RFC 2488 (rfc2488) - Page 2 of 19

Enhancing TCP Over Satellite Channels using Standard Mechanisms

RFC 2488         Enhancing TCP Over Satellite Channels      January 1999


   currently well understood and on the IETF standards track (or are
   compliant with IETF standards).

   This document is divided up as follows: Section 2 provides a brief
   outline of the characteristics of satellite networks.  Section 3
   outlines two non-TCP mechanisms that enable TCP to more effectively
   utilize the available bandwidth.  Section 4 outlines the TCP
   mechanisms defined by the IETF that may benefit satellite networks.
   Finally, Section 5 provides a summary of what modern TCP
   implementations should include to be considered "satellite friendly".

2.  Satellite Characteristics

   There is an inherent delay in the delivery of a message over a
   satellite link due to the finite speed of light and the altitude of
   communications satellites.

   Many communications satellites are located at Geostationary Orbit
   (GSO) with an altitude of approximately 36,000 km [Sta94].  At this
   altitude the orbit period is the same as the Earth's rotation period.
   Therefore, each ground station is always able to "see" the orbiting
   satellite at the same position in the sky.  The propagation time for
   a radio signal to travel twice that distance (corresponding to a
   ground station directly below the satellite) is 239.6 milliseconds
   (ms) [Mar78].  For ground stations at the edge of the view area of
   the satellite, the distance traveled is 2 x 41,756 km for a total
   propagation delay of 279.0 ms [Mar78].  These delays are for one
   ground station-to-satellite-to-ground station route (or "hop").
   Therefore, the propagation delay for a message and the corresponding
   reply (one round-trip time or RTT) could be at least 558 ms.  The RTT
   is not based solely on satellite propagation time.  The RTT will be
   increased by other factors in the network, such as the transmission
   time and propagation time of other links in the network path and
   queueing delay in gateways.  Furthermore, the satellite propagation
   delay will be longer if the link includes multiple hops or if
   intersatellite links are used.  As satellites become more complex and
   include on-board processing of signals, additional delay may be
   added.

   Other orbits are possible for use by communications satellites
   including Low Earth Orbit (LEO) [Stu95] [Mon98] and Medium Earth
   Orbit (MEO) [Mar78].  The lower orbits require the use of
   constellations of satellites for constant coverage.  In other words,
   as one satellite leaves the ground station's sight, another satellite
   appears on the horizon and the channel is switched to it.  The
   propagation delay to a LEO orbit ranges from several milliseconds
   when communicating with a satellite directly overhead, to as much as
   80 ms when the satellite is on the horizon.  These systems are more



Allman, et. al.          Best Current Practice