Hi Thomas and Brian,
Thanks for the help. I will discuss the matter with the implementor.
My understanding is that this was a software decision based on there reading of
4443.
Regards,
Tim
On Nov 3, 2010, at 8:34 AM, Brian Haberman wrote:
> Hi Tim,
>
> On 11/3/10 8:00 AM, Thomas Narten wrote:
>> Hi Tim
>>
>>> At the UNH-IOL we recently received a router implementation that
>>> discards a packet when it receives a packet with a hop
>>> limit of zero. Based on the following quote from RFC 2460,
>>> "The packet is discarded if Hop Limit is decremented to
>>> zero." If router is the end-node it should still process
>>> the the packet, as the hop-limit isn't decremented until
>>> the packet is forwarded.
>>
>> That is the intended behavior. You only discard a packet if you
>> decrement the TTL and it reaches zero.
>
> Correct.
>
>>
>>> According to RFC 4443 Section 3.3 "If a router receives a
>>> packet with a Hop Limit of zero, or if a router decrements a
>>> packet's Hop Limit to zero, it MUST discard the packet and originate
>>> an ICMPv6 Time Exceeded message with Code 0 to the source of the
>>> packet." The UNH-IOL had interpreted router to be a device that is
>>> forwarding a packet, therefore the packet should still be processed
>>> when it's the end receiver. The implementation viewed this quote as
>>> stating that a router should discard the packet regardless of being
>>> the end receiver.
>>
>> This section refers to the generation of a Time Exceeded Message. You
>> shouldn't be executing this section of the spec unless you had already
>> decided to generate such a message. (That said, the wording above
>> could be better.)
>
> I agree the wording could be better. I would also note that it is
> possible that this particular router implementation may have a different
> design than other routers. Is it possible that the forwarding logic is
> used generically to move locally destined traffic to the main processor?
> In this case, the forwarding ASIC still thinks it is routing the packet.
>
>>
>>> So I would like to ask the working group should a router always
>>> discard a packet with a hop limit of zero even when it's
>>> the end receiver of the packet?
>>
>> IMO, only if it decrements the TTL and it reaches zero. One only
>> decrements if one is about to forward a packet...
>>
>> Also, RFC 2460 says:
>>
>>> Hop Limit 8-bit unsigned integer. Decremented by 1 by
>>> each node that forwards the packet. The packet
>>> is discarded if Hop Limit is decremented to
>>> zero.
>
> Right. So, is this router under test using a different design that
> doesn't fit the generic router model used to develop this text?
>
> Regards,
> Brian
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