Attachments in SOAP Messages

Web Services rely on SOAP, an XML-based communication protocol for exchanging messages between computers regardless of their operating systems, programming environment. SOAP is the de facto standard messaging protocol used by web services and codifies the use of XML as an encoding scheme for request and response parameters using HTTP as a means for transport. For better memory requirements, smaller message size, smaller process times for SOAP Messages, Attachments are used to prevent large volume of Data being send as part of SOAP Message or non-XML Data ( for e.g. media files etc) also need to be part of SOAP Message. Following different approaches were used to send attachments in SOAP Messages:
1. WS-Attachments over DIME(Direct Internet Message Encapsulation)
DIME is a packaging mechanism that allows multiple records of arbitrarily formatted data to be streamed together. Records are serialized into the stream one after the other and are delineated with an efficient binary header. For large records or records where the size of the data is not initially known, DIME has defined a "record chunk”. WS-Attachments indicate that the Primary SOAP Message Part (Main Message) must be contained in the first record of a DIME message. WS-Attachments define the use of the HREF attribute for making a reference to Attachment. For the most part it is similar to simply sending the primary SOAP message part on its own, except that the HTTP Content-Type header must be set to "application/dime" and the body of the HTTP request is the DIME message instead of the SOAP message.
2. SOAP with Attachments ( SwA)
SwA defines a way for binding attachments to a SOAP envelope using the multipart/related MIME type. MIME cannot be represented as an XML Infoset – this effectively breaks the web services model since attachments cannot be secured using WS-Security.
3. Message Transmission and Optimization Mechanism(MTOM)
MTOM is based over MIME and includes attachments as part of the Infoset (since SOAP 1.2 is built around Infoset), thus making the SOAP 1.2 processing model applicable to the attachments as well. MTOM combines the composability of Base 64 encoding with the transport efficiency of SOAP with Attachments. Non-XML data is processed just as it is with SOAP with Attachments SWA – the data is simply streamed as binary data in one of the MIME message parts.
MTOM is composed of three distinct specifications:
• MTOM CORE describes an abstract feature for optimizing the transmission and/or wire format of a SOAP 1.2 message by selectively encoding portions of the message, while still presenting an XML Infoset to the SOAP application.
• XOP (XML-binary Optimization Packaging) specifies the method for serializing XML Infosets with non-XML content into MIME packages.
• The Resource Representation SOAP Header Block specification defines a SOAP header block that can carry resource representations within SOAP messages.
MTOM attachments are streamed as binary data within a MIME message part, making it fairly easy to pass MTOM attachments to SWA or receive SWA attachments into an MTOM implementation.Image1 describes the attachment using MTOM.

MTOM specification is in fact part of Messaging (includes SOAP, WS-Addressing, and MTOM) part of WSIT & Web Services Enhancements (WSE) for Microsoft .NET specifications. Image-2 compares the different attachment approaches in SOAP messages.

MTOM support is fully interoperable with .NET clients and servers using the Metro distribution and is currently the best option for sending the attachments in SOAP Messages.