Subject: [Anima] 2 questions about the prefix management draft

From: Brian E Carpenter <[email-masked]>
Date: Fri, 12 May 2017 15:15:09 +1200

I have two questions about draft-ietf-anima-prefix-management-03
before we move it forward. As a reminder, the main GRASP objective is
defined like this:

     objective = ["PrefixManager", objective-flags, loop-count,
                  [PD-support, length, ?prefix]]

     loop-count = 0..255         ; as in the GRASP specification
     objective-flags /=          ; as in the GRASP specification
     PD-support = true / false   ; indicates whether sender supports PD
     length = 0..128             ; requested or offered prefix length
     prefix = bytes .size 16     ; offered prefix in binary format

Question 1: Should we add an IP version indicator? In this case the
value of the objective would be something like:

[IP-version, PD-support, length, ?prefix]

(and the prefix would be 4 or 16 bytes according to the IP-version).

This seems quite simple to define (and to implement: I have figured
out how to do it for my demonstration code). It makes the objective
more versatile at low cost.

Question 2: What is the value of the PD-support parameter?

I have been thinking about use cases, and I can't find one. Lets's
assume that an ASA supports "PrefixManager" in order to obtain
address space from a pool, and then hands out prefixes to non-autonomic
devices such as CE routers. It may use DHCPv6/PD for that, or RADIUS,
or some other method. However, that is quite irrelevant for the
autonomic operations where this ASA communicates with other ASAs to
obtain address space. 

We introduced this parameter a long time ago and the text says:

4.3.  Behavior after Successful Negotiation

   Upon receiving a GRASP Negotiation-ending message that indicates that
   an acceptable prefix length is available, the requesting device may
   request the prefix using DHCPv6 PD, if both ASAs have indicated that
   they are within a device that supports PD.  Otherwise, it is
   permissible for the initiating ASA to use the negotiated prefix
   without further messages.

What's the point? Why would we need to use PD when we've already
negotiated a prefix?

Regards,
    Brian