[Ndn-interest] HopLimit vs Interest aggregation

[David R. Oran]

On 30 Mar 2018, at 12:38, Junxiao Shi wrote:

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

> What do others think?
>
> Yours, Junxiao
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DaveO