Space Communication Demonstration Using Internet Technology [^1]

Dave Israel

NASA Goddard Space Flight Center

Ron Parise, Keith Hogie, Ed Criscuolo

Computer Sciences Corp

ABSTRACT

This paper presents work being done at NASA/GSFC by the Operating
Missions as Nodes on the Internet (OMNI) project to demonstrate the
application of Internet communication technologies to space
communication. The goal is to provide global addressability and standard
network protocols and applications for future space missions. It
describes the communication architecture and operations concepts that
will be deployed and tested on a Space Shuttle flight in July 2002. This
is a NASA Hitchhiker mission called Communication and Navigation
Demonstration On Shuttle (CANDOS).

The mission will be using a small programmable transceiver mounted in
the Shuttle bay that can communicate through NASA\'s ground tracking
stations as well as NASA\'s space relay satellite system. The
transceiver includes a processor running the Linux operating system and
a standard synchronous serial interface that supports the High-level
Data Link Control (HDLC) framing protocol. One of the main goals will be
to test the operation of the Mobile IP protocol (RFC 2002) for automatic
routing of data as the Shuttle passes from one contact to another. Other
protocols to be utilized onboard CANDOS include secure login (SSH),
UDP-based reliable file transfer (MDP), and blind commanding using UDP.

The paper describes how each of these standard protocols available in
the Linux operating system can be used to support communication with a
space vehicle. It will discuss how each protocol is suited to support
the range of special communication needs of space missions.

keywords

Internet Protocol (IP), High-level Data Link Control (HDLC), Operating
Missions as Nodes on the Internet (OMNI), Space, Shuttle, Mobile IP, Low
Power Transceiver (LPT), HitchHiker, Multicast Dissemination Protocol
(MDP)

INTRODUCTION

The Operating Missions as Nodes on the Internet (OMNI) project\[[^2]\]
at NASA\'s Goddard Space Flight Center (GSFC) has been defining and
demonstrating end-to-end communication systems for future space missions
based on standard Internet technology. In late 2001 the OMNI project was
presented with an opportunity to support Internet Protocol (IP)
demonstrations with a HitchHiker payload flying in the Shuttle bay on
flight STS-107 in July 2002. The mission is called Communication and
Navigation Demonstration On Shuttle (CANDOS).

The primary goal of the mission is to test the newly developed Low Power
Transceiver (LPT) on-orbit. The LPT is a small, multi-channel,
programmable transceiver that was developed by ITT Industries under
contract to NASA/GSFC. The OMNI project has been involved in another
goal of the mission to investigate the use of standard Internet
protocols for communicating with a space vehicle. This paper covers the
Internet technologies incorporated in the mission and discusses the
operational data transfer scenarios to be examined during the mission.

CANDOS SPACE SYSTEM

The LPT consists of circuit boards in a PC104 form factor and mounted in
metal rings. The boards and their associated mounting rings stack
together to form the shiny rectangular block in figure 1. For CANDOS,
the stack of boards also contains a 686 processor running Linux 6.1, a
FastComm ESCC synchronous serial interface card, and a DC-to-DC power
supply to convert the Shuttle 28 VDC into 5 and 12 VDC.

![](media/image1.pct){width="4.05625in"
height="2.165277777777778in"}Figure 1 -- LPT Transceiver and Processor
Slices under High-gain Antenna

The Linux operating system on the CANDOS processor provides standard IP
services built into the operating system to provide the following:

-   Internet Protocol (IP)\[[^3]\] addressing support

-   User Datagram Protocol (UDP)\[[^4]\] sockets application programming
    interface (API)

-   Transport Control Protocol (TCP)\[[^5]\] sockets as another API

The following applications either come standard with the Linux operating
system or can be easily downloaded and installed:

-   Multicast Dissemination Protocol (MDP)\[[^6]\] reliable file
    transfer operating over UDP

-   Network Time Protocol (NTP)\[[^7]\] operating over UDP

-   File Transfer Protocol (FTP)\[[^8]\] operating over TCP

-   Telnet\[[^9]\] remote access over TCP

-   Secure Shell (SSH)\[[^10]\] encrypted remote login and Secure Copy
    (SCP) file transfer operating over TCP

A key component in the system is a synchronous serial interface card
with its RS-422 clock and data lines connected to the data input and
output ports on the LPT transceivers. The OMNI project developed a
network device driver that allows this serial interface card to be
accessed as a network device with IP addresses. When the Linux system
wants to transmit a packet, the network driver receives the packet from
the Linux IP stack, adds the appropriate Frame Relay\[[^11]\]/
HDLC\[[^12]\] header, queues the packet for output, initiates the data
transfer, and processes interrupts to complete the transfer.

The combination of a standard Linux operating system with IP and a
standard synchronous serial interface with a Frame Relay network driver
is the key to a simple space communication system. Using standard IP
packet and HDLC frame formats on the spacecraft means the resulting
bitstream is easy to interface directly into standard serial ports on
commercial routers from vendors such as Cisco, Nortel, and 3com. These
ground station routers then use standard Ethernet interfaces to pass
data directly on and off the NASA operational communication IP backbone.

One other special piece of software added to the onboard system is a
Mobile IP\[[^13]\] daemon to support the spacecraft end of the protocol.
This is a software process that runs continually on the processor and
listens on a raw socket for Internet Control Message Protocol (ICMP)
packets with Mobile IP advertisements coming up from the ground. Once
the Mobile IP daemon responds to the advertisement, the rest of the IP
routing work is handled by the ground components. This process is
described in more detail at the end of this paper.

-   

![](media/image2.pct){width="6.0in" height="2.625in"}Figure 2 -- LPT and
Antennas on HitchHiker Truss

The LPT has a high-gain, fixed position, circular 18" antenna shown on
the left in figure 2. The three other small disks on columns are a
low-gain transmit antenna, a low-gain receive antenna and one GPS
antenna.

CANDOS Ground SYSTEMs

The ground systems for the CANDOS mission will use existing antennas,
transmitters, and receivers at NASA's Ground Network (GN) and Space
Network (SN) stations. However, the stations will be upgraded by
connecting standard routers to the transmitter/receiver to provide
direct connectivity to NASA's operational IP backbone. Modifications
will be made at both Tracking and Data Relay Satellite System (TDRSS)
stations in White Sands as well as at stations at Wallops (WLP) and
Merritt Island Launch Area (MILA).

The main component of the upgrade is the addition of standard Cisco
routers between the clock and data bitstream interfaces on the
transmitters and receivers and NASA's operational IP backbone. Standard
router options will be configured to process the HDLC frames and the
Frame Relay headers on the router's serial interface. The serial
interfaces will make the link to the spacecraft look like any other wide
area network (WAN) interface on NASA's backbone.

However, the actual bitstream interfaces at NASA stations vary from
station to station and are not always fully compatible with a standard
router interface. A conversion device developed at NASA/GSFC will be
inserted between the router and the transmitter/receiver interfaces to
perform functions such as signal level shifting, convolutional
encoding/decoding, and data scrambling/de-scrambling (pseudorandom noise
coding/decoding). This process is identical to current satellite modems
that are widely used to interface standard routers with commercial
communication relay satellites, although some of the coding algorithms
are different. A key issue is to segregate the satellite specific coding
and forward error correction (FEC) techniques in an external conversion
box that is separate from the router. This approach simplifies future
growth and change by allowing new coding and FEC solutions to be
transparently inserted in the bitstream interface between the RF system
and the router with no impact on the router.

Once the HDLC bitstream from the spacecraft enters a router serial
interface, the interface locates the frames and extracts the
encapsulated IP packets. These packets are them passed out a router
Ethernet interface onto NASA's operational IP backbone network. This
network is already in place and connects NASA's ground stations.

The control center for the CANDOS mission consists of a collection of
workstation and laptop computers with both PC and Macintosh processors.
The machines will used the Linux, Window, and MacOS operating systems
which all support standard IP software.

end-to-end data flow

Significant benefits of using IP technology for future space
communications are the extensive end-to-end network architecture
available and the wide range of hardware and software components readily
available. Internet technologies include an extensive suite of protocols
that have all been crafted to work together to provide a very powerful
and flexible communication system. Today's space communication systems
often use custom protocols on the spacecraft and to the ground and then
use Internet technologies to move data around on the ground. A major
change on CANDOS is to start all data flows with standard Internet
protocols onboard. It will use standard TCP or UDP sockets on the
spacecraft and continue the standard Internet formats over the
space-to-ground link as well as on the ground. This results in a simple
and flexible end-to-end data system.

Figure 3 shows the key technologies used for end-to-end data flows. The
LPT processor in the upper left of the diagram is running standard Red
Hat Linux version 6.1 with applications such as MDP, NTP, Telnet, FTP,
SSH, and SCP. These applications all use UDP and TCP sockets for their
application programming interface (API) to send and receive data. They
will be interacting with similar applications on the computer systems on
the ground.

The TCP and UDP sockets hide lower layer communication details from the
applications so the applications don't need to know whether they are
running over an Ethernet LAN or a serial space link. The sockets pass
packets of data down to the IP layer which adds source and destination
addresses.

![](media/image3.pct){width="5.99375in"
height="2.740972222222222in"}Figure 3 -- End-to-End Data Flow

The next step is to send the packets out a synchronous serial interface
where they are delivered to data ports of the LPT using clock and data
lines with RS-422 signal levels. Getting from the IP layer to the serial
interface requires a Linux network device driver that can support the
Commtech FastComm PC104 serial interface card used in CANDOS. The card
came with a character device driver that supported opening a serial port
and sending and receiving individual characters. In order to provide
standard IP support, the OMNI project wrote a network device driver that
supports a standard Linux interface to accept IP packets from the IP
stack. The driver also adds the proper Frame Relay header bytes and
sends the resulting frame out the serial card where it gets wrapped in
HDLC framing.

The clock and data lines from the serial interface deliver a continual
stream of bits to and from the transmit and receive interfaces on the
LPT. The LPT performs its coding and modulation functions by operating
on this bitstream without any knowledge of the format of the frames
being transmitted.

Another major difference between CANDOS and conventional spacecraft is
that it uses exactly the same framing and upper layer protocols on both
its transmit and receive links. The receive path just reverses the
processing of the transmit path. This concept of identical framing on
both directions of the communication link is important for future
missions which are looking at implementing cross-links between
spacecraft.

Today's spacecraft communicate with ground stations and often refer to
their data links as the forward link, uplink, or command link, to the
spacecraft, and return link, downlink, or telemetry link back to the
ground. With space and ground end points this concept works. However, on
a spacecraft cross-link, the concept of one format on the forward link
and a different format on the return link does not work. This results
from the fact that one spacecraft's downlink, or transmit interface, is
trying to send data to the forward link, or receive interface on the
other spacecraft.

Also, the concept of "commands" to the spacecraft and "telemetry" to the
ground does not properly describe spacecraft communication with many
Internet protocols. For example, a TCP session transferring data to the
ground would have TCP ACKs going back up to the spacecraft. However,
these ACKs are not really a "command".

CANDOS data flows

The goal of the Internet protocol tests on CANDOS is to analyze the
performance of numerous standard Internet applications and protocols to
operate the LPT. Operations will include monitoring and controlling the
processor and radio, as well as retrieving data from GPS experiments
performed with some of the LPT receivers. The protocols and applications
to be analyzed include:

-   Mobile IP -- automated route management to the spacecraft

-   Multicast Dissemination Protocol (MDP) -- UDP based, reliable file
    transfers from space

-   Network Time Protocol (NTP) -- UDP based spacecraft clock
    synchronization

-   UDP packets - real-time spacecraft telemetry

-   UDP packets -- spacecraft blind commanding

-   Secure Shell (SSH) and Telnet -- TCP based remote login to the
    spacecraft

-   Secure Copy (SCP) and FTP -- TCP based reliable file transfer to and
    from the spacecraft

An important aspect of these protocol investigations is that these will
not be highly controlled tests run in a precise laboratory environment.
The main goal for all tests will be to record time-stamped performance
data on the protocols. This data will then be correlated with Shuttle
position and attitude data and RF link measurements to characterize the
operational performance of the various protocols. The following
subsections describe each of the protocols to be examined.

Mobile IP

In today\'s spacecraft communication, control centers normally send
commands to the ground station/antenna the spacecraft is passing over
and the command is uplinked to the spacecraft. The major issue is that
the control center must know where to send the commands and address them
accordingly. However, as large constellations of spacecraft are
deployed, advance planning and scheduling of contacts becomes more
complex and expensive and an automated solution for delivering commands
to the proper ground station is desirable.

Figure 4 shows an example where a spacecraft has an Internet address
(e.g. 100.10.10.18), that is part of a ground based subnet (e.g.
100.10.10.x). Any IP datagrams addressed to the spacecraft address from
anywhere on the Internet will be routed using standard Internet routing
and will be delivered to the Home Agent ground subnet. However, the Home
Agent router needs to know how to forward the packet to the proper
ground station to get it to its final destination. This is exactly the
same problem encountered by other mobile devices on the Internet such as
laptops, PDAs, and eventually automobiles.

The Internet Engineering Task Force (IETF) has developed standards
called Mobile IP (RFC 3220) to deal with this problem. These protocols
use an initial protocol exchange to allow the mobile device or
spacecraft to determine if it is in direct contact with its home subnet
and associated home agent software or a foreign subnet and its foreign
agent software. If the mobile device is in contact with a foreign
subnet, the foreign agent establishes an IP encapsulation tunnel from
the home agent to the foreign agent. Then when the control center sends
a datagram to the spacecraft address, the packet goes to the home router
where the home agent notices that there is a tunnel to the spacecraft
via a foreign router. The packet is then sent through the tunnel to the
foreign agent which passes it out its serial interface and up to the
spacecraft.

This sort of Mobile IP scenario is primarily an issue for sending data
to the spacecraft. When any packets are sent from the spacecraft to any
ground station, the ground station simply uses the destination address
to forward the packets using standard Internet routing rules. One
possible exception is if the foreign ground station has additional
routing rules, for security reasons, which prevent it from forwarding
packets whose source address is not within the foreign subnet. Then the
tunneling features of Mobile IP would be needed to encapsulate the
spacecraft packets for delivery to their home subnet.

![](media/image4.pct){width="5.995833333333334in"
height="3.0243055555555554in"}Figure 4 -- Mobile IP Routing

These cases have only addressed a spacecraft or mobile host with a
single IP address. If the mobile device or spacecraft has a LAN with
multiple IP addresses then the problem gets more complex. One solution
is for each node on the spacecraft with an IP address to perform Mobile
IP registration and set up tunnels for each. However, this does not
scale well and causes additional traffic for all of the registrations
and additional software for each node. The solution currently being
worked on in the IETF is called Mobile Routing. It involves a router
that performs all of the Mobile IP operations and none of the nodes on
the LAN even realize they are mobile. They simply operate just like they
do on a fixed LAN. The research and development in this area is being
driven by concepts in which all future automobiles will have onboard
LANs with Internet addresses and full mobile Internet connectivity. The
size of the automobile market, a potential market for mobile routers, is
huge and the commercial research and development investments are
substantial. A version of the Mobile Routing protocol is currently
available in Cisco routers in version 12.2.4 of the IOS software.

During the CANDOS mission, Mobile IP operation will be carefully
monitored to analyze its performance during real space mission
conditions. All Mobile IP packets will be captured and router Mobile IP
authentication and tunnel management activities will be logged. The time
stamps on the data will be compared to the Shuttle position and antenna
orientation to understand overall protocol performance especially around
the beginning and end of the communication contacts.

MDP reliable file transfers

Future spacecraft system designers are very interested in moving to
storage systems that use a standard file system concept and pass data
files around using reliable file transfer mechanisms. This approach
simplifies spacecraft data storage designs and eliminates most of the
data reassembly functions performed in today's level-zero processing
(LZP) systems.

Many standard Internet data transfers such as web surfing, email, and
file downloads use reliable file transfer mechanisms operating over the
TCP transport protocol. However, using a TCP transport layer in space
has problems due to TCP's use of continual acknowledgements (ACKs)
flowing in the opposite direction of data flow. In searching for ways
around this problem the OMNI project found an attractive solution from
work by the Naval Research Laboratory (NRL) in reliable multicast file
delivery. The OMNI project is also working with NRL to incorporate some
additions to the MDP application to provide more robust and automated
operation in space environments \[[^14]\].

In multicast data delivery, a packet is sent out to potentially hundreds
or thousands of receivers. If each of those receivers sent an ACK back
for every few packets, the original sender would see a flood of
thousands of ACKs coming back for every few packets out. To solve this
problem, NRL developed the Multicast Dissemination Protocol (MDP) which
uses UDP to allow primarily one-way data delivery with only negative
acknowledgements (NACKs) as needed.

On CANDOS, GPS experiments will be performed with the LPT's GPS
receivers. These experiments will generate files of data that need to be
delivered to the ground. MDP will be used to transfer these data files
to the ground. The MDP throttle data rate will be set to maximize
bandwidth on the current RF link. File transfers will be monitored to
analyze the actual performance of MDP and how well it utilizes the
available link bandwidth.

MDP file transfers over primarily one-way, SN demand access link

In NASA's TDRSS system there are often one-way, return-only data paths
available. This resource is more available and more easily scheduled
than two-way links. MDP is built to primarily send data in one direction
with minimal ACKs or NACKs returned. During the CANDOS mission, this
capability will be used to deliver files to the ground over a one-way
link. When a two-way contact is available any NACKs will flow back up,
lost data will be retransmitted, and the reliable file transfer will
complete. This usage of a one-way link with intermittent two-way contact
is exactly the type of data delivery currently being considered for the
Global Precipitation Mission (GPM).

Network Time Protocol (NTP) - time sync

A common operational issue for spacecraft is to keep the onboard clock
synchronized to UTC so data can be accurately time stamped on the
spacecraft. For the CANDOS mission initial clock management will be
performed manually but in later passes an NTP application onboard will
be tested with ground NTP servers to automatically set the LPT clock and
verify its stability. Initial time setting will be done using an
existing time server on NASA's backbone. All of the CANDOS foreign agent
routers will be synchronized to the same NTP server and may be used as
NTP servers for the spacecraft. This will allow comparing NTP operation
across short communication paths at each station versus longer paths by
traversing the ground network to get to central NTP servers. Results are
expected to be in the 1's to 10's of milliseconds precision.

UDP real-time telemetry and housekeeping data

A normal operation for all spacecraft is to send telemetry and
housekeeping data in TDM or CCSDS frames. On the CANDOS mission, UDP
packets in HDLC frames will be used instead of CCSDS frames to
accomplish the identical function. The LPT processor will normally be
sending out a low-rate stream of UDP packets containing status
information such as receiver lock, signal levels, temperatures, etc. By
sending this data in UDP packets, it can flow to the ground during both
one-way and two-way communication contacts. Each packet is completely
self contained with one set of status data samples and full IP
addressing to allow the ground network to deliver it to its proper
destination. If a UDP packet is lost due to bit errors, it will simply
disappear. However, since each packet is standalone and there is no
session state as there is with TCP, there is no retransmission and no
corresponding delay of following data. The next packet propagates on
down with minimal network delay.

UDP blind commanding

The LPT transceiver is configured with its receiver normally ON but its
transmitter OFF. Typically, spacecraft have either an onboard schedule
that turns on their transmitter at a specified time or they receive a
command from the ground to turn on their transmitter. For the CANDOS
mission, the transmitter can be turned on by manually configuring an IP
tunnel to route packets to the proper ground station and sending a UDP
packet addressed to the LPT on its blind commanding port. This process
is identical to the "blind commanding" scenarios used with today's
spacecraft when they do not have a two-way link available.

SSH/Telnet remote login

All of the previous applications described herein use UDP to operate
over intermittent and noisy space links. But tests will also be
performed with applications that operate over TCP. Standard SSH and
Telnet applications will be used to log into the LPT processor from the
CANDOS control center to monitor its operation and make software
changes. The packets from these interactive sessions will be captured
and analyzed with programs like tcptrace to analyze TCP performance over
the space link.

SCP/FTP file transfers

Standard SCP and FTP applications will also be used for reliable file
transfers when a two-way link is available. These packets will also be
captured and analyzed with tcptrace to compare performance with similar
MDP file transfers.

conclusions

This paper is being prepared before the actual mission so measured
results are not currently available. The tests performed and results
collected during the mission will focus primarily on examining protocol
operation in a space environment. Actual performance of the protocols is
expected to vary widely based on each contact's RF link error
characteristics. Since the LPT antennas are not steerable, they are
always pointed directly up out of the Shuttle bay. Much of the time they
will not be aimed directly at TDRSS or a ground station antenna
depending on the Shuttle orientation. This should provide a wide range
of RF link performance that will provide an excellent variety of test
conditions.

The use of HDLC framing in space is expected to work very well based on
current experience from tests performed using Internet protocols through
TDRSS, on the UoSAT-12 spacecraft, and in simulations performed by ITT
Industries for NASA/GSFC. The CANDOS mission will allow collecting more
information on HDLC performance with both SN and GN stations across a
full range of orbital and antenna pointing scenarios.

CANDOS will also provide an excellent opportunity to collect performance
information on the operation of the Mobile IP protocol in these same
orbital scenarios. An area of special interest will be to monitor Mobile
IP performance at the beginning and end of contacts to see how it
performs when the spacecraft is close to the horizon. Current missions
normally wait until the spacecraft is at least 5 degrees above the
horizon before attempting communication. Since Mobile IP operates over
UDP and only requires a few packets it should be able to operate closer
to the horizon.

Tests performed using MDP over a one-way, return link configuration will
provide useful information for future missions like GPM. A primarily
goal of testing applications like MDP and NTP is to identify ways to
allow future spacecraft to operate more automatically with less
commanding required from ground personnel.

Using TCP based applications such as SSH/SCP and Telnet/FTP will provide
insight into their performance over noisy and intermittent links. During
the middle of a contact they should operate fine. The more interesting
tests will be to determine if it is reasonable to use them near the
beginning and end of contacts.

acknowledgements

The end-to-end Internet protocol data flow design and software
configuration described in this paper was carried out by personnel from
Computer Sciences Corporation working for NASA's Goddard Space Flight
Center under contract GS-35F-4381G S-36130-G, with additional efforts
and support contributed by individuals from ITT Industries and other
GSFC organizations. The work was performed for the LPT CANDOS project
through funding provided by NASA Headquarters, Code M. The authors would
also like to thank GSFC code 450 for their pioneering IP work on the
South Pole TDRSS Relay (SPTR) project\[[^15]\].

references

[^1]: U.S. Government work not protected by U.S. copyright

[^2]: http://ipinspace.gsfc.nasa.gov/

[^3]: \"Internet Protocol, DARPA Internet Program Protocol
    Specification\", Internet Engineering Task Force RFC-791, September
    1981

[^4]: \"User Datagram Protocol\", Internet Engineering Task Force
    RFC-768, August 1980

[^5]: \"Transmission Control Protocol\", Internet Engineering Task Force
    RFC-793, September 1981

[^6]: J Macker, R B Adamson, "The Multicast Dissemination Protocol (MDP)
    Toolkit", IEEE, 1999,
    [[http://mdp.pf.itd.nrl.navy.mil/MdpToolkitOverview.ps.gz]{.underline}](http://mdp.pf.itd.nrl.navy.mil/)

[^7]: \"Network Time Protocol (Version 3) Specification, Implementation
    and Analysis\", Internet Engineering Task Force RFC-1305, March 1992

[^8]: \"File Transfer Protocol\", Internet Engineering Task Force
    RFC-959, October 1985

[^9]: \"Telnet Protocol Specification\", Internet Engineering Task Force
    RFC-854, May 1983

[^10]: "SSH Protocol Architecture", Internet Engineering Task Force --
    draft-ietf-secsh-architecture-12.txt, January 2002

[^11]: \"Multiprotocol Interconnect over Frame Relay\", Internet
    Engineering Task Force RFC-2427, September 1998

[^12]: \"High-level Data Link Control (HDLC) - Frame Structure\",
    ISO-3309

[^13]: \"IP Mobility Support for IPv4\", Internet Engineering Task Force
    RFC-3220, January 2002

[^14]: Jim Rash, Ed Crisuolo, Keith Hogie, Ron Parise, "MDP: reliable
    file transfer for space missions", Earth Science Technology
    Conference 2002, Pasadena CA*,* June 2002

[^15]: Israel, David j., "The South Pole TDRSS Relay (SPTR)."
    Astrophysics From Antarctica, ASP Conference Series Volume 141,
    1998: 319+