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From: JBVB@AI.AI.MIT.EDU ("James B. VanBokkelen")
Newsgroups: comp.protocols.tcp-ip
Subject: Re: ..."layering violations"
Message-ID: <276112.871027.JBVB@AI.AI.MIT.EDU>
Date: Tue, 27-Oct-87 22:40:41 EST
Article-I.D.: AI.276112.871027.JBVB
Posted: Tue Oct 27 22:40:41 1987
Date-Received: Sat, 31-Oct-87 00:45:22 EST
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 27

One has to be at least a little suspicious of layering violations, if
for no other reason that if you go blithely installing hooks and odd
interdependencies, you will wind up with a tangled morass of code that
can't be enhance, or ported, or even maintained.  Layering usually
exists because the designers wanted modularity, and relatively clean
interface specifications on module boundaries.

Handling RIF-cache flush on TCP timeouts means adding another hook,
and dummy routines to the other low-level routing layers.  Maybe
some of the other routing layers could use it, too.  It certainly
represents more code, and more complexity (and work, and money for
initial purchase and support).

Even something like ICMP illustrates this:  Essentially all TCPs will
return an error to their caller whenb they receive a Reset.  An ICMP
Destination Unreachable message implies much the same thing, but many
TCP/IPs won't return an error to the caller.  I won't defend this, but
I certainly understand why:  Handling ICMP Destination Unreachable requires
a 2nd, parallel demultiplexing path through IP and into the TCP, and it
is not absolutely required during the initial rush to get on the air.

A seer employed by one large network user I know of has pronounced that
1 Mb of memory will be necessary to implement ISO.  I don't know if he/she/it
is right, but the pronouncement certainly made more than one manufacturer
jump...

jbvb