IP in Space FAQ

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  1. General
       1. Define what you mean by "IP".
       2. What do you mean by "layers"?
       3. Doesn't IP fail in space due to long propagation delays?
       4. Isn't IP designed for clean terrestrial links instead of noisy,
          intermittant space links?
       5. What is required to adapt a ground station for IP to a spacecraft?
       6. Isn't the Space Communication Protocol Standards (SCPS) suite of
          protocols designed to overcome the shortcomings of IP used in
          space networks?

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  1. General
       1. Define what you mean by "IP".
          "IP" stands for Internet Protocol. Strictly speaking, this is a
          layer 3 (networking) protocol that is only concerned with
          end-to-end network addressing. However, when speaking generally,
          the term "IP" is oftern used to refer to the entire suite of
          Internet protocols, including TCP, UDP, FTP, NTP, SMTP, etc., and
          not just the network layer protocol.
       2. What do you mean by "layers"?
          "Layers" refers to a set of Open Systems Interconnect (OSI)
          network services that was originally defined by ISO in the mid
          1980's. Each layer has a well defined subset of services it
          provides to the layer above, and a well defined interface to the
          layers above and below. The OSI model had 7 layers, but as the
          Internet evolved, layers 5 and 6 were rarely used.
          The layers are:
               Layer 7: Application Layer
               High-level protocols that use lower layer services.
               (FTP, Telnet, SMTP, NFS, DNS, SNMP)
               Layer 6: Presentation Layer - not generally used
               Layer 5: Session Layer - not generally used
               Layer 4: Transport Layer
               End-to-end stream multiplexing Protocols.
               (TCP - reliable byte-stream delivery, UDP - unreliable packet
               delivery)
               Layer 3: Network Layer
               End-to-end addressing, packet routing, simple packet
               forwarding.
               (IP, RIP, OSPF, IGRP, BGP)
               Layer 2: Data Link Layer
               Data unit delivery across a single data link, data unit
               framing, link level addressing.
               (Ethernet, HDLC, ATM, PPP, SLIP)
               Layer 1: Physical Layer
               Physical media and signal levels. coding specifications,
               bit-level operations.
               (10baseT, RS-422, UTP, Reed-Solomon)
       3. Doesn't IP fail in space due to long propagation delays?
          This is a misconception that is brought about by confusing IP, a
          layer 3 network protocol, with TCP, a layer 4 transport protocol.
          IP is strictly an end-to-end addressing mechanism. IP packet
          headers have a source address, an destination address, and a few
          bytes of options. These get slapped on a packet which is then sent
          on it's merry way and forgotten about. Network layer protocols do
          not perform handshaking or retransmission. These functions get
          performed at a higher layer. As a result, IP is insensitive to
          transmission delay. It will work equally well across the room or
          out to Pluto. It will even work over uindirectional links. UDP, a
          layer 4 transport protocol, will also work over unidirectional
          links, allowing applications to be built that are not sensitive to
          delay. TCP, on the other hand, requires a bidirectional link to
          perform handshaking. The round-trip-time and the TCP "window"
          buffer size combine to restrict the maximum throughput possible
          over a TCP connection. TCP experiments have been done with the
          ACTS satellite in geosynchronous orbit that achieved a throughput
          of 540 Mbits/sec. It appears that TCP is quite useable in a
          planetary neighborhood, out to about lunar distance. Beyond that,
          UDP-based applications are called for.
       4. Isn't IP designed for clean terrestrial links instead of noisy,
          intermittant space links?
          This is another misconception. One of the commonest physical links
          for the terrestrial Internet, the telephone line, has a bit error
          rate (BER) than is similar to most spacecraft RF links! The CCITT
          recommendations for voice circuits that have been conditioned to
          carry data is a BER of 10-5 (Source: ITT Reference Data for Radio
          Engineers). On the other hand, NASA typically specs it's
          spacecraft RF links at a BER of 10-5! The reason IP works over the
          phone lines is that the modem applies error correction down at the
          physical layer, transparently to the upper layers. This allows the
          upper layers to behave as though they have a clean link.
          Similarly, NASA applies error correction to it's space links,
          achieving BERs down to 10-7 or better. At 10-7, handshaking
          protocols, such as TCP/IP, work well.
       5. What is required to adapt a ground station for IP to a spacecraft?

          Connect the clock and data lines from the groundstation RF system
          to a synchronous serial WAN port on a router.
       6. Isn't the Space Communication Protocol Standards (SCPS) suite of
          protocols designed to overcome the shortcomings of IP used in
          space networks?
          At the time the SCPS Rationale, Requirements, and Application
          Notes document (CCSDS SCPS-710-0-G-0.4) was generated, that was
          true; however, in the two years since then, while ongoing industry
          research has solved, implemented, standardized, and deployed IP
          solutions to many of these problems, and are continuing to do so.
          For example, IP now supports constrained bandwidth channels via
          the header compression standard defined in RFC-2507, which was
          SPECIFICALLY designed for channels with non-trival packet loss
          rates. And asymetric channels are better supported by Selective
          Acknowledgement (SACK) RFC-2018, which is now deployed standard
          with most operating systems. Selectable routing, such as flood
          routing, and managed connection operation is being handled under
          the evolving IETF draft for the Dynamic Source Routing (DSR)
          protocol, and congestion signaling is being handled by Explicit
          Congestion Notification (ECN) and Random Early Detection (RED)
          spelled out in RFC-2481.
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              Last Modified: Monday, 24-Jul-2000 10:25 EDT