[Ndn-interest] HopLimit vs Interest aggregation

[Junxiao Shi]

Dear folks

NDN Packet Format v0.3 introduces a HopLimit element in Interest packet.
The spec says:

If element is present: if the HopLimit value is larger than or equal to 1,
a node should accept the packet and decrease the encoded value by 1.


My question is, how should a node proceed if multiple Interests with same
name but different HopLimit values are aggregated?

Consider the following topology:

A--\
    G---P
B--/

First, G receives an Interest /P HopLimit=10 from A and forwards it to P
with HopLimit=9.
Later, after any suppression period has elapsed but before P replies and
before A's Interest expires, G receives another Interest /P HopLimit=5 from
B.
Should G forward the Interest to P with HopLimit=4, or HopLimit=9?

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.
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.

What do others think?

Yours, Junxiao
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