prototype IPv6 echo host available

[Kevin M. Lahey]

A prototype IPv6 echo host is now availble at echo.ipv6.nas.nasa.gov
(5f01:2900:8163:cd00::2).  The implementation is based on Craig Partridge's
description of echo hosts in draft-rfced-exp-partridge-00.txt:

   An IP echo host returns IP datagrams to their original source host,
   with the IP source and destination addresses reversed, so that the
   returning datagram appears to be coming from the echo host to the
   original source.  IP echo hosts are tremendously useful for debugging
   applications and protocols.  They allow researchers to create looped
   back conversations across the Internet, exposing their traffic to all
   the vagaries of Internet behavior (congestion, cross traffic,
   variable round-trip times and the like) without having to distribute
   prototype software to a large number of test machines.

The prototype implementation is intentionally conservative:  It rejects
packets with loopback, local, site, multicast, IPv4 compatibility or
IPv4 mapped source addresses,  packets with next header fields other
than UDP or TCP, and packets with non-zero flow ids.

If the 6bone community finds this service useful, I'll be happy to put
in the work to remove most of these restrictions.  If no one uses it,
I'll probably drop support for it when I next upgrade the system, in a
few more weeks.

Enjoy,

Kevin