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From: braden@VENERA.ISI.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re:  TCP requesting ARP flushes and other "layering violations"
Message-ID: <8710271619.AA00832@braden.isi.edu>
Date: Tue, 27-Oct-87 11:19:26 EST
Article-I.D.: braden.8710271619.AA00832
Posted: Tue Oct 27 11:19:26 1987
Date-Received: Thu, 29-Oct-87 23:04:19 EST
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 37

Mike O'Dell writes:

	
	Another way to consider the problem is that at some point,
	for reasons it cannot know about, TCP decides correctly that
	the path to its destination is failing.  There needs to be
	a way for TCP to register a complaint with the lower levels that
	it isn't happy with the level of service its getting and would
	like the lower levels to "try harder."  One could argue that
	the lower levels should always "try their hardest," but their
	connectionless nature often precludes them from getting enough
	feedback to really evaluate the effectiveness of their efforts.
	So, if TCP could say - "The path to host XY.Z.Z.Y seems to be
	screwed - please do anything you can to remedy the situation,"
	several useful scenarios become possible.  Among them are 
	redunantly reliable local cables.
	
	...
	
	Currently, this kind of feedback is considered a "layering
	violation" by some.  I suggest that either this notion of layering
	is wrong, or people have a very stilted view of the interaction between
	layers.
	
		-Mike O'Dell
	
	
Actually, this particular "creative" layer "violation" is very much a part
of the long-accepted requirements for a well-designed TCP/IP
implementation.  It is explicitly discussed in one of the "Dave Clark
Five" papers, entitled "Fault Isolation and Recovery" (RFC816).  It is
unfortunately true that there are some TCP/IP implementations extent
in the Internet which do not have this important feature; however, the
requirement was clearly laid out in Dave's paper.  You don't have to
apologize for it.

Bob Braden