RFC 2760 (rfc2760) - Page 3 of 46

Ongoing TCP Research Related to Satellites

RFC 2760       Ongoing TCP Research Related to Satellites  February 2000


   some point, the mechanisms discussed in this memo prove to be safe
   and appropriate to be recommended for general use, the appropriate
   IETF documents will be written.

   It should be noted that non-TCP mechanisms that help performance over
   satellite links do exist (e.g., application-level changes, queueing
   disciplines, etc.).  However, outlining these non-TCP mitigations is
   beyond the scope of this document and therefore is left as future
   work.  Additionally, there are a number of mitigations to TCP's
   performance problems that involve very active intervention by
   gateways along the end-to-end path from the sender to the receiver.
   Documenting the pros and cons of such solutions is also left as
   future work.

2   Satellite Architectures

   Specific characteristics of satellite links and the impact these
   characteristics have on TCP are presented in RFC 2488 [AGS99].  This
   section discusses several possible topologies where satellite links
   may be integrated into the global Internet.  The mitigation outlined
   in section 3 will include a discussion of which environment the
   mechanism is expected to benefit.

2.1 Asymmetric Satellite Networks

   Some satellite networks exhibit a bandwidth asymmetry, a larger data
   rate in one direction than the reverse direction, because of limits
   on the transmission power and the antenna size at one end of the
   link.  Meanwhile, some other satellite systems are unidirectional and
   use a non-satellite return path (such as a dialup modem link).  The
   nature of most TCP traffic is asymmetric with data flowing in one
   direction and acknowledgments in opposite direction.  However, the
   term asymmetric in this document refers to different physical
   capacities in the forward and return links.  Asymmetry has been shown
   to be a problem for TCP [BPK97,BPK98].

2.2 Satellite Link as Last Hop

   Satellite links that provide service directly to end users, as
   opposed to satellite links located in the middle of a network, may
   allow for specialized design of protocols used over the last hop.
   Some satellite providers use the satellite link as a shared high
   speed downlink to users with a lower speed, non-shared terrestrial
   link that is used as a return link for requests and acknowledgments.
   Many times this creates an asymmetric network, as discussed above.






Allman, et al.               Informational