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From: braden@ISI.EDU.UUCP
Newsgroups: comp.protocols.tcp-ip
Subject: Re:  ICMP echo
Message-ID: <8704272025.AA00279@braden.isi.edu>
Date: Mon, 27-Apr-87 16:25:43 EDT
Article-I.D.: braden.8704272025.AA00279
Posted: Mon Apr 27 16:25:43 1987
Date-Received: Wed, 29-Apr-87 01:06:42 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: world
Organization: The ARPA Internet
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Hi.  You have asked a question about ICMP Echo semantics that ought to be,
but isn't, contained in the RFC985-revision.  Sigh.

Here is my totally-prejudiced view on how ICMP Echo should behave (it is
the way my own TCP/IP code implemented ICMP Echo, of course!).  Anyone
who wants to beat me up about this should do so now... otherwise, this
interpretation is likely to find its way into the gateway specification
RFC.

   A host or gateway D that receives an ICMP Echo Request addressed to
   itself from host S should send an ICMP Echo Reply back to S,
   using the following rules:
   
      1.  If the Echo Request contained no source route, the Reply should
          be sent to S using the normal routing rules of D.  
          
	      As a result, the Reply may come back by a different path
	      than the Request took. 
	      
	  2.  If the Echo Request did contain a source route, the Echo
	      Reply will be sent using the return-route built up in the
	      source-routing option of the Echo Request.
	      
	      As a result, it is not possible to force a route for the
	      Reply which is different from the route used for the Echo.       

   A result of these rules is that ICMP Echo can be used to sample the
   complete round-trip path which any other higher-level protocol e.g.,
   TCP) will use for its data and ACK packets between D and S.  The
   rules generally follow from the assumption that ICMP is a protocol
   parallel to TCP and above IP, and needs no special routing mechanism.

   
Bob Braden