NFD

# Namespace-based Scope Control

NFD performs scope control of Interest and Data according to their namespace.

Namespace-based scope control is the only kind of scope control supported by NFD.

[Scope Guider](http://named-data.net/doc/ndn-tlv/interest.html#scope) in NDN-TLV Interest packet is not supported, and is ignored.

## localhost Scope

The localhost scope limits propagation to the applications on the originating host.

It is equivalent to [Scope=1](http://named-data.net/doc/ndn-tlv/interest.html#scope).

Interest and Data packets under prefix `ndn:/localhost` are restricted by these rules:

* Interest namespace can come from and go to local faces only.

* Data namespace can come from and go to local faces only.

### local face

Whether a face is considered "local" for scope control purpose is determined by the face implementation.

`Face::isLocal()` attribute returns true if a face is local for scope control purpose.

Generally, the following faces are local:

* the `InternalFace` (in NFD management module)

* UNIX-domain socket face

* TCP face whose remote address is a loopback address

* WebSocket face whose remote address is a loopback address

A local process may connect as non-local by establishing a TCP or WebSocket connection to the IP address on a physical NIC of the local machine.

Since this IP address is not a loopback address, the face would be non-local.

This is desirable in scenarios such as a protocol translator.

## localhop Scope

The localhop scope limits propagation to no further than the next node.

It is equivalent to [Scope=2](http://named-data.net/doc/ndn-tlv/interest.html#scope).

Interest packets under prefix `ndn:/localhop` are restricted by these rules:

* Interest can come from a local face or a non-local face.

* If PIT entry has at least one InRecord from a local face, it can be forwarded to local faces and non-local faces.

* If PIT entry has all InRecords from non-local faces, it can only be forwarded to local faces.

* PIT entry can be satisfied by Data from any source.

Data packets under prefix `ndn:/localhop` are unrestricted.

The localhop scope allows an Interest packet to travel only one hop from the consumer host, but a Data packet may travel multiple hops from the producer host.

Suppose we have three hosts A-B-C in a linear topology, B can retrieve a Data under /localhop namespace from C, and this Data is cached in B's ContentStore.

Later A sends an Interest to B; although B cannot forward this Interest to C, the cached Data can satisfy the Interest and would be returned to A.