Re: [Tsvwg] SCTP applicability statement

[Allison Mankin <[e-mail]>]

Brian,

> So, is your answer no?

Yes, my answer is no :) - SCTP is not intended to fail over between
different associations - the protocol reliability is within
the association.  With the endpoints being on different boxes,
that is, in arbitrary network locations, it would be a different
protocol to do what you request.

But multiple SCTP associations can still be used as failovers
for each other, through use of an application layer watchdog and
a short list of duplicate transactions that might need to be sent.
The section on aaa transport that I recommended (3.2 in
draft-ietf-aaa-transport-04.txt) is about a robust way to do this.

> Is it fair to say that SCTP does not support failover between
> multi-homed hosts, between associations, your second case?
> 
Yes, that's fair.  But to repeat, those different associations have
arbitrary endpoints.   How do SIGTRAN applications know what to make
of a set of message that start arriving suddenly because failover has
occurred - there must be application support of backup server use.

> Are you saying that applications such as SIGTRAN, RSERPOOL, AAA,
> must implement their own sequence numbers and acknowledgements
> and retransmission queues between associations?
> 
No, I think SIGTRAN and RSERPOOL apps (and aaa, by way of the draft I
recommended you look at) can use multiple associations with an application
layer algorithm that is different (and simpler) than sequence numbers,
acks and transport retransmission queues.  

There needs to be duplicate detection of transactions, but failing over
within the SCTP association can result in duplicates too.  

I think you should look at the algorithm I've recommended and compare it
with the specific requirements.  

Allison

> 
> Allison Mankin wrote:                                     (Sun, 04 Nov 2001 16:
> 22:14)
> >
> > > Does SCTP support failover between multi-homed hosts?
> > > 
> > > 
> > > Is failover between multi-homed hosts within the scope of SCTP?
> > 
> > Brian,
> > 
> > SCTP's multi-homing has no relation to anything between*
> > hosts, only between interfaces of one host.  There might be
> > some kind of cluster that appears to its SCTP peer be one host 
> > with multiple interfaces, but it would not be within the scope
> > of SCTP to know about the cluster. 
> > 
> > In draft-ietf-aaa-transport-04.txt, section 3.2. there's a 
> > good discussion of failover/failback, which distinguishes
> > among:
> > 
> > - what a cluster does in its cluster OS or cluster communication
> >   system 
> > - what application-level watchdogs do, where the failover is among
> >   multiple potential peers (here AAA servers)
> > - what SCTP does
> > 
> > If all interfaces of an SCTP host are down, that's the second case.
> > 

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