[Ndn-interest] Selector Protocol over exact matching

Burke, Jeff jburke at remap.ucla.edu
Fri Oct 3 19:17:32 PDT 2014

Sorry about the delay in returning to this.  The idea of a protocol that
provides these features at a higher layer definitely draws out the design
issues; thanks for writing it out.

A couple of questions that occur to me about the approach:

- Data names become about the query rather than about the data; this seems
to be a fundamental design difference.  If the response to each interest
includes a hash of the query, it can't be used to fulfill different
queries that would have the same result. So some caching benefit is lost.
(This seems like the basic tradeoff of exact match vs lpm+selectors.)

- In the case where there are multiple publishers with different answers
to the same selector protocol query, they would return different data for
the same request.   In the current NDN case, this is disambiguated because
the response includes the name of the data itself.  But in the selector
protocol case, couldn't you have many answers to the question all sharing
the same name?  

- Further, without exclusions and without more specific names, how do you
retrieve multiple answers to the same query?  Let's assume that in some
cases this is a desired feature.

- How do you envision applications interacting with the selector protocol?
 Would they implement it directly or publish "normally named" content to a
stack that then implements the protocol?


On 9/25/14, 1:10 AM, "Ignacio.Solis at parc.com" <Ignacio.Solis at parc.com>

>On 9/25/14, 9:17 AM, "Marc.Mosko at parc.com" <Marc.Mosko at parc.com> wrote:
>>In the CCNx 1.0 spec, one could also encode this a different way.  One
>>could use a name like ³/mail/inbox/selector_matching/<hash of payload>²
>>and in the payload include "exclude_before=(t=version, l=2, v=279) &
>I want to highlight this.
>There is a role that selectors can play in a network.  However, our
>biggest issue with selectors is that they are mandated at the forwarder
>level.  This means that every node must support selectors.
>We want to make sure that the core protocol is simple and efficient.
>Exact matching gives us that.  If you¹re interested in selector matching
>and searching, then create that protocol over exact matching.
>Marc just described a simple ³Selector Protocol", basically:
>- Encode selectors (or any query you want) in the interest payload.
>- Add a name segment to indicate that this is a selector based query
>- Add a name segment to uniquely identify the query (a hash of the payload
>for example)
>name    = /mail/inbox/list/selector_matching/<hash of interest payload>
>payload = version > 100
>A ‹‹ B ‹‹ C
>A and C run the Selector Protocol
>B does not run the Selector Protocol
>- Any node that does not understand the Selector Protocol (B) forwards
>normally and does exact matching.
>- Any node that understands the Selector Protocol (C) can parse the
>payload to find a match.
>If no match is found, forward the interest.
>If a match is found, create a reply.
>The reply can contain 2 types of data:
>- Structured data with links to the actual content objects
>- Encapsulated content objects
>So, in our example, the Selector Protocol reply could be:
>name = /mail/inbox/list/selector_matching/<hash of interest payload>
>payload =
>  [  matching name = /mail/inbox/list/v101 ]
>  [  embedded object < name = /mail/inbox/list/v101, payload = list,
>signature = mail server > ]
>signature = responding cache
>A few notes:
>- Malicious nodes could inject false replies.  So, if C is malicious, it
>can insert a reply linking to some random object or just return junk.
>Well, this would be the case with regular selectors as well.  C could
>reply with random crap or it could reply with a valid answer that is not
>the optimal answer (so, for example, not the right-most child or
>This is something that we can¹t prevent.
>In the case of CCN, our fast path does not check signatures, so you
>wouldn¹t be able to check the signature of the reply no matter what.  I¹m
>unsure if NDN is still advocating that every node checks signatures.  If
>you are, then this approach might not work for you.
>Nodes that DO understand the Selector Protocol can check the signature of
>the encapsulated reply (if they wanted to).
>Nodes that DO understand the Selector Protocol can unpack the reply, and
>add the corresponding object to their cache, effectively enabling them to
>answer other Selector Protocol queries.
>- The reply from the Selector Protocol enabled node (C), could:
>‹  include a list of all valid answers
>‹  embed no objects
>‹  embed more than 1 object
>‹  process complex queries, regex, etc.
>The Selector Protocol could also:
>- include a method for authentication
>- include a cursor or some other state between queries
>I think this sort of protocol gives you everything you want while still
>maintaining an exact match protocol as the core protocol.
>What is this protocol missing to satisfy your needs?
>Can we create a protocol that will satisfy your needs on top of exact
>Nacho (Ignacio) Solis
>Protocol Architect
>Principal Scientist
>Palo Alto Research Center (PARC)
>Ignacio.Solis at parc.com

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