A Last Call Working Draft for SOAP 1.2 Attachment Feature has been released by the W3C XML Protocol Working Group as part of the W3C Web Services Activity. The specification is based upon the IETF Internet-Draft "WS-Attachments" and "defines a SOAP feature that represents an abstract model for SOAP attachments. The compound SOAP structure model is abstract in the sense that it does not define an actual means by which compound SOAP structures are represented or transmitted by a SOAP binding. [The specification] provides the basis for the creation of SOAP bindings that transmit such attachments along with a SOAP envelope, and provides for reference of those attachments from the envelope. SOAP attachments are described using the notion of a compound document structure consisting of a primary SOAP message part and zero or more related documents parts known as attachments. The compound SOAP structure model is independent of the underlying protocol used for transmitting the primary SOAP message part and any of the secondary parts. That is, there is no requirement that all parts of a compound SOAP structure representation be transmitted within the same unit of the underlying protocol." The last call review period for the WD ends on 15-October-2002.

Bibliographic information: SOAP 1.2 Attachment Feature. W3C Working Draft 24-September-2002. Edited by Henrik Frystyk Nielsen (Microsoft) and Hervé Ruellan (Canon). Version URL: http://www.w3.org/TR/2002/WD-soap12-af-20020924. Latest version URL: http://www.w3.org/TR/soap12-af. Previous version URL: http://www.w3.org/TR/2002/WD-soap12-af-20020814.

Motivation (from the Introduction):

SOAP 1.2 part 1 provides a flexible and extensible envelope for describing structured information intended for exchange between SOAP nodes. Even though SOAP 1.2 is described in terms of the XML Infoset, it is expected that XML 1.0 will be a widely used representation for SOAP data.

The following problems can arise when using such an XML 1.0 representation for SOAP data:

1. Encapsulation of binary data in the form of image files etc. cannot be done without additional encoding/decoding of the data. The overhead of encoding binary data in a form acceptable to XML (for example using base64 as defined by RFC 2045) is often significant both in terms of bytes added because of the encoding as well as processor overhead performing the encoding/decoding.
2. Encapsulation of other XML documents as well as XML fragments is cumbersome within a SOAP message--especially if the XML parts do not use the same character encoding.
3. Although SOAP messages inherently are self-delimiting, the message delimiter can only be detected by parsing the complete message. This can imply a significant overhead in generic message processing as well as parsing and memory allocation.

The compound message structures provided by this specification may be used to create SOAP bindings, or other specifications to be used by bindings, that avoid some or all such problems.