Re: [tcpm] draft-ananth-tcpm-persist-00.txt as a WG document

Ted Faber <faber@ISI.EDU> Fri, 03 October 2008 18:38 UTC

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Date: Fri, 3 Oct 2008 11:37:26 -0700
From: Ted Faber <faber@ISI.EDU>
To: Murali Bashyam <MBashyam@OcarinaNetworks.com>
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Cc: "Anantha Ramaiah \(ananth\)" <ananth@cisco.com>, "tcpm@ietf.org" <tcpm@ietf.org>, David Borman <david.borman@windriver.com>
Subject: Re: [tcpm] draft-ananth-tcpm-persist-00.txt as a WG document
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I don't mean to be answering from John, but your comments interested me.

On Thu, Oct 02, 2008 at 05:13:50PM -0700, Murali Bashyam wrote:
> 
> > From: John Heffner [mailto:johnwheffner@gmail.com]
> > I have reservations about moving forward with this draft as a wg
> > document.  While the information is technically correct, it is overly
> > specific to the point of possibly being misleading rather than
> > clarifying.  An operating system may terminate a tcp connection at any
> > time, not just when the connection is in the persist state.  Further,
> > I would actually argue that terminating a connection *because* it's in
> > the persist state is a bad idea and should be discouraged.  (The
> > reason for termination should be for its use of resources, not TCP's
> > state.)
> 
> No implementation today (the well-known ones BSD, Windows and Linux)
> terminates the TCP connection in that persist state as long as ACKs
> are being reliably received from the peer. Those three implementations
> are not doing what you are saying they should be doing, and if it's
> crystal clear from the standard that they should be doing so, why
> aren't they?

Your comment is a little ambiguous.  John says that he believes that
persist state is not necessarily a good marker for aborting connections
when resources are scarce.  To take him to task because implementations
are not using it as a marker - because the implementers agreed with
his position - is a little unfair, IMHO.

On the WG item:

As a bigger picture issue, I find it helpful to remember that
protocol standards, like 1122, are primarily interoperation documents.
As such, they tend to focus on what protocol implementers must do to
talk with one another successfully, not to nail down all possible
choices.

Once we start offering advice (or stronger) to developers, we're leaving
the interoperability domain.  We, as a standards body, have to assume
that implementers know their application and their environment better
than we do, and that they can and will make appropriate decisions if we
give them the room to do so.

Where a standard has unnecessarily tied the hands of implementers, that
standard should be changed, IMHO.  I am also sympathetic to clarifying
the standards body's intent where poor language has obscured it.  I
don't see that kind of confusion in the text the authors are addressing
(that is RFC 1122 Section 4.2.2.17), but reasonable people can disagree
on such things.

As such, taking on this work item - clarifying 1122 4.2.2.17 - doesn't
excite me, but I don't oppose it.

The draft should, IMHO, point out that 1122 has nothing to say about
resource management.  Designers and implementers are free to be as
clever or foolish as they'd like.

End of thoughts on the WG item.

As for the draft itself, I'm concerned that it seems to be advocating a
particular resource management position more strongly than simply
pointing out that RFC1122 should not affect resource allocation
decisions.  I also see some misplaced standards language that is
somewhat confusing.  I'm looking at this paragraph in Section 2:


	An extensive discussion took place recently about this issue on
	the TCPM WG mailing list [TCPM].  The general opinion seemed to
	be that terminating a TCP connection in persist condition does
	not violate RFC 1122.  In particular the operating system, a
	resource manager, or an application can instruct TCP to abort a
	connection in the persist condition.  TCP itself SHOULD not take
	any action and continue to keep the connection open as mandated
	by RFC 1122 unless otherwise instructed to do so.  The exact
	mechanism by which the instruction to abort the connection is
	conveyed to TCP is an implementation decision and falls beyond
	the scope of the current memo. 

There's no instruction going on, and one doesn't request an abort.
Abort is a command in the TCP interface (defined in RFC 793, p. 50) that
destroys a TCP connection.  Abort works in any state.  Anything with
access to the TCP interface, including the OS, a resource manager, or an
application, can call it.  A TCP abort is "the exact mechanism by which
the instruction to abort the connection is conveyed to TCP", and the
only reason that it's outside the scope of this document is that it's
defined in 793.

In light of that I think that the sentence about "TCP itself SHOULD
not[sic] take any action..." is more confusing than the 1122 section.
It seems to imply that TCP might react differently to an abort than
the single paragraph in 793, and I don't think that's the case.

Now, a strictly defined abort command seems to somewhat contradict my
earlier comments about allowing implementers some freedom.  This is a
spot where the standards body has required a specific tool to be
available to applications (and OSes and resource managers): there must
be a way to annihilate a TCP connection from outside without regard to
its state or anything else.  The standard is silent about how the
command is used; that's the flexibility here.


-- 
Ted Faber
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