Subject: [savi] Summary of FCFS-versus-CPS discussion

From: Christian Vogt <[email-masked]>
Date: Thu, 21 May 2009 14:00:14 -0700

Dear all -

There has recently been a discussion on whether a SAVI device should,
for a statelessly auto-configured address, create a binding either based
on any first packet sent from this address, or based on the result of
the DAD procedure for the address.  The former approach is followed by
FCFS; the latter approach is followed by CPS.  The discussion so far has
shown that both FCFS and CPS have strengths and weaknesses.  I would
like to share with you my summary of the discussion so far, to ensure
that nothing is missed, and to provide a basis for moving forward:

- The advantage of FCFS is that the probability of false packet drops is
   very low.  In general, a risk for a false packet drop exists when a
   host has an address, but the SAVI device guarding the host does not
   have a binding for this address.  FCFS can deal with this situation
   because it can create bindings at any time.  On the other hand, CPS
   cannot easily deal with this situation because it can create
   bindings only during a DAD procedure.  Hence, CPS has a higher
   probability for false packet drops.

- The advantage of CPS is that, in most scenarios, there is no need for
   a designated protocol for binding verification.  Since CPS creates
   bindings only during DAD, it can re-use DAD for binding
   verification. FCFS cannot re-use DAD for binding verification
   because it may create bindings when DAD is not performed.  This
   said, there are scenarios in which a designated binding verification
   protocol may be required also for CPS.  DAD is not reliable, and
   hence cannot provide reliable binding verification either. Reliable
   binding verification, in turn, may be desired in some scenarios,
   such as in DSL networks.  A designated binding verification protocol
   may hence be necessary for those scenarios.

Do folks agree with this summary?  Please speak up if you think the
summary above is missing something.

- Christian

---

From: marcelo bagnulo braun <[email-masked]>
Date: Fri, 22 May 2009 06:21:47 +0200

Christian,

Christian Vogt escribió:
> FCFS;the latter approach is followed by CPS.

I don't think this description is accurate.
FCFS as defined in 
http://www.ietf.org/internet-drafts/draft-ietf-savi-fcfs-01.txt creates 
binding using _both_ data packets and the DAD packets.
I understand that the distinction is the following:
- FCFS uses _both_ data and DAD packets to create the bindings
- CPS uses _only_ DAD packets to create the bindings (i.e. does not uses 
data packets)

The main difference imho is not that one:
the main difference imho is the default behaviour of the mechanism.
In FCFS, if a data packet arrives for which there is no binding, the 
FCFS SAVI device learns the binding and forwards the packet.
In CPS, if a data packet arrives for which there is no binding, the 
packet is discarded.

there are several implications of such behaviour, as you mention below...

I agree with the description you make below

regards, marcelo

> The discussion so far has
>   verification.FCFS cannot re-use DAD for binding verification