To actually get SOAP messages from one node to another, you must use some underlying transport protocol, such as HTTP [RFC 2616] or SMTP [RFC 2821, 2822]. The extent to which you must augment or restrict the transport protocol varies depending on how closely the native mechanism of the protocol matches the needs of SOAP.
By convention, most TCP/IP-based protocols have a default "port" number for example, port 80 for HTTP and port 25 for SMTP. Servers that support a protocol normally have, listening on the default port, a process that handles incoming service requests. The SOAP specification recommends the use of a different port number when you use an existing transport protocol with SOAP. This recommendation reflects the fact that the service being requested is SOAP service, rather than the service for which the transport protocol was originally intended. For example, HTTP was originally developed to support Web browsing. Although you can accommodate both that use and SOAP on the default HTTP port 80, it is simpler and more efficient to have separate processes servicing and optimized for these different kinds of requests on different ports.
8.4.1 Transport Message Exchange Patterns
The XML Protocol Working Group is developing a general model for transport mechanism patterns and their binding to SOAP. Such patterns describe the following aspects of transport:
The Transport Message Exchange Pattern (TMEP) model is complex; readers are referred to [SOAP] for its detail. The only TMEP specified thus far is the Single-Request-Response TMEP, named in the current draft by the URI "http://www.example.org/2001/12/soap/transport-mep-single-request-response."
8.4.2 The SOAP HTTP Binding
The Hypertext Transfer Protocol [RFC 2616] is a convenient protocol to use for SOAP. HTTP is based on a request response model that works well when the single request response TMEP is being used in SOAP. However, the semantic match is not complete, so some interpretation is required [SOAP]. For example, HTTP intermediaries (such as proxies) are not necessarily SOAP intermediaries.
When carrying SOAP in HTTP, you must take three considerations into account:
You use the SOAPAction Header as a optional hint to optimize processing. It takes a URI, which can be relative or absolute. If the URI is relative, interpret it using the HTTP Request-URI as a base. If a SOAP receiver does not require it, it must ignore the absence of this Header or the presence of an incorrect SOAPAction Header. Conversely, if a SOAP receiver requires this Header, and none is present or the receiver does not recognize the one provided, it must respond with the following HTTP error status code:
427 "SOAPAction Required"
An HTTP response with this 427 status code may have a Required-SOAPAction Header that gives a URI that a SOAPAction Header can use in a resubmission of the HTTP request. The following example shows a SOAPAction request Header:
The following example shows a Required-SOAPAction response Header:
[SOAP] contains detailed tables describing how to create and respond to other HTTP error codes. These responses generally preserve the existing HTTP semantics of the codes.