[Ndn-interest] HopLimit vs Interest aggregation

Christos Papadopoulos christos at colostate.edu
Sun Apr 1 07:42:31 PDT 2018

On 04/01/2018 08:19 AM, David R. Oran wrote:
> 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.

In addition, Hop Limit (or the equivalent TTL in IP) has proven useful 
in scoped communication, especially in multicast. I think the wishes of 
the Interest sender should be respected, there may be a reason for the 
selected value of Hop Limit. Moreover, changing the value in the middle 
of the network without notifying the sender makes things very hard to debug.


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