Communications of the ACM
Communications of the ACM
The Interplanetary Internet
Imagine a future where human intelligence is scattered all over the solar system. In some places, say, the Moon and on and around Mars, there are thousands or millions of intelligent systems that have to exchange information not only with intelligence back on Earth but among themselves. How would such communication occur? How would it differ from more familiar forms of information dissemination across the terrestrial Internet?
Humans have been voyaging into space, either personally or through robotic presence, for almost 45 years. Space is an all-round harsh environment, and communicating with remote spacecraft has required development of highly specialized techniques for handling very long propagation delays, extremely weak radio signal levels, and stringent onboard power-generation limitations inherent once we leave Earth.
Each early space mission implemented a unique and literally handcrafted "protocol" for passing data back and forth between ground and spacecraft. As time and technology progressed and the number of missions increased, we also had to drastically reduce costs and allow missions to share international ground support infrastructure. In the late 1970s, around the time Vinton Cerf and his colleagues at the Defense Advanced Research Projects Agency were experimenting with the early precursors to the Internet, the various national space agencies around the world responded to this need by establishing the Consultative Committee for Space Data Systems (www.ccsds.org) to develop common and internationally agreed-on space communications standards. Adopting packet-switching concepts, CCSDS began standardizing the protocols used for sending commands to spacecraft and transmitting measurements back to Earth. Today, 20 years later, about 200 space projects with spacecraft dispersed across the solar system have opted to use these international packetized standards.
In 1998, the CCSDS's founders, working at the California Institute of Technology's Jet Propulsion Laboratory (JPL), NASA's lead center for deep-space exploration, teamed with Cerf, now senior vice president for Internet architecture at WorldCom, Inc., and began studying how to more closely unite the space and terrestrial Internet communities. The result was a concept called "InterPlaNetary Internet," or IPN. For the past three years, a team of JPL engineers, with Cerf frequently lending his expert advice in his new role as a JPL Distinguished Visiting Scientist, has been working on a top-level IPN architecture, funded in part by the DARPA Next-Generation Internet initiative (see the figure here).
Deceptively Simple Architecture
The preliminary fruits of the IPN architectural work were published May 2001 as an Internet Draft (www.ietf.org/internet-drafts/draft-irtf-ipnrg-arch-00.txt). The suggested architecture is deceptively simple, involving just four principal components:
Follow standard Internet rules. Wherever space operations are conducted in a local short-delay communications environmentaround Earth, within a free-flying spacecraft, on and around another planetdata-handling protocols can be used that either follow standard Internet rules or are closely related to their terrestrial Internet counterparts.
Specialized deep-space backbone network. These local Internets, distributed across the solar system (each conceptually autonomous) are then interconnected through a specialized deep-space backbone network of long-haul wireless links. This interplanetary backbone is expected to evolve to include multiple space-based data-relay satellites.
New overlay protocol. The resulting interplanetary Internet thus consists of a "network of Internets." Just as the Internet Protocol (IP) suite unites the Earth's network of networks into the Internet, the Interplanetary Internet needs a new family of overlay protocols, called Bundling, to tie together a set of heterogeneous Internets to support end-to-end user dialogue.
Multiple data-protection mechanisms. Embedded in the layered architecture of the new IPN protocols are multiple data-protection mechanisms, enabling implementation of strong measures assuring the security of both the backbone and the end-to-end exchange of user data.
The set of Bundling protocols is functionally analogous to the familiar TCP/IP used on Earth. From the perspective of Bundling, the Earth's entire Internet looks like a single link. Therefore, a routing component of Bundling is required to progressively move bundles of user data through a concatenated series of independent Internets, just like IP routes data through a series of independent networks on Earth. In order to guarantee the reliability of the end-to-end transfer, the Bundles also need retransmission mechanisms functionally similar to those provided by TCP.
While the Earth's backbone network is wiredlarge numbers of fiber or copper circuits interconnecting fixed hubsthe interplanetary backbone depends on fragile wireless links.
However, the similarity between the Bundling protocols and TCP/IP ends there. Whereas users of the Earth's Internet are usually continuously connected, the Interplanetary Internet will rarely present a continuous end-to-end path. While the Earth's backbone network is wiredlarge numbers of fiber or copper circuits interconnecting fixed hubsthe interplanetary backbone depends on fragile wireless links. The hubs on the interplanetary backbone that provide routing between remote local Internets all move with respect to one another. Planets travel in fixed orbits, and sometimes large bodies like the Sun cause line-of-sight occultations that last for long periods of time. Moreover, vehicles landed on remote planetary surfaces move out of sight of Earth as the body rotates; they may have to communicate through local relay satellites that provide data transmission contacts for only a few minutes at each contact.
Unlike the Earth's backbone environment of relatively continuous connectivity, negligible delay, and clean data channels, the hallmarks of the interplanetary backbone are intermittent connectivity, huge propagation delays, and noisy data channels.
Handling the Environment
Unlike the Earth's backbone environment of relatively continuous connectivity, negligible delay, and clean data channels, the hallmarks of the interplanetary backbone are intermittent connectivity, huge propagation delays, and noisy data channels. The Bundling protocols handle this environment in two ways:
Store-and-forward mode. Operating in store-and-forward mode, similar to email, Bundles are held at routers along the way until such time as a forward path is established.
Custodial mode. Operating in custodial mode, Bundling avoids the need for senders to store data until an acknowledgment is received from the other end. Bundles are simply handed to the next forwarding node where an agent takes custody of the next hop in the data transfer and allows the sender to free-up local resources.
Getting Bundles delivered from source to destination across multiple Internets and the interplanetary backbone involves unusual challenges. The suggested concept, as outlined by the IPN team at JPL, is that rather than have a single address space across the entire solar system, thus requiring every part to evolve at the same pace, routing is done in two stages via a two-part naming scheme. One part of the name of a particular Bundlethe routing handlegets the Bundle delivered to the destination Internet. The secondadministrativepart contains the information needed to resolve to a local Internet address. For example, a business on Mars may have a two-part name consisting of the routing handle "mars.sol" concatenated with the administrative part "joes-rockshop.com." If the related Bundle were sent by a user on Earth, the ".sol" part of the routing handle would resolve to the Internet address of an interplanetary gateway on Earth; the "mars" part of the handle is then used to route the Bundle across the interplanetary backbone to the Mars entry gateway. At this point, the routing handle has done its job; the Bundle then enters the Mars "region," where standard Internet technologies may be available. In such a transmission, the "joes-rockshop.com" administrative part of the name comes into play and may resolve to a local IP address of Joe's server on the Martian Internet.
This scheme may sound far-fetched, but JPL is well along toward deploying these new capabilities. Internet-like CCSDS protocols using packet-switching techniques are already widely used on the point-to-point links connecting spacecraft to NASA's Deep Space Network of large tracking stations (in Australia, California, and Spain).
As early as next year, it is hoped a new custodial, store-and-forward CCSDS file-delivery protocol will be tested on its first NASA mission. CCSDS will then progressively scale-up this capabilityan early version of Bundlingto support a richer suite of user services.
Interplanetary Infrastructure
As new mission requirements emerge in the coming decades, the suite of IPN protocols will thus be able to evolve to meet them. In particular, the armada of international missions scheduled to swarm on and around Mars over the next 10 years will themselves contribute to building the Interplanetary Internet. By taking the time now to establish a robust and flexible architecture, NASA and JPL will thus be able to plan the investments in new and reusable interplanetary infrastructure with a clear view of the functions they'll have to perform, as well as how they'll fit into the evolution of the traditional Earth-based Internet.
Figures
Figure. Long-term vision. Intermittently connected fragments of terrestrial Internet technology, scattered throughout the solar system, and beyond.
The Digital Library is published by the Association for Computing Machinery. Copyright © 2001 ACM, Inc.
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