NFD

Environment:

A---B
|   |
C---D

• consumer on A, producer on D
• A-B-D link RTT is 10ms
• A-C-D link RTT is 15ms

Steps to reproduce:

1. send Interests from A to D
2. observe the behavior of NCC strategy

Expected: A-B-D remains the preferred path; A-C-D may be explored but doesn't become the preferred path

Actual: A-C-D becomes the preferred path after it's explored and Data comes back

This bug is caused by:

In ccnd,

1. When first Data comes back that satisfies the PIT entry, the incoming face of that Data is remembered.
2. The PIT entry is deleted as soon as it's satisfied.
3. When second Data comes back, it's discarded as unsolicited.

In NFD,

1. When first Data comes back that satisfies the PIT entry, the incoming face of that Data is remembered.
2. The PIT entry is kept, and will be deleted after straggler timer.
3. When second Data comes back, the strategy is notified, and remembers the incoming face.

NCC strategy incorrectly assumes the incoming Data is from the fastest upstream, causing the bug.