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TCP/IP for Gateway

TCP/IP for Gateway?

We’ve been hearing that there are some within NASA advocating for the use of TCP/IP for space data communications on the Gateway lunar missions. At first glance, this might seem to make sense— after all, the Near-Rectilinear Halo Orbit planned for Gateway would guarantee that its line-of-sight with earth would never be interrupted. While it would seem that RTT between ground stations on earth and Gateway’s communications relay module would be problematic for latency-sensitive applications, TCP/IP would probably work. Kind of.

However, we think it would be ill advised for reasons other than interactive voice and video performance issues: the whole point of Gateway is to meet the objectives laid out in Space Policy Directive-1:

Beginning with missions beyond low-Earth orbit, the United States will lead the return of humans to the Moon for long-term exploration and utilization, followed by human missions to Mars and other destinations.

If the reason for Gateway is to support eventual human missions to Mars and other interplanetary destinations, it makes no sense to use anything other than the communication protocols designed to support those missions: in particular the DTN suite. Benefits resulting from the use of DTN for Gateway include:

  • Gaining experience with a protocol suite suitable for further exploration beyond lunar orbit.
  • Built-in resilience to communications disruptions that do occur, e.g. due to weather effects on Earth if Ka-band or optical links are used.
  • More efficient use of the Gateway-to-Ground links, since Bundle Protocol convergence layers can be tuned for the characteristics of the links (e.g. using alternate congestion control mechanisms and hence avoiding the issues with running TCP over high bandwidth-delay-product paths).
  • Flexibility in the use of ground stations to communicate with Gateway, especially when humans are not present.  DTN’s store-and-forward capability will enable more flexible allocation of ground stations to communicate with Gateway, since data will be stored if all available ground stations need to be used for other tasks.

Because so many operational benefits accrue from the use of DTN vs. TCP/IP, and because DTN aligns with the overall objectives specified in Space Policy Directive-1, we at IPNSIG highly recommend that NASA elect to use DTN for all space data communications on the Gateway missions.

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