In the frame of the CCSDS (Consultative Committee for Space Data Systems) cooperation between DLR and NASA an operational DTN link was established between the new DTN node at GSOC and the Marshall Space Flight Center in Huntsville, Alabama. For the very first time, operational data flows over the IGS (Interconnected Ground Subnet) Network.
The CCSDS DTN Working Group is working already for a while to define an international Standard.
The Delay Tolerant network is a protocol that is able to overcome the difficulties related to networks that are not always connected or have different and mixed protocols and architectures. DTN is based on the NASA development for the interplanetary internet (IPN) used for Space Communications. The big difference between DTN and other networks is the seldom connections between the source and the target points.
In this particular case the usage of the store and forward principle make perfect sense. DTN protocol divides the data in little packets (so called Bundles) and sent them to the next reachable node. The Bundle will be sent the data to the next reachable node as long as the link is available and will keep them in a buffer till a confirmation from the target node reach the source one telling him that all the data is safely transmitted.
There are two different DTN protocols is use now, ION and DTN2. DLR has already experience using ION due to the testing carried out by the institute of Communication and Navigation (KN). In frame of the cooperation with the KN experts, with their support and after a good exchange of information and experience between both institutes in OP, GSOC decided to use DTN2 for the testing with Marshall.
The current configuration gets GSE packets from the Huntsville Operations Support Center (HOSC) and sent to the Columbus Control Center (COL-CC) at GSOC. In order to not affect the real time operations a new parallel connection was established having data requests and transport only over DTN simultaneously with the real time ones.
To carry out this testing a very good planning with the real time operations is mandatory. If the network over the Atlantic is not overloaded with other traffic, the parallel request can be allocated. In this way is possible to make comparison about the data transmission using or not the Bundle protocol. Due the fact that the IGS network is very stable and reliable, the DTN protocol basics can be tested. To test the connections interruptions, the firewall ports must be closed for a period of time (seconds, minutes and hours ) and then evaluate the data integrity and data lost, if any.
The bundle protocol is a group of data blocks that are connected to each other like a bundle. Each of these bundle (not the individual blocks) have enough semantic information to run an application over the network as long as the link exists, like for example a payload on the ISS. The bundles will be transmitted through the network when the connection between two nodes is established. The store and forward principle is used. The transmission can be done using different transport protocols like the IP ones. In this test UDP and TCP were tested.
To have a secure connection without having permanent link is quite difficult. A cryptographic protocol or a key exchange between the nodes that are only temporary connected is complex. During the DTN development the ID-based cryptography was created. Using it, allows the different nodes with publics ID to communicate and transmit encrypted data to each other’s.
The future plans foreseen to test using DTN over the internet and the compatibility of ION and DTN2 with JSC in Houston and JPL in Pasadena.