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From: infopath@well.UUCP ("Jay W. Melvin")
Newsgroups: comp.protocols.tcp-ip
Subject: ICMP HOST_UNREACH Param: 1777
Message-ID: <9101012157.AA06468@well.sf.ca.us>
Date: 1 Jan 91 21:57:11 GMT
Sender: usenet@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 45

Greetings, and thanks in advance to anyone willing and able to address this 
question.

BACKGROUND:  I am implementing ICMP for the first time and need clarification
on the topic of Error_to_Source messages.

ISSUE:  MIL-STD 1777, Reassembling state decision table (pg. 1-119).
The test "Where_to?"  (pg. 1-126) tests the following to determine if the 
datagram is "destined for this site":
        1.  Datagram options for completion of source route address visits,
        2.  Datagram destination address for identity with this host's id.
If either test fails, the decision table specifies that the HOST_UNREACH 
parameter should be passed to the Error_to_Source routine (pg. 1-131).

PROBLEM 1:  The Error_to_Source routine does not accept the HOST_UNREACH
parameter.  Instead, it accepts the PROTOCOL_UNREACH parameter, which has
not been tested for by "Where_to?".

PROBLEM 2:  Consulting RFC 792 (pg. 5) deepens the apparent inconsistency 
by stating that the HOST_UNREACH parameter is generated by gateways, not hosts.

Additionally, D. P. Sidhu's execllent RFC 963 (pg. 16) reinforces the
PROTOCOL_UNREACH parameter by mentioning it in reference to another issue,
but does not cite it as a problem in itself.

QUESTIONS:
Is there really an inconsistency here, or have I missed something?
What information does the Internet Community expect the HOST_UNREACH ICMP 
  message to communicate?  
Is it actually acceptable for a host to issue the HOST_UNREACH message 
  under the conditions described above?  
Is there a discussion in the ICMP literature which would clarify the
  usage of the HOST_UNREACH parameter, since it appears to be the correct 
  parameter to expect from a failed "Where_to?" test in a host environment.


Many thanks,  

Linda Melvin 
Senior programmer 
infoPATH Communications Software Services, La Honda, CA
infopath@well.com.sf.ca.us

P.S.  This is also my first attempt to use the TCP-IP forum.  
Please advise if I've not done it correctly.