Subject: [ANCP] Ambiguity in draft-ietf-ancp-mc-extensions-06.txt section 5.3

From: Tom Taylor <[email-masked]>
Date: Mon, 23 Apr 2012 20:46:52 -0400

Section 5.3 of draft-ietf-ancp-mc-extensions-06.txt defines the 
List-Action TLV, which is quite complex. Near the end of the section, 
the text allows prefix lengths of zero to signify wildcards. If a zero 
prefix length is used, the corresponding prefix is to be omitted. The 
text also says that a zero value for the source prefix length indicates ASM.

This text fails to cover a couple of cases where the operator is using 
SSM, because it conflates the absence of a specific source address with 
ASM. The first case is when the operator wants to specify the 
combination of an SSM multicast group index and wildcarded source 
prefix. The second case is when the operator wants to wildcard 
everything (i.e. group and source prefix lengths both zero), but is 
using SSM.

My proposed solution is to delete the following sentence:

   "In particular, a value of 0x00 for the
    Source PrefLen indicates an ASM multicast entry, and the Source
    Prefix will be absent."

Obviously this means that in the case of a zero-length source address 
prefix, the AN does not know from the profile whether ASM or SSM is 
being used. I don't think the AN needs to know that to do its job -- it 
just has to admit or block flows based on the criteria given to it.

Comments?

Tom Taylor