Routing Protocols for InterPlaNetary Internet 
 

• Locate the elements in a hierarchical way in the InterPlaNetary Internet architecture, support for efficient routing through different subnetworks and under node movement.
• Allow the InterPlaNetary Internet to expand while maintaining the addressability of previously-deployed elements.
• Dynamically allocate addresses, i.e., retrieve addresses from nodes under power failure or physical damage, and assign new addresses to newly deployed elements.

InterPlaNetary Backbone Network
The main challenges that affect network layer design in the InterPlaNetary Backbone Network are:

• Long and variable delay: Routing protocols that require timely dissemination of state are most severely affected by the extremely long propagation delays. Without timely distribution of topology information, routing computations will  fail to converge to a common solution, resulting in route inconsistency or  oscillation. The movement of nodes during  propagation must be considered while computing the routes or while scheduling the packet forwarding time. 
• Intermittent connectivity: Intermittent links cause several  challenging problems: Determining the predicted time and duration of  intermittent links and the degree of uncertainty of these estimates, obtaining  knowledge of the state of pending messages, the scheduling of their transmission when the link becomes available, and decision of the time at which to abandon  the wait for a predicted link.
  

• Distribution of topology information: One-hop link state information and a distance-vector type of aggregation beyond one hop can be maintained to obtain a probabilistic view of the overall topology. Moreover, the trajectory and velocity information of backbone nodes can be distributed. However, the trajectory information does not reflect the link quality, which is affected by environmental interference and traffic distribution. 
• Path calculation: Hop-by-hop routing is expected using incomplete topology information and probabilistic estimation. Moreover, adaptive algorithms are needed to decide when and how to reroute packets, whether or not to keep a local copy of forwarded packets and when to drop it. When a packets arrives at the border of a PlaNetary Network, specific routing protocols adaptive to the local circumstances can be initiated to continue forwarding the packet to its final destination.
• Interaction with transport layer protocols: The performance of routing protocols will be affected by the transport layer protocols. The interaction between transport and network layers needs to be considered to achieve better performance.

Proposed solution: Space Backbone Routing (SBR)
A new routing framework, called Space Backbone Rouiting (SBR), is proposed for  routing through different autonomous regions (ARs) in the InterPlaNetary Internet.  SBR has two integral parts:   SBR-external (SBR-e) and SBR-interior (SBR-i). 
SBR-e addresses the delivery of remote control message and scientific data  through ARs in the IPN Internet.  Location-Predicted Directional Broadcast (LPDB) is proposed for reliable delivery of remote control messages and automatic data delivery.  In LPDB, paths are calculated en route based on the predictable AR locations.   These paths are used to direct and limit the control message broadcast.  For controlled data delivery, Receiver-Initiated On-demand Routing (RIOR) is proposed. In RIOR, the route discovery is initiated on-demand by the receiver and  routing tables are maintained in soft state at the intermediate AR nodes.  SBR-i exchanges inter-AR routing information among backbone nodes within an AR  and schedule the inter-AR message transmission.  Two important functionalities of SBR-i, i.e., contact allocation and traffic dispatching, are defined. As a first attempt, the Longest Queues (LQ) policy is proposed for contact allocation for AR border routers and the Minimum Waiting (MW) policy is introduced for scheduling inter-regrion messages through an AR.

The proposed routing framework of SBR is shown as follows:  

Space Backbone Routing (SBR)
SBR-external		SBR-interior
remote control & automatic data delivery	controlled data delivery	contact allocation	traffic dispatching
Location-Predicted Directional Broadcast (LPDB)	Receiver-Initiated On-demand Routing (RIOR)	Longest Queues (LQ)	Minimum Waiting (MW)

• Extreme Power Constraints: Space elements mainly depend on rechargeable battery using solar energy. Therefore, the power availability is of overriding importance to the PlaNetary Surface Network.
• Frequent Network Partitioning: The network can be partitioned due to environmental factors, such as meteoroid shower, high electromagnetic radiation,
  sand storm, and node malfunction.
• Adaptive Routing Through Heterogeneous Networks: The PlaNetary Network includes fixed elements (e.g., landers), satellites with scheduled movement, mobile elements with slow movement (e.g., rovers, balloons), mobile elements with fast movement (e.g., powered spacecraft), and low-power sensor nodes in clusters.
  Adaptive protocols are needed to maintain the connectivity and achieve seamless
  routing among these elements.
  

• Routing Support from Satellites: PlaNetary Satellite Network plays an important role to support the end-to-end routing between Earth and outer-space planet, as well as between community regions within PlaNetary Surface Networks. Network integration of routing protocols through the satellites is needed. Meanwhile, new protocol support from the satellites is called to help maximize the connectivity of the PlaNetary Surface Network.
• Topology Maintenance and Re-configuration: The frequent network partitioning in the PlaNetary Surface Network calls for network reconfiguration mechanisms to reconstruct network topology to achieve better performance. It is preferable that these mechanisms are executed locally due to the long distance of interplanetary links to the Earth control center. Local decisions can be made to update network topology by node mobility, power adjustment, and adaptive clustering, etc.
• Power Efficiency: Power efficiency is of great importance in developing mechanisms in the PlaNetary Surface Networks. Therefore, routing decisions should consider power availability at each node; network reconfiguration should reduce power dissipation in topology set up and maintenance; surface elements can switch to sleep mode temporarily when no mission is planned and under unfavorable environmental conditions.
• Cross-layer Interaction: The adaptive protocol design that works efficiently on an end-to-end basis in the PlaNetary Network is challenging. However, due to the extreme environment characteristics, different layers are likely to use the same information (such as the location and energy information) in making layer-specific desicions. Moreover, in a dynamic wireless network with nodes of different capabilities and mobility levels, different layers need to cooperate closely to meet the QoS requirements of the applications.