Interest NACKJunxiao Shi, 2014-07-18

Introduction• Interest NACK, aka "negative acknowledgement", is sent from

upstream to downstream to inform that Data could not be retrieved in response to an Interest.• Interest NACK is useful in forwarding strategy as a explicit signal, for

congestion control and other purposes.• This document is a proposal for including Interest NACK in NDN-TLV

packet format, and in NFD forwarding pipelines.

Semantics

Interest NACK• An Interest NACK is a packet sent by upstream to inform the

downstream that Data cannot be retrieved or delivered in response to an Interest.• An Interest NACK contains an Interest and a reason code.

Interest in Interest NACK• The Interest in an Interest NACK is the Interest that triggers the

Interest NACK.• After an Interest packet is forwarded from downstream to upstream,

the upstream can either return a Data, or return an Interest NACK that contains this Interest.• On each point to point link, with absence of packet loss, there is a flow

balance between Interests vs Data + Interest NACKs.

Reason in Interest NACK• The reason code in an Interest NACK indicates why the upstream

wants to send an Interest NACK in response to the enclosed Interest.• Commonly used reason codes include Duplicate, Congestion, NoData,

Busy, etc (defined in this section)

Reason: Duplicate• An Interest NACK with reason code Duplicate informs the

downstream that the Interest is a duplicate, because the combination of Name and Nonce is previously seen by the upstream.• This means either the Interest is looped, or the Interest has reached

the upstream via another path.

Reason: Congestion• An Interest NACK with reason code Congestion informs the

downstream that there is a congestion on the forwarding path of the Interest.• The congestion can occur on the link between downstream and

upstream, or on the path from upstream toward the content source(s).• The congestion can occur in either direction.

Reason: NoData• An Interest NACK with reason code NoData informs the downstream

that the upstream has no route to forward the Interest.

Reason: Busy• An Interest NACK with reason code Busy informs the downstream that

the Interest has reached a producer, but the producer is too busy to handle this Interest.

Packet Format

Packet FormatInterestNack ::= INTEREST-NACK-TYPE TLV-LENGTH Interest NackCode

NackCode ::= NACK-CODE-TYPE TLV-LENGTH nonNegativeInteger

Interest NACK is not an Interest• An Interest is intended to retrieve some content.• An Interest NACK is to inform the downstream about a problem, and

upstream is not expecting a response from the downstream.

Interest NACK is not a Data• A Data is a piece of content in response to an Interest, that should be

forwarded toward the consumer.• An Interest NACK is in response to an Interest, but it's a hop-by-hop

message that is useful in the network, but useless for the consumer.

Interest NACK is a third packet type• The processing path for Interest NACKs is different from either

Interest or Data, so it should be a third packet type.

Interest NACK is unsigned• Interest NACK is hop-by-hop, so it cannot be used to confuse a

faraway node.• Malicious nodes can gain nothing by sending bogus Interest NACKs.

Sending an Interest NACK informs the downstream that the upstream doesn't work for the Interest, while the malicious node may as well silently drop the Interest.

Forwarding Pipelines

Incoming NACK pipeline

PIT match

receive NACK

trigger strategy: after receive NACK

(drop)

mark out-record as NACKed

N

has out-record?

Y

(drop)

N

Y

Outgoing NACK pipeline

send NACKstrategy action: send NACK

mark in-record as NACKed

has in-record?

(drop)

N

Y

NACKed field in PIT in-record• A NACKed field is added to PIT in-record.• This field is cleared in Incoming Interest pipeline.• This field is set to the reason code in Outgoing NACK pipeline.• The purpose of this field is to help forwarding strategy make

decisions.

NACKed field in PIT out-record• A NACKed field is added to PIT out-record.• This field is cleared in Outgoing Interest pipeline.• This field is set to the reason code in Incoming NACK pipeline.• The purpose of this field is to help forwarding strategy make

decisions.