[Ndn-interest] HopLimit vs Interest aggregation

Junxiao Shi shijunxiao at email.arizona.edu
Mon Apr 2 15:07:17 PDT 2018

Dear folks

20180402 NFD call discussed this issue.

> I can see arguments both ways.
> > The argument for choosing HopLimit=4 is: it is possible that the Interest
> > from B is the previous Interest looped back. There could be a path from P
> > to B (not shown in the topology, as G does not know the global topology),
> > and one of the routers on that path has changed the nonce for probing.

This point is invalid. If a router N elsewhere changes the nonce for
probing, the Interest coming to G from B has not completed a full cycle. G
should not treat it as “looping”. If it reaches N again, N could detect a
duplicate nonce.

> > The argument for choosing HopLimit=9 is: when G retries the Interest, it
> > should use the maximum HopLimit among unexpired downstream nodes, to
> > maximize the possibility of reaching the content.

This is consistent with the choice of InterestLifetime: a forwarder should
use an InterestLifetime that reflects the latest expiration time among
downstream’s Interests.

> Safety over optimality. Hop Limit is a protection mechanism. Be
> conservative.
> Always decrement the hop limit when forwarding. NEVER increase it.

G is not “increasing” HopLimit. When “Interest /P HopLimit=5” comes from B,
G could decide to forward “Interest /P HopLimit=10” from A again and thus
use HopLimit=9, as long as that downstream’s Interest is not yet expired.

Yours, Junxiao
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