NeXT – Network Crossing via Translation

As the next generation Internet Protocol, IPv6, becomes mainstream, many aircraft communication services including air traffic control (ATC) applications are using the new protocol.

However, no satellite IPv6-based networks have been deployed to day and, given the time to bring new satellite networks into service, this is likely to be the situation for at least a number of years. To overcome this obstacle, Network Crossing via Translation (NeXT) has been designed as an efficient way of transporting IPv6 information over an IPv4 network.

The development of NeXT involved modifying the standard Network Address Port Translation-Protocol Translation (NAPT-PT). The modification is required because of NAPT-PT’s limitation that allows IPv6 to IPv4 communication and vice versa, but not IPv6 to IPv6 with an IPv4 segment in between, as is the case with aircraft communicating to the ground using a satellite link.

NeXT, like NAPT-PT, works by translating IPv6 to IPv4 using translation tables. The five stages of the process are:

  1. The first packet of the session, which is destined for the outside IPv6 world, is generated on the aircraft and sent to the on-board NeXT router
  2. The router receives it, assigns it an address and port pair, and creates a new entry in the translation table
  3. The packet is then translated and sent, with the IPv6 addresses and ports
  4. The packet passes through the satellite network as IPv4 and is received by the terrestrial NeXT router, which reads the IPv6 addresses and ports and creates a new entry in its translation table
  5. Finally, the packet is translated back to IPv6 and sent to its destination

A very similar process is used for packets being sent to the aircraft, the only difference being that the terrestrial NeXT router cannot assign the IPv4 addresses and ports, so has to request them to the on-board NeXT router.

NeXT application scenario

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The current state-of-the-art alternative to NeXT is the Layer 2 Tunnelling Protocol (L2TP). However, NeXT provides much more efficient satellite use, and therefore lower costs, by using significantly smaller overheads.

Using a tunnel interface, each packet typically has 77 bytes of overhead:

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By comparison, NeXT overheads are significantly lower, because the addresses and ports are only sent with the first packet of the session. The first packet has 68 bytes of overhead, and all subsequent packets only 32 bytes:

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The table below shows the bandwidth used for a voice call, comparing NeXT and L2TP.  NeXT’s significantly better performance is represented by the bottom line:

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For more information on NeXT, click here