#### Interoperability Toolkit
Core

### Overview

The Distribution Envelope provides a standardised structure to carry information regarding common end-to-end distribution requirements. The Distribution Envelope is used to wrap one or more payloads in an ITK transaction.

#### Distribution Envelope Components

The Distribution Envelope is a lightweight structure to carry information relating to the end-to-end technical distribution of a message. The diagram below shows a schematic of the Distribution Envelope.

Schematic of the Distribution Envelope

The diagram shows that the Distribution Envelope is split into a Header section and a Payloads section.

#### The Header Section

The Header Section contains details that pertain to all the payload content.

- The Original Service Request contains a copy of the service that was originally requested (for example SendCDADocument-v2-0). This is useful for a Web Services environment, where the Web Service request would normally be transmitted in the Simple Open Access Protocol (SOAP) header. However, over a multi-hop transmission the SOAP header may be stripped and re-formatted by routers along the way, bringing the danger that the original service request would be lost. Putting the original service request in the Distribution Envelope (as well as the SOAP header) mitigates this risk. Over other transports such as email or the Data Transfer Service (DTS) where no SOAP header is required, the original service request acts as a consistent place holder for the service details.
- The Tracking Id acts an identifier for the transmission. As its name suggests this is useful for tracking transmission (for example its route to a destination).
- The Address List contains the distribution list. The ITK architecture treats all recipients equally (for example, there are no Primary Recipients and Tracker Recipients - there are just recipients).
- The Audit Identity is a resolvable reference for audit purposes.
- The Manifest contains technical details for each of the payloads.
- The Sender Address is the address of the message originator. This address is used to route transmission acknowledgements.
- The Handling Specifications act as a list of instructions for the recipient. For example, there may be a Handling Specification to send a Business Acknowledgement for the transmission.

#### The Payload Section

The Payload Section contains the actual payloads being transmitted. Generally the type of payload carried by an ITK transmission will depend on the Service definition used. However, the Content Metadata Payload may be used by multiple Service definitions.

The Content Metadata Payload can be used to carry metadata about each payload in the transmission. There will only ever be one Content Metadata Payload. Not all payloads will attract metadata, but for payloads that do, the metadata is expressed in an IHE compliant format.