2.3 Comparison of TCP/IP and DTN

2.3  Comparison of TCP/IP and DTN

2.3.1     DTN vs. TCI/IP: Protocol Architecture

Internet layers and DTN layers are identical except that DTN architecture implements store and forward message switching by overlaying a new protocol layer called bundle layer. It is placed on top of Transport layers. The bundle layer ties together the Transport layers and below with application programs to communicate across multiple regions. Bundles are also called messages switched. The bundle layer stores and forwards entire bundles or bundle fragments between nodes. A single bundle layer protocol is used across all networks that make up a DTN. By contrast, the layers below the bundle layer which is transport layer and below are chosen for their appropriateness to the communication environment of each region.

Figure 2.14 shows the DTN layer compares to the Internet layer [9]. Bundle consists of a source-application user data, Control Information provided by source application and destination application which describes how to process, store, dispose and handle user data and a Bundle header. Bundles extend the hierarchy of data-object encapsulation performed by the Internet protocols.

Figure 2.14: Comparison Internet Protocol Layers with DTN Protocol Layers

Figure 2.15 shows how bundle-layer encapsulation works in the context of TCP/IP protocols. A bundle layer may break whole bundles or messages into fragments as an IP layer may break a whole datagram into fragments. If bundles are fragmented, the bundle layer at the final destination reassembles them.

Figure 2.15: Bundle Encapsulation [9]

            On intermittently connected links with long delays, conversational protocols such as TCP/IP that involves many end to end round trips may take impractical amounts of times or connectivity failure. To overcome this problem, DTN bundle layers communicate between themselves using simple sessions with minimal or no round trips as shown in Figure 2.16. Acknowledgement from the receiving node is depending on the class of service selected as shown in Figure 2.17. The lower layer protocols that support bundle layer exchange may of course follow the existing TCP. But on intermittently connected links with long delays, minimal conversational lower layer protocols can be implemented [9].

Figure 2.16: Minimal conversational lower layer protocols [9]

The bundle layer provides six classes of Bundle Services:

i.                    Custody Transfer: Responsible of data retransmission to an accepting node. The sending node can recover its retransmission resources. The accepting node returns a custodial acceptance acknowledgement to the previous custodian.

ii.                  Return Receipt: Confirmation to the source. It is a reply to the sender application that the bundle has been received by the destination application.

iii.                Custody Transfer Notification: Notify the source when a node accepts a custody transfer of the bundle.

iv.                Bundle Forwarding Notification: Notify the source when a bundle is forwarded to another node.

v.                  Priority of Delivery

vi.                Authentication: Verify sender identity and message integrity.

In this thesis, the bundle service class (i), (v) and (vi) is used to form a store and forward overlay network and provide the key capabilities of bundle protocol which is custody based retransmission and able to cope with intermittent connectivity [10].

Figure 2.17: Class of Bundle Service [9]