Subject: (ION) Multiple ARP servers

From: Jean-Michel Pittet <[email-masked]>
Date: Wed, 25 Mar 1998 12:09:44 -0800

I see that classic 2 provides for a list for ATMARP servers (but only defines
the behaviour for a single server). Then I see SCSP detailing the behaviour
of the servers to each other, with atmarp extensions talking about the 
ATMARP specific packet format.

What I can't find anywhere is some details on the clients behaviour to:

1. choose an initial address from the list of possible candidates
2. choose an alternate address after (maybe an outside agent helps) the client
   find(s) out that the current ARPSERVER is dead.

The impression I'm getting is that the client can chose any address for 1. 
and any address except the one it is using for 2. since all ATMARP servers
being part of that list and supposedly doing SCSP should have up to date 
caches.

What SCSP implementations are there currently or planned?

Cheers
JM 
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Jean-Michel Pittet,     [email-masked]     Phone:650-933-6149      FAX:933-3542
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From: Albert Manfredi <[email-masked]>
Date: Wed, 25 Mar 1998 15:54:07 -0500

>    find(s) out that the current ARPSERVER is dead.

I think that two answers would be given to this most often:

a. As you say in 1, you can have Primary and Secondary ATMARP
servers preconfigured, same as you can do now with DNS servers, but
much better,

b. Use ATM anycast to find these ATMARP or other servers. Anycast
can be used to find the topologically closest server (ATM topology),
which would certainly make sense here.

Not sure why you would need an outside agent to tell you what is
dead. Isn't a simple timeout enough?

> What SCSP implementations are there currently or planned?

Je n'sais pas.

Bert
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From: Jean-Michel Pittet <[email-masked]>
Date: Wed, 25 Mar 1998 13:19:35 -0800

> > I see that classic 2 provides for a list for ATMARP servers (but only 
> > defines the behaviour for a single server). Then I see SCSP detailing
> > the behaviour of the servers to each other, with atmarp extensions 
> > talking about the ATMARP specific packet format.
> > 
> > 2. choose an alternate address after (maybe an outside agent helps)
> >    the client find(s) out that the current ARPSERVER is dead.
> 

OK if preconfigured: 
1. is it important for two or more clients in the LIS to use the same
   ATMARP server (since there might be some histeresis in SCSP for the
   slave to become completely up to date, or because of some other
   reason)

   if so? then _how_ do the clients decide which is primary and which is 
   secondary? (This is the question that I have, is there any detail on 
   the how or doesn't it matter)

2. it is NOT important which of the 2 addresses the clients use (i.e. 
   assumption  mentioned above (I'm asuming that it doesn't matter since
   I don't know any better :(

It seems to me that it should matter, since there are some states in SCSP 
where the server know's it is in the process of updating its cache and 
it is likely that its cache will change with the next update package. In 
that case I would think it wrong to use that "stale" information to 
answer a clients request. So the server can choose not to answer any 
reqeusts until it "feels" up to date (again this is not very quantitative 
but qualitative).

The client on the other hand doesn't have (to my knowledge) any way to
start with the "right" (where that means the initial, the server expected 
to be most up to date etc) server.

> which would certainly make sense here.

What exactly are you refering to with "Anycasts"? Did I miss something in 
classic 2 ?

You are assuming that the topologically closest is the "right" one to be 
the primary one. Is that the desired behaviour for clients? 

> dead. Isn't a simple timeout enough?

Depending on your application you do not want to wait 20 minutes but 
would like to find out failure faster. On the other hand you might not 
want or you can't decrease the ARP timouts to that granularity level. 
Then you will use a seperate NW watchdog type falure detection on the 
hosts you care about.

Cheers
JM

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From: Albert Manfredi <[email-masked]>
Date: Wed, 25 Mar 1998 16:45:14 -0500

[Alternate ATMARP servers]

>    I don't know any better :(

I think each net designer can decide. In my opinion, I would favor
hosts sharing the load among the two or more servers, and SCSP
hysteresis be damned. I mean, if you have humongous LISs where this
would be a real potential problem, you should probably rethink the
size of that LIS anyway. Let's keep in mind what we're talking about
here.

> What exactly are you refering to with "Anycasts"? Did I miss something in

See ATM UNI Signaling 4.0, available at the ATM Forum FTP site,
under approved-specs. Anycast is used to find a particular service
in the ATM network, without knowing the AESA of the servers. For
example, an anycast number would define "the ATMARP server," to
variable sets of criteria the designer can choose to apply. Using
anycast would mean that the hosts do less work, while the ATM
network takes over this chore of finding a healthy server.
 
> the primary one. Is that the desired behaviour for clients?

Typically, yes. Atypically maybe not.
 
> Then you will use a seperate NW watchdog type falure detection on the

Why 20 minutes? Can't you detect a non-response in fractions of a
second? You don't need for the SVC to be torn down by default or
anything. I mean, when my primary DNS server doesn't respond, my PC
goes to the next two choices in short order. Why not the same here?

Bert