[Nfd-dev] review request: NDNLPv2 NACK, Fragmentation, LocalControlHeader
christian.tschudin at unibas.ch
christian.tschudin at unibas.ch
Mon Jun 1 03:56:03 PDT 2015
Hi Junxiao,
thanks for these answers.
So it seems that NDNLPv2 will be an index-fragment protocol only, for
the link-types mentioned in "Goals" only, and no B-E variant ever?
My assumption was that NDNLP would be a "framework" that various
sub-network-layer protocols would share for their task-specific headers.
But it seems that this is quite wrong.
What would you suggest as header structure for bringing LP variants into
the type-value-space of NDN packets? A forth outer type?
best, christian
On Thu, 28 May 2015, Junxiao Shi wrote:
> Hi Christian
> NdnlpHeader wrapper
> In the full plan NDNLPv2_20150513.pptx page 17-20, we have a header and a
> trailer. The header wrapper makes it easier to define the portion covered by
> HMAC signature (page 53), and permits a padding at the end of header (page
> 24).
> 2-octet overhead is negligible on the link types mentioned in "Goals"
> section. NDNLPv2 is not designed for 20-octet MTU links; those links need a
> different link adaptation layer protocol.
>
> B-E Fragmentation and extra fragment types
> No, only NdnlpFragment type is allowed.
> 3-octet overhead is negligible on the link types mentioned in "Goals"
> section. NDNLPv2 is not designed for 20-octet MTU links; those links need a
> different link adaptation layer protocol.
>
> Concatenation in single frame
> NDNLPv2 is able to operation on a stream socket, so it can handle
> concatenation in a single frame.
> However, NDNLPv2 does not define whether concatenation is allowed, and this
> is up to face implementation.
> Seealso http://www.lists.cs.ucla.edu/pipermail/ndn-lib/2014-November/000215.ht
> ml on a related topic about WebSocket frame vs network layer packets.
>
> NdnlpSequence width
> The width is chosen by link implementation.
> Suppose EthernetFace implementation for 100Mbps link has chosen 48-bit, all
> NdnlpSequence on this link must be 48-bit.
> There's no negotiation.
>
> Unknown Fields
> Excerpt from NDNLPv2_20150513.pptx page 27:
> Rationale: NdnlpPacket is hop-by-hop. It's feasible to ensure
> everyone to understand all fields.
>
>
> Ignoring an unknown field will cause incorrect operations, see #2520
> note-11.
>
> A vendor-specific extension can claim a TLV-TYPE as "known field but
> relevant feature is disabled".
> This is not a "unknown field".
>
> Extension fields on first fragment only
> This mainly applies to extension fields that decorate a whole network layer
> packet.
> For example, it's sufficient to add a Network NACK on the first fragment,
> and it's unnecessary to repeat the Network NACK field on subsequent
> fragments.
>
> Fields that do not decorate a network layer packet, such as Automated Repeat
> reQuest, can be added to any fragment.
>
> Yours, Junxiao
>
> On Thu, May 28, 2015 at 7:33 AM, <christian.tschudin at unibas.ch> wrote:
> 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
>
>
>
>
>
>
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