Comments within and below.
> Subject: Re: Subject: Re: TCP/IP question [7:17343]
>
> >This subject is not as clear as it ought to be. If you look
at
> >the subject of loopbacks generically, there are two RFCs that
> >come to mind. The first deals with RFC 1122 "Requirements
for
> >Internet Hosts". The second deals with RFC
1122 "Requirements
> >for IPv4 Routers".
>
> Kind of a nit, but 1122 has been superceded by 1812.
>
I must respectfully disagree.
The lineage of the RFC progression of Internet Gateways is
documented very well in Fred Baker's hyperlinked RFC pages and
other sources. On the first page he notes in the preface the
following:
"PREFACE
This document is an updated version of RFC 1716, the historical
Router Requirements document. That RFC preserved the
significant work that went into the working group, but failed
to adequately describe current technology for the IESG to
consider it a current standard."
This disclaimer statement is likely necessary because RFC 1716
was categorized as informational, not standards track.
If you go to RFC 1716 and look at the introductory paragraph it
states the following:
"1 INTRODUCTION
The goal of this work is to replace RFC-1009, Requirements for
Internet Gateways ([INTRO:1]) with a new document."
If you go to RFC 1009, that appears to be more or less the
first defined RFC named, "Requirements for Internet Gateways".
Although RFC 985 does deserve mention, it was only a draft
standard. The RFCs that are referenced in this document number
62 references, including the original RFCs governing IP (700
series RFCs).
If you look at RFC 1122, it states the following:
"Status of This Memo
This RFC is an official specification for the Internet
community. It incorporates by reference, amends, corrects, and
supplements the primary protocol standards documents relating
to hosts. Distribution of this document is unlimited."
There are no listed or named successor standards that supercede
RFC 1122 in the standards track (as they relate exclusively to
Internet Hosts).
The general point of confusion exists around this specific
statement in RFC 1812, para. 1.2 appropriately
titled, "Relationship to other standards". It states,
"Host Requirements - This pair of documents reviews the
specifications that apply to hosts and supplies guidance and
clarification for any ambiguities. Note that these requirements
also apply to routers, except where otherwise specified in this
memo. As of this writing, the current versions of these
documents are RFC 1122 and RFC 1123 (STD 3), [INTRO:2] and
[INTRO:3].
This is saying not that the standard has been superceded, but
rather it has been incoporated by reference. Any areas of
ambiguities (as they apply to Internet gateways) are to be
resolved explicitly in RFC 1812.
Okay, if you have made it this far, you are naturally
asking, "what is my point"? There are discontinuities in
certain areas of RFC 1122 and RFC 1812. If the device is an
Internet host (not a router), it is only required to comply
with the requirrements in RFC 1122. If it is an IPv4 router,
than it should comply with the requirements in RFC 1812. So
where's the discontinuity?
Let's try subnet zero for starters. Look at this statement
from RFC 1812, page 49, para. 4.2.2.11:
"DISCUSSION
Previous versions of this document also noted that subnet
numbers must be neither 0 nor -1, and must be at least two bits
in length. In a CIDR world, the subnet number is clearly an
extension of the network prefix and cannot be interpreted
without the remainder of the prefix. This restriction of subnet
numbers is therefore meaningless in view of CIDR and may be
safely ignored. "
This says that subnet zero is allowed and is considered a good
practice to use in the CIDR world (why waste address space?)
Here's the rub. Go to RFC 1122 and see what it says about
subnet zero:
"From the Assigned Numbers memo [9]:
"In certain contexts, it is useful to have fixed
addresses with functional significance rather than as
identifiers of specific hosts. When such usage is called for,
the address zero is to be interpreted as meaning "this", as
in "this network". The address of all ones are to be
interpreted as meaning "all", as in "all hosts". For example,
the address 128.9.255.255 could be interpreted as meaning all
hosts on the network 128.9. Or, the address 0.0.0.37 could be
interpreted as meaning host 37 on this network."
It is useful to preserve and extend the interpretation
of these special addresses in subnetted networks. This means
the values of all zeros and all ones in the subnet field should
not be assigned to actual (physical) subnets.
So, what is the issue and what are the differences? RFC 1812
indicates that subnet zero is allowed, useful in CIDR, and
should be used. RFC 1122 clearly indicates that Internet hosts
should not be placed in subnet zero networks.
One could easily ask, what relevance does this have to anything
I do today? In my particular case, I walked into a situation
where I was using a terminal server configured by somebody
else's equipment. Everything that I checked seem to indicate
that it should work. In the process of deductive analysis, I
finally ended up connecting the host directly into the ethernet
interface of the router via crossover cable, bypassing all hubs
and switches. The router's IP addr was 172.16.0.1/24 My PC's
IP addr was 172.16.0.8/24. The PC **could not* ping the
router, period. I then dug back into my memory banks on this
very issue of subnet zero and decided to change the IP addr of
the router to 172.16.1.1/24 and I changed the host to
172.16.1.8/24. Everything worked flawlessly. Although MS
claimed that Win98 (don't remember the exact iteration) would
recognize subnet zero, it didn't.
As a general rule of thumb, it is safe to say that the use of
subnet zero is safely used on any given router, particularly if
there are no Internet hosts on that segment (such as WAN
links). It is not a good idea however, to use subnet zero as a
subnetwork populated with Internet hosts. This is just one of
several incosistencies between RFC 1812 and RFC 1122. If an
issue pertains to an Internet host explicitly, RFC 1122 is the
governing Internet standard. If the issue involves an IPv4
router (Internet Gateway), then RFC 1812 governs. In the case
of devices that resemble both (NT4 server running RRAS among
other possibilities), it is not really clear which RFC would
govern. My bet would be on the more stringent requirement
between the two standards.
v/r,
Paul Werner
p.s. In this particular instance (loopbacks), there are no
incosistencies between the two standards.
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