Subject: Re: [SUA issue 17 & M3UA issue #11] IPSP cases.

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Thu, 6 Sep 2001 05:47:00 -0500

Javier,

Please see comments below...

On Thu, 06 Sep 2001, Javier Pastor-Balbas (ECE) wrote:

> Hi, 
> 
> Regarding a few concerns expressed by Brian in a previous mail about
> handling of links, linksets, routes, routesets, the changeover procedure and
> management inhibiting, here is some reassuring material:

I don't find any of this reassuring.

> The use of several links in a linkset, several routes and the changeover
> procedure, provides redundancy in MTP networks, and also increases the
> signalling capacity of a single link.
> 
> What must be achieved by M3UA is a message delivery without duplication or
> loss, in sequence when required, and also some redundancy in order to avoid
> a single point of failure.
> 
> In an IP network, the message delivery without duplication or loss is
> achieved by the SCTP, which also grants the sequence of the messages, refer
> to RFC 2960.

SCTP does provides for interface redundancy on multihomed hosts, but does
not provide for host redundancy.  The method used by M3UA to obtain host
redundancy is to use multiple SCTP associations.  SCTP provides only poor
mechanisms for failing over between SCTP associations (retrieval).  So,
unfortunately, SCTP doesn't provide all that is needed.  Changeover between
SCTP associations (and between hosts) requires additional protocol and
procedure.

> 
> Network redundancy is provided by having several IP paths accessed through
> the corresponding IP routers for a given point to point communication, and
> IP has very powerful dynamic routing protocols (e.g. OSPF) among the routers
> to handle these paths, much better than the static methods of the MTP.

Compare the QoS guarantees of IP and MTP and then look again at what you just
wrote.

> 
> In order to provide nodal redundancy, SCTP provides multi-homing procedures,
> which apply to a single SCTP association, so multiple SCTP associations are
> not needed at all to provide redundancy, and applications don't have to be
> coordinated to achieve this, since SCTP takes care of it on its own.

Unfortunately, M3UA provides multiple associations to achieve redundancy
between hosts.  SCTP does not provide for this.  There are no reassurances
in your statements here.  More than a single association between SG and ASP
is even recommended by M3UA.

> 
> And last, but not least, in order to provide the management inhibiting, an
> IPSP only needs to send an ASPIA message to prevent its peer from sending
> any traffic.

I think your statement here may be a little bit premature: we haven't handled
ASPIA cases for IPSP yet.

--brian

-- 
Brian F. G. Bidulock
[email-masked]
http://www.openss7.org/

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