RFC 2075 (rfc2075) - Page 2 of 5

IP Echo Host Service

RFC 2075                  IP Echo Host Service              January 1997


   The most straightforward situation is when an echo host receives an
   IP datagram with no options and whose protocol field has a value
   other than 1 (ICMP).  In this case, the echo host modifies the header
   by exchanging the source and destination addresses, decrements the
   TTL by one and updates the IP header checksum.  The host then
   transmits the updated IP datagram back to the original source of the
   datagram.

      NOTE: If the TTL is zero or less after decrementing, the datagram
      MUST not be echoed.  In general, an echo host is required to do
      all the various sanity checks that a router or host would do to an
      IP datagram before accepting the datagram for echoing (see STD 3,
      RFC 1122, and RFC 1812).

      The TTL MUST be decremented for security reasons noted below.
      Observe, however, that the effect is that hosts using an echo path
      through an echo host SHOULD set their TTL to twice the normal
      value to be sure of achieving connectivity over the echo path.

   If an arriving IP datagram has options, the echo host's
   responsibilities are more complex.  In general, the IP source and
   destination are always exchanged and TTL and checksum updated, but in
   certain situations, other special actions may have to take place.

   If the datagram contains an incomplete source route option (i.e. the
   echo host is not the final destination), the datagram MUST be
   discarded.  If the datagram contains a complete source route option,
   the source route option MUST be reversed, and the datagram (with
   source and destination IP addresses exchanged and updated TTL) MUST
   be sent back along the reverse source route.

   More generally, the goal with any option is to update the option such
   that when the echoed packet is received at the original source, the
   option fields will contain data which makes sense for a datagram
   originating at the echo host.

   There is one option for which it is unclear what the correct action.
   The timestamp option is sometimes used for round-trip time
   estimation.  If the option is reset at the echo host, then a history
   of roughly half of the trip delay will be lost.  But if the option is
   not reset, then the timestamp option will appear inconsistent with
   the source and destination addresses of the datagram.  To try to
   balance these two issues, the following rules are suggested:

      1. If the first entry in the timestamp option contains the IP
      address of the source host, the entry SHOULD be rewritten to
      contain the IP address of the echo host, and the timestamp option
      pointer SHOULD be truncated so that this timestamp is the only one



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