[Nfd-dev] Handling new content in app for pending interest in NFD

Junxiao Shi shijunxiao at email.arizona.edu
Wed Apr 1 10:21:02 PDT 2015

Hi Jeff

The recommendation for "soft realtime app" is given below.
Please note that some features are not yet implemented, but the
specifications are already approved.
Watch each listed feature for implementation progress.

The *forwarder MUST NOT cache unsolicited Data*, regardless of its origin.
Feature: http://redmine.named-data.net/issues/2181

A soft realtime app MUST maintain an *in-memory storage*.
It MUST NOT rely on the forwarder to cache its Data, even if they are sent
to the forwarder in response to an unexpired Interest.
The data structure is referred to as "in-memory storage" rather than
"cache", because it SHOULD be fully managed by the app to keep all Data
that the app wants to serve. It's not a cache where useful Data can be
Reason: the forwarder is free to evict any Data.

A soft realtime app SHOULD set *CachingPolicy=NoCache* in
LocalControlHeader (or NDNLPv2 header) on a Data served from an in-memory
The local forwarder ContentStore SHOULD NOT cache a Data with
CachingPolicy field is not carried to remote forwarders, so that remote
forwarders can still cache the Data. It avoids duplicate Data copies on
Feature: http://redmine.named-data.net/issues/2183

A soft realtime app MAY maintain its PIT to *avoid sending unsolicited Data*
When an app chooses not to maintain a PIT, sending unsolicited Data has no
harmful effect.

Yours, Junxiao

On Wed, Apr 1, 2015 at 8:38 AM, Burke, Jeff <jburke at remap.ucla.edu> wrote:

> Thanks for all of the discussion. I am not sure if I have been able to
> extract an answer to the original question I was asking.  Let me try again
> with more information:
> - The original question was intended to be only about a local NFD (for
> now).
> - The use case is a (soft) real-time application, like videoconferencing
> or streaming sensor data for interactive applications. Specifically, we
> have encountered the issue in 1) ndnrtc/ndncon
> <https://github.com/remap/ndncon/> for videoconferencing and 2) ndn-opt,
> which streams data from OpenPTrack <http://openptrack.org/>, a person
> tracking system built on the robot operating system (ROS).
> - In these use cases, consumers want to receive data with low latency, so
> they send interests intended to arrive before the time the data is
> produced, but not so long before that the Interests expire before
> corresponding data is produced.
> - As they are born, samples are signed and put into a repository by the
> publisher.  Currently these apps use an in-memory repo specific to the
> application, via the MemoryContentCache class provided by the NDN-CCL
> library. But this could just as well be a repo outside the application.
> - So, when Interests arrive, they go in the local  NFD's PIT, and are
> forwarded to the application.   We are interested in the case where the
> data is not born *yet*, so the publisher cannot respond to the Interest.
> - Then, before the Interest expires from the local NFD's PIT, the
> corresponding data is born in the application and *should* be used to
> answer the Interest.  But how does that data get back to the local
> forwarder?
> - What should the application do?  1) Maintain its own PIT?   2) Or, push
> the data to the local NFD, letting it drop it if it doesn't satisfy
> anything.  3) Or, as we have previously discussed about the
> MemoryContentCache, do we need to consider a more sophisticated interaction
> model between the application and the daemon's content store and/or PIT
> that reduces duplication of these capabilities in applications?
> - For now, we have implemented an in-application PIT in ndnrtc.  (I am not
> sure what method we ended up using in ndn-opt, perhaps the unsolicited data
> method.)
> - Discussion on this topic will help inform what library support will be
> provided in the short term.  (I don't think we want every application
> author to have to write a PIT, so am thinking in the short term we should
> provide one to complement the MemoryContentCache class.  But this is
> duplicating data structures and functionality in NFD – is that natural, or
> should we look to handle it through a different interaction with the local
> forwarder?)
> Thanks,
> Jeff
> On Mon, Mar 30, 2015 at 10:28 AM, Burke, Jeff <jburke at remap.ucla.edu>

>  Hi folks,
>  We are facing this scenario in a few applications:
>  1) Interest received by NFD, passed to an application
> 2) Application not able to respond to interest, so interest stays in NFD
> 3) Some time passes, but not enough for the Interest to expire
> 4) Application generates data (e.g., from a sensor reading) that would
> answer the Interest in the NFD PIT
>  Question: How does app know to inform NFD it has the data after step 4,
> and how should it do that?
>  - In this type of app, should it push the data unsolicited to the NFD
> and let it decide if there is something to do?
> - Is it recommended to implement an application-level PIT so the app is
> sure this data is solicited?  (Why add *another* PIT?)
>  Thank you,
> Jeff
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