.