[ndnSIM] NDN Routing question

[Lixia Zhang]

> On Jun 3, 2018, at 12:57 AM, John Baugh <jpbaugh at umich.edu> wrote:
> 
> Greetings all,
> 
> 
> I've been studying and working with ICN, NDN, and ndnSIM for quite some time now.  I've read tons of papers on many of the different characteristics and issues within NDN.
> 
> One thing I haven't really seen addressed is the following:  in NDN, a Data packet will take the reverse route that the Interest packet took, correct?
> 
> So, what happens if one of the intermediate routers goes down?  Does the entry in the PIT just time out, and the consumer eventually re-transmits the Interest packet?  Or is there any way to route around the malfunctioning/dead router?

John, there seem two separable questions here:

1/ which entity would retransmit the interest

2/ whether the retransmitted interest would take a different path (that get around the failed node)

for 1/: the early CCN code let routers perform aggressive interest retransmissions -- I vaguely recall that if no data packet comes back in 5 or 8msec, a router would retransmit the interest to another interface (so this answered 2/ as well)

In the current NFD implementation, I believe if a router receives a NACK for an interest, it'd try the interest with a different face; if just a PIT entry timeout, then 
a)the timeout event has an impact on that face's status (that impacts later interest forwarding) 
b) the end consumer needs to retransmit the interest if it still wants the data, the interest is likely going along a different path, I assume, but those more familiar with the code can tell you more.

Lixia