[Nfd-dev] review request: NDNLPv2 NACK, Fragmentation, LocalControlHeader

[christian.tschudin at unibas.ch]

Dear Junxiao,

thanks for sharing your design plans and the possibility to comment.

A packet layout question: Isn't the NdnlpHeader wrapper redundant? You 
can see this in the grammar where no other field can ever follow the 
outer NDNLP-PACKET-TYPE and -length.

Or are there plans for other fields coming before or instead of the 
NdnlpHeader? Else I guess that this was introduced for parsing speed 
reasons? But the price is considerable: at least 2 mandatory bytes for 
every Ndnlp packet for carrying no additional information.

Regarding fragmentation, I'm wondering about plans for the B+E style 
mentioned in the slides. Could additional fragment types be forseen?

   fragmentBegin, fragmentMiddle, fragmendEnd and fragmentBeginEnd

This would bring the overhead from 2 or 3 bytes for a B+E header 
extension down to 0 additional bytes. *)

Finally some suggestions where the packet format description could 
benefit from clarifications regarding operational aspects:

- NDNLP packet stuffing (concatenation) inside the same frame:
   ok or not?

- mixed NDNLP/Interest/Data stuffing inside the same frame: ok or not?

- fixed width of sequence number field: can the sender choose
   unilateraly or will be there a link negotiation protocol? Can
   the sender change the width on the fly?

- Isn't the statement that "If an incoming NdnlpPacket contains unknown
   fields, the receiver MUST drop the packet" evolution-hostile, also
   against the old principle of "be liberal in what you accept"?

   Or expresses this a stance against the feature interaction problem
   and wants to prevent vendor-specific extensions? Asked differently:
   Shouldn't there be a type range reserved for experimental and
   vendor-specific extensions with an explicit semantics on how to
   process such packets with unknown extensions (without dropping the
   packet alltogether)?

- Related: Does the statement (in the indexed fragmentation section)
   reading "Unless otherwise specified, header extension fields from
   other features shall only appear on the first fragment." mean that
   in general a link feature can/will squat the link for some
   self-chosen duration?
   For example, a hypothetical "urgent msg feature" (or signals like
   a fragment ack or nack feature running in the other direction) would
   have to wait until a fragment train has ended?

best, christian


*) Looking at the "minimum Ndnlp packet fragment overhead" might be
    an interesting metric to observe in the design. Currently, it seems
    that a B+E scheme in NDNLP will vary between 12 and 7 bytes of
    overhead, depending on the final design and type allocation choices
    that will be made. In a BT low energy context with 20B MTU this
    means a change from 8 to 13 bytes payload, a 60% capacity increase.


On Tue, 26 May 2015, Junxiao Shi wrote:

> 
> Dear folks
> 
> I have written the protocol spec of first batch of features in NDNLPv2, and
> I need someone to review the design.
> 
> http://redmine.named-data.net/projects/nfd/wiki/NDNLPv2
> 
> If you don't know what this is about, see #2520, #2763, andhttp://redmine.named-data.net/attachments/download/301/NDNLPv2_20150417.ppt
> x
> 
> I'll appreciate all review comments.
> You don't have to be an expert in order to do a design review.
> 
> Yours, Junxiao
> 
> 
>