Subject: Re: [OPSAWG] definitions of continuity and connectivity

From: t.petch <[email-masked]>
Date: Fri, 1 Jun 2012 12:50:18 +0100

Stepping back (and top-posting), I think that there is a difference
between literary style and technical specification, that good literary
style calls for the use of continuity, connectivity, liveness and all
the other riches that the English language has to offer, whereas
technical specification calls for the boring, repetitive use of the same
old words.

In the context of checking that a link is operational, many words would
do; I use keepalive, others heart beat etc but for me, there is little
difference in the semantics.

In the MPLS-TP context, which trill is quoting, there is a requirement
to detect a failure, switch to protection and restore service to the
client within 50mS, so a rate of 3.3mS with three failures causing a
Loss Of Continuity event is a typical example, and this is referred to
as proactive monitoring, or Continuity Check.  (With BGP, for example,
three strikes and you're out still applies but now the Hold Time is
three seconds, or more).

In MPLS-TP, on-demand checks are referred to as Connectivity
Verification, and are used when the overhead of CC is too great, or to
trouble shoot further a fault.  It could have been called Continuity
Verification, but the originator of it chose otherwise, so Continuity
Check and Connectivity Verification it is:-(

Any attempt to understand CC and CV by resorting to a knowledge of
English is ... well, futile - why aren't they Keep Alive and Liveness
Verification? rather,  accept that they are what they are, proactive and
on-demand (although some MPLS RFC muddy the waters with references to
'proactive CV').

I think that 'tissa-trill' has failed to grasp this.

I would use oam-framework, RFC6371, as my main reference, with more
detail in such as RFC6426.

As you may know, after the IETF had defined protocols to meet all the
MPLS-TP requirements, the ITU-T decided to use a different set, in part
at least.

Tom Petch

----- Original Message -----
From: "Thomas Narten" <[email-masked]>
To: "t.petch" <[email-masked]>
Cc: <[email-masked]>
Sent: Thursday, May 31, 2012 8:37 PM
Subject: Re: [OPSAWG] definitions of continuity and connectivity

> Hi Tom.
>
> > > I have a very practical question. Are the two MUST requirements
above
> > > the same, or are they different? I can't tell without knowing what
the
> > > difference between continuity and connectivity is.
>
> > Different.
>
> > This was hammered out in MPLS but RFC5860 is sloppy in this regard.
>
> > Continuity Check is proactive, at a rate commensurate with the time
> > within which failures should be detected, which could be mS or could
be
> > seconds.  Thus it must be low cost, simple, basic.
>
> Does this also imply multiple messages sent, and getting a percentage
> of packets lost? As opposed to a single request/response?
>
> > Connectivity Verification is on demand, run when it is needed,
perhaps
> > as a result of an alarm; it may not be run for months, so can afford
to
> > be heavyweight, gathering everything that might be needed.
>
> Does this also imply a single message/response transaction? Or can
> this also imply something more sophisticated, involving multiple
> messages?
>
> What I'm trying to do is separate what "continuity" and "connectivity"
> are intended to achieve, vs. mechanics for acheiving the desired
> semantics. But a lot of the discussion about the terms seems to be
> implying a solution rather than what the solution is required to
> achieve.
>
> And while I understand that on-demand vs. proactive is an important
> distinction for when/how to invoke functionality, I'd like to
> understand the distinction between the two in terms of what they are
> supposed to achieve (independently of whether invoked proactively or
> on-demand).
>
> Thomas
>
>