[Ndn-interest] Loop Detection Issue

Lan Wang (lanwang) lanwang at memphis.edu
Thu Aug 4 07:25:26 PDT 2016


Marc,

> On Aug 3, 2016, at 10:16 AM, Marc.Mosko at parc.com wrote:
> 
> My understanding is that X should send a “duplicate NACK” to Z.  As long as the NACK is not lost, C2 will get the NACK and re-try its Interest with a difference nonce (if using BestRoute v2 strategy).  If the NACK is lost, then you still have the described problematic behavior.  Note that the BestRoute v2 strategy will not re-transmit the C2 Interest at Z with a different nonce, it will propagate the NACK to C2.

The NACK will not be propagated from Z to C2 because the NACKed nonce is difference from the nonce in C2’s interest.
> 
> I also think that the duplicate NACK would be propagated along the whole reverse route to Y and then back to X, if using BestRoute v2.  

This is correct.
> 
> Another thing to consider is that NFD uses an aggregation hold-down timer.  The Interest from C2 would only be aggregated at Z if it arrived within 250 msec of the Interest from Y.  If it arrived after that, it would not be aggregated. This is strategy dependent.  I think there’s also current work on using an exponential back off rather than a fixed 250 sec.

Yes. 
> 
> Yet another another thing to consider is that there are two ways X can detect a duplicate nonce.  First is what we’ve talked about, it has a pending PIT entry.  The second case is if the duplicate Interest from Z arrives after the content from S has cleared the PIT entry at X, but there is still a “dead nonce list” entry at X.  If you read section 4.2.1 (Incoming Interest Pipeline) of version 6 (I believe current) NFD guide, step 2 is to check the Dead Nonce List.

Yes.

>  In this case, even though X may have a content store entry, the Interest from Z is given to the “Interest loop pipeline”.  It explicitly does not create an in-record for the Interest from Z.  Section 4.2.2 (Interest loop pipeline) will generate a NACK and drop the Interest.  At this point Z would forward the NACK to C2 (via BestRoute) and C2 would re-try.
> 
I don’t think the NACK will be forwarded to C2.

> Finally, one should also note that a duplicate NACK is only generated on a point-to-point face.  If the incoming face is multiple access, no NACK is generated.  C2 would need to timeout in that case.

Right now NACK is only generated on a point-to-point face in NFD, because we don’t have a final NACK design for multi-access links yet.  But NACK is still applicable to multi-access links in general.

Lan
> 
> Marc
> 
> 
>> On Aug 3, 2016, at 6:43 AM, marxer at claudio.li wrote:
>> 
>> 
>> On 08/03/16 14:28, Lan Wang (lanwang) wrote:
>>> They have different nonces but if the PIT entry created by C1 has not been removed, then C2 may be suppressed.   This is the dangling PIT entries problem.The solutions (already implemented in NFD) are the following:
>> 
>> Yes, this is what we meant in the example.
>> 
>>> 1. some strategies, e.g., recent versions of BestRoute) allow interest retransmission: when an interest is forwarded, a suppression timer is set on the PIT.  When the timer expires, it  may forward any new Interest matching the PIT (and then set the suppression timer again).
>>> 2. when X detects an Interest with a duplicate nonce, it sends a duplicate NACK  which may be further forwarded to clear the dangling PIT entries at the nodes that sent the interest.  When the PIT entry is cleared, new interest with the same name can be forwarded.  Right now some strategies (not all) in NFD handle the NACKs correctly.
>> 
>> Yes, this works. A question regarding the implementation: According to the NFD developer guide, the PIT does not hold nonce values. Thus Z can not fully reconstruct the interest from C2. Does Z generate a new nonce? If yes: Not sure but intuitively I think that loop partially detection might be broken if the green interest was also part of a loop (because Z changes its nonce).
>> 
>>> Lan
>> 
>> Thanks to all of you for the explanations,
>> Claudio
>> 
>>>> On Aug 3, 2016, at 4:54 AM, Lixia Zhang <lixia at cs.ucla.edu> wrote:
>>>> I have a few other questions to the specifics of the scenario.
>>>> for a short answer quick: interests form C1 and C2 carry different nonces, so your concern on the last slide should not happen as I see.
>>>>> On Aug 3, 2016, at 1:15 AM, marxer at claudio.li wrote:
>>>>> Hi all,
>>>>> Urs and I constructed a situation in which we think the network does not behave as expected (attached PDF). Can this happen in the wild or did we oversee something?
>>>>> Thanks,
>>>>> Urs and Claudio
>>>>> <interest-looping.pdf>_______________________________________________
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