Essay: The Bundle Layer
Essay: The Bundle Layer
The DTN architecture uses the store-and-forward message switching to deal with the problems related to long delay frequent disruptions and high error rates. It implements this by overlaying the Bundle protocol layer in a new layer called the Bundle protocol, on top of heterogeneous region-specific lower layers. This layer ties together the lower layers so that application programs can communicate over multiple regions. The Bundle protocol stores and forwards whole bundles (messages) or fragments between devices called nodes. A single Bundle layer is used across the all networks that make up a DTN. Each bundle consists of three parts, a source-application’s user data, control information for destination application provided by source and a bundle header which is inserted by the Bundle protocol.
On intermittent connections with long delay, protocols such as TCP, fail completely because it implements a short round-trip time. DTNs, however, communicate with each other via Bundle protocol using simple sessions with minimal or no round trips. Any acknowledgment from receiving node is optional.
Another point to mention is that the TCP protocol ensures end-to-end reliability by retransmitting any segment that is not acknowledged by the destination. The physical, link and network layers have responsibility for ensuring other types of data integrity. In a DTN, however, the Bundle protocol is dependent on lower layer protocols for ensuring the reliability of the communication. The Bundle protocol, though, is able to forward bundles within or between DTN regions (defined later), thus acting as surrogates for end-to-end sources and destinations. This isolates the conversational lower layer protocols at the Bundle layer from long delays in other regions of the end to end path.
The Bundle Layer Classes of Service (CoS)
The Bundle protocol offers six classes of service(CoS) for a bundle:
This service allows for delegation of retransmission responsibility to an accepting node. The accepting node acknowledges with a custodian acceptance to the previous custodian.
This allows for confirmation to the source, or its reply-to entity that the bundle has arrived at destination in an intact form.
Custody Transfer Notification
Custody Notification allows for the notification to the source or its reply-to entity when a node accepts a custody transfer of the bundle.
Bundle Forwarding Notification
This service allows for the notification of the source or its reply-to entity, when the bundle is forwarded to another node.