SOAP Message Structure

A SOAP message is an ordinary XML document containing the following elements.

• Envelope: ( Mandatory )
  Defines the start and the end of the message.
  
• Header: ( Optional )
  Contains any optional attributes of the message used in processing the message, either at an intermediary point or at the ultimate end point.
  
• Body: ( Mandatory )
  Contains the XML data comprising the message being sent.
  
• Fault: ( Optional )
  An optional Fault element that provides information about errors that occurred while processing the message
  

All these elements are declared in the default namespace for the SOAP envelope:
http://www.w3.org/2001/12/soap-envelope
and the default namespace for SOAP encoding and data types is:
http://www.w3.org/2001/12/soap-encoding

NOTE: All these specifications are subject to change. So keep updating yourself with the latest specifications available W3 website.


Syntax Rules
Here are some important syntax rules:

• A SOAP message MUST be encoded using XML
• A SOAP message MUST use the SOAP Envelope namespace
• A SOAP message MUST use the SOAP Encoding namespace
• A SOAP message must NOT contain a DTD reference
• A SOAP message must NOT contain XML Processing Instructions

A SOAP Message Structure-

<?xml version="1.0"?>
<SOAP-ENV:Envelope
xmlns:SOAP-ENV="http://www.w3.org/2001/12/soap-envelope"
SOAP-ENV:encodingStyle="http://www.w3.org/2001/12/soap-encoding">

<SOAP-ENV:Header>
  ...
  ...
</SOAP-ENV:Header>

<SOAP-ENV:Body>

  ...
  ...
  <SOAP-ENV:Fault>
    ...
    ...
  </SOAP-ENV:Fault>
</SOAP-ENV:Body>

</SOAP_ENV:Envelope>

References
1. Wikipedia for SOAP