NFD

# Namespace-based Scope Control

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

## localhost Scope

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

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

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

* Data 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.

[[FaceMgmt#Static-Face-Attributes|FaceScope]] attribute indicates whether a face is local or non-local.

Generally, the following faces are local:

* internal face for NFD [[Management]]

* 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 of a non-loopback 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.

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.