NFD

Lixia had a related idea: it could be useful to inform the downstream how many hops away were the Nack generated. This can be achieved by, either keep the HopLimit element in the payload indicating the HopLimit value seen at Nack generation node, or add a HopCount field inside Nack header field.

Lixia withdraws this idea on 20200421 NFD call.

I was reminded about this issue and same as Davide, have a major concern with the current direction. One big question is what’s the point of this interest in nack. If I’m not mistaken, the original decision was about having the interest, so the downstream doesn’t need to have it jn PIT (such as pit may use something more compact). If this still is our decision, then the whole issue here is moot.

If we are changing the decision, requiring nodes keeping the original interest (and in case of parametrized interest I’m having doubts that this would be the right requirement), then what is included is NOT interest. It is something that can identify a PIT entry: special structure defined by NDNLP and respected by NFD or interest digest. Coding this stuff as a special way to encode interest is quite wrong.

Forwarders have to store the Interest, including any parameters, in the PIT. Forwarder needs to do so anyway if they want to support timer based strategy retries. Moreover, memory is cheaper than bandwidth.

Nack packet carries enough information to:

• Identity the PIT entry: Name, CBP, MBF, FH.
• Verify that the Nack is against latest forwarded Interest, not an earlier transmission: Nonce.

It reuses the general structure of the Interest, but is not an Interest.

Junxiao Shi wrote:

Forwarders have to store the Interest, including any parameters, in the PIT.

Why?

Forwarder needs to do so anyway if they want to support timer based strategy retries.

That's not a requirement.

It reuses the general structure of the Interest, but is not an Interest.

Then why are we calling it Interest and why does it use the same TLV type as an Interest?

I could redefine as a completely different type, like this:

Nack
  NackReason
  Name
  CanBePrefix
  MustBeFresh
  ForwardingHint
  Nonce

This would appear inside the Fragment/Payload field of NDNLP, rather than as a header field.