Subject: [ANCP] ANCP base protocol issues: Tech Type in the Port Up/Down and Port Management messages

From: Tom Taylor <[email-masked]>
Date: Tue, 16 Nov 2010 15:56:45 -0500

Stefaan de Cnodder's review questioned the continued use of Tech Type in 
the Port Up/Down and Port Management messages, given that we have 
discontinued its use in the adjacency messages.

Taking a look at the messages, I note the following:

  - the NAS needs to know the Tech Type for a given access line, so it 
can know what TLVs to expect in the Port Up message. This seems like a 
good reason for Port Up to continue to carry the Tech Type. Port Down 
would carry it just because it uses the same format.

  - the Port Management message is used for several capabilities:

   -- DSL Line Provisioning
   -- DSL Line Testing
   -- NAS-initiated Replication
   -- Conditional Access with White and Black Lists
   -- Conditional Access With Grey Lists
   -- Delegated Bandwidth

The specification language for what the NAS has to expect in a given 
Port Management message instance is a little tricky, but the Function 
field helps. Basically a Function value of 8 warns the NAS to be 
prepared to accept configuration data for DSL Line Configuration (in the 
base protocol specification) plus any configuration data defined in 
extensions to the base protocol that reuse the Port Management message.

Nowhere here does there seem to be a need for Tech Type. The multicast 
capabilities all specify the inclusion of the Bandwidth-Allocation TLV, 
and two of them specify the inclusion of the Multicast-Service-Profile 
TLV, but I can't see different capabilities having different values of 
these TLVs. Hence there is no need to tag the TLVs with the capability 
type with which they are associated. The conclusion is that the Tech 
Type and Block Length fields in the Port Management message should be 
replaced by a single 16-bit Reserved field. (See Figure 13 in the 
current draft. Block Length is there for historical reasons and is unused.)

It would be nice to get rid of the whole 32-bit word containing these 
fields, since Message Type is redundant, but I expect people want to 
keep it for compatibility with pre-standard implementations.

Conclusion: retain Tech Type in the Port Up/Down messages, drop it in 
the Port Management message.

Please let me have any comments by the end of the week.

Tom Taylor