Figure 1.1 The TCP relational structure. If the server offers both incoming and outgoing communication, as with TCP/IP, then the relationship between the entities is bilateral, or mutual. If the  server  is  nothing  more  than  a  receiver  and  processor  of  information without returning a confirmation of processes success or failure back to the client post-process, as with UDP  (User Datagram Protocol), then the relationship  is  parasitic,  or  unilateral.  In  the  case  of  XML  Web  services, were virtually always dealing with bilateral relationships. SOAP is not a first-tier or second-tier protocol (see Figure 1.1) because it depends  on  a  second-tier  protocol  for  translation  capabilities  across  the Internet—HTTP. In turn, the HTTP protocol depends on the TCP transport. Instead, SOAP sits atop the other protocols. It uses its own standard speci- fications supported by its own specific reserved words, designated num- bers, and so forth. What makes SOAP really powerful and noteworthy is that  it  supports  language  and  platform  interoperability  through  XML. Another factor that makes the SOAP protocol remarkable is that the devel- opers of the protocol brought structure and reusability to an age-old prac- tice where standards were not always possible. SOAP - Simple Object Access Protocol FTP - File Transfer Protocol HTTP - Hyper- Text Transfer Protocol All Others - (e.g., SMTP, POP3, and Telnet) TCP/IP Transmission Control Protocol/ Internet Protocol Third-Tier Protocol Second-Tier Protocols First-Tier Protocol TCP Relational Structure What Are Web Services? 5