[Nfd-dev] Interest equality for for Nack

[Junxiao Shi]

Hi Klaus

How an incoming Nack matches a PIT entry is determined by NFD’s incoming Nack pipeline.
Its specification is in NFD developer guide.

“retransmitted Interest” can only be created by the consumer application.
Cheng Yi’s thesis has clarified the difference between “retry” and “retransmit”: the forwarding strategy on a forwarder can retry an Interest (without changing the Nonce); only the consumer application can retransmit an Interest (with a new Nonce).
NDNLPv2 retransmission is invisible in forwarding pipelines and strategy, so it’s irrelevant to Nack.

When the original Nack has reason “duplicate”, in most cases, this condition will not apply to a retransmitted Interest.
“no route” and “congestion” may still apply to a retransmitted Interest, but it’s better to adopt the same rule regardless of Nack type, to simplify the protocol.

Yours, Junxiao

> On Apr 26, 2016, at 9:44 PM, Klaus Schneider <klaus at email.arizona.edu> wrote:
> 
> Hi Junxiao,
> 
>>> The Nack must also carry the latest Nonce coming from the downstream.
>>> Otherwise, in case a Nack and a retransmitted Interest are in-flight at
>>> the same time between an upstream and a downstream, if the downstream
>>> ignores the Nonce and accepts the Nack, it would incorrectly conclude
>>> that the upstream cannot answer its retransmitted Interest while the
>>> upstream is actively trying to find content for its retransmitted Interest.
> 
> - Is there any specification of this design decision?
> 
> - Is the "retransmitted Interest" created by the link layer (NDNLP) or by the NDN network layer?
> 
> I guess that the downstream router retransmits a packet because it thinks that the upstream has lost the packet. Shouldn't this retransmission timer be much longer than it usually takes for the upstream to send a NACK back? Thus, the retransmitted Interest and the NACK are unlikely to be in-flight at the same time.
> 
> You send an interest to the upstream and either get a NACK back quickly, or not at all.
> 
> Moreover, whatever the reason for the original NACK was (let's say "no path"), there is a good chance that the condition will still apply for the re-transmitted interest, so the downstream would make no mistake in accepting the NACK for the old packet. (If necessary, I'll continue the discussion on the other two types of NACKs).
> 
> I'm not saying that "putting the latest nonce from downstream interest into NACK packets" is a bad choice. It might just be unnecessary for most cases.
> 
> 
> Best regards,
> Klaus
> 
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